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Robust, Ultrathin, and Highly Sensitive Reduced Graphene Oxide/Silk Fibroin Wearable Sensors Responded to Temperature and Humidity for Physiological Detection
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  • Hyeonho Cho
    Hyeonho Cho
    School of Mechanical Engineering, Chung-Ang University, Dongjak-gu, Seoul 06974, Korea
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  • Chanui Lee
    Chanui Lee
    School of Mechanical Engineering, Chung-Ang University, Dongjak-gu, Seoul 06974, Korea
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  • ChaBum Lee
    ChaBum Lee
    J. Mike Walker ’66 Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843-3123, United States
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  • Sangmin Lee
    Sangmin Lee
    School of Mechanical Engineering, Chung-Ang University, Dongjak-gu, Seoul 06974, Korea
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    Orcidhttps://orcid.org/0000-0002-6713-4797
  • Sunghan Kim*
    Sunghan Kim
    School of Mechanical Engineering, Chung-Ang University, Dongjak-gu, Seoul 06974, Korea
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0003-0597-5512
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Biomacromolecules

Cite this: Biomacromolecules 2023, 24, 6, 2606–2617
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https://doi.org/10.1021/acs.biomac.3c00106
Published April 19, 2023

Copyright © 2023 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Skin temperature and skin humidity are used for monitoring physiological processes, such as respiration. Despite advances in wearable temperature and humidity sensors, the fabrication of a durable and sensitive sensor for practical uses continues to pose a challenge. Here, we developed a durable, sensitive, and wearable temperature and humidity sensor. A reduced graphene oxide (rGO)/silk fibroin (SF) sensor was fabricated by employing a layer-by-layer technique and thermal reduction treatment. Compared with rGO, the elastic bending modulus of rGO/SF could be increased by up to 232%. Furthermore, an evaluation of the performance of an rGO/SF sensor showed that it had outstanding robustness: it could withstand repeatedly applied temperature and humidity loads and repeated bending. The developed rGO/SF sensor is promising for practical applications in healthcare and biomedical monitoring.

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1. Introduction

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The skin’s temperature and humidity are representative parameters used for monitoring physiological processes and conditions such as respiration, fever, and perspiration. (1−3) While real-time continuous monitoring of physiological signals is important for doctors to comprehensively understand a patient’s condition, traditional methods for detecting the body’s condition, such as the use of thermometers, cannot provide continuous information. For real-time detection of physiological signals, a wearable sensor can be used, and many wearable temperature and humidity sensors have been developed. (4−7) In particular, wearable sensors based on biomaterials, such as mycelium and cellulose, have been developed to detect various environmental conditions such as humidity. (8−10)
Reduced graphene oxide (rGO) is one of the materials favored for use as a temperature and humidity sensor, owing to its rich oxygen-containing functional groups, high electrical conductivity, and good thermal properties. (11−14) The oxygen-containing functional groups, such as hydroxyl, epoxy, and carboxyl groups, are responsible for its hydrophilic nature. (15,16) Notably, rGO-based materials can be used as temperature sensors in a wide temperature range owing to their high electrical conductivity and good thermal properties. (17,18) However, the development of an rGO-based sensor with superior mechanical properties, a facile fabrication process, and high sensitivity poses a challenge.
An rGO film is easily delaminated because of its low interfacial strength and brittle nature, and hence, it is not suitable for practical use. (19,20) Recently, silk proteins, which are biomaterials, have been found to have the potential to be used in carbon-based sensors for physiological monitoring. (21−23) In particular, the addition of silk fibroin (SF), which is a silk protein and which can interact strongly with rGO flakes through hydrogen bonding, polar–polar bonding, and hydrophobic–hydrophobic bonding, can prevent delamination and enhance the flexibility of rGO films. (24) An rGO/SF composite with a nacre-like structure can be prepared by a layer-by-layer (LbL) self-assembly method. (25,26) This method is facile and can be used to obtain composites with mechanical properties superior to those of rGO. Nacre is 1000 times tougher than its constituents because of strong interfacial interaction between its constituents, which results from its brick-and-mortar structure. (27−30) Because of its nacre-like structure, a GO–SF composite prepared by an LbL method has higher strength and toughness than one prepared using a simple mixing and casting method. (24,31,32) Xu et al. prepared a GO–SF composite with high mechanical properties (elastic modulus: 145 GPa; tensile strength: 300 MPa) by using a spin-assisted LbL method. (16) By contrast, a GO-SF composite prepared by Huang et al. by simply mixing a GO solution and an SF solution and casting the solution at room temperature had a tensile modulus of 17.2 GPa and a tensile strength of 221 MPa. (27) These results showed that LbL fabrication methods can enhance the overall mechanical properties of GO/SF and rGO/SF materials. In particular, the mechanical properties of rGO can be adjusted by controlling the defect content. (33,34) Several methods, such as dip, spin, spray, electromagnetic, and fluidic coating, have been used to fabricate LbL structures. (35,36) The LbL dip-coating technique for fabricating a nacre-like structure with nanoscale thickness is facile and cost-effective. To the best of our knowledge, researchers have elucidated the sensing mechanism of rGO-based temperature sensors: an increase in the temperature leads to increased charge transport because of enhanced hopping and tunneling effect. (37−39) Furthermore, studies have shown that the rGO structure exhibits negative thermal expansion (NTE). (40) The rGO network connection changes with the temperature because of this NTE, resulting in changes in electrical parameters such as resistance and current of the rGO structure. However, there are few studies on the effect of physical changes in the rGO structure on the electrical signals.
Although GO-based humidity sensors show good performance, rGO-based humidity sensors are more advantageous. (41,42) Owing to its hydrophilic properties resulting from the abundance of oxygen-containing functional groups, GO swells or shrinks when the humidity changes. However, because GO has low electrical conductance, measuring the electrical signal of a GO humidity sensor is difficult. Hence, GO should be combined with other conductive materials for realizing a humidity sensor with high sensitivity. (43,44) On the other hand, owing to its high electrical conductance and hydrophilic surface functional groups, rGO has the potential to be used as a temperature and humidity sensor. To use GO as a low-electrical-conductance humidity sensor, we should use the capacitance change rather than the resistance change to detect humidity changes. However, a capacitive-signal-based humidity sensor has a complex structure, unlike a resistive-signal-based humidity sensor, which has a single conductive layer as the sensing layer. (41,45,46) Furthermore, since the number of functional groups in rGO is tailorable, the performance of an rGO sensor can be adjusted by varying the number of functional groups, which would influence the interaction of the sensor with moisture in the air. (47,48) Since SF has low electrical conductance, the advantage of rGO/SF-based sensors over GO/SF-based sensors can be attributed to the rGO/SF composite’s significantly improved electrical conductance owing to rGO.
Here, we developed an rGO/SF multifunctional sensor with superior mechanical properties by using a facile LbL dip-coating technique and thermal reduction treatment. The mechanical properties of the rGO/SF sensor were determined by conducting the atomic force microscope (AFM) bending test. The rGO/SF sensor was found to have superior mechanical properties because of the strong hierarchical interaction between rGO and SF. The elastic bending modulus of an rGO/SF film could be enhanced by up to 232% compared with that of an rGO film. Furthermore, the mechanical properties of rGO/SF films could be tailored by adjusting the reduction temperature since (1) the number of defects in the rGO lattice structure varies with the temperature and (2) SF can be recrystallized. The sensing mechanism of the rGO/SF sensor was that its structure varied with the amount of moisture adsorbed and temperature. When the temperature and humidity of the rGO/SF sensor’s environment changed, the interaction between the adsorbed moisture and rGO flakes altered the rGO networking in the sensor, which caused the electrical conductance of the sensor to change. The sensitivity of the rGO/SF sensor could be tailored by varying the number of hydrophilic functional groups in the rGO flakes, which caused the degree of moisture–rGO flake interaction to change. The sensitivity of the humidity and temperature sensor decreased with an increase in the reduction temperature. The sensitivity changes of the sensors are attributed to changes in its structure, which can be monitored by temperature coefficient of resistance (TCR). The TCR of the rGO/SF sensor was adjusted to 2.20, 1.87, 1.73, 1.66, 1.50, and 1.39% K–1 for reduction temperatures of 155, 170, 185, 200, 215, and 230 °C, respectively. To evaluate the stability and durability of the temperature and humidity sensor, we performed a cyclic test for 1000 cycles. The rGO/SF sensor showed excellent stability and durability. Although its performance in terms of parameters such as the TCR slightly deteriorated, its outstanding durability resulted in high robustness to repeated temperature and humidity loads and repeated bending, without any degradation of the sensing performance. Since the developed rGO/SF sensor can be used for monitoring physiological parameters and processes such as respiration and skin temperature, it has the potential to be used in practical applications in the fields of healthcare and biomedical monitoring.

2. Experimental Details

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2.1. Material Preparation

2.1.1. Preparation of GO and SF

GO was prepared by the well-established Hummer’s method. (49) Briefly, graphite power (0.8 g) was oxidized and then exfoliated using sodium nitrate (0.4 g), sulfuric acid (18.5 mL), and potassium permanganate (2.4 g) for 3 h in an ice bath. The oxidized and exfoliated graphite solution was then placed in a water bath and stirred for 1 h. Deionized (DI) water (37.5 mL) was added dropwise very slowly to the solution, and the solution was boiled in the water bath that was maintained at approximately 90 °C for 2 h. The solution was then cooled in air, and DI water (112.5 mL) was added to it under stirring for 1 h. Next, hydrogen peroxide (30%, 12.5 mL) was added very slowly to the solution, and the solution was then sealed with aluminum foil and stirred for 1 h. The resulting solution was precipitated for more than 6 h. The supernatant of the solution was replaced with new DI water, and the solution was precipitated again. The replacement with DI water and precipitation were repeated twice, and the solution was subsequently filtered by using a vacuum filtration system and 1 μm filter paper. Finally, GO was obtained by drying the filter paper at 60 °C.
Aqueous SF solution was prepared from Bombyx mori cocoons. (50) Briefly, silk cocoons (5 g) were degummed by immersing them in boiling sodium carbonate solution (0.02 M) for 30 min. They were then squeezed and washed with DI water. The degumming and washing processes were repeated thrice. Subsequently, the silk cocoons were dried in a fume hood for 12 h and then dissolved in lithium bromide (9.3 M) at 60 °C in an oven for 4 h. The silk solution was repeatedly dialyzed by using a hydrated dialysis cassette (3500 MW cutoff), and it was then repeatedly refined through centrifugation (9000 rpm, 20 min). The SF solution that was finally obtained was stored in a refrigerator at 4 °C.

2.1.2. Preparation of the Temperature Sensor (rGO and rGO/SF)

An rGO-based temperature sensor was prepared as follows (Scheme 1). An office paper was cut to have dimensions of 1 cm × 3 cm. GO paper was prepared by LbL dip-coating the paper with GO aqueous suspension (0.3 wt %), and GO/SF paper was prepared by LbL dip-coating an office paper alternately with GO aqueous suspension (0.3 wt %) and SF solution (0.1 wt %). Next, the two papers (GO and GO/SF papers) were washed by immersing each of them in DI water to remove the excess GO coating, and they were dried at 60 °C in an oven for 30 min. The LbL dip-coating process was repeated until the desired number of bilayers was formed (generally three bilayers). Thereafter, the GO and GO/SF papers were thermally reduced at 155, 170, 185, 200, 215, and 230 °C in an oven for 3 h. For the measurement of electrical signals of the thermal reduced GO and GO/SF (hereafter referred to as rGO and rGO/SF) sensors, electrical wires were attached to the ends of the rGO and rGO/SF sensors using a silver paste adhesive (P-100, CANS). The distance between electrodes (i.e., the wires at the two ends of the rGO and rGO/SF sensors) was approximately 2.3 cm.

Figure 1

Figure 1. (a) FE-SEM images showing morphologies of rGO and rGO/SF films. The carbon to oxygen (C/O) ratio of (b) rGO and (c) rGO/SF films determined using the EDS. All scale bars are 40 μm.

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2.2. Method

2.2.1. Characterization of rGO and rGO/SF Films

The morphology of rGO and rGO/SF films was observed using AFM. The soft tapping mode was used to obtain morphological images with a NANOSENSORS tip (PPP-NCHR), and the scan rate was 0.5 Hz. According to manufacturer specifications, the tip had a force constant of 42 N/m and a radius below 10 nm. The chemical analysis of the rGO and rGO/SF films was conducted using field emission scanning electron microscopy (FE-SEM; SIGMA, Carl Zeiss) and energy-dispersive X-ray spectrometry (EDS) at an accelerating voltage of 15 kV. The Fourier transform infrared (FTIR) spectra of the rGO/SF films obtained at reduction temperatures of 155, 200, and 230 °C was recorded using an FTIR spectrometer (Nicolet 6700, Thermo Scientific).

2.2.2. Characterization of Sensors

The performance of the rGO and rGO/SF sensors as temperature sensors and humidity sensors was evaluated. The performance of a temperature sensor was evaluated by placing it on a hot plate and measuring the sensor’s temperature (Figure S1a) with a thermometer (FLUKE 52 II, FLUKE) at different hot plate temperatures. The temperature range was 30–100 °C. The performance of a humidity sensor was evaluated by using saturated salt solutions, namely, LiCl, MgCl2, NaCl, and K2SO4, that could control the relative humidity at 11, 33, 75, and 97%, respectively (Figure S1b). The sensor was placed in a closed humidity-controlled vessel containing the desired saturated salt solution, and a hygrometer (DHT22, Arduino) was used to measure the relative humidity. While measuring the temperature and relative humidity of a sensor’s environment, a source meter (Keithley 2450, Keithley Instruments) was used to measure the electrical signals of the sensors.

2.2.3. AFM Bending Test

The AFM bending test was performed to determine the mechanical properties of the rGO and rGO/SF films; AFM (NX-10, Park Systems) was used in the test. The rGO and rGO/SF films were prepared on a silicon (Si) wafer (100 mm diameter) by using an LbL dip-coating method (Scheme 1). The wafer was spin-coated to form a sacrificial layer of cellulose acetate (CA). Before the AFM bending test, the rGO and rGO/SF films were transferred on a copper mesh grid after removing the CA layer with acetone solution. During the AFM bending test, an AFM cantilever (HQ:XSC11, MikroMasch) with a spring constant of 0.2 N/m was used, and the z-scanner speed was 0.1 μm/s. The AFM bending test specimen was prepared by using an LbL dip-coating method similar to that used for the sensor.

Scheme 1

Scheme 1. Schematics of the Fabrication Process of (a) rGO Paper and (b) rGO/SF Paper
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3. Results and Discussion

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3.1. Fabrication of rGO and rGO/SF Films

The rGO and rGO/SF films were prepared using an LbL dip-coating method and thermal reduction treatment (Scheme 1), and their morphologies were observed using FE-SEM (Figure 1a). For all reduction temperatures, the rGO and rGO/SF films were well coated on papers. EDS analysis was performed to determine the effect of the reduction temperature on the chemical properties of the films. For both films, the carbon to oxygen (C/O) ratio increased with the reduction temperature (Figure 1b,c). This increase in the C/O ratio was attributed to a decrease in the number of oxygen groups of GO with an increase in the reduction temperature. (51) The tailorable C/O ratio allowed for the interaction of the rGO and rGO/SF films with various materials (such as moisture) to be engineered. An increase in the C/O ratio of the films increased their electrical conductivity (Figure S2). Representative EDS results for rGO and rGO/SF films for a reduction temperature of 230 °C are shown in Figure S3. Since the thin films were on a Si wafer, the intensity of the Si peak in the EDS results was large. However, despite the large intensity of the Si peak, the carbon and oxygen peaks could be clearly discerned.

3.2. Mechanical Properties of rGO and rGO/SF Films

The overall mechanical properties of LbL films were influenced by the interfacial interaction between layers. (52−55) For the accurate determination of the mechanical properties of rGO and rGO/SF films with abundant interfacial interactions between layers, the films were prepared using a spin-assisted LbL technique. (52,56) The mechanical properties were investigated using the AFM bending test. (57) The rGO and rGO/SF films were transferred on an aperture grid to perform the test. Before the test, the surface of the film was scanned using the AFM to determine the aperture (Figure S4). The location of the aperture grid supporting the films was determined from a force image and a morphological image of the films.
The AFM bending test was conducted at the center of a hole, which was identified from an AFM image. The AFM tip approached the film at the center of the hole and indented the film. While the AFM tip caused the bending of the film, the AFM cantilever was also bent by the reaction force associated with the bending of the film. Considering the deflection of the AFM cantilever (δ), we expressed the deflection of the film (w) as (57)
w=Δz−δ
(1)
where Δz is the distance of the z-scanner between the instants when the AFM tip came in contact with the film (with no deflection of the AFM cantilever) and when the film was bent by the AFM tip. Because the deflection of the AFM cantilever (δ) and the distance of the z-scanner (Δz) were easily obtained from AFM system, the deflection of the film (w) could be calculated from eq 1. Figure S5 shows the force–displacement (Δz) curves (FD curves) for an rGO film and an rGO/SF film obtained from AFM bending tests. The continuous FD curves show the absence of delamination and damage during the AFM bending test. Since the rGO and rGO/SF films were immersed in acetone solution during their transfer from an Si wafer to a copper mesh grid, a residual stress could have been generated after the evaporation of the acetone solution. The absence of residual stress was shown by applying a force in the opposite direction (Figure S6c; retracting direction) to the rGO and rGO/SF films during the AFM bending test; the load–deflection curves in cases of approaching and retracting were almost identical for the opposite force in the AFM bending test (Figure S6).
From AFM bending tests, load–deflection plots were obtained for the rGO and rGO/SF films for reduction temperatures of 155, 200, and 230 °C (Figure 2a). For both types of films, the slope of the load–deflection curves decreased with an increase in the reduction temperature. The thicknesses of the rGO and rGO/SF films used in the AFM bending tests were obtained to determine the mechanical properties of the films (Figure 2b). The thicknesses of the rGO film for reduction temperatures of 155, 200, and 230 °C were 33.8, 32.0, and 32.1 nm, respectively, and the thicknesses of the rGO/SF film for these reduction temperatures were 33.3, 33.2, and 31.7 nm, respectively. The thicknesses of all films were very similar. The root mean square (RMS) roughness of the rGO and rGO/SF films for the reduction temperature of 200 °C was obtained as 2.52 and 3.95 nm, respectively. These values indicated that the surface of the films used in the AFM bending test was sufficiently flat (Figures 2c,d and S7). (57,58)

Figure 2

Figure 2. AFM load–deflection plots of (a) rGO and rGO/SF sensors. (b) Thickness of different types of bilayer films. Morphological AFM images of (c) rGO and (d) rGO/SF sensors for a reduction temperature of 200 °C.

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The reciprocal of the slope of the load–defection plots was defined as the effective bending stiffness. (59) The effective bending stiffness values of the rGO film were 0.43, 0.71, and 1.04 N/m for reduction temperatures of 155, 200, and 230 °C, respectively (Figure 3a), and the parameter’s values for the rGO/SF film were 0.83, 1.61, and 1.81 N/m for these reduction temperatures, respectively. The effective bending stiffness of both types of films increased with the reduction temperature. In particular, for a given reduction temperature, the effective bending stiffness of the rGO film was less than that of the rGO/SF film. Since the thicknesses of the rGO and rGO/SF films were very similar, it was apparent that compared with the rGO film, the rGO/SF film showed enhanced mechanical properties.

Figure 3

Figure 3. Calculated values of the (a) effective bending stiffness, (b) elastic bending modulus, (c) flexural rigidity, and (d) normalized flexural rigidity of rGO and rGO/SF sensors for reduction temperatures of 155, 200, and 230 °C.

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The elastic bending modulus of the rGO and rGO/SF films for reduction temperatures of 155, 200, and 230 °C was calculated from the following expression: (57)
Ebending=12(1−v2)t32r2ln(r/a)+(a2−r2)16πPw
(2)
where t is the film thickness, v is Poisson’s ratio (assumed as 0.3), r is the distance between the loading point in the AFM bending test and the center of the aperture grid, P is the force applied to the film, w is the deflection of the film, and a is the aperture of the grid. Because the AFM bending test was conducted at the center of the aperture grid, r was set to zero. The thickness (t) obtained from Figure 2b was used to calculate the elastic bending modulus. From eq 2, the elastic bending modulus of the rGO film for reduction temperatures of 155, 200, and 230 °C was calculated to be 21.9 ± 7.3, 25.8 ± 8.6, and 61.1 ± 32.8 GPa, respectively, and that of the rGO/SF film for these reduction temperatures was 43.7 ± 26.0, 85.8 ± 32.7, and 110.9 ± 49.0 GPa, respectively (Figure 3b). The elastic bending modulus of both types of films increased with the reduction temperature. Notably, compared with the rGO film, the rGO/SF film’s elastic bending modulus was enhanced by 99.5, 232.6, and 81.5% for reduction temperatures of 155, 200, and 230 °C, respectively.
Flexural rigidity and normalized flexural rigidity are important mechanical properties that provide useful information in the design of wearable devices. Flexural rigidity can be calculated as (57)
D=P16πw(2r2ln⁡ra+a2−r2)
(3)
where P/w is the effective bending stiffness. The calculated flexural rigidity of the rGO film for reduction temperatures of 155, 200, and 230 °C was 0.08, 0.13, and 0.19 μN·μm, respectively, and for the rGO/SF film, the values were 0.15, 0.29, and 0.32 μN·μm at these reduction temperatures, respectively (Figure 3c). The normalized flexural rigidity (D/t3) of the rGO film was calculated to be to 2.00, 3.89, and 5.60 GPa for reduction temperatures of 155, 200, and 230 °C, respectively, and for the rGO/SF film, the values at these reduction temperatures were 4.00, 7.86, and 10.15 GPa, respectively (Figure 3d). The enhanced flexural mechanical properties of the rGO/SF film indicate that the film will show more robust performance than the rGO film when used as a sensor.
The enhanced mechanical properties of the rGO/SF film were attributed to its nacre-like structure (Figure 4). In particular, the rGO and SF layers can interact strongly through polar–polar, hydrophobic, and hydrogen bonding interactions. (58,60) The mechanical properties of an rGO film can be enhanced by adding SF layers because of the interaction between the film and SF layers. (57) The enhanced mechanical properties at a high reduction temperature was attributed to the reduction in the number of defects in the lattice structure of an rGO flake. (61,62) Because the number of defects in the lattice structure of an rGO flake decreased with an increase in the reduction temperature, the mechanical properties of rGO/SF films were enhanced to a greater extent for a higher reduction temperature. (63,64)

Figure 4

Figure 4. Schematic of the process by which a nacre-like rGO/SF structure is formed from a GO/SF structure.

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The secondary structure of SF was changed by the thermal reduction treatment, and this led to a change in the mechanical properties of the rGO/SF film. (65,66) The amorphous region of SF can be transformed into a crystalline region (β-sheet) through high temperature reduction treatment. Since the β-sheet can strongly interact with a hydrophobic part of rGO, the β-sheet formed at the interface between SF and rGO was strongly bound to the adjacent rGO layers (Figure 4). (60) The strong binding between the β-sheet and adjacent rGO layers increased the interfacial shear strength and thereby enhanced the mechanical properties of the rGO/SF film. (60) The increased number of β-sheet could be monitored by using FTIR spectra of rGO/SF films obtained at reduction temperatures of 155, 200, and 230 °C (Figure S8). Owing to its enhanced mechanical properties and the change in its secondary structure after thermal reduction treatment, the rGO/SF film can provide robust performance when used as a sensor.
An rGO/SF film with a structure that was not nacre-like was prepared to compare it with an rGO/SF film with a nacre-like structure. The former rGO/SF film was fabricated by mixing rGO and SF solutions and dip-coating the resulting solution on a Si wafer. The morphology of rGO/SF films with and without a nacre-like structure was investigated by using a scanning AFM tip (Figure S9a,b). The surface of the rGO/SF film whose structure was not nacre-like was rough. On the other hand, the surface of the rGO/SF film with a nacre-like structure was smooth, which implied alternately stacked rGO and SF layers; both the rGO and SF films had a smooth and uniform surface (Figure S9c,d). A nacre-like structure can transfer stress from one location to the surrounding regions and thereby dissipate stress. (67) Accordingly, the mechanical properties of an rGO/SF film with a nacre-like structure can be enhanced, unlike an rGO/SF film with a structure that is not nacre-like.

3.2.1. Performance of rGO and rGO/SF Sensors

We evaluated the rGO and rGO/SF temperature-humidity sensors by heating them and measuring their electrical signals and temperature with a source meter and thermometer, respectively (Figure S1a). The normalized resistance (ΔR/R0) of both sensors decreased with an increase in their temperature (Figure 5a). Furthermore, the normalized resistance of both sensors was more strongly affected by the temperature for a lower reduction temperature. Thus, both rGO and rGO/SF films have the potential to be used as temperature sensors with tunable performance.

Figure 5

Figure 5. (a) Variation of the responsivity of rGO and rGO/SF temperature sensors (prepared at different reduction temperatures) in terms of resistance change, ΔR/R0, as a function of the temperature. (b) Variation of the TCR at 300 K with the reduction temperature for the rGO and rGO/SF sensors.

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The relationship between the resistance and the temperature of a sensor is expressed by the following Arrhenius-type equation: (68)
R=R0exp(−EakT)
(4)
where R is the resistance of the sensor, R0 is initial resistance of the sensor, T is the temperature of the sensor, Ea is the activation energy, and k is the Boltzmann constant. All the rGO and rGO/SF sensors, irrespective of the reduction temperature, were well fitted by eq 4 (Figures S10 and S11). The TCR of the rGO and rGO/SF sensors was calculated from the equation (68)
TCR=dRdT1R
(5)
From eqs 4 and 5, the TCR of the rGO sensor was obtained to be 2.77, 2.30, 2.05, 1.77, 1.75, and 1.54%/K for reduction temperatures of 155, 170, 185, 200, 215, and 230 °C, respectively, and the TCR of the rGO/SF sensor was calculated to be 2.20, 1.87, 1.73, 1.66, 1.50, and 1.39%/K for the same reduction temperatures, respectively. The TCR of the rGO and rGO/SF sensors could be tailored by varying the reduction temperature. The TCR of the rGO and rGO/SF sensors obtained in the present study was higher than the values reported in previous studies (Table S1).
The TCR of the rGO sensor was reduced by 20.6, 18.7, 15.6, 7.9, 14.3, and 9.7% for reduction temperatures of 155, 170, 185, 200, 215, and 230 °C when SF layers were added (i.e., for an rGO/SF sensor), respectively. Although the rGO/SF film had a slightly lower TCR than the rGO film, the TCR of the rGO/SF film resulted in the film showing considerably higher performance as a temperature and humidity sensor (Table S1). The reason for the slightly lower TCR, which is related to the sensor’s working mechanism, is discussed later.
The performance of the rGO and rGO/SF sensors as humidity sensors was evaluated by controlling the humidity by using saturated salt solutions (Figure S1b). Figure 6a shows that the resistance of the rGO and rGO/SF films increased with the relative humidity (11, 33, 75, and 97%). This increase in the resistance indicated the high potential of the films for use as humidity sensors. Furthermore, the slopes in Figure 6a decrease with an increase in the reduction temperature. Since the slopes reflect the sensitivity of the humidity sensor, the sensitivity of the rGO and rGO/SF films can be tailored through an appropriate choice of the reduction temperature (Figure 6b,c). The performance of our humidity sensors was comparable to that of previously reported sensors (Table S2).

Figure 6

Figure 6. (a) Responsivity of rGO and rGO/SF humidity sensors (fabricated at different reduction temperatures) in terms of resistance change as a function of humidity change, and the gradient of a linear fit to the resistance-RH plot for (b) rGO sensor and (c) rGO/SF sensor.

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For real-time temperature and humidity detection, the response and recovery time are crucial factors. The temperature response and recovery time were measured by moving the rGO/SF sensor between hot plates heated to 25 and 27 °C. The humidity response and recovery time were measured by moving the sensor away from and toward a water-filled container. As shown in Figure S12, the response and recovery time for the temperature change were 5.5 and 5.1 s, respectively, and those for the humidity change were 40 and 45 s, respectively. These values are comparable to previously obtained values presented in Table S2.
The mechanisms of the rGO and rGO/SF films were related to the NTE of rGO and SF (Figure 7a). (40,69,70) In the rGO/SF film, moisture was present between the laminated layers owing to the abundance of hydrophilic surface groups that interacted with water molecules. When the temperature of the sensors increased, water molecules in the laminated layers were desorbed because of debonding between the water molecules and the hydrophilic surface groups of the laminated structure. Moreover, thermally activated hydrogen bonding in SF caused the contraction of β-sheet crystallites, which decreased the thickness of the structure. (69,70) The desorption of water molecules reduced the interval between the laminated layers and thereby enhanced electrical contact between rGO flakes. On the other hand, when the temperature of the sensors decreased, the adsorption of water on the laminated layers led to reduced electrical contact between rGO flakes. Thus, the desorption and adsorption of water induced by changes in the temperature altered the electrical signals of the rGO/SF sensors.

Figure 7

Figure 7. Schematics of (a) mechanism of temperature and humidity sensors and (b) tailorable sensitivity.

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The mechanism of humidity sensors based on the rGO and rGO/SF films was similar to that of the temperature sensors. When the relative humidity decreased or increased, water was desorbed from or adsorbed on rGO and SF, which caused the expansion or contraction of the sensor’s structure. (71,72) The swelling of the structure altered the interval between the laminated layers and changed the degree of electrical contact between rGO flakes, which in turn resulted in a change in the electrical signals of the sensors. Thus, the swelling of the sensor structure because of an increase in the relative humidity altered the electrical signals of the rGO and rGO/SF sensors.
The possibility of tailoring the sensitivity of the temperature and humidity sensors by varying the reduction temperature can be explained on the basis of the change in the number of hydrophilic surface functional groups of rGO flakes with the reduction temperature (Figure 7b). For a higher reduction temperature, the deoxidation of rGO flakes was stronger, which reduced the number of hydrophilic groups (Figure 1b,c). This reduction in the number of hydrophilic groups in rGO flakes resulted in a smaller number of water molecules interacting with rGO flakes. Furthermore, the densely formed SF on rGO during the reduction treatment interacted strongly with rGO through hydrogen bonding, which inhibited the interaction between water molecules and rGO. (54,61,62) Because the change in the interval between the laminated layers of the rGO/SF film for a high reduction temperature was less than that for a lower reduction temperature, the electrical signals of sensors were less sensitive to the environment’s temperature and humidity. Accordingly, the sensitivity of the sensors decreased with an increase in the reduction temperature.
The TCR of the rGO sensor decreased when SF was added (i.e., the rGO/SF sensor) (Figure 5b). The hydrophilic groups in rGO flakes bonded with SF through hydrogen bonding, which increased the interfacial shear strength of the sensor. (54,56) In this case, bonding of water molecules with the available hydrophilic groups in rGO flakes could significantly alter the electrical signal of the sensor. Because the hydrophilic groups that had already bonded with SF could not interact with water molecules, the TCR of the rGO/SF sensor was slightly lower than that of the rGO sensor.
The variation of the thickness of the rGO and rGO/SF films with the temperature and relative humidity was investigated using AFM. Dip-coated rGO and rGO/SF films on CA layers were transferred to Si wafers by using sacrificial layers (Section 2.2.2). While the rGO and rGO/SF films were heated by using a heating stage, their edges were scanned with the AFM to measure their thickness (Figure S13). Figure S13 shows representative AFM images of a film (rGO film for a reduction temperature of 155 °C) for different heating stage temperatures.
Figure 8a shows that the thickness of the rGO and rGO/SF films decreased with an increase in the temperature. The temperature dependence of the thickness resulted from the NTE of the rGO and rGO/SF films and the interaction between water molecules and the laminated structure. The change in the thickness for different reduction temperature is shown in Figure 8b. The change in the thickness of the rGO film for reduction temperatures of 155, 200, and 230 °C was calculated to be −0.050, −0.032, and – 0.023 nm/K, respectively, and the change in the thickness of the rGO/SF film for these reduction temperatures was calculated to be −0.042, −0.027, and −0.015 nm/K, respectively. The change in the thickness decreased with an increase in the reduction temperature. This decrease resulted from the decrease in the number of hydrophilic functional groups. The reduced number of hydrophilic groups led to reduced interaction between water molecules and the laminated structure, thereby reducing the amount of thickness change. The change in the thickness of the rGO/SF film was smaller than that in the case of the rGO film since the interaction between rGO and water molecules in the former film was hindered by the hydrogen bonding between rGO and SF.

Figure 8

Figure 8. (a) Thickness of rGO and rGO/SF films at temperatures of 30, 40, 50, 60, 70, 75, and 80 °C for reduction temperatures of 155, 200, and 230 °C. (b) Change in the thickness of the two films for reduction temperatures of 155, 200, and 230 °C. (c) Coefficient of thermal expansion of the two films for reduction temperatures of 155, 200, and 230 °C.

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The coefficient of thermal expansion (α) can be expressed as (73)
α=ΔL/L0L0ΔT
(6)
where ΔL is the change in thickness, L0 is the initial thickness, and ΔT is the change in temperature. The coefficient of thermal expansion of the rGO film for reduction temperatures of 155, 200, and 230 °C was calculated to be −1.03 × 10–3, −0.73 × 10–3, and −0.60 × 10–3/K (Figure 8c), respectively, and that of the rGO/SF film for the same reduction temperatures was calculated to be −0.80 × 10–3, −0.62 × 10–3, and −0.55 × 10–3/K, respectively. The coefficient of thermal expansion of both films decreased with an increase in the reduction temperature, and the decrease is attributed to the reduction in the number of hydrophilic functional groups of rGO flakes, which interact with moisture. Because the thermo-induced thickness change was strongly related to the sensitivity of the sensor, the relationship between the coefficient of thermal expansion and the reduction temperature for the sensors was consistent with that between the sensitivity of the sensors and the reduction temperature.
Reversible thermal expansion was investigated by comparing the thickness of a film before and after heating (Figure S14). The thickness of the film at 30 °C was 45.6 nm, and after being heated to 80 °C, its thickness decreased to 43.6 nm. When the film heated to 80 °C was cooled to 30 °C, its thickness increased to 45.7 nm. Thus, the thickness of the film before being heated to 80 °C was almost identical to that after being heated to 80 °C and cooled, which showed the outstanding reversibility of the film. Because the mechanism of the sensor was based on its electrical signal changing with its thickness, reversible thermal expansion endowed the sensor with reversibility of electrical signals.
To investigate the stability, reliability, and durability of the rGO/SF sensor, we investigated its responsivity (resistance change ΔR/R0) under repetitive temperature and humidity changes. Figure S15 shows the method used for temperature and humidity cycling. The temperature cycling test was performed repeatedly by heating and cooling the sensors with a heating pad, and the humidity cycling test was conducted using a linear motor with which the sensor could be moved toward and away from water to give rise to repetitive relative humidity changes. The temperature responsivity of the sensor to heat produced by the heating pad was evaluated for 1000 cycles for the temperature range 40–60 °C. The humidity responsivity of the sensor to humidity varied by using the linear motor was evaluated for 1000 cycles for the relative humidity range 50–85%. Figure 9 shows the responsivity of the temperature and humidity sensor for 0, 500, and 1000 cycles. The responsivity of the sensor in the cycling tests was nearly unchanged, which indicated high stability, reliability, and durability of the rGO/SF sensor.

Figure 9

Figure 9. (a) Result of the temperature cycling test for the rGO/SF sensor for a reduction temperature of 170 °C, performed to ascertain the sensor’s stability and durability. (b) Result of the humidity cycling test for the humidity sensor, performed to assess the stability and durability of the sensor.

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The durability of the rGO/SF sensor was higher than that of the rGO sensor. Furthermore, the rGO/SF film had higher mechanical properties, such as elastic bending modulus and flexural rigidity, than the rGO film (Figure 3). The mechanical bending test was performed for the rGO and rGO/SF films, and their surface was monitored during the test with an optical microscope with a 388.5× magnification (Figure 10). Both films were placed on a clamp to apply a bending displacement (Figure 10; inset images). The bending of the films was increased by using the clamp. When the films were bent by the clamp (the mechanical bending test), the region with the smallest bending diameter in the films (Figure 10b,d; a green arrow) was observed with an optical microscope. After the films were delaminated, the bending diameter of the films was measured. While the rGO film was delaminated at a bending diameter of 2.1 mm, the rGO/SF film was delaminated at a bending diameter of 0.8 mm. The delaminated area of the rGO and rGO/SF films is indicated by a red arrow in Figure 10b,d. The mechanical bending test results indicated that failure occurred because of interfacial delamination between the top and bottom layers. A smaller bending diameter without delamination represented higher interfacial strength between layers, and it indicated high wearability. Thus, the rGO/SF sensor is more suitable than the rGO sensor for use as a wearable sensor. The good wearability of the rGO/SF sensor was attributed to its high interfacial shear strength resulting from strong interaction between the rGO and SF layers. (58,60)

Figure 10

Figure 10. Surface of the rGO sensor (a) before and (b) after the bending test (diameter: 2.1 mm), and the surface of the rGO/SF sensor (c) before and (d) after the bending test (diameter: 0.8 mm). The delamination area is shown by a red arrow. All scale bars are 200 μm.

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The high durability of the rGO/SF sensor resulted in the sensor showing high stability of electrical signals. The rGO and rGO/SF sensors fabricated with a reduction temperature of 200 °C were repeatably bent 50 times with a displacement of 6 mm and a bending rate of 1 mm/min by using a tensile test machine. While the electrical signal of the rGO sensor changed after being bent 50 times (compared with the electrical signal before the bending test), the rGO/SF sensor withstood repeated bending and its electrical signal was almost unchanged before and after the bending test (Figure 11). Furthermore, while the rGO sensor was delaminated after repeated bending, the rGO/SF film withstood the bending test (Figure S16). The higher durability of the rGO/SF sensor makes it promising for practical external use as a wearable sensor without failure.

Figure 11

Figure 11. Evaluation of the performance of sensors after the bending test through a comparison with the performance before the bending test: (a) rGO and (b) rGO/SF sensors fabricated with a reduction temperature of 200 °C.

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The durable, sensitive, and wearable rGO/SF temperature and humidity sensor can be used as a physiological signal monitoring sensor, such as a respiratory rate sensor. The rGO/SF sensor was attached to the philtrum of a person to detect respiration (Figure 12a; inset image), and it detected the respiratory rate for normal breaths to be 18.7 breaths/min (Figure 12a). The detected breaths were mostly typical of the normal healthy state of adults. When the respiratory rate was excessively increased intentionally, the rGO/SF sensor detected a rapid respiration rate of 51.3 breaths/min. It is known that adults with a respiratory rate of over 24 breaths/min are generally diagnosed as being critically ill. (74) It is noteworthy that the proposed rGO/SF sensor could detect a respiratory rate of over 24 breaths/min. Thus, it can be used to provide alerts during emergencies for patients with a critical disease.

Figure 12

Figure 12. (a) Respiration rates detected by the rGO/SF sensor. (b) Electrical signal of the rGO/SF sensor in the philtrum region induced by the inhaled air (upper red dashed line; temperature of 29.7 °C and relative humidity of 39.9%) and exhaled air (lower red dashed line; temperature of 31.6 °C and relative humidity of 76.7%).

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The respiration sensing mechanism of the rGO/SF sensor was attributed to respiration-associated changes in the temperature and humidity in the philtrum region (Figure 12b). The temperature and humidity conditions of the philtrum region were altered by the inhaled and exhaled air. (75,76) The temperature and humidity monitored by commercial temperature and humidity sensors in one cycle of inspiration and expiration are between 29.7 °C/39.9% RH and 31.6 °C/76.7% RH (red dashed lines in Figure 12b). It was found that the rGO/SF sensor could detect changes in the temperature and humidity in the philtrum region. The rGO/SF sensors worked properly and continuously monitored changes in the temperature and humidity in the philtrum region during breathing (blue waveform in Figure 12b). It is concluded that the proposed rGO/SF sensor has the potential to be used in the fields of healthcare and biomedical monitoring and in physiological signal monitoring devices.

4. Conclusions

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In this work, a wearable rGO/SF-based temperature and humidity sensor was fabricated using an LbL technique and thermal reduction treatment. The mechanical properties of the rGO/SF film were determined by performing the AFM bending test. Compared with an rGO film, rGO/SF films fabricated with reduction temperatures of 155, 200, and 230 °C showed a bending modulus that was higher by 99.5, 232.6, and 81.5%, respectively. The mechanically reinforced rGO/SF film was characterized by strong interaction between rGO and SF because of SF binding strongly with rGO during recrystallization, and the strong interaction increased the interfacial shear strength of the film’s structure. The mechanism of the sensor is attributed to its electrical signal changing in response to absorption/desorption of water molecules onto/from the LbL structure. The sensitivity of the rGO/SF sensor could be tuned by varying the reduction temperature. The TCR of the sensor was adjusted to 2.20, 1.87, 1.73, 1.66, 1.50, and 1.39%/K for reduction temperatures of 155, 170, 185, 200, 215, and 230 °C, respectively. The sensitivity of the humidity sensor decreased with an increase in the reduction temperature. Furthermore, compared with the TCR of the rGO sensor, the TCR of the rGO/SF sensor was slightly degraded by 20.6, 18.7, 15.6, 7.9, 14.3, and 9.7% for reduction temperatures of 155, 170, 185, 200, 215, and 230 °C, respectively. Despite this degradation, the durable rGO/SF sensor showed outstanding robustness against repeatedly applied temperature, humidity, and bending loads, without any performance degradation, unlike the rGO sensor. The rGO/SF temperature and humidity sensor can be used in the fields of healthcare and biomedical monitoring, for instance, as a respiration sensor. The results of this study demonstrate that the rGO/SF temperature and humidity sensor has high potential for use in next-generation wearable devices. Furthermore, the results can serve as a guide for the fabrication of rGO-based devices that can be used in complex environmental conditions. In future work, we intend to develop an rGO/SF sensor with strain-insensitive electrical properties by using origami and kirigami structures for better use in healthcare and human–machine interface applications.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.biomac.3c00106.

  • Schematic of the sensor evaluation method, sensor resistance, EDS plots of films, AFM images of films on Cu mesh, force–distance curves, load–deflection plots during approaching and retracting, surface roughness, FTIR spectra, AFM image of films with and without an LbL structure, resistance–temperature curve of sensors, response and recovery curves, specifications and comparison of various wearable temperature and humidity sensors, experimental setup for measuring film thickness at various temperatures, AFM images of films at various temperature, methods for cyclic responsivity measurements of sensors, and film delamination after repeated bending tests (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Hyeonho Cho - School of Mechanical Engineering, Chung-Ang University, Dongjak-gu, Seoul 06974, Korea
    • Chanui Lee - School of Mechanical Engineering, Chung-Ang University, Dongjak-gu, Seoul 06974, Korea
    • ChaBum Lee - J. Mike Walker ’66 Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843-3123, United States
    • Sangmin Lee - School of Mechanical Engineering, Chung-Ang University, Dongjak-gu, Seoul 06974, Korea;  Orcidhttps://orcid.org/0000-0002-6713-4797
  • Author Contributions

    H.C. and C.L. contributed equally to this work.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by National Research Foundation of Korea (NRF) grants funded by the Korean Government (MSIT) [Grant Nos. NRF-2021R1A2C4001717 and NRF-2021R1A4A3030268].

References

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    In the emerging Internet of Things (IoT) society, there is a significant need for low-cost, high-performance flexible humidity sensors in wearable devices. However, com. available humidity sensors lack flexibility or require expensive and complex fabrication methods, limiting their application and widespread use. We report a high-performance printed flexible humidity sensor using a cellulose nanofiber/carbon black (CNF/CB) composite. The cellulose nanofiber enables excellent dispersion of carbon black, which facilitates the ink prepn. and printing process. At the same time, its hydrophilic and porous nature provides high sensitivity and fast response to humidity. Significant resistance changes of 120% were obsd. in the sensor at humidity ranging from 30% RH to 90% RH, with a fast response time of 10 s and a recovery time of 6 s. Furthermore, the developed sensor also exhibited high-performance uniformity, response stability, and flexibility. A simple humidity detection device was fabricated and successfully applied to monitor human respiration and noncontact fingertip moisture as a proof-of-concept.
  11. 11
    Stobinski, L.; Lesiak, B.; Malolepszy, A.; Mazurkiewicz, M.; Mierzwa, B.; Zemek, J.; Jiricek, P.; Bieloshapka, I. Graphene oxide and reduced graphene oxide studied by the XRD, TEM and electron spectroscopy methods. J. Electron Spectrosc. Relat. Phenom. 2014, 195, 145,  DOI: 10.1016/j.elspec.2014.07.003
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    11
    Graphene oxide and reduced graphene oxide studied by the XRD, TEM and electron spectroscopy methods
    Stobinski, L.; Lesiak, B.; Malolepszy, A.; Mazurkiewicz, M.; Mierzwa, B.; Zemek, J.; Jiricek, P.; Bieloshapka, I.
    Journal of Electron Spectroscopy and Related Phenomena (2014), 195 (), 145-154CODEN: JESRAW; ISSN:0368-2048. (Elsevier B.V.)
    The com. and synthesized few-layer graphene oxide, prepd. using oxidn. reactions, and few-layer reduced graphene oxide samples were structurally and chem. investigated by the X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron spectroscopy methods, i.e. XPS and reflection electron energy loss spectroscopy (REELS). The com. graphene oxide (FL-GOc) shows a stacking nanostructure of about 22 × 6 nm av. diam. by height with the distance of 0.9 nm between 6-7 graphene layers, whereas the resp. reduced graphene oxide (FL-RGOc)-about 8 × 1 nm av. diam. by height stacking nanostructure with the distance of 0.4 nm between 2-3 graphene layers (XRD). The REELS results are consistent with those by the XRD indicating 8 (FL-GOc) and 4 layers (FL-RGOc). In graphene oxide and reduced graphene oxide prepd. from the graphite the REELS indicates 8-11 and 7-10 layers. All graphene oxide samples show the C/O ratio of 2.1-2.3, 26.5-32.1 at% of C sp3 bonds and high content of functional oxygen groups (hydroxyl-C-OH, epoxy-C-O-C, carbonyl-C=O, carboxyl-C-OOH, water) (XPS). Redn. increases the C/O ratio to 2.8-10.3, decreases C sp3 content to 11.4-20.3 at% and also the content of C-O-C and C=O groups, accompanied by increasing content of C-OH and C-OOH groups. Formation of addnl. amt. of water due to functional oxygen group redn. leads to layer delamination. Removing of functional oxygen groups and water mols. results in decreasing the distance between the graphene layers.
  12. 12
    Olowojoba, G.-B.; Eslava, S.; Gutierrez, E.-S.; Kinloch, A.-J.; Mattevi, C.; Rocha, V.-G.; Taylor, A.-C. In situ thermally reduced graphene oxide/epoxy composites: Thermal and mechanical properties. Appl. Nanosci. 2016, 6, 1015,  DOI: 10.1007/s13204-016-0518-y
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    12
    In situ thermally reduced graphene oxide/epoxy composites: thermal and mechanical properties
    Olowojoba, Ganiu B.; Eslava, Salvador; Gutierrez, Eduardo S.; Kinloch, Anthony J.; Mattevi, Cecilia; Rocha, Victoria G.; Taylor, Ambrose C.
    Applied Nanoscience (2016), 6 (7), 1015-1022CODEN: ANPACY; ISSN:2190-5517. (Springer GmbH)
    Graphene has excellent mech., thermal, optical and elec. properties and this has made it a prime target for use as a filler material in the development of multifunctional polymeric composites. However, several challenges need to be overcome to take full advantage of the aforementioned properties of graphene. These include achieving good dispersion and interfacial properties between the graphene filler and the polymeric matrix. In the present work, we report the thermal and mech. properties of reduced graphene oxide/epoxy composites prepd. via a facile, scalable and com. viable method. Electron micrographs of the composites demonstrate that the reduced graphene oxide (rGO) is well dispersed throughout the composite. Although no improvements in glass transition temp., tensile strength and thermal stability in air of the composites were obsd., good improvements in thermal cond. (about 36%), tensile and storage moduli (more than 13%) were recorded with the addn. of 2 wt% of rGO.
  13. 13
    Chen, Q.; Liu, Y.; Gu, K.; Yao, J.; Shao, Z.; Chen, X. Silk-based electrochemical sensor for the detection of glucose in sweat. Biomacromolecules 2022, 23, 3928,  DOI: 10.1021/acs.biomac.2c00753
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    13
    Silk-Based Electrochemical Sensor for the Detection of Glucose in Sweat
    Chen, Qianying; Liu, Yi; Gu, Kai; Yao, Jinrong; Shao, Zhengzhong; Chen, Xin
    Biomacromolecules (2022), 23 (9), 3928-3935CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
    The development of reliable glucose sensors for noninvasive monitoring is highly desirable and essential for diabetes detection. As a testing sample, sweat is voluminous and is easy to collect compared to blood. However, the application of sweat glucose sensors is generally limited because of their low stability and sensitivity compared to com. glucometers. In this manuscript, a silk nanofibril (SNF)/reduced graphene oxide (RGO)/glucose oxidase (GOx) composite was developed as the working electrode of the sweat glucose sensor. The SNF/RGO/GOx composite was prepd. via a facile two-step process, which involved the self-assembly of SNF from silk fibroin while reducing graphene oxide to RGO and immobilizing GOx on SNF. The SNF/RGO/GOx glucose sensor exhibited a low limit of detection (300 nM) and high sensitivity (18.0μA/mM) in the sweat glucose range, covering both healthy people and diabetic patients (0-100μM). Moreover, the SNF/RGO/GOx glucose sensors showed a long stability for at least 4 wk. Finally, the SNF/RGO/GOx glucose sensor was applied to test the actual sweat samples from two volunteers and two sweating methods (by dry sauna and exercise). The results indicate the glucose data tested by the SNF/RGO/GOx glucose sensor were reliable, which correlated well to the data obtained from the com. glucometer. Therefore, the SNF/RGO/GOx glucose sensor developed in this study may have a great potential for glucose control in personalized healthcare monitoring and chronic disease management.
  14. 14
    Cho, H.; Lee, H.; Lee, S.; Kim, S. Reduced graphene oxide-based wearable and bio-electrolyte triggered pressure sensor with tunable sensitivity. Ceram. Int. 2021, 47, 17702,  DOI: 10.1016/j.ceramint.2021.03.090
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    14
    Reduced graphene oxide-based wearable and bio-electrolyte triggered pressure sensor with tunable sensitivity
    Cho, Hyeonho; Lee, Hyoeun; Lee, Sangmin; Kim, Sunghan
    Ceramics International (2021), 47 (12), 17702-17710CODEN: CINNDH; ISSN:0272-8842. (Elsevier Ltd.)
    Herein, we introduce a wearable, self-powered pressure sensor with tailorable sensitivity using a reduced graphene oxide-coated porous polyurethane (rGO-PP) composite. The micropores of the porous polyurethane (incorporating rGO flakes) create a conductive path which changes based on the applied pressure, resulting in corresponding changes in the elec. signal. The rGO-PP pressure sensor's power is generated using a self-powered galvanic cell and bio-electrolyte extd. from human skin. The rGO-PP pressure sensor circumvents complicated elec. circuits, facilitating its ease of use as a wearable device. Through the serial connection of galvanic cells, the sensitivity of the rGO-PP pressure sensor can be tailored within an extraordinarily wide range between 0.46 to 3.05 kPa-1. The rGO-PP pressure sensors display rapid response times (16 ms). Consequently, they can be used in the development of applications including braille recognition, human-machine interaction, and the restoration of touch perception.
  15. 15
    Hu, X.; Xia, X.-X.; Huang, S.-C.; Qian, Z.-G. Development of adhesive and conductive resilin-based hydrogels for wearable sensors. Biomacromolecules 2019, 20, 3283,  DOI: 10.1021/acs.biomac.9b00389
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    15
    Development of Adhesive and Conductive Resilin-Based Hydrogels for Wearable Sensors
    Hu, Xiao; Xia, Xiao-Xia; Huang, Sheng-Chen; Qian, Zhi-Gang
    Biomacromolecules (2019), 20 (9), 3283-3293CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
    Integrating multifunctionality such as stretchability, adhesiveness, and electrocond. on a single protein hydrogel is highly desirable for various applications, and remains a challenge. Here we present the development of such multifunctional hydrogels based on resilin, a natural rubber-like material with remarkable extensibility and resilience. First, genetically engineered resilin-like proteins (RLPs) with varying mol. wt. were biosynthesized to tune mech. strength and stiffness of the crosslinked RLP hydrogels. Second, glycerol was incorporated into the hydrogels to endow adhesive properties. Next, a graphene-RLP conjugate was synthesized for crosslinking with the unmodified, pristine RLP to form an integrated network. The obtained hybrid hydrogel could be stretched to over four times of its original length, and self-adhered to diverse substrate surfaces due to its high adhesion strength of ∼24 kPa. Furthermore, the hybrid hydrogel showed high sensitivity, with a gauge factor of 3.4 at 200% strain, and was capable of real-time monitoring human activities such as finger bending, swallowing, and phonating. Due to these favorable attributes, the graphene/resilin hybrid hydrogel was a promising material for use in wearable sensors. In addn., the above material design and functionalization strategy may provide intriguing opportunities to generate innovative materials for broad applications.
  16. 16
    Xu, L.; Zhai, H.; Chen, X.; Liu, Y.; Wang, M.; Liu, Z.; Umar, M.; Ji, C.; Chen, Z.; Jin, L.; Liu, Z.; Song, Q.; Yue, P.; Li, Y.; Ye, T.-T. Coolmax/graphene-oxide functionalized textile humidity sensor with ultrafast response for human activities monitoring. Chem. Eng. J. 2021, 412, 128639  DOI: 10.1016/j.cej.2021.128639
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    16
    Coolmax/graphene-oxide functionalized textile humidity sensor with ultrafast response for human activities monitoring
    Xu, Lulu; Zhai, Heng; Chen, Xiao; Liu, Yulong; Wang, Miao; Liu, Zhangchi; Umar, Muhammad; Ji, Chengyu; Chen, Zhongda; Jin, Lu; Liu, Zekun; Song, Qingwen; Yue, Pengfei; Li, Yi; Ye, Terry T.
    Chemical Engineering Journal (Amsterdam, Netherlands) (2021), 412 (), 128639CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
    Among different fibrous e-textile sensors, humidity sensor has a particular significance in respiration monitoring, water presence alert, and skin contact/non-contact indication. These applications not only demand sensors to have fast response and recovery speed, robustness to attrition and friction, the sensors responses also need to be unimodal, i.e., only sensitive to humidity and insensitive to other impacts, such as temp. variation, folding and stretching of the fabric. Previously reported e-textile humidity sensors, when being used as a quick humidity indicator, suffered from slow response/recovery time, interference from multimodal sensitivities, or devices are non-fabric based and cannot be seamlessly integrated with apparels. In this paper, we have studied textile-based humidity sensors constructed from different natural and synthetic fibers (cotton, wool and Coolmax) and discovered that the graphene-oxide (GO) functionalized Coolmax humidity sensor (GO-Coolmax) exhibits ultrafast response/recovery time (less than 0.6 s). The senor is also insensitive to external pressure and temp. changes, i.e., the resistance variations caused by these impacts are within 10%, much smaller than that from humidity variations (above 80%). We have also demonstrated the use of the sensor as a humidity indicator/alert in various wearable applications through prototyping and expts.
  17. 17
    Zeng, Y.; Li, T.; Yao, Y.; Li, T.; Hu, L.; Marconnet, A. Thermally conductive reduced graphene oxide thin films for extreme temperature sensors. Adv. Funct. Mater. 2019, 29, 1901388  DOI: 10.1002/adfm.201901388
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    Ni, Y.; Huang, J.; Li, S.; Dong, X.; Zhu, T.; Cai, W.; Chen, Z.; Lai, Y. Robust superhydrophobic rGO/PPy/PDMS coatings on a polyurethane sponge for underwater pressure and temperature sensing. ACS Appl. Mater. Interfaces 2021, 13, 53271,  DOI: 10.1021/acsami.1c17165
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    18
    Robust Superhydrophobic rGO/PPy/PDMS Coatings on a Polyurethane Sponge for Underwater Pressure and Temperature Sensing
    Ni, Yimeng; Huang, Jianying; Li, Shuhui; Dong, Xiuli; Zhu, Tianxue; Cai, Weilong; Chen, Zhong; Lai, Yuekun
    ACS Applied Materials & Interfaces (2021), 13 (44), 53271-53281CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Flexible wearable pressure sensors have attracted great interest from researchers in recent years because of their important applications in human-machine interaction, human behavior detection, medical diagnosis, and other fields. At present, integrating multiple functions such as pressure and temp. sensing and self-cleaning into a single material remains a challenging task. Here, by in situ redn. of graphene oxide (GO) grown on a sponge surface and deposition of polypyrrole (PPy) nanoparticles, we have built a highly sensitive, stable, and multifunctional rGO/PPy/poly(dimethylsiloxane) (PDMS) polyurethane (PU) sponge (GPPS) sensor for the detection of pressure, water level, and temp. This multifunctional sensor shows excellent pressure-sensing performance, ultrasensitive loading sensing of a leaf (98 mg), and outstanding reproducibility over 5000 cycles. Due to the stability of the superhydrophobic surface water contact angle (WCA) = 153.3°, our sensor can work in an underwater environment, which can sense water levels from 1 cm (~ 98 Pa) to 40 cm and also a variety of underwater behaviors (knock, ultrasonication, blow, etc.) with high stability. In addn., the sensor can be integrated into a circuit for the water level and pressure detection. The sensor can also be used as a smart underwater-temp. sensor; it shows a linear temp. coeff. of resistance (TCR) of 0.48% °C-1 in a temp. range of 35-80°C. This multifunctional sensor shows potential application prospects in wearable electronic devices for sensing.
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    Wan, S.; Chen, Y.; Fang, S.; Wang, S.; Xu, Z.; Jiang, L.; Baughman, R. H.; Cheng, Q. High-strength scalable graphene sheets by freezing stretch-induced alignment. Nat. Mater. 2021, 20, 624,  DOI: 10.1038/s41563-020-00892-2
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    High-strength scalable graphene sheets by freezing stretch-induced alignment
    Wan, Sijie; Chen, Ying; Fang, Shaoli; Wang, Shijun; Xu, Zhiping; Jiang, Lei; Baughman, Ray H.; Cheng, Qunfeng
    Nature Materials (2021), 20 (5), 624-631CODEN: NMAACR; ISSN:1476-1122. (Nature Portfolio)
    Efforts to obtain high-strength graphene sheets by near-room-temp. assembly have been frustrated by the misalignment of graphene layers, which degrades mech. properties. While in-plane stretching can decrease this misalignment, it reappears when releasing the stretch. Here we use covalent and π-π inter-platelet bridging to permanently freeze stretch-induced alignment of graphene sheets, and thereby increase isotropic in-plane sheet strength to 1.55 GPa, in combination with a high Young's modulus, elec. cond. and wt.-normalized shielding efficiency. Moreover, the stretch-bridged graphene sheets are scalable and can be easily bonded together using a com. resin without appreciably decreasing the performance, which establishes the potential for practical applications.
  20. 20
    Mao, L.; Park, H.; Soler-Crespo, R. A.; Espinosa, H. D.; Han, T. H.; Nguyen, S. T.; Huang, J. Stiffening of graphene oxide films by soft porous sheets. Nat. Commun. 2019, 10, 3677,  DOI: 10.1038/s41467-019-11609-8
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    20
    Stiffening of graphene oxide films by soft porous sheets
    Mao Lily; Nguyen SonBinh T; Park Hun; Han Tae Hee; Park Hun; Huang Jiaxing; Soler-Crespo Rafael A; Espinosa Horacio D; Espinosa Horacio D
    Nature communications (2019), 10 (1), 3677 ISSN:.
    Graphene oxide (GO) sheets have been used as a model system to study how the mechanical properties of two-dimensional building blocks scale to their bulk form, such as paper-like, lamellar-structured thin films. Here, we report that the modulus of multilayer GO films can be significantly enhanced if some of the sheets are drastically weakened by introducing in-plane porosity. Nanometer-sized pores are introduced in GO sheets by chemical etching. Membrane-deflection measurements at the single-layer level show that the sheets are drastically weakened as the in-plane porosity increases. However, the mechanical properties of the corresponding multilayer films are much less sensitive to porosity. Surprisingly, the co-assembly of pristine and etched GO sheets yields even stiffer films than those made from pristine sheets alone. This is attributed to the more compliant nature of the soft porous sheets, which act as a binder to improve interlayer packing and load transfer in the multilayer films.
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    Liang, X.; Zhu, M.; Li, H.; Dou, J.; Jian, M.; Xia, K.; Li, S.; Zhang, Y. Hydrophilic, breathable, and washable graphene decorated textile assisted by silk sericin for integrated multimodal smart wearables. Adv. Funct. Mater. 2022, 32, 2200162  DOI: 10.1002/adfm.202200162
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    21
    Hydrophilic, Breathable, and Washable Graphene Decorated Textile Assisted by Silk Sericin for Integrated Multimodal Smart Wearables
    Liang, Xiaoping; Zhu, Mengjia; Li, Haifang; Dou, Jinxin; Jian, Muqiang; Xia, Kailun; Li, Shuo; Zhang, Yingying
    Advanced Functional Materials (2022), 32 (42), 2200162CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Achieving integrated systems with comfortability and durability comparable to traditional textiles is one of the ultimate pursuits of smart wearables. This work reports a hydrophilic, breathable, biocompatible, and washable graphene-decorated electronic textile that is achieved with the assistance of silk sericin and enables the fabrication of comfortable and integrated multisensing textiles. The graphene-decorated textile is prepd. by dyeing com. textile with an aq. graphene ink, which contains natural silk sericin-coated graphene and is free of any artificial chems. The conformally coated hydrophilic sericin-graphene flakes and the well-reserved knitted structure endow the textile with good cond., excellent hydrophilicity, biocompatibility, breathability, and flexibility, ensuring its electronic performance and wearing-comfort. Moreover, the obtained textile is washable after processed with a crosslinking agent. Based on the obtained textile, an integrated multisensing textile that can simultaneously collect and analyze myoelec. and mech. signals is developed, enabling the recognition and distinguishment of complex human motions. With the combined features of hydrophilicity, breathability, biocompatibility, washability, and versatility, this strategy of fabricating electronic textiles based on conventional textiles and an aq. sericin-graphene ink provides a scalable and sustainable way to construct smart wearables.
  22. 22
    Liang, X.; Li, H.; Dou, J.; Wang, Q.; He, W.; Wang, C.; Li, D.; Lin, J.-M.; Zhang, Y. Stable and biocompatible carbon nanotube ink mediated by silk protein for printed electronics. Adv. Mater. 2020, 32, 2000165  DOI: 10.1002/adma.202000165
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    22
    Stable and Biocompatible Carbon Nanotube Ink Mediated by Silk Protein for Printed Electronics
    Liang, Xiaoping; Li, Haifang; Dou, Jinxin; Wang, Qi; He, Wenya; Wang, Chunya; Li, Donghang; Lin, Jin-Ming; Zhang, Yingying
    Advanced Materials (Weinheim, Germany) (2020), 32 (31), 2000165CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Ink-based processes, which enable scalable fabrication of flexible devices based on nanomaterials, are one of the practical approaches for the prodn. of wearable electronics. However, carbon nanotubes (CNTs), which possess great potential for flexible electronics, are facing challenges for use in inks due to their low dispersity in most solvents and suspicious cytotoxicity. Here, a stable and biocompatible CNT ink, which is stabilized by sustainable silk sericin and free from any artificial chems., is reported. The ink shows stability up to months, which can be attributed to the formation of sericin-CNT (SSCNT) hybrid through non-covalent interactions. It is demonstrated that the SSCNT ink can be used for fabricating versatile circuits on textile, paper, and plastic films through various techniques. As proofs of concept, ECG electrodes, breath sensors, and electrochem. sensors for monitoring human health and activity are fabricated, demonstrating the great potential of the SSCNT ink for smart wearables.
  23. 23
    Li, S.; Zhang, Y.; Liang, X.; Wang, H.; Lu, H.; Zhu, M.; Wang, H.; Zhang, M.; Song, Y.; Zhang, Y. Humidity-sensitive chemoelectric flexible sensors based on metal-air redox reaction for health management. Nat. Commun. 2022, 13, 5416,  DOI: 10.1038/s41467-022-33133-y
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    23
    Humidity-sensitive chemoelectric flexible sensors based on metal-air redox reaction for health management
    Li, Shuo; Zhang, Yong; Liang, Xiaoping; Wang, Haomin; Lu, Haojie; Zhu, Mengjia; Wang, Huimin; Zhang, Mingchao; Qiu, Xinping; Song, Yafeng; Zhang, Yingying
    Nature Communications (2022), 13 (1), 5416CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
    Numerous studies have shown flexible electronics play important roles in health management. The way of power supply is always an essential factor of devices and self-powered ones are very attractive because of the fabrication easiness, usage comfort and aesthetics of the system. In this work, based on the metal-air redox reaction, which is usually used in designing metal-air batteries, we design a self-powered chemoelec. humidity sensor where a silk fibroin (SF) and LiBr gel matrix contg. parallel aligned graphene oxide (GO) flakes serve as the electrolyte. The abundant hydrophilic groups in GO/SF and the hygroscopicity of LiBr lead to tight dependence of the output current on the humidity, enabling the sensor high sensitivity (0.09μA/s/1%), fast response (1.05 s) and quick recovery (0.80 s). As proofs of concept, we design an all-in-one respiratory monitoring-diagnosing-treatment system and a non-contact human-machine interface, demonstrating the applications of the chemoelec. humidity sensor in health management.
  24. 24
    Hu, K.; Gupta, M.-K.; Kulkarni, D.-D.; Tsukruk, V.-V. Ultra-robust graphene oxide-silk fibroin nanocomposite membranes. Adv. Mater. 2013, 25, 2301,  DOI: 10.1002/adma.201300179
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    24
    Ultra-Robust Graphene Oxide-Silk Fibroin Nanocomposite Membranes
    Hu, Kesong; Gupta, Maneesh K.; Kulkarni, Dhaval D.; Tsukruk, Vladimir V.
    Advanced Materials (Weinheim, Germany) (2013), 25 (16), 2301-2307CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    In this communication, novel ultrathin, robust nanocomposite membranes by incorporating graphene oxide sheets into silk fibroin matrix through heterogeneous surface interactions in an organized layer-by-layer (LbL) manner are reported. The outstanding values of the mech. properties achieved here are manyfold higher than those reported to date, which include a tensile modulus of 145 G Pa, an ultimate stress of more than 300 M Pa, and a toughness of above 2.2 MJ m~3 for silk nanocomposites with a silk content around 80%. The authors suggest that the outstanding performance that well exceeds the theor. values predicted by conventional mech. models is facilitated by the effective 2D graphene oxide filler which maximize all hydrogen bonding, polar-polar, and hydrophobic-hydrophobic interactions of the defective graphene oxide sheets with the silk fibroin matrix composed of polar random silk domains and the hydrophobic /3-sheet nanocrystals. Also the dense network of weak interactions has been suggested between the silk fibroin domains and graphene oxide sheets, which are in intimate contact within a 5 nm-thick bilayer, facilitates the formation of mol. interphase zones, thus effectively increasing the reinforcing effect and allowing a record high mech. strength and toughness, unheard of for biopolymer based nanocomposite films, to be acheived. The outstanding values of the mech. properties achieved here are manyfold higher than those reported to date, which include a tensile modulus of 145 G Pa, an ultimate stress of more than 300 M Pa, and a toughness of above.
  25. 25
    Xiong, R.; Hu, K.; Zhang, S.; Lu, C.; Tsukruk, V. V. Ultrastrong freestanding graphene oxide nanomembranes with surface-enhanced Raman scattering functionality by solvent-assisted single-component layer-by-layer assembly. ACS Nano 2016, 10, 6702,  DOI: 10.1021/acsnano.6b02012
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    25
    Ultrastrong Freestanding Graphene Oxide Nanomembranes with Surface-Enhanced Raman Scattering Functionality by Solvent-Assisted Single-Component Layer-by-Layer Assembly
    Xiong, Rui; Hu, Kesong; Zhang, Shuaidi; Lu, Canhui; Tsukruk, Vladimir V.
    ACS Nano (2016), 10 (7), 6702-6715CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    We report single-component ultrathin reduced graphene oxide (rGO) nanomembranes fabricated via nonconventional layer-by-layer assembly (LbL) of graphene oxide flakes, using org. solvent instead of water to provide strong complementary interactions and to ensure the uniform layered growth. This unique approach does not require regular polymeric from the assembly process or intermediate surface chem. modification. The resulting ultrastrong freestanding graphene oxide (rGO) LbL nanomembranes with a very low thickness of 3 nm (three GO monolayers) can be transferred over a large surface area across tens of square centimeters by using a facile surface-tension-assisted release technique. These uniform and ultrasmooth nanomembranes with high transparency (up to 93% at 550 nm) and high elec. cond. (up to 3000 S/m) also exhibit outstanding mech. strength of 0.5 GPa and a Young's modulus of 120 GPa, which are several times higher than that of other reported regular rGO films. Furthermore, up to 94 wt % of silver nanoplates can be sandwiched between 5 nm GO layers to construct a flexible freestanding protected noble metal monolayer with surface-enhanced Raman scattering properties. These flexible rGO/Ag/rGO nanomembranes can be transferred and conformally coat complex surfaces and show a cleaner Raman signature, enhanced wet stability, and lower oxidn. compared to bare Ag nanostructures.
  26. 26
    Choi, H. K.; Lee, A.; Park, M.; Lee, D. S.; Bae, S.; Lee, S.-K.; Lee, S. H.; Lee, T.; Kim, T.-W. Hierarchical porous film with layer-by-layer assembly of 2D copper nanosheets for ultimate electromagnetic interference shielding. ACS Nano 2021, 15, 829,  DOI: 10.1021/acsnano.0c07352
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    26
    Hierarchical Porous Film with Layer-by-Layer Assembly of 2D Copper Nanosheets for Ultimate Electromagnetic Interference Shielding
    Choi, Ho Kwang; Lee, Aram; Park, Mina; Lee, Dong Su; Bae, Sukang; Lee, Seoung-Ki; Lee, Sang Hyun; Lee, Takhee; Kim, Tae-Wook
    ACS Nano (2021), 15 (1), 829-839CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The emergence of technologies, such as 5G telecommunication, elec. vehicles, and wearable electronics, has prompted demand for ultrahigh-performance and cost-effective shielding materials to protect against both the potentially harmful effects of electromagnetic interference (EMI) on human health and electronic device operation. Here, we report hierarchical porous Cu foils via an assembly of single-cryst., nanometer-thick, and micrometer-long copper nanosheets and their use in EMI shielding. Layer-by-layer assembly of Cu nanosheets enabled the formation of a hierarchically structured porous Cu film with features such as multilayer stacking; two-dimensional networking; and a layered, sheetlike void architecture. The hierarchical-structured porous Cu foil exhibited outstanding EMI shielding performance compared to the same thickness of dense copper and other materials, exhibiting EMI shielding effectiveness (SE) values of 100 and 60.7 dB at thicknesses of 15 and 1.6μm, resp. In addn., the EMI SE of the hierarchical porous Cu film was maintained up to 18 mo under ambient conditions at room temp. and showed negligible changes after thermal annealing at 200°C for 1 h. These findings suggest that Cu nanosheets and their layer-by-layer assembly are one of the promising EMI shielding technologies for practical electronic applications.
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    Huang, L.; Li, C.; Yuan, W.; Shi, G. Strong composite films with layered structures prepared by casting silk fibroin–graphene oxide hydrogels. Nanoscale 2013, 5, 3780,  DOI: 10.1039/c3nr00196b
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    Strong composite films with layered structures prepared by casting silk fibroin-graphene oxide hydrogels
    Huang, Liang; Li, Chun; Yuan, Wenjing; Shi, Gaoquan
    Nanoscale (2013), 5 (9), 3780-3786CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Composite films of graphene oxide (GO) sheets and silk fibroin (SF) with layered structures were prepd. by facile soln. casting of SF-GO hydrogels. The as-prepd. composite film contg. 15% (by wt.) of SF shows a high tensile strength of 221 ± 16 MPa and a failure strain of 1.8 ± 0.4%, which partially surpass those of natural nacre. Particularly, this composite film also has a high modulus of 17.2 ± 1.9 GPa. The high mech. properties of this composite film can be attributed to its high content of GO (85 wt%), compact layered structure and the strong hydrogen bonding interaction between SF chains and GO sheets.
  28. 28
    Zhao, H.; Yue, Y.; Zhang, Y.; Li, L.; Guo, L. Ternary artificial nacre reinforced by ultrathin amorphous alumina with exceptional mechanical properties. Adv. Mater. 2016, 28, 2037,  DOI: 10.1002/adma.201505511
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    Ternary Artificial Nacre Reinforced by Ultrathin Amorphous Alumina with Exceptional Mechanical Properties
    Zhao, Hewei; Yue, Yonghai; Zhang, Youwei; Li, Lidong; Guo, Lin
    Advanced Materials (Weinheim, Germany) (2016), 28 (10), 2037-2042CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Herein, a new type of ternary artificial nacre (graphene oxide (GO)/ultrathin amorphous alumina (AA)/sodium CM-cellulose (SCMC)) has successfully been synthesized by a vacuum-assisted filtration method. In order to guarantee the uniform layered nacre-like structure of this new product for the first time, the ultrathin amorphous alumina was grown in situ on the surface of GO with a thickness varying from around 1 to 10 nm before mixing this together with the polymer SCMC. Amorphous alumina was chosen as the third reinforcement agent because of its superior strength and toughness compared to crystal alumina.
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    Cheng, Q.; Duan, J.; Zhang, Q.; Jiang, L. Learning from nature: Constructing integrated graphene-based artificial nacre. ACS Nano 2015, 9, 2231,  DOI: 10.1021/acsnano.5b01126
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    Learning from Nature: Constructing Integrated Graphene-Based Artificial Nacre
    Cheng, Qunfeng; Duan, Jianli; Zhang, Qi; Jiang, Lei
    ACS Nano (2015), 9 (3), 2231-2234CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    A review. Natural nacre supplies a no. of properties that can be used in designing high-performance bioinspired materials. Likewise, due to the extraordinary properties of graphene, a series of bioinspired graphene-based materials have recently been demonstrated. Compared to other approaches for constructing graphene-based materials, bioinspired concepts result in high-loading graphene, and the resultant high-performance graphene-based artificial nacres demonstrate isotropic mech. and elec. properties. In this Perspective, we describe how to construct integrated graphene-based artificial nacre through the synergistic relationship between interface interactions and building blocks. These integrated graphene-based artificial nacres show promising applications in many fields, such as aerospace, flexible supercapacitor electrodes, artificial muscle, and tissue engineering.
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    Shakil, A.; Kim, S.; Polycarpou, A. A. Creep behavior of graphene oxide, silk fibroin, and cellulose nanocrystal bionanofilms. Adv. Mater. Interfaces 2022, 2101640  DOI: 10.1002/admi.202101640
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    Creep behavior of graphene oxide, silk fibroin, and cellulose nanocrystal bionanofilms
    Shakil, Ahmad; Kim, Sunghan; Polycarpou, Andreas A.
    Advanced Materials Interfaces (2022), 9 (18), 2101640CODEN: AMIDD2; ISSN:2196-7350. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Graphene oxide (GO), silk fibroin (SF), and cellulose nanocrystal (CNC) nanocomposite is a novel biomaterial with superior mech. properties. Elevated temp. nanoindentation expts. using const. load hold method are performed to investigate temp.-dependent mech. and creep behavior of the GO-SF-CNC nanocomposite. Hardness and reduced modulus of GO-SF-CNC are detd. from expts. at 25, 40, 60, 80, and 100°C, and yield strength and creep coeffs. are predicted from finite element anal. using two-layer viscoplasticity theory. Results show that increasing the temp. from 25 to 80°C, hardness, reduced modulus, and yield strength of GO-SF-CNC nanocomposite dramatically increase by 112%, 40%, and 140% resp., and creep displacements during const. load hold reduce by 53%. It is attributed to increasing in crystns. in the nanocomposite because of increasing in β-sheet formations of SF material and redn. in water mols. in CNC material. However, at 100°C, the mech. properties deteriorate, and creep displacements increase because of water evapn. from the nanocomposite, making it weaker. Hardness-to-yield strength ratio is found within 1.84-2.06. Maximum creep exponent is 2.9 at 40°C, which reduces to 2.06 at 80°C and again increases to 2.27 at 100°C.
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    Ye, C.; Combs, Z. A.; Calabrese, R.; Dai, H.; Kaplan, D. L.; Tsukruk, V. V. Robust microcapsules with controlled permeability from silk fibroin reinforced with graphene oxide. Small 2014, 10, 5087,  DOI: 10.1002/smll.201401119
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    Robust Microcapsules with Controlled Permeability from Silk Fibroin Reinforced with Graphene Oxide
    Ye, Chunhong; Combs, Zachary A.; Calabrese, Rossella; Dai, Hongqi; Kaplan, David L.; Tsukruk, Vladimir V.
    Small (2014), 10 (24), 5087-5097CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Robust and stable microcapsules are assembled from poly-amino acid-modified silk fibroin reinforced with graphene oxide flakes using layer-by-layer (LbL) assembly, based on biocompatible natural protein and carbon nanosheets. The composite microcapsules are extremely stable in acidic (pH 2.0) and basic (pH 11.5) conditions, accompanied with pH-triggered permeability, which facilitates the controllable encapsulation and release of macromols. Furthermore, the graphene oxide incorporated into ultrathin LbL shells induces greatly reinforced mech. properties, with an elastic modulus which is two orders of magnitude higher than the typical values of original silk LbL shells and shows a significant, three-fold redn. in pore size. Such strong nanocomposite microcapsules can provide solid protection of encapsulated cargo under harsh conditions, indicating a promising candidate with controllable loading/unloading for drug delivery, reinforcement, and bioengineering applications.
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    Xie, W.; Tadepalli, S.; Park, S.-H.; Kazemi-Moridani, A.; Jiang, Q.; Singamaneni, S.; Lee, J.-H. Extreme mechanical behavior of nacre-mimetic graphene-oxide and silk nanocomposites. Nano Lett. 2018, 18, 987,  DOI: 10.1021/acs.nanolett.7b04421
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    Extreme Mechanical Behavior of Nacre-Mimetic Graphene-Oxide and Silk Nanocomposites
    Xie, Wanting; Tadepalli, Sirimuvva; Park, Sang Hyun; Kazemi-Moridani, Amir; Jiang, Qisheng; Singamaneni, Srikanth; Lee, Jae-Hwang
    Nano Letters (2018), 18 (2), 987-993CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Biol. materials have the ability to withstand extreme mech. forces due to their unique multilevel hierarchical structure. Here, we fabricated a nacre-mimetic nanocomposite comprised of silk fibroin and graphene oxide that exhibits hybridized dynamic responses arising from alternating high-contrast mech. properties of the components at the nanoscale. Dynamic mech. behavior of these nanocomposites is assessed through a microscale ballistic characterization using a 7.6 μm diam. silica sphere moving at a speed of approx. 400 m/s. The vol. fraction of graphene oxide in these composites is systematically varied from 0 to 32 vol. % to quantify the dynamic effects correlating with the structural morphologies of the graphene oxide flakes. Specific penetration energy of the films rapidly increases as the distribution of graphene oxide flakes evolves from noninteracting, isolated sheets to a partially overlapping continuous sheet. The specific penetration energy of the nanocomposite at the highest graphene oxide content tested here is significantly higher than that of Kevlar fabrics and close to that of pure multilayer graphene. This study evidently demonstrates that the morphologies of nanoscale constituents and their interactions are crit. to realize scalable high-performance nanocomposites using typical nanomaterial constituents having finite dimensions.
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    Zandiatashbar, A.; Lee, G.-H.; An, S.-J.; Lee, S.; Mathew, N.; Terrones, M.; Hayashi, T.; Picu, C.-R.; Hone, J.; Koratkar, N. Effect of defects on the intrinsic strength and stiffness of graphene. Nat. Commun. 2014, 5, 3186,  DOI: 10.1038/ncomms4186
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    Effect of defects on the intrinsic strength and stiffness of graphene
    Zandiatashbar Ardavan; Lee Gwan-Hyoung; An Sung Joo; Hone James; Lee Sunwoo; Mathew Nithin; Picu Catalin R; Terrones Mauricio; Hayashi Takuya; Koratkar Nikhil
    Nature communications (2014), 5 (), 3186 ISSN:.
    It is important from a fundamental standpoint and for practical applications to understand how the mechanical properties of graphene are influenced by defects. Here we report that the two-dimensional elastic modulus of graphene is maintained even at a high density of sp(3)-type defects. Moreover, the breaking strength of defective graphene is only ~14% smaller than its pristine counterpart in the sp(3)-defect regime. By contrast, we report a significant drop in the mechanical properties of graphene in the vacancy-defect regime. We also provide a mapping between the Raman spectra of defective graphene and its mechanical properties. This provides a simple, yet non-destructive methodology to identify graphene samples that are still mechanically functional. By establishing a relationship between the type and density of defects and the mechanical properties of graphene, this work provides important basic information for the rational design of composites and other systems utilizing the high modulus and strength of graphene.
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    Bai, Y.; Cai, H.; Qiu, X.; Fang, X.; Zheng, J. Effects of graphene reduction degree on thermal oxidative stability of reduced graphene oxide/silicone rubber nanocomposites. High Perform. Polym. 2015, 27, 997,  DOI: 10.1177/0954008315604205
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    Effects of graphene reduction degree on thermal oxidative stability of reduced graphene oxide/silicone rubber nanocomposites
    Bai, Yulian; Cai, Hai; Qiu, Xingna; Fang, Xin; Zheng, Junping
    High Performance Polymers (2015), 27 (8), 997-1006CODEN: HPPOEX; ISSN:0954-0083. (Sage Publications Ltd.)
    In this work, graphene oxide (GO) was prepd. with a modified Hummers method and then reduced to different redn. degree by thermal treatment. The GO and reduced GO (RGO) samples were introduced into silicone rubber (SR) matrix to prep. nanocomposites. XPS, Raman spectra, X-ray diffraction, and transmission electron microscopy measurement were performed to detect the structure and morphol. changes of GO and RGO sheets. The results showed that the redn. removed most of the oxygen-contg. functional groups on the surface of GO, esp. C-O-C group, and thus reestablished a graphitic network of sp2 hybrid; by increasing the redn. temp., the redn. degree of GO was increased, and meanwhile, the extent of exfoliation was increased. More importantly, tensile testing and thermogravimetric anal. revealed that RGO improved the mech. properties and thermal oxidative stability of SR nanocomposites. The improvement enhanced with increasing the redn. degree of GO simultaneously.
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    Richardson, J.-J.; Björnmalm, M.; Caruso, F. Technology-driven layer-by-layer assembly of nanofilms. Science 2015, 348, 2491,  DOI: 10.1126/science.aaa24
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    Alongi, J.; Carosio, F.; Frache, A.; Malucelli, G. Layer by layer coatings assembled through dipping, vertical or horizontal spray for cotton flame retardancy. Carbohydr. Polym. 2013, 92, 114,  DOI: 10.1016/j.carbpol.2012.08.086
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    Layer by Layer coatings assembled through dipping, vertical or horizontal spray for cotton flame retardancy
    Alongi, Jenny; Carosio, Federico; Frache, Alberto; Malucelli, Giulio
    Carbohydrate Polymers (2013), 92 (1), 114-119CODEN: CAPOD8; ISSN:0144-8617. (Elsevier Ltd.)
    Silica-based assemblies have been deposited on cotton fibers through Layer by Layer technique in order to enhance their flame retardant properties. To this aim, three different deposition procedures (namely, dipping, vertical and horizontal sprays) have been considered and compared. The resulting morphologies of the deposited assemblies have been thoroughly investigated by SEM and elemental anal. SEM observations have demonstrated that only the horizontal spray allows obtaining the deposition of a very homogeneous silica coating when compared to vertical spray or dipping. As a consequence, horizontal spray has proved to ensure the best flame resistance, promoting a substantial increase of the total burning time and final residue, as assessed by flammability tests. Furthermore, cone calorimetry measurements have shown a remarkable increase of the time to ignition, and a significant decrease of heat release rate and total heat release for the fabrics treated by horizontal spray.
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    Trung, T.-Q.; Ramasundaram, S.; Hwang, B.-U.; Lee, N.-E. An all-elastomeric transparent and stretchable temperature sensor for body-attachable wearable electronics. Adv. Mater. 2016, 28, 502,  DOI: 10.1002/adma.201504441
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    An All-Elastomeric Transparent and Stretchable Temperature Sensor for Body-Attachable Wearable Electronics
    Trung, Tran Quang; Ramasundaram, Subramaniyan; Hwang, Byeong-Ung; Lee, Nae-Eung
    Advanced Materials (Weinheim, Germany) (2016), 28 (3), 502-509CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A transparent stretchable (TS) gated sensor array with high optical transparency, conformality, and high stretchability of up to 70% is demonstrated. The TS-gated sensor array has high responsivity to temp. changes in objects and human skin. This unprecedented TS-gated sensor array, as well as the integrated platform of the TS-gated sensor with a transparent and stretchable strain sensor, shows great potential for application to wearable skin electronics for recognition of human activity.
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    Liu, Q.; Tai, H.; Yuan, Z.; Zhou, Y.; Su, Y.; Jiang, Y. A high-performance flexible temperature sensor composed of polyethyleneimine/reduced graphene oxide bilayer for real-time monitoring. Adv. Mater. Technol. 2019, 4, 1800594  DOI: 10.1002/admt.201800594
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    Sehrawat, P.; Islam, S.-S.; Mishra, P. Reduced graphene oxide based temperature sensor: Extraordinary performance governed by lattice dynamics assisted carrier transport. Sens. Actuators, B 2018, 258, 424,  DOI: 10.1016/j.snb.2017.11.112
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    Reduced graphene oxide based temperature sensor: Extraordinary performance governed by lattice dynamics assisted carrier transport
    Sehrawat, Poonam; Abid; Islam, S. S.; Mishra, Prabhash
    Sensors and Actuators, B: Chemical (2018), 258 (), 424-435CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
    In this article, we report the sensing performance of reduced graphene oxide (rGO) based resistive type temp. sensor fabricated by spin coating. A detailed anal. is presented for understanding the combined effect of lattice vibrational properties and temp. dependent elec. cond. while considering charge carrier scattering with phonons, impurities, defects, and edge boundaries of rGO flakes. The purpose of this anal. is to find out how together they influence the temp. coeff. of resistance (TCR) and thermal hysteresis (HTh) of rGO based films. TCR and Hth are the core factors for efficient operation of a temp. sensor as these govern important sensing characteristics such as sensitivity, resoln., drift, response- and recovery-time. Exptl. results show that the proposed sensor exhibits TCR ∼ -0.801%/K (in 303K-373K) and negligible thermal hysteresis (∼0.7%) resulting in high resoln. (∼0.1 K), response- and recovery-time of ∼52 s and ∼285 s resp. Besides, TCR and Hth are also found to depend on rGO concn. and working temp. range of sensors. By lowering the sensing temp. range to 303K-77K region, TCR was found to increase abruptly from -0.801%/K to -32.04%/K. All this optimized data were obtained for the sensor with 3 wt.% of rGO. Dynamic plot shows its sensitivity to respond to even ∼0.1 K change in temp. Cyclic testing demonstrates good stability in 77K-573K temp. range with negligible drift. These studies are significant towards the fabrication of simple, highly sensitive, and cost effective temp. sensor with high reproducibility. There is still enough room to improve TCR of rGO based sensors through synthesis, advanced sensor design and development; higher TCR will definitely lead to far better temp. sensing performance as theory predicts.
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    Zhu, J.; Andres, C.-M.; Xu, J.; Ramamoorthy, A.; Tsotsis, T.; Kotov, N.-A. Pseudonegative thermal expansion and the state of water in graphene oxide layered assemblies. ACS Nano 2012, 6, 8357,  DOI: 10.1021/nn3031244
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    Pseudonegative Thermal Expansion and the State of Water in Graphene Oxide Layered Assemblies
    Zhu, Jian; Andres, Christine M.; Xu, Jiadi; Ramamoorthy, Ayyalusamy; Tsotsis, Thomas; Kotov, Nicholas A.
    ACS Nano (2012), 6 (9), 8357-8365CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Unraveling the complex interplay between thermal properties and hydration is a part of understanding the fundamental properties of many soft materials and very essential for many applications. Here we show that graphene oxide (GO) demonstrates a highly neg. thermal expansion (NTE) coeff. owing to unique thermohydration processes related with fast transport of water between the GO sheets, the amphiphilic nature of nanochannels, and close-to-zero intrinsic thermal expansion of GO. The humidity-dependent NTE of GO layered assemblies, or "pseudoneg. thermal expansion" (PNTE), differs from that of other hygroscopic materials due to its relatively fast and highly reversible expansion/contraction cycles and occurrence at low humidity levels while bearing similarities to classic NTE. Thermal expansion of polyvinyl alc./GO composites is easily tunable with addnl. intricacy of thermohydration effects. PNTE combined with isotropy, nontoxicity, and mech. robustness is an asset for applications of actuators, sensors, MEMS devices, and memory materials and crucial for developing methods of thermal/photopatterning of GO devices.
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    Borini, S.; White, R.; Wei, D.; Astley, M.; Haque, S.; Spigone, E.; Harris, N.; Kivioja, J.; Ryhanen, T. Ultrafast graphene oxide humidity sensors. ACS Nano 2013, 7, 11166,  DOI: 10.1021/nn404889b
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    Ultrafast Graphene Oxide Humidity Sensors
    Borini, Stefano; White, Richard; Wei, Di; Astley, Michael; Haque, Samiul; Spigone, Elisabetta; Harris, Nadine; Kivioja, Jani; Ryhanen, Tapani
    ACS Nano (2013), 7 (12), 11166-11173CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Sensors allow an electronic device to become a gateway between the digital and phys. worlds, and sensor materials with unprecedented performance can create new applications and new avenues for user interaction. Graphene oxide can be exploited in humidity and temp. sensors with a no. of convenient features such as flexibility, transparency and suitability for large-scale manufg. Here we show that the two-dimensional nature of graphene oxide and its superpermeability to water combine to enable humidity sensors with unprecedented response speed (∼30 ms response and recovery times). This opens the door to various applications, such as touchless user interfaces, which we demonstrate with a 'whistling' recognition anal.
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    Adib, M. R.; Lee, Y.; Kondalkar, V. V.; Kim, S.; Lee, K. A highly sensitive and stable rGO: MoS2-based chemiresistive humidity sensor directly insertable to transformer insulating oil analyzed by customized electronic sensor interface. ACS Sens. 2021, 6, 1012,  DOI: 10.1021/acssensors.0c02219
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    A Highly Sensitive and Stable rGO:MoS2-Based Chemiresistive Humidity Sensor Directly Insertable to Transformer Insulating Oil Analyzed by Customized Electronic Sensor Interface
    Adib, Md Ridwan; Lee, Yongbum; Kondalkar, Vijay V.; Kim, Sihyeok; Lee, Keekeun
    ACS Sensors (2021), 6 (3), 1012-1021CODEN: ASCEFJ; ISSN:2379-3694. (American Chemical Society)
    Reduced graphene oxide and molybdenum disulfide (rGO:MoS2) are the most representative two-dimensional materials, which are promising for a humidity sensor owing to its high surface area, a large no. of active sites, and excellent mech. flexibility. Herein, we introduced a highly sensitive and stable rGO:MoS2-based humidity sensor integrated with a low-power in-plane microheater and a temp. sensor, directly insertable to transformer insulating oil, and analyzed by a newly developed customized sensor interface electronics to monitor the sensor's output variations in terms of relative humidity (RH) concn. rGO:MoS2 sensing materials were synthesized by simple ultrasonication without using any additives or addnl. heating and selectively deposited on titanium/platinum (Ti/Pt) interdigitated electrodes on a SiO2 substrate using the drop-casting method. The significant sensing capability of p-n heterojunction formation between rGO and MoS2 was obsd. both in the air and transformer insulating oil environment. In air testing, the sensor exhibited an immense sensitivity of 0.973 kΩ/%RH and excellent linearity of ~ 0.98 with a change of humidity from 30 to 73%RH, and a const. resistance deviation with an inaccuracy rate of 0.13% over 400 h of continual measurements. In oil, the sensor showed a high sensitivity of 1.596 kΩ/%RH and stable repeatability for an RH concn. range between 34 and 63%RH. The obtained results via the sensor interface were very similar to those measured with a digital multimeter, denoting that our developed total sensor system is a very promising candidate for real-time monitoring of the operational status of power transformers.
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    Wang, Y.; Zhang, L.; Zhang, Z.; Sun, P.; Chen, H. High-sensitivity wearable and flexible humidity sensor based on graphene oxide/non-woven fabric for respiration monitoring. Langmuir 2020, 36, 9443,  DOI: 10.1021/acs.langmuir.0c01315
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    High-Sensitivity Wearable and Flexible Humidity Sensor Based on Graphene Oxide/Non-Woven Fabric for Respiration Monitoring
    Wang, Yamei; Zhang, Liwen; Zhang, Zhenwei; Sun, Pengyuan; Chen, Huawei
    Langmuir (2020), 36 (32), 9443-9448CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The popularity of humidity sensing for respiratory anal. of patients is gradually increasing because of its portability and cost-effectiveness. However, current flexible humidity sensors are mainly made of polymer films, whose poor hygroscopicity and breathability reduce their sensitivity and comfort. A highly sensitive humidity sensor was developed using non-woven fabric (NWF) coated with graphene oxide (GO). Bovine serum albumin was used to improve the adsorption of GO onto the NWF, and its effect on sensitivity was studied by adjusting its concn. High-humidity sensitivity was exptl. validated by testing different relative humidity levels, and its fast response and excellent feasibility under diverse breathing conditions were verified by successful monitoring of fast and deep breathing, differentiating nose and mouth breathing, and even identifying simple spoken words. This study developed a breathable and skin-friendly humidity sensor based on GO/NWF, which is a promising device for human healthcare.
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    Jia, G.; Zheng, A.; Wang, X.; Zhang, L.; Li, L.; Li, C.; Zhang, Y.; Cao, L. Flexible, biocompatible and highly conductive MXene-graphene oxide film for smart actuator and humidity sensor. Sens. Actuators, B 2021, 346, 130507  DOI: 10.1016/j.snb.2021.130507
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    Flexible, biocompatible and highly conductive MXene-graphene oxide film for smart actuator and humidity sensor
    Jia, Guangwen; Zheng, Ao; Wang, Xiao; Zhang, Lu; Li, Ling; Li, Chenxing; Zhang, Yan; Cao, Lingyan
    Sensors and Actuators, B: Chemical (2021), 346 (), 130507CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
    The evapn. of water occurs ubiquitously on earth. Hence, smart materials that can directly convert signals generated via water stimulation into mech. motion have attracted wide attention. However, it is still a challenge to develop novel functional materials with fast response, large scale deformation, and long-term stability for moisture-gradient actuators. Here, a flexible, conductive, layer-structured homogenous Ti3C2TX MXene-graphene oxide (MGO) film-based moisture-driven actuator and humidity sensor were fabricated. The oxygen groups and d-spacing could be effectively adjusted by MXene/GO compn. ratio, thereby tuning the actuation performance. MGO3 (MXene/GO = 3) displayed a large bending angle, and reversible deformation. And the bending speed of MGO3 is up to 32°s-1. Furthermore, MGO3 actuation displayed long-term stability via suppression of MXene oxidn. by the introduction of GO and showed good cycling stability. MGO3 actuators are constructed, which could mimic the blooming of flower, lifting and carrying objects, and be used as a non-contact control switch. In addn., MGO3 showed a linear sensitive response to humidity and excellent biocompatibility which make it suitable for respiratory monitoring. This work demonstrated that flexible, biocompatibility and conductive MGO films have broad application prospects in the fields of smart actuators, sensing devices, and biol. and health care.
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    Jason, N. N.; Ho, M. D.; Cheng, W. Resistive electronic skin. J. Mater. Chem. C 2017, 5, 5845,  DOI: 10.1039/C7TC01169E
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    Resistive electronic skin
    Jason, Naveen N.; Ho, My D.; Cheng, Wenlong
    Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2017), 5 (24), 5845-5866CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
    Devices made from traditional conductive bulk materials using complex microfabrication methods often are restricted to being rigid and in some cases, flexible but not strethcable. The main reason is the mismatching mechanics between these traditional materials and the elastomeric materials they were bonded with, which causes materials delimination and/or cracks at soft/hard materials interfaces under strains. Conductive nanomaterials potentially offer new opportunity to tackle this challenge. Their availability in various sizes and shapes enables us to create composites with various dimensions, such as 1D conductive traces, 2D film, and 3D sponge-like architectures. These have opened the door for fabrication of stretchable interconnects, circuits, energy storage devices, antennas, LEDs, etc. The basis of using conductive nanomaterials composites in sensors is that any stimulus or change will generate a measurable elec. impulse. These impulses can be broadly classified as piezoelec., triboelec., capacitive, and resistive responses. Depending on the sensitivity required and the preference of elec. impulse to be measured, the device construction maybe tailored to give one of the four kinds of elec. responses. Resistive sensors in addn. to being the easiest to construct are also the easiest to measure, which is the crucial reason for a large no. of publications in this area. The working mechanism of resistive sensors based on the constituent conductive materials and their percolation network will be discussed in detail. Compn. of conductive inks fabricated using wet chem. methods, and nanomaterials using dry methods, their subsequent applications are covered as well. The exciting applications relating to human health and well-being will also be described. Finally a brief outlook of the future of wearable sensors as "invisibles" will be presented.
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    Guan, X.; Hou, Z.; Wu, K.; Zhao, H.; Liu, S.; Fei, T.; Zhang, T. Flexible humidity sensor based on modified cellulose paper. Sens. Actuators, B 2021, 339, 129879  DOI: 10.1016/j.snb.2021.129879
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    Flexible humidity sensor based on modified cellulose paper
    Guan, Xin; Hou, Zhaonan; Wu, Ke; Zhao, Hongran; Liu, Sen; Fei, Teng; Zhang, Tong
    Sensors and Actuators, B: Chemical (2021), 339 (), 129879CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
    It is of great significance to exploit a simple, cost-effective and environmentally friendly prepn. method of multifunctional humidity sensors. However, most humidity sensors need complex manufg. processes, high cost and narrow usage range. Herein, humidity sensors based on glycidyl tri-Me ammonium chloride (EPTAC) modified cellulose paper via a facile soln. method were fabricated, among which the paper is used for the purpose of the humidity sensing material and the sensor substrate. The sensitivity of the obtained sensor was improved by the modification of EPTAC, the response time was decreased to 25 s, which is equiv. to the first-rate paper-based humidity sensors. In addn., the paper-based humidity sensor is provided with well flexibility and biocompatibility, and it exhibits multifunctional applications in respiratory monitoring, non-contact switch and skin humidity monitoring. The low cost and facile prepn. technique in this work could provide a useful strategy for developing multifunctional humidity sensor.
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    Jiao, S.; Li, Y.; Li, J.; Abrha, H.; Liu, M.; Cui, J.; Wang, J.; Dai, Y.; Liu, X. Graphene oxide as a versatile platform for emerging hydrovoltaic technology. J. Mater. Chem. A 2022, 10, 18451,  DOI: 10.1039/D2TA04830B
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    Graphene oxide as a versatile platform for emerging hydrovoltaic technology
    Jiao, Shipu; Li, Yang; Li, Jiaxuan; Abrha, Halayit; Liu, Miao; Cui, Jinran; Wang, Jiao; Dai, Yexin; Liu, Xianhua
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2022), 10 (36), 18451-18469CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    A review. Hydrovoltaic technol. can harvest sustainable energy and clean water directly from various environments, providing a novel way to alleviate global environmental problems and the energy crisis. A wide variety of hydrovoltaic materials with distinctly different morphol., mech. and functional features have been created by using GO as a versatile building block. However, there is still a lack of comprehensive knowledge regarding the involvement of GO in the hydrovoltaic technol. and its future perspectives. In this review, the latest progress in the prepn. of GO-based hydrovoltaic materials and their various applications are summarized. The working mechanisms for the hydrovoltaic power generation and some remaining challenges are also discussed. Finally, some suggestions are given for further development of GO-based hydrovoltaic technol.
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    Joshi, P.; Yadav, R.; Hara, M.; Inoue, T.; Motoyama, Y.; Yoshimura, M. Contribution of B, N-co-doped reduced graphene oxide as a catalyst support to the activity of iridium oxide for oxygen evolution reaction. J. Mater. Chem. A 2021, 9, 9066,  DOI: 10.1039/D1TA00158B
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    Contribution of B,N-co-doped reduced graphene oxide as a catalyst support to the activity of iridium oxide for oxygen evolution reaction
    Joshi, Prerna; Yadav, Rohit; Hara, Masanori; Inoue, Tetsunari; Motoyama, Yukihiro; Yoshimura, Masamichi
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2021), 9 (14), 9066-9080CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    The current research deals with the study of boron and nitrogen co-doped reduced graphene oxide (BN-rGO) as a support material for iridium oxide (IrO2) nanoparticles for oxygen evolution reaction (OER) catalysis. The synthetic approach for the IrO2-BN-rGO catalyst involves the combination of pyrolysis and hydrothermal methods used for hierarchical nanostructures. BN-rGO possesses B-N, B-C, and N-C functional groups to support and stabilize the IrO2 catalyst nanoparticles. The altered electronic states of IrO2 on the BN-rGO support are compared with those of IrO2 on a non-doped support, rGO (IrO2-rGO), and on com. BN sheets (IrO2-c-BN). The catalyst shows a low overpotential (300 mV at 10 mA cm-2), high c.d. (55 mA cm-2 at 1.65 V), and significantly high durability (12 350 cycles; 45 h) in an acidic environment. The high stability of IrO2-BN-rGO may result from the presence of a chem. and electrochem. stable B-N bond. We confirm that other functional groups (B-C and N-C) and the rGO framework are equally crucial for better attachment of IrO2 nanoparticles.
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    Hummers, W. S., Jr.; Offeman, R. E. Preparation of graphitic oxide. J. Am. Chem. Soc. 1958, 80, 1339,  DOI: 10.1021/ja01539a017
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    Preparation of graphitic oxide
    Hummers, Wm. S., Jr.; Offeman, Richard E.
    Journal of the American Chemical Society (1958), 80 (), 1339CODEN: JACSAT; ISSN:0002-7863.
    See U.S. 2,798,878 (C.A. 51, 15080a).
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    Rockwood, D.-N.; Preda, R.-C.; Yücel, T.; Wang, X.-Q.; Lovett, M.-L.; Kaplan, D.-L. Materials fabrication from Bombyx mori silk fibroin. Nat. Protoc. 2011, 6, 1612,  DOI: 10.1038/nprot.2011.379
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    Materials fabrication from Bombyx mori silk fibroin
    Rockwood, Danielle N.; Preda, Rucsanda C.; Yucel, Tuna; Wang, Xiao-Qin; Lovett, Michael L.; Kaplan, David L.
    Nature Protocols (2011), 6 (10), 1612-1631CODEN: NPARDW; ISSN:1750-2799. (Nature Publishing Group)
    Silk fibroin, derived from Bombyx mori cocoons, is a widely used and studied protein polymer for biomaterial applications. Silk fibroin has remarkable mech. properties when formed into different materials, demonstrates biocompatibility, has controllable degrdn. rates from hours to years and can be chem. modified to alter surface properties or to immobilize growth factors. A variety of aq. or org. solvent-processing methods can be used to generate silk biomaterials for a range of applications. In this protocol, the authors include methods to ext. silk from B. mori cocoons to fabricate hydrogels, tubes, sponges, composites, fibers, microspheres and thin films. These materials can be used directly as biomaterials for implants, as scaffolding in tissue engineering and in vitro disease models, as well as for drug delivery.
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    Yin, K.; Li, H.; Xia, Y.; Bi, H.; Sun, J.; Liu, Z.; Sun, L. Thermodynamic and kinetic analysis of low temperature thermal reduction of graphene oxide. Nano–Micro Lett. 2011, 3, 51,  DOI: 10.1007/BF03353652
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    Thermodynamic and kinetic analysis of low-temperature thermal reduction of graphene oxide
    Yin, Kuibo; Li, Haitao; Xia, Yidong; Bi, Hengchang; Sun, Jun; Liu, Zhiguo; Sun, Litao
    Nano-Micro Letters (2011), 3 (1), 51-55CODEN: NLAEBV; ISSN:2150-5551. (Nano-Micro Letters)
    The thermodn. state and kinetic process of low-temp. deoxygenation reaction of graphene oxide (GO) were investigated for better understanding the redn. mechanism by using Differential Scanning Calorimetry (DSC), Thermogravimetry-Mass Spectrometry (TG-MS), and XPS. It was found that the thermal redn. reaction of GO is exothermic with degassing of CO2, CO and H2O. The graphene is thermodynamically more stable than GO. The deoxygenation reaction of GO is kinetically controlled and the activation energy for GO is calcd. as 167 kJ/mol (1.73 eV/atom).
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    Kim, S.; Geryak, R.-D.; Zhang, S.; Ma, R.; Calabrese, R.; Kaplan, D.-L.; Tsukruk, V.-V. Interfacial shear strength and adhesive behavior of silk ionomer surfaces. Biomacromolecules 2017, 18, 2876,  DOI: 10.1021/acs.biomac.7b00790
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    Interfacial shear strength and adhesive behavior of silk ionomer surfaces
    Kim, Sunghan; Geryak, Ren D.; Zhang, Shuaidi; Ma, Ruilong; Calabrese, Rossella; Kaplan, David L.; Tsukruk, Vladimir. V.
    Biomacromolecules (2017), 18 (9), 2876-2886CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
    The interfacial shear strength between different layers in multilayered structures of layer-by-layer (LbL) microcapsules is a crucial mech. property to ensure their robustness. In this work, we investigated the interfacial shear strength of modified silk fibroin ionomers utilized in LbL shells, an ionic-cationic pair with complementary ionic pairing, (SF)-poly-L-glutamic acid (Glu) and SF-poly-L-lysine (Lys), and a complementary pair with partially screened Coulombic interactions due to the presence of poly(ethylene glycol) (PEG) segments and SF-Glu/SF-Lys[PEG] pair. Shearing and adhesive behavior between these silk ionomer surfaces in the swollen state were probed at different spatial scales and pressure ranges by using functionalized at. force microscopy (AFM) tips as well as functionalized colloidal probes. The results show that both approaches were consistent in analyzing the interfacial shear strength of LbL silk ionomers at different spatial scales from a nanoscale to a fraction of a micron. Surprisingly, the interfacial shear strength between SF-Glu and SF-Lys[PEG] pair with partially screened ionic pairing was greater than the interfacial shear strength of the SF-Glu and SF-Lys pair with a high d. of complementary ionic groups. The difference in interfacial shear strength and adhesive strength is suggested to be predominantly facilitated by the interlayer hydrogen bonding of complementary amino acids and overlap of highly swollen PEG segments.
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    Geryak, R.; Quigley, E.; Kim, S.; Korolovych, V. F.; Calabrese, R.; Kaplan, D. L.; Tsukruk, V. V. Tunable interfacial properties in silk ionomer microcapsules with tailored multilayer interactions. Macromol. Biosci. 2019, 19, 1800176  DOI: 10.1002/mabi.201800176
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    Xiong, R.; Kim, H. S.; Zhang, S.; Kim, S.; Korolovych, V. F.; Ma, R.; Yingling, Y. G.; Lu, C.; Tsukruk, V. V. Template-guided assembly of silk fibroin on cellulose nanofibers for robust nanostructures with ultrafast water transport. ACS Nano 2017, 11, 12008,  DOI: 10.1021/acsnano.7b04235
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    Template-Guided Assembly of Silk Fibroin on Cellulose Nanofibers for Robust Nanostructures with Ultrafast Water Transport
    Xiong, Rui; Kim, Ho Shin; Zhang, Shuaidi; Kim, Sunghan; Korolovych, Volodymyr F.; Ma, Ruilong; Yingling, Yaroslava G.; Lu, Canhui; Tsukruk, Vladimir V.
    ACS Nano (2017), 11 (12), 12008-12019CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The authors report a spontaneous formation of peculiar "shish kebab" nanostructures with the periodic arrangement of silk fibroin domains along straight segments of cellulose nanofibers. The authors suggest that the formation of these shish kebab nanostructures is facilitated by the preferential organization of heterogeneous (β-sheets and amorphous silk) domains along the cellulose nanofiber driven by modulated axial distribution of cryst. planes, hydrogen bonding, and hydrophobic interactions as suggested by all-atom mol. dynamic simulations. Such shish kebab nanostructures enable the ultrathin membrane to possess open, transparent, mech. robust interlocked networks with high mech. performance with up to 30 GPa in stiffness and 260 MPa in strength. These nanoporous robust membranes allow for the extremely high water flux, up to 3.5 × 104 L h-1 m-2 bar-1 combined with high rejection rate for various org. mols., capability of capturing heavy metal ions and their further redn. into metal nanoparticles for added SERS detection capability and catalytic functionalities.
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    Cranston, E. D.; Eita, M.; Johansson, E.; Netrval, J.; Salajkova, M.; Arwin, H.; Wagberg, L. Determination of Young’s modulus for nanofibrillated cellulose multilayer thin films using buckling mechanics. Biomacromolecules 2011, 12, 961,  DOI: 10.1021/bm101330w
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    Determination of Young's Modulus for Nanofibrillated Cellulose Multilayer Thin Films Using Buckling Mechanics
    Cranston, Emily D.; Eita, Mohamed; Johansson, Erik; Netrval, Julia; Salajkova, Michaela; Arwin, Hans; Wagberg, Lars
    Biomacromolecules (2011), 12 (4), 961-969CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
    The Young's modulus of multilayer films contg. nanofibrillated cellulose (NFC) and polyethyleneimine (PEI) was detd. using the strain-induced elastic buckling instability for mech. measurements (SIEBIMM) technique. Multilayer films were built up on polydimethylsiloxane substrates using electrostatic layer-by-layer assembly. At 50% relative humidity, SIEBIMM gave a const. Young's modulus of 1.5 ± 0.2 GPa for 35-75 nm thick films. Conversely, in vacuum, the Young's modulus was 10 times larger, at 17.2 ± 1.2 GPa. A slight decrease in buckling wavelength with increasing strain was obsd. by SEM with in situ compression, and above 10% strain, extensive cracking parallel to the compressive direction occurred. We conclude that whereas PEI acts as a "glue" to hold multiple layers of NFC together, it prevents full development of hydrogen bonding and specific fibril-fibril interactions, and at high humidity, its hygroscopic nature decreases the elastic modulus when compared with pure NFC films.
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    Yin, Y.; Hu, K.; Grant, A.-M.; Zhang, Y.; Tsukruk, V.-V. Biopolymeric nanocomposites with enhanced interphases. Langmuir 2015, 31, 10859,  DOI: 10.1021/acs.langmuir.5b02744
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    Biopolymeric Nanocomposites with Enhanced Interphases
    Yin, Yi; Hu, Kesong; Grant, Anise M.; Zhang, Yuhong; Tsukruk, Vladimir V.
    Langmuir (2015), 31 (39), 10859-10870CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Ultrathin and robust nanocomposite membranes were fabricated by incorporating graphene oxide (GO) sheets into a silk fibroin (SF) matrix by a dynamic spin-assisted layer-by-layer assembly (dSA-LbL). We obsd. that in contrast to traditional SA-LbL reported earlier fast soln. removal during dropping of soln. on constantly spinning substrates resulted in largely unfolded biomacromols. with enhanced surface interactions and suppressed nanofibril formation. The resulting laminated nanocomposites possess outstanding mech. properties, significantly exceeding those previously reported for conventional LbL films with similar compn. The tensile modulus reached extremely high values of 170 GPa, which have never been reported for graphene oxide-based nanocomposites, the ultimate strength was close to 300 MPa, and the toughness was above 3.4 MJ m-3. The failure modes obsd. for these membranes suggested the self-reinforcing mechanism of adjacent graphene oxide sheets with strong 2 nm thick silk interphase composed mostly from individual backbones. This interphase reinforcement leads to the effective load transfer between the graphene oxide components in reinforced laminated nanocomposite materials with excellent mech. strength that surpasses those known today for conventional flexible laminated carbon nanocomposites from graphene oxide and biopolymer components.
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    Cho, H.; Shakil, A.; Polycarpou, A.-A.; Kim, S. Enabling selectively tunable mechanical properties of graphene oxide/silk fibroin/cellulose nanocrystal bionanofilms. ACS Nano 2021, 15, 19546,  DOI: 10.1021/acsnano.1c06573
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    Enabling Selectively Tunable Mechanical Properties of Graphene Oxide/Silk Fibroin/Cellulose Nanocrystal Bionanofilms
    Cho, Hyeonho; Shakil, Ahmad; Polycarpou, Andreas A.; Kim, Sunghan
    ACS Nano (2021), 15 (12), 19546-19558CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Enhancing and manipulating the mech. properties of graphene oxide (GO)-based structures are challenging because the GO assembly is easily delaminated. We develop nacre-like bionanofilms whose in-plane mech. properties can be manipulated through water vapor annealing without influencing their mech. properties in the thickness direction. These bionanofilms are prepd. from GO, silk fibroin (SF), and cellulose nanocrystals (CNCs) via a spin-assisted layer-by-layer assembly. The postannealing mech. properties of the films are detd. with at. force microscopy (AFM) bending and nanoindentation, and it is confirmed that the mech. properties of the bionanofilms are altered only in the in-plane direction. While AFM bending shows Young's moduli of 26.9, 36.3, 24.3, and 41.4 GPa for 15, 15 annealed, 30, and 30 annealed GO/SF/CNC trilayers, nanoindentation shows reduced moduli of 19.5 ± 2.6 and 19.5 ± 2.5 GPa before and after annealing, resp. The unaltered mech. properties of the bionanofilms along the thickness direction after annealing can be attributed to the CNC frame in the SF matrix acting as a support against stress in the thickness direction, while annealing reorganizes the bionanofilm structure. The tunability of the bionanofilms' mech. properties in only one direction through structure manipulation can lead to various applications, such as e-skin, wearable sensors, and human-machine interaction devices.
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    Cho, H.; Lee, J.; Hwang, H.; Hwang, W.; Kim, J.-G.; Kim, S. Mechanical properties of graphene oxide–silk fibroin bionanofilms via nanoindentation experiments and finite element analysis. Friction 2022, 10, 282,  DOI: 10.1007/s40544-021-0490-8
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    Kim, S.; Xiong, R.; Tsukruk, V.-V. Probing flexural properties of cellulose nanocrystal–graphene nanomembranes with force spectroscopy and bulging test. Langmuir 2016, 32, 5383,  DOI: 10.1021/acs.langmuir.6b01079
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    Probing Flexural Properties of Cellulose Nanocrystal-Graphene Nanomembranes with Force Spectroscopy and Bulging Test
    Kim, Sunghan; Xiong, Rui; Tsukruk, Vladimir V.
    Langmuir (2016), 32 (21), 5383-5393CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The flexural properties of ultrathin freely standing composite nanomembranes from reduced graphene oxide (rGO) and cellulose nanocrystals (CNC) have been probed by combining force spectroscopy for local nanomech. properties and bulging test for global mech. properties. We obsd. that the flexural properties of these rGO-CNC nanomembranes are controlled by rGO content and deformational regimes. The nanomembranes showed the enhanced mech. properties due to the strong interfacial interactions between interwoven rGO and CNC components. The presence of weak interfacial interactions resulted in time-dependent behavior with the relaxation time gradually decreased with increasing the deformational rate owing to the reducing viscous damping at faster probing regimes close to 10 Hz. We obsd. that the microscopic elastic bending modulus of 141 GPa from local force spectroscopy is close to the elastic tensile modulus evaluated from macroscopic bulging test, indicating the consistency of both approaches for analyzing the ultrathin nanomembranes at different spatial scales of deformation. We showed that the flexible rGO-CNC nanomembranes are very resilient in terms of their capacity to recover back into original shape.
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    Wang, Y.; Ma, R.; Hu, K.; Kim, S.; Fang, G.; Shao, Z.; Tsukruk, V.-V. Dramatic enhancement of graphene oxide/silk nanocomposite membranes: Increasing toughness, strength, and Young’s modulus via annealing of interfacial structures. ACS Appl. Mater. Interfaces 2016, 8, 24962,  DOI: 10.1021/acsami.6b08610
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    Dramatic Enhancement of Graphene Oxide/Silk Nanocomposite Membranes: Increasing Toughness, Strength, and Young's modulus via Annealing of Interfacial Structures
    Wang, Yaxian; Ma, Ruilong; Hu, Kesong; Kim, Sunghan; Fang, Guangqiang; Shao, Zhengzhong; Tsukruk, Vladimir. V.
    ACS Applied Materials & Interfaces (2016), 8 (37), 24962-24973CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    We demonstrate that stronger and more robust nacre-like laminated GO (graphene oxide)/SF (silk fibroin) nanocomposite membranes can be obtained by selectively tailoring the interfacial interactions between "bricks"-GO sheets and "mortar"-silk interlayers via controlled water vapor annealing. This facial annealing process relaxes the secondary structure of silk backbones confined between flexible GO sheets. The increased mobility leads to a significant increase in ultimate strength (by up to 41%), Young's modulus (up to 75%) and toughness (up to 45%). We suggest that local silk recrystn. is initiated in the proximity to GO surface by the hydrophobic surface regions serving as nucleation sites for β-sheet domains formation and followed by SF assembly into nanofibrils. Strong hydrophobic-hydrophobic interactions between GO layers with SF nanofibrils result in enhanced shear strength of layered packing. This work presented here not only gives a better understanding of SF and GO interfacial interactions, but also provides insight on how to enhance the mech. properties for the nacre-mimic nanocomposites by focusing on adjusting the delicate interactions between heterogeneous "brick" and adaptive "mortar" components with water/temp. annealing routines.
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    Sehrawat, P.; Islam, S.-S.; Mishra, P.; Ahmad, S. Reduced graphene oxide (rGO) based wideband optical sensor and the role of temperature, defect states and quantum efficiency. Sci. Rep. 2018, 8, 3537,  DOI: 10.1038/s41598-018-21686-2
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    Reduced graphene oxide (rGO) based wideband optical sensor and the role of Temperature, Defect States and Quantum Efficiency
    Abid; Sehrawat Poonam; Islam S S; Mishra Prabhash; Ahmad Shahab
    Scientific reports (2018), 8 (1), 3537 ISSN:.
    We report a facile and cost-effective approach to develop self-standing reduced Graphene Oxide (rGO) film based optical sensor and its low-temperature performance analysis where midgap defect states play a key role in tuning the crucial sensor parameters. Graphite oxide (GO) is produced by modified Hummers' method and reduced thermally at 250 °C for 1 h in Argon atmosphere to obtain rGO. Self-standing rGO film is prepared via vacuum filtration. The developed film is characterized by HRTEM, FESEM, Raman, and XRD techniques. The developed sensor exhibits highest sensitivity towards 635 nm illumination wavelength, irrespective of the operating temperature. For a given excitation wavelength, photoresponse study at low temperature (123K-303K) reveals inverse relationship between sensitivity and operating temperature. Highest sensitivity of 49.2% is obtained at 123 K for 635 nm laser at power density of 1.4 mW/mm(2). Unlike sensitivity, response- and recovery-time demonstrate directly proportional dependence with operating temperature. Power dependent studies establish linear relation between power-density and sensitivity, and a safe limit beyond which sample heating prolongs the recovery time. Wavelength-dependent studies shows that proposed sensor can efficiently operate from visible to near NIR region. To the best of our knowledge such rGO based optical sensor performance at low temperature had not been reported earlier.
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    Chang, H.; Sun, Z.; Saito, M.; Yuan, Q.; Zhang, H.; Li, J.; Wang, Z.; Fujita, T.; Ding, F.; Zheng, Z.; Yan, F.; Wu, H.; Chen, M.; Ikuhara, Y. Regulating infrared photoresponses in reduced graphene oxide phototransistors by defect and atomic structure control. ACS Nano 2013, 7, 6310,  DOI: 10.1021/nn4023679
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    Regulating Infrared Photoresponses in Reduced Graphene Oxide Phototransistors by Defect and Atomic Structure Control
    Chang, Haixin; Sun, Zhenhua; Saito, Mitsuhiro; Yuan, Qinghong; Zhang, Han; Li, Jinhua; Wang, Zhongchang; Fujita, Takeshi; Ding, Feng; Zheng, Zijian; Yan, Feng; Wu, Hongkai; Chen, Mingwei; Ikuhara, Yuichi
    ACS Nano (2013), 7 (7), 6310-6320CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Defects play significant roles in properties of graphene and related device performances. Most studies of defects in graphene focus on their influences on electronic or luminescent optical properties, while controlling IR optoelectronic performance of graphene by defect engineering remains a challenge. In the meantime, pristine graphene has very low IR photoresponses of ∼0.01 A/W due to fast photocarrier dynamics. Here the authors report regulating IR photoresponses in reduced graphene oxide phototransistors by defect and at. structure control for the 1st time. The IR optoelectronic transport and photocurrent generation are significantly influenced and well controlled by oxygenous defects and structures in reduced graphene oxide. Also, remarkable IR photoresponses are obsd. in photoconductor devices based on reduced graphene oxide with an external responsivity of ∼0.7 A/W, at least over one order of magnitude higher than that from pristine graphene. External quantum efficiencies of IR devices reach ultrahigh values of ∼97%, which to the authors' knowledge is one of the best efficiencies for IR photoresponses from nonhybrid, pure graphene or graphene-based derivs. The flexible IR photoconductor devices demonstrate no photoresponse degrdn. even after 1000 bending tests. The results open up new routes to control optoelectronic behaviors of graphene for high-performance devices.
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    Lee, C.; Wei, X.; Kysar, J.-W.; Hone, J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008, 321, 385,  DOI: 10.1126/science.1157996
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    Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene
    Lee, Changgu; Wei, Xiaoding; Kysar, Jeffrey W.; Hone, James
    Science (Washington, DC, United States) (2008), 321 (5887), 385-388CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    We measured the elastic properties and intrinsic breaking strength of free-standing monolayer graphene membranes by nanoindentation in an at. force microscope. The force-displacement behavior is interpreted within a framework of nonlinear elastic stress-strain response, and yields second- and third-order elastic stiffnesses of 340 newtons per m (N m-1) and -690 N m-1, resp. The breaking strength is 42 N m-1 and represents the intrinsic strength of a defect-free sheet. These quantities correspond to a Young's modulus of E = 1.0 terapascals, third-order elastic stiffness of D = -2.0 terapascals, and intrinsic strength of σint = 130 gigapascals for bulk graphite. These expts. establish graphene as the strongest material ever measured, and show that atomically perfect nanoscale materials can be mech. tested to deformations well beyond the linear regime.
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    Suk, J. W.; Piner, R.-D.; An, J.; Ruoff, R.-S. Mechanical properties of monolayer graphene oxide. ACS Nano 2010, 4, 6557,  DOI: 10.1021/nn101781v
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    Mechanical Properties of Monolayer Graphene Oxide
    Suk, Ji Won; Piner, Richard D.; An, Jinho; Ruoff, Rodney S.
    ACS Nano (2010), 4 (11), 6557-6564CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Mech. properties of ultrathin membranes consisting of one layer, two overlapped layers, and three overlapped layers of graphene oxide platelets were investigated by at. force microscopy (AFM) imaging in contact mode. To evaluate both the elastic modulus and prestress of thin membranes, the AFM measurement was combined with the finite element method (FEM) in a new approach for evaluating the mechanics of ultrathin membranes. Monolayer graphene oxide was found to have a lower effective Young's modulus (207.6 ±23.4 GPa when a thickness of 0.7 nm is used) as compared to the value reported for "pristine" graphene. The prestress (39.7-76.8 MPa) of the graphene oxide membranes obtained by soln.-based deposition was found to be 1 order of magnitude lower than that obtained by others for mech. cleaved graphene. The novel AFM imaging and FEM-based mapping methods presented here are of general utility for obtaining the elastic modulus and prestress of thin membranes.
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    Magoshi, J.; Magoshi, Y.; Nakamura, S.; Kasai, N.; Kakudo, M. Physical properties and structure of silk. V. Thermal behavior of silk fibroin in the random-coil conformation. J. Polym. Sci. Polym. Phys. 1977, 15, 1675– 1683,  DOI: 10.1002/pol.1977.180150915
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    Physical properties and structure of silk. V. Thermal behavior of silk fibroin in the random-coil conformation
    Magoshi, Jun; Magoshi, Yoshiko; Nakamura, Shigeo; Kasai, Nobutami; Kakudo, Masao
    Journal of Polymer Science, Polymer Physics Edition (1977), 15 (9), 1675-83CODEN: JPLPAY; ISSN:0098-1273.
    Evaluation of the thermal behavior of films of amorphous silk fibroin in the random-coil conformation by differential scanning calorimetry, thermal expansion, dynamic mech. measurements, x-ray diffraction, and ir spectroscopy showed that water was lost up to 100°, inter- and intramol. H bonds were broken at 150-80°, glass transition occurred at 173°, and the random coil → β-form transition, accompanied by reformation of H bonds, occurred at >180°. Thermally-induced crystn. to β-form crystal started at ∼190°.
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    Freddi, G.; Monti, P.; Nagura, M.; Gotoh, Y.; Tsukada, M. Structure and molecular conformation of tussah silk fibroin films: Effect of heat treatment. J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 841,  DOI: 10.1002/(SICI)1099-0488(19970415)35:5<841::AID-POLB13>3.0.CO;2-A
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    Structure and molecular conformation of tussah silk fibroin films: effect of heat treatment
    Freddi, Giuliano; Monti, Patrizia; Nagura, Masanobu; Gotoh, Yohko; Tsukada, Masuhiro
    Journal of Polymer Science, Part B: Polymer Physics (1997), 35 (5), 841-847CODEN: JPBPEM; ISSN:0887-6266. (Wiley)
    Structural changes of tussah (Antheraea pernyi) silk fibroin films induced by heat treatment were studied as a function of the treatment temp. in the range 200-250°. The DSC curve of tussah films with α-helix mol. conformation displayed characteristic endo and exo peaks at 216 and 226°, resp. These peaks first weakened and then completely disappeared after heating at 230°. Accordingly, the TMA thermal shrinkage at 206° disappeared when the films were heated at 230°. The onset of wt. loss was monitored at 210° by means of TG measurements. X-ray diffraction profiles gradually changed from α-helix to β-sheet cryst. structure as the treatment temp. increased from 200 to 250°. On raising the heating temp. above 200°, the intensity of IR and Raman bands characteristic fo β-sheet conformation increased in the whole ranges of amide and skeletal modes. The sample treated at 200° showed a spectral pattern intermediate between α-helix and β-sheet mol. conformation. The IR marker band for random coil structure, still detectable at 200°, disappeared at higher treatment temps. Spectral changes attributable to the onset of thermal degrdn. appeared at 230°.
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    Yin, Z.; Hannard, F.; Barthelat, F. Impact-resistant nacre-like transparent materials. Science 2019, 364, 1260,  DOI: 10.1126/science.aaw8988
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    Impact-resistant nacre-like transparent materials
    Yin, Z.; Hannard, F.; Barthelat, F.
    Science (Washington, DC, United States) (2019), 364 (6447), 1260-1263CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Glass has outstanding optical properties, hardness, and durability, but its applications are limited by its inherent brittleness and poor impact resistance. Lamination and tempering can improve impact response but do not suppress brittleness. We propose a bioinspired laminated glass that duplicates the 3-dimensional brick and mortar arrangement of nacre from mollusk shells, with periodic 3-dimensional architectures and interlayers made of a transparent thermoplastic elastomer. This material reproduces the tablet sliding mechanism, which is key to the toughness of natural nacre but has been largely absent in synthetic nacres. Tablet sliding generates nonlinear deformations over large vols. and significantly improves toughness. This nacre-like glass is also 2-3-fold more impact resistant than laminated glass and tempered glass while maintaining high strength and stiffness.
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    Trung, T.-Q.; Le, H.-S.; Dang, T.-M.-L.; Ju, S.; Park, S.-Y.; Lee, N.-E. Freestanding, fiber-based, wearable temperature sensor with tunable thermal index for healthcare monitoring. Adv. Healthcare Mater. 2018, 7, 1800074  DOI: 10.1002/adhm.201800074
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    Kim, B.-K.; Kwon, O.-H.; Park, W.-H.; Cho, D. Thermal, mechanical, impact, and water absorption properties of novel silk fibroin fiber reinforced poly (butylene succinate) biocomposites. Macromol. Res. 2016, 24, 734,  DOI: 10.1007/s13233-016-4102-9
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    Thermal, mechanical, impact, and water absorption properties of novel silk fibroin fiber reinforced poly(butylene succinate) biocomposites
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  • Abstract

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    Figure 1

    Figure 1. (a) FE-SEM images showing morphologies of rGO and rGO/SF films. The carbon to oxygen (C/O) ratio of (b) rGO and (c) rGO/SF films determined using the EDS. All scale bars are 40 μm.

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    Scheme 1

    Scheme 1. Schematics of the Fabrication Process of (a) rGO Paper and (b) rGO/SF Paper
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    Figure 2

    Figure 2. AFM load–deflection plots of (a) rGO and rGO/SF sensors. (b) Thickness of different types of bilayer films. Morphological AFM images of (c) rGO and (d) rGO/SF sensors for a reduction temperature of 200 °C.

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    Figure 3

    Figure 3. Calculated values of the (a) effective bending stiffness, (b) elastic bending modulus, (c) flexural rigidity, and (d) normalized flexural rigidity of rGO and rGO/SF sensors for reduction temperatures of 155, 200, and 230 °C.

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    Figure 4

    Figure 4. Schematic of the process by which a nacre-like rGO/SF structure is formed from a GO/SF structure.

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    Figure 5

    Figure 5. (a) Variation of the responsivity of rGO and rGO/SF temperature sensors (prepared at different reduction temperatures) in terms of resistance change, ΔR/R0, as a function of the temperature. (b) Variation of the TCR at 300 K with the reduction temperature for the rGO and rGO/SF sensors.

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    Figure 6

    Figure 6. (a) Responsivity of rGO and rGO/SF humidity sensors (fabricated at different reduction temperatures) in terms of resistance change as a function of humidity change, and the gradient of a linear fit to the resistance-RH plot for (b) rGO sensor and (c) rGO/SF sensor.

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    Figure 7

    Figure 7. Schematics of (a) mechanism of temperature and humidity sensors and (b) tailorable sensitivity.

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    Figure 8

    Figure 8. (a) Thickness of rGO and rGO/SF films at temperatures of 30, 40, 50, 60, 70, 75, and 80 °C for reduction temperatures of 155, 200, and 230 °C. (b) Change in the thickness of the two films for reduction temperatures of 155, 200, and 230 °C. (c) Coefficient of thermal expansion of the two films for reduction temperatures of 155, 200, and 230 °C.

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    Figure 9

    Figure 9. (a) Result of the temperature cycling test for the rGO/SF sensor for a reduction temperature of 170 °C, performed to ascertain the sensor’s stability and durability. (b) Result of the humidity cycling test for the humidity sensor, performed to assess the stability and durability of the sensor.

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    Figure 10

    Figure 10. Surface of the rGO sensor (a) before and (b) after the bending test (diameter: 2.1 mm), and the surface of the rGO/SF sensor (c) before and (d) after the bending test (diameter: 0.8 mm). The delamination area is shown by a red arrow. All scale bars are 200 μm.

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    Figure 11

    Figure 11. Evaluation of the performance of sensors after the bending test through a comparison with the performance before the bending test: (a) rGO and (b) rGO/SF sensors fabricated with a reduction temperature of 200 °C.

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    Figure 12

    Figure 12. (a) Respiration rates detected by the rGO/SF sensor. (b) Electrical signal of the rGO/SF sensor in the philtrum region induced by the inhaled air (upper red dashed line; temperature of 29.7 °C and relative humidity of 39.9%) and exhaled air (lower red dashed line; temperature of 31.6 °C and relative humidity of 76.7%).

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  • References

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      Liu, Xiaohua; Zhang, Dongzhi; Wang, Dongyue; Li, Tingting; Song, Xiaoshuang; Kang, Zhanjia
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2021), 9 (25), 14524-14533CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      In this work, a quartz crystal microbalance (QCM) humidity sensor based on a chitosan/polypyrrole (CS/PPy) composite film was prepd. by a facile phys. modification method. The as-synthesized CS/PPy composite was characterized using XPS, SEM (SEM), water contact angle test and Fourier transform IR spectroscopy (FT-IR) to confirm its successful prepn. The exptl. results showed that the CS/PPy QCM humidity sensor had high sensitivity, low hysteresis (1.68% RH), short response/recovery time (13 s/2 s), excellent selectivity, and good repeatability and stability. The overall frequency offset was -5132.12 Hz (0-97% RH). The mechanism of the enhanced humidity sensing performance was attributed to the hydrophilic groups and incomplete coating structure. In addn., the sensor could quickly and accurately sense changes in rate, depth, and rhythm of breathing. Furthermore, an app was designed to accurately det. the type of breathing and demonstrate the potential respiratory monitoring of patients.
    7. 7
      Zhang, S.; Geryak, R.; Geldmeier, J.; Kim, S.; Tsukruk, V. V. Synthesis, assembly, and applications of hybrid nanostructures for biosensing. Chem. Rev. 2017, 117, 12942,  DOI: 10.1021/acs.chemrev.7b00088
      7
      Synthesis, Assembly, and Applications of Hybrid Nanostructures for Biosensing
      Zhang, Shuaidi; Geryak, Ren; Geldmeier, Jeffrey; Kim, Sunghan; Tsukruk, Vladimir V.
      Chemical Reviews (Washington, DC, United States) (2017), 117 (20), 12942-13038CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review on the recent advances in the synthesis, assembly, and applications of nanoengineered reporting and transducing components crit. for efficient biosensing; including inorg. reporters and org. transducers.
    8. 8
      Danninger, D.; Pruckner, R.; Holzinger, L.; Koeppe, R.; Kaltenbrunner, M. MycelioTronics: Fungal mycelium skin for sustainable electronics. Sci. Adv. 2022, 8, eadd7118  DOI: 10.1126/sciadv.add7118
      There is no corresponding record for this reference.
    9. 9
      Han, M.; Shen, W. Nacre-inspired cellulose nanofiber/MXene flexible composite film with mechanical robustness for humidity sensing. Carbohydr. Polym. 2022, 298, 120109  DOI: 10.1016/j.carbpol.2022.120109
      9
      Nacre-inspired cellulose nanofiber/MXene flexible composite film with mechanical robustness for humidity sensing
      Han, Mimi; Shen, Wenhao
      Carbohydrate Polymers (2022), 298 (), 120109CODEN: CAPOD8; ISSN:0144-8617. (Elsevier Ltd.)
      Inspired by nacre-layered nanostructure, 1D 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)-oxidized cellulose nanofibers (TOCNFs) were used as the template to assemble 2D MXene nanosheets into a layered TOCNF/MXene nanocomposite film by a vacuum-assisted filtration strategy. The synergistic effect of the MXene "brick" and TOCNFs "mortar" endowed the composite film with excellent flexibility and a tensile strength of 128.13 MPa, which were attributed to interactions between the interconnected three-dimensional network and multiple hydrogen bonds between TOCNFs and MXene. The humidity-sensing mechanism of the sensor involved the swelling/contraction of channels between MXene interlayers induced by adsorbed H2O and the swelling of TOCNFs. The TOCNF/MXene sensor showed a max. response (-ΔI/I0) of 90% under 97% RH, outstanding bending and folding durability (up to 50 cycles), and long-term stability. Lastly, the sensor could dynamically monitor human respiration, skin, and fingertip humidity, suggesting its promising applications in smart wearable electronics.
    10. 10
      Tachibana, S.; Wang, Y. F.; Sekine, T.; Takeda, Y.; Hong, J.; Yoshida, A.; Abe, M.; Miura, R.; Watanabe, Y.; Kumaki, D.; Tokito, S. A printed flexible humidity sensor with high sensitivity and fast response using a cellulose nanofiber/carbon black composite. ACS Appl. Mater. Interfaces 2022, 14, 5721– 5728,  DOI: 10.1021/acsami.1c20918
      10
      A Printed Flexible Humidity Sensor with High Sensitivity and Fast Response Using a Cellulose Nanofiber/Carbon Black Composite
      Tachibana, Shogo; Wang, Yi-Fei; Sekine, Tomohito; Takeda, Yasunori; Hong, Jinseo; Yoshida, Ayako; Abe, Mai; Miura, Reo; Watanabe, Yushi; Kumaki, Daisuke; Tokito, Shizuo
      ACS Applied Materials & Interfaces (2022), 14 (4), 5721-5728CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      In the emerging Internet of Things (IoT) society, there is a significant need for low-cost, high-performance flexible humidity sensors in wearable devices. However, com. available humidity sensors lack flexibility or require expensive and complex fabrication methods, limiting their application and widespread use. We report a high-performance printed flexible humidity sensor using a cellulose nanofiber/carbon black (CNF/CB) composite. The cellulose nanofiber enables excellent dispersion of carbon black, which facilitates the ink prepn. and printing process. At the same time, its hydrophilic and porous nature provides high sensitivity and fast response to humidity. Significant resistance changes of 120% were obsd. in the sensor at humidity ranging from 30% RH to 90% RH, with a fast response time of 10 s and a recovery time of 6 s. Furthermore, the developed sensor also exhibited high-performance uniformity, response stability, and flexibility. A simple humidity detection device was fabricated and successfully applied to monitor human respiration and noncontact fingertip moisture as a proof-of-concept.
    11. 11
      Stobinski, L.; Lesiak, B.; Malolepszy, A.; Mazurkiewicz, M.; Mierzwa, B.; Zemek, J.; Jiricek, P.; Bieloshapka, I. Graphene oxide and reduced graphene oxide studied by the XRD, TEM and electron spectroscopy methods. J. Electron Spectrosc. Relat. Phenom. 2014, 195, 145,  DOI: 10.1016/j.elspec.2014.07.003
      11
      Graphene oxide and reduced graphene oxide studied by the XRD, TEM and electron spectroscopy methods
      Stobinski, L.; Lesiak, B.; Malolepszy, A.; Mazurkiewicz, M.; Mierzwa, B.; Zemek, J.; Jiricek, P.; Bieloshapka, I.
      Journal of Electron Spectroscopy and Related Phenomena (2014), 195 (), 145-154CODEN: JESRAW; ISSN:0368-2048. (Elsevier B.V.)
      The com. and synthesized few-layer graphene oxide, prepd. using oxidn. reactions, and few-layer reduced graphene oxide samples were structurally and chem. investigated by the X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron spectroscopy methods, i.e. XPS and reflection electron energy loss spectroscopy (REELS). The com. graphene oxide (FL-GOc) shows a stacking nanostructure of about 22 × 6 nm av. diam. by height with the distance of 0.9 nm between 6-7 graphene layers, whereas the resp. reduced graphene oxide (FL-RGOc)-about 8 × 1 nm av. diam. by height stacking nanostructure with the distance of 0.4 nm between 2-3 graphene layers (XRD). The REELS results are consistent with those by the XRD indicating 8 (FL-GOc) and 4 layers (FL-RGOc). In graphene oxide and reduced graphene oxide prepd. from the graphite the REELS indicates 8-11 and 7-10 layers. All graphene oxide samples show the C/O ratio of 2.1-2.3, 26.5-32.1 at% of C sp3 bonds and high content of functional oxygen groups (hydroxyl-C-OH, epoxy-C-O-C, carbonyl-C=O, carboxyl-C-OOH, water) (XPS). Redn. increases the C/O ratio to 2.8-10.3, decreases C sp3 content to 11.4-20.3 at% and also the content of C-O-C and C=O groups, accompanied by increasing content of C-OH and C-OOH groups. Formation of addnl. amt. of water due to functional oxygen group redn. leads to layer delamination. Removing of functional oxygen groups and water mols. results in decreasing the distance between the graphene layers.
    12. 12
      Olowojoba, G.-B.; Eslava, S.; Gutierrez, E.-S.; Kinloch, A.-J.; Mattevi, C.; Rocha, V.-G.; Taylor, A.-C. In situ thermally reduced graphene oxide/epoxy composites: Thermal and mechanical properties. Appl. Nanosci. 2016, 6, 1015,  DOI: 10.1007/s13204-016-0518-y
      12
      In situ thermally reduced graphene oxide/epoxy composites: thermal and mechanical properties
      Olowojoba, Ganiu B.; Eslava, Salvador; Gutierrez, Eduardo S.; Kinloch, Anthony J.; Mattevi, Cecilia; Rocha, Victoria G.; Taylor, Ambrose C.
      Applied Nanoscience (2016), 6 (7), 1015-1022CODEN: ANPACY; ISSN:2190-5517. (Springer GmbH)
      Graphene has excellent mech., thermal, optical and elec. properties and this has made it a prime target for use as a filler material in the development of multifunctional polymeric composites. However, several challenges need to be overcome to take full advantage of the aforementioned properties of graphene. These include achieving good dispersion and interfacial properties between the graphene filler and the polymeric matrix. In the present work, we report the thermal and mech. properties of reduced graphene oxide/epoxy composites prepd. via a facile, scalable and com. viable method. Electron micrographs of the composites demonstrate that the reduced graphene oxide (rGO) is well dispersed throughout the composite. Although no improvements in glass transition temp., tensile strength and thermal stability in air of the composites were obsd., good improvements in thermal cond. (about 36%), tensile and storage moduli (more than 13%) were recorded with the addn. of 2 wt% of rGO.
    13. 13
      Chen, Q.; Liu, Y.; Gu, K.; Yao, J.; Shao, Z.; Chen, X. Silk-based electrochemical sensor for the detection of glucose in sweat. Biomacromolecules 2022, 23, 3928,  DOI: 10.1021/acs.biomac.2c00753
      13
      Silk-Based Electrochemical Sensor for the Detection of Glucose in Sweat
      Chen, Qianying; Liu, Yi; Gu, Kai; Yao, Jinrong; Shao, Zhengzhong; Chen, Xin
      Biomacromolecules (2022), 23 (9), 3928-3935CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
      The development of reliable glucose sensors for noninvasive monitoring is highly desirable and essential for diabetes detection. As a testing sample, sweat is voluminous and is easy to collect compared to blood. However, the application of sweat glucose sensors is generally limited because of their low stability and sensitivity compared to com. glucometers. In this manuscript, a silk nanofibril (SNF)/reduced graphene oxide (RGO)/glucose oxidase (GOx) composite was developed as the working electrode of the sweat glucose sensor. The SNF/RGO/GOx composite was prepd. via a facile two-step process, which involved the self-assembly of SNF from silk fibroin while reducing graphene oxide to RGO and immobilizing GOx on SNF. The SNF/RGO/GOx glucose sensor exhibited a low limit of detection (300 nM) and high sensitivity (18.0μA/mM) in the sweat glucose range, covering both healthy people and diabetic patients (0-100μM). Moreover, the SNF/RGO/GOx glucose sensors showed a long stability for at least 4 wk. Finally, the SNF/RGO/GOx glucose sensor was applied to test the actual sweat samples from two volunteers and two sweating methods (by dry sauna and exercise). The results indicate the glucose data tested by the SNF/RGO/GOx glucose sensor were reliable, which correlated well to the data obtained from the com. glucometer. Therefore, the SNF/RGO/GOx glucose sensor developed in this study may have a great potential for glucose control in personalized healthcare monitoring and chronic disease management.
    14. 14
      Cho, H.; Lee, H.; Lee, S.; Kim, S. Reduced graphene oxide-based wearable and bio-electrolyte triggered pressure sensor with tunable sensitivity. Ceram. Int. 2021, 47, 17702,  DOI: 10.1016/j.ceramint.2021.03.090
      14
      Reduced graphene oxide-based wearable and bio-electrolyte triggered pressure sensor with tunable sensitivity
      Cho, Hyeonho; Lee, Hyoeun; Lee, Sangmin; Kim, Sunghan
      Ceramics International (2021), 47 (12), 17702-17710CODEN: CINNDH; ISSN:0272-8842. (Elsevier Ltd.)
      Herein, we introduce a wearable, self-powered pressure sensor with tailorable sensitivity using a reduced graphene oxide-coated porous polyurethane (rGO-PP) composite. The micropores of the porous polyurethane (incorporating rGO flakes) create a conductive path which changes based on the applied pressure, resulting in corresponding changes in the elec. signal. The rGO-PP pressure sensor's power is generated using a self-powered galvanic cell and bio-electrolyte extd. from human skin. The rGO-PP pressure sensor circumvents complicated elec. circuits, facilitating its ease of use as a wearable device. Through the serial connection of galvanic cells, the sensitivity of the rGO-PP pressure sensor can be tailored within an extraordinarily wide range between 0.46 to 3.05 kPa-1. The rGO-PP pressure sensors display rapid response times (16 ms). Consequently, they can be used in the development of applications including braille recognition, human-machine interaction, and the restoration of touch perception.
    15. 15
      Hu, X.; Xia, X.-X.; Huang, S.-C.; Qian, Z.-G. Development of adhesive and conductive resilin-based hydrogels for wearable sensors. Biomacromolecules 2019, 20, 3283,  DOI: 10.1021/acs.biomac.9b00389
      15
      Development of Adhesive and Conductive Resilin-Based Hydrogels for Wearable Sensors
      Hu, Xiao; Xia, Xiao-Xia; Huang, Sheng-Chen; Qian, Zhi-Gang
      Biomacromolecules (2019), 20 (9), 3283-3293CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
      Integrating multifunctionality such as stretchability, adhesiveness, and electrocond. on a single protein hydrogel is highly desirable for various applications, and remains a challenge. Here we present the development of such multifunctional hydrogels based on resilin, a natural rubber-like material with remarkable extensibility and resilience. First, genetically engineered resilin-like proteins (RLPs) with varying mol. wt. were biosynthesized to tune mech. strength and stiffness of the crosslinked RLP hydrogels. Second, glycerol was incorporated into the hydrogels to endow adhesive properties. Next, a graphene-RLP conjugate was synthesized for crosslinking with the unmodified, pristine RLP to form an integrated network. The obtained hybrid hydrogel could be stretched to over four times of its original length, and self-adhered to diverse substrate surfaces due to its high adhesion strength of ∼24 kPa. Furthermore, the hybrid hydrogel showed high sensitivity, with a gauge factor of 3.4 at 200% strain, and was capable of real-time monitoring human activities such as finger bending, swallowing, and phonating. Due to these favorable attributes, the graphene/resilin hybrid hydrogel was a promising material for use in wearable sensors. In addn., the above material design and functionalization strategy may provide intriguing opportunities to generate innovative materials for broad applications.
    16. 16
      Xu, L.; Zhai, H.; Chen, X.; Liu, Y.; Wang, M.; Liu, Z.; Umar, M.; Ji, C.; Chen, Z.; Jin, L.; Liu, Z.; Song, Q.; Yue, P.; Li, Y.; Ye, T.-T. Coolmax/graphene-oxide functionalized textile humidity sensor with ultrafast response for human activities monitoring. Chem. Eng. J. 2021, 412, 128639  DOI: 10.1016/j.cej.2021.128639
      16
      Coolmax/graphene-oxide functionalized textile humidity sensor with ultrafast response for human activities monitoring
      Xu, Lulu; Zhai, Heng; Chen, Xiao; Liu, Yulong; Wang, Miao; Liu, Zhangchi; Umar, Muhammad; Ji, Chengyu; Chen, Zhongda; Jin, Lu; Liu, Zekun; Song, Qingwen; Yue, Pengfei; Li, Yi; Ye, Terry T.
      Chemical Engineering Journal (Amsterdam, Netherlands) (2021), 412 (), 128639CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
      Among different fibrous e-textile sensors, humidity sensor has a particular significance in respiration monitoring, water presence alert, and skin contact/non-contact indication. These applications not only demand sensors to have fast response and recovery speed, robustness to attrition and friction, the sensors responses also need to be unimodal, i.e., only sensitive to humidity and insensitive to other impacts, such as temp. variation, folding and stretching of the fabric. Previously reported e-textile humidity sensors, when being used as a quick humidity indicator, suffered from slow response/recovery time, interference from multimodal sensitivities, or devices are non-fabric based and cannot be seamlessly integrated with apparels. In this paper, we have studied textile-based humidity sensors constructed from different natural and synthetic fibers (cotton, wool and Coolmax) and discovered that the graphene-oxide (GO) functionalized Coolmax humidity sensor (GO-Coolmax) exhibits ultrafast response/recovery time (less than 0.6 s). The senor is also insensitive to external pressure and temp. changes, i.e., the resistance variations caused by these impacts are within 10%, much smaller than that from humidity variations (above 80%). We have also demonstrated the use of the sensor as a humidity indicator/alert in various wearable applications through prototyping and expts.
    17. 17
      Zeng, Y.; Li, T.; Yao, Y.; Li, T.; Hu, L.; Marconnet, A. Thermally conductive reduced graphene oxide thin films for extreme temperature sensors. Adv. Funct. Mater. 2019, 29, 1901388  DOI: 10.1002/adfm.201901388
      There is no corresponding record for this reference.
    18. 18
      Ni, Y.; Huang, J.; Li, S.; Dong, X.; Zhu, T.; Cai, W.; Chen, Z.; Lai, Y. Robust superhydrophobic rGO/PPy/PDMS coatings on a polyurethane sponge for underwater pressure and temperature sensing. ACS Appl. Mater. Interfaces 2021, 13, 53271,  DOI: 10.1021/acsami.1c17165
      18
      Robust Superhydrophobic rGO/PPy/PDMS Coatings on a Polyurethane Sponge for Underwater Pressure and Temperature Sensing
      Ni, Yimeng; Huang, Jianying; Li, Shuhui; Dong, Xiuli; Zhu, Tianxue; Cai, Weilong; Chen, Zhong; Lai, Yuekun
      ACS Applied Materials & Interfaces (2021), 13 (44), 53271-53281CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Flexible wearable pressure sensors have attracted great interest from researchers in recent years because of their important applications in human-machine interaction, human behavior detection, medical diagnosis, and other fields. At present, integrating multiple functions such as pressure and temp. sensing and self-cleaning into a single material remains a challenging task. Here, by in situ redn. of graphene oxide (GO) grown on a sponge surface and deposition of polypyrrole (PPy) nanoparticles, we have built a highly sensitive, stable, and multifunctional rGO/PPy/poly(dimethylsiloxane) (PDMS) polyurethane (PU) sponge (GPPS) sensor for the detection of pressure, water level, and temp. This multifunctional sensor shows excellent pressure-sensing performance, ultrasensitive loading sensing of a leaf (98 mg), and outstanding reproducibility over 5000 cycles. Due to the stability of the superhydrophobic surface water contact angle (WCA) = 153.3°, our sensor can work in an underwater environment, which can sense water levels from 1 cm (~ 98 Pa) to 40 cm and also a variety of underwater behaviors (knock, ultrasonication, blow, etc.) with high stability. In addn., the sensor can be integrated into a circuit for the water level and pressure detection. The sensor can also be used as a smart underwater-temp. sensor; it shows a linear temp. coeff. of resistance (TCR) of 0.48% °C-1 in a temp. range of 35-80°C. This multifunctional sensor shows potential application prospects in wearable electronic devices for sensing.
    19. 19
      Wan, S.; Chen, Y.; Fang, S.; Wang, S.; Xu, Z.; Jiang, L.; Baughman, R. H.; Cheng, Q. High-strength scalable graphene sheets by freezing stretch-induced alignment. Nat. Mater. 2021, 20, 624,  DOI: 10.1038/s41563-020-00892-2
      19
      High-strength scalable graphene sheets by freezing stretch-induced alignment
      Wan, Sijie; Chen, Ying; Fang, Shaoli; Wang, Shijun; Xu, Zhiping; Jiang, Lei; Baughman, Ray H.; Cheng, Qunfeng
      Nature Materials (2021), 20 (5), 624-631CODEN: NMAACR; ISSN:1476-1122. (Nature Portfolio)
      Efforts to obtain high-strength graphene sheets by near-room-temp. assembly have been frustrated by the misalignment of graphene layers, which degrades mech. properties. While in-plane stretching can decrease this misalignment, it reappears when releasing the stretch. Here we use covalent and π-π inter-platelet bridging to permanently freeze stretch-induced alignment of graphene sheets, and thereby increase isotropic in-plane sheet strength to 1.55 GPa, in combination with a high Young's modulus, elec. cond. and wt.-normalized shielding efficiency. Moreover, the stretch-bridged graphene sheets are scalable and can be easily bonded together using a com. resin without appreciably decreasing the performance, which establishes the potential for practical applications.
    20. 20
      Mao, L.; Park, H.; Soler-Crespo, R. A.; Espinosa, H. D.; Han, T. H.; Nguyen, S. T.; Huang, J. Stiffening of graphene oxide films by soft porous sheets. Nat. Commun. 2019, 10, 3677,  DOI: 10.1038/s41467-019-11609-8
      20
      Stiffening of graphene oxide films by soft porous sheets
      Mao Lily; Nguyen SonBinh T; Park Hun; Han Tae Hee; Park Hun; Huang Jiaxing; Soler-Crespo Rafael A; Espinosa Horacio D; Espinosa Horacio D
      Nature communications (2019), 10 (1), 3677 ISSN:.
      Graphene oxide (GO) sheets have been used as a model system to study how the mechanical properties of two-dimensional building blocks scale to their bulk form, such as paper-like, lamellar-structured thin films. Here, we report that the modulus of multilayer GO films can be significantly enhanced if some of the sheets are drastically weakened by introducing in-plane porosity. Nanometer-sized pores are introduced in GO sheets by chemical etching. Membrane-deflection measurements at the single-layer level show that the sheets are drastically weakened as the in-plane porosity increases. However, the mechanical properties of the corresponding multilayer films are much less sensitive to porosity. Surprisingly, the co-assembly of pristine and etched GO sheets yields even stiffer films than those made from pristine sheets alone. This is attributed to the more compliant nature of the soft porous sheets, which act as a binder to improve interlayer packing and load transfer in the multilayer films.
    21. 21
      Liang, X.; Zhu, M.; Li, H.; Dou, J.; Jian, M.; Xia, K.; Li, S.; Zhang, Y. Hydrophilic, breathable, and washable graphene decorated textile assisted by silk sericin for integrated multimodal smart wearables. Adv. Funct. Mater. 2022, 32, 2200162  DOI: 10.1002/adfm.202200162
      21
      Hydrophilic, Breathable, and Washable Graphene Decorated Textile Assisted by Silk Sericin for Integrated Multimodal Smart Wearables
      Liang, Xiaoping; Zhu, Mengjia; Li, Haifang; Dou, Jinxin; Jian, Muqiang; Xia, Kailun; Li, Shuo; Zhang, Yingying
      Advanced Functional Materials (2022), 32 (42), 2200162CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Achieving integrated systems with comfortability and durability comparable to traditional textiles is one of the ultimate pursuits of smart wearables. This work reports a hydrophilic, breathable, biocompatible, and washable graphene-decorated electronic textile that is achieved with the assistance of silk sericin and enables the fabrication of comfortable and integrated multisensing textiles. The graphene-decorated textile is prepd. by dyeing com. textile with an aq. graphene ink, which contains natural silk sericin-coated graphene and is free of any artificial chems. The conformally coated hydrophilic sericin-graphene flakes and the well-reserved knitted structure endow the textile with good cond., excellent hydrophilicity, biocompatibility, breathability, and flexibility, ensuring its electronic performance and wearing-comfort. Moreover, the obtained textile is washable after processed with a crosslinking agent. Based on the obtained textile, an integrated multisensing textile that can simultaneously collect and analyze myoelec. and mech. signals is developed, enabling the recognition and distinguishment of complex human motions. With the combined features of hydrophilicity, breathability, biocompatibility, washability, and versatility, this strategy of fabricating electronic textiles based on conventional textiles and an aq. sericin-graphene ink provides a scalable and sustainable way to construct smart wearables.
    22. 22
      Liang, X.; Li, H.; Dou, J.; Wang, Q.; He, W.; Wang, C.; Li, D.; Lin, J.-M.; Zhang, Y. Stable and biocompatible carbon nanotube ink mediated by silk protein for printed electronics. Adv. Mater. 2020, 32, 2000165  DOI: 10.1002/adma.202000165
      22
      Stable and Biocompatible Carbon Nanotube Ink Mediated by Silk Protein for Printed Electronics
      Liang, Xiaoping; Li, Haifang; Dou, Jinxin; Wang, Qi; He, Wenya; Wang, Chunya; Li, Donghang; Lin, Jin-Ming; Zhang, Yingying
      Advanced Materials (Weinheim, Germany) (2020), 32 (31), 2000165CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Ink-based processes, which enable scalable fabrication of flexible devices based on nanomaterials, are one of the practical approaches for the prodn. of wearable electronics. However, carbon nanotubes (CNTs), which possess great potential for flexible electronics, are facing challenges for use in inks due to their low dispersity in most solvents and suspicious cytotoxicity. Here, a stable and biocompatible CNT ink, which is stabilized by sustainable silk sericin and free from any artificial chems., is reported. The ink shows stability up to months, which can be attributed to the formation of sericin-CNT (SSCNT) hybrid through non-covalent interactions. It is demonstrated that the SSCNT ink can be used for fabricating versatile circuits on textile, paper, and plastic films through various techniques. As proofs of concept, ECG electrodes, breath sensors, and electrochem. sensors for monitoring human health and activity are fabricated, demonstrating the great potential of the SSCNT ink for smart wearables.
    23. 23
      Li, S.; Zhang, Y.; Liang, X.; Wang, H.; Lu, H.; Zhu, M.; Wang, H.; Zhang, M.; Song, Y.; Zhang, Y. Humidity-sensitive chemoelectric flexible sensors based on metal-air redox reaction for health management. Nat. Commun. 2022, 13, 5416,  DOI: 10.1038/s41467-022-33133-y
      23
      Humidity-sensitive chemoelectric flexible sensors based on metal-air redox reaction for health management
      Li, Shuo; Zhang, Yong; Liang, Xiaoping; Wang, Haomin; Lu, Haojie; Zhu, Mengjia; Wang, Huimin; Zhang, Mingchao; Qiu, Xinping; Song, Yafeng; Zhang, Yingying
      Nature Communications (2022), 13 (1), 5416CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
      Numerous studies have shown flexible electronics play important roles in health management. The way of power supply is always an essential factor of devices and self-powered ones are very attractive because of the fabrication easiness, usage comfort and aesthetics of the system. In this work, based on the metal-air redox reaction, which is usually used in designing metal-air batteries, we design a self-powered chemoelec. humidity sensor where a silk fibroin (SF) and LiBr gel matrix contg. parallel aligned graphene oxide (GO) flakes serve as the electrolyte. The abundant hydrophilic groups in GO/SF and the hygroscopicity of LiBr lead to tight dependence of the output current on the humidity, enabling the sensor high sensitivity (0.09μA/s/1%), fast response (1.05 s) and quick recovery (0.80 s). As proofs of concept, we design an all-in-one respiratory monitoring-diagnosing-treatment system and a non-contact human-machine interface, demonstrating the applications of the chemoelec. humidity sensor in health management.
    24. 24
      Hu, K.; Gupta, M.-K.; Kulkarni, D.-D.; Tsukruk, V.-V. Ultra-robust graphene oxide-silk fibroin nanocomposite membranes. Adv. Mater. 2013, 25, 2301,  DOI: 10.1002/adma.201300179
      24
      Ultra-Robust Graphene Oxide-Silk Fibroin Nanocomposite Membranes
      Hu, Kesong; Gupta, Maneesh K.; Kulkarni, Dhaval D.; Tsukruk, Vladimir V.
      Advanced Materials (Weinheim, Germany) (2013), 25 (16), 2301-2307CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      In this communication, novel ultrathin, robust nanocomposite membranes by incorporating graphene oxide sheets into silk fibroin matrix through heterogeneous surface interactions in an organized layer-by-layer (LbL) manner are reported. The outstanding values of the mech. properties achieved here are manyfold higher than those reported to date, which include a tensile modulus of 145 G Pa, an ultimate stress of more than 300 M Pa, and a toughness of above 2.2 MJ m~3 for silk nanocomposites with a silk content around 80%. The authors suggest that the outstanding performance that well exceeds the theor. values predicted by conventional mech. models is facilitated by the effective 2D graphene oxide filler which maximize all hydrogen bonding, polar-polar, and hydrophobic-hydrophobic interactions of the defective graphene oxide sheets with the silk fibroin matrix composed of polar random silk domains and the hydrophobic /3-sheet nanocrystals. Also the dense network of weak interactions has been suggested between the silk fibroin domains and graphene oxide sheets, which are in intimate contact within a 5 nm-thick bilayer, facilitates the formation of mol. interphase zones, thus effectively increasing the reinforcing effect and allowing a record high mech. strength and toughness, unheard of for biopolymer based nanocomposite films, to be acheived. The outstanding values of the mech. properties achieved here are manyfold higher than those reported to date, which include a tensile modulus of 145 G Pa, an ultimate stress of more than 300 M Pa, and a toughness of above.
    25. 25
      Xiong, R.; Hu, K.; Zhang, S.; Lu, C.; Tsukruk, V. V. Ultrastrong freestanding graphene oxide nanomembranes with surface-enhanced Raman scattering functionality by solvent-assisted single-component layer-by-layer assembly. ACS Nano 2016, 10, 6702,  DOI: 10.1021/acsnano.6b02012
      25
      Ultrastrong Freestanding Graphene Oxide Nanomembranes with Surface-Enhanced Raman Scattering Functionality by Solvent-Assisted Single-Component Layer-by-Layer Assembly
      Xiong, Rui; Hu, Kesong; Zhang, Shuaidi; Lu, Canhui; Tsukruk, Vladimir V.
      ACS Nano (2016), 10 (7), 6702-6715CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      We report single-component ultrathin reduced graphene oxide (rGO) nanomembranes fabricated via nonconventional layer-by-layer assembly (LbL) of graphene oxide flakes, using org. solvent instead of water to provide strong complementary interactions and to ensure the uniform layered growth. This unique approach does not require regular polymeric from the assembly process or intermediate surface chem. modification. The resulting ultrastrong freestanding graphene oxide (rGO) LbL nanomembranes with a very low thickness of 3 nm (three GO monolayers) can be transferred over a large surface area across tens of square centimeters by using a facile surface-tension-assisted release technique. These uniform and ultrasmooth nanomembranes with high transparency (up to 93% at 550 nm) and high elec. cond. (up to 3000 S/m) also exhibit outstanding mech. strength of 0.5 GPa and a Young's modulus of 120 GPa, which are several times higher than that of other reported regular rGO films. Furthermore, up to 94 wt % of silver nanoplates can be sandwiched between 5 nm GO layers to construct a flexible freestanding protected noble metal monolayer with surface-enhanced Raman scattering properties. These flexible rGO/Ag/rGO nanomembranes can be transferred and conformally coat complex surfaces and show a cleaner Raman signature, enhanced wet stability, and lower oxidn. compared to bare Ag nanostructures.
    26. 26
      Choi, H. K.; Lee, A.; Park, M.; Lee, D. S.; Bae, S.; Lee, S.-K.; Lee, S. H.; Lee, T.; Kim, T.-W. Hierarchical porous film with layer-by-layer assembly of 2D copper nanosheets for ultimate electromagnetic interference shielding. ACS Nano 2021, 15, 829,  DOI: 10.1021/acsnano.0c07352
      26
      Hierarchical Porous Film with Layer-by-Layer Assembly of 2D Copper Nanosheets for Ultimate Electromagnetic Interference Shielding
      Choi, Ho Kwang; Lee, Aram; Park, Mina; Lee, Dong Su; Bae, Sukang; Lee, Seoung-Ki; Lee, Sang Hyun; Lee, Takhee; Kim, Tae-Wook
      ACS Nano (2021), 15 (1), 829-839CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The emergence of technologies, such as 5G telecommunication, elec. vehicles, and wearable electronics, has prompted demand for ultrahigh-performance and cost-effective shielding materials to protect against both the potentially harmful effects of electromagnetic interference (EMI) on human health and electronic device operation. Here, we report hierarchical porous Cu foils via an assembly of single-cryst., nanometer-thick, and micrometer-long copper nanosheets and their use in EMI shielding. Layer-by-layer assembly of Cu nanosheets enabled the formation of a hierarchically structured porous Cu film with features such as multilayer stacking; two-dimensional networking; and a layered, sheetlike void architecture. The hierarchical-structured porous Cu foil exhibited outstanding EMI shielding performance compared to the same thickness of dense copper and other materials, exhibiting EMI shielding effectiveness (SE) values of 100 and 60.7 dB at thicknesses of 15 and 1.6μm, resp. In addn., the EMI SE of the hierarchical porous Cu film was maintained up to 18 mo under ambient conditions at room temp. and showed negligible changes after thermal annealing at 200°C for 1 h. These findings suggest that Cu nanosheets and their layer-by-layer assembly are one of the promising EMI shielding technologies for practical electronic applications.
    27. 27
      Huang, L.; Li, C.; Yuan, W.; Shi, G. Strong composite films with layered structures prepared by casting silk fibroin–graphene oxide hydrogels. Nanoscale 2013, 5, 3780,  DOI: 10.1039/c3nr00196b
      27
      Strong composite films with layered structures prepared by casting silk fibroin-graphene oxide hydrogels
      Huang, Liang; Li, Chun; Yuan, Wenjing; Shi, Gaoquan
      Nanoscale (2013), 5 (9), 3780-3786CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Composite films of graphene oxide (GO) sheets and silk fibroin (SF) with layered structures were prepd. by facile soln. casting of SF-GO hydrogels. The as-prepd. composite film contg. 15% (by wt.) of SF shows a high tensile strength of 221 ± 16 MPa and a failure strain of 1.8 ± 0.4%, which partially surpass those of natural nacre. Particularly, this composite film also has a high modulus of 17.2 ± 1.9 GPa. The high mech. properties of this composite film can be attributed to its high content of GO (85 wt%), compact layered structure and the strong hydrogen bonding interaction between SF chains and GO sheets.
    28. 28
      Zhao, H.; Yue, Y.; Zhang, Y.; Li, L.; Guo, L. Ternary artificial nacre reinforced by ultrathin amorphous alumina with exceptional mechanical properties. Adv. Mater. 2016, 28, 2037,  DOI: 10.1002/adma.201505511
      28
      Ternary Artificial Nacre Reinforced by Ultrathin Amorphous Alumina with Exceptional Mechanical Properties
      Zhao, Hewei; Yue, Yonghai; Zhang, Youwei; Li, Lidong; Guo, Lin
      Advanced Materials (Weinheim, Germany) (2016), 28 (10), 2037-2042CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Herein, a new type of ternary artificial nacre (graphene oxide (GO)/ultrathin amorphous alumina (AA)/sodium CM-cellulose (SCMC)) has successfully been synthesized by a vacuum-assisted filtration method. In order to guarantee the uniform layered nacre-like structure of this new product for the first time, the ultrathin amorphous alumina was grown in situ on the surface of GO with a thickness varying from around 1 to 10 nm before mixing this together with the polymer SCMC. Amorphous alumina was chosen as the third reinforcement agent because of its superior strength and toughness compared to crystal alumina.
    29. 29
      Cheng, Q.; Duan, J.; Zhang, Q.; Jiang, L. Learning from nature: Constructing integrated graphene-based artificial nacre. ACS Nano 2015, 9, 2231,  DOI: 10.1021/acsnano.5b01126
      29
      Learning from Nature: Constructing Integrated Graphene-Based Artificial Nacre
      Cheng, Qunfeng; Duan, Jianli; Zhang, Qi; Jiang, Lei
      ACS Nano (2015), 9 (3), 2231-2234CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      A review. Natural nacre supplies a no. of properties that can be used in designing high-performance bioinspired materials. Likewise, due to the extraordinary properties of graphene, a series of bioinspired graphene-based materials have recently been demonstrated. Compared to other approaches for constructing graphene-based materials, bioinspired concepts result in high-loading graphene, and the resultant high-performance graphene-based artificial nacres demonstrate isotropic mech. and elec. properties. In this Perspective, we describe how to construct integrated graphene-based artificial nacre through the synergistic relationship between interface interactions and building blocks. These integrated graphene-based artificial nacres show promising applications in many fields, such as aerospace, flexible supercapacitor electrodes, artificial muscle, and tissue engineering.
    30. 30
      Shakil, A.; Kim, S.; Polycarpou, A. A. Creep behavior of graphene oxide, silk fibroin, and cellulose nanocrystal bionanofilms. Adv. Mater. Interfaces 2022, 2101640  DOI: 10.1002/admi.202101640
      30
      Creep behavior of graphene oxide, silk fibroin, and cellulose nanocrystal bionanofilms
      Shakil, Ahmad; Kim, Sunghan; Polycarpou, Andreas A.
      Advanced Materials Interfaces (2022), 9 (18), 2101640CODEN: AMIDD2; ISSN:2196-7350. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Graphene oxide (GO), silk fibroin (SF), and cellulose nanocrystal (CNC) nanocomposite is a novel biomaterial with superior mech. properties. Elevated temp. nanoindentation expts. using const. load hold method are performed to investigate temp.-dependent mech. and creep behavior of the GO-SF-CNC nanocomposite. Hardness and reduced modulus of GO-SF-CNC are detd. from expts. at 25, 40, 60, 80, and 100°C, and yield strength and creep coeffs. are predicted from finite element anal. using two-layer viscoplasticity theory. Results show that increasing the temp. from 25 to 80°C, hardness, reduced modulus, and yield strength of GO-SF-CNC nanocomposite dramatically increase by 112%, 40%, and 140% resp., and creep displacements during const. load hold reduce by 53%. It is attributed to increasing in crystns. in the nanocomposite because of increasing in β-sheet formations of SF material and redn. in water mols. in CNC material. However, at 100°C, the mech. properties deteriorate, and creep displacements increase because of water evapn. from the nanocomposite, making it weaker. Hardness-to-yield strength ratio is found within 1.84-2.06. Maximum creep exponent is 2.9 at 40°C, which reduces to 2.06 at 80°C and again increases to 2.27 at 100°C.
    31. 31
      Ye, C.; Combs, Z. A.; Calabrese, R.; Dai, H.; Kaplan, D. L.; Tsukruk, V. V. Robust microcapsules with controlled permeability from silk fibroin reinforced with graphene oxide. Small 2014, 10, 5087,  DOI: 10.1002/smll.201401119
      31
      Robust Microcapsules with Controlled Permeability from Silk Fibroin Reinforced with Graphene Oxide
      Ye, Chunhong; Combs, Zachary A.; Calabrese, Rossella; Dai, Hongqi; Kaplan, David L.; Tsukruk, Vladimir V.
      Small (2014), 10 (24), 5087-5097CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Robust and stable microcapsules are assembled from poly-amino acid-modified silk fibroin reinforced with graphene oxide flakes using layer-by-layer (LbL) assembly, based on biocompatible natural protein and carbon nanosheets. The composite microcapsules are extremely stable in acidic (pH 2.0) and basic (pH 11.5) conditions, accompanied with pH-triggered permeability, which facilitates the controllable encapsulation and release of macromols. Furthermore, the graphene oxide incorporated into ultrathin LbL shells induces greatly reinforced mech. properties, with an elastic modulus which is two orders of magnitude higher than the typical values of original silk LbL shells and shows a significant, three-fold redn. in pore size. Such strong nanocomposite microcapsules can provide solid protection of encapsulated cargo under harsh conditions, indicating a promising candidate with controllable loading/unloading for drug delivery, reinforcement, and bioengineering applications.
    32. 32
      Xie, W.; Tadepalli, S.; Park, S.-H.; Kazemi-Moridani, A.; Jiang, Q.; Singamaneni, S.; Lee, J.-H. Extreme mechanical behavior of nacre-mimetic graphene-oxide and silk nanocomposites. Nano Lett. 2018, 18, 987,  DOI: 10.1021/acs.nanolett.7b04421
      32
      Extreme Mechanical Behavior of Nacre-Mimetic Graphene-Oxide and Silk Nanocomposites
      Xie, Wanting; Tadepalli, Sirimuvva; Park, Sang Hyun; Kazemi-Moridani, Amir; Jiang, Qisheng; Singamaneni, Srikanth; Lee, Jae-Hwang
      Nano Letters (2018), 18 (2), 987-993CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Biol. materials have the ability to withstand extreme mech. forces due to their unique multilevel hierarchical structure. Here, we fabricated a nacre-mimetic nanocomposite comprised of silk fibroin and graphene oxide that exhibits hybridized dynamic responses arising from alternating high-contrast mech. properties of the components at the nanoscale. Dynamic mech. behavior of these nanocomposites is assessed through a microscale ballistic characterization using a 7.6 μm diam. silica sphere moving at a speed of approx. 400 m/s. The vol. fraction of graphene oxide in these composites is systematically varied from 0 to 32 vol. % to quantify the dynamic effects correlating with the structural morphologies of the graphene oxide flakes. Specific penetration energy of the films rapidly increases as the distribution of graphene oxide flakes evolves from noninteracting, isolated sheets to a partially overlapping continuous sheet. The specific penetration energy of the nanocomposite at the highest graphene oxide content tested here is significantly higher than that of Kevlar fabrics and close to that of pure multilayer graphene. This study evidently demonstrates that the morphologies of nanoscale constituents and their interactions are crit. to realize scalable high-performance nanocomposites using typical nanomaterial constituents having finite dimensions.
    33. 33
      Zandiatashbar, A.; Lee, G.-H.; An, S.-J.; Lee, S.; Mathew, N.; Terrones, M.; Hayashi, T.; Picu, C.-R.; Hone, J.; Koratkar, N. Effect of defects on the intrinsic strength and stiffness of graphene. Nat. Commun. 2014, 5, 3186,  DOI: 10.1038/ncomms4186
      33
      Effect of defects on the intrinsic strength and stiffness of graphene
      Zandiatashbar Ardavan; Lee Gwan-Hyoung; An Sung Joo; Hone James; Lee Sunwoo; Mathew Nithin; Picu Catalin R; Terrones Mauricio; Hayashi Takuya; Koratkar Nikhil
      Nature communications (2014), 5 (), 3186 ISSN:.
      It is important from a fundamental standpoint and for practical applications to understand how the mechanical properties of graphene are influenced by defects. Here we report that the two-dimensional elastic modulus of graphene is maintained even at a high density of sp(3)-type defects. Moreover, the breaking strength of defective graphene is only ~14% smaller than its pristine counterpart in the sp(3)-defect regime. By contrast, we report a significant drop in the mechanical properties of graphene in the vacancy-defect regime. We also provide a mapping between the Raman spectra of defective graphene and its mechanical properties. This provides a simple, yet non-destructive methodology to identify graphene samples that are still mechanically functional. By establishing a relationship between the type and density of defects and the mechanical properties of graphene, this work provides important basic information for the rational design of composites and other systems utilizing the high modulus and strength of graphene.
    34. 34
      Bai, Y.; Cai, H.; Qiu, X.; Fang, X.; Zheng, J. Effects of graphene reduction degree on thermal oxidative stability of reduced graphene oxide/silicone rubber nanocomposites. High Perform. Polym. 2015, 27, 997,  DOI: 10.1177/0954008315604205
      34
      Effects of graphene reduction degree on thermal oxidative stability of reduced graphene oxide/silicone rubber nanocomposites
      Bai, Yulian; Cai, Hai; Qiu, Xingna; Fang, Xin; Zheng, Junping
      High Performance Polymers (2015), 27 (8), 997-1006CODEN: HPPOEX; ISSN:0954-0083. (Sage Publications Ltd.)
      In this work, graphene oxide (GO) was prepd. with a modified Hummers method and then reduced to different redn. degree by thermal treatment. The GO and reduced GO (RGO) samples were introduced into silicone rubber (SR) matrix to prep. nanocomposites. XPS, Raman spectra, X-ray diffraction, and transmission electron microscopy measurement were performed to detect the structure and morphol. changes of GO and RGO sheets. The results showed that the redn. removed most of the oxygen-contg. functional groups on the surface of GO, esp. C-O-C group, and thus reestablished a graphitic network of sp2 hybrid; by increasing the redn. temp., the redn. degree of GO was increased, and meanwhile, the extent of exfoliation was increased. More importantly, tensile testing and thermogravimetric anal. revealed that RGO improved the mech. properties and thermal oxidative stability of SR nanocomposites. The improvement enhanced with increasing the redn. degree of GO simultaneously.
    35. 35
      Richardson, J.-J.; Björnmalm, M.; Caruso, F. Technology-driven layer-by-layer assembly of nanofilms. Science 2015, 348, 2491,  DOI: 10.1126/science.aaa24
      There is no corresponding record for this reference.
    36. 36
      Alongi, J.; Carosio, F.; Frache, A.; Malucelli, G. Layer by layer coatings assembled through dipping, vertical or horizontal spray for cotton flame retardancy. Carbohydr. Polym. 2013, 92, 114,  DOI: 10.1016/j.carbpol.2012.08.086
      36
      Layer by Layer coatings assembled through dipping, vertical or horizontal spray for cotton flame retardancy
      Alongi, Jenny; Carosio, Federico; Frache, Alberto; Malucelli, Giulio
      Carbohydrate Polymers (2013), 92 (1), 114-119CODEN: CAPOD8; ISSN:0144-8617. (Elsevier Ltd.)
      Silica-based assemblies have been deposited on cotton fibers through Layer by Layer technique in order to enhance their flame retardant properties. To this aim, three different deposition procedures (namely, dipping, vertical and horizontal sprays) have been considered and compared. The resulting morphologies of the deposited assemblies have been thoroughly investigated by SEM and elemental anal. SEM observations have demonstrated that only the horizontal spray allows obtaining the deposition of a very homogeneous silica coating when compared to vertical spray or dipping. As a consequence, horizontal spray has proved to ensure the best flame resistance, promoting a substantial increase of the total burning time and final residue, as assessed by flammability tests. Furthermore, cone calorimetry measurements have shown a remarkable increase of the time to ignition, and a significant decrease of heat release rate and total heat release for the fabrics treated by horizontal spray.
    37. 37
      Trung, T.-Q.; Ramasundaram, S.; Hwang, B.-U.; Lee, N.-E. An all-elastomeric transparent and stretchable temperature sensor for body-attachable wearable electronics. Adv. Mater. 2016, 28, 502,  DOI: 10.1002/adma.201504441
      37
      An All-Elastomeric Transparent and Stretchable Temperature Sensor for Body-Attachable Wearable Electronics
      Trung, Tran Quang; Ramasundaram, Subramaniyan; Hwang, Byeong-Ung; Lee, Nae-Eung
      Advanced Materials (Weinheim, Germany) (2016), 28 (3), 502-509CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A transparent stretchable (TS) gated sensor array with high optical transparency, conformality, and high stretchability of up to 70% is demonstrated. The TS-gated sensor array has high responsivity to temp. changes in objects and human skin. This unprecedented TS-gated sensor array, as well as the integrated platform of the TS-gated sensor with a transparent and stretchable strain sensor, shows great potential for application to wearable skin electronics for recognition of human activity.
    38. 38
      Liu, Q.; Tai, H.; Yuan, Z.; Zhou, Y.; Su, Y.; Jiang, Y. A high-performance flexible temperature sensor composed of polyethyleneimine/reduced graphene oxide bilayer for real-time monitoring. Adv. Mater. Technol. 2019, 4, 1800594  DOI: 10.1002/admt.201800594
      There is no corresponding record for this reference.
    39. 39
      Sehrawat, P.; Islam, S.-S.; Mishra, P. Reduced graphene oxide based temperature sensor: Extraordinary performance governed by lattice dynamics assisted carrier transport. Sens. Actuators, B 2018, 258, 424,  DOI: 10.1016/j.snb.2017.11.112
      39
      Reduced graphene oxide based temperature sensor: Extraordinary performance governed by lattice dynamics assisted carrier transport
      Sehrawat, Poonam; Abid; Islam, S. S.; Mishra, Prabhash
      Sensors and Actuators, B: Chemical (2018), 258 (), 424-435CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
      In this article, we report the sensing performance of reduced graphene oxide (rGO) based resistive type temp. sensor fabricated by spin coating. A detailed anal. is presented for understanding the combined effect of lattice vibrational properties and temp. dependent elec. cond. while considering charge carrier scattering with phonons, impurities, defects, and edge boundaries of rGO flakes. The purpose of this anal. is to find out how together they influence the temp. coeff. of resistance (TCR) and thermal hysteresis (HTh) of rGO based films. TCR and Hth are the core factors for efficient operation of a temp. sensor as these govern important sensing characteristics such as sensitivity, resoln., drift, response- and recovery-time. Exptl. results show that the proposed sensor exhibits TCR ∼ -0.801%/K (in 303K-373K) and negligible thermal hysteresis (∼0.7%) resulting in high resoln. (∼0.1 K), response- and recovery-time of ∼52 s and ∼285 s resp. Besides, TCR and Hth are also found to depend on rGO concn. and working temp. range of sensors. By lowering the sensing temp. range to 303K-77K region, TCR was found to increase abruptly from -0.801%/K to -32.04%/K. All this optimized data were obtained for the sensor with 3 wt.% of rGO. Dynamic plot shows its sensitivity to respond to even ∼0.1 K change in temp. Cyclic testing demonstrates good stability in 77K-573K temp. range with negligible drift. These studies are significant towards the fabrication of simple, highly sensitive, and cost effective temp. sensor with high reproducibility. There is still enough room to improve TCR of rGO based sensors through synthesis, advanced sensor design and development; higher TCR will definitely lead to far better temp. sensing performance as theory predicts.
    40. 40
      Zhu, J.; Andres, C.-M.; Xu, J.; Ramamoorthy, A.; Tsotsis, T.; Kotov, N.-A. Pseudonegative thermal expansion and the state of water in graphene oxide layered assemblies. ACS Nano 2012, 6, 8357,  DOI: 10.1021/nn3031244
      40
      Pseudonegative Thermal Expansion and the State of Water in Graphene Oxide Layered Assemblies
      Zhu, Jian; Andres, Christine M.; Xu, Jiadi; Ramamoorthy, Ayyalusamy; Tsotsis, Thomas; Kotov, Nicholas A.
      ACS Nano (2012), 6 (9), 8357-8365CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Unraveling the complex interplay between thermal properties and hydration is a part of understanding the fundamental properties of many soft materials and very essential for many applications. Here we show that graphene oxide (GO) demonstrates a highly neg. thermal expansion (NTE) coeff. owing to unique thermohydration processes related with fast transport of water between the GO sheets, the amphiphilic nature of nanochannels, and close-to-zero intrinsic thermal expansion of GO. The humidity-dependent NTE of GO layered assemblies, or "pseudoneg. thermal expansion" (PNTE), differs from that of other hygroscopic materials due to its relatively fast and highly reversible expansion/contraction cycles and occurrence at low humidity levels while bearing similarities to classic NTE. Thermal expansion of polyvinyl alc./GO composites is easily tunable with addnl. intricacy of thermohydration effects. PNTE combined with isotropy, nontoxicity, and mech. robustness is an asset for applications of actuators, sensors, MEMS devices, and memory materials and crucial for developing methods of thermal/photopatterning of GO devices.
    41. 41
      Borini, S.; White, R.; Wei, D.; Astley, M.; Haque, S.; Spigone, E.; Harris, N.; Kivioja, J.; Ryhanen, T. Ultrafast graphene oxide humidity sensors. ACS Nano 2013, 7, 11166,  DOI: 10.1021/nn404889b
      41
      Ultrafast Graphene Oxide Humidity Sensors
      Borini, Stefano; White, Richard; Wei, Di; Astley, Michael; Haque, Samiul; Spigone, Elisabetta; Harris, Nadine; Kivioja, Jani; Ryhanen, Tapani
      ACS Nano (2013), 7 (12), 11166-11173CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Sensors allow an electronic device to become a gateway between the digital and phys. worlds, and sensor materials with unprecedented performance can create new applications and new avenues for user interaction. Graphene oxide can be exploited in humidity and temp. sensors with a no. of convenient features such as flexibility, transparency and suitability for large-scale manufg. Here we show that the two-dimensional nature of graphene oxide and its superpermeability to water combine to enable humidity sensors with unprecedented response speed (∼30 ms response and recovery times). This opens the door to various applications, such as touchless user interfaces, which we demonstrate with a 'whistling' recognition anal.
    42. 42
      Adib, M. R.; Lee, Y.; Kondalkar, V. V.; Kim, S.; Lee, K. A highly sensitive and stable rGO: MoS2-based chemiresistive humidity sensor directly insertable to transformer insulating oil analyzed by customized electronic sensor interface. ACS Sens. 2021, 6, 1012,  DOI: 10.1021/acssensors.0c02219
      42
      A Highly Sensitive and Stable rGO:MoS2-Based Chemiresistive Humidity Sensor Directly Insertable to Transformer Insulating Oil Analyzed by Customized Electronic Sensor Interface
      Adib, Md Ridwan; Lee, Yongbum; Kondalkar, Vijay V.; Kim, Sihyeok; Lee, Keekeun
      ACS Sensors (2021), 6 (3), 1012-1021CODEN: ASCEFJ; ISSN:2379-3694. (American Chemical Society)
      Reduced graphene oxide and molybdenum disulfide (rGO:MoS2) are the most representative two-dimensional materials, which are promising for a humidity sensor owing to its high surface area, a large no. of active sites, and excellent mech. flexibility. Herein, we introduced a highly sensitive and stable rGO:MoS2-based humidity sensor integrated with a low-power in-plane microheater and a temp. sensor, directly insertable to transformer insulating oil, and analyzed by a newly developed customized sensor interface electronics to monitor the sensor's output variations in terms of relative humidity (RH) concn. rGO:MoS2 sensing materials were synthesized by simple ultrasonication without using any additives or addnl. heating and selectively deposited on titanium/platinum (Ti/Pt) interdigitated electrodes on a SiO2 substrate using the drop-casting method. The significant sensing capability of p-n heterojunction formation between rGO and MoS2 was obsd. both in the air and transformer insulating oil environment. In air testing, the sensor exhibited an immense sensitivity of 0.973 kΩ/%RH and excellent linearity of ~ 0.98 with a change of humidity from 30 to 73%RH, and a const. resistance deviation with an inaccuracy rate of 0.13% over 400 h of continual measurements. In oil, the sensor showed a high sensitivity of 1.596 kΩ/%RH and stable repeatability for an RH concn. range between 34 and 63%RH. The obtained results via the sensor interface were very similar to those measured with a digital multimeter, denoting that our developed total sensor system is a very promising candidate for real-time monitoring of the operational status of power transformers.
    43. 43
      Wang, Y.; Zhang, L.; Zhang, Z.; Sun, P.; Chen, H. High-sensitivity wearable and flexible humidity sensor based on graphene oxide/non-woven fabric for respiration monitoring. Langmuir 2020, 36, 9443,  DOI: 10.1021/acs.langmuir.0c01315
      43
      High-Sensitivity Wearable and Flexible Humidity Sensor Based on Graphene Oxide/Non-Woven Fabric for Respiration Monitoring
      Wang, Yamei; Zhang, Liwen; Zhang, Zhenwei; Sun, Pengyuan; Chen, Huawei
      Langmuir (2020), 36 (32), 9443-9448CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The popularity of humidity sensing for respiratory anal. of patients is gradually increasing because of its portability and cost-effectiveness. However, current flexible humidity sensors are mainly made of polymer films, whose poor hygroscopicity and breathability reduce their sensitivity and comfort. A highly sensitive humidity sensor was developed using non-woven fabric (NWF) coated with graphene oxide (GO). Bovine serum albumin was used to improve the adsorption of GO onto the NWF, and its effect on sensitivity was studied by adjusting its concn. High-humidity sensitivity was exptl. validated by testing different relative humidity levels, and its fast response and excellent feasibility under diverse breathing conditions were verified by successful monitoring of fast and deep breathing, differentiating nose and mouth breathing, and even identifying simple spoken words. This study developed a breathable and skin-friendly humidity sensor based on GO/NWF, which is a promising device for human healthcare.
    44. 44
      Jia, G.; Zheng, A.; Wang, X.; Zhang, L.; Li, L.; Li, C.; Zhang, Y.; Cao, L. Flexible, biocompatible and highly conductive MXene-graphene oxide film for smart actuator and humidity sensor. Sens. Actuators, B 2021, 346, 130507  DOI: 10.1016/j.snb.2021.130507
      44
      Flexible, biocompatible and highly conductive MXene-graphene oxide film for smart actuator and humidity sensor
      Jia, Guangwen; Zheng, Ao; Wang, Xiao; Zhang, Lu; Li, Ling; Li, Chenxing; Zhang, Yan; Cao, Lingyan
      Sensors and Actuators, B: Chemical (2021), 346 (), 130507CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
      The evapn. of water occurs ubiquitously on earth. Hence, smart materials that can directly convert signals generated via water stimulation into mech. motion have attracted wide attention. However, it is still a challenge to develop novel functional materials with fast response, large scale deformation, and long-term stability for moisture-gradient actuators. Here, a flexible, conductive, layer-structured homogenous Ti3C2TX MXene-graphene oxide (MGO) film-based moisture-driven actuator and humidity sensor were fabricated. The oxygen groups and d-spacing could be effectively adjusted by MXene/GO compn. ratio, thereby tuning the actuation performance. MGO3 (MXene/GO = 3) displayed a large bending angle, and reversible deformation. And the bending speed of MGO3 is up to 32°s-1. Furthermore, MGO3 actuation displayed long-term stability via suppression of MXene oxidn. by the introduction of GO and showed good cycling stability. MGO3 actuators are constructed, which could mimic the blooming of flower, lifting and carrying objects, and be used as a non-contact control switch. In addn., MGO3 showed a linear sensitive response to humidity and excellent biocompatibility which make it suitable for respiratory monitoring. This work demonstrated that flexible, biocompatibility and conductive MGO films have broad application prospects in the fields of smart actuators, sensing devices, and biol. and health care.
    45. 45
      Jason, N. N.; Ho, M. D.; Cheng, W. Resistive electronic skin. J. Mater. Chem. C 2017, 5, 5845,  DOI: 10.1039/C7TC01169E
      45
      Resistive electronic skin
      Jason, Naveen N.; Ho, My D.; Cheng, Wenlong
      Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2017), 5 (24), 5845-5866CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
      Devices made from traditional conductive bulk materials using complex microfabrication methods often are restricted to being rigid and in some cases, flexible but not strethcable. The main reason is the mismatching mechanics between these traditional materials and the elastomeric materials they were bonded with, which causes materials delimination and/or cracks at soft/hard materials interfaces under strains. Conductive nanomaterials potentially offer new opportunity to tackle this challenge. Their availability in various sizes and shapes enables us to create composites with various dimensions, such as 1D conductive traces, 2D film, and 3D sponge-like architectures. These have opened the door for fabrication of stretchable interconnects, circuits, energy storage devices, antennas, LEDs, etc. The basis of using conductive nanomaterials composites in sensors is that any stimulus or change will generate a measurable elec. impulse. These impulses can be broadly classified as piezoelec., triboelec., capacitive, and resistive responses. Depending on the sensitivity required and the preference of elec. impulse to be measured, the device construction maybe tailored to give one of the four kinds of elec. responses. Resistive sensors in addn. to being the easiest to construct are also the easiest to measure, which is the crucial reason for a large no. of publications in this area. The working mechanism of resistive sensors based on the constituent conductive materials and their percolation network will be discussed in detail. Compn. of conductive inks fabricated using wet chem. methods, and nanomaterials using dry methods, their subsequent applications are covered as well. The exciting applications relating to human health and well-being will also be described. Finally a brief outlook of the future of wearable sensors as "invisibles" will be presented.
    46. 46
      Guan, X.; Hou, Z.; Wu, K.; Zhao, H.; Liu, S.; Fei, T.; Zhang, T. Flexible humidity sensor based on modified cellulose paper. Sens. Actuators, B 2021, 339, 129879  DOI: 10.1016/j.snb.2021.129879
      46
      Flexible humidity sensor based on modified cellulose paper
      Guan, Xin; Hou, Zhaonan; Wu, Ke; Zhao, Hongran; Liu, Sen; Fei, Teng; Zhang, Tong
      Sensors and Actuators, B: Chemical (2021), 339 (), 129879CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
      It is of great significance to exploit a simple, cost-effective and environmentally friendly prepn. method of multifunctional humidity sensors. However, most humidity sensors need complex manufg. processes, high cost and narrow usage range. Herein, humidity sensors based on glycidyl tri-Me ammonium chloride (EPTAC) modified cellulose paper via a facile soln. method were fabricated, among which the paper is used for the purpose of the humidity sensing material and the sensor substrate. The sensitivity of the obtained sensor was improved by the modification of EPTAC, the response time was decreased to 25 s, which is equiv. to the first-rate paper-based humidity sensors. In addn., the paper-based humidity sensor is provided with well flexibility and biocompatibility, and it exhibits multifunctional applications in respiratory monitoring, non-contact switch and skin humidity monitoring. The low cost and facile prepn. technique in this work could provide a useful strategy for developing multifunctional humidity sensor.
    47. 47
      Jiao, S.; Li, Y.; Li, J.; Abrha, H.; Liu, M.; Cui, J.; Wang, J.; Dai, Y.; Liu, X. Graphene oxide as a versatile platform for emerging hydrovoltaic technology. J. Mater. Chem. A 2022, 10, 18451,  DOI: 10.1039/D2TA04830B
      47
      Graphene oxide as a versatile platform for emerging hydrovoltaic technology
      Jiao, Shipu; Li, Yang; Li, Jiaxuan; Abrha, Halayit; Liu, Miao; Cui, Jinran; Wang, Jiao; Dai, Yexin; Liu, Xianhua
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2022), 10 (36), 18451-18469CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      A review. Hydrovoltaic technol. can harvest sustainable energy and clean water directly from various environments, providing a novel way to alleviate global environmental problems and the energy crisis. A wide variety of hydrovoltaic materials with distinctly different morphol., mech. and functional features have been created by using GO as a versatile building block. However, there is still a lack of comprehensive knowledge regarding the involvement of GO in the hydrovoltaic technol. and its future perspectives. In this review, the latest progress in the prepn. of GO-based hydrovoltaic materials and their various applications are summarized. The working mechanisms for the hydrovoltaic power generation and some remaining challenges are also discussed. Finally, some suggestions are given for further development of GO-based hydrovoltaic technol.
    48. 48
      Joshi, P.; Yadav, R.; Hara, M.; Inoue, T.; Motoyama, Y.; Yoshimura, M. Contribution of B, N-co-doped reduced graphene oxide as a catalyst support to the activity of iridium oxide for oxygen evolution reaction. J. Mater. Chem. A 2021, 9, 9066,  DOI: 10.1039/D1TA00158B
      48
      Contribution of B,N-co-doped reduced graphene oxide as a catalyst support to the activity of iridium oxide for oxygen evolution reaction
      Joshi, Prerna; Yadav, Rohit; Hara, Masanori; Inoue, Tetsunari; Motoyama, Yukihiro; Yoshimura, Masamichi
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2021), 9 (14), 9066-9080CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      The current research deals with the study of boron and nitrogen co-doped reduced graphene oxide (BN-rGO) as a support material for iridium oxide (IrO2) nanoparticles for oxygen evolution reaction (OER) catalysis. The synthetic approach for the IrO2-BN-rGO catalyst involves the combination of pyrolysis and hydrothermal methods used for hierarchical nanostructures. BN-rGO possesses B-N, B-C, and N-C functional groups to support and stabilize the IrO2 catalyst nanoparticles. The altered electronic states of IrO2 on the BN-rGO support are compared with those of IrO2 on a non-doped support, rGO (IrO2-rGO), and on com. BN sheets (IrO2-c-BN). The catalyst shows a low overpotential (300 mV at 10 mA cm-2), high c.d. (55 mA cm-2 at 1.65 V), and significantly high durability (12 350 cycles; 45 h) in an acidic environment. The high stability of IrO2-BN-rGO may result from the presence of a chem. and electrochem. stable B-N bond. We confirm that other functional groups (B-C and N-C) and the rGO framework are equally crucial for better attachment of IrO2 nanoparticles.
    49. 49
      Hummers, W. S., Jr.; Offeman, R. E. Preparation of graphitic oxide. J. Am. Chem. Soc. 1958, 80, 1339,  DOI: 10.1021/ja01539a017
      49
      Preparation of graphitic oxide
      Hummers, Wm. S., Jr.; Offeman, Richard E.
      Journal of the American Chemical Society (1958), 80 (), 1339CODEN: JACSAT; ISSN:0002-7863.
      See U.S. 2,798,878 (C.A. 51, 15080a).
    50. 50
      Rockwood, D.-N.; Preda, R.-C.; Yücel, T.; Wang, X.-Q.; Lovett, M.-L.; Kaplan, D.-L. Materials fabrication from Bombyx mori silk fibroin. Nat. Protoc. 2011, 6, 1612,  DOI: 10.1038/nprot.2011.379
      50
      Materials fabrication from Bombyx mori silk fibroin
      Rockwood, Danielle N.; Preda, Rucsanda C.; Yucel, Tuna; Wang, Xiao-Qin; Lovett, Michael L.; Kaplan, David L.
      Nature Protocols (2011), 6 (10), 1612-1631CODEN: NPARDW; ISSN:1750-2799. (Nature Publishing Group)
      Silk fibroin, derived from Bombyx mori cocoons, is a widely used and studied protein polymer for biomaterial applications. Silk fibroin has remarkable mech. properties when formed into different materials, demonstrates biocompatibility, has controllable degrdn. rates from hours to years and can be chem. modified to alter surface properties or to immobilize growth factors. A variety of aq. or org. solvent-processing methods can be used to generate silk biomaterials for a range of applications. In this protocol, the authors include methods to ext. silk from B. mori cocoons to fabricate hydrogels, tubes, sponges, composites, fibers, microspheres and thin films. These materials can be used directly as biomaterials for implants, as scaffolding in tissue engineering and in vitro disease models, as well as for drug delivery.
    51. 51
      Yin, K.; Li, H.; Xia, Y.; Bi, H.; Sun, J.; Liu, Z.; Sun, L. Thermodynamic and kinetic analysis of low temperature thermal reduction of graphene oxide. Nano–Micro Lett. 2011, 3, 51,  DOI: 10.1007/BF03353652
      51
      Thermodynamic and kinetic analysis of low-temperature thermal reduction of graphene oxide
      Yin, Kuibo; Li, Haitao; Xia, Yidong; Bi, Hengchang; Sun, Jun; Liu, Zhiguo; Sun, Litao
      Nano-Micro Letters (2011), 3 (1), 51-55CODEN: NLAEBV; ISSN:2150-5551. (Nano-Micro Letters)
      The thermodn. state and kinetic process of low-temp. deoxygenation reaction of graphene oxide (GO) were investigated for better understanding the redn. mechanism by using Differential Scanning Calorimetry (DSC), Thermogravimetry-Mass Spectrometry (TG-MS), and XPS. It was found that the thermal redn. reaction of GO is exothermic with degassing of CO2, CO and H2O. The graphene is thermodynamically more stable than GO. The deoxygenation reaction of GO is kinetically controlled and the activation energy for GO is calcd. as 167 kJ/mol (1.73 eV/atom).
    52. 52
      Kim, S.; Geryak, R.-D.; Zhang, S.; Ma, R.; Calabrese, R.; Kaplan, D.-L.; Tsukruk, V.-V. Interfacial shear strength and adhesive behavior of silk ionomer surfaces. Biomacromolecules 2017, 18, 2876,  DOI: 10.1021/acs.biomac.7b00790
      52
      Interfacial shear strength and adhesive behavior of silk ionomer surfaces
      Kim, Sunghan; Geryak, Ren D.; Zhang, Shuaidi; Ma, Ruilong; Calabrese, Rossella; Kaplan, David L.; Tsukruk, Vladimir. V.
      Biomacromolecules (2017), 18 (9), 2876-2886CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
      The interfacial shear strength between different layers in multilayered structures of layer-by-layer (LbL) microcapsules is a crucial mech. property to ensure their robustness. In this work, we investigated the interfacial shear strength of modified silk fibroin ionomers utilized in LbL shells, an ionic-cationic pair with complementary ionic pairing, (SF)-poly-L-glutamic acid (Glu) and SF-poly-L-lysine (Lys), and a complementary pair with partially screened Coulombic interactions due to the presence of poly(ethylene glycol) (PEG) segments and SF-Glu/SF-Lys[PEG] pair. Shearing and adhesive behavior between these silk ionomer surfaces in the swollen state were probed at different spatial scales and pressure ranges by using functionalized at. force microscopy (AFM) tips as well as functionalized colloidal probes. The results show that both approaches were consistent in analyzing the interfacial shear strength of LbL silk ionomers at different spatial scales from a nanoscale to a fraction of a micron. Surprisingly, the interfacial shear strength between SF-Glu and SF-Lys[PEG] pair with partially screened ionic pairing was greater than the interfacial shear strength of the SF-Glu and SF-Lys pair with a high d. of complementary ionic groups. The difference in interfacial shear strength and adhesive strength is suggested to be predominantly facilitated by the interlayer hydrogen bonding of complementary amino acids and overlap of highly swollen PEG segments.
    53. 53
      Geryak, R.; Quigley, E.; Kim, S.; Korolovych, V. F.; Calabrese, R.; Kaplan, D. L.; Tsukruk, V. V. Tunable interfacial properties in silk ionomer microcapsules with tailored multilayer interactions. Macromol. Biosci. 2019, 19, 1800176  DOI: 10.1002/mabi.201800176
      There is no corresponding record for this reference.
    54. 54
      Xiong, R.; Kim, H. S.; Zhang, S.; Kim, S.; Korolovych, V. F.; Ma, R.; Yingling, Y. G.; Lu, C.; Tsukruk, V. V. Template-guided assembly of silk fibroin on cellulose nanofibers for robust nanostructures with ultrafast water transport. ACS Nano 2017, 11, 12008,  DOI: 10.1021/acsnano.7b04235
      54
      Template-Guided Assembly of Silk Fibroin on Cellulose Nanofibers for Robust Nanostructures with Ultrafast Water Transport
      Xiong, Rui; Kim, Ho Shin; Zhang, Shuaidi; Kim, Sunghan; Korolovych, Volodymyr F.; Ma, Ruilong; Yingling, Yaroslava G.; Lu, Canhui; Tsukruk, Vladimir V.
      ACS Nano (2017), 11 (12), 12008-12019CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The authors report a spontaneous formation of peculiar "shish kebab" nanostructures with the periodic arrangement of silk fibroin domains along straight segments of cellulose nanofibers. The authors suggest that the formation of these shish kebab nanostructures is facilitated by the preferential organization of heterogeneous (β-sheets and amorphous silk) domains along the cellulose nanofiber driven by modulated axial distribution of cryst. planes, hydrogen bonding, and hydrophobic interactions as suggested by all-atom mol. dynamic simulations. Such shish kebab nanostructures enable the ultrathin membrane to possess open, transparent, mech. robust interlocked networks with high mech. performance with up to 30 GPa in stiffness and 260 MPa in strength. These nanoporous robust membranes allow for the extremely high water flux, up to 3.5 × 104 L h-1 m-2 bar-1 combined with high rejection rate for various org. mols., capability of capturing heavy metal ions and their further redn. into metal nanoparticles for added SERS detection capability and catalytic functionalities.
    55. 55
      Cranston, E. D.; Eita, M.; Johansson, E.; Netrval, J.; Salajkova, M.; Arwin, H.; Wagberg, L. Determination of Young’s modulus for nanofibrillated cellulose multilayer thin films using buckling mechanics. Biomacromolecules 2011, 12, 961,  DOI: 10.1021/bm101330w
      55
      Determination of Young's Modulus for Nanofibrillated Cellulose Multilayer Thin Films Using Buckling Mechanics
      Cranston, Emily D.; Eita, Mohamed; Johansson, Erik; Netrval, Julia; Salajkova, Michaela; Arwin, Hans; Wagberg, Lars
      Biomacromolecules (2011), 12 (4), 961-969CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
      The Young's modulus of multilayer films contg. nanofibrillated cellulose (NFC) and polyethyleneimine (PEI) was detd. using the strain-induced elastic buckling instability for mech. measurements (SIEBIMM) technique. Multilayer films were built up on polydimethylsiloxane substrates using electrostatic layer-by-layer assembly. At 50% relative humidity, SIEBIMM gave a const. Young's modulus of 1.5 ± 0.2 GPa for 35-75 nm thick films. Conversely, in vacuum, the Young's modulus was 10 times larger, at 17.2 ± 1.2 GPa. A slight decrease in buckling wavelength with increasing strain was obsd. by SEM with in situ compression, and above 10% strain, extensive cracking parallel to the compressive direction occurred. We conclude that whereas PEI acts as a "glue" to hold multiple layers of NFC together, it prevents full development of hydrogen bonding and specific fibril-fibril interactions, and at high humidity, its hygroscopic nature decreases the elastic modulus when compared with pure NFC films.
    56. 56
      Yin, Y.; Hu, K.; Grant, A.-M.; Zhang, Y.; Tsukruk, V.-V. Biopolymeric nanocomposites with enhanced interphases. Langmuir 2015, 31, 10859,  DOI: 10.1021/acs.langmuir.5b02744
      56
      Biopolymeric Nanocomposites with Enhanced Interphases
      Yin, Yi; Hu, Kesong; Grant, Anise M.; Zhang, Yuhong; Tsukruk, Vladimir V.
      Langmuir (2015), 31 (39), 10859-10870CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Ultrathin and robust nanocomposite membranes were fabricated by incorporating graphene oxide (GO) sheets into a silk fibroin (SF) matrix by a dynamic spin-assisted layer-by-layer assembly (dSA-LbL). We obsd. that in contrast to traditional SA-LbL reported earlier fast soln. removal during dropping of soln. on constantly spinning substrates resulted in largely unfolded biomacromols. with enhanced surface interactions and suppressed nanofibril formation. The resulting laminated nanocomposites possess outstanding mech. properties, significantly exceeding those previously reported for conventional LbL films with similar compn. The tensile modulus reached extremely high values of 170 GPa, which have never been reported for graphene oxide-based nanocomposites, the ultimate strength was close to 300 MPa, and the toughness was above 3.4 MJ m-3. The failure modes obsd. for these membranes suggested the self-reinforcing mechanism of adjacent graphene oxide sheets with strong 2 nm thick silk interphase composed mostly from individual backbones. This interphase reinforcement leads to the effective load transfer between the graphene oxide components in reinforced laminated nanocomposite materials with excellent mech. strength that surpasses those known today for conventional flexible laminated carbon nanocomposites from graphene oxide and biopolymer components.
    57. 57
      Cho, H.; Shakil, A.; Polycarpou, A.-A.; Kim, S. Enabling selectively tunable mechanical properties of graphene oxide/silk fibroin/cellulose nanocrystal bionanofilms. ACS Nano 2021, 15, 19546,  DOI: 10.1021/acsnano.1c06573
      57
      Enabling Selectively Tunable Mechanical Properties of Graphene Oxide/Silk Fibroin/Cellulose Nanocrystal Bionanofilms
      Cho, Hyeonho; Shakil, Ahmad; Polycarpou, Andreas A.; Kim, Sunghan
      ACS Nano (2021), 15 (12), 19546-19558CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Enhancing and manipulating the mech. properties of graphene oxide (GO)-based structures are challenging because the GO assembly is easily delaminated. We develop nacre-like bionanofilms whose in-plane mech. properties can be manipulated through water vapor annealing without influencing their mech. properties in the thickness direction. These bionanofilms are prepd. from GO, silk fibroin (SF), and cellulose nanocrystals (CNCs) via a spin-assisted layer-by-layer assembly. The postannealing mech. properties of the films are detd. with at. force microscopy (AFM) bending and nanoindentation, and it is confirmed that the mech. properties of the bionanofilms are altered only in the in-plane direction. While AFM bending shows Young's moduli of 26.9, 36.3, 24.3, and 41.4 GPa for 15, 15 annealed, 30, and 30 annealed GO/SF/CNC trilayers, nanoindentation shows reduced moduli of 19.5 ± 2.6 and 19.5 ± 2.5 GPa before and after annealing, resp. The unaltered mech. properties of the bionanofilms along the thickness direction after annealing can be attributed to the CNC frame in the SF matrix acting as a support against stress in the thickness direction, while annealing reorganizes the bionanofilm structure. The tunability of the bionanofilms' mech. properties in only one direction through structure manipulation can lead to various applications, such as e-skin, wearable sensors, and human-machine interaction devices.
    58. 58
      Cho, H.; Lee, J.; Hwang, H.; Hwang, W.; Kim, J.-G.; Kim, S. Mechanical properties of graphene oxide–silk fibroin bionanofilms via nanoindentation experiments and finite element analysis. Friction 2022, 10, 282,  DOI: 10.1007/s40544-021-0490-8
      There is no corresponding record for this reference.
    59. 59
      Kim, S.; Xiong, R.; Tsukruk, V.-V. Probing flexural properties of cellulose nanocrystal–graphene nanomembranes with force spectroscopy and bulging test. Langmuir 2016, 32, 5383,  DOI: 10.1021/acs.langmuir.6b01079
      59
      Probing Flexural Properties of Cellulose Nanocrystal-Graphene Nanomembranes with Force Spectroscopy and Bulging Test
      Kim, Sunghan; Xiong, Rui; Tsukruk, Vladimir V.
      Langmuir (2016), 32 (21), 5383-5393CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The flexural properties of ultrathin freely standing composite nanomembranes from reduced graphene oxide (rGO) and cellulose nanocrystals (CNC) have been probed by combining force spectroscopy for local nanomech. properties and bulging test for global mech. properties. We obsd. that the flexural properties of these rGO-CNC nanomembranes are controlled by rGO content and deformational regimes. The nanomembranes showed the enhanced mech. properties due to the strong interfacial interactions between interwoven rGO and CNC components. The presence of weak interfacial interactions resulted in time-dependent behavior with the relaxation time gradually decreased with increasing the deformational rate owing to the reducing viscous damping at faster probing regimes close to 10 Hz. We obsd. that the microscopic elastic bending modulus of 141 GPa from local force spectroscopy is close to the elastic tensile modulus evaluated from macroscopic bulging test, indicating the consistency of both approaches for analyzing the ultrathin nanomembranes at different spatial scales of deformation. We showed that the flexible rGO-CNC nanomembranes are very resilient in terms of their capacity to recover back into original shape.
    60. 60
      Wang, Y.; Ma, R.; Hu, K.; Kim, S.; Fang, G.; Shao, Z.; Tsukruk, V.-V. Dramatic enhancement of graphene oxide/silk nanocomposite membranes: Increasing toughness, strength, and Young’s modulus via annealing of interfacial structures. ACS Appl. Mater. Interfaces 2016, 8, 24962,  DOI: 10.1021/acsami.6b08610
      60
      Dramatic Enhancement of Graphene Oxide/Silk Nanocomposite Membranes: Increasing Toughness, Strength, and Young's modulus via Annealing of Interfacial Structures
      Wang, Yaxian; Ma, Ruilong; Hu, Kesong; Kim, Sunghan; Fang, Guangqiang; Shao, Zhengzhong; Tsukruk, Vladimir. V.
      ACS Applied Materials & Interfaces (2016), 8 (37), 24962-24973CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      We demonstrate that stronger and more robust nacre-like laminated GO (graphene oxide)/SF (silk fibroin) nanocomposite membranes can be obtained by selectively tailoring the interfacial interactions between "bricks"-GO sheets and "mortar"-silk interlayers via controlled water vapor annealing. This facial annealing process relaxes the secondary structure of silk backbones confined between flexible GO sheets. The increased mobility leads to a significant increase in ultimate strength (by up to 41%), Young's modulus (up to 75%) and toughness (up to 45%). We suggest that local silk recrystn. is initiated in the proximity to GO surface by the hydrophobic surface regions serving as nucleation sites for β-sheet domains formation and followed by SF assembly into nanofibrils. Strong hydrophobic-hydrophobic interactions between GO layers with SF nanofibrils result in enhanced shear strength of layered packing. This work presented here not only gives a better understanding of SF and GO interfacial interactions, but also provides insight on how to enhance the mech. properties for the nacre-mimic nanocomposites by focusing on adjusting the delicate interactions between heterogeneous "brick" and adaptive "mortar" components with water/temp. annealing routines.
    61. 61
      Sehrawat, P.; Islam, S.-S.; Mishra, P.; Ahmad, S. Reduced graphene oxide (rGO) based wideband optical sensor and the role of temperature, defect states and quantum efficiency. Sci. Rep. 2018, 8, 3537,  DOI: 10.1038/s41598-018-21686-2
      61
      Reduced graphene oxide (rGO) based wideband optical sensor and the role of Temperature, Defect States and Quantum Efficiency
      Abid; Sehrawat Poonam; Islam S S; Mishra Prabhash; Ahmad Shahab
      Scientific reports (2018), 8 (1), 3537 ISSN:.
      We report a facile and cost-effective approach to develop self-standing reduced Graphene Oxide (rGO) film based optical sensor and its low-temperature performance analysis where midgap defect states play a key role in tuning the crucial sensor parameters. Graphite oxide (GO) is produced by modified Hummers' method and reduced thermally at 250 °C for 1 h in Argon atmosphere to obtain rGO. Self-standing rGO film is prepared via vacuum filtration. The developed film is characterized by HRTEM, FESEM, Raman, and XRD techniques. The developed sensor exhibits highest sensitivity towards 635 nm illumination wavelength, irrespective of the operating temperature. For a given excitation wavelength, photoresponse study at low temperature (123K-303K) reveals inverse relationship between sensitivity and operating temperature. Highest sensitivity of 49.2% is obtained at 123 K for 635 nm laser at power density of 1.4 mW/mm(2). Unlike sensitivity, response- and recovery-time demonstrate directly proportional dependence with operating temperature. Power dependent studies establish linear relation between power-density and sensitivity, and a safe limit beyond which sample heating prolongs the recovery time. Wavelength-dependent studies shows that proposed sensor can efficiently operate from visible to near NIR region. To the best of our knowledge such rGO based optical sensor performance at low temperature had not been reported earlier.
    62. 62
      Chang, H.; Sun, Z.; Saito, M.; Yuan, Q.; Zhang, H.; Li, J.; Wang, Z.; Fujita, T.; Ding, F.; Zheng, Z.; Yan, F.; Wu, H.; Chen, M.; Ikuhara, Y. Regulating infrared photoresponses in reduced graphene oxide phototransistors by defect and atomic structure control. ACS Nano 2013, 7, 6310,  DOI: 10.1021/nn4023679
      62
      Regulating Infrared Photoresponses in Reduced Graphene Oxide Phototransistors by Defect and Atomic Structure Control
      Chang, Haixin; Sun, Zhenhua; Saito, Mitsuhiro; Yuan, Qinghong; Zhang, Han; Li, Jinhua; Wang, Zhongchang; Fujita, Takeshi; Ding, Feng; Zheng, Zijian; Yan, Feng; Wu, Hongkai; Chen, Mingwei; Ikuhara, Yuichi
      ACS Nano (2013), 7 (7), 6310-6320CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Defects play significant roles in properties of graphene and related device performances. Most studies of defects in graphene focus on their influences on electronic or luminescent optical properties, while controlling IR optoelectronic performance of graphene by defect engineering remains a challenge. In the meantime, pristine graphene has very low IR photoresponses of ∼0.01 A/W due to fast photocarrier dynamics. Here the authors report regulating IR photoresponses in reduced graphene oxide phototransistors by defect and at. structure control for the 1st time. The IR optoelectronic transport and photocurrent generation are significantly influenced and well controlled by oxygenous defects and structures in reduced graphene oxide. Also, remarkable IR photoresponses are obsd. in photoconductor devices based on reduced graphene oxide with an external responsivity of ∼0.7 A/W, at least over one order of magnitude higher than that from pristine graphene. External quantum efficiencies of IR devices reach ultrahigh values of ∼97%, which to the authors' knowledge is one of the best efficiencies for IR photoresponses from nonhybrid, pure graphene or graphene-based derivs. The flexible IR photoconductor devices demonstrate no photoresponse degrdn. even after 1000 bending tests. The results open up new routes to control optoelectronic behaviors of graphene for high-performance devices.
    63. 63
      Lee, C.; Wei, X.; Kysar, J.-W.; Hone, J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008, 321, 385,  DOI: 10.1126/science.1157996
      63
      Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene
      Lee, Changgu; Wei, Xiaoding; Kysar, Jeffrey W.; Hone, James
      Science (Washington, DC, United States) (2008), 321 (5887), 385-388CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      We measured the elastic properties and intrinsic breaking strength of free-standing monolayer graphene membranes by nanoindentation in an at. force microscope. The force-displacement behavior is interpreted within a framework of nonlinear elastic stress-strain response, and yields second- and third-order elastic stiffnesses of 340 newtons per m (N m-1) and -690 N m-1, resp. The breaking strength is 42 N m-1 and represents the intrinsic strength of a defect-free sheet. These quantities correspond to a Young's modulus of E = 1.0 terapascals, third-order elastic stiffness of D = -2.0 terapascals, and intrinsic strength of σint = 130 gigapascals for bulk graphite. These expts. establish graphene as the strongest material ever measured, and show that atomically perfect nanoscale materials can be mech. tested to deformations well beyond the linear regime.
    64. 64
      Suk, J. W.; Piner, R.-D.; An, J.; Ruoff, R.-S. Mechanical properties of monolayer graphene oxide. ACS Nano 2010, 4, 6557,  DOI: 10.1021/nn101781v
      64
      Mechanical Properties of Monolayer Graphene Oxide
      Suk, Ji Won; Piner, Richard D.; An, Jinho; Ruoff, Rodney S.
      ACS Nano (2010), 4 (11), 6557-6564CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Mech. properties of ultrathin membranes consisting of one layer, two overlapped layers, and three overlapped layers of graphene oxide platelets were investigated by at. force microscopy (AFM) imaging in contact mode. To evaluate both the elastic modulus and prestress of thin membranes, the AFM measurement was combined with the finite element method (FEM) in a new approach for evaluating the mechanics of ultrathin membranes. Monolayer graphene oxide was found to have a lower effective Young's modulus (207.6 ±23.4 GPa when a thickness of 0.7 nm is used) as compared to the value reported for "pristine" graphene. The prestress (39.7-76.8 MPa) of the graphene oxide membranes obtained by soln.-based deposition was found to be 1 order of magnitude lower than that obtained by others for mech. cleaved graphene. The novel AFM imaging and FEM-based mapping methods presented here are of general utility for obtaining the elastic modulus and prestress of thin membranes.
    65. 65
      Magoshi, J.; Magoshi, Y.; Nakamura, S.; Kasai, N.; Kakudo, M. Physical properties and structure of silk. V. Thermal behavior of silk fibroin in the random-coil conformation. J. Polym. Sci. Polym. Phys. 1977, 15, 1675– 1683,  DOI: 10.1002/pol.1977.180150915
      65
      Physical properties and structure of silk. V. Thermal behavior of silk fibroin in the random-coil conformation
      Magoshi, Jun; Magoshi, Yoshiko; Nakamura, Shigeo; Kasai, Nobutami; Kakudo, Masao
      Journal of Polymer Science, Polymer Physics Edition (1977), 15 (9), 1675-83CODEN: JPLPAY; ISSN:0098-1273.
      Evaluation of the thermal behavior of films of amorphous silk fibroin in the random-coil conformation by differential scanning calorimetry, thermal expansion, dynamic mech. measurements, x-ray diffraction, and ir spectroscopy showed that water was lost up to 100°, inter- and intramol. H bonds were broken at 150-80°, glass transition occurred at 173°, and the random coil → β-form transition, accompanied by reformation of H bonds, occurred at >180°. Thermally-induced crystn. to β-form crystal started at ∼190°.
    66. 66
      Freddi, G.; Monti, P.; Nagura, M.; Gotoh, Y.; Tsukada, M. Structure and molecular conformation of tussah silk fibroin films: Effect of heat treatment. J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 841,  DOI: 10.1002/(SICI)1099-0488(19970415)35:5<841::AID-POLB13>3.0.CO;2-A
      66
      Structure and molecular conformation of tussah silk fibroin films: effect of heat treatment
      Freddi, Giuliano; Monti, Patrizia; Nagura, Masanobu; Gotoh, Yohko; Tsukada, Masuhiro
      Journal of Polymer Science, Part B: Polymer Physics (1997), 35 (5), 841-847CODEN: JPBPEM; ISSN:0887-6266. (Wiley)
      Structural changes of tussah (Antheraea pernyi) silk fibroin films induced by heat treatment were studied as a function of the treatment temp. in the range 200-250°. The DSC curve of tussah films with α-helix mol. conformation displayed characteristic endo and exo peaks at 216 and 226°, resp. These peaks first weakened and then completely disappeared after heating at 230°. Accordingly, the TMA thermal shrinkage at 206° disappeared when the films were heated at 230°. The onset of wt. loss was monitored at 210° by means of TG measurements. X-ray diffraction profiles gradually changed from α-helix to β-sheet cryst. structure as the treatment temp. increased from 200 to 250°. On raising the heating temp. above 200°, the intensity of IR and Raman bands characteristic fo β-sheet conformation increased in the whole ranges of amide and skeletal modes. The sample treated at 200° showed a spectral pattern intermediate between α-helix and β-sheet mol. conformation. The IR marker band for random coil structure, still detectable at 200°, disappeared at higher treatment temps. Spectral changes attributable to the onset of thermal degrdn. appeared at 230°.
    67. 67
      Yin, Z.; Hannard, F.; Barthelat, F. Impact-resistant nacre-like transparent materials. Science 2019, 364, 1260,  DOI: 10.1126/science.aaw8988
      67
      Impact-resistant nacre-like transparent materials
      Yin, Z.; Hannard, F.; Barthelat, F.
      Science (Washington, DC, United States) (2019), 364 (6447), 1260-1263CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Glass has outstanding optical properties, hardness, and durability, but its applications are limited by its inherent brittleness and poor impact resistance. Lamination and tempering can improve impact response but do not suppress brittleness. We propose a bioinspired laminated glass that duplicates the 3-dimensional brick and mortar arrangement of nacre from mollusk shells, with periodic 3-dimensional architectures and interlayers made of a transparent thermoplastic elastomer. This material reproduces the tablet sliding mechanism, which is key to the toughness of natural nacre but has been largely absent in synthetic nacres. Tablet sliding generates nonlinear deformations over large vols. and significantly improves toughness. This nacre-like glass is also 2-3-fold more impact resistant than laminated glass and tempered glass while maintaining high strength and stiffness.
    68. 68
      Trung, T.-Q.; Le, H.-S.; Dang, T.-M.-L.; Ju, S.; Park, S.-Y.; Lee, N.-E. Freestanding, fiber-based, wearable temperature sensor with tunable thermal index for healthcare monitoring. Adv. Healthcare Mater. 2018, 7, 1800074  DOI: 10.1002/adhm.201800074
      There is no corresponding record for this reference.
    69. 69
      Kim, B.-K.; Kwon, O.-H.; Park, W.-H.; Cho, D. Thermal, mechanical, impact, and water absorption properties of novel silk fibroin fiber reinforced poly (butylene succinate) biocomposites. Macromol. Res. 2016, 24, 734,  DOI: 10.1007/s13233-016-4102-9
      69
      Thermal, mechanical, impact, and water absorption properties of novel silk fibroin fiber reinforced poly(butylene succinate) biocomposites
      Kim, Byung Kuk; Kwon, Oh Hyeong; Park, Won Ho; Cho, Donghwan
      Macromolecular Research (2016), 24 (8), 734-740CODEN: MRAECT; ISSN:1598-5032. (Polymer Society of Korea)
      Novel silk fibroin fiber reinforced poly(butylene succinate) (PBS) biocomposites have been developed by twin-screw extrusion and injection molding processes, varying the fiber content from 10 to 40 % by wt. The thermal stability, thermal expansion, dynamic mech., tensile, flexural, and impact properties and water absorption behavior of the biocomposites with various silk fiber contents are investigated. The obtained results are consistent with each other. The thermal stability of the biocomposites depends on the silk fibroin fiber content, reflecting the thermal characteristics of both PBS and silk fiber. The coeff. of thermal expansion is somewhat decreased by incorporating silk fibroin fibers into the PBS. The storage modulus, the tensile modulus, and the flexural properties are gradually increased with increasing the fiber content up to 40 wt%. The Izod impact strength is the highest at 20-25 wt% fiber content, as similarly found in the tensile strength. The percentage of water absorption is gradually increased with increasing the fiber content as well as the water immersion time.
    70. 70
      Aksakal, B.; Akdere, Ü.; Günay, S.-D.; Çağın, T.; Taşseven, Ç. Influence of repeating sequence on structural and thermal stability of crystalline domain of Bombyx mori silk fibroin. Mater. Res. Express 2019, 6, 125356,  DOI: 10.1088/2053-1591/ab6548
      70
      Influence of repeating sequence on structural and thermal stability of crystalline domain of bombyx mori silk fibroin
      Aksakal, Baki; Akdere, Unsal; Gunay, Seckin D.; Cagin, Tahir; Tasseven, Cetin
      Materials Research Express (2019), 6 (12), 125356CODEN: MREAC3; ISSN:2053-1591. (IOP Publishing Ltd.)
      Thermophys. properties of polar-antiparallel β-sheet cryst. domains of B.mori silk fibroin has been investigated via mol. dynamics (MD) simulations between 300 K and 700 K. In general, the type of interactions dets. the character of the thermal expansion in corresponding directions except that the cryst. domains with serine residue contracts along the chain direction resulting with a neg. thermal expansion(NTE) coeff. of αy ≈ -10-4K-1 at 300 K. The heat capacity at const. pressure CP increases sublinearly up to about 650 K with an almost continuous decrease in the rate of change of entropy. (dS/TP)p Energetic behavior of the β-sheet cryst. units is detd. by the main chain below 400 K then thermally activated motion of side chain (HG1) of serine residue becomes effective up to the degrdn. point. These are particularly important in designing thermally controlled composite materials.
    71. 71
      Zheng, S.; Tu, Q.; Urban, J.-J.; Li, S.; Mi, B. Swelling of graphene oxide membranes in aqueous solution: Characterization of interlayer spacing and insight into water transport mechanisms. ACS Nano 2017, 11, 6440,  DOI: 10.1021/acsnano.7b02999
      71
      Swelling of Graphene Oxide Membranes in Aqueous Solution: Characterization of Interlayer Spacing and Insight into Water Transport Mechanisms
      Zheng, Sunxiang; Tu, Qingsong; Urban, Jeffrey J.; Li, Shaofan; Mi, Baoxia
      ACS Nano (2017), 11 (6), 6440-6450CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Graphene oxide (GO) has recently emerged as a promising 2D nanomaterial to make high-performance membranes for important applications. However, the aq.-phase sepn. capability of a layer-stacked GO membrane can be significantly limited by its natural tendency to swell, i.e., absorb water into the GO channel and form an enlarged interlayer spacing (d-spacing). In this study, the d-spacing of a GO membrane in an aq. environment was exptl. characterized using an integrated quartz crystal microbalance with dissipation and ellipsometry. This method can accurately quantify a d-spacing in liq. and well beyond the typical measurement limit of ∼2 nm. Mol. simulations were conducted to fundamentally understand the structure and mobility of water in the GO channel, and a theor. model was developed to predict the d-spacing. It was found that, as a dry GO membrane was soaked in water, it initially maintained a d-spacing of 0.76 nm, and water mols. in the GO channel formed a semiordered network with a d. 30% higher than that of bulk water but 20% lower than that of the rhombus-shaped water network formed in a graphene channel. The corresponding mobility of water in the GO channel was much lower than in the graphene channel, where water exhibited almost the same mobility as in the bulk. As the GO membrane remained in water, its d-spacing increased and reached 6 to 7 nm at equil. In comparison, the d-spacing of a GO membrane in NaCl and Na2SO4 solns. decreased as the ionic strength increased and was ∼2 nm at 100 mM.
    72. 72
      Lee, M.; Jeon, H.; Kim, S. A highly tunable and fully biocompatible silk nanoplasmonic optical sensor. Nano Lett. 2015, 15, 3358,  DOI: 10.1021/acs.nanolett.5b00680
      72
      A Highly Tunable and Fully Biocompatible Silk Nanoplasmonic Optical Sensor
      Lee, Myungjae; Jeon, Heonsu; Kim, Sunghwan
      Nano Letters (2015), 15 (5), 3358-3363CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Novel concepts for manipulating plasmonic resonances and the biocompatibility of plasmonic devices offer great potential in versatile applications involving real-time and in vivo monitoring of analytes with high sensitivity in biomedical and biol. research. Here we report a biocompatible and highly tunable plasmonic bio/chem. sensor consisting of a natural silk protein and a gold nanostructure. Our silk plasmonic absorber sensor (SPAS) takes advantage of the strong local field enhancement in the metal-insulator-metal resonator in which silk protein is used as an insulating spacer and substrate. The silk insulating spacer has hydrogel properties and therefore exhibits a controllable swelling when exposed to water-alc. mixts. We exptl. and numerically show that drastic spectral shifts in reflectance min. arise from the changing phys. vol. and refractive index of the silk spacer during swelling. Furthermore, we apply this SPAS device as a glucose sensor with a very high sensitivity of 1200 nm/RIU (refractive index units) and high relative intensity change.
    73. 73
      Su, Y.; Wei, H.; Gao, R.; Yang, Z.; Zhang, J.; Zhong, Z.; Zhang, Y. Exceptional negative thermal expansion and viscoelastic properties of graphene oxide paper. Carbon 2012, 50, 2804,  DOI: 10.1016/j.carbon.2012.02.045
      73
      Exceptional negative thermal expansion and viscoelastic properties of graphene oxide paper
      Su, Yanjie; Wei, Hao; Gao, Rungang; Yang, Zhi; Zhang, Jing; Zhong, Zhaohui; Zhang, Yafei
      Carbon (2012), 50 (8), 2804-2809CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
      Reported are unusual neg. thermal expansion and viscoelastic properties in graphene oxide paper. The paper was prepd. from aq. GO dispersions using a simple vacuum evapn. technique. From room temp. to 150°, this paper-like graphene oxide sheet exhibits a const. neg. thermal expansion coeff. of -67 × 10-6/K along the in-plane direction. Peculiar hysteresis loops of thermal expansion-temp. curves are obsd., which are affected by the cooling rate and starting temp. of cooling. The tensile tests on GO paper show clear hysteresis loops, revealing the viscoelastic property of the paper. The viscoelastic properties show excellent frequency-stability in the range of 1.0-60 Hz. In the low temp. range -150 to 25°, however, they show a strong temp. dependence. The storage modulus of the graphene oxide paper continuously increases with decreasing temp.
    74. 74
      Cretikos, M.-A.; Bellomo, R.; Hillman, K.; Chen, J.; Finfer, S.; Flabouris, A. Respiratory rate: The neglected vital sign. Med. J. Aust. 2008, 188, 657,  DOI: 10.5694/j.1326-5377.2008.tb01825.x
      74
      Respiratory rate: the neglected vital sign
      Cretikos Michelle A; Bellomo Rinaldo; Hillman Ken; Chen Jack; Finfer Simon; Flabouris Arthas
      The Medical journal of Australia (2008), 188 (11), 657-9 ISSN:0025-729X.
      The level of documentation of vital signs in many hospitals is extremely poor, and respiratory rate, in particular, is often not recorded. There is substantial evidence that an abnormal respiratory rate is a predictor of potentially serious clinical events. Nurses and doctors need to be more aware of the importance of an abnormal respiratory rate as a marker of serious illness. Hospital systems that encourage appropriate responses to an elevated respiratory rate and other abnormal vital signs can be rapidly implemented. Such systems help to raise and sustain awareness of the importance of vital signs.
    75. 75
      AL-Khalidi, F.-Q.; Saatchi, R.; Burke, D.; Elphick, H.; Tan, S. Respiration rate monitoring methods: A review. Pediatr. Pulmonol. 2011, 46, 523,  DOI: 10.1002/ppul.21416
      75
      Respiration rate monitoring methods: a review
      Al-Khalidi F Q; Saatchi R; Burke D; Elphick H; Tan S
      Pediatric pulmonology (2011), 46 (6), 523-9 ISSN:.
      Respiration rate is an important indicator of a person's health, and thus it is monitored when performing clinical evaluations. There are different approaches for respiration monitoring, but generally they can be classed as contact or noncontact. For contact methods, the sensing device (or part of the instrument containing it) is attached to the subject's body. For noncontact approaches the monitoring is performed by an instrument that does not make any contact with the subject. In this article a review of respiration monitoring approaches (both contact and noncontact) is provided. Concerns related to the patient's recording comfort, recording hygiene, and the accuracy of respiration rate monitoring have resulted in the development of a number of noncontact respiration monitoring approaches. A description of thermal imaging based and vision based noncontact respiration monitoring approaches we are currently developing is provided.
    76. 76
      Pereira, C.-B.; Yu, X.; Goos, T.; Reiss, I.; Orlikowsky, T.; Heimann, K.; Venema, B.; Blazek, V.; Leonhardt, S.; Teichmann, D. Noncontact monitoring of respiratory rate in newborn infants using thermal imaging. IEEE Trans. Biomed. Eng. 2018, 66, 1105,  DOI: 10.1109/TBME.2018.2866878
      There is no corresponding record for this reference.

  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.biomac.3c00106.

    • Schematic of the sensor evaluation method, sensor resistance, EDS plots of films, AFM images of films on Cu mesh, force–distance curves, load–deflection plots during approaching and retracting, surface roughness, FTIR spectra, AFM image of films with and without an LbL structure, resistance–temperature curve of sensors, response and recovery curves, specifications and comparison of various wearable temperature and humidity sensors, experimental setup for measuring film thickness at various temperatures, AFM images of films at various temperature, methods for cyclic responsivity measurements of sensors, and film delamination after repeated bending tests (PDF)


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