Download Hi-Res ImageDownload to MS-PowerPointCite This:ACS Catal. 2023, 13, 13, 9090-9101

Research ArticleJune 26, 2023

Low-Volume Reaction Monitoring of Carbon Dot Light Absorbers in Optofluidic Microreactors
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Takashi Lawson
Takashi Lawson
NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
Yusuf Hamied Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
More by Takashi Lawson
https://orcid.org/0000-0002-2571-4216
Alexander S. Gentleman
Alexander S. Gentleman
NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
Yusuf Hamied Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
More by Alexander S. Gentleman
Ava Lage
Ava Lage
Yusuf Hamied Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
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Carla Casadevall
Carla Casadevall
Yusuf Hamied Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
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https://orcid.org/0000-0002-3090-4938
Jie Xiao
Jie Xiao
Helmholtz-Zentrum Berlin für Materialien und Energy GmbH, Albert-Einstein-Straße 15, 12489 Berlin, Germany
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https://orcid.org/0000-0002-2320-6111
Tristan Petit
Tristan Petit
Helmholtz-Zentrum Berlin für Materialien und Energy GmbH, Albert-Einstein-Straße 15, 12489 Berlin, Germany
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https://orcid.org/0000-0002-6504-072X
Michael H. Frosz
Michael H. Frosz
Max Planck Institute for the Science of Light, Staudtstr. 2, 91058 Erlangen, Germany
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Erwin Reisner*
Erwin Reisner
Yusuf Hamied Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
*Email: [email protected]
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https://orcid.org/0000-0002-7781-1616
Tijmen G. Euser*
Tijmen G. Euser
NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
*Email: [email protected]
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https://orcid.org/0000-0002-8305-9598

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ACS Catalysis

Cite this: ACS Catal. 2023, 13, 13, 9090–9101

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https://doi.org/10.1021/acscatal.3c02212

Published June 26, 2023

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Abstract

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Optical monitoring and screening of photocatalytic batch reactions using cuvettes ex situ is time-consuming, requires substantial amounts of samples, and does not allow the analysis of species with low extinction coefficients. Hollow-core photonic crystal fibers (HC-PCFs) provide an innovative approach for in situ reaction detection using ultraviolet–visible absorption spectroscopy, with the potential for high-throughput automation using extremely low sample volumes with high sensitivity for monitoring of the analyte. HC-PCFs use interference effects to guide light at the center of a microfluidic channel and use this to enhance detection sensitivity. They open the possibility of comprehensively studying photocatalysts to extract structure–activity relationships, which is unfeasible with similar reaction volume, time, and sensitivity in cuvettes. Here, we demonstrate the use of HC-PCF microreactors for the screening of the electron transfer properties of carbon dots (CDs), a nanometer-sized material that is emerging as a homogeneous light absorber in photocatalysis. The CD-driven photoreduction reaction of viologens (XV²⁺) to the corresponding radical monocation XV^•+ is monitored in situ as a model reaction, using a sample volume of 1 μL per measurement and with a detectability of <1 μM. A range of different reaction conditions have been systematically studied, including different types of CDs (i.e., amorphous, graphitic, and graphitic nitrogen-doped CDs), surface chemistry, viologens, and electron donors. Furthermore, the excitation irradiance was varied to study its effect on the photoreduction rate. The findings are correlated with the electron transfer properties of CDs based on their electronic structure characterized by soft X-ray absorption spectroscopy. Optofluidic microreactors with real-time optical detection provide unique insight into the reaction dynamics of photocatalytic systems and could form the basis of future automated catalyst screening platforms, where samples are only available on small scales or at a high cost.

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Subjects

Keywords

Introduction

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Carbon dots (1,2) (CDs) are quasi-spherical particles, generally below 10 nm in size, (3,4) which can exist in two forms: crystalline with sp² character (graphitic and nitrogen-doped graphitic, gCDs and NgCDs, respectively) or amorphous (aCDs) with a majority contribution of sp³ carbons. They are proposed as potential next-generation photosensitizers as they offer a unique combination of properties. They are robust, environmentally benign, visible-light-active, biocompatible, and stable. (5) CDs are surface-functionalized with oxygen-containing functional groups such as carboxyls and alcohols, enabling exceptional water solubility and the ability to tune their surface functionalities. (2,6,7) Moreover, CD synthesis is inexpensive, simple, and scalable, and can even be produced from waste such as lignocellulosic biomass. (2,8)

When implemented in photochemical processes, photoexcited CDs can undergo electron transfer in the presence of an electron acceptor (EA), such as viologens or molecular catalysts, (9,10) or an electron donor. Viologen redox couples are established electron relays in a range of hydrogen-evolving systems. (11,12) Strauss et al. have postulated that CDs and viologens form preorganized bound complexes in solution due to the electrostatic attraction between the negatively charged CD surface and viologen dication, which results in a decrease in the measured ζ-potential. (13) Sacrificial electron donors (SEDs) are often used in place of an oxygen evolution counter reaction to simplify the study of a photocatalytic half-reaction. As such, the ideal SED should neither rate-limit the desired reaction nor interfere with the reductive chemistry. Commonly used SEDs in solar fuel synthesis include tertiary amines such as ethylenediaminetetraacetic acid (EDTA), triethylamine (TEA), and triethanolamine (TEOA), which form known noninnocent species upon oxidation (14)

The combination of CDs with synthetic Ni- and Co-based molecular catalysts in hybrid photocatalytic systems has been shown to evolve hydrogen in water, (8,9,15) and the use of CDs has been reported in organocatalysis, for example, acid–base catalysis, hydrogen-bond catalysis, and aminocatalysis. (16−19) Graphitization of CDs enhances light absorption, and nitrogen doping of graphitic CDs (NgCDs) can prolong the lifetime of photogenerated charges, thereby enhancing their overall photocatalytic performance. (5)

Despite the advantages of CDs, the quantum yield of CD-based systems is relatively low and the photophysical properties are not fully understood. Understanding and optimization of these photocatalytic systems require comprehensive knowledge of the reaction kinetics that can only be obtained through screening of different types of CDs, EAs, SEDs, and other reaction conditions. Unfortunately, screening processes for photocatalysts are challenging as they typically rely on ex situ methods with high sample concentrations for analysis, which may not necessarily reflect the system under real catalytic conditions and therefore lead to poor reaction optimization decisions. Additionally, the paucity of detailed cuvette-based studies aimed at varying reaction parameters for photocatalytic reaction optimization highlights the cumbersome nature of performing such investigations, with relatively large (mL) sample volumes, long sampling intervals, and difficulty in modulating the irradiance. As such, there is a growing need for microreactors that allow rapid changes in reaction conditions to be monitored in situ within tiny reaction volumes and with enhanced time resolution. (20,21)

Hollow-core photonic crystal fibers (HC-PCFs) have emerged as an attractive optofluidic platform for in situ monitoring within microreactors. A key advantage is that they allow light to be guided along a microfluidic channel over extended lengths while keeping the internal sample volume very low. HC-PCFs thus enable highly sensitive liquid-phase spectroscopy on sub-μL sample volumes. (22−29) The dead volume, accounting for tubing and the pressure cell in this system, is <55 μL. Once a measurement is completed, a fresh sample can infiltrate the fiber by flowing ca. 10 μL of the sample through the fiber (corresponding to 10× the internal volume contained in the fiber core). The dead volume can be minimized to below 1 μL by replacing the pressure cell with a microfluidic chip. (30)

We have previously demonstrated that HC-PCF microreactors can be used to monitor in situ CD-driven photoreduction of viologens (XV²⁺) to the radical monocation XV^•+ on volumes less than 36 nL. (31) Alternative types of hollow waveguides, such as a large-diameter (>800 μm) Teflon-coated capillary fiber, (32−34) have also been used for reaction monitoring. However, homogeneous excitation of strongly absorbing CD samples can only be achieved by side-excitation, requiring the waveguide to be made of ultraviolet (UV)-transparent materials such as fused silica.

Here, we build upon our previous proof-of-principle study and use UV–visible (UV–vis) spectroscopy performed with HC-PCF microreactors to demonstrate the ability to screen for a wide range of reaction conditions (Figure 1a), generating new kinetic insights into the photoactivity of CDs and improving our understanding of structure–function relations. We use viologens as indicators of electron transfer from CDs due to their distinctive UV–vis absorption peaks and their ability to provide insight into single electron transfers. CD-Viologen reactions thus form an excellent test system to understand redox mechanisms independently from catalysis requirements. The performance of CD light absorbers is benchmarked against the conventional photosensitizer [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine), with the electron transfer properties of each CD type also discussed, as characterized by soft X-ray absorption spectroscopy (XAS). This work demonstrates how fiber-based optofluidic microreactors can provide comprehensive kinetic insight into an exemplar photocatalytic platform, i.e. CDs, which hold promise for driving a wide range of sustainable organic transformations. (16,35)

Figure 1. Fiber-based CD screening. [a] Schematic of CD-driven photoreduction of viologens and the four screening factors explored in this study: the irradiance, sacrificial electron donor (SED) used, CD photosensitizer type, and viologen species (XV). [b] Scanning electron microscopy (SEM) image of the core region of the kagomé-style hollow-core photonic crystal fiber (HC-PCF) overlaid with the measured mode intensity profile (450–600 nm). [c] Optical setup enabling the transmitted mode to be simultaneously analyzed by a camera (see [b]) and its spectrum by a spectrometer (see [d]). BS, beamsplitter; LP, long-pass; MMF, multimode fiber (300 μm core diameter); SC, supercontinuum. O1 corresponds to the in-coupling objective and O2 corresponds to the out-coupling objective. A1 is an aperture used for alignment. [d] Time-resolved spectra are converted to absorbance with a trace taken at 600 nm to deduce the XV^•+ concentration in [e]. The spectral shape of the absorbance profile is characteristic of an XV^•+ species. [e] Fitting method for calculating the delay time, *t_d*, and photoreduction rate constant, k. The time at which the electron transfer starts, t₀, is defined by the x-intercept of the linear fitting function.

Carbon Dot Screening Scheme

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The proposed anaerobic reaction mechanism is summarized in eqs 1–6 and is based on studies by Mandal et al. and Suzuki et al. who used [Ru(bpy)₃]²⁺ as a photosensitiser. (36,37)

CD + h ν ⇌ {CD}^{*}

(1)

{CD}^{*} + SED \to {CD}^{-} + {SED}_{ox}^{+}

(2)

{CD}^{-} + {XV}^{2 +} \to CD + {XV}^{• +}

(3)

{CD}^{-} + {XV}^{• +} \to CD + {XV}^{0}

(4)

{SED}_{ox}^{+} \to {SED}^{'}

(5)

{XV}^{0} \to {XV}^{'}

(6)

Reaction 1 represents the formation of the photoexcited carbon dot, CD*, which is then reductively quenched by a SED in reaction 2 according to the literature. (14,15) This results in a reduced carbon dot, CD^–, and an oxidized SED species, SED_ox⁺, whereby the SED_ox⁺ ideally comprises innocent decomposition products after electron transfer. Reaction 3 represents the electron transfer from CD^– to XV²⁺ to form a radical monocation that we can detect by UV–vis absorption spectroscopy (λ_max = 600 nm). Reaction 4 represents an electron transfer from CD^– to XV^•+ to form a neutral XV⁰. (37) The SED_ox⁺ species, in the cases of EDTA, TEA, and TEOA, refers to the aminyl radical species, R₂N^•+CH₂–, which is readily deprotonated to form SED′, a carbon-centered radical species and strong reducing agent, R₂NC^•H–, (36) in reaction 5. XV⁰ is readily protonated and reacts with water to form XV′, a decomposition product without a UV–vis absorbance feature in reaction 6. (38)

There are a few pathways that can lead to the depletion of XV^•+. For instance, the concentration of XV^•+ can be depleted in interaction with CD^– through a second electron transfer in reaction 4 or with highly reducing decomposed SED (SED′). Another possible route for XV^•+ depletion is via direct interaction with non-photoexcited and nonreduced CDs; a pathway not considered in the proposed reaction mechanism. Evidence for this pathway is presented in the Results and Discussion section.

Experimental Section

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Hollow-Core Photonic Crystal Fiber (HC-PCF)

This study utilizes kagomé-style HC-PCFs (Figure 1b), whose guidance properties are well described by the anti-resonant reflection theory. (39,40) By controlling the thickness of the glass struts surrounding the hollow-core cladding structure, this guidance mechanism permits broadband transmission extending from the ultraviolet (UV) to near-infrared, and hence, kagomé-style HC-PCFs are the preferred choice for UV–vis spectral measurements. (41) The fiber used in this study was fabricated with a strut thickness, t, of 160 nm to suppress the first resonant loss, λ₁, to 183 nm (see eq 7). (40) The first resonant loss was calculated for a fiber infiltrated with water, assuming a glass refractive index, n_glass, of 1.45. Additionally, a core region of 30 μm diameter facilitated a UV–vis detection volume of ca. 7 nL cm^–1.

λ_{q = 1} = 2 t \sqrt{n_{glass}^{2} - n_{water}^{2}}

(7)

Kagomé-style HC-PCFs were interfaced with standard polyether ether ketone (PEEK) microfluidic tubing through custom-designed stainless steel pressure cells with sapphire windows (see Supporting Information, Figure S1), enabling the HC-PCF to be loaded with a liquid solution while remaining optically accessible. HC-PCFs were sealed with PEEK sleeves (IDEX F-240 Blue) and finger-tight fittings, as in previous work. (31) Samples were thoroughly purged under nitrogen for at least 15 min before loading into the fiber and inserted through the out-coupling facet to minimize disturbances to the optical coupling. All samples were injected into the pressure cell using gas-tight syringes and a syringe pump (Aladdin AL-1000), having passed through 0.2 μm pore size cellulose acetate filters to prevent fiber blockages.

Repeat measurement runs were loaded successively into the fiber and started once the spectrum returned to the reference baseline. This ensured that residual reaction products from the previous run had been pushed clear of the fiber. The fiber was cleaned with 200 μL of water (200× the internal fiber volume) between sample sets of different reaction conditions. Subsequent samples were loaded once the spectrum matched an internal water reference.

UV–Vis Absorption Spectroscopy Setup

A broadband fiber-coupled supercontinuum (SC) laser (NKT Photonics SuperK Compact─Figure 1c) was used to perform UV–vis absorption spectroscopy. A 450 nm long-pass filter was placed in the probe beam path to prevent excitation of the photochemical reaction by the probe light. Furthermore, the probe power coupled into the fiber was reduced to 2.2 μW using neutral density filters. The beam was launched into the core of the liquid-filled fiber, exciting a fundamental optical mode (Figure 1b). We achieve transmission of 48%T at 600 nm for 20 cm of fiber infiltrated with water, indicating a good coupling efficiency (see Supporting Information, Figures S2 and S3).

A beamsplitter (BS) cube divides the transmitted probe light over an imaging CCD camera (IDS UI-3240LE-NIR-GL) and a fiber-coupled spectrometer (Ocean Optics QE Pro 65000). The guided light was strongly confined to the core. An aperture (A1) was placed in the optical setup to aid optical alignment.

UV–vis absorption spectra were generated by dividing the spectrometer counts by a reference spectrum, which in this case is the spectrum of the initial unirradiated sample (Figure 1d). Concentration was then calculated from the absorption peak at 600 nm via the Beer–Lambert law (Figure 1e), as detailed in the Supporting Information. Experiments reveal a delay time, t_d, between the excitation source being switched on and the formation of XV^•+. We have previously reported on this finding. (31)

A 365 nm UV light-emitting diode (LED) source (HepatoChem EvoluChem 365PF) and an optical diffuser (Thorlabs DG20-220-MD) were used to side-irradiate a 5 cm section of the HC-PCF, ensuring a homogeneous excitation profile across the 36 nL UV–vis detection volume. UV excitation was chosen due to the significant absorption of CDs at 365 nm (see Supporting Information, Figure S4). Significantly higher concentrations of CDs would be required for irradiation at visible wavelengths, which would, in turn, reduce transmission of the broadband probe light, resulting in much noisier UV–vis data. The irradiance was varied between 1.6 and 88.4 mW cm^–2 by placing 2″ × 2″ absorptive neutral density filters (Thorlabs NE2 series) between the excitation source and fiber. A 450 nm blue LED source (HepatoChem EvoluChem 450PF) was additionally utilized to benchmark the performance of nitrogen-doped graphitic carbon dots (NgCDs) against [Ru(bpy)₃]²⁺ at an irradiance of 99.1 mW cm^–2.

X-ray Absorption Spectroscopy (XAS)

Experiments were conducted at the U49-2_PGM1 soft X-ray beamline of the BESSY II synchrotron using the LiXEdrom endstation. The CDs were drop-cast on a conductive Si substrate, and XAS was acquired in vacuum using total electron yield (TEY) mode detecting the sample drain current by a Keithley 6514 ammeter at the C and O K-edge and total fluorescence yield (TFY) collecting emitted photons by a photodiode placed in front of samples at the N K-edge.

Materials

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All chemicals and reagents were purchased from commercial suppliers and used as received unless otherwise noted. Laboratory-grade reagents were used for synthesis, and chemicals for the analytical part were of the highest available purity. Phosphate buffer solutions (0.2 M) and SED stock solutions (0.2 M) were prepared and verified using a pH electrode (Mettler Toledo, FiveEasy Plus) at pH 6 and pH 8. The pH of SED stock solutions was adjusted using sodium hydroxide and hydrochloric acid. Methyl viologen (MV²⁺), ethyl viologen (EV²⁺), and benzyl viologen (BV²⁺) stock solutions were made up by dissolving methyl viologen dichloride, ethyl viologen dibromide, and benzyl viologen dichloride salts, respectively. A stock solution of [Ru(bpy)₃]²⁺ was made up by dissolving tris(bipyridine)ruthenium(II) chloride. All stock solutions were made up using Milli-Q purified water.

Synthesis of CDs

All CD types were synthesized and characterized according to previously reported procedures. (5,9) In brief, amorphous carbon dots (aCD) were synthesized by pyrolysis of citric acid at 180 °C for 40 h and graphitic carbon dots (gCD) were similarly synthesized by pyrolysis of citric acid at 180 °C for 40 h followed by pyrolysis at 320 °C for a further 100 h. Nitrogen-doped graphitic carbon dots (NgCD) were obtained by pyrolysis of aspartic acid at 320 °C for 100 h. To enhance water solubility, unless otherwise noted, all CD types were neutralized to pH 7 using NaOH and then freeze-dried to obtain the final product as a brown powder. All CDs were characterized by UV–vis, Fourier transform infrared (FTIR), and Raman spectroscopy (see Supporting Information, Figures S4 and S5).

Results and Discussion

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To ascertain whether XV^•+ interacts with the CDs, MV²⁺ (160 μM) was chemically reduced to MV^•+ with one equivalent of sodium dithionite (SDT, Na₂S₂O₄) under nitrogen in a gas-tight cuvette. Further reduction of MV^•+ to MV⁰ by Na₂S₂O₄ is excluded based on reduction potentials (E^{MV^•+/MV⁰} = −1.15 V vs normalized hydrogen electrode (NHE) and E^SDT/SDT⁺ = −0.66 V vs NHE). Solutions were then prepared with and without the presence of a SED (i.e., EDTA). Subsequent titration with aCDs comprising different surface moieties (NMe₂⁺, COO^–, and COOH) under nitrogen revealed a reduction in the intensity of the characteristic UV–vis absorption band of MV^•+ upon addition of small equivalents of the aCDs under nitrogen (see Supporting Information, Figure S7), with corresponding NMR data showing electron transfer occurring from MV^•+ to the aCDs to yield MV²⁺ (see Supporting Information, Figure S8). This indicates that there may be an interaction with CDs that depletes the MV^•+ concentration in solution when a SED is not present. The addition of the NMe₂⁺-functionalized aCDs shows the slowest depletion rate of MV^•+ compared to the COOH- and COO^–-functionalized dots, which is most likely due to the electrostatic repulsive interactions between MV^•+ and the positive surface charge on the NMe₂⁺-functionalized aCDs. (42,43)

In comparison with titrations with the SED (EDTA) present, it is found that the SED forms a complex with MV^•+, resulting in a smaller absorbance at 600 nm. Subsequent titration after the SED is added leads to a reversal in the trend of depletion rate, that is, positively charged NMe₂⁺-functionalized aCDs deplete MV^•+ faster than the negatively charged COO^–-functionalized aCDs. This is most likely due to the cooperative electrostatic interactions of the negatively charged EDTA, which is expected for solutions of pH 4–7 (refer to Supporting Information, Table S1). The cooperative electrostatic interactions bring together MV^•+ and the positively charged NMe₂⁺-functionalized aCDs in solution, thus making them interact more quickly than the negatively charged COO^–-functionalized aCDs. All forms of aCDs considered here are involved in the depletion of XV^•+ and as such, an additional reaction for the proposed reaction mechanism must be considered reaction 8. It is likely that this reaction is mediated by the filling of trap states within the CDs, resulting in a sufficient driving force for this reaction to proceed.

{XV}^{• +} + CD \to {XV}^{2 +} + {CD}^{-}

(8)

Following confirmation of this additional pathway to deplete XV^•+, mechanistic insight into the photoactivity of CDs was obtained by the systematic screening of four variables (irradiance, photosensitizer, EAs, and SEDs), as shown in Figure 1a. A 20 cm length of kagomé-style HC-PCF was infiltrated with various aqueous solutions of CDs, viologens, and sacrificial electron donors (SEDs).

Effect of aCD Concentration

Samples of 40 μM MV²⁺ and 0.1 M EDTA were prepared in an aqueous pH 6 phosphate buffer solution and infiltrated into the kagomé-style HC-PCF, with various concentrations of COO^–-functionalized aCDs. These samples were subject to UV irradiation (λ = 365 nm, 90.2 mW cm^–2), with the corresponding concentration and rate profiles shown in Figure 2a,b. As observed, peak radical monocation generation (λ_max = 600 nm) is higher when the CD concentration is greater, suggesting a dynamic equilibrium exists between the photoinduced rate of formation of the radical monocation and the depletion rate of the radical monocation. At higher concentrations of CDs, the position of the equilibrium is shifted toward the radical monocation given the higher [CD*]. However, a rise in [CD] is also associated with a rise in [CD^–], with CD^– interacting with MV^•+ as in reaction 4. This results in a greater depletion rate following the peak, as observed in Figure 2b.

Figure 2. Photoreduction dependence on CD concentration. [a] MV^•+ concentration, and [b] rate profiles, both illustrating the dependence of the reaction kinetics on amorphous carbon dot (aCD) concentration. The MV^•+ concentration was calculated from the UV–vis absorption peak at 600 nm. Each sample consists of aCDs with 40 μM MV²⁺ and 0.1 M EDTA in an aqueous pH 6 phosphate buffer. Samples were initially kept in the dark and, from t = 60 s, were subject to continuous UV irradiation (λ = 365 nm) at an irradiance of 90.2 mW cm^–2. [c] Delay time and [d] photoreduction rate constant, both plotted as a function of aCD concentration, with error bars signifying the standard deviation of triplicates.

The peak in the reaction profile (Figure 2) results from the competition between the generation of MV^•+ reaction 3 and its depletion reaction 4. As MV²⁺ is depleted during the reaction, the rate of formation of MV^•+ falls. In contrast, the rate of formation of MV⁰ increases. This manifests as a relatively sudden and significant decrease in the rate of MV^•+ depletion when MV^•+ reaches a critical amount. As observed in Figure 2, such rate-switching behavior is expected to arise earlier as [CD] increases, as MV²⁺ is consumed faster via reaction 3 inevitably resulting in a critical value of MV^•+ being reached more rapidly. The origin of this rate-switching behavior is further verified by the [MV^•+] peak being reached much later when the pH is lowered (see Supporting Information, Figure S10), which is most likely due to the rate of reaction 2 decreasing with increased solution acidity, which is consistent with protonation of the SED lowering its ability to reduce photosensitisers. (44,45)

The delay time (Figure 1e), which is the period between the excitation source being switched on and the formation of XV^•+, is inversely proportional to CD concentration (Figure 2c). This implies that the mechanism behind the delay time is dependent on the number of photons absorbed. Potentially, this phenomenon is driven by interactions between CDs, such as the collision-induced filling of trap states within the CDs with excited electrons from the valence band before the CDs can transfer electrons to MV²⁺. (31) Additionally, the photoreduction rate is linearly proportional to the CD concentration, with each addition of 0.1 g L^–1 of aCDs contributing 3.5 × 10^–3 s^–1 to the photoreduction rate constant (Figure 2d).

Effect of MV Concentration

To provide further insight into the radical monocation depletion and delay time phenomenon, the starting concentration of MV²⁺, [MV²⁺]_t=0, was varied to ascertain the point at which the aCD surface becomes saturated with pre-assembled MV species. This set of samples was made up of 0.5 g L^–1 aCD and 0.1 M EDTA in an aqueous pH 6 phosphate buffer. Once samples were loaded into the fiber, they were subject to UV excitation as before, with the corresponding concentration and percentage conversion profiles shown in Figure 3a,b. Here, the greater concentration of MV^•+ produced and corresponding strong UV–vis absorption meant that negligible transmitted counts were detectable at the spectrometer at 600 nm. Therefore, the conversion from absorbance to concentration was performed by averaging the absorbance within a 10 nm wide band around 670 nm, with a scaled absorption coefficient of 6.2 × 10³ M^–1 cm^–1 (see Supporting Information, Figure S11).

Figure 3. Photoreduction dependence on MV²⁺ concentration. [a] MV^•+ concentration, and [b] percentage conversion profiles, illustrating the dependence of the reaction kinetics on the starting concentration of the methyl viologen dication, [MV²⁺]_t=0. The MV^•+ concentration was calculated from the UV–vis absorption tail at 670 nm. The conversion was calculated by dividing the concentration profile by the starting concentration of the methyl viologen dication. Each sample consists of MV²⁺ with 0.5 g L^–1 aCDs and 0.1 M EDTA in an aqueous pH 6 phosphate buffer. Samples were subject to continuous UV irradiation (λ = 365 nm) from t = 60 s at an irradiance of 90.2 mW cm^–2. [c] Delay time and [d] photoreduction rate constant, both plotted as a function of [MV²⁺]_t=0, with error bars signifying the standard deviation of at least triplicates.

The depletion rate of MV^•+ is constant at all starting concentrations of MV²⁺ (Figure 3a), suggesting that the CD^– species is rate-limiting the depletion. The rate equation takes the form of eq 9. If so, it suggests the rate of electron donation to the CDs from the SED is not coupled to the rate of electron transfer from the CDs to the MV²⁺, and we have similar amounts of CD^– at all starting concentrations of MV²⁺.

\frac{d [{MV}^{• +}]}{d t} = k^{'} [{MV}^{2 +}]

(9)

In contrast to the sweep in CD concentration, no clear correlation between [MV²⁺]_t=0 and delay time is observed, signifying that the delay time phenomenon is not regulated by MV²⁺, but is intrinsic to the CDs themselves or dependent on the SED (Figure 3c). A drop in the photoreduction rate constant, k′, was observed above a starting concentration of 100 μM MV²⁺ (Figure 3d), with a maximum conversion to the radical monocation of 38% (Figure 3b). This drop indicates the switch to the photon-limited regime, where the local volumetric rate of photon absorption (L_p, LVRPA) limits the photoreduction process. Above 100 μM MV²⁺, the photoreduction rate is no longer pseudo-first-order with respect to MV²⁺, and the LVRPA should be considered as part of the rate equation. This new second-order rate equation takes the form of eq 10.

\frac{d [{MV}^{• +}]}{d t} = k^{'} [{MV}^{2 +}] L_{p}

(10)

In general, a decrease in the photoreduction rate constant could be linked to the available surface area on each CD that MV²⁺ molecules can adsorb to. To give insight into this, we convert the mass concentration of aCDs to an approximate molar concentration, using their average diameter deduced by TEM (8 nm), (5) together with their density, 1.5 g cm^–3, (46) yielding a molar concentration of 2 μM. This indicates that each carbon dot can drive 50 conversions to MV^•+ per second before limitation by the number of photoexcited electrons (photon absorption rate is ca. 450 μM s^–1). This corresponds to an average period of 20 ms for an MV²⁺ to attach to the carbon dot, accept an electron to form MV^•+, then escape the solvent cage.

MV²⁺ adsorption competes with other adsorbing species, such as the SED and its oxidized counterparts, together with MV^•+ that has not been able to escape the solvent cage, given the negatively charged surface of the carbon dots.

Effect of EDTA Concentration

To confirm whether the SED (EDTA) was rate-limiting and to validate our pseudo-first-order rate equation eq 9, the concentration of EDTA was varied between 1 and 100 mM. This set of samples was prepared with 0.5 g L^–1 aCD and 40 μM MV²⁺ in an aqueous pH 6 phosphate buffer, loaded into the HC-PCF, and subsequently subjected to UV excitation as before. The corresponding concentration and rate profiles are shown in Figure 4a,b.

Figure 4. Photoreduction dependence on EDTA concentration. [a] MV^•+ concentration, and [b] rate profiles, illustrating the dependence of the reaction kinetics on EDTA concentration. The MV^•+ concentration was calculated from the UV–vis absorption peak at 600 nm. Each sample consists of EDTA with 0.5 g L^–1 aCDs and 40 μM MV²⁺ in an aqueous pH 6 phosphate buffer. Samples were subject to continuous UV irradiation (λ = 365 nm) from t = 60 s at an irradiance of 90.2 mW cm^–2. [c] Delay time and [d] photoreduction rate constant, both plotted as a function of EDTA concentration, with error bars signifying the standard deviation of at least triplicates.

The delay time is relatively consistent in the 10–100 mM range (Figure 4c), with a significant increase noticeable at 1 mM. This behavior suggests the delay time is linked to the electron donation phenomenon to the CDs, and EDTA limits this phenomenon at 1 mM. Additional experiments substituting the CDs for the photosensitizer [Ru(bpy)₃]²⁺ found that the delay time was not intrinsic to the CDs. We hypothesize that although great efforts were taken to purge oxygen in our measurement samples, a small amount of residual oxygen trapped at the photosensitizer surface must be depleted before the MV²⁺ photoreduction can begin. This residual oxygen is depleted by accepting photoexcited electrons from the CD, with a SED required to fill the remaining holes in the CD valence band.

The concentration profiles of the 25, 50, and 100 mM measurement runs are similar in Figure 4a, with the photoreduction rate constant starting to saturate above 25 mM in Figure 4d. This suggests there is a sufficient reservoir of EDTA above 25 mM for the photoreduction not to be rate-limited by the SED, and as such, we can attribute the rate-limiting step to photon absorption when the [EDTA] >25 mM. This exceeds the aCD concentration by 10⁴ times. However, given EDTA competes with MV²⁺ to access the CD surface, excessively large EDTA concentrations may counter-productively reduce the photoreduction rate constant. It is challenging to deduce whether this is the case at 100 mM within the margins of error.

The 1 and 10 mM samples appear to be limited by the EDTA reservoir, and the depletion rate is noticeably lower at these concentrations (Figure 4a,b). The decrease in the rate of depletion at 1 and 10 mM is a result of less CD^– being available to reduce MV^•+ to MV⁰. Given the EDTA ions are negatively charged, the immediate surroundings of the negatively charged CD surface likely consist of an inner sphere of positively charged methyl viologen species and an outer sphere of negatively charged EDTA. Therefore, the electron transfer from the CD to the MV²⁺ is likely an inner-sphere electron transfer, and the electron donation from the EDTA to the CD is likely an outer-sphere transfer event.

Effect of Irradiance

The irradiance dependence on the photoreduction reaction kinetics was explored using 6 different irradiances in the range 1.6–88.4 mW cm^–2 with aCDs, MV, and EDTA. Reaction kinetic profiles were used to determine whether the photoreduction process was photon-limited and to calculate a quantum yield. This set of samples was made up of 0.5 g/L aCD, 40 μM MV²⁺, and 0.1 M EDTA in an aqueous pH 6 phosphate buffer. The results are shown in Figure 5a.

Figure 5. Photoreduction dependence on the local volumetric rate of photon absorption (LVRPA). [a] MV^•+ concentration profile illustrating the dependence of the reaction kinetics on irradiance. The MV^•+ concentration was calculated from the UV–vis absorption peak at 600 nm. Each sample consists of MV²⁺ with 0.5 g L^–1 aCDs, 40 μM MV²⁺, and 0.1 M EDTA in an aqueous pH 6 phosphate buffer. Samples were subject to continuous UV irradiation (λ = 365 nm) from t = 60 s. [b] Delay time (inverse) and [c] photoreduction rate, both plotted as a function of LVRPA.

Irradiances, I, were measured using a Thorlabs S130 photodiode at the same distance below the optical diffuser as the HC-PCF (1 cm), with the values converted to an LVRPA by considering the CD absorption coefficient, ε_λ, (see Supporting Information, Figure S4a) together with the CD concentration, [CD], and the excitation wavelength, λ, in eq 11.

LVRPA = \ln 10 ε_{λ} [CD] \frac{λ I}{hc N_{a}}

(11)

It is worth noting that adjustments to the irradiance are equivalent to adjustments to the CD concentration, as both quantities are directly proportional to the LVRPA. As such, the effect of increasing the irradiance on the shape of the concentration profile is similar to increasing the CD concentration, as depicted in Figure 2a. However, high LVRPA regimes are challenging to reach with highly concentrated solutions of CDs. At high [CD], viologen photoreduction cannot be analyzed in the fiber due to substantial absorption at 600 nm from the CDs over the 20 cm optical path length. In these cases, high LVRPA regimes can be accessed by increasing the irradiance.

The delay time (t_d) was found to be inversely proportional to the LVRPA (Figure 5b), suggesting the delay time phenomenon is also light-driven (in addition to being dependent on CD concentration). The strong correlation supports the hypothesis that MV²⁺ photoreduction begins once a fixed amount of residual oxygen in the solution is depleted. A clear linear trend is observed between LVRPA and the initial photoreduction rate, indicating that the photoreduction process is indeed photo-limited, giving a quantum yield of 0.107% (see Figure 5c). This low yield relates to the fast relaxation of photoexcited CDs on the picosecond timescale, with only a small proportion of photoexcited CDs remaining long enough to take part in an electron transfer to the viologen dication. (15)

Comparing Carbon Dots, Sacrificial Electron Donors, and Different Viologens

The concentration sweeps of each component in the photosystem provide insight into the reaction kinetics of CD-driven photoreduction and support the reaction scheme presented. However, to further corroborate and understand the reaction mechanism, individual components of the photosystem were altered to probe their effects on the photoreduction rate constant and delay time, with the results shown in Figure 6.

Figure 6. Photoreduction dependence on individual photosystem components. [a, b] Varying the viologen species: [a] photoreduction rate constants and [b] delay time. Each sample consists of 40 μM XV²⁺, 0.1 M EDTA, and 0.5 g L^–1 aCD. [c, d] Varying the sacrificial electron donor and pH: [c] photoreduction rate constants and [d] delay time. Each sample consists of 40 μM MV²⁺, 0.1 M SED, and 0.5 g L^–1 aCD. [e, f] Varying the photosensitizer type and excitation wavelength: [e] photoreduction rate constants and [f] delay time. Each sample consists of 40 μM MV²⁺, 0.1 M EDTA, and 0.01 g L^–1 light absorber. Samples were subject to continuous UV irradiation (λ = 365 nm, 88.4 mW cm^–2) or blue irradiation (λ = 450 nm, 99.1 mW cm^–2). Ru = [Ru(bpy)₃]²⁺, with Ru* and NgCD* denoting excitation at 450 nm. Error bars signify the standard deviation of at least triplicates.

Small changes in the molecular structure of the viologen species were used to gain insight into the electron transfer process at the surface of aCDs, with EDTA as the sacrificial electron donor (Figure 6a,b). The viologens studied comprised MV, EV, and BV, each expected to form electrostatically bound complexes together with aCDs, but with modified binding affinities.

Based on the redox potentials of methyl viologen (E^{MV²⁺/MV^•+} = −0.446 V vs NHE), (47) ethyl viologen (E^{EV²⁺/EV^•+} = −0.449 V vs NHE), (47) and benzyl viologen (E^{BV²⁺/BV^•+} = −0.359 V vs NHE), (47) the photoreduction rate should be fastest for BV²⁺, followed by MV²⁺ then EV²⁺ considering the average excited-state potential of COOH-functionalized CDs is −0.55 V vs reversible hydrogen electrode at pH 5. (48) However, we do not observe this trend in Figure 6a, with electron transfer being the fastest for EV²⁺. We hypothesize that competing factors, such as the cage escape process and CD binding affinity, contribute to the reaction rate. For example, the bulkiness of the BV²⁺ species restricts the ease of cage escape.

The effect of varying the SED was investigated with EDTA, TEA, and TEOA at pH 6 and pH 8, in conjunction with aCDs and MV. Changes in the molecular structure of the SEDs were used to understand the depletion of XV^•+ observed in all our measurements. Adjustments to pH also allowed us to tune the electron-donating ability of the SED.

The electron donor has a significant effect on both the photoreduction rate constant (Figure 6c) and delay time (Figure 6d). EDTA is well known as an electron donor that can operate in neutral conditions, with TEA and TEOA working best under alkaline conditions. The electron donation to CD* in reaction 2 is pH sensitive, with more basic conditions lessening the ability of the SED to donate electrons to CD*. Therefore, increasing the pH results in a greater rate of MV^•+ production with all of the electron donors tested. The poor performance of TEA and TEOA at pH 6 is attributed to the inferior electron-donating ability of the protonated forms of TEA and TEOA (TEA pK_a = 10.7, TEOA pK_a = 7.9). (14) EDTA displays an improved performance as its relevant pK_a value is 6.1, (14) with both singly protonated and doubly protonated forms of EDTA coexisting at pH 6 in solution. It is surprising to see a ×7 higher photoreduction rate with TEA over TEOA, given that TEOA has a lower pK_a. We postulate that the cage escape efficiency is faster with TEA over TEOA, thus producing the radical monocation more rapidly.

Overall, the delay time is dependent on the type of SED used (Figure 6d, Supporting Information Figure S10). At pH 6, TEA and TEOA have considerably longer delay times before the onset of viologen photoreduction. This is linked to the poor electron-donating ability of both these SEDs at pH 6, resulting in a longer amount of time to remove residual oxygen in the sample. When viologen photoreduction was performed at pH 8 with the different SEDs, the delay times are observed to be more similar, meaning that each SED was not inactivated under slightly alkaline conditions.

The performance of the photosensitizer was probed by comparing the rate of photoreduction of aCDs, gCDs, and NgCDs. NgCDs were benchmarked against [Ru(bpy)₃]²⁺ under UV 365 nm and blue 450 nm irradiation. For this set of measurements, methyl viologen was utilized as the electron acceptor and EDTA as the electron donor (Figure 6e,f).

By comparing CDs at the same mass loading (10 mg L^–1), we find that NgCDs perform the best out of the CDs by a considerable margin─with a ×10 higher photoreduction rate observed over its un-doped graphitic counterpart (Figure 6e). This increase corroborates well with the increase in reported turnover frequencies (TOF) for proton reduction when gCDs and NgCDs are used in conjunction with the molecular [Ni(P₂N₂)₂]²⁺ DuBois-type catalyst (NiP). (5) Given that the size distributions of both gCDs and NgCDs are similar, (5) the difference in delay time cannot be attributed to a difference in the number density of CDs. The absorption coefficients of both gCDs and NgCDs are also similar (see Supporting Information, Figure S4a), which suggests that the excited-state lifetime differentiates the two. Therefore, we conclude that nitrogen doping promotes a greater proportion of long-lived species, which is also corroborated by transient absorption spectroscopy measurements. (15)

Benchmarking

Benchmarking against [Ru(bpy)₃]²⁺ at the same mass loading shows that although NgCDs drive higher rates of reaction under UV (365 nm) irradiation, [Ru(bpy)₃]²⁺ performs best under blue (450 nm) irradiation (Figure 6e). This is partly because NgCDs have a ×2.3 lower absorbance at 450 nm, whereas [Ru(bpy)₃]²⁺ has ×3.1 higher absorbance at this wavelength (see Supporting Information, Figure S4a). Consequently, a ×7.1 increase in rate is expected by changes in UV–vis absorption. Applying a small correction to account for the different photon fluxes incident with the 365 nm source and 450 nm sources, we arrive at an expected increase of ×9.8. However, we observe a ×13 increase in rate, highlighting the superior ability of [Ru(bpy)₃]²⁺ in donating an electron to MV²⁺ in this photosystem. This is thought to be due to the extended excited-state lifetime of [Ru(bpy)₃]²⁺ over the NgCDs, as the 450 nm transition corresponds to the metal-to-ligand charge transfer (MLCT) excited state that is ideally suited for facilitating outer-sphere electron transfer to substrates or co-catalysts.

Effect of CD Surface

To interrogate the influence of CD surface charge on the rate constant and delay time, the performance of aCDs with protonated (COOH moieties), deprotonated (COO^– moieties) and tertiary-capped amine (NMe₂⁺) surfaces were compared under the same conditions (0.5 g L^–1 aCDs, 40 μM MV, 0.1M EDTA, pH 6).

Our findings (Figure 7, Supporting Information Figure S12) show that a negative CD surface charge, and subsequent electrostatic attraction to MV²⁺, is advantageous for improving the rate of electron transfer, even overcoming the requirement for frontier orbitals to be largely present on the aCDs to facilitate the electron transfer to MV²⁺. This is corroborated by X-ray Absorption Spectroscopy (XAS) data.

Figure 7. Comparison of protonated, deprotonated, and tertiary amine-capped aCDs. [a] photoreduction rate constant and [b] delay time. Each sample consists of 40 μM MV²⁺, 0.1 M EDTA, and 0.5 g L^–1 aCD. Samples were subject to continuous UV irradiation (λ = 365 nm, 88.4 mW cm^–2). Error bars signify the standard deviation of at least triplicates.

X-ray absorption (XA) spectra of all three types of aCDs at the carbon K-edge (see Supporting Information, Figure S9), show a much lower contribution from transitions from C1s to π*_C═C (285.0 eV) and π*_C═O (288.6 eV) bonds in both the deprotonated and tertiary-capped amine aCDs, relative to their protonated analogue. The higher peak observed at the O K-edge XA spectra for the transition from O1s to π*_C═O (532.2 eV─see Supporting Information, Figure S9) for protonated aCDs also validates this further. Overall, this implies that the electrostatic attraction between deprotonated aCDs and MV²⁺ facilitates increased rates of electron transfer despite the much lower proportion of frontier π/π*orbitals in comparison to the protonated aCDs. In previous work, it was shown that electrons excited to π*_C═C in aCDs cannot be transferred to water due to the amorphous nature of the core, in contrast to the graphitic CDs. (49) Electronic states related to surface groups are therefore more likely to be involved in electron transfer. Interestingly, despite the lack of frontier π/π*orbitals on the tertiary-capped amine dots, they display a higher electron transfer rate constant than the protonated aCDs. This is most likely due to increased ion-dipole interactions between the solvent cage and slightly positive surface charge on the tertiary-capped amine dots, as evidenced by the C–NHMe₂⁺ signal in their N K-edge XA spectrum (399.9 eV─Supporting Information, Figure S9). Such interactions would constrain the aCD-NMe₂⁺ dots within the solvent cage, thus facilitating faster electron transfer rates (despite the likely electrostatic repulsion with MV²⁺) in comparison to the aCD-COOH, which will have a distinct lack of ion-dipole interactions.

Conclusions

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We have shown that comprehensive in situ spectrokinetic analysis can be performed on a CD-driven photoreduction reaction with a total cumulative sample volume of <1.5 mL. This study builds upon our previous proof-of-principle work demonstrating the use of HC-PCF microreactors for studying photochemistry. (31) Systematic, low-volume screening of 29 reaction conditions enabled us to understand the complex interplays between each component in our homogeneous photosystem, consisting of an electron acceptor, electron donor, and photosensitizer. We highlight the significant role of the electron donor, specifically the level of protonation, in controlling the kinetics of these viologen photoreduction reactions. Optofluidic microreactors with real-time optical detection provide unprecedented insight into the reaction dynamics of photocatalytic systems, and once measurements are performed in continuous flow, they are likely to become part of the automated catalyst screening systems of the future.

Data Availability

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The data that support the findings of this study are openly available at the University of Cambridge Apollo Repository. (50)

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscatal.3c02212.

Data analysis methodology; optical setup diagrams; HC-PCF transmission characterization; cuvette-based measurements; concentration profiles with different SEDs and aCD surface moieties; and XAS data (PDF)

cs3c02212_si_001.pdf (2.03 MB)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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Corresponding Authors
- Erwin Reisner - Yusuf Hamied Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.; https://orcid.org/0000-0002-7781-1616; Email: [email protected]
- Tijmen G. Euser - NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.; https://orcid.org/0000-0002-8305-9598; Email: [email protected]
Authors
- Takashi Lawson - NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.; Yusuf Hamied Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.; Present Address: Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, United Kingdom; https://orcid.org/0000-0002-2571-4216
- Alexander S. Gentleman - NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.; Yusuf Hamied Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
- Ava Lage - Yusuf Hamied Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
- Carla Casadevall - Yusuf Hamied Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.; https://orcid.org/0000-0002-3090-4938
- Jie Xiao - Helmholtz-Zentrum Berlin für Materialien und Energy GmbH, Albert-Einstein-Straße 15, 12489 Berlin, Germany; https://orcid.org/0000-0002-2320-6111
- Tristan Petit - Helmholtz-Zentrum Berlin für Materialien und Energy GmbH, Albert-Einstein-Straße 15, 12489 Berlin, Germany; https://orcid.org/0000-0002-6504-072X
- Michael H. Frosz - Max Planck Institute for the Science of Light, Staudtstr. 2, 91058 Erlangen, Germany
Author Contributions
T.L., A.S.G., E.R., and T.G.E. designed the project. T.L. performed the fiber-based measurements and analyzed the data. A.L. synthesized the carbon dots. A.L. and C.C. performed the CD titration measurements. M.H.F. fabricated the kagomé-style HC-PCFs. J.X. and T.P. performed and analyzed the XAS measurements. T.L. and A.S.G. drafted the manuscript. All authors contributed to the discussion, revision, and completion of the manuscript. T.G.E. supervised the work.
Notes
The authors declare no competing financial interest.

Acknowledgments

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T.G.E. acknowledges the support from the Winton Programme for the Physics of Sustainability and the Isaac Newton Trust. T.L. and E.R. acknowledge the Cambridge NanoDTC (EPSRC Grant EP/L015978/1 and EP/S022953/1). C.C. and E.R. acknowledge the UKRI Cambridge Creative Circular Plastics Centre (EP/S025308/1). C.C. acknowledges the European Commission’s H2020 program for an MSCA IF (SmArtC, No.890745). A.L. acknowledges the EPSRC for a DTA studentship. T.G.E, A.S.G., and E.R. acknowledge the Leverhulme Trust (Research Project Grant RPG-2018-256). The authors acknowledge the kind support from the staff of the BESSY II synchrotron facility, especially Ronny Golnak, and thank the Helmholtz-Zentrum Berlin for the allocation of beamtime at the U49/2 PGM-1 beamline.

Abbreviations

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aCD	amorphous carbon dot
BV	benzyl viologen
CD	carbon dot
EA	electron acceptor
EDTA	ethylenediaminetetraacetic acid
EV	ethyl viologen
gCD	graphitic carbon dot
HC-PCF	hollow-core photonic crystal fiber
LVRPA	local volumetric rate of photon absorption
MLCT	metal-to-ligand charge transfer
MV	methyl viologen
NgCD	nitrogen-doped graphitic carbon dot
NHE	normalized hydrogen electrode
PEEK	polyether ether ketone
TEA	triethylamine
TEOA	triethanolamine
TEY	total electron yield
TFY	total fluorescence yield
UV	ultraviolet
XAS	X-ray absorption spectroscopy

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Labelling Proteins with Carbon Nanodots
Rao, Chethana; Khan, Syamantak; Verma, Navneet C.; Nandi, Chayan Kanti
ChemBioChem (2017), 18 (24), 2385-2389CODEN: CBCHFX; ISSN:1439-4227. (Wiley-VCH Verlag GmbH & Co. KGaA)
We present efficient labeling of several proteins with orange-emissive carbon dots. N-Hydroxysuccinimide was used to activate the carboxyl groups of carbon dots, which subsequently reacted with the lysine groups present on the protein. Labeling was confirmed by UV absorption spectroscopy, PAGE and fluorescence correlation spectroscopy. Protein-conjugated carbon dots showed an enhancement in fluorescence lifetime and intensity owing to reduced intramol. dynamic fluctuations. Single-mol. fluorescence measurements showed reduced fluorescence fluctuations and higher photon budget after protein tagging. Our study opens up opportunities to use carbon dots as highly precise biolabelling probes.
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Jia, W.; Tang, B.; Wu, P. Carbon Dots with Multi-Functional Groups and the Application in Proton Exchange Membranes. Electrochim. Acta 2018, 260, 92– 100, DOI: 10.1016/J.ELECTACTA.2017.11.047
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Carbon dots with multi-functional groups and the application in proton exchange membranes
Jia, Wei; Tang, Beibei; Wu, Peiyi
Electrochimica Acta (2018), 260 (), 92-100CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
It is of great necessity to achieve a proton exchange membrane (PEM) with high proton cond. and low MeOH permeability for the practical applications. Nafion modified C dots (NCDs) with multi-functional groups were successfully synthesized via the pyrolysis of citric acid (CA) with the existence of Nafion. The modification process is motivated by the hydrophilicity-to-hydrophobicity transformation of CA as well as the noncovalent hydrophilic-hydrophobic interaction during the pyrolysis procedure. Multi-functionalized NCDs with moderate hydrophilicity influence the aggregation structure of Nafion matrix of the composite membranes and effectively enhance the high-temp. H2O retention ability. Both the proton cond. and the MeOH resistance ability of the composite PEMs are significantly enhanced. 0.5-NCD-0.5 Nafion composite PEM presents a 5-10 times increase in proton cond. and 50% percent decrease in MeOH permeability than that of recast Nafion.
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Achilleos, D. S.; Kasap, H.; Reisner, E. Photocatalytic Hydrogen Generation Coupled to Pollutant Utilisation Using Carbon Dots Produced from Biomass. Green Chem. 2020, 22, 2831– 2839, DOI: 10.1039/D0GC00318B
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Photocatalytic hydrogen generation coupled to pollutant utilisation using carbon dots produced from biomass
Achilleos, Demetra S.; Kasap, Hatice; Reisner, Erwin
Green Chemistry (2020), 22 (9), 2831-2839CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
Photocatalysis is deemed as an appealing strategy to exploit solar energy for simultaneous fuel prodn. and pollutant utilization. However, current photocatalytic systems rarely couple both processes and commonly suffer from restricted scalability and sustainability as they use toxic or UV light harvesters, combined with noble-metal co-catalysts under corrosive conditions. Here, the synthesis is shown of ultra-scalable and low-cost carbon nanodots from lignocellulosic waste, which when combined with a non-precious Ni-based co-catalyst, use visible light to drive H2 prodn. in untreated river and sea water. Org. pollutants and chloride anions in these untreated media do not only allow unhindered photocatalytic activities, but also function as electron donors for the photoexcited carbon dots to enable proton redn. This system combines Earth's most abundant resources (biomass, solar energy, untreated water) and functions at ambient temp., pressure and benign pH, thereby creating perspectives for simultaneous fuel synthesis as well as sustainable and practical pollutant utilization.
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Martindale, B. C. M.; Hutton, G. A. M.; Caputo, C. A.; Reisner, E. Solar Hydrogen Production Using Carbon Quantum Dots and a Molecular Nickel Catalyst. J. Am. Chem. Soc. 2015, 137, 6018– 6025, DOI: 10.1021/jacs.5b01650
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Solar Hydrogen Production Using Carbon Quantum Dots and a Molecular Nickel Catalyst
Martindale, Benjamin C. M.; Hutton, Georgina A. M.; Caputo, Christine A.; Reisner, Erwin
Journal of the American Chemical Society (2015), 137 (18), 6018-6025CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Carbon quantum dots (CQDs) are established as excellent photosensitizers in combination with a mol. catalyst for solar light driven hydrogen prodn. in aq. soln. The inexpensive CQDs can be prepd. by straightforward thermolysis of citric acid in a simple one-pot, multigram synthesis and are therefore scalable. The CQDs produced reducing equiv. under solar irradn. in a homogeneous photocatalytic system with a Ni-bis(diphosphine) catalyst, giving an activity of 398 μmolH2 (gCQD)-1 h-1 and a "per Ni catalyst" turnover frequency of 41 h-1. The CQDs displayed activity in the visible region beyond λ > 455 nm and maintained their full photocatalytic activity for at least 1 day under full solar spectrum irradn. A high quantum efficiency of 1.4% was recorded for the noble- and toxic-metal free photocatalytic system. Thus, CQDs are shown to be a highly sustainable light-absorbing material for photocatalytic schemes, which are not limited by cost, toxicity, or lack of scalability. The photocatalytic hybrid system was limited by the lifetime of the mol. catalyst, and intriguingly, no photocatalytic activity was obsd. using the CQDs and 3d transition metal salts or platinum precursors. This observation highlights the advantage of using a mol. catalyst over commonly used heterogeneous catalysts in this photocatalytic system.
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Dalle, K. E.; Warnan, J.; Leung, J. J.; Reuillard, B.; Karmel, I. S.; Reisner, E. Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes. Chem. Rev. 2019, 119, 2752– 2875, DOI: 10.1021/acs.chemrev.8b00392
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Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes
Dalle, Kristian E.; Warnan, Julien; Leung, Jane J.; Reuillard, Bertrand; Karmel, Isabell S.; Reisner, Erwin
Chemical Reviews (Washington, DC, United States) (2019), 119 (4), 2752-2875CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based mol. catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aq. media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive assocns. between mol. catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The mol.-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 redn. to C1 products, by summarizing cases for higher-value products from N2 redn., Cx>1 products from CO2 utilization, and other reductive org. transformations.
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Prasad, D. R.; Hoffman, M. Z. Photodynamics of the Tris(2,2′-bipyrazine)ruthenium(2+)/Methylviologen/EDTA System in Aqueous Solution. J. Am. Chem. Soc. 1986, 108, 2568– 2573, DOI: 10.1021/ja00270a013
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Photodynamics of the tris(2,2'-bipyrazine)ruthenium(2+)/methylviologen/EDTA system in aqueous solution
Prasad, Dasari R.; Hoffman, Morton Z.
Journal of the American Chemical Society (1986), 108 (10), 2568-73CODEN: JACSAT; ISSN:0002-7863.
Luminescence quenching and flash photolysis (pulsed laser and conventional) techniques were employed to study the photodynamics of the Ru(bpz)32+/MV2+/EDTA system in aq. soln. (bpz = 2,2'-bipyrazine; MV2+ = methylviologen). The quenching of *Ru(bpz)32+ by EDTA in alk. soln. via Stern-Volmer kinetics (kq = 6.9 × 108 and 7.7 × 108 M-1 s-1 at pH 8.7 and 11.0, resp.) generates Ru(bpz)3+ which reacts with MV2+ (k = 4.5 × 108 M-1 s-1) to yield MV+·; the reducing EDTA radical, formed from the irreversible transformation of the species obtained from the oxidn. of EDTA, reacts with MV2+ (k = 1.5 × 109 M-1 s-1) to yield a 2nd equiv. of MV+·. At pH 4.7, the quenching of *Ru(bpz)32+ by EDTA does not follow Stern-Volmer kinetics; kq approaches ∼2 × 108 and ∼2 × 107 M-1 s-1 in the limits of low and high [EDTA], resp. The quantum yields of MV+· in acidic soln. are dramatically lower than in neutral and alk. soln. due to the lower efficiencies of the quenching and cage release processes and the decreased reactivities of the protonated forms of Ru(bpz)3+ and reducing EDTA radicals toward MV2+.
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Yatsuzuka, K.; Yamauchi, K.; Kawano, K.; Ozawa, H.; Sakai, K. Improving the Overall Performance of Photochemical H₂ Evolution Catalyzed by the Co-NHC Complex via the Redox Tuning of Electron Relays. Sustainable Energy Fuels 2021, 5, 740– 749, DOI: 10.1039/D0SE01597K
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Improving the overall performance of photochemical H2 evolution catalyzed by the Co-NHC complex via the redox tuning of electron relays
Yatsuzuka, Koichi; Yamauchi, Kosei; Kawano, Ken; Ozawa, Hironobu; Sakai, Ken
Sustainable Energy & Fuels (2021), 5 (3), 740-749CODEN: SEFUA7; ISSN:2398-4902. (Royal Society of Chemistry)
The catalytic performance of a photochem. H2 evolution system made up of EDTA (EDTA disodium salt), [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) and a macrocyclic N-heterocyclic carbene cobalt (Co-NHC-1) catalyst has been examd. at pH 5.0 E(2H+/H2) = -0.54 V vs. SCE by using six electron relays (ERs) having different first redn. potentials (Ered) in the range -0.69 < Ered < -1.08 V. Compared to the photosystem using the conventional methylviologen (i.e., N,N'-dimethyl-4,4'-bipyridinium; MV2+), the overall catalytic performance is dramatically improved by employing the ERs having the redn. potentials by 0.08-0.24 V more neg. than that of MV2+ (Ered = -0.69 V), revealing that the overall rate is limited by the electron transfer (ET) from the one-electron reduced ER to Co-NHC-1, correlated to hydrogen evolution reaction (HER), rather than that from [Ru*(bpy)3]2+ (triplet excited state) to ER, since the driving force for the HER (DFHER) predominates that for the ET from [Ru*(bpy)3]2+ to ER (DFET). The optimum condition was realized by selecting one of the viologen derivs. with a medium redn. potential (N,N',2,2',6,6'-hexamethyl-4,4'-bipyridinium; tmMV2+; Ered = -0.85 V), leading to afford the initial rate of HER (55-57 μmol h-1) 70 times higher than that obtained by using MV2+ (0.79 μmol h-1). The stability of each one-electron reduced ER under the photolysis conditions has been also examd. spectrophotometrically, clarifying that some ERs rather decomp. rapidly upon redn. and cannot effectively participate in HER. This study successfully demonstrates for the first time that the overall catalytic performance of the present photosystem cannot be only controlled by the tuning of DFET and DFHER but also be affected by the stability of the one-electron reduced form of ER.
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Strauss, V.; Margraf, J. T.; Dolle, C.; Butz, B.; Nacken, T. J.; Walter, J.; Bauer, W.; Peukert, W.; Spiecker, E.; Clark, T.; Guldi, D. M. Carbon Nanodots: Toward a Comprehensive Understanding of Their Photoluminescence. J. Am. Chem. Soc. 2014, 136, 17308– 17316, DOI: 10.1021/ja510183c
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Carbon Nanodots: Toward a Comprehensive Understanding of Their Photoluminescence
Strauss, Volker; Margraf, Johannes T.; Dolle, Christian; Butz, Benjamin; Nacken, Thomas J.; Walter, Johannes; Bauer, Walter; Peukert, Wolfgang; Spiecker, Erdmann; Clark, Timothy; Guldi, Dirk M.
Journal of the American Chemical Society (2014), 136 (49), 17308-17316CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The characterization is reported of C nanodots (NDs) synthesized under mild and controlled conditions, i.e., in a microwave reactor. The synthesized C NDs exhibit homogeneous and narrowly dispersed optical properties. They are suited as a testbed for studies of the photophysics of C-based nanoscopic emitters. In addn. to steady-state studies, time-correlated single-photon counting, fluorescence up-conversion, and transient pump probe absorption spectroscopy were used to elucidate the excited-state dynamics. Quenching the C ND-based emission with electron donors or acceptors helped shed light on the nature of individual states. D. functional theory and semiempirical CI calcns. on model systems helped understand the fundamental structure-property relations for this novel type of material.
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Pellegrin, Y.; Odobel, F. Sacrificial Electron Donor Reagents for Solar Fuel Production. C. R. Chim. 2017, 20, 283– 295, DOI: 10.1016/J.CRCI.2015.11.026
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Sacrificial electron donor reagents for solar fuel production
Pellegrin, Yann; Odobel, Fabrice
Comptes Rendus Chimie (2017), 20 (3), 283-295CODEN: CRCOCR; ISSN:1631-0748. (Elsevier Masson SAS)
A review is given. Although justly considered as a cumbersome component in artificial photosystems, these simple mols. are a necessary evil to drive photo-induced reactions aiming at producing high added value mols. by photo-induced redn. of low energy value substrates. This paper 1rst presents the specifications of sacrificial electron donors. Then the various families of sacrificial donors used from the early 1970s to nowadays are reviewed, such as aliph. and arom. amines, benzyl-dihydronicotinamide (BNAH), dimethylphenylbenzimidazoline (BIH), ascorbic acid, oxalate and finally thiols. Exptl. conditions (pH, solvent) are immensely versatile but important trends are given for adequate operation of a three-component system. Although literature abounds with various, very different artificial photosystems, we will realize that virtually the same sacrificial donors are used over and over again.
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Hutton, G. A. M.; Martindale, B. C. M.; Reisner, E. Carbon Dots as Photosensitisers for Solar-Driven Catalysis. Chem. Soc. Rev. 2017, 46, 6111– 6123, DOI: 10.1039/c7cs00235a
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Carbon dots as photosensitisers for solar-driven catalysis
Hutton, Georgina A. M.; Martindale, Benjamin C. M.; Reisner, Erwin
Chemical Society Reviews (2017), 46 (20), 6111-6123CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
A review. Artificial photosynthesis is the mimicry of the natural process of solar energy conversion into chem. energy carriers. Photocatalytic systems that combine light-harvesting materials and catalysts in soln. or suspension provide a promising route towards this goal. A key requirement for a sustainable solar fuel prodn. system is a low-cost, stable and non-toxic light harvester. Photoluminescent carbon nanoparticles, carbon dots (CDs), are promising emerging light-harvesters for photocatalytic fuel prodn. systems. CDs possess many desirable properties for this purpose, such as inexpensive, scalable synthetic routes, low-toxicity and tuneable surface chem. In this tutorial review, the integration of CDs in photocatalytic fuel generation systems with metallic, mol. and enzymic catalysts is discussed. An overview of CD types, synthesis and properties is given along with a discussion of tuneable CD properties that can be optimized for applications in photocatalysis. Current understanding of the photophys. electron transfer processes present in CD photocatalytic systems is outlined and various avenues for their further development are highlighted.
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Rosso, C.; Filippini, G.; Prato, M. Carbon Dots as Nano-Organocatalysts for Synthetic Applications. ACS Catal. 2020, 10, 8090– 8105, DOI: 10.1021/acscatal.0c01989
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Carbon Dots as Nano-Organocatalysts for Synthetic Applications
Rosso, Cristian; Filippini, Giacomo; Prato, Maurizio
ACS Catalysis (2020), 10 (15), 8090-8105CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
A review. Over the last decades, org. chem. has taken a resolute step toward green catalytic synthesis. This tries to ensure efficient and sustainable base chem. prodn., while also safeguarding human health and the environment. To this end, the development of novel, nontoxic, and effective catalytic systems that are capable of driving value-added chem. transformations in environmentally benign solvents (e.g., water) is highly desirable. Moreover, these new catalysts need to be metal-free, easy-to-prep., and potentially recyclable. Carbon dots, which are relatively new carbon-based nanoparticles, fulfill all these requirements because of their outstanding physicochem. features and thus have emerged as promising nanocatalytic platforms. This Perspective highlights the recent advances in synthesis of carbon dots and their applications in org. catalysis and photocatalysis, with particular attention to green nonmetal-doped systems. Finally, forward-looking opportunities within this field are mentioned here.
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Cailotto, S.; Negrato, M.; Daniele, S.; Luque, R.; Selva, M.; Amadio, E.; Perosa, A. Carbon Dots as Photocatalysts for Organic Synthesis: Metal-Free Methylene–Oxygen-Bond Photocleavage. Green Chem. 2020, 22, 1145– 1149, DOI: 10.1039/C9GC03811F
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Han, Y.; Huang, H.; Zhang, H.; Liu, Y.; Han, X.; Liu, R.; Li, H.; Kang, Z. Carbon Quantum Dots with Photoenhanced Hydrogen-Bond Catalytic Activity in Aldol Condensations. ACS Catal. 2014, 4, 781– 787, DOI: 10.1021/cs401118x
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Carbon quantum dots with photoenhanced hydrogen-bond catalytic activity in aldol condensations
Han, Yuzhi; Huang, Hui; Zhang, Hengchao; Liu, Yang; Han, Xiao; Liu, Ruihua; Li, Haitao; Kang, Zhenhui
ACS Catalysis (2014), 4 (3), 781-787CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
Carbon quantum dots (CQDs) were synthesized by an electrochem. etching method. The CQDs are well-dispersed with uniform size about 5 nm. FT-IR spectra suggest the presence of many hydroxyl groups on the surface of CQDs. Here, CQDs with diam. approx. 5 nm, directly used as effective heterogeneous nanocatalysts for H-bond catalysis in aldol condensations, show excellent photoenhanced catalytic ability (89% yields when 4-cyanobenzaldehyde is used). It demonstrated that aldol condensation between acetone and arom. aldehydes resulted in higher yields with visible light irradn. than in the dark, confirming visible light is necessary for good conversion. The H-bond catalytic activities of CQDs can be significantly enhanced with visible light irradn. The high catalytic activities of CQDs are due to highly efficient electron-accepting capabilities. Repeated catalytic expts. suggest that the CQD catalyst can be easily recycled as a heterogeneous catalyst with a long catalyst life.
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Li, H.; Sun, C.; Ali, M.; Zhou, F.; Zhang, X.; MacFarlane, D. R. Sulfated Carbon Quantum Dots as Efficient Visible-Light Switchable Acid Catalysts for Room-Temperature Ring-Opening Reactions. Angew. Chem. 2015, 127, 8540– 8544, DOI: 10.1002/ange.201501698
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Wirth, T. Microreactors in Organic Chemistry and Catalysis, 2nd ed.; Wiley: Weinheim, 2013.
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Liu, X.; Ünal, B.; Jensen, K. F. Heterogeneous Catalysis with Continuous Flow Microreactors. Catal. Sci. Technol. 2012, 2, 2134– 2138, DOI: 10.1039/c2cy20260c
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Heterogeneous catalysis with continuous flow microreactors
Liu, Xiaoying; Uenal, Baris; Jensen, Klavs F.
Catalysis Science & Technology (2012), 2 (10), 2134-2138CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
Packed-bed microreactors are employed under flow conditions for studies of heterogeneous catalysis: oxidn. of 4-isopropylbenzaldehyde and hydrogenation of 2-methylfuran. They have been demonstrated to be a valuable platform for rapid screening of catalytic materials, efficient optimization of reaction conditions, inline monitoring of reaction progress, and extn. of kinetic parameters.
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Schmidt, M.; Cubillas, A. M.; Taccardi, N.; Euser, T. G.; Cremer, T.; Maier, F.; Steinrück, H.-P.; Russell, P. S. J.; Wasserscheid, P.; Etzold, B. J. M. Chemical and (Photo)-Catalytical Transformations in Photonic Crystal Fibers. ChemCatChem 2013, 5, 641– 650, DOI: 10.1002/cctc.201200676
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Chemical and (Photo)-Catalytical Transformations in Photonic Crystal Fibers
Schmidt, Matthias; Cubillas, Ana M.; Taccardi, Nicola; Euser, Tijmen G.; Cremer, Till; Maier, Florian; Steinrueck, Hans-Peter; Russell, Philip St. J.; Wasserscheid, Peter; Etzold, Bastian J. M.
ChemCatChem (2013), 5 (3), 641-650CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
The concept of employing photonic crystal fibers for chem. and (photo)-catalytic transformations is presented. These optofluidic microdevices represent a versatile platform where light and fluids can interact for spectroscopic or photoactivation purposes. The use of photonic crystal fibers in chem. and sensing is reviewed and recent applications as catalytic microreactors are presented. Results on homogeneous catalysis and the immobilization of homogeneous and heterogeneous catalysts in the fiber channels are discussed. The examples demonstrate that combining catalysis and the excellent light guidance of photonic crystal fibers provides unique features for example, for photocatalytic activation and quant. photospectroscopic reaction anal.
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Cubillas, A. M.; Schmidt, M.; Euser, T. G.; Taccardi, N.; Unterkofler, S.; Russell, P. S. J.; Wasserscheid, P.; Etzold, B. J. M. In Situ Heterogeneous Catalysis Monitoring in a Hollow-Core Photonic Crystal Fiber Microflow Reactor. Adv. Mater. Interfaces 2014, 1, 1300093 DOI: 10.1002/admi.201300093
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McQuitty, R. J.; Unterkofler, S.; Euser, T. G.; Russell, P. St. J.; Sadler, P. J. Rapid Screening of Photoactivatable Metallodrugs: Photonic Crystal Fibre Microflow Reactor Coupled to ESI Mass Spectrometry. RSC Adv. 2017, 7, 37340– 37348, DOI: 10.1039/C7RA06735F
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Rapid screening of photoactivatable metallodrugs: photonic crystal fibre microflow reactor coupled to ESI mass spectrometry
McQuitty, Ruth J.; Unterkofler, Sarah; Euser, Tijmen G.; Russell, Philip St. J.; Sadler, Peter J.
RSC Advances (2017), 7 (59), 37340-37348CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
We explore the efficacy of a hyphenated photonic crystal fiber microflow reactor - high-resoln. mass spectrometer system as a method for screening the activity of potential new photoactivatable drugs. The use of light to activate drugs is an area of current development as it offers the possibility of reduced side effects due to improved spatial and temporal targeting and novel mechanisms of anticancer activity. The di-nuclear ruthenium complex [{(η6-indan)RuCl}2(μ-2,3-dpp)](PF6)2, previously studied by Magennis et al. (Inorg. Chem., 2007, 46, 5059) is used as a model drug to compare the system to std. irradn. techniques. The photodecompn. pathways using blue light radiation are the same for PCF and conventional cuvette methods. These findings are consistent with studies using conventional methods. The dinuclear complex also binds strongly to GSH after irradn., a possible explanation for its lack of potency in cell line testing. The use of the PCF-MS system dramatically reduced the sample vol. required and reduced the irradn. time by four orders of magnitude from 14 h to 12 s. However, the reduced sample vol. also results in a reduced MS signal intensity. The dead time of the combined system is 15 min, limited by the intrinsic dead vol. of the HR-MS.
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Gentleman, A. S.; Lawson, T.; Ellis, M. G.; Davis, M.; Turner-Dore, J.; Ryder, A. S. H.; Frosz, M. H.; Ciaccia, M.; Reisner, E.; Cresswell, A. J.; Euser, T. G. Stern–Volmer Analysis of Photocatalyst Fluorescence Quenching within Hollow-Core Photonic Crystal Fibre Microreactors. Chem. Commun. 2022, 58, 10548– 10551, DOI: 10.1039/D2CC03996F
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Stern-Volmer analysis of photocatalyst fluorescence quenching within hollow-core photonic crystal fibre microreactors
Gentleman, Alexander S.; Lawson, Takashi; Ellis, Matthew G.; Davis, Molly; Turner-Dore, Jacob; Ryder, Alison S. H.; Frosz, Michael H.; Ciaccia, Maria; Reisner, Erwin; Cresswell, Alexander J.; Euser, Tijmen G.
Chemical Communications (Cambridge, United Kingdom) (2022), 58 (75), 10548-10551CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
We report the use of optofluidic hollow-core photonic crystal fibers as microreactors for Stern-Volmer (SV) luminescence quenching anal. of visible-light photocatalytic reactions. This technol. enables measurements on nanolitre vols. and paves the way for automated SV analyses in continuous flow that minimise catalyst and reagent usage. The method is showcased using a recently developed photoredox-catalyzed α-C-H alkylation reaction of unprotected primary alkylamines.
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Williams, G. O. S.; Euser, T. G.; Russell, P. S. J.; MacRobert, A. J.; Jones, A. C. Highly Sensitive Luminescence Detection of Photosensitized Singlet Oxygen within Photonic Crystal Fibers. ChemPhotoChem 2018, 2, 616– 621, DOI: 10.1002/cptc.201800028
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Highly Sensitive Luminescence Detection of Photosensitized Singlet Oxygen within Photonic Crystal Fibers
Williams, Gareth O. S.; Euser, Tijmen G.; Russell, Philip St. J.; MacRobert, Alexander J.; Jones, Anita C.
ChemPhotoChem (2018), 2 (7), 616-621CODEN: CHEMYH ISSN:. (Wiley-VCH Verlag GmbH & Co. KGaA)
Highly sensitive, quant. detection of singlet oxygen (1O2) is required for the evaluation of newly developed photosensitizers and the elucidation of the mechanisms of many processes in which singlet oxygen is known or believed to be involved. The direct detection of 1O2 through its intrinsic phosphorescence at 1270 nm is challenging, because of the extremely low intensity of this emission, coupled with the low quantum efficiency of currently available photodetectors at this wavelength. We introduce hollow-core photonic crystal fibers (HC-PCF) as a novel optofluidic modality for photosensitization and detection of 1O2. We report the use of this approach to achieve highly sensitive detection of the luminescence decay of 1O2 produced by using two common photosensitizers, Rose Bengal and Hypericin, within the 60-μm diam. core of a 15 cm length of HC-PCF. We demonstrate the feasibility of directly detecting sub-picomole quantities of 1O2 by using this methodol., and identify some aspects of the HC-PCF technol. that can be improved to yield even higher detection sensitivity.
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Schorn, F.; Aubermann, M.; Zeltner, R.; Haumann, M.; Joly, N. Y. Online Monitoring of Microscale Liquid-Phase Catalysis Using in-Fiber Raman Spectroscopy. ACS Catal. 2021, 11, 6709– 6714, DOI: 10.1021/acscatal.1c01264
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Online Monitoring of Microscale Liquid-Phase Catalysis Using in-Fiber Raman Spectroscopy
Schorn, Florian; Aubermann, Manfred; Zeltner, Richard; Haumann, Marco; Joly, Nicolas Y.
ACS Catalysis (2021), 11 (11), 6709-6714CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
We report on the use of hollow-core photonic crystal fibers to monitor the evolution of chem. reactions. The combination of tight confinement and long interaction length allows single-pass spectroscopic measurements using less than a microliter vol. of chems. with good accuracy. As a proof of principle, we used here nonlinear Raman spectroscopy for a reaction screening of the acidic catalyzed esterification of methanol and acetic acid.
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Miele, E.; Dose, W. M.; Manyakin, I.; Frosz, M. H.; Ruff, Z.; De Volder, M. F. L.; Grey, C. P.; Baumberg, J. J.; Euser, T. G. Hollow-Core Optical Fibre Sensors for Operando Raman Spectroscopy Investigation of Li-Ion Battery Liquid Electrolytes. Nat. Commun. 2022, 13, 1651 DOI: 10.1038/s41467-022-29330-4
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Hollow-core optical fibre sensors for operando Raman spectroscopy investigation of Li-ion battery liquid electrolytes
Miele, Ermanno; Dose, Wesley M.; Manyakin, Ilya; Frosz, Michael H.; Ruff, Zachary; De Volder, Michael F. L.; Grey, Clare P.; Baumberg, Jeremy J.; Euser, Tijmen G.
Nature Communications (2022), 13 (1), 1651CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
Improved anal. tools are urgently required to identify degrdn. and failure mechanisms in Li-ion batteries. However, understanding and ultimately avoiding these detrimental mechanisms requires continuous tracking of complex electrochem. processes in different battery components. Here, we report an operando spectroscopy method that enables monitoring the chem. of a carbonate-based liq. electrolyte during electrochem. cycling in Li-ion batteries with a graphite anode and a LiNi0.8Mn0.1Co0.1O2 cathode. By embedding a hollow-core optical fiber probe inside a lab-scale pouch cell, we demonstrate the effective evolution of the liq. electrolyte species by background-free Raman spectroscopy. The anal. of the spectroscopy measurements reveals changes in the ratio of carbonate solvents and electrolyte additives as a function of the cell voltage and show the potential to track the lithium-ion solvation dynamics. The proposed operando methodol. contributes to understanding better the current Li-ion battery limitations and paves the way for studies of the degrdn. mechanisms in different electrochem. energy storage systems.
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Lawson, T.; Gentleman, A. S.; Pinnell, J.; Eisenschmidt, A.; Antón-García, D.; Frosz, M. H.; Reisner, E.; Euser, T. G. In Situ Detection of Cobaloxime Intermediates During Photocatalysis Using Hollow-Core Photonic Crystal Fiber Microreactors. Angew. Chem., Int. Ed. 2023, 62, e202214788 DOI: 10.1002/anie.202214788
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In situ Detection of Cobaloxime Intermediates During Photocatalysis Using Hollow-Core Photonic Crystal Fiber Microreactors
Lawson, Takashi; Gentleman, Alexander S.; Pinnell, Jonathan; Eisenschmidt, Annika; Anton-Garcia, Daniel; Frosz, Michael H.; Reisner, Erwin; Euser, Tijmen G.
Angewandte Chemie, International Edition (2023), 62 (9), e202214788CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
Hollow-core photonic crystal fibers (HC-PCFs) provide a novel approach for in situ UV/Vis spectroscopy with enhanced detection sensitivity. Here, we demonstrate that longer optical path lengths than afforded by conventional cuvette-based UV/Vis spectroscopy can be used to detect and identify the CoI and CoII states in hydrogen-evolving cobaloxime catalysts, with spectral identification aided by comparison with DFT-simulated spectra. Our findings show that there are two types of signals obsd. for these mol. catalysts; a transient signal and a steady-state signal, with the former being assigned to the CoI state and the latter being assigned to the CoII state. These observations lend support to a unimol. pathway, rather than a bimol. pathway, for hydrogen evolution. This study highlights the utility of fiber-based microreactors for understanding these and a much wider range of homogeneous photocatalytic systems in the future.
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Unterkofler, S.; McQuitty, R. J.; Euser, T. G.; Farrer, N. J.; Sadler, P. J.; Russell, P. S. J. Microfluidic Integration of Photonic Crystal Fibers for Online Photochemical Reaction Analysis. Opt. Lett. 2012, 37, 1952– 1954, DOI: 10.1364/OL.37.001952
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Microfluidic integration of photonic crystal fibers for online photochemical reaction analysis
Unterkofler, S.; McQuitty, R. J.; Euser, T. G.; Farrer, N. J.; Sadler, P. J.; Russell, P. St. J.
Optics Letters (2012), 37 (11), 1952-1954CODEN: OPLEDP; ISSN:0146-9592. (Optical Society of America)
Liq.-filled hollow-core photonic crystal fibers (HC-PCFs) are perfect optofluidic channels, uniquely providing low-loss optical guidance in a liq. medium. As a result, the overlap of the dissolved specimen and the intense light field in the micron sized core is increased manyfold compared to conventional bioanal. techniques, facilitating highly-efficient photoactivation processes. Here we introduce a novel integrated anal. technol. for photochem. by microfluidic coupling of a HC-PCF nanoflow reactor to supplementary detection devices. Applying a continuous flow through the fiber, we deliver photochem. reaction products to a mass spectrometer in an online and hence rapid fashion, which is highly advantageous over conventional cuvette-based approaches.
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Koehler, P.; Lawson, T.; Neises, J.; Willkomm, J.; Martindale, B. C. M.; Hutton, G. A. M.; Antón-García, D.; Lage, A.; Gentleman, A. S.; Frosz, M. H.; Russell, P. S. J.; Reisner, E.; Euser, T. G. Optofluidic Photonic Crystal Fiber Microreactors for In Situ Studies of Carbon Nanodot-Driven Photoreduction. Anal. Chem. 2021, 93, 895– 901, DOI: 10.1021/acs.analchem.0c03546
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Optofluidic photonic crystal fiber microreactors for in situ studies of carbon nanodot-driven photoreduction
Koehler, Philipp; Lawson, Takashi; Neises, Julian; Willkomm, Janina; Martindale, Benjamin C. M.; Hutton, Georgina A. M.; Anton-Garcia, Daniel; Lage, Ava; Gentleman, Alexander S.; Frosz, Michael H.; Russell, Philip St. J.; Reisner, Erwin; Euser, Tijmen G.
Analytical Chemistry (Washington, DC, United States) (2021), 93 (2), 895-901CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
Performing quant. in situ spectroscopic anal. on minuscule sample vols. is a common difficulty in photochem. To address this challenge, we use a hollow-core photonic crystal fiber (HC-PCF) that guides light at the center of a microscale liq. channel and acts as an optofluidic microreactor with a reaction vol. of less than 35 nL. The system was used to demonstrate in situ optical detection of photoredn. processes that are key components of many photocatalytic reaction schemes. The photoredn. of viologens (XV2+) to the radical XV•+ in a homogeneous mixt. with carbon nanodot (CND) light absorbers is studied for a range of different carbon dots and viologens. Time-resolved absorption spectra, measured over several UV irradn. cycles, are interpreted with a quant. kinetic model to det. photoredn. and photobleaching rate consts. The powerful combination of time-resolved, low-vol. absorption spectroscopy and kinetic modeling highlights the potential of optofluidic microreactors as a highly sensitive, quant., and rapid screening platform for novel photocatalysts and flow chem. in general.
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Groeneveld, I.; Schoemaker, S. E.; Somsen, G. W.; Ariese, F.; van Bommel, M. R. Characterization of a Liquid-Core Waveguide Cell for Studying the Chemistry of Light-Induced Degradation. Analyst 2021, 146, 3197– 3207, DOI: 10.1039/D1AN00272D
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Liquid core waveguide cell chemistry light induced degradation of organic dye
Groeneveld, Iris; Schoemaker, Suzan E.; Somsen, Govert W.; Ariese, Freek; van Bommel, Maarten R.
Analyst (Cambridge, United Kingdom) (2021), 146 (10), 3197-3207CODEN: ANALAO; ISSN:0003-2654. (Royal Society of Chemistry)
Many org. compds. undergo changes under the influence of light. This might be beneficial in, for example, water purifn., but undesirable when cultural-heritage objects fade or when food ingredients (e.g., vitamins) degrade. It is often challenging to establish a strong link between photodegrdn. products and their parent mols. due to the complexity of the sample. To allow effective study of light-induced degrdn. (LID), a low-vol. exposure cell was created in which solutes are efficiently illuminated (esp. at low concns.) while simultaneously analyzed by absorbance spectroscopy. The new LID cell encompasses a gas-permeable liq.-core waveguide (LCW) connected to a spectrograph allowing collection of spectral data in real-time. The aim of the current study was to evaluate the overall performance of the LID cell by assessing its transmission characteristics, the abs. photon flux achieved in the LCW, and its capacity to study solute degrdn. in presence of oxygen. The potential of the LID set-up for light-exposure studies was successfully demonstrated by monitoring the degrdn. of the dyes eosin Y and crystal violet.
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Groeneveld, I.; Bagdonaite, I.; Beekwilder, E.; Ariese, F.; Somsen, G. W.; van Bommel, M. R. Liquid Core Waveguide Cell with In Situ Absorbance Spectroscopy and Coupled to Liquid Chromatography for Studying Light-Induced Degradation. Anal. Chem. 2022, 94, 7647– 7654, DOI: 10.1021/acs.analchem.2c00886
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Liquid Core Waveguide Cell with In Situ Absorbance Spectroscopy and Coupled to Liquid Chromatography for Studying Light-Induced Degradation
Groeneveld, Iris; Bagdonaite, Ingrida; Beekwilder, Edwin; Ariese, Freek; Somsen, Govert W.; van Bommel, Maarten R.
Analytical Chemistry (Washington, DC, United States) (2022), 94 (21), 7647-7654CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
In many areas, studying photostability or the mechanism of photodegrdn. is of high importance. Conventional methods to do so can be rather time-consuming, laborious, and prone to exptl. errors. In this paper we evaluate an integrated and fully automated system for the study of light-induced degrdn., comprising a liq. handler, an irradn. source and exposure cell with dedicated optics and spectrograph, and a liq. chromatog. (LC) system. A liq. core waveguide (LCW) was used as an exposure cell, allowing efficient illumination of the sample over a 12 cm path length. This cell was coupled to a spectrograph, allowing in situ absorbance monitoring of the exposed sample during irradn. The LCW is gas-permeable, permitting diffusion of air into the cell during light exposure. This unit was coupled online to LC with diode array detection for immediate and automated anal. of the compn. of the light-exposed samples. The anal. performance of the new system was established by assessing linearity, limit of detection, and repeatability of the in-cell detection, sample recovery and carryover, and overall repeatability of light-induced degrdn. monitoring, using riboflavin as the test compd. The applicability of the system was demonstrated by recording a photodegrdn. time profile of riboflavin.
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Islam, M. S.; Cordeiro, C. M. B.; Franco, M. A. R.; Sultana, J.; Cruz, A. L. S.; Abbott, D. Terahertz Optical Fibers. Opt. Express 2020, 28, 16089– 16117, DOI: 10.1364/OE.389999
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Terahertz optical fibers [Invited]
Islam, Md. Saiful; Cordeiro, Cristiano M. B.; Franco, Marcos A. R.; Sultana, Jakeya; Cruz, Alice L. S.; Abbott, Derek
Optics Express (2020), 28 (11), 16089-16117CODEN: OPEXFF; ISSN:1094-4087. (Optical Society of America)
A review. Lying between optical and microwave ranges, the terahertz band in the electromagnetic spectrum is attracting increased attention. Optical fibers are essential for developing the full potential of complex terahertz systems. In this manuscript, we review the optimal materials, the guiding mechanisms, the fabrication methodologies, the characterization methods and the applications of such terahertz waveguides. We examine various optical fiber types including tube fibers, solid core fiber, hollow-core photonic bandgap, anti-resonant fibers, porous-core fibers, metamaterial-based fibers, and their guiding mechanisms. The optimal materials for terahertz applications are discussed. The past and present trends of fabrication methods, including drilling, stacking, extrusion and 3D printing, are elaborated. Fiber characterization methods including different optics for terahertz time-domain spectroscopy (THz-TDS) setups are reviewed and application areas including short-distance data transmission, imaging, sensing, and spectroscopy are discussed.
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Testa, C.; Zammataro, A.; Pappalardo, A.; Sfrazzetto, G. T. Catalysis with Carbon Nanoparticles. RSC Adv. 2019, 9, 27659– 27664, DOI: 10.1039/C9RA05689K
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Catalysis with carbon nanoparticles
Testa, Caterina; Zammataro, Agatino; Pappalardo, Andrea; Trusso Sfrazzetto, Giuseppe
RSC Advances (2019), 9 (47), 27659-27664CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
A review. Carbon nanoparticles (CNPs) represent a recent class of nanomaterials, based on carbon sp2 atoms in the inner core. These new nano-dots cover a wide range of application fields: anal., sensing and biosensing, bioimaging, theranostic, and mol. communication. However, their use as nanocatalysts is relatively new. Although CNPs can be easily synthesized and obtained in good amts., few reports on their catalytic applications have been reported. This minireview collects the use of these nanoparticles as catalysts highlighting the improvements with respect to the classic catalytic systems. In particular, due to their unique optical and elec. properties, and due to the possibility to cover the external shell with a wide variety of functional groups, CNPs have found catalytic applications in three main classes of reactions: (i) photocatalysis, (ii) acid-base catalysis and (iii) electro catalysis.
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Mandal, K.; Hoffman, M. Z. Quantum Yield of Formation of Methylviologen Radical Cation in the Photolysis of the Ru(bpy)₃²⁺/Methylviologen/EDTA System. J. Phys. Chem. A 1984, 88, 5632– 5639, DOI: 10.1021/j150667a035
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Suzuki, M.; Morris, N. D.; Mallouk, T. E. Photosensitized Production of Doubly Reduced Methylviologen Followed by Highly Efficient Methylviologen Radical Formation Using Self-Assembling Ruthenium(II) Complexes. Chem. Commun. 2002, 14, 1534– 1535, DOI: 10.1039/b205076p
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Kim, J. Y.; Lee, C.; Park, J. W. The Kinetics of Neutral Methyl Viologen in Acidic H2O+DMF Mixed Solutions Studied by Cyclic Voltammetry. J. Electroanal. Chem. 2001, 504, 104– 110, DOI: 10.1016/S0022-0728(01)00417-X
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The kinetics of neutral methyl viologen in acidic H2O+DMF mixed solutions studied by cyclic voltammetry
Kim, J. Y.; Lee, C.; Park, J. W.
Journal of Electroanalytical Chemistry (2001), 504 (1), 104-110CODEN: JECHES ISSN:. (Elsevier Science S.A.)
The chem. of the two-electron redn. product of viologen (1,1'-dialkyl-4,4'-bipyridinium, V2+) neutral species, is important in understanding the electrochem. behavior of viologens and their use. The kinetics for the reactions of neutral Me viologen (V0) in the presence of H+ (from HCl), CH3COOH (pKa = 4.75), ClCH2CH2COOH (pKa = 4.00), HCOOH (pKa=3.75) in aq. media was examd. by cyclic voltammetry according to the EECi mechanism. To avoid the electrodeposition of V0, the authors used a 9:1 (vol./vol.%) H2O+DMF mixt. as the solvent medium. To evaluate the rate consts. for the chem. reaction followed by the 2nd electron transfer step of V2+, the ratio of the anodic and cathodic peak current (Ipa2/Ipc2) corresponding to V0 - e- .dblharw. V•+ was plotted against log τ, where τ is the time between E1/2 and the switching potential, at various scan rates of 0.02-3.5 V s-1. The chem. reaction is a parallel reaction consisting of H+-catalyzed and general-acid (HA) catalyzed reactions. The 2nd-order rate consts. are detd. as kH+ = 3.5 × 103 M-1 s-1, kCH3COOH = 5.7 M-1 s-1, kHCOOH = 4.6 × 101 M-1 s-1, and kClCH2CH2COOH = 3.2 × 101 M-1 s-1 using the Nicholson-Shain method and kH2O was estd. as <3 × 10-6 M-1 s-1. The CVs were digitally simulated under the assumption of a two-step reaction of V0 following the two-step electrode reactions of V2+ to V0. The simulated CVs show good agreement with those obtained exptl., when the 1st-step reaction of V0 is a relatively fast reversible reaction and the 2nd-step reaction is a slow irreversible one. Based on these results, probably V0 is in pseudo-equil. with H+ or HA to produce VH+ which undergoes a reaction with H2O.
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Couny, F.; Benabid, F.; Roberts, P. J.; Light, P. S.; Raymer, M. G. Generation and Photonic Guidance of Multi-Octave Optical-Frequency Combs. Science 2007, 318, 1118– 1121, DOI: 10.1126/science.1149091
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Generation and Photonic Guidance of Multi-Octave Optical-Frequency Combs
Couny, F.; Benabid, F.; Roberts, P. J.; Light, P. S.; Raymer, M. G.
Science (Washington, DC, United States) (2007), 318 (5853), 1118-1121CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
Ultrabroad coherent comb-like optical spectra spanning several octaves are a chief ingredient in the emerging field of attoscience. We demonstrate generation and guidance of a three-octave spectral comb, spanning wavelengths from 325 to 2300 nm, in a hydrogen-filled hollow-core photonic crystal fiber. The waveguidance results not from photonic a band gap but from the inhibited coupling between the core and cladding modes. The spectrum consists of up to 45 high-order Stokes and anti-Stokes lines and is generated by driving the confined gas with a single, moderately powerful (10-kW) IR laser, producing 12-ns-duration pulses. This represents a redn. by six orders of magnitude in the required laser powers over previous equiv. techniques and opens up a robust and much simplified route to synthesizing attosecond pulses.
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Archambault, J.-L.; Black, R. J.; Lacroix, S.; Bures, J. Loss Calculations for Antiresonant Waveguides. J. Lightwave Technol. 1993, 11, 416– 423, DOI: 10.1109/50.219574
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Cubillas, A. M.; Unterkofler, S.; Euser, T. G.; Etzold, B. J. M.; Jones, A. C.; Sadler, P. J.; Wasserscheid, P.; Russell, P. S. J. Photonic Crystal Fibres for Chemical Sensing and Photochemistry. Chem. Soc. Rev. 2013, 42, 8629– 8648, DOI: 10.1039/c3cs60128e
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Photonic crystal fibers for chemical sensing and photochemistry
Cubillas, Ana M.; Unterkofler, Sarah; Euser, Tijmen G.; Etzold, Bastian J. M.; Jones, Anita C.; Sadler, Peter J.; Wasserscheid, Peter; Russell, Philip St. J.
Chemical Society Reviews (2013), 42 (22), 8629-8648CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
In this review, photonic crystal fiber is introduced as a novel optofluidic microdevice that can be employed as both a versatile chem. sensor and a highly efficient microreactor. It provides an excellent platform in which light and chem. samples can strongly interact for quant. spectroscopic anal. or photoactivation purposes. The use of photonic crystal fiber in photochem. and sensing is discussed and recent results on gas and liq. sensing as well as on photochem. and catalytic reactions are reviewed. These developments demonstrate that the tight light confinement, enhanced light-matter interaction and reduced sample vol. offered by photonic crystal fiber make it useful in a wide range of chem. applications.
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Hutton, G. A. M.; Reuillard, B.; Martindale, B. C. M.; Caputo, C. A.; Lockwood, C. W. J.; Butt, J. N.; Reisner, E. Carbon Dots as Versatile Photosensitizers for Solar-Driven Catalysis with Redox Enzymes. J. Am. Chem. Soc. 2016, 138, 16722– 16730, DOI: 10.1021/jacs.6b10146
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Carbon Dots as Versatile Photosensitizers for Solar-Driven Catalysis with Redox Enzymes
Hutton, Georgina A. M.; Reuillard, Bertrand; Martindale, Benjamin C. M.; Caputo, Christine A.; Lockwood, Colin W. J.; Butt, Julea N.; Reisner, Erwin
Journal of the American Chemical Society (2016), 138 (51), 16722-16730CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Light-driven enzymic catalysis is enabled by the productive coupling of a protein to a photosensitizer. Photosensitizers used in such hybrid systems are typically costly, toxic, and/or fragile, with limited chem. versatility. Carbon dots (CDs) are low-cost, nanosized light-harvesters that are attractive photosensitizers for biol. systems as they are water-sol., photostable, nontoxic, and their surface chem. can be easily modified. We demonstrate here that CDs act as excellent light-absorbers in two semibiol. photosynthetic systems utilizing either a fumarate reductase (FccA) for the solar-driven hydrogenation of fumarate to succinate or a hydrogenase (H2ase) for redn. of protons to H2. The tunable surface chem. of the CDs was exploited to synthesize pos. charged ammonium-terminated CDs (CD-NHMe2+), which were capable of transferring photoexcited electrons directly to the neg. charged enzymes with high efficiency and stability. Enzyme-based turnover nos. of 6000 mol succinate (mol FccA)-1 and 43,000 mol H2 (mol H2ase)-1 were reached after 24 h. Neg. charged carboxylate-terminated CDs (CD-CO2-) displayed little or no activity, and the electrostatic interactions at the CD-enzyme interface were detd. to be essential to the high photocatalytic activity obsd. with CD-NHMe2+. The modular surface chem. of CDs together with their photostability and aq. soly. make CDs versatile photosensitizers for redox enzymes with great scope for their utilization in photobiocatalysis.
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C dots (CDs) exhibit outstanding physicochem. properties that render them excellent materials for various applications, often occurring in an aq. environment, such as light harvesting and fluorescence bioimaging. Here the authors characterize the electronic structures of CDs and H2O mols. in aq. dispersions using in situ x-ray absorption spectroscopy. Three types of CDs with different core structures (amorphous vs. graphitic) and compns. (undoped vs. N-doped) were studied. Depending on the CD core structure, different ionic currents generated upon x-ray irradn. of the CD dispersions at the C K-edge were detected, which are interpreted in terms of different charge transfer to the surrounding solvent mols. The H bonding networks of H2O mols. upon interaction with the different CDs were also probed at the O K-edge. Both core graphitization and N doping endow the CDs with enhanced electron transfer and H bonding capabilities with the surrounding H2O mols.
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Stuart Macpherson, Takashi Lawson, Anna Abfalterer, Paolo Andrich, Ava Lage, Erwin Reisner, Tijmen G. Euser, Samuel D. Stranks, Alexander S. Gentleman. Influence of Electron Donors on the Charge Transfer Dynamics of Carbon Nanodots in Photocatalytic Systems. ACS Catalysis 2024, 14 (16) , 12006-12015. https://doi.org/10.1021/acscatal.4c02327
Shannon A. Bonke, Giovanni Trezza, Luca Bergamasco, Hongwei Song, Santiago Rodríguez-Jiménez, Leif Hammarström, Eliodoro Chiavazzo, Erwin Reisner. Multi-Variable Multi-Metric Optimization of Self-Assembled Photocatalytic CO2 Reduction Performance Using Machine Learning Algorithms. Journal of the American Chemical Society 2024, 146 (22) , 15648-15658. https://doi.org/10.1021/jacs.4c01305

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Abstract
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Figure 1
Figure 1. Fiber-based CD screening. [a] Schematic of CD-driven photoreduction of viologens and the four screening factors explored in this study: the irradiance, sacrificial electron donor (SED) used, CD photosensitizer type, and viologen species (XV). [b] Scanning electron microscopy (SEM) image of the core region of the kagomé-style hollow-core photonic crystal fiber (HC-PCF) overlaid with the measured mode intensity profile (450–600 nm). [c] Optical setup enabling the transmitted mode to be simultaneously analyzed by a camera (see [b]) and its spectrum by a spectrometer (see [d]). BS, beamsplitter; LP, long-pass; MMF, multimode fiber (300 μm core diameter); SC, supercontinuum. O1 corresponds to the in-coupling objective and O2 corresponds to the out-coupling objective. A1 is an aperture used for alignment. [d] Time-resolved spectra are converted to absorbance with a trace taken at 600 nm to deduce the XV^•+ concentration in [e]. The spectral shape of the absorbance profile is characteristic of an XV^•+ species. [e] Fitting method for calculating the delay time, t_d, and photoreduction rate constant, k. The time at which the electron transfer starts, t₀, is defined by the x-intercept of the linear fitting function.
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Figure 2
Figure 2. Photoreduction dependence on CD concentration. [a] MV^•+ concentration, and [b] rate profiles, both illustrating the dependence of the reaction kinetics on amorphous carbon dot (aCD) concentration. The MV^•+ concentration was calculated from the UV–vis absorption peak at 600 nm. Each sample consists of aCDs with 40 μM MV²⁺ and 0.1 M EDTA in an aqueous pH 6 phosphate buffer. Samples were initially kept in the dark and, from t = 60 s, were subject to continuous UV irradiation (λ = 365 nm) at an irradiance of 90.2 mW cm^–2. [c] Delay time and [d] photoreduction rate constant, both plotted as a function of aCD concentration, with error bars signifying the standard deviation of triplicates.
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Figure 3
Figure 3. Photoreduction dependence on MV²⁺ concentration. [a] MV^•+ concentration, and [b] percentage conversion profiles, illustrating the dependence of the reaction kinetics on the starting concentration of the methyl viologen dication, [MV²⁺]_t=0. The MV^•+ concentration was calculated from the UV–vis absorption tail at 670 nm. The conversion was calculated by dividing the concentration profile by the starting concentration of the methyl viologen dication. Each sample consists of MV²⁺ with 0.5 g L^–1 aCDs and 0.1 M EDTA in an aqueous pH 6 phosphate buffer. Samples were subject to continuous UV irradiation (λ = 365 nm) from t = 60 s at an irradiance of 90.2 mW cm^–2. [c] Delay time and [d] photoreduction rate constant, both plotted as a function of [MV²⁺]_t=0, with error bars signifying the standard deviation of at least triplicates.
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Figure 4
Figure 4. Photoreduction dependence on EDTA concentration. [a] MV^•+ concentration, and [b] rate profiles, illustrating the dependence of the reaction kinetics on EDTA concentration. The MV^•+ concentration was calculated from the UV–vis absorption peak at 600 nm. Each sample consists of EDTA with 0.5 g L^–1 aCDs and 40 μM MV²⁺ in an aqueous pH 6 phosphate buffer. Samples were subject to continuous UV irradiation (λ = 365 nm) from t = 60 s at an irradiance of 90.2 mW cm^–2. [c] Delay time and [d] photoreduction rate constant, both plotted as a function of EDTA concentration, with error bars signifying the standard deviation of at least triplicates.
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Figure 5
Figure 5. Photoreduction dependence on the local volumetric rate of photon absorption (LVRPA). [a] MV^•+ concentration profile illustrating the dependence of the reaction kinetics on irradiance. The MV^•+ concentration was calculated from the UV–vis absorption peak at 600 nm. Each sample consists of MV²⁺ with 0.5 g L^–1 aCDs, 40 μM MV²⁺, and 0.1 M EDTA in an aqueous pH 6 phosphate buffer. Samples were subject to continuous UV irradiation (λ = 365 nm) from t = 60 s. [b] Delay time (inverse) and [c] photoreduction rate, both plotted as a function of LVRPA.
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Figure 6
Figure 6. Photoreduction dependence on individual photosystem components. [a, b] Varying the viologen species: [a] photoreduction rate constants and [b] delay time. Each sample consists of 40 μM XV²⁺, 0.1 M EDTA, and 0.5 g L^–1 aCD. [c, d] Varying the sacrificial electron donor and pH: [c] photoreduction rate constants and [d] delay time. Each sample consists of 40 μM MV²⁺, 0.1 M SED, and 0.5 g L^–1 aCD. [e, f] Varying the photosensitizer type and excitation wavelength: [e] photoreduction rate constants and [f] delay time. Each sample consists of 40 μM MV²⁺, 0.1 M EDTA, and 0.01 g L^–1 light absorber. Samples were subject to continuous UV irradiation (λ = 365 nm, 88.4 mW cm^–2) or blue irradiation (λ = 450 nm, 99.1 mW cm^–2). Ru = [Ru(bpy)₃]²⁺, with Ru* and NgCD* denoting excitation at 450 nm. Error bars signify the standard deviation of at least triplicates.
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Figure 7
Figure 7. Comparison of protonated, deprotonated, and tertiary amine-capped aCDs. [a] photoreduction rate constant and [b] delay time. Each sample consists of 40 μM MV²⁺, 0.1 M EDTA, and 0.5 g L^–1 aCD. Samples were subject to continuous UV irradiation (λ = 365 nm, 88.4 mW cm^–2). Error bars signify the standard deviation of at least triplicates.
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  Carbon dots with multi-functional groups and the application in proton exchange membranes
  Jia, Wei; Tang, Beibei; Wu, Peiyi
  Electrochimica Acta (2018), 260 (), 92-100CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
  It is of great necessity to achieve a proton exchange membrane (PEM) with high proton cond. and low MeOH permeability for the practical applications. Nafion modified C dots (NCDs) with multi-functional groups were successfully synthesized via the pyrolysis of citric acid (CA) with the existence of Nafion. The modification process is motivated by the hydrophilicity-to-hydrophobicity transformation of CA as well as the noncovalent hydrophilic-hydrophobic interaction during the pyrolysis procedure. Multi-functionalized NCDs with moderate hydrophilicity influence the aggregation structure of Nafion matrix of the composite membranes and effectively enhance the high-temp. H2O retention ability. Both the proton cond. and the MeOH resistance ability of the composite PEMs are significantly enhanced. 0.5-NCD-0.5 Nafion composite PEM presents a 5-10 times increase in proton cond. and 50% percent decrease in MeOH permeability than that of recast Nafion.
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8. 8
  Achilleos, D. S.; Kasap, H.; Reisner, E. Photocatalytic Hydrogen Generation Coupled to Pollutant Utilisation Using Carbon Dots Produced from Biomass. Green Chem. 2020, 22, 2831– 2839, DOI: 10.1039/D0GC00318B
  8
  Photocatalytic hydrogen generation coupled to pollutant utilisation using carbon dots produced from biomass
  Achilleos, Demetra S.; Kasap, Hatice; Reisner, Erwin
  Green Chemistry (2020), 22 (9), 2831-2839CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
  Photocatalysis is deemed as an appealing strategy to exploit solar energy for simultaneous fuel prodn. and pollutant utilization. However, current photocatalytic systems rarely couple both processes and commonly suffer from restricted scalability and sustainability as they use toxic or UV light harvesters, combined with noble-metal co-catalysts under corrosive conditions. Here, the synthesis is shown of ultra-scalable and low-cost carbon nanodots from lignocellulosic waste, which when combined with a non-precious Ni-based co-catalyst, use visible light to drive H2 prodn. in untreated river and sea water. Org. pollutants and chloride anions in these untreated media do not only allow unhindered photocatalytic activities, but also function as electron donors for the photoexcited carbon dots to enable proton redn. This system combines Earth's most abundant resources (biomass, solar energy, untreated water) and functions at ambient temp., pressure and benign pH, thereby creating perspectives for simultaneous fuel synthesis as well as sustainable and practical pollutant utilization.
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9. 9
  Martindale, B. C. M.; Hutton, G. A. M.; Caputo, C. A.; Reisner, E. Solar Hydrogen Production Using Carbon Quantum Dots and a Molecular Nickel Catalyst. J. Am. Chem. Soc. 2015, 137, 6018– 6025, DOI: 10.1021/jacs.5b01650
  9
  Solar Hydrogen Production Using Carbon Quantum Dots and a Molecular Nickel Catalyst
  Martindale, Benjamin C. M.; Hutton, Georgina A. M.; Caputo, Christine A.; Reisner, Erwin
  Journal of the American Chemical Society (2015), 137 (18), 6018-6025CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Carbon quantum dots (CQDs) are established as excellent photosensitizers in combination with a mol. catalyst for solar light driven hydrogen prodn. in aq. soln. The inexpensive CQDs can be prepd. by straightforward thermolysis of citric acid in a simple one-pot, multigram synthesis and are therefore scalable. The CQDs produced reducing equiv. under solar irradn. in a homogeneous photocatalytic system with a Ni-bis(diphosphine) catalyst, giving an activity of 398 μmolH2 (gCQD)-1 h-1 and a "per Ni catalyst" turnover frequency of 41 h-1. The CQDs displayed activity in the visible region beyond λ > 455 nm and maintained their full photocatalytic activity for at least 1 day under full solar spectrum irradn. A high quantum efficiency of 1.4% was recorded for the noble- and toxic-metal free photocatalytic system. Thus, CQDs are shown to be a highly sustainable light-absorbing material for photocatalytic schemes, which are not limited by cost, toxicity, or lack of scalability. The photocatalytic hybrid system was limited by the lifetime of the mol. catalyst, and intriguingly, no photocatalytic activity was obsd. using the CQDs and 3d transition metal salts or platinum precursors. This observation highlights the advantage of using a mol. catalyst over commonly used heterogeneous catalysts in this photocatalytic system.
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10. 10
  Dalle, K. E.; Warnan, J.; Leung, J. J.; Reuillard, B.; Karmel, I. S.; Reisner, E. Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes. Chem. Rev. 2019, 119, 2752– 2875, DOI: 10.1021/acs.chemrev.8b00392
  10
  Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes
  Dalle, Kristian E.; Warnan, Julien; Leung, Jane J.; Reuillard, Bertrand; Karmel, Isabell S.; Reisner, Erwin
  Chemical Reviews (Washington, DC, United States) (2019), 119 (4), 2752-2875CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based mol. catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aq. media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive assocns. between mol. catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The mol.-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 redn. to C1 products, by summarizing cases for higher-value products from N2 redn., Cx>1 products from CO2 utilization, and other reductive org. transformations.
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11. 11
  Prasad, D. R.; Hoffman, M. Z. Photodynamics of the Tris(2,2′-bipyrazine)ruthenium(2+)/Methylviologen/EDTA System in Aqueous Solution. J. Am. Chem. Soc. 1986, 108, 2568– 2573, DOI: 10.1021/ja00270a013
  11
  Photodynamics of the tris(2,2'-bipyrazine)ruthenium(2+)/methylviologen/EDTA system in aqueous solution
  Prasad, Dasari R.; Hoffman, Morton Z.
  Journal of the American Chemical Society (1986), 108 (10), 2568-73CODEN: JACSAT; ISSN:0002-7863.
  Luminescence quenching and flash photolysis (pulsed laser and conventional) techniques were employed to study the photodynamics of the Ru(bpz)32+/MV2+/EDTA system in aq. soln. (bpz = 2,2'-bipyrazine; MV2+ = methylviologen). The quenching of *Ru(bpz)32+ by EDTA in alk. soln. via Stern-Volmer kinetics (kq = 6.9 × 108 and 7.7 × 108 M-1 s-1 at pH 8.7 and 11.0, resp.) generates Ru(bpz)3+ which reacts with MV2+ (k = 4.5 × 108 M-1 s-1) to yield MV+·; the reducing EDTA radical, formed from the irreversible transformation of the species obtained from the oxidn. of EDTA, reacts with MV2+ (k = 1.5 × 109 M-1 s-1) to yield a 2nd equiv. of MV+·. At pH 4.7, the quenching of *Ru(bpz)32+ by EDTA does not follow Stern-Volmer kinetics; kq approaches ∼2 × 108 and ∼2 × 107 M-1 s-1 in the limits of low and high [EDTA], resp. The quantum yields of MV+· in acidic soln. are dramatically lower than in neutral and alk. soln. due to the lower efficiencies of the quenching and cage release processes and the decreased reactivities of the protonated forms of Ru(bpz)3+ and reducing EDTA radicals toward MV2+.
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12. 12
  Yatsuzuka, K.; Yamauchi, K.; Kawano, K.; Ozawa, H.; Sakai, K. Improving the Overall Performance of Photochemical H₂ Evolution Catalyzed by the Co-NHC Complex via the Redox Tuning of Electron Relays. Sustainable Energy Fuels 2021, 5, 740– 749, DOI: 10.1039/D0SE01597K
  12
  Improving the overall performance of photochemical H2 evolution catalyzed by the Co-NHC complex via the redox tuning of electron relays
  Yatsuzuka, Koichi; Yamauchi, Kosei; Kawano, Ken; Ozawa, Hironobu; Sakai, Ken
  Sustainable Energy & Fuels (2021), 5 (3), 740-749CODEN: SEFUA7; ISSN:2398-4902. (Royal Society of Chemistry)
  The catalytic performance of a photochem. H2 evolution system made up of EDTA (EDTA disodium salt), [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) and a macrocyclic N-heterocyclic carbene cobalt (Co-NHC-1) catalyst has been examd. at pH 5.0 E(2H+/H2) = -0.54 V vs. SCE by using six electron relays (ERs) having different first redn. potentials (Ered) in the range -0.69 < Ered < -1.08 V. Compared to the photosystem using the conventional methylviologen (i.e., N,N'-dimethyl-4,4'-bipyridinium; MV2+), the overall catalytic performance is dramatically improved by employing the ERs having the redn. potentials by 0.08-0.24 V more neg. than that of MV2+ (Ered = -0.69 V), revealing that the overall rate is limited by the electron transfer (ET) from the one-electron reduced ER to Co-NHC-1, correlated to hydrogen evolution reaction (HER), rather than that from [Ru*(bpy)3]2+ (triplet excited state) to ER, since the driving force for the HER (DFHER) predominates that for the ET from [Ru*(bpy)3]2+ to ER (DFET). The optimum condition was realized by selecting one of the viologen derivs. with a medium redn. potential (N,N',2,2',6,6'-hexamethyl-4,4'-bipyridinium; tmMV2+; Ered = -0.85 V), leading to afford the initial rate of HER (55-57 μmol h-1) 70 times higher than that obtained by using MV2+ (0.79 μmol h-1). The stability of each one-electron reduced ER under the photolysis conditions has been also examd. spectrophotometrically, clarifying that some ERs rather decomp. rapidly upon redn. and cannot effectively participate in HER. This study successfully demonstrates for the first time that the overall catalytic performance of the present photosystem cannot be only controlled by the tuning of DFET and DFHER but also be affected by the stability of the one-electron reduced form of ER.
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13. 13
  Strauss, V.; Margraf, J. T.; Dolle, C.; Butz, B.; Nacken, T. J.; Walter, J.; Bauer, W.; Peukert, W.; Spiecker, E.; Clark, T.; Guldi, D. M. Carbon Nanodots: Toward a Comprehensive Understanding of Their Photoluminescence. J. Am. Chem. Soc. 2014, 136, 17308– 17316, DOI: 10.1021/ja510183c
  13
  Carbon Nanodots: Toward a Comprehensive Understanding of Their Photoluminescence
  Strauss, Volker; Margraf, Johannes T.; Dolle, Christian; Butz, Benjamin; Nacken, Thomas J.; Walter, Johannes; Bauer, Walter; Peukert, Wolfgang; Spiecker, Erdmann; Clark, Timothy; Guldi, Dirk M.
  Journal of the American Chemical Society (2014), 136 (49), 17308-17316CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The characterization is reported of C nanodots (NDs) synthesized under mild and controlled conditions, i.e., in a microwave reactor. The synthesized C NDs exhibit homogeneous and narrowly dispersed optical properties. They are suited as a testbed for studies of the photophysics of C-based nanoscopic emitters. In addn. to steady-state studies, time-correlated single-photon counting, fluorescence up-conversion, and transient pump probe absorption spectroscopy were used to elucidate the excited-state dynamics. Quenching the C ND-based emission with electron donors or acceptors helped shed light on the nature of individual states. D. functional theory and semiempirical CI calcns. on model systems helped understand the fundamental structure-property relations for this novel type of material.
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14. 14
  Pellegrin, Y.; Odobel, F. Sacrificial Electron Donor Reagents for Solar Fuel Production. C. R. Chim. 2017, 20, 283– 295, DOI: 10.1016/J.CRCI.2015.11.026
  14
  Sacrificial electron donor reagents for solar fuel production
  Pellegrin, Yann; Odobel, Fabrice
  Comptes Rendus Chimie (2017), 20 (3), 283-295CODEN: CRCOCR; ISSN:1631-0748. (Elsevier Masson SAS)
  A review is given. Although justly considered as a cumbersome component in artificial photosystems, these simple mols. are a necessary evil to drive photo-induced reactions aiming at producing high added value mols. by photo-induced redn. of low energy value substrates. This paper 1rst presents the specifications of sacrificial electron donors. Then the various families of sacrificial donors used from the early 1970s to nowadays are reviewed, such as aliph. and arom. amines, benzyl-dihydronicotinamide (BNAH), dimethylphenylbenzimidazoline (BIH), ascorbic acid, oxalate and finally thiols. Exptl. conditions (pH, solvent) are immensely versatile but important trends are given for adequate operation of a three-component system. Although literature abounds with various, very different artificial photosystems, we will realize that virtually the same sacrificial donors are used over and over again.
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15. 15
  Hutton, G. A. M.; Martindale, B. C. M.; Reisner, E. Carbon Dots as Photosensitisers for Solar-Driven Catalysis. Chem. Soc. Rev. 2017, 46, 6111– 6123, DOI: 10.1039/c7cs00235a
  15
  Carbon dots as photosensitisers for solar-driven catalysis
  Hutton, Georgina A. M.; Martindale, Benjamin C. M.; Reisner, Erwin
  Chemical Society Reviews (2017), 46 (20), 6111-6123CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
  A review. Artificial photosynthesis is the mimicry of the natural process of solar energy conversion into chem. energy carriers. Photocatalytic systems that combine light-harvesting materials and catalysts in soln. or suspension provide a promising route towards this goal. A key requirement for a sustainable solar fuel prodn. system is a low-cost, stable and non-toxic light harvester. Photoluminescent carbon nanoparticles, carbon dots (CDs), are promising emerging light-harvesters for photocatalytic fuel prodn. systems. CDs possess many desirable properties for this purpose, such as inexpensive, scalable synthetic routes, low-toxicity and tuneable surface chem. In this tutorial review, the integration of CDs in photocatalytic fuel generation systems with metallic, mol. and enzymic catalysts is discussed. An overview of CD types, synthesis and properties is given along with a discussion of tuneable CD properties that can be optimized for applications in photocatalysis. Current understanding of the photophys. electron transfer processes present in CD photocatalytic systems is outlined and various avenues for their further development are highlighted.
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16. 16
  Rosso, C.; Filippini, G.; Prato, M. Carbon Dots as Nano-Organocatalysts for Synthetic Applications. ACS Catal. 2020, 10, 8090– 8105, DOI: 10.1021/acscatal.0c01989
  16
  Carbon Dots as Nano-Organocatalysts for Synthetic Applications
  Rosso, Cristian; Filippini, Giacomo; Prato, Maurizio
  ACS Catalysis (2020), 10 (15), 8090-8105CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  A review. Over the last decades, org. chem. has taken a resolute step toward green catalytic synthesis. This tries to ensure efficient and sustainable base chem. prodn., while also safeguarding human health and the environment. To this end, the development of novel, nontoxic, and effective catalytic systems that are capable of driving value-added chem. transformations in environmentally benign solvents (e.g., water) is highly desirable. Moreover, these new catalysts need to be metal-free, easy-to-prep., and potentially recyclable. Carbon dots, which are relatively new carbon-based nanoparticles, fulfill all these requirements because of their outstanding physicochem. features and thus have emerged as promising nanocatalytic platforms. This Perspective highlights the recent advances in synthesis of carbon dots and their applications in org. catalysis and photocatalysis, with particular attention to green nonmetal-doped systems. Finally, forward-looking opportunities within this field are mentioned here.
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17. 17
  Cailotto, S.; Negrato, M.; Daniele, S.; Luque, R.; Selva, M.; Amadio, E.; Perosa, A. Carbon Dots as Photocatalysts for Organic Synthesis: Metal-Free Methylene–Oxygen-Bond Photocleavage. Green Chem. 2020, 22, 1145– 1149, DOI: 10.1039/C9GC03811F
  There is no corresponding record for this reference.
18. 18
  Han, Y.; Huang, H.; Zhang, H.; Liu, Y.; Han, X.; Liu, R.; Li, H.; Kang, Z. Carbon Quantum Dots with Photoenhanced Hydrogen-Bond Catalytic Activity in Aldol Condensations. ACS Catal. 2014, 4, 781– 787, DOI: 10.1021/cs401118x
  18
  Carbon quantum dots with photoenhanced hydrogen-bond catalytic activity in aldol condensations
  Han, Yuzhi; Huang, Hui; Zhang, Hengchao; Liu, Yang; Han, Xiao; Liu, Ruihua; Li, Haitao; Kang, Zhenhui
  ACS Catalysis (2014), 4 (3), 781-787CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  Carbon quantum dots (CQDs) were synthesized by an electrochem. etching method. The CQDs are well-dispersed with uniform size about 5 nm. FT-IR spectra suggest the presence of many hydroxyl groups on the surface of CQDs. Here, CQDs with diam. approx. 5 nm, directly used as effective heterogeneous nanocatalysts for H-bond catalysis in aldol condensations, show excellent photoenhanced catalytic ability (89% yields when 4-cyanobenzaldehyde is used). It demonstrated that aldol condensation between acetone and arom. aldehydes resulted in higher yields with visible light irradn. than in the dark, confirming visible light is necessary for good conversion. The H-bond catalytic activities of CQDs can be significantly enhanced with visible light irradn. The high catalytic activities of CQDs are due to highly efficient electron-accepting capabilities. Repeated catalytic expts. suggest that the CQD catalyst can be easily recycled as a heterogeneous catalyst with a long catalyst life.
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19. 19
  Li, H.; Sun, C.; Ali, M.; Zhou, F.; Zhang, X.; MacFarlane, D. R. Sulfated Carbon Quantum Dots as Efficient Visible-Light Switchable Acid Catalysts for Room-Temperature Ring-Opening Reactions. Angew. Chem. 2015, 127, 8540– 8544, DOI: 10.1002/ange.201501698
  There is no corresponding record for this reference.
20. 20
  Wirth, T. Microreactors in Organic Chemistry and Catalysis, 2nd ed.; Wiley: Weinheim, 2013.
  There is no corresponding record for this reference.
21. 21
  Liu, X.; Ünal, B.; Jensen, K. F. Heterogeneous Catalysis with Continuous Flow Microreactors. Catal. Sci. Technol. 2012, 2, 2134– 2138, DOI: 10.1039/c2cy20260c
  21
  Heterogeneous catalysis with continuous flow microreactors
  Liu, Xiaoying; Uenal, Baris; Jensen, Klavs F.
  Catalysis Science & Technology (2012), 2 (10), 2134-2138CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
  Packed-bed microreactors are employed under flow conditions for studies of heterogeneous catalysis: oxidn. of 4-isopropylbenzaldehyde and hydrogenation of 2-methylfuran. They have been demonstrated to be a valuable platform for rapid screening of catalytic materials, efficient optimization of reaction conditions, inline monitoring of reaction progress, and extn. of kinetic parameters.
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22. 22
  Schmidt, M.; Cubillas, A. M.; Taccardi, N.; Euser, T. G.; Cremer, T.; Maier, F.; Steinrück, H.-P.; Russell, P. S. J.; Wasserscheid, P.; Etzold, B. J. M. Chemical and (Photo)-Catalytical Transformations in Photonic Crystal Fibers. ChemCatChem 2013, 5, 641– 650, DOI: 10.1002/cctc.201200676
  22
  Chemical and (Photo)-Catalytical Transformations in Photonic Crystal Fibers
  Schmidt, Matthias; Cubillas, Ana M.; Taccardi, Nicola; Euser, Tijmen G.; Cremer, Till; Maier, Florian; Steinrueck, Hans-Peter; Russell, Philip St. J.; Wasserscheid, Peter; Etzold, Bastian J. M.
  ChemCatChem (2013), 5 (3), 641-650CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The concept of employing photonic crystal fibers for chem. and (photo)-catalytic transformations is presented. These optofluidic microdevices represent a versatile platform where light and fluids can interact for spectroscopic or photoactivation purposes. The use of photonic crystal fibers in chem. and sensing is reviewed and recent applications as catalytic microreactors are presented. Results on homogeneous catalysis and the immobilization of homogeneous and heterogeneous catalysts in the fiber channels are discussed. The examples demonstrate that combining catalysis and the excellent light guidance of photonic crystal fibers provides unique features for example, for photocatalytic activation and quant. photospectroscopic reaction anal.
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23. 23
  Cubillas, A. M.; Schmidt, M.; Euser, T. G.; Taccardi, N.; Unterkofler, S.; Russell, P. S. J.; Wasserscheid, P.; Etzold, B. J. M. In Situ Heterogeneous Catalysis Monitoring in a Hollow-Core Photonic Crystal Fiber Microflow Reactor. Adv. Mater. Interfaces 2014, 1, 1300093 DOI: 10.1002/admi.201300093
  There is no corresponding record for this reference.
24. 24
  McQuitty, R. J.; Unterkofler, S.; Euser, T. G.; Russell, P. St. J.; Sadler, P. J. Rapid Screening of Photoactivatable Metallodrugs: Photonic Crystal Fibre Microflow Reactor Coupled to ESI Mass Spectrometry. RSC Adv. 2017, 7, 37340– 37348, DOI: 10.1039/C7RA06735F
  24
  Rapid screening of photoactivatable metallodrugs: photonic crystal fibre microflow reactor coupled to ESI mass spectrometry
  McQuitty, Ruth J.; Unterkofler, Sarah; Euser, Tijmen G.; Russell, Philip St. J.; Sadler, Peter J.
  RSC Advances (2017), 7 (59), 37340-37348CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
  We explore the efficacy of a hyphenated photonic crystal fiber microflow reactor - high-resoln. mass spectrometer system as a method for screening the activity of potential new photoactivatable drugs. The use of light to activate drugs is an area of current development as it offers the possibility of reduced side effects due to improved spatial and temporal targeting and novel mechanisms of anticancer activity. The di-nuclear ruthenium complex [{(η6-indan)RuCl}2(μ-2,3-dpp)](PF6)2, previously studied by Magennis et al. (Inorg. Chem., 2007, 46, 5059) is used as a model drug to compare the system to std. irradn. techniques. The photodecompn. pathways using blue light radiation are the same for PCF and conventional cuvette methods. These findings are consistent with studies using conventional methods. The dinuclear complex also binds strongly to GSH after irradn., a possible explanation for its lack of potency in cell line testing. The use of the PCF-MS system dramatically reduced the sample vol. required and reduced the irradn. time by four orders of magnitude from 14 h to 12 s. However, the reduced sample vol. also results in a reduced MS signal intensity. The dead time of the combined system is 15 min, limited by the intrinsic dead vol. of the HR-MS.
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25. 25
  Gentleman, A. S.; Lawson, T.; Ellis, M. G.; Davis, M.; Turner-Dore, J.; Ryder, A. S. H.; Frosz, M. H.; Ciaccia, M.; Reisner, E.; Cresswell, A. J.; Euser, T. G. Stern–Volmer Analysis of Photocatalyst Fluorescence Quenching within Hollow-Core Photonic Crystal Fibre Microreactors. Chem. Commun. 2022, 58, 10548– 10551, DOI: 10.1039/D2CC03996F
  25
  Stern-Volmer analysis of photocatalyst fluorescence quenching within hollow-core photonic crystal fibre microreactors
  Gentleman, Alexander S.; Lawson, Takashi; Ellis, Matthew G.; Davis, Molly; Turner-Dore, Jacob; Ryder, Alison S. H.; Frosz, Michael H.; Ciaccia, Maria; Reisner, Erwin; Cresswell, Alexander J.; Euser, Tijmen G.
  Chemical Communications (Cambridge, United Kingdom) (2022), 58 (75), 10548-10551CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
  We report the use of optofluidic hollow-core photonic crystal fibers as microreactors for Stern-Volmer (SV) luminescence quenching anal. of visible-light photocatalytic reactions. This technol. enables measurements on nanolitre vols. and paves the way for automated SV analyses in continuous flow that minimise catalyst and reagent usage. The method is showcased using a recently developed photoredox-catalyzed α-C-H alkylation reaction of unprotected primary alkylamines.
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26. 26
  Williams, G. O. S.; Euser, T. G.; Russell, P. S. J.; MacRobert, A. J.; Jones, A. C. Highly Sensitive Luminescence Detection of Photosensitized Singlet Oxygen within Photonic Crystal Fibers. ChemPhotoChem 2018, 2, 616– 621, DOI: 10.1002/cptc.201800028
  26
  Highly Sensitive Luminescence Detection of Photosensitized Singlet Oxygen within Photonic Crystal Fibers
  Williams, Gareth O. S.; Euser, Tijmen G.; Russell, Philip St. J.; MacRobert, Alexander J.; Jones, Anita C.
  ChemPhotoChem (2018), 2 (7), 616-621CODEN: CHEMYH ISSN:. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Highly sensitive, quant. detection of singlet oxygen (1O2) is required for the evaluation of newly developed photosensitizers and the elucidation of the mechanisms of many processes in which singlet oxygen is known or believed to be involved. The direct detection of 1O2 through its intrinsic phosphorescence at 1270 nm is challenging, because of the extremely low intensity of this emission, coupled with the low quantum efficiency of currently available photodetectors at this wavelength. We introduce hollow-core photonic crystal fibers (HC-PCF) as a novel optofluidic modality for photosensitization and detection of 1O2. We report the use of this approach to achieve highly sensitive detection of the luminescence decay of 1O2 produced by using two common photosensitizers, Rose Bengal and Hypericin, within the 60-μm diam. core of a 15 cm length of HC-PCF. We demonstrate the feasibility of directly detecting sub-picomole quantities of 1O2 by using this methodol., and identify some aspects of the HC-PCF technol. that can be improved to yield even higher detection sensitivity.
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27. 27
  Schorn, F.; Aubermann, M.; Zeltner, R.; Haumann, M.; Joly, N. Y. Online Monitoring of Microscale Liquid-Phase Catalysis Using in-Fiber Raman Spectroscopy. ACS Catal. 2021, 11, 6709– 6714, DOI: 10.1021/acscatal.1c01264
  27
  Online Monitoring of Microscale Liquid-Phase Catalysis Using in-Fiber Raman Spectroscopy
  Schorn, Florian; Aubermann, Manfred; Zeltner, Richard; Haumann, Marco; Joly, Nicolas Y.
  ACS Catalysis (2021), 11 (11), 6709-6714CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  We report on the use of hollow-core photonic crystal fibers to monitor the evolution of chem. reactions. The combination of tight confinement and long interaction length allows single-pass spectroscopic measurements using less than a microliter vol. of chems. with good accuracy. As a proof of principle, we used here nonlinear Raman spectroscopy for a reaction screening of the acidic catalyzed esterification of methanol and acetic acid.
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28. 28
  Miele, E.; Dose, W. M.; Manyakin, I.; Frosz, M. H.; Ruff, Z.; De Volder, M. F. L.; Grey, C. P.; Baumberg, J. J.; Euser, T. G. Hollow-Core Optical Fibre Sensors for Operando Raman Spectroscopy Investigation of Li-Ion Battery Liquid Electrolytes. Nat. Commun. 2022, 13, 1651 DOI: 10.1038/s41467-022-29330-4
  28
  Hollow-core optical fibre sensors for operando Raman spectroscopy investigation of Li-ion battery liquid electrolytes
  Miele, Ermanno; Dose, Wesley M.; Manyakin, Ilya; Frosz, Michael H.; Ruff, Zachary; De Volder, Michael F. L.; Grey, Clare P.; Baumberg, Jeremy J.; Euser, Tijmen G.
  Nature Communications (2022), 13 (1), 1651CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
  Improved anal. tools are urgently required to identify degrdn. and failure mechanisms in Li-ion batteries. However, understanding and ultimately avoiding these detrimental mechanisms requires continuous tracking of complex electrochem. processes in different battery components. Here, we report an operando spectroscopy method that enables monitoring the chem. of a carbonate-based liq. electrolyte during electrochem. cycling in Li-ion batteries with a graphite anode and a LiNi0.8Mn0.1Co0.1O2 cathode. By embedding a hollow-core optical fiber probe inside a lab-scale pouch cell, we demonstrate the effective evolution of the liq. electrolyte species by background-free Raman spectroscopy. The anal. of the spectroscopy measurements reveals changes in the ratio of carbonate solvents and electrolyte additives as a function of the cell voltage and show the potential to track the lithium-ion solvation dynamics. The proposed operando methodol. contributes to understanding better the current Li-ion battery limitations and paves the way for studies of the degrdn. mechanisms in different electrochem. energy storage systems.
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29. 29
  Lawson, T.; Gentleman, A. S.; Pinnell, J.; Eisenschmidt, A.; Antón-García, D.; Frosz, M. H.; Reisner, E.; Euser, T. G. In Situ Detection of Cobaloxime Intermediates During Photocatalysis Using Hollow-Core Photonic Crystal Fiber Microreactors. Angew. Chem., Int. Ed. 2023, 62, e202214788 DOI: 10.1002/anie.202214788
  29
  In situ Detection of Cobaloxime Intermediates During Photocatalysis Using Hollow-Core Photonic Crystal Fiber Microreactors
  Lawson, Takashi; Gentleman, Alexander S.; Pinnell, Jonathan; Eisenschmidt, Annika; Anton-Garcia, Daniel; Frosz, Michael H.; Reisner, Erwin; Euser, Tijmen G.
  Angewandte Chemie, International Edition (2023), 62 (9), e202214788CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Hollow-core photonic crystal fibers (HC-PCFs) provide a novel approach for in situ UV/Vis spectroscopy with enhanced detection sensitivity. Here, we demonstrate that longer optical path lengths than afforded by conventional cuvette-based UV/Vis spectroscopy can be used to detect and identify the CoI and CoII states in hydrogen-evolving cobaloxime catalysts, with spectral identification aided by comparison with DFT-simulated spectra. Our findings show that there are two types of signals obsd. for these mol. catalysts; a transient signal and a steady-state signal, with the former being assigned to the CoI state and the latter being assigned to the CoII state. These observations lend support to a unimol. pathway, rather than a bimol. pathway, for hydrogen evolution. This study highlights the utility of fiber-based microreactors for understanding these and a much wider range of homogeneous photocatalytic systems in the future.
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30. 30
  Unterkofler, S.; McQuitty, R. J.; Euser, T. G.; Farrer, N. J.; Sadler, P. J.; Russell, P. S. J. Microfluidic Integration of Photonic Crystal Fibers for Online Photochemical Reaction Analysis. Opt. Lett. 2012, 37, 1952– 1954, DOI: 10.1364/OL.37.001952
  30
  Microfluidic integration of photonic crystal fibers for online photochemical reaction analysis
  Unterkofler, S.; McQuitty, R. J.; Euser, T. G.; Farrer, N. J.; Sadler, P. J.; Russell, P. St. J.
  Optics Letters (2012), 37 (11), 1952-1954CODEN: OPLEDP; ISSN:0146-9592. (Optical Society of America)
  Liq.-filled hollow-core photonic crystal fibers (HC-PCFs) are perfect optofluidic channels, uniquely providing low-loss optical guidance in a liq. medium. As a result, the overlap of the dissolved specimen and the intense light field in the micron sized core is increased manyfold compared to conventional bioanal. techniques, facilitating highly-efficient photoactivation processes. Here we introduce a novel integrated anal. technol. for photochem. by microfluidic coupling of a HC-PCF nanoflow reactor to supplementary detection devices. Applying a continuous flow through the fiber, we deliver photochem. reaction products to a mass spectrometer in an online and hence rapid fashion, which is highly advantageous over conventional cuvette-based approaches.
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31. 31
  Koehler, P.; Lawson, T.; Neises, J.; Willkomm, J.; Martindale, B. C. M.; Hutton, G. A. M.; Antón-García, D.; Lage, A.; Gentleman, A. S.; Frosz, M. H.; Russell, P. S. J.; Reisner, E.; Euser, T. G. Optofluidic Photonic Crystal Fiber Microreactors for In Situ Studies of Carbon Nanodot-Driven Photoreduction. Anal. Chem. 2021, 93, 895– 901, DOI: 10.1021/acs.analchem.0c03546
  31
  Optofluidic photonic crystal fiber microreactors for in situ studies of carbon nanodot-driven photoreduction
  Koehler, Philipp; Lawson, Takashi; Neises, Julian; Willkomm, Janina; Martindale, Benjamin C. M.; Hutton, Georgina A. M.; Anton-Garcia, Daniel; Lage, Ava; Gentleman, Alexander S.; Frosz, Michael H.; Russell, Philip St. J.; Reisner, Erwin; Euser, Tijmen G.
  Analytical Chemistry (Washington, DC, United States) (2021), 93 (2), 895-901CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
  Performing quant. in situ spectroscopic anal. on minuscule sample vols. is a common difficulty in photochem. To address this challenge, we use a hollow-core photonic crystal fiber (HC-PCF) that guides light at the center of a microscale liq. channel and acts as an optofluidic microreactor with a reaction vol. of less than 35 nL. The system was used to demonstrate in situ optical detection of photoredn. processes that are key components of many photocatalytic reaction schemes. The photoredn. of viologens (XV2+) to the radical XV•+ in a homogeneous mixt. with carbon nanodot (CND) light absorbers is studied for a range of different carbon dots and viologens. Time-resolved absorption spectra, measured over several UV irradn. cycles, are interpreted with a quant. kinetic model to det. photoredn. and photobleaching rate consts. The powerful combination of time-resolved, low-vol. absorption spectroscopy and kinetic modeling highlights the potential of optofluidic microreactors as a highly sensitive, quant., and rapid screening platform for novel photocatalysts and flow chem. in general.
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32. 32
  Groeneveld, I.; Schoemaker, S. E.; Somsen, G. W.; Ariese, F.; van Bommel, M. R. Characterization of a Liquid-Core Waveguide Cell for Studying the Chemistry of Light-Induced Degradation. Analyst 2021, 146, 3197– 3207, DOI: 10.1039/D1AN00272D
  32
  Liquid core waveguide cell chemistry light induced degradation of organic dye
  Groeneveld, Iris; Schoemaker, Suzan E.; Somsen, Govert W.; Ariese, Freek; van Bommel, Maarten R.
  Analyst (Cambridge, United Kingdom) (2021), 146 (10), 3197-3207CODEN: ANALAO; ISSN:0003-2654. (Royal Society of Chemistry)
  Many org. compds. undergo changes under the influence of light. This might be beneficial in, for example, water purifn., but undesirable when cultural-heritage objects fade or when food ingredients (e.g., vitamins) degrade. It is often challenging to establish a strong link between photodegrdn. products and their parent mols. due to the complexity of the sample. To allow effective study of light-induced degrdn. (LID), a low-vol. exposure cell was created in which solutes are efficiently illuminated (esp. at low concns.) while simultaneously analyzed by absorbance spectroscopy. The new LID cell encompasses a gas-permeable liq.-core waveguide (LCW) connected to a spectrograph allowing collection of spectral data in real-time. The aim of the current study was to evaluate the overall performance of the LID cell by assessing its transmission characteristics, the abs. photon flux achieved in the LCW, and its capacity to study solute degrdn. in presence of oxygen. The potential of the LID set-up for light-exposure studies was successfully demonstrated by monitoring the degrdn. of the dyes eosin Y and crystal violet.
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33. 33
  Groeneveld, I.; Bagdonaite, I.; Beekwilder, E.; Ariese, F.; Somsen, G. W.; van Bommel, M. R. Liquid Core Waveguide Cell with In Situ Absorbance Spectroscopy and Coupled to Liquid Chromatography for Studying Light-Induced Degradation. Anal. Chem. 2022, 94, 7647– 7654, DOI: 10.1021/acs.analchem.2c00886
  33
  Liquid Core Waveguide Cell with In Situ Absorbance Spectroscopy and Coupled to Liquid Chromatography for Studying Light-Induced Degradation
  Groeneveld, Iris; Bagdonaite, Ingrida; Beekwilder, Edwin; Ariese, Freek; Somsen, Govert W.; van Bommel, Maarten R.
  Analytical Chemistry (Washington, DC, United States) (2022), 94 (21), 7647-7654CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
  In many areas, studying photostability or the mechanism of photodegrdn. is of high importance. Conventional methods to do so can be rather time-consuming, laborious, and prone to exptl. errors. In this paper we evaluate an integrated and fully automated system for the study of light-induced degrdn., comprising a liq. handler, an irradn. source and exposure cell with dedicated optics and spectrograph, and a liq. chromatog. (LC) system. A liq. core waveguide (LCW) was used as an exposure cell, allowing efficient illumination of the sample over a 12 cm path length. This cell was coupled to a spectrograph, allowing in situ absorbance monitoring of the exposed sample during irradn. The LCW is gas-permeable, permitting diffusion of air into the cell during light exposure. This unit was coupled online to LC with diode array detection for immediate and automated anal. of the compn. of the light-exposed samples. The anal. performance of the new system was established by assessing linearity, limit of detection, and repeatability of the in-cell detection, sample recovery and carryover, and overall repeatability of light-induced degrdn. monitoring, using riboflavin as the test compd. The applicability of the system was demonstrated by recording a photodegrdn. time profile of riboflavin.
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34. 34
  Islam, M. S.; Cordeiro, C. M. B.; Franco, M. A. R.; Sultana, J.; Cruz, A. L. S.; Abbott, D. Terahertz Optical Fibers. Opt. Express 2020, 28, 16089– 16117, DOI: 10.1364/OE.389999
  34
  Terahertz optical fibers [Invited]
  Islam, Md. Saiful; Cordeiro, Cristiano M. B.; Franco, Marcos A. R.; Sultana, Jakeya; Cruz, Alice L. S.; Abbott, Derek
  Optics Express (2020), 28 (11), 16089-16117CODEN: OPEXFF; ISSN:1094-4087. (Optical Society of America)
  A review. Lying between optical and microwave ranges, the terahertz band in the electromagnetic spectrum is attracting increased attention. Optical fibers are essential for developing the full potential of complex terahertz systems. In this manuscript, we review the optimal materials, the guiding mechanisms, the fabrication methodologies, the characterization methods and the applications of such terahertz waveguides. We examine various optical fiber types including tube fibers, solid core fiber, hollow-core photonic bandgap, anti-resonant fibers, porous-core fibers, metamaterial-based fibers, and their guiding mechanisms. The optimal materials for terahertz applications are discussed. The past and present trends of fabrication methods, including drilling, stacking, extrusion and 3D printing, are elaborated. Fiber characterization methods including different optics for terahertz time-domain spectroscopy (THz-TDS) setups are reviewed and application areas including short-distance data transmission, imaging, sensing, and spectroscopy are discussed.
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35. 35
  Testa, C.; Zammataro, A.; Pappalardo, A.; Sfrazzetto, G. T. Catalysis with Carbon Nanoparticles. RSC Adv. 2019, 9, 27659– 27664, DOI: 10.1039/C9RA05689K
  35
  Catalysis with carbon nanoparticles
  Testa, Caterina; Zammataro, Agatino; Pappalardo, Andrea; Trusso Sfrazzetto, Giuseppe
  RSC Advances (2019), 9 (47), 27659-27664CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
  A review. Carbon nanoparticles (CNPs) represent a recent class of nanomaterials, based on carbon sp2 atoms in the inner core. These new nano-dots cover a wide range of application fields: anal., sensing and biosensing, bioimaging, theranostic, and mol. communication. However, their use as nanocatalysts is relatively new. Although CNPs can be easily synthesized and obtained in good amts., few reports on their catalytic applications have been reported. This minireview collects the use of these nanoparticles as catalysts highlighting the improvements with respect to the classic catalytic systems. In particular, due to their unique optical and elec. properties, and due to the possibility to cover the external shell with a wide variety of functional groups, CNPs have found catalytic applications in three main classes of reactions: (i) photocatalysis, (ii) acid-base catalysis and (iii) electro catalysis.
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36. 36
  Mandal, K.; Hoffman, M. Z. Quantum Yield of Formation of Methylviologen Radical Cation in the Photolysis of the Ru(bpy)₃²⁺/Methylviologen/EDTA System. J. Phys. Chem. A 1984, 88, 5632– 5639, DOI: 10.1021/j150667a035
  There is no corresponding record for this reference.
37. 37
  Suzuki, M.; Morris, N. D.; Mallouk, T. E. Photosensitized Production of Doubly Reduced Methylviologen Followed by Highly Efficient Methylviologen Radical Formation Using Self-Assembling Ruthenium(II) Complexes. Chem. Commun. 2002, 14, 1534– 1535, DOI: 10.1039/b205076p
  There is no corresponding record for this reference.
38. 38
  Kim, J. Y.; Lee, C.; Park, J. W. The Kinetics of Neutral Methyl Viologen in Acidic H2O+DMF Mixed Solutions Studied by Cyclic Voltammetry. J. Electroanal. Chem. 2001, 504, 104– 110, DOI: 10.1016/S0022-0728(01)00417-X
  38
  The kinetics of neutral methyl viologen in acidic H2O+DMF mixed solutions studied by cyclic voltammetry
  Kim, J. Y.; Lee, C.; Park, J. W.
  Journal of Electroanalytical Chemistry (2001), 504 (1), 104-110CODEN: JECHES ISSN:. (Elsevier Science S.A.)
  The chem. of the two-electron redn. product of viologen (1,1'-dialkyl-4,4'-bipyridinium, V2+) neutral species, is important in understanding the electrochem. behavior of viologens and their use. The kinetics for the reactions of neutral Me viologen (V0) in the presence of H+ (from HCl), CH3COOH (pKa = 4.75), ClCH2CH2COOH (pKa = 4.00), HCOOH (pKa=3.75) in aq. media was examd. by cyclic voltammetry according to the EECi mechanism. To avoid the electrodeposition of V0, the authors used a 9:1 (vol./vol.%) H2O+DMF mixt. as the solvent medium. To evaluate the rate consts. for the chem. reaction followed by the 2nd electron transfer step of V2+, the ratio of the anodic and cathodic peak current (Ipa2/Ipc2) corresponding to V0 - e- .dblharw. V•+ was plotted against log τ, where τ is the time between E1/2 and the switching potential, at various scan rates of 0.02-3.5 V s-1. The chem. reaction is a parallel reaction consisting of H+-catalyzed and general-acid (HA) catalyzed reactions. The 2nd-order rate consts. are detd. as kH+ = 3.5 × 103 M-1 s-1, kCH3COOH = 5.7 M-1 s-1, kHCOOH = 4.6 × 101 M-1 s-1, and kClCH2CH2COOH = 3.2 × 101 M-1 s-1 using the Nicholson-Shain method and kH2O was estd. as <3 × 10-6 M-1 s-1. The CVs were digitally simulated under the assumption of a two-step reaction of V0 following the two-step electrode reactions of V2+ to V0. The simulated CVs show good agreement with those obtained exptl., when the 1st-step reaction of V0 is a relatively fast reversible reaction and the 2nd-step reaction is a slow irreversible one. Based on these results, probably V0 is in pseudo-equil. with H+ or HA to produce VH+ which undergoes a reaction with H2O.
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39. 39
  Couny, F.; Benabid, F.; Roberts, P. J.; Light, P. S.; Raymer, M. G. Generation and Photonic Guidance of Multi-Octave Optical-Frequency Combs. Science 2007, 318, 1118– 1121, DOI: 10.1126/science.1149091
  39
  Generation and Photonic Guidance of Multi-Octave Optical-Frequency Combs
  Couny, F.; Benabid, F.; Roberts, P. J.; Light, P. S.; Raymer, M. G.
  Science (Washington, DC, United States) (2007), 318 (5853), 1118-1121CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
  Ultrabroad coherent comb-like optical spectra spanning several octaves are a chief ingredient in the emerging field of attoscience. We demonstrate generation and guidance of a three-octave spectral comb, spanning wavelengths from 325 to 2300 nm, in a hydrogen-filled hollow-core photonic crystal fiber. The waveguidance results not from photonic a band gap but from the inhibited coupling between the core and cladding modes. The spectrum consists of up to 45 high-order Stokes and anti-Stokes lines and is generated by driving the confined gas with a single, moderately powerful (10-kW) IR laser, producing 12-ns-duration pulses. This represents a redn. by six orders of magnitude in the required laser powers over previous equiv. techniques and opens up a robust and much simplified route to synthesizing attosecond pulses.
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40. 40
  Archambault, J.-L.; Black, R. J.; Lacroix, S.; Bures, J. Loss Calculations for Antiresonant Waveguides. J. Lightwave Technol. 1993, 11, 416– 423, DOI: 10.1109/50.219574
  There is no corresponding record for this reference.
41. 41
  Cubillas, A. M.; Unterkofler, S.; Euser, T. G.; Etzold, B. J. M.; Jones, A. C.; Sadler, P. J.; Wasserscheid, P.; Russell, P. S. J. Photonic Crystal Fibres for Chemical Sensing and Photochemistry. Chem. Soc. Rev. 2013, 42, 8629– 8648, DOI: 10.1039/c3cs60128e
  41
  Photonic crystal fibers for chemical sensing and photochemistry
  Cubillas, Ana M.; Unterkofler, Sarah; Euser, Tijmen G.; Etzold, Bastian J. M.; Jones, Anita C.; Sadler, Peter J.; Wasserscheid, Peter; Russell, Philip St. J.
  Chemical Society Reviews (2013), 42 (22), 8629-8648CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
  In this review, photonic crystal fiber is introduced as a novel optofluidic microdevice that can be employed as both a versatile chem. sensor and a highly efficient microreactor. It provides an excellent platform in which light and chem. samples can strongly interact for quant. spectroscopic anal. or photoactivation purposes. The use of photonic crystal fiber in photochem. and sensing is discussed and recent results on gas and liq. sensing as well as on photochem. and catalytic reactions are reviewed. These developments demonstrate that the tight light confinement, enhanced light-matter interaction and reduced sample vol. offered by photonic crystal fiber make it useful in a wide range of chem. applications.
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42. 42
  Hutton, G. A. M.; Reuillard, B.; Martindale, B. C. M.; Caputo, C. A.; Lockwood, C. W. J.; Butt, J. N.; Reisner, E. Carbon Dots as Versatile Photosensitizers for Solar-Driven Catalysis with Redox Enzymes. J. Am. Chem. Soc. 2016, 138, 16722– 16730, DOI: 10.1021/jacs.6b10146
  42
  Carbon Dots as Versatile Photosensitizers for Solar-Driven Catalysis with Redox Enzymes
  Hutton, Georgina A. M.; Reuillard, Bertrand; Martindale, Benjamin C. M.; Caputo, Christine A.; Lockwood, Colin W. J.; Butt, Julea N.; Reisner, Erwin
  Journal of the American Chemical Society (2016), 138 (51), 16722-16730CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Light-driven enzymic catalysis is enabled by the productive coupling of a protein to a photosensitizer. Photosensitizers used in such hybrid systems are typically costly, toxic, and/or fragile, with limited chem. versatility. Carbon dots (CDs) are low-cost, nanosized light-harvesters that are attractive photosensitizers for biol. systems as they are water-sol., photostable, nontoxic, and their surface chem. can be easily modified. We demonstrate here that CDs act as excellent light-absorbers in two semibiol. photosynthetic systems utilizing either a fumarate reductase (FccA) for the solar-driven hydrogenation of fumarate to succinate or a hydrogenase (H2ase) for redn. of protons to H2. The tunable surface chem. of the CDs was exploited to synthesize pos. charged ammonium-terminated CDs (CD-NHMe2+), which were capable of transferring photoexcited electrons directly to the neg. charged enzymes with high efficiency and stability. Enzyme-based turnover nos. of 6000 mol succinate (mol FccA)-1 and 43,000 mol H2 (mol H2ase)-1 were reached after 24 h. Neg. charged carboxylate-terminated CDs (CD-CO2-) displayed little or no activity, and the electrostatic interactions at the CD-enzyme interface were detd. to be essential to the high photocatalytic activity obsd. with CD-NHMe2+. The modular surface chem. of CDs together with their photostability and aq. soly. make CDs versatile photosensitizers for redox enzymes with great scope for their utilization in photobiocatalysis.
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43. 43
  Badiani, V. M.; Casadevall, C.; Miller, M.; Cobb, S. J.; Manuel, R. R.; Pereira, I. A. C.; Reisner, E. Engineering Electro- and Photocatalytic Carbon Materials for CO₂ Reduction by Formate Dehydrogenase. J. Am. Chem. Soc. 2022, 144, 14207– 14216, DOI: 10.1021/jacs.2c04529
  43
  Engineering Electro- and Photocatalytic Carbon Materials for CO2 Reduction by Formate Dehydrogenase
  Badiani, Vivek M.; Casadevall, Carla; Miller, Melanie; Cobb, Samuel J.; Manuel, Rita R.; Pereira, Ines A. C.; Reisner, Erwin
  Journal of the American Chemical Society (2022), 144 (31), 14207-14216CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Semi-artificial approaches to renewable fuel synthesis exploit the integration of enzymes with synthetic materials for kinetically efficient fuel prodn. Here, a CO2 reductase, formate dehydrogenase (FDH) from Desulfovibrio vulgaris Hildenborough, is interfaced with C nanotubes (CNTs) and amorphous C dots (a-CDs). Each C substrate, tailored for electro- and photocatalysis, is functionalized with pos. (-NHMe2+) and neg. (-COO-) chem. surface groups to understand and optimize the electrostatic effect of protein assocn. and orientation on CO2 redn. Immobilization of FDH on pos. charged CNT electrodes results in efficient and reversible electrochem. CO2 redn. via direct electron transfer with >90% faradaic efficiency and -250μA cm-2 at -0.6 V vs. SHE (pH 6.7 and 25°) for formate prodn. In contrast, neg. charged CNTs only result in marginal currents with immobilized FDH. Quartz crystal microbalance anal. and attenuated total reflection IR spectroscopy confirm the high binding affinity of active FDH to CNTs. FDH has subsequently been coupled to a-CDs, where the benefits of the pos. charge (-NHMe2+-terminated a-CDs) were translated to a functional CD-FDH hybrid photocatalyst. High rates of photocatalytic CO2 redn. (turnover frequency: 3.5 × 103 h-1; AM 1.5 G) with DL-dithiothreitol as the sacrificial electron donor were obtained after 6 h, providing benchmark rates for homogeneous photocatalytic CO2 redn. with metal-free light absorbers. This work provides a rational basis to understand interfacial surface/enzyme interactions at electrodes and photosensitizers to guide improvements with catalytic biohybrid materials.
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44. 44
  Miller, D.; McLendon, G. Model Systems for Photocatalytic Water Reduction: Role of pH and Metal Colloid Catalysts. Inorg. Chem. 1981, 20, 950– 953, DOI: 10.1021/ic50217a065
  44
  Model systems for photocatalytic water reduction: role of pH and metal colloid catalysts
  Miller, Deborah; McLendon, George
  Inorganic Chemistry (1981), 20 (3), 950-3CODEN: INOCAJ; ISSN:0020-1669.
  H quantum yields were measured in solns. contg. a phosphate or borate buffer, tris(2,2'-bipyridine)ruthenium(2+) [15158-62-0] 2.0 × 10-4, Na2EDTA 2.0 × 10-2, an electron-transfer mediator such as paraquat [4685-14-7] 2.0 × 10-3 M, and poly(vinyl alc.)-stabilized Pt catalyst. The roles of pH, mediator potential, and EDTA [60-00-4] oxidn. rate in these H2O redn. systems are reported.
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45. 45
  Mulazzani, Q. G.; Venturi, M.; Hoffman, M. Z. Radiolytically Induced One-Electron Reduction of Methylviologen in Aqueous Solution. Reactivity of EDTA Radicals toward Methylviologen. J. Phys. Chem. B 1985, 89, 722– 728, DOI: 10.1021/j100250a032
  There is no corresponding record for this reference.
46. 46
  Siddique, A. B.; Pramanick, A. K.; Chatterjee, S.; Ray, M. Amorphous Carbon Dots and Their Remarkable Ability to Detect 2,4,6-Trinitrophenol. Sci. Rep. 2018, 8, 9770 DOI: 10.1038/s41598-018-28021-9
  46
  Amorphous Carbon Dots and their Remarkable Ability to Detect 2,4,6-Trinitrophenol
  Siddique Abu Bakar; Chatterjee Subrata; Ray Mallar; Pramanick Ashit Kumar
  Scientific reports (2018), 8 (1), 9770 ISSN:.
  Apparently mundane, amorphous nanostructures of carbon have optical properties which are as exotic as their crystalline counterparts. In this work we demonstrate a simple and inexpensive mechano-chemical method to prepare bulk quantities of self-passivated, amorphous carbon dots. Like the graphene quantum dots, the water soluble, amorphous carbon dots too, exhibit excitation-dependent photoluminescence with very high quantum yield (~40%). The origin and nature of luminescence in these high entropy nanostructures are well understood in terms of the abundant surface traps. The photoluminescence property of these carbon dots is exploited to detect trace amounts of the nitro-aromatic explosive - 2,4,6-trinitrophenol (TNP). The benign nanostructures can selectively detect TNP over a wide range of concentrations (0.5 to 200 μM) simply by visual inspection, with a detection limit of 0.2 μM, and consequently outperform nearly all reported TNP sensor materials.
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47. 47
  Bird, C. L.; Kuhn, A. T. Electrochemistry of the Viologens. Chem. Soc. Rev. 1981, 10, 49– 82, DOI: 10.1039/cs9811000049
  47
  Electrochemistry of the viologens
  Bird, C. L.; Kuhn, A. T.
  Chemical Society Reviews (1981), 10 (1), 49-82CODEN: CSRVBR; ISSN:0306-0012.
  Half-wave polarog. redn. potentials of 66 viologens are reported (detd. by B. G. White et al.). The electrochem. of the viologens is discussed fully.
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48. 48
  Martindale, B. C. M.; Joliat, E.; Bachmann, C.; Alberto, R.; Reisner, E. Clean Donor Oxidation Enhances the H 2 Evolution Activity of a Carbon Quantum Dot–Molecular Catalyst Photosystem. Angew. Chem., Int. Ed. 2016, 55, 9402– 9406, DOI: 10.1002/anie.201604355
  48
  Clean Donor Oxidation Enhances H2 Evolution Activity of a Carbon Quantum Dot-Molecular Catalyst Photosystem
  Martindale, Benjamin C. M.; Joliat, Evelyne; Bachmann, Cyril; Alberto, Roger; Reisner, Erwin
  Angewandte Chemie, International Edition (2016), 55 (32), 9402-9406CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Carbon quantum dots (CQDs) are new-generation light absorbers for photocatalytic H2 evolution in aq. soln., but the performance of CQD-mol. catalyst systems is currently limited by the decompn. of the mol. component. Clean oxidn. of the electron donor by donor recycling prevents the formation of destructive radical species and non-innocent oxidn. products. This approach allowed a CQD-mol. nickel bis(diphosphine) photocatalyst system to reach a benchmark lifetime of more than 5 days and a record turnover no. of 1094±61 molH2 (molNi)-1 for a defined synthetic mol. nickel catalyst in purely aq. soln. under AM1.5G solar irradn.
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49. 49
  Ren, J.; Achilleos, D. S.; Golnak, R.; Yuzawa, H.; Xiao, J.; Nagasaka, M.; Reisner, E.; Petit, T. Uncovering the Charge Transfer between Carbon Dots and Water by In Situ Soft X-Ray Absorption Spectroscopy. J. Phys. Chem. Lett. 2019, 10, 3843– 3848, DOI: 10.1021/acs.jpclett.9b01800
  49
  Uncovering the Charge Transfer between Carbon Dots and Water by In Situ Soft X-ray Absorption Spectroscopy
  Ren, Jian; Achilleos, Demetra S.; Golnak, Ronny; Yuzawa, Hayato; Xiao, Jie; Nagasaka, Masanari; Reisner, Erwin; Petit, Tristan
  Journal of Physical Chemistry Letters (2019), 10 (14), 3843-3848CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  C dots (CDs) exhibit outstanding physicochem. properties that render them excellent materials for various applications, often occurring in an aq. environment, such as light harvesting and fluorescence bioimaging. Here the authors characterize the electronic structures of CDs and H2O mols. in aq. dispersions using in situ x-ray absorption spectroscopy. Three types of CDs with different core structures (amorphous vs. graphitic) and compns. (undoped vs. N-doped) were studied. Depending on the CD core structure, different ionic currents generated upon x-ray irradn. of the CD dispersions at the C K-edge were detected, which are interpreted in terms of different charge transfer to the surrounding solvent mols. The H bonding networks of H2O mols. upon interaction with the different CDs were also probed at the O K-edge. Both core graphitization and N doping endow the CDs with enhanced electron transfer and H bonding capabilities with the surrounding H2O mols.
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50. 50
  Lawson, T.; Gentleman, A. S.; Lage, A.; Casadevall, C.; Xiao, J.; Petit, T.; Frosz, M. H.; Reisner, E.; Euser, T. G. Research Data Supporting “Low-Volume Reaction Monitoring of Carbon Dot Light Absorbers in Optofluidic Microreactors”; Apollo Repository. University of Cambridge, 2023, DOI: 10.17863/CAM.97100 .
  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/acscatal.3c02212.
- Data analysis methodology; optical setup diagrams; HC-PCF transmission characterization; cuvette-based measurements; concentration profiles with different SEDs and aCD surface moieties; and XAS data (PDF)
- cs3c02212_si_001.pdf (2.03 MB)
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Introduction

Figure 1

Carbon Dot Screening Scheme

Experimental Section

Hollow-Core Photonic Crystal Fiber (HC-PCF)

UV–Vis Absorption Spectroscopy Setup

X-ray Absorption Spectroscopy (XAS)

Materials

Synthesis of CDs

Results and Discussion

Effect of aCD Concentration

Figure 2

Effect of MV Concentration

Figure 3

Effect of EDTA Concentration

Figure 4

Effect of Irradiance

Figure 5

Comparing Carbon Dots, Sacrificial Electron Donors, and Different Viologens

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Benchmarking

Effect of CD Surface

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Conclusions

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Low-Volume Reaction Monitoring of Carbon Dot Light Absorbers in Optofluidic Microreactors
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