PNIPAM Brushes in Colloidal Photonic Crystals Enable Ex Situ Ethanol Vapor Sensing

Structural colors are formed by the periodic repetition of nanostructures in a material. Upon reversibly tuning the size or optical properties of the repetitive unit inside a nanostructured material, responsive materials can be made that change color due to external stimuli. This paper presents a simple method to obtain films of ethanol vapor-responsive structural colors based on stacked poly(N-isopropylacrylamide) (PNIPAM)-grafted silica nanoparticles. Our materials show clear, reversible color transitions in the presence of near-saturated ethanol vapor. Moreover, due to the absorption of ethanol in the PNIPAM brushes, relatively long recovery times are observed (∼30 s). Materials based on bare or poly(methyl methacrylate) (PMMA) brush-grafted silica nanoparticles also change color in the presence of ethanol vapor but possess significantly shorter recovery times (∼1 s). Atomic force microscopy reveals that the delayed recovery originates from the ability of PNIPAM brushes to swell in ethanol vapor. This renders the films highly suitable for ex situ ethanol vapor sensing.

Table S1 Determination of the dry polymer thickness by SEM and TEM, using ImageJ image analysis software.The method, sample type, SI ATRP reaction time (t SI-ATRP ) and dry brush height (h dry ) are denoted for five different samples.The values of h dry were obtained by averaging > 15 measurements per at least 2 SEM or TEM images.Figure S3 FTIR spectra of the PMMA-grafted nanoparticles (left) and the PNIPAM-grafted nanoparticles (right).

Characterization type Sample t SI-ATRP (h) h dry (nm
The bare and BiBB-functionalized nanoparticles are shown in grey and black, respectively, to mark differences between the FTIR peaks.Relevant peaks at 1350-1750 cm -1 are added in a separate zoom-in.

Ethanol vapor-responsive characteristics
The swelling characteristics of PNIPAM-g-SiNPs and PMMA-g-SiNPs in ethanol vapor were determined via DLS/SEM and AFM.In addition, ellipsometry measurements were performed on PNIPAM and PMMA brushes grafted onto SiO 2 wafers.From the DLS/SEM and ellipsometry results, the swelling ratio α could be calculated via the following equation: with h swell being the swollen height of a polymer brush in nm, and h dry being the corresponding dry height in nm.In the DLS/SEM methodology h swell is extracted from DLS data, whereby the nanoparticles are being dissolved in a solvent with a high affinity to the polymer brush in order to avoid aggregation.The h dry value is extracted from SEM, as was previously shown in Table S2.
The following section contains an overview of the swelling characteristics of PNIPAM and PMMA brushes (Table S2), AFM images from the structurally colored material studies in ethanol vapor (Figure S4), a comparison study between the Bragg-Snell's theorem and observed coloration (Figure S5), a parameter influence study for the prediction of the reflected wavelength (Table S3), a qualitative material study of the sensitivity towards ethanol vapor (Figure S6), a data overview of the long-term optical stability (Table S4) and ellipsometry data obtained during a swelling experiment in ethanol vapor (Figure S7).Table S3 Parameter influence study of colloidal spacing D and effective refractive index n eff on the calculation of the reflected wavelength λ calc via Bragg-Snell's law.The pre-determined variable is θ 0 = 0 o .For the comparison study between Bragg-Snell law's theoretical predictions and our experimental findings, we estimate the effective refractive index n eff .Our structural colors consist of three different materials for vapor sensing, which are Stöber silica (nanoparticles), PNIPAM (brushes) and air.The bulk refractive indices are 1.475, 1 1.5031 2 and 1.0003, 3 respectively.A modified Lorentz-Lorentz relation is used to estimate the value of n eff :

D (nm
where f denotes the respective volume fractions of air, SiO 2 and PNIPAM.In our calculations for the effective refractive index, we assume a FCC colloidal packing.Our assumption is supported by the AFM images in Figure S4, which show a tight packing of the core-shell nanoparticles.For a FCC packing, the volume fraction f air is equal to 0.26. 4 The core-shell particles, consisting of a SiO 2 core and PNIPAM shell, are thus assumed to take up a total of 74% of the material's volume.The corresponding volume fractions for SiO 2 and PNIPAM are predicted on the basis of Table S1 and the following formula: Table S4 Optical stability of a PNIPAM-g-SiNP structural color over time.The material properties, including the delay time, relaxation time, reflection peak in air (λ max,air ) and reflection peak in saturated ethanol vapor (λ max,EtOH ) were measured fresh (t = 0 days) and after a period of ∼8 months (t = 250 days).The thickness of the brush is determined via a Cauchy optical model and the swelling ratio is determined by considering h swell = 69.5 nm and h dry = 31.7 nm and using eq. 1.The equilibrated swollen height h swell was determined via an exponential fit, indicated by the dotted red line.

Figure
Figure S1 a) SEM image of PNIPAM-g-SiNPs, obtained after a SI-ATRP reaction of t = 1 hour.The image is taken at 70,000x magnification and 1.5 kV acceleration voltage.b) SEM image of PMMA-g-SiNPs, obtained after a SI-ATRP reaction of t = 1 hour.The image is taken at 70,000x magnification and 1.5 kV acceleration voltage.c) DLS spectra of non-functionalized and functionalized SiNPs.To obtain stable dispersions, DLS samples of SiNPs and PNIPAM-g-SiNPs were prepared in ethanol solvent and PMMA-g-SiNPs in acetone solvent.

Figure S2
Figure S2 TEM images of PNIPAM-g-SiNPs magnified at 145.000x (left) and 380.000x (right).With a SI-ATRP reaction time of t = 0.5 hour, a small polymeric shell of PNIPAM can be recognized.

Figure S4
Figure S4Top surface AFM images of PNIPAM-g-SiNP films before exposure to near-saturated ethanol vapor.The tapping mode in air was used to image a thin dipcoated film (PNIPAM-g-SiNP(2), left) and a thick self-assembled film (PNIPAM-g-SiNP(1), right).The mono-to bi-layer formation is clearly visible in the left PNIPAM-g-SiNP(2) material, with the nanoparticle closely packed in a FCC(100) ordering.The PNIPAM-g-SiNP(1) sample shows an identical close packing of the nanoparticles, with terrace formation on the micrometer scale.

Figure S5
Figure S5 Comparison study to verify the origin of structural coloration of our materials.The observed reflection peak (white line) is put next to the expected Bragg-Snell reflection (double black line) and reflection peaks due to thin film interference (dotted black lines).Thin film interference values were calculated via λ = 2 n eff t m +0.5 Assumed is n eff = 1.39,D = 223.5 nm, t = 1152 nm, and θ = 0 o .From this study, it is clear that the materials can be considered a colloidal photonic crystal rather than a thin film.

Figure S6
Figure S6Qualitative sensitivity study of PNIPAM-g-SiNP(1) and PNIPAM-g-SiNP(2) films towards ethanol vapor.A gentle ethanol vapor flow was positioned d = 5 cm and d = 0.5 cm from the structurally color samples to mimic a different ethanol vapor concentration.The original color without ethanol vapor exposure is depicted on the left pictures.In this comparison, it can be seen that the thick PNIPAM-g-SiNP(1) films show a gradual red-shift color transition for decreasing exposure distances.PNIPAM-g-SiNP(2) film do not show this gradual color transition.

18 Figure S7
Figure S7Swelling response of a PNIPAM polymer brush in saturated ethanol vapor (flow: 800 ml/min), as measured by ellipsometry.The flow is switched between 800 ml/min N 2 to ethanol vapor at t ∼ 2 minutes.The thickness of the brush is determined via a Cauchy optical model and the swelling ratio is determined by considering h swell = 69.5 nm and h dry = 31.7 nm and using eq. 1.The equilibrated swollen height h swell was determined via an exponential fit, indicated by the dotted red line.

Table S2
Swelling characteristics of PNIPAM and PMMA brushes in different geometries.The swelling medium, characterization method and swelling ratio (α) were denoted for PNIPAM and PMMA brushes on SiNP or SiO 2 substrates.The difference between PNIPAM and PMMA in their affinity towards ethanol as a swelling medium is noticeable by the difference in α values.