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Dynamic Optical Switching of Polymer/Plasmonic Nanoparticle Hybrids with Sparse Loading Zhaoxia Qian, Kathryn N. Guye, David J. Masiello, and David S Ginger J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b00013 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 20, 2017

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The Journal of Physical Chemistry

Dynamic Optical Switching of Polymer/Plasmonic Nanoparticle Hybrids with Sparse Loading Zhaoxia Qian, Kathryn N. Guye, David J. Masiello*, David S. Ginger* Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States ABSTRACT. Responsive nanomaterials composed of gold nanoparticles (AuNPs) and temperature-responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogels offer the promise of designing smart materials that can change color in response to varying thermal, or photothermal stimuli. Typical PNIPAM/AuNP hybrids are heavily loaded with AuNPs. Here we demonstrate hybrids with an average loading of 3 to 5 AuNPs per PNIPAM sphere exhibit peak extinction shifts of over 150 nm and color change from red to purple to gray as the temperature increases from 25oC to 50oC. We observe that the timescale for spectral shifts is offset from that for hydrophobic collapse of the PNIPAM spheres. Facilitated by the low loading density, we combine kinetic studies of the extinction spectra changes with finite-difference time-domain (FDTD) simulations to show that the location of AuNPs relative to the PNIPAM sphere at different stages of collapse is a key variable accounting for the time- and temperature-dependence of the experimental data.

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INTRODUCTION Smart materials that respond to external stimuli in a prescribed manner have attracted tremendous attention due to their demonstrated applications in fields as diverse as sensing1, drug delivery2, microrobotics3, and smart coatings for energy efficiency or camouflage4. Plasmonic nanoparticles especially gold and silver nanoparticles exhibit various colors due to their localized surface plasmon resonances (LSPR). Combining optically active plasmonic nanoparticles and stimuli-responsive materials, responsive plasmonic nanomaterials offer the potential to design smart materials that change color over a wide range.5-6 A number of efforts have combined plasmonic nanoparticles with responsive interconnects such as DNA, hydrogels, and azobenzene photoswitches, to build stimuli-responsive nanostructures which are responsive to ionic strength,78

temperature,9 and light10-11. Among these strategies, temperature responsive poly(N-

isopropylacrylamide)(PNIPAM) hydrogels have been widely used due to their well-studied syntheses and physical properties.6, 12 PNIPAM undergoes a reversible phase transition from a hydrophilic swollen state to hydrophobic shrunken state in water when heated above its lower critical solution temperature (LCST) around 32oC.13 As such, it can serve as a powerful scaffold to control the distance and coupling of plasmonic nanoparticles. Typically, metal nanoparticles (NPs) and PNIPAM are put together via either wrapping the NPs with PNIPAM polymer,14-16 or synthesizing NPs in-situ inside of the PNIPAM matrix,17 or tethering the NPs onto the surface of PNIPAM spheres.18-20 Among these approaches, tethering NPs onto the surface of PNIPAM spheres allows easy control of the structural parameters of both the PNIPAM spheres and the NPs. More importantly, this geometry makes it possible to tune the extinction spectra of the PNIPAM/NP hybrids in a wide wavelength range.


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Tremendous efforts have been put into tethering the plasmonic NPs onto the surface of PNIPAM spheres in order to gain wide optical tunability via varying the temperature.6 A common strategy is to increase the loading density of the NPs on the PNIPAM spheres so that the plasmonic coupling between the NPs can be maximized above the LCST of the PNIPAM sphere.18-21 For example, Lim et al21 demonstrated that PNIPAM spheres heavily loaded with tens of gold nanospheres per PNIPAM micelle display significant spectral shifts ( ~ 300 nm) with increasing temperature. However, PNIPAM spheres heavily loaded with NPs do not always result in dramatic resonance shifts in extinction so that obvious color change of the PNIPAM/NP hybrid can be observed. For example, Dong et al17 reported silver nanoparticles synthesized in-situ inside of the PNIPAM matrix that exhibited only a ~15 nm extinction spectra shift as the temperature increased from 25oC and 45oC. Similarly, a ~50 nm extinction spectra shift was observed on PNIPAM-coallylacetic acid microspheres heavily loaded with nanorods.19, 22 In all these examples, the NPs were tethered to the PNIPAM spheres strongly via electrostatic interactions18-19 or chemical binding,17, 20 raising the possibility that their movements relative to the PNIPAM spheres could be hindered. Here, we take a different approach and consider PNIPAM spheres with low AuNP loadings (a few AuNPs per polymer sphere). We show that this low-loading limit is attractive both in that it can exhibit significant shifts in the extinction peak wavelength (~150 nm) and color change, and that it is highly amenable to both experimental observations and simulation of the optical properties of the hybrid. Importantly, this loading limit also allows us to observe that the PNIPAM/AuNP hybrids exhibit time-dependent extinction spectra on timescales exceeding that of the PNIPAM volume change. We propose a model emphasizing the location of AuNPs relative to the PNIPAM sphere at different stages to explain these experimental observations.



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EXPERIMENTAL SECTION Materials. N-isopropylacrylamide (≥ 99%), N, N’-methylene bis(acrylamide) (99%), potassium persulfate (ACS reagent, ≥ 99.0%), allylamine (≥ 99%) were purchased from Sigma Aldrich. Gold nanoparticles with a 50-nm diameter were purchased from BBI Solutions. Synthesis of the Poly(N-isopropylacrylamide) (PNIPAM) Hydrogel. PNIPAM hydrogels were synthesized via free radical polymerization.21 Typically, 0.25 g of N-isopropylacrylamide (NIPAM) and 0.01 g of N, N’-methylene bis(acrylamide) (BIS) were dissolved into 50 mL of nanopure water and transferred into a 100 mL three-neck flask. The solution was heated to 80oC and purged with N2 gas for 30 min. Then, 70 µL of allylamine and 0.25 mL of potassium persulfate aqueous solution (0.025 g/mL) were injected into the solution sequentially. The reaction proceeded for 2 hrs while stirring at 800 rpm under N2 protection. Then the solution was centrifuged at 8000 rpm for 30 min and redispersed into nanopure water. The same washing procedure was repeated another two times. The final product was dissolved into 40 mL of nanopure water and stored at 4oC. Fabrication of PNIPAM/AuNP Hybrid. Typically, 25 µL to 100 µL of PNIPAM solution and 500 µL of as-purchased 50 nm gold nanoparticles were mixed in a 4-mL glass vial. The solution was mixed well and allowed to sit undisturbed at room temperature for 30 min before further characterization. Materials Characterization. Scanning electron microscope (SEM) images were obtained using a FEI Sirion XL30 SEM at a 5 keV accelerating voltage. Typically, 20 ~ 50 µL of the PNIPAM/AuNP hybrid solution was drop casted onto a precleaned ITO substrate. After 5 ~ 10 minutes, the substrate was gently blown dry using nitrogen gas. Extinction spectra were measured


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with an Agilent 8453 UV-Vis spectrometer. Dynamic light scattering measurement was conducted using a Malvern Zetasizer with a 633-nm laser. Finite-Difference Time-Domain (FDTD) Simulation. The extinction spectra of PNIPAM/AuNP hybrids were calculated via finite-difference time-domain (FDTD) simulations using Lumerical Solutions, Inc. FDTD package 8.16. The AuNPs were either embedded inside or on the surface of a PNIPAM sphere in the model (Scheme S1). Their positions were generated randomly using a Python 3.5 package and a minimum interparticle distance of 3 nm is ensured to mimic the experimental observation that the PNIPAM branches wrap around the AuNPs. A mesh size of 3 nm was used for all simulations, which guaranteed that the simulations converge. A broadband total-field scatter-field (TFSF) pulse was injected into the rectangular simulation region and a perfectly matched layer (PML) absorbing boundary condition was used. The dielectric function provided by Johnson and Christy was used for AuNPs. It is reported that the dielectric constant of PNIPAM spheres change slightly between 1.34 and 1.37 below and above LCST,23 thus a dielectric constant of 1.37 is used for all simulations between 400 nm and 1000 nm. We believe the error introduced by slight overestimation of the dielectric constant is negligible to the simulation result presented to the manuscript. In order to mimic a range of AuNP loadings for each type of PNIPAM/Au hybrid fabricated with different volume ratio between PNIPAM and AuNP, 50 models with a range of AuNP number on the PNIPAM sphere were generated. For PNIPAM/AuNP hybrid prepared via mixing every 500 µL AuNP with 100 µL, 85 µL and 70 µL PNIPAM solution, the number of AuNPs in each set of 50 models are shown in array Na, Nb and Nc, respectively. Here Na = [1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 5, 6, 6, 6, 7, 10], Nb = [1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 8, 8, 8, 8, 9], Nc = [2, 2, 2, 2, 3, 3, 5 ACS Paragon Plus Environment


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3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6, 7, 7, 7, 7, 8, 8, 8, 8, 9, 10, 10, 10, 10, 11, 13]. All these values are obtained from SEM images of corresponding PNIPAM/AuNP hybrid.

RESULTS & DISCUSSION Figure 1A-C show SEM images of temperature-responsive poly(N-isopropylacrylamide) (PNIPAM) /AuNP hybrids we synthesized with low loadings of ~3-5 AuNPs per PNIPAM sphere. We prepared these PNIPAM spheres using free radical polymerization by adapting literature methods,21, 24 and then mixed the PNIPAM spheres with 50-nm-diameter AuNPs to form the hybrids (see Experimental Section for details). We varied the AuNP loading on the PNIPAM spheres by adjusting the volume ratio between the added AuNP solution and the PNIPAM sphere solution (insets in Figure 1A-C). The histograms in the insets to Figure 1A-C were produced by counting the AuNP number on individual PNIPAM spheres from the SEM images (Figure S1, S2). The average AuNP number on individual PNIPAM spheres increases from 3 ± 2 in Figure 1A to 4 ± 2 and to 5 ± 3 in Figure 1B and Figure 1C, respectively. These values are in good agreement with estimated values calculated based on the mole ratio of the AuNPs and PNIPAM spheres (See Supporting Information, Figure S3). Despite having relatively low average AuNP loadings per PNIPAM sphere compared with previous reports,20-22 Figure 1D-G shows that these PNIPAM/AuNP hybrids exhibit significant peak spectral shifts (up to ~150 nm) upon heating above the PNIPAM LCST as a result of the increasing coupling between the AuNPs that has been attributed to the hydrophobic collapse of the polymer at elevated temperatures.25-26 Using dynamic light scattering (DLS) we confirmed that our PNIPAM spheres undergo a collapse from a hydrophilic state with an effective hydrodynamic diameter of 427 ± 12 nm at 25oC, to a 6 ACS Paragon Plus Environment

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hydrophobic state with a hydrodynamic radius of 140 ± 1 nm as the temperature is increased to 50 o

C (Figure 1I). This diameter change corresponds to a swelling ratio (volume ratio between high

temperature and room temperature) of 0.035, consistent with previous reports.27-28 While the PNIPAM/AuNP hybrids in Figure 1A-C show seemingly small differences in AuNP loading number as judged from the SEMs, we nevertheless observe significant differences in the temperature-dependent optical properties of these colloidal suspensions as seen in Figure 1D-H. Before heating, all three hybrids show a single peak at around 536 nm (Figure 1D-F) at 25oC. This peak is close to the LSPR of the individual AuNPs (Figure S1), and indicates the AuNPs are relatively far apart and only weakly coupled in the polymer/water matrix at this stage.29 However, as the PNIPAM spheres start to collapse close to the PNIPAM/AuNP hybrid LCST, (which is the same as the LCST of the PNIPAM spheres ~37oC, Figure 1I, S4, S5), the spectra of the PNIPAM/AuNP hybrids begin to red shift, with the samples with higher AuNP loadings exhibiting more dramatic changes, in terms of wavelength and intensity, in the extinction spectra (Figure 1D-F, S6). For instance, the peak wavelength at 50oC is only 561 nm for the sample with an average loading number of ~3, but increases to 690 nm for the sample with an average loading number of 5. Similarly, the peak extinction values start to show differences beginning around 37oC, with the larger loading densities showing a 73% drop in absorbance at 540 nm by the time the sample reaches 50oC, and the lowest loading density showing only a 38% change in absorbance at 540 nm when raised to 50oC (Figure 1H). Notably, the samples go from essentially zero extinction in the NIR at 900 nm at room temperature, to an appreciable value of 0.32 at 50oC for the sample with an average loading density of 5 (Figure 1H). The samples with lower loading density also exhibit a smaller change in extinction in the NIR, rising from zero at room temperature, to only 0.1 at 50oC (Figure 1H). Given the very low NIR extinction of the samples before heating, the



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relative increases in the NIR extinction are dramatic for these samples (see Figure 1H), suggesting that these hybrids could be of interest for NIR color-shifting technologies such as smart windows. At 25oC, the hydrodynamic diameter of all three PNIPAM/AuNP hybrids is about 330 nm, which is significantly smaller than that of the PNIPAM sphere (~ 414 nm), while this value (~ 160 nm) is quite slightly higher than that of the PNIPAM spheres (~ 140 nm) at 50oC above the PNIPAM LCST (Figure 1I). This decrease of the hydrodynamic diameter of the PNIPAM upon addition of AuNPs implies that the PNIPAM may partially collapsed when interacting with the AuNPs, probably via conformation change to facilitate absorption on the gold surfaces, as has been speculated by Gawlitza et al.28

Figure 1. (A-C) SEM images of the PNIPAM/AuNP hybrids with increasing loading of AuNPs. The white dots denote AuNPs and the darker spheres underneath the AuNPs denote PNIPAM spheres. For every 500 µL of AuNPs, volume of PNIPAM spheres used to form the hybrid is 100 8 ACS Paragon Plus Environment

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µL (A), 85 µL (B) and 70 µL (C), respectively. Insets are histograms of the number of AuNPs anchored on each PNIPAM sphere. Average number of AuNPs on individual PNIPAM sphere is 3 ± 2 (A), 4 ± 2 (B) and 5 ± 3 (C), respectively. The total number of counts in each histogram is 110. (D-F) Extinction spectra of the PNIPAM/AuNP hybrid solution shown in (A), (B), (C) as the temperature increases from 25oC to 50oC at a step size of 1oC and an incubation time at every temperature of 2 min. (G) Wavelength of the extinction peak (λmax) versus the temperature, derived from plots shown in (A) (black square), (B) (red sphere) and (C) (blue triangle). (H) Extinction peak intensity of PNIPAM/AuNP hybrids shown in (A) (black), (B) (red) and (C) (blue) at 540 nm (solid dots) and 900 nm (void dots) versus temperature. (I) Change of the hydrodynamic diameter of PNIPAM spheres (pink square) and PNIPAM/AuNP hybrids shown in (A) (black rectangle), (B) (red sphere) and (C) (blue triangle) as the temperature increases from 25oC to 50oC.

Figure 2. (A) Extinction spectra of the PNIPAM/AuNP hybrid solution as its temperature changes from 25oC (black curve) to 50oC (red curve) to 25oC (blue curve) to 50oC (dark yellow curve) to 9 ACS Paragon Plus Environment


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25oC (olive curve). For every 500 µL of AuNPs, 70 µL of PNIPAM spheres is used to fabricate the hybrid. The equilibrium time at each temperature is 10 min. (B) The extinction peak wavelength change as the PNIPAM/AuNP solution was heated up to 50oC followed by cooling down to 25oC and this process was repeated for multiple cycles. Images on the right show the color of the PNIPAM/AuNP solution at 25oC (bottom left, enclosed in a red rectangle), 50oC (right top, enclosed in a blue rectangle) and back to 25oC (bottom right, enclosed in a pink rectangle). (C) Hydrodynamic diameter of the PNIPAM/AuNP hybrid solution cycled between 25oC and 50oC measured by DLS. (D, E) SEM images of the as-prepared PNIPAM/AuNP hybrid (D) and after annealing at 50oC for 10 min and cooling down to 25oC (E). The scale bar is 1 µm. Schematics for each structure are below the SEM images. The reversibility of the optical switching of the PNIPAM/AuNP hybrid is of critical importance for their practical applications and has been examined for high-loading density composites by several research groups.19, 21-22 Here, we turn to study the reversibility of the optical changes in our low-loading-density composites. Figure 2A shows the extinction spectra of the asprepared PNIPAM/AuNP hybrid solution as it is temperature cycled, beginning at 25oC, then heated up to 50oC followed by cooling down to 25oC and then cycled again. Initially, at 25oC the extinction spectrum exhibits a single peak at 536 nm, which shifts to 688 nm upon heating up to 50oC (Figure 2A). After cooling back to 25oC, however the spectrum does not return to its position at 536 nm, but rather equilibrates at 560 nm -- 24 nm red shifted compared to the initial peak wavelength (Figure 2A, S7). Subsequent heating and cooling stages cycle reproducibly between these two endpoints (688 nm at 50oC, and 560 nm at 25oC). We consider two possibilities that may lead to the irreversible change of the extinction spectrum of the PNIPAM/AuNP hybrid in the first heating and cooling cycle: (A) the hybrid collapses when initially heated up to 50oC and does not 10 ACS Paragon Plus Environment

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return to its original size at 25oC in the following heating and cooling cycles; (B) the AuNPs move closer to each other when initially heated up to 50oC, however they do not completely move back to their original positions when cooled down to 25oC; the hybrid does return to its initial size at 25oC in the heating and cooling cycles. The DLS results in Figure 2C show that the hydrodynamic diameter Dh of the PNIPAM/AuNP hybrid changes reversibly when cycled between 25oC and 50oC, which indicates that hypothesis (A) is not the main cause of the irreversible extinction spectra change of the hybrid in the first heating and cooling cycle. Instead, the SEM images in Figure 2D and 2E seem to support hypothesis (B). Figure 2D shows that the AuNPs are mostly located on the surface of the PNIPAM spheres in the as-synthesized PNIPAM/AuNP hybrid. However, after annealing the hybrid at 50oC and then cooling down to 25oC, the AuNPs move inside of the PNIPAM spheres (Figure 2E) which allows stronger interparticle plasmonic coupling. We found similar results in PNIPAM/AuNP hybrids with higher AuNP loading (Figure S8). Based on the reproducible cycling after that point, we conclude that the AuNPs stay inside of the PNIPAM during subsequent heating and cooling cycles, with the interparticle distances increasing or decreasing as the PNIPAM swells or shrinks when the temperature is cycled between 25oC and 50oC. Given that the AuNPs penetrate inside of the PNIPAM spheres when initially heated up to 50oC, the different extinction responses of the PNIPAM/AuNP hybrids with different AuNP loading shown in Figure 1 may be attributed to a combined effect of varying AuNP loading and more AuNPs residing inside the PNIPAM spheres for the hybrids with higher AuNP loading.



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Figure 3. (A-B) Extinction spectra of the PNIPAM/AuNP hybrid as the temperature increases from 25oC to 50oC at a step size of 1oC. The incubation time at every temperature is 3 min (A) and 4 min (B), respectively. For every 500 µL of AuNPs, 100 µL of the PNIPAM spheres is used to fabricate the PNIPAM/AuNP hybrid. (C) Extinction peak wavelength of the PNIPAM/AuNP hybrids versus temperature derived from (A-B) and Figure 1D. (D) The dependence of the hydrodynamic diameter of the PNIPAM/AuNP hybrid on temperature measured with an incubation time of 2.5 min (black rectangle) and 5 min (red sphere) at every step with a step size of 1oC. (E) Time-dependent correlation of the temperature (blue triangle), hydrodynamic diameter (black rectangle) and the peak wavelength of the extinction spectra (red sphere) of the PNIPAM/AuNP hybrid. (F) (Top) Images of the PNIPAM/AuNP hybrids solution incubating at 50oC for 5 min and 30 min after the initial annealing cycle; (Bottom) schematics of corresponding PNIPAM/AuNP hybrid model.

To understand the seemingly large extinction spectra shifts of the PNIPAM/AuNP hybrids at such low AuNP loading, the dependence of their extinction spectra with incubation time at 12 ACS Paragon Plus Environment

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varying temperatures is examined. Figure 3A-B shows the extinction spectra of the hybrids with an incubation time of 3 min (Figure 3A) or 4 min (Figure 3B) at every step from 25oC to 50oC with a step size of 1oC. Their peak wavelengths versus temperature is plotted in Figure 3C. The extinction spectra start to show more red shift above the PNIPAM LCST at 37oC, ending in ~57 nm greater red shift from 577 nm to 634 nm at 50oC, as the incubation time increases from 3 min to 4 min. One might speculate that the dependence of the spectral shift on heating rate and time reflects differences in the hydrophobic collapse of the PNIPAM sphere, however the DLS data show that this hypothesis cannot be true, as the hydrodynamic diameter of the PNIPAM/AuNP hybrid is essentially identical, whether the incubation time is 2.5 min, or even 5 min at each step from 25oC to 50oC (Figure 3D). In fact, the DLS results show that the size of the PNIPAM sphere reaches its steady-state less than 2.5 min after each step between 25oC and 50oC. Since the spectral shifts take much longer, we can infer that the AuNPs require longer time to reorganize, and are rearranging inside of the PNIPAM sphere to reach a new steady state. Figure 3E shows the correlation of the temperature, hydrodynamic diameter and maximum wavelength of the extinction peak (λmax) of the PNIPAM/AuNP hybrid (Figure S9). As the temperature increases from 25oC to 50oC at t = 5 min, the hydrodynamic diameter of the hybrid decreases from 340 ± 10 nm to 151 ± 4 nm and stays constant in the next 25 min at 50oC. In contrast, λmax increases quickly from 536 nm to 559 nm in the first 5 min followed by slow increase to 570 nm at t = 30 min. The hysteresis of λmax change compared to the hydrodynamic diameter of the hybrid shown in Figure 3E further confirms that the PNIPAM collapse dynamics are much faster than the reorganization dynamics of the AuNPs inside of the PNIPAM spheres. This slower AuNP reorganization is probably because the slightly crosslinked PNIPAM branches entangle with the AuNP surfaces via gold-amine interaction20 and this entanglement may hinder the movement of AuNPs inside of the PNIPAM



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spheres. Figure 3F shows that the PNIPAM/AuNP hybrid solution turns from purple after 5 min into gray after 10 min at 50oC. Based on the above results, we attribute the reorganization of the AuNPs in the PNIPAM spheres to be responsible for the time-dependent optical properties of the hybrid (Figure 3F bottom). The increasingly red shift of the extinction spectra above the LCST of the PNIPAM sphere indicates that the AuNPs gradually move closer to each other, probably because the van der Waals attraction surpasses the repulsive force between the AuNPs as the PNIPAM branches crash on the AuNP surface above the LCST. This result is consistent with recent work by Ding et al11 who reported the fast switching of AuNPs surface functionalized with PNIPAM between assembled and disassembled state under laser illumination which induced temperature increase of both the AuNPs and the PNIPAM coatings. Next, we use finite-difference time-domain (FDTD) simulations of the optical response of the clusters to test the validity of our hypothesized model against the observed data. To begin, we computed the extinction spectra of three different structural models of the PNIPAM/AuNP hybrids using the FDTD method (see Experimental Section for details) and compared these simulations with our experimental results (Figure 4). Because of the dispersity in loading density and particle position seen in our SEM images, we simulated the properties of 50 different randomly generated clusters for each structural model. For the three likely structures, we examined a 330 nm diameter PNIPAM/AuNP hybrid with the AuNPs attached to the surface (Figure 4A), a 330 nm diameter PNIPAM/AuNP hybrid with the AuNPs embedded throughout the volume of the polymer (Figure 4B), and a 160-nm collapsed hybrid with the AuNPs also embedded within the volume of the sphere.


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Figure 4. (A-C) Simulated extinction spectra of single PNIPAM/AuNP hybrid using FDTD method. The cartoon of the models used for the simulation are shown on the left upper corners of each plot. The models used for the calculation are AuNPs randomly decorating the surface of a PNIPAM sphere with a diameter of 330 nm (A) and AuNPs randomly embedded inside of a PNIPAM sphere with a diameter of 330 nm (B) or 160 nm (C). Here the diameter of the PNIPAM spheres is obtained from DLS measurements shown in Figure 1I. The number of AuNPs used in every set of 50 simulations is obtained from counts of the PNIPAM/AuNP hybrid sample shown in Figure 1C (see Experimental Section for details). Each set of data shown in (A-C) is calculated five times using models with randomly generated AuNP positions (Figure S10-S13). (D) 15 ACS Paragon Plus Environment


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Comparing average simulated spectra (top) derived from (A-C) with experimental spectra (bottom) at 25oC (black) followed by heating up to 50oC (blue) and cooling down to 25oC (red). (E) Schematic description of the structures of the PNIPAM/AuNP hybrid during the heating and cooling processes. The first structure in the black box represents the initial structure before heating in which the AuNPs are mostly anchored on the surface of the PNIPAM spheres. The middle one in the blue box represents the structure after heating to 50oC. The right one in the red box represents the structure after cooling back to 25oC.

Figure 4A shows 50 calculated extinction spectra of the PNIPAM/AuNP hybrid in which the AuNPs were randomly anchored on the surface of a 330 nm PNIPAM sphere (see Experimental Section for detailed information) chosen to match the hydrodynamic radius and average loading density observed in our experiments. The calculated extinction spectra vary with structure, but generally exhibit one major peak at around 536 nm, with a small shoulder peak above 600 nm (Figure 4A). The average spectrum (Figure 4D, black curve, top) has a main peak at 536 nm and agrees fairly well with the experimental extinction spectra of the as-synthesized hybrid solution at 25oC (Figure 4D, black curve, bottom), in terms of both the overall spectral profile and peak position. Figure 4B shows 50 calculated extinction spectra of the model with AuNPs randomly embedded inside of a 330 nm PNIPAM sphere (rather than attached to its surface). Their average spectrum (Figure 4D, red curve, top) exhibits a major peak around 544 nm and the overall spectral profile also matches the measured extinction spectra of the PNIPAM/AuNP hybrid solution after cooling down to 25oC (Figure 4D, red curve, top). Figure 4C shows 50 calculated extinction spectra of AuNPs randomly embedded inside of a 160 nm PNIPAM sphere – simulating a collapsed


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PNIPAM micelle above the LCST. Most spectra have multiple peaks, reflecting the structural heterogeneity as manifested by different degrees of near-field coupling giving rise to different hybridized AuNP modes. Despite the individual heterogeneity, the average calculated extinction spectrum (Figure 4D, blue curve, top) covers a broad range in the visible region and is close to the profile of the experimentally measured extinction spectra of the PNIPAM/AuNP hybrid solution at 50oC (Figure 4D, blue curve, bottom). Similar agreement between experimentally measured extinction spectra and calculated extinction cross section is also observed in PNIPAM/AuNP hybrids with other AuNP loadings (Figure S14). In contrast, the simulated average extinction spectrum for the AuNPs randomly anchored on the surface of a 160 nm PNIPAM sphere exhibits noticeably bluer peaks compared to the experimental spectra (Figure S15). Thus, the comparatively good agreement between the calculated and experimentally measured extinction spectra of the hybrid seems consistent with our proposed structure of the PNIPAM/AuNP hybrid at different stages in the thermal heating and cooling cycles, as shown in Figure 4E. We note that these proposed models are consistent with previous work. For example, Gawlitza et al.28 studied the influence of the crosslinking density of PNIPAM spheres on the optical properties of the PNIPAM/AuNP hybrids and noticed that with matched AuNP loading densities, the hybrids with the lowest crosslinking density exhibited the largest plasmon shifts above the LCST of the PNIPAM spheres. They proposed that the AuNPs penetrated easier and deeper into the mesh of the PNIPAM spheres with lower crosslinking density. The crosslinking density of PNIPAM spheres used in this work is 3%, which is in the lower regime compared to the work by Gawlitza et al 28 and make the AuNP penetration into the PNIPAM matrix possible. Our work here provides detailed structural analysis of the PNIPAM/AuNP hybrids at each stage during the thermal heating and cooling cycles.



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CONCLUSIONS Here, we synthesize and characterize PNIPAM/AuNP composites with a low loading density of ~ 3 to 5 AuNPs per polymer sphere. We show that these responsive materials exhibit significant extinction peak shifts – of over 150 nm – resulting in a color change from red to purple to gray as the temperature increases from 25oC to 50oC. The optical switching of these hybrids is reversible after an initial annealing above the PNIPAM LCST. Our kinetic measurements show that the spectra and color change of typical PNIPAM/AuNP hybrid reported here is time-dependent and takes over 30 min, a timescale much longer than that for the temperature-induced collapse of the PNIPAM sphere, and which we associate with the time scale for the AuNPs inside the PNIPAM spheres to reorganize and reach steady state. Based on our experimental observations and FDTD simulations, we show that our data are broadly consistent with a structural model where the AuNPs are initially randomly anchored on the PNIPAM sphere surface and then penetrate inside the polymer volume after thermal annealing at 50oC. This work is important because: (1) it provides an effective route to fabricate three-dimensional plasmonic nanostructures in which the interparticle distances and plasmonic coupling can be reversibly and dynamically controlled. This ability to dynamically control the structural parameters of plasmonic nanostructuress is critical for their applications, especially in spectroscopy and biosensing;30 and (2) it offers a novel platform to control the spatial distribution of nanoparticles in polymers without stringent regulation of the NP size, loading density and polymer size, which is quite challenging in designing functional polymer nanoparticle composites.31-32 We envision that by carefully controlling the interactions of AuNPs inside of the stimuli-responsive polymer through their structural parameters, future plasmonic nanomaterials with novel properties and applications can be fabricated.


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ASSOCIATED CONTENT Supporting Information Available: Calculation of the Average Number of AuNPs on individual PNIPAM spheres. Additional TEM images, extinction spectra and FDTD simulations of the PNIPAM/AuNP hybrid. AUTHOR INFORMATION Corresponding Author *Email: [email protected] (D.S.G.) *Email: [email protected] (D.J.M) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS D.S.G acknowledges that this research is based in part on work supported by AFOSR FA9550-14-1-0250. D.S.G also acknowledges additional support from the University of Washington Kwiram Endowment. Z.Q acknowledges the Washington Research Foundation Innovation Fellowships in Clean Energy offered by the Clean Energy Institute at the University of Washington. D.J.M. acknowledges funding from the Department of Chemistry at the University of Washington (CHE-1253775).



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