A Simple and Fast Compression‐Based Method to Fabricate Responsive Gold‐pNIPAM Hybrid Materials: From Thin Films to Anisotropic Microgels

In recent years, hydrogel‐based soft materials with hybrid properties have found widespread use in various technological fields, including tissue engineering, soft actuators, and flexible electronics. The proper implementation of these smart multifunctional materials into real‐world applications requires the development of simple, cost‐effective, and large‐scale fabrication methods. Herein, a simple compression‐ and colloid‐based method is presented to fabricate responsive Au‐poly(N‐isopropylacrylamide) (pNIPAM) hybrid films using photopolymerizable resin containing Au‐pNIPAM core–shell microgels as building blocks. Uniform Au‐pNIPAM hybrid films of 25 × 25 mm with adjustable thickness in the micron‐size range (2.3–1.2 µm) w ere successfully fabricated on glass substrates and flexible commercial acetate sheets. The resulting flexible Au‐pNIPAM films exhibit robust optical and mechanical properties, even after repeated edge‐to‐edge bending cycle tests. Additionally, using patterned light to polymerize the Au‐pNIPAM films allows synthesizing of anisotropic Au‐pNIPAM microgels with high width‐to‐height aspect ratios, such as square, circular, and rectangular microgels, adding a new dimension to the proposed fabrication method.


Introduction
Hydrogel-based soft materials with hybrid properties are increasingly used in the development of novel technologies attaining DOI: 10.1002/admi.202300453[11] Most of these hydrogels are typically considered smart materials due to their ability to experience changes in their physical and/or chemical properties upon external stimuli, such as temperature, light, pH, and ionic strength. [4,12]mong the existing stimuli-sensitive hydrogels, those made from poly(Nisopropylacrylamide) (pNIPAM) have been widely studied due to the volume phase transition (VPT) behavior when heated above the VPT temperature. [13,14]his response to changes in temperature is related to the lower critical solution temperature (LCST) of pNIPAM in water, which is 32 °C. [15]Surpassing the VPT temperature leads to volumetric shrinkage of the pNIPAM hydrogel due to changes in solvation caused by the disruption of hydrogen bonds between water molecules and polymer chains. [16]he incorporation of light-absorbing nanomaterials within the network of thermo-responsive hydrogels, e.g., graphene oxide, [17,18] carbon nanotubes, [19,20] and plasmonic gold nanoparticles, [21][22][23][24] has been an interesting alternative for locally and remotely triggering the shrinkage of pNIPAM hydrogels.In particular, gold nanoparticles supporting localized surface plasmon resonances (LSPR) exhibit good photothermal properties due to their large optical extinction cross-section spectrum at visible or near-infrared wavelengths, [25][26][27] which can be easily tuned by controlling parameters such as the size and morphology of gold nanoparticles. [28]For instance, thin hybrid films composed of gold nanoparticles and pNIPAM hydrogels (Au-pNIPAM) have been developed and applied as light-actuated hydrogel platforms to manipulate and exert mechanical stimulations on living cell cultures with a great spatiotemporal resolution; [29] as dynamic-and light-switchable plasmonic metafilms for sensing, imaging optics, and image displays applications; [30,31] and as light-responsive shape-adaptable flexible electronics films. [23]34] On that account, simple, cost-effective, and large-scale fabrication techniques are pivotal to synthesizing advanced hydrogel films with hybrid properties, controllable thickness/morphology, and scalable protocols. [35]Although several methods have been used to fabricate hydrogel films, such as spin-coating, [36] layer-bylayer dip-coating, [37] molding, [38] interfacial polymerization, [39] and 3D printing, [40] among others, [41,42] most of these methods still present some limitations regarding the precise control of the thickness of thin films and the fabrication of sophisticated structures. [41]48][49] This work presents a simple compression-and colloid-based method to fabricate photopolymerized Au-pNIPAM hybrid films with adjustable thickness in the micron-size range.Our method allowed the fabrication of Au-pNIPAM films on glass substrates with a homogeneous distribution of gold nanoparticles by preparing a photopolymerizable pNIPAM-based resin containing Au-pNIPAM core-shell microgels as building blocks.Furthermore, flexible Au-pNIPAM films fabricated on commercial acetate sheets as supporting substrates showed robust optical and mechanical properties after repeated edge-to-edge bending cycle tests.As for the hybrid properties of the films, both the plasmonic properties and thermo-responsivity were studied.Finally, the use of light to polymerize the Au-pNIPAM films permitted the patterning -employing a photomask -of films and hence obtaining organized anisotropic Au-pNIPAM microgel arrays with high width-to-height aspect ratios on glass substrates, as well as the production of anisotropic microgel aqueous suspensions through the subsequent detachment of microgels from the substrate.

Fabrication of Light-Sensitive Thin Au-pNIPAM Films
Uniform and thin Au-pNIPAM films were fabricated through a simple compression-based approach of a photopolymerizable resin (or photo resin) composed of Au-pNIPAM core-shell microgels, additional NIPAM monomer, and BIS cross-linker, as well as TPO as the photoinitiator.Here, the photo-resin was pipetted onto an organosilane-functionalized glass substrate, compressed with an upper cleaned-glass substrate, and then photopolymerized for 3 min before removing the top substrate, as depicted in Figure 1.The dispersion of Au-pNIPAM core-shell microgels employed in the photo-resin was synthesized via seeded precipitation polymerization. [50,51]Figure S2 (Supporting Information) shows the extinction spectrum of the Au-pNIPAM coreshell microgels in water with a plasmon peak at 524 nm.The inset image of Figure S2 (Supporting Information) corresponds to a TEM image showing the successful encapsulation of gold cores (14 ± 1 nm in diameter) with a pNIPAM shell.The mean hydrodynamic diameter of the core-shell microgels in water is 189 ± 3 nm at 15 °C, as determined by dynamic light scattering experiments.54] In fact, the addition of uncoated gold nanoparticles into the prepolymer solution of NIPAM, BIS, and TPO dissolved in a solution mixture of 1 Milli-Q water : 3 n-propanol results in almost instantaneous particle aggregation.The deposition of the upper piranha-cleaned glass substrate compressing the photo-resin allowed the spreading of a uniform and thin layer over the 3-MPS-functionalized glass substrate.With 3 min of illumination at 405 nm, a completely photopolymerized Au-pNIPAM thin film was obtained.This Au-pNIPAM film remained firmly adhered to the bottom substrate, even after taking off the top substrate, due to the chemical bonding between the pNIPAM hydrogel and the methacrylate functional groups of the bottom glass surface. [7,55]Figure 2a shows an image of the red-colored thin Au-pNIPAM film obtained.In order to characterize the optical properties and uniformity of the Au-pNIPAM film, UV-vis spectrophotometry measurements were performed at nine different positions on the 25 × 25 mm film (Figure 2b).The extinction spectra (normalized at 400 nm) for the nine different positions presented in Figure 2b show slight variations in terms of plasmon peak intensities at 530 nm, resulting in a relative standard deviation of 1.5%.This low plasmon peak intensity variation agrees with the homogeneous distribution of gold nanoparticles within the film, as exhibited by Figure 2c.The latter is a top-view SEM image of the film, where the bright points represent the gold nanoparticles with a mean number distribution of particles along the film of 34 ± 4 NPs/μm 2 .Figure 2d is a cross-section SEM image of the Au-pNIPAM film obtained through FIB milling, where it is possible to identify the glass substrate, the Au-pNIPAM film, and on top, the Pt-coating to protect the hydrogel during the milling process.The zoomed-in crosssection image (Figure 2e) shows a uniform particle distribution along the thickness plane, pointed out with red arrows.It is worth mentioning that the redshifting of the plasmon peak of the film at 530 nm, compared to that of the core-shell microgel dispersion at 524 nm (Figure S2, Supporting Information), corresponds to the differences in the refractive index of the medium. [56,57]o assess the repeatability of this method to fabricate Au-pNIPAM films, three replicas were prepared (S1.1, S1.2, and S1.3) with the same conditions as described above.Figure S3a (Supporting Information) in the supporting information shows the mean extinction spectra of the three replicas normalized at 400 nm with their corresponding standard deviation, obtained from the extinction spectra of the nine different positions for each film.The three extinction spectra reveal good repeatability of the method with uniform intensities for each Au-pNIPAM film, as shown in Figure S3b (Supporting Information) of the supporting information, where the intra-film variation of plasmon peak intensities is <1.5% and <2% for the inter-film variation.
Moreover, this compression-based method can be applied to fabricate Au-pNIPAM films on flexible substrates, thus forming responsive hybrid hydrogel films with robust mechanical and malleable properties.][60] Figure 3a shows an image of a uniform and bendable Au-pNIPAM film that was prepared on a cellulose acetate sheet.Experimental details on the fabrication process are described in the Experimental Section and Section S2 (Supporting Information).The benefits of using cellulose acetate sheets as malleable supports stem from their high optical transparency, relatively low cost, good mechanical strength, and biocompatibility. [61]As for the optical property of the acetate sheet-supported Au-pNIPAM film, this malleable film maintained the plasmonic properties of gold nanoparticles with a plasmon peak at 532 nm, as shown in Figure 3b.The extinction spectrum (normalized at 400 nm) corresponds to the mean extinction spectrum with the standard deviation as error bars, determined from extinction spectra measured at nine different positions of the film.The coefficient of variation of the plasmon peak intensity for this sample remained <2%, as the glass substrate-supported Au-pNIPAM films presented above.Additionally, in order to demonstrate the mechanical robustness of the malleable Au-pNIPAM film, consecutive cycles of mechanical bending were performed through manual edge-to-edge bending tests (≈180°deformation).Figure 3c presents the normalized extinction spectrum of the malleable film after every bending cycle out of a total of twenty cycles.The inner plot displays the plasmon peak intensity at 532 nm as a function of the bending cycle.No significant changes in the optical properties are observed, as neither the plasmon peak position nor intensity.This suggests that the film remained structurally intact, with no changes in the number of gold nanoparticles per area and maintained the refractive index conditions of the film after mechanical stress.Although no additional mechanical tests were conducted to determine, for instance, the tensile strength of the fabricated acetate sheet-supported Au-PNIPAM films, and thus make a direct comparison with other pNIPAM-based flexible films found in the literature, [58][59][60]62,63] the optical characterization after edge-to-edge bending cycles provides valuable insights into the potential of the method for fabricating flexible and thin light-responsive actuators possessing good mechanical durability. Indeed through the method proposed in this work, robust thin films of a few microns of thickness could be easily fabricated onto substrates of diverse materials.[64] Also, the fine thickness modulation of films below tens of microns in thickness represents an advantage of this compression-based method over other molding-based or blade-coating fabrication techniques.[65]

Thickness Modulation of Au-pNIPAM Films
Owing to the compression-based nature of the proposed method, the thickness of Au-pNIPAM films can be easily modulated by increasing the pressure on top of the compressing substrate.For this, four loading masses of 1.60, 58.90, 130.00, and 305.65 g were used to modulate the hydrogel thickness.These weight values include the weight of the 25 × 25 mm glass substrates employed to compress the photo-resin.Figure 4 shows the thickness of dried Au-pNIPAM films as a function of the loading mass weight measured by AFM.The upper x-axis represents the equivalent applied pressure with each loading mass on a 25 × 25 mm area.Three replica samples of Au-pNIPAM films were prepared for each mass.The AFM measurements were performed at three positions per sample, as illustrated in Figure S4a (Supporting Information).Then, at each position, three profile lines were considered for analysis as shown in Figure S4b-d   mation).Hence, the thickness values presented in Figure 4 correspond to the mean thickness of the replica samples, while the error bars were calculated as the propagation of standard deviation obtained for each sample.As expected, the thickness of Au-pNIPAM films is inversely proportional to the applied compressing pressure, [55] permitting modulation of the thickness from 2.3 to 1.2 μm with a variation of <3%.

In Situ Overgrowth of the Gold Cores
In order to enhance the plasmonic properties of Au-pNIPAM films, an in situ overgrowth protocol to increase the size of the gold cores -described in the Experimental Section -was carried out. [66]To this end, six Au-pNIPAM films were prepared on 3-MPS-functionalized glass substrates of 7 × 25 mm in size.Then, five of the six prepared Au-pNIPAM films were completely dipped into the gold growing, and subsequently removed from the solution at 5, 10, 15, 25, and 45 min of growing time, respectively, to interrupt the reaction.Figure 5a shows an image of the grown-core Au-pNIPAM films, where, from left to right, it is possible to see an increase in the film coloration as the overgrowth time increases.This increase in color intensity is attributed to both a rise redshift of the extinction spectrum of the overgrown gold cores. [67]Growing times varying from 0 min (reference film) to 45 min resulted in plasmon peak wavelength shifting from 530 to 540 nm, as presented in Figure 5b.The error bars correspond to the standard deviation calculated from the extinction spectra measured at three different positions along the film.Regarding the morphology of the grown gold cores, Figure 5c shows three SEM images of the reference film (green frame), the film after a reaction time of 10 min (light blue frame), and after a reaction time of 45 min (red frame).As observed in the images, there was an increase in the size of gold nanoparticles with increasing reaction time in the growth solution, and the particles mostly grew quasi-isotropic (spherical nanoparticles).The mean particle diameter -determined from the SEM images -increased from 14 ± 1 nm in the reference film to 46 ± 16 nm during 45 min of reaction time.For 10 min of reaction time, the mean particle size was 34 ± 8 nm.In Figure S5 (Supporting Information), SEM images for every studied overgrowing time and their corresponding particle size distribution and extinction spectrum can be found.The particle size distributions in Figure S5 (Supporting Information) may suggest a polydisperse population of grown particles.However, it is worth noting that the particles are not necessarily on the same plane when imaging, which affects the calculation of the mean diameter of the particles without implying a large polydispersity of these.On top of that, the LSPR peaks measured by extinction spectroscopy for each sample in Figure S5 (Supporting Information) are narrow and do not show a shoulder or additional resonance peak at higher wavelengths, as would be the case for anisotropically grown gold nanoparticles. [68]The mean particle size obtained in the films at the end of the overgrowth process is comparable to those reported in the literature for self-assembled monolayers of Au-pNIPAM core-shell nanoparticles under similar experimental conditions, such as HAuCl 4 and CTAB concentrations, and growing times.By plotting the LSPR peak positions against the mean diameter of the grown gold nanoparticles, a similar trend to that published by Müller, et al. is observed. [66]This is mainly because the overgrowth process is strongly diffusion-limited due to the surrounding pNIPAM hydrogel matrix.Figure S6 (Supporting Information) shows the LSPR peak positions as a function of the mean diameter of the grown gold nanoparticles.The error bars correspond to the standard deviation calculated for the LSPR peak positions and the particle size distributions.

Thermal-Response of Au-pNIPAM Films
The hybrid nature of the fabricated Au-pNIPAM films allows these materials to display unique properties, including plas- monic properties from the gold nanoparticles the thermoresponsive behavior from the pNIPAM hydrogel.This latter, expressing the volume phase transition of pNIPAM-based hydrogels (usually at 32 °C or slightly higher in water), was investigated by scattering measurements performed at different temperatures using a spectrofluorometer equipped with a temperaturecontrolled Peltier cell holder. [69]The 7 × 25 mm Au-pNIPAM film prepared as a reference in the previous section was used to perform the temperature-dependent measurements.For this, the film was placed in the cuvette containing Milli-Q water and let hydrate for 24 h before starting measurements.Figure 6a shows the variation of the scattering intensity at 400 nm as a function of temperature in both heating and cooling processes.Three temperature scans from 15 to 61 °C and 61 to 15 °C were performed for each heating and cooling process, respectively, in order to obtain mean scattering intensities with error bars corresponding to the standard deviation.As observed in the heating process (red curve), the scattering intensity slowly increases between 15 and 30 °C before showing a steep increase from 30 to 40 °C, i.e., in the range where the VPT of the pNIPAM hydrogel is expected.The VPPT was determined at ≈37 °C.During further heating, above 40 °C, the scattering intensity starts continuously decreasing until reaching 61 °C, where the cooling back process begins.During the cooling process (blue curve), a hysteresis behavior -particularly in the 40 to 61 °C temperature range -is observed.Below 40 °C, the hysteresis is less pronounced.This hysteresis behavior is attributed to the formation of microglobules in the collapsed state due to intra-and inter-chain hydrogen bond interactions occurring at higher temperatures, [70] limiting thus the hydration of polymer chains in the cooling process. [71,72]Also, such hysteresis in hydrogel thin films is amplified due to the constraints imposed by the cross-linking points, the gold nanoparticles, and the substrate. [72,73]These constraints slow down the reorganization kinetics of polymer chains in the collapsed state to reach the equilibrium state, which would explain the decrease in scattering intensity above 40 °C (Figure 6a) due to the short equilibration time employed (5 min). [73]Figure 6b shows the reversible behavior of the Au-pNIPAM film under continuous thermocycling between 20 and 50 °C with 30 min of equilibration time.A larger equilibration time resulted in a fully reversible behavior of the film with no signs of hysteresis in the scattering intensity upon heating and cooling cycles.Here it is noted that the scattering intensity at 50 °C in Figure 6b falls between the curves shown in Figure 6a, which suggests that the equilibrium condition in the collapsed state has been reached.
A complementary study was conducted to validate the hypothesis about the relationship between a short equilibration time and the kinetics of polymer chains, in the decrease of the scattering signal above the VPTT.This study employed UV-vis spectroscopy and similar experimental conditions.Figure S7a (Supporting Information) shows the extinction spectrum of the Au-pNIPAM as a function of temperature.At first glance, the extinction spectrum increases with temperature, and the LSPR peak redshifts as expected due to the shrinkage of the pNIPAM hydrogel. [74]However, by examining the evolution of the extinction spectrum at 400 nm as a function of temperature (Figure S7b, Supporting Information), a similar trend as that of Figure 6a was obtained, i.e., the extinction signal decreases above the VPTT, suggesting that the polymer chains slowly reorganize in the collapsed state before reaching the equilibrium state.

Photopatterning of Au-pNIPAM Films
Au-pNIPAM microgel structures were fabricated onto 3-MPSfunctionalized glass substrates by using photomasks for the photopolymerization step. [75]Acetate photomasks of 25 × 25 mm with square, circular, and rectangular micron-size shapes were employed.Figure S8 (Supporting Information) shows optical microscopy images of the photomasks with their corresponding shapes and dimensions.The arrayed Au-pNIPAM microgel structures were fabricated with the previously described method to obtain Au-pNIPAM films.Only 2 min of illumination at 405 nm with the collimated light beam (see Experimental Section for more details) followed by thorough rinsing with Milli-Q water resulted in well-defined and isolated Au-pNIPAM microgel structures.Figure 7 presents bright-field microscopy images of photopatterned Au-pNIPAM microgels, including squares (Figure 7a), disks (Figure 7b), and rectangles (Figure 7c).The inner images of Figure 7a-c correspond to SEM images captured at a tilted angle of 52°to evince the anisotropy (high width-to-height aspect ratio) of the obtained microgels.As observed from the images, the Au-pNIPAM microgels are completely isolated from each other and separated at ≈100 μm.The measured dimensions for the three distinct microgel shapes reveal a narrow size distribution, namely (78 ± 4) × (72 ± 4) μm per side for the square microgels (Figure 7a), (78 ± 2) μm in diameter for the disk microgels (Figure 7b), and (123 ± 6) × (53 ± 4) μm in length and width, respectively, for the rectangle microgels (Figure 7c).It is worth noting that the microgels were completely dehydrated when imaging, which explains the difference in size between the microgels and their respective photomask. [76]long with the fabrication of patterned Au-pNIPAM microgel structures on solid substrates, the proposed method may be applied to synthesize suspending anisotropic microgels.To demonstrate this, the photopatterning of square and rectangular microgel shapes was conducted on Piranha-cleaned glass substrates to allow the detachment of microgels from the substrate.After 2 min of photopolymerization, the micro-structured substrates were immersed in a 1 m NaCl aqueous solution to split off the anisotropic microgels from the glass. [77]It is to be noted that no additional purification steps were performed for these tests.Figure S9 (Supporting Information) shows bright-field images of square and rectangular microgels in solution at room temperature.As shown, free-floating microgels were obtained, maintaining the original anisotropic shapes without showing signs of aggregation.Figure 8 displays images of the microgels in water at 22 and 37 °C, where an evident decrease in size is observed.Indeed, the measured dimensions shown in the images exhibit a reduction -in terms of area -of ≈30% for the square microgels and ≈50% for the rectangular microgels.It is worth noting that the rectangular (rod-like shape) microgels maintained their aspect ratio at the collapsed state, resulting in a rod-to-rod collapse. [78]lso, the sizes of the hydrated microgels at room temperature (22 °C) coincide with that of their corresponding photomask shown in the Figure S8 (Supporting Information).

Conclusion
In summary, this work presented a simple compression-based method to fabricate thin Au-pNIPAM hybrid films through the photopolymerization of a photo-resin containing Au-pNIPAM core-shell microgels as light-sensitive nanocomposites.The proposed fabrication method allowed the synthesis of uniform Au-pNIPAM films as determined by UV-vis spectroscopy.The co- efficients of variation of the LSPR intensity obtained from various samples and various positions per sample were <2% in both cases.Also, flexible Au-pNIPAM films with pronounced plasmonic and robust mechanical properties were fabricated by using commercial acetate sheets as supporting substrates.The thickness of the Au-pNIPAM films was adjusted from 2.3 to 1.2 μm (at dried state) with a variation of <3% per sample by increasing the pressure applied to compress the photo-resin.Moreover, the plasmonic properties of the hybrid films were successfully modulated via an in situ overgrowth protocol that was used to increase the size of the embedded gold nanoparticles.The results showed an increase in the size of gold nanoparticles from 14 to 46 nm in diameter for 45 min of reaction time in the growth solution, leading to a plasmon peak shift from 530 to 540 nm.Mostly gold nanoparticles of spherical shape were obtained as observed from SEM images and extinction spectra.As for the thermo-responsivity of the Au-pNIPAM films, both temperature-controlled scattering and UV-vis spectroscopy measurements confirmed the thermoresponsive behavior of the films with a transition temperature of ≈37 °C.Despite the hysteresis observed during heating and cooling cycles due to constraints in the reorganization kinetics of the polymer in the collapsed state, the responsivity of the film remained reversible between the swollen and collapsed states.On the other hand, the use of a photomask during the photopolymerization process of Au-pNIPAM films permitted the patterning of anisotropic Au-pNIPAM microgels, including squares, disks, and rectangles; as well as obtaining anisotropic microgel aqueous suspensions.These free-floating anisotropic microgels maintained their thermo-sensitivity by decreasing in size when increasing the temperature of the water, as confirmed by brightfield microscopy images.To conclude, our compression-based method permitting the fabrication of thin hydrogel films and anisotropic microgel composites contributes to the development of novel, multifunctional hydrogel-based soft materials.
Synthesis of pNIPAM-Encapsulated Gold Nanoparticles: Gold seed nanoparticles of ≈15 nm in diameter were synthesized by the onestep Turkevich method using sodium citrate dehydrate as a reducing agent. [50,79]Briefly, in a clean 1000 mL round bottom flask equipped with a magnetic stirrer and a reflux condenser, 500 mL of a 0.5 mm aqueous HAuCl 4 solution was brought to a boil using an oil bath at 120 °C under vigorous stirring (900 rpm).Then, 25 mL of a 1 wt.% hot aqueous solution (85 °C) of SCTD was quickly added to the reaction vessel and left to stir for 20 min before cooling down to room temperature at a lower stirring speed (600 rpm).Next, the gold seeds were functionalized with B-en-A to render the particles hydrophobic and hence promote the seeded precipitation polymerization of NIPAM on their surface. [50]To this, 3 mL of a 1 mm aqueous SDS solution was added into the gold nanoparticle stock solution to increase their stability during subsequent centrifugation steps.After 20 min of continuous stirring (600 rpm) at room temperature, 1.63 mL of a 1.4 mm ethanolic B-en-A solution was dropwise added to the solution and let stir for another 20 min.The functionalized gold nanoparticles were then centrifugated for 14 h at 1000 RCF (Relative Centrifugal Force) and redispersed in 5 mL of Milli-Q water.The final concentration of particles (in particles/mL) was determined by nanoparticle tracking analysis (NTA).Finally, the encapsulation of gold seeds nanoparticles with a pNI-PAM shell was performed via seeded precipitation polymerization. [50,51]In a 250 mL three-neck round-bottom flask equipped with a magnetic stirrer and a reflux condenser, 228 mg of NIPAM and 48 mg of BIS (15 mol.% with respect to NIPAM) were dissolved in 98.3 mL of Milli-Q water under continuous stirring (600 rpm) at room temperature.Upon complete mixing, the solution was heated to 70 °C and purged with nitrogen to remove oxygen.After 20 min, 1.7 mL of butenylamine-functionalized gold seeds were added dropwise to the solution.Here, the volume of functionalized gold nanoparticles was adjusted to obtain the right concentration of nucleation points during the polymerization process as described by Sepúlveda et al. in order to control the encapsulation yield of gold cores. [51]After 15 min of equilibration time, the polymerization was initiated with the quick addition of 1 mL of a 1.85 mm aqueous KPS solution and allowed to proceed for 2 h before cooling down to room temperature.The resulting Au-pNIPAM core-shell microgels were purified by three consecutive centrifugations at 9400 RCF until obtaining a clear supernatant to then redisperse them in 5 mL of Milli-Q water and freeze-dry them.
Preparation of Pre-Polymer Solution: The pre-polymer solution or resin was prepared by the direct addition of freeze-dried Au-pNIPAM core-shell microgels into the solution mixture containing the NIPAM monomer, BIS cross-linker, and TPO photoinitiator reactants.To this, in a clean 20 mL scintillation vial equipped with a magnetic stirrer, 1.132 g of NIPAM (1 m), 0.231 g of BIS (0.15 m), and 1 wt.% of TPO were mixed in a 10 mL solution mixture of 1 Milli-Q water : 3 n-propanol under continuous stirring (600 rpm) and away from room light.After 20 min, 5 wt.% of freeze-dried pNIPAM-encapsulated gold nanoparticles were added to the solution mixture and let to stir in for 1 h.Then, the red-colored resin was stored away from the room light at 4 °C.
Cleaning and Functionalization of Microscope Glass Slides: Before cleaning and functionalizing microscope glass slides, 25 × 75 × 1.0 mm microscope slides (Fisher Scientific) were cut into three pieces.The 25 × 25 mm glass substrates were immersed in a piranha solution (3 H 2 SO 4 :1 H 2 O 2 ) for 30 min at room temperature and then thoroughly rinsed with Milli-Q water and isopropanol.The substrates were dried in the oven at 70 °C for 15 min.Half of the piranha-cleaned glass substrates were functionalized with the organosilane coupling agent 3-MPS to graft the hydrogel film onto the glass substrate. [55]To this end, the clean glass substrates were completely immersed for 1 h in a 10-vol.%acetone solution of 3-MPS.Next, the substrates were rinsed with acetone, methanol, and isopropanol to finally bake at 120 °C for 10 min.
Fabrication of Thin Au-pNIPAM Films: Light-sensitive thin Au-pNIPAM films were fabricated using a simple and compression-based approach.Briefly, in a homemade photopolymerization setup (see Section S1, Supporting Information for more details), 14 μL of resin was directly pipetted on the 3-MPS-functionalized glass substrate to then spread over the surface uniformly with the aid of the piranha-cleaned substrate, forming a sandwich-like structure.Then, the 405-nm LED (1.5 mW) was turned on for 3 min to let the process of polymerization take place to peel off the piranha-cleaned substrate later gently.The resulting thin Au-pNIPAM film was immersed in Milli-Q water for 30 min and then left overnight to get rid of any unreacted compound.Lastly, the film was dried at room temperature.
In Situ Overgrowth of Gold Nanoparticles: For the in situ gold core overgrowth of gold nanoparticles in the hydrogel film, the protocol reported by Müller et al. was followed with some slight modifications. [66]In a clean beaker equipped with a magnetic stirrer, 100 mL of an aqueous 0.1 M CTAB solution was continuously stirred (300 rpm) for 45 min and at 30 °C for better solubilization and foam decreasing.Next, 417 μL of an aqueous 0.1 m HAuCl 4 solution was added dropwise into the solution.After 5 min, 588 μL of a freshly prepared aqueous 0.1 m AA solution was added dropwise.To overgrow the gold cores, the substrate-supported thin films were dipped into the gold-growing solution, allowing the reaction to take place for the desired growing time without stirring.Then, the grown-core films were thoroughly rinsed with Milli-Q water to stop the reaction and then left overnight in Milli-Q water to remove any unreacted compound.Finally, the films were dried at room temperature.
Photo-Masked Thin Au-pNIPAM Films: The patterning of Au-pNIPAM films was carried out using acetate photomasks of 25 × 25 mm in size (printed by LG Chabot, Inc.) that allowed the photopolymerization of micron-sized structures onto the substrates.In order to obtain welldefined features, a 4f Köhler system was implemented to collimate the light illumination from the 405-nm LED source.More details about the implemented optical system can be found in ref. [80] As well as the original photopolymerization setup (Figure S1, Supporting Information), illumination occurs from the bottom to the top for employing the same fabrication procedure described above.Here, the photo-patterned Au-pNIPAM films were cured for 2 min and then thoroughly rinsed with Milli-Q water to get rid of the unreacted resin.At last, the films were dried at room tempera-Optical Characterization: pNIPAM-encapsulated gold nanoparticles in solution were characterized by UV-vis spectrophotometry (Cary 50, Agilent Technologies) using 10 mm path-length quartz cuvettes.Au-pNIPAM films were characterized by UV-vis-NIR spectrophotometry with a scanning dual-beam spectrophotometer (Cary 5000, Agilent Technologies).Dried substrate-supported Au-pNIPAM films were mounted on a homemade 3D-printed holder (Ultimaker S3, Ultimaker BV), allowing measurements in nine different positions for 25 × 25 mm substrates.
Thickness Characterization: To characterize the thickness of Au-pNIPAM films, atomic force microscopy (AFM) images were carried out on a scanning probe microscope (NanoScope V, Veeco) in tapping mode using antimony-doped silicon cantilevers (TESPW-V2, Bruker).Data visualization and analysis were performed with Gwyddion software.
Thermo-Responsive Characterization: The thermo-responsive behavior of Au-pNIPAM films was characterized via temperature-controlled scattering measurements on a Fluorolog 3 spectrofluorometer (Horiba Scientific).The parameters used were: front-face detection,  exc = 400 nm (0.5 nm slit),  em = 390-410 nm (0.5 nm slit).Temperature scans were set from 15 to 61 °C and 61 to 15 °C in 2 °C steps with 5 min of equilibration time, while thermo-cycling measurements were set between 20 and 50 °C with 30 min of equilibration time.Temperature-dependent extinction spectra were recorded with a Specord S 600 spectrophotometer equipped with a Peltier temperature-controlled sample changer (Analytik Jena AG).Spectra were recorded in the wavelength range of 183-1019 nm in transmission geometry with the film facing toward the detector.The temperature measurements were performed over the range between 15 and 61 °C in 1 °C steps with equilibration times of 3 min.The glass substrates with the attached films were placed in a 1 cm PMMA cuvette containing water.A glass substrate placed in a 1 cm PMMA cuvette filled with water was used as a reference for the background correction.
Transmission Electron Microscopy: The morphology and gold core size distribution of pNIPAM-encapsulated gold nanoparticles were determined by transmission electron microscopy (TEM; model JEM-1230, JOEL) at an accelerating voltage of 80 kV.Samples were prepared by drop-casting a dispersion of particles on carbon-coated copper TEM grids (200 mesh, Electron Microscopy Sciences) and letting them dry in the air prior to analysis.Image analysis was performed for ≥200 particles using ImageJ software.
Scanning Electron Microscopy: Imaging of Au-pNIPAM films was characterized by scanning electron microscopy on a dual-beam SEM/FIB microscope (Quanta 3D FEG, FEI) in a backscattering electron (BSE) signal with an accelerating voltage of 5 kV to minimize artifacts and film damages due to surface charging.Cross-section images of Au-pNIPAM films were obtained by focused ion beam (FIB) milling.To this, samples were sputter-coated with a thin Pt layer (≈10 nm) prior to analysis.
Optical Microscopy: Photopatterned Au-pNIPAM microgel structures were characterized by bright-field microscopy on an upright optical microscope (Olympus BX53, Olympus).Arrayed Au-pNIPAM microgels on glass substrates were let dry overnight at room temperature prior to imaging, while Au-pNIPAM microgels in suspension were overnight hydrated in a Petri-dished with Milli-Q water for direct imaging.For the temperaturecontrolled images, a water bath setup using Petri dishes was employed to control the temperature of the microgels in liquid suspension.

Figure 1 .
Figure 1.Schematic representation of the compression-based method to fabricate light-sensitive thin Au-pNIPAM films.

Figure 2 .
Figure 2. a) Photograph of the photopolymerized Au-pNIPAM film on a glass substrate, b) normalized extinction spectra of the Au-pNIPAM film measured at nine different positions, c) top-view SEM image of the Au-pNIPAM film, and d,e) cross-section SEM images of the Au-pNIPAM film.

Figure 3 .
Figure 3. a) Malleable Au-pNIPAM film prepared on a cellulose acetate sheet, b) mean extinction spectrum (normalized at 400 nm) of the acetatesupported Au-pNIPAM film,and c) its extinction spectra after twenty consecutive bending cycles (normalized at 400 nm).The inset plot shows the variation of the plasmon peak intensity (at 532 nm) as a function of the bending cycle.

Figure 4 .
Figure 4. Thickness of dried Au-pNIPAM films as a function of the loading mass weight.

Figure 5 .
Figure 5. a) Photographs of the Au-pNIPAM films after different reaction times in the growth solution, b) plasmon peak wavelength of the Au-pNIPAM films as a function of the growing time, and c) SEM images of the films at 0 min (green frame), 10 min (light blue frame), and 45 min (red frame) of growing time.Scale bars are 250 nm.

Figure 6 .
Figure 6.Thermo-responsive behavior of the Au-pNIPAM film.a) Scattering intensity as a function of temperature during heating and cooling scans, and b) reversible behavior under thermocycling between 20 and 50 °C.

Figure 7 .
Figure 7. Bright-field microscopy images of a) square, b) circular, and c) rectangular Au-pNIPAM microgels on glass substrates.Inset images correspond to SEM images at a tilted angle of 52°.Black scale bars are 500 μm white scale bars are 300 μm.