Patchy and Multiregion Janus Particles with Tunable Optical

Jan 11, 2010 - Bulk synthesis of Janus objects and asymmetric patchy particles. Gabriel Loget , Alexander Kuhn. Journal of Materials Chemistry 2012 22...
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Patchy and Multiregion Janus Particles with Tunable Optical Properties Marla D. McConnell, Matthew J. Kraeutler, Shu Yang,* and Russell J. Composto* Department of Materials Science and Engineering and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania 19104 ABSTRACT Multiregion and patchy optically active Janus particles were synthesized via a hierarchical self-assembly process. Gold nanoparticles were assembled on the top surfaces of nano- and submicrometer silica particles, which were selectively protected on their bottom surfaces by covalent attachment to a copolymer film. The morphologies of the gold particle layer, and the resulting optical properties of the Janus particles, were tuned by changing the surface energy between the silica and gold particles, followed by annealing. KEYWORDS Nanoparticle, polymer, Janus, wetting, plasmon

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anus particles are traditionally composed of two chemically distinct regions, making them suitable for applications as biological sensors, nanomotors, antireflection coatings, optical sensing devices, and two-phase stabilizers as well as for fundamental studies in asymmetric particle assembly.1-4 Many techniques have been established to synthesize these asymmetric particles, including the photopolymerization of precursors in a microfluidic channel,5,6 layer-by-layer assembly,4,7 and biphasic electrified jetting.8 Janus particles have also been fabricated by the application of an evaporated metal coating onto the top surfaces of particles in a close-packed monolayer9-11 or of particles trapped in a solid medium.12 Recently, interest has emerged in the synthesis of complex patchy and multiregion Janus particles. Such particles provide additional chemical surface variation, relative to tworegion Janus particles, which results in an increased number of available assembly mechanisms and morphologies. Control over the percent modification of the Janus particle has been achieved by embedding 500 nm silica particles into one-dimensional fibers, followed by application of a sparse coating of gold nanoparticles.13 Micrometer-sized particles enclosed in a polymer film have been exposed to an oxygen plasma for varying times to reveal different amounts of the particle surfaces, which were subsequently modified with 60 nm silica particles.14 Micrometer-sized “patchy” Janus particles, in which the top surface of the particles are patterned with gold patches of various shapes and sizes, have been assembled by sputter coating gold via glancing angle deposition onto a close packed monolayer of colloidal PS latex particles.15 Multiregion Janus particles have been synthesized via a 2-fold microcontact printing method, in which

each pole of the parent particle is functionalized with a different type of molecular monolayer, which allowed the particles to be modified further with smaller secondary particles.16 Because metal nanoparticles (NPs) have dimensionally dependent surface plasmon resonances that absorb radiation in the visible region of the spectrum, there has been much focus on manipulating their size, shape, and architecture.17-21 The formation of gold nanoshells around a different core material has garnered much attention, because these hierarchical structures allow for the plasmon resonance to be tuned into the near-IR wavelength.18,19 Janus particles that incorporate optically active species such as gold are attractive for many applications, including as sensors. Here we develop a new approach to create self-assembled, optically tunable, multiregion and patchy gold-onsilica Janus particles. Multiregion particles with gold caps on the top surface, gold patches around the particle equator and silica bottoms, and Janus particles with tunable gold patch sizes can be synthesized on the nanometer and submicrometer scale. The multiregion and patchy Janus particles can both be synthesized from a single self-assembly method, as seen in Figure 1. We have previously shown that amine-modified silica NPs can be covalently attached to styrene-acrylic acid random copolymer films.22 As the reaction progresses, the reaction solvent swells the polymer substrates, causing the silica particles to sink into the films, while the polymer chains wet up the sides of the particles.23 We utilize this spontaneous process to protect the bottom regions of 106, 230, and 460 nm silica particles. Gold NPs are then electrostatically assembled on the top surfaces of the amine-modified silica particles. On the 230 and 460 nm silica particles, the gold NPs form a closely packed network. This packing is dependent on the dimensions of the underlying silica particles and results in a red shift of the gold NP optical absorption peak, relative to their absorption in solu-

* Corresponding authors: R. J. Composto, [email protected]; S. Yang, [email protected]. Received for review: 10/30/2009 Published on Web: 01/11/2010 © 2010 American Chemical Society

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FIGURE 1. Schematic representation of the self-assembled formation of patchy and multiregion Janus particles. 230 nm silica particles were covalently attached to the surface of P(S-r-AA) and subsequently sunk into the polymer film as a function of reaction time. 15 nm gold NPs were then electrostatically assembled onto the surface of 230 nm amine modified silica particles. These hybrid particles were then annealed with and without compatibilizing agents to form the Janus particles.

tion. On the 106 nm silica particles, the gold NPs do not form a closely packed network, so their optical absorption is unshifted, relative to their absorption in solution. The surface energy of the silica particles was then manipulated in order to control the dewetting of gold nanoparticles to form patches or gold hemispherical caps upon thermal annealing. By manipulating the surface energy of the silica particles and the annealing time, it is possible to tune the absorption of these Janus particles from the visible to the near-IR region, making them ideal candidates for optical temperature sensors. Due to their chemical and optical tunability, these particles can also be used as biological sensors. The Janus particles created in this study were sequentially self-assembled from silica and gold NPs. Silica NPs were synthesized according to the Sto¨ber method.24 Particles of 230 ( 19 nm were synthesized by adding 5 mL of ethanol solution containing 2.4 M TEOS to a 35 mL ethanol solution of water and ammonia. Particles of 460 ( 20 nm were synthesized similarly, and 106 ( 7.3 nm particles were obtained from Nissan Chemical (IPA-ST-ZL). Particle diameters were characterized by scanning electron microscopy (SEM), and reported diameters are the average of at least 100 particles measured with ImageJ, ( one standard deviation. SEM images and the histograms of frequency versus particle diameter can be seen in Figure S1 of the Supporting Information. Particles were then solvent-exchanged into anhydrous DMSO and amine-functionalized via reaction with (aminopropyl)triethoxysilane (APTES). An excess of APTES was added to the NP suspension in anhydrous DMSO and allowed to react for 8 h. The NPs were then purified via centrifugation and solvent-exchanged into ethanol. To prepare films of poly(styrene-ran-acrylic acid) (P(S-ranAA)), first a 70:30 mol:mol poly(styrene-random-tert-butyl acrylate) (P(S-ran-tBA)) polymer was synthesized using RAFT polymerization.22 The polymer was spin cast onto silicon support substrates that had been cleaned in piranha solution (3:7 v:v H2O2:H2SO4) at 80 °C for 20 min and exposed to UV-ozone for 10 min to form a uniform oxide layer. The P(S-ran-tBA) film thicknesses were measured with ellipsom© 2010 American Chemical Society

etry to be 200 nm. The P(S-ran-tBA) was converted to P(Sran-AA) by annealing the polymer films at 185 °C for 15 h under Ar. These annealing conditions resulted in complete deprotection of the tert-butyl groups, as characterized using attenuated total reflectance Fourier transform infrared (ATRFTIR) spectroscopy (Nicolet Nexus 470 with MCT-B detector, Harrick GATR). The spectra of the as-cast P(S-ran-tBA) films show the characteristic bending and stretching modes associated with tert-butyl acrylate and styrene. The absorption bands originating from poly(tBA) are located at 1730 cm-1 (νCdO, ester), 1394/1368 cm-1 (νCH3), 1277/1258 cm-1 (νC-O-O), and 1160 cm-1 (νC-O). The bands originating from polystyrene are located between 1600 and 1400 cm-1 (νCdC, aromatic). After the P(S-ran-tBA) films are annealed, the peaks associated with the tert-butyl protecting group disappeared completely, and the carbonyl stretching mode broadened and underwent a blue shift of ∼30 cm-1. These changes in the carbonyl peak are consistent with deprotection of the tert-butyl group to form acrylic acid.22 The covalent attachment of silica NPs to P(S-ran-AA) films was previously described.22 There are two steps in the covalent attachment of amine-modified silica NPs to the acid groups in the P(S-ran-AA). In the first step, the P(S-ran-AA) films were immersed in a solution of ethylene dichloride (0.1 M) and N-hydroxysuccinimde (NHS) (0.2 M) in deionized ultrafiltered (DIUF) water for 1 h, in order to activate the acrylic acid groups with NHS. The FTIR spectrum of NHSactivated acrylic acid in the P(S-ran-AA) has a strong band at 1740 cm-1, which is attributed to the succinimidyl carbonyl (νCdO). In the second step, the activated substrates were immersed, upside-down, into suspensions of the aminemodified particles in ethanol. This geometry minimized the number of particles that nonspecifically adhered to the substrates and is responsible, in part, for the reproducibility of particle coverage over macroscopic (micrometer) areas, as well as uniform particle spacing. Once the substrates were removed from the nanoparticle suspensions, they were immediately swirled in ethanol for 5 min, washed vigorously with DIUF water, and dried in a stream of nitrogen. Covalent 604

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FIGURE 3. SEM and corresponding optical images of untreated, uncoated 230 nm hybrid structures after annealing for (A) 0 s, (B) 5 s, (C) 30 s, (D) 2 min, (E) 5 min, (F) 1 h, (G) 8 h, and (H) 24 h. at 350 °C. Upon annealing the gold, NPs fuse (B) and then begin to dewet (C). After 1 h the gold patch size remains essentially unchanged, indicating that the gold has fully dewetted from the silica surface. SEM image scale bars are 100 nm and optical image scale bars are 1 cm.

FIGURE 2. SEM images of 15 nm gold particles assembled on the surfaces of (A) 106, (B) 230, and (C) 460 nm amine-modified silica particles. The network of highly clustered gold particles can be seen in (B) and (C). SEM scale bars are 100 nm, and optical image scale bars are 1 cm. (D) UV-vis absorption spectra of the 15 nm gold NPs in solution, assembled into a closely packed network on 230 nm silica particles, and assembled on 106 nm silica particles.

because of the variation in curvature of the silica particles. On smooth planar surfaces, NPs have been shown to have a maximum coverage of 30%;22,26 however, on surfaces with roughness features on the order of the size of the assembling particles, coverages up to 70% have been observed.22,23 Although UV-vis characterization of 15 nm gold NPs in solution shows a peak at 520 nm, the formation of a network of randomly packed gold NPs on the surface of the 230 nm silica particles induces a red shift in the absorption peak to 673 nm, as seen in Figure 2D. This shift of 153 nm results from plasmon coupling between adjacent NPs. Because the gold NPs do not closely pack on the surface of the 106 nm silica particles, the absorption peak does not undergo a shift and remains at 520 nm, as seen in Figure 2D. UV-vis data are not available for the 460 nm particles, because the silica particles strongly scatter the incident radiation. This packing of the gold NPs causes the 230 and 460 nm hybrid particle films to appear blue, as seen in the insets in parts B and C of Figure 2, while the 106 nm hybrid films appear red, Figure 2A. Additionally, it was found that 800 nm silica particles do not induce close packing of 15 nm gold particles, and therefore also appear red (data not shown). Gold NPs have been shown to reshape at temperatures below their melting temperature.27 Lu et al. showed that the evaporated gold caps on silica Janus particles will dewet at the edges from the adhesion layer and form gold crystals ranging in size from 300 to 650 nm.11 Additionally, Xiu et al. showed that sputter-coated gold films on top of silica particles formed gold NPs after annealing at 620 °C.28 Therefore, by annealing the hybrid particles at relatively low

attachment of the nanoparticles by amide bond formation was confirmed by ATR-FTIR of the P(S-ran-AA)-nanoparticle substrate, where the band at 1653 cm-1 is attributed to the amide carbonyl (νCdO). We have previously shown that amine-modified silica nanoparticles ranging from 15 to 230 nm undergo sinking into P(S-ran-AA) films as a function of reaction time.23 Additionally, P(S-ran-AA) has been shown to wet the surface of the amine-modified NPs. By choosing an appropriate reaction time, it is possible to shield the bottom of the silica particles (106, 230, 460 nm) with P(S-ran-AA), leaving the top surfaces of the silica NPs in the original, amine-modified state and available for further reaction with gold NPs, as shown in Figure 1. This method is versatile, allowing for the attachment of any negatively charged nanoparticle. Gold NPs (15 ( 3 nm) were synthesized by adding 18 mL of 38.8 mM trisodium citrate to a rapidly boiling solution of 182 mL of 1 mM HAuCl4.25 SEM images and the histograms of frequency versus particle diameter can be seen in Figure S1 in Supporting Information. Because the gold NPs are negatively charged, due to the surfactant layer, and the exposed portions of the amine-modified silica particles are positively charged, it is possible to assemble the gold NPs onto the surface of the silica particles via electrostatic interactions. SEM images of these hybrid particles (15 nm gold NPs assembled on the surface of 106, 230, and 460 nm silica particles) can be seen in Figure 2A-C. Differences in packing of the gold NPs on the larger silica NPs likely occur © 2010 American Chemical Society

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ran-AA) revealed that the polymer was completely removed after 4 h at 350 °C. SEM images of the annealed 230 nm hybrid particle samples, Figure 3, show that the randomly close-packed network of gold NPs on the surface of the silica NPs fuse at early time points (5 s) and then begin to dewet from the silica particle surface (t < 2 min). At later times, this dewetting process generates discrete patches of gold that are 44 ( 10, 38 ( 10, and 36 ( 7 nm in diameter upon annealing for 1, 8, and 24 h, respectively. Because the network of randomly packed gold nanoparticles fuses together and then breaks up into discrete patches upon annealing, the corresponding optical absorption peak, λmax, undergoes a blue shift as a function of annealing time, as seen in Figure 4 (spectra can be seen in Figure S2 in Supporting Information). Initially, the absorption peak increases from 673 (0 s) to 683 nm (5 s), as NPs fuse, and then, as the gold dewets, decreases rapidly to 548 nm (1 h), and then approaches a constant value, 535 nm (24 h). Correspondingly, samples annealed for 1 to 24 h appear red, as seen in the insets in Figure 3. This thermally induced dewetting occurs because the high surface energy of gold (γAu ) 1.5 J/m2)28 relative to silica (γSiO2 ) 0.04 J/m2)29,30 makes it energetically unfavorable for the gold to wet the surface of the silica particles. In order to improve the wettability and increase the tunability of the morphologies of gold on the silica surfaces, three treatments were applied to the surface of the hybrid particles. In the first treatment, a thin layer (∼10 Å) of Au was evaporated onto the 230 nm hybrid particle surface. However, the results of this treatment were essentially identical to those observed for the untreated, annealed hybrid particles, where the gold NPs fused together and then dewetted from the surface of the silica particle, as seen in Figure 3. Because chromium is a known compatibilizer for

FIGURE 4. λmax vs annealing time for the untreated 230 nm hybrid particles at 350 °C. The maximum absorption wavelength initially increases after annealing for 5s, due to the presence of the fused NPs, and begins to decrease rapidly once the gold dewets from the surface of the silica particles. The circled data point is t ) 0.

temperatures, the morphologies, and consequently the plasmon resonances, of the gold NPs can be thermally controlled. By combining an annealing step and different wetting agents (Au, Au + Cr, and Au:Pd), a wide array of gold morphologies, from gold patches to hemispherical caps, can be attained. Because the 15 nm gold NPs form a network of randomly packed particles on the 230 nm silica particles, these hybrid particles were exclusively used in the annealing study. The 230 nm hybrid particles were annealed in a muffle furnace at 350 °C for 5 s to 24 h. This step simultaneously altered the morphology of the gold NPs and removed the protective polymer layer on the bottom surface of the silica NPs. Thermal gravimetric analysis (TGA) of P(S-

FIGURE 5. SEM images of untreated hybrid particle structures after annealing at 350 °C for (A) 1, (B) 8, and (C) 24 h. SEM images of hybrid particles treated with 5 Å Cr + 10 Å Au after annealing for (D) 1, (E) 8, and (F) 24 h. SEM images of hybrid particles treated with 5 Å Cr + 30 Å Au after annealing for (G) 8 and (H) 24 h. Insets are the corresponding optical image for each sample. SEM scale bars are 100 nm and optical image scale bars are 1 cm. (I) λmax as a function of annealing time for the ([) unannealed, (9) 5 Å Cr + 30 Å Au treated, (b) 5 Å Cr + 10 Å Au treated, and (2) no coating samples. Selective studies showed excellent reproducibility in λmax across multiple samples treated identically, with shifts (1-2 nm. Note: lines are guides to the eye. © 2010 American Chemical Society

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silica and gold,31 the second and third treatments involved the evaporation of a thin layer of Cr onto the hybrid particle surfaces, followed by the evaporation of another thin layer of Au to prevent oxidation of the Cr. In the first of these treatments, a 5 Å layer of Cr and 10 Å layer of Au were evaporated sequentially onto the 230 nm hybrid particle surfaces. The treated samples were subsequently annealed for 1, 8, and 24 h at 350 °C. In the second of these treatments, a 5 Å layer of Cr was evaporated onto the 230 nm hybrid particle surface followed by a 30 Å layer of Au. These samples were then annealed at 350 °C for 8 and 24 h. For both thicknesses of evaporated Au, SEM images show that the Cr compatibilizing layers slow the dewetting of the gold particles upon annealing, Figure 5D-H. In order to understand how the two treatments influence wetting, the kinetics of the gold dewetting from the silica particle surface were investigated. UV-vis absorption spectra of both of the Cr + Au treated, annealed samples show that the absorption peaks shift to smaller wavelengths and narrow as a function of annealing time (Figure S3 in Supporting Information). This behavior is indicative of the formation of individual gold patches as the dewetting process progresses. Additionally, a decrease in the peak absorption wavelength corresponds to a decrease in gold patch size. A comparison of the peak absorption wavelength as a function of annealing time for the untreated hybrid particles as well as the 5 Å Cr + 10 Å Au and 5 Å Cr + 30 Å Au treated hybrid particle surfaces can be seen in Figure 5I. The maximum wavelength of the untreated hybrid particles, “no coating”, quickly decreases with annealing time and approaches a constant value of 535 nm after 8 h. By comparison, the addition of a 5 Å Cr + 10 Å Au or a 5 Å Cr + 30 Å Au bilayer slows the dewetting, with λmax approaching 555 and 560 nm, respectively, after 24 h. This slowed blue shift in the maximum wavelength vs time, compared to the untreated samples, corresponds well with the SEM images (Figure 5A-H). The 5 Å Cr + 10 Å Au treated samples show both interconnected gold patches and larger discrete gold patches after 1 and 8 h (Figure 5D,E), as compared to the untreated samples, which are fully dewetted after 5 min (e.g., Figure 5A). Subsequently, at each annealing time, the maximum wavelength increases (i.e., the gold patch size increases) as a function of coating in the following manner: untreated ≈ (10 Å Au) < 5 Å Cr + 10 Å Au < 5 Å Cr + 30 Å, due to slowing of the dewetting process. The Au patch size as a function of annealing time confirms that the blue shift in the maximum wavelengths is due to the network of gold particles breaking up into individual gold patches (Supporting Information, S4). At each time point, the Cr + Au treated samples exhibit larger gold patch sizes than the untreated samples. By tuning the surface energy between the silica and gold particles, i.e., changing the wetting agent, the morphology and size of the patchy Janus particles can be easily tuned. © 2010 American Chemical Society

FIGURE 6. SEM images of the Au:Pd treated hybrid particles after annealing at 350 °C for (A) 1, (B) 4, (C) 8, and (D) 24 h. Scale bars are 100 nm. (E) UV-vis absorption spectra of the Au:Pd coated, hybrid particle before annealing and after 24 h of annealing at 350 °C. The UV-vis of the Au:Pd coated and annealed sample is a spectrum from three substrates worth of particles. The peak and discontinuity at 800 nm are an instrumental artifact.

Although the Cr + Au wetting agent did not prevent dewetting of the gold from the silica surface, it significantly affected the energetics of gold wetting on the silica surface, compared to the untreated surfaces. This is particularly true during the early stages of the annealing process (t < 24 h). Previous studies have shown that, upon annealing a 200 nm Au layer on top of a 6 nm layer of Cr at 400 °C for 8 h, Cr atoms diffuse into the gold layer. After the sample was annealed for 16 h, the Cr layer had completely diffused into the gold layer, forming a solid solution with the gold. Additionally, Cr had diffused through the gold layer to reach the surface and oxidized, forming Cr2O3.31 Thus, in our Cr + Au system, Cr diffusion into Au and the resulting formation of Cr2O3 is the likely cause of the slowed dewetting of the gold from the silica surface. Because of the strong shift of the absorption wavelength (and color change) in response 607

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FIGURE 7. SEM images of the multiregion Janus particles after coating with Au:Pd and annealing for 24 h at 350 °C. Images of the tops and bottoms of the 106 (A and B) and 230 nm (C and D) Janus particles show the gold cap and protected silica surface, as well as the unincorporated gold patches along the equator of the silica particle. (E) SEM image of a side view of the 460 nm Janus particle showing all three regions in the multiregion particle. (F) SEM image of a group of multiregion, 230 nm Janus particles. The gold caps and rings of individual gold particles around the equator of the silica particles are both visible. Scale bars are 100 nm for A-E and 200 nm for F.

atoms are provided a path along the surface of the silica particles on which to wet. The UV-vis spectrum for the Au: Pd-capped 230 nm Janus particles can be seen in Figure 6E. Before annealing, the untreated 230 nm hybrid particles have an absorption peak near 673 nm; however, after annealing, the particles treated with Au:Pd alloy show an absorption peak around 750 nm and a shoulder around 550 nm. The tops (i.e., hemispherical gold caps) of the multiregion Janus particles (106, 230, and 460 nm) prepared using the Au:Pd coating-annealing process can be seen in Figure 7. Although the gold NPs on top of the 106 nm silica particles were originally well separated, addition of the Au:Pd wetting agent enabled the formation of a continuous hemispherical gold cap. The bottom surfaces of the silica particles were protected by P(S-ran-AA) during the initial assembly of the gold nanoparticles and, consequently, contain no gold particles, Figure 7. Around the circumference of the hybrid particles, a ring of isolated gold particles that are not part of the gold cap is observed as shown in Figure 7E. These individual particles can also be seen along the equator of the 230 nm particles in Figure 7F. These regions exist because the Au:Pd surface treatment is a line-of-sight process and does not fully coat the sides of the hybrid particles. Consequently, there are few Pd atoms present on the sides of the silica particles, and the clustered gold particles dewet in these areas. Thus, these particles exhibit three distinct regions: silica on the bottom, Au patches along the equator, and a Au:Pd hemispherical cap on top. The two goldcontaining regions are accounted for in the UV-vis spectra, Figure 6E, with a peak due to the gold cap (750 nm) and a shoulder (∼550 nm) due to the individual gold particles around the equator of the 230 nm silica particles. We have demonstrated a simple method for creating optically active, silica-gold, core-shell, patchy and multi-

to annealing conditions, these hybrid Janus particles could be utilized as temperature sensors. Specifically, at 300 °C the untreated hybrid particles undergo a blue shift in maximum absorption after about 2 min, at 350 °C a noticeable shift occurs after only 30 s, and at 600 °C the network of randomly packed gold particles dewets very rapidly, within 5 s of annealing. The addition of a chromium layer slows the gold dewetting process, enabling greater control over the morphology and size of the gold patches on the silica core. However, the Cr wetting layer does not allow the Au NPs to wet the silica and form a continuous Au layer. Thus, a different treatment was chosen to obtain multiregion Janus particles with gold caps on the surface of the silica particles. Because palladium atoms have a strong interaction with silica,32,33 we hypothesized that Pd would act as an adhesion layer between the gold NPs and the silica surface. A thin layer (∼15 Å, determined by AFM) of 60:40 Au:Pd alloy was sputtered onto the surface of the hybrid particle samples. The 230 nm hybrid samples were then annealed for 1, 4, 8, and 24 h at 350 °C in a muffle furnace. SEM images of the annealed 230 nm hybrid particle samples, Figure 6A-D, show the evolution of the Au:Pd coated, gold NP/silica particle hybrid structure into a continuous cap of gold. No further changes in the gold cap were observed upon annealing for an additional 24 h. Although Pd and SiO2 also have a large mismatch in surface energies (γPd ) 2 J/m2, γSiO2 ) 0.04 J/m2), Komiyama and Shimaguchi found that Pd easily occupies oxygen vacancies in SiO2 thin films.33 This suggests that Pd is much less likely than Cr to diffuse away from the SiO2 interface upon annealing. Additionally, Yu et al. found that Pt, a metal which also easily occupies oxygen vacancies in SiO2 thin films, wet a silica surface upon annealing at 500 °C.34 Because of the strong affinity between Pd and SiO2, the gold © 2010 American Chemical Society

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region Janus particles and investigated their optical responses to thermal annealing. During covalent attachment of amine-modified silica particles (106, 230, and 460 nm) to a P(S-ran-AA) film, the silica particles partially sink into the film. This process protected the bottom portions of the particles, while negatively charged 15 nm gold NPs were assembled onto the positively charged silica surfaces. The curvature of the larger 230 and 460 nm silica particles induced the formation of a network of closely packed gold particles, which caused a large red shift in the UV-vis absorption, relative to the gold nanoparticle solution. By treating these hybrid particles with a series of thin metal coatings (Au, Cr + Au, and Au:Pd) then annealing them at 350 °C, it was possible to manipulate their optical properties. Both the Au and Cr + Au treatments led to the dewetting of the gold particles from the surface of the 230 nm silica particles, resulting in a blue shift in the absorption peak. Treatment with Au:Pd, followed by annealing, led to the formation of a continuous cap of gold on the top surface of all the silica particles. These Janus particles exhibited a shift in the optical absorption peak to the near-IR region.

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Acknowledgment. This research is supported in part by the National Science Foundation (NSF) CAREER award (DMR-0548070) (S.Y., M.D.M.), NSF/MRSEC (DMR05-20020) (S.Y.,R.J.C.,andM.D.M.),andNSF/PolymerProgram(DMR0549307) and NSF/NSEC (DMR04-25780) (R.J.C., M.D.M.). M.D.M. acknowledges partial support from the Aston Fellowship (UPenn) and the Pennsylvania Muscle Institute (UPenn) for seed grant. M.J.K. acknowledges NBIC-REU.

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Supporting Information Available. Figures showing SEM images of Au and silica particles and histograms of frequency versus particle diameter, UV-vis absorption spectra for the untreated hybrid particles and hybrid particles treated with 5 Å Cr + 10 Å Au, and average particle size as a function of annealing time. This material is available free of charge via the Internet at http://pubs.acs.org.

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DOI: 10.1021/nl903636r | Nano Lett. 2010, 10, 603-609