Incorporation of Gold Nanorods and Their Enhancement of ...

J. Phys. Chem. C 2008, 112, 18895–18903

18895

Incorporation of Gold Nanorods and Their Enhancement of Fluorescence in Mesostructured Silica Thin Films Zhi Yang,† Weihai Ni,† Xiaoshan Kou,† Shuzhuo Zhang,‡ Zhenhua Sun,† Ling-Dong Sun,‡ Jianfang Wang,*,† and Chun-Hua Yan‡ Department of Physics, The Chinese UniVersity of Hong Kong, Shatin, Hong Kong SAR, People’s Republic of China and State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking UniVersity, Beijing 100871, People’s Republic of China ReceiVed: August 5, 2008; ReVised Manuscript ReceiVed: September 27, 2008

Gold nanorods with longitudinal surface plasmon resonance wavelengths ranging from 544 to 840 nm have been coated with silica at varying thicknesses. Silica-coated Au nanorods are vitreophilic, and mesostructured composite thin films have thereafter been prepared by dispersing silica-coated Au nanorods in the silica precursor solution. Au nanorods are distributed relatively uniformly in the resultant films and their incorporation amounts can be readily varied. Mesostructured composite films containing both Au nanorods and fluorophores have been further fabricated. Each Au nanorod in these films can be treated as being embedded in a uniform array of fluorophores. Dark-field and fluorescence imaging have been carried out in the same area of the films to correlate the scattering from single nanorods with the fluorescence from the surrounding fluorophores. An exact correspondence between the bright spots on the dark-field image and those on the fluorescence image indicates that the fluorescence of the fluorophores is enhanced by nearby Au nanorods. The fluorescence enhancement is found to be dependent on different fluorophores and gets larger as the thickness of the precoated silica layer on Au nanorods becomes thinner. 1. Introduction Noble metal nanoparticles exhibit extraordinary catalytic and plasmonic properties.1-6 A large number of their applications have been proposed or demonstrated, including ultrafast data communication, optical data storage, nonlinear optics, solar energy conversion, catalysis, enhancement of Raman scattering and fluorescence, chemical and biological sensing, bioimaging, and phototherapy. Many of these applications require that noble metal nanoparticles be uniformly distributed in and stably supported by robust matrices. Mesostructured and mesoporous materials are among the best candidates as a host matrix for stabilizing metal nanoparticles because of their adjustable pore sizes, pore wall thicknesses, pore architectures, and overall morphologies.7-10 Among various morphologies (e.g., powders, fibers, films, and monoliths), thin films of mesostructured and mesoporous materials with embedded metal nanoparticles are often highly desirable. For example, large-area coatings are generally required for practical applications in light harvesting, photocatalytic cleaning and water splitting.11,12 Thin films incorporating metal nanoparticles are preferable for making chemical and biological sensing devices based on optical excitation and detection.13 Arrays of metal nanoparticles formed in mesostructured thin films are compatible with standard microelectronics processing and patterning for fabricating electronic devices and circuits.14-16 Methods that have been employed to embed noble metal nanoparticles in mesostructured and mesoporous thin films can be divided into two major categories. One is the loading of metal precursor salts into mesoporous thin films, followed by the * To whom correspondence should be addressed. E-mail: jfwang@ phy.cuhk.edu.hk. † The Chinese University of Hong Kong. ‡ Peking University.

production of metal nanoparticles through thermal treatment, hydrogen reduction, UV irradiation, or ultrasonication.16-25 By this method, spherical metal nanoparticles can be readily obtained, but the generation of nonspherical metal nanoparticles has remained difficult, although there have been a few examples demonstrating the preparation of metal ellipsoids and nanowires with broad length distributions in mesoporous silica materials.26-30 The other is the presynthesis of metal nanoparticles, followed by surface modification to make them dispersible in the precursor solutions of mesostructured materials.13-15,31 The surface-modified metal nanoparticles are then incorporated in mesostructured thin films through strong binding interactions with the organic or inorganic components in the precursor solutions. This method allows for the incorporation of metal nanoparticles of varying shapes and sizes into mesostructured thin films, which is important because the plasmonic properties of metal nanoparticles are strongly dependent on their shapes and sizes. Despite this attractive advantage, the incorporation of nonspherical metal nanoparticles in mesostructured thin films has rarely been explored. Noble metal nanoparticles can cause a significant change in the fluorescence behavior of adjacent fluorophores. Depending on specific fluorophores and the metal-fluorophore separation, fluorescence enhancement or quenching has been experimentally observed. The fluorescence enhancement by metal nanoparticles has recently attracted increasing attention due to the probability of creating bright and photostable fluorescent markers for fluorescence microscopy, sensing, and microarray technologies. The origin of the fluorescence enhancement has been understood as arising from two contributions. The first is the increased light absorption by nearby fluorophores due to the local electric field enhancement near the surface of metal nanoparticles. The second is the alteration of the radiative and nonradiative decay rates of adjacent fluorophores, which causes changes in both the

10.1021/jp8069699 CCC: $40.75  2008 American Chemical Society Published on Web 11/08/2008

18896 J. Phys. Chem. C, Vol. 112, No. 48, 2008 fluorescence lifetime and quantum yield. The fluorescence enhancement has been observed in a number of experiments on roughened metal surfaces,32 dense layer of metal nanoparticles deposited on planar substrates,33,34 and individual metal nanoparticles.35-42 It will also be useful to realize the fluorescence enhancement by incorporating metal nanoparticles together with fluorophores in thin film materials. Fluorophorecontaining thin films have been used to fabricate waveguides, lasers, sensors, optical data storage devices, and solar cells. The realization of the fluorescence enhancement in thin film materials will have potential for the development of novel photonic devices. We report here on the incorporation of presynthesized Au nanorods of varying longitudinal surface plasmon resonance wavelengths (LSPRWs) into mesostructured silica thin films and their enhancement of the fluorescence from coincorporated organic fluorophores. The LSPRWs of Au nanorods can be tuned in a much wider range than the plasmon resonance wavelengths of Au nanospheres. The wide-range LSPRW tunability of Au nanorods is beneficial for plasmon-enhanced fluorescence, where the plasmon resonance wavelength is generally required to be close to the fluorescence excitation and/ or emission wavelength. The incorporation of Au nanorods is achieved by silica coating, which makes Au nanorods vitreophilic and therefore readily dispersible in the precursor solution of mesostructured silica. The amount of Au nanorods embedded in mesostructured thin films can be controlled by varying the amount of silica-coated Au nanorods added in the precursor solution. Because mesostructured silica materials are composed of ordered nanoscale organic and inorganic domains, the organic and inorganic nanoscale phase separation can provide chemically different environments for loading organic molecules.43-46 Organic fluorophores are therefore incorporated together with Au nanorods into mesostructured silica films. Dark-field and fluorescence imaging performed on exactly the same region of the films show that for every bright spot on the dark-field image, there is a corresponding bright spot on the fluorescence image at exactly the same location. This result indicates unambiguously that the fluorescence from organic fluorophores is enhanced by nearby Au nanorods in the films. Moreover, the fluorescence enhancement is found to be dependent on different fluorophores and the thickness of the silica shell coated on Au nanorods. 2. Experimental Section 2.1. Chemicals. All commercially available chemicals were used as received. Cetyltrimethylammonium bromide (CTAB), gold(III) chloride trihydrate (HAuCl4 · 3H2O), sodium borohydride (NaBH4), ascorbic acid, silver nitrate (AgNO3), glutathione, poly(sodium 4-styrenesulfonate) (PSS, MW 7 × 104 g mol-1), poly(allylamine hydrochloride) (PAH, MW 1.5 × 104 g mol-1), poly(vinylpyrrolidone) (PVP, MW 5.5 × 104 g mol-1), sodium chloride (NaCl), tetraethyl orthosilicate (TEOS), and hydrogen peroxide (H2O2, 35 wt %) were purchased from Aldrich. Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (EO20PO70EO20, P123) was a gift from BASF. Sulfuric acid (H2SO4, 98 wt %) and hydrochloric acid (HCl, 37 wt %) were obtained from Fisher. Oxazine 725 perchlorate (OX725, CAS No.: 24796-94-9) and rhodamine 640 perchlorate (R640, CAS No.: 72102-91-1) were purchased from Exciton. Ammonium hydroxide solution (32 wt % NH3 in H2O) and ethanol were obtained from Merck. Isopropanol was purchased from Labscan. Deionized water with a resistivity of 18.1 MΩ cm was used in all the preparations. 2.2. Growth of Gold Nanocubes and Nanorods. Au nanocubes can be thought of as nanorods with an aspect ratio

Yang et al. of 1. They were prepared using a seeded growth method reported by Sau and Murphy47 with slight modification. Briefly, the seeds were made by the addition of a freshly prepared, ice-cold aqueous NaBH4 solution (0.01 mM, 0.3 mL) into an aqueous mixture solution composed of HAuCl4 (0.01 M, 0.125 mL) and CTAB (0.1 M, 3.75 mL), followed by rapid inversion mixing for 2 min. The resultant seed solution was kept at room temperature for 1 h before use. The growth solution was prepared by the sequential addition of CTAB (0.1 M, 6.4 mL), HAuCl4 (0.01 M, 0.8 mL), and ascorbic acid (0.1 M, 3.8 mL) into water (32 mL). Twenty microliters of the CTAB-stabilized seed solution diluted 10× with water was then added into the growth solution. The resultant solution was mixed by gentle inversion for 10 s and then left undisturbed overnight. Gold nanorods with LSPRWs of 840 and 768 nm were prepared using the seeded growth method previously described.48,49 For the preparation of Au nanorods with a LSPRW of 840 nm, the seed solution was prepared by the addition of a freshly prepared, ice-cold aqueous NaBH4 solution (0.01 M, 0.6 mL) into an aqueous mixture composed of HAuCl4 (0.01 M, 0.25 mL) and CTAB (0.1 M, 9.75 mL), followed by rapid inversion mixing for 2 min. The resultant seed solution was kept at room temperature and used 2 h after its preparation. The growth solution was made by mixing together aqueous solutions of HAuCl4 (0.01 M, 2 mL), AgNO3 (0.01 M, 0.4 mL), and CTAB (0.1 M, 40 mL), followed by the sequential addition of a freshly prepared aqueous ascorbic acid solution (0.1 M, 0.32 mL) and HCl (1.0 M, 0.8 mL). The resultant solution was mixed by inversion and the seed solution (0.096 mL) was then added. The reaction mixture was mixed by gentle inversion for 10 s and left undisturbed overnight. The same procedure was followed to grow Au nanorods with a LSPRW of 768 nm, with changes in the amounts of the chemical reagents. Specifically, the seed solution was prepared using HAuCl4 (0.01 M, 0.125 mL), CTAB (0.1 M, 3.75 mL), and NaBH4 (0.01 M, 0.3 mL). The growth solution was made using HAuCl4 (0.01 M, 1.8 mL), AgNO3 (0.01 M, 0.27 mL), CTAB (0.1 M, 42.75 mL), and ascorbic acid (0.1 M, 0.288 mL), but no HCl. 0.189 mL of the seed solution was added in the growth solution. Gold nanorods with shorter LSPRWs were prepared using a transverse overgrowth method that we previously reported.48,49 Glutathione was selectively bound to the ends of Au nanorods to block longitudinal growth and thus induce transverse overgrowth. The Au nanorods with a LSPRW of 840 nm were employed as starting nanorods for transverse overgrowth. The growth solution was composed of CTAB (0.1 M, 190 mL), HAuCl4 (0.01 M, 8 mL), AgNO3 (0.01 M, 1.2 mL), and ascorbic acid (0.1 M, 1.28 mL). Glutathione (0.01 M) was added into the starting nanorod solution to reach a concentration of 120 µM. After 2 h, four aliquots (8.0 mL) of the starting nanorod solution were placed into plastic tubes. Varying volumes of the growth solution (11.2, 12.8, 44.0, and 80.0 mL) were then added. The growth was left undisturbed for more than 10 h. The LSPRWs of the resultant nanorods are 764, 748, 620, and 574 nm, respectively. 2.3. Silica Coating. The formation of silica coating on Au nanorods was based on a combination of the polyelectrolyte layer-by-layer assembly and the hydrolysis and condensation of TEOS.50 Typically, for PSS coating, as-prepared Au nanorods (20 mL) were centrifuged at 7000 ×g for 20 min. The precipitate was redispersed in water (10 mL). The resultant nanorod dispersion was then added dropwise to an aqueous PSS solution (2 g L-1, 10 mL, containing 6 mM NaCl, presonicated for 30 min) under vigorous stirring. Stirring was continued for 3 h.

Enhancement of Fluorescence in Silica Thin Films The excess PSS was removed by performing two cycles of centrifugation at 3000 ×g for 20 min and redispersion in water (10 mL). For PAH coating, the PSS-coated Au nanorods were added dropwise to an aqueous PAH solution (2 g L-1, 10 mL, containing 6 mM NaCl, presonicated for 30 min) under vigorous stirring. Stirring was continued for 3 h. The mixture solution was centrifuged at 3000 ×g for 20 min to remove excess PAH and the precipitate was redispersed in water (10 mL). For PVP coating, the Au nanorods coated with PSS and PAH (10 mL) were mixed with an aqueous PVP solution (4 g L-1, 10 mL) and stirred overnight. The mixture was centrifuged at 3000 ×g for 20 min and the precipitate was redispersed in water (0.4 mL). The resultant Au nanorod dispersion was added dropwise to isopropanol (2 mL) under vigorous stirring. Silica coating was carried out by the sequential addition of water (0.92 mL), ammonium hydroxide solution in isopropanol (0.11 mL of ammonium hydroxide solution and 2.75 mL of isopropanol) under vigorous stirring and TEOS in isopropanol (0.97 vol %, 0.10 mL) under gentle stirring. The reaction was allowed to continue for 2 h. At this point, the thickness of the obtained silica shell is ∼8 nm. Silica shells of different thicknesses were obtained by the addition of varying amounts of TEOS. The resultant silica-coated Au nanorods were precipitated by centrifugation at 7000 ×g for 10 min. 2.4. Incorporation of Gold Nanorods in Mesostructured Silica Thin Films. For the preparation of the precursor solution for mesostructured silica films, TEOS (5.2 g) was mixed with ethanol (6.0 g) and water (2.7 g, containing HCl at pH ) 2.0). The mixture was stirred until it became homogeneous, which usually took ∼20 min. Separately, P123 (1.38 g) was dissolved in ethanol (4.0 g). The two solutions were mixed, followed by the addition of an appropriate amount of silica-coated Au nanorods dispersed in ethanol and a calculated amount of fluorophores (OX725 or R640). The mixture solution was stirred at room temperature for 2 h before use. The molar composition of the precursor solution was 1 TEOS: 6 H2O: 0.001 HCl: 0.0095 P123: 8.8 EtOH: 0.003 fluorophore. Glass slides were used as film substrates. They were cleaned by soaking them in piranha solution (3:1 v/v of 98 wt % H2SO4 and 35 wt % H2O2) for 3 h and then rinsing sequentially with water, acetone, and ethanol. (Note, extreme caution must be exercised when working with piranha solution.) Mesostructured silica films containing Au nanorods and fluorophores were prepared by spin coating onto cleaned glass slides at 3000 rpm. The resultant films were kept at room temperature overnight for drying. The same procedure was followed for the preparation of mesostructured silica films containing no Au nanorods or fluorophores. 2.5. Characterization. Transmission electron microscopy (TEM) imaging was performed on a FEI CM120 microscope operating at 120 kV. For TEM characterization, silica-coated Au nanorods were washed twice by centrifugation, redispersed in ethanol, and then deposited on TEM copper grids coated with lacey Formvar and stabilized with carbon. Mesostructured silica films containing Au nanorods were carefully scraped off glass slides, dispersed in ethanol by ultrasonication, and then deposited on TEM grids. Scanning electron microscopy (SEM) imaging was performed on a FEI Quanta 400 FEG microscope. Extinction spectra of Au nanorod samples and absorption spectra of fluorophores were recorded using a Hitachi U-3501 UV-vis/ NIR spectrophotometer. As-prepared and silica-coated Au nanorods were dispersed in water for extinction spectral measurements. Mesostructured silica films of 3-5 mm in thickness were made by drop-casting the precursor solution on glass slides and drying in an oven at 65 °C for measuring the

J. Phys. Chem. C, Vol. 112, No. 48, 2008 18897 extinction properties of embedded Au nanorods. Fluorescence emission spectra of fluorophores were acquired using a Hitachi F-4500 spectrofluorometer. Fluorophores were dissolved in ethanol for absorption and fluorescence measurements. Smallangle X-ray diffraction (SAXRD) patterns were obtained on a Bruker D8 X-ray diffractometer with Cu KR radiation. The film thickness was measured using an Alpha-Step 500 surface profiler. Dark-field imaging and spectroscopy of individual Au nanorods embedded in mesostructured silica thin films were carried out on an upright Olympus BX60 optical microscope integrated with an Acton SpectraPro 2300i monochromator and a Princeton Instruments Pixis 512B charge-coupled device (CCD), which was thermoelectrically cooled to -50 °C. Mesostructured silica films were illuminated by white light from a 100-W tungsten lamp through the back aperture of a dark-field objective (100×, NA ) 0.8). The scattered light was collected with the same objective and reflected to the entrance slit of the monochromator. Scattering spectra from individual Au nanorods were corrected by subtracting background spectra taken from the nearby regions containing no Au nanorods. Fluorescence imaging and spectroscopy were performed on the same optical microscope so that the same regions in the mesostructured films could be probed by either the dark-field or fluorescence mode. For fluorescence characterization, the light from a 100-W tungsten lamp filtered with a band-pass filter (Thorlabs, center wavelength ) 600 nm, full width at half-maximum ) 40 nm) illuminated the films through glass slide substrates from the opposite side of the objective for excitation. The fluorescence signal was collected by the objective, reflected by a mirror, and passed through a long-pass filter (Thorlabs, 50% cut-on wavelength ) 650 nm) before entering the monochromator. 3. Results and Discussion 3.1. Silica Coating of Gold Nanorods. Au nanorods were prepared using the transverse overgrowth method previously reported.48,49 In this method, small thiol molecules are selectively bonded to the ends of starting nanorods to block longitudinal overgrowth and induce transverse overgrowth. As more Au precursor is supplied, Au nanorods grow fatter and therefore the length-to-diameter aspect ratio becomes smaller. Their shape also undergoes a gradual transformation from rods and peanuts to irregularly faceted particles. Au nanorods exhibit two surface plasmon resonance modes, which are known as transverse and longitudinal surface plasmon resonances. While the transverse surface plasmon resonance wavelength is almost independent of the aspect ratio, the LSPRW varies linearly as a function of the aspect ratio. Therefore, the LSPRWs of Au nanorods obtained from the transverse overgrowth method become shorter as more Au precursor is supplied. Figure 1 shows the representative extinction spectra of the overgrown Au nanorods. The starting nanorod sample has an LSPRW of 840 nm (Figure 1, curve f). Au nanorods with LSPRWs decreasing to ∼570 nm were produced. In addition, Au nanocubes were also prepared according to a procedure slightly modified from that reported previously.47 They have an average edge length of (50 ( 3) nm (see Figure S1 in Supporting Information), and their surface plasmon resonance wavelength is 544 nm (Figure 1, curve a). They can be treated as nanorods with an aspect ratio of 1. All of the Au nanorods were prepared in the presence of CTAB. After preparation, they are encapsulated with a CTAB bilayer and therefore positively charged.51 They remain welldispersed in aqueous solutions without aggregation for several months even after excess CTAB is removed by two cycles of

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Figure 1. Normalized extinction spectra of (a) Au nanocubes with a surface plasmon resonance wavelength of 544 nm, and (b)-(f) Au nanorods with LSPRWs of 574, 620, 748, 768, and 840 nm, respectively. Figure 3. TEM images of silica-coated Au nanorods: (A) Au nanocubes with a surface plasmon resonance wavelength of 544 nm, (B)-(F) Au nanorods with LSPRWs of 620, 748, 764, 768, and 840 nm, respectively. The thicker nanorods become peanut-like and irregularly faceted. The surface plasmon resonance wavelengths were measured from aqueous dispersions of the Au nanorods before silica coating. The thicknesses of the coated silica from (A) to (F) are (8.6 ( 0.2), (13.3 ( 0.6), (11.8 ( 0.8), (11.8 ( 0.9), (20.5 ( 0.9), and (16.8 ( 0.7) nm, respectively.

Figure 2. (A) Extinction spectra recorded at different stages of coating silica on irregularly faceted Au nanoparticles: (a) as-grown, CTABstabilized nanoparticles, (b) the nanoparticles after PSS coating, (c) the nanoparticles after PAH coating, (d) the nanoparticles after PVP coating, (e) the nanoparticles coated with a thin layer of silica, and (f) the nanoparticles coated with a thick layer of silica. The LSPRWs of the samples from (a) to (f) are 574, 570, 570, 570, 582, and 590 nm, respectively. (B) and (C) TEM images of the Au nanoparticles coated with a silica layer. The thicknesses of the silica layer are (7.9 ( 0.6) and (23.8 ( 0.6) nm, respectively. The extinction spectra of these two nanoparticle samples are plotted as (e) and (f) in (A).

centrifugation and redispersion. We tried the direct addition of the CTAB-stabilized Au nanorods into the precursor solution for making mesostructured silica films. Despite their high stability in water, the Au nanorods were quickly aggregated in the precursor solution. Surface functionalization is therefore necessary for their incorporation into mesostructured silica thin films. The CTAB-stabilized Au nanorods were coated with silica to make them vitreophilic so that they can be dispersed in the precursor solution of mesostructured silica. The silica coating method is based on a combination of the polyelectrolyte layerby-layer assembly technique and the hydrolysis and condensation of TEOS in isopropanol-water mixtures.50 The positively charged Au nanorods were first coated sequentially with negatively charged PSS, positively charged PAH, and PVP through the layer-by-layer assembly technique. The polymer coating allows the Au nanorods to be redispersed in isopropanolwater mixtures for subsequent silica coating through the Sto¨ber method.52 The thickness of the coated silica shell was controlled by the amount of TEOS. As an example, Figure 2A shows the evolution of the extinction spectra of irregularly faceted Au nanoparticles at different stages of coating. The CTAB-stabilized Au nanoparticles have an LSPRW of 574 nm. The polymer coating causes a slight blue shift in the surface plasmon

resonance peak. After being coated with silica, the nanorods exhibit a red shift in the surface plasmon resonance peak, due to the increase in the refractive index of the surrounding medium.53 The red shift in the surface plasmon resonance peak is also found to increase with increasing thickness of the silica shell. Figure 2, parts B and C, shows the representative TEM images of the same Au nanoparticles coated with silica at two different thicknesses. For the same batch of silica-coated Au nanoparticles, the thickness of the silica shell is relatively uniform at different locations of a single nanoparticle and on different nanoparticles. Figure 3 shows the TEM images of Au nanorods that have LSPRWs ranging from 544 to 840 nm and are coated with silica at varying thicknesses. The thinnest silica layer coating that can be made on Au nanorods by this coating method is ∼5 nm. In general, the isotropic hydrolysis and condensation of TEOS follow the anisotropic shapes of Au nanorods, leading to the production of uniformly thick silica layers over individual nanorods. 3.2. Incorporation of Gold Nanorods in Mesostructured Silica Thin Films. The silica-coated Au nanorods are dispersible in the precursor solution of mesostructured silica and can therefore be readily incorporated in mesostructured silica films. A triblock copolymer, P123, was employed as the mesostructure-directing agent. The precursor solution was prepared by mixing an ethanolic solution of P123, a hydrolyzed TEOS solution, and a certain amount of silica-coated Au nanorods and stirring the mixture solution at room temperature for 2 h prior to film deposition. TEOS hydrolysis was carried out by stirring the mixture of calculated amounts of TEOS, ethanol, water, and HCl at room temperature for ∼20 min. Mesostructured silica films were produced by depositing the precursor solution onto clean glass slides by spin coating. The resultant mesostructured composite thin films are transparent and continuous with smooth surfaces. The film thickness is dependent on the viscosity of the coating solution and the spin rate. It typically ranges from 500 to 1000 nm. The mesostructural ordering of the resultant thin films were characterized using SAXRD and TEM. The SAXRD pattern of the film prepared without the addition of Au nanorods exhibits two peaks (see Figure S2A in Supporting Information). TEM imaging (see Figure S2, parts B and C in Supporting Informa-

Enhancement of Fluorescence in Silica Thin Films

Figure 4. (A) SAXRD pattern of mesostructured silica films containing Au nanorods. The nanorods have an LSPRW of 840 nm before silica coating and the thickness of the coated silica layer is (16.8 ( 0.7) nm. The pattern exhibits two sharp peaks at 2θ ) 1.14° and 2.28°, which can be indexed as the (100) and (200) reflections of a two-dimensional hexagonal mesostructure. The lattice constant of the mesostructure is determined to be a ) 8.9 nm. (B)-(F) TEM images showing Au nanorods embedded in mesostructured silica films. Shown in (B) are embedded Au nanocubes. Shown in (C) are embedded Au nanorods with an LSPRW of 748 nm. Shown in (D)-(F) are embedded Au nanorods with an LSPRW of 840 nm. (E) and (F) are magnified images recorded from the central and edge region of the same mesostructured film as in (D). Mesostructured pore channels are visible in (B) and (F). (G) Normalized extinction spectra of mesostructured silica films containing Au nanorods of different LSPRWs. The LSPRWs of the Au nanorods before silica coating are 544, 574, 620, and 840 nm for (a)-(d), respectively. They become 558, 590, 650, and 888 nm, respectively, after being embedded in mesostructured silica films.

tion) shows that the mesostructural channels form a twodimensional hexagonal mesostructure. The lattice constant determined from the SAXRD pattern is a ) 11.2 nm. The SAXRD pattern of the film containing Au nanorods also exhibits two peaks (Figure 4A). TEM images show that the mesostructural channels also form a two-dimensional hexagonal mesostructure (Figure 4, parts B and F). The lattice constant determined from the SAXRD pattern is a ) 8.9 nm for the film containing Au nanorods. The incorporation of Au nanorods causes a reduction in the lattice constant but still preserves the formation of the two-dimensional hexagonal mesostructure. The incorporation of Au nanorods in mesostructured silica films is revealed by TEM imaging (Figure 4B-F). Au nanorods of varying sizes can be readily embedded in mesostructured silica. The distribution of Au nanorods in the films is relatively uniform. Almost no aggregation of Au nanorods is observed. This is important for the use of these mesostructured composite films in optical applications because the plasmonic properties of aggregated Au nanorods will be substantially different from those of individual ones and they are generally difficult to be controlled. In addition, Au nanorods are embedded tightly in mesostructured silica. No gaps are seen between Au nanorods and their surrounding silica or between the precoated silica layer and the newly formed mesostructured silica. This tight embedding is believed to result from the cross-linking between the outer surface of the precoated silica layer and the mesostructured silica formed during film deposition. The uniform incorporation of Au nanorods in mesostructured silica films without aggregation is further confirmed by the extinction spectra (Figure 4G) measured from the films that were drop-cast on glass slides. These films are 3-5 mm thick, which

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Figure 5. (A)-(D) Dark-field scattering images of Au nanorods embedded in mesostructured silica films. The exposure time is 1 s. The number densities of Au nanorods determined from dark-field scattering images are 7, 22, 97, and 173 per 1000 µm2, respectively. (E) Three representative scattering spectra of individual Au nanorods. The exposure time is 15 s. The peak wavelengths are 594, 609, and 616 nm, respectively. The Au nanorods have an LSPRW of 574 nm before being incorporated in mesostructured silica films.

allows for extinction spectral measurements with common spectrophotometers. The extinction spectra measured from these mesostructured composite films are similar in shape to those measured from aqueous dispersions of Au nanorods, suggesting that Au nanorods are well-dispersed in the films without aggregation. The major surface plasmon resonance peaks of the Au nanorod samples embedded in mesostructured silica are shifted in the red direction compared to those of the corresponding Au nanorod samples dispersed in aqueous solutions. This red shift is ascribed to the increase in the refractive index from water to mesostructured silica.53 The magnitude of the red shift ranges from 10 to 50 nm. It increases with increasing LSPRWs, which is consistent with the previous finding that the refractive index sensitivity generally increases with increasing surface plasmon resonance wavelengths for nanoparticles of the same metal and similar shapes.53 Gold nanorods embedded in mesostructured silica films have also been characterized using dark-field imaging and spectroscopy. Figure 5A-D shows the dark-field scattering images of Au nanorods embedded in mesostructured films at increasing amounts. Each bright dot corresponds to a single Au nanorod. The number densities of embedded Au nanorods can be readily determined from these dark-field scattering images. The number densities of Au nanorods determined for the composite films shown in Figure 5A-D are 7, 22, 97, and 173 per 1000 µm2, respectively. The increase in the number density of Au nanorods in the films is realized by adding more silica-coated Au nanorods in the precursor solution. Moreover, the scattering spectra from single nanorods can be obtained by aligning individual bright spots to the central slit of a spectrometer and using a grating to disperse the scattered light onto a CCD camera. Figure 5E shows three representative scattering spectra recorded for three individual Au nanorods shown in the dark-field images. The nanorods in aqueous solutions have an LSPRW of 574 nm (Figure 1, curve b). Their LSPRW becomes 590 nm when embedded in mesostructured silica films (Figure 4G, curve b). For comparison, the LSPRWs obtained from the scattering spectra of individual Au nanorods are 594, 609, and 616 nm,


Figure 6. (A) Molecular structure of OX725. (B) Molecular structure of R640. (C) Absorption and fluorescence spectra of OX725 and R640 dissolved in ethanol. (a) Absorption spectrum of R640 with a peak wavelength of 574 nm. (b) Fluorescence spectrum of R640 with a peak wavelength of 600 nm. (c) Absorption spectrum of OX725 with a peak wavelength of 645 nm. (d) Fluorescence spectrum of OX725 with a peak wavelength of 661 nm.

respectively, indicating an inhomogeneous size distribution of the Au nanorods. 3.3. Gold Nanorod-Enhanced Fluorescence. Mesostructured silica materials are composed of highly ordered nanoscale organic and inorganic domains. These nanoscale domains provide different environments for hosting organic molecules. Depending on their polarity, organic molecules can be occluded in organic domains, inorganic domains, or even the interfacial regions between organic and inorganic domains. The unique architectures provided by mesostructured silica materials can therefore allow for the protective packaging of organic fluorophores into three-dimensional arrays at high concentrations without concentration quenching.43-46 We have therefore further prepared mesostructured silica films containing both Au nanorods and organic fluorophores in order to investigate the interaction between the fluorescence and surface plasmon resonance. Noble metal nanoparticles exhibit local electric field enhancements under resonant illumination. Such local electric field enhancements can lead to the enhancement of the fluorescence from nearby fluorophores. A host of recent experiments have demonstrated the enhancement of the fluorescence from organic fluorophores and semiconductor nanocrystals by individual metal nanoparticles of different shapes.35-42 While the fundamental understanding of the effect of metal nanoparticles on the fluorescence of nearby fluorophores has been improved from these single-particle experiments, it is also highly desired to fabricate macroscopic composite materials of properly positioned metal nanoparticles and fluorophores and achieve enhanced fluorescence for optical device applications. Two fluorophores, R640 and OX725, were chosen (Figure 6, parts A and B). R640 has a maximum absorption wavelength of 574 nm and an emission wavelength of 600 nm. Its fluorescence quantum yield is 1.0.54 OX725 has a maximum absorption wavelength of 645 nm and an emission wavelength of 661 nm (Figure 6C). Its fluorescence quantum yield is 0.11.55 From the incorporated amount of the fluorophores, the volume occupied by each molecule in the mesostructured films is estimated to be 43 nm3, which corresponds to a cube with an edge length of 3.5 nm. For comparison, the nearest-neighbor distance between embedded Au nanorods estimated from the dark-field scattering images (Figure 5A-D) decreases from ∼12 to ∼2.4 µm as the incorporated amount of Au nanorods is

Yang et al. increased. Since the neighboring distance between embedded Au nanorods is much larger than that between fluorophores, each nanorod can be treated as being embedded in a uniform array of fluorophores. The widths and lengths of the nanorods used in this experiment are in the range of 20-50 nm and 50-80 nm, respectively. For a typical nanorod of 45 by 65 nm, there are about 43 000 fluorophores in the surrounding region within a distance of 50 nm from the nanorod surface. An upright optical microscope that is capable of both darkfield and fluorescence imaging/spectroscopy in the same area of the films was used to characterize the mesostructured films containing both Au nanorods and fluorophores. For dark-field imaging, white light is introduced to the back aperture of a darkfield objective and focused from the top on the film sample. For fluorescence imaging, white light is passed through a bandpass filter (center wavelength ) 600 nm, full width at halfmaximum ) 40 nm) and illuminates the film from the bottom. The light collected by the objective is passed through a longpass filter (50% cut-on wavelength ) 650 nm) and introduced to the entrance slit of a spectrometer. We chose Au nanorods with LSPRWs being close to the excitation or emission wavelengths so that the surface plasmon resonance can interact with the excitation or emission process. Figure 7 shows representative dark-field (left side) and corresponding fluorescence images (right side) of the mesostructured silica films containing both Au nanorods and fluorophores. The ensemble LSPRW of Au nanorods used here is 574 nm when they are dispersed in water and 590 nm when they are embedded in the mesostructured silica films. These nanorods are slightly elongated with irregular facets. Each bright spot on the darkfield scattering images indicates a single Au nanorod. For each bright scattering spot, there is a corresponding bright spot at the same spatial location on the fluorescence image. Because the fluorophores are uniformly distributed in the composite films, there is a relatively uniform fluorescence background. The appearance of a bright spot over the uniform fluorescence background at the location of each Au nanorod indicates that the fluorescence from the fluorophores nearby each Au nanorod is enhanced. In order to ascertain if the fluorescence bright spots are an imaging artifact of the optical system, we prepared mesostructured films containing Au nanorods but no fluorophores and carried out dark-field and fluorescence imaging. Bright spots are observed only on the dark-field images but not on the fluorescence images (Figure 7, parts I and J). Moreover, Au nanorods with an LSPRW of 748 nm were incorporated together with OX725 into mesostructured films. The longitudinal surface plasmon resonance of the nanorods is far away in energy from the absorption and emission of OX725 (see Figure S3A in Supporting Information). There is no interaction between the longitudinal surface plasmon resonance and the excitation or emission process of the fluorophore. As expected, bright spots are observed only on the dark-field images but not on the fluorescence images (Figure 7, parts K and L). These control experiments demonstrate unambiguously that the enhanced fluorescence from the fluorophores in mesostructured films is due to the presence of Au nanorods instead of the accumulation of the fluorophores on the interface between Au nanorods and mesostructured silica. Gold nanorods precoated with different thicknesses of silica were incorporated together with the fluorophores in mesostructured silica films. It is qualitatively observed that Au nanorods precoated with a thinner silica shell cause a larger fluorescence enhancement for both OX725 (compare Figure 7, B and D) and R640 (compare Figure 7, parts F and H). This is because the

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Figure 7. Dark-field scattering images (A, C, E, G, I, and K, exposure time, 1 s) and corresponding fluorescence images (B, D, F, H, J, and L, exposure time, 20 s) of mesostructured silica films containing both Au nanorods and fluorophores. The Au nanorods in (A)-(J) have an LSPRW of 574 nm before silica coating, and those in (K) and (L) have an LSPRW of 748 nm before silica coating. (A) and (B) Mesostructured film containing OX725 and Au nanorods precoated with 23.8-nm silica. (C) and (D) Mesostructured film containing OX725 and Au nanorods precoated with 7.9-nm silica. (E) and (F) Mesostructured film containing R640 and Au nanorods precoated with 23.8-nm silica. (G) and (H) Mesostructured film containing R640 and Au nanorods precoated with 7.9-nm silica. (I) and (J) Mesostructured film containing only Au nanorods precoated with 7.9-nm silica. (K) and (L) Mesostructured film containing OX725 and Au nanorods precoated with 11.8-nm silica.

electric field enhancement associated with noble metal nanoparticles decays rapidly with the distance away from the metal surface.56 There are no fluorophores in the precoated silica layer around Au nanorods. As the thickness of the precoated silica layer gets thinner, the spacing between the nanorod surface and the nearest fluorophores becomes smaller and, therefore, the overall fluorescence enhancement becomes larger. Moreover, at the same thickness of silica coating, the fluorescence enhancement for R640 is larger than that for OX725 (compare Figure 7, part B versus F and Figure 7, part D versus H). This can be ascribed to that the energy difference between the surface plasmon resonance of the Au nanorods embedded in the mesostructured silica and the emission of R640 is smaller than that for OX725 (Figure 8). Previous experiments on fluorophores attached to Ag nanoprisms at a fixed distance of 5.5 nm have shown that the largest fluorescence enhancement occurs for Ag nanoprisms with the surface plasmon resonance energies only slightly larger than the emission energy of the fluorophore.40 Au nanorods with LSPRWs being close to the emission wavelength of OX725 can presumably be used, but the surface plasmon resonance of these Au nanorods will not enhance the

fluorophore excitation because their LSPRWs will be nearly out of the window of the band-pass filter used for fluorescence excitation. We further prepared mesostructured silica films containing the fluorophores and mixtures of Au nanorods that have LSPRWs of 544, 574, 606, and 620 nm when dispersed in water. Background-corrected scattering spectra of individual Au nanorods were obtained by subtracting the background spectrum that was taken from the nearby region without Au nanorods from the raw spectra. A large portion of the background spectrum is contributed by the fluorescence from the fluorophores (see Figure S3, parts B,C in Supporting Information). Figure 8 shows two representative extinction spectra of individual Au nanorods embedded in the mesostructured composite films. The LSPRWs of more than 10 nanorods were obtained in order to quantitatively correlate the fluorescence enhancement of the fluorophores with the LSPRWs of Au nanorods. However, no clear trend was found, which we believe is mainly due to two reasons. One is that the depths and orientations of Au nanorods embedded in the films are different. The other is that a band-pass filter was used for the excitation of the fluorophores.


Yang et al. ded in mesostructured silica films. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 8. Representative scattering spectra from individual Au nanorods and fluorescence spectra from individual bright spots acquired from the samples shown in Figure 7. The exposure time is 15 s for both types of spectra. (A) Mesostructured film containing OX725 and Au nanorods. The scattering peak wavelength is 609 nm. (B) Mesostructured film containing R640 and Au nanorods. The scattering peak wavelength is 585 nm. The asymmetry of the fluorescence peaks is due to the use of a long-pass filter with a 50% cut-on wavelength at 650 nm.

The excitation light intensity is nonuniform at different wavelengths. 4. Conclusions We have coated Au nanorods that exhibit different LSPRWs with silica at varying thicknesses by combining the polyelectrolyte layer-by-layer assembly and the Sto¨ber method. Silicacoated Au nanorods are vitreophilic and can then be dispersed in the precursor solution for making mesostructured composite films. Au nanorods are distributed relatively uniformly in the mesostructured silica films without aggregation. The incorporation of Au nanorods preserves the formation of the twodimensional hexagonal mesostructure. The incorporation amount of Au nanorods can be varied. We have further prepared mesostructured composite silica films containing both Au nanorods and fluorophores, where each Au nanorod can be treated as being embedded in a uniform array of fluorophores. An optical microscope that is capable of dark-field and fluorescence imaging/spectroscopy has been employed to record scattering and fluorescence images in exactly the same area of the films. For each bright spot on the dark-field image, which indicates a single Au nanorod, there is a corresponding bright spot over the relatively uniform fluorescence background on the fluorescence image. These bright fluorescence spots are ascribed to the fluorescence enhanced by nearby Au nanorods. The fluorescence enhancement is dependent on different fluorophores and gets larger as the thickness of the precoated silica layer on Au nanorods becomes thinner. Acknowledgment. This work was supported by the RGC Research Grant Direct Allocation (Project Code: 2060332) and the Joint Research Scheme between the National Natural Science Foundation of China and the Research Grants Council of Hong Kong SAR (Reference: N_CUHK448/06, Project Code: 2900318). Supporting Information Available: SEM image of Au nanocubes, SAXRD pattern of mesostructured silica thin films without embedded Au nanorods, and scattering and fluorescence spectra associated with individual Au nanorods that are embed-

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