Au Composite Nanoarrays As Substrates for Surface-Enhanced

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J. Phys. Chem. C 2010, 114, 93–100

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ZnO/Au Composite Nanoarrays As Substrates for Surface-Enhanced Raman Scattering Detection Limiao Chen, Linbao Luo, Zhenhua Chen, Mingliang Zhang, Juan Antonio Zapien, Chun Sing Lee, and Shuit Tong Lee* Center of Super-Diamond and AdVanced Films (COSDAF) and Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong SAR, China ReceiVed: September 1, 2009; ReVised Manuscript ReceiVed: October 13, 2009

Well-aligned ZnO nanoneedle and nanorod arrays were grown on an aluminum-doped zinc oxide (AZO) buffer layer by chemical vapor deposition and used as templates for making gold-coated ZnO (ZnO/Au) composite nanoarrays. The coverage of Au nanoparticles on ZnO nanoneedle and nanorod was controlled by varying the concentration of Au precursor. High coverage led to the formation of ZnO/Au nanoneedle bundles. The ZnO/Au composite nanoarrays were applied as substrates in surface-enhanced Raman scattering (SERS) measurement. The SERS enhancement factor is mainly controlled by gold coverage and morphologies of the ZnO/Au nanoarrays. Their application in rapid detection of melamine in egg white is demonstrated. This SERS-based melamine detection is advantageous in that sample pretreatment is not needed. It showed the potential of the ZnO/Au nanoarrays as a convenient and robust SERS-active substrate for food safety monitoring. 1. Introduction Surface-enhanced Raman scattering (SERS) integrates high sensitivity with spectroscopic precision, presenting tremendous potential for chemical and biomolecular sensing.1–4 The key to a wider application of SERS is the development of an efficient SERS substrate that not only can provide strong enhancement factors, but also can be stable, reproducible, inexpensive, and easy to fabricate and handle. Various approaches have been developed to fabricate SERS substrates, but with limited successes.5–13 Structurally, most SERS substrates are made from pure metallic nanostructures such as nanoparticles or particle arrays, core-shell nanoparticles, nanowires, and thin films.5–13 Recently, template-based fabrication methods have been used to fabricate SERS substrates, which exhibited promising potentials.14–17 In particular, one-dimensional nanostructures such as nanowires and nanorods have been used as templates for preparing SERS substrate. Prokes et al.14 fabricated Ga2O3/Ag and ZnO/Ag composite nanowires by the vapor-liquid-solid (VLS) growth mechanism and vacuum electron beam evaporation. Fan et al.15 used a glancing angle deposition method to deposit an aligned Si nanorod array; and after coating with a thin layer of Au, an aligned array of Au nanorods was formed on the surface. These substrates showed high sensitivity, uniformity, and reproducibility. However, metal nanoparticles produced from physical evaporation are unstable, difficult to reproduce, and unsuitable for high-volume production. Recently, an electroless plating/seeding method was used to synthesize noble metal-coated silicon nanowire arrays and silica nanofibers for SERS-based sensing.16,17 In this method, the nanowires and nanofibers were modified with linking molecules such as 3-aminopropyltrimethoxysilane and (3-aminopropyl)triethoxysilane in order to prepare them as templates for the assembly of noble metal nanoparticles. This method is tedious and timeconsuming for practical application. Moreover, the linking molecules would exist in the final composites as impurities, which would degrade SERS detection. Therefore, development * To whom correspondence should be addressed.

of a facile method to prepare one-dimensional nanostructures suitable for SERS substrates remains of interest and a challenge. Recently, intentional incorporation of a high melamine level has been discovered in wheat gluten, eggs, and various milk products.18 These adulterated food products can contain melamine in concentrations as high as ∼3300 ppm,18 posing serious health danger to consumers. The combination of melamine at high doses with another triazine and cyanuric acid may lead to the formation of insoluble crystals in kidneys, causing renal failure in humans.19 Consequently, monitoring the level of melamine has become an important issue for public health and food safety. Several methods, including a low-temperature plasma probe combined with tandem mass spectrometry,20 liquid chromatography/tandem mass spectrometry,21 and mass spectrometry22 have been developed for the detection of melamine in complex mixtures such as porcine muscle tissue and food. However, these methods require time-consuming and complex sample pretreatment such as extraction, preconcentration, and derivatization. The “fingerprint-like” SERS detection is capable of providing overall and specific information on various chemical and biochemical components in a complex system without destroying the sample, and requires little or no sample preparation.1 However, up to now, there are no reports on directly detecting melamine in food such as milk, egg, and wheat gluten by SERS. As a semiconductor material, ZnO is a practically nontoxic and biosafe material, and thus of special interest for biomolecular applications.23 In this work, we select aligned ZnO nanoneedle and nanorod arrays as a template to fabricate ZnO/Au composite nanoarrays using a hydrothermal method without adding any linking molecules. First, [Au(OH)4]- ions were adsorbed on the surfaces of ZnO nanoneedle or nanorod via electrostatic attraction between the positively charged ZnO surface and the negatively charged ions,24,25 and then the adsorbed ions were reduced by methanol to form ZnO/Au composites.26 This method should be much simpler than the current typical fabrication process for noble metal coated composite nanowires.16,17 Further, the coverage degree of Au nanoparticles on the ZnO nanoneedle or nanorod surfaces can be readily tuned via the Au precursor

10.1021/jp908423v  2010 American Chemical Society Published on Web 12/07/2009

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Figure 1. (a-c) SEM images of ZnO nanoneedle arrays (tilted view (a, b) and cross-sectional view (c)); the inset in part a is a top-view SEM image. (d) A representative TEM image of a single nanoneedle. Insets are the corresponding SAED patterns taken at spots I and II. (e, f) SEM images of ZnO nanorod arrays (tilted view); the inset in part e is a top-view SEM image.

concentration. Raman measurements showed that the composite nanoneedle arrays could be used as good SERS-active substrates with Rhodamine 6G (R6G) as test probe molecules, and the enhancement factor was estimated to be as high as 1.2 × 107. The optimized SERS substrate was employed successfully to detect melamine in egg white. 2. Experimental Section Morphologies and structures of the samples were characterized with scanning electron microscopy (SEM, Philips XL 30 FEG), transmission electron microscopy (TEM, Philips CM20, operated at 200 kV), and X-ray diffraction (XRD, Philips X′Pert MRD with Cu KR radiation). Ultraviolet-visible (UV-vis) spectra were recorded on a UV-2501 PC Spectrometer (Shimadzu). Raman measurements were conducted with a Renishaw 2000 laser Raman microscope equipped with a 633 nm argon ion laser of 2 µm spot size for excitation. 2.1. Synthesis of ZnO Arrays. The ZnO nanoneedles were grown on AZO substrates by chemical vapor transport and condensation technique. The AZO polycrystalline layer was deposited onto a (100) Si wafer at 500 °C with use of radio frequency magnetron sputtering with a 2 wt % Al2O3:ZnO target. The Si substrate was cleaned and degreased prior to film deposition. Then ZnO arrays were synthesized in a small quartz tube system.27 In brief, an AZO substrate was placed 4-2 cm away from the open end of a small tube. After evacuating, the

source materials (ZnO and graphite powder, 1:1 by weight) and substrate were respectively heated to 950 and 700 °C at a heating rate of 20-50 deg min-1 and kept at these temperatures for 10-60 min. The chamber pressure was maintained constant (300 mbar) by a continuous flow of pure argon gas (99.995%). A small amount of oxygen gas (99.99%) was admitted after the target temperature was reached. 2.2. Synthesis of ZnO/Au Composite Arrays. ZnO/Au composites were synthesized by a hydrothermal method. First, a suitable amount (0.3-1.5 mL) of HAuCl4 aqueous solution (0.01 M) and 1 mL of methanol were added into 20 mL of deionized water. The pH of the solution was adjusted to 7-8 with 0.01 M sodium hydroxide (NaOH) solution under stirring. The final concentration of HAuCl4 was in the range of 0.1-0.5 mM. Then the solution was transferred to a stainless steel autoclave (40 mL in volume). The ZnO arrays together with the AZO/Si substrate were immersed in the above solution. After slow stirring for about 1 h, hydrothermal synthesis was carried out at 120 °C for 1 h in an electric oven without stirring. After cooling to room temperature naturally, samples were taken out from the solution, washed with deionized water several times to remove residual ions and molecules, and dried at 80 °C under vacuum. 2.3. Solution Preparation. Using R6G stock solutions of 10-3-10-4 M, we prepared solutions with concentrations down to 10-10 M via successive dilution by factors of 10 or 100.

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Figure 2. SEM images of ZnO/Au nanoneedle arrays prepared at different concentrations of HAuCl4: (a) 0.1, (b) 0.3, and (c) 0.5 mM. The insets are corresponding high-magnification SEM images. (d) EDX spectrum of ZnO/Au nanoneedle arrays prepared at 0.5 mM HAuCl4.

Melamine is poorly soluble in water with a solubility of about 2.5 × 10-2 M. A series of concentrations (10-4, 10-5, 10-6, 10-7, and 10-8 M) of melamine in deionized water were prepared. A stock solution of egg white was prepared as follows: 3 mL of egg white was first added into 500 mL of distilled water under vigorous stirring at room temperature (23 °C) to obtain a homogeneous solution, and then the solution was filtered through 4 layers of gauge. The tainted egg white solution was prepared by adding melamine solution into the egg white solution. The final concentration of melamine and egg white in the mixture is 1.0 × 10-5 M and about 6 g/L, respectively. 3. Results and Discussion 3.1. Morphological Characterization of ZnO Nanoarrays. Panels a-c of Figure 1 show SEM images of ZnO nanoneedle arrays synthesized at 950 °C by ramping up the temperature of the furnace at a rate of 50 deg min-1. Panels a and b of Figure 1 are respectively low- and high-magnification SEM images, which show that the nanoneedles are well-aligned and uniform over a large scale. The inset in Figure 1a is a top-view SEM image. The density of nanoneedles was estimated to be about 6 per µm2. Figure 1c shows a cross-sectional view image. The diameters at the nanoneedle root and tip are respectively in the range of 100-300 and 50-120 nm. A typical TEM image of a single nanoneedle is shown in Figure 1d, and the corresponding selected area electronic diffraction (SAED) patterns at locations marked “I” and “II” are in the insets. The SAED patterns show that the nanoneedle is a single crystal with a growth direction of [0001]. Panels e and f of Figure 1 show SEM images of ZnO nanorod arrays synthesized at 950 °C by ramping up the temperature of the furnace at a rate of 20 deg min-1. The nanorods were all straight and standing vertically on the substrate. The diameters of the nanorods are in the range of 100-450 nm. The inset in Figure 1e is a top-view SEM image and the density of the ZnO nanorod arrays was about 10 per µm2. The effects of temperature ramping rates upon the morphology and density are attributed to differing Zn supersaturation during initial growth stages, which alters the growth mode from two-dimension, layer by layer, growth under high

Zn supersaturation, to one-dimension, anisotropic, nanorod growth under lower Zn supersaturation.28,29 3.2. Morphological Characterization of ZnO/Au Composite Nanoarrays. Panels a-c of Figure 2 show the morphologies of the ZnO/Au nanoneedle arrays prepared at different concentrations of HAuCl4. Energy dispersive X-ray (EDX) spectroscopy (Figure 2d) confirms that the samples consist primarily of Zn and Au. When the concentration of HAuCl4 was 0.1 mM, the Au nanoparticles had a relatively low density on the surfaces of ZnO nanoneedles (Figure 2a). As the concentration of HAuCl4 increased, higher density Au nanoparticles were coated on the surfaces of ZnO nanoneedles (Figure 2b). When the concentration of HAuCl4 was further increased to 0.5 mM, a much denser Au nanoshell formed on the surfaces of the nanoneedles (Figure 2c). Similar results could be observed with use of ZnO nanorods (shown in Figure 1e,f) as the cores (Figure 3). The results shown in Figures 2 and 3 suggest that the density of Au nanoparticles on the surface of ZnO nanoarrays could be easily tuned by altering the concentration of HAuCl4. Panels a-c of Figure 2 also show that as more Au nanoparticles were attached onto the surfaces of the ZnO nanoneedles, most of the tips of the nanoneedles bended and attracted to form bundles. This observation can be explained as follows. Since the density and the particle size of Au nanoparticles attached onto the surfaces of the ZnO nanoneedles are not uniform, so the center of gravity of each individual ZnO/ Au nanoneedle is not aligned with those of other nanoneedles. The longer tip of the nanoneedle with the center of gravity off axis to gravitation will bend. The van der Waal attractions between the tips would cause the bended tips to form closely interacting bundles.11 The diameter of the tips may also play a role in forming the bundles. The nanorod with a larger diameter would show higher rigidity than one with a smaller diameter, and would not bend and form bundles. This can be seen from Figure 3. When using the ZnO nanorods with larger diameters (in Figure 1e,f) as cores, there is no bending and formation of bundles (Figure 3a-c) even if the concentration of HAuCl4 was increased to 0.5 mM. These results illustrate that the coverage degree of Au nanoparticles on the ZnO nanoneedles and the

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Figure 3. SEM images of ZnO/Au nanorod arrays prepared at different concentrations of HAuCl4: 0.5 (a), 0.3 (b), and 0.1 mM (c). (d) EDX spectrum of ZnO/Au nanoneedle arrays prepared with 0.5 mM HAuCl4.

Figure 4. TEM images of ZnO/Au nanoneedles prepared at different concentrations of HAuCl4: (a) 0.1, (b) 0.3, and (c) 0.5 mM. (d) HRTEM image of a ZnO/Au nanoneedle.

morphology of ZnO/Au nanoneedle arrays can be tuned by altering the concentration of HAuCl4. Panels a-c of Figure 4 show the representative TEM images of the ZnO/Au nanoneedles synthesized with different concentrations of HAuCl4. Obviously, as the concentration of HAuCl4 increases, the density and size of the Au nanoparticles on the ZnO nanoneedle surfaces increase. Figure 4d shows the highresolution TEM (HRTEM) images of crystallographic structures of the nanoparticles and nanoneedles at the interface area. The HRTEM image taken at the interface clearly reveals two distinct sets of lattice fringes. An interlayer spacing of 0.267 nm was observed in the nanoneedle region, in good agreement with the d spacing of the (0001) lattice planes of the hexagonal wurtzite ZnO crystal,27 while in the particle region an interlayer spacing of 0.235 nm was obtained, which matches well to the lattice spacing of the (001) planes of the fcc Au.30 Note that no oxide layer is observed on the surfaces of the Au nanoparticles, indicating that the products did not suffer from observable oxidation. The corresponding electron diffraction (ED) patterns, shown in the inset of Figure 4d, further verify the presence of Au nanocrystals, with the dot patterns contributed by the single crystalline ZnO nanoneedle and ring patterns from the Au nanocrystal. 3.3. Crystal Structure of ZnO and ZnO/Au Nanoneedle Arrays. The crystallinity of the as-prepared ZnO and ZnO/Au nanoneedle arrays was studied with XRD analysis. Figure 5a

Figure 5. XRD patterns of ZnO nanoneedles (a) and ZnO/Au nanoneedles prepared at different concentrations of HAuCl4: (b) 0.1, (c) 0.3, and (d) 0.5 mM.

shows that the ZnO nanoneedle arrays are crystalline and there is only one intense (002) peak, confirming that the ZnO needles have a preferred [0001] growth direction. Spectra b-d of Figure 5 show the XRD spectra of ZnO/Au nanoneedle arrays prepared at different concentrations of HAuCl4. All spectra show two sets of diffraction peaks, indicating that the as-synthesized products are composite materials. The peaks marked with “/” can be indexed to hexagonal wurtzite ZnO, while those marked with “#” can be indexed to the face-centered-cubic (fcc) structure of Au; no other crystalline impurities can be observed.

ZnO/Au Composite Nanoarrays

Figure 6. Absorption spectra of ZnO (a). ZnO/Au nanoneedles prepared at different concentrations of HAuCl4: 0.1 (b), 0.3 (c), and 0.5 mM (d). The inset shows the absorption spectra of gold nanoparticles in colloid solution.

Moreover, with increasing concentration of HAuCl4 the intensities of those peaks indexed to Au increase gradually, indicating more Au nanoparticles were attached on the ZnO nanoneedle surface. 3.4. Optical Absorption of ZnO and ZnO/Au Nanoneedle Arrays. The UV-vis absorption spectra of ZnO and ZnO/Au nanoneedle arrays are shown in Figure 6. All the samples were collected and then redispersed in ethanol for the UV-vis scan measurement. The strong adsorption between 200 and 400 nm is attributed to the absorption of ZnO in the UV light region. The bare ZnO nanorods did not show any absorption between 400 and 800 nm, but the composite arrays displayed an obvious absorption band with a maximum absorption wavelength of 532 ( 6 nm due to the localized surface plasmon resonance (LSPR) of Au nanoparticles. In comparison with the surface plasmon band (about 520 nm) of pure Au nanoparticles in solution (inset in Figure 6), the characteristic Au plasmon peak in ZnO/Au composite is distinctly broadened and red-shifted by 6-18 nm. Furthermore, as the concentration of HAuCl4 increases, the plasmon resonance peak shows a red-shift from 526 to 538 nm, and begins to broaden due to the larger size and higher coverage of Au particles as shown in Figures 2 and 4. The reason for the broadening and red-shift of the surface plasmon absorption is attributed to the strong interfacial coupling between Au nanoparticles.31,32 3.5. SERS Property of ZnO/Au Composite Nanoarrays. As the SERS effect is very sensitive to the roughness of metal surfaces, the different as-synthesized ZnO/Au composite nanoneedle arrays were used as substrates to examine the SERS effect. R6G was chosen as the probe molecule owing to its wellestablished vibrational features. Figure 7 shows the SERS spectra of R6G solution (1 × 10-7 M) dispersed onto ZnO/Au nanoneedle arrays, while the spectrum of the same R6G solution dispersed on pure ZnO substrate was also included for comparison. Figure 7a reveals that no Raman peaks could be observed when the R6G solution of 1 × 10-5 M was dispersed on pure ZnO substrate, but for ZnO/Au nanoneedles prepared from different HAuCl4 solutions, strong Raman signals were observed even for 1 × 10-7 M R6G, as shown in Figure 7b-d. The strong and medium-strong Raman bands at 1648, 1575, 1510, 1361, 1310, 1187, 774, and 611 cm-1 are assigned to the xanthene ring stretch, ethylamine group wag, and carbon-oxygen stretch of R6G.33 An obvious trend shows that the intensity of the Raman signal at 1510 cm-1 increased strongly with increasing concentration of HAuCl4 and reached a maximum at 0.5 mM. The change of SERS intensity is attributed to the

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Figure 7. SERS spectra of R6G adsorbed on pure ZnO (a). ZnO/Au nanoneedles prepared at different concentrations of HAuCl4: 0.1 (b), 0.3 (c), and 0.5 mM (d). Laser power 25 mW, 1%; integration time, 20 s.

Figure 8. SERS spectra of R6G adsorbed on ZnO/Au nanoneedle (a, d) and nanorod (b, c) arrays prepared at different concentrations of HAuCl4: 0.1 (a, b) and 0.5 mM (c, d). Laser power 25 mW, 1%; integration time, 20 s.

different coverage of Au nanoparticles on the ZnO nanoneedle obtained from different concentrations of HAuCl4. In our experiments, the SERS intensity was dependent on the density of Au nanoparticles, which can be controlled by varying the concentration of HAuCl4. It appears that the density of Au nanoparticles and morphology of ZnO/Au nanoneedles are two main factors affecting SERS enhancement activity. It can be seen from Figures 2 and 4 that the density of the Au nanoparticles increased and more Au nanoparticles aggregated on the ZnO nanoneedle surfaces with increasing concentration of HAuCl4. According to theoretical and experimental studies,34–36 the largest SERS enhancement occurs at the junction between two metal nanoparticles. As the number density of Au nanoparticles increases, the distance between the nanoparticles decreases, therefore the number of junctions increases providing a larger SERS enhancement. As the distance between the nanoparticles decreases, the coupled plasmon resonance shifts to the red, the enhanced electromagnetic field increases in the junction of the aggregated nanoparticles. The junction thus can be considered as an electromagnetic hot spot similar to those predicted to exist in Ag clusters.37,38 Alternatively, the morphology of ZnO/Au nanoneedle arrays could be another cause of the dependency of the SERS activity on concentration of HAuCl4. With increasing concentration of HAuCl4, the coverage degree of Au nanoparticles increased, and more nanoneedle bundles were formed and the size of the bundles increased (see in Figure 2). As the bundles formed, the gap between tips of neighboring nanoneedles decreased greatly. Therefore the junctions between Au nanoparticles adsorbed onto different nanon-

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Figure 9. 40 SERS spectra (a-c) and the peak height (d) of the 1510 cm-1 line for the 40 SERS spectra of R6G (1 × 10-7 M) collected on ZnO/Au nanoneedle arrays prepared at different concentrations: 0.1 (a, i), 0.3 (b, ii), and 0.5 mM (c, iii). Laser power 25 mW, 1%; integration time, 20 s.

Figure 10. SERS spectra of melamine of various concentrations: (a) 1.0 × 10-4, (b) 1.0 × 10-5, (c) 1.0 × 10-6, (d)1.0 × 10-7, and (e) 1.0 × 10-8 M on ZnO/Au nanoneedles prepared at 0.5 mM HAuCl4. Laser power 25 mW, 1%; integration time, 20 s.

eedles were formed. As the size of the bundles increased, the junctions increased in number and also provided a higher SERS enhancement. To further demonstrate the dependence of the SERS enhancement on morphology of ZnO/Au nanoneedle arrays, the SERS spectra of R6G (1.0 × 10-7 M) from the ZnO/Au nanoneedle and nanorod arrays (as shown in Figures 2 and 3, respectively) under identical experimental conditions were obtained. The comparison results were shown in Figure 8. It can be seen that, at low concentration of HAuCl4 (0.1 mM) the ZnO/Au nanoneedle and nanorod arrays exhibit nearly the same SERS signal intensities, whereas at high concentration (0.5 mM) the SERS signal intensities of nanoneedle bundles are about 2 times stronger than those obtained from ZnO/Au nanorod arrays (using the Raman band at 1510 cm-1 as a reference). The SERS enhancement factors (EFs) for R6G adsorbed on the ZnO/Au

Figure 11. SERS spectra of egg white solution (6 g/L) (a) and the melamine-tainted egg white solution (b) on ZnO/Au nanoneedles prepared at 0.5 mM HAuCl4. The concentration of melamine and egg white in the mixture is 1.0 × 10-5 M and about 6 g/L. Laser power 25 mW, 1%; integration time, 20 s.

nanoneedles and nanorod arrays prepared at 0.5 mM HAuCl4 are calculated according to the equation39 EF) (ISERS/Ibulk)(Nbulk/ Nsurface), where ISERS and Ibulk denote the integrated intensities for the 1510 cm-1 band of the 100 nM R6G adsorbed on the ZnO/Au surface and 10 mM R6G on glass, respectively, whereas NSERS and Nbulk represent the corresponding number of R6G molecules excited by the laser beam. The EFs of ZnO/Au nanoneedle and nanorod arrays are evaluated to be 1.2 × 107 and 6.4 × 106, respectively. The EFs are much higher than those with Au nanoparticle aggregates.40–42 To assess the reproducibility of SERS signals obtained from the as-prepared ZnO/Au nanoneedle arrays, the SERS spectra of R6G (1.0 × 10-7 M) from 40 randomly selected positions on each substrate under identical experimental conditions were measured, and the heights of the 1510 cm-1 peak in these spectra

ZnO/Au Composite Nanoarrays are shown in Figure 9. Obviously, the Raman spectra of R6G are enhanced greatly at each point, indicating good SERS activity and reproducibility of the ZnO/Au nanoneedle arrays. However, for the same substrate the signal intensities vary with the sampling position. For different substrates, intensity fluctuations in the spectra taken from ZnO/Au nanoneedle arrays prepared at 0.1 and 0.5 mM HAuCl4 are much smaller than those in the spectra taken from ZnO/Au nanoneedle arrays prepared at 0.3 mM HAuCl4. A reasonable explanation for this observation is the nonuniform morphology of the ZnO/Au nanoneedle arrays, which is an important factor affecting SERS enhancement. As mentioned, there may be more hot spot sites in the nanoneedle bundles, and the number of hot spots would increase with increasing size of the bundles. From Figure 2a-c we can find that, at low and high concentration of HAuCl4 (0.1 and 0.5 mM, respectively), the size of the ZnO/Au nanoneedle bundles is more uniform than that of the ZnO/Au nanoneedle bundles prepared at 0.3 mM HAuCl4. Consequently, relatively more uniform SERS intensity would be achieved from the ZnO/ Au nanoneedle arrays prepared at 0.1 and 0.5 mM HAuCl4. 3.6. Application of SERS-Active Substrates for Melamine Detection. As discussed above, the optimized ZnO/Au nanoneedle arrays may be excellent candidates for SERS-based ultrasensitive molecular sensing. To evaluate their possible applications in human health and food safety issues, we have examined the use of the present SERS substrates for melamine detection. Figure 10 shows the SERS spectra of melamine with various concentrations from 1.0 × 10-4 to 1.0 × 10-8 M. The most intense Raman peak of melamine is at 682 cm-1, which is assigned to the ring breathing 2 mode involving an in-plane deformation of the triazine ring.43 The medium-strong Raman peak at 987 cm-1 is the ring breathing mode 1 of the triazine ring.43 Figure 10 shows that while the Raman peak at 682 cm-1 decreases in intensity strongly with decreasing concentration of melamine, it remained clearly observable at concentrations of melamine solution as low as 1 × 10-8 M (Figure 10e). This indicates that the ZnO/Au nanoneedle arrays as SERS substrates provided strong Raman signals of melamine. The possibility of directly detecting melamine in egg white solution is explored. For comparison, the spectra of egg white solution dispersed on ZnO/Au nanoneedle arrays were also measured. Figure 11a shows the SERS spectra of egg white solution from three randomly selected positions on the substrate. Slight variation in SERS spectra is observed, which may be caused by different molecular conformation, orientation, and binding specificity to the substrate surface.4 The bands in the SERS spectrum can be assigned to tryptophan (Trp), tyrosine (Tyr), aspartic acid (Asp), histidine (His), phenylalanine (Phe), and glutamine (Glu) based on the reports for amino acid,44 peptides,45 and proteins.46 Figure 11b shows the SERS spectrum of egg white solution tainted by melamine. In comparing these spectra with those of untainted egg white solution in Figure 11a, peaks are apparently different for the untainted egg white solution and melamine-tainted egg white solution in the range of 600 to 1000 cm-1. A sharp peak at about 682 cm-1 and a small peak at 987 cm-1 in the spectrum of melamine-tainted egg white solution appeared but were absent in that of egg white solution. The peaks at 682 and 987 cm-1 can be attributed to the 1 and 2 modes of melamine, respectively.43 This difference indicates the presence of melamine in the egg white. Significantly, the 682 cm-1 peak also remained clearly detectable even when the concentration of melamine in the mixture decreased to 1.0 × 10-6 M. The result demonstrates the possibility for

J. Phys. Chem. C, Vol. 114, No. 1, 2010 99 facile, high-sensitivity detection of melamine in the complex egg white solution without the need of sample pretreatments. 4. Conclusion We have presented a simple method to prepare large-area ZnO/Au composite nanoarrays for SERS-based sensing. SEM, TEM, and UV-visible spectroscopy confirmed the formation of ZnO/Au composite nanoarrays. The density of Au nanoparticles on the ZnO nanoneedle surface can be easily controlled by altering the concentration of HAuCl4. High-density coating of Au nanoparticles leads to the formation of ZnO/Au nanoneedle bundles. Raman analyses showed that the composite nanoneedle arrays are highly SERS active substrates. The enhancement factor was as high as 1.2 × 107 with Rhodamine 6G (R6G) as the probe molecule. The Raman enhancement factor was affected by the density of Au nanoparticles on ZnO nanoneedle surfaces as well as the morphology of ZnO/Au nanoneedle arrays. The ZnO/Au composite nanoarrays yielded high SERS activity for melamine detection, suggesting their high potential as a convenient SERS substrate for biological and food safety monitoring. Acknowledgment. This work was supported by the Innovation and Technology Commission (Grant No. ITS/029/08), and Research Grants Council of Hong Kong SAR (Grant No. CityU 103208). References and Notes (1) Yonzon, C. R.; Haynes, C. L.; Zhang, X. Y.; Walsh, J. T., Jr.; Van Duyne, R. P. Anal. Chem. 2004, 76, 78–85. (2) Mulvihill, M.; Tao, A.; Benjauthrit, K.; Arnold, J.; Yang, P. Angew. Chem., Int. Ed. 2008, 47, 6456–6460. (3) Lin, M.; He, L.; Awika, J.; Yang, L.; Ledoux, D. R.; Li, H.; Mustapha, A. J. Food Sci. 2008, 73, T129–134. (4) Barhoumi, A.; Zhang, D.; Tam, F.; Halas, N. J. J. Am. Chem. Soc. 2008, 130, 5523–5529. (5) Shen, C.; Hui, C.; Yang, T.; Xiao, C.; Tian, J.; Bao, L.; Chen, S.; Ding, H.; Gao, H. Chem. Mater. 2008, 20, 6939–6944. (6) Luo, W.; van der Veer, W.; Chu, P.; Mills, D. L.; Penner, R. M.; Hemminger, J. C. J. Phys. Chem. C 2008, 112, 11609–11613. (7) Bao, F.; Li, J. F.; Ren, J.; Yao, J. L.; Gu, R.-A.; Tian, Z. Q. J. Phys. Chem. C 2008, 112, 345–350. (8) Fang, P. P.; Li, J. F.; Yang, Z. L.; Li, L. M.; Ren, B.; Tian, Z. Q. J. Raman Spectrosc. 2008, 39, 1679–1687. (9) Jeong, D. H.; Zhang, Y. X.; Moskovits, M. J. Phys. Chem. B 2004, 108, 12724–12728. (10) Mohanty, P.; Yoon, I.; Kang, T.; Seo, K.; Varadwaj, K. S. K.; Choi, W.; Park, Q. H.; Ahn, J. P.; Suh, Y. D.; Ihee, H.; Kim, B. J. Am. Chem. Soc. 2007, 129, 9576–9577. (11) Lee, S. J.; Morrill, A. R.; Moskovits, M. J. Am. Chem. Soc. 2006, 128, 2200–2201. (12) Yoon, I.; Kang, T.; Choi, W.; Kim, J.; Yoo, Y.; Joo, S. W.; Park, Q. H.; Ihee, H.; Kim, B. J. Am. Chem. Soc. 2009, 131, 758–762. (13) Lu, L.; Eychmller, A.; Kobayashi, A.; Hirano, Y.; Yoshida, K.; Kikkawa, Y.; Tawa, K.; Ozaki, Y. Langmuir 2006, 22, 2605–2609. (14) Prokes, S. M.; Glembocki, O. J.; Rendell, R. W.; Ancona, M. G. Appl. Phys. Lett. 2007, 90, 093105. (15) Fan, J. G.; Zhao, Y. P. Langmuir 2008, 24, 14172–14175. (16) Zhang, B.; Wang, H.; Lu, L.; Ai, K.; Zhang, G.; Cheng, X. AdV. Funct. Mater. 2008, 18, 2348–2355. (17) Zhang, S.; Ni, W.; Kou, X.; Yeung, M.; Sun, L.; Wang, J.; Yan, C. A. AdV. Funct. Mater. 2007, 17, 3258–3266. (18) http://en.wikipedia.org/wiki/2008_baby_milk_scandal. (19) Brown, C. A.; Jeong, K.-S.; Poppenga, R. H.; Puschner, B.; Miller, D. M.; Ellis, A. E.; Kang, K. I.; Sum, S.; Cistola, A. M.; Brown, S. A.; Vet, J. Diagn. InVest. 2007, 19, 525–531. (20) Huang, G.; Ouyang, Z.; Cooks, R. G. Chem. Commun. 2009, 556– 558. (21) Filigenzi, M. S.; Tor, E. R.; Poppenga, R. H.; Aston, L. A.; Puschner, B. Rapid Commun. Mass Spectrom. 2007, 21, 4027–4032. (22) Vail, T. M.; Jones, P. R.; Sparkman, O. D. J. Anal. Toxicol. 2007, 31, 304–312. (23) Shan, G.; Wang, S.; Fei, X.; Liu, Y.; Yang, G. J. Phys. Chem. B 2009, 113, 1468–1472.

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