ARTICLE pubs.acs.org/JPCC
Silver Nanoparticles Coated Zinc Oxide Nanorods Array as Superhydrophobic Substrate for the Amplified SERS Effect Fugang Xu,†,‡ Yue Zhang,†,‡ Yujing Sun,† Yan Shi,†,‡ Zhiwei Wen,†,‡ and Zhuang Li*,† †
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P.R. China ‡ Graduate School of the Chinese Academy of Sciences, Beijing 100039, P.R. China
bS Supporting Information ABSTRACT: A superhydrophobic substrate that combines the superhydrophobic condensation effect and high enhancement ability of silver nanoparticle coated zinc oxide nanorods array (Ag@ZnO) is explored for surface enhanced Raman scattering (SERS). The effects of water contact angle and droplet volume on the final SERS signal intensity are also investigated for the first time. Our results indicate the superhydrophobic substrate could exhibit 3-fold signal enhancement more than the ordinary hydrophilic Ag@ZnO substrate due to the superhydrophobic condensation effect. This signal amplification effect is affected by the water contact angle and water droplet volume on the substrate, i.e., (1) the higher the contact angle is, the higher the SERS signal is; (2) the SERS intensity fluctuates as the droplet volume increases, and proper volume, not the largest one, should be chosen to achieve a stronger signal. Most importantly, this superhydrophobic substrate with high signal reproducibility is successfully applied to detect small molecules such as adenine and melamine, with the detection limits of 1 order of magnitude less than those on the hydrophilic Ag@ZnO substrate. It is expected this superhydrophobic SERS substrate can be widely used in the trace analysis in the future.
’ INTRODUCTION Surface enhanced Raman scattering (SERS) is a giant enhancement of the Raman cross section of molecules when these molecules are adsorbed on a rough metal surface.1,2 Due to its high sensitivity and spectroscopic precision, SERS has been used as a powerful analytical tool in various fields of chemistry, material science, and biophysics.35 One of the key issues for the practical applications of SERS is the preparation of active substrates with high enhancement ability, good stability, and well reproducibility. Up to now, various SERS active substrates have been fabricated, which ranged from roughened silver electrodes to silver/gold nanoparticle aggregates and multiform assemblies.615 It is noticeable that most of these fabrications are focused on the materials and structures of hydrophilic substrates.11,16 Consequently, the effects of surface properties (e.g., wettability) on SERS signal have rarely been reported. Since the giant enhancement of SERS only happens on/near the metal surface,1,2 it is expected that tailing the surface properties may heavily influence the intensity of SERS, and thus provide another way to fabricate efficient substrate for SERS application. Recently, gold or silver nanoparticle decorated nanorods array, such as Ag@ZnO, Ag@SiO2, and Ag@TiO2, are reported as SERS active substrates due to their high enhancement ability and improved reproducibility.1720 Moreover, nanorod arrays are also widely used to construct superhydrophobic surfaces.2123 One characteristic of the superhydrophobic surface is that it could r 2011 American Chemical Society
dramatically reduce the contact area between the droplet and the underlying surface than that on the hydrophilic surface (step1 in Scheme 1). Thus the diluted analyte in the droplet could be highly concentrated after the droplet evaporated on the superhydrophobic surface (step 2 in Scheme 1). This is known as the superhydrophobic condensation effect,24 which may further amplify the SERS signal (step 3 in Scheme 1) to achieve more sensitive detection. To the best of our knowledge, up to now, there is only one paper that reported the fabrication of a superhydrophobic substrate for molecule sensing.25 However, the involvement of optical lithography and ion etching makes their fabrication method too technologically demanding, complex, and expensive. Particularly, the effects of several important parameters, such as the contact angle and the droplet volume, on the SERS signal are not explored, and the SERS enhancement mainly comes from silver nanoparticle aggregates, which may lead to poor reproducibility. In this work, a new superhydrophobic substrate (s-substrate) based on stearic acid modified Ag@ZnO (silver nanoparticle coated zinc oxide nanorod array) is fabricated for amplified SERS detection. The surface wettability and enhancement ability of the substrate and the effects of water contact angle and droplet volume on the final SERS signal are investigated. Our solution Received: February 27, 2011 Revised: April 16, 2011 Published: April 28, 2011 9977
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the Supporting Information. The as-prepared Ag@ZnO was a hydrophilic substrate (i.e., h-substrate). Superhydrophobic Modification of Ag@ZnO. The h-substrate was immersed into the ethanol solution of 1 mM stearic acid for 12 h, and then the substrates were thoroughly rinsed with ethanol and water, respectively, and dried with nitrogen blowing. Characterization. SERS spectra were obtained using a Renishaw Raman system model 2000 spectrometer. The 514 nm radiation from an Arþ ion laser was used as the excitation source. The spectra were collected using a 20 microscope objective (N.A. = 0.4) with 25% amplitude of 25 mW laser power. High laser power (50100% amplitude) will lead to a low signal-to-noise ratio, and low power (10% amplitude) will result in a weak characteristic signal. The data acquisition time was 10 s for one accumulation. The Raman band of a silicon wafer at 520 cm1 was used to calibrate the spectrometer. The morphologies of the samples were investigated using a FEI/ Philips XL30 ESEM FEG field-emission scanning electron microscope. The water contact angles on glass slides with as-prepared coatings were measured using the SL-2800 contact angle system.
’ RESULTS AND DISCUSSION based fabrication method is more accessible, and the obtained s-substrate exhibits high signal reproducibility. The present results indicate that a 3-fold higher SERS signal could be achieved on the s-substrate than that on the ordinary hydrophilic Ag@ZnO substrate (h-substrate) due to the condensation effect. This signal amplification effect of the hydrophobic substrate is affected by the contact angle and the volume of water droplet coated on the substrate, which demonstrates that higher contact angle and proper droplet volume should be manipulated to obtain a stronger SERS signal. The s-substrate could be used to detect small molecules with the detection limits of 1 order of magnitude less than those on the h-substrate. The present results clearly indicate that (1) the preconcentration technique (e.g., superhydrophobic condensation proposed here) could be used to further amplify SERS signals to achieve more sensitive detection and (2) tailing the surface properties may provide another way in addition to structure controlling to fabricate an efficient substrate for SERS application.
’ EXPERIMENTAL SECTION Chemicals and Materials. Zinc acetate dehydrate was purchased from Sigma-Aldrich (St. Louis, USA). Zinc nitrate, stearic acid, and potassium sodium tartrate were obtained from Beijing Chemical Co. (Beijing, China). Hexamethylene tetraamine (HMT) was bought from Shanghai Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 1,3-Diaminopropane (DAP), 4-aminothiophenol (4-ATP), melamine, and adenine were purchased from Alfa-Aesar. All chemicals were used as received without further purification. Glassware were cleaned in aqua regia (Caution: very corrosive liquid) and rinsed with deionized water thoroughly. Ultrapure water (18.2 MΩ 3 cm, produced by a Milli-Q system) was used as solvent throughout this work except for special indication. Synthesis of Ag@ZnO Nanorod Arrays. Ag@ZnO were prepared using a previously reported method with some modification.26 Briefly, a chemical bath deposition approach was used to fabricate the ZnO nanorods array on a glass slide, and then silver nanoparticle decoration was accomplished through the silver mirror reaction. Details of the fabrication can be found in
Morphology and Wettability of the As-Prepared Substrate. The fabrication of a vertical aligned ZnO nanorod array
and its application in construction of a superhydrophobic interface have been widely reported by other researchers.2123 Figure 1 shows the SEM images of the ZnO nanorod array (Figure 1a,b), hydrophilic Ag@ZnO (i.e., h-substrate, Figure 1c,d), and stearic acid modified Ag@ZnO (i.e., s-substrate, Figure 1e,f), respectively. From Figure 1a, it is observed that well ordered ZnO nanorods are densely and uniformly distributed on the glass surface. EDX results (Figure S1a in the Supporting Information) confirm that this composite is mainly Zn and O elements. The magnified image (Figure 1b) clearly displays that ZnO nanorods with tapered tips are successfully synthesized.26 The contact angle θ of water droplet on this substrate is about 0° (inset of Figure 1a), being in accordance with the previous result.27 Figure 1c presents the low magnified SEM image of ZnO nanorod array after silver plating (i.e., h-substrate). The morphology of h-substrate is similar to the ZnO nanorod array without plating, which suggests the decoration of silver nanoparticles does not change the structure of ZnO nanorod array significantly. The higher magnified image (Figure 1d) clearly shows that lots of silver nanoparticles are coated on the tops and sides of ZnO nanorods. The EDX result further confirms the presence of silver nanoparticles (Figure S1b in the Supporting Information). Notably, the interparticle distance of these silver nanoparticles is small due to the densely arranged ZnO nanorods and the aggregation of these nanoparticles on these rods. So, lots of hot spots and thus large electromagnetic enhancement are expected on this substrate. After silver plating, the water contact angle on this substrate increases to about 35° (inset of Figure 1c), demonstrating it is a hydrophilic substrate. Figure 1e, f show the Ag@ZnO nanorods array modified by the stearic acid (i.e., s-substrate). The morphology of this s-substrate is almost the same as that without modification, which implies the high enhancement ability of Ag@ZnO may be retained. However, the wettability of the substrate distinctly changes from hydrophilic (θ = 35°, inset of Figure 1c) to superhydrophobic (θ = 152°, inset of Figure 1e). Comparison of SERS Intensity on Different Substrates. To compare the enhancement ability of these substrates, 10 μL of 9978
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Figure 1. SEM images of different substrates. (a and b) ZnO; (c and d) Ag@ZnO (h-substrate); (e and f) Ag@ZnO modified by stearic acid (s-substrate). The insets show the corresponding shapes of water droplets (10 μL) on these substrates.
1 μM and 0.1 μM 4-ATP were drop-coated onto these substrates and dried in air, and then SERS spectra were collected. First, the Raman spectrum of solid 4-ATP was collected (Figure S2a in the Supporting Information) as reference. The result is identical to previous reports.2830 The ZnO nanorod array without silver coating does not show any response for 4-ATP (data not shown). After being modified with stearic acid, the ZnO array became superhydrophobic but still not active for Raman enhancement of 1 mM 4-ATP (Figure S2b in Supporting Information). For 1 μM 4-ATP, both the h-substrate (curve a in Figure 2a) and the s-substrate (curve a in Figure 2b) display good enhancement ability, since strong characteristic peaks of 4-ATP are clearly observed. The main peaks at 1576 (νCC), 1436 (νCC þ δCH), 1390 (δCH þ νCC), and 1142 cm1 (δCH) can be assigned to the b2 vibration mode, indicating that the enhancement via chargetransfer resonance mechanism is significant.2830 Two other main peaks at 1190 (δCH) and 1072 cm1 (δCS) are ascribed to the a1 vibration mode, implying that electromagnetic enhancement is also important.2830 Although the peaks are well-defined for both substrates, the main peak intensities of 4-ATP obtained from the s-substrate (curve a in Figure 2b) are about 3 fold higher than that obtained from the h-substrate (curve a in Figure 2a). It is believed
this enhancement mainly comes from the effect of superhydrophobic condensation as we predicted in Scheme 1.24 To validate this hypothesis, the surface coverage of the 4-ATP was estimated. After the droplet evaporated in air, a dried spot with rough circle outline was formed on the s-substrates. It was found that strong SERS signals of 4-ATP were always obtained within the spot area, and no characteristic signals were observed when the detected sites were outside the spot. Therefore, the size of the spot could be estimated by monitoring the SERS intensities of 4-ATP along one diameter of the spot. According to this method, the local surface coverage is calculated as 4.97 μM/m2 on the s-substrate and 1.24 μM/m2 on the h-substrate for 10 μL of 1 μM 4-ATP (Table 1). So it is obvious that the condensation effect leads to the increased molecule concentration in local area (i.e., increased surface coverage), which directly results in the amplified SERS signal. This amplification is more important for detection of analytes at low concentration. For example, the SERS spectrum from h-substrate does not show obvious peaks corresponding to 0.1 μM 4-ATP (curve b in Figure 2a), whereas that from the s-substrate gives well discernible peaks of 0.1 μM 4-ATP at 1576, 1436, 1390, and 1072 cm1 (curve b in Figure 2b). This result demonstrates that enhanced signal and thus lower detection limit can be obtained on 9979
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Figure 3. Variations of water contact angle and SERS intensity of 10 μL 4-ATP (1 μM) on substrates with different immersion times. Corresponding error bars are embedded to show the standard deviation of the measurement value.
Figure 2. SERS intensities of 10 μL 4-ATP on different substrates: (a) h-substrate and (b) s-substrate. Concentration of 4-ATP: 1 μM for curve a, and 0.1 μM for curve b in both spectra.
Table 1. Estimated Spot Diameter and Calculated Surface Coverage of 4-ATP for Different Volumes of Droplets after They Dried on Different Substrates diameter of dried spot (mm)
surface coverage (μM/m2)
h-substrate
h-substrate
droplet volume (μL)
s-substrate
s-substrate
5
2.5 ( 0.2
1.3 ( 0.1
1.02 ( 0.06
3.76 ( 0.29
10
3.2 ( 0.3
1.6 ( 0.2
1.24 ( 0.12
4.97 ( 0.54
15
3.8 ( 0.3
2.1 ( 0.3
1.32 ( 0.14
4.33 ( 0.55
the superhydrophobic SERS substrate, which is attractive to trace analysis. Effect of Contact Angle on SERS Intensity. The above results reveal that the superhydrophobic condensation could further amplify the SERS signal. Since the condensation effect is related to the contact angle, the final SERS signal will be related to the droplet contact angle on these substrates. To prove this thoery, SERS intensities of 10 μL 4-ATP (1 μM) on Ag@ZnO composite substrate with different contact angles were investigated. The substrates with different contact angles were obtained by immersing the hydrophilic Ag@ZnO substrates into the ethanol solution of 1 mM stearic acid for different periods. Figure 3 depicts the changes of contact angle and SERS intensity versus the immersion period. It is found that the water
Figure 4. SERS responses of different volumes of 1 μM 4-ATP on (a) h-substrate and (b) s-substrate. Volumes: 5 μL for curve a, 10 μL for curve b, and 15 μL for curve c in both spectra.
contact angle and SERS intensity of 4-ATP on these substrates increase synchronously as the immersion period elongates, which confirms our hypothesis. This result is reasonable. As the immersion time extends, more stearic acid molecules are adsorbed onto the Ag@ZnO substrate due to the strong chelating bond between carboxyl and zinc atom.31 The continuous adsorption of this molecule with long alkyl chain makes the substrate more hydrophobic, which is indicated by an increased contact angle. The 9980
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Figure 5. (a) SERS spectra of 10 μL 4-ATP (1 μM) obtained from 20 sites of the same substrate. (b) The SERS intensities of the strongest peak (1436 cm1) at 20 sites and the calculated RSD.
higher contact angle means the same amount of probe molecule in the droplet could be concentrated in a smaller area,32,33 and thus, increased surface coverage is obtained. The later directly leads to the enhanced SERS signal on substrate with high contact angle. Effect of Droplet Volume on SERS Intensity. Considering that the contact angle is affected by the droplet volume,34 it is supposed the SERS intensity of analytes on the s-substrate is dependent on the droplet volume. Here, different volumes of water droplets with same concentration of probe molecule (5, 10, and 15 μL of 1 μM 4-ATP) were drop-coated on the same s-substrate. A control experiment was performed on the h-substrate by the same procedure. Figure 4 displays the investigated result. For the h-substrate, the SERS signal of 4-ATP increases all the time as the droplet volume increases (Figure 4a). However, different result is obtained for the s-substrate (Figure 4b). As can be seen, the SERS intensity of 4-ATP first increases when droplet volume changes from 5 μL (curve a) to 10 μL (curve b), and then decreases when droplet volume further increases to 15 μL (curve c). Qualitative analysis may have difficulty in explaining theses results, so quantitative analysis is performed. Basically, the SERS intensity is directly related to the surface coverage of molecules on the substrate, so the distributed area of probe molecule on the s-substrate was determined using the method described above. The results are summarized in Table 1. It is observed that the surface coverage of 4-ATP on the h-substrate keeps increasing when the droplet volume changes from 5 to 10
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Figure 6. SERS responses of 10 μL adenine on (a) h-substrate and (b) s-substrate. Curves a: 100 μM adenine; curves b: 1 μM adenine. Background subtraction was applied for the spectra.
and 15 μL, leading to the continuously rising SERS signal. For s-substrate, the surface coverage first increases and then decreases, resulting in the fluctuation of SERS intensity. This result indicates that proper droplet volume, not the largest one, should be selected in order to achieve high SERS intensity. Reproducibility. The reproducibility of SERS signal is a very important parameter for a SERS active substrate. Usually, nanoparticle aggregates could produce large enhancement ability, but the signal reproducibility is poor.35 Therefore, self-assembled 2-D nanoparticle films and 3-D nanoparticle arrays have been developed as SERS substrate. The ordered nanostructures endow these substrates with high enhancement ability and improved reproducibility. To test the reproducibility of our s-substrates, the SERS spectra of 1 μM 4-ATP were collected from 20 random sites of the same substrate. As can be seen in Figure 5a, well discernible SERS spectra of 4-ATP with similar intensity are obtained at all of the sample sites. For the strongest peak at 1436 cm1, the relative standard deviation (RSD) of SERS intensity at 20 different sites on the same substrate is about 11% (Figure 5b), which is comparable with the Ag@SiO2 substrate reported by Zhang.18 This low RSD indicates the structure and surface property of the s-substrate is relatively uniform, which is important to generate reproducible SERS signal. Application in Molecule Sensing. Due to its high sensitivity and abundant chemical and structural information, SERS have been widely used in the ultrasensitive detection of various chemical and biological molecules.3638 As discussed above, 9981
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to the ring breathing II mode of the triazine45,46 is obtained on both substrates for 100 μM melamine (curves a in Figure 7) . The weak Raman peak at 987 cm1 is ascribed to the ring breathing mode I of the triazine. This peak can be found from the s-substrate (curve a in Figure 7b) but not from the h-substrate (curve a in Figure 7a). When the concentration of melamine decreases to 0.1 μM, no characteristic peak of melamine is observed from the h-substrate (curve b in Figure 7a), whereas the intense Raman peak at 685 cm1 is still obtained from the s-substrate (curve b in Figure 7b). Based on these results, it can be concluded that SERS based on the s-substrate could further extend its detection range in trace analysis.
’ CONCLUSIONS We have successfully fabricated a superhydrophobic substrate based on stearic acid modified Ag@ZnO for SERS application. In addition to the high enhancement from the Ag@ZnO, the combination with the condensation effect of the superhydrophobic surface could further amplify the SERS signal to achieve more sensitive detection. This signal amplification effect is affected by the contact angle and the volume of water droplet coated on the substrate. As a result, higher contact angle and proper droplet volume should be manipulated to obtain stronger SERS signal. The developed superhydrophobic substrate with high signal reproducibility has been successfully used for the sensitive detection of molecules with improved detection limit. The present study may provide new insight in fabricating efficient substrate for SERS, and it is expected this superhydrophobic SERS substrate can be widely used in the trace analysis in the future. Figure 7. SERS responses of 10 μL melamine on (a) h-substrate and (b) s-substrate. Curves a: 100 μM melamine; curves b: 0.1 μM melamine. Background subtraction was applied for the spectra.
the s-substrate exhibited an amplification effect for SERS signal. Here, adenine and melamine were chosen to evaluate its potential application in bioanalysis and food safety issue. On one hand, the sensitive determination of adenine may provide useful information for the rapid DNA detection and DNA sequencing, because the SERS peaks of nucleotide bases could be easily distinguished from other components (e.g., sugar or phosphate groups) in DNA backbones.39 On the other hand, the fast and sensitive detection of melamine in dairy products has attracted much attention since the illegally adulterating melamine into dairy products seriously threatens the public health and food safety.40 Figure 6 shows the SERS spectra of adenine obtained from different substrates. Two distinct Raman bands are observed on both h-substrate and s-substrates for 100 μM adenine (curves a in Figure 6a,b). The peak at 737 cm1 can be assigned to the purine ring breathing mode and the peak at 1336 cm1 is assigned to the CN stretching mode.41,42 Also, enhanced signal is obtained from s-substrate (curve a in Figure 6b). For 1 μM adenine, no signal of adenine is observed from the h-substrate (curve b in Figure 6a), while these two main peaks at 737 and 1336 cm1 are still clearly observed from the s-substrate (curve b in Figure 6b). Thus more sensitive detection of adenine can be achieved on the s-substrate, and this detection limit (1 μM) is better than previous results.4244 Similar results are observed for melamine as shown in Figure 7. The most intense Raman peak of melamine at 685 cm1 assigned
’ ASSOCIATED CONTENT
bS
Supporting Information. Fabrication details of ZnO nanorods array, EDAX results of as-prepared substrate in Figure 1, Raman spectrum of solid 4-ATP, and SERS spectrum of 1 mM 4-ATP obtained from stearic acid modified ZnO nanorod array. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Fax: þ86 431 85262057. Tel.: þ86 431 85262057. E-mail: zli@ ciac.jl.cn.
’ ACKNOWLEDGMENT Financial support by the National Natural Science Foundation of China (20775077), the National Basic Research Program of China (973 Program, No.2010CB933600), and the Chinese Academy of Sciences (KJCX2-YW-H11) is gratefully acknowledged. ’ REFERENCES (1) Van Duyne, R. P.; Jeanmaire, D. L. J. Electroanal. Chem. 1977, 84, 1–20. (2) Fleischmann, M.; Hendra, P. J.; Mcquillan, A. J. Chem. Phys. Lett. 1974, 26, 163–166. (3) Kneipp, K.; Moskovits, M.; Kneipp, H. E. Surface-Enhanced Raman Scattering. Physics and Applications; Springer-Verlag: Germany, 2006. (4) Lombardi, J. R.; Birke, R. L. Acc. Chem. Res. 2009, 42, 734–742. 9982
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