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May 20, 2014 - ABSTRACT: We report on a system of surface-enhanced Raman scattering (SERS) ... chloride (CuCl2·H2O, 99.8%) and silver nitrate (AgNO3,...
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Improved Performance of Solution-Phase Surface-Enhanced Raman Scattering at Ag/CuO Nanocomposite Surfaces Shuchen Hsieh,*,†,‡ Pei-Ying Lin,† and Ling-Ya Chu† †

Department of Chemistry and Center for Nanoscience and Nanotechnology, National Sun Yat-sen University, 70 Lien-Hai Road, Kaohsiung 80424, Taiwan ‡ School of Pharmacy, College of Pharmacy, Kaohsiung Medical University, 100 Shih-Chuan first Road, Kaohsiung 80708, Taiwan ABSTRACT: We report on a system of surface-enhanced Raman scattering (SERS) using solution-phase Ag decorated CuO nanoparticles. The Ag/CuO nanocomposite exhibits up to a 105 Raman signal enhancement as determined using the target molecule 4-aminothiophenol (4-ATP) at concentrations down to 10 μM. The surface plasmon absorption of Ag shifted slightly toward higher binding energy, which is attributed to electron transfer (charge transfer) from Ag to CuO. Further, metallic particles deposited on semiconductor materials formed a local electromagnetic field which altered the interfacial charge distribution. Using solution-phase SERS detection, the dynamic hot spot density was increased significantly, allowing for highly efficient analyte adsorption onto the Ag/CuO nanocomposite with increased electromagnetic field. The observed SERS signal enhancement in this solution-phase Ag/CuO nanocomposite system is attributed to a combination of chemical charge-transfer effect and electromagnetic enhancement occurring simultaneously at the metal−semiconductor interface. Because SERS detection is performed in the solution phase, this method provides a system for in-situ detection in aqueous environment and is thus well suited for monitoring analyte concentrations.

1. INTRODUCTION

CuO is an important narrow band gap p-type semiconductor that is well-known for diverse practical applications in catalysis, solar energy conversion, magnetic storage media, gas sensing, and field emission.5,11,12 Because of the unique properties and diverse applications of CuO, we have investigated the improved SERS activity of solution-phase Ag/CuO nanocomposites as a function of Ag content, using 4-aminothiophenol (4-ATP) as the probe molecule. The origin of the enhancement on Ag/ CuO nanocomposites may be partially ascribed to a CT transition between the semiconductor and metallic nanoparticles. Further, because the Ag/CuO particles are in an aqueous environment, a higher density of dynamic multi-hot spots in a unit area can be achieved, resulting in highly efficient adsorption of analyte onto the Ag/CuO nanocomposite substrate with increasing electromagnetic field. In this system, a combination of EM and CT mechanisms may be present which leads to a greater net signal enhancement effect. Compared to other strategies, solution-phase substrates can provide a rapid and simple method for in-situ SERS detection and may lead to deeper insight into the metal−semiconductor interface.

Semiconductor/noble metal hybrid nanocomposites exhibit tunable shape- and composition-dependent properties and have been explored for a broad range of applications in catalysis, biology, optoelectronic, and surface-enhanced Raman scattering (SERS).1−4 Although previous SERS has been limited to studies on noble metals (Au, Ag, and Cu) and transition metals (Fe, Co, Ni, Pt, Pd, Ru, and Rh), it was recently reported that semiconductor oxide materials such as ZnO, TiO2, and Cu2O also generate weak SERS signals with enhancement factors (EF) ranging from 101 to 103.5−7 Semiconductor/noble metal nanocomposites have more recently been found that exhibit stronger Raman enhancement associated with tuning of the localized surface plasmon resonance (LSPR) of the metallic nanoparticles, inducing a charge transfer (CT) effect at the semiconductor/metal interface.5,8,9 Common metals such as Cu and Zn may be used in this type of composite material; thus, significant effort has been directed toward the design of various semiconductor/noble metal nanocomposites for improved SERS activity. For example, Yang et al. observed SERS on the TiO2/Ag nanocomposite which was contributed to a TiO2to-molecule CT mechanism rather than the contribution of electromagnetic (EM) effects from the surface-modified Ag itself.6 Shan et al. synthesized multifunctional ZnO/Ag nanorod arrays to construct SERS-active and photocatalytic substrates based on the formation of an interfacial electric field between ZnO nanorods and Ag.10 © 2014 American Chemical Society

Received: April 1, 2014 Revised: May 16, 2014 Published: May 20, 2014 12500

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2. EXPERIMENTAL SECTION 2.1. Materials. All chemical reagents were analytical grade and used as obtained without further purification. Copper chloride (CuCl2·H2O, 99.8%) and silver nitrate (AgNO3, 99.8%) were purchased from Sigma-Aldrich (St. Louis, MO), sodium hydroxide (NaOH, 99.8%) was purchased from Merck, and ethanol (99.5%) was purchased from Echo Chemical. MilliQ reagent-grade (type I) water (18.2 MΩ·cm at 25 °C) was used for all synthesis processing steps. 2.2. Ag/CuO Nanocomposite Synthesis. A solution of 0.25 M CuCl2 was heated to 40 °C and then added to a 2.5 M NaOH solution at a volume ratio of 1:1 with continued heating at 40 °C for 1 h. Samples were then rinsed five times with MilliQ water, dried under vacuum for 24 h, and then plasma treated for 1 h, yielding a CuO powder. To prepare Ag/CuO nanocomposites, 0.01 g of CuO powder was reacted with 100 μL of AgNO3 (at 0.25, 0.5, 0.75, and 1 M) in a 1.5 mL microcentrifuge tube for 1, 3, 5, and 10 min under ultrasonic vibration. The ultrasonication (DC200H ultrasonic water bath, 200 W, Taiwan Delta New Instrument Co. Ltd.) is operated in continuous mode at a 40 kHz operating frequency and 25 °C to increase the frequency of two particle collisions.13 Samples were then washed with ethanol, centrifuged to remove water, and then dried to yield the final Ag/CuO nanocomposite products. 2.3. Sample Characterization. Samples were prepared for analysis by transmission electron microscopy (TEM) by first depositing a droplet of solution onto a 200-mesh copper grid (Ted Pella Inc., CA) and allowing the sample to dry in air. TEM images were acquired using a JEOL JEM-2100 operated at 200 kV. Crystal structure characterization was carried out by power X-ray diffraction (XRD) on a Bruker D8 Advance X-ray diffractometer. Samples were prepared for analysis by X-ray photoelectron spectroscopy (XPS) by depositing an aliquot of the sample solution onto clean gold substrates at room temperature and drying in air. XPS spectra were acquired using a JEOL JAMP 9010 MX equipped with a monochromatic Mg Kα X-ray (1253.6 eV) radiation source. The spectra were calibrated using the C 1s peak at 285 eV from adventitious surface carbon as an internal reference. The Cu and Ag peak curve fitting was performed with a Gaussian/Lorentzian ratio of 0.7 using peak fitting software by JEOL SpecSurf XPS, after a Shirley-type background subtraction. 2.4. SERS Measurement. The target analyte, 4-aminothiophenol (4-ATP, 97%), was purchased from Acros (Geel, Belgium) and used without further purification. 4-ATP was dissolved in ethanol (Echo Chemical, ROC) at a series of concentrations (10−3, 10−4, 10−5, and 10−6 M). CuO/Ag (0.0025 g) was dissolved with 1 mL of Milli-Q water, mixed with 4-ATP solution at a volume ratio of 1:1, and finally deposited by droplet on separate silicon substrates. All Raman and SERS experiments were performed on a Raman microscope (WiTec alpha 300R) with a 633 nm incident laser operating at a power of 21.9 mW. A holographic grating (1800 grooves mm−1) and a 1024 × 127 pixel back-illuminated CCD detector with total accumulation times of 30 s were used.

composites. These results are shown in Figure 1. The asprepared CuO crystals (Figure 1a) were approximately tens to

Figure 1. TEM images of CuO and Ag/CuO nanocomposites at different reaction times for (a) CuO, (b) 5 min, and (c) 10 min. (d) An HRTEM image shows the lattice spacing between CuO (110) and Ag (111) planes.

hundreds of nanometers in length and tended to aggregate during the sample preparation process. In Figure 1b, we show the TEM image for Ag/CuO nanocomposite samples prepared with a CuO−AgNO3 reaction time of 5 min. Here one can clearly see that nanoparticles (NPs) decorate the edges of the CuO nanocrystals. These NPs were well dispersed with minimal observed aggregation. Longer reaction times lead to Ag/CuO nanocomposites with higher Ag NP densities, as shown in Figure 1c for a 10 min reaction time. Structural analysis of the nanocomposites revealed that they were crystalline with distinct lattice fringes, as observed in HRTEM images (Figure 1d). A measured lattice spacing of 0.27 nm corresponds to the (110) plane of CuO,4 and 0.23 nm corresponds to the (111) face of Ag.14 The XRD patterns for samples of CuO nanocrystals and Ag/ CuO nanocomposites are shown in Figure 2. The patterns indicate a crystallized structure at 2θ: 32.5°, 35.5°, 38.7°, 48.8°, 53.5°, 58.2°, 61.5°, 66.2°, 68.1°, 72.1°, and 75.3°, which are assigned to the (110), (002, 1̅11), (111, 200), (2̅02), (020), (202), (1̅13), (3̅11), (220), (311,) and (004, 2̅22) crystallographic faces of CuO (reference JCPDS card no. 05-0661). As shown, new peaks were observed at 38.1° and 77.3° of Ag/ CuO nanocomposites compared to that of CuO nanocrystals, which could be attributed to the (111) and (311) crystallographic faces of metallic Ag (reference JCPDS card no. 040783). XRD analysis has confirmed the presence of both CuO nanocrystals and Ag nanoparticles in the formed nanocomposites. The elemental composition and chemical state of Ag/CuO nanocomposites were characterized by XPS spectra analysis. Figure 3 shows XPS spectral regions for Cu 2p and Ag 3d at three stages of Ag/CuO formation (0, 1, and 5 min reaction

3. RESULTS AND DISCUSSION 3.1. Structural Characterization and Elemental Analysis. Transmission electron microscopy was used to characterize the structure of CuO nanocrystals and Ag/CuO nano12501

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with CuO for 1, 3, 5, and 10 min to determine the optimal preparation conditions for SERS. The SERS activity was characterized using 4-ATP as the target probe molecule. The resulting spectra for this series of measurements are shown in Figure 4. The Raman peak intensities observed for 4-ATP

Figure 2. XRD patterns of the synthesized CuO nanocrystals (a) and Ag/CuO nanocomposites (b).

Figure 3. XPS spectra showing the (a) Cu 2p and (b) Ag 3d peak regions of an Ag/CuO nanocomposite prepared with reaction times of 0, 1, and 5 min.

Figure 4. SERS spectra of 4-ATP acquired on Ag/CuO nanocomposite substrates prepared under different reaction conditions (increasing AgNO3 concentrations): (a) 0.25, (b) 0.5, (c) 0.75, and (d) 1.0 M AgNO3. Each graph presents acquired in situ at time intervals of 1, 3, 5, and 10 min. SERS spectra from 4-ATP and CuO are also displayed as benchmark data.

times). The high-resolution Cu 2P XPS spectra (Figure 3a) show individual peaks at 937 and 957 eV, which correspond to Cu 2p3/2 and Cu 2p1/2, respectively, and two satellite peaks at 946 and 965 eV.15 The Ag 3d XPS spectra exhibits two peaks at 368.2 and 374.2 eV, which are assigned to the doublet of Ag 3d5/2 and Ag 3d3/2 (Figure 3b).16 The XPS results confirm that Ag NPs were anchored on the CuO nanocrystals, which is consistent with TEM results. Further, a downward shift in the Cu 2p spectra and a corresponding upward shift in the Ag 3d spectrum were observed with increased Ag loading of the Ag/ CuO nanocomposite. This suggests that the interfacial surface charge distribution of the Ag/CuO nanocomposites has changed and indicates formation of a charge-transfer complex as described below. 3.2. Optimal Preparation Conditions of Ag/CuO Nanocomposites for SERS. As mentioned above, the AgNO3 concentration and reaction time with CuO directly controlled the Ag content of the resulting Ag/CuO nanocomposite material. To evaluate the SERS activity of different Ag/CuO nanocomposites, we prepared samples using a series of AgNO3 concentrations (0.25, 0.5, 0.75, and 1 M) reacted

solution on clean silicon and on CuO substrates were negligible and are included in each pane of Figure 4 as a benchmark. At low Ag loading (0.25 and 0.5 M), the 4-ATP Raman signal was weak and the intensities changed only slightly with increased reaction time. At higher Ag loading (0.75 and 1 M), the Raman peak intensities were significantly enhanced and also varied with reaction time. For the 0.75 M AgNO3 sample (Figure 4c), the intensity increased with reaction time up to 5 min but then decreased for a longer reaction time of 10 min. The 1 M AgNO3 sample SERS peak intensity was constant for reaction times of 1, 3, and 5 min but decreased for a reaction time of 10 min. We suspect that the observed decrease in SERS intensity for the longest (10 min) reaction time was due to overloading the CuO surface with Ag−which would effectively cover the most interfacial Ag/CuO SERS active sites and reduce the enhancement. On the basis of our matrix, we propose that the optimal concentration and reaction time for preparing SERS 12502

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Figure 5. SERS spectra as a function of 4-ATP concentration while adsorbed on Ag/CuO nanocomposites. Inset shows the linear relationship between the Raman intensity of the 1077 cm−1 peak and the 4-ATP concentration.

shifted downward while the Ti 2p binding energy shifted upward.29 They suggested a charge transfer occurred from the TiO2 support to the Ag particles at the TiO2/Ag interface inducing a large electromagnetic field. In our case, Figure 3 shows the XPS binding energy shift of the Ag 3d peaks to higher binding energy revealing a decrease of electron density at Ag. Simultaneously, the Cu 2p binding energy band shifted lower, indicating a higher electron density at the Cu atoms in the Ag/CuO nanocomposite. This result suggests that the electrons might migrate from Ag to CuO, thus changing the charge distribution between the two at the interface. Previous studies have reported that the predominance of 4ATP a1 modes may be attributed to an electromagnetic (EM) enhancement.7,22 The intensities of the b2-bands are associated with a charge transfer (CT) mechanism.22,23 The Raman spectra presented in Figure 5 suggest that both EM and CT mechanisms contributed to the observed SERS spectra of 4ATP on the Ag/CuO nanocomposites. To further confirm our speculation, Figure 6a shows the results of Raman spectra of 4-ATP adsorbed on solution-phase Ag/CuO, a pure Ag film, and on solution-phase CuO. Note that on the solution-phase CuO alone there was no Raman signal, whereas a sharp difference in the Raman spectra between the Ag film and Ag/CuO solution was observed. It is known that a pure Ag film creates a very weak electromagnetic field that can lead to a weakly enhanced SERS signal for some molecules. In our experiment, we observed a significant enhancement for the Ag/CuO nanocomposite over the pure Ag film alone. This shows that the SERS enhancement is not due to the Ag or CuO materials individually but occurs only on the combined Ag/CuO nanocomposite. This can be attributed to a CT transition occurring at the metal−semiconductor interface which induces a strong local electromagnetic field and results in effective Raman enhancement. A schematic diagram for this mechanism is shown in Figure 6b,c.

active Ag/CuO nanocomposites for 4-ATP detection as 0.75 M AgNO3 and 5 min, respectively. To determine the performance of Ag/CuO nanocomposite for SERS, a serial dilution of 4-ATP was prepared and Raman spectra obtained at an excitation wavelength of 633 nm. The SERS spectra exhibited several peaks relevant to 4-ATP which were slightly shifted in frequency relative to other literature reports.17−19 Figure 5 shows that the Raman intensity decreases with decreasing 4-ATP concentration, becoming negligible at 10−6 M 4-ATP. This suggests a SERS detection limit for 4-ATP on Ag/CuO nanocomposite of as low as 10−5 M. The SERS spectra obtained on Ag/CuO nanocomposites exhibited peaks at 1077, 1144, 1175, 1218, 1395, 1442, 1508, 1570, and 1632 cm−1, which are consistent with literature reports for Raman spectra of 4-ATP molecules.19−21 Among these, the peaks at around 1077, 1175, and 1508 cm−1 were assigned to the ν(C−S), δ(C−H), and ν(C−C) stretching vibration, respectively, which are dominated by characteristic a1 vibrational modes.22−24 The δ(C−H) at 1144 cm−1, [ν(C−C) + δ(C−H)] at 1395 and 1442 cm−1, and ν(C−C) at 1570 cm−1 were interpreted as b2 modes.25−27 To quantify the effectiveness for SERS enhancement by Ag/ CuO nanocomposites, the enhancement factor (EF) was calculated using the Raman peak at 1077 cm−1 according to the formula22,28 EF = ISERS/Isubstrate × (concentration factor), where ISERS and Isubstrate are the SERS intensities of the selected band in the SERS and ordinary Raman spectra on Ag/CuO nanocomposite, respectively. The EF values for 4-ATP concentrations from 10−3 to 10−5 M were 1.6 × 103, 2.6 × 104, and 1.0 × 105, respectively. Accordingly, we plotted Raman intensities versus concentrations (10−5, 10−4, 2.5 × 10−4, 5 × 10−4, and 10−3 M) for 4-ATP at 1077 cm−1 (Figure 5 inset) to illustrate this concentration dependence. 3.3. SERS Enhanced Mechanism. In a previous study on TiO2/Ag materials, Yu et al. reported that with increased Ag content in the composite the Ag 3d binding energy in XPS 12503

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Figure 6. Raman spectra (a) at 1000−2000 cm−1 of 4-ATP adsorbed on solution-phase Ag/CuO, a pure Ag film, and CuO in solution. Schematic (b, c) of charge transfer transition from Ag to CuO. Figure 7. Solution-phase Ag/CuO nanocomposite (a) and solid-phase Ag/CuO nanocomposite on silicon wafer (b) as SERS-active substrate.

The energy level diagram in Figure 6b describes the CT in more detail. The work function of Ag is ∼4.1 eV, and the Fermi level of CuO (∼5.3 eV) is lower than that of Ag. Thus, electrons may transfer from Ag to CuO until the two systems achieve equilibration.5 This charge redistribution results in the junction of positively charged Ag and negatively charged CuO, where the highest charge density may excite a more intense LSPR, as shown in Figure 6c. A similar Fermi level equilibration has been observed at a Ag/ZnO interface.30 In that study, the authors indicated that the electromagnetic field at the interface can be strongly improved by charge transfer induced polarization, thus enhancing the SERS intensity. 3.4. Enhanced SERS Performance Using SolutionPhase Substrate. Previous studies have shown that deposition of SERS-active substrates from aqueous phase avoids many harsh conditions common to conventional assays, such as high temperature, organic solvents, and high vacuum. Thus, aqueous phase processing is better suited for biological component analysis.31−33 In this experiment, we designed the solutionphase SERS substrate of Ag/CuO nanocomposite to provide a much higher spatial density of hotspots than a typical adsorbed film substrate structure. A comparison of the Raman intensities using the solution-phase and solid-phase Ag/CuO nanocomposite as a SERS substrate is shown in Figure 7. The solution phase Ag/CuO nanocomposite substrate produced a 10-fold Raman enhancement for 10−3 M 4-ATP compared to a solid-phase Ag/CuO nanocomposite substrate. The increase in sensitivity may be due in part to the large effective surface area resulting from a solution-based process. The charge transfer from Ag to CuO at the interface may induce a large electromagnetic field enhancement. This, assisted by the aqueous environment, provides a higher density of dynamic hot spots in a localized region leading to improved SERS performance on the solution-phase Ag/CuO nanocomposite. This substrate has the potential for rapid screening and sensitive analysis and further provides a built mechanism for tuning the preparation conditions to achieve the optimal nanocomposite composition for SERS analysis.

demonstrated an overall enhancement of 105, which is sufficient for single molecule detection. Quantitative detection of 4-ATP was demonstrated for concentrations as low as 10 μM, and the Raman intensities exhibited a linear relationship to 4-ATP concentration. The higher SERS activity of Ag/CuO nanocomposites can be attributed to an EM- and CT-enhancement mechanism occurring simultaneously at the metal−semiconductor interface. Additionally, solution-phase SERS detection provides high density of dynamic hot spots, resulting in strong enhancement of the SERS signal on Ag/CuO nanocomposites. This work demonstrates the improved performance of a solution-phase SERS system that is thus well suited for real-time analyte monitoring.

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AUTHOR INFORMATION

Corresponding Author

*Tel 011-886-7-525-2000 ext 3931; Fax 011-886-7-525-3908; e-mail [email protected] (S.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Ministry of Science and Technology, Taiwan (NSC 101-2113-M-110-013-MY3), and the National Sun Yat-sen University Biochip Research Group for financial support of this work. Prof. Hsieh also thanks Dr. David Beck for helpful discussions and proofreading, Mr. Hsien-Tsan Lin of Regional Instruments Center at National Sun Yat-Sen University for his help in TEM experiments, and Pei-Chuan Tsai for assistant measurement.

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REFERENCES

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4. CONCLUSIONS A method for detecting 4-ATP using Ag/CuO as a solutionphase SERS-active substrate was established. This SERS system 12504

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