Self-Assembled Metal Colloid Films: Two Approaches for Preparing

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Self-Assembled Metal Colloid Films: Two Approaches for Preparing New SERS Active Substrates Xiaoling Li,† Weiqing Xu,† Junhu Zhang,† Huiying Jia,† Bai Yang,† Bing Zhao,*,† Bofu Li,† and Yukihiro Ozaki*,‡ Key Laboratory for Supermolecular Structure and Materials of the Ministry of Education, Jilin University, Changchun 130023, People’s Republic of China, and Department of Chemistry, School of Science and Technology, Kwansei-Gakuin University, Sanda, Hyogo 669-1337, Japan Received September 2, 2003. In Final Form: November 20, 2003 In this paper, we propose two new approaches for preparing active substrates for surface-enhanced Raman scattering (SERS). In the first approach (method 1), one transfers AgI nanoparticles capped by negatively charged mercaptoacetic acid from a AgI colloid solution onto a quartz slide and then deoxidizes AgI to Ag nanoparticles on the substrate. The second approach (method 2) deoxidizes AgI to Ag nanoparticles in a colloid solution and then transfers the Ag nanoparticles capped by negatively charged mercaptoacetic acid onto a quartz slide. By transfer of the AgI/Ag nanoparticles from the colloid solutions to the solid substrates, the problem of instability of the colloid solutions can largely be overcome. The films thus prepared by both approaches retain the merits of metal colloid solutions while they discharge their shortcomings. Accordingly, the obtained Ag particle films are very suitable as SERS active substrates. SERS active substrates with different coverages can be formed in a layer-by-layer electrostatic assembly by exposing positively charged surfaces to the colloid solutions containing oppositely charged AgI/Ag nanoparticles. The SERS active substrates fabricated by the two novel methods have been characterized by means of atomic force microscopy (AFM) and ultraviolet-visible (UV-vis) spectroscopy. The results of AFM and UV-vis spectroscopy show that the Ag nanoparticles grow with the increase in the number of coverage and that most of them remain isolated even at high coverages. Consequently, the surface optical properties are dominated by the absorption due to the isolated Ag nanoparticles. The relationship between SERS intensity and surface morphology of the new active substrates has been investigated for Rhodamine 6G (R6G) adsorbed on them. It has been found that the SERS enhancement depends on the size and aggregation of the Ag particles on the substrates. Especially, we can obtain a stronger SERS signal from the substrate prepared by method 1, implying that for the metal nanoparticles capped with stabilizer molecules such as mercaptoacetic acid, the in situ deoxidization in the film is of great use in preparing SERS active substrates. Furthermore, we have found that the addition of Cl- into the AgI colloid solution changes the surface morphology of the SERS active substrates and favors stronger SERS enhancement.

Introduction Since the discovery of the remarkable enhancement of Raman scattering of molecules adsorbed on properly prepared metal surfaces, surface-enhanced Raman scattering (SERS) spectroscopy has attracted great interest because of the potential applications in various fields as well as the curious physical principle.1-15 Recent reports * To whom correspondence should be addressed. Fax: +81-79565-9077. E-mail: [email protected]. † Jilin University. ‡ Kwansei-Gakuin University. (1) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163. (2) Jeanmaire, D. L.; Van Duyne, P. R. J. Electroanal. Chem. 1977, 84, 1. (3) Yogev, D.; Efrima, S. J. Phys. Chem. 1988, 92, 5761. (4) Dutta, P. K.; Robins, D. Langmuir 1991, 7, 2004. (5) Sibblad, M. S.; Chumanov, G.; Cotton, T. M. J. Phys. Chem. 1996, 100, 4672. (6) Albrecht, M. G.; Creighten, A. S. J. Am. Chem. Soc. 1977, 99, 5215. (7) Xue, G.; Dong, J. Anal. Chem. 1991, 63, 2393. (8) Ni, F.; Cotton, T. M. Anal. Chem. 1986, 58, 3159. (9) Bello, J. M.; Stokes, D. L.; Vo-Dinh, T. Anal. Chem. 1989, 61, 1779. (10) Aroca, R.; Jennings, C.; Kovacs, G. J.; Loutfy, R. O.; Vincett, P. S. J. Phys. Chem. 1985, 89, 4051. (11) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391. (12) Matsui, H.; Pan, S.; Douberly, G. E., Jr. J. Phys. Chem. B 2001, 105, 1683. (13) Chumanov, G.; Sokolov, K.; Gregory, B. W.; Cotton, T. M. J. Phys. Chem. 1995, 99, 9466.

on single-molecule probing by means of SERS have further enhanced the interest in SERS.16-20 Various kinds of colloidal and solid support based substrates have been widely used for SERS measurements.21-31 Suspended (14) Tessier, P. M.; Velev, O. D.; Kalambur, A. T.; Rabolt, J. F.; Lenhoff, A. M.; Kaler, E. W. J. Am. Chem. Soc. 2000, 122, 9554. (15) Tian, Z. Q.; Ren, B.; Mao, B. W. J. Phys. Chem. B 1997, 101, 1338. (16) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (17) Kenipp, K.; Wang, Y.; Kneipp, H.; Perelaman, L. T.; Itzkan, I.; Dasari, R. R.; Fled, M. S. Phys. Rev. Lett. 1997, 78, 1667. (18) Michaels, A. M.; Nirmal, M.; Brus, L. E. J. Am. Chem. Soc. 1999, 121, 9932. (19) Doering, W. E.; Nie, S. J. Phys. Chem. B 2002, 106, 311. (20) Taylor, J. R.; Fang, M. M.; Nie, S. Anal. Chem. 2000, 72, 1979. (21) Walls, D. J.; Bohn, P. W. J. Phys. Chem. 1989, 93, 2976. (22) Huang, Q. J.; Yao, J. L.; Mao, B. W.; Gu, R. A.; Tian, Z. Q. Chem. Phys. Lett. 1999, 306, 314. (23) Carron, K.; Peitersen, L.; Lewis, M. Environ. Sci. Technol. 1992, 26, 1950. (24) Akbarain, F.; Dunn, B. S.; Zink, J. I. J. Raman Spectrosc. 1996, 27, 775. (25) Prochazka, M.; Stepanek, J.; Turpin, P.-Y.; Bok, J. J. Phys. Chem. B 2002, 106, 1543. (26) Li, Y. S.; Wang, Y. Appl. Spectrosc. 1992, 46, 142. (27) Mullen, K. E.; Carron, K. T. Anal. Chem. 1991, 63, 2196. (28) Zou, S.; Weaver, M. J.; Li, X. Q.; Ren, B.; Tian, Z. Q. J. Phys. Chem. B 1999, 103, 4218. (29) Cao, P. G.; Yao, J. L.; Zheng, J. W.; Gu, R. A.; Tian, Z. Q. Langmuir 2002, 18, 100. (30) Leverette, C. L.; Shubert, V. A.; Wade, T. L.; Varazo, K.; Dluhy, R. A. J. Phys. Chem. B 2002, 106, 8747. (31) Hu, J. W.; Zhao, B.; Xu, W. Q.; Fan, Y. G.; Li, B. F.; Ozaki, Y. J. Phys. Chem. B 2002, 106, 6500.

10.1021/la0356396 CCC: $27.50 © 2004 American Chemical Society Published on Web 01/23/2004

Self-Assembled Metal Colloid Films

metal colloid with a high specific surface area is advantageous for emerging SERS from low concentrations of an adsorbate.11,32,33 However, a major problem with the use of metal colloid is the tendency for colloidal aggregation after the addition of analyte, which makes the colloid unstable and leads often to poor reproducibility of the SERS spectra. To avoid variations in the SERS enhancement due to the changes in the colloid aggregation, a great number of SERS investigations employ solid support based SERS substrates, such as metal island films,21 electrochemically roughened metals,22 chemically etched metal foils,23 sol-gel silicate glass with trapped gold particles,24 chemically reduced Ag on alumina-coated glass slides,26 abrasively roughened optical fiber,27 and so on. In general, solid SERS substrates require controlledsize nanoscale roughness to allow the excitation of surface plasmon. In SERS active films, the surface morphology is critical to determine the properties of the substrates and the adsorption mode of an adsorbate. The surface morphology of the SERS substrates is more or less predetermined by their preparation procedures and is not strongly affected by the chemical nature of the deposited adsorbates.34,35 A number of studies have been carried out to investigate the dependence of the properties of the substrates on their preparation procedures.19,25,31,34-39 For example, we recently reported a new method for preparing a SERS active substrate by employing the aggregation of Ag particles trapped at an air-water interface.31 An elaborately devised U-shaped capillary device was used to form the Ag particle films quickly from the Ag particles at the air-water interface. The Ag particle film method for SERS shows a high enhancement factor and high stability. Natan and co-workers40,41 reported that Au colloids formed in an aqueous solution can be immobilized onto a solid support to use as a SERS substrate. This procedure enables one to prepare a large number of substrates simultaneously to provide a uniform batch of SERS active surfaces having the same enhancement characteristics. The space of the Au nanoparticles on the surface can be controlled by the coverage of surface functional groups used to capture the colloids and also by the presence of adsorbate46,47 that can shield electrostatic repulsion between the particles.44 The use of an amineor mercapto-functionalized glass surface to capture the metal particles from a metal solution allows their particle (32) Muniz-Miranda, M.; Sbrana, G. J. Phys. Chem. B 1999, 103, 10639. (33) Aroca, R. F.; Clavijo, R. E.; Halls, M. D.; Schlegel, H. B. J. Phys. Chem. A 2000, 104, 9500. (34) Sutherland, W. S.; Laserna, J. J.; Winefordner, J. D. Spectrochim. Acta 1991, 47A, 329. (35) Vlckova, B.; Gu, X. J.; Tsai, D. P.; Moskovits, M. J. Phys. Chem. 1996, 100, 3169. (36) Roark, S. E.; Semin, D. J.; Lo, A.; Skodje, R. T.; Rowlen, K. L. Anal. Chim. Acta 1995, 307, 341. (37) Inhyung, L.; Sang Woo, H.; Kwan, K. J. Raman Spectrosc. 2001, 32 (11), 947. (38) Lecomte, S.; Matejka, P.; Baron, M. H. Langmuir 1998, 14 (16), 4373. (39) Muniz-Miranda, M.; Sbrana, G. J. Phys. Chem. B 1999, 103 (48), 10639. (40) Bright, R. M.; Walter, D. G.; Musick, M. D.; Jackson, M. A.; Allison, K. J.; Natan, M. J. Langmuir 1996, 12, 810. (41) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629. (42) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148. (43) Musick, M. D.; Keating, C. D.; Keefe, M. H.; Natan, M. J. Chem. Mater. 1997, 9, 1499. (44) Inoue, M.; Ohtaka, K. J. Phys. Soc. Jpn. 1983, 52, 3853. (45) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313. (46) Chumanow, G.; Sokolov, K.; Cotton, T. M. J. Phys. Chem. 1996, 100, 5166.

Langmuir, Vol. 20, No. 4, 2004 1299 Scheme 1. Two Proposed Approaches for the Preparation of New SERS Active Substrates

size, spacing, and aggregation to be controlled to a greater degree than many other techniques.40,45,46 This method has been widely used to prepare SERS active substrates during the past decade. The purpose of the present study is to propose two novel techniques for the fabrication of SERS active substrates with immobilized Ag colloid particles. Both techniques use electrostatic interaction to capture the metal particles from metal colloid (see Scheme 1). In the first method (method 1), AgI nanoparticles capped by negatively charged mercaptoacetic acid are transferred from the AgI colloid onto a quartz substrate and then the AgI nanoparticles are deoxidized into Ag nanoparticles on the quartz substrate, while the second method (method 2) deoxidizes AgI colloid into Ag colloid and then transfers the Ag nanoparticles capped by negatively charged mercaptoacetic acid from the Ag colloid onto a quartz substrate. We briefly mentioned the basic idea of method 1 in our previous paper.47 The advantages of the two new approaches lie in the ease of preparation of Ag particle film and in the fact that the surface morphology can be controlled by the different number of depositions. Especially, the Ag nanoparticles prepared by method 1 are just closely packed, grow in the film, and keep their characters. Furthermore, the applications of the two new approaches to SERS show strong enhancement ability with low excitation power. By using the two techniques, we can explore how the aggregation of captured colloids influences their ability of SERS enhancement and optical properties. We also investigate variations in the SERS enhancement with the aggregation of immobilized Ag nanoparticles on the surfaces prepared by the two methods. In this paper, we report the details of the new approaches. The structures and optical properties of the substrates prepared by the two methods and the relationship between the SERS intensity and the surface morphology are investigated by means of atomic force (47) Zhang, J. H.; Li, X. L.; Liu, K.; Cui, Z. C.; Zhang, G.; Zhao, B.; Yang, B. J. Colloid Interface Sci. 2002, 255, 115.

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microscopy (AFM), ultraviolet-visible (UV-vis) spectroscopy, and actual SERS measurements of Rhodamine 6G (R6G). With the increase in the depositions, the Ag nanoparticles grow and become closely packed. The emergence of SERS clearly depends on the size, shape, and spacing of the Ag particles. Changes in the SERS intensity with the variation in the wavelength of the absorption maximum from the multiparticle resonance are investigated. Using the R6G as a probe molecule, the SERS enhancement ability of the two methods is compared. Furthermore, we explore the effect of the addition of Cl- on the surface morphology of the SERS active substrates and their SERS enhancement ability. Experimental Section 1. Sample Preparation. AgNO3 (99.5%) was obtained from Wako Pure Chemical Industries, Ltd. NaI (99%), NaBH4 (98%), and mercaptoacetic acid (98%) were obtained from Sigma Chemical Co. Poly(diallydimethylammonium chloride) (PDDA) with medium molecular weight (200 000-350 000) was purchased from Aldrich Chemical Co., Inc. Rhodamine 6G was purchased from Exction Chemical Co. H2SO4 and 30% H2O2 were obtained from Beijing Chemical Plant. Quartz slides were cleaned by immersing them in a boiling solution prepared by mixing 30% H2O2 and concentrated H2SO4 with a volume ratio of 3-7. After cooling, the slides were rinsed repeatedly with triply distilled water. They were then immersed in a 0.5% PDDA solution for about 30 min and finally rinsed with triply distilled water. AgI colloid was prepared as follows: 2.5 mL of 10-2 M AgNO3 was added to 75 mL of triply distilled water, and 5 mL of 10-2 M mercaptoacetic acid was added as a stabilizer to the solution with stirring. After 10 min of mixing, 2.5 mL of 10-2 M NaI was dropped into the mixed solution slowly. After the addition of NaI, the solution was stirred for about 20 min. 2. Preparation of Two Kinds of New SERS Active Substrates. The two novel methods for preparing SERS active substrates are illustrated in Scheme 1. Method 1. The quartz slide with OH- and PDDA prepared by the above method was immersed into the AgI colloid for about 20 min and then rinsed with triply distilled water. The obtained substrate was immersed into a NaBH4 solution for about 20 min and then rinsed with triply distilled water. Method 2. NaBH4 (20 mg) was added to the AgI colloid, and the obtained colloid solution was stirred for about 15 min. The quartz slide was immersed into the above Ag colloid solution for about 20 min and then rinsed with triply distilled water. 3. Characterization of SERS Active Substrates. UVvisible spectra were measured with a Shimadzu UV-3100 spectrophotometer. SERS measurements were performed by a Renishaw 1000 model confocal microscopy Raman spectrometer with a CCD detector and a holographic notch filter. The SERS excitation wavelength was provided by the 514.5 nm line of a Coherent Radiation Innova Ar+ laser. The laser power at the sample position was typically 3.97 mW for R6G on the SERS substrates. All the spectra reported here were the results of a single 10-s accumulation. AFM images of the substrates were measured with a Digital Instruments Nanoscope IIIA by a multimode using Si cantilevers purchased from DI and Nanosensor Co. Ltd. E and J scanners were selected for the multimode.

Results and Discussion 1. Characterization of Ag Colloid SERS Substrates. As is well-known, the enhancement of Raman scattering from most metal colloid substrates derives principally from metal colloid aggregates, which greatly increase the amplitude of the plasmon resonance and corresponding electromagnetic enhancement compared to isolated metal particles.47-49 It was reported that SERS (48) Anno, E.; Hoshino, R. J. Phys. Soc. Jpn. 1982, 51, 1185. (49) Gotschy, W.; Vonmetz, K.; Leitner, A.; Aussenegg, F. R. Appl. Phys. B 1996, 63, 381.

Figure 1. UV-visible spectra of 1-19 layer immobilized Ag films prepared by method 1. The arrow indicates the increase in the surface coverage.

from isolated metal particles with the diameter of 1 µm exhibits an enhancement factor with the order of 104 while clusters of the same size particles provide an additional 100-fold increase in the enhancement.50 To explore how the aggregation of captured colloids influences their Raman scattering enhancement and optical properties and also to investigate the trend of SERS enhancement with the aggregation of immobilized Ag colloids on the surface, self-assembled Ag colloid films with different numbers of coverages were prepared by the two novel methods. In the first method (method 1, see Scheme 1), a quartz slide with OH- is immersed into a 0.5% PDDA solution for 30 min, AgI colloid suspension for 20 min, and then a NaBH4 solution for 20 min. Additional layers of Ag particles can be deposited by the alternate immersion of the substrate in the 0.5% PDDA, AgI colloid suspension, and NaBH4 solution, allowing the deposition of a multilayer on the surface.51,52 Figure 1 shows UV-vis spectra of 1-19 layer immobilized Ag films fabricated by method 1. At the low coverage, a band due to the Mie resonance of the isolated Ag particles appears at around 380 nm. This maximum shifts to the longer wavelength side by a few nanometers as more Ag nanoparticles are bound to the surface. The wavelength of maximum absorption and the bandwidth of the plasmon resonance depend on the size and shape of the metal particles or aggregates on the substrates.47,53 It is obvious that the peak from the plasmon resonance of isolated particles persists even at the high colloid coverages. The AFM measurements of the substrates (see below) reveal that the Ag nanoparticles grow with the increase in the depositions but most of them remain isolated even at the high coverages. Notice that in the procedure of method 1, we fabricate 1-layer AgI nanoparticles on a quartz substrate and then deoxidize them by NaBH4 and then other layers are deposited one by one. The fact that the band due to the isolated particles persists with the increase in the depositions implies that the Ag (50) Blatchford, C. G.; Campbell, J. R.; Creighton, J. A. Surf. Sci. 1982, 102, 435. (51) Xiao, T.; Yi, Q.; Sun, L. J. Phys. Chem. B 1997, 101, 632. (52) Kudelski, A.; Buknowska, J. Vib. Spectrosc. 1996, 10, 335. (53) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148.

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Figure 2. UV-visible spectra of 1-15 layer immobilized Ag films prepared by method 2. The arrow indicates the increase in the surface coverage.

nanoparticles prepared by this method are just closely packed, grow in the film, and keep their characters. This is one of the advantages of method 1. In addition, with the increase in the depositions, a new broad band appears in the 500-560 nm region as a shoulder. This band arises from the plasmon resonance of aggregates and emerges only when the colloids become close together enough to interact electronically.54 Similar SERS substrates were also prepared by the other method (method 2), which uses alternate exposure of the substrates to the 0.5% PDDA solution for 30 min and the suspension of Ag colloid for 20 min. Figure 2 shows UV-vis spectra of the 1-15 layer immobilized Ag films prepared by method 2. A broad band due to the singleparticle absorption appears near 380 nm in the spectrum of the 1-layer film. This wavelength is almost the same as the corresponding wavelength of the substrate prepared by method 1, suggesting the average particle size is similar for the substrates with 1 deposition prepared by both methods.47,53 For the substrates with the high coverages, the absorbance of the absorption maximum of the method 2 substrate is less than that of the method 1 substrate shown in Figure 1, implying that more colloid nanoparticles can be readily bound to the substrate prepared by method 1. In addition, as more colloids are bound to the surface, the band due to the single-particle absorption for the method 2 substrate shifts much to a longer wavelength. Another broad band due to the multiparticle resonance grows and also shifts to a longer wavelength with the final maximum wavelength of about 610 nm. As the number of immersions into the colloid suspension is increased, the absorption arising from the multipleparticle resonance grows and shifts to a longer wavelength with the increasing characteristic aggregate size. This trend is also observed for the UV-vis spectra of metal island films.36,48 With the increase in the average thickness of an evaporated film, the average size of the islands increases while the distance between them decreases, causing a red-shift of the maximum of absorption.36,48 2. AFM Images of Immobilized Ag Colloid on the Quartz Slides. To clarify how the optical properties of the immobilized colloids are related to the physical (54) Olson, L. G.; Lo, Y. S.; Beebe, T. P.; Harris, J. M. Anal. Chem. 2001, 73, 4268.

Figure 3. AFM images of immobilized Ag colloids on the quartz slides prepared by method 1: (a) 1 deposition and (b) 18 depositions.

structure of such films, AFM images were measured for both method 1 and method 2 substrates with two different surface coverages. Figure 3 and Figure 4 depict AFM images of immobilized colloids prepared by method 1 (1 and 18 depositions) and method 2 (1 and 14 depositions), respectively. Individual colloids can be readily observed in the images of the lowest coverage samples (Figures 3a and 4a). The particle size and shape are uniform on the substrates prepared by the two methods. The average particle diameter on both the method 1 and method 2 substrates with 1 deposition is about 30 nm. This result is consistent with the results of UV-vis spectra, which present similar absorption bands of the Ag films with 1 deposition prepared by method 1 and method 2. We can easily observe the Ag aggregates in the image of the higher coverage sample prepared by method 2 (Figure 4b); however, we find few Ag aggregates in the image of the higher coverage sample prepared by method 1 (Figure 3b). The difference between Figure 3 and Figure 4 indicates that Ag nanoparticles are easier to aggregate than the AgI nanoparticles during their transfer from the colloid onto the quartz substrates. Additionally, we hardly observe the truncated particles in the images of higher coverage accumulated-films prepared by both method 1 and method 2. These indicate that the films with the accumulated Ag particles have more stable structures due to the multiple points of contact and adhesion. It has generally been accepted that “surface roughness” is very important for SERS. The magnitude of the SERS effect depends not only on the nature of the noble metal and the excitation wavelength but also on the size, shape,

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Figure 5. SERS spectra of R6G on 1-layer immobilized Ag colloid substrates prepared by (a) method 1 and (b) method 2.

Figure 4. AFM images of immobilized Ag colloids on the quartz slides prepared by method 2: (a) 1 deposition and (b) 14 depositions.

and spacing of metal nanostructures for electromagnetic field enhancement.1,2,6 The AFM images from the substrates with the single exposure prepared by the two methods (Figures 3a and 4a) exhibit a relatively smooth surface as indicated by the surface roughness of 2.0 ( 0.5 nm and 8.0 ( 0.5 nm for the method 1 and method 2 substrates, respectively. The deviations from the average roughness for the two surfaces are much less than the corresponding isolated particle sizes due to the limited coverage of the surface by the particles; the uncovered, smooth surface contributes significantly to the average. These results for the 1-layer Ag particle films are consistent with the results of the UV-vis spectra shown in Figures 1 and 2, which imply that the surface optical properties are dominated by the band due to the isolated particle absorption. For the substrates with the multiple deposition steps, the AFM images (Figures 3b and 4b) are different from each other in appearance, but they exhibit similar surface roughness. The average roughness for the two different substrates is 8.0 ( 0.5 nm and 11.0 ( 0.5 nm, respectively. These values indicate that for the method 1 and method 2 substrates, the well-accumulated films with few protrusions are formed even with the high coverages. 3. SERS Spectra of R6G on the New Ag Particle Substrates. One of the major goals of the present study is to prepare SERS substrates that can be useful for investigating the molecules adsorbed on them. Figure 5 shows SERS spectra of R6G on the 1-layer Ag colloid films prepared by method 1 (a) and method 2 (b). Note that the

substrate prepared by method 1 (Figure 5a) shows a much stronger SERS signal from R6G than that from the substrate prepared by method 2 (Figure 5b), indicating that method 1 is more powerful than method 2 for the preparation of SERS active substrates. The AFM images show that the surface morphology of the substrates with 1 deposition prepared by both methods is similar, but the SERS enhancement is different for these two substrates. The reason is as follows: In the preparation of the method 2 substrate, we deoxidize the AgI nanoparticles in the colloid solution and thus the Ag nanoparticles were capped by HSCH2COO-. We transferred the Ag nanoparticles from the colloid solution onto the substrate just by the electrostatic interaction of the negative charge of HSCH2COO- and positive charge of PDDA. Consequently, the HSCH2COO- occupied the active sites on the surface of the Ag particle film, and we obtained rather weak SERS signals. However, for the method 1 substrate, we deoxidized the AgI nanoparticles in the film (in situ deoxidization) and thus enhanced the active sites on the surface of the Ag particle film. Consequently, the SERS signals were enhanced. This fact shows that for metal nanoparticles capped by stabilizer molecules such as mercaptoacetic acid, the in situ deoxidization method holds great potential and also creates a new dimension in the preparation of SERS active substrates. The enhancement of Raman scattering was also investigated for the immobilized Ag colloid films with different depositions exposed to a 1 × 10-4 M R6G solution. The excitation wavelength for the maximum SERS enhancement is generally coincident with the wavelength of the absorption maximum of the aggregate excitation band.55-58 A correlation between the SERS intensity from R6G and the wavelength of the absorption maximum of the multiparticle resonance has been investigated for the substrates prepared by method 1. Figure 6 illustrates the maximum SERS intensity at 1647 cm-1 (left axis) and the wavelength of the multiparticle absorption maximum (right axis) versus the number of colloid depositions. The (55) Gersten, J. I.; Nitzan, A. Surf. Sci. 1985, 158, 165. (56) Fornasiero, D.; Greiser, F. J. Chem. Phys. 1987, 5, 3213. (57) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 1979, 75, 790. (58) Siiman, O.; Bumm, L. A.; Callaghan, R.; Blatchford, C. G.; Kerker, M. J. Phys. Chem. 1983, 87, 1014.

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Figure 6. The maximum SERS intensity at 1647 cm-1 (left axis, b) and the wavelength of the multiparticle absorbance maximum (right axis, 9) versus the number of colloid depositions (method 1).

solid and the dotted lines through the points are drawn only to guide the eyes. It can be seen from Figure 6 that the SERS intensity increases gradually with the increase in the number of colloid depositions and then levels down after 11 depositions as the peak of the multiparticle plasmon band shifts to a longer wavelength than the excitation wavelength (514.5 nm).54-57 That is to say, when the wavelength of the plasmon resonance is shorter than the excitation wavelength, the SERS intensity increases with the number of depositions of colloid. When the resonance maximum shifts to a wavelength longer than that of the excitation, the SERS intensity falls off. Olson et al.54 have also found this trend in their research on a silane-modified immobilized gold colloid film as a SERS substrate. We have performed a similar investigation for method 2. However, after several depositions, SERS signals disappear. The above results show that the surface morphology is critical to determine an enhancement factor of the substrate. 4. Effect of Cl- on the Surface Morphology of a SERS Active Substrate and SERS Enhancement. As is well-known, screening of the repulsive interactions between the colloidal particles by the addition of Cl- into a colloid solution should enhance the rate of formation of aggregates and would thus provide higher SERS enhancement.58-60 We added a 0.5 M KCl solution into the AgI colloid with a volume ratio of 1 to 30 in advance and then prepared a SERS active substrate by method 1. UVvis spectra of a layer of Ag nanoparticles on a quartz substrate before and after the addition of Cl- are shown in Figure 7. The Ag film obtained by method 1 without adding Cl- shows a band at 380 nm, which is characteristic of the isolated Ag nanoparticles. However, the corresponding band of the Ag film after the addition of Clappears at 392 nm. Also, a band originating from the longitudinal plasmon resonance is observed around 500 nm, indicating that some Ag aggregates emerge on the substrate. Thus, we conclude that the surface morphology of the SERS active substrate prepared by method 1 is modified by the addition of Cl- into the AgI colloid. On the substrate prepared without Cl-, the Ag nanoparticles are (59) Siiman, O.; Feilchenfeld, H. J. Phys. Chem. 1988, 92, 453. (60) Dou, X.; Jung, Y. M.; Cao, Z.; Ozaki, Y. Appl. Spectrosc. 1999, 53, 1440.

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Figure 7. UV-visible spectra of 1-layer Ag films on the quartz slides obtained by method 1 (a) after and (b) before the addition of 0.5 M KCl solution.

Figure 8. SERS spectra of R6G on 1-layer immobilized Ag colloid substrates prepared by method 1 (a) after and (b) before the addition of 0.5 M KCl solution.

isolated, while on the substrate prepared after the addition of Cl- into the AgI colloid, the isolated Ag nanoparticles and their aggregates coexist. Curves a and b of Figure 8 compare SERS spectra of R6G on the substrates prepared with and without the addition of the KCl solution, respectively. It is clear that the SERS signals from the substrate prepared with the addition of Cl- into the AgI colloid are stronger than those from the substrate prepared without the addition of Cl-. This may be explained as follows: after the Cl- ions are added into the AgI colloid, they accelerate the cross-linking of the colloidal particles, and then the AgI particles form some aggregates in the colloid. When the slide is taken out from the AgI colloid, the isolated AgI nanoparticles and aggregates can be transferred onto the substrate together, and after they are deoxidized, they change into isolated Ag nanoparticles and Ag aggregates. Since the Ag aggregates favor great SERS emergence for most molecules, we obtained stronger SERS signals from the substrate prepared with the addition of Cl- into the AgI colloid.

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Conclusion The immobilized SERS active Ag films were prepared by the two novel methods. The surface optical properties of the Ag films are dominated by the adsorption due to the isolated Ag nanoparticles. By transfer of the colloid nanoparticles onto the solid substrates, the problem of instability of the colloid can be overcome. The films formed by the two methods retain the merits of Ag colloid while they discharge the shortcoming of Ag colloid. The application of the Ag films to SERS shows great enhancement, needs low excitation power, is easy to prepare, and is convenient to use. Using R6G as a probe molecule, we obtained much stronger SERS signals from the substrates prepared by method 1 than from those obtained by method 2, indicating that for metal nanoparticles capped by stabilizer molecules such as mercaptoacetic acid, the in situ deoxidization method holds great promise and provides a new vision in the preparation of SERS active

Li et al.

substrates. The optical properties and SERS enhancement of the substrates vary systematically with the accumulation of immobilized colloid particles. We compared the effects of the Ag colloid aggregation on the plasmon resonance and the wavelength dependence of the SERS enhancement and confirmed that the SERS enhancement is related to the surface morphology of the substrate. Furthermore, it was found that the addition of Cl- into the AgI colloid can enhance the rate of formation of aggregates; the surface morphology of the SERS substrate was thus changed by the addition of Cl- into the AgI colloid. Stronger SERS signal can be obtained from the substrate with the addition of the KCl solution. Acknowledgment. This research was supported by the National Natural Science Foundation (Grant No. 20173019, 20273022, 20375014) of P. R. China. LA0356396