Mercaptoacetic Acid-Capped Silver Nanoparticles Colloid: Formation

Apr 8, 2003 - Abstract. Mercaptoacetic acid-capped spherical silver nanoparticles with a diameter of about 17 nm were prepared by a simple chemical re...
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Langmuir 2003, 19, 4285-4290

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Mercaptoacetic Acid-Capped Silver Nanoparticles Colloid: Formation, Morphology, and SERS Activity Xiaoling Li,† Junhu Zhang,† Weiqing Xu,† Huiying Jia,† Xu Wang,† Bai Yang,† Bing Zhao,*,† Bofu Li,† and Yukihiro Ozaki*,‡ Key Laboratory for Supermolecular Structure and Materials of 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 February 2, 2003 Mercaptoacetic acid-capped spherical silver nanoparticles with a diameter of about 17 nm were prepared by a simple chemical reaction. The formation process of the silver nanoparticles was investigated by UV-visible (UV-vis) spectroscopy and transmission electron microscopy. The results show that the spherical and rodlike particles were formed at the beginning of the reaction, and then the rodlike particles were gradually converted into spherical particles with the reaction continuing. Finally, the content of the rodlike particles was less than 3% in the silver colloid. Thus, the final obtained silver nanoparticles were uniform in their shapes and showed little variation in their sizes. This silver colloid can remain stable for several weeks, which makes it convenient for use in practice. We also investigated the effect of Cl- on the rate of changes in the optical properties of the silver colloid by UV-vis absorption. The results indicate that Cl- accelerates the aggregation of the colloidal particles by effectively screening the repulsive electrostatic interactions between the negatively charged silver particles. We used the self-assembled technique to transfer the silver nanoparticles onto solid substrates from the colloid with and without Cl- in the solution. The UV-vis spectra show that the absorption band red shifts and a new band appears at a longer wavelength when the silver nanoparticles are transferred onto the substrate from the colloid with Cl- in the solution, indicating that the silver nanoparticles can grow and aggregate on the substrate. This was further confirmed by the atomic force microscopy measurements. Both the silver colloid and the substrates prepared by transferring the silver nanoparticles from the silver colloidal solution onto the quartz slides can serve as surface-enhanced Raman spectroscopy (SERS)-active substrates. It was found that the SERS enhancement depends on the sizes and aggregation of the silver particles, and the addition of Cl- generates much stronger SERS signals.

Introduction In recent years, research on nanoparticles of noble metal has become extremely active because of their sizedependent optical, magnetic, and catalytic properties and the potential of the emergence of surface-enhanced Raman spectroscopy (SERS).1-9 Most of studies on metal colloids * To whom all correspondence should be sent. Fax: +81-79565-9077. E-mail: [email protected]. jp. † Jilin University. ‡ Kwansei-Gakuin University. (1) (a) Yonezawa, T.; Onoue, S.; Kimizuka, N. Langmuir 2001, 17 (8), 2291. (b) Hanamura, E. Phys. Rev. B: Condens. Matter 1988, 37, 1273. (c) Esumi, K.; Hosoya, T.; Suzuki, A.; Torigoe, K.; Langmuir 2000, 16 (6), 2978. (d) Van Duyne, P. R. In Chemical and Biochemical Applications of Lasers; Moore, C. B., Ed.; Academic Press: New York, 1979; Vol. IV, p 101. (2) (a) Ozin, G. A. Adv. Mater. 1993, 5, 412. (b) Hirai, T.; Bando, Y.; Komasawa, I. J. Phys. Chem. B 2002, 106 (35), 8967. (c) Kim, H. S.; Ryu, J. H.; Jose, B.; Lee, B. G.; Ahn, B. S.; Kang, Y. S. Langmuir 2001, 17 (19), 5817. (d) Liu, J.; Ong, W.; Kaifer, A. E.; Peinador, C. Langmuir 2002, 18 (16), 5981. (3) (a) Bright, R. B.; Musick, M. D.; Natan, M. J. Langmuir 1998, 14, 5695. (b) Henglein, A.; Giersig, M.; J. Phys. Chem. B 1999, 103 (44), 9533. (c) Okitsu, K.; Yue, A.; Tanabe, S.; Matsumoto, H.; Yobiko, Y. Langmuir 2001, 17 (25), 7717. (d) Cotton, T. M. In Spectroscopy of Surface; Clark, R. J. H., Hester, R. E., Eds.; John Wiley & Sons: Chichester, U.K., 1993; p 91. (4) (a) Yonezawa, T.; Yasui, K.; Kimizuka, N. Langmuir 2001, 17 (2), 271. (b) Wang, W.; Efrima, S.; Regev, O. Langmuir 1998, 14, 602. (c) Vo-Dinh, T. In Photonic Probes of Surfaces; Halevi, P., Ed.; Elsevier: Amsterdam, The Netherlands, 1995; p 67. (d) Xu, W.; Liao, Y.; Akins, D. L. J. Phys. Chem. B 2002, 106 (43), 11127. (5) (a) Sun, T.; Seff, K. Chem. Rev. 1994, 94, 857. (b) Duff, D. G.; Baiker, A. Langmuir 1993, 9, 2301. (6) Zhou, X. F.; Chu, D. B.; Wang, S. W.; Lin, C. J.; Tian, Z. Q. Mater. Res. Bull. 2002, 37 (11), 1851.

have been concerned with silver, gold, and copper colloids. The chemical reduction of metal ions is most commonly employed in the preparation of metal nanoparticles in a solution. However, the sizes, shapes, and stability of these systems are rather difficult to control despite the ease of preparation of the noble metal colloids and particles. To prepare the small and stable nanoparticles, capped organic molecules are necessary for preventing the nanoparticles from irreversible aggregation in a solvent and making the particles soluble in a given solvent. Consequently, studies of the nanoparticle formation process become important for understanding the particle growth and also for controlling the size distribution.9,10 In fact, many research groups carried out investigations on the control of the sizes of the particles and the formation of the nanoparticles.9-12 For example, Zhang et al.9 studied the formation of silver clusters and nanoparticles prepared in polyacrylate and inverse micellar solutions. Manna et al.11 reported the capability for forming silver nanoparticles from a single source of the N-hexadecylethylene(7) Aihara, N.; Torigoe, K.; Esumi, K. Langmuir 1998, 14, 4945. (8) Liz-Marzan, P.; Vlckova, B.; Vohidal, J.; Pancoska, P.; Baumrunk, V. J. Phys. Chem. 1992, 96, 1361. (9) Zhang, Z.; Patel, R. C.; Kotharl, R.; Johnson, C. P.; Friberg, S. E.; Alkens, P. A. J. Phys. Chem. B 2000, 104, 1176. (10) (a) Chen, S.; Templeton, A. C.; Murray, R. W. Langmuir 2000, 16, 3543. (b) Chen, S.; Huang, K.; Stearms, J. A. Chem. Mater. 2000, 12, 540. (c) Shiraishi, Y; Toshima, N. J. Mol. Catal. A: Chem. 1999, 141, 187. (11) Manna, A.; Imae, T.; Iida, M.; Hisamatsu, N. Langmuir 2001, 17 (19), 6000. (12) He, S. T.; Yao, J. N.; Jiang, P.; Shi, D. X.; Zhang, H. X.; Xie, S. S.; Pang, S. J.; Gao, H. J. Langmuir 2001, 17, 1571.

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diamine silver nitrate complex that acts as a metal-ion provider as well as a particle protector. He et al.12 used a liquid-liquid two-phase method to prepare 1-nonanethiol-capped silver nanoparticles and explored the formation of silver nanoparticles. In the present work, we report the formation, morphology, and SERS activity of the mercaptoacetic acid-capped silver nanoparticles. The silver nanoparticles with uniform shapes and sizes can be obtained by a new method proposed in this paper. This method utilizes a simple chemical reaction of silver idodide and sodium borohydride. The advantages of the method are the ease of preparation, convenience in use, and, especially, that the obtained silver nanoparticles are uniform in their shapes and sizes. The last point is important for SERS measurements because the shapes and sizes of the metal nanoparticles are significant parameters in terms of the electromagnetic theory of SERS. Furthermore, we employed UV-visible (UV-vis) spectroscopy to determine the optimum conditions for the preparation of stable and highly SERS-active silver colloids. Time-dependent UVvis spectroscopy and transmission electron microscopy (TEM) are employed to monitor the silver formation process of the nanoparticles. Specifically, we observed changes in the shapes of the silver nanoparticles during the formation of the nanoparticles. This may be helpful in understanding the growth of the nanoparticles and creates a new dimension in controlling the shapes of the nanoparticles. We also investigated the effect of Cl- on the rate of a change in the optical properties of the silver colloid by using UV-vis spectroscopy. In addition, we used a self-assembled technique to transfer the silver nanoparticles from the colloid onto the quartz slides. The addition of Cl- can change the surface morphology of the SERS-active substrate. Both the silver colloid and the silver film on the substrate are SERS-active, and the SERS results indicate that the SERS enhancement depends on the sizes and aggregation of the silver particles and the existence of Cl-. Experimental Section Materials. AgNO3 (99.5%) was obtained from Wako Pure Chemical Industries, Ltd. NaI (99%), NaBH4 (98%), mercaptoacetic acid (98%), and poly(dialyldimethylammonium chloride) (PDDA) were obtained from Sigma Chemical Company. H2SO4 and 30% H2O2 were obtained from Beijing Chemical Plant. 1,4Bis[2-(4-pyridyl)ethenyl]benzene (BPENB) was provided by another group in our lab.14 Rhodamine 6G (R6G) was obtained from Exciton Chemical Co., Inc. Silver Colloid Preparation. A total of 2.5 mL of 10-2 M AgNO3 was added to 75 mL of triply distilled water. A total of 5 mL of 10-2 M mercaptoacetic acid was added as stabilizer to the solution with stirring. After 10 min of mixing, 2.5 mL of 10-2 M NaI was dropped into the solution slowly, yielding a greenyellow AgI colloid. A total of 20 mg of NaBH4 was added to the AgI colloidal solution, and the reaction mixture was continually stirred for about 20 min. The silver colloid was finally obtained. During the whole reaction, the color of the colloidal solution changed from green-yellow to nut-brown at the beginning, then to brown, and finally to orange. Preparation of SERS Substrates. The quartz slides were cleaned by immersion in a boiling solution prepared by mixing 30% H2O2 and concentrated H2SO4 with a volume ratio of 3:7. After cooling, the substrates were rinsed repeatedly with triply distilled water. The slides were then immersed in a 0.5% PDDA solution for about 30 min and finally rinsed with triply distilled water. The slides were continually immersed into the silver colloid (13) Jin, R. C.; Cao, Y. W.; Mirkin, A. C.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (14) Yam, V. W.-W.; Lau, V. C.-Y.; Wu, L. X. J. Chem. Soc., Dalton Trans. 1998, 1461.

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Figure 1. UV-vis spectra of silver colloidal solutions with different concentrations of NaBH4: (a) 0.66 × 10-3, (b) 0.99 × 10-3, (c) 1.7 × 10-3, (d) 3.3 × 10-3, (e) 5.0 × 10-3, (f) 6.6 × 10-3, and (g) 9.9 × 10-3 M. The inset illustrates the dependence of fwhm (left axis, 9) and absorbance (right axis, 1) on the concentration of NaBH4. or were immersed into the silver colloid after the addition of a 0.5 M KCl solution with a volume ratio of 30:1 for about 20 min. Finally, the slides were rinsed by triply distilled water. Instruments. UV-vis spectra were recorded on a Shimadzu UV-3100 spectrophotometer. SERS measurements were performed by a Renishaw 1000 model confocal microscopy Raman spectrometer with a charge-coupled device detector and a holographic notch filter. The SERS excitation was provided by the 514.5-nm line of a Coherent Radiation Innova Ar+ laser. The laser power at the sample position was typically 4.0 mW for BPENB on the silver colloid and 2.2 mW for R6G on the SERS substrates. Data acquisition was the result of five 50-s accumulations and a single 10-s accumulation for BPENB and R6G, respectively. TEM was measured with a Hitachi H-8100 IV operating at 200 kV. The 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. Silver Colloid Formation and SERS Measurements. 1.1. Effect of the Concentration of NaBH4 in the Reaction Mixture on the Silver Particle Size Distribution. To determine the optimum conditions for the preparation of stable and highly SERS-active silver colloids with the narrowest particle size distribution, a large number of experiments were carried out, varying the concentration of NaBH4 in the reaction mixture described in the Experimental Section (see Silver Colloid Preparation). Figure 1 shows UV-vis spectra of silver colloidal solutions with different concentrations of NaBH4. It can be seen from Figure 1 that, with the increase in the concentration of NaBH4, the full width at half-maximum (fwhm) of the absorption band due to the silver nanoparticles decreases from 200 to 75 nm. This result means that the particle size distribution becomes narrower and the colloid system changes from polydispersion to monodispersion with the increase in the concentration of NaBH4. Note that the strongest intensity and the narrowest particle size distribution can be obtained at the NaBH4 concentration of 6.6 × 10-3 M. 1.2. Formation of the Silver Nanoparticles. The formation process of mercaptoacetic acid-capped silver nanoparticles was traced by the UV-vis spectra and TEM micrographs. Figure 2 shows absorption spectra of the

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Figure 2. UV-vis spectra of the silver particles measured at different reaction times: (a) 0, (b) 1, (c) 10, (d) 20, and (e) 60 min. The inset illustrates the content of rodlike particles versus time.

silver nanoparticles measured during the whole reaction process at different times. When the concentration of NaBH4 in the reaction mixture reached 6.6 × 10-3 M, that is, the addition of the reducing agent was finished, a very remarkable absorption band appeared immediately (Figure 2b), showing that the silver particles were formed at that time. When the reaction continued, the peak shape and position changed obviously (Figure 2b-e). Figure 3 shows TEM micrographs of the silver particles at different reaction times. It is clear that the particle size distribution is narrow and the average diameter is about 17 nm. In the whole reaction process, the particle size does not show obvious change but the particle shape does change remarkably. In the initial stage, the spherical and rodlike particles coexisted in the colloidal solution. However, the number of the rodlike particles decreased gradually with time, suggesting that these particles had changed into spherical particles. In the final stage, the percentage of the rodlike particles was less than 3%. The content of the rodlike silver nanoparticles was estimated by the observational TEM. The inset of Figure 2 illustrates the content of the rodlike particles versus time. This result for the particle shapes is significantly different from those of widely used colloids prepared, for example, according to the procedure of Lee and Meisel,15 where one finds a great variety of shapes, from spheres and cubes to rods and needles. In other words, in the colloid prepared by the conventional methods one cannot control the variations in the shapes of the silver particles. One of the noted advantages of our novel method is that one can prepare solely the silver particles with spherical shapes and little variation in their sizes. By combining the results of the time-dependent UVvis spectra and TEM, we can infer the conversion process of the nanoparticle shape. In the initial stage, the surface plasmon band appears at 382 nm, which is characteristic of spherical silver particles, but at the same time, a band with a long tail is also observed, implying that the rodlike particles also coexist in the colloidal solution.13,16-18 With (15) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391. (16) Klasu, T.; Joerger, R.; Olsson, E.; Granqvist, C. G. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 13611. (17) Brown, K. R.; Walter, D. G.; Natan, M. J. Chem. Mater. 2000, 12, 306. (18) Bohren, C. F.; Huffman, D. R. Adsorption and Scattering of Light by Small Particles; John Wiley & Sons: New York, 1983.

Figure 3. TEM micrographs of silver particles at different reaction times: (a) 3, (b) 20, and (c) 60 min.

the reaction continuing, the long tail disappeared gradually, and the shape of the plasmon band changed to be more and more symmetric, as can be seen in Figure 2c-e, suggesting that the shapes of the silver particles became uniform. As mentioned in the Experimental Section, the color change of the reaction mixture also showed the formation process of the nanoparticles. When the NaBH4 was added to the silver iodide colloid, the color of the solution changed quickly from greenish-yellow to nutbrown, which indicates the formation of the silver particles. Then, the color gradually changed to brown and finally to orange, which was characteristic of the spherical silver particles. To monitor the stability of the final prepared silver colloid, we measured the absorption spectra of the colloid on different days (not shown here). There was no obvious change in the shape, position, and symmetry of the absorption peak during the initial 20 days, except for the decrease of the absorbance. After the 20th day, the fwhm of the spectrum started to become wider than before, and the peak maximum showed a slight red shift, implying the onset of nanoparticle aggregation. These results demonstrate that the silver nanoparticles colloidal solution can remain stable for about 3 weeks. It is of interest to know the formation process of the nanoparticles in more detail. From these limited experimental data, it is not easy to provide detailed information on the process. However, on the basis of our experimental

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Figure 4. UV-vis spectra of the silver colloidal solution measured at (a) 0, (b) 2, (c) 30, and (d) 60 min after the addition of a 0.5 M KCl solution.

results we can describe this formation process as follows: At the beginning of the reaction, silver ions were reduced to atoms from the solution during a very short period just after the reducing agent was added. The silver atoms aggregated rapidly, forming spherical and rodlike nanoparticles, and then the rodlike nanoparticles were gradually changed into spherical particles. Moreover, mercaptoacetic acid as the stabilizer prevailingly linked the surfaces of the silver nanoparticles, which could make the silver nanoparticles separate from each other. Thus, the observed spherical nanoparticles were separated in the solution. 1.3. Effect of Cl- on the Rate of Change in the Optical Properties of the Silver Colloid. Screening of the repulsive interactions between the colloidal particles by the addition of salt should enhance the rate of formation of aggregates and would, thus, provide further insight into the kinetics of the aggregation process.19-25 Figure 4 shows UV-vis spectra collected at different times after the addition of a 0.5 M KCl solution to the mercaptoacetic acid-capped silver colloidal solution. These measurements were performed on freshly capped silver colloidal solutions (and, thus, not aged) to see the effect of added salt on the rate of change in the optical properties of the colloidal solution. A clearly resolved longitudinal plasma resonance centered at 550 nm is observed to grow immediately after the addition of salt to the silver colloidal solution. Furthermore, this resonance shifts to red at larger time intervals (about 30 min, see Figure 4b-d). This result indicates that the sizes of the aggregates increases with time. It is clear from Figure 4 that the salt accelerates the aggregation of the colloidal particles by changing the interactions between the negatively charged silver particles. (19) (a) Mandal, S.; Gole, A.; Lala, N.; Gonnade, R.; Ganvir, V.; Sastry, M. Langmuir 2001, 17, 6262. (b) Siiman, O.; Bumm, L. A.; Callaghan, R.; Blatchford, C. G.; Kerker, M. J. Phys. Chem. 1983, 87, 1014. (20) Siiman, O.; Feilchenfeld, H. J. Phys. Chem. 1988, 92, 453. (21) Dou, X.; Jung, Y. M.; Cao, Z.; Ozaki, Y. Appl. Spectrosc. 1999, 53, 1440. (22) Creighton, J. A. In Surface Enhanced Raman Scattering; Chang, R. K., Furtak, T. E., Eds.; Plenum Press: New York, 1982; p 315. (23) Nickel, U.; Castell, A.-Z.; Poppl, K.; Schneider, S. Langmuir 2000, 16, 9087. (24) Schneider, S.; Grau, H.; Halbig, P.; Freunsche, P.; Nickel, U. J. Raman Spectrosc. 1996, 27, 57. (25) Sanchez-Cortes, S.; Garcia-Ramos, J. V.; Morcillo, G.; Tinti, A. Z. J. Colloid Interface Sci. 1995, 175, 358.

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Figure 5. SERS spectra of BPENB on the silver colloid (a) before and (b) after the addition of a 0.5 M KCl solution.

1.4. SERS Measurements. As is known, the particles’ shapes are important parameters for the surface enhancement in terms of the electromagnetic theory of SERS.22,23 From the TEM results, we know that the silver nanoparticles in the colloid are spherical with little variation in their sizes. In general, a stable SERS signal can be obtained if the particles’ shapes and sizes are more uniform. Figure 5 shows the SERS spectra of BPENB adsorbed on the silver colloid before and after the addition of Cl-. It is obvious that the SERS signal becomes stronger after the addition of Cl-, suggesting that Cl- ions favor the SERS emergence. In the literature,24,25 Cl- ions are often used for two apparently conflicting reasons. Most authors insist that they have stabilizing action under acidic or salt conditions and they coadsorb on particular sites that favor the SERS detection of molecules.26 In other works, chloride salts are used as aggregating agents, probably through the cation property or the high concentration of the salts.27 In our study, the Cl- ion effect on the silver colloid confirms the latter opinion. 2. SERS-Active Solid Substrate. In general, suspended metal colloids with a high specific surface area can be advantageous for use in observing SERS from low concentrations of adsorbate.28 However, colloidal suspensions will often aggregate spontaneously depending on the composition of an added sample, which can change their SERS enhancement.28 To avoid a variation in enhancement due to the changes in aggregation, a large number of SERS investigations employ solid supportbased SERS substrates.29-39 Thus, we transferred the (26) Kamishny, A. L.; Kakharov, V. N.; Fedorov, Y. V.; Galashin, A. E.; Aslanov, A. L. J. Colloid Interface Sci. 1993, 158, 171. (27) Griffith, W.; Koh, T. Spectrochim. Acta, Part A 1995, 51, 253. (28) Rodger, C.; Rutherford, V.; White, P. C.; Smith, W. E. J. Raman Spectrosc. 1998, 29, 601. (29) Vo-Dihn, T.; Stokes, D. L. Appl. Spectrosc. 1993, 47, 1728. (30) Mullen, K. I.; Carron, K. T. Anal. Chem. 1991, 63, 2196. (31) Lacy, W. B.; Williams, J. M.; Wenzler, L. A.; Beebe, T. P.; Harris, J. M. Anal. Chem. 1996, 68, 1003. (32) Huang, Q. J.; Yao, J. L.; Mao, B. W.; Gu, R. A.; Tian, Z. Q. Chem. Phys. Lett. 1999, 306, 314. (33) Carron, K.; Peitersen, L.; Lewis, M. Environ. Sci. Technol. 1992, 26, 1950. (34) Akbarain, F.; Dunn, B. S.; Zink, J. I. J. Raman Spectrosc. 1996, 27, 775. (35) Bello, J.; Stokes, D. L.; Vo-Dinh, T. Anal. Chem. 1989, 61, 1779. (36) Akbarain, F.; Dunn, B. S.; Zink, J. I. J. Raman Spectrosc. 1996, 27, 775. (37) Lee, A. S. L.; Li, Y. S. J. Raman Spectrosc. 1994, 25, 209.

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Figure 6. UV-vis spectra of the one-layer silver films on quartz slides obtained by transferring the silver nanoparticles from the silver colloid (a) before and (b) after the addition of a 0.5 M KCl solution.

silver nanoparticles from the colloid to the solid substrates by the electrostatic interaction (see Experimental Section). 2.1. UV-Vis Spectra. The UV-vis spectra of a layer of silver nanoparticles on the quartz slides transferred from the silver colloid before and after the addition of a 0.5 M KCl solution with a volume ratio of 30:1 are shown in Figure 6. A band due to the silver nanoparticles transferred from the silver colloid without Cl- appears at 385 nm, which is characteristic of the isolated silver nanoparticles. The corresponding band of the silver nanoparticles transferred from the silver colloid with Cl- is observed at 393 nm. The 8-nm red shift indicates that the silver nanoparticles grow after the addition of Cl- to the silver colloid. Also, a longitudinal plasmon resonance centered at 580 nm is observed in Figure 6b, indicating that some silver aggregates appear on the substrate prepared by transferring the silver nanoparticles from the silver colloid with Cl-. 2.2. AFM Images. Parts a and b of Figure 7 presents AFM images of silver films on quartz slides prepared by transferring silver nanoparticles from the silver colloidal solution before and after the addition of Cl-, respectively. The average diameter of the particles on the substrates prepared by transferring the silver nanoparticles from the silver colloid without and with Cl- was 30 and 40 nm, respectively. It is noted that the silver nanoparticles grow, and more silver nanoparticles are bound to the substrate after the addition of Cl-. Thus, it is very likely that Clcan induce the silver colloid aggregation. This result is consistent with the results of Figure 6. 2.3. SERS Spectra. The SERS spectra of R6G on the substrates prepared by transferring the silver nanoparticles from the silver colloid with and without Cl- are compared in Figure 8. Of note is that the SERS signals from the substrate prepared by transferring the silver nanoparticles from the silver colloid with Cl- are much stronger than those from the substrate prepared without Cl-. This can be explained as follows: The chloride ions added to the silver colloid screen the repulsive interactions between the colloidal particles and, hence, accelerate the cross-linking of the colloidal particles. Then, the silver (38) Mullen, K. I.; Carron, K. T. Anal. Chem. 1991, 63, 2196. (39) Xue, G.; Dong, J. Anal. Chem. 1991, 63, 2393.

Figure 7. AFM images of the one-layer silver films on quartz slides obtained by transferring the silver nanoparticles from the silver colloid (a) before and (b) after the addition of a 0.5 M KCl solution.

Figure 8. SERS spectra of R6G on the silver substrates obtained by transferring the silver nanoparticles from the silver colloid (a) before and (b) after the addition of a 0.5 M KCl solution.

particles grow and form some aggregates in the colloid. When the slide is taken out from the silver colloid, the grown silver nanoparticles and some aggregates are transferred onto the substrate together. This can be

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confirmed by the UV-vis and AFM results. As is known, the SERS enhancement depends on the sizes and aggregation of the silver particles.40-42 In general, the silver aggregates favor the SERS detection of most molecules.19 Accordingly, we obtained much stronger SERS signals from the substrate prepared by transferring silver nanoparticles from the colloid with Cl-. Conclusions Mercaptoacetic acid-capped silver nanoparticles with uniform shapes and sizes were prepared by a new method. The narrowest particle size distribution can be obtained at the reducing-agent concentration of 6.6 × 10-3 M. We employed UV-vis spectroscopy and TEM to monitor the formation process of the silver nanoparticles, and the results showed that the spherical and rodlike particles were formed at the beginning of the chemical reaction, and then the rodlike particles were converted to spherical (40) Gersten, J. I.; Nitzan, A. Surf. Sci. 1985, 158, 165. (41) Fornasiero, D.; Greiser, F. J. Chem. Phys. 1987, 5, 3213. (42) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75 (5), 790.

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particles as the reaction progressed. Finally, we obtained spherical silver nanoparticles with a diameter of about 17 nm. The silver nanoparticles with uniform shapes may be used as building blocks to construct ordered mesoscopic structural materials, which will provide the possibilities of detecting the collective physical properties of the ensemble and further exploring the size-tunable optical and electronic properties of the nanocrystals in devices such as single-electron transistors in the future. In addition, it has been found that the addition of Clenhances the rate of formation of aggregates of the silver nanoparticles. The silver nanoparticles can be transferred onto a quartz substrate by electrostatic attraction. Both the silver colloid and the silver particle film on the quartz substrate can serve as SERS-active substrates. Acknowledgment. This research was supported by the Major State Basic Research Development Program (Grant G2000078102) and the National Natural Science Foundation (Grants 29975011, 20173019, 20273022, and 200340062) of the People’s Republic of China. LA0341815