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J. Phys. Chem. 1995,99, 3892-3894
Surface-Enhanced Raman Spectroscopy Using Photodeposited Gold Particles in Porous Sol-Gel Silicates Fatemeh Akbarian,t Bruce S. Dunn,t and Jeffrey I. Zink*3t Department of Chemistry and Biochemistry and Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, California 90024 Received: November 18. 1994@
A new optically transparent, porous material is prepared that acts as a substrate for surface-enhanced Raman spectroscopy (SERS). The material is a silica matrix synthesized by the sol-gel method containing gold particles. Small molecules, such as pyrazine, diffuse into the matrix and are detected by using SERS. The gold particles are produced by a photochemical method that allows systematic and simple optimization which is appropriate for the SERS applications. The silica matrix stabilizes the gold particles compared to colloids in liquid media. SERS is verified by the enhancement of the Raman bands of pyrazine. The effect is further characterized by studying the change in the Raman depolarization ratio. Organometallic gold precursor compounds are dissolved in the sol and encapsulated in growing silicate network. Irradiation of doped monoliths with ultraviolet light causes the photodeposition of gold particles within the silica gel or xerogel. The particle size is controlled by the irradiation time and by the form of the matrix. The particles are characterized by electronic absorption spectra, X-ray diffraction, and transmission electron microscopy.
Surface-enhanced Raman spectroscopy (SERS)’ is the increase in the intensity of Raman bands that occurs when molecules interact with specific metal surface^.^.^ The technique offers the potential for development of new optically-based chemical sensors. Most of the SERS studies have involved silver surfaces, but gold surfaces and colloidal gold also produce enhancement.“’ Theoretical calculations suggest that the optimum size of gold particles should be in the 60-80 nm range.* However, gold colloids in solution are not stable for periods of longer than a few weeks. Sol-gel materials offer a unique environment for stabilizing gold colloids for SERS applications because they are transparent in the visible region of the spectrum, porous enough to allow molecules to diffuse to the physically trapped gold particles, and dimensionally and chemically stable. The porosity and transport of small molecules in sol-gel glasses have been well ~ t u d i e d . ~ - ’ ~ In this paper we report the synthesis of a silica monoliths containing the gold precursor compounds dimethyl(trifluor0acetylacetonato)gold, (CH3)2Au(tfac), or dimethyl(hexafluor0acetylacetonato)gold, (CH3)2Au(hfac),and their irradiation with UV light to produce gold particles in the interior of the monoliths. Other techniques for preparation of gold colloid in silica glass are known and include the traditional melt method,l3.l4 radio-frequency ~puttering,’~ ion bombardment,I6 pyrolysis of precursor molecules in sol-gel films,”.I8 and pyrolysis of precursor molecules in sol-gel ORMOSILS.’9 Our photodeposition method combines the advantages of the avoidance of heat treatment, the ability to pattern the deposit, and, most importantly, systematic and simple control of the particle size. This control allows the optimization for SERS measurements. The SERS effect is demonstrated by the enhancement of the intensities and changes in the depolarization ratios of Raman bands of a test molecule, pyrazine, that diffused into the pores of the sol-gel monolith. Using the room-temperature sol-gel p r o c e s ~ , ~silicate . ’ ~ gel glass monoliths were prepared by mixing 15 mL of tetraethylDepartment of Chemistry and Biochemistry.
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’ Department of Materials Science and Engineering. Abstract published in Advance ACS Abstracts, March 1, 1995.
orthosilicate, 5 mL of distilled water, and 0.04 mL of 6 N HCl. The mix was ice-cooled and stirred for 15-20 min. The resulting sol was doped with the gold precursor compounds dissolved in 2-propanol. The doped sol was poured into polystyrene cuvettes, sealed with parafilm, and allowed to gel. Some monoliths were used while fresh and others were allowed to age. Xerogels were prepared by exposing the monoliths to air and allowing them to dry over a period of a month. Irradiation of the doped monoliths by 351 nm laser light or light from an unfiltered 100 W Hg-vapor lamp leads to photodeposition of gold colloid particles in only the irradiated areas of the monoliths. Prior to irradiation, the gel monoliths doped with the precursor compounds are colorless or pale yellow. Formation of gold colloids by photodeposition results in coloration of the irradiated portion of the monoliths that varies progressively with the irradiation time. The growth of the particle size can be sequentially monitored by absorption spectroscopy. The irradiation time provides control of the particle size. Increasing the irradiation time promotes particle aggregation, leading to an increase in mean particle size and hence a red shift in the absorption maximum (vide infra). The optimum method of making a sample with a desired particle size is to photolyze a freshly prepared gel sample (with large, solvent-filled pores) until the absorption spectrum shows that the desired size has been achieved and then either dry or store the sample for further use. When small ( < 100 A) particles are desired, irradiation of xerogels is the preferred method. In all cases, after irradiation the particle size remains constant even when further drying and shrinkage of the monolith take place. Under ambient conditions these samples are stable for over 2 years. The monoliths containing colloidal gold were characterized by using absorption spectroscopy, X-ray diffraction, and transmission electron microscopy. The most convenient method is absorption spectroscopy. The wavelength of the absorption band maximum increases, and the band becomes broader as the particle size increases. The average radius of spherical gold particles can be calculated from these properties.I8 X-ray diffraction confirms the presence of elemental gold in the
0022-365419512099-3892$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 12, 1995 3893
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Figure 1. Raman spectrum of a sol-gel monolith containing photodeposited colloidal gold after immersion in a 0.1 M solution of pyrazine in ethanol. The enhanced ring-breathing mode of pyrazine occurs at 1020 cm-'. Inset: absorption spectrum of a sol-gel monolith containing photodeposited colloidal gold that was used in the SERS studies.
irradiated sample. Peaks are observed at 28 = 38.19", 44.39", 64.58", and 77.55" corresponding to the ( l l l ) , (200), (220), and (222) planes, respectively. The average radius can be calculated from the peak widths by using the Scherrer equation." Particle sizes calculated in this way are in reasonable agreement with those estimated from absorption spectroscopy. Examination of samples under TEM revealed gold particles of varying morphologies. Average particle sizes measured by this method for different samples were in good agreement with those estimated by the above methods. Of the samples that have been studied, the ones producing the best SERS effect are those with absorption maxima at 570 nm (Figure 1). These samples are prepared by 45 min irradiation of fresh gels (less than 3 days old) containing (CH3)2Au(hfac). These samples have an average particle radius calculated from the absorption spectrum of 475 A, close to the 600-800 A theoretical optimal radii for SERS.8 The absorption band maximum and hence the gold particle size remain unchanged as the gels age further in sealed containers or are allowed to air-dry. The average radius calculated from XRD studies of these samples is 209 A. TEM examination reveals irregularly-shaped gold with dimensions on the order of several hundred angstroms. Both the monoliths stored in sealed containers and those that were air-dried are used in the SERS studies. SERS activity was probed by using pyrazine as the test molecule. Gel glass monoliths containing gold colloids, and undoped monoliths as controls, were immersed in 0.1 M solutions of pyrazine in ethanol. The clearest evidence of the enhancement of the intensity of the Raman bands of pyrazine is the comparison of the intensity of the totally symmetric ring breathing mode of pyrazine22to that of the 976 cm-' band from the sol-gel matrix. The latter peak is a broad band arising from the bulk gel. A typical Raman spectrum of a monolith that was immersed in the ethanol solution of pyrazine for 3 h is shown in Figure 1. The spectrum was obtained by exciting the monolith at 647.1 nm with about 75 mW of power from a Kr ion laser. The spectrum was accumulated by using a CCD detector in 2 min. The pyrazine peak at 1020 cm-' has greater height than the peak from the bulk monolith at 976 cm-I. The other peaks in the spectrum arise from normal modes of ethanol.
The control experiments involved immersing similarly prepared sol-gel monoliths that do not contain colloidal gold in the solutions of pyrazine in ethanol. A sample immersed for the same amount of time as the SERS active sample described above did not show any pyrazine peaks in the Raman spectrum. The enhancement of Raman intensity caused by interaction of the pyrazine with the gold particles enables detection of the molecule even when only a small amount has diffused into the pores of the gel. A second measure of SERS activity is the depolarizationratio, e, equal to the ratio of the Raman scattering intensity polarized perpendicular to and parallel to that of the laser light.22,23The depolarization ratio was measured for the 1020 cm-' totally symmetric ring breathing mode of pyrazine. This band is polarized (e = 0) in solution spectra. When the pyrazine molecule interacts with the gold colloid, an asymmetry is introduced, causing e > 0. A nonzero value of e is additional evidence for the SERS effect. In ethanolic solutions containing colloidal gold,24 e = 0.44 f 0.08. In monoliths that were immersed in the ethanol solution of pyrazine for 3 h, e = 0.59 f 0.1. These depolarization ratio changes, together with the intensity enhancements, provide unambiguous evidence for SERS. In summary, photodeposition of gold from sol-gel monoliths containing (CH&Au(tfac) or (CH&Au(hfac) is demonstrated. The sol-gel materials containing photodeposited colloidal gold are used as matrices for SERS. Photodeposition allows the size of the gold particles to be controlled and for spatial control of the particles in the matrix. The SERS effect is demonstrated by the intensity enhancement of pyrazine bands in the Raman spectra of the monoliths and by the change in the depolarization ratio. The stability, transparency, and porosity of the new materials make them interesting new substrates for surfaceenhanced Raman spectroscopy and for potential sensor applications.
Acknowledgment. This work was made possible by a grant from the National Science Foundation (DMR 92-02 182). References and Notes (1) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163-166. (2) Jeanmaire, D. L.; Van Duyne, R. P. J . Electroanal. Chem. 1977, 1 , 84. (3) Albrecht, M. G.; Creighton, J. A. J. Am. Chem. Soc. 1977,99,5215. (4) Blatchford, C. G.; Campbell, J. R.; Creighton, J. A. Surf: Sci. 1982, 120,435-455. (5) Schatz, G. C. Ace. Chem. Res. 1984, 17, 370-376. (6) Zeman, E. J.; Schatz, G. C. J . Am. Chem. Soc. 1987, 91, 634643. (7) Creighton, J. A,; Blatchford, C. G.; Albrecht, M. G. J . Chem. Soc., Faraday Trans. 2 1979, 75, 790. (8) Zeman, E. J.; Schatz, G. C. In Proceedings of the 17th Jerusalem Symposium; Pullman, B., Jortner, J., Gerber, B., Eds.; Nitzan Dyn. SurJ 1984, 413-424. (9) Klein, L. C. Annu. Rev. Mater. Sci. 1985, 15, 227. (10) Brinker, C. J.; Scherer, G. Sol-Gel Science: The Physics and Chemisrry of Sol-Gel Processing; Academic Press: San Diego, 1989. (11) Dunn, B.; Zink, J. I. J . Mater. Chem. 1991, I , 903. (12) Avnir, D.; Braun, S.; Otolenghi, M. In Supramolecular Aichitecture; Bein, T., Ed.; ACS Symposium Series 499; American Chemical Society: Washington, DC, 1992, pp 384-404. (13) Weyle, W. A. Coloured Glasses; The Society of Glass Technology: Sheffield, 1951. (14) McMillan, R. W. Glass-Ceramics; Academic Press: London, 1964. (15) Wakabayashi, H.; Yamanaka, H.; Kadono, K.; Sakaguchi, T.; Miya, M. Science and Technology of New Glasses, Proceedings of Intenational Conference on Science and Technology of New Glasses; Sakka, S . , Ed.; 1991; p 412. (16) Fukumi, K.; Chayahara, A,; Kadono, K.; Sakaguchi, T.; Horino, Y . ;Miya, M.; Hayakawa, J.; Saton, M. Jpn. J . Appl. Phys. 1991, 30, L742.
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