Optical materials for surface enhanced Raman applications based on sol-gel encapsulated gold particles Fatemeh Akbarian, Bruce S. Dunn and Jeffrey I. Zink Department of Chemistry and Biochemistry and Department ofmaterials Science and Engineering University of California Los Angeles Los Angeles, CA 90024 ABSTRACT A photochemical method of producing nanometer gold particles in optically transparent solgel silicate materials is described. Organometallic gold precursor compounds are dissolved in the sol and encapsulated in the growing silicate network. Irradiation of the doped monoliths with ultraviolet light causes the photodeposition of gold particles within the silica gel or xerogel. The particles are characterized by their electronic absorption spectra and by ThM. The transparent, porous monoliths are excellent substrates for Surface Enhanced Raman Spectroscopy (SERS). Small molecules such as pyrazine diffuse into the monoliths and are detected by using SERS. The sol-gel matrix stabilizes the gold particles (in comparison to colloids in liquid media) and SERS can be used to detect molecules that penetrate the matrix. 2. INTRODUCTION This paper reports the photoproduction of gold particles in silicate sol-gel matrices. Gold particles are of interest for non-linear optical materials1'2 and, as discussed here, for Surface Enhanced Raman Spectroscopy (SERS).37 Other techniques for preparation of gold colloids in silica glass are known and include the traditional melt method,8'9 RF-sputtering,10 ion 1 pyrolysis of precursor molecules in sol-gel films,12'13 and pyrolysis of precursor molecules in sol-gel ORMOSILS.14 The photodeposition method described in this paper combines the advantages of the avoidance of heat treatment, attainment of very small particle size, and the systematic and simple control of the particle size and thus the sample color by means of irradiation time. Most importantly, deposition of gold by using a focused laser beam or irradiation through a mask allows the gold to be deposited in a patterned structure. Surface Enhanced Raman Spectra (SERS) is the 10 to 106 increase in the intensity of Raman bands that occurs when molecules interact with specific metal surfaces. The technique offers the potential for development of optically-based chemical sensors. Most of the SERS studies have involved silver surfaces, and the favored system of SERS investigators has been pyridine on silver. However, related compounds such as pyrazine also yield enhancement. 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.15 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. Photoproduction of gold particles in the pores of the sol-gel material offers the means of controlling particle size. In this paper we report the synthesis of sol-gel monoliths containing the gold precursor compounds dimethyltrifluoroacetylacetonategold(ill), (CH3)2Au(tfac), or 140 ISPIE Vol. 2288 Sol-Gel Optics Ill (1994) 0-8194-1612-6/94/$6.00
dimethythexafluoroacetylacetonategold(ffl), (CH3)2Au(hfac), and the irradiation of the monoliths with UV light to produce gold particles in the interior of the materials. The gold particles are characterized by optical absorption methods and by ThM. The SERS effect is demonstrated by the enhancement of the intensities of Raman bands of a test molecule, pyrazine, that diffused into the pores of the sol-gel monolith. 3. SYNTHESIS Using the room-temperature sol-gel process,16'17 silicate gel glass monoliths were prepared with 15 ml of tetraethylorthosilicate (TEOS), 5 ml distilled water, and one drop of 6 N HC1. The mixture was ice-cooled and stirred for 15-20 mm. The resulting sol was doped with the gold precursor compounds (CH3)2Au(tfac), and (CH3)2Au(hfac), dissolved in isopropanol. The sol was poured into polystyrene cuvettes and the material allowed to gel, age and dry. Irradiation of the doped monoliths by 35 1 nm laser light or light from an unfiltered 100 W Hg-vapor lamp led to the photodeposition of gold colloid particles in the irradiated areas of the monoliths. Under ambient conditions these samples remain stable for over 2 years. In addition, the unirradiated samples are also stable for more than two years. Prior to irradiation, TEOS gel monoliths doped with the gold compounds are colorless. Gold colloid formation by photodeposition results in the irradiated portion of the monoliths taking on colors that varied with irradiation time. Monoliths doped with (CH3)2Au(tfac) and irradiated for 15 minutes became blue whereas samples inadiated for 45 minutes become red. Monoliths doped with (CH3)2Au(hfac) and irradiated for 15 minutes became violet whereas monoliths irradiated for 45 minutes or longer become a metallic golden color. Increasing the irradiation time promotes particle aggregation leading to an increase in the mean particle size and hence a red-shift in the absorption maximum. Thus, the irradiation time provides one method of controlling the particle size. Table 1. Sample Color, Absorption Maxima, and Processing Stage of TEOS Gel Glass Monoliths Doped with Photodeposited Gold Colloid Dopant initial Au conc. processing stage color max (nm) (CH3)2Au(tfac) 0. 14 mm aged gel blue 653 (CH3)2Au(tfac) 0. 14 mm xerogel dark violet 540 (CH3)2Au(hfac) 2.2 mm gel gold 572 (CH3)2Au(hfac) 2.6 mm gel violet 591 4. OPTICAL CHARACTERIZATION The presence of gold colloid in the irradiated monoliths was evident by the formation of colored glass and supported by the results of absorption spectroscopy. Average radii (for spherical gold particles) may be calculated from the band widths in the absorption spectra. The radius of the particles is inversely proportional to the absorption band width, and increases as the wavelength of the band maximum increases. Color, absorption maxima, and the state of the material (fresh gel, aged gel or xerogel) for a variety of monoliths are summarized in Table I. Absorption spectra of two of the monoliths listed in Table 1, one red and the other blue, are shown in Figure 1. SPIE Vol. 2288 Sol-Gel Optics III (1994) / 141
1.2 1.1 0 1.0 0(ID 0.9 0.8 0.7 450 500 550 600 650 700 750 800 2(nm) Figure 1. Absorption spectra of two monoliths containing colloidal gold. The sample with the absorption maximum at 535 nm was prepared by irradiating a monolith containing (CH3)2Au(hfac); the sample with the maximum at 653 nm was prepared by irradiating a monolith containing (CH3)2Au(tfac). The absorbance scale on the left corresponds to the 535nm peak. ThM investigation confirms the presence of gold particles. For example, a blue sample similar to that whose spectrum is shown in figure 1 was examined.0 The ThM images showed the presence of small particles with dimensions on the order of 20-50 A. Diffraction was observed from the larger particles suggesting that they consist of crystalline gold. % range of particle sizes was observed including particles that were too small to be resolved (< 1OA) and some clusters of total dimension of several hundred A. 5. SURFACE ENHANCED RAMAN STUDfES 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. Evidence for the SERS effect was provided by measurement of the intensity enhancement of peaks from pyrazine compared with those of the bulk sol-gel matrix. 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 pyrazine to that of the silanol band from the sol-gel matrix. The latter peak is a broad band arising from the bulk gel. 142 ISPIE Vol. 2288 Sol-Gel Optics Ill (1994)
A typical Raman spectrum of a monolith that was immersed in the ethanol solution of pyrazine for three hours is shown in figure 2. 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 in two minutes by using a CCD detector. The pyrazine peak at 1020 cm1 has greater intensity than the silanol peak from the bulk monolith at 976 cm1. The other peaks in the spectrum arise from normal modes of ethanol. 300- CD >- 200-100- 0- I I I I 900 950 1000 1050 1100 wavenumber (cm1) 1150 Figure 2. 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 cm1. The control experiments involved immersing 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. Only after immersion for one week could the pyrazine peak be observed with intensity comparable to that of the reference silanol peak. These experiments show that 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. SPIE Vol. 2288 So/-Ge/ Optics Ill (1994) / 143
In summary, photodeposition of gold from sol-gel monoliths containing (CH3)2Au(tfac) or (CH3)2Au(hfac) is demonstrated. The sol-gel materials containing photodeposited colloidal gold are used as matrixes for SERS spectroscopy. Photodeposition allows the size of the gold particles to be controlled and in addition allows spatial control of the particles in the matrix. The SERS effect is demonstratedby the intensity enhancement of pyrazine bands in the Raman spectra of the monoliths. The stability, transparency and porosity of the new materials make them interesting new substrates for surface enhanced Raman spectroscopy and for potential sensor applications. 6. ACKNOWLEDGMENT This work was made possible by a grant from the National Science Foundation (DMR 92-02182) 1974. 7. REFERENCES 1. D. Ricard, P. Roussignol, and C. Flytzanis, Opt. Letters. 10, 51 1, 1985. 2. F. Hache, D. Ricard, and C. Flytzanis, J. Opt. Soc. Am. B3, 1647, 1986. 3. M.Fleischmann, P. J.Hendra, A.J. McQuillan, Chem. Phys. Lett. 26(2), 163-166, 4. C.G. Blatchford, J. R. Campbell, J. A. Creighton, Surface Science. 120, 435-455, 1982. 5. G. C. Schatz, Acc. Chem. Res. 17, 370-376, 1984. 6. E. J. Zeman, G. C. Schatz, J. Am. Chem. Sco. 91, 634-643, 1987. 7. J. A. Creighton, C. G. Blatchford, M. G. Albrecht, J. Chem. Soc. Faraday II, 75, 790, 1979. 8. W. A. Weyle, "Coloured Glasses", The Society of Glass Technology, Sheffield, 1951. 9. R. W. McMillan, "Glass-Ceramics", Academic Press, London,,1964. 10. H. Wakabayashi, H. Yamanaka, K. Kadono, T. Sakaguchi, M. Miya, "Science and Technology of New Glasses", Proceedings of International Conference on Science and Technology of New Glasses, S. Sakka, Eds., p412, 1991. 1 1. K. Fukumi, A. Chayahara, K. Kadono, T. Sakaguchi, Y. Horino, M. Miya, J. Hayakawa, M. Saton, Jpn J. Appi. Phys., 30, L742, 1991. 12. J. Matsuoka, R. Mizutani, H. Nasu, K. Kamiya, J. Ceram. Soc. Jpn., 100, 599, 1992. 13. H. Kozuka, S. Sakka, Chem. Mater., 5, 222-228, 1993. 14. C. Y. Li, J. Y. Tseng, C. Lechner and J. D. Mackenzie, private communication. 15. E. J. Zeman, G. C. Schatz, "In Proceedings of the 17th Jerusalem Symposium," B. Pullman, J. Jortner, B. Gerber, Nitzan Dynamics on Surfaces 4 13-424, 1984. 16. L. C. Klein, Ann. Rev. Mater. Sci., 15, 227, 1985. 17. C. J. Brinker and G. Scherer, "Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing", Academic Press, San Diego, 1989. 144 ISPIE Vol. 2288 So/-Gel Optics ill (1994)