Preparation of substrates for surface‐enhanced Raman microprobe spectroscopy

Abstract

The results obtained using a variety of new solid supports for the preparation of surface-enhanced Raman-active substrates via direct chemical reduction of silver salts and by vacuum vapor deposition of metallic silver are presented. These supports include two small pore size cellulose filter papers and frosted glass slides. These supports yield silver substrates with a much more uniform silver particle size and distribution than observed on previously used supports such as Whatman No. 1 and No. 5 filter papers. Results obtained using a solid silver membrane filter are also presented. These substrates provide metal particle morphology which is uniform on a submicrometer scale. This degree of uniformity is necessary for enhancing the reproducibility of SERS using a Raman microprobe since the area probed by this microprohe is 5-10 pm'. By using filter papers and glass slides, coupled with the vapor deposition and chemical reduction techniques, the resulting substrates are also fast, simple, and inexpensive, making them ideal candidates for routine analysis. INTRODUCTJONWhen analytical chemists turned their attention toward the possibility of using the technique of surfaceenhanced Raman spectroscopy (SERS) for routine analysis, one of their major concerns was the lack of preparation methods for SERS-active substrates. The most important factors for such methods were that they be inexpensive, simple and fast, with reproducible, strong SERS activity, especially at the argon ion laser wavelength of 514.5 nm, the most common excitation source for routine Raman spectroscopy. Little concern was given to the details of the surface morphology of the prepared substrates and their ability to be modeled with the electrodynamic theory. These analytically motivated surface preparation methods include the coating of fumed silica with silver by vacuum vapor deposition,' the coating of silver onto frosted glass slides using a Tollen's reduction of silver nitrate,' the coating of silver onto smooth microscope slides by direct chemical reduction using both the Rochelle salt process and the Brashear process3 and the coating of silver onto Whatman No. 1 cellulose filter paper4s5 and Whatman No. 5 cellulose filter paper6 by direct chemical reduction of an aqueous silver nitrate solution with an aqueous sodium borohydride solution.Another method of note involves evaporating silver onto polystyrene spheres which have been coated onto Whatman No. 50 filter paper' and glass slides8 This method allows the control of particle size by the choice of the polystyrene sphere size and control of the particle density by control of the initial concentration of spheres in solution. Experimental optimization of the sphere size, density and coating thickness was reported by Vo-Dinh et aL9 and this surface has been shown to have practical use in the detection of organophosphorus chemical agents" and nitro polycyclic aromatic hydrocarbons." Two of the more unique surface preparation methods, mentioned here for completeness, involve the observation of a sp...

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