Formation of SERS-active silver structures on the surface of mesoporous silicon

Abstract

UDC 543. 424+535.375 We have optimized the procedure for preparation of nanostructured silver films on the surface of mesoporous silicon (PSi) to use them as active substrates in surface-enhanced Raman scattering (SERS) spectroscopy. The greatest enhancement of the SERS signal was observed for samples obtained when the silver was deposited on PSi from an aqueous AgNO 3 solution with concentration 1⋅10 -2 M over a 10-15 minute period. The detection limit for rhodamine 6G on SERS-active substrates prepared by the optimized procedure was 1⋅10 -10 M. The enhancement factor for the SERS signal on these surfaces was estimated as ≈2⋅10 8 . We have shown that SERS-active substrates based on mesoporous silicon are promising for detection and study of complex organic compounds, in particular tetrapyrrole molecules.Introduction. SERS spectroscopy, having all the advantages inherent to conventional Raman scattering (specificity or selectivity, characteristicity of spectra, noninvasiveness), is characterized by extremely high sensitivity, which is achieved as a result of a significant increase in the scattering cross section for molecules adsorbed on "rough" metallic surfaces, so-called "SERS-active" substrates [1,2]. In this case, the signal is enhanced a million-fold or more compared with conventional Raman scattering, which makes it possible to use SERS to obtain detailed information about the structure and composition of a material, including biological specimens, at the lowest possible concentrations (all the way down to recording the spectra of single molecules) [3]. Today, methods for rapid quantitative medical analysis have been developed based on SERS: analysis for lactic acid (one of the most important metabolic parameters of blood [4]) and creatine [5], and real-time and high-sensitivity determination of glucose [6,7]. A method has been proposed for measurement of the concentration of the drug 5-fluorouracil in the body of patients undergoing chemotherapy from analysis of a drop of saliva [8], etc.Despite the rather large number of existing applications of SERS in biomedicine [9, 10], there is a need for further development of highly sensitive techniques and consequently for discovery of new effective SERS substrates. Until recently, the most widely used SERS-active materials were colloids of Ag and Au aggregates, and also electrodes made of noble metals with nanosized surface asperities. However, the low stability and difficulties involved in preparation of colloidal solutions with reproducible characteristics, and also the rather weak enhancement of the Raman signal on the electrodes hinder their broad application for practical purposes.Recently publications have appeared on obtaining SERS-active nanostructures based on metal-plated porous silicon (PSi) [11,12]. In this case, the active surfaces are rather easily formed by immersion plating of silver from solution on porous silicon substrates [12]. These solid-state materials proved to be comparable in sensitivity with the best colloidal silver substrates, and...

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Fabrication of Non-woven Fabric-Based SERS Substrate for Direct Detection of Pesticide Residues in Fruits

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