Improvement of Surface Enhanced Raman Spectroscopy By Analyte Molecules Covering with Graphene
Laser exitation at surface enhanced Raman spectroscopy (SERS) has been found to damage analyte molecules causing changes in their spectra and making it difficult to perform clear qualitative and quantitative analysis . In the present work, we have prevented the destruction of different organic molecules during SERS analysis by their covering with graphene (Gr).
The fabrication of solid substrates demonstrating SERS-activity included two steps of liquid processing: (i) formation of porous silicon (PS) template by anodization of n+-Si wafer in HF-based solution; (ii) electroless deposition of Ag on PS from the aqueous solution of AgNO3 and C2H5OH . Ag was chosen due to its strongest plasmonic properties while the PS template provided nanostructuring Ag deposit. Therefore, combination of these materials led to the enormous enhancement of Raman signal during scattering [2, 3].
Several organic analytes were used to be studied by SERS: dye R6G, porphyrine CuTMpyP4 and human tear proteins (lactoferrine and lizocime). The PS/Ag substrates were immersed into 10-6 – 10-12M solutions of the analytes for 2 h to provide an adsorption of the organic molecules.
Gr was grown on Cu foil by the thermal decomposition of methane and acetylene and then transferred onto the PS/Ag and PS/Ag/analyte substrates.
Raman spectra were registered on the 3D scanning confocal Raman microscope Confoteq NR500 equipped with 473 nm laser (SOL Instruments, Belarus).
Morphology study of the experimental samples was performed with a scanning electron microscope (SEM) Hitachi-4800 (Chiyoda-ku, Japan) provided 1 nm resolution.
Results and Discussion
Mapping at the Raman scattering of PS/Ag/Gr showed that transferred Gr presents an alternation of single- and multilayered sheets.
SEM analysis revealed dimensions and spatial distribution of the Ag nanostructures in the SERS-active substrates are not affected by the covering with Gr (fig. 1). However, there are some Gr wrinkles, which can influence on SERS-activity.
Figure 2 shows SERS-spectra of R6G molecules adsorbed on the PS/Ag substrate and covered with the single and multilayered Gr. It is well seen, the multilayered Gr hinders bands of the analyte in contrast to the single Gr.
The areas of the PS/Ag/R6G/single Gr were studied by Raman scattering in comparison with the PS/Ag/R6G (fig. 3). Spectrum of the samples free of Gr demonstrate disappearing/fluctuating some R6G characteristic peaks due to photoinduced desorption or chemical reactions with the analyte molecules. Moreover, there are observed broad bands related to the reaction of Ag and products of analyte combustion under laser excitation . On the other hand, Gr-covered samples demonstrate clear peaks typical for R6G molecules. Since Gr presents material, which is inert and resistant to oxygen from environment, it prevents destruction, physical and chemical changes of the underlying organic molecules.
The similar results were obtained for CuTMpyP4, lactoferrine and lizocime at all concentrations used in the present work.
Single Gr cover of organic molecules adsorbed on the SERS-active substrates prevents their destruction or physic-chemical changes. It provides clear determination and concentration estimation of the analyte in aqueous solution. The results are very promising for the analysis of liquids of human body containing molecules, which have similar SERS-spectra.
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