Abstract
Citrate-reduced silver nanoparticles (Ag-NPs) are used extensively for surface-enhanced Raman scattering (SERS) studies, but are typically found to aggregate using an aggregation agent. This study is aimed at developing a simple, stable, and reproducible aggregated method for Ag-NPs without any aggregation agents in aqueous solutions. The aggregation is induced by the process of centrifugation, water washing and ultrasonication. A mechanism based on the nonuniform distribution of capping ligands is proposed to account for the aggregated structure formation. UV-Vis-NIR extinction spectra and TEM allowed us to identify the existence of Ag-NPs aggregation. Further, due to the polydisperse mixture of Ag-NPs (20~65 nm) used in the present work, Ag-NPs are aggregated closely, which contribute to the observation of low-concentration SERS from the residual citrate layer or even the single-molecule SERS of R6Gon aggregation. After the evaporation of droplet of Ag-NPs aggregation on the Si substrate, citrate or R6Gcould also be detected but with marked redor blue-shifts.
Highlights
Surface-enhanced Raman scattering (SERS) technique has proved to be a very effective analytical tool due to its high sensitivity, high selectivity, and fluorescence-quenching properties [1,2,3,4], which is a nondestructive technique that provides rich molecular information about molecules and molecular structures in the close vicinity of noble metal surfaces such as gold and silver [5,6,7,8]
transition electron microscopy (TEM) measurements (Figure 1A) ensured us that various shapes of prepared particles such as ellipsoid, rod, sphere or their aggregation were contained with wide range of sizes, 20 - 65 nm (Figure 1B), as supported by a little local surface plasmon peak at 415 nm with accompanying a distinct shoulder at longer wavelength 470 nm formed by the local aggregation of Ag-NPs in extinction spectra (Figure 2A-a)
In the present of appropriate AgNPs aggregation, citrate group should be examined by surface-enhanced Raman scattering (SERS)
Summary
Surface-enhanced Raman scattering (SERS) technique has proved to be a very effective analytical tool due to its high sensitivity, high selectivity, and fluorescence-quenching properties [1,2,3,4], which is a nondestructive technique that provides rich molecular information about molecules and molecular structures in the close vicinity of noble metal surfaces such as gold and silver [5,6,7,8]. One highly promising approach for the design of SERS substrates involves the positioning or fabrication of nearly adjacent metallic nanostructures with nanoscale gaps [16]. Roughened metal surfaces and dried colloids are common used to generate the “hot spots”, but they cannot yield reproducible SERS spectra because of irreproducible SERS substrates or different orientation of analytical molecules on a metal surface [8]. Noble metal NPs aggregation structure in aqueous solution is a better SERS substrate for reliable and convenient detection for any analytes compared to solid nanostructures.
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