Synthesis of wool roll-like silver nanoflowers in an ethanol/water mixture and their application to detect traces of the fungicide carbendazim by SERS technique

Abstract

The Raman signal enhancement ability of the surface-enhanced Raman scattering (SERS) technique is largely determined by the SERS substrate, which is usually a collection of precious metal (such as silver or gold) nanoparticles. For use in the SERS substrate, anisotropic metal nanoparticles, e.g. flower-like, will be preferred over the isotropic ones since they will give higher Raman enhancement. The problem is that it is very difficult to fabricate anisotropic metal nanoparticles as small as the isotropic ones that are best suited for use as SERS substrates. This study deals with the synthesis of wool roll-like silver nanoflowers (AgNFs) in a mixed ethanol/water solution instead of the usual aqueous solution when reducing silver nitrate with ascorbic acid in the presence of citric acid, which acts as a structure-directing agent. The size of the wool roll-shaped AgNFs was reduced from about 700 nm when the solution was purely aqueous to about 280 nm when in the mixed solution the ethanol/water volume ratio was 75/25. Thanks to the size reduction of AgNFs, the enhancement factor of SERS substrates made from them has increased dramatically, from 2.7 × 10⁶ when the size of AgNFs is 700 nm to 5.4 × 10⁹ when their size is 280 nm (the calculation is based on rhodamine 6G Raman and SERS spectroscopy). The application of the above AgNFs to recording the SERS spectrum of carbendazim (CBZ), a typical fungicide, at low concentrations has also shown that the smaller the size of the AgNFs, the higher the intensity of the CBZ characteristic bands. The wool roll-shaped AgNFs with a size of 280 nm allowed CBZ to be detected down to a concentration of 0.01 ppm (4.2 × 10⁻⁸ M) with a detection limit of 3.2 ppb (13.4 × 10⁻⁹ M).