Facile tuning of tip sharpness on gold nanostars by the controlled seed-growth method and coating with a silver shell for detection of thiram using surface enhanced Raman spectroscopy (SERS)

Abstract

Developing SERS substrates based on individual gold and silver metals, either with rough surfaces or bare nanoparticles, has certain limitations in practical analysis applications. In order to improve the range of applications of the noble metallic substrates, a comprehensive approach has been proposed for preparing non-traditional SERS nano-substrates by combining tip-enhanced gold nanostars and Raman signal amplification of the silver layer. This preparation process is conducted in two steps, including tuning the sharpness and length of tips by a modified seed growth method followed by coating the silver layer on the formed star-shaped nanoparticles. The obtained AuNS-Ag covered with an average size of around 100 nm exhibited interesting properties as a two-component nano-substrate to amplify the activities in SERS for detecting thiram. The controllable and convenient preparation route of gold nanostars is based on the comproportionation reaction of Au seed particles with Au(III) ions, achieved by governing the stirring times of the mixture of the Au seed and the growth solution. Thus, the citrate-seed particles decreased in size (below 2 nm) and grew into nanostars with sharp tips. The thickness of Ag covering the Au particles' surface also was appropriately controlled and the tips were still exposed to the outside, which is a benefit for matching with the source excitation wavelength to achieve good SERS performance. The Raman signals of thiram can be instantly and remarkably detected with the enhancement of the substrates. Thiram can be determined without any pretreatment. It was found that the limit of detection for thiram is 0.22 ppm, and the limit of quantification is 0.73 ppm. These experimental results shed some light on developing the SERS method for detecting pesticide residue.