Synthesis of silver hierarchical superstructures in bicontinuous microemulsion and their application as SERS substrates

Abstract

Hierarchical superstructures (HSS) are three-dimensional structures with a significant improvement in some properties as compared to isolated nanoparticles. In the case of plasmonic materials, such as silver, a 3-D arrangement induces higher hot spots concentration which increase the electric field in the entire superstructure. This behavior can be exploited in Surface Enhanced Raman Spectroscopy (SERS) to detect analytes of concern at very low concentrations. However, the obtention of 3-D structures has been conducted by intricate methods that usually involves the synthesis of the building blocks and then, the assembly in superstructures in a second step. As an alternative one-pot procedure, we propose here the use of a bicontinuous microemulsion (BCME), since they are a confined reaction media with a nanostructured liquid morphology. We analyze the parameters that influence the formation of silver HSS and its application as SERS substrates for Rhodamine 6G sensing. By using sodium citrate as stabilizing agent before the total silver reduction in the water channels of the BCME, we were able to obtain an interconnected fibrous material composed of a silver citrate layer and connected 10 nm spherical silver nanoparticles. The decomposition of the silver citrate layer revealed the 3-D hierarchical arrangement of silver nanoparticles. The silver HSS showed and enhancement factor of 2.4x109 for the sensing of Rhodamine 6G in nM concentration, using a 633nm laser operating at 5% of the nominal power (17mW). This promising result indicates that this silver HSS are ideal SERS substrates to sense analytes that could be destroyed under harsh irradiation conditions.