Large-Area, Reproducible Ag-Film-Functionalized Mixed Polystyrene Spheres on Silicon for Surface-Enhanced Raman Spectroscopy

Abstract

Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical tool which presents high sensitivity in detection of the low concentration molecules and analytes. Its physical mechanism is based on strong confined electric fields in the tiny gaps between metallic nanostructures. In this work, two different sizes of polystyrene (PS) spheres were mixed to fabricate large-area quasi-random nanoarray on silicon decorated with Ag film for SERS. With the tuning of the volume ratio of two different sizes of spheres and the thickness of Ag film, it will always exhibit higher SERS intensity than periodic nanoarray with only one size. The Finite-Difference Time-Domain (FDTD) simulation shows that this hybrid structure provides high density of hot spots around the hybrid nanostructure due to surface plasmons, and the produced long decay length of the enhanced electric field makes the SERS substrate a zero-gap system for sensitive detection of large biomolecules. And this SERS system achieved an enhancement factor of the order of 105-106 in the detection of R6G molecules. The hybrid nanostructure is reproducible with a relative standard deviation of 10% .And the SERS substrates also produced sensitive Raman signals on long Aβ and hIAPP fibrils, which is potential for sensitive detection of amyloid aggregates that are related to Alzheimer's disease and type 2 diabetes (T2D). Our study demonstrates that the Ag-Film-Functionalized mixed PS spheres substrates may serve as good SERS substrates in the detection for biomedical and chemical analytes.