Universal substrates based on Ag colloidal particles for routine surface-enhanced Raman scattering spectral measurements

Abstract

We developed a method to fabricate sensitive and stable substrates that can be used in routine surface-enhanced Raman scattering (SERS) spectral measurements. The substrates were fabricated by immobilization of Ag colloidal particles on a glass plate coated with poly(4-vinyl pyridine) (P4VP), adsorption of molecules on the immobilized colloidal particles, and second immobilization. In the second step, we adsorbed weakly adsorbed molecules such as aniline instead of target molecules. We purified pristine silver sols by centrifuging to increase the surface charge of the colloidal particles and optimized Ag colloidal particle diameter to achieve high Raman signal enhancement from their dimers. When purified silver sols were used in the immobilizations, dimers were predominantly formed with very few trimers. When target molecular solutions were dropped in arrays on the substrate prepared by adsorbing aniline and dried, a good SERS spectrum was observed from each spot without any peaks of aniline and P4VP. The average enhancement was found to be 1.0 × 10⁷ and the enhancement of the dimers was ∼1.6 × 10⁷. The latter corresponds to the enhancement outside the hotspot regions, since target molecules could not be adsorbed at the junctions after the formation of dimers. About one hundred samples could be tested on the substrate fabricated on a cover glass when target molecular solutions were dropped by using a micropipette. Since dropping and drying are very simple and easy processes, and many samples could be tested on a substrate, our substrates can be used in routine SERS spectral measurements and assays like bioassays.