Femtomolar response of a plasmon-coupled ZnO/graphene/silver hybrid whispering-gallery mode microcavity for SERS sensing

Abstract

A whispering-gallery mode (WGM) microcavity provides a natural structure to strengthen the light–matter interactions. The surface plasmon (SP) of a metal generates tremendous electromagnetic near-fields which contributes to the detection of Raman signals. Moreover, graphene can improve the Raman signals through charge transfer between the graphene and the analyte molecules, namely chemical enhancement. Combining these three kinds of enhancement factors, a femtomolar SERS response with an enhancement factor of 0.95 × 10¹² and an ultralow detection limit down to 10⁻¹⁵ M was achieved in a hybrid ZnO/graphene/Ag WGM microcavity for probe molecule detection. It can be attributed not only to the optical field confinement effect of the natural WGM structure but also to the graphene-assisted charge transfer and Ag SPs. This label-free SERS substrate has further been employed to detect a neurotransmitter and also to obtain high sensing performance.