Electrochemical Surface Plasmon Resonance Sensing using a van der Waals Heterostructure

Coupling surface plasmon resonance (SPR) sensing with electrochemistry (EC) is a promising analytical strategy to obtain in situ information about interfacial phenomena in heterogeneous reactions. Typical EC-SPR sensors utilize a metal film both as the plasmonic material and as the working electrode. In this configuration, the eigenmodulation of the plasmonic properties of the metal film under applied potential results in a background signal, which hampers the unambiguous interpretation of the sensor response due to redox reactions. Here, we present a new strategy to overcome this disadvantage by using a van der Waals heterostructure (vdW-HS) as the working electrode on a plasmonic gold film. The vdW-HS comprises of a graphene sheet on top of a hexagonal boron nitride (hBN) sheet transferred onto a gold film of a standard SPR sensor. We show that the background signal is completely suppressed using such an architecture enabling the unambiguous analysis of SPR sensor response due to electrochemical reactions. Moreover, parasitic electrocatalytic responses due to intercalated or embedded impurities in the gold film can be efficiently avoided using hBN as the interlayer. We further observe that the potential dependent plasmonic signals are not just a reproduction of the electrochemical current when probing diffusive redox active species in solution. Subtle differences between the current and the plasmonic signal can be traced back to the diffusive nature of the redox active species. Finally, we show that EC-SPR can be used as a complementary method to directly infer if the heterogeneous reaction takes place predominantly via diffusion or adsorption of the redox active species.