Biomolecular charges influence the response of surface plasmon resonance biosensors through electronic and ionic mechanisms

Surface plasmon resonance (SPR) biosensors have become an important label-free optical biomolecular sensing technology and a "gold standard" for retrieving information on the kinetics of biomolecular interactions. Even though biomolecules typically contain an abundance of easily ionizable chemical groups, there is a gap in understanding of whether (and how) the electrostatic charge of a biomolecular system influences the SPR biosensor response. In this work we show that negative static charge present in a biomolecular layer on the surface of an SPR sensor results in significant SPR spectral shifts, and we identify two major mechanisms responsible for such shifts: 1) the formation of an electrical double layer (ionic mechanism), and 2) changes in the electron density at the surface of a metal (electronic mechanism). We show that under low ionic strength conditions, the electronic mechanism is dominant and the SPR wavelength shift is linearly proportional to the surface concentration of biomolecular charges. At high ionic strength conditions, both electric and ionic mechanisms contribute to the SPR wavelength shift. Using the electronic mechanism, we estimated the pKa of surface-bound carboxylic groups and the relative concentration of the carboxyl-terminated alkanethiols in a binary self-assembled monolayer of alkanethiols. The reported sensitivity of SPR to surface charge is especially important in the context of biomolecular sensing. Moreover, it provides an avenue for the application of SPR sensors for fast, label-free determination of the net charge of a biomolecular coating, which is of interest in material science, surface chemistry, electrochemistry, and other fields.