Ion-specific quantitative measurement scheme using transit-time surface plasmon resonance

A sensitive, ion-specific measurement scheme is proposed and analysed theoretically. It is based on transit-time surface plasmon resonance (SPR) sensing in an electrolytic conductivity cell comprising an electrode supporting surface plasmons (SPs). The transittime SPR response involves dynamics of ion transport in the electrolytic conductivity cell and a highly sensitive SPR response from the electrode supporting SPs. This is modeled by solving the modified-Poisson Nernst Planck equations and the Fresnel equations. It is shown that the magnitude of the transit-time SPR detection signal scales with the square root of ion concentration (C0), whilst bulk SPR detection scales linearly with ion concentration (C0). Transit-time SPR is, therefore, a superior detection scheme at low ion concentrations. Analyses by Tikhonov regularization show that transit-time SPR response enables specific ion detection through differences in ion mobility and charge. The proposed method is purely physical, not requiring chemical affinity layers or chemical reduction-oxidation processes at the electrodes. It is based on a differential measurement scheme with and without voltage applied. Compared to conductivity sensors, the method adds specificity, while compared to bulk SPR sensors, it improves the resolution at low concentration levels and enables specificity without use of surface chemistry or biochemistry.