Figure 2.

Electrochemical impedance spectroscopy (EIS) and surface plasmon resonance (SPR) provide orthogonal measurements for the study of membrane protein-based biochips. (a) In EIS, a sinusoidal potential difference is applied between the reference and working electrodes. The current is monitored and impedance modulus (E/I) and phase shift (ϕ) are recorded as a function of frequency. (b) EIS data is analyzed by fitting the data to an equivalent circuit model (see text for details); (c) SPR measurement schematic. Light is coupled into the plasmon modes of an ultra-thin Au film through a high index of refraction prism, and (d) the reflected light intensity is recorded (top). The angle at which the reflected light intensity reaches a minimum is observed as a function of time as analyte is added at t = 0, and washed from the cell at t = t₁ (see text for details).