QUANTITATIVE INTERPRETATION OF THE RESPONSE OF SURFACE-PLASMON RESONANCE SENSORS TO ADSORBED FILMS

A simple but quantitative mathematical formalism for interpretation of surface plasmon resonance (SPR) signals from adsorbed films of a wide variety of structures is presented. It can be used to estimate adsorb ed film thicknesses, surface coverages, or surface concentrations from the SPR response over the entire range of film thicknesses without re lying on calibration curves of response versus known thicknesses or su rface concentrations. This formalism is compared to more complex optic al simulations. It is further tested by (1) calibrating the response o f two SPR spectrometers to changes in bulk index of refraction, (2) us ing these calibrations with this formalism to predict responses to sev eral well-characterized adlayer structures (alkanethiolates and serum albumin on gold, propylamine on COOH-functionalized gold), and then (3 ) comparing these predictions to measured SPR responses. Methods for e stimating the refractive index of the adlayer material are also discus sed. Detection limits in both bulk and adsorption-based analyses are d iscussed. The planar system used here has a detection limit of similar to 0.003 nm in average film thickness for adsorbates whose refractive index differs from that of the solvent by only 0.1. The temperature s ensitivities of these two SPR spectrometers are characterized and disc ussed in terms of detection limits.