Sensitivity and detection limit of concentration and adsorption measurements by laser-induced surface-plasmon resonance.

The shot noise limitation as well as other factors that influence the sensitivity of measurements with a surface plasmon resonance (SPR) sensor are considered. It is demonstrated that minute changes in the refractive index of a medium close to the surface of a metal film can be detected owing to a shift in the resonance angle. In particular, changes in the adsorption layer of only a fraction of a biomolecular monolayer could be measured. Data for SPR are presented with adjacent media of air, water, as well as aqueous solutions of ethanol and sodium chloride at different concentrations. The immobilization of the protein bovine serum albumin to a specially prepared surface was monitored with the SPR technique. Specific responses to changes in the concentration and thickness of the adsorption layer were determined. The angular resolution of the present apparatus is approximately 1 millidegree, corresponding to a detection limit for an adsorbed protein layer of 15 pg/mm(2), which is still 2 to 3 orders of magnitude larger than the shot-noise limit, and therefore a further improvement in sensitivity is possible.