Quantitative Interpretation of the Response of Surface Plasmon Resonance Sensors to Adsorbed Films

TLDR

The paper presents a simple quantitative formalism for interpreting SPR signals from adsorbed films of diverse structures. The formalism is benchmarked against detailed optical simulations and validated experimentally by calibrating two SPR spectrometers, predicting responses for known adlayers, and comparing to measurements, while also addressing refractive index estimation and detection limits. The method achieves a detection limit of about 0.003 nm in film thickness for adsorbates with a 0.1 refractive index contrast, and temperature sensitivity is quantified in terms of its impact on detection limits.

Abstract

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 adsorbed film thicknesses, surface coverages, or surface concentrations from the SPR response over the entire range of film thicknesses without relying on calibration curves of response versus known thicknesses or surface concentrations. This formalism is compared to more complex optical simulations. It is further tested by (1) calibrating the response of two SPR spectrometers to changes in bulk index of refraction, (2) using these calibrations with this formalism to predict responses to several 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 estimating the refractive index of the adlayer material are also discussed. Detection limits in both bulk and adsorption-based analyses are discussed. The planar system used here has a detection limit of ∼0.003 nm in average film thickness for adsorbates whose refractive index differs from that of the solvent by only 0.1. The temperature sensitivities of these two SPR spectrometers are characterized and discussed in terms of detection limits.