Distance-determined sensitivity in attenuated total reflection-surface enhanced infrared absorption spectroscopy: aptamer-antigen compared to antibody-antigen.

Distance-dependent signal intensity in immunoassay by attenuated total reflection-surface enhanced infrared absorption spectroscopy is demonstrated by controlling the distance of target proteins away from the enhancement substrate. Based on this optical near-field effect, sensitive detection of protein molecules with a detection limit of 0.6 nM and investigation of the kinetics and thermodynamics of protein-aptamer/antibody interactions can be achieved.