Surface plasmon resonance enhanced optical absorption spectroscopy for studying molecular adsorbates

We present an automated setup to measure the surface plasmon resonance (SPR )-enhanced optical absorption spectra of molecular adsorbates. The setup de tects the reflectivity at the SPR resonance angle as a function of the inci dent light wavelength. Because the resonance angle is wavelength dependent, a feedback mechanism adjusts the photodetector position to follow the reso nance angle when the wavelength varies. Both theoretical calculations and e xperimental measurements show a signal enhancement of up to similar to 40 t imes over the conventional absorption spectroscopy. The SPR-based absorptio n spectroscopy is surface specific because the optical field is localized n ear the surface at resonance. In addition, the SPR angular shift is simulta neously measured, which provides adsorbate coverage and adsorption kinetic information. We anticipate that with our automated system, the method could be used in the study of adsorbed molecules and in chemical and biosensor a pplications. (C) 2001 American Institute of Physics.