The Laser Thermal Effect. Surface Plasmon Resonance Detection

Surface plasmon resonance (SPR) is an optical technique that can monitor interfacial reactions at solid-liquid interfacese. SPR is attractive owing to several inherent advantages, such as label-free analysis, rapid and high sensitivity.Thermal effect inherent in long-term irradiation of a focused laser beam can generate appreciable changes in the surface plasmon resonance (SPR) resonance angle. Such an effect in some cases may become a major source of error in SPR measurements, especially when infinitesimal signals (e.g.,reorganization of monolayers or conformational changes of immobilized proteins) are measured. In this work, the laser-induced thermal effect was examined by continuously monitoring changes brought about by injecting 2% ethanol solution into a flow cell housing bare and chemically modified gold films that had been irradiated by a laser beam for various times. Factors affecting the SPR response, viz.,solution flow rate, compactness of the adsorbate layer, and power of the laser beam, were examined. The SPR dip shift changes were shown to be dependent not only on the magnitude of the laser power but also on the thermal dissipation at the metal-solution interface. With a better understanding of the parameters affecting the thermal energy dissipation and the temperature dependence of the SPR signals, measures can be taken to improve the accuracy of SPR data. We show as an example that, if the laser-induced thermal effect is not considered or reduced, accuracy in the determination of redox-triggered reorganization of a pre-immobilized 11-ferrocenylundecanethiol self-assembled monolayer(SAM) may be seriously affected, leading to an erroneous conclusion.