Laser-induced thermal effect in surface plasmon resonance

In this work, thermal effect inherent in long-term irradiation of a focused laser beam may become a major source of error in surface plasmon resonance (SPR) measurements, especially when infinitesimal signals (e.g., reorganization of monolayers or conformational changes of immobilized proteins) are measured. 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, and a maximum relative error of 21% was obtained. Factors affecting the SPR response, namely solution flow rate, compactness of the adsorbate layer, and power of the laser beam, were examined. The SPR dip shift (SPR angular shift) was 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 determining redox-triggered reorganization of a pre-immobilized 11-ferrocenylundecanethiol self-assembled monolayer (SAM) may be decreased by more than 40%, leading to an erroneous conclusion.