On the Mechanism of the Plasmonic Field Enhancement of the Solar-to-Electric Energy Conversion by the Other Photosynthetic System in Nature (Bacteriorhodopsin): Kinetic and Spectroscopic Study

We have recently reported Ag nanoparticles (AgNPs) plasmonic field enhancement of the Bacteriorhodopsin (bR) photocurrent observed during its proton pump photocycle in solution. We proposed a mechanism based on the plasmonic field enhancement of the blue light effect which bypasses the slow part of the photocycle and increases the rate of proton production and thus the observed photocurrent. In this present work, we studied the AgNPs plasmonic field effect on the spectroscopy and kinetics of the bR proton pumping photocycle. We examined the blue light effect on both the recovery rate of bR and the decay rate of the M intermediate by using 532-nm short-pulsed laser excitation of bR in the presence of AgNPs and continuous-wave blue light exposure. Our observation showed that the recovery of bR and the decay of the M intermediate are both greatly accelerated in the presence of both AgNPs and blue light simultaneously. This gives support for the proposed mechanism of the enhanced proton current in the presence of AgNPs with a plasmon band in the blue region. It was found experimentally that the 40 nm-AgNPs enhancement of the blue light effect on the decay rate is around 400x larger than that of 8 nm-AgNPs. This is found to be in agreement with the known dependence of the plasmonic field on size and the overlap of the plasmonic extinction band with the absorption band of the M intermediates.