Tuning Photocurrent Responses from Photosystem I via Microenvironment Alterations: Effect of Plasmonic Electric Fields and Membrane Confinements

Robust photoelectrochemical activities of PSI make it an ideal candidate for bio-hybrid photovoltaic and optoelectronic devices. This dissertation focuses on role of microenvironment alterations around PSI in tuning its photocurrent responses when assembled with tailored plasmonic metal nanostructures and biomimetic lipid interfaces. To this end, a series of systematic studies aimed at tuning the plasmon enhanced photocurrent responses from PSI assembled with gold and silver metal nanopatterns tailored for different plasmonic absorption wavelengths. The experimental observation of plasmon-induced photocurrent enhancements in PSI is investigated using Fischer patterns of silver nanopyramids (Ag-NPs) wherein the resonant peaks were tuned to match the PSI absorption peaks at ~450 and ~680 nm. A conservative estimate for the enhancement factors were found to be ~ 5.8 – 6.5 when compared to PSI on planar Ag substrate assemblies. Furthermore, spatially localized and spectrally resolved wavelength-dependent plasmon-enhanced photocurrents from PSI are investigated by specifically assembling the protein units in regions around highly ordered Au (AuND) and Ag (AgND) nano-discs where the dipolar plasmon resonance modes from the respective NDs are tuned to the wavelengths of ~680 nm and ~560 nm, respectively. Specifically, we report plasmon-enhancement factors of ~6.8 and ~17.5 for the PSI photocurrents recorded under the excitation wavelengths of ~680 nm and ~565 nm respectively as compared to PSI assembled on planar ITO substrates. The results indicate: 1) direct correlations between the photocurrent enhancement spectra from the PSI assemblies and the plasmonic resonance modes for the respective nanopatterned substrates, and 2) broadband photocurrent enhancements due to plasmon-coupled photoactivation in the otherwise blind chlorophyll regions of the native PSI absorption spectra. In our continuing efforts to investigate the alterations in the photoexcitation/dissipation pathways in PSI due to characteristic changes in their optical and structural properties under biomimetic membrane confinements, , the PSI complexes are reconstituted in synthetic lipid membranes of 1,2-diphytanoyl-sn-glycero-3-phospho-(1ʹ-rac-glycerol) (DPhPG) and 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC). The results presented here from absorption, fluorescence and circular dichroism indicate unique changes around the carotenoid/chlorophyll spectral bands leading to attainment of broad-band light harvesting via enhanced absorption in the otherwise non-absorptive green region (500 – 580 nm) of unconfined PSI absorption spectra.