(187ah) Understanding the Deposition Dynamics of Photosystem I (PS I) Onto Thiol Activated Au Substrates

Photosystem I (PS I), a supra-molecular protein complex (MW~300 kDa) responsible for natural photosynthesis mechanism, is a nano-scale photodiode that charge separates upon exposure to light. Our overall objective is to use the photo-induced electrochemical activities of PS I to fabricate hybrid photovoltaic (PV) devices. To accomplish this, a number of challenges have to be overcome. Specifically, these proteins have to be extracted from their natural thylakoid membranes and encapsulated in an organic/ inorganic substrate as a first step towards PV device fabrication. We present results indicating various experimental parameters that alter the surface attachment of PSI deposited from a colloidal suspension in aqueous buffer solution onto hydroxyl-terminated alkanethiolate self-assembled monolayer (SAM)/Au substrates. Previous studies have suggested that PSI preferentially attaches onto OH-terminated alkanethiols with the electron vectors pointing outward thereby enabling easy electron capture pathways. But, our present studies indicate that depending on various experimental parameters, the attachment dynamics of PSI onto alkanthiolate SAM/ Au substrates can result in complex structural arrangements of PSI onto these surfaces which can either hinder or facilitate the photo-electrochemistry of such systems.

To investigate the deposition dynamics of PSI, we assembled the protein on SAM/ Au substrates with two different mechanisms: 1) gravity-driven sedimentation and 2) electric field assisted deposition. In the first case, we deposited the PSI by treating the SAM/ Au substrates to a wide range of PSI concentrations in aqueous buffer solution balanced with Triton X-100 and Dodecyl Maltoside (DM) as detergents. We used AFM and Spectroscopic Ellipsometry (SE) to characterize the topology and thickness of their surfaces. With Triton X-100 as the detergent, our results indicate that at higher concentrations, PSI agglomerates into large number of columnar structures whose dimensions are higher than the characteristic length scales of PSI trimers. But, these structures gradually dissipate into monolayer of PSI at lower concentrations. Also, results from DM as the detergent indicates much lesser agglomerated clusters. To further elucidate and understand the attachment dynamics of PSI onto SAM, we carried out the experiments under various conditions like using PSI monomers and trimers extracted from Thermosynechococcus elongatus and Synechocystis PCC6803 species and different adsorption temperatures. In each of the cases, the surface topology indicated distinct morphological and phase characteristics.

In the second stage of this study, we carried out an electric field assisted deposition of PSI onto thiol activated Au surfaces. SAM/Au substrates were mounted on the cathode and anode of a parallel plate electrode system across which the electric field was applied. In comparison to the gravity-driven systems, the transport of protein in field assisted assembly is convection dominated resulting in uniform deposition of PS I on the surfaces. The protein complexes mostly deposited on the surface mounted on the anode (negative electrode) with very few depositing on the cathode at lower voltages that diminishes to almost none at higher voltages thereby indicating a net positive charge distribution on the protein surfaces. We believe that the protein agglomerations in the gravity-driven sedimentation process are largely driven by Brownian diffusion that gets overpowered by the electrical mobility of the proteins under the field assisted deposition system. The uniformity and packing density of the deposition layer showed marked improvement with increasing voltage of the field.

Such studies would equip us with better tools in future to monitor, tailor and optimize the attachment dynamics of PSI onto various materials and surfaces. In turn, we would be able to outline a unified approach for directional attachment of PS I onto specially tailored surfaces for photovoltaic device fabrication.