(581d) Electrochemical Characterization of Photosystem I (PS I)/Self-Assembled Monolayer (SAM)/Au Substrates: The Critical Bottle-Necks in Electron Transfer

Photosystem I (PS I), a supra-molecular protein complex, functions as a biological photodiode

that charge separates upon exposure to light and is responsible for driving natural photosynthesis.

The highly efficient photo-electrochemical activities of PSI (100% quantum efficiency

over 54% of the solar spectrum) make it an ideal candidate for next-generation bio-hybrid

electronic devices. Our specific interest in using PS I for future hybrid photovoltaic (PV)

device fabrications requires optimal encapsulation of these proteins onto chemically grafted

SAM substrates. Our previous results indicate that various experimental parameters alter the

surface attachment dynamics of PS I, deposited from colloidal aqueous buffer suspensions

onto OH-terminated alkanethiolate/Au SAM substrates, thereby resulting in complex structural

arrangements which affect the electron transfer and capture pathway of PSI. We present surface

topographical and electrochemical characterizations of PS I/Au SAM substrates to elucidate

photo-activated electronic activities of PS I monolayer. Specifically, light induced directional

electron transfer by surface immobilized PS I is demonstrated via direct electrochemistry

measurements from PS I complexes assembled on alkanethiolate SAM/Au surfaces prepared

with varying thiol chain lengths (C2, C4, C6, C8). The results reveal the effect of interfacial

thiol brush density and chain length on limiting the electron transfer rate at the donor side

from Au surface to oxidized P700+ reaction center in PS I, due to non-optimal packing and/or,

large tunneling distance through the SAM. On the other hand, at the acceptor side, the electron

donation rates from reduced FB- sites into the solution is significantly increased with the addition

of a soluble electron acceptor, Methyl Viologen (MV2+

previous work shone light on the critical role of oxygen in the electron uptake process, both via

direct mediation from FB-

O2. To this end, the present work takes a step towards detailed electrochemical characterizations

that elucidate and optimize the true bottleneck of electron mediation through PS I monolayer by

separately analyzing the interfacial electron transfer from SAM substrate to P700+ and from FB-

to the solution.

) in the presence of dissolved oxygen. Our

to O2 and through a secondary complex formation between MV2+ and