16048 DOI: 10.1021/la102832x Langmuir 2010, 26(20), 16048–16054 Published on Web 09/16/2010 pubs.acs.org/Langmuir © 2010 American Chemical Society Controlling the Morphology of Photosystem I Assembly on Thiol-Activated Au Substrates Dibyendu Mukherjee, †,‡ Mark May, †,‡ Michael Vaughn, †,§, ) Barry D. Bruce, †,‡,§ and Bamin Khomami* ,†,‡ † Sustainable Energy Education and Research Center (SEERC), ‡ Department of Chemical and Biomolecular Engineering, § Department of Biochemistry, Cellular and Molecular Biology, University of Tennessee, Knoxville, Tennessee 37996. ) Current address: Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287. Received July 15, 2010. Revised Manuscript Received August 17, 2010 Morphological variations of Photosystem I (PS I) assembly on hydroxyl-terminated alkanethiolate self-assembled monolayer (SAM)/Au substrates with various deposition techniques is presented. Our studies indicate that deposition conditions such as PS I concentration and driving force play a central role in determining organization of immobilized PS I on thiol-activated Au surfaces. Specifically, atomic force microscopy (AFM) and ellipsometry analyses indicate that gravity-driven deposition from concentrated PS I solutions results in a large number of columnar PS I aggregates, which assemble perpendicular to the Au surface. PS I deposition yields much more uniform layers when deposited at lower concentrations, suggesting preassembly of the aggregate formation in the solution phase. Moreover, in electric- field assisted deposition at high field strengths, columnar self-assembly is largely prevented, thereby allowing a uniform, monolayer-like deposition even at very high PS I concentrations. In situ dynamic light scattering (DLS) studies of solution-phase aggregation dynamics of PS I suspensions in both the presence and absence of an applied electric field support these observations and clearly demonstrate that the externally imposed electric field effectively fragments large PS I aggregates in the solution phase, thereby permitting a uniform deposition of PS I trimers on SAM/Au substrates. Introduction In nature, plants and algae have evolved an advanced photo- synthesis mechanism that harnesses solar energy in a highly effi- cient manner with nearly 100% quantum efficiency. Photosystem I (PS I) is a supramolecular protein complex 1 which functions as a biological photodiode 2 and undergoes photochemical charge separation resulting in unidirectional electron transfer between the reaction center (P700) electron donor on the lumenal side and Fe-S clusters (F A ,F B ,F X ) 3 at the stromal side. For PS I isolated from the thermophilic cyanobacteria, Thermosynechococcus elon- gatus (TE), the structural and dimensional characteristics of PS I are well-documented 4 (see Figure 1 for details). These structural properties combined with the strong electrochemical properties makes PS I an ideal candidate for incorporation into solid-state bioelectronic or hybrid photovoltaic devices. 5-8 However, ra- tional design and optimization of these devices require clear elucidation of the surface attachment dynamics and properties of these protein complexes on various substrates. Gravity-driven sedimentation of detergent-solubilized PS I onto alkanethiolate self-assembled monolayer (SAM)/Au surfaces 9,10 has received attention in recent years. More recently, assisted deposition techniques including attachment of PS I layers on Au electrodes using solvent evaporation under vacuum 11 has been explored. The main limiting step in converting these relatively simple, lab-based concepts into practical, optimized devices lies in the difficulty of producing a uniform and densely packed PS I monolayer on organic/inorganic interfaces. Earlier studies have suggested that PS I attaches favorably onto OH-terminated thiols without denaturation 9-11 and with ∼70% of the electron-transfer vectors pointing outward. 12 Among earlier attempts toward direct attachment of PS I to Au substrates for photovoltaic applications, chemical platinization of PS I to facilitate PS I welding to Au via PS I-Pt-Au bonding 13 affinity is note- worthy. However, the fact that the mechanism for such chemical “welding” is not very well understood raises questions about the repeatability and uniformity of deposition in these more complex approaches. Other immobilization approaches have utilized bioengineered polyhistidine tags that can self-assemble onto a thiol-coupled Ni-NTA complex on Au surface. 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