Progress 12/01/04 to 11/30/07
Outputs
OUTPUTS: Activities: The activities required to meet the proposed objectives include the conduction and analyses of various experiments as well as teaching. Specific experiments included (1) backfilling around adsorbed Photosystem I (PSI) with long-chain alkanethiols to produce mimics of biological membranes, and the detailed characterization of these thin films with infrared spectroscopy, electrochemical impedance spectroscopy, and contact angles; (2) investigating the effect of chain length of the intervening self-assembled monolayer (SAM) on electron transfer rates to adsorbed PSI; (3) investigating the effect of different covalent attachments between PSI and the SAM on the measured current using photochronoamperometry; (4) measuring the stability of PSI that is adsorbed via covalent versus physical approaches; (5) photoreduce Pt on the acceptor site of adsorbed PSI; (6) prepare Langmuir-Blodgett and Langmuir-Shaeffer monolayers of PSI to determine relative orientation of the
electron transfer vector of PSI; (7) enhance the assembly rates of PSI by applying a vacuum to pull off water and drive the protein into a dense monolayer or multilayer configuration and measure this rate enhancement by spectroscopic ellipsometry and photochronoamperometry; (8) investigate photocurrent of PSI multilayers; (9) assemble PSI into nanoporous supports. Dr. Jennings used results from this project to develop curricula for and to teach a freshman-level engineering course on chemical engineering and a summer lecture and laboratory course for gifted high school students through the Vanderbilt Summer Academy. As part of the laboratory component, the students, some as young as rising 8th graders, fabricated their own photoelectrochemical cells based on PSI. The winning group achieved a current of ~300 nanoamperes per square cm. Dr. Jennings also presented results from this project to K-12 teachers on four different occasions, twice at Vanderbilt and twice at Fisk University
(HBCU). Susan Lees, a chemistry teacher at White House High School, worked on this project for the summers of 2006 and 2007 through a Vanderbilt Research Experiences for Teachers (RET)Program. Products: The tangible product developed from this work is a first-generation PSI-based photoelectrochemical cell based on both monolayers and multilayers of PSI. The multilayer PSI cell can power a hand calculator. Other products include significant advances in the fundamental knowledge related to the wiring and assembly of protein complexes at electrode surfaces as well as the measurement of photo-induced current with mediator systems. In addition, Dr. Jennings developed the above-mentioned curricula for two different courses, as well as K-12 outreach.
PARTICIPANTS: Dr. Kane Jennings (PD), Associate Professor of Chemical Engineering, coordinated the project research and verified that project objectives were achieved. He also oversaw surface modification as well as attachment and characterization of the PSI layers. Dr. David Cliffel (co-PD) led the electrochemical characterizations and design of device architectures. Professors Cliffel and Jennings have worked together for six years at Vanderbilt, creating a vibrant interdisciplinary environment for discovery. Current Ph.D. students Christopher Faulkner and Peter Ciesielski (not supported on this grant) have desks within Jennings' laboratory in Chemical Engineering but spend many hours per week in the Cliffel laboratory in Chemistry to test PSI-based photoelectrochemical cells. Dr. Madalina Ciobanu (post-doc in Chemistry) was briefly supported on this project to help characterize PSI monolayers with photochronoamperometry and cyclic voltammetry. Dr. Helen Kincaid (graduate student
in ChE) and Tom Niedgringhaus (undergraduate student in ChE) were funded on this project to prepare membrane mimics to stabilize and insulate PSI monolayers on the electrode surface. Dr. Kincaid is now a post-doctoral associate at the Wake Forest Institute for Regenerative Medicine whereas Mr. Niedringhaus is a graduate student at Stanford. Brad Berron (graduate student in ChE) was briefly funded on this grant to prepare loosely packed self-assembled monolayers that were useful to construct the membrane mimics developed by Kincaid. Morgan Krim (graduate student in Chemistry) worked to photoprecipitate Pt nanodeposits atop adsorbed PSI centers. Christopher Faulkner (graduate student in ChE) has developed covalent attachment strategies to bind PSI to surfaces as well as vacuum-assisted deposition strategies to greatly enhance the speed and quality of PSI monolayer and multilayer preparation. Mr. Faulkner's contributions have helped increase photocurrent by 3 orders of magnitude since
early 2006. In total, this project helped train 1 post-doctoral associate, 4 Ph.D. students, and 1 undergraduate student with an enhanced intuition regarding the interfacing of biological nanomaterials at electrode surfaces.
TARGET AUDIENCES: Target audiences for our efforts included the scientific communities in photosynthesis, surfaces and thin films, biomimetic solar energy conversion, bioelectroanalytical chemistry, and nanotechnology for new, alternative energies; K-12 teachers who participated in workshops offered at Vanderbilt University and Fisk University (HBCU); gifted middle- and high-school students who participated in the Nanotechnology course through the Vanderbilt Summer Academy and constructed solar cells from PSI; Freshmen engineering students at Vanderbilt who learned about PSI-based photoelectrochemical cells; various members of the Vanderbilt community through articles and information written up in news letters and magazines (such as Vanderbilt Magazine; 93,000 circulation).
PROJECT MODIFICATIONS: There were no major changes to our original approaches and objectives that were established in the proposal.
Impacts
The USDA-CSREES provided funds to support student and faculty time and enable the following developments and discoveries: 1) We have developed a unique approach to insulate and stabilize adsorbed PSI complexes within a hydrophobic mimic of the thylakoid membrane of green plants that is chemically bound to the underlying metal for enhanced stability. This approach consists of adsorbing PSI onto a thin SAM followed by exposure of the PSI layer to an aqueous solution containing a long-chain alkanethiol, which exchanges with the shorter-chained thiol at exposed areas to provide covalently immobilized insulation and hydrophobic confinement of PSI while greatly reducing background current in electrochemical measurements. 2) We discovered that hydroxyl-terminated SAMs of intermediate chain length (n = 6, 8) are optimal to facilitate electron transfer from gold to PSI. Using these SAMs of intermediate chain length, we have observed direct photoconversion from a monolayer of
PSI to redox species in solution. 3) We discovered that a covalent attachment between PSI and an underlying SAM results in an order-of-magnitude enhancement in photocurrent over the use of physical interactions. This dramatic enhancement in current density is attributed to the use of a covalent anchoring to provide preferred orientations of PSI on the surface and more direct electron pathways into the active center of PSI. The covalent approach also improves the stability of PSI monolayer films. 4) We have photoreduced Pt from solution on the acceptor site of an adsorbed monolayer of PSI. These Pt nanodeposits can aid in wiring up the protein to collect photocurrent. 5) We have compressed PSI monolayers at the air-water interface to a surface footprint of 80 square nm/PSI and transferred them to a hydroxyl-terminated SAM through both Langmuir-Blodgett and Langmuir-Schaefer methods to generate PSI monolayers that are mirror images of each other. Photocurrent results are consistent with
mirror-image monolayers from the LB versus LS transfers and indicate that LB and LS methods result in 57% and 43% orientation with P700 nearest the solid surface. 6) Applying a vacuum above the aqueous PSI solution during assembly drives off water and causes PSI to phase separate from the mediating surfactants and salts in the residual aqueous solution. Using this approach, we can achieve photocurrents upon 0.5 h assembly times that are 12 times higher than those obtained by allowing PSI to self-assemble from solution and ~100 times faster than the time required for solution-based assembly to yield comparable currents. 7) The vacuum-based approach also enables the flexibility to prepare a multilayered film of PSI. Using a multilayered film of PSI to increase light absorption and enhance mediator partitioning, we routinely observe 1 to 2 microamps per sq cm of current that is pulled by a single oriented monolayer of PSI. We have successfully used this larger current to replace
batteries with a PSI multilayer to power a hand calculator.
Publications
- B. Berron and G. K. Jennings; "Loosely Packed, Hydroxyl-Terminated SAMs on Gold," Langmuir, 22, 7235-7240, 2006.
- M. Ciobanu, H. A. Kincaid, V. Lo, G. K. Jennings, and D. E. Cliffel; "Voltammetric Studies of PSI Direct Electrochemistry," J. Electroanal. Chem., 599, 72-78, 2007.
- R. D. Weinstein, J. Richards, S. D. Thai, C. A. Bessel, D. M. Omiatek, C. J. Faulkner, S. Othman, G. K. Jennings; "Characterization of Self-Assembled Monolayers from Lithium Dialkyldithiocarbamate Salts," Langmuir, 23, 2887-2891, 2007.
- H. A. Kincaid, T. P. Niedringhaus, M. Ciobanu, D. E. Cliffel, and G. K. Jennings; "Entrapment of Photosystem I Within Self-Assembled Films," Langmuir, 22, 8114-8120, 2006.
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Progress 12/01/04 to 11/30/05
Outputs
A major goal of this project is to prepare a photoelectrochemical device that uses Photosystem I (PSI), extracted from plants, as the active nanostructural component. The development of such a biomimetically inspired device requires several steps: extraction of PSI from plants, adsorption and orientation of extracted PSI onto surfaces without loss of its biological function, stabilization of PSI on the surface to mimic the thylakoid membrane of green plants, direct electron transfer from gold to the P700 reaction center, and interfacing the acceptor side of PSI to provide a sink for photocurrent. Prior published work from the Jennings and Cliffel groups has established that PSI can be extracted using literature methods and that its adsorption can be controlled and directed by the pre-modification of gold surfaces with self-assembled monolayers (SAMs) that contain prescribed surface groups. Specific progress toward the proposed USDA objectives is discussed below: (i)
Stabilization of PSI within a Multicomponent Film. We have developed a unique strategy to stabilize and insulate individual PSI complexes on the surface and thereby mimic the thylakoid membrane of green plants (Langmuir, in preparation). Exposure of a PSI submonolayer atop a short, hydroxyl-terminated SAM to a solution containing a long-chain alkanethiol results in displacement of the short thiolates by the longer competing thiol at exposed regions, filling the interprotein domains with long hydrocarbon chains of the competing thiol. This backfilling results in greatly decreased interfacial capacitance and should enable higher resolution in the measurement of photocurrent. (ii) Direct Electron Transfer. We have demonstrated the first direct electron transfer from an underlying electrode to the P700 reaction center of a PSI monolayer (J. Am. Chem. Soc., submitted). These results are important in that they represent a critical feat for the preparation of PSI-based photoelectrochemical
cells. Specifically, we observe direct electron transfer when the intervening hydroxyl-terminated monolayer is of intermediate chain length (n = 6 to 8 methylene units) but not when the monolayer is short (n = 2, 4) due to monolayer disorder or long (n = 11) due to increased distances for electron tunneling. (iii) Interfacing the Acceptor Side of PSI. Once PSI is photoexcited, an electron must transfer out of the FB complex, and a new electron must transfer to the P700+ center before the PSI can be re-excited by another photon. We have recently observed direct photoconversion from a monolayer of PSI on the electrode to methyl viologen, an electron acceptor compatible with the FB center (J. Am. Chem. Soc., submitted). In the presence of light, PSI will draw electrons from the electrode, leading to an increased electrode current corresponding to the reduction of the P700+ center. Importantly, the enhanced photoreduction current demonstrates the feasibility of direct energy conversion
from light to electrical current using PSI. While our photocurrent density in this experiment is only 10 nA/cm2, we plan to optimize this direct energy conversion in the near future.
Impacts
This research project is developing an environmentally friendly, biomimetic approach to convert solar energy to useful electrical current. The proposed photoelectrochemical device contains nanoscale protein complexes, harvested from spinach, as the biologically active components that are molecularly integrated with organic and metallic materials. Successful completion of the project will result in a new biologically inspired device and an enhanced molecular-level understanding of protein-materials interfaces.
Publications
- As of February 7, 2006, one manuscript has been submitted to J. Am. Chem. Soc., and two are in progress (most likely to Langmuir). We expect that these manuscripts will all be published by the end of the grant period and that 1-2 others will be in preparation based on ongoing work.
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