This award by the Biomaterials program in the Division of Materials Research to Vanderbilt University is to study PhotoSystem I (PSI) for potential applications in solar energy conversion. PSI is the primary macromolecular system in plants and photosynthetic bacteria for the conversion of sun light to useable energy. The main goal of the project will be to understand the fundamental mechanisms of electron transfer to and from immobilized PSI monolayer and multilayer films during the photosynthesis. With this award, methods will be developed to study higher order oriented multilayer structures of PSI with that mimic larger scale biological structures. The deposited films of PSI will be linked through molecular wires for better connectivity and electron conductivity. The following studies will be carried out with this project: 1) attachment of PSI to gold electrodes that are used as molecular wires; 2) optimization of the photoconversion efficiency for PSI monolayers by electrochemical catalysis; 3) functional imaging of PSI by scanning electrochemical microscopy; and 4) study the mechanism of functional multilayer structures that mimic the larger scale biological structures like the thylakoid membranes. The proposed studies are expected to integrate functional biological systems with molecular and mesoscale materials through ?bottom-up? processing, while maintaining the function of the PSI system. The biomimetic approaches proposed could result in converting sunlight to be a potential useable source of energy.
The proposed project is expected to enhance the fundamental understanding of the interface between biological structures and well-controlled, model organic materials. New covalent and biomimetic attachments, along with 2-D crystals and strategies will be investigated and their effects measured through electrochemical procedures. This multidisciplinary research team consists of chemical and biomolecular engineers and chemists from Vanderbilt University and Tennessee State University, the largest HBCU in Tennessee. In addition, a large number of graduate and undergraduate students from these institutions will be trained by the research team. Participation of high school teachers and students in the research activities through different programs such as NSF-funded Research Experiences for Teachers and Vanderbilt Summer Academy is another aspect of the project.
In terms of broader impacts, this project has resulted in 11 publications and two under review. In total, 5 Ph.D. students (one PhD graduate), 2 Masters students( 1 Graduate), and 6 undergraduates (2 at TSU) have participated in this project. We have worked with the Vanderbilt Center for Technology Transfer and Commercialization to submit a provisional patent application on our PSI-semiconductor devices, and have explored with their help potential licensing partners both foreign and domestic, including executing a new CDA agreement with one. Below, we describe specific intellectual merit accomplishments and outcomes: The vacuum-assisted approach described above enables the flexibility to prepare much thicker, multilayered films of PSI that absorb far more light (Figure 3b) than a monolayer of PSI and function to dramatically increase photocurrent (Figure 3a) up to ~8 mA/cm2 for a 2.7 mm-thick film on Au at a light intensity of 95 mW/cm2. The PSI in these films remained active over an entire 280 day study in which the cell was stored in the dark and periodically tested in the light. This finding demonstrates that isolated PSI exhibits active lifetimes in a solar cell that are well beyond those in vivo. We have developed a simulation and kinetic model for the photoelectrochemical behavior of a PSI monolayer on an electrode. The kinetic parameters for the simulation were extracted from experimental data, and the resulting simulation is capable of predicting the photochronoamperometric behavior of the system over a range of overpotentials. The model is used to investigate the various contributions to the photocurrent production of the system as well as the effects of the orientation of PSI complexes adsorbed to the electrode surface. The model predicts that improving PSI orientation from 60% to 90% would result in over an order-of-magnitude enhancement in photocurrent. Through current NSF support, we have photoreduced platinum within a PSI film for the first time and located via SECM the Pt photocatalytic "hot spots" for hydrogen production from aqueous protons. We have compressed PSI monolayer films at the air-water interface and transferred them to electrodes via Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) approaches that yield opposite orientations of the protein. Using a pure water subphase, the LB approach yields 57% of the proteins with the electron transfer vector pointed away from the electrode, whereas the LS method results in 57% of the proteins oriented with the vector toward the electrode. A 1 mm thick PSI film on p-doped silicon produces far more photocurrent than p-doped silicon by itself and 1,000 times more photocurrent than a similar PSI film on a gold electrode, due to band alignment between PSI and silicon (Figure 2) that enables unidirectional electron flow from the substrate to the protein. This finding represents a quantum leap in the performance of PSI-based electrodes, and with further improvements as proposed herein, could place these biohybrid systems near the performance level of mature, commercial technologies.
