Photosynthesis demonstrates the most efficient and optimized photoconversion performance seen in nature with a quantum yield of nearly 100%. The performance is realized via a carefully tuned electron transfer pathway, through which photoelectrons are transferred via a redox cascade in a stepwise fashion after photoinduced charge separation occurs in the chlorophyll reaction center. The performance efficiency of the photosynthetic process has been achieved through natural selection over four billion years, and such levels of performance have not yet been acquired in artificial approaches. The presenters have investigated the performance of a bio-conjugated photosensing system that combines PSI, molecular wire, gold nanoparticle, and Au electrodes (Fig. 1) with Prof. Nakazato (Nagoya Univ.), Prof. Inoue (Tokyo Univ. of Science), and their co-workers.

The photosensing system in this report consisted of a Au microelectrode, coated by a self-assembled monolayer (SAM) of molecular wires that were terminally conjugated to gold nanoparticles. The gold nanoparticles were functionalized with a naphthoquinone derivative that could connect to PSI. PSI was isolated and purified from the thermophilic cyanobacterium, “Thermosynechococcus elongatus”, whose structure and electron transfer reactions have been systematically characterized. The full naphthoquinone-functionalized gold nanoparticles acted as a molecular wire connecting PSI to the Au electrode. The average diameter of the resulting gold nanoparticles was 1.7±0.1 nm, such that the nanoparticle size and distribution were suitable for controlled electron storage. The naphthoquinone derivative structure was similar to VK1 and connected to a vacated quinone pocket on PSI. The square-shaped Au microelectrode, originally developed for this study, is 17mm on a side (Fig. 2).

The addition of two kinds of surfactants to the system significantly improved the photoconversion performance, causing a faster and larger change in voltage (Fig. 3). The hydrophobic moiety of the surfactants around the gold nanoparticles helped to keep water molecules away whose high dielectric constant could interfere with capacitance of particles and reduces the voltage response of the system. This proves that the nanoparticle plays a crucial part of the system acting as electron storage.

Although the dimensions of the Au electrodes are currently on the microscale, a decrease in the electrode size to a few hundred nanometers will enable the detection and counting of small numbers of photons in the future.

1. Mariko Miyachi, Yoshinori Yamanoi, Yusuke Shibata, Hirokazu Matsumoto, Kazuo Nakazato, Masae Konno, Kohsuke Ito, Yasunori Inoue and Hiroshi Nishihara, Chemical Communications, 2010, 46, 2557-2559 (Published online on March 12, 2010).
2. Highlighted in NPG Asia Materials research highlight, in press.