Photosystem I (PSI) is a photoactive electron-transport protein found in plants that participates in the process of photosynthesis. Because of PSI's abundance in nature and its efficiency with charge transfer and separation, there is a great interest in applying the protein in photoactive electrodes. Here, we developed a completely organic, transparent, conductive electrode using reduced graphene oxide (RGO) on which a multilayer of PSI could be deposited. The resulting photoactive electrode demonstrated current densities comparable to that of a gold electrode modified with a multilayer film of PSI and significantly higher than that of a graphene electrode modified with a monolayer film of PSI. The relatively large photocurrents produced by integrating PSI with RGO and using an opaque, organic mediator can be applied to the facile production of more economic solar energy conversion devices.

matrixes, [ 5 ] the use of thick protein fi lms, [ 9 ] and the use of semiconductor electrode materials. [ 10,11 ] The integration of PSI with carbon nano-materials, however, has been limited. The work of Carmeli and co-workers demonstrated how PSI could be covalently attached to carbon nanotubes. [ 12 ] Additionally, our research group demonstrated how PSI could be interfaced as a monolayer on a graphene electrode. [ 13 ] The use of carbon nanostructures such as graphene and carbon nanotubes to develop nano-composite materials has become an area of great research interest due to the unique properties of these materials. [ 14 ] The time-consuming and challenging preparation of graphene, however, has resulted in the increased interest in graphene oxide (GO). [ 15 ] GO, the oxidized form of graphene, is easily generated through the oxidation and subsequent exfoliation of graphite. The oxygen functional groups enable GO to be dispersed in polar solvents (i.e., water) and further functionalized. However, because the conjugation of the system is disrupted, the conductivity of GO is fi ve orders of magnitude lower than graphite. [ 16 ] To regain conductivity, GO is commonly reduced to generate reduced graphene oxide (RGO). [ 17 ] Both GO and RGO have been incorporated with various materials to develop new functional composites. [ 18,19 ] The addition of GO and RGO is particularly attractive for PSIbased devises for a number of reasons. GO and RGO are both water-soluble, making the integration of these materials with PSI straightforward. Furthermore, the low cost and facile synthesis of GO and RGO make these materials ideal candidates for making inexpensive composite materials. The functional groups present on GO make possible the interaction with both the polar groups of PSI as well as the electrochemical mediator. However, the improved conductivity of RGO can facilitate electron transfer between the PSI complexes and the underlying electrode. Here we describe how incorporating either GO or RGO with PSI can improve the photoelectrochemical properties of p-doped silicon. Experimental SectionExtraction and Isolation of Photosystem I : Photosystem I complexes were isolated from commercially available baby spinach leaves as described previously. [ 11 ] Briefl y, the baby spinach was Photosystem I is a photoactive membrane protein used in nature to photoexcite electrons with nearly unit internal quantum effi ciency, sparking interest in using this biomaterial for solar energy conversion. Films of PSI deposited on p-doped silicon have previously demonstrated signifi cant photocurrents with an electrochemical mediator; however, improvement in electron transfer is needed. Here, it is investigated how PSI can be combined with graphene oxide (GO) or reduced graphene oxide (RGO) to generate composite fi lms capable of improved photoelectrochemical performance. It is found that both composite fi lms outperformed the PSI fi lm alone, and the PSI-GO composite fi lm is found to perform the best. The enhancement is attributed to the d...

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