Photosystem I/molecular wire/metal nanoparticle bioconjugates for the photocatalytic production of H₂

Photocatalysis of H₂ from alcohol reforming on gold or platinum nanoparti- cles requires semiconductor materials, such as titania, that supply electrons needed in the reduction of H⁺ to H₂. However, this photocatalytic system requires photons that provide energy greater than the band gap of the semiconductor. The band gap of titania (∼3.2 eV) requires radiation with wavelengths shorter than 350 nm. Unfortunately, only a small fraction of the solar radiation meets this requirement. However, photosystem I (PS I) absorbs all wavelengths of visible radiation shorter than -700 nm, which is 43%-46% of the solar radiation. PS I can provide a charge-separated P700 ⁺-F_B state that is stable for ∼100 ms. Thus, PS I linked to Au or Pt nanoparticles is an alternative to titania-supported photocatalytic production of H₂.In this study, PS I was covalently linked to nanoparticles using a molecular wire that enables transfer of electrons at a rate faster than that of the recombination of the charge-separated state. PS I was formed as shown in Scheme 1 (steps 1-3). This rebuilt PS I was covalently linked either to 12-nm Au or 3-nm Pt nanoparticles using 1,6-hexanedithiol as the molecular wire (Scheme 1, step 4). After illuminating with white light at an intensity of 2500 μE for 12-16 h, samples were taken and analyzed for H². It was found that both PS I conjugated to Au and to Pt nanoparticles produced hydrogen gas (Table 1). To evaluate if H² evolution is limited by electron transfer through the molecular wire or through the donor side reduction of P700⁺, cytochrome c₆ (Cyt c₆) was added to PS I/nanoparticle bioconjugates. The rate of hydrogen production was higher upon addition of Cyt c₆ compared to the bioconjugate that used DCPlP as the electron donor. This proved that hydrogen evolution is limited by reduction of P700 ⁺. It is also noteworthy that there was no hydrogen gas observed using wild-type PS I and in experiments lacking one or more components for bioconjugation, indicating that the light provides the necessary charge-separated state for H² production. In addition, the electron transfer to the nanoparticle surface occurred only when the rebuilt PS I was covalently attached to the nanoparticle through 1,6-hexanedithiol, which captures the iron-sulfur cluster and connects the protein to the nanoparticle.