Photochemical Generation of H2 using a Platinum Nanoparticle/Porphyrin Photosensitizer/Protein Scaffold
Splitting of water into dihydrogen and dioxygen by visible light in aqueous solutions constitutes the holy grail of artificial solar energy conversion. One approach is to focus on one of the two half-reactions: 2 H+ + 2 e− → H2 2 H2O → O2 + 4 e− + 4 H+ The following research presents two unique systems combining platinum nanoparticles (NPs), porphyrin photosensitizers, and a protein scaffold to photochemically reduce H+ to H2 in aqueous solutions at neutral pH. Zinc(II) porphyrin (ZnP) photosensitizers were incorporated in place of the naturally occurring heme in the 24-subunit iron storage protein bacterioferritin (Bfr). In addition to wild-type Bfr, several engineered variants were created to optimize access to the photosensitizer and/or improve Pt NP incorporation. The first system employs a Bfr dimer variant designed to increase solvent and/or redox agent access to ZnP. An optimized "one-pot" system consisting of a ~50:50 mixture of Pt-free ZnP-Bfr and 24-mers surrounding Pt NPs generates H2 upon visible light irradiation in the presence of triethanolamine as sacrificial electron donor and methyl viologen as electron relay. The second system eliminates the need for an electron relay by using anionic citrate-stabilized Pt NPs capable of loose association with the ZnP-Bfr dimer. The close proximity between the photosensitizer and the Pt NP allows for direct electron transfer. The unique combination of ascorbate as the electron donor and a zwitterionic buffer results in better stabilization of the ZnP, preventing degradation of the porphyrin resulting in higher yields of H2.