A Living System for Robust Photocatalytic Hydrogen Evolution from CdSe Quantum Dots

Developing systems that interface nanomaterials and microorganisms is an attractive approach to solar fuels production that can take advantage of the extraordinary properties of both components. For example, nanomaterials have been utilized to provide electrons to microorganisms in photo(electro)chemical processes to fuel metabolic pathways for desired products. While semiconductor nanocrystals are known to be highly active and robust photocatalysts for hydrogen evolution, their use is for all practical purposes severely hampered by the need for an inexpensive and sustainable source of electrons from oxidative side of the reaction. Direct water oxidation is thermodynamically challenging and inefficient, and consequently, easily oxidized electron donors are typically supplemented in high concentrations to drive photocatalysis, which is a major limitation. Here, an alternative approach to constructing living bio-nano artificial photosynthetic systems will be discussed.<br/> <br/>Rather than catalysis regulated by electron transfer <i>from</i> the nanomaterial <i>to</i> the microorganism, we show that exceptional photocatalytic activity of semiconductor nanocrystals (NCs) is sustained by extracellular electron transfer (EET) <i>from</i> electrogenic bacteria <i>to</i> the NCs. In our fully light-driven system, <i>Shewanella oneidensis </i>MR-1 (MR-1), a non-photosynthetic facultative anaerobe, respires CdSe NCs, via EET. Under illumination from visible light (530 nm LEDs), we observe sustained H₂ evolution from the NCs for over a week without the requirement of an external voltage. EET from MR-1 is a natural anaerobic respiratory process, and thus, replenishing the growth medium allows catalysis to continue over a second week. To test the hypothesis that the H₂ production in the complete system results from bacterial respiration of CdSe NCs, the MR-1 gene knockout variant <i>hyaA</i> <i>(ΔhyaA</i>) was used. This variant of MR-1 has impaired hydrogenase activity, and thus is not expected to produce H₂ on its own. Indeed, <i>ΔhyaA </i>cultures produced no detectable H₂ over the course of one week. However, the system consisting of <i>ΔhyaA </i>and CdSe NCs, produced H₂ at a similar rate and efficiency to the wild type MR-1, confirming that respiration of CdSe NCs was essential and sufficient to complete the oxidative portion of the catalytic cycle. There exists many possible barriers to MR-1 EET directly to colloidal NCs: surface area needed for bacterial adhesion, the oxidative damage from photocatalytic processes, biocompatibility of the NCs, and efficiency of electron transfer. The strategy we will outline demonstrates that these challenges are surmountable in a remarkably simple system and suggests a new solution to the oxidative roadblock in NC photocatalysis. Success here opens the door to pairing this strategy with a host of different NC photocatalytic reactions to yield sustainable systems for NC-mediated photocatalysis.