Biocatalytic transformation has received increasing attention in the green synthesis of chemicals because of the diversity of enzymes, their high catalytic activities and specificities, and mild reaction conditions. The idea of solar energy utilization in chemical synthesis through the combination of photocatalysis and biocatalysis provides an opportunity to make the "green" process greener. Oxidoreductases catalyze redox transformation of substrates by exchanging electrons at the enzyme's active site, often with the aid of electron mediator(s) as a counterpart. Recent progress indicates that photoinduced electron transfer using organic (or inorganic) photosensitizers can activate a wide spectrum of redox enzymes to catalyze fuel-forming reactions (e.g., H evolution, CO reduction) and synthetically useful reductions (e.g., asymmetric reduction, oxygenation, hydroxylation, epoxidation, Baeyer-Villiger oxidation). This Review provides an overview of recent advances in light-driven activation of redox enzymes through direct or indirect transfer of photoinduced electrons.

Natural photosynthesis is an effective route for the clean and sustainable conversion of CO into high-energy chemicals. Inspired by the natural process, a tandem photoelectrochemical (PEC) cell with an integrated enzyme-cascade (TPIEC) system was designed, which transfers photogenerated electrons to a multienzyme cascade for the biocatalyzed reduction of CO to methanol. A hematite photoanode and a bismuth ferrite photocathode were applied to fabricate the iron oxide based tandem PEC cell for visible-light-assisted regeneration of the nicotinamide cofactor (NADH). The cell utilized water as an electron donor and spontaneously regenerated NADH. To complete the TPIEC system, a superior three-dehydrogenase cascade system was employed in the cathodic part of the PEC cell. Under applied bias, the TPIEC system achieved a high methanol conversion output of 220 μm h , 1280 μmol g h using readily available solar energy and water.

Surface ozone is a severe air pollution problem in the North China Plain, which is home to 300 million people. Ozone concentrations are highest in summer, driven by fast photochemical production of hydrogen oxide radicals (HOx) that can overcome the radical titration caused by high emissions of nitrogen oxides (NOx) from fuel combustion. Ozone has been very low during winter haze (particulate) pollution episodes. However, the abrupt decrease of NOx emissions following the COVID-19 lockdown in January 2020 reveals a switch to fast ozone production during winter haze episodes with maximum daily 8-h average (MDA8) ozone concentrations of 60 to 70 parts per billion. We reproduce this switch with the GEOS-Chem model, where the fast production of ozone is driven by HOx radicals from photolysis of formaldehyde, overcoming radical titration from the decreased NOx emissions. Formaldehyde is produced by oxidation of reactive volatile organic compounds (VOCs), which have very high emissions in the North China Plain. This remarkable switch to an ozone-producing regime in January–February following the lockdown illustrates a more general tendency from 2013 to 2019 of increasing winter–spring ozone in the North China Plain and increasing association of high ozone with winter haze events, as pollution control efforts have targeted NOx emissions (30% decrease) while VOC emissions have remained constant. Decreasing VOC emissions would avoid further spreading of severe ozone pollution events into the winter–spring season.

Z-scheme in natural photosynthesis are promising for solar-driven CO 2 conversion. [2] By combining multiple photoelectrodes or photovoltaics (PV), the Z-scheme PEC cells can provide sufficient photopotential to simultaneously drive water oxidation and CO 2 reduction under minimal or no external bias. [3] Nevertheless, lowering the kinetic barrier of thermodynamically inert CO 2 remains a hurdle for efficient CO 2 reduction. The development of CO 2reducing biocatalyst-conjugated cathodes can improve chemoselectivity and increase yield under mild conditions. [4] Compared to synthetic catalysts that often require extreme conditions such as high pressure, pH, or temperature, enzymes show high catalytic activities and specificities under mild conditions, making them a valuable catalyst for sustainable and green applications. In particular, formate dehydrogenase (FDH) is an attractive redox enzyme that reduces CO 2 to formate, an alternative water-soluble feedstock that can be easily converted to other common fuels. [5] Previous studies have focused on mediated electron transfer (MET)-type reactions, [6] in which redox mediators such as nicotinamide adenine dinucleotide cofactor (NADH) and Rh-based complexes shuttle electrons between an electrode and FDH. However, the MET-based biocatalysis requires costly electron mediators and multiple electron transfer steps that cause side reactions and significant losses in efficiency. [7] Here, we report the development of 3D titanium nitride nanoshell (3D TiN) electrodes for biocatalytic PEC cells that convert CO 2 to formate through direct electron transfer (DET), as depicted in Scheme 1a. A highly ordered, porous TiN structure is employed as an electrically conductive scaffold for efficient DET to a W-containing FDH from Clostridium ljungdahlii (ClFDH) (inset, Scheme 1a). TiN was chosen as a scaffold for DET-based bioelectrode because it is highly conductive, electrochemically stable and exhibit high chemical and thermal resistance, as well as exceptional hardness. [8] The 3D TiN electrode simultaneously provides (i) a large electroactive surface area generated from an ultrathin (≈30 nm), 3D nanoshell structure with high porosity (92.1%) for high enzyme loading per geometric area, (ii) a continuous electron transfer network with high electrical Z-scheme-inspired tandem photoelectrochemical (PEC) cells have received attention as a sustainable platform for solar-driven CO 2 reduction. Here, continuously 3D-structured, electrically conductive titanium nitride nanoshells (3D TiN) for biocatalytic CO 2 -to-formate conversion in a bias-free tandem PEC system are reported. The 3D TiN exhibits a periodically porous network with high porosity (92.1%) and conductivity (6.72 × 10 4 S m −1 ), which allows for high enzyme loading and direct electron transfer (DET) to the immobilized enzyme. It is found that the W-containing formate dehydrogenase from Clostridium ljungdahlii (ClFDH) on the 3D TiN nanoshell is electrically activated through DET for CO 2 reduction. At a low overpotential...

Redox enzymes catalyze fascinating chemical reactions with excellent regio- and stereo-specificity. Nicotinamide adenine dinucleotide cofactor is essential in numerous redox biocatalytic reactions and needs to be regenerated because it is consumed as an equivalent during the enzymatic turnover. Here we report on unbiased photoelectrochemical tandem assembly of a photoanode (FeOOH/BiVO4) and a perovskite photovoltaic to provide sufficient potential for cofactor-dependent biocatalytic reactions. We obtain a high faradaic efficiency of 96.2% and an initial conversion rate of 2.4 mM h−1 without an external applied bias for the photoelectrochemical enzymatic conversion of α-ketoglutarate to l-glutamate via l-glutamate dehydrogenase. In addition, we achieve a total turnover number and a turnover frequency of the enzyme of 108,800 and 6200 h−1, respectively, demonstrating that the tandem configuration facilitates redox biocatalysis using light as the only energy source.