Inspired by natural photosynthesis, biocatalytic photoelectrochemical (PEC) platforms are gaining prominence for the conversion of solar energy into useful chemicals by combining redox biocatalysis and photoelectrocatalysis. Herein, we report a dual biocatalytic PEC platform consisting of a molybdenum (Mo)-doped BiVO₄ (Mo:BiVO₄) photoanode and an inverse opal ITO (IO-ITO) cathode that gives rise to the coupling of peroxygenase and ene-reductase-mediated catalysis, respectively. In the PEC cell, the photoexcited electrons generated from the Mo:BiVO₄ are transferred to the IO-ITO and regenerate reduced flavin mononucleotides to drive ene-reductase-catalyzed trans-hydrogenation of ketoisophrone to (R)-levodione. Meanwhile, the photoactivated Mo:BiVO₄ evolves H₂O₂ in situ via a two-electron water-oxidation process with the aid of an applied bias, which simultaneously supplies peroxygenases to drive selective hydroxylation of ethylbenzene into enantiopure (R)-1-phenyl-1-hydroxyethane. Thus, the deliberate integration of PEC systems with redox biocatalytic reactions can simultaneously produce valuable chemicals on both electrodes using solar-powered electrons and water.

Peroxygenases catalyze selective oxyfunctionalization of hydrocarbons with high conversion efficiencies using H₂O₂ as a key cosubstrate. Here, we report an unbiased photoelectrochemical (PEC) tandem structure consisting of a FeOOH/BiVO₄ photoanode, a Cu(In,Ga)Se₂ solar absorber, and a graphitic carbon nitride/reduced graphene oxide hybrid cathode for light-driven peroxygenase catalysis. Powered by sufficient photovoltage generated by the solar absorber, the PEC platform generates H₂O₂ in situ through reductive activation of molecular oxygen using water as an electron donor in the absence of external bias. The peroxygenase from Agrocybe aegerita catalyzed the stereoselective hydroxylation of ethylbenzene to (R)-1-phenylethanol with total turnover numbers over 43 300 and high enantioselectivity (ee > 99%) in the unbiased PEC tandem system.

Peroxygenases are very interesting catalysts for specific oxyfunctionalization chemistry. Instead of relying on complicated electron transport chains, they rely on simple hydrogen peroxide as the stoichiometric oxidant. Their poor robustness against H₂O₂ can be addressed via in situ generation of H₂O₂. Here we report that simple graphitic carbon nitride (g-C₃N₄) is a promising photocatalyst to drive peroxygenase-catalyzed hydroxylation reactions. The system has been characterized by outlining not only its scope but also its current limitations. In particular, spatial separation of the photocatalyst from the enzyme is shown as a solution to circumvent the undesired inactivation of the biocatalyst. Overall, very promising turnover numbers of the biocatalyst of more than 60.000 have been achieved.

Light-driven activation of redox enzymes is an emerging route for sustainable chemical synthesis. Among redox enzymes, the family of Old Yellow Enzyme (OYE) dependent on the nicotinamide adenine dinucleotide cofactor (NADH) catalyzes the stereoselective reduction of α,β-unsaturated hydrocarbons. Here, we report OYE-catalyzed asymmetric hydrogenation through light-driven regeneration of NADH and its analogues (mNADHs) by N-doped carbon nanodots (N-CDs), a zero-dimensional photocatalyst. Our spectroscopic and photoelectrochemical analyses verified the transfer of photo-induced electrons from N-CDs to an organometallic electron mediator (M) for highly regioselective regeneration of cofactors. Light triggered the reduction of NAD⁺ and mNAD⁺s with the cooperation of N-CDs and M, and the reduction behaviors of cofactors were dependent on their own reduction peak potentials. The regenerated cofactors subsequently delivered hydrides to OYE for stereoselective conversions of a broad range of substrates with excellent biocatalytic efficiencies.

Peroxygenases are very promising catalysts for oxyfunctionalization reactions. Their practical applicability, however, is hampered by their sensitivity against the oxidant (H₂O₂), therefore necessitating in situ generation of H₂O₂. Here, we report a photoelectrochemical approach to provide peroxygenases with suitable amounts of H₂O₂ while reducing the electrochemical overpotential needed for the reduction of molecular oxygen to H₂O₂. When tethered on single-walled carbon nanotubes (SWNTs) under illumination, flavins allowed for a marked anodic shift of the oxygen reduction potential in comparison to pristine SWNT and/or nonilluminated electrodes. This flavin-SWNT-based photoelectrochemical platform enabled peroxygenases-catalyzed, selective hydroxylation reactions.

Enoate reductases from the family of old yellow enzymes (OYEs) can catalyze stereoselective trans-hydrogenation of activated C=C bonds. Their application is limited by the necessity for a continuous supply of redox equivalents such as nicotinamide cofactors [NAD(P)H]. Visible light-driven activation of OYEs through NAD(P)H-free, direct transfer of photoexcited electrons from xanthene dyes to the prosthetic flavin moiety is reported. Spectroscopic and electrochemical analyses verified spontaneous association of rose bengal and its derivatives with OYEs. Illumination of a white light-emitting-diode triggered photoreduction of OYEs by xanthene dyes, which facilitated the enantioselective reduction of C=C bonds in the absence of NADH. The photoenzymatic conversion of 2-methylcyclohexenone resulted in enantiopure (ee>99 %) (R)-2-methylcyclohexanone with conversion yields as high as 80–90 %. The turnover frequency was significantly affected by the substitution of halogen atoms in xanthene dyes.

In green plants, solar-powered electrons are transferred through sophistically arranged photosystems and are subsequently channelled into the Calvin cycle to generate chemical energy. Inspired by the natural photosynthetic scheme, a photoelectrochemical cell (PEC) is constructed configured with protonated graphitic carbon nitride (p-g-C₃N₄) and carbon nanotube hybrid (CNT/p-g-C₃N₄) film cathode, and FeOOH-deposited bismuth vanadate (FeOOH/BiVO₄) photoanode for the production of industrially useful chiral alkanes using an old yellow enzyme homologue from Thermus scotoductus (TsOYE). In the biocatalytic PEC platform, photoexcited electrons provided by the FeOOH/BiVO₄ photoanode are transferred to the robust and self-standing CNT/p-g-C₃N₄ hybrid film that electrocatalytically reduces flavin mononucleotide (FMN) mediator. The p-g-C₃N₄ promotes a two-electron reduction of FMN coupled with an accelerated electron transfer by the conductive CNT network. The reduced FMN subsequently delivers the electrons to TsOYE for the highly enantioselective conversion of ketoisophorone to (R)-levodione. Under light illumination (>420 nm) and external bias, (R)-levodione is synthesized with the enantiomeric excess value of above 83%, not influenced by the scale of applied bias, simultaneously exhibiting stable and high current efficiency. The results suggest that the biocatalytic PEC made up of economical materials can selectively synthesize high-value organic chemicals using water as an electron donor.