The increasing levels of carbon dioxide (CO₂) in the atmosphere and the growing global need for energy necessitate the development of renewable methods for electricity production that can help reduce carbon emissions and of green energy-based CO₂ utilization technologies. In this context, sustainable and clean energy sources have attracted a lot of attention. In particular, solar energy-based techniques can be used to convert CO₂ into valuable chemicals and fuels, such as CO, formate or methanol [1]. Currently, one established way to utilize CO₂ is to use the coenzyme-dependent activity of formic acid dehydrogenase (FDH) to catalyze the reduction of CO₂ to formate [2]. Nevertheless, the high cost of the coenzymes (i.e., NADH and MV^•+) have so far limited the application of this technology [3].

Therefore, efficient and inexpensive coenzymes regeneration methods using economic energy sources would provide a greener and sustainable pathway for CO₂ reduction.

In recent years, different groups have been focused their activity on the improvement of visible-light-driven photoredox reactions to implement artificial photosynthesis systems for the efficient regeneration of enzymatic cofactors, in order to lower the cost of the CO₂ reduction process [4-7]. These systems are usually formed by a photosensitizer, an electron donor, and an electron acceptor [4-7]. Upon illumination and light absorption, the photosensitizer is converted from the ground state to the excited state. The photo-excited electrons from the triplet state are then transferred to the electron acceptor (i.e., NAD⁺ or MV²⁺), and at the same time, the triplet state of the photosensitizer regains the electrons from the electron donor (usually, triethanolamine or TEOA). Moreover, for NADH regeneration, rhodium-based compounds are commonly added to the reaction and used as electron mediators to guarantee the regiospecific reduction of NAD⁺ to the enzyme active form of NADH (i.e., 1,4-NADH) [8, 9].

So far, different materials have been used as photosensitizers, from inorganic species [10] to organic dyes such as porphyrins and their derivatives [11]. Compared to the inorganic materials, the organic molecules are cheaper and easier to obtain and have a wide variety of chemical and physical properties [12]. Nevertheless, there are some practical limitations to their application, including instability, slightly high cost and difficulty of recovery [12].

Here we report about the use of conjugated-polymer-based water-processable nanoparticles (WPNPs) as photosensitizer in a photoredox system for the regeneration of both NADH and MV^•+. Such materials show excellent thermo- and photo-stability, good visible absorption properties, low cost/low temperature solution processing and easy removal [13]. Moreover, the use of non-toxic solvents (i.e., water) makes them appealing in biomedical and biocompatible applications [14].

For this purpose, we synthetized a derivative of the semiconducting polymer P3HT, i.e., poly[2,2''''-bis[[(2-butyloctyl)oxy]carbonyl][2,2':5',2'':5'',2'''-quaterthiophene]-5,5'''-diyl] (PDCBT), and prepared the corresponding WPNPs through a miniemulsion approach [15-17]. Then, the PBDTTPD WPNPs preparation was employed in a visible-light-driven NADH/MV^•+ regeneration system consisting of TEOA as electron donor and NAD⁺ or MV²⁺ as electron acceptors and enzyme-catalyzed redox reactions were used to validate the production of the regenerated cofactor.

Our results show a successful example of conjugated-polymer WPNPs used as a photosensitizer for selective coenzyme regeneration in an artificial photosynthesis system, which is easy to build and usable under mild conditions for the facile regeneration of different coenzymes. These results could pave the way for other photoredox reaction systems that enable the selective and sustainable production of chemicals and fuels through the use of solar light.