The Sticky World of Polydopamine in Biohybrid Devices

In a low-carbon future, the development of solar power technologies using photosynthetic bacteria is considered one of the best options to meet the increasing future energy demand and to pioneer the cutting-edge novel strategies for environmentally safe and cost-effective energy production. Polydopamine (PDA), a covalent, biocompatible polymer obtained via self-oxidative reaction of dopamine, has been utilized to embed the photoactive reaction centre (RC) from purple non-sulphur bacteria into a thin film, while preserving its structure and therefore its activity. This RC/PDA thin film can be generated over electron surfaces, to provide photocurrent in photoelectrochemical cells [1]. The RC/PDA matrix can be further doped with specific diamine groups, rendering the composite transparent, for enhanced light transmission and photoconversion [2].<br/>The presence of O₂ as an oxidant is crucial for the self-polymerization of dopamine and alternative routes may be of interest for the formation of adhesive PDA layers in environmental conditions where oxygen is not available. Recently, PDA polymerization was achieved exploiting the metabolism of the extremely sensitive to oxygen mutant strains R26 of <i>R</i><i>hodobacter </i>(<i>R</i><i>.</i>) <i>sphaeroides</i>. The increase of the optical density recorded at different wavelengths, when bacteria are grown in dopamine supplemented medium, implied the contemporary biomass gain and the formation of PDA which absorbs at 588 nm [3]. Using whole, metabolically active microorganisms greatly simplifies the preparation of the biocatalyst avoiding enzyme isolation and purification and potentially enhances stability of the system thanks to their self-repairing and replication features. PDA conductive coatings were used as biotic-abiotic interfaces in biohybrid photoelectrochemical devices through the encapsulation of entire bacterial cells of <i>R. sphaeroides</i> [4] and <i>R. capsulatus </i>[5]<i>,</i> ensuring electronic communication of the biological component with the electrodes’ surfaces in photoelectrochemical cells, using one-pot or single-cells coating methods.<br/>A biophotoelectrode was obtained with polyhydroxybutyrate (PHB), a biopolymer, which purple non-sulphur bacteria produce as an energy stock under specific environmental conditions. The composite material was modified with PDA and thermally treated to obtain a hydrophilic electrode having stable and reproducible electrochemical behavior. The bio-based electrode was tested with metabolically active cells of <i>R. capsulatus</i> embedded in a biohybrid matrix of PDA. The system achieved enhanced catalytic activity under illumination, with an 18-fold increase in photocurrent production compared to biophotoelectrodes based on glassy carbon [6].<br/><br/>This work was funded by the Fonds National Suisse de la Recherche Scientifique, project <i>Phosbury - Photosynthetic bacteria in Self-assembled Biocompatible coatings for the transduction of energy</i> - (Project Nr CRSII5_205925/1)<br/><br/>[1] M. Lo Presti et al. <i>Adv. Electron. Mater.</i>, 6, 2000140 (2020).<br/>[2] G. Buscemi et al. <i>Adv. Sustain. Syst.</i>, 5, 2000303 (2021).<br/>[3] R. Labarile et al. <i>MRS Advances 8(8), 423–428 </i>(2023).<br/>[4] R. Labarile et al. <i>Nano Research</i> 17, 875-881(2023).<br/>[5] G. Buscemi et al. <i>ACS Applied Materials and Interfaces</i> 14(23), 26631–26641(2022).<br/>[6] L. Torquato et al. <i>Journal of The Electrochemical Society </i>171 055502 (2024).