Carbon quantum dots boost microbial biofilm-based hydrovoltaic electricity generation

Microbial biofilm-based hydrovoltaic electricity generators (BioHEGs) have recently been developed as promising and readily available platforms for green energy harvesting, despite their unsatisfactory output performances and unspecified mechanisms regarding electric current production. Herein, carbon quantum dots (CQDs) were used to construct a nano-biohybrid system with Shewanella oneidensis MR-1 (S. oneidensis), through which the CQDs/S. oneidensis BioHEG achieved a maximum open-circuit voltage of ca. 0.65 V and short-circuit current density of ca. 5.23 μA·cm^-2. In addition, both the hydrovoltaic effect and electrical conductivity of CQDs/S. oneidensis nano-biohybrids were noticeably improved due to enhanced secretion of extracellular polymeric substances (EPS) and accelerated electron transfer upon CQDs implantation, thereby leading to a nearly 14-fold increase in output power density compared to the bare S. oneidensis cells. Studies aimed to elucidate the underlying mechanism indicated that the hybridization of CQDs and S. oneidensis greatly promoted the metabolic synthesis of outer membrane c-type cytochromes (OM c-Cyts) and the extracellular secretion of riboflavin (RF), which was demonstrated to be decisive in the current producing process of the CQDs/S. oneidensis BioHEG. This work thus proposes a viable strategy to boost the hydrovoltaic electricity generation capacity of microbial biofilms and provides a new perspective on the mechanism of accelerated electron transfer pathways inside BioHEGs.