Carbon dots internalization enhances electroactive biofilm formation and microbial acetate synthesis

This study investigated how carbon dots (CDs) and N-doped CDs modification of acetogenic bacteria enhanced the extracellular electron transfer (EET) and the formation of electroactive biofilm, which subsequently improved acetate production from CO₂ in microbial electrosynthesis (MES). Characteristic fluorescence emission peaks of CDs and N-doped CDs were detected inside and outside of bacterial cells, respectively, suggesting that the CDs were successfully internalized by the cells. The (N-doped) CDs modified acetogens increased current output by 89–95%, and acetate production rate by 57–71%. The highest acetate production rate and electron recovery efficiency reached 2.1 mM d⁻¹ and 91.3%, respectively. Acetoanaerobium and Sporomusa, which are considered to have different EET strategies, dominated the biocathodes modified with CDs and N-doped CDs, respectively. The CDs modification increased biofilm thickness by 86% and reached 41.0 ± 1.3 μm, which could be related to the increased secretion of flavins that enhanced the signal transduction interspecies. With the N-doped CDs modified, the biofilm was occupied with active pyrrolic N/graphitic N (52%), and the expression of functional genes encoding cytochrome (cydB, cybH, CcdA) were enhanced significantly. This study offers insights into the EET mechanisms by acetogens and a viable approach to increase acetate production from CO₂.