Carbon dots enhance biofilm-electrode electron transfer for nitrite-independent ammonia oxidation

Low biofilm conductivity limits electron transfer at the cell-electrode interface, constraining nitrogen removal in microbial electrolysis cell (MEC)-anammox systems. Herein, we developed a carbon dots (CDs)-modified biofilm to improve electron transfer and support sustained nitrogen removal. Nanoscale CDs bearing abundant surface functional groups were incorporated into the extracellular matrix, forming conductive connections between microbial cells and the anode. CDs incorporation reduced interfacial electron-transfer barriers and facilitated electron extraction. Internalized CDs further enhance intracellular metabolism to increase heme c, NADH, and ATP levels, which in turn improves the biosynthesis of extracellular polymers (EPS). EPS accumulation may provide an anchoring matrix that supports CDs retention and conductive network stability. The engineered system achieved 95% total nitrogen removal and a stable current density of 1.15 mA cm^-2. This work demonstrates a simple, cost-effective and efficient route to improve bioelectricity generation of anammox, offering promising opportunities for sustained wastewater treatment and energy recovery.