Single Atoms Functioning as Catalysts Inside Living Matter.

Single-atom catalysts represent the ultimate limit of materials miniaturization, yet their functionality has been confined to well-defined abiotic environments. Whether atomically dispersed metal centers can preserve catalytic identity within the chemically crowded, dynamically regulated milieu of living matter remains unknown. Here we show that Fe-Nx single atoms embedded in graphene quantum dots operate as catalytic entities inside the cytoplasm of bacteria. Following cellular internalization, these isolated sites establish a light-driven intracellular redox cycle that accelerates NADH oxidation while maintaining cellular viability. Time-resolved fluorescence measurements reveal pronounced excited-state quenching in the biohybrid, supporting close material-cell coupling and light-driven charge consumption within the cellular environment. The resulting perturbation propagates through endogenous biochemical networks, producing a programmable redistribution of reducing equivalents and enhanced succinate biosynthesis without genetic modification. Structural analyses confirm that atomic dispersion of Fe centers is preserved under biological conditions. These findings demonstrate that atomically defined materials can function within living systems while retaining both structural integrity and catalytic activity, thereby extending single-atom catalysis from abiotic interfaces to biological environments. Living matter thus emerges as a viable reaction field for atomic-scale materials, opening opportunities for designing functional materials capable of operating within complex biological settings.