Can Bioelectrocatalytic Films With Open Boundaries Confine Redox Cofactors?

Electrochemically driven enzyme cascades require efficient transfer of intermediates and cofactors between enzymes. However, bioelectrocatalytic films have open boundaries, where diffusive loss across the film-solution interface may impede catalytic efficiency. By systematically exploring the parameter space, we identify the precise conditions that allow such films to confine redox cofactors. Counterintuitively, confinement in electrochemical films arises not from the mere local proximity of enzymes, but from the global interplay between cofactor reaction and diffusion. At steady state, confinement fundamentally results from reaction–diffusion boundary layers within the film, which isolate reactions from the film-electrolyte interface, dynamically trapping cofactors and intermediates even when open boundaries and large gradients provide a driving force for their diffusive loss to the electrolyte solution. Conversely, while confinement is not achievable under purely transient conditions, tuning cofactor reaction–diffusion kinetics yields order-of-magnitude increases in turnover numbers. These findings provide a general strategy for spatiotemporal control of intermediates and cofactors, broadly applicable beyond enzymatic reactions to diverse forms of heterogeneous cascade catalysis.