Redox hydrogel films are commonly applied for bioelectrode fabrication and we recently demonstrated that such matrices can be engineered to efficiently protect the hydrogenases from O₂ damage to enable their integration into H₂/O₂ biofuel cells [1,2]. The formation of these films is highly reproducible for thicknesses between 30 to 300 µm. However, the most relevant thickness range for technological applications is in the low micrometer thicknesses which are more challenging to produce due to film thickness heterogeneities. Here, we propose the in-situ gelation of monodispersed viologen-modified macromolecules to achieve homogeneous films with thicknesses down to 100 nm. The relative standard deviations of the mean film thickness, was found to be only 4.8%. Electroanalytical investigations reveal defect-free, efficient electron transfer pathways over the complete film. We illustrate the advantage of this method and the importance of the film homogeneity for bioelectrocatalytic applications by investigating the behavior of bioanodes built from ferredoxin-NADP⁺-oxidoreductase or NiFe-hydrogenase for NADPH and H₂ oxidation, respectively. The ability of forming thin films reveal that peak catalytic performances are reached at 10-fold lower catalyst loading compared to conventional approaches owing to the extreme homogeneity in film morphologies. This method will be generally transposable to other related applications, involving catalyst deposition by drop-casting with strict demands for film homogeneity.

[1] N. Plumeré, O. Rüdiger, A. Alsheikh Oughli, R. Williams, J. Vivekananthan, S. Pöller, W. Schuhmann, W. Lubitz, Nature Chemistry, 2014, 6, 822–827.

[2] V. Fourmond, S. Stapf, H. Li, D. Buesen, J. Birrell, O. Rüdiger, W. Lubitz, W. Schuhmann, N. Plumeré, C. Léger, J. Am. Chem. Soc., 2015, 137, 5494-5505.

Financial support by the Cluster of Excellence RESOLV (EXC 1069) and by the ERC Starting grant REDOX SHIELDS is gratefully acknowledged.