Comparative investigation of small laccase immobilized on carbon nanomaterials for direct bioelectrocatalysis of oxygen reduction

In this study, different carbon nanomaterials (carbon nanotubes, solid carbon spheres and graphene nanosheets) for immobilizing bacterial small laccase (SLAC) to enable direct bioelectrocatalysis of oxygen reduction were comparatively investigated. Highest bioelectrocatalytic current response was achieved with ethanol-assisted SLAC adsorption on single-walled carbon nanotubes. Direct electron transfer (DET) rates were determined and generally shown to be larger with carbon nanomaterials in smaller dimensions or higher surface curvature, i.e., 1.92, 1.05, 1.52, 0.62 and 0.63 s−1 for adsorbed SLAC on single-walled carbon nanotubes (d = ca. 1.5 nm), double-walled carbon nanotubes (d = 2–4 nm), thinner multi-walled carbon nanotubes (d = 8–15 nm), thicker multi-walled carbon nanotubes (d = 20–30 nm) and graphene nanosheets, respectively. No obvious catalytic current response was obtained with solid carbon spheres (d > 200 nm). Surface nanostructures on electrodes with decreased cylindrical diameter could enable a favorable protein orientation of immobilized SLAC for efficient electronic communication between the catalytic copper sites and carbon nanomaterials as revealed by surface vibrational spectroscopy. Moreover, such a difference in DET kinetics of SLAC could be an outcome of synergic effects of surface properties (curvature, oxygen-containing groups, etc.) and conductivity of carbon nanomaterials. It is expected to offer useful information to fabricating carbon-based oxygen-reducing biocathodes of wide working pH range and high tolerance to chloride. •A first comparative investigation of immobilized SLAC on carbon nanomaterials of different morphologies.•DET efficiency of SLAC in oxygen reduction is mainly related to the surface curvature of carbon nanomaterials.•Apparent electron transfer rate constant of SLAC on curved SWCNTs is three times of that on planar graphene nanosheets.•Structural insights are provided to unravel the relationship between DET kinetics of SLAC and properties of carbon materials.