Biofunctionalisation of p-doped silicon with cytochrome c553 minimises charge recombination and enhances photovoltaic performance of the all-solid-state photosystem I-based biophotoelectrode

Abstract

Surface-directed passivation of p-doped silicon (Si) substrate was achieved by its biofunctionalisation with hexahistidine (His₆)-tagged cytochrome c₅₅₃ (cyt c₅₅₃), a soluble electroactive photosynthetic protein responsible for electron donation to photooxidised photosystem I (PSI). Five distinct variants of cyt c₅₅₃ were genetically engineered by introducing the specific linker peptides of 0–19 amino acids (AA) in length between the cyt c₅₅₃ holoprotein and a C-terminal His₆-tag, the latter being the affinity ‘anchor’ used for the specific immobilisation of this protein on the semiconductor surface. Calculation of 2D Gibbs free energy maps for the five cyt c₅₅₃ variants showed a significantly higher number of thermodynamically feasible conformations of immobilised cyt c variants containing longer linker peptides. Here we show that the distinct cyt c₅₅₃-based Si bioelectrodes display some characteristics of the p–n-type diodes, albeit varying in the level of dark saturation current J₀ considered as the charge recombination parameter. These combined bioinformatic and electrochemical analyses indicate that the cyt c₅₅₃ variants with longer linker peptides, up to 19AA in length, allow for more structural flexibility of immobilised cyt c₅₅₃ in terms of both, orientation and distance of the haem group with respect to the Si surface, and promote the efficient biopassivation of the semiconductor substrate. Incorporation of the specifically immobilised 19AA cyt c₅₅₃ variant into the all-solid-state biophotoelectrodes containing light harvesting PSI module enhanced biophotovoltaic performance of the PSI biophotoelectrode compared to the analogous device devoid of cyt c₅₅₃.