Self-Assembly of Topologically Networked Protein–Ti₃C₂T_x MXene Composites

Hierarchical organization plays an important role in the stunning physical properties of natural and synthetic composites. Limits on the physical properties of such composites are generally defined by percolation theory and can be systematically altered using the volumetric filler fraction of the inorganic/organic phase. In natural composites, organic materials such as proteins that interact with inorganic filler materials can further alter the hierarchical order and organization of the composite via topological interactions, expanding the limits of the physical properties defined by percolation theory. However, existing polymer systems do not offer a topological parameter that can systematically modulate the assembly characteristics of composites. Here, we present a composite based on proteins and titanium carbide (Ti 3 C 2 T x ) MXene that manifests a topological network that regulates the organization, and hence physical properties, of these biomimetic composites. We designed, recombinantly expressed, and purified synthetic proteins consisting of polypeptides with repeating amino acid sequences (tandem repeats) that have the ability to self-assemble into topologically networked biomaterials. We demonstrated that the interlayer distance between MXene sheets can be controlled systematically by the number of tandem repeat units. We varied the filler fraction and number of tandem repeat units to regulate the in-plane and out-of-plane electrical conductivities of these composites. Once Ti 3 C 2 T x MXene sheets are separated enough to facilitate formation of cross-links in our proteins with the number of tandem repeat units reaching 11, the linear I–V characteristics of the composites switched into nonlinear I–V curves with a distinct hysteresis for out-of-plane electron transport, while the in-plane I–V characteristics remained linear. This highlights the impact of synthetic protein templates, which can be designed to modulate electronic transport in composites both isotropically and anisotropically.