Structural Factors Tuning Peptide Proton Transport via Self-Assembly Monolayers

Proton transport (PT) in solid-state materials is crucial for applications in energy conversion and protonic devices. Nevertheless, the highly complex and disordered structures of conventional proton-conducting materials, such as polymers and proteins, hinder a clear understanding of the mechanisms underlying PT, particularly the formation of hydrogen bond (H-bond) networks and their role in mediating PT. Here, we show that self-assembling monolayers (SAMs) of oligopeptides provide a promising platform for elucidating the key factors that modulate PT related H-bonds, including amide bond interactions, peptide sequence, and chain length. Combined with structural characterizations of SAMs, the electrical measurements under both direct and alternating current modes demonstrate that longer and more extended oligopeptide chains in SAMs result in an ordered molecular arrangement, leading to a more pronounced response of current density (J) to increasing relative humidity (RH). Moreover, this increase in molecular order also shifts the transition from electron-dominated to proton-dominated charge transport to a higher RH. The synergy between carrier concentration and mobility is a key factor contributing to the increase in J. This study not only elucidates the critical role of ordered H-bonds in PT but also expands the application of SAM technology in controlling molecular conformation and enhancing proton conduction.