Nanomechanics of π-cation-π interaction with implications for bio-inspired wet adhesion

Cation-pi interactions play a vital role in modulating various biological processes, e.g., potassium-selective channel, protein folding and adhesion of marine organism. Previous studies mainly focus on binary cation-pi interaction, whereas due to the complexity of biological systems and surrounding environments, a single cation is often in close proximity with more than one pi-conjugated unit, which could exhibit essentially different binding behavior. Herein, the first experimental evidence of ternary pi-cation-pi interaction is reported through direct nanomechanical force measurement in a model pi-conjugated poly(catechol) (PC) system coexisting with K+. Ternary pi-cation-pinteractions can bridge pi-conjugated moieties, resulting in robust adhesion and promoting PC assembly and deposition. Particularly, these ternary complexes are discovered to transit to binary binding pairs by increasing K+ concentration, undermining adhesion and assembly due to lack of bridging. The pi-cation-pi binding strength follows the trend of NMe 4 + > K+ > Na+ > Li+. Employing the pi-cation-pi interaction, a deposition strategy to fabricate pi-conjugated moiety based adhesive coatings on different substrates is realized. Our findings provide useful insights in engineering wet adhesives and coatings with reversible adhesion properties, and more broadly, with implications on rationalizing biological assembly. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.