The two facets of the synergic effect of amine cation and catechol on the adhesion of catechol in underwater conditions

• Published in: Applied surface science 2020-11, Vol.530, p.146973, Article 146973
• Main Authors: Mu, Youbing, Mu, Pengzhou, Wu, Xiao, Wan, Xiaobo
• Format: Article
• Language: English
• Subjects: AFM; Amine cation; Catechol; Synergic effect; Underwater adhesion
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2020.146973

Spatial proximity might not be a prerequisite for the synergic effect between amino and catechol groups in underwater adhesion. However, the synergic effect is greatly enhanced at lower pH conditions when the catechol and amino groups are integrated into one molecule. The role of proton is to interfere with the formation of the hydration layer, which helps the amine cation to evict the absorbed metallic cations and the nearby catechol to coordinate sequentially. [Display omitted] •Spatial proximity might not be a prerequisite for the synergic effect between NH2 and catechol groups.•The synergic effect is enhanced at lower pH values when NH2 and catechol groups are next to each other.•The synergic effect is not senstive to pH value changes when NH2 and catechol groups are separated.•Na+ hydration layer could be more easily removed from the surface than K+ hydration layer. Many factors govern the underwater adhesion of mussels, among which the synergic effect of charged amino group and catechol moiety in mussel foot-byssus proteins has been discovered recently. Here we show that although very important, spatial proximity might not be the prerequisite for the synergic effect between amino and catechol groups in underwater adhesion. By designing probes grafted with monomer integrated with both groups and with monomers carrying the two groups separately, we revisted the synergic effect at nanoscale level. It is found that the synergic effect has two facets: first, the presence of both groups enable stronger adhesion in electrolyte solutions than the presence of catechols alone, and this effect does not require catechols and amine cations to be next to each other in space. Second, only when the catechol and cation are paired and next to each other, the adhesion force could be further enhanced in low pH solutions, which means the dependence of synergic effect on the spatial proximity of catechols and amine cations is conditional to the acidity of the electrolyte solution. We reason that besides the already discovered effect in protecting catechol from oxidation, acid condition contributes to underwater adhension by interfering with the formation of the hydration layer, which helps the eviction of absorbed cations and the coordination of catechol sequentially when the catechol and amino cation are spatially next to each other.