pH-tunable gradients of wettability and surface potential

TLDR

Smart materials that sense and respond to environmental changes are sought for diverse applications, including guiding charged particles, proteins, or bacteria. The study reports pH‑responsive gradient surfaces whose wettability and surface potential vary with pH. The gradients were created by spatially varying amine and carboxyl acid concentrations, and their pH‑dependent wettability and surface potential were characterized via contact angle and AFM‑based surface force measurements. The surface potential shifts from neutral to positive at the amine end and from negative to neutral at the acid end at low pH, reverses at high pH, and transitions from negative to positive across the gradient at intermediate pH.

Abstract

Smart materials that can sense and respond to changes in the environment are of interest in numerous and diverse applications. In this paper, we report gradient surfaces where wettability and surface potential respond to changes in the pH. The gradients are produced by controlling the concentration of amine and carboxyl acid groups across the surface. The response of surface wettability to pH changes was studied by water contact angle measurements. The potential across the surface was determined by atomic force microscopy-based surface force measurements. These studies showed that at low pH the surface potential changes from "no charge" at the acid end to a positive charge at the amine end. At high pH the surface potential changed from negative at the acid end to "no charge" at the amine side. At an intermediate pH the charge across the surface changes from negative at the acid end to positive at the amine end. Potential applications include separation or guidance of charged entities such as particles, proteins or bacteria.