Amine-Phosphate Specific Interactions within Nanochannels: Binding Behavior and Nanoconfinement Effects

Abstract

In the last years, the ionic conductance behavior of solid-state nanochannels (SSN) has been extensively studied with both basic and applied purposes. In particular, the interactions between confined groups and dissolved species have been widely used for the design of biosensors and smart devices. Being the species confined to the small volume of the SSN, the ionic equilibrium usually differs from that in the solution bulk and nanoconfinement effects appear. In this work, we study the binding equilibrium between surface-confined amine groups and phosphate anions taking place within SSN by measuring the changes in the iontronic transmembrane current response of single nanochannels at different phosphate concentrations. Phosphate binding is higher compared with other divalent anions and takes place even in electrostatically hindered conditions, which reinforces the idea of chemical specificity of the amine-phosphate interaction. The sensitivity of the iontronic response of asymmetric SSN to changes in the surface charge allowed the interpretation of the experimental results in terms of a simple binding model, which reveals that the nanoconfinement effects are responsible for a one order of magnitude increase in the effective constants for the anion binding to the surface amine groups in the nanochannel walls. Furthermore, polyphosphates show a more pronounced binding tendency toward amine moieties, which allows the detection and quantification of ATP in the micromolar range from the analysis of the iontronic response.