Advanced nanostructured membranes with high ion flux and selectivity bring new opportunities for generating clean energy by exploiting the osmotic pressure difference between water sources of different salinities.
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.
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