Calcium Binding and Ionic Conduction in Single Conical Nanopores with Polyacid Chains: Model and Experiments

Abstract

Calcium binding to fixed charge groups confined over nanoscale regions is relevant to ion equilibrium and transport in the ionic channels of the cell membranes and artificial nanopores. We present an experimental and theoretical description of the dissociation equilibrium and transport in a single conical nanopore functionalized with pH-sensitive carboxylic acid groups and phosphonic acid chains. Different phenomena are simultaneously present in this basic problem of physical and biophysical chemistry: (i) the divalent nature of the phosphonic acid groups fixed to the pore walls and the influence of the pH and calcium on the reversible dissociation equilibrium of these groups; (ii) the asymmetry of the fixed charge density; and (iii) the effects of the applied potential difference and calcium concentration on the observed ionic currents. The significant difference between the carboxylate and phosphonate groups with respect to the calcium binding is clearly observed in the corresponding current-voltage (I-V) curves and can be rationalized by using a simple molecular model based on the grand partition function formalism of statistical thermodynamics. The I-V curves of the asymmetric nanopore can be described by the Poisson and Nernst-Planck equations. The results should be of interest for the basic understanding of divalent ion binding and transport in biological ion channels, desalination membranes, and controlled drug release devices.

Transport mechanisms in nanopores and nanochannels: can we mimic nature?

“…In principle, any process able to modify the charge on the walls of the channel can be used for gating, for example electrostatic gating was achieved in phosphate-modified pores by Ca 2+ addition [46,47] and in photoacidmodified channels by light [48,49]. It is worthwhile to note, however, that gating in biological channels do not rely on the electrostatic gating mechanism discussed so far.…”

Conductivity-based detection techniques in nanofluidic devices

“…6A. 136 pH-dependent rectification is similarly achieved by modifying PET nanopores with ss-DNA to create electrostatic meshes. 137 Nanopore surfaces functionalized with zinc finger peptides bind Zn 2+ to induce a conformation change and cause a fluctuation in ion current.…”

“…Subsequently, the ligand binds streptavidin, which minimizes the excess surface charge and results in decreased ICR as well as decreased total ion current. Panel A adapted with permission from Ali et al 136 Copyright 2012 American Chemical Society. Panels B-C adapted with permission from Ali et al 146 Copyright 2008 American Chemical Society.…”

The facile surface chemical modification of a single glass nanopore and its use in the nonenzymatic detection of uric acid

“…15,18,19 But the ion-beam or electron-beam based techniques require complex/large apparatuses like the transmission electron microscope and He ion microscope, and the fabrication process of solid-state nanopores in polymer membranes (e.g. 8,17,20,[24][25][26][27][28][29] The relatively facile functionalization of the glass surface further provides a glass nanopore platform which is promising for potential applications. 15 Recently a glass capillary-based nanopipette or nanopore technology has been proven to be an exciting new approach for biosensing applications and is attracting more and more attention from researchers.…”