Towards explicit regulating-ion-transport: nanochannels with only function-elements at outer-surface

Abstract

Function elements (FE) are vital components of nanochannel-systems for artificially regulating ion transport. Conventionally, the FE at inner wall (FEIW) of nanochannel−systems are of concern owing to their recognized effect on the compression of ionic passageways. However, their properties are inexplicit or generally presumed from the properties of the FE at outer surface (FEOS), which will bring potential errors. Here, we show that the FEOS independently regulate ion transport in a nanochannel−system without FEIW. The numerical simulations, assigned the measured parameters of FEOS to the Poisson and Nernst-Planck (PNP) equations, are well fitted with the experiments, indicating the generally explicit regulating-ion-transport accomplished by FEOS without FEIW. Meanwhile, the FEOS fulfill the key features of the pervious nanochannel systems on regulating-ion-transport in osmotic energy conversion devices and biosensors, and show advantages to (1) promote power density through concentrating FE at outer surface, bringing increase of ionic selectivity but no obvious change in internal resistance; (2) accommodate probes or targets with size beyond the diameter of nanochannels. Nanochannel-systems with only FEOS of explicit properties provide a quantitative platform for studying substrate transport phenomena through nanoconfined space, including nanopores, nanochannels, nanopipettes, porous membranes and two-dimensional channels.

Revealing Ionic Signal Enhancement with Probe Grafting Density on the Outer Surface of Nanochannels

“…The grafting density of PNA probes was adjusted by adding the same volume but different concentrations of PNA within the same area of the Au layer (6 mm × 6 mm). The grafting density of DNA can be quantitatively measured by the electrochemical method of Ru­(III) ions absorbed on their phosphate backbones. ,, Here, the grafting density of the PNA probes was roughly obtained by measuring the grafting density of thiol-modified DNA with the same sequence and concentration in buffer solution. With the increase of the PNA probe concentration in buffer, the grafting density of PNA increased, and the transmembrane ion current ( I probe ) decreased, which is indicated by the current–voltage ( I – V ) curves (Figures c and S2).…”

“…Recently, multiscale target recognitions from ions to cells without any limitation of the pore sizes of nanochannels have been achieved by the probe−target recognition on the outer surface of solid-state nanochannels. 33 Through the rejection by the nanochannel's mouth and the Au−thiol interaction, 34,35 in a Au-coated nanochannel with narrow pore size, the thiolmodified probes were unable to go deep into cavities and were immobilized on the outer surface and the mouth of the nanochannels. The targets captured by these probes would change the potential of the outer surface and the steric hindrance of the pore mouth, thus producing the ionic signal.…”

“…The targets captured by these probes would change the potential of the outer surface and the steric hindrance of the pore mouth, thus producing the ionic signal. 33 Compared with the narrow space inside nanochannels, the outer surface and the mouth are exposed to the solution of targets, which can not only accept probes or capture targets larger than pore sizes but also greatly improve the recognition efficiency between probes and targets. 36−40 now, the signaling mechanism of probe−target recognition on the outer surface of nanochannels is unclear.…”

“…Through the rejection by the nanochannel’s mouth and the Au–thiol interaction, , in a Au-coated nanochannel with narrow pore size, the thiol-modified probes were unable to go deep into cavities and were immobilized on the outer surface and the mouth of the nanochannels. The targets captured by these probes would change the potential of the outer surface and the steric hindrance of the pore mouth, thus producing the ionic signal . Compared with the narrow space inside nanochannels, the outer surface and the mouth are exposed to the solution of targets, which can not only accept probes or capture targets larger than pore sizes but also greatly improve the recognition efficiency between probes and targets. − However, until now, the signaling mechanism of probe–target recognition on the outer surface of nanochannels is unclear.…”

Nanopore Sensing Technique for Studying the Hofmeister Effect

“…Xia et al reported a series of surface functionalization strategies for regulating ion behaviors; i) outer-surface enhancement and inner-wall ion gating, ii) outer-surface anti-interference and inner-wall ion gating, and iii) independent outer-surface functionalization. [94][95][96] Among all the surface functionalization methods of solid-state nanopores, modification with COOH and NH 2 groups is the most widely used for fabricating cationselective (negatively charged) and anion-selective (positively charged) nanopores, and this method shows excellent selectivity in monovalent alkali and halogen ions. [97] Siwy developed COOH-modified Si 3 N 4 with substantially high Ca 2+ selectivity in mixtures of KCl and CaCl 2 , which could be explained by the local charge inversion.…”