Liuxuan Cao

A Biomimetic Potassium Responsive Nanochannel: G-Quadruplex DNA Conformational Switching in a Synthetic Nanopore

Potassium is especially crucial in modulating the activity of muscles and nerves whose cells have specialized ion channels for transporting potassium. Normal body function extremely depends on the regulation of potassium concentrations inside the ion channels within a certain range. For life science, undoubtedly, it is significant and challenging to study and imitate these processes happening in living organisms with a convenient artificial system. Here we report a novel biomimetic nanochannel system which has an ion concentration effect that provides a nonlinear response to potassium ion at the concentration ranging from 0 to 1500 microM. This new phenomenon is caused by the G-quadruplex DNA conformational change with a positive correlation with ion concentration. In this work, G-quadruplex DNA was immobilized onto a synthetic nanopore, which undergoes a potassium-responsive conformational change and then induces the change in the effective pore size. The responsive ability of this system can be regulated by the stability of G-quadruplex structure through adjusting potassium concentration. The situation of the grafting G-quadruplex DNA on a single nanopore can closely imitate the in vivo condition because the G-rich telomere overhang is attached to the chromosome. Therefore, this artificial system could promote a potential to conveniently study biomolecule conformational change in confined space by the current measurement, which is significantly different from the nanopore sequencing. Moreover, such a system may also potentially spark further experimental and theoretical efforts to simulate the process of ion transport in living organisms and can be further generalized to other more complicated functional molecules for the exploitation of novel bioinspired intelligent nanopore machines.

Current Rectification in Temperature-Responsive Single Nanopores

Herein we demonstrate a fully abiotic smart single-nanopore device that rectifies ionic current in response to the temperature. The temperature-responsive nanopore ionic rectifier can be switched between a rectifying state below 34 degrees C and a non-rectifying state above 38 degrees C actuated by the phase transition of the poly(N-isopropylacrylamide) [PNIPAM] brushes. On the rectifying state, the rectifying efficiency can be enhanced by the dehydration of the attached PNIPAM brushes below the LCST. When the PNIPAM brushes have sufficiently collapsed, the nanopore switches to the non-rectifying state. The concept of the temperature-responsive current rectification in chemically-modified nanopores paves a new way for controlling the preferential direction of the ion transport in nanofluidics by modulating the temperature, which has the potential to build novel nanomachines with smart fluidic communication functions for future lab-on-chip devices.

Towards understanding the nanofluidic reverse electrodialysis system: well matched charge selectivity and ionic composition

The widespread use of tiny electrical devices, from microelectromechanical systems (MEMS) to portable personal electronics, provides a new challenge in the miniaturization and integration of power supply systems. Towards this goal, we have recently demonstrated a bio-inspired nanofluidic energy harvesting system that converts salinity gradient energy from the ambient environment into sustainable electricity with single ion-selective nanopores (Adv. Funct. Mater. 2010, 20, 1339). The nanofluidic reverse electrodialysis system (NREDS) significantly improves the performance of conventional membrane-based reverse electrodialysis systems due to a higher ionic flux and a lower fluidic resistance. However, the fundamental working mechanism of the NREDS has been largely unexplored in the literature. In this work we have systematically investigated the performance of the NREDS in relation to the electrolyte type and the charge selectivity of the nanofluidic channel using both experimental and theoretical approaches. Experimental results show that the short-circuit current, the open-circuit voltage, and the resulting electric power of the NREDS are very sensitive to the ionic composition of the electrolyte solution. Through an in-depth theoretical analysis, two dominant factors that govern the charge separation and ion selectivity of the nanochannels were identified. The results prove that, with well-matched electrolyte types and nanopore charge selectivity, the harvested electric power and energy conversion efficiency can be improved by nearly two orders of magnitude.

Concentration-gradient-dependent ion current rectification in charged conical nanopores.

Ion current rectification (ICR) in negatively charged conical nanopores is shown to be controlled by the electrolyte concentration gradient depending on the direction of ion diffusion. The degree of ICR is enhanced with the increasing forward concentration difference. An unusual rectification inversion is observed when the concentration gradient is reversely applied. A numerical simulation based on the coupled Poisson and Nernst-Planck (PNP) equations is proposed to solve the ion distribution and ionic flux in the charged and structurally asymmetric nanofluidic channel with diffusive ion flow. Simulation results qualitatively describe the diffusion-induced ICR behavior in conical nanopores suggested by the experimental data. The concentration-gradient-dependent ICR enhancement and inversion is attributed to the cooperation and competition between geometry-induced asymmetric ion transport and the diffusive ion flow. The present study improves our understanding of the ICR in asymmetric nanofluidic channels associated with the ion concentration difference and provides insight into the rectifying biological ion channels.

On the Origin of Ionic Rectification in DNA-Stuffed Nanopores: The Breaking and Retrieving Symmetry

The discovery of ionic current rectification (ICR) phenomena in synthetic nanofluidic systems elicits broad interest from interdisciplinary fields of chemistry, physics, materials science, and nanotechnology; and thus, boosts their applications in, for example, chemical sensing, fluidic pumping, and energy related aspects. So far, it is generally accepted that the ICR effect stems from the broken symmetry either in the nanofluidic structures, or in the environmental conditions. Although this empirical regularity is supported by numerous experimental and theoretical results, great challenge still remains to precisely figure out the correlation between the asymmetric ion transport properties and the degree of symmetry breaking. An appropriate and quantified measure is therefore highly demanded. Herein, taking DNA-stuffed nanopores as a model system, we systematically investigate the evolution of dynamic ICR in between two symmetric states. The fully stuffed and fully opened nanopores are symmetric; therefore, they exhibit linear ion transport behaviors. Once the stuffed DNA superstructures are asymmetrically removed from one end of the nanopore via aptamer-target interaction, the nanofluidic system becomes asymmetric and starts to rectify ionic current. The peak of ICR is found right before the breakthrough of the stuffed DNA forest. After that, the nanofluidic system gradually retrieves symmetry, and becomes non-rectified. Theoretical results by both the coarse-grained Poisson-Nernst-Planck model and the 1D statistic model excellently support the experimental observations, and further establish a quantified correlation between the ICR effect and the degree of asymmetry for different molecular filling configurations. Based on the ICR properties, we develop a proof-of-concept demonstration for sensing ATP, termed the ATP balance. These findings help to clarify the origin of ICR, and show implications to other asymmetric transport phenomena for future innovative nanofluidic devices and materials.

Enhancing the rectification efficiency by changing ion species and introducing concentration gradients

Synthetic nanopores with ion selectivity have transport characteristics similar to biological ion channels [1, 2] and semiconductor electron devices [1]. For example, conical nanopores with fixed surface charge and small openings have rectification properties, which depend on the surface charge density and can be regulated by surface chemical modification. Here we show a simple and effective method to significantly enhance the rectification ratio by changing the ion species and introducing concentration gradient across the nanopores. The experimental results show that the rectification ratio (Q=I−/I+) of a 100nm nanopore in polyimide film without chemical modification approaches 50 in KF solution with the electrolyte concentration on each side of the membrane is 1mM and 300mM, respectively.