Juan Liu

Surface Charge Density Determination of Single Conical Nanopores Based on Normalized Ion Current Rectification

Current rectification is well known in ion transport through nanoscale pores and channel devices. The measured current is affected by both the geometry and fixed interfacial charges of the nanodevices. In this article, an interesting trend is observed in steady-state current-potential measurements using single conical nanopores. A threshold low-conductivity state is observed upon the dilution of electrolyte concentration. Correspondingly, the normalized current at positive bias potentials drastically increases and contributes to different degrees of rectification. This novel trend at opposite bias polarities is employed to differentiate the ion flux affected by the fixed charges at the substrate-solution interface (surface effect), with respect to the constant asymmetric geometry (volume effect). The surface charge density (SCD) of individual nanopores, an important physical parameter that is challenging to measure experimentally and is known to vary from one nanopore to another, is directly quantified by solving Poisson and Nernst-Planck equations in the simulation of the experimental results. The flux distribution inside the nanopore and the SCD of individual nanopores are reported. The respective diffusion and migration translocations are found to vary at different positions inside the nanopore. This knowledge is believed to be important for resistive pulse sensing applications because the detection signal is determined by the perturbation of the ion current by the analytes.

Transmembrane Potential across Single Conical Nanopores and Resulting Memristive and Memcapacitive Ion Transport

Memristive and memcapacitive behaviors are observed from ion transport through single conical nanopores in SiO(2) substrate. In i-V measurements, current is found to depend on not just the applied bias potential but also previous conditions in the transport-limiting region inside the nanopore (history-dependent, or memory effect). At different scan rates, a constant cross-point potential separates normal and negative hysteresis loops at low and high conductivity states, respectively. Memory effects are attributed to the finite mobility of ions as they redistribute within the negatively charged nanopore under applied potentials. A quantative correlation between the cross-point potential and electrolyte concentration is established.

Quantification of Steady-State Ion Transport through Single Conical Nanopores and a Nonuniform Distribution of Surface Charges

Electrostatic interactions of mobile charges in solution with the fixed surface charges are known to strongly affect stochastic sensing and electrochemical energy conversion processes at nanodevices or devices with nanostructured interfaces. The key parameter to describe this interaction, surface charge density (SCD), is not directly accessible at nanometer scale and often extrapolated from ensemble values. In this report, the steady-state current-voltage (i-V) curves measured using single conical glass nanopores in different electrolyte solutions are fitted by solving Poisson and Nernst-Planck equations through finite element approach. Both high and low conductivity state currents of the rectified i-V curve are quantitatively fitted in simulation at less than 5% error. The overestimation of low conductivity state current using existing models is overcome by the introduction of an exponential SCD distribution inside the conical nanopore. A maximum SCD value at the pore orifice is determined from the fitting of the high conductivity state current, while the distribution length of the exponential SCD gradient is determined by fitting the low conductivity state current. Quantitative fitting of the rectified i-V responses and the efficacy of the proposed model are further validated by the comparison of electrolytes with different types of cations (K(+) and Li(+)). The gradient distribution of surface charges is proposed to be dependent on the local electric field distribution inside the conical nanopore.

Noninvasive Surface Coverage Determination of Chemically Modified Conical Nanopores that Rectify Ion Transport

Surface modification will change the surface charge density (SCD) at the signal-limiting region of nanochannel devices. By fitting the measured i-V curves in simulation via solving the Poisson and Nernst-Planck equations, the SCD and therefore the surface coverage can be noninvasively quantified. Amine terminated organosilanes are employed to chemically modify single conical nanopores. Determined by the protonation-deprotonation of the functional groups, the density and polarity of surface charges are adjusted by solution pH. The rectified current at high conductivity states is found to be proportional to the SCD near the nanopore orifice. This correlation allows the noninvasive determination of SCD and surface coverage of individual conical nanopores.

Physical Origin of Dynamic Ion Transport Features through Single Conical Nanopores at Different Bias Frequencies

Ionic transport through nanometer scale channels or interfaces is the physical origin of the detection signals in stochastic single molecular sensing, DNA sequencing and nano-structured electrodes. Dynamic transport regulated by systematically varying the bias frequency has not been explored. In this report, ion transport through single conical nanopore platforms is studied by applying an alternating electrical field at a wide range of scan rates. Rich frequency-dependent features of the measured transport current are discovered. The complete transition of characteristic transport features from low to high scan rates or bias frequencies is demonstrated experimentally. Key dynamic features include: multiple hysteresis loops separated by one or two non-zero cross points in the I–V curves, shifts in cross point potentials at different scan rates, and growth and diminishment in the hysteresis loops with normal and negative phase shifts. Combined theoretical and experimental studies reveal different processes contributing to and dominating the total current responses on different time scales. The respective contributions of each type of transport process to the overall measured current signals are quantitatively separated based on the fundamental insights gained. Although these exciting experimental observations are successfully simulated using an optimized model by solving PNP equations, the experimental studies at this nanoscale dimension suggest substantial deviations from a continuum regime. Inputs from molecular theory are needed to further validate the proposed physical mechanism.

Correlation of Ion Transport Hysteresis with the Nanogeometry and Surface Factors in Single Conical Nanopores

Better understanding in the dynamics of ion transport through nanopores or nanochannels is important for sensing, nucleic acid sequencing and energy technology. In this paper, the intriguing nonzero cross point, resolved from the pinched hysteresis current-potential (i-V) curves in conical nanopore electrokinetic measurements, is quantitatively correlated to the surface and geometric properties by simulation studies. The analytical descriptions of the conductance and potential at the cross point are developed: the cross-point conductance includes both the surface and volumetric conductance; the cross-point potential represent the overall/averaged surface potential difference across the nanopore. The impacts by individual parameter such as pore radius, half cone angle, and surface charges are systematically studied in the simulation that would be convoluted and challenging in experiments. The elucidated correlation is supported by and offer predictive guidance for experimental studies. The results also offer more quantitative and systematic insights in the physical origins of the concentration polarization dynamics in addition to ionic current rectification inside conical nanopores and other asymmetric nanostructures. Overall, the cross point serves as a simple yet informative analytical parameter to analyze the electrokinetic transport through broadly defined nanopore-type devices.

Cationic and Anionic Transport through Single Quartz Nanopipettes

Ionic transport through single quartz nanopipettes is studied by cyclic voltammetry and multi-frequency impedance spectroscopy. DC current rectification is correlated to the features in the two dimensional impedance Nyquist plots, by varying the ionic strength tailored by solution electrolyte concentration. Normalized conductivity is relatively comparable in different electrolyte concentrations at negative potentials, but is drastically enhanced at lower concentration at positive bias potentials. By employing electrolytes with common anions but different cations, the respective contribution of cation and anion transport to the overall ionic current is qualitatively differentiated. The phenomena are attributed to the fixed charges at the substrate-solution interface. Interestingly, the amount of fixed interfacial charges, or surface charge density at the mass transport limiting region, was found to develop over time, which results in more significant current rectification.