Vishal V. R. Nandigana

In this article we measure the surface potential of the polycarbonate track etched membrane having multiple nanopores and graphene/copper membrane having multiple nanopores. We use potassium chloride elec trolyte solution (KCl) with 0.6 mol in the high concentration inlet source reservoir. We use 0.6 mmol potassium chloride electrolyte solution in the low concentration outlet sink reservoir. We provide theory to match the surface potential of the membranes. We develop model to understand the potassium ion transference number inside the multiple nanopores for our membranes. We develop GUI simulation to understand the concentration of the potassium chloride electrolyte solution inside single nanochannel, single nanopore for different surface potential of the membranes. We use Gouy-Chapman equation to calculate the concentration and Smolu chowski model is used to calculate the velocity of the potassium chloride electrolyte solution. Here we present a detailed comparative analysis of the structural, transport mechanisms of nanochannels, nanopores to ex plain the several orders of magnitude difference in the current inside the nanochannel and the nanopore. The volume of the single nanochannel is larger than the volume of the single nanopore resulting in larger charge of the potassium ions, chlorine ions and potassium chloride electrolyte solution inside the single nanochannel compared to the single nanopore resulting in the significant difference in the current. We develop GUI based simulation of nanofluidics electronics calculator. Each button of the calculator is related to their own current of the potassium chloride electrolyte solution inside the nanochannel and nanopore, respectively.

Membrane surface charge induced electro-osmotic flow (EOF) is key to create negative differential resistance (NDR). Charge polarization induced EOF dominates over diffusio-osmosis, causing the backflow of low concentration/conductivity solution into the pore, causing NDR.

The ion transport measurements using various ion-exchange membranes (IEMs) face several challenges, including controllability, reproducibility, reliability, and accuracy. This is due to the manual filling of the solutions in two different reservoirs in a typical diffusion cell experiment with a random flow rate, which results in the diffusion through the IEM even before turning on the data acquisition system as reported so far. Here, we report the design and development of an automated experimental setup for ion transport measurements using IEMs. The experimental setup has been calibrated and validated by performing ion transport measurements using a standard nanoporous polycarbonate membrane. We hope that the present work will provide a standard tool for realizing reliable ion transport measurements using ion-exchange membranes and can be extended to study other membranes of various pore densities, shapes, and sizes.

In this paper, we perform all-atom molecular dynamics (AA-MD) simulations to predict noise in solid-state nanopores. The simulation system consists of ∼70,000 to ∼350,000 atoms. The simulations are carried out for ∼1.3 µs over ∼6500 CPU hours in 128 processors (Intel^® E5-2670 2.6 GHz Processor). We observe low and high frequency noise in solid-state nanopores. The low frequency noise is due to the surface charge density of the nanopore. The high frequency noise is due to the thermal motion of ions and dielectric material of the solid-state nanopore. We propose a generalised noise theory to match both the low and high frequency noise. The study may help ways to study noise in solid-state nanoporous membranes using MD simulations.

We demonstrate a novel nanofluidic diode that produces rectification factors in excess of 1000. The nanofluidic diode consists of ion permselective nanopores that connect two reservoirs of different diameters- a micropore reservoir and a macropore reservoir. On the application of +100 V to the micropore, a low OFF state current is observed. The OFF state is caused by formation of the ion depleted zone in the micropore because the anions are prevented from entering the nanopores from the micropore and the cations are depleted in this region to maintain charge neutrality. On the application of −100 V, we observe a high ON state current. The ON state is caused by formation of the ion enriched zone in the microchannel because the anions cannot pass through the nanopores and accumulate in the microchannel. To maintain charge neutrality the cations also become enriched in the microchannel. The ratio of ON state current to the OFF state current gives the rectification of current. Here, plasma oxidation is used to achieve a nanopore with a large wall surface charge density of σ_n = −55 mC/m² which yields a rectification of current on the order of 3500 that is nearly two orders of magnitude higher than those reported thus far. In contrast to the other nanofluidic diodes, this nanofluidic diode does not introduce asymmetry to the nanopore, but asymmetry is produced by having the nanopores join a micropore and a macropore. Introduction of asymmetry into the fluidic reservoirs which the nanopores connect is quite simple. Hence, the nanofluidic diode is easy to scale up to industrial level.

Nanopores have been used for myriad applications ranging from water desalination, gas separation, fluidic circuits, DNA sequencing, and preconcentration of ions. In all of these applications noise is an important factor during signal measurement. Noisy signals disrupt the exact measuring signal in almost all of these applications. In this paper, we rationalize whether current oscillations should be classified only as noise or the physical disturbance in ionic charges has some other meaning. We infer that the physical disturbance in ionic charges and the current oscillations are not noise but can be chaos. Chaos is present in the system due to depletion of the ions, created by nonequilibrium anharmonic distribution in the electrostatic potential. In other words, multiple electric potential wells are observed in the nanoporous system. The multiple electric potential wells leads to bi-directional hopping of ions as the ions transport through the pore. The bi-directional hopping results in current oscillations. This paper suggests that chaos exists from a deterministic perspective and that there is no stochastic element leading to current oscillations. We prove this case by considering a simple oscillator model involving the electrostatic and dissipative forces in order to model ionic current. We observed current oscillations even in the absence of a stochastic noise force. Hence, we state that current oscillations in nanopores can be due to chaos as well and not necessarily due to noise. Furthermore, the color associated with the chaotic spectrum is not brown but pink, with 1/f type dynamics similar to the 1/f type pink noise presented by theorists and experimentalists. However, the 1/f type pink chaos exists due to deterministic current oscillations and not due to a stochastic fluctuating noise force.

Making use of the osmotic pressure difference between fresh water and seawater is an attractive, renewable and clean way to generate power and is known as 'blue energy'. Another electrokinetic phenomenon, called the streaming potential, occurs when an electrolyte is driven through narrow pores either by a pressure gradient or by an osmotic potential resulting from a salt concentration gradient. For this task, membranes made of two-dimensional materials are expected to be the most efficient, because water transport through a membrane scales inversely with membrane thickness. Here we demonstrate the use of single-layer molybdenum disulfide (MoS2) nanopores as osmotic nanopower generators. We observe a large, osmotically induced current produced from a salt gradient with an estimated power density of up to 10(6) watts per square metre--a current that can be attributed mainly to the atomically thin membrane of MoS2. Low power requirements for nanoelectronic and optoelectric devices can be provided by a neighbouring nanogenerator that harvests energy from the local environment--for example, a piezoelectric zinc oxide nanowire array or single-layer MoS2 (ref. 12). We use our MoS2 nanopore generator to power a MoS2 transistor, thus demonstrating a self-powered nanosystem.