The effect of the charge pattern on the applicability of a nanopore as a sensor

Abstract

We investigate a model nanopore sensor that is able to detect analyte ions that are present in the electrolyte solution in very small concentrations. The nanopore selectively binds the analyte ions with which the local concentrations of the ions of the background electrolyte (KCl), and, thus, the ionic current flowing through the pore is changed. Analyte concentration can be determined from calibration curves. In our previous study (Mádai et al. J. Chem. Phys., 147(24):244702, 2017.), we proposed a symmetric model (surface charge is negative all along the pore). The mechanism of sensing was a competition between K + and positive analyte ions, so increasing analyte concentration decreased K + current. Here we allow asymmetric charge patterns on the pore wall (positive/negative/neutral along the pore), thus, gaining an additional device function, rectification, resulting in a dual responsive device. We find that a bipolar nanopore is an efficient geometry with Cl − ions being the main charge carriers. The mechanism of sensing is that more positive analyte ions attract more Cl − ions into the pore thus increasing the current. Also they make the pore less asymmetric and, thus, decrease rectification. We use a hybrid computer simulation method, where a generalization of the grand canonical Monte Carlo method to non-equilibrium (Local Equilibrium Monte Carlo) is coupled to the Nernst-Planck equation with which the flux is computed.

Application of a bipolar nanopore as a sensor: rectification as an additional device function

“…Because the shapes of the I−U curves are similar for every case, we can characterize the ON and OFF states by two chosen voltages, ±200 mV as in our previous studies [44,45,1,48]. Therefore, we define our device functions as the ON-…”

“…The left 2/3 of the pore is a buffer region whose charge determines the main charge carrier of the nanopore, while the right 1/3 of the pore is a region where analyte ions (denoted by X z X with z X being the valence of the analyte ion; we consider only positively charged X ions) are bound by binding sites modeled with the square-well (SW) potential here. This work is a direct continuation of our previous paper [1] in which we allowed different charge densities (positive, negative, or zero) in the two regions and studied the effect of varying charge patterns on the applicability of the nanopore as a sensor. Surface charge pattern is a powerful tunable structural feature because it can be manipulated with chemical methods relatively easily [2,3,4,5,6,7].…”

“…In our previous work [1], we established that the most efficient asymmetric charge pattern from the point of view of the efficiency of sensing is the bipolar one where the left region is positively, while the right region is negatively charged. In this case, the main charge carrier is the anion (Cl − ) and the sensor works on the basis of the increasing Cl − current caused by the increased accumulation of Cl − attracted by the positive analyte ions into the binding region.…”

“…We apply the Nernst-Planck (NP) transport equation and couple it to LEMC resulting in a hybrid method, called NP+LEMC. This method was successfully applied for various problems in the last couple of years such as particle transport through model membranes [39,40], ion channels [41,42,43], and nanopores [44,45,8,46,47,1,48].…”

Rectification of bipolar nanopores in multivalent electrolytes: effect of charge inversion and strong ionic correlations

“…[55,56] LEMC has successfully described transport through membranes [55,57], calcium channels, [56,58] bipolar nanopores [30,28,59,32], transistors [31,34], and sensors. [29,33,35] Poisson-Nernst-Planck theory…”