Ion Current Rectification in Nanopipettes: From Understanding Ion Transport Dynamics to the Development of Optimized Sensing Systems

Ion current rectification is an interesting non-faradic ion transport property of nanopores that exhibit asymmetry. The interplay of nanoscale geometry, surface charges that significantly dominate at the nanoscale, applied bias and electrical double layer rearrangement creates substantial ion flux selectivity to electrolyte transport, leading towards the emergence of concentration polarization and diode-like current-voltage characteristics. This body of work presents a detailed study of ion transport behaviour within quartz nanopipettes, as well as the exploitation of this ion transport behaviour for the design of selective and highly sensitive sensors. Initial work focuses on the effect of electrolyte concentration on the ion-rectifying properties of nanopipettes, including the observation of inversion of the rectification at very low electrolyte concentrations. The experimental observations are explained in terms of the positional ion accumulation and depletion within the pore as a function of electrolyte concentration and pore size. Later, a three-step solid-phase synthesis procedure is prepared to graft single stranded DNA probes onto the internal walls of the quartz nanopipettes, with the modification process monitored by studying changes to the rectification response. DNA probes are utilized for sensing DNA corresponding to pathogenic methicillin resistant Staphylococcus aureus bacterial strain. Interestingly, upon optimization of this sensor to achieve ultra-low detection limits, we discovered the importance of probe loading on the surface and how it can tune the range of the to be detected target analyte. Lower probe loadings of the MRSA probe leads to a wider dynamic range and limit of detection as low as 350 fM. Finally, an optimized ion-rectifying apta-sensor for the pesticide imidacloprid was developed, and it was discovered that utilization of a dipping approach to modify nanopipette surface, as opposed to adding reagents to the back of the nanopipette, can improve sensitivity 100-fold and enable maximization of response at a specified target analyte concentration. Limits of detection as low as 2.4 pM were determined for this sensor system, and real-world applicability were demonstrated in lake-water and celery samples. Overall, the findings presented herein contribute towards our fundamental understanding of ion transport in nanoscale systems, and highlights how careful surface modification can be used to manipulate the output of ion-rectifying nanopipette sensors as well as provide fundamental insights into the ion transport properties of nanopipettes for a variety of sensing applications.