Nanopore sensing has emerged as a versatile approach to detecting and identifying biomolecules. Within this frame of reference, the fast-responding ionic current is considered an essential criterion for accurately measuring small objects with a nanopore.

An article published in the journal IScience discussed the role of ion diffusion kinetics at the liquid-electrode interface in nanopore sensing. Here, a slow and large reduction in ionic current through a nanopore was observed using platinum (Pt) electrodes in a salt solution, suggesting the significant influence of impedance generated at the metal-liquid interface via Cottrell diffusion.

During the nanoparticle detection, the resistive pulses became weak, followed by a constant increase in the resistance at the partially polarizable electrodes. Moreover, the interfacial impedance coupled with the nanopore chip capacitance degraded the ionic current’s temporal resolution in a time-varying manner. The findings of the present work can help choose the ideal size and material of electrodes for analyzing single particles and molecules by the ionic current.

Nanopore Towards Analyte Detection

Nanopore helps analyze biological samples at a single molecule level. Nanopore sensing is developing into a powerful label-free approach to investigating the features of biomolecules at the single-molecule level.

Here, the translocation of species residing within a nanopore effectively changes the physical and chemical properties of the nanopore interior (conductance or refractive index), detected in a label-free manner.

When a charged molecule is captured within a nanopore, it modulates the ionic current, which is recorded in real-time to reveal the properties of the target molecule. Thus, the nanopore serves as a conductometer that detects a relative change in ion flow at the nanoscale level.

Electrochemistry in a confined space has attracted significant interest because of the intriguing effects of nanoconfinement on mass transport, electrochemical kinetics, and electric field. The nanopore electrochemistry provides a powerful method to address scientific challenges in nanoscience, biochemistry, and energy conversion and storage.

Nanopores providing the electrochemically confined space for the accommodation of single analytes directly convert the single-molecule behaviors into the measurable electrochemical read-outs with a high signal-to-noise ratio.

In the nanopore-based electrochemical reaction, the electric current reveals the dynamics at the electrode-liquid interfaces. Here, the application of voltage results in the over-consumption of reactants, disturbing the local ion distribution and subsequently inducing motions in the bulk that ultimately leads to the relaxation of the acute ion concentration gradient near the electrode surface. The ionic current gradually declines due to the Cottrell diffusion, and its features reveal information about the nature of ions.

Role of Electrodes in Nanopore Sensing

In the present study, the resistive pulse measurements of various polymer nanoparticles were compared using different kinds of electrodes to investigate the relevance of Cottrell diffusion in nanopore sensing. The findings in the present work proved the role of electrode materials in nanopore sensing.

Using a silver (Ag)/ silver chloride (AgCl) electrode system prevented fluctuations in ionic current flow in chloride solution, which otherwise were associated with variation in concentration of reactants and products due to their adsorption or precipitation at the surface of the electrode. The persistent ionic current consequently helped in detecting the particles and molecules.

On the other hand, replacing Ag/AgCl with Pt electrodes resulted in different ionic current characteristics. Here, the open pore current (I_pore) showed a large decrease compared to Ag/AgCl electrodes. Moreover, unlike Ag/AgCl electrodes, the electrochemical reactions in the chloride solution involved no precipitation or adsorption of the reactants, which induced a growing interfacial impedance.

While using Ag resulted in a reduced I_pore and the resistive pulse heights over time, using a titanium (Ti) electrode resolved the issue by maintaining a stable ionic current and uniform height resistive pulses of the polystyrene nanoparticles, demonstrating the superior usefulness of Ti compared to Ag/AgCl for nanopore sensing.

Conclusion and Limitation of the Study

Overall, the results of this study demonstrated the significance of electrode materials in nanopore sensing. It has been demonstrated that Ag/AgCl is especially helpful for obtaining persistent ionic current in a chloride solution for reliable resistive pulse detections of particles and molecules.

Electrochemical reactions at the Pt surfaces, in contrast to those at non-polarizable electrodes, did not result in the precipitation or adsorption of reactants, resulting in an increased interfacial impedance.

It has been shown that this Cottrell diffusion-derived resistance significantly reduced the temporal resolution of ionic current measurements and altered the translocation dynamics of analytes in a time-varying manner, making it impossible to distinguish between analytes like viruses and proteins based on the differences in the ionic signal waveforms.

Although the present work demonstrated the roles of electrode materials, the study was restricted to only nanopores of 300-nanometer diameter. Moreover, since the smaller nanopores possess a larger resistor of resistance at the nanopore (R_pore), the role of Cottrell diffusion changes as the voltage division at the resistor of resistance at the electrode (R_ele) becomes smaller.