How can ion diffusion kinetics affect nano-hole sensing?

Nanopore sensing has emerged as a versatile approach for the discovery and identification of biomolecules. Within this frame of reference, a fast responsive ionic current is an essential criterion for accurately measuring small objects using a nanopore.

​​​​​​the study: Electrochemical ion diffusion interference in nanosensing. Image credit: Unwind / Shutterstock.com

Article published in the magazine IScience Discuss the role of ion diffusion kinetics at the electrode-liquid interface in nanoporous sensing. Here, a slow and significant decrease in the ionic current through a nanoporous hole was observed using platinum (Pt) electrodes in salt solution, indicating the significant effect of the resistance generated at the metal-liquid interface via Cottrell diffusion.

During nanoparticle detection, the impedance pulses become weak, followed by a steady increase in resistance at the partially polarizable electrodes. Moreover, the interfacial impedance coupled with the capacity of the nanochip degraded the temporal resolution of the ionic current in a time-varying manner. The results of the present work can help to choose the ideal size and material for the electrodes for the analysis of individual particles and particles by ionic current.

Nanopore toward detection analytics

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

Here, transfer of resident species inside a nano-hole alters the physical and chemical properties of the interior of the nanoparticle (conductivity or refractive index), which was detected in a label-free manner.

When a charged molecule is captured inside a nano-hole, it modulates the ionic current, which is recorded in real time to reveal the properties of the target molecule. Thus, the nano-hole acts as a conductance meter that detects a relative change in ion flux at the nanoscale level.

Confined space electrochemistry has attracted significant interest due to the intriguing effects of nanoscale effects on mass transport, electrochemical kinetics, and the electric field. Nanohole electrochemistry provides a powerful method for addressing scientific challenges in nanoscience, biochemistry, and energy conversion and storage.

The nanoswabs provide an electrochemically narrow space to accommodate single analytes, and directly convert single-molecule behaviors into measurable electrochemical readouts with a high signal-to-noise ratio.

In the nano-based electrochemical reaction, electric current reveals the dynamics at the electrode-liquid interfaces. Here, the application of voltage leads to an over-consumption of the reactants, which disturbs the local ion distribution and thus leads to bulk motions that eventually lead to the relaxation of the steep ion concentration gradient near the electrode surface. The ion current gradually decreases due to Cottrell diffusion, and its parameters reveal information about the nature of the ions.

The role of electrodes in sensing nanopores

In this study, impedance pulse measurements of several polymeric nanoparticles using different types of electrodes were compared to verify the importance of Cottrell diffusion in nanoporous sensing. The results in the present work prove the role of electrode materials in nanosensing.

The use of a silver (Ag)/silver chloride (AgCl) electrode system prevented fluctuations in ionic current flow in the chloride solution, which were otherwise related to variation in the concentration of reactants and products due to their adsorption or deposition on the electrode surface. Thus DC ionic current helped in the discovery of particles and molecules.

On the other hand, the replacement of Ag/AgCl with Pt electrodes resulted in different ionic current properties. Here, the open pore current (Ipore) significantly reduced compared to the Ag/AgCl electrodes. Moreover, in contrast to the Ag/AgCl electrodes, the electrochemical reactions in the chloride solution did not involve any precipitation or adsorption of the reactants, resulting in an increased interfacial resistance.

During the use of Ag led to a decrease in Ipore And the impedance pulse heights over time, using a titanium (Ti) electrode solved the problem by maintaining a constant ionic current and uniform spike impedance pulses for polystyrene nanoparticles, demonstrating the superior utility of Ti compared to Ag/AgCl for sensing nanopores.

Conclusion and limits of the study

Overall, the results of this study demonstrated the importance of electrode materials in nanosensing. It has been shown that Ag/AgCl is particularly useful for obtaining a stable ionic current in a chloride solution for detecting reliable impedance pulses of particles and molecules.

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

This resistance derived from Cottrell diffusion has been shown to significantly reduce the temporal resolution of ionic current measurements and alter the transmission dynamics of analytes in a time-varying manner, making it impossible to distinguish analytes such as viruses and proteins based on differences in ionic signal waveforms.

Although the present work demonstrated the roles of electrode materials, the study was limited to nanopores with a diameter of 300 nm. Moreover, since smaller nanopores possess greater impedance resistance at nanoporous perforation (R.pore), the role of the Cottrell diffusion changes as the voltage division at the resistor of the resistance at the electrode (Rhe is) becomes smaller.

reference

Leong, I.W., Kishimoto, S, Tsutsui, M, Taniguchi, M (2022). Electrochemical ion diffusion interference in nanosensing. IScience.

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