A Fourier Transform-Induced Data Process for Label-Free Selective Nanopore Analysis under Sinusoidal Voltage Excitations

Emerging Data Processing Methods for Single-Entity Electrochemistry

Single-entity electrochemistry is a powerful tool that enables the study of electrochemical processes at interfaces and provides insights into the intrinsic chemical and structural heterogeneities of individual entities. Signal processing is a critical aspect of single-entity electrochemical measurements and can be used for data recognition, classification, and interpretation. In this review, we summarize the recent five-year advances in signal processing techniques for single-entity electrochemistry and highlight their importance in obtaining high-quality data and extracting effective features from electrochemical signals, which are generally applicable in single-entity electrochemistry. Moreover, we shed light on electrochemical noise analysis to obtain single-molecule frequency fingerprint spectra that can provide rich information about the ion networks at the interface. By incorporating advanced data analysis tools and artificial intelligence algorithms, single-entity electrochemical measurements would revolutionize the field of single-entity analysis, leading to new fundamental discoveries.

Ion transport based structural description for in situ synthesized SBA-15 nanochannels in a sub-micropipette

Construction of nanoporous arrays can greatly facilitate their development in the fields of sensing, energy conversion, and nanofluidic devices. It is important to characterize the structure and understand the ion transport behaviour of a nanoporous array, especially those prepared by in situ synthesis, which are difficult to be characterized by conventional methods. Herein, an inorganic and non-crystalline mesoporous silica SBA-15 is selected as a template, where a combination (GP-SBA-15) of a sub-micropipette and SBA-15 is constructed by in situ synthesis, and the multichannel array structure of GP-SBA-15 is illustrated by its ion transport properties from current-voltage responses. Experiments of linear scan voltammetry and chronoamperometry show a rapid accumulation and slow redistribution of ions in the surface-charged nanochannels, and the high/low currents originate from the accumulation/depletion of ions in the channels. The finite element simulation is introduced to calculate the effects of surface charge and pore size on ion rectification and ion concentration distribution. In addition, the short straight channels and long bending channels present in GP-SBA-15 are demonstrated by the voltage-independent resistance pulse signals in the translocation of BSA. This study shows that electrochemical means effectively provide insight into ion transport, achieve structural description and reveal the sensing potential of GP-SBA-15.

Galvani Potential-Dependent Single Collision/Fusion Impacts at Liquid/Liquid Interface: Faradic or Capacitive?

A new subtype of nano-impacts by emulsion droplets via reorganization of the electric double layer (EDL) at the liquid/liquid interface (LLI) is reported. This subtype shows anodic, bipolar, and cathodic transient currents with a potential of zero charge (PZC) dependence, revealing the non-faradic characteristic of single fusion impacts. In addition, the absolute integrated mean charge is proportional to the Galvani potential at the ITIES, indicating that the EDL at the LLI may obey the discrete Helmholtz model. The exact PZC point is interpolated from the fitting curve, and the droplet size distribution is estimated from the integrated charge distribution. Moreover, the different values of E_pzc between single fusion impacts of MgCl₂ droplets and pure water droplets is due to the specific absorption between Mg²⁺ and antagonistic anion in the organic phase. The influence of the concentration of the supporting electrolyte is also investigated. The above work gives physicochemical insights into the EDL at the micropipette-supported LLI and provides potential application to measure micro/nanoscale heterogeneous media without catalytic, reactive, or charge-transfer activity via impact experiments at LLI.

Studies on the Morphology Effect on Catalytic Ability of a Single MnO2 Catalyst Particle with a Solid Nanopipette

Investigating the catalytic ability of an individual catalyst particle helps to understand heterogeneity and can provide new insights into the synthesis of high-efficiency catalysts. Solid-state nanopores have become a promising tool for detecting single molecules/particles due to their high temporal and spatial resolution. Here, we report a nanopore-based strategy for the evaluation and comparison of a single MnO₂ catalyst particle with different morphologies by monitoring the generated O₂ bubbles from the catalytic decomposition of H₂O₂. The finite element simulation was introduced to account for the flow velocity and bubble-induced current variation in the nanopore. In particular, the differences in catalytic ability of spherical and cubic MnO₂ have been studied by calculating the production rate and volume of O₂. It demonstrates that the shape of a single MnO₂ catalyst particle has a significant effect on its catalytic activity indeed.

A Guide to Signal Processing Algorithms for Nanopore Sensors

Nanopore technology holds great promise for a wide range of applications such as biomedical sensing, chemical detection, desalination, and energy conversion. For sensing performed in electrolytes in particular, abundant information about the translocating analytes is hidden in the fluctuating monitoring ionic current contributed from interactions between the analytes and the nanopore. Such ionic currents are inevitably affected by noise; hence, signal processing is an inseparable component of sensing in order to identify the hidden features in the signals and to analyze them. This Guide starts from untangling the signal processing flow and categorizing the various algorithms developed to extracting the useful information. By sorting the algorithms under Machine Learning (ML)-based versus non-ML-based, their underlying architectures and properties are systematically evaluated. For each category, the development tactics and features of the algorithms with implementation examples are discussed by referring to their common signal processing flow graphically summarized in a chart and by highlighting their key issues tabulated for clear comparison. How to get started with building up an ML-based algorithm is subsequently presented. The specific properties of the ML-based algorithms are then discussed in terms of learning strategy, performance evaluation, experimental repeatability and reliability, data preparation, and data utilization strategy. This Guide is concluded by outlining strategies and considerations for prospect algorithms.

Confined Synthesis of Silver Wire at the Nanopipette-Liquid/Liquid Interface

Herein, silver wire is synthesized electrochemically within a nanopipette using the nanopipette-liquid/liquid interface. The i-t curve characterizes the growth state of the silver wire. The higher rate of current increase indicates the faster electron transfer and the faster growth of the silver wire; conversely, when the current does not increase significantly with time, i.e., the rate of increase of the current is small, the growth rate of the silver wire is slow. The main driving force for the growth of silver into a linear structure is the theoretical current differential between the water and oil, caused by the concentration difference between the silver nitrate and ferrocene. The growth of the silver wire is also influenced by the shape of the nanopipette. If the diameter of the pipet increases quickly, silver wire tends to produce multibranched structures, while a smaller diameter makes it easier to obtain silver wire with fewer branches due to the confinement effect. This method is also applicable to the synthesis of gold within a nanopipette. The combination of nanopipette and metallic material using a liquid-liquid interface results in a broader application of nanopipettes for nanopore sensors, nanopore electrodes, bipolar electrodes, etc.