Toward the Detection and Identification of Single Bacteria by Electrochemical Collision Technique

Revisiting the catalytic activity of single horseradish peroxidase clusters through electrochemical collision technique: Effect of electrolyte and substrate

Horseradish peroxidase (HRP) is a versatile biosensing label and signal reporter owing to its broad-spectrum catalytic ability. In present work, we characterized HRP's catalytic performance with various substrates using electrochemical collision technique and analyzed the associated electron transfer processes. Different electrolyte solutions greatly affected enzyme dispersibility and zeta potential, thereby impacting HRP collision dynamics in single H2O2 substrate system. The maximum turnover number (kcat) for single HRP molecules was calculated to be 3.611 ± 0.149 × 103 s-1 in 0.85 % NaCl and 2.967 ± 0.286 × 103 s-1 in 0.1 M PBS solution, reflecting differences in cluster size induced by the electrolyte conditions. More severe agglomeration of HRP molecules was observed in double-substrate systems, where the hydrophilic mediator (K4Fe(CN)6) and lipophilic mediator (ABTS) served as electron donors and signal reporters. The calculated kcat value of single HRP molecules in ABTS-H2O2 was 7.6 times higher than that in K4Fe(CN)6-H2O2. This difference is attributed to mediators' solubility, lipophilicity, and HRP's affinity for different substrates, with HRP demonstrated stronger affinity for ABTS-H2O2 substrates, which realized more efficient electron transfer and compensated for the low diffusion coefficient of ABTS. This work provides a comprehensive analysis of the effects of electrolytes and substrates on HRP collision and catalytic behavior, offering valuable insights for the advanced design of HRP-based biosensors and diagnostic platforms.

Savitzky-Golay processing and bidimensional plotting of current-time signals from stochastic blocking electrochemistry to analyze mixtures of rod-shaped bacteria

In stochastic blocking electrochemistry, adsorptive collisions of nano and micro-particles with an ultramicroelectrode (UME) generate steps of decreasing current overlaid on the current-time (i-t) baseline of an electroactive mediator reacting at the UME. The step amplitude (Δi) induced by particle blockage informs about its size, while collision frequency correlates with particle transport. However, because most particles arrive at the UME faster than the acquisition speed of conventional electrochemical instruments, current steps appear vertical. Recently, when analyzing rod-shape bacteria (bacilli), we detected slanted steps of duration Δt (∼0.6 to 1.1 s) that were found to scale up with bacillus length (∼1 to 5 μm, respectively). In this work, we apply a Savitzky-Golay (SG) algorithm coded in MATLAB to convert experimental i-t recordings into derivative plots of Δi/Δt versus t. As a result, current steps become peaks on a flat baseline. Unlike the original values of Δi and Δt that require manual gauging, the coded SG-algorithm generates both parameters automatically from peak integration. We then display Δi and Δt in bidimensional scatter plots comparing mixtures of A. erythreum (∼1 μm) and B. subtilis (∼5 μm). The spread of Δi and Δt values complies with the size distribution observed using scanning electron microscopy. By introducing SG-processing and bidimensional plotting of i-t recordings from stochastic blocking data we broaden the scope of the technique. The approach facilitates distinguishing bacilli in mixtures because both Δt and Δi increase with bacillus length and now they can be displayed in a single graph along with adsorption frequency. Moreover, density distribution and proportion of data points from groups of bacteria are also discernible from the plots.

Analyzing bacterial detection and transport using redox impact electrochemistry

Understanding bacterial transport dynamics, particularly at the single-particle level, is crucial across diverse fields from environmental science to biomedical research. In recent times, the emerging impact electrochemistry method offers a transformative approach for detection of bacteria at the single-particle level. The method employs the principle of single-entity electrochemistry to scrutinize electrochemical processes during interaction with the working electrode. In this study, we utilized redox impact electrochemistry to detect bacteria and analyze their transport processes towards the working electrode. Stochastic detection using redox reactions at the ultramicroelectrode enabled the detection of individual bacteria, with collision resulting in a current spike signal due to charge transfer. Notably, the detection of bacteria was demonstrated at an exceptionally low concentration (100 CFU/mL), with recorded current spikes reaching approximately 8.1 nA. Analysis of integrated areas under these spikes unveiled a diverse distribution of charge transfer at the ultramicroelectrode during redox reactions, implying variations in bacterial sizes, collision positions on the electrode surface, and redox activity among bacteria. Remarkably, the average charge transfer per bacterium between E. coli and the electrode was found to be (244 ± 24) pC, underscoring the intrinsic redox activity of the bacteria, equivalent to (2.52 ± 0.25) × 10-15 mol. Additionally, our investigation explored the effects of cell transport mechanisms, including diffusion, migration, convection, and settlement on stochastic interactions of the bacteria at the ultramicroelectrode. Through the collision frequency calculations, we found that migration is the primary factor shaping bacterial transport, with gravitational cell settlement also exerting a significant influence.

Enhancing low-concentration cell detection in single entity electrochemical systems through forced convection

This study advances the detection of bacteria at low concentrations in single-entity electrochemistry (SEE) systems by integrating forced convection. Our results show that forced convection significantly improves the mass transfer rate of electrolyte, with the mass transfer coefficient demonstrating a proportional relationship to the flow rate to the power of 1.37. Notably, while the collision frequency of E. coli initially increases with the flow rate, a subsequent decrease is observed at higher rates. This pattern is attributed to the mechanics of cell collision under forced convection. Specifically, while forced convection propels cells towards the ultra-microelectrode (UME), it does not aid in their penetration through the boundary layer, leading to cells being driven away from the UME at higher flow rates. This hypothesis is supported by the statistical analysis of collision data, including signal heights and rise times. By optimizing the flow rate to 2 mL/min, we achieved enhanced detection of E. coli in concentrations ranging from 0.9 × 107 to 5.0 × 107 cells/mL. This approach significantly increased collision frequency by elevating the mass transfer of cells, with the mass transfer coefficient rising from 0.1 × 10-5 m/s to 0.9 × 10-5 m/s. It provides a viable solution to the challenges of detecting bacteria at low concentrations in SEE systems.

Average collision velocity of single yeast cells during electrochemically induced impacts

We recorded current-time (i-t) profiles for oxidizing ferrocyanide (FCN) while spherical yeast cells of radius (rc ≈ 2 μm) collided with disk ultramicroelectrodes (UMEs) of increasing radius (re ≈ 12-45 μm). Collision signals appear as minority steps and majority blips of decreased current overlayed on the i-t baseline when cells block ferrocyanide flux (JFCN). We assigned steps to adsorption events and blips to bouncing collisions or contactless passages. Yeast cells exhibit impact signals of long duration (Δt ≈ 15-40 s) likely due to sedimentation. We assume cells travel a threshold distance (T) to generate collision signals of duration Δt. Thus, T represents a distance from the UME surface, at which cell perturbations on JFCN blend in with the UME noise level. To determine T, we simulated the UME current, while placing the cell at increasing distal points from the UME surface until matching the bare UME current. T-Values at 90°, 45°, and 0° from the UME edge and normal to the center were determined to map out T-regions in different experimental conditions. We estimated average collision velocities using the formula T/Δt, and mimicked cells entering and leaving T-regions at the same angle. Despite such oversimplification, our analysis yields average velocities compatible with rigorous transport models and matches experimental current steps and blips. We propose that single-cells encode collision dynamics into i-t signals only when cells move inside the sensitive T-region, because outside, perturbations of JFCN fall within the noise level set by JFCN and rc/re (experimentally established). If true, this notion will enable selecting conditions to maximize sensitivity in stochastic blocking electrochemistry. We also exploited the long Δt recorded here for yeast cells, which was undetectable for the fast microbeads used in early pioneering work. Because Δt depends on transport, it provides another analytical parameter besides current for characterizing slow-moving cells like yeast.

Effect of cell settlement on the electrochemical collision behaviors of single microbes

Electrochemical collision technique has emerged as a powerful approach to detect the intrinsic properties of single entities. The diffusion model, together with migration and convection processes are generally used to describe the transport and collision processes of single entities. However, things become more complicated concerning microbes because of their relatively large size, inherent motility and biological activities. In this work, the electrochemical collision behaviors of four different microorganisms: Escherichia coli (Gram-negative bacteria), Staphylococcus aureus, Bacillus subtilis (Gram-positive bacteria) and Saccharomyces cerevisiae (fungus) were systematically detected and compared using a blocking strategy. By using K4Fe(CN)6 as redox probe, the downwards step-like signals were recorded in the collision process of all the three bacteria, whereas the collision of S. cerevisiae was rarely detected. To further investigate the underlying reason for the abnormal collision behavior of S. cerevisiae, the effect of cell settlement was discussed. The results indicated that ellipsoidal S. cerevisiae with a cell size larger than 2 μm exhibited a cell sedimentation rate of 261.759 nm s-1, which is dozens of times higher than the other three bacteria. By further enhanced convection near the microelectrode or positioned the microelectrode at the bottom of electrochemical cell, the collision signals of S. cerevisiae were successfully detected, indicating cell sedimentation is a nonnegligible force in large cell transport. This study fully addressed the effect of cell settlement on the transport of microbial cells and provided two strategies to counteract this effect, which benefit for the deeper understanding and further application of electrochemical collision technique in single-cell detection.

When microplastics meet electroanalysis: future analytical trends for an emerging threat

Microplastics are a major modern challenge that must be addressed to protect the environment, particularly the marine environment. Microplastics, defined as particles ≤5 mm, are ubiquitous in the environment. Their small size for a relatively large surface area, high persistence and easy distribution in water, soil and air require the development of new analytical methods to monitor their presence. At present, the availability of analytical techniques that are easy to use, automated, inexpensive and based on new approaches to improve detection remains an open challenge. This review aims to outline the evolution and novelties of classical and advanced methods, in particular the recently reported electroanalytical detectors, methods and devices. Among all the studies reviewed here, we highlight the great advantages of electroanalytical tools over spectroscopic and thermal analysis, especially for the rapid and accurate detection of microplastics in the sub-micron range. Finally, the challenges faced in the development of automated analytical methods are discussed, highlighting recent trends in artificial intelligence (AI) in microplastics analysis.

A portable smartphone-based colorimetric sensor that utilizes dual amplification for the on-site detection of airborne bacteria

Over the past few years, infections caused by airborne pathogens have spread worldwide, infecting several people and becoming an increasingly severe threat to public health. Therefore, there is an urgent need for developing airborne pathogen monitoring technology for use in confined environments to enable epidemic prevention. In this study, we designed a colorimetry-based bacterial detection platform that uses a clustered regularly interspaced short palindromic repeat-associated protein 12a system to amplify signals and a urease enzyme to induce color changes. Furthermore, we have developed a smartphone application that can distinguish colors under different illumination conditions based on the HSV model and detect three types of disease-causing bacteria. Even synthetic oligomers of a few picomoles of concentration and genomic DNA of airborne bacteria smaller than several nanograms can be detected with the naked eye and using color analysis systems. Furthermore, in the air capture model system, the bacterial sample generated approximately a 2-fold signal difference compared with that in the control group. This colorimetric detection method can be widely applied for public safety because it is easy to use and does not require complex equipment.

Sensitive quantification of mercury ions in real water systems based on an aggregation-collision electrochemical detection

Nanoparticle impact electrochemistry (NIE) is an emerging electroanalytical technique that has been utilized to the sensitive detection of a wide range of biological species. So far, the NIE based trace ion detection is largely unexplored due to the lack of effective signal amplification strategies. We herein develop an NIE-based electrochemical sensing platform that utilizes T-Hg2+-T coordination induced AgNP aggregation to detect Hg2+ in aqueous solution. The proposed aggregation-collision strategy enables highly sensitive and selective detection. A dual-mode analysis based on the change in impact frequency and oxidative charge of the anodic oxidation of the AgNPs in NIE allows for more accurate self-validated quantification. Furthermore, the current NIE-based sensor demonstrates reliable analysis of Hg2+ of real water samples, showing great potential for practical environmental monitoring and point-of-care testing (POCT) applications.

Label-Free Detection of Single Living Bacteria: Single-Entity Electrochemistry Targeting Metabolic Products

This study presents a novel approach employing single-entity electrochemistry for the label-free detection of living Escherichia coli. By examination of the collision signals generated from the reduction of hydrogen peroxide, a metabolic product of E. coli that accumulates on the cell surface, the concentration of living bacteria can be determined. Within a broad concentration range from 3.0 × 107 to 1.0 × 109 cells/mL, cell aggregation was not observed. Cell migration in the solution was primarily governed by diffusion, exhibiting a diffusion coefficient of 6.8 × 10-9 cm2/s. The collision frequency exhibits a linear relationship with the cell concentration, aligning well with theoretical predictions. Through statistical analysis of each collision signal's integrated charge quantity, the metabolic activity of single cells can be assessed. This method was applied to a cytotoxicity assay, where it monitored the decline in living cell numbers and metabolic activities in addition to identifying potential cell damage during antibiotic treatment.

Crystalline silica particle functionalized by PEG for its collision-enhanced detection at ultramicroelectrode

Detecting individual particulate matter is highly significant in many areas, such as mine safety, environment, and human health. The analytical method based on single entity electrochemistry (SEE) has shown great potential in detecting, counting, and measuring individual particles, especially conductive metals or carbon particles, based on their unique charge transfer reactions at an ultramicroelectrode (UME). In this study, we report an innovative SEE method for improving the sensitivity of the detection of electrochemical inert crystalline silica particles by functionalizing silica particles with polyethylene glycol (PEG) molecules. The PEG surface functionalization of the silica was characterized by Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) techniques. The morphology of silica particles was characterized by a scanning electron microscope (SEM), and a transmission electron microscope (TEM) was employed to calibrate size distribution and determine the elemental composition of silica particles. The surface charges of silica particles were measured by dynamic light scattering techniques. The collision behaviors of crystalline silica particles with UME were investigated by cyclic voltammetric experiments, which are rarely reported in the literature. The crystalline silica particles were detected based on electrochemically blocking the flux of the redox mediator at the surface of UME, which showed significant signal amplification in the proposed method. Our method was demonstrated for detecting crystalline silica functionalized with or without PEG, acquiring the limit of quantification (LOQ) values of 0.391 μM (23.45 μg/L) and 0.824 μM (49.45 μg/L), respectively, which confirmed that a more than two times improvement in LOQ could be achieved over the PEG functionalized silica particles. We further presented a theoretical model using finite element simulations with COMSOL Multiphysics. We deduced a quantitative relation between the distribution of the current step size and the size distribution of silica particles. Therefore, the reported method here provides a paradigm for SEE-based detection of electrochemically inert crystalline silica particles, which extends the previous report substantially concerning particle detection.

Impedimetric nano-collision Escherichia coli analysis based on Silver-Gold bimetallic nanoparticles

An impedimetric detection of E. coli was developed using chemically synthesised bimetallic Ag-Au (1:2) nanoparticles (NPs). The UV-visible spectra of the NPs had absorption bands at 470 and 580 nm for Ag NPs and Au NPs, respectively. In the presence of E. coli, a negative potential shift and a blue shift was observed in the voltammograms and spectra respectively. The complex formed had an oxidation potential at + 0.95 V. Technique choice was based on sensitivity comparison of Differential pulse voltammetry, cyclic voltammetry and impedance spectroscopy in 0.1 M PBS with Impedance being the best choice. Optimum sensing conditions of the NPs-E. coli complex for NPs concentration, incubation period, method modulation amplitude and applied potential were 5 mM, 20 min, 10 mV and + 0.5 V, respectively. The sensor's linearity range, lower limits of detection and quantification were found to be 10¹-10⁷, 1.88 × 10¹, 2.34 × 10² cells/mL, respectively. The sensor's applicability was validated by repeatability, stability and selectivity studies showing minimum changes in signal. Potential usage of the sensor in real samples was demonstrated by standard addition analysis of sea and River water samples as well as recovery of spiked water and fruit juices with acceptable % RSD < 2%.

Enhanced Single-Particle Collision Electrochemistry at Polysulfide-Functionalized Microelectrodes for SARS-CoV-2 Detection

Single-particle collision electrochemistry (SPCE) has shown great promise in biosensing applications due to its high sensitivity, high flux, and fast response. However, a low effective collision frequency and a large number of interfering substances in complex matrices limit its broad application in clinical samples. Herein, a novel and universal SPCE biosensor was proposed to realize sensitive detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) based on the collision and oxidation of single silver nanoparticles (Ag NPs) on polysulfide-functionalized gold ultramicroelectrodes (Ps-Au UMEs). Taking advantage of the strong interaction of the Ag-S bond, collision and oxidation of Ag NPs on the Ps-Au UME surface could be greatly promoted to generate enhanced Faraday currents. Compared with bare Au UMEs, the collision frequency of Ps-Au UMEs was increased by 15-fold, which vastly improved the detection sensitivity and practicability of SPCE in biosensing. By combining magnetic separation, liposome encapsulation release, and DNAzyme-assisted signal amplification, the SPCE biosensor provided a dynamic range of 5 orders of magnitude for spike proteins with a detection limit of 6.78 fg/mL and a detection limit of 21 TCID50/mL for SARS-CoV-2. Furthermore, SARS-CoV-2 detection in nasopharyngeal swab samples of infected patients was successfully conducted, indicating the potential of the SPCE biosensor for use in clinically relevant diagnosis.

The Microelectrode Insulator Influences Water Nanodroplet Collisions

Studying chemical reactions in very small (attoliter to picoliter) volumes is important in understanding how chemistry proceeds at all relevant scales. Stochastic electrochemistry is a powerful tool to study the dynamics of single nanodroplets, one at a time. Perhaps the most conceptually simple experiment is that of the current blockade, where the collision of an insulating particle is observed electrochemically as a stepwise decrease in current. Here, we demonstrate that nanodroplet collisions on microelectrodes are not as simple as water droplets adsorbing to the electrode to block current and that the environment immediately around the microelectrode (glass insulator) plays a pivotal role in the electrochemical collision response. We use correlated opto-electrochemical measurements to understand a variety of electrochemical responses when water nanodroplets collide with a microelectrode during the heterogeneous oxidation of decamethylferrocene in oil. The amperometric current reports not only on current blockades but also on nanodroplet coalescence events and preferential wetting to the glass around the microelectrode. Treating the glass with dichlorodimethylsilane creates a hydrophobic environment around the working electrode, and the simple current blockade response expected from the absorption of insolating nanoparticles is observed. These results highlight the importance of the environment around the working electrode for nanodroplet collision studies.

NIR as a "trigger switch" for situ distinguish superbacteria and photothermal synergistic antibacterial treatment with Ag2O particles/lignosulfonate/cationic guar gum hybrid hydrogel

The in situ identification of superbugs with the simultaneous killing of it is key to preventing human health. Here, a one-stop identification and killing platform for near-infrared (NIR) triggering was designed and constructed using lignosulfonate (LS), cationic guar gum (CG) and Ag₂O NPs hydrogels (LS/CG/Ag₂O). The hydrogel network is used as a fixed matrix for Ag₂O NPs and a nano reactor, meanwhile 3,3', 5,5'-tetramethylbenzidine (TMB) as a single probe sensor array for bacterial identification. In contrast to conventional methods, hybrid hydrogels have catalytic qualities through which TMB be catalyzed to generate oxidized TMB (oxTMB). The drug resistance of the same strain can be distinguished based on the different inhibition abilities of drug-resistant superbacteria in TMB and hydrogel reactions. Then, the employing of oxTMB photothermal characteristics, it can be efficiently killed in real time while being driven by a near-infrared laser. The proposed one-stop hydrogel platform paves a way for the rapid identification and killing of drug-resistant superbacteria.

Trends in single-impact electrochemistry for bacteria analysis

Single-impact electrochemistry for the analysis of bacteria is a powerful technique for biosensing applications at the single-cell scale. The sensitivity of this electro-analytical method has been widely demonstrated based on chronoamperometric measurements at an ultramicroelectrode polarized at the appropriate potential of redox species in solution. Furthermore, the most recent studies display a continuous improvement in the ability of this sensitive electrochemical method to identify different bacterial strains with better selectivity. To achieve this, several strategies, such as the presence of a redox mediator, have been investigated for detecting and identifying the bacterial cell through its own electrochemical behavior. Both the blocking electrochemical impacts method and electrochemical collisions of single bacteria with a redox mediator are reported in this review and discussed through relevant examples. An original sensing strategy for virulence factors originating from pathogenic bacteria is also presented, based on a recent proof of concept dealing with redox liposome single-impact electrochemistry. The limitations, applications, perspectives, and challenges of single-impact electrochemistry for bacteria analysis are briefly discussed, based on the most significant published data.

Homogeneous Electrochemical Immunoassay Using an Aggregation-Collision Strategy for Alpha-Fetoprotein Detection

Homogeneous immunoassays represent an attractive alternative to traditional heterogeneous assays due to their simplicity and high efficiency. Homogeneous electrochemical assays, however, are not commonly accessed due to the requirement of electrode immobilization of the recognition elements. Herein, we demonstrate a new homogeneous electrochemical immunoassay based on the aggregation-collision strategy for the quantification of tumor protein biomarker alpha-fetoprotein (AFP). The detection principle relies on the aggregation of AgNPs induced by the molecular biorecognition between AFP and AgNPs-anti-AFP probes, which leads to an increased AgNP size and decreased AgNP concentration, allowing an accurate self-validated dual-mode immunoassay by performing nanoimpact electrochemistry (NIE) of the oxidation of AgNPs. The intrinsic one-by-one analytical capability of NIE as well as the participation of all of the atoms of the AgNPs in signal transduction greatly elevates the detection sensitivity. Accordingly, the current sensor enables a limit of detection (LOD) of 5 pg/mL for AFP analysis with high specificity and efficiency. More importantly, reliable detection of AFP in diluted human sera of hepatocellular carcinoma (HCC) patients is successfully achieved, indicating that the NIE-based homogeneous immunoassay shows great potential in HCC liquid biopsy.

Influence of Charged Self-Assembled Monolayers on Single Nanoparticle Collision

Studying nanoparticle (NP)-electrode interactions in single nanoparticle collision events is critical to understanding dynamic processes such as nanoparticle motion, adsorption, oxidation, and catalytic activity, which are abundant on electrode surfaces. Herein, NP-electrode electrostatic interactions are studied by tracking the oxidation of AgNPs at Au microelectrodes functionalized with charged self-assembled monolayers (SAMs). Tuning the charge of short alkanethiol-based monolayers and selecting AgNPs that can be partially or fully oxidized upon impact enabled probing the influence of attractive and repulsive NP-electrode electrostatic interactions on collision frequency, electron transfer, and nanoparticle sizing. We find that repulsive electrostatic interactions lead to a significant decrease in collision frequency and erroneous nanoparticle sizing. In stark difference, attractive electrostatic interactions dramatically increase the collision frequency and extend the sizing capability to larger nanoparticle sizes. Thus, these findings demonstrate how NP-monolayer interactions can be studied and manipulated by combining nanoimpact electrochemistry and functionalized SAMs.

Micro- and nano-devices for electrochemical sensing

Electrode miniaturization has profoundly revolutionized the field of electrochemical sensing, opening up unprecedented opportunities for probing biological events with a high spatial and temporal resolution, integrating electrochemical systems with microfluidics, and designing arrays for multiplexed sensing. Several technological issues posed by the desire for downsizing have been addressed so far, leading to micrometric and nanometric sensing systems with different degrees of maturity. However, there is still an endless margin for researchers to improve current strategies and cope with demanding sensing fields, such as lab-on-a-chip devices and multi-array sensors, brain chemistry, and cell monitoring. In this review, we present current trends in the design of micro-/nano-electrochemical sensors and cutting-edge applications reported in the last 10 years. Micro- and nanosensors are divided into four categories depending on the transduction mechanism, e.g., amperometric, impedimetric, potentiometric, and transistor-based, to best guide the reader through the different detection strategies and highlight major advancements as well as still unaddressed demands in electrochemical sensing.

Graphical Abstract

Electronic Circuit Simulations as a Tool to Understand Distorted Signals in Single-Entity Electrochemistry

Electrochemical analysis relies on precise measurement of electrical signals, yet the distortions caused by potentiostat circuitry and filtering are rarely addressed. Elucidation of these effects is essential for gaining insights behind sensitive low-current and short-duration electrochemical signals, e.g., in single-entity electrochemistry. We present a simulation approach utilizing the Electrical Simulation Program with Integrated Circuit Emphasis (SPICE), which is extensively used in electronic circuit simulations. As a proof-of-concept, we develop a universal electrical circuit model for single nanoparticle impact experiments, incorporating potentiostat and electronic filter circuitry. Considering these alterations, the experimentally observed transients of silver nanoparticle oxidation were consistently shorter and differently shaped than those predicted by established models. This reveals the existence of additional processes, e.g., migration, partial or asymmetric oxidation. These results highlight the SPICE approach's ability to provide valuable insights into processes occurring during single-entity electrochemistry, which can be applied to various electrochemical experiments, where signal distortions are inevitable.

Clinically Applicable Homogeneous Assay for Serological Diagnosis of Alpha-Fetoprotein by Impact Electrochemistry

Tumor protein quantification with high specificity, sensitivity, and efficiency is of great significance to enable early diagnosis and effective treatment. The existing methods for protein analysis usually suffer from high cost, time-consuming operation, and insufficient sensitivity, making them not clinically friendly. In this work, a label-free homogeneous sensor based on the nano-impact electroanalytic (NIE) technique was proposed for the detection of tumor protein marker alpha-fetoprotein (AFP). The detection principle is based on the recovery of current of single PtNP catalyzed hydrazine oxidation due to the release of the pre-adsorbed passivating aptamers on PtNPs from the competition of the stronger binding between the specific interaction of the AFP aptamer and AFP. The intrinsic one-by-one analytical ability of NIE allows highly sensitive detection, which can be further improved by reducing the reaction/incubation volume. Meanwhile, the current sensor avoids a laborious labeling procedure as well as the separation and washing steps due to the in situ characteristic of NIE. Accordingly, the current sensor enables efficient, highly sensitive, and specific AFP analysis. More importantly, the reliable detection of AFP in diluted real sera from hepatocellular carcinoma (HCC) patients is successfully achieved, indicating that the impact electrochemistry-based sensing platform has great potential to be applied in point-of-care devices for HCC liquid biopsy.

Rapid Quantitative Detection of Live Escherichia coli Based on Chronoamperometry

The rapid quantitative detection of Escherichia coli (E. coli) is of great significance for evaluating water and food safety. At present, the conventional bacteria detection methods cannot meet the requirements of rapid detection in water environments. Herein, we report a method based on chronoamperometry to rapidly and quantitatively detect live E. coli. In this study, the current indicator i0 and the electricity indicator A were used to record the cumulative effect of bacteria on an unmodified glassy carbon electrode (GCE) surface during chronoamperometric detection. Through the analysis of influencing factors and morphological characterization, it was proved that the changes of the two set electrochemical indicator signals had a good correlation with the concentration of E. coli; detection time was less than 5 min, the detection range of E. coli was 104−108 CFU/mL, and the error range was <30%. The results of parallel experiments and spiking experiments showed that this method had good repeatability, stability, and sensitivity. Humic acid and dead cells did not affect the detection results. This study not only developed a rapid quantitative detection method for E. coli in the laboratory, but also realized a bacterial detection scheme based on the theory of bacterial dissolution and adsorption for the first time, providing a new direction and theoretical basis for the development of electrochemical biosensors in the future.

Magnetic Rolling Circle Amplification-Assisted Single-Particle Collision Immunosensor for Ultrasensitive Detection of Cardiac Troponin I

Owing to its simplicity, high throughput, and ultrasensitivity, single-particle collision electrochemistry (SPCE) has attracted great attention in biosensing, especially labeled SPCE. However, the low signal conversion efficiency and much interference from complex samples limit its wide application. Here, a new and robust SPCE immunosensor was proposed for ultrasensitive cardiac troponin I (cTnI) detection by combining target-driven rolling circle amplification (RCA) with magnetic beads (MBs). Antibody-modified MBs have good stability, dispersity, and magnetic response capacity in complex samples, enabling efficient capture and separation of cTnI with high specificity and anti-interference ability. The presence of cTnI could specifically drive the formation of magnetic immunocomplexes followed by triggering RCA and enzyme digestion reaction. By using Pt nanoparticles (Pt NPs)-modified ssDNA as signal probes, one cTnI molecule could induce the release of 4.5 × 10⁴ Pt NPs for collision experiments, greatly enhancing signal conversion efficiency and detection sensitivity. Based on the integration of MBs with RCA, the SPCE immunosensor realized 0.57 fg/mL cTnI detection with a wide linear range of 1 fg/mL to 50 ng/mL. Furthermore, cTnI detection in serum samples of myocardial infarction patients was successfully performed, demonstrating great application prospect of the SPCE immunosensor in clinical diagnosis.

Smartphone-based surface plasmon resonance sensing platform for rapid detection of bacteria

Bacterial infection poses severe threats to public health, and early rapid detection of the pathogen is critical for controlling bacterial infectious diseases. Current methods are commonly labor intensive, time consuming or dependent on lab-based equipment. In this study, we proposed a novel and practical method for bacterial detection based on smartphones using the surface plasmon resonance (SPR) phenomena of gold nanoparticles (AuNPs). The proposed smartphone-based SPR sensing method is achieved by utilizing color development that arises from the change in interparticle distance of AuNPs induced by bacterial lysate. The pictures of bacteria/AuNPs color development were captured, and their color signals were acquired through a commercial smartphone. The proposed method has a detection range between 2.44 × 10⁵ and 1.25 × 10⁸ cfu mL⁻¹ and a detection limit of 8.81 × 10⁴ cfu mL⁻¹. Furthermore, this method has acceptable recoveries (between 85.7% and 95.4%) when measuring spiked real waters. Combining smartphone-based signal reading with AuNP-dependent color development also offers the following advantages: easy-to-use, real-time detection, free of complex equipment and low cost. In view of these features, this sensing platform would have widespread applications in the fields of medical, food, and environmental sciences.

In this study, we propose a novel and practical method for bacterial detection based on smartphones using the surface plasmon resonance (SPR) phenomena of gold nanoparticles (AuNPs).

Single Probe-Based Chemical-Tongue Sensor Array for Multiple Bacterial Identification and Photothermal Sterilization in Real Time

Simple and efficient identification of multiple bacteria and sterilization in real time is of considerable significance for clinical diagnostics and quality control in food. Herein, a novel chemical-tongue sensor array with 3,3',5,5'-tetramethylbenzidine (TMB) as a single probe was developed for bacterial identification and photothermal elimination. The synthesized bimetallic palladium/platinum nanoparticles (Pd/Pt_NPs) present excellent catalytic capability that can catalyze TMB into oxidized TMB (oxTMB) with four feature absorption peaks. Bacteria have different ability on inhibiting the reaction between TMB and Pd/Pt_NPs. With the absorbance intensity of oxTMB at the four feature peaks as readout, nine kinds of bacteria including two drug-resistant bacteria can be successfully distinguished via linear discriminant analysis. Remarkably, oxTMB exhibits excellent photothermal properties and can effectively kill bacteria in real time under near-infrared laser irradiation. The strategy of selecting TMB as a single probe simplifies the experimental operation and reduces the time cost. Furthermore, the developed sensing system was used to promote the wound healing process of MRSA-infected mice in vivo. The investigation provides a promising simple and efficient strategy for bacterial identification and sterilization with a universal platform, which has great potential application in clinical diagnosis and therapy.

Electrochemical Sensors for Antibiotic Susceptibility Testing: Strategies and Applications

Increasing awareness of the impacts of infectious diseases has driven the development of advanced techniques for detecting pathogens in clinical and environmental settings. However, this process is hindered by the complexity and variability inherent in antibiotic-resistant species. A great deal of effort has been put into the development of antibiotic-resistance/susceptibility testing (AST) sensors and systems to administer proper drugs for patient-tailored therapy. Electrochemical sensors have garnered increasing attention due to their powerful potential to allow rapid, sensitive, and real-time monitoring, alongside the low-cost production, feasibility of minimization, and easy integration with other techniques. This review focuses on the recent advances in electrochemical sensing strategies that have been used to determine the level of antibiotic resistance/susceptibility of pathogenic bacteria. The recent examples of the current electrochemical AST sensors discussed here are classified into four categories according to what is detected and quantitated: the presence of antibiotic-resistant genes, changes in impedance caused by cell lysis, current response caused by changes in cellular membrane properties, and changes in the redox state of redox molecules. It also discusses potential strategies for the development of electrochemical AST sensors, with the goal of broadening their practical applications across various scientific and technological fields.

Electrochemical and spectroelectrochemical characterization of bacteria and bacterial systems

Microbes, such as bacteria, can be described, at one level, as small, self-sustaining chemical factories. Based on the species, strain, and even the environment, bacteria can be useful, neutral or pathogenic to human life, so it is increasingly important that we be able to characterize them at the molecular level with chemical specificity and spatial and temporal resolution in order to understand their behavior. Bacterial metabolism involves a large number of internal and external electron transfer processes, so it is logical that electrochemical techniques have been employed to investigate these bacterial metabolites. In this mini-review, we focus on electrochemical and spectroelectrochemical methods that have been developed and used specifically to chemically characterize bacteria and their behavior. First, we discuss the latest mechanistic insights and current understanding of microbial electron transfer, including both direct and mediated electron transfer. Second, we summarize progress on approaches to spatiotemporal characterization of secreted factors, including both metabolites and signaling molecules, which can be used to discern how natural or external factors can alter metabolic states of bacterial cells and change either their individual or collective behavior. Finally, we address in situ methods of single-cell characterization, which can uncover how heterogeneity in cell behavior is reflected in the behavior and properties of collections of bacteria, e.g. bacterial communities. Recent advances in (spectro)electrochemical characterization of bacteria have yielded important new insights both at the ensemble and the single-entity levels, which are furthering our understanding of bacterial behavior. These insights, in turn, promise to benefit applications ranging from biosensors to the use of bacteria in bacteria-based bioenergy generation and storage.

Particle Impact Electrochemistry

Experiments involving collisions between a single entity and the electrode surface have become an active area of research. The electrochemical contribution of individual nanoparticles (NPs), enzymes, and other entities, such as aggregates or agglomerates, can be determined using particle impact experiments. Destructive nanoimpact experiments of materials, such as Ag, and the electrocatalytic amplification (ECA) are used to detect the NP/electrode interactions. This review covers the seminal work, critical theoretical studies, and some recent applications. The applications to electrocatalysis include measurements of electron transfer rate constants on individual nanoparticles. Applications in analytical chemistry have allowed the detection of nonelectroactive species by detecting the collisions of soft materials, e.g. micellar suspensions and proteins have increased the technique's analytical possibilities. With ECA, NPs can be used as tags for the electrochemical detection of bioanalytes such as DNA, proteins, and liposomes. The theory of ECA collisions, including frequency of collision and the size of the electrochemical current transients, are also covered. For nanoimpacts, the charge measured during a NP electrolysis, such as Ag NP, is used to detect the NP. Measurements of NP diameter are possible, but limitations to this analysis are covered. The electron transfer studies to the electrolysis of Ag and of metal oxides are discussed. Finally, key experimental instrumentations are discussed, including instrumentation techniques for the small currents inherent to single NP measurement. The effect of filtering, instrumentations rise time, and sampling frequency are also covered.

Electrochemical observation of individual collision-blocking events of TX-100 nanomicelles: An accurate and universal approach for the critical micelle concentration determination of surfactants

The electrochemical collision-blocking technique, equipped with the nanoelectrode of Pt was proposed for determination of the critical micelle concentration (CMC) of non-ionic surfactant TX-100. The approach was found on detection of individual collided nanomicelles in amperometric measurements of the oxidation of K₄Fe(CN)₆ varying the titrated concentration of TX-100 whereas the formed micelles above the CMC stick on the electrode surface during collision to locally block the flux of electroactive species and further to change the faradaic current. The step-like current transients observed in i-t curves have been demonstrated corresponding to electrochemical collision events of individual TX-100 micelles and micelle aggregates by 3D COMSOL simulations. The logarithm relations between the collision frequency of micelle(s) and the concentration of TX-100 were derived by regression analysis to give the corresponding values of CMC in salt solutions. Further, an 'ideal' CMC of TX-100 without influence of additional salts was estimated to be 0.194 mM using the McDevit-Long theory. The more accurate CMC determined in this work has shown less than the previously reported, mainly due to the detection limit for micelle as low as 0.41 fM. Also, we determined the second CMC of 1.21 mM as the first observation of the collision response of micelle aggregates during TX-100 titration. Owing to its analytical characteristics in single-particle tracking and material insensitivity, the approach we proposed is potentially to be a universal tool for accurate determination of CMC of surfactants, and also for studying the formation of polymer particles at a single-particle level, which is not easily accessible using conventional ensemble measurements.

Rapid and Accurate Data Processing for Silver Nanoparticle Oxidation in Nano-Impact Electrochemistry

In recent years, nano-impact electrochemistry (NIE) has attracted widespread attention as a new electroanalytical approach for the analysis and characterization of single nanoparticles in solution. The accurate analysis of the large volume of the experimental data is of great significance in improving the reliability of this method. Unfortunately, the commonly used data analysis approaches, mainly based on manual processing, are often time-consuming and subjective. Herein, we propose a spike detection algorithm for automatically processing the data from the direct oxidation of sliver nanoparticles (AgNPs) in NIE experiments, including baseline extraction, spike identification and spike area integration. The resulting size distribution of AgNPs is found to agree very well with that from transmission electron microscopy (TEM), showing that the current algorithm is promising for automated analysis of NIE data with high efficiency and accuracy.

Modern Sensing Approaches for Predicting Toxicological Responses of Food- and Drug-Based Bioactives on Microbiomes of Gut Origin

Recent scientific findings have correlated the gut microbes with homeostasis of human health by delineating their role in pathogen resistance, bioactive metabolization, and immune responses. Foreign materials, like xenobiotics, that induce an altering effect to the human body also influence the gut microbiome to some extent and often limit their use as a result of significant side effects. Investigating the xenobiotic effect of new therapeutic material or edible could be quite painstaking and economically non-viable. Thus, the use of predictive toxicology methods can be an innovative strategy in the food, pharma, and agriculture industries. There are reported in silico, ex vivo, in vitro, and in vivo methods to evaluate such effects but with added drawbacks, such as lower predictability, physiological dissimilarities, and high cost of associated invasive procedures. This review highlights the current and future possibilities with newer modern sensing approaches of economic and time-scale advantages for predicting toxicological responses on gut microbiomes.

Dynamics of Collisions and Adsorption in the Stochastic Electrochemistry of Emulsion Microdroplets

Current-time recordings of emulsified toluene microdroplets containing 20 mM Ferrocene (Fc), show electrochemical oxidation peaks from individual adsorption events on disk microelectrodes (5 μm diameter). The average droplet diameter (∼0.7 μm) determined from peak area integration was close to Dynamic Light Scattering measurements (∼1 μm). Random walk simulations were performed deriving equations for droplet electrolysis using the diffusion and thermal velocity expressions from Einstein. The simulations show that multiple droplet-electrode collisions, lasting ∼0.11 μs each, occur before a droplet wanders away. Updating the Fc-concentration at every collision shows that a droplet only oxidizes ∼0.58% of its content in one collisional journey. In fact, it would take ∼5.45 × 10⁶ collisions and ∼1.26 h to electrolyze the Fc in one droplet with the collision frequency derived from the thermal velocity (∼0.52 cm/s) of a 1 μm-droplet. To simulate adsorption, the droplet was immobilized at first contact with the electrode while the electrolysis current was computed. This approach along with modeling of instrumental filtering, produced the best match of experimental peaks, which were attributed to electrolysis from single adsorption events instead of multiple consecutive collisions. These results point to a heightened sensitivity and speed when relying on adsorption instead of collisions. The electrochemical current for the former is limited by the probability of adsorption per collision, whereas for the latter, the current depends on the collision frequency and the probability of electron transfer per collision (J. Am. Chem. Soc. 2017, 139, 16923-16931).

Advances in the Microbiology of Stenotrophomonas maltophilia

Stenotrophomonas maltophilia is an opportunistic pathogen of significant concern to susceptible patient populations. This pathogen can cause nosocomial and community-acquired respiratory and bloodstream infections and various other infections in humans. Sources include water, plant rhizospheres, animals, and foods. Studies of the genetic heterogeneity of S. maltophilia strains have identified several new genogroups and suggested adaptation of this pathogen to its habitats. The mechanisms used by S. maltophilia during pathogenesis continue to be uncovered and explored. S. maltophilia virulence factors include use of motility, biofilm formation, iron acquisition mechanisms, outer membrane components, protein secretion systems, extracellular enzymes, and antimicrobial resistance mechanisms. S. maltophilia is intrinsically drug resistant to an array of different antibiotics and uses a broad arsenal to protect itself against antimicrobials. Surveillance studies have recorded increases in drug resistance for S. maltophilia, prompting new strategies to be developed against this opportunist. The interactions of this environmental bacterium with other microorganisms are being elucidated. S. maltophilia and its products have applications in biotechnology, including agriculture, biocontrol, and bioremediation.

Single enzyme electroanalysis

Traditional studies of enzymatic activity rely on the combined kinetics of millions of enzyme molecules to produce a product, an experimental approach that may wash out heterogeneities that exist between individual enzymes. Evaluating these properties on an enzyme-by-enzyme basis represents an unambiguous means of elucidating heterogeneities; however, the quantification of enzymatic activity at the single-enzyme level is fundamentally limited by the maximum catalytic rate, k_cat, inherent to a given enzyme. For electrochemical methods measuring current, single enzymes must turn over greater than 10⁷ molecules per second to produce a measurable signal on the order of 10^-12 A. Enzymes with this capability are extremely rare in nature, with typical k_cat values for biologically relevant enzymes falling between 1 and 10 000 s^-1. Thus, clever amplification strategies are necessary to electrochemically detect the vast majority of enzymes. This review details the progress toward the electroanalytical detection and evaluation of single enzyme kinetics largely focused on the nanoimpact method, a chronoamperometric detection strategy that monitors the change in the current-time profile associated with stochastic collisions of freely diffusing entities (e.g., enzymes) onto a microelectrode or nanoelectrode surface. We discuss the experimental setups and methods developed in the last decade toward the quantification of single molecule enzymatic rates. Special emphasis is given to the limitations of measurement science in the observation of single enzyme activity and feasible methods of signal amplification with reasonable bandwidth.

Stochastic Collision Electrochemistry from Single G-Quadruplex/Hemin: Electrochemical Amplification and MicroRNA Sensing

Stochastic collision electrochemistry is a hot topic in single molecule/particle research, which provides an opportunity to investigate the details of the single molecule/particle reaction mechanism that is always masked in ensemble-averaged measurements. In this work, we develop an electrochemical amplification strategy to monitor the electrocatalytic behavior of single G-quadruplex/hemin (GQH) for the reaction between hydrogen peroxide and hydroquinone (HQ) through the collision upon a gold nanoelectrode. The intrinsic peroxidase activities of single GQH were investigated by stochastic collision electrochemical measurements, giving further insights into understanding biocatalytic processes. Based on the unique catalytic activity of GQH, we have also designed a hybridization chain reaction strategy to detect miRNA-15 with good selectivity and sensitivity. This work provided a meaningful strategy to investigate the electrochemical amplification and the broad application for nucleic acid sensing at the single molecule/particle level.

Redox activity of single bacteria revealed by electrochemical collision technique

This paper reports on an innovative strategy based on the electrochemical collision technique to quantify the redox activity of two bacterial species: the Gram-negative Escherichia coli and the Gram-positive Bacillus subtilis. Thionine (TH), as a redox mediator, was electrostatically adsorbed on bacterial surface and formed the bacterium-TH complexes. TH can receive electrons from bacterial metabolic pathways and be reduced. When a single bacterium-TH complex collides on the ultramicroelectrode, the reduced TH will be re-oxidized at certain potential and generate current spike. The frequency of the spikes is linearly proportional to the living bacteria concentration, and the redox activity of individual bacterium can be quantified by the charges enclosed in the current spike. The redox ability of Gram-negative E.coli to the TH mediator was 6.79 ± 0.26 × 10^-18 mol per bacterial cell in 30 min, which is relatively more reactive than B. subtilis (3.52 ± 0.31 × 10^-18 mol per cell). The spike signals, fitted by 3D COMSOL Multiphysics simulation, revealed that there is inherent redox ability difference of two bacterial strains besides the difference in bacterial size and collision position. This work successfully quantified the bacterial redox activity to mediator in single cells level, which is of great significance to improve understanding of heterogeneous electron transfer process and build foundations to the microorganism selection in the design of microbial electrochemical devices.

Colorimetric and electrochemical detection of pathogens in water using silver ions as a unique probe

The manuscript highlights the efficacy of silver ions to act as a unique probe for the detection of bacterial contamination in water samples. The bacterial cell membrane adherence property of the silver ions was employed to develop two different bacterial detection assays employing colorimetric and electrochemical techniques. In one of the schemes, silver ion was used directly as a detector of bacteria in a colorimetric assay format, and in the other scheme surface-functionalized antibodies were used as a primary capture for specific detection of Salmonella enterica serovar Typhi. The colorimetric detection is based on silver-induced inhibition of urease activity and silver ion utilization by bacteria for the rapid screening of enteric pathogens in water. The specific detection of bacteria uses an antibody-based electrochemical method that employs silver as an electrochemical probe. The ability of silver to act as an electrochemical probe was investigated by employing Anodic Stripping Voltammetry (ASV) for targeted detection of Salmonella Typhi. For further insights into the developed assays, inductively coupled plasma mass spectrometry (ICP-MS) and transmission electron microscopy (TEM) studies were performed. The sensitivity of the developed assay was found to be 100 cfu mL⁻¹ for colorimetric and 10 cfu mL⁻¹ for electrochemical assay respectively.

Smart Chip for Visual Detection of Bacteria Using the Electrochromic Properties of Polyaniline

Finding fast and reliable ways to detect pathogenic bacteria is crucial for addressing serious public health issues in clinical, environmental, and food settings. Here, we present a novel assay based on the conversion of an electrochemical signal into a more convenient optical readout for the visual detection of Escherichia coli. Electropolymerizing polyaniline (PANI) on an indium tin oxide screen-printed electrode (ITO SPE), we achieved not only the desired electrochromic behavior but also a convenient way to modify the electrode surface with antibodies (taking advantage of the many amine groups of PANI). Applying a constant potential to the PANI-modified ITO SPE induces a change in their oxidation state, which in turn generates a color change on the electrode surface. The presence of E. coli on the electrode surface increases the resistance in the circuit affecting the PANI oxidation states, producing a different electrochromic response. Using this electrochromic sensor, we could measure concentrations of E. coli spanning 4 orders of magnitude with a limit of detection of 10² colony forming unit per 1 mL (CFU mL^-1) by the naked eye and 10¹ CFU mL^-1 using ImageJ software. In this work we show that merging the sensitivity of electrochemistry with the user-friendliness of an optical readout can generate a new and powerful class of biosensors, with potentially unlimited applications in a variety of fields.

Rapid Antimicrobial Susceptibility Testing of Patient Urine Samples Using Large Volume Free-Solution Light Scattering Microscopy

The emergence of antibiotic resistance has prompted the development of rapid antimicrobial susceptibility testing (AST) technologies that will enable evidence-based treatment and promote antimicrobial stewardship. To date, many rapid AST methods have been developed, but few are able to be performed on clinical samples directly. Here we developed a large volume light scattering microscopy technique that tracks phenotypic features of single bacterial cells directly in clinical urine samples without sample enrichment or culturing. The technique demonstrated rapid (90 min) detection of Escherichia coli in 24 clinical urine samples with 100% sensitivity and 83% specificity and rapid (90 min) AST in 12 urine samples with 87.5% categorical agreement with two antibiotics, ampicillin and ciprofloxacin.