Blocking electrochemical collisions of single E. coli and B. subtilis bacteria at ultramicroelectrodes elucidated using simultaneous fluorescence microscopy

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.

Surface-Enhanced Raman Spectroscopy and Electrochemistry: The Ultimate Chemical Sensing and Manipulation Combination

One of the lessons we learned from the COVID-19 pandemic is that the need for ultrasensitive detection systems is now more critical than ever. While sensors' sensitivity, portability, selectivity, and low cost are crucial, new ways to couple synergistic methods enable the highest performance levels. This review article critically discusses the synergetic combinations of optical and electrochemical methods. We also discuss three key application fields-energy, biomedicine, and environment. Finally, we selected the most promising approaches and examples, the open challenges in sensing, and ways to overcome them. We expect this work to set a clear reference for developing and understanding strategies, pros and cons of different combinations of electrochemical and optical sensors integrated into a single device.

Collision Oxidation Behavior of Silver Nanoparticles in Alkaline Solution

In recent years, silver nanoparticles (Ag NPs) have been used as positive electrode material for zinc/silver batteries, and the silver oxides formed during the charging process determine the discharge performance of batteries. Therefore, it is important to study the oxidation behavior of Ag NPs in alkaline solution. Single-nanoparticle collision is an important tool for analyzing oxidation behavior of individual nanoparticles. Based on thermodynamic information from collision events, it is known that oxidation products are potential-dependent and size-dependent. Based on dynamic information, including collisional peak shapes and duration time, it was observed that the Ag NP collision oxidation process changed from stepwise oxidation to direct oxidation as the potential increased or size decreased. This work provides guidance for application of Ag NPs in zinc/silver batteries and proposed a strategy for oxidation behavior of individual NP that could be tracked in situ through an all-encompassing view of thermodynamic and dynamic information.

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.

The role of applied potential on particle sizing precision in single-entity blocking electrochemistry

Blocking electrochemistry, a subfield of single-entity electrochemistry, enables in-situ sizing of redox-inactive particles. This method exploits the adsorptive impact of individual insulating particles on a microelectrode, which decreases the electrochemically active surface area of the electrode. Against the background of an electroactive redox reaction in solution, each individual impacting particle results in a discrete current drop, with the magnitude of the drop corresponding to the size of the blocking particle. One significant limitation of this technique is "edge effects", resulting from the inhomogeneous flux of the redox species' diffusion due to increased mass transport to the edge of the disk electrode surface. "Edge effects" cause increased errors in size detection, resulting in poor analytical precision. Here, we use computational simulations to demonstrate that inhomogeneous diffusional edge flux of quasi-reversible redox species is mitigated at lowered overpotentials. This phenomenon is further illustrated experimentally by lowering the applied potential such that the system is operating under a kinetically-controlled regime instead of a diffusion-limited regime, which mitigates edge effects and increases particle sizing precision significantly. In addition, we found this method to be generalizable, as the precision enhancement is not limited to geometrically spherical particles but also occurs for cubic particles. This work presents a simple, novel methodology for edge effect mitigation with general applicability across different particle types.

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.

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.

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%.

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.

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.

Three-Mediator Enhanced Collisions on an Ultramicroelectrode for Selective Identification of Single Saccharomyces cerevisiae

Selective detection of colliding entities, especially cells and microbes, is of great challenge in single-entity electrochemistry. Herein, based on the different cellular electron transport pathways between microbes and mediators, we report a three-mediator system [K₃Fe(CN)₆, K₄Fe(CN)₆, and menadione] to achieve redox activity analysis and selective identification of single Saccharomyces cerevisiae without the usage of antibodies. K₄Fe(CN)₆ in the three-mediator system will oxidize near the electrode surface and increase the local concentration of K₃Fe(CN)₆, which will promote the redox reaction of S. cerevisiae. The hydrophobic mediator─menadione─can selectively penetrate through the S. cerevisiae membrane and get access to its intracellular redox center and can further react with K₃Fe(CN)₆ in the bulk solution. In contrast, the mediator can only get access to the bacterial membranes of Escherichia coli and Staphylococcus aureus, which results in different electrochemical collision signals between the above microbes. In the three-mediator system, upward step-like collision signals were observed in S. cerevisiae suspension, which are related to their microbial redox activity. In comparison, E. coli or S. aureus only generated downward current steps because the blockage effect of mediator diffusion suppresses their redox activities. When S. cerevisiae co-existed with E. coli or S. aureus, transients generated by both blockage and redox activity were observed. The approach enables us to trace the collision behaviors of different microbes and distinguish their simultaneous collisions, which is the foundation for further application of electrochemical collision technique in the specific identification of single biological entities.

Ring Ultramicroelectrodes for Current-Blockade Particle-Impact Electrochemistry

In current-blockade impact electrochemistry, insulating particles are detected amperometrically as they impinge upon a micro- or nanoelectrode via a decrease in the faradaic current caused by a redox mediator. A limit of the method is that analytes of a given size yield a broad distribution of response amplitudes due to the inhomogeneities of the mediator flux at the electrode surface. Here, we overcome this limitation by introducing microfabricated ring-shaped electrodes with a width that is significantly smaller than the size of the target particles. We show that the relative step size is somewhat larger and exhibits a narrower distribution than at a conventional ultramicroelectrode of equal diameter.

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.

Recent advancement in nano-optical strategies for detection of pathogenic bacteria and their metabolites in food safety

Pathogenic bacteria and their metabolites are the leading risk factor in food safety and are one of the major threats to human health because of the capability of triggering diseases with high morbidity and mortality. Nano-optical sensors for bacteria sensing have been greatly explored with the emergence of nanotechnology and artificial intelligence. In addition, with the rapid development of cross fusion technology, other technologies integrated nano-optical sensors show great potential in bacterial and their metabolites sensing. This review focus on nano-optical strategies for bacteria and their metabolites sensing in the field of food safety; based on surface-enhanced Raman scattering (SERS), fluorescence, and colorimetric biosensors, and their integration with the microfluidic platform, electrochemical platform, and nucleic acid amplification platform in the recent three years. Compared with the traditional techniques, nano optical-based sensors have greatly improved the sensitivity with reduced detection time and cost. However, challenges remain for the simple fabrication of biosensors and their practical application in complex matrices. Thus, bringing out improvements or novelty in the pretreatment methods will be a trend in the upcoming future.

Single-Particle Electrochemical Biosensor with DNA Walker Amplification for Ultrasensitive HIV-DNA Counting

Single-particle electrochemical collision has gained great achievements in fundamental research, but it is challenging to use in practice on account of its low collision frequency and the interference of the complex matrix in actual samples. Here, magnetic separation and DNA walker amplification were integrated to build a robust and sensitive single-particle electrochemical biosensor. Magnetic nanobeads (MBs) can specifically capture and separate targets from complex samples, which not only ensures the anti-interference capability of this method but also avoids the aggregation of platinum nanoparticles (Pt NPs) caused by numerous coexisting substances. A low amount of targets can lead to the release of more Pt NPs and the generation of more collision current transients, realizing cyclic amplification. Compared with simple hybridization, a DNA walker can improve the collision frequency by about 3-fold, greatly enhancing detection sensitivity, and a relationship between collision frequency and target concentration is used to realize quantification. The biosensor realized an ultrasensitive detection of 4.86 fM human immunodeficiency virus DNA (HIV-DNA), which is 1-4 orders of magnitude lower than that of traditional methods. The successful HIV-DNA detection in complex systems (serum and urine) demonstrated a great promising application in real samples and in the development of new single-entity biosensors.

Revealing Dynamic Rotation of Single Graphene Nanoplatelets on Electrified Microinterfaces

Nanoparticles interact with a variety of interfaces, from cell walls for medicinal applications to conductive interfaces for energy storage and conversion applications. Unfortunately, quantifying dynamic changes of nanoparticles near interfaces is difficult. While optical techniques exist to study nanoparticle dynamics, motions smaller than the diffraction limit are difficult to quantify. Single-entity electrochemistry has high sensitivity, but the technique suffers from ambiguity in the entity's size, morphology, and collision location. Here, we combine optical microscopy, single-entity electrochemistry, and numerical simulations to elucidate the dynamic motion of graphene nanoplatelets at a gold ultramicroelectrode (radius ∼5 μm). The approach of conductive graphene nanoplatelets, suspended in 10 μM NaOH, to an ultramicroelectrode surface was tracked optically during the continuous oxidation of ferrocenemethanol. Optical microscopy confirmed the nanoplatelet size, morphology, and collision location on the ultramicroelectrode. Nanoplatelets collided on the ultramicroelectrode at an angle, θ, enhancing the electroactive area, resulting in a sharp increase in current. After the collision, the nanoplatelets reoriented to lay flat on the electrode surface, which manifested as a return to the baseline current in the amperometric current-time response. Through correlated finite element simulations, we extracted single nanoplatelet angular velocities on the order of 0.5-2°/ms. These results are a necessary step forward in understanding nanoparticle dynamics at the nanoscale.

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.

Self-Induced Convection at Microelectrodes via Electroosmosis and Its Influence on Impact Electrochemistry

Faradaic reactions at low supporting electrolyte concentrations induce convection via electroosmotic flows. Here we combine finite-element simulations and electrochemical measurements on microparticles at ultramicroelectrodes to explore this effect. We show that convection becomes the dominant form of mass transport for experiments at low salt concentrations, violating the common assumption that convection can be neglected.

Advanced Nanoscale Approaches to Single-(Bio)entity Sensing and Imaging

Individual (bio)chemical entities could show a very heterogeneous behaviour under the same conditions that could be relevant in many biological processes of significance in the life sciences. Conventional detection approaches are only able to detect the average response of an ensemble of entities and assume that all entities are identical. From this perspective, important information about the heterogeneities or rare (stochastic) events happening in individual entities would remain unseen. Some nanoscale tools present interesting physicochemical properties that enable the possibility to detect systems at the single-entity level, acquiring richer information than conventional methods. In this review, we introduce the foundations and the latest advances of several nanoscale approaches to sensing and imaging individual (bio)entities using nanoprobes, nanopores, nanoimpacts, nanoplasmonics and nanomachines. Several (bio)entities such as cells, proteins, nucleic acids, vesicles and viruses are specifically considered. These nanoscale approaches provide a wide and complete toolbox for the study of many biological systems at the single-entity level.