Collision of Aptamer/Pt Nanoparticles Enables Label-Free Amperometric Detection of Protein in Rat Brain

Single-Particle Collision Electrochemical Biosensor Developed by a Typical Alkaline Phosphatase-Catalyzed Silver Deposition Reaction

Considering the intrinsic importance of alkaline phosphatase (ALP) as a biomarker in disease monitoring and as a mostly widely used biolabel for signal transmission in bioanalysis, the development of a new ALP assay method is highly pursued. Herein, a well-known ALP-catalyzed silver deposition reaction onto gold nanoparticles (Au NPs) was developed into a single-particle-collision-based electrochemical biosensor. ALP-catalyzed dephosphorylation of ascorbic acid 2-phosphate (AA-P) resulted in ascorbic acid (AA), which in turn reduced the silver ion to form a silver nanoshell on the surface of Au NPs (Au@Ag NPs). The generated Au@Ag NPs could stochastically collide with the microelectrode to produce transient current spikes. The collision frequency and charge could concurrently indicate the amount of produced Au@Ag NPs and then the ALP activity. Thus, a new single-particle collision-based electrochemical biosensing platform for ALP was constructed. It operates homogeneously and does not require electrode modification, nanoparticle biofunctionalization, and washing and separation steps. It showed good detection sensitivity toward ALP activity with a quantification limit of 2 mU/mL in 10 μL. The background-free feature endows it with absolute selectivity. It could also be used for inhibitor screening and applied for the ALP assay in the serum. In addition, the proposed collision-based electrochemical strategy was developed for a new enzyme-linked immunosorbent assay. With the human immunoglobulin G (IgG) as a model target, it could effortlessly evaluate 5 ng/mL analytes. It thus opens a new avenue toward the development of single-particle collision-based electrochemical biosensors for a wide range of applications in disease diagnosis and bioanalysis.

Stochastic Impact Electrochemistry of Alkanethiolate-Functionalized Silver Nanoparticles

This study uses single-impact experiments to explore how the nanoparticles' surface chemistry influences their redox activity. 20 and 40 nm-sized silver nanoparticles are functionalized with alkanethiol ligands of various chain lengths (n = 3, 6, 8, and 11) and moieties (carboxyl ─COOH / hydroxyl ─OH), and the critical role of the particle shell is systematically examined. Short COOH-terminated ligands enable efficient charge transfer, resulting in higher impact rates and fast, high-amplitude transients. Even elevated potentials fail to overcome tunneling barriers for ligand lengths of n ≥ 6 and risk oxidizing the electrode, forming an insulating layer. Electrostatic interactions play a key role in governing reaction dynamics. In general, particles with a COOH-group exhibit higher impact rates and current amplitudes in KCl than those with an OH-group. This effect is more pronounced for 40 nm-sized particles; although, they rarely oxidize completely. The influence of electrolyte composition-concentration, pH, and a biologically relevant electrolyte-reveals that its impact on the redox activity can be as critical as that of the particle shell, with both determining particle adsorption and electron tunneling. These findings provide insights into the complex interdependencies at the electrode-particle-electrolyte interface, aiding the design of custom redox-active (silver) nanoparticles for ultrasensitive electrochemical sensing.

Critical Role of Molecular Adsorption on Electrocatalysis at Single Nanoparticles

We report a mechanistic study of the electrocatalytic response of single Pt nanoparticles (NPs) on a carbon ultramicroelectrode (UME) in a hydrazine (N2H4) solution. Using a NP collision approach, our study shows their catalytic response is characterized by a sharp, <50 μs-long current spike followed by a steady step-current signal. Our results suggest that the current spike is due to the quick oxidation of N2H4 molecules preadsorbed onto the NP surface, while the step current reflects the continuous catalytic oxidation of protonated hydrazine (N2H5+), which goes through a deprotonation and adsorption step on Pt. Since each N2H5+ molecule releases five H+ upon complete oxidation, a drastic decrease in local pH can be expected in the vicinity of the NP. This pH shift in turn limits the rate of adsorption and the steady-state oxidation current one can observe from each colliding particle. Our study reveals the key importance of molecular adsorption and the changing local chemical environment (e.g., pH) to the observed catalytic response of single NPs and highlights that steady-state currents in their measurement may be chemically or mass-transport limited.

Aptamer-based sensors for fluid biopsies of protein disease markers

Fluid biopsy technology, characterized by its minimally invasive nature, speed, and continuity, has become a rapidly advancing and widely applied real-time diagnostic technique. Among various biomarkers, proteins represent the most abundant class of disease indicators. The sensitive and accurate detection of protein markers in bodily fluids is significantly influenced by the control exerted by recognition ligands. Aptamers, which are structurally dynamic functional oligonucleotides, exhibit high affinity, specific recognition of targets, and notable characteristics of high editability and modularity. These features make aptamer universal "recognition-capture" components, contribute to a significant leap in their applications within the biosensor domain. In this context, we provide a comprehensive review of the extensive application of aptamer-based biosensors in fluid biopsy. We systematically compile the characteristics and construction strategies of aptamer-based biosensors tailored for fluid biopsy, including aptamer sequences, affinity (KD), fluid background, sensing technologies, sensor construction strategies, incubation time, detection performance, and influencing factors. Furthermore, a comparative analysis of their advantages and disadvantages was conducted. In conclusion, we delineate and deliberate on prospective research trajectories and challenges that lie ahead in the realm of aptamer-based biosensors for fluid biopsy.

Universal Covalent Grafting Strategy of an Aptamer on a Carbon Fiber Microelectrode for Selective Determination of Dopamine In Vivo

The chemical information on brain science provided by electrochemical sensors is critical for understanding brain chemistry during physiological and pathological processes. A major challenge is the selectivity of electrochemical sensors in vivo. This work developed a universal covalent grafting strategy of an aptamer on a carbon fiber microelectrode (CFE) for selective determination of dopamine in vivo. The universal strategy was proposed by oxidizing poly(tannic acid) (pTA) to form an oxidized state (pTAox) and then coupling a nucleophilic sulfhydryl molecule of the dopamine-binding mercapto-aptamer with the o-quinone moiety of pTAox based on click chemistry for the interfacial functionalization of the CFE surface. It was found that the universal strategy proposed could efficiently graft the aptamer on a glassy carbon electrode, which was verified by using electroactive 6-(ferrocenyl) hexanethiol as a redox reporter. The amperometric method using a fabricated aptasensor for the determination of dopamine was developed. The linear range of the aptasensor for the determination of dopamine was 0.2-20 μM with a sensitivity of 0.09 nA/μM and a limit of detection of 88 nM (S/N = 3). The developed method has high selectivity originating from the specific recognition of the aptamer in concert with the cation-selective action of pTA and could be easily applicable to probe dopamine dynamics in the brain. Furthermore, complex vesicle fusion modes were first observed at the animal level. This work demonstrated that the covalently grafted immobilization strategy proposed is promising and could be extended to the in vivo analysis of other neurochemicals.

Impact electrochemistry for biosensing: advances and future directions

Impact electrochemistry allows for the investigation of the properties of single entities, ranging from nanoparticles (NPs) to soft bio-particles. It has introduced a novel dimension in the field of biological analysis, enhancing researchers' ability to comprehend biological heterogeneity and offering a new avenue for developing novel diagnostic devices for quantifying biological analytes. This review aims to summarize the recent advancements in impact electrochemistry-based biosensing over the past two to three years and provide insights into the future directions of this field.

Aptamer-Protein Interactions: From Regulation to Biomolecular Detection

Serving as the basis of cell life, interactions between nucleic acids and proteins play essential roles in fundamental cellular processes. Aptamers are unique single-stranded oligonucleotides generated by in vitro evolution methods, possessing the ability to interact with proteins specifically. Altering the structure of aptamers will largely modulate their interactions with proteins and further affect related cellular behaviors. Recently, with the in-depth research of aptamer-protein interactions, the analytical assays based on their interactions have been widely developed and become a powerful tool for biomolecular detection. There are some insightful reviews on aptamers applied in protein detection, while few systematic discussions are from the perspective of regulating aptamer-protein interactions. Herein, we comprehensively introduce the methods for regulating aptamer-protein interactions and elaborate on the detection techniques for analyzing aptamer-protein interactions. Additionally, this review provides a broad summary of analytical assays based on the regulation of aptamer-protein interactions for detecting biomolecules. Finally, we present our perspectives regarding the opportunities and challenges of analytical assays for biological analysis, aiming to provide guidance for disease mechanism research and drug discovery.

Functionalized nanopores based on hybridization chain reaction: Fabrication and microRNA sensing

Enzyme-free hybridization chain reaction (HCR) technology is often used as a signal amplification tool for the detection of different targets. In this study, an ultrasensitive and label-free method for detecting miRNA-21 was developed using the nanopore ionic current rectification (ICR) technology coupled with HCR technology. The probe oligonucleotide (DNA1) was combined with the gold-coated nanopore through the Au-S bond to form a DNA1-functionalized gold-coated nanopore (DNA1-Au-coated nanopore). Since miRNA-21 is partially complementary to DNA1, it can be selectively recognized by DNA1-functionalized gold-coated nanopores. The target (miRNA-21) can induce the opening of hairpin DNA and HCR reaction after the introduction of hairpin DNA H1 and H2. The concentration of miRNA-21 will affect the combination of H1 and H2 on the inner wall of the nanopore, and its surface charge will change with the internal modification, thereby changing the ion current rectification ratio. Under the condition that the concentration of H1, H2 and HCR reaction time are constant, the change of ICR ratio is linearly correlated with the logarithm of miRNA-21 concentration within a certain range, which shows that the sensing strategy we designed can achieve target miRNA-21 detection. This ultrasensitive miRNA holds great promise in the field of cancer diagnosis.

Multiplex Assays of MicroRNAs by Using Single Particle Electrochemical Collision in a Single Run

It is important to quantify multiple biomarkers in a single run due to the advantages of precious samples and diagnostic accuracy. Based on the distinguishability of two types of current signals from single particle electrochemical collision (SPEC), step-type current transients produced by Pt nanoparticles (PtNPs) catalyzed hydrazine oxidation and peak-type current transients produced by Ag nanoparticles (AgNPs) oxidation, a kind of multiplex immunoassay of target microRNAs (miRNA-21 and Let-7a) have been established during SPEC in a single run. When the single-stranded DNA (ssDNA1) that was perfectly complementary to miRNA-21 was coupled to the surface of PtNPs, the SPEC of PtNPs electrocatalysis was inhibited and the step-type current transients disappeared, while the single-stranded DNA (ssDNA2) that was perfectly complementary to Let-7a was coupled to the surface of AgNPs, the SPEC of AgNPs oxidation was inhibited, and the peak-type current transients disappeared, thus the signals were in the "off" state at this time. After that, miRNA-21 and Let-7a were added into solution, complementary base pairing disrupted the weak DNA-NP interaction and restored the electrocatalysis of PtNPs and the electrooxidation of AgNPs, and the step-type current signals and peak-type current signals were in the "on" state. Moreover, the frequencies from two different recovered signals (PtNPs catalysis and AgNPs oxidation) corresponded to the amount of added miRNA-21 and Let-7a, thus a multiplex immunoassay method for dual quantification of miRNA-21 and Let-7a in a single run was established.

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.

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.

Recent progress and perspectives of continuous in vivo testing device

Devices for continuous in-vivo testing (CIVT) can detect target substances in real time, thus providing a valuable window into a patient's condition, their response to therapeutics, metabolic activities, and neurotransmitter transmission in the brain. Therefore, CIVT devices have received increased attention because they are expected to greatly assist disease diagnosis and treatment and research on human pathogenesis. However, CIVT has been achieved for only a few markers, and it remains challenging to detect many key markers. Therefore, it is important to summarize the key technologies and methodologies of CIVT, and to examine the direction of future development of CIVT. We review recent progress in the development of CIVT devices, with consideration of the structure of these devices, principles governing continuous detection, and nanomaterials used for electrode modification. This detailed and comprehensive review of CIVT devices serves three purposes: (1) to summarize the advantages and disadvantages of existing devices, (2) to provide a reference for development of CIVT equipment to detect additional important markers, and (3) to discuss future prospects with emphasis on problems that must be overcome for further development of CIVT equipment. This review aims to promote progress in research on CIVT devices and contribute to future innovation in personalized medical treatments.

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.

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.

Development of Carcinoembryonic Antigen Rapid Detection System Based on Platinum Microelectrode

Rapid and highly sensitive detection of carcinoembryonic antigen (CEA) in blood could effectively improve the diagnostic sensitivity of colorectal cancer. In this work, a platinum microelectrode (PtμE) modified with gold nanoparticles was developed as a microsensor for the detection of CEA. As the recognition element, a CEA aptamer modified with sulfhydryl could be conjugated onto the surface of the PtμEs/Au. The quantitative analysis of the concentration of CEA [CEA] by the prepared PtμEs/Au aptasensor was carried out through square wave voltammetry. Under the optimized conditions, the PtμEs/Au aptasensor exhibits a linear response toward [CEA] in the range of 1.0 × 10^–11—1.0 × 10^–7 g/ml (S = 5.5 nA/dec, R² = 0.999), and the detection limit is 7.7 × 10^–12 g/ml. The PtμEs/Au aptasensor also has good selectivity against other types of proteins existing in blood. The availability of the developed assay toward [CEA] in blood samples was investigated, and the results agreed well with those obtained through electrochemiluminescence provided by the hospital, and the volume of the blood sample for detection is only 20 μl. Herein, the proposed detection system could be used for the quantitative analysis of CEA in blood, with the advantages of high sensitivity, short time, and low cost. Moreover, the PtμEs/Au aptasensor has a potential application in clinical diagnosis.

Stochastic Collision Electrochemistry from Single Pt Nanoparticles: Electrocatalytic Amplification and MicroRNA Sensing

Single-particle collisions have made many achievements in basic research, but challenges still exist due to their low collision frequency and selectivity in complex samples. In this work, we developed an "on-off-on" strategy based on Pt nanoparticles (PtNPs) that catalyze N₂H₄ collision signals on the surface of carbon ultramicroelectrodes and established a new method for the detection of miRNA21 with high selectivity and sensitivity. PtNPs catalyze the reduction of N₂H₄ on the surface of carbon ultramicroelectrodes to generate a stepped collision signal, which is in the "on" state. The single-stranded DNA paired with miRNA21 is coupled with PtNPs to form the complex DNA/PtNPs. Because PtNPs are covered by DNA, the electrocatalytic collision of N₂H₄ oxidation is inhibited. At this time, the signal is in the "off" state. When miRNA21 is added, the strong complementary pairing between miRNA21 and DNA destroys the electrostatic adsorption of DNA/PtNP conjugates and restores the electrocatalytic performance of PtNPs, and the signal is in the "on" state again. Based on this, a new method for detecting miRNA21 was established. It provides a new way for small-molecule sensing and has a wide range of applications in electroanalysis, electrocatalysis, and biosensing.

A general controllable release amplification strategy of liposomes for single-particle collision electrochemical biosensing

As an important component of the COVID-19 mRNA vaccines, liposomes play a key role in the efficient protection and delivery of mRNA to cells. Herein, due to the controllable release amplification strategy of liposomes, a reliable and robust single-particle collision electrochemical (SPCE) biosensor was constructed for H9N2 avian influenza virus (H9N2 AIV) detection by combining liposome encapsulation-release strategy with immunomagnetic separation. The liposomes modified with biotin and loaded with platinum nanoparticles (Pt NPs) were used as signal probes for the first time. Biotin facilitated the coupling of biomolecules (DNA or antibodies) through the specific reaction of biotin-streptavidin. Each liposome can encapsulate multiple Pt NPs, which were ruptured under the presence of 1 × PBST (phosphate buffer saline with 0.05% Tween-20) within 2 min, and the encapsulated Pt NPs were released for SPCE experiment. The combination of immunomagnetic separation not only improved the anti-interference capabilities but also avoided the agglomeration of Pt NPs, enabling the SPCE biosensor to realize ultrasensitive detection of 18.1 fg/mL H9N2 AIV. Furthermore, the reliable SPCE biosensor was successfully applied in specific detection of H9N2 AIV in complex samples (chicken serum, chicken liver and chicken lung), which promoted the universality of SPCE biosensor and its application prospect in early diagnosis of diseases.

Graphene and graphene-related materials as brain electrodes

Neural electrodes are used for acquiring neuron signals in brain-machine interfaces, and they are crucial for next-generation neuron engineering and related medical applications. Thus, developing flexible, stable and high-resolution neural electrodes will play an important role in stimulation, acquisition, recording and analysis of signals. Compared with traditional metallic electrodes, electrodes based on graphene and other two-dimensional materials have attracted wide attention in electrophysiological recording and stimulation due to their excellent physical properties such as unique flexibility, low resistance, and high optical transparency. In this review, we have reviewed the recent progress of electrodes based on graphene, graphene/polymer compounds and graphene-related materials for neuron signal recording, stimulation, and related optical signal coupling technology, which provides an outlook on the role of electrodes in the nanotechnology-neuron interface as well as medical diagnosis.

Quantification of Tumor Protein Biomarkers from Lung Patient Serum Using Nanoimpact Electrochemistry

Protein quantification with high throughput and high sensitivity is essential in the early diagnosis and elucidation of molecular mechanisms for many diseases. Conventional approaches for protein assay often suffer from high costs, long analysis time, and insufficient sensitivity. The recently emerged nanoimpact electrochemistry (NIE), as a contrast, allows in situ detection of analytes one at a time with simplicity, fast response, high throughput, and the potential of reducing the detection limits down to the single entity level. Herein, we propose a NIE-enabled electrochemical immunoassay using silver nanoparticles (AgNPs) as labels for the detection of CYFRA21-1, a typical protein marker for lung carcinoma. This strategy is based on the measurement of the impact frequency and the charge intensity of the electrochemical oxidation of individual AgNPs before and after they are modified with anti-CYFRA21-1 and in turn immunocomplexed with CYFRA21-1. Both the frequency and intensity modes of single-nanoparticle electrochemistry correlate well with each other, resulting in a self-validated immunoassay that provides linear ranges of two orders of magnitude and a limit of detection of 0.1 ng/mL for CYFRA21-1 analysis. The proposed immunoassay also exhibits excellent specificity when challenged with other possible interfering proteins. In addition, the CYFRA21-1 content is validated by a conventional, well-known enzyme-linked immunosorbent assay and successfully quantified in a diluted healthy serum with a satisfactory recovery. Moreover, CYFRA21-1 detection in serum samples of lung cancer patients is successfully demonstrated, suggesting the feasibility of the NIE-based immunoassay in clinically relevant diagnosis. To the best of our knowledge, this is the first report to construct NIE-based electrochemical immunoassays for the specific detection of tumor protein biomarkers.

Size-Resolved Single Entity Collision Biosensing for Dual Quantification of MicroRNAs in a Single Run

Limited to the accuracy of size resolution, single entity collision biosensing (SECBS) for multiplex immunoassays remains challenging, because it is difficult to get the true value of nanoparticle (NP) sizes based on the current intensity due to the complex movement of NPs on the electrode surface. Considering that the size-dependent movement of NPs meanwhile will generate a characteristic current shape, in this work, the huge difference in the current rise time of 5 and 15 nm Pt NPs colliding on an Au ultramicroelectrode (d = 30 μm) was originally used to develop a size-resolved SECBS for multiplex immunoassays of miRNAs. The limit concentration that can be detected was 0.5 fM. Compared with conventional electrochemical biosensors for multiplex immunoassays, for the size-resolved SECBS, one does not need to worry about potential overlapping. Therefore, the proposed method demonstrates a promising potential for the application of SECBS in multiplex immunoassays.

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.

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.

Sizing Single Particles at the Orifice of a Nanopipette

Developing new methods and techniques for the size analysis of particles in a solution is highly desirable not only for the industrial screening of particles but also for single biological entity analysis (e.g., single cells or single vesicles). Herein, we report a new technique for sizing single particles in a solution with a nanopipette. The rationale is essentially based on ion-current blockage when the particles approach the proximity of a nanopipette orifice. By rationally controlling the geometry of the nanopipette and the applied potential, the spike-type ion current transient generated from the motion of particles in the process of "collision and departure" is employed for sizing single particles. The results show that both the relative ion-current change (ΔI/I₀) and the dwell time (Δt) of spike-type transient are dependent on particle size. Differently, Δt is also related to an externally applied voltage. Statistical analysis shows that ΔI/I₀ is proportional to the particle diameter, and this linear relationship is further understood by finite-element simulations. This study not only provides a new principle for sizing single particles in a solution but also is helpful to understand the motion of a particle near the orifice of the nanopipette.

One-to-Many Single Entity Electrochemistry Biosensing for Ultrasensitive Detection of microRNA

Single-entity electrochemistry (SEEC), a promising method for biosensing, has an intrinsic limitation on sensitivity since at most one colliding entity can be successfully triggered by one target. Here, we take advantage of one-to-many (1:n) signal amplification to develop a new single-entity electrochemistry biosensing (SEECBS), integrating satellite magnetic nanoparticle (MN)-DNA-Pt nanoparticle (NP) conjugates, duplex-specific nuclease (DSN) assisted Pt NPs releasing with stabilization, and effective collision of small sized and nearly naked Pt NPs. Compared with conventional SEECBS, the 1:n SEECBS can successfully enrich ∼2 nM Pt NPs by adding 50 aM microRNA (miRNA), in other words, ∼4 × 10⁷ Pt NPs can be triggered by one target. The proposed SEECBS allows the detection of 47 aM miRNA-21, nearly 6 orders of magnitude lower than the previous work, and discrimination of nontarget miRNAs containing even single-nucleotide mismatch. Besides, this method has also been successfully demonstrated for quantification of miRNA in different cell lines. Therefore, the proposed method holds great potential for the application of SEECBS in early diagnosis and prognosis monitoring of cancer.