Temporal Sensing Platform Based on Bipolar Electrode for the Ultrasensitive Detection of Cancer Cells

Fe-single-atom catalysts boosting electrochemiluminescence via bipolar electrode integrated with its peroxidase-like activity for bioanalysis

Multifunctional single-atom catalysts (SACs) have been extensively investigated as outstanding signal amplifiers in bioanalysis field. Herein, a type of Fe single-atom catalysts with Fe-nitrogen coordination sites in nitrogen-doped carbon (Fe-N/C SACs) was synthesized and demonstrated to possess both catalase and peroxidase-like activity. Utilizing Fe-N/C SACs as dual signal amplifier, an efficient bipolar electrode (BPE)-based electrochemiluminescence (ECL) immunoassay was presented for determination of prostate-specific antigen (PSA). The cathode pole of the BPE-ECL platform modified with Fe-N/C SACs is served as the sensing side and luminol at the anode as signal output side. Fe-N/C SACs could catalyze decomposition of H2O2 via their high catalase-like activity and then increase the Faraday current, which can boost the ECL of luminol due to the electroneutrality in a closed BPE system. Meanwhile, in the presence of the target, glucose oxidase (GOx)-Au NPs-Ab2 was introduced through specific immunoreaction, which catalyzes the formation of H2O2. Subsequently, Fe-N/C SACs with peroxidase-like activity catalyze the reaction of H2O2 and 4-chloro-1-naphthol (4-CN) to generate insoluble precipitates, which hinders electron transfer and then inhibits the ECL at the anode. Thus, dual signal amplification of Fe-N/C SACs was achieved by increasing the initial ECL and inhibiting the ECL in the presence of target. The assay exhibits sensitive detection of PSA linearly from 1.0 pg/mL to 100 ng/mL with a detection limit of 0.62 pg/mL. The work demonstrated a new ECL enhancement strategy of SACs via BPE system and expands the application of SACs in bioanalysis field.

Ultrasensitive Electrochemiluminescence Imaging Detection of Multiple miRNAs in Single Cells with a Closed Bipolar Electrode Array Chip

In situ sensitive detection of multiple biomarkers in a single cell was highly necessary for understanding the pathogenesis mechanism and facilitating disease diagnosis. Herein, a bipolar electrode (BPE)-electrochemiluminescence (ECL) imaging chip was designed for ultrasensitive in situ detection of multiple miRNAs in single cells based on a dual-signal amplification strategy. A single cell was trapped and lysed within the microtrap of the cathode chamber and an HCR amplification process and nanoprobes (Fc/DNA/Fe3O4) were introduced, leading to a large number of electroactive molecules (Fc) being modified on the surface. Under a suitable potential, Fc+ in the cathodic chamber was reduced to Fc and L-012 was oxidized in the anodic chamber according to the electric neutrality principle of the bipolar electrode system, resulting in the ECL signal recorded by EMCCD. Ascribed to the dual-signal amplification, sensitive visual detection of miRNA-21 and miRNA-155 in single cells was achieved. For MCF-7 cells, miRNA-21 and miRNA-155 were calculated to be 4385 and 1932 copies/cell (median), respectively. For HeLa cells, miRNA-21 and miRNA-155 were calculated to be 1843 and 1012 copies/cell (median), respectively. The comprehensive evaluation of two kinds of miRNA could effectively eliminate error signals, and the detection precision was improved by 10%.

Electrochemiluminescence Analysis of Multiple Glycans on Single Living Cell with a Closed Bipolar Electrode Array Chip

The profiling of multiple glycans on a single cell is important for elucidating glycosylation mechanisms and accurately identifying disease states. Herein, we developed a closed bipolar electrode (BPE) array chip for live single-cell trapping and in situ galactose and sialic acid detection with the electrochemiluminescence (ECL) method. Methylene blue-DNA (MB-DNA) as well as biotin-DNA (Bio-DNA) codecorated AuNPs were prepared as nanoprobes, which were selectively labeled on the cell surface through chemoselective labeling techniques. The individual cell was captured and labeled in the microtrap of the cathodic chamber, under an appropriate potential, MB molecules on the cellular membrane underwent oxidation, triggering the reduction of [Ru(bpy)3]2+/TPA and consequently generating ECL signals in the anodic chamber. The abundance of MB groups on the single cell enabled selective monitoring of both sialic acid and galactosyl groups with high sensitivity using ECL. The sialic acid and galactosyl content per HepG2 cell were detected to be 0.66 and 0.82 fmol, respectively. Through comprehensive evaluation of these two types of glycans on a single cell, tumor cells, and normal cells could be effectively discriminated and the accuracy of single-cell heterogeneous analysis was improved. Additionally, dynamic monitoring of variations in galactosyl groups on the surface of the single cell was also achieved. This work introduced a straightforward and convenient approach for heterogeneity analysis among single cells.

Wireless electrochemical actuation of soft materials towards chiral stimuli

Different areas of modern chemistry, require wireless systems able to transfer chirality from the molecular to the macroscopic event. The ability to recognize the enantiomers of a chiral analyte is highly desired, since in the majority of cases such molecules present different physico-chemical properties that could lead, eventually, to dangerous or harmful interactions with the environment or the human body. From an electrochemical point of view, enantiomers have the same electrochemical behavior except when they interact in a chiral environment. In this Feature Article, different approaches for the electrochemical recognition of chiral information based on the actuation of conducting polymers are described. Such a dynamic behavior of π-conjugated materials is based on an electrochemically induced shrinking/swelling transition of the polymeric matrix. Since all the systems, described so far in the literature, are achiral and require a direct connection to a power supply, new strategies will be presented in the manuscript, concerning the implementation of chirality in electrochemical actuators and their use in a wireless manner through bipolar electrochemistry. Herein, the synergy between the wireless unconventional actuation and the outstanding enantiorecognition of inherent chiral oligomers is presented as an easy and straightforward read out of chiral information in solution. This approach presents different advantages in comparison to classic electrochemical systems such as its wireless nature and the possible real-time data acquisition.

Monitoring Bipolar Electrochemistry and Hydrogen Evolution Reaction of a Single Gold Microparticle under Sub-Micropipette Confinement

Herein, an approach to track the process of autorepeating bipolar reactions and hydrogen evolution reaction (HER) on a micro gold bipolar electrode (BPE) is established. Once blocking the channel of the sub-micropipette tip, the formed gold microparticle is polarized into the wireless BPE, which induces the dissolution of the gold at the anode and the HER at the cathode. The current response shows a periodic behavior with three regions: the bubble generation region (I), the bubble rupture/generation region (II), and the channel opening region (III). After a stable low baseline current of region I, a series of positive spike signals caused by single H2 nanobubbles rupture/generation are recorded standing for the beginning of region II. Meanwhile, the dissolution of the gold blocking at the orifice will create a new channel, increasing the baseline current for region III, where the synthesis of gold occurs again, resulting in another periodic response. Finite element simulations are applied to unveil the mechanism thermodynamically. In addition, the integral charge of the H2 nanobubbles in region II corresponds to the consumption of the anode gold. It simultaneously monitors autorepeating bipolar reactions of a single gold microparticle and HER of a single H2 nanobubble electrochemically, which reveals an insightful physicochemical mechanism in nanoscale confinement and makes the glass nanopore an ideal candidate to further reveal the heterogeneity of catalytic capability at the single particle level.

Bipolar Electrochemistry - A Powerful Tool for Micro/Nano-Electrochemistry

The understanding of areas for "classical" electrochemistry (including catalysis, electrolysis and sensing) and bio-electrochemistry at the micro/nanoscale are focus on the continued performance facilitations or the exploration of new features. In the recent 20 years, a different mode for driving electrochemistry has been proposed, which is called as bipolar electrochemistry (BPE). BPE has garnered attention owing to the interesting properties: (i) its wireless nature facilitates electrochemical sensing and high throughput analysis; (ii) the gradient potential distribution on the electrodes surface is a useful tool for preparing gradient surfaces and materials. These permit BPE to be used for modification and analytical applications on a micro/nanoscale surface. This review aims to introduce the principle and classification of BPE and BPE at micro/nanoscale; sort out its applications in electrocatalysis, electrosynthesis, electrophoresis, power supply and so on; explain the confined BPE and summarize its analytical application for single entities (single cells, single particles and single molecules), and discuss finally the important direction of micro/nanoscale BPE.

Gold Nanowires Array-Based Closed Bipolar Nanoelectrode System for Electrochemiluminescence Detection of α-Fetoprotein on Cell Surface

Inspired by the promising applications of a closed bipolar electrodes (c-BPEs) system in electrochemiluminescence (ECL) detection of cell adhesion and disease-related biomarkers, here, a gold nanowires array-based c-BPEs system was constructed for cell surface protein detection. Regular and uniform gold nanowires array were prepared by intermittent potentiostatic deposition. Then, two poly(dimethylsiloxane) (PDMS) chips with a hole diameter of 2 mm as a reservoir were placed at both sides of Au nanowires array to construct c-BPEs system. Thionine-functionalized silicon dioxide nanoparticles conjugated to antibody (Ab2-Th@SiO₂) were used as the electrochemical probe, while [Ru(bpy)₃]²⁺-wrapped SiO₂ nanoparticles (Ru(II)@SiO₂) were employed as the ECL signal readout. Taking α-fetoprotein (AFP) as model, the gold nanowires array-based c-BPEs system allowed sensitive detection of AFP at a linear range from 0.002 to 50.0 ng/mL and at least 6 living cells ascribing to the synergetic amplification effect at both sensing and reporting chambers. Besides, the amount of AFP expressed by HepG2 cells was calculated to be 6.71 pg/cell. The presented strategy with high sensitivity provided a promising and universal platform for the detection of other cancer cells and disease-related biomarkers (such as proteins, glycan, miRNA).

Plasmon-Enhanced Nitrogen Vacancy-Rich Carbon Nitride Electrochemiluminescence Aptasensor for Highly Sensitive Detection of miRNA

The development of biosensors for biologically important substances with ultralow content such as microRNA is of great significance. Herein, a novel surface plasmon-enhanced electrogenerated chemiluminescence-based aptasensor was developed for ultrasensitive sensing of microRNA by using nitrogen vacancy-rich carbon nitride nanosheets as effective luminophores and gold nanoparticles as plasmonic sources. The introduction of nitrogen vacancies improved the electrochemiluminescence behavior due to improved conductance and electrogenerated chemiluminescence activity. The introduction of plasmonic gold nanoparticles increased the electrochemiluminescence signal intensity by more than eightfold. The developed surface plasmon-enhanced electrogenerated chemiluminescence aptasensor exhibited good selectivity, ultrasensitivity, excellent stability, and reproducibility for the determination of microRNA-133a, with a dynamic linear range of 1 aM to 100 pM and a limit of detection about 0.87 aM. Moreover, the surface plasmon-enhanced electrogenerated chemiluminescence sensor obtained a good recovery when detecting the content of microRNA in actual serum.

Bipolar (Bio)electroanalysis

This contribution reviews a selection of the most recent studies on the use of bipolar electrochemistry in the framework of analytical chemistry. Despite the fact that the concept is not new, with several important studies dating back to the middle of the last century, completely novel and very original approaches have emerged over the last decade. This current revival illustrates that scientists still (re)discover some exciting virtues of this approach, which are useful in many different areas, especially for tackling analytical challenges in an unconventional way. In several cases, this "wireless" electrochemistry strategy enables carrying out measurements that are simply not possible with classic electrochemical approaches. This review will hopefully stimulate new ideas and trigger scientists to integrate some aspects of bipolar electrochemistry in their work in order to drive the topic into yet unexplored and eventually completely unexpected directions.

Electrofluorochromic Imaging Analysis of Glycan Expression on Living Single Cell with Bipolar Electrode Arrays

The in situ glycan profiling of a single tumor cell plays an important role in personalized cancer treatment. Herein, an integrated microfluidic system was designed for living single-cell trapping and real-time monitoring of galactosyl expression on the surface, combining closed bipolar electrode (BPE) arrays and electrofluorochromic (EFC) imaging. Galactosyl groups on human liver cancer HepG2 cells were used as the model analysts, galactose oxidase (GAO) could selectively oxidize hydroxyl sites of galactosyl groups on the cell surface to aldehydes, and then biotin hydrazide (BH) was used to label the aldehydes by aniline-catalyzed hydrazone ligation. With the biotin-avidin system, nanoprobes were finally introduced to the galactosyl groups on the cell surface with avidin as a bridge, which was prepared by simultaneously assembling ferrocene-DNA (Fc-DNA) and biotin-DNA (Bio-DNA) on gold nanoparticles (AuNPs) due to their large surface area and excellent electrical conductivity. After a labeled single cell was captured in the anodic microchannel, the Fc groups attached on the cell surface were oxidized under suitable potential, and the nonfluorescent resazurin on the cathode was correspondingly reduced to produce highly fluorescent resorufin, collected by fluorescence confocal microscope. The combination of EFC imaging and BPE realized monitoring galactosyl group expression of 5.0 × 10⁸ molecules per cell. Furthermore, the proposed platform had the ability to distinguish a single cancer cell from a normal cell according to the expression level of galactosyl groups and to dynamically monitor the galactosyl group variation on the cell surface, providing a simple and accessible method for the single-cell analysis.

Electrogenerated chemiluminescence detection of single entities

Electrogenerated chemiluminescence, also known as electrochemiluminescence (ECL), is an electrochemically induced production of light by excited luminophores generated during redox reactions. It can be used to sense the charge transfer and related processes at electrodes via a simple visual readout; hence, ECL is an outstanding tool in analytical sensing. The traditional ECL approach measures averaged electrochemical quantities of a large ensemble of individual entities, including molecules, microstructures and ions. However, as a real system is usually heterogeneous, the study of single entities holds great potential in elucidating new truths of nature which are averaged out in ensemble assays or hidden in complex systems. We would like to review the development of ECL intensity and imaging based single entity detection and place emphasis on the assays of small entities including single molecules, micro/nanoparticles and cells. The current challenges for and perspectives on ECL detection of single entities are also discussed.

Paper-Based Bipolar Electrode Electrochemiluminescence Platform for Detection of Multiple miRNAs

This paper introduces a novel potential-resolved paper-based biosensor for simultaneous detection of multiple microRNAs (miRNAs) (taking miRNA-155 and miRNA-126 as examples) based on the bipolar electrode (BPE) electrochemiluminescence (ECL) strategy. The proposed multiple-channel paper-based sensing microfluidic platform was prepared by wax-printing technology, screen-printing method, and in situ Au nanoparticles (AuNPs) growth to form hydrophilic areas, hydrophobic boundaries, waterproof electronic bridge, driving electrode regions, and parallel bipolar electrode regions. CdTe quantum dots (QDs)-H2 and Au@g-C₃N₄ nanosheets (NSs)-DNA1 were used as dual electrochemiluminescence signal probes, and carboxylated Fe₃O₄ magnetic nanoparticles existed as carriers. CdTe QDs-H2/S₂O₈^2- and Au@g-C₃N₄ NSs-DNA1/S₂O₈^2- could exhibit two strong and stable ECL emissions at a drive voltage of 9 and 12 V, respectively, which can be used as effective potential-resolved signal tags. In addition, the proposed three-dimensional (3D) DNA nanomachine model and the target miRNA cycle strategy were used to achieve double amplification of electrochemiluminescence intensity. More importantly, the combination of the bipolar electrode system and the potential-resolved multitarget electrochemiluminescence method can greatly reduce the spatial interference between substances. The prepared ECL biosensor showed a favorable linear response for the detection of miRNA-155 and miRNA-126 with relatively low detection limits of 5.7 and 4.2 fM, respectively. With excellent sensitivity, the strategy may provide an efficient method for clinical application, especially in detection of trace multiple targets.

Functionalized reduced graphene oxide with aptamer macroarray for cancer cell capture and fluorescence detection

An integrated aptamer macroarray functionalized with reduced graphene oxide (rGO) to specifically capture and sensitively detect cancer cells is reported. The capture for cancer cells is based on effective recognition of the modified rGO surface through the aptamer against epithelial cell adhesion molecule (EpCAM). The rough structure of rGO enhances morphologic interactions between rGO film interface and the cancer cells, while super-hydrophilicity of modified rGO hinders nonspecific cell capture. The synergistic interactions offer the aptamer macroarray high efficiency of cancer cell capture. By means of a turn-on fluorescence strategy based on the conformation change of the aptamer induced by the target recognition, the enriched cancer cells can be directly read out at excitation/emission wavelengths of 550/680 nm without washing, separation, and dying steps. The working range is 1 × 10² to 2 × 10⁴ cells per mL with a detection limit of 22 cells per mL. The results indicate that the aptamer macroarray has a considerable foreground for early diagnosis, therapy, and monitoring of cancer. Graphical abstract.

Recent Advances in Electrochemiluminescence-Based Systems for Mammalian Cell Analysis

Mammalian cell analysis is essential in the context of both fundamental studies and clinical applications. Among the various techniques available for cell analysis, electrochemiluminescence (ECL) has attracted significant attention due to its integration of both electrochemical and spectroscopic methods. In this review, we summarize recent advances in the ECL-based systems developed for mammalian cell analysis. The review begins with a summary of the developments in luminophores that opened the door to ECL applications for biological samples. Secondly, ECL-based imaging systems are introduced as an emerging technique to visualize single-cell morphologies and intracellular molecules. In the subsequent section, the ECL sensors developed in the past decade are summarized, the use of which made the highly sensitive detection of cell-derived molecules possible. Although ECL immunoassays are well developed in terms of commercial use, the sensing of biomolecules at a single-cell level remains a challenge. Emphasis is therefore placed on ECL sensors that directly detect cellular molecules from small portions of cells or even single cells. Finally, the development of bipolar electrode devices for ECL cell assays is introduced. To conclude, the direction of research in this field and its application prospects are described.

Closed Bipolar Electrode Array for On-Chip Analysis of Cellular Respiration by Cell Aggregates

Cell aggregates have attracted much attention owing to their potential applications in tissue engineering and drug screening. To evaluate cellular respiration of individual cell aggregates in these applications, noninvasive and on-chip high-throughput analytical tools are necessary. Electrochemical methods for detecting oxygen concentrations are useful because of their noninvasiveness. However, these conventional methods may be unsuitable for high-throughput detection because it is difficult to prepare many electrodes on a small chip owing to the limitation of area for connecting electrodes. Alternatively, a bipolar electrode (BPE) system offers clear advantages. In this system, electrochemical reactions are induced at both ends of a BPE without complex wiring. In this study, we present a BPE array for detecting the respiratory activity of cell aggregates. Oxygen concentrations near cell aggregates at cathodic poles of BPEs were converted to electrochemiluminescence (ECL) signals of [Ru(bpy)₃]²⁺/tripropylamine at anodic poles of BPEs. To separate ECL chemicals from cell aggregates, we fabricated a closed BPE device containing analytical and reporter chambers. As a proof of concept, 32 BPEs were controlled wirelessly using a pair of driving electrodes, and the respiratory activities of individual MCF-7 cell aggregates as a cancer model were successfully detected by monitoring ECL signals. Compared with conventional electrode arrays for cell analysis, the wiring of the current device was simple because the multiple BPEs functioned with only a single power supply. To the best of our knowledge, this is the first study of on-chip analysis of cellular activity using a BPE system.

Near-Infrared Light-Switched MoS2 Nanoflakes@Gelatin Bioplatform for Capture, Detection, and Nondestructive Release of Circulating Tumor Cells

The integrative bioplatform for capture, detection and release of circulating tumor cells (CTCs) is of great significance in clinical diagnosis and biomedical research. To fulfill this demand, we introduced a near-infrared (NIR) light-switched bioplatform for efficient isolation and downstream analysis of CTCs. The platform was created by first modifying the PEG-MoS₂ nanoflakes (NFs)@gelatin nanocomposite on the ITO surface, and then introducing the MUC1 aptamer as a specific recognition element via coupling reaction between aptamer and gelatin to achieve the specific capture for CTCs. Subsequently, the captured cells are released under a NIR light irradiation (808 nm) by using MoS₂ NFs as the NIR-regulated control element. Significantly, this platform could capture and release of CTCs with an excellent capture/release efficiency of 89.5% and 92.5%, respectively. Furthermore, the electrochemical bioplatform exhibited a wide linear range for the detection of CTCs from 50 to 1 × 10⁶ cells mL^-1 with a detection limit of 15 cells mL^-1. After 5 days of reculture, the released cells still maintain good cell shape and proliferation capacity. Moreover, the bioplatfrom is a simple, versatile, and universal system for the recognition, capture, release, and detection of different types of CTCs. Therefore, this bioplatform shows potential applications on the early diagnosis of cancers.

Screen printed bipolar electrode for sensitive electrochemiluminescence detection of aflatoxin B1 in agricultural products

In order to avoid the occurrence of false positives and false negatives caused by improper pretreatment during the detection of aflatoxin B1 by enzyme linked immunosorbent assay (ELISA). In this paper, we developed a screen printed bipolar electrode (BPE) for sensitive electrochemiluminescence (ECL) detection of aflatoxin B1 in agricultural products. The sensor uses a cathode of closed BPE as a functional sensing interface and an anode as a signal collection interface. In this way, the analyte does not need to participate in the ECL reaction of the anode. It avoids direct contact of photoactive molecules with complex reaction systems and greatly broadens the range of applications for ECL. After mixing the test sample with a known fixed concentration of horseradish peroxidase-labeled AFB1 (HRP-AFB1), they compete for binding to monoclonal antibodies. HRP catalyzes the polymerization of aniline to form polyaniline (PANI). Thereby causing a change in the oxidation-reduction potential and the ECL intensity in the electrochemical system, and then achieve the purpose of detecting the AFB1 concentration in the sample. As a result, the sensor has a good analytical performance for AFB1 with a linear range of 0.1-100 ng mL^-1 and a detection limit of 0.033 ng mL^-1. The sensor avoids the direct contact between the reaction system and the signal measurement system. In recovery experiment for six grains, the results demonstrate that the recovery rate and accuracy of this sensor is better than that of ELISA. This method provides a new idea for the detection of other mycotoxins in grains.

Bipolar electrode-electrochemiluminescence (ECL) biosensor based on a hybridization chain reaction

A novel electrochemiluminescence (ECL) closed bipolar electrode (BPE) chip was designed based on a hybridization chain reaction (HCR)-induced ECL amplification strategy for the detection of both DNA and H₂O₂. Without the utilization of a patterned ITO bipolar electrode (BPE), this chip platform consisted of an ITO glass coated with two layers of PDMS slices. The ITO cathode was modified with Au nanoparticles for further functionalization of biomolecules, which could also amplify the ECL signal at the anode of the BPE. Based on the specific hybridization and hybridization chain reaction (HCR), DNA sequences were greatly extended, leading to a significant increase in the resistance of the cathode. The reduction of H₂O₂ was inhibited on the cathode of the BPE, resulting in a quenching effect on the ECL intensity on the anode of the BPE. The designed biosensor displayed a satisfactory linear relationship for the detection of both DNA and H₂O₂. Therefore, the biosensor could not only be employed for DNA assays but also used in enzyme reactions based on the generation of H₂O₂.

Visual Electrofluorochromic Detection of Cancer Cell Surface Glycoprotein on a Closed Bipolar Electrode Chip

This work reports an electrofluorochromic strategy on the basis of electric field control of fluorescent signal generation on bipolar electrodes (BPEs) for visualizing cancer cell surface glycoprotein (mucin 1). The device included two separate cells: anodic sensing cell and cathodic reporting cell, which were connected by a screen-printing electrode patterned on poly(ethylene terephthalate) (PET) membrane. In the sensing cell, anti-MUC1 antibody immobilized on a chitosan-multiwalled carbon nanotube (CS-MWCNT)-modified anodic BPE channel was used for capturing mucin-1 (MUC1) or MCF-7 cancer cells. Then ferrocene (Fc)-labeled mucin 1 aptamers were introduced through hybridization. Under an applied voltage, the ferrocene was oxidized and the electroactive molecules of 1,4-benzoquinone (BQ) in the cathodic reporting cell were reduced according to electroneutrality. This produced a strongly basic 1,4-benzoquinone anion radical (BQ^•-), which turned on the fluorescence of pH-responsive fluorescent molecules of (2-(2-(4-hydroxystyryl)-6-methyl-4 H-pyran-4-ylidene)malononitrile) (SPM) coexisting in the cathode reporting cell for both spectrophotometric detection and imaging. This strategy allowed sensitive detection of MUC1 at a concentration down to 10 fM and was capable of detecting a minimum of three MCF-7 cells. Furthermore, the amount of MUC1 on MCF-7 cells was calculated to be 6.02 × 10⁴ molecules/cell. Our strategy also had the advantages of high temporal and spatial resolution, short response time, and high luminous contrast and is of great significance for human health and the promotion of life science development.

An ultrasensitive electrochemical cytosensor for highly specific detection of HL-60 cancer cells based on metal ion functionalized titanium phosphate nanospheres

Facile and sensitive detection methods of cancer cells in the early stage are beneficial for monitoring cancers and treating patients in time to reduce the death rate. In this work, an ultrasensitive cytosensor was constructed using aptamers as cell capturers and metal ion-exchanged titanium phosphate nanospheres as electrochemical probes. KH1C12 can specifically recognize HL-60 cells and distinguish them from other cell lines, K562 and CCRF-CEM, to obtain high selectivity. Cadmium ion functionalized titanium phosphate nanospheres show large quantities of electroactive cadmium ion output and a highly sensitive electrochemical signal. This proposed cytosensor showed a wide dynamic linear range from 102 cells per mL to 107 cells per mL with a low detection limit of 35 cells per mL, providing a new, simple and ultrasensitive platform for cancer diagnosis in biomedical and clinical research.

Direct Observation of Oxidation Reaction via Closed Bipolar Electrode-Anodic Electrochemiluminescence Protocol: Structural Property and Sensing Applications

In this work, we developed an innovative closed bipolar electrode (BPE)-electrochemiluminescence (ECL) sensing strategy with generality for target detection. Based on charge balance and 100% current efficiency between the closed BPE poles and the driving electrodes, one of the driving electrodes in one cell of the closed BPE system was employed as ECL sensing surface to reflect the target on the BPE pole in the opposite cell. Compared with traditional BPE-ECL sensing method, which in general adopted the anodic ECL reagents such as Ru(bpy)₃²⁺ and its coreactant on one pole (anode) to reflect the target (occurring reduction reaction) on the other pole (cathode), the difference was that the targets occurring oxidation reaction could be detected by the anodic ECL reagents based on this strategy. To verify the feasibility of this strategy, the detection principle was stated first, and Fe(CN)₆^4- as model target at anodic BPE pole were detected by anodic ECL reagents (Ru(bpy)₃²⁺ and TprA) on the driving electrode first. The ECL signals showed good performance for target detection. By changing the size and the material of the BPE pole where the targets were located, the detection of l-ascorbic acid (AA), uric acid (UA), and dopamine (DA) as other model targets with higher detection limit were accomplished. Visual and high-throughput detection of AA, UA, and DA were also successfully realized by an array of the closed BPE system. This closed BPE (array) system is an effective supplement of traditional BPE-ECL sensing and could greatly expand the scope of the detection target.

Potential-resolved Faraday cage-type electrochemiluminescence biosensor for simultaneous determination of miRNAs using functionalized g-C3N4 and metal organic framework nanosheets

Here, a novel Faraday cage-type electrochemiluminescence (ECL) biosensor was presented for simultaneous determination of miRNA-141 and miRNA-21 based on the potential-resolved strategy. In this work, capture units were prepared by immobilizing hairpin DNA1 (HP1) and hairpin DNA2 (HP2) on Fe₃O₄ @Au nanocomposites, while g-C₃N₄ @AuNPs nanocomposites labelled by signal DNA1 (sDNA1) and ruthenium-based metal organic framework (Ru-MOF) nanosheets labelled by signal DNA2 (sDNA2) were used as signal units. In this proposed biosensor, signal units g-C₃N₄ @AuNPs-sDNA1 and Ru-MOF-sDNA2 could exhibit two strong and stable ECL emissions at - 1.4 V and + 1.5 V respectively, which could be used as effective potential-resolved signal tags. Moreover, taking advantage of the proposed Faraday cage-type model, all electrochemiluminophores in the signal units could take part in electrode reactions, the signal units became part of the electrode surface and extended the outer Helmholtz plane (OHP) of the proposed electrode, and then the detection sensitivity was improved greatly. Accordingly, dual targets miRNA-141 and miRNA-21 could be detected within the linear range of 1 fM to 10 pM, with the detection limit of 0.3 fM. Meanwhile, the proposed miRNA assay exhibited high selectivity and sensitivity, even for practical analysis in human serum. So, this potential-resolved ECL biosensor is proved to be a feasible tool for dual targets detection of miRNAs in clinical diagnosis.

Patchy gold coated Fe3O4 nanospheres with enhanced catalytic activity applied for paper-based bipolar electrode-electrochemiluminescence aptasensors

In this work, novel multifunctional patchy gold coated Fe₃O₄ hybrid nanoparticles (PG-Fe₃O₄ NPs) have been successfully synthesized in aqueous medium via a facile adsorption-reduction method. A rational formation mechanism has been proposed by monitoring the morphological evolution. The PG-Fe₃O₄ NPs retained the good magnetic property and exhibited excellent catalytical effeciency towards the electrochemical reduction of hydrogen peroxide. Chronoamperometric and amperometric experiments indicated a relatively high catalytic rate constant of 3.13 × 10⁵ M^-1 s^-1, a high sensitivity of 578.87 µA mM^-1 cm^-2 and a low Michaelis-Menten constant of 462 µM. Meanwhile, the introduction of patchy gold could help biofunctionalization via Au-S bond for different biodetection and biosensing purposes. Here, as an example, thiol-terminated aptamers were immobilized onto the patchy gold part as a signal probe to detect carcinoembryonic antigen (CEA). A related paper-based bipolar electrode-electrochemiluminescence (pBPE-ECL) aptasensor was fabricated as the low-cost, disposable and miniature platform. To improve the sensitivity, Au nanodendrites were electrodeposited at the BPE cathode as the matrix for Apt1 immobilization. This aptasensor showed a wide linear range of 0.1 pg mL^-1-15 ng mL^-1 with a low detection limit of 0.03 pg mL^-1, remaining competitive against other ones, and also demonstrating the PG-Fe₃O₄ NPs have promising potential for catalysis and bioassays.

Cerium Dioxide-Mediated Signal "On-Off" by Resonance Energy Transfer on a Lab-On-Paper Device for Ultrasensitive Detection of Lead Ions

In this report, a 3D microfluidic lab-on-paper device for ultrasensitive detection of lead cation was designed using phoenix tree fruit-shaped CeO₂ nanoparticles (PFCeO₂ NPs) as the catalyst and 50 nm silver NPs (Ag NPs) as the quencher. First, snowflake-like Ag NPs were grown on the paper working electrode through an in situ growth method and used as a matrix for DNAzymes that were specific for lead ions (Pb²⁺). After the addition of Ag NP-labeled substrate strands, the Ag NPs restrained the electrochemiluminescence (ECL) intensity of luminol greatly through the resonance energy transfer from luminol to Ag NPs. However, under the existence of Pb²⁺, the substrate strands were separated, and then PFCeO₂ NP-labeled signal strands were hybridized with the DNAzymes. The ECL signal was improved greatly under the fast catalytic reaction between PFCeO₂ NPs and H₂O₂, which converted the response from signal off to signal on state, resulting in sensitive detection of Pb²⁺. Under the optimal conditions, the ECL signal response exhibited a good linear relationship with the logarithm of lead cation in a wide linear range of 0.05-2000 nM and an ultralow detection limit of 0.016 nM. Meanwhile, a sensor featured with good specificity, acceptable stability, reproducibility, and low cost provides a promising portable, simple, and effective strategy for Pb²⁺ detection.

Nanotechnology-Enhanced No-Wash Biosensors for in Vitro Diagnostics of Cancer

In vitro biosensors have been an integral component for early diagnosis of cancer in the clinic. Among them, no-wash biosensors, which only depend on the simple mixing of the signal generating probes and the sample solution without additional washing and separation steps, have been found to be particularly attractive. The outstanding advantages of facile, convenient, and rapid response of no-wash biosensors are especially suitable for point-of-care testing (POCT). One fast-growing field of no-wash biosensor design involves the usage of nanomaterials as signal amplification carriers or direct signal generating elements. The analytical capacity of no-wash biosensors with respect to sensitivity or limit of detection, specificity, stability, and multiplexing detection capacity is largely improved because of their large surface area, excellent optical, electrical, catalytic, and magnetic properties. This review provides a comprehensive overview of various nanomaterial-enhanced no-wash biosensing technologies and focuses on the analysis of the underlying mechanism of these technologies applied for the early detection of cancer biomarkers ranging from small molecules to proteins, and even whole cancerous cells. Representative examples are selected to demonstrate the proof-of-concept with promising applications for in vitro diagnostics of cancer. Finally, a brief discussion of common unresolved issues and a perspective outlook on the field are provided.

Bipolar Electrodes with 100% Current Efficiency for Sensors

A bipolar electrode (BPE) is an electron conductor that is embedded in the electrolyte solution without the direct connection with the external power source (driving electrode). When the sufficient voltage was provided, the two poles of BPE promote different oxidation and reduction reactions. During the past few years, BPEs with wireless feature and easy integration showed great promise in the various fields including asymmetric modification/synthesis, motion control, targets enrichment/separation, and chemical sensing/biosensing combined with the quantitative relationship between two poles of BPE. In this perspective paper, we first describe the concept and history of the BPE for analytical chemistry and then review the recent developments in the application of BPEs for sensing with ultrahigh current efficiency (η_c = i_BPE/i_channel) including the open and closed bipolar system. Finally, we offer the guide for possible challenge faced and solution in the future.

A novel metal–organic framework loaded with abundant N-(aminobutyl)-N-(ethylisoluminol) as a high-efficiency electrochemiluminescence indicator for sensitive detection of mucin1 on cancer cells

Abundant luminophore N-(aminobutyl)-N-(ethylisoluminol) functionalized metal–organic framework was synthesized as a novel and high-efficiency electrochemiluminescence indicator.