Analysis of Intracellular Glucose at Single Cells Using Electrochemiluminescence Imaging

An electrochemiluminescence sensor for 17β-estradiol detection based on resonance energy transfer in α-FeOOH@CdS/Ag NCs

An electrochemiluminescence (ECL) resonance energy transfer system is constructed for 17β-estradiol (E2) detection using α-FeOOH@CdS nanospheres as the ECL-active substrates and Ag NCs as an efficient quencher. CdS QDs loaded onto three-dimensional (3D) urchin-like α-FeOOH nanospheres (α-FeOOH@CdS nanospheres) exhibited excellent ECL responses, which is attributed to dual-amplification of α-FeOOH frameworks. The 3D hierarchical structure of the α-FeOOH nanospheres provided abundant sites for loading ECL-active species, thus significantly improving the ECL performance of substrates; While Fe³⁺ presented on surface of α-FeOOH nanospheres could be reduced to Fe²⁺ in negative potentials, after which might activate persulfate in a Fenton-like process, resulting in more sulfate free radicals for more effective ECL responses via electron transfer reactions. Additionally, Ag nanoclusters (Ag NCs) stabilized by single stranded oligonucleotide were introduced as quenching probes for CdS QDs owing to the well-matched donor-acceptor spectrum for efficient energy transfer, which makes them appropriate for detection of E2. The proposed strategy displayed a desirable dynamic range from 0.01 to 10 pg mL^-1 with a limit of detection of 0.003 pg mL^-1. The proposed strategy based on the ECL-RET strategy offered an ideal way for E2 detection, and also revealed an alternative platform for detection of other small molecules.

Titanium carbide MXenes combined with red-emitting carbon dots as a unique turn-on fluorescent nanosensor for label-free determination of glucose

Titanium carbides (Ti3C2), a new family of two-dimensional (2D) nanomaterials, have attracted extensive attention due to their unique structure and desirable physiochemical properties. Herein, we developed an effective and selective fluorescent turn-on nanosensor for glucose detection based on Ti3C2 nanosheets combined with red-emitting carbon dots (RCDs). The fluorescence intensity of RCDs could be effectively quenched (>96%) by Ti3C2 nanosheets through the inner-filter effect (IFE). In the presence of H2O2, the quenched fluorescence of the RCDs can remarkably recover due to the Ti3C2 nanosheets which were oxidized into Ti(OH)4 by H2O2. Based on H2O2 generated from oxidation of glucose catalyzed by glucose oxidase, the nanosensor can also be exploited for monitoring glucose. Under optimal conditions, a linear relationship between the increased fluorescence intensity of RCDs and the concentration of glucose was established in the range from 0.1 to 20 mM. The detection limit was 50 μM (S/N = 3). The proposed nanosensor also represented excellent selectivity for glucose analysis in biological fluid samples, providing a valuable platform for glucose sensing in clinical diagnostics.

Electrochemiluminescence Imaging for the Morphological and Quantitative Analysis of Living Cells under External Stimulation

In this work, a simple electrochemiluminescence (ECL) imaging method based on the cell shield of the ECL emission was developed for the morphological and quantitative analysis of living cells under external stimulation. ECL images of MCF-7 cells cultured on or captured at the glassy carbon electrode (GCE) surface in a solution of tris(2,2'-bipyridyl)ruthenium(II)-tri-n-propylamine were recorded. Important morphological characteristics of living cells, including cell shape, cell area, average cell boundary, and junction distance between two adjacent cells, were directly obtained using the developed negative ECL imaging method. The ECL images revealed gradual morphological changes in cells on the GCE surface. During the course of H₂O₂ stimulation of cells on the GCE surface, cells shrunk, rounded up, disengaged from surrounding cells, and finally detached from the electrode surface. During the course of electrical stimulation (0.8 V), the cells on the GCE surface exhibited aggregation as demonstrated by increases in the average cell boundary and decreases in the junction distance between two adjacent cells. Additionally, a quantitative method for the sensitive determination of MCF-7 cells with a limit of detection of 29 cells/mL was developed using the negative ECL imaging strategy. This work demonstrates that the proposed negative ECL imaging strategy is a promising approach to assess important morphological characteristics of living cells during the course of external stimulation and to obtain quantitative information on cell concentrations in solution.

Amplified electrochemiluminescence signals promoted by the AIE-active moiety of D-A type polymer dots for biosensing

Three-component conjugated polymers of a strong donor-acceptor (D-A) type could be synthesized by Pd-catalyzed Suzuki coupling polymerization reaction of 1,2-bis(4-bromophenyl)-1,2-diphenylethene (M-1) with 9-octyl-3,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole (M-2) and 4,6-bis((E)-4-bromostyryl)-2,2-difluoro-5-phenyl-2H-1l3,3,2l4-dioxaborinine (M-3). Among them, P-1 and P-2 with high TPE ratios at 0.95 and 0.9 showed obvious aggregation-induced emission (AIE) behavior; in contrast P-3 with a low TPE ratio at 0.8 showed an aggregation-caused quenching (ACQ) phenomenon. In particular, the three resulting polymer dots (P-1 to P-3 Pdots) exhibited a 200 mV lower electrochemiluminescence (ECL) potential due to their strong D-A electronic structure. Most importantly, the ECL signals of Pdots could be enhanced as high as 3 times by increasing their AIE-active TPE moiety ratios from 0.8 (P-3) to 0.95 (P-1) via the band gap emission process. Herein, P-1 Pdots with the strongest ECL signal were successfully used as ECL biosensors for the detection of catechol, epinephrine and dopamine with detection limits of 1, 7 and 3 nM, respectively. This work provides a new strategy for developing highly sensitive ECL biosensors by the smart structure design of the AIE-active Pdots.

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.

Triple Amplification of 3,4,9,10-Perylenetetracarboxylic Acid by Co2+-Based Metal-Organic Frameworks and Silver-Cysteine and Its Potential Application for Ultrasensitive Assay of Procalcitonin

In this work, a triple-amplified biosensor with a bioactivity-maintained peculiarity was constructed for quantitative procalcitonin (PCT) detection. As everyone knows, a strong electrochemiluminescence (ECL) signal is the premise to ensure high sensitivity for trace target detection. Hence, a valid tactic was developed to achieve signal amplification of luminophor by using Co²⁺-based metal-organic frameworks (ZIF-67) and silver-cysteine (AgCys). The ZIF-67 particles, which have more atomically dispersed Co²⁺, could play the role of a co-reaction accelerator to catalyze S₂O₈^2- to generate abundant Co³⁺ and sulfate radical anions (SO₄^•-). Afterward, a mass of Co³⁺ was reduced to more hydroxyl radicals (OH^•) by H₂O, thus ulteriorly reducing S₂O₈^2- to generate more SO₄^•-. Remarkably, S₂O₈^2- was reduced to SO₄^•- continuously with the recycling of Co²⁺ and Co³⁺, which realized an effective signal amplification. Meanwhile, the AgCys complex with superior catalysis and biocompatibility was prepared to further improve the ECL signal and maintain the bioactivity of the biomolecule. Furthermore, HWRGWVC, a heptapeptide that was used for combining the Fc fragments of an antibody by Au-S bonding to achieve the fixed point fixation, could not only maintain bioactivity of an antibody but also improved its incubation efficiency, thus further enhancing biosensor sensitivity. Under optimum conditions, the proposed biosensor realized highly sensitive assay for PCT with a wide dynamic range from 10 fg/mL to 100 ng/mL and a detection limit as low as 3.67 fg/mL. With superior stability, selectivity, and repeatability, the prepared biosensor revealed immense potential application of ultrasensitive assay for PCT in human serum.

High throughput profiling drug response and apoptosis of single polar cells

The drug response of single polar cells was evaluated via single cell trapping on anisotropic microwells for tumor heterogeneity.

Nanomaterials in electrochemical cytosensors

Nano-electrochemical cytosensors have attracted intensive attention and achieved huge progress in the biomedical field owing to their stability, rapidity, accuracy, and low-cost properties.

Recent advances in co-reaction accelerators for sensitive electrochemiluminescence analysis

In electrochemiluminescence sensing platforms, co-reaction accelerators are specific materials used to catalyze the dissociation of co-reactants into active radicals, which can significantly boost the ECL emission of luminophores.

Gold nanoparticles decorated carbon nitride nanosheets as a coreactant regulate the conversion of the dual-potential electrochemiluminescence of Ru(bpy)32+ for Hg2+ detection

Dual-potential electrochemiluminescence of Ru(bpy)₃²⁺ for Hg²⁺ assay using Au–g-C₃N₄ NSs as on-electrode coreactant.

Electrogenerated Chemiluminescence Biosensing

This Feature simply introduces the history and mechanism of classical electrogenerated chemiluminescence (ECL) systems for the detection of biomolecules, highlights new advances and emerging fields of the ECL biosensing with recent illustrative examples, and presents the challenges and perspectives of ECL biosensing.

Fluorescence quantification of intracellular materials at the single-cell level by an integrated dual-well array microfluidic device

We present an integrated microfluidic device for quantifying intracellular materials at the single-cell level. This device combines a dual-well structure and a microfluidic control system. The dual-well structure includes capture wells (20 μm in diameter) for trapping a single cell and reaction wells (200 μm in diameter) for confining reagents. The control system enables a programmable procedure for single-cell analysis. This device achieves highly efficient trapping of single cells, overcoming the Poisson distribution, while affording sufficient biochemical reagents for each isolated reactor. We successfully utilized the presented device to monitor the catalytic interaction between intracellular alkaline phosphatase enzyme and a fluorogenic substrate and to quantify the intracellular glucose concentration of a single K562 cell based on an external standard method. The results demonstrate the feasibility and convenience of our dual-well array microfluidic device as a practical single-cell research tool.

Electrochemiluminescence Imaging for Bioanalysis

Electrochemiluminescence (ECL) is a widely used analytical technique with the advantages of high sensitivity and low background signal. The recent and rapid development of electrochemical materials, luminophores, and optical elements significantly increases the ECL signals and, thus, ECL imaging with enhanced spatial and temporal resolutions is realized. Currently, ECL imaging is successfully applied to high-throughput bioanalysis and to visualize the distribution of molecules at single cells. Compared with other optical bioassays, no optical excitation is involved in imaging, so the approach avoids a background signal from illumination and increases the detection sensitivity. This review highlights some of the most exciting developments in this field, including the mechanisms, electrode designs, and the applications of ECL imaging in bioanalysis and at single cells and particles.

Electrochemiluminescence as emerging microscopy techniques

The use of electrochemiluminescence (ECL), i.e., chemiluminescence triggered by electrochemical stimulus, as emitting light source for microscopy is an emerging approach with different applications ranging from the visualization of nanomaterials to cell mapping. In this trend article, we give an overview of the state of the art in this new field with the purpose to illustrate all the possible applications so far explored as well as describing the mechanism underlying this transduction technique. The results discussed here would highlight the great potential of the combination between ECL and microscopy and how this marriage can turn into an innovative approach with specific application in analytical sciences. Graphical abstract.

Surface-Confined Electrochemiluminescence Microscopy of Cell Membranes

Herein is reported a surface-confined microscopy based on electrochemiluminescence (ECL) that allows to image the plasma membrane of single cells at the interface with an electrode. By analyzing photoluminescence (PL), ECL and AFM images of mammalian CHO cells, we demonstrate that, in contrast to the wide-field fluorescence, ECL emission is confined to the immediate vicinity of the electrode surface and only the basal membrane of the cell becomes luminescent. The resulting ECL microscopy reveals details that are not resolved by classic fluorescence microscopy, without any light irradiation and specific setup. The thickness of the ECL-emitting regions is ∼500 nm due to the unique ECL mechanism that involves short-lifetime electrogenerated radicals. In addition, the reported ECL microscopy is a dynamic technique that reflects the transport properties through the cell membranes and not only the specific labeling of the membranes. Finally, disposable transparent carbon nanotube (CNT)-based electrodes inkjet-printed on classic microscope glass coverslips were used to image cells in both reflection and transmission configurations. Therefore, our approach opens new avenues for ECL as a surface-confined microscopy to develop single cell assays and to image the dynamics of biological entities in cells or in membranes.

A high precision MUA-spaced single-cell sensor for cellular receptor assay based on bifunctional Au@Cu-PbCQD nanoprobes

A single-cell sensor with a spatial architecture was firstly fabricated for realizing high precision single-cell analysis using an 11-mercaptoundecanoic acid (MUA)-spaced sensing interface to prop up single cells and provide a suitable space for effective nanoprobe labeling. Mercapto acids (MA) with different carbon chain lengths were optimized and MUA was selected to provide optimal interspace on the electrodeposited PANI/AuNP substrates, and its carboxyl could couple with folic acid to capture cancer cells. Bifunctional Au@Cu-PbCQD nanoprobes, in which the AuNP cores were linked with lead-coadsorbed carbon quantum dots (PbCQDs) by a copper(ii) ion bridge, were firstly synthesized and applied as highly sensitive electrochemiluminescence (ECL) probes and electrochemical probes. Hyaluronic acid (HA)-functionalized Au@Cu-PbCQD nanoprobes were labelled on MCF-7 cells via specific recognition to the CD44 receptor, which served as the research model. The ECL response of the sensor was applied to evaluate the validity of nanoprobe labeling. With MUA modified, the sensor was able to enhance the ECL intensity by 37.5 ± 3.9%, indicating the remarkable amelioration of the accuracy of single-cell analysis. To take advantage of the bifunctional nanoprobes, differential pulse voltammetry (DPV) was further applied to confirm the feasibility of the proposed single-cell sensor with a spatial architecture. Therefore, the novel strategy provides a single-cell analysis platform to acquire high-precision analytical results, and more accurately to elucidate cellular heterogeneity and biological function.

Non-invasive monitoring of blood glucose using optical methods for skin spectroscopy-opportunities and recent advances

Diabetes mellitus is a widespread disease with greatly rising patient numbers expected in the future, not only for industrialized countries but also for regions in the developing world. There is a need for efficient therapy, which can be via self-monitoring of blood glucose levels to provide tight glycemic control for reducing the risks of severe health complications. Advancements in diabetes technology can nowadays offer different sensor approaches, even for continuous blood glucose monitoring. Non-invasive blood glucose assays have been promised for many years and various vibrational spectroscopy-based methods of the skin are candidates for achieving this goal. Due to the small spectral signatures of the glucose hidden among a largely variable background, the largest signal-to-noise ratios and multivariate calibration are essential to provide the method applicability for self-monitoring of blood glucose. Besides multiparameter approaches, recently presented devices based on photoplethysmography with wavelengths in the visible and near-infrared range are evaluated for their potential of providing reliable blood glucose concentration predictions. Graphical abstract ᅟ.

Hybrid Mesoporous Nanocarriers Act by Processing Logic Tasks: Toward the Design of Nanobots Capable of Reading Information from the Environment

Here, we present the design of smart nanodevices capable of reading molecular information from the environment and acting accordingly by processing Boolean logic tasks. As proof of concept, we prepared Au-mesoporous silica (MS) nanoparticles functionalized with the enzyme glucose dehydrogenase (GDH) on the Au surface and with supramolecular nanovalves as caps on the MS surface, which is loaded with a cargo (dye or drug). The nanodevice acts as an AND logic gate and reads information from the solution (presence of glucose and nicotinamide adenine dinucleotide (NAD⁺)), which results in cargo release. We show the possibility of coimmobilizing GDH and the enzyme urease on nanoparticles to mimic an INHIBIT logic gate, in which the AND gate is switched off by the presence of urea. We also show that such nanodevices can deliver cytotoxic drugs in cancer cells by recognizing intracellular NAD⁺ and the presence of glucose.

Gold-coated polydimethylsiloxane microwells for high-throughput electrochemiluminescence analysis of intracellular glucose at single cells

In this communication, a gold-coated polydimethylsiloxane (PDMS) chip with cell-sized microwells was prepared through a stamping and spraying process that was applied directly for high-throughput electrochemiluminescence (ECL) analysis of intracellular glucose at single cells. As compared with the previous multiple-step fabrication of photoresist-based microwells on the electrode, the preparation process is simple and offers fresh electrode surface for higher luminescence intensity. More luminescence intensity was recorded from cell-retained microwells than that at the planar region among the microwells that was correlated with the content of intracellular glucose. The successful monitoring of intracellular glucose at single cells using this PDMS chip will provide an alternative strategy for high-throughput single-cell analysis. Graphical abstract ᅟ.

New Frontiers and Challenges for Single-Cell Electrochemical Analysis

Previous measurements of cell populations might obscure many important cellular differences, and new strategies for single-cell analyses are urgently needed to re-examine these fundamental biological principles for better diagnosis and treatment of diseases. Electrochemistry is a robust technique for the analysis of single living cells that has the advantages of minor interruption of cellular activity and provides the capability of high spatiotemporal resolution. The achievements of the past 30 years have revealed significant information about the exocytotic events of single cells to elucidate the mechanisms of cellular activity. Currently, the rapid developments of micro/nanofabrication and optoelectronic technologies drive the development of multifunctional electrodes and novel electrochemical approaches with higher resolution for single cells. In this Perspective, three new frontiers in this field, namely, electrochemical microscopy, intracellular analysis, and single-cell analysis in a biological system (i.e., neocortex and retina), are reviewed. The unique features and remaining challenges of these techniques are discussed.

Aggregation-induced emission of luminol: a novel strategy for fluorescence ratiometric detection of ALP and As(v) with high sensitivity and selectivity

Luminol is introduced as a ligand for Tb³⁺ which, combined with GMP, leads to a sensor which is more robust, sensitive, and efficient.

Electrochemiluminescence biosensing based on different modes of switching signals

Electrochemiluminescence (ECL) has attracted much attention in various fields of analysis owing to low background signals, high sensitivity, and excellent controllability.