Functional Three-Dimensional Porous Conductive Polymer Hydrogels for Sensitive Electrochemiluminescence in Situ Detection of H2O2 Released from Live Cells

General and fast synthesis of graphene frameworks using sugars for high-performance hydrogen peroxide nonenzymatic electrochemical sensor

3D graphene frameworks (GFs) are fast and scalably synthesized via a general and facile method from the rich biomass of sugars with the aid of molten salts, using glucose as the prototype, to obtain an effective sensing platform for sensitive nonenzymatic hydrogen peroxide (H₂O₂) detection. The electroactive area of the GFs/GCE (0.1437 cm²) is obviously higher than that of bare GCE (0.0653 cm²). The GFs are found to exhibit remarkable electrocatalytic activity toward H₂O₂ reduction while avoiding enzyme loading. The electrochemical sensor for H₂O₂ based on GFs displays a low detection limit of 0.032 ± 0.005 μM (S/N = 3) at a working potential of - 0.55 V in 0.01 M N₂-saturated phosphate-buffered saline (PBS, pH = 7.4) by an amperometric method. The sensor has good selectivity over other compounds such as ascorbic acid, dopamine, uric acid, NaCl, citric acid, and glucose. Moreover, the sensor shows excellent reproducibility with a relative standard deviation of 3.7% and acceptable stability after 30 days of usage. Furthermore, it can detect H₂O₂ released from living tumorigenic cells in real time. Most importantly, it is demonstrated that such GFs can be obtained from a variety of sugars (sucrose, fructose, lactose, and maltose). This work may offer a new general avenue for the synthesis of 3D GFs and promote the development of electrochemical sensors. Graphical abstract We have reported a general and fast method to synthesize GFs from sugars (glucose, sucrose, fructose, lactose, and maltose) with the addition of molten Na₂CO₃ salt as a template. The developed GFs can be applied as excellent electrode materials for efficient electrochemical sensing of H₂O₂.

Applications of advanced materials in bio-sensing in live cells: Methods and applications

A wide variety of species, such as different ions, reactive oxygen species, and biomolecules play critical roles in many cell functions. These species are responsible for a range of cellular functions such as signaling, and disturbed levels could be involved in many diseases, such as diabetes, cancer, neurodegeneration etc. Thus, sensitive and specific detection methods for these biomarkers could be helpful for early disease detection and mechanistic investigations. New ultrasensitive sensors for detection of markers within living cells are a growing field of research. The present review provides updates in live cell-based biosensing, which have been published within the last decade. These sensors are mainly based on carbon, gold and other metals, and their physicochemical advantages and limitations are discussed. Advanced materials can be incorporated into probes for the detection of various analytes in living cells. The sensitivity is strongly influenced by the intrinsic properties of the nanomaterials as well their shape and size. The mechanisms of action and future challenges in the developments of new methods for live cell based biosensing are discussed.

Fluorescent Noble Metal Nanoclusters Loaded Protein Hydrogel Exhibiting Anti-Biofouling and Self-Healing Properties for Electrochemiluminescence Biosensing Applications

Electrochemiluminescence (ECL) showed great potential in various analytical applications, especially in the sensing of biotargets, taking advantage of its high sensitivity, selectivity, ease of spatial and temporal control, and simplified optical setup. However, during the sensing of complex biological samples, ECL sensors often suffered severe interferences from unavoidable nonspecific-binding of biomacromolecules and physical damages of ECL sensing interfaces. Herein, a hydrogel based ECL biosensing system exhibiting excellent anti-biofouling and self-healing properties is developed. A protein hydrogel composed of bovine serum albumin (BSA) directed fluorescent Au/Ag alloy nanoclusters (Au/Ag NCs) is applied in building ECL sensing systems. The hydrogel matrix facilitates the immobilization of fluorescent Au/Ag NCs as excellent ECL probes, and the porous hydrophilic structure allows the free diffusion of small molecular biotargets while rejecting macromolecular interferences. Moreover, the hydrogel exhibits excellent self-healing property, with the ECL intensity recovered rapidly in 10 min after cutting. The hydrogel ECL system is successfully applied in sensing glutathione (GSH) in serum, confirming the applicability of the hydrogel based anti-biofouling ECL sensing system in sensing complex biological samples. This research may inspire the development of novel anti-biofouling and self-healing ECL biosensors for biosensing applications.

A non-enzymatic electrochemical biosensor based on Au@PBA(Ni-Fe):MoS2 nanocubes for stable and sensitive detection of hydrogen peroxide released from living cells

Hydrogen peroxide (H2O2) is the main product of enzymatic reactions and plays an important role in biological processes. The detection of H2O2 inside organisms or cells is critical. Here, we report a nickel-iron Prussian blue analogue nanocube doped with molybdenum disulfide and Au nanoparticles (Au@PBA(Ni-Fe):MoS2) as an electrochemical sensing material for the stable detection of H2O2 in neutral solutions for a long time. First, the Prussian blue analogue (PBA(Ni-Fe)) is synthesized by a simple charge-assembly technology, and then etched into PBA(Ni-Fe):MoS2 hollow nanocubes by a high-temperature hydrothermal reaction. Finally, Au nanoparticles are reduced inside the PBA(Ni-Fe):MoS2in situ to generate Au@PBA(Ni-Fe):MoS2 nanocubes. Ni-doping enhances the nanocube's stability in neutral solutions; as a result, the sensor can maintain a stable current response towards H2O2 reduction for more than 1 h. The sensing material can meet the needs of a long-time test. The introduction of Au enhances the electron transfer efficiency, which endows the sensor with good reduction ability for H2O2 at 0 V over a wide linear range (0.5-200 μM and 210-3000 μM) and with a low detection limit (0.23 μM (S/N = 3)), which fulfills the requirements for the detection of H2O2 in a biological system. The sensor can sense H2O2 released from cells stimulated by ascorbic acid. Au@PBA(Ni-Fe):MoS2 provides good guidance for the future development of efficient biosensors to be applied in cell biology.

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.

DNA-AgNCs as a fluorescence turn-off probe for dual functional detection of H2O2 and Fe(II) ions

A novel fluorescence probe (DNA-AgNCs) was synthesized for dually detecting hydrogen peroxide (H₂O₂) and ferrous ion (Fe(II)) in water samples. The assay is carried out through a dramatic "turn-off" fluorescence response of AgNCs by hydroxyl radical (OH), which is produced when H₂O₂ and Fe(II) are present simultaneously. Under the optimal conditions, the degree of fluorescence quenching of the DNA-AgNCs at 525 nm is linearly related to the concentration of H₂O₂ in the range of 0.3 to 450 pM, and Fe(II) in the range of 0.2 to 6.0 μM. The limit of detection (LOD) of H₂O₂ and Fe(II) are as low as 99 fM and 60 nM, respectively. Moreover, this method has the advantages of good specificity, high sensitivity, and can be successfully applied for detecting H₂O₂ and Fe(II) in the real water samples.

Organic Electrochemical Transistor for in Situ Detection of H2O2 Released from Adherent Cells and Its Application in Evaluating the In Vitro Cytotoxicity of Nanomaterial

Detection of hydrogen peroxide (H₂O₂) produced by living cells is very significant to fully understand its roles in cellular physiology, as well as providing reliable diagnosis of pathological conditions. However, in situ detection of H₂O₂ released from adherent cells in cellular culture medium is still insufficiently achieved. Here, we report an electrochemical platform for in situ detection of H₂O₂ produced by adherent cells in cellular culture medium. It is based on the use of organic electrochemical transistor (OECT) fabricated on a flexible poly(ethylene terephthalate) substrate and Transwell support. A screen-printed carbon paste electrode was modified with carbon nanotubes and platinum nanoparticles and served as the gate of the device. Under optimal conditions, this device exhibits good modulation and sensitivity. It works in the 0.5 μM to 0.1 mM H₂O₂ concentration range and has a 0.2 μM detection limit. The cells were seeded and grew on the Transwell membrane. Upon being stimulated by N-formylmethionyl-leucyl-phenylalanine peptide, H₂O₂ produced by the adherent cells diffused into the bottom chamber of the Transwell and was in situ detected by OECT. Moreover, evaluating in vitro cytotoxicity of the nanomaterial using the OECT-Transwell platform was realized. This simple electrochemical platform would be of great interest for in vitro cytotoxicity, cellular physiology study, and diagnosis of pathological conditions.

Fe-N-C Single-Atom Nanozymes for the Intracellular Hydrogen Peroxide Detection

Recently, in situ detection of hydrogen peroxide (H₂O₂) generated from live cells have caused tremendous attention, because it is of great significance in the control of multiple biological processes. Herein, Fe-N-C single-atom nanozymes (Fe-N-C SAzymes) with intrinsic peroxidase-like activity were successfully prepared via high-temperature calcination using FeCl₂, glucose, and dicyandiamide as precursors. The Fe-N-C SAzymes with FeN_x as active sites were similar to natural metalloproteases, which can specifically enhance the peroxidase-like activity rather than oxidase-like activity. Accordingly, owing to the excellent catalytic efficiency of the Fe-N-C SAzymes, colorimetric biosensing of H₂O₂ in vitro was performed via a typical 3,3',5,5'-tetramethylbenzidine induced an allochroic reaction, demonstrating the satisfactory specificity and sensitivity. With regard to the practical application, in situ detection of H₂O₂ generated from the Hela cells by the Fe-N-C SAzymes was also performed, which can expand the applications of the newborn SAzymes.

ATMP-induced three-dimensional conductive polymer hydrogel scaffold for a novel enhanced solid-state electrochemiluminescence biosensor

Reliable and sensitive detection of xanthine has important medical and biological significance. In this work, a novel three-dimensional (3D) conductive polymer hydrogel of polyaniline (PAni) was feasibly prepared using aniline (Ani), amino trimethylene phosphonic acid (ATMP) and ammonium persulfate ((NH₄)₂S₂O₈) as monomer, gelatinizing agent and oxidizing agent, respectively. Protonation of aniline can be achieved by ATMP, inducing good conductivity of the obtained hydrogel. ATMP remained the chelating abilities in the conductive hydrogel, enabling further immobilization with silver nanoparticles (AgNPs) functionalized by a luminol derivative, N-(aminobutyl)-N-(ethylisoluminol) (ABEI). ABEI-Ag@PAni-ATMP exhibited an enhanced performance of solid-state electrochemiluminescence (ECL). Integrated with xanthine oxidase (XOD), the proposed biosensor can be applied in the detection of xanthine via in-situ generated hydrogen peroxide (H₂O₂), and present a low detection limit of 9.6 nM, a wide linear range (from 0.01 to 200 μM) and excellent stability.

Recent Advances in Aggregation-Induced Electrochemiluminescence

The emergence of the rising alliance between aggregation-induced emission (AIE) and electrochemiluminescence (ECL) is defined as aggregation-induced electrochemiluminescence (AIECL). The booming science of AIE has proved to be not only distinguished in luminescent materials but could also inject new possibility into ECL analysis. Especially in the aqueous phase and solid state for hydrophobic materials, AIE helps ECL circumvent the dilemma between substantial emission intensity and biocompatible media. The wide range of analytes makes ECL an overwhelmingly interesting analytical technique. Therefore, AIECL has gained potential in clinical diagnostics, environmental assays, and biomarker detections. This review will focus on introduction of the novel concept of AIECL, current applied luminophores, and related applications developed in recent years.

A multifunctional self-healing G-PyB/KCl hydrogel: smart conductive, rapid room-temperature phase-selective gelation, and ultrasensitive detection of alpha-fetoprotein

A multifunctional G-PyB/KCl hydrogel showed outstanding self-healability, high conductivity, and rapid room-temperature phase-selective gelation capacity, and was developed as an electrochemical aptamer sensing platform for the ultrasensitive detection of alpha-fetoprotein.

Three-Dimensional TiO2@Cu2O@Nickel Foam Electrodes: Design, Characterization, and Validation of O2-Independent Photocathodic Enzymatic Bioanalysis

This work reports the innovative design and application of a three-dimensional (3D) TiO₂@Cu₂O@nickel foam electrode synergized with enzyme catalysis toward the proof-of-concept study for oxygen-independent photocathodic enzymatic detection. Specifically, a 3D-nanostructured photoelectrode has great potential in the semiconductor-based photoelectrochemical (PEC) biological analysis. On the other hand, using various photocathodes, cathodic PEC bioanalysis, especially the photocathodic enzymatic detection, represents an attractive frontier in the field. Different from state-of-the-art photocathodic enzymatic studies that are oxygen-dependent, herein, we present the ingenious design, characterization, and implementation of 3D TiO₂@Cu₂O@nickel foam photocathodes for the first oxygen-independent example. In such a configuration, the Cu₂O acted as the visible-light absorber, while the TiO₂ shell would simultaneously function as a protective layer for Cu₂O and as a desirable substrate for the immobilization of enzyme biomolecules. Especially, because of the proper band positions, the as-designed photocathode exhibited unique O₂-independent PEC property. Exemplified by glucose oxidases, the as-developed sensor exhibited positive response to glucose with good performance. Because various oxidases could be integrated with the system, this protocol could serve as a universal O₂-independent platform for many other targets. This work is also anticipated to catalyze more studies in the advanced 3D photoelectrodes toward innovative enzymatic applications.

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.

Tunable Three-Dimensional Nanostructured Conductive Polymer Hydrogels for Energy-Storage Applications

Three-dimensional (3D) nanostructured conducting polymer hydrogels represent a group of high-performance electrochemical energy-storage materials. Here, we demonstrate a molecular self-assembly approach toward controlled synthesis of nanostructured polypyrrole (PPy) conducting hydrogels, which was "cross-linked" by a conjugated dopant molecule trypan blue (TB) to form a 3D network with controlled morphology. The protonated TB by ion bonding aligns the free sulfonic acid groups into a certain spatial structure. The sulfonic acid group and the PPy chain are arranged by a self-sorting mechanism to form a PPy nanofiber structure by electrostatic interaction and hydrogen bonding. It is found that PPy hydrogels doped with varying dopant concentrations and changing dopant molecules exhibited controllable morphology and tunable electrochemical properties. In addition, the conjugated TB dopants promoted interchain charge transport, resulting in higher electrical conductivity (3.3 S/cm) and pseudocapacitance for the TB-doped PPy, compared with PPy synthesized without TB. When used as supercapacitor electrodes, the TB-doped PPy hydrogel reaches maximal specific capacitance of 649 F/g at the current density 1 A/g. The result shows that PPy nanostructured hydrogels can be tuned for potential applications in next-generation energy-storage materials.

Ultrasensitive sandwich-type immunosensor for cardiac troponin I based on enhanced electrocatalytic reduction of H2O2 using β-cyclodextrins functionalized 3D porous graphene-supported Pd@Au nanocubes

A signal amplification principle based on increased electrocatalytic reduction of H₂O₂ by the CDs-3D-PG-Pd@Au NCs using the mediated effect of Th.

A Novel Non-Enzymatic Electrochemical Hydrogen Peroxide Sensor Based on a Metal-Organic Framework/Carbon Nanofiber Composite

A co-based porous metal-organic framework (MOF) of zeolitic imidazolate framework-67 (ZIF-67) and carbon nanofibers (CNFs) was utilized to prepare a ZIF-67/CNFs composite via a one-pot synthesis method. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) were employed to investigate the morphology, structure, and composition of the resulting composite. A novel high-performance non-enzymatic electrochemical sensor was constructed based on the ZIF-67/CNFs composite. The ZIF-67/CNFs based sensor exhibited enhanced electrocatalytic activity towards H₂O₂ compared to a pure ZIF-67-based sensor, due to the synergistic effects of ZIF-67 and CNFs. Meanwhile, chronoamperometry was utilized to explore the detection performance of the sensor. Results showed the sensor displayed high-efficiency electrocatalysis towards H₂O₂ with a detection limit of 0.62 μM (S/N = 3), a sensitivity of 323 µA mM⁻¹ cm⁻², a linear range from 0.0025 to 0.19 mM, as well as satisfactory selectivity and long-term stability. Furthermore, the sensor demonstrated its application potential in the detection of H₂O₂ in food.