Ultrasensitive detection of hydrogen peroxide and dopamine using copolymer-grafted metal-organic framework based electrochemical sensor

Enzymeless detection and real-time analysis of intracellular hydrogen peroxide released from cancer cells using gold nanoparticles embedded bimetallic metal organic framework

Abnormal cell growth and proliferation can lead to tumor formation and cancer, one of the most fatal diseases worldwide. Hydrogen peroxide (H2O2) has emerged as a cancer biomarker, with its concentration being crucial for distinguishing cancer cells from normal cells. Herein, a cost-effective and enzymeless electrochemical sensing system for the monitoring of intracellular H2O2 has been constructed. The sensor is fabricated using gold nanoparticles embedded bimetallic copper/nickel metal organic framework (Au-CNMOF) immobilized reduced graphene oxide (RGO) modified screen printed electrode (SPE). The synthesized materials were characterized and confirmed by XRD, FTIR, SEM with EDS, and electrochemical analysis. The fabricated sensor displayed a redox peak at a formal potential (E°) of -0.155 V, corresponding to CuII/I redox couple of CNMOF in 0.1 M phosphate buffer. Electrochemical investigations revealed that the proposed sensor has a large electrochemical active surface area (1.113 cm2) and a higher surface roughness (5.67). Additionally, the sensor demonstrated excellent electrocatalytic activity towards H2O2 at -0.3 V, over a wide linear detection range from 28.5 µM to 4.564 mM with a limit of detection of 4.2 µM (S/N=3). Furthermore, the proposed sensor exhibits excellent stability, repeatability, reproducibility, and good anti-interference activity. Ultimately, the sensor was validated through real-time analysis of H2O2 released from cancer cells, successfully quantifying the released H2O2. The developed sensor holds great promise for real-time H2O2 analysis, with potential applications in clinical diagnostics, biological research and environmental monitoring.

Selective and Comparative Study of B/nZVCu-Fe and B/nZVCu-Zn Nanoparticles as Fluorescent Probe for Dopamine in Presence of its Interference Molecules

This work focuses on the synthesis of Bentonite supported nano zero valent bimetallic nanoparticles (B/nZVCu-M NPs) to be utilized for fast and highly sensitive, reversible, fluorescent determination of dopamine (DA) in the presence of dopamine, other biomolecules and ions. The X-ray Photoelectron Spectroscopy(XPS), Powder X-Ray Diffraction(PXRD) and Scanning Electron Microscopy(SEM) revealed the formation of nanoparticles with size ranging from 15 to 20 nm. The composition was revealed by Fourier Transform Infrared(FTIR) Spectoscopy and Energy Dispersive X-Ray (EDX) Analysis. The Limits of Detection(LOD) were noted to be 5.57nM and 6.07nM. The binding of DA is noted to be reversible with respect to EDTA2-. Furthermore, the developed sensor exhibited good repeatability, satisfactory long-term stability, and was successfully used for the selective detection of dopamine sample with desired recoveries or reversibilities. The main aim of our work is to selectively detect dopamine in presence of its major interferents and biomolecules that are normally present/ co-exist with dopamine in biological systems.

Zirconium-based metal-organic frameworks and their roles in electrocatalysis

Due to their exceptional chemical stability in water and high structural tunability, zirconium(IV)-based MOFs (Zr-MOFs) have been considered attractive materials in the broad fields of electrocatalysis. Numerous studies published since 2015 have attempted to utilise Zr-MOFs in electrocatalysis, with the porous framework serving as either the active electrocatalyst or the scaffold or surface coating to further enhance the performance of the actual electrocatalyst. Herein, the roles of Zr-MOFs in electrocatalytic processes are discussed, and some selected examples reporting the applications of Zr-MOFs in various electrocatalytic reactions, including several studies from our group, are overviewed. Challenges, limitations and opportunities in using Zr-MOFs in electrocatalysis in future studies are discussed.

The Role of Catecholamines in the Pathogenesis of Diseases and the Modified Electrodes for Electrochemical Detection of Catecholamines: A Review

Catecholamines (CAs), which include adrenaline, noradrenaline, and dopamine, are neurotransmitters and hormones that critically regulate the cardiovascular system, metabolism, and stress response in the human body. The abnormal levels of these molecules can lead to the development of various diseases, including pheochromocytoma and paragangliomas, Alzheimer's disease, and Takotsubo cardiomyopathy. Due to their low cost, high sensitivity, flexible detection strategies, ease of integration, and miniaturization, electrochemical techniques have been extensively employed in the detection of CAs, surpassing traditional analytical methods. Electrochemical detection of CAs in real samples is challenging due to the tendency of poisoning electrode. Chemically modified electrodes have been widely used to solve the problems of poor sensitivity and selectivity faced by bare electrodes. There are a few articles that provide an overview of electrochemical detection and efficient enrichment of CAs, but there is a dearth of updates on the role of CAs in the pathogenesis of diseases. Additionally, there is still a lack of systematic synthesis with a focus on modified electrodes for electrochemical detection. Thus, this review provides a summary of the recent clinical pathogenesis of CAs and the modified electrodes for electrochemical detection of CAs published between 2017 and 2022. Moreover, challenges and future perspectives are also highlighted. This work is expected to provide useful guidance to researchers entering this interdisciplinary field, promoting further development of CAs pathogenesis, and developing more novel chemically modified electrodes for the detection of CAs.

Advanced Hybrid materials in electrochemical sensors: Combining MOFs and conducting polymers for environmental monitoring

The integration of conducting polymers (CPs) with metal-organic frameworks (MOFs) has arisen as a dynamic and innovative approach to overcome some intrinsic limitations of both materials, representing a transformative method to address the pressing need for high-performance environmental monitoring tools. MOFs, with their intricate structures and versatile functional groups, provide tuneable porosity and an extensive surface area, facilitating the selective adsorption of target analytes. Conversely, CPs, characterized by their exceptional electrical conductivity and redox properties, serve as proficient signal transducers. By combining these two materials, a novel class of hybrid materials emerges, capitalizing on the unique attributes of both components. These MOF/CP hybrids exhibit heightened sensitivity, selectivity, and adaptability, making them primordial in detecting and quantifying environmental contaminants. This review examines the synergy between MOFs and CPs, highlighting recent advancements, challenges, and prospects, thus offering a promising solution for developing advanced functional materials with tailored properties and multifunctionality to be applied in electrochemical sensors for environmental monitoring.

Highly efficient electrochemical detection of H2O2 utilizing an innovative copper porphyrinic nanosheet decorated bismuth metal-organic framework modified electrode

The ability to detect hydrogen peroxide is important due to the presence in biological systems. Researchers are highly interested in developing efficient electrochemical hydrogen peroxide sensors. Metal-organic frameworks (MOFs) with their composites, an emerging class of porous materials, are ideal candidates for heterogeneous catalysts because of their versatile functionalities. Using a facile solvothermal reaction, we fabricated a 2D Cu-TCPP nanosheet uniformly grown on a 3D Bi-MOF. The process takes advantage of the large surface area and pore volume of the Bi-MOF while maintaining the crystallinity of Bi-BTC when Cu-TCPP is added to the surface. The sensor was fabricated by depositing the Bi-BTC-Cu-TCPP nanocomposites on a glassy carbon electrode to conduct electrochemical measurements such as cyclic voltammetry and electrochemical impedance spectroscopy. Finally, differential pulse voltammetry was utilized to investigate the effect of hydrogen peroxide on the electrochemical activity of Bi-BTC-Cu-TCPP deposited on a glassy carbon electrode. This electrode showed high electrochemical performance activity for the reduction of hydrogen peroxide. The sensor showed a linear response to H2O2 in the 10-120 μM concentration range, with a detection limit of 0.20 μM. The sensor was also stable and selective for H2O2 in the presence of other interfering species. This work demonstrates the potential of nanocomposite-based electrochemical sensors for sensitive and selective detection of H2O2. Besides, the modified electrode has many advantages, including remarkable catalytic activity, long-term stability, good reproducibility, and a good signal response during H2O2 reduction.

Detection mechanism and the outlook of metal-organic frameworks for the detection of hazardous substances in milk

Milk has a high nutritional value. However, milk is easily contaminated in the production, processing, and storage processes, which harms consumers' health. Therefore, the harmful substances' detection in milk is important. Metal-organic frameworks (MOFs) have proven high potential in food safety detection due to their unique porous structure, large effective surface area, large porosity, and structural tunability. This article systematically describes the detection mechanism of fluorescence, electrochemical, colorimetric, and enzyme-linked immunosorbent assay based on MOFs. The progress of the application of MOFs in the detection of antibiotics, harmful microorganisms and their toxins, harmful ions, and other harmful substances in milk in recent years is reviewed. The structural tunability of MOFs enables them to be functionalized, giving the ability to be applied to different detection methods or substances. Therefore, MOFs can be used as an advantageous sensing material for detecting harmful substances in the complex environment of milk.

Ultrasensitive Electrochemical Platform for Dopamine Detection Based on CoNi-MOF@ERGO Composite

An electrochemical sensor applied for dopamine (DA) detection was constructed. An easy static way was used to synthesize bimetallic CoNi-MOF. Next, it was mixed with graphene oxide (GO) under ultrasound to get a uniform suspension. Subsequently, the solution was coated on the glassy carbon electrode (GCE) to form CoNi-MOF@ERGO/GCE by the electrochemical reduction method. The interaction between CoNi-MOF and electrochemically reduced graphene oxide (ERGO) enhances the electrocatalytic performance for DA detection. CoNi-MOF@ERGO/GCE has a wider linear range (0.1-400 μM) and a lower detection limit (0.086 μM) under optimum conditions. Furthermore, it has been applied to test DA in human serum samples. The results reveal that the DA sensor shows excellent performance, which will provide a novel idea for more sensitive and quicker DA detection.

Metal-Organic Frameworks Meet Polymers: From Synthesis Strategies to Healthcare Applications

Metal-organic frameworks (MOFs) have been at the forefront of nanotechnological research for the past decade owing to their high porosity, high surface area, diverse configurations, and controllable chemical structures. They are a rapidly developing class of nanomaterials that are predominantly applied in batteries, supercapacitors, electrocatalysis, photocatalysis, sensors, drug delivery, gas separation, adsorption, and storage. However, the limited functions and unsatisfactory performance of MOFs resulting from their low chemical and mechanical stability hamper further development. Hybridizing MOFs with polymers is an excellent solution to these problems, because polymers-which are soft, flexible, malleable, and processable-can induce unique properties in the hybrids based on those of the two disparate components while retaining their individuality. This review highlights recent advances in the preparation of MOF-polymer nanomaterials. Furthermore, several applications wherein the incorporation of polymers enhances the MOF performance are discussed, such as anticancer therapy, bacterial elimination, imaging, therapeutics, protection from oxidative stress and inflammation, and environmental remediation. Finally, insights from the focus of existing research and design principles for mitigating future challenges are presented.

UiO-66-Derived PBA Composite as Multifunctional Electrochemical Non-Enzymatic Sensor Realizing High-Performance Detection of Hydrogen Peroxide and Glucose

In this work, a highly efficient multifunctional non-enzymatic electrochemical sensor is successfully fabricated based on a facile two-step synthetic strategy. It resolves two important challenges of poor stability and low reproducibility compared to conventional electrochemical enzyme-based sensors. Herein, a metal-organic framework (UiO-66) is selected as a sacrificial template to construct the corresponding Prussian blue analogue (PBA) target to improve its stability and conductivity, namely, PBA/UiO-66/NF. Target PBA/UiO-66/NF exhibits excellent electrochemical sensing performance as hydrogen peroxide (H₂O₂) and glucose sensors with ultrahigh sensitivity of up to 1903 μA mM^-1 cm^-2 for H₂O₂ and 22,800 μA mM^-1 cm^-2 for glucose, as well as a very low detection limit of 0.02 μM (S/N = 3) for H₂O₂ and 0.28 μM for glucose. Especially, extremely high stability can be observed, which will be beneficial for practical application.

An electrochemical determination strategy for miRNA based on bimetallic nanozyme and toehold-mediated DNA replacement procedure

An electrochemical strategy based on bimetallic nanozyme in collaboration with toehold-mediated DNA replacement effect is proposed for the sensitive determination of miRNA-21. The AuPt nanoparticles (AuPt NPs) are prepared as a catalytic beacon; it shows favorable peroxidase properties with a Michaelis contant (K_m) of 0.072 mM for H₂O₂, which is capable of catalyzing H₂O₂ to induce an intense redox reaction, and causing a measurable electrochemical signal. To further enhance the strength of the signal response, a novel toehold-mediated DNA replacement strategy is employed. DNA strands with specific sequences are modified on electrodes and AuPt NPs, respectively. In the presence of miRNA-21, a cyclic substitution effect is subsequently activated via a specific toehold sequence and leads to a large accumulation of AuPt NPs on the electrodes. Subsequently, a strong signal depending on the amount of miRNA-21 is obtained after adding a small amount of H₂O₂. The analytical range of this determination method is from 0.1 pM to 1.0 nM, and the LOD is 84.1 fM. The spike recoveries for serum samples are 95.0 to 102.4% and the RSD values are 3.7 to 5.8%. The results suggests a promising application of the established method in clinical testing and disease diagnosis.

Electrochemical-Based Sensing Platforms for Detection of Glucose and H2O2 by Porous Metal-Organic Frameworks: A Review of Status and Prospects

Establishing enzyme-free sensing assays with great selectivity and sensitivity for glucose and H₂O₂ detection has been highly required in biological science. In particular, the exploitation of nanomaterials by using noble metals of high conductivity and surface area has been widely investigated to act as selective catalytic agents for molecular recognition in sensing platforms. Several approaches for a straightforward, speedy, selective, and sensitive recognition of glucose and H₂O₂ were requested. This paper reviews the current progress in electrochemical detection using metal-organic frameworks (MOFs) for H₂O₂ and glucose recognition. We have reviewed the latest electrochemical sensing assays for in-place detection with priorities including straightforward procedure and manipulation, high sensitivity, varied linear range, and economic prospects. The mentioned sensing assays apply electrochemical systems through a rapid detection time that enables real-time recognition. In profitable fields, the obstacles that have been associated with sample preparation and tool expense can be solved by applying these sensing means. Some parameters, including the impedance, intensity, and potential difference measurement methods have permitted low limit of detections (LODs) and noticeable durations in agricultural, water, and foodstuff samples with high levels of glucose and H₂O₂.

Ligand influence in electrocatalytic properties of Cu(II) triazole complexes for hydrogen peroxide detection in aqueous media

In this work, electrocatalytic changes of Cu(II) triazole complexes (Cu(L)₂) resulting from inductive effects were evaluated to fabricate a sensor for hydrogen peroxide (H₂O₂) determination. Three copper(II) complexes with electronically differentiated ligands were synthesized by slow diffusion method and characterized by X-ray crystallography, Fourier transformed infrared (FTIR), UV-Vis, scanning electron microscopy (SEM) and voltammetry cyclic (CV). Cu(LOMe)2/GC, Cu(LBr)2/GC and Cu(LNO2)2/GC sensors were then prepared. Under optimal conditions (pH = 11), the optimal sensor presented a response at -0.5 V, good linear range of 1-32 μM, reproducibility (1.7%), repeatability (1.2%), LOD of 0.0246 μM (S/N = 5), LOQ of 0.0747 μM (S/N = 5) and selectivity. Additionally, Cu(LNO2)2/GC sensor has been successfully applied in commercial substances, such as mouthwash, milk and tea.

Rapid and selective detection of dopamine in human serum using an electrochemical sensor based on zinc oxide nanoparticles, nickel phthalocyanines, and carbon nanotubes

Composite materials have gained significant attention owing to the synergistic effects of their constituent materials, thereby facilitating their utilization in new applications or in improving the existing ones. In this study, a composite based on nickel phthalocyanine (NiTsPc), zinc oxide nanoparticles (ZnONPs), and carbon nanotubes (CNT) was developed and subsequently immobilized on a pyrolytic graphite electrode (PGE). The PGE/NiTsPc-ZnONPs-CNT was identified as a selective catalytic hybrid system for detection of neurotransmitter dopamine (DA). The electrochemical and morphological characterizations were conducted using atomic force microscopy (AFM). Chronoamperometry and differential pulse voltammetry (DPV) were used to detect DA and detection limits of 24 nM and 7.0 nM was found, respectively. In addition, the effects of some possible DA interferents, such as ascorbic acid, uric acid, and serotonin, on DA response were evaluated. Their presence did not show significant variations in the DA electrochemical response. The high specificity and sensitivity of PGE/NiTsPc-ZnONPs-CNT for DA enabled its direct detection in human serum without sample pretreatment as well as in DA-enriched serum samples, whose recovery levels were close to 100%, thereby confirming the effectiveness of the proposed method. In general, PGE/NiTsPc-ZnONPs-CNT is a promising candidate for future applications in clinical diagnosis.

Recent advancements in metal-organic frameworks composites based electrochemical (bio)sensors

Metal-organic frameworks (MOFs) are a novel class of crystalline materials which find widespread applications in the field of microporous conductors, catalysis, separation, biomedical engineering, and electrochemical sensing. With a specific emphasis on the MOF composites for electrochemical sensor applications, this review summarizes the recent construction strategies on the development of conductive MOF composites (post-synthetic modification of MOFs, in situ synthesis of functional materials@MOFs composites, and incorporating electroactive ligands). The developed composites are revealed to have excellent electrochemical sensing activity better than their pristine forms. Notably, the applicable functionalized MOFs to electrochemical sensing/biosensing of various target species are discussed. Finally, we highlight the perspectives and challenges in the field of electrochemical sensors and biosensors for potential directions of future development.

Structure engineering of silicon nanoparticles with dual signals for hydrogen peroxide detection

Fluorescent silicon nanoparticles (SiNPs) were synthesized by a one-step, simple, and green method with 3-Aminopropyltriethoxysilane (APTES) and ascorbic acid (AA) as reaction agents. Subsequently, the SiNPs and AgNPs nanocomplex (SiNPs@AgNPs) was constructed as the probe for hydrogen peroxide (H₂O₂) detection. The fluorescence of SiNPs was quenched due to the surface plasmonic-enhanced energy transfer between SiNPs and AgNPs. Meanwhile, the color tends to be yellow due to the existence of AgNPs. As the AgNPs were etched by H₂O₂, the fluorescence recovers and color fadings. Based on the well-designed structure, the "off-on" fluorescence sensing and "on-off" color sensing platforms for H₂O₂ were fabricated. The as-synthesized materials were characterized by Fourier transform infrared (FT-IR) spectroscopy, dynamic light scattering (DLS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Fluorescence and UV-vis absorption spectra were used to evaluate the optical performance. The fabricated sensor exhibited a linear range of 1.0-100.0 μM, with a limit of detection of 0.36 μM for the fluorescence sensing of H₂O₂. Additionally, a linear range of 1.0-50.0 μM and a limit of detection of 0.45 μM were displayed for the detection of H₂O₂ by colorimetric assay. The feasibility in complex medium of the fabricated fluorescent and colorimetric dual-signal sensor was evaluated by the detection of H₂O₂ in phosphate buffer saline (PBS) and lake water samples.

Facile synthesis of flower-like CePO4 with a hierarchical structure for the simultaneous electrochemical detection of dopamine, uric acid and acetaminophen

A flower-like CePO4 with a hierarchical structure was hydrothermally prepared for electrochemical sensing of dopamine, uric acid and acetaminophen.

Facile preparation of urchin-like NiCo2O4 microspheres for efficient hydrogen peroxide detection

The fabricated NiCo2O4 microspheres acted as excellent sensors for H2O2, with much better performance than the reported NiCo2O4-based H2O2 sensors.

Synthesis and characterization of MXene (Ti3C2Tx)/Iron oxide composite for ultrasensitive electrochemical detection of hydrogen peroxide

Due to the widespread usage of hydrogen peroxide (H₂O₂) in various consumer and industrial products (Examples: fuel cells and antibacterial agents), it became important to accurately detect H₂O₂ concentration in environmental, medical and food samples. Herein, titanium carbide Ti₃C₂T_x (MXene) was synthesized by using Ti, Al and C powders at high-temperature. Then, nanocrystalline iron oxide (α-Fe₂O₃) was obtained from a single solid-phase method. Using Ti₃C₂T_x and Fe₂O₃ powders, Ti₃C₂T_x-Fe₂O₃ nanocomposite was prepared by ultrasonication. As-synthesized, Ti₃C₂T_x-Fe₂O₃ composite had been characterized by UV-Visible (UV-Vis), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and Raman spectroscopy. The Fe₂O₃ nanoparticles (NPs) were decorated on the surface of Ti₃C₂T_x as observed by high resolution scanning electron microscopy (HR-SEM) and high resolution transmission electron microscopy (HR-TEM). The Ti₃C₂T_x nanosheets were formed with the average size of 400-500 nm. HR-SEM images of α-Fe₂O₃ showed that the coral-like particles with the average length ~5 μm were obtained_. The electrochemical properties of the individual (Ti₃C₂T_x and α-Fe₂O₃) and composite materials (Ti₃C₂T_x-Fe₂O₃) were investigated by cyclic voltammetry (CV). Ti₃C₂T_x-Fe₂O₃ nanocomposite modified electrode had exhibited potent electro-catalytic activity for H₂O₂ reduction by reducing the overpotential about 320 mV and a linear response was recorded from 10 nM to 1 μM H₂O₂. The optimization of various parameters such as material composition ratio, amount of catalyst, effects of pH, scan rate and interference effects with other biomolecules were carried out. In addition, the kinetic parameters such as rate constant, diffusion coefficient and the active surface area of the electrodes were calculated. Moreover, the Ti₃C₂T_x-Fe₂O₃ composite modified electrode was used successfully to detect H₂O₂ in food and urine samples. We believe that Ti₃C₂T_x-Fe₂O₃ composite based materials could be used for the fabrication of non-enzymatic H₂O₂ sensors for medical diagnosis, food safety and environmental monitoring applications.

Gold Nanorods (AuNRs) and Zeolitic Imidazolate Framework-8 (ZIF-8) Core-Shell Nanostructure-Based Electrochemical Sensor for Detecting Neurotransmitters

The development of novel electrode materials for rapid and sensitive detection of neurotransmitters in the human body is of great significance for early disease diagnosis and personalized therapy. Herein, gold nanorod@zeolitic imidazolate framework-8 (AuNR@ZIF-8) core-shell nanostructures were prepared by controlled encapsulation of gold nanorods within a ZIF-8 assembly. The designed AuNR@ZIF-8 nanostructures have uniform morphology, good dispersion, a large specific surface area, and an average size of roughly 175 nm. Compared with individual ZIF-8 and AuNR-modified electrodes, the obtained core-shell-structured AuNR@ZIF-8 nanocomposite structure-modified electrode shows excellent electrocatalytic performance in the determination of dopamine (DA) and serotonin (ST). The designed AuNR@ZIF-8 exhibited a wide linear range of 0.1-50 μM and low detection limit (LOD, 0.03 μM, S/N = 3) for the determination of DA, as well as a linear range of 0.1-25 μM and low LOD (0.007 μM, S/N = 3) for monitoring ST. The improved performance is attributed to the synergistic effect of the high conductivity of AuNRs and multiple catalytic sites of ZIF-8. The good electroanalytical ability of AuNR@ZIF-8 for detection of DA and ST can provide a guide to efficiently and rapidly monitor other neurotransmitters and construct novel electrochemical sensors.

Graphene and Carbon Nanotube-based Electrochemical Sensing Platforms for Dopamine

Dopamine (DA) is an important neurotransmitter, which is created and released from the central nervous system. It plays a crucial role in human activities, like cognition, emotions, and response to anything. Maladjustment of DA in human blood serum results in different neural diseases, like Parkinson's and Schizophrenia. Consequently, researchers have started working on DA detection in blood serum, which is undoubtedly a hot research area. Electrochemical sensing techniques are more promising to detect DA in real samples. However, utilizing conventional electrodes for selective determination of DA encounters numerous problems due to the coexistence of other materials, such as uric acid and ascorbic acid, which have an oxidation potential close to DA. To overcome such problems, researchers have put their focus on the modification of bare electrodes. The aim of this review is to present recent advances in modifications of most used bare electrodes with carbonaceous materials, especially graphene, its derivatives, and carbon nanotubes, for electrochemical detection of DA. A brief discussion about the mechanistic phenomena at the electrode interface has also been included in this review.

Bimetallic MOFs-derived coral-like Ag-Mo2C/C interwoven nanorods for amperometric detection of hydrogen peroxide

Coral-like Ag-Mo₂C/C-I and blocky Ag-Mo₂C/C-II composites were obtained from one-step in situ calcination of [Ag(HL)₃(Mo₈O₂₆)]_n·nH₂O [L: N-(pyridin-3-ylmethyl) pyridine-2-amine] under N₂/H₂ and N₂ atmospheres, respectively. The coral-like morphology of Ag-Mo₂C/C-I is composed of interwoven nanorods embedded with small particles, and the nano-aggregate of Ag-Mo₂C/C-II is formed by cross-linkage of irregular nanoparticles. The above composites are decorated on glassy carbon electrode (GCE) drop by drop to generate two enzyme-free electrochemical sensors (Ag-Mo₂C/C/GCE) for amperometric detection of H₂O₂. In particular, the coral-like Ag-Mo₂C/C-I/GCE sensor possesses rapid response (1.2 s), high sensitivity (466.2 μA·mM^-1·cm^-2), and low detection limit (25 nM) towards trace H₂O₂ and has wide linear range (0.08 μM~4.67 mM) and good stability. All these sensing performances are superior to Ag-Mo₂C/C-II/GCE, indicating that the calcining atmosphere has an important influence on microstructure and electrochemical properties. The excellent electrochemical H₂O₂ sensing performance of Ag-Mo₂C/C-I/GCE sensor is mainly attributed to the synergism of unique microstructure, platinum-like electron structure of Mo₂C, strong interaction between Mo and Ag, as well as the increased active sites and conductivity caused by co-doped Ag and carbon. Furthermore, this sensor has been successfully applied to the detection of H₂O₂ in human serum sample, contact lens solution, and commercial disinfector, demonstrating the potential in related fields of environment and biology. Graphical abstract.

Reasonable design of an MXene-based enzyme-free amperometric sensing interface for highly sensitive hydrogen peroxide detection

Sensitive detection of H₂O₂ in the nano- to micromolar range is critical for health monitoring and disease diagnosis. Two-dimensional transition metal carbides or/and nitrides (called MXenes, MXs) have excellent potential applications in the electrochemical field due to their outstanding electrical conductivity and catalytic properties. In this work, Ti₃C₂T_x (MX) was employed for the construction of a sensitive and enzyme-free electrochemical sensing interface for the detection of hydrogen peroxide (H₂O₂) through a simple and effective method. Prussian blue (PB) was electrochemically deposited on the surface of a glassy carbon electrode (GCE). Chitosan (CS) and MX were sequentially dripped onto the PB modified GCE surface. The reasonable fabrication of the MX/CS/PB/GCE sensing interface presented good electrochemical sensing performance towards H₂O₂ with a low limit of detection (4 nM), a wide linear range from 50 nM to 667 μM and good selectivity. The proposed MX/CS/PB/GCE has been proven to monitor H₂O₂ in food samples and biological samples with recoveries between 94.7% and 100.3%. This work has made a beneficial attempt and research for exploring and expanding the application of MXs in the field of electrochemical sensing.

3D nitrogen-doped carbon nanofoam arrays embedded with PdCu alloy nanoparticles: Assembling on flexible microelectrode for electrochemical detection in cancer cells

In this work, we developed a novel and facile strategy for the synthesis of a highly active and stable electrocatalyst based on PdCu alloy nanoparticles (PdCu-ANPs) embedded in 3D nitrogen-doped carbon (NC) nanofoam arrays (NFAs), which were assembled on flexible carbon fiber (CF) microelectrode for in situ sensitive electrochemical detection of biomarker H₂O₂ in cancer cells. Our results showed that NC-NFAs support possessed a unique hierarchically porous architecture by integrating the macrospores in arrays scaffold within mesopores in individual NC nanofoam, which offered exceptionally large surface area for embedding high-density PdCu-ANPs in it as well as facilitated the mass transfer and molecular diffusion during the electrochemical reaction. Taking the advantages of the unique structural merit of NC-NFAs support and excellent electrocatalyitc properties of PdCu-ANPs that embedded in it, the resultant PdCu-ANPs/NC-NFAs modified CF microelectrode exhibited good electrochemical sensing performances towards H₂O₂ including a wide linear range from 2.0 μM to 3.44 mM, a low detection limit of 500 nM, as well as good reproducibility, stability and anti-interference ability. When used in real-time in situ tracking H₂O₂ secreted from different types of human colorectal cancer cells, i.e., HCT116, HT29, SW48 and LoVo, it can distinguish the types of cancer cells by measuring the number of extracellular H₂O₂ molecules released per cell, which demonstrates its great promise in cancer diagnose and management.

Novel neuron-network-like Cu-MoO2/C composite derived from bimetallic organic framework for highly efficient detection of hydrogen peroxide

Fabrication of non-enzymatic electrochemical sensors based on metal oxides with low valence-state for nanomolar detection of H₂O₂ has been a great challenge. In this work, a novel neuron-network-like Cu-MoO₂/C hierarchical structure was simply prepared by in-situ pyrolysis of 3D bimetallic-organic framework [Cu(Mo₂O₇)L]_n [L: N-(pyridin-3-ylmethyl)pyridine-2-amine] crystals. Meanwhile, the MoO₂/C nano-aggregates were also obtained by liquid phase copper etching. Subsequently, two non-enzymatic electrochemical sensors were fabricated by simple drop-coating of the above two materials on the surface of glassy carbon electrode (GCE). Electrochemical measurements indicate that the Cu-MoO₂/C/GCE possesses highly efficient electrocatalytic H₂O₂ property during wider linear range of 0.24 μM-3.27 mM. At room temperature, the Cu-MoO₂/C composite displays higher sensitivity (233.4 μA mM^-1 cm^-2) and lower limit of detection (LOD = 85 nM), which are 1 and 2.5 times larger than those of MoO₂/C material, respectively. Such excellent ability for trace H₂O₂ detection mainly originates from the synergism of neuron-network-like structure, enhanced electrical conductivity and increased active sites caused by low valence-state MoO₂ and co-doping of Cu and carbon, and even the interaction between Cu and Mo. In addition, the H₂O₂ detection in spiked human serum and commercially real samples indicates that the Cu-MoO₂/C/GCE sensor has certain potential application in the fields of environment and biology.

Ultrasensitive Electrochemical Sensor for Luteolin Based on Zirconium Metal-Organic Framework UiO-66/Reduced Graphene Oxide Composite Modified Glass Carbon Electrode

Luteolin is a kind of natural flavonoid with many bioactivities purified from a variety of natural herbs, fruits and vegetables. Electrochemical sensing has become an outstanding technology for the detection of luteolin in low concentration due to its fast response, easy operation and low cost. In this study, electroreduced graphene oxide (ErGO) and UiO-66 were successively modified onto a glassy carbon electrode (UiO-66/ErGO/GCE) and applied to the detection of luteolin. A combination of UiO-66 and ErGO showed the highest promotion in the oxidation peak current for luteolin compared with those of a single component. The factors affecting the electrochemical behavior of UiO-66/ErGO/GCE were evaluated and optimized including pH, accumulation potential, accumulation time and scan rate. Under optimum conditions, UiO-66/ErGO/GCE showed satisfactory linearity (from 0.001 μM to 20 μM), reproducibility and storage stability. The detection limit of UiO-66/ErGO/GCE reached 0.75 nM of luteolin and was suitable for testing real samples. Stable detection could be provided at least eight times by one modified electrode, which guaranteed the practicability of the proposed sensor. The fabricated UiO-66/ErGO/GCE showed a rapid electrochemical response and low consumption of materials in the detection of luteolin. It also showed satisfactory accuracy for real samples with good recovery. In conclusion, the modification using MOFs and graphene materials made the electrode advanced property in electrochemical sensing of natural active compounds.