Graphene blended with SnO2 and Pd-Pt nanocages for sensitive non-enzymatic electrochemical detection of H2O2 released from living cells

Sensitive electrochemical flow injection analysis of H2O2 released from cells with a pass-through mode

Conventional plate electrodes were commonly used in electrochemical flow injection analysis and only part of molecules diffused to the plane of electrodes could be detected, which would limit the performance of electrochemical detection. In this study, a low-cost native stainless steel wire mesh (SSWM) electrode was integrated into a 3D-printed device for electrochemical flow injection analysis with a pass-through mode, which is different compared with previous flow-through mode. This strategy was applied for sensitive analysis of hydrogen peroxide (H2O2) released from cells. Under the optimal conditions (the applied potentials, the flow rate and the sample volume), the device exhibits high sensitivity toward H2O2. Linear relationships could be achieved between electrochemical responses and the concentration of H2O2 ranging from 1 nM to 1 mM. The excellent analytical performance of the SSWM-based device could be attributed to the pass-through mode based on the mesh microstructure and intrinsic catalytic properties for H2O2 by stainless steel. This approach could be further successfully extended for screening of H2O2 released from HeLa cells with electrochemical responses linear to the number of cells in a range of 3 - 1.35 × 104 cells with an injection volume of 30 μL. This study revealed the potential of mesh electrodes in electrochemical flow injection analysis for cellular function and pathology and its possible extension in cell counting and on-line analysis.

Enhancing electrochemical sensing through the use of functionalized graphene composites as nanozymes

Graphene-based nanozymes possess inherent nanomaterial properties that offer not only a simple substitute for enzymes but also a versatile platform capable of bonding with complex biochemical environments. The current review discusses the replacement of enzymes in developing biosensors with nanozymes. Functionalization of graphene-based materials with various nanoparticles can enhance their nanozymatic properties. Graphene oxide functionalization has been shown to yield graphene-based nanozymes that closely mimic several natural enzymes. This review provides an overview of the classification, current state-of-the-art development, synthesis routes, and types of functionalized graphene-based nanozymes for the design of electrochemical sensors. Furthermore, it includes a summary of the application of functionalized graphene-based nanozymes for constructing electrochemical sensors for pollutants, drugs, and various water and food samples. Challenges related to nanozymes as electrocatalytic materials are discussed, along with potential solutions and approaches for addressing these shortcomings.

Electrochemical monitoring of hydrogen peroxide by a signal-amplified microfluidic chip coupled with colloidal VO2 nanostructures as a peroxidase enzyme mimic

We present a novel electrochemical microfluidic device for the sensitive and selective detection of hydrogen peroxide (H₂O₂) through a VO₂ nanostructure enzyme mimic. The low-cost ($0.50) microfluidic chip was fabricated using a simple and rapid prototyping technique via three syringe needles. Each needle played the role of an electrode (working, reference, and counter), and was connected by micro-hoses to a construction of the electrochemical microfluidic chip. The colloidal VO₂ nanoflakes with peroxidase-like activity could be easily transferred on to the electrodes by a syringe, for development of a novel electrochemical platform to enable the detection of H₂O₂. The unique microfluidic electrochemical sensor delivered a wide linear dynamic range from 0.5 to 300 μM, with a limit of detection of 0.14 μM. The facile, rapid, sensitive, and selective as-fabricated H₂O₂ sensors were proven to be appropriate for the real-time monitoring of H₂O₂ released from PC12 cells. The integration of a microfluidic sensor with an enzyme mimic nanostructure is essentially a promising strategy for the low-cost and accurate monitoring of physiological processes.

A non-enzymatic, biocompatible electrochemical sensor based on N-doped graphene quantum dot-incorporated SnS2 nanosheets for in situ monitoring of hydrogen peroxide in breast cancer cells

The current study reports the design and construction of enzyme-free sensor using N-doped graphene quantum dots (N-GQDs)-decorated tin sulfide nanosheets (SnS₂) for in situ monitoring of H₂O₂ secreted by human breast cancer cells. N-GQDs nanoparticles having a size of less than 1 nm were incorporated into SnS₂ nanosheets to form an N-GQDs@SnS₂ nanocomposite using a simple hydrothermal approach. The resulting hybrid material was an excellent electrocatalyst for the reduction of H₂O₂, owing to the combined properties of highly conductive N-GQDs and SnS₂ nanosheets. The N-GQDs@SnS₂-based sensing platform demonstrated substantial sensing ability, with a detection range of 0.0125-1128 µM and a limit of detection of 0.009 µM (S/N = 3). The sensing performance of N-GQDs@SnS₂ was highly stable, selective, and reproducible. The practical application of the N-GQDs@SnS₂ sensor was successfully demonstrated by quantifying H₂O₂ in lens cleaner, human urine, and saliva samples. Finally, the N-GQDs@SnS₂ electrode was successfully applied for the real-time monitoring of H₂O₂ released from breast cancer cells and mouse fibroblasts. This study paves the way to designing efficient non-enzymatic electrochemical sensors for various biomolecule detection using a simple method.

Electrochemical Sensors for the Detection of Reactive Oxygen Species in Biological Systems: A Critical Review

Reactive oxygen species (ROS) involving superoxide anion, hydrogen peroxide and hydroxyl radical play important role in human health. ROS are known to be the markers of oxidative stress associated with different pathologies including neurodegenerative and cardiovascular diseases, as well as cancer. Accordingly, ROS level detection in biological systems is an essential problem for biomedical and analytical research. Electrochemical methods seem to have promising prospects in ROS determination due to their high sensitivity, rapidity, and simple equipment. This review demonstrates application of modern electrochemical sensors for ROS detection in biological objects (e.g., cell lines and body fluids) over a decade between 2011 and 2021. Particular attention is paid to sensors materials and various types of modifiers for ROS selective detection. Moreover, the sensors comparative characteristics, their main advantages, disadvantages and their possibilities and limitations are discussed.

Copper-based metal-organic frameworks for biomedical applications

Metal-organic frameworks (MOFs) are a class of important porous, crystalline materials composed of metal ions (clusters) and organic ligands. Owing to the unique redox chemistry, photochemical and electrical property, and catalytic activity of Cu^2+/+, copper-based MOFs (Cu-MOFs) have been recently and extensively explored in various biomedical fields. In this review, we first make a brief introduction to the synthesis of Cu-MOFs and their composites, and highlight the recent synthetic strategies of two most studied representatives, three-dimensional HKUST-1 and two-dimensional Cu-TCPP. The recent advances of Cu-MOFs in the applications of cancer treatment, bacterial inhibition, biosensing, biocatalysis, and wound healing are summarized and discussed. Furthermore, we propose a prospect of the future development of Cu-MOFs in biomedical fields and beyond.

Pt–Pd Bimetallic Aerogel as High-Performance Electrocatalyst for Nonenzymatic Detection of Hydrogen Peroxide

Hydrogen peroxide (H2O2) plays an indispensable role in the biological, medical, and chemical fields. The development of an effective H2O2 detecting method is of great importance. In the present work, a series of PtxPdy bimetallic aerogels and Pt, Pd monometallic aerogels were controllably synthesized by one-step gelation method. Their morphologies and compositions were characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, and so forth. These aerogels were used as nonenzyme electrocatalysts for the detection of H2O2. The cyclic voltammetric and amperometric results demonstrated that the performance of the metal aerogels showed volcano-type behavior, with the Pt50Pd50 aerogel sitting on top. The Pt50Pd50 aerogel-based electrochemical sensor exhibited excellent comprehensive performance, with a low overpotential of −0.023 V vs. Ag/AgCl, a broad linear range from 5.1 to 3190 μM (R2 = 0.9980), and a high sensitivity of 0.19 mA mM−1 cm−2, in combination with good anti-interference ability and stability. A comprehensive study indicated that the superior sensing performance of the Pt50Pd50 aerogel is closely related to its optimized d-band center and larger cumulative pore volume. This work first applied Pt–Pd bimetallic aerogels into the detection of H2O2 and shows the promising application of noble metal aerogels in the electrochemical sensing area.

Recent Advances in Electrochemical Sensing of Hydrogen Peroxide (H2O2) Released from Cancer Cells

Cancer is by far the most common cause of death worldwide. There are more than 200 types of cancer known hitherto depending upon the origin and type. Early diagnosis of cancer provides better disease prognosis and the best chance for a cure. This fact prompts world-leading scientists and clinicians to develop techniques for the early detection of cancer. Thus, less morbidity and lower mortality rates are envisioned. The latest advancements in the diagnosis of cancer utilizing nanotechnology have manifested encouraging results. Cancerous cells are well known for their substantial amounts of hydrogen peroxide (H₂O₂). The common methods for the detection of H₂O₂ include colorimetry, titration, chromatography, spectrophotometry, fluorimetry, and chemiluminescence. These methods commonly lack selectivity, sensitivity, and reproducibility and have prolonged analytical time. New biosensors are reported to circumvent these obstacles. The production of detectable amounts of H₂O₂ by cancerous cells has promoted the use of bio- and electrochemical sensors because of their high sensitivity, selectivity, robustness, and miniaturized point-of-care cancer diagnostics. Thus, this review will emphasize the principles, analytical parameters, advantages, and disadvantages of the latest electrochemical biosensors in the detection of H₂O₂. It will provide a summary of the latest technological advancements of biosensors based on potentiometric, impedimetric, amperometric, and voltammetric H₂O₂ detection. Moreover, it will critically describe the classification of biosensors based on the material, nature, conjugation, and carbon-nanocomposite electrodes for rapid and effective detection of H₂O₂, which can be useful in the early detection of cancerous cells.

Electrochemical sandwich-type immunosensor for the detection of PSA based on a trimetallic AgAuPt nanocomposite synthesized using the galvanic replacement reaction

A sandwich-type electrochemical immunoassay was introduced for the determination of the prostate-specific antigen (PSA) biomarker. A direct and simple galvanic replacement reaction was performed between the Ag framework and metallic salts of tetrachloroauric(iii) acid trihydrate and chloroplatinic acid to produce a trimetallic composite of AgAuPt. The trimetallic composite of AgAuPt was applied to the preparation of the capture layer of the immunoassay for stabilizing the primary Ab at the surface of the prepared composite. The immunoassay detection layer was also prepared using a labeled antibody containing a bimetallic composite of AgPt as a label. The various procedures in the immunoassay fabrication were monitored step by step using cyclic voltammetry and electrochemical impedance spectroscopy. Also, the electrochemical determination of PSA was performed using differential pulse voltammetry in the presence of the ferrocene redox probe and H₂O₂. Furthermore, the effective parameters in the fabrication of the immunoassay included the drop volume of the AgAuPt trimetallic composite and the incubation time for the immobilization of biomolecules (i.e., Ab₁, BSA, PSA, and labeled Ab₂), and the concentration of H₂O₂ were optimized during the determination of PSA. Then, the determination of PSA was performed under optimized conditions. It could be seen that there was a linear relation between the PSA concentration and DPV responses in the concentration range of 50 pg mL^-1 to 500 ng mL^-1 and the limit of detection (LOD) for the proposed immunoassay was calculated as 17.0 pg mL^-1. In the following investigation, the cross-reactivity of the proposed immunoassay was studied in the presence of BSA, CEA, IgG, and human hepatitis surface antigen, in which the results showed a negligible change in the performance of the immunoassay.

In Situ Single-Cell Stimulation and Real-Time Electrochemical Detection of Lactate Response Using a Microfluidic Probe

Metabolism of a single cell, even within the same organization, differs from other cells by orders of magnitude. Single-cell analysis provides key information for early diagnosis of cancer as well as drug screening. Any slight change in the microenvironment may affect the state of a single cell. Timely and effective cell monitoring is conducive to better understand the behavior of single cells. The immediate response of a single cell described in this study is a liquid transfer-based approach for real-time electrochemical detection. The cell was in situ stimulated by continuous flow with glucose, and lactate secreted from the cell would diffuse into the microflow. The microflow was aspirated into the detection channel where lactate was then decomposed by coupled enzyme reactions and detected by an electrode. This work provides a novel approach for detecting lactate response from a single cell by noninvasive measurements, and the position resolution of the microfluidic probe reaches the level of a single cell and permits individual heterogeneity in cells to be explored in the diagnosis and treatment of cancer as well as in many other situations.

A comprehensive review on the applications of nano-biosensor-based approaches for non-communicable and communicable disease detection

The outstretched applications of biosensors in diverse domains has become the reason for their attraction for scientific communities. Because they are analytical devices, they can detect both quantitative and qualitative biological components through the generation of detectable signals. In the recent past, biosensors witnessed significant changes and developments in their design as well as features. Nanotechnology has revolutionized sensing phenomena by increasing biodiagnostic capacity in terms of specificity, size, and cost, resulting in exceptional sensitivity and flexibility. The steep increase of non-communicable diseases across the world has emerged as a matter of concern. In parallel, the abrupt outbreak of communicable diseases poses a serious threat to mankind. For decreasing the morbidity and mortality associated with various communicable and non-communicable diseases, early detection and subsequent treatment are indispensable. Detection of different biological markers generates quantifiable signals that can be electrochemical, mass-based, optical, thermal, or piezoelectric. Speculating on the incumbent applicability and versatility of nano-biosensors in large disciplines, this review highlights different types of biosensors along with their components and detection mechanisms. Moreover, it deals with the current advancements made in biosensors and the applications of nano-biosensors in detection of various non-communicable and communicable diseases, as well as future prospects of nano-biosensors for diagnostics.

3D Flower-like NiCo Layered Double Hydroxides: An Efficient Electrocatalyst for Non-Enzymatic Electrochemical Biosensing of Hydrogen Peroxide in Live Cells and Glucose in Biofluids

Herein, a hierarchical structure of flower-like NiCo layered double hydroxides (NiCo LDH) microspheres composed of three-dimensional (3D) ultrathin nanosheets was successfully synthesized via a facile hydrothermal approach. The formation of NiCo LDH was confirmed by various physicochemical studies, and the NiCo LDH-modified glassy carbon electrode was used as an efficient dual-functional electrocatalyst for non-enzymatic glucose and hydrogen peroxide (H₂O₂) biosensor. The host matrix of hydrotalcite NiCo LDH exhibits the enhanced electrocatalytic sensing performances with a quick response time (<3 s), wide linear range (50 nM-18.95 mM and 20 nM-11.5 mM) and lowest detection limits (S/N = 3) (10.6 and 4.4 nM) toward glucose and H₂O₂, and also it exhibits good stability, selectivity, and reproducibility. In addition, this biosensor was successfully utilized to the real-time detection of endogenous H₂O₂ produced from live cells and glucose in various biological fluids, and demonstrates that the as synthesized NiCo LDH may provide a successful pathway for physiological and clinical pathological diagnosis.

Fabrication of Cu-hemin metal-organic-frameworks nanoflower supported on three-dimensional reduced graphene oxide for the amperometric detection of H2O2

A novel electrochemical sensor based on Cu-hemin metal-organic-frameworks nanoflower/three-dimensional reduced graphene oxide (Cu-hemin MOFs/3D-RGO) was constructed to detect H₂O₂ released from living cells. The nanocomposite was synthesized via a facile co-precipitation method using hemin as the ligand, then decorated with 3D-RGO. The prepared Cu-hemin MOFs showed a 3D hollow spherical flower-like structure with a large specific surface area and mesoporous properties, which could load more biomolecules and greatly enhance the stability by protecting the activity of hemin. In addition, the introduction of 3D-RGO effectively enhanced the conductivity of Cu-hemin MOFs. Thus, the proposed sensor (Cu-hemin MOFs/3D-RGO/GCE) showed excellent electrochemical performances towards H₂O₂ with a wide linear range (10-24,400 μM), high sensitivity (207.34 μA mM^-1 cm^-2), low LOD (0.14 μM), and rapid response time (less than 3 s). Most importantly, we prepared a Cu-hemin MOFs/3D-RGO/ITO electrode with cells growing on it. Compared with detecting H₂O₂ in cell suspension by GCE-based electrode, adhesion of cells on ITO could shorten the diffusion distance of H₂O₂ from solution to the surface of the electrode and achieve in situ and a real-time monitor of H₂O₂ released by living cells. This self-supported sensing electrode showed great potential applications in monitoring the pathological and physiological dynamics of cancer cells.

Molybdenum-containing polypyrrole self-supporting hollow flexible electrode for hydrogen peroxide detection in living cells

A flexible electrode based on polypyrrole-supported free-standing molybdenum oxide-molybdenum disulfide/polypyrrole nanostructure (MoO₃-MoS₂/PPy) was synthesized. The petal-like MoO₃-MoS₂ sheets grown on PPy were prepared step by step through simple electrodeposition and hydrothermal methods. The corresponding surface morphological and structural characterizations were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results showed that the prepared petal MoO₃-MoS₂ hybrid nanomaterials were uniformly distributed on the PPy skeleton and exhibited a three-dimensional porous network structure. The flexible electrode was used for non-enzymatic detection of hydrogen peroxide (H₂O₂), and the developed MoO₃-MoS₂/PPy nanomaterials exhibited high electrochemical sensing performance in the range of 0.3-150 μM, with the detection limit of 0.18 μM (S/N = 3). The excellent detection properties enabled the MoO₃-MoS₂/PPy flexible electrode to detect H₂O₂ released by living cells. The resulting MoO₃-MoS₂/PPy flexible electrode also has the advantages of customizable shape and adjustability, which provides a potential platform for constructing clinically diagnosed in vivo portable instruments and real-time environmental monitoring.

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.

Detection Technologies for Reactive Oxygen Species: Fluorescence and Electrochemical Methods and Their Applications

Reactive oxygen species (ROS) have been found in plants, mammals, and natural environmental processes. The presence of ROS in mammals has been linked to the development of severe diseases, such as diabetes, cancer, tumors, and several neurodegenerative conditions. The most common ROS involved in human health are superoxide (O2•-), hydrogen peroxide (H2O2), and hydroxyl radicals (•OH). Organic and inorganic molecules have been integrated with various methods to detect and monitor ROS for understanding the effect of their presence and concentration on diseases caused by oxidative stress. Among several techniques, fluorescence and electrochemical methods have been recently developed and employed for the detection of ROS. This literature review intends to critically discuss the development of these techniques to date, as well as their application for in vitro and in vivo ROS detection regarding free-radical-related diseases. Moreover, important insights into and further steps for using fluorescence and electrochemical methods in the detection of ROS are presented.

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.

Determination of hydrogen peroxide released from cancer cells by a Fe-Organic framework/horseradish peroxidase-modified electrode

Multi-walled carbon nanotubes (MWCNTs) were used as conductive carrier on the glassy carbon electrode (GCE), and the hybrid of metal organic framework [NH₂-MIL-53(Fe)] and horseradish peroxidase (HRP) was prepared by simple physical mechanical mixture. The GCE modified by the above material with immobilization, namely NH₂-MIL-53(Fe)/HRP/MWCNTs/GCE, was used to construct an electrochemical biosensor toward H₂O₂. The results indicated that the addition of NH₂-MIL-53(Fe) had a good synergistic effect on the electron transfer of HRP and the detection of H₂O₂. Under the optimized condition, the biosensor exhibited excellent electrochemical performances such as low detection limit, high sensitivity, good stability and so on. The H₂O₂ biosensor showed two linear ranges of 0.1-1 μM and 1-600 μM with a calculated detection limit of 0.028 μM (signal-to-noise ratio, S/N = 3). In addition, the stability of the hybrid of NH₂-MIL-53(Fe) and HRP were discussed by SEM, XRD and UV-vis methods. Furthermore, the reported biosensors were practically used in direct detection of H₂O₂ released from HeLa and HepG2 cells successfully. Thus, this work provides a new strategy to fabricate electrochemical biosensors using MOFs and biomolecules.

One-Step Electrodeposition of Silver Nanostructures on 2D/3D Metal-Organic Framework ZIF-67: Comparison and Application in Electrochemical Detection of Hydrogen Peroxide

Metal-organic frameworks (MOFs) have been widely used as supporting materials to load or encapsulate metal nanoparticles for electrochemical sensing. Herein, the influences of morphology on the electrocatalytic activity of Co-containing zeolite imidazolate framework-67 (ZIF-67) as supporting materials were studied. Three types of morphologies of MOF ZIF-67 were facilely synthesized by changing the solvent because of the influence of the polar solvent on the nucleation and preferential crystal growth. Two-dimensional (2D) ZIF-67 with microplate morphology and 2D ultrathin ZIF-67 nanosheets were obtained from pure H₂O (H-ZIF-67) and a mixed solution of dimethylformamide and H₂O (D-ZIF-67), respectively. Three-dimensional ZIF-67 with rhombic dodecahedron morphology was obtained from pure methanol (M-ZIF-67). Then, one-step electrodeposition of silver nanostructures on ZIF-67-modified glassy carbon electrode (Ag/ZIF-67/GCE) was performed for the reduction of hydrogen peroxide (H₂O₂). Cyclic voltammetry can be used to investigate the electrocatalytic activity of Ag/ZIF-67/GCE, and Ag/H-ZIF-67/GCE displayed the best electrocatalytic property than Ag/D-ZIF-67/GCE and Ag/M-ZIF-67/GCE. The electrochemical H₂O₂ sensor showed two wide linear ranges of 5 μM to 7 mM and 7 to 67 mM with the sensitivities of 421.4 and 337.7 μA mM^-1 cm^-2 and a low detection limit of 1.1 μM. In addition, the sensor exhibited good selectivity, high reproducibility, and stability. Furthermore, it has been utilized for real-time detection of H₂O₂ from HepG2 human liver cancer cells. This work provides a novel strategy for enhancing the detection performance of electrochemical sensors by changing the crystalline morphologies of supporting materials.

2D materials in electrochemical sensors for in vitro or in vivo use

Individual cells and cell populations are at the present time investigated with a myriad of analytical tools. While most of them are commercially available, some of these analytical tools are just emerging from research laboratories and are in the developmental phase. Electrochemical sensors which allow the monitoring of low molecular weight compounds released (and / or uptaken) by cells are among these emerging tools. Such sensors are increasingly built using 2D materials (e.g. graphene-based materials, transition metal dichalcogenides, etc.) with the aim of conferring better analytical performances to these devices. The present work critically reviews studies published during the last 10 years describing electrochemical sensors made with 2D materials and exploited to monitor small compounds (e.g. H₂O₂, ·NO, glucose, etc.) in living biological systems. It also discusses the very few 2D material-based electrochemical sensors which are wearable or usable in vivo. Finally, the present work includes a specific section about 2D material biocompatibility, a fundamental requirement for 2D material-based sensor applications in vitro and in vivo. As such, the review provides a critical view on the state of the art of electrochemical sensors made with 2D materials and used at cellular level and it evaluates the possibility that such sensors will be used on / in the human body on a wider scale.

Direct chemical vapor deposition of graphene on plasma-etched quartz glass combined with Pt nanoparticles as an independent transparent electrode for non-enzymatic sensing of hydrogen peroxide

In this work, chemical vapor deposition (CVD) method-grown graphene on plasma-etched quartz glass supported platinum nanoparticles (PtNPs/eQG) was constructed as an independent transparent electrode for non-enzymatic hydrogen peroxide (H2O2) detection. Graphene grown on quartz glass by the CVD method can effectively reduce the wrinkles and pollution caused by traditional transfer methods. The addition of the CF4 plasma-etched process accelerates the growth rate of graphene on quartz glass. The platinum nanoparticles (PtNPs) prepared by in situ sputtering have favorable dispersibility and maximize exposed active catalytic sites on graphene, providing performance advantages in the application of H2O2 detection. The resulting sensor's detection limit (3.3 nM, S/N = 3), detection linear range (10 nM to 80 μM) and response time (less than 2 s) were significantly superior to other graphene supported PtNPs materials in sensing of H2O2. In addition, the material preparation method was related to the non-transfer CVD method and in situ sputtering technology, allowing for the creation of independent electrodes without additional electrode modification processes. This primitive material preparation and electrode assembly process were promoted for the application and development of practical H2O2 sensors.

Revealing Facet Effects of Palladium Nanocrystals on Electrochemical Biosensing

Noble-metal nanocrystals (NCs) are functional segments of biosensing platforms, but their sensitivity and facet effects are still challenging. Conventional synthesis using surfactants to direct crystal growth unfortunately causes adsorbate-surface hindrance, which not only reduces sensing responses but also leads to misunderstanding on facet-dependence. Herein, we utilize electrochemical CO displacement to remove residual surfactants from facet-engineered Pd NCs, and further investigate the structure-activity relationship on specific facets, for example, {100} in cubes, {111} in octahedrons, and {110} in rhombic dodecahedrons. Along with the remarkably boosted response, facet dependence is obvious for H₂O₂ sensing after surface cleaning. The Pd{100} shows high sensitivity, low detection limit, and wide applicable concentration range, superior to the {110} and {111}. This can be theoretically interpreted by the befitting *OH binding on {100} and thereby the facilitated H₂O₂ reduction kinetics. The outstanding selectivity to H₂O₂ ensures the high efficiency of Pd NCs to measure intracellular H₂O₂ and recognize different types of cancer cells. Moreover, facet effects are also evidenced in glucose detection, highlighting that this work can provide guidelines to design efficient sensing platforms.

Highly dispersive Pt-Pd nanoparticles on graphene oxide sheathed carbon fiber microelectrodes for electrochemical detection of H2O2 released from living cells

We report a facile strategy for the synthesis of surfactant-free, small and highly dispersive Pt-Pd nanoparticles on graphene oxide (Pt-Pd NPs/GO) by an electroless deposition method, which is sheathed on carbon fiber microelectrodes (CFMs) as an electrochemical sensing platform for highly sensitive and selective detection of hydrogen peroxide (H₂O₂) released from the living cells. GO serves as the reducing agent and stabilizer for electroless deposition of Pd NPs on the surface of GO owing to its low work function (4.38 eV) and highly conjugated electronic structure. The obtained Pd NPs/GO have a relatively high work function (4.64 eV), and thereby could be used as stabilizer for synthesis of surfactant-free, small and highly dispersive Pt-Pd NPs/GO by chemical reduction of K₂PtCl₄. The obtained Pt-Pd NPs have a uniform size of 4.0 ± 0.6 nm on the surface of GO. Moreover, the Pt-Pd NPs/GO sheathed CFMs exhibit an excellent electrocatalytic activity for the reduction of H₂O₂ with a low detection limit of 0.3 μM and good selectivity. These good properties enable the modified microelectrode to detect the H₂O₂ released from living cells.

Facile Synthesis of ZnMn2O4@rGO Microspheres for Ultrasensitive Electrochemical Detection of Hydrogen Peroxide from Human Breast Cancer Cells

Mixed transition-metal oxides have witnessed increasing attention in catalysts and electrocatalysts. Herein, reduced graphene oxide-wrapped ZnMn₂O₄ microspheres (ZnMn₂O₄@rGO) were facilely synthesized through the solvothermal technique. The microstructure and morphology of ZnMn₂O₄@rGO microspheres were analyzed under Raman, X-ray photoelectron, X-ray diffraction, and energy-dispersive spectroscopies and scanning electron microscopy. The synthesized ZnMn₂O₄@rGO was employed as an excellent electrocatalyst for the reduction of hydrogen peroxide (H₂O₂). The ZnMn₂O₄@rGO-modified glassy carbon electrode (ZnMn₂O₄@rGO/GCE) exhibited a linear detection to H₂O₂ in a wide concentration range of 0.03-6000 μM with a detection limit of 0.012 μM. The biosensor was evaluated to determine H₂O₂ secreted by human breast cancer cells (MCF-7), indicating its promising applications in physiology and diagnosis.

Nanomaterials for Healthcare Biosensing Applications

In recent years, an increasing number of nanomaterials have been explored for their applications in biomedical diagnostics, making their applications in healthcare biosensing a rapidly evolving field. Nanomaterials introduce versatility to the sensing platforms and may even allow mobility between different detection mechanisms. The prospect of a combination of different nanomaterials allows an exploitation of their synergistic additive and novel properties for sensor development. This paper covers more than 290 research works since 2015, elaborating the diverse roles played by various nanomaterials in the biosensing field. Hence, we provide a comprehensive review of the healthcare sensing applications of nanomaterials, covering carbon allotrope-based, inorganic, and organic nanomaterials. These sensing systems are able to detect a wide variety of clinically relevant molecules, like nucleic acids, viruses, bacteria, cancer antigens, pharmaceuticals and narcotic drugs, toxins, contaminants, as well as entire cells in various sensing media, ranging from buffers to more complex environments such as urine, blood or sputum. Thus, the latest advancements reviewed in this paper hold tremendous potential for the application of nanomaterials in the early screening of diseases and point-of-care testing.

Electrochemical performance of ruthenium nanoparticles decorated on nitride carbon for non-enzymatic detection of hydrogen peroxide

Development of high-performance Pt-free non-enzymatic hydrogen peroxide (H2O2) sensors on the basis of supported metal nanoparticles (NPs) is important for industrial and biological applications. Here, we report the preparation of ultrafine, surface-clean, and well-distributed Ru NPs and concomitant formation of nitride carbon (Ru/NC) by pyrolyzing tris(2,2'-bipyridyl)ruthenium(ii) chloride (TBRC) with carbon. The use of the nitrogen (N)-containing Ru complex of TBRC as the metal precursor is essential for the preparation of ultrafine and highly dispersed Ru NPs (1.20 nm in diameter) on a NC support. The as-synthesized Ru/NC-800 displays superior analytical performance for non-enzymatic detection of H2O2 with a low detection limit of 0.468 μM, high sensitivity of 698 μA mM-1 cm-2, excellent linear detection ranging from 0.001 to 10.000 mM, good stability, and high selectivity. The control experiment results indicate that the high-performance of Ru/NC-800 must be ascribed to the ultrasmall and highly dispersed Ru NPs and N-doping, which can supply a higher density of active sites available for H2O2 detection. This study provides a facile strategy to synthesize ultrafine metal NPs and for concomitant production of NC for electrocatalytic non-enzymatic sensing.

Future Applications of MXenes in Biotechnology, Nanomedicine, and Sensors

The past few years have seen significant developments in the chemistry and potential biological applications of 2D materials. This review focuses on recent advances in the biotechnological and biomedical applications of MXenes, which are 2D carbides, nitrides, and carbonitrides of transition metals. Nanomaterials based on MXenes can be used as therapeutics for anticancer treatment, in photothermal therapy as drug delivery platforms, or as nanodrugs without any additional modification. Furthermore, we discuss the potential use of these materials in biosensing and bioimaging, including magnetic resonance and photoacoustic imaging techniques. Finally, we present the most significant examples of the use of MXenes as efficient agents for environmental and antimicrobial treatments, as well as a brief discussion of their future prospects and challenges.

Nanoporous platinum-copper flowers for non-enzymatic sensitive detection of hydrogen peroxide and glucose at near-neutral pH values

Multimodal nanoporous PtCu flowers (np-PtCu) were prepared via a two-step dealloying strategy under mild conditions. The np-PtCu alloy possesses an interconnected flower-like network skeleton with multiscale pore distribution. This material was placed on a glassy carbon electrode where it shows outstanding detection performance towards hydrogen peroxide and glucose in near-neutral pH solutions. It can be attributed to the specific structure in terms of interconnected nanoscaled ligaments, rich pore openings and a synergistic alloying effect. Figures of merit for detection H₂O₂ assay include (a) a working voltage of 0.7 V (vs. the reversible hydrogen electrode); (b) a wide linear response range (from 0.01 to 1.7 mM), and (c) a low detection limit (0.1 μM). The respective data for the glucose assay are (a) 0.4 V, (b) 0.01-2.0 mM, and (c) 0.1 μM. The method is not interfered in the presence of common concentrations of dopamine, acetaminophen and ascorbic acid. Graphical abstract Multimodal nanoporous (np) PtCu alloy was prepared via a two-step dealloying strategy under mild conditions. Np-PtCu exhibits superior electrocatalytic activity. The assay is highly sensitive, selective, and it allows for a long-term detection of H₂O₂ and glucose.

A sandwich-type electrochemical aptasensor for the carcinoembryonic antigen via biocatalytic precipitation amplification and by using gold nanoparticle composites

A sandwich-type electrochemical aptasensor is described for detecting the carcinoembryonic antigen (CEA) with high sensitivity and accuracy. Two kinds of nanomaterials are used. The first was obtained by modifying gold nanoparticles with reduced graphene oxide and hemin (Hemin-rGO-AuNPs). The second consists of horseradish peroxidase-modified organic-inorganic hybrid nanoflowers linked to gold nanoparticles to obtain an architecture of type HRP-Cu₃(PO₄)₂-HNF-AuNPs). These serve as carriers for two aptamers (apt1 and apt2) against CEA. Simultaneously, they were used to catalyze the precipitation reaction between 4-chloro-1-naphthol(4-CN) and H₂O₂. A sandwich-type assay linked to enzyme inhibition amplification was established for electrochemical determination of CEA. Under optimal experimental conditions and by using differential pulse voltammetry, the response peak currents (best measured at -0.34 V vs. Ag/AgCl) increases linearly with the logarithm of the CEA concentration in the range between 100 fg mL^-1 and 100 ng mL^-1. The detection limit is as low as 29 fg mL^-1. Graphical abstract Schematic representation of the sandwich-type electrochemical aptasensor based on signal inhibition amplification from biocatalytic precipitation reaction. (HRP-Cu₃(PO₄)₂ hybrid nanoflowers: Horseradish Peroxidase-Cu₃(PO₄)₂ hybrid nanoflowers; AuNPs: Gold Nanoparticles; Hemin-rGO-AuNPs: Hemin-Reduced Graphene Oxide-Gold Nanoparticles; BSA: Bovine Serum Albumin; CEA: Carcinoembryonic Antigen; CEA_apt1: 5'-SH-(CH₂)₆-ATA CCA GCT TAT TCA ATT-3'; CEA_apt2: 5'-NH₂-(CH₂)₆-AGG GGG TGA AGG GAT ACC C-3'; GCE: Glassy carbon electrode; 4-CN: 4-Chloro-1-naphthol; DPV: Differential pulse voltammetry).

A facile preparation of FePt-loaded few-layer MoS2 nanosheets nanocomposites (F-MoS2-FePt NCs) and their application for colorimetric detection of H2O2 in living cells

BACKGROUND

Rapid and sensitive detection of H2O2 especially endogenous H2O2 is of great importance for series of industries including disease diagnosis and therapy. In this work, uniform FePt nanoparticles are successfully anchored onto Few-layer molybdenum disulfide nanosheets (F-MoS2 NSs). The powder X-ray diffraction, transmission electron microscopy, UV-Vis spectra and atomic force microscopy were employed to confirm the structure of the obtained nanocomposites (F-MoS2-FePt NCs). The prepared nanocomposites show efficient peroxidase-like catalytic activities verified by catalyzing the peroxidation substrate 4,4'-diamino-3,3',5,5'-tetramethylbiphenyl (TMB) with the existence of H2O2.

RESULTS

The optimal conditions of the constructed colorimetric sensing platform is proved as 35 °C and pH 4.2. Under optimal catalytic conditions, the detection limit for H2O2 detection reaches 2.24 μM and the linear ranger is 8 μM to 300 μM. Furthermore, the proposed colorimetric sensing platform was successfully utilized to detect the intracellular H2O2 of cancer cells (MCF-7).

CONCLUSIONS

These findings indicated that the F-MoS2-FePt-TMB-H2O2 system provides a potential sensing platform for hydrogen peroxide monitoring in living cells.

Reduced Graphene Oxide Nanohybrid-Assembled Microneedles as Mini-Invasive Electrodes for Real-Time Transdermal Biosensing

A variety of nanomaterial-based biosensors have been developed to sensitively detect biomolecules in vitro, yet limited success has been achieved in real-time sensing in vivo. The application of microneedles (MN) may offer a solution for painless and minimally-invasive transdermal biosensing. However, integration of nanostructural materials on microneedle surface as transdermal electrodes remains challenging in applications. Here, a transdermal H₂ O₂ electrochemical biosensor based on MNs integrated with nanohybrid consisting of reduced graphene oxide and Pt nanoparticles (Pt/rGO) is developed. The Pt/rGO significantly improves the detection sensitivity of the MN electrode, while the MNs are utilized as a painless transdermal tool to access the in vivo environment. The Pt/rGO nanostructures are protected by a water-soluble polymer layer to avoid mechanical destruction during the MN skin insertion process. The polymer layer can readily be dissolved by the interstitial fluid and exposes the Pt/rGO on MNs for biosensing in vivo. The applications of the Pt/rGO-integrated MNs for in situ and real-time sensing of H₂ O₂ in vivo are demonstrated both on pigskin and living mice. This work offers a unique real-time transdermal biosensing system, which is a promising tool for sensing in vivo with high sensitivity but in a minimally-invasive manner.

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.

A porous polyaniline nanotube sorbent for solid-phase extraction of the fluorescent reaction product of reactive oxygen species in cells, and its determination by HPLC

A method is described for extracting and detecting the fluorescent reaction product (2',7'-dichlorofluorescein, DCF) that is formed by reaction of reactive oxygen species (ROS) with dichlorodihydrofluorescein diacetate (DCFH-DA). DCF is extracted by using porous polyaniline nanotubes (PPN) which have a large specific surface and pore volume which favor the adsorption capacity. Additional attractive features include an appropriate pore size distribution, hydrophobic surface, and electron-attracting groups which contribute to DCF adsorption. A variety of methods was applied to characterize the morphology of PPN. Under optimal conditions and by performing DCF in 0.08-1.0 μM concentrations, the correlation coefficient of the calibration plot is 0.999. The limits of detection for standard DCF solutions is 20 nM. Compared with commercial sorbents for solid-phase extraction (SPE) such as commercially available carbon or Welchrom® C18, the use of the new sorbent results in better retraction recovery (92%) and longer reuse times (30 times). Doxorubicin and X-ray radiation were used to externally stimulate the ROS production in HepG2 and Hela cells. ROS was stabled by DCFH-DA and quantified by DCF. Following SPE, DCF was detected by HPLC and the concentration ROS was calculated. Graphical abstract ᅟ.

Carbon quantum dots encapsulated in super small platinum nanocrystals core-shell architecture/nitrogen doped graphene hybrid nanocomposite for electrochemical biosensing of DNA damage biomarker-8-hydroxy-2'-deoxyguanosine

In this work, carbon quantum dots (CQD) encapsulated in super small platinum nanocrystals core-shell architecture/nitrogen doped graphene hybrid nanocomposite (CQD@PDA@PtNCs-NGR) was design synthesized. Without using any capping reagent, stabilizer and surfactant, very small CQD was served as template and anchoring point for the synthesis of Pt NCs with a super small size (2.25 nm) and a uniform distribution. Meanwhile, dopamine (DA) was used as bridging agent, positioning agent and weak reducing agent to make Pt²⁺ grow on the CQD. Combine the high dispersed Pt NCs with high specific surface area and high conductivity of NGR, the CQD@PDA@PtNCs-NGR shows excellent electrocatalytic performance towards the biosensing of DNA damage biomarker- 8-Hydroxy-2'-deoxyguanosine (8-OH-dG). A very low detection limit of 0.45 nM and 0.85 nM (S/N = 3), a wide linear range of 0.013 μM-109.78 μM and a high sensitivity of 7.912 μA μM^-1cm^-2 and 4.190 μA μM^-1cm^-2 were obtained. The fabricated CQD@PDA@PtNCs-NGR realized the detection of 8-OH-dG in human urine practical sample. Furthermore, CQD@PDA@PtNCs-NGR was applied for the determination of 8-OH-dG generated from damaged DNA and damaged guanine (G), respectively. This work effectively combines the electrochemical signal of 8-OH-dG with DNA damage, confirms the mechanism of DNA damage, which might pave a new way to establish the associations between degree of DNA damage and 8-OH-dG.