Non-enzymatic hydrogen peroxide sensor based on graphene quantum dots-chitosan/methylene blue hybrid nanostructures

Ultrasensitive electrochemiluminescence biosensor based on polymethylene blue nanoparticles and DNA network for Staphylococcus aureus detection

A novel bipolar electrode (BPE)-electrochemiluminescence (ECL) device was constructed for the ultra-sensitive detection of Staphylococcus aureus (S. aureus) by combining polymerase chain reaction (PCR) amplification and DNA network-loaded polymethylene blue nanoparticles (pMB NPs). The presence of target triggered the dissociation of double-stranded DNA on Fe3O4 NPs and the release of T strand, which initiated the PCR. The PCR product contains two protruding single-stranded DNA fragments that serve as bridges to connect Au NPs labeled probes. The PCR-Au products were captured by the probes on cathode of BPE to form three-dimensional DNA networks, which offer multiple adsorption sites for pMB NPs, leading to the remarkable enhancement of ECL intensity. Under optimal circumstances, a wide linear range from 10 to 108 CFU/mL and a low detection limit of 0.78 CFU/mL were achieved. This research opens new horizons for the application of PCR-based biosensors for the accurate and sensitive measurement of pathogenic bacteria.

Electrical Wiring of Malarial Parasite Intermediate Hematin on a Tailored N-Doped Carbon Nanomaterial Surface and Its Bioelectrocatalytic Hydrogen Peroxide Reduction and Sensing

Hematin, an iron-containing porphyrin compound, plays a crucial role in various biological processes, including oxygen transport, storage, and functionality of the malarial parasite. Specifically, hematin-Fe interacts with the nitrogen atom of antimalarial drugs, forming an intermediate step crucial for their function. The electron transfer functionality of hematin in biological systems has been scarcely investigated. In this study, we developed a biomimicking electrical wiring of hematin-Fe with a model N-drug system, represented as {hematin-Fe---N-drug}. We achieved this by immobilizing hematin on a multiwalled carbon nanotube (MWCNT)/N-graphene quantum dot (N-GQD) modified electrode (MWCNT/N-GQD@Hemat). N-GQD serves as a model molecular drug system containing nitrogen atoms to mimic the {hematin-Fe---N-drug} interaction. The prepared bioelectrode exhibited a distinct redox peak at a measured potential (E1/2) of -0.410 V vs Ag/AgCl, accompanied by a surface excess value of 3.54 × 10-9 mol cm-2. This observation contrasts significantly with the weak or electroinactive electrochemical responses documented in literature-based hematin systems. We performed a comprehensive set of physicochemical and electrochemical characterizations on the MWCNT/N-GQD@Hemat system, employing techniques including FESEM, TEM, Raman spectroscopy, IR spectroscopy, and AFM. To evaluate the biomimetic electrode's electroactivity, we investigated the selective-mediated reduction of H2O2 as a model system. As an important aspect of our research, we demonstrated the use of scanning electrochemical microscopy to visualize the in situ electron transfer reaction of the biomimicking electrode. In an independent study, we showed enzyme-less electrocatalytic reduction and selective electrocatalytic sensing of H2O2 with a detection limit of 319 nM. We achieved this using a batch injection analysis-coupled disposable screen-printed electrode system in physiological solution.

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.

Recent advancement in quantum dot-based materials for energy storage applications: a review

The need for energy storage and conversion is growing as a result of the worsening consequences of climate change and the depletion of fossil fuels. Energy conversion and storage requirements are rising as a result of environmental problems including global warming and the depletion of fossil fuels. The key to resolving the energy crisis is anticipated to be the quick growth of sustainable energy sources including solar energy, wind energy, and hydrogen energy. In this review, we have focused on discussing various quantum dots (QDs) and polymers or nanocomposites used for SCs and have provided examples of each type's performance. Effective QD use has really led to increased performance efficiency in SCs. The use of quantum dots in energy storage devices, batteries, and various quantum dots synthesis have all been emphasized in a number of great literature articles. In this review, we have homed in on the electrode materials based on quantum dots and their composites for storage and quantum dot based flexible devices that have been published up to this point.

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.

A Core–Shell Au@TiO2 and Multi-Walled Carbon Nanotube-Based Sensor for the Electroanalytical Determination of H2O2 in Human Blood Serum and Saliva

A hydrogen peroxide (H₂O₂) sensor was developed based on core–shell gold@titanium dioxide nanoparticles and multi-walled carbon nanotubes modified glassy carbon electrode (Au@TiO₂/MWCNTs/GCE). Core–shell Au@TiO₂ material was prepared and characterized using a scanning electron microscopy and energy dispersive X-ray analysis (SEM/EDX), transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman spectroscopy, X-ray diffraction (XRD) and Zeta-potential analyzer. The proposed sensor (Au@TiO₂/MWCNTs/GCE) was investigated electrochemically using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The analytical performance of the sensor was evaluated towards H₂O₂ using differential pulse voltammetry (DPV). The proposed sensor exhibited excellent stability and sensitivity with a linear concentration range from 5 to 200 µM (R² = 0.9973) and 200 to 6000 µM (R² = 0.9994), and a limit of detection (LOD) of 1.4 µM achieved under physiological pH conditions. The practicality of the proposed sensor was further tested by measuring H₂O₂ in human serum and saliva samples. The observed response and recovery results demonstrate its potential for real-world H₂O₂ monitoring. Additionally, the proposed sensor and detection strategy can offer potential prospects in electrochemical sensors development, indicative oxidative stress monitoring, clinical diagnostics, general cancer biomarker measurements, paper bleaching, etc.

Graphene quantum dots: synthesis, properties, and applications to the development of optical and electrochemical sensors for chemical sensing

GQDs exhibits exceptional electrochemical activity owing to their active edge sites that make them very attractive for biosensing applications. However, their use in the design of new biosensing devices for application to the detection and quantification of toxins, pathogens, and clinical biomarkers has so far not investigated in detail. In this regard, herein we provide a detailed review on various methodologies employed for the synthesis of GQDs, including bottom-up and top-down approaches, with a special focus on their applications in biosensing via fluorescence, photoluminescence, chemiluminescence, electrochemiluminescence, fluorescence resonance energy transfer, and electrochemical techniques. We believe that this review will shed light on the critical issues and widen the applications of GQDs for the design of biosensors with improved analytical response for future applications. HIGHLIGHTS: • Properties of GQDs play a critical role in biosensing applications. • Synthesis of GQDs using top-down and bottom-up approaches is discussed comprehensively. • Overview of advancements in GQD-based sensors over the last decade. • Methods for the design of selective and sensitive GQD-based sensors. • Challenges and opportunities for future GQD-based sensors.

A graphene-printed paper electrode for determination of H2O2 in municipal wastewater during the COVID-19 pandemic

Graphene prepared through exfoliation process was printed on paper substrate using inkjet-printer and then printed paper electrode was used as an electrochemical sensor for analysis of H2O2 in cyclic voltammetry.

New Methylene Blue Covalently Functionalized Graphene Oxide Nanocomposite as Interfacial Material for the Electroanalysis of Hydrogen Peroxide

New methylene blue (NMB), a phenothiazine dye, was covalently bonded to graphene oxide (GO) using glutaraldehyde as a crosslinking agent, which was characterized by spectroscopic techniques and electrochemistry. The obtained GO-NMB nanocomposite was used as interface material to construct a novel electrochemical sensor for the determination of hydrogen peroxide (H₂O₂). The electrochemical sensor based on GO-NMB nanocomposite exhibited excellent electrocatalytic activity for the reduction of hydrogen peroxide (H₂O₂), which was also enhanced by GO within the GO-NMB nanocomposite. With the optimized experimental conditions, the developed sensor showed high sensitivity (79.4 μA mM^-1 cm^-2) for electrocatalytic determination of H₂O₂ at the applied potential of -0.50 V in the concentration range of 0.000333 to 2.28 mΜ. The low limit of detection (1.35 μM), good reproducibility, and high stability of the sensor suggests that the electrochemical sensor based on the GO-NMB nanocomposite possesses obvious advantages, which paves a new avenue to functionalize GO for obtaining electrode interface materials.

Forensic electrochemical presumptive blood test based on the voltammetric behaviour of methylene blue and whole blood

The ability to identify the presence of blood residues is important in a number of fields, such as in the forensic and archaeological sciences. A number of tests presently exist; however, these suffer drawbacks, such as difficulties with the interpretation of positive results and interferences from common chemicals and reagents. In this present study, for the first time, we demonstrate the possibility of applying an electrochemical technique as a semi-quantitative presumptive test for the detection of blood residues. Our method is based on the cyclic voltammetric behaviour of the methylene blue mediated detection of haemoglobin present in blood residues. Initial studies investigated the voltammetric behaviour of methylene blue and the possibility of using it for the mediated detection of haemoglobin. Using this approach, it was shown to be possible to detect haemoglobin and hence the presence of blood. We have shown the possibility of successfully identifying the presence of whole blood residues recovered from cloth gaining a coefficient of variation of 5.3%. Our method was shown to overcome many of the commonly reported interferences and interpretation issues. The results demonstrate that the developed method could be successfully used for the detection of blood residues in such samples requiring only simple dilution of the sample.

A Review on Metal- and Metal Oxide-Based Nanozymes: Properties, Mechanisms, and Applications

Since the ferromagnetic (Fe3O4) nanoparticles were firstly reported to exert enzyme-like activity in 2007, extensive research progress in nanozymes has been made with deep investigation of diverse nanozymes and rapid development of related nanotechnologies. As promising alternatives for natural enzymes, nanozymes have broadened the way toward clinical medicine, food safety, environmental monitoring, and chemical production. The past decade has witnessed the rapid development of metal- and metal oxide-based nanozymes owing to their remarkable physicochemical properties in parallel with low cost, high stability, and easy storage. It is widely known that the deep study of catalytic activities and mechanism sheds significant influence on the applications of nanozymes. This review digs into the characteristics and intrinsic properties of metal- and metal oxide-based nanozymes, especially emphasizing their catalytic mechanism and recent applications in biological analysis, relieving inflammation, antibacterial, and cancer therapy. We also conclude the present challenges and provide insights into the future research of nanozymes constituted of metal and metal oxide nanomaterials.

Chitosan-based carbon nanoparticles as a heavy metal indicator and for wastewater treatment

Removal of heavy metal ions by carbon nanoparticles synthesized from chitosan.

Gold nanoparticles plated porous silicon nanopowder for nonenzymatic voltammetric detection of hydrogen peroxide

A voltammetric approach was developed for the selective and sensitive determination of hydrogen peroxide using Au plated porous silicon (PSi) nanopowder modified glassy carbon electrode (GCE). The AuNPs-PSi hybrid structure was synthesized via stain etching procedure followed by an immersion plating method to deposit AuNPs onto PSi via a simple galvanic displacement reaction with no external reducing agent to convert Au³⁺ to Au⁰. The as-fabricated AuNPs-PSi catalyst was successfully characterized by XRD, Raman, FTIR, XPS, SEM, TEM and EDS techniques. Well crystalline nature of the as-fabricated hybrid structure with AuNPs size ranging from 5 to 40 nm was observed. The specific surface area and total pore volume for both PSi and AuNPs plated PSi were evaluated using N₂ adsorption isotherm technique. Cyclic voltammetry and electrochemical impedance spectroscopy techniques were applied to investigate the catalytic efficiency of AuNPs-PSi modified electrode compared to pure PSi/GCE and unmodified GCE. The sensing performance of the active material modified GCE was thoroughly examined with linear sweep voltammetry (LSV) and square wave voltammetry (SWV) techniques. The AuNPs-PSi/GCE exhibited a remarkable linear dynamic range between 2.0 and 13.81 mM (for LSV) and 0.5-6.91 mM for (SWV) with high sensitivity and low detection limit of 10.65 μAmM^-1cm^-2 and 14.84 μM for LSV, whereas 10.41 μAmM^-1cm^-2 and 15.16 μM using SWV techniques, respectively. The fabricated sensor electrode showed excellent anti-interfering ability in the presence of several common biomolecules as well as demonstrated good operational stability and reproducibility with low relative standard deviation. Moreover, the modified electrode showed acceptable recovery of H₂O₂ in a real sample analysis. Thus, the developed AuNPs-PSi hybrid nanomaterial represents an excellent electrocatalyst for the efficient detection and quantification of H₂O₂ by the electrochemical approach.

Biomolecules and Electrochemical Tools in Chronic Non-Communicable Disease Surveillance: A Systematic Review

Over recent three decades, the electrochemical techniques have become widely used in biological identification and detection, because it presents optimum features for efficient and sensitive molecular detection of organic compounds, being able to trace quantities with a minimum of reagents and sample manipulation. Given these special features, electrochemical techniques are regularly exploited in disease diagnosis and monitoring. Specifically, amperometric electrochemical analysis has proven to be quite suitable for the detection of physiological biomarkers in monitoring health conditions, as well as toward the control of reactive oxygen species released in the course of oxidative burst during inflammatory events. Besides, electrochemical detection techniques involve a simple and swift assessment that provides a low detection-limit for most of the molecules enclosed biological fluids and related to non-transmittable morbidities.

Electrochemical assays for determination of H2O2 and prostate-specific antigen based on a nanocomposite consisting of CeO2 nanoparticle-decorated MnO2 nanospheres

A nanocomposite consisting of CeO₂ nanoparticle-decorated MnO₂ nanospheres (CeO₂@MnO₂) was synthesized for the first time via a hydrothermal method. CeO₂@MnO₂ was exploited to construct an electrochemical assays for detecting H₂O₂ and prostate-specific antigen (PSA) with square wave voltammetry (SWV). The electrochemical results proved that CeO₂@MnO₂ owned a better electrocatalytic effect towards H₂O₂ reduction than pure MnO₂ NS and CeO₂ NP due to the synergistic effect between MnO₂ NS and CeO₂ NP. Under optimized conditions, CeO₂@MnO₂-based assay can be applied to detect H₂O₂ in the range 1 to 3.0 × 10³ μmol L^-1. The label-free electrochemical immunoassay based on CeO₂@MnO₂ displayed linearly with concentrations of PSA from 0.005 to 50.0 ng mL^-1. The electrochemical assays also possessed acceptable sensitivity, selectivity, and stability. The study showed that CeO₂@MnO₂ hold great potential as a biosensing platform and the clinical determination of tumor markers in human serum. Graphical abstract A nanocomposite consisting of CeO₂ nanoparticles decorated MnO₂ nanospheres (CeO₂ @MnO₂) was firstly synthesized via a hydrothermal method. CeO₂@MnO₂ was firstly exploited to construct electrochemical assays for detecting H2O₂ and prostate-specific antigen (PSA) with square wave voltammetry (SWV), respectively. The electrochemical results proved that CeO₂@MnO₂ owned better electrocatalysis towards H₂O₂ reduction than pure MnO₂ NS and CeO₂ NP due to the synergistic effect between MnO₂ NS and CeO₂ NP. Under optimized conditions, CeO₂@MnO₂ based assay relative to the H₂O₂ system can be applied to detect H₂O₂ with range from 1 to 3.0 × 10³ μmol L^-1. The label-free electrochemical immunoassay based on CeO₂@MnO₂ relative to the H₂O₂ system displayed linearly with concentrations of PSA from 0.005 to 50.0 ng mL^-1. The electrochemical assays also possessed acceptable sensitivity, selectivity and stability. The study showed that CeO₂@MnO₂ hold great potential for biosensing platform and the clinic determination of tumor markers in human serum.

A Sensitive Electrochemical Ascorbic Acid Sensor Using Glassy Carbon Electrode Modified by Molybdenite with Electrodeposited Methylene Blue

A non-enzymatic amperometric sensor using natural molybdenite (MLN) electrodeposited with methylene blue (MB) has been fabricated and characterized and its analytical performances were investigated for the determination of ascorbic acid (AA). The surface morphology of the electrode modified by electrodeposited MB was studied by use of the Advanced Mineral Identification and Characterization System (AMICS) and laser confocal high-temperature scanning microscope (LCSM). The poly(MB) and MLN immobilized sensor showed good stability, reproducibility, sensitivity, and selectivity. It exhibited a linear performance range from 3 to 1000 μM, with a lower detection limit of 0.083 μM (signal/noise = 3) and short response time (< 5 s). No obvious decrease in the current was observed after 20 days storage. The methodology reproducibility of this sensor was 2.6%. It showed good anti-interference ability for the potential interfering compounds. The poly(MB) film not only can enhance the electron-transfer rate but also increase the lifetime of the sensor. This study demonstrated the applicability of natural molybdenite for the fabrication of non-enzymatic electrochemical AA sensor.

Electrochemical biosensing to move forward in cancer epigenetics and metastasis: A review

Early detection and effective treatment are crucial to reduce the physical, emotional, and financial pressure exerted by growing cancer burden on individuals, families, communities, and health systems. Currently, it is clear that the accurate analysis of emerging cancer epigenetic and metastatic-related biomarkers at different molecular levels is envisaged as an exceptional solution for early and reliable diagnosis and the improvement of therapy efficiency through personalized treatments. Within this field, electrochemical biosensing has demonstrated to be competitive over other emerging and currently used methodologies for the determination of these biomarkers accomplishing the premises of user-friendly, multiplexing ability, simplicity, reduced costs and decentralized analysis, demanded by clinical oncology, thus priming electrochemical biosensors to spark a diagnostic revolution for cancer prediction and eradication. This review article critically discusses the main characteristics, opportunities and versatility exhibited by electrochemical biosensing, through highlighting representative examples published during the last two years, for the reliable determination of these emerging biomarkers, with great diagnostic, predictive and prognostic potential. Special attention is paid on electrochemical affinity biosensors developed for the single or multiplexed determination of methylation events, non-coding RNAs, ctDNA features and metastasis-related protein biomarkers both in liquid and solid biopsies of cancer patients. The main challenges to which further work must be addressed and the impact of these advances should have in the clinical acceptance of these emerging biomarkers are also discussed which decisively will contribute to understand the molecular basis involved in the epigenetics and metastasis of cancer and to apply more efficient personalized therapies.

Enzyme-less amperometric sensor manufactured using a Nafion–LaNiO3 nanocomposite for hydrogen peroxide

In the present study, an enzyme-less amperometric sensor based on Nafion (NF) and a LaNiO₃ (LNO) nanocomposite was constructed for H₂O₂ detection. LNO from the perovskite group was mixed with NF as an effective solubilizing and stabilizing agent that was used as a novel modifier for modification of the glassy carbon electrode (GCE). The designed sensor showed a desirable electrocatalytic response toward H₂O₂ reduction. The calibration curve revealed two linear portions in the concentration ranges of 0.2–50 μM and 50–3240 μM, and the detection limit was 0.035 μM. The accuracy of the interference-free sensor was checked by recovery analysis in serum samples.

In the present study, an enzyme-less amperometric sensor based on Nafion (NF) and a LaNiO₃ (LNO) nanocomposite was constructed for H₂O₂ detection.

Design of Ni(OH)2 nanocages@MnO2 nanosheets core-shell architecture to jointly facilitate electrocatalytic dynamic for highly sensitive detection of dopamine

In this work, Ni(OH)₂ nanocages@MnO₂ nanosheets core-shell architecture (Ni(OH)₂ NCs@MnO₂ NSs CSA) was successfully prepared through coordinated etching and precipitation (CEP) route followed by hydrothermal reaction, and then tested as sensitive electrode material for detection of dopamine (DA). The three dimensional (3D) hollow Ni(OH)₂ core effectively prevented the aggregation of MnO₂ NSs, leading to high utilization rate of MnO₂ NSs. Meanwhile, the two dimensional (2D) MnO₂ shell endowed Ni(OH)₂ NCs with larger specific area and abundant diffusion channels, facilitating mass transport. Ni(OH)₂ NCs@MnO₂ NSs CSA modified glassy carbon electrode (GCE) exhibited two satisfying sensitivities of 467.1 and 1249.9 μA mM^-1 cm^-2 within the two linear ranges of 0.02-16.30 μM and 18.30-118.58 μM, respectively. Furthermore, Ni(OH)₂ NCs@MnO₂ NSs CSA/GCE presented low detection limit of 1.75 nM and short response time of 1.14 s. Overall, Ni(OH)₂ NCs@MnO₂ NSs/GCE looks promising for analytical sensing of DA thanks to its prominent electrocatalytic dynamic issued from the 3D hollow structure@2D nanosheets core-shell architecture.

Fabrication of hexahedral Au-Pd/graphene nanocomposites biosensor and its application in cancer cell H2O2 detection

Currently, real time monitoring of chemical substances in vivo and in vitro has gained enormous attraction, and many researches reports have been focused on the design and construction of high-performance biosensor devices. In this work, a high-performance sensor was constructed by taking advantage of the excellent electrochemical activity and high-index facets of Au-Pd nanocubes and the large surface of rGO. Glassy carbon electrodes (GCEs) were modified by both Au-Pd nanocubes and rGO nanocomposites via physical adsorption. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were utilized to characterize and identify this unique nanostructure. These three-dimensional nanocomposites possess a high electroactive surface area and an excellent electrical conductivity, which resulted in favorable electroreduction activity toward H₂O₂ with a lower detection limit of 4 nM, a wide linear range from 0.005 μM to 3.5 mM and a rapid response time. Furthermore, the proposed sensor exhibited desirable performance in the detection of endogenous H₂O₂ in human serum samples and real-time monitoring of H₂O₂ released from living breast cancer cell lines. In summary, this work not only provides a potential method to construct a physiological and pathological H₂O₂ biosensor but also makes a valuable contribution to the early diagnosis of different cancers.

Carbon Dots and Graphene Quantum Dots in Electrochemical Biosensing

Graphene quantum dots (GQDs) and carbon dots (CDs) are among the latest research frontiers in carbon-based nanomaterials. They provide interesting attributes to current electrochemical biosensing due to their intrinsic low toxicity, high solubility in many solvents, excellent electronic properties, robust chemical inertness, large specific surface area, abundant edge sites for functionalization, great biocompatibility, low cost, and versatility, as well as their ability for modification with attractive surface chemistries and other modifiers/nanomaterials. In this review article, the use of GQDs and CDs as signal tags or electrode surface modifiers to develop electrochemical biosensing strategies is critically discussed through the consideration of representative approaches reported in the last five years. The advantages and disadvantages arising from the use of GQDs and CDs in this context are outlined together with the still required work to fulfil the characteristics needed to achieve suitable electrochemical enzymatic and affinity biosensors with applications in the real world.

Enzymeless electrochemical determination of hydrogen peroxide at a heteropolyanion-based composite film electrode

For the first time, a sensitive and efficient composite film of [PB/WV–Pt@Pd]₆was constructed for H₂O₂detection.

N-Arylated bisferrocene pyrazole for the dual-mode detection of hydrogen peroxide: an AIE-active fluorescent “turn ON/OFF” and electrochemical non-enzymatic sensor

N-Arylated bisferrocene pyrazoles for the dual-mode detection of H₂O₂ by AIE-active fluorescence and non-enzymatic electrochemical methods.

Ultrasensitive determination of receptor tyrosine kinase with a label-free electrochemical immunosensor using graphene quantum dots-modified screen-printed electrodes

A new label-free electrochemical immunosensor is constructed for the selective and sensitive determination of the clinically relevant biomarker receptor tyrosine kinase (AXL) in human serum. The disposable immunosensing platform is prepared by immobilization of the specific anti-AXL antibody onto amine functionalized graphene quantum dots (fGQDs)-modified screen-printed carbon electrodes (SPCEs). The affinity reactions were monitored by measuring the decrease in the differential pulse voltammetric (DPV) response of the redox probe Fe(CN)₆^3-/4-. All the experimental variables involved in the preparation of the modified electrodes and in the immunosensor performance were optimized. The as prepared immunosensor exhibits an improved analytical performance with respect to other electrochemical immunosensors reported so far, with a wider range of linearity and a lower detection limit, 0.5 pg mL^-1, which is more than one hundred thousand times lower than the established cut-off value for heart failure (HF) diagnosis in serum (71 ng mL^-1). The developed immunosensor was successfully applied to the determination of the endogenous content of AXL in serum of HF patients without any matrix effect observed after just a sample dilution.

A blood-serum sulfide selective electrochemical sensor based on a 9,10-phenanthrenequinone-tethered graphene oxide modified electrode

The sulfide ion and its associated species (H₂S and HS^-) are widely referred to as toxic chemicals. However, at concentrations of ∼10-100 μM, it serves as a neurotransmitter and signaling agent in biological systems. Abnormalities in blood serum sulfide can be an indication of several diseases, including diabetes, wherein there is a significant reduction in the sulfide ion concentration (<10 μM). Herein, we wish report a 9,10-phenanthrenequinone (PQn) tethered graphene oxide (GO) modified glassy carbon electrode (GCE/GO@PQn) for the highly selective and stable electrocatalytic oxidation and flow injection analysis (FIA) of sulfide ions. The electrode exhibits a detection range of 1-100 μM, and is suitable for the common biochemical interference-free detection of blood serum sulfide in pH 7 phosphate buffer solution. The modified electrode was found to be tolerant of interfering chemicals such as cysteine, uric acid, ascorbic acid, nitrate, glucose, hydrogen peroxide, nitrate, nitrite and dissolved oxygen. This is unlike conventional redox mediator modified electrodes, which all show marked interference with the above-mentioned chemicals during sulfide detection. A constructed FIA calibration plot (applied potential, E_app = 0.15 V vs. Ag/AgCl) was linear in the sulfide concentration ranges of 1-100 μM (1^st region) and 300 μM-5 mM (2^nd region) with a detection limit value of 700 nM. The selective and quick FIA of sulfide ions in three diabetic patient blood samples along with a control was demonstrated as a proof of concept.