Electrochemical sensor based on nitrogen doped graphene: Simultaneous determination of ascorbic acid, dopamine and uric acid

Electrochemical sensor based on novel two-dimensional nanohybrids: MoS2nanosheets conjugated with organic copper nanowires for simultaneous detection of hydrogen peroxide and ascorbic acid

MoS₂nanosheets were conjugated with organic copper nanowires for fabrication of electrochemical hydrogen peroxide and ascorbic acid sensors.

An efficient electrode for simultaneous determination of guanine and adenine using nano-sized lead telluride with graphene

Herein, lead telluride (PbTe) nanocrystals were chemically synthesized at room temperature via reduction of homogeneous mixtures of tartrate complexes of Pb²⁺ and Te⁴⁺ with sodium borohydride.

Ultrafine Pt–Ni bimetallic nanoparticles anchored on reduced graphene oxide nanocomposites for boosting electrochemical detection of dopamine in biological samples

The present report demonstrates the development of a Pt–Ni/rGO composite electrochemical sensor for the detection of dopamine.

A. AH, Elshikh MS, Farah MA. Hierarchically structured CuFe2O4 ND@RGO composite for the detection of oxidative stress biomarker in biological fluids

In this work, stable and catalytically active copper ferrite nanodots (CuFe₂O₄) entrapped by porous RGO nanosheets were prepared via a facile condensation process using a green reducing agent.

Synthesis and properties of phosphorus and sulfur co-doped graphene

The derivatisation of graphene significantly extends its application potential beyond just a highly conductive material.

Chemical sensing with 2D materials

During the last decade, two-dimensional materials (2DMs) have attracted great attention due to their unique chemical and physical properties, which make them appealing platforms for diverse applications in sensing of gas, metal ions as well as relevant chemical entities.

Fluorescent sensing of ascorbic acid based on iodine induced oxidative etching and aggregation of lysozyme-templated silver nanoclusters

In this work, we developed a sensitive and highly selective fluorescent approach for the detection of ascorbic acid (AA) by taking advantage of the oxidative etching effect of iodine (I₂) on the lysozyme-stabilized silver nanoclusters (dLys-AgNCs) with fluorescence quenching. I₂ could be produced from the redox reaction between iodate (IO₃^-) and AA, and thus the fluorescence intensity of dLys-AgNCs was turned off significantly in the coexistence of IO₃^- and AA. The fluorescence quenching of dLys-AgNCs had a good linear relationship with AA concentration, which allowed the detection of AA in the range from 0.05 to 45.0 μmol L^-1 with a detection limit of 20 nmol L^-1. The quenching mechanism was elucidated by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), zeta potential, and dynamic light scattering (DLS) measurements, confirming that the fluorescence quenching of the dLys-AgNCs was attributed to the oxidative etching of the in situ generated I₂, inducing aggregation of the dLys-AgNCs probe by forming Ag@AgI nanocomposite. The dLys-AgNCs probe exhibited excellent selectivity for AA sensing over several common reducing agents tested. Moreover, this approach was extended to the detection of AA in orange juice and urine with recovery rates in the range of 96.0% (RSD: 4.11) to 100.9% (RSD: 3.28) and 94.5% (RSD: 6.40) to 99.2% (RSD: 5.36), respectively.

One-Pot Synthesis of Reduced Graphene Oxide/Metal (Oxide) Composites

Graphene, one of the most attractive two-dimensional nanomaterials, has demonstrated a broad range of applications because of its excellent electronic, mechanical, optical, and chemical properties. In this work, a general, environmentally friendly, one-pot method for the fabrication of reduced graphene oxide (RGO)/metal (oxide) (e.g., RGO/Au, RGO/Cu₂O, and RGO/Ag) composties was developed using glucose as the reducing agent and the stabilizer. The glucose not only reduced GO effectively to RGO but also reduced the metal precursors to form metal (oxide) nanoparticles on the surface of RGO. Moreover, the RGO/metal (oxide) composites were stabilized by gluconic acid on the surface of RGO. The developed RGO/metal (oxide) composites were characterized using STEM, FE-SEM, EDS, UV-vis absorption spectroscopy, XRD, FT-IR, and Raman spectroscopy. Finally, the developed nanomaterials were successfully applied as an electrode catalyst to simultaneous electrochemical analysis of l-ascorbic acid, dopamine, and uric acid.

Graphene oxide electrochemistry: the electrochemistry of graphene oxide modified electrodes reveals coverage dependent beneficial electrocatalysis

The modification of electrode surfaces is widely implemented in order to try and improve electron transfer kinetics and surface interactions, most recently using graphene related materials. Currently, the use of 'as is' graphene oxide (GO) has been largely overlooked, with the vast majority of researchers choosing to reduce GO to graphene or use it as part of a composite electrode. In this paper, 'as is' GO is explored and electrochemically characterized using a range of electrochemical redox probes, namely potassium ferrocyanide(II), N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), dopamine hydrochloride and epinephrine. Furthermore, the electroanalytical efficacy of GO is explored towards the sensing of dopamine hydrochloride and epinephrine via cyclic voltammetry. The electrochemical response of GO is benchmarked against pristine graphene and edge plane-/basal plane pyrolytic graphite (EPPG and BPPG respectively) alternatives, where the GO shows an enhanced electrochemical/electroanalytical response. When using GO as an electrode material, the electrochemical response of the analytes studied herein deviate from that expected and exhibit altered electrochemical responses. The oxygenated species encompassing GO strongly influence and dominate the observed voltammetry, which is crucially coverage dependent. GO electrocatalysis is observed, which is attributed to the presence of beneficial oxygenated species dictating the response in specific cases, demonstrating potential for advantageous electroanalysis to be realized. Note however, that crucial coverage based regions are observed at GO modified electrodes, owing to the synergy of edge plane sites and oxygenated species. We report the true beneficial electrochemistry of GO, which has enormous potential to be beneficially used in various electrochemical applications 'as is' rather than be simply used as a precursor to making graphene and is truly a fascinating member of the graphene family.

Nitrogen-Rich Polyacrylonitrile-Based Graphitic Carbons for Hydrogen Peroxide Sensing

Catalytic substrate, which is devoid of expensive noble metals and enzymes for hydrogen peroxide (H₂O₂), reduction reactions can be obtained via nitrogen doping of graphite. Here, we report a facile fabrication method for obtaining such nitrogen doped graphitized carbon using polyacrylonitrile (PAN) mats and its use in H₂O₂ sensing. A high degree of graphitization was obtained with a mechanical treatment of the PAN fibers embedded with carbon nanotubes (CNT) prior to the pyrolysis step. The electrochemical testing showed a limit of detection (LOD) 0.609 µM and sensitivity of 2.54 µA cm^-2 mM^-1. The promising sensing performance of the developed carbon electrodes can be attributed to the presence of high content of pyridinic and graphitic nitrogens in the pyrolytic carbons, as confirmed by X-ray photoelectron spectroscopy. The reported results suggest that, despite their simple fabrication, the hydrogen peroxide sensors developed from pyrolytic carbon nanofibers are comparable with their sophisticated nitrogen-doped graphene counterparts.

Ultrasensitive electrochemical detection of tumor cells based on multiple layer CdS quantum dots-functionalized polystyrene microspheres and graphene oxide - polyaniline composite

In this work, a novel ultrasensitive electrochemical biosensor was developed for the detection of K562 cell by a signal amplification strategy based on multiple layer CdS QDs functionalized polystyrene microspheres(PS) as bioprobe and graphene oxide(GO) -polyaniline(PANI) composite as modified materials of capture electrode. Due to electrostatic force of different charge, CdS QDs were decorated on the surface of PS by PDDA (poly(diallyldimethyl-ammonium chloride)) through a layer-by-layer(LBL) assemble technology, in which the structure of multiple layer CdS QDs increased the detection signal intensity. Moreover, GO-PANI composite not only enhanced the electron transfer rate, but also increased tumor cells load ratio. The resulting electrochemical biosensor was used to detect K562 cells with a lower detection limit of 3 cellsmL^-1 (S/N = 3) and a wider linear range from 10 to 1.0 × 10⁷ cellsmL^-1. This sensor was also used for mannosyl groups on HeLa cells and Hct116 cells, which showed high specificity and sensitivity. This signal amplification strategy would provide a novel approach for detection, diagnosis and treatment for tumor cells.

Synthesis of Multifunctional Electrically Tunable Fluorine-Doped Reduced Graphene Oxide at Low Temperatures

Doping with heteroatoms is a well-established method to tune the electronic properties and surface chemistry of graphene. Herein, we demonstrate the synthesis of a fluorine-doped reduced graphene oxide (FrGO) at low temperatures that offers multiple opportunities in applied fields. The as-synthesized FrGO product shows a better electrical conductivity of 750 S m^-1 than that of undoped rGO with an electrical conductivity of 195 S m^-1. To demonstrate the multifunctional applications of the as-synthesized FrGO, it was examined for electromagnetic interference shielding and electrochemical sensing of histamine as an important food biomarker. A laminate of FrGO delivered an EMI shielding effectiveness value of 22 dB in Ku band as compared with 11.2 dB for an rGO laminate with similar thickness. On the other hand, an FrGO modified sensor offered an excellent sensitivity (∼7 nM), wide detection range, and good selectivity in the presence of similar biomarkers. This performance originates from the better catalytic ability of FrGO as compared with rGO, where fluorine atoms play the role of catalytic active sites owing to their high electronegativity. The fluorination reaction also helps to improve the reduction degree of the chemically synthesized graphene, consequently enhancing the electrical conductivity, which is a prime requirement for increasing the electromagnetic and electrochemical properties of graphene.

A redox-modulated fluorescent strategy for the highly sensitive detection of metabolites by using graphene quantum dots

In this paper, a redox-modulated fluorescent strategy based on the transformation of Fe²⁺/Fe³⁺ couple and enzymatic reaction for rapid monitoring glucose and uric acid using graphene quantum dots (GQDs) as fluorescent probe was developed. Hydrogen peroxide (H₂O₂) can be produced by the enzymatic reaction of a series of metabolites, such as glucose and uric acid. In the presence of hydrogen peroxide, Fe²⁺ can be oxidized and converted to Fe³⁺, which have a significant quenching difference in the fluorescence of graphene quantum dots (GQDs). Thus, a sensitive and label-free biosensor for the detection of uric acid and glucose was developed. Under the optimized experimental conditions, the fluorescence intensity was linearly correlated with the concentration of uric acid and glucose in the range of 0.1-45 μmolL^-1 and 0.1-30 μmolL^-1 with a detection limit of 0.026 μmolL^-1and 0.021 μmolL^-1, respectively. The proposed method was applied to the determination of uric acid and glucose in human serum samples with satisfactory results, which had potential application to detect metabolites associated with H₂O₂ release.

Nitrogen-doped graphene: effect of graphite oxide precursors and nitrogen content on the electrochemical sensing properties

Graphene, produced via chemical methods, has been widely applied for electrochemical sensing due to its structural and electrochemical properties as well as its ease of production in large quantity. While nitrogen-doped graphenes are widely studied materials, the literature showing an effect of graphene oxide preparation methods on nitrogen quantity and chemical states as well as on defects and, in turn, on electrochemical sensing is non-existent. In this study, the properties of nitrogen-doped graphene materials, prepared via hydrothermal synthesis using graphite oxide produced by various classical methods using permanganate or chlorate oxidants Staudenmaier, Hummers, Hofmann and Brodie oxidation methods, were studied; the resulting nitrogen-doped graphene oxides were labeled as ST-GO, HU-GO, HO-GO and BR-GO, respectively. The electrochemical oxidation of biomolecules, such as ascorbic acid, uric acid, dopamine, nicotinamide adenine nucleotide and DNA free bases, was carried out using cyclic voltammetry and differential pulse voltammetry techniques. The nitrogen content in doped graphene oxides increased in the order ST-GO < BR-GO < HO-GO < HU-GO. In the same way, the pyridinic form of nitrogen increased and the electrocatalytic effect of N-doped graphene followed this trend, as shown in the cyclic voltammograms. This is a very important finding that provides insight into the electrocatalytic effect of N-doped graphene. The nitrogen-doped graphene materials exhibited improved sensitivity over bare glassy carbon for ascorbic acid, uric acid and dopamine detection. These studies will enhance our understanding of the effects of graphite oxide precursors on the electrochemical sensing properties of nitrogen-doped graphene materials.

Seed-mediated grown silver nanoparticles as a colorimetric sensor for detection of ascorbic acid

A simple and sensitive approach was demonstrated for detection of ascorbic acid (AA) based on seed-mediated growth of silver nanoparticles (Ag NPs). According to the seeding strategy, silver ions existing in the growth solution were reduced to silver atoms on the surface of silver seeds via redox reaction between silver ions and AA. This process -led to appear an absorption band in near 420nm owing to the localized surface plasmon resonance peak of the generated Ag NPs. This change in absorption spectra of Ag NPs caused a change in color of the mixture from colorless to yellow. It was found that the changes in absorption intensity at 420nm have a good relationship with the concentration of AA. Also, detection of AA was achieved through the established colorimetric sensor in the range of 0.25-25μM with detection limit of 0.054μM. Moreover, the selectivity of the method was evaluated with considering potential interferences. The method showed high selectivity toward AA rather than potential interferences and coexisted molecules with AA. It was successfully applied for detection and determination of AA in pharmaceutical tablets and commercial lemonade.

Enabling Inkjet Printed Graphene for Ion Selective Electrodes with Postprint Thermal Annealing

Inkjet printed graphene (IPG) has recently shown tremendous promise in reducing the cost and complexity of graphene circuit fabrication. Herein we demonstrate, for the first time, the fabrication of an ion selective electrode (ISE) with IPG. A thermal annealing process in a nitrogen ambient environment converts the IPG into a highly conductive electrode (sheet resistance changes from 52.8 ± 7.4 MΩ/□ for unannealed graphene to 172.7 ± 33.3 Ω/□ for graphene annealed at 950 °C). Raman spectroscopy and field emission scanning electron microscopy (FESEM) analysis reveals that the printed graphene flakes begin to smooth at an annealing temperature of 500 °C and then become more porous and more electrically conductive when annealed at temperatures of 650 °C and above. The resultant thermally annealed, IPG electrodes are converted into potassium ISEs via functionalization with a poly(vinyl chloride) (PVC) membrane and valinomycin ionophore. The developed potassium ISE displays a wide linear sensing range (0.01-100 mM), a low detection limit (7 μM), minimal drift (8.6 × 10^-6 V/s), and a negligible interference during electrochemical potassium sensing against the backdrop of interfering ions [i.e., sodium (Na), magnesium (Mg), and calcium (Ca)] and artificial eccrine perspiration. Thus, the IPG ISE shows potential for potassium detection in a wide variety of human fluids including plasma, serum, and sweat.

Simultaneous determination of ascorbic acid and caffeine in commercial soft drinks using reversed-phase ultraperformance liquid chromatography

A new reversed-phase ultraperformance liquid chromatography method with a photodiode array detector was developed for the quantification of ascorbic acid (AA) and caffeine (CAF) in 11 different commercial drinks consisting of one energy drink and 10 ice tea drinks. Separation of the analyzed AA and CAF with an internal standard, caffeic acid, was performed on a Waters BEH C₁₈ column (100 mm × 2.1 mm, 1.7 μm i.d.), using a mobile phase consisting of acetonitrile and 0.2M H₃PO₄ (11:89, v/v) with a flow rate of 0.25 mL/min and an injection volume of 1.0 μL. Calibration graphs for AA and CAF were computed from the peak area ratio of AA/internal standard and CAF/internal standard detected at 244.0 nm and 273.6 nm, respectively. The developed reversed-phase ultraperformance liquid chromatography method was validated by analyzing standard addition samples. The proposed reversed-phase ultraperformance liquid chromatography method gave us successful results for the quantitative analysis of commercial drinks containing AA and CAF substances.

Investigation on the ability of heteroatom-doped graphene for biorecognition

Doped graphene platforms have been attracting considerable attention due to their improved electrochemical performances. Recent studies have shown the advantage of using either p-type or n-type doped graphene materials as transducers for the detection of various electroactive probes. Here we wanted to take a step forward and extend the study to investigate the ability of heteroatom doped graphene as an electrochemical platform for biorecognition. To this aim, a boron-doped graphene, a nitrogen-doped graphene and an undoped graphene material prepared under similar conditions were employed for the detection of fumonisin B₁, a highly carcinogenic mycotoxin found in food commodities. We found that the material structural features, such as the amount of oxygen functionalities, had a stronger influence on the sensitivity of biorecognition rather than the kind and amount of dopant. Our findings may be essential for the choice of a proper platform for the assessment of food safety.

Doping two-dimensional materials: ultra-sensitive sensors, band gap tuning and ferromagnetic monolayers

The successful isolation of graphene from graphite in 2004 opened up new avenues to study two-dimensional (2D) systems from layered materials. Since then, research on 2D materials, including graphene, hexagonal-BN (h-BN), transition metal dichalcogenides (TMDs) and black phosphorous, has been extensive, thus leading to various possible applications in the fields of optoelectronics, biomedicine, spintronics, electrochemistry, energy storage and catalysis. However, certain barriers still need to be overcome when dealing with real applications, such as graphene's lack of a bandgap, restricting its use in semiconductor electronics. In this context, a possible solution is to tailor the electronic and optical properties of 2D materials by introducing defects or elemental doping. Although defects play a major role in modifying materials properties, the fact that we call them "defects" might have a negative impact. There has been a long debate on whether structurally perfect materials are equally relevant for modifying the properties and for applications. In this focus article, we clarify that although extra large amounts of defects could be detrimental to the materials properties, well-designed defects might lead to unprecedented properties and interesting applications that pristine materials do not have. Given the relatively short history of research on doped 2D layered materials, our objective is to answer and clarify the following fundamental questions: why does nanomaterial doping offer improved physico-chemical properties? What new applications arise from doping? And what are the current challenges along this line?

Dendritic unzipped carbon nanofibers enable uniform loading of surfactant-free Pd nanoparticles for the electroanalysis of small biomolecules

Illustration of the mechanisms of SCNF and preparation of Pd/GNF composites and Pd/GNF sensors for the simultaneous determination of small biomolecules.

Surfactant exfoliated 2D hexagonal Boron Nitride (2D-hBN) explored as a potential electrochemical sensor for dopamine: surfactants significantly influence sensor capabilities

The surfactant utilised in the exfoliated synthesis of 2D hexagonal Boron Nitride (2D-hBN) significantly influences sensor capabilities towards the detection of dopamine.