•Nitrogen (N) and sulfur (S) co-doped reduced graphene oxide (NS-rGO) was prepared by a one-step thermal annealing procedure.•NS-rGO is expected to replace precious metal nanoparticles as electrode materials because its comparable electrocatalytic activity, low cost, and straightforward synthetic steps.•NS-rGO modified glassy carbon electrode (NS-rGO/GCE) showed superior electrochemical performance in H2O2 determination than some noble metal nanoparticles based electrochemical sensors.•NS-rGO/GCE had desirable selectivity, repeatability and stability, and was utilized for H2O2 determination in human serum.Hydrogen peroxide (H2O2) is becoming significant due to its extensive applications, so determination of H2O2 is very important topic in analytical chemistry. Metal-free “graphene alloy” – nitrogen (N) and sulfur (S) heteroatoms co-doped reduced graphene oxide (NS-rGO) was produced via a simple one-step thermal annealing procedure using a mixture of 5-amino-2-mercapto-1,3,4-thiadiazole (AMT) and graphene oxide (GO). The obtained metal-free NS-rGO composite showed better electrocatalytic activity toward the reduction of H2O2 compared with the reduced graphene oxide (rGO). The enhanced performance was caused by the synergistic effect of N and S co-doping. Under optimum conditions, the constructed sensor demonstrated a linear response to H2O2 in the range of 7–18000 μM, with a lower detection limit of 0.45 μM (S/N=3), even better than some reported sensors based on noble metal nanoparticles. Moreover, the proposed sensor exhibited excellent analytical performance in terms of acceptable selectivity, excellent reproducibility and long-time stability. These results indicated that the NS-rGO composite was a promising metal-free electrocatalytic material for constructing H2O2 sensors. Additionally, NS-rGO composite was expected to be applied as catalysts for fuel cell applications, even for applications beyond fuel cells.

The surface and interface could be designed to enhance properties of electrocatalysts, and they are regarded as the key characteristics. This report describes surface modification of a bifunctional rht-type metal–organic framework (MOF, Cu-TDPAT) with nanosized electrochemically reduced graphene oxide (n-ERGO). The hybrid strategy results in a Cu-TDPAT–n-ERGO sensor with sensitive and selective response toward hydrogen peroxide (H2O2). Compared with Cu-TDPAT, Cu-TDPAT–n-ERGO exhibits significantly enhanced electrocatalytic activities, highlighting the importance of n-ERGO in boosting their electrocatalytic activity. The sensor shows a wide linear detection range (4–12 000 μM), and the detection limit is 0.17 μM (S/N = 3) which is even lower than horseradish peroxidase or recently published noble metal nanomaterial based biosensors. Moreover, the sensor displays decent stability, excellent anti-interference performance, and applicability in human serum and urine samples. Such good sensing performance can be explained by the synergetic effect of bifunctional Cu-TDPAT (open metal sites and Lewis basic sites) and n-ERGO (excellent conductive property). It is expected that rht-type MOF-based composites can provide wider application potential for the construction of bioelectronics devices, biofuel cells, and biosensors.Keywords: electrochemical; hydrogen peroxide; nanosized graphene; rht-type metal−organic framework; synergetic effect;

AbstractWe report a facile approach to prepare an artificial enzyme system for tandem catalysis. NiPd hollow nanoparticles and glucose oxidase (GOx) were simultaneously immobilized on the zeolitic imidazolate framework 8 (ZIF-8) via a co-precipitation method. The as-prepared GOx@ZIF-8(NiPd) nanoflower not only exhibited the peroxidase-like activity of NiPd hollow nanoparticles but also maintained the enzymatic activity of GOx. A colorimetric sensor for rapid detection of glucose was realized through the GOx@ZIF-8(NiPd) based multi-enzyme system. Moreover, the GOx@ZIF-8(NiPd) modified electrode showed good bioactivity of GOx and high electrocatalytic activity for the oxygen reduction reaction (ORR), which could also be used for electrochemical detection of glucose.

A novel one-pot strategy is proposed to fabricate 3D porous graphene (3D GN) decorated with Fe3O4 nanoparticles (Fe3O4 NPs) by using hemin as iron source. During the process, graphene oxide was simultaneously reduced and self-assembled to form 3D graphene hydrogel while Fe3O4 NPs synthesized from hemin distributed uniformly on 3D GN. The preparation process is simple, facile, economical, and green. The obtained freeze-dried product (3D GH-5) exhibits outstanding peroxidase-like activity. Compared to the traditional 2D graphene-based nanocomposites, the introduced 3D porous structure dramatically improved the catalytic activity, as well as the catalysis velocity and its affinity for substrate. The high catalytic activity could be ascribed to the formation of Fe3O4 NPs and 3D porous graphene structures. Based on its peroxidase-like activity, 3D GH-5 was used for colorimetric determination of glucose with a low detection limit of 0.8 μM.Keywords: 3D graphene nanocomposites; colorimetry; glucose detection; one-pot; peroxidase mimics;

•A novel mimetic enzyme, poly(Cu-AMT), was synthesized via a simple electropolymerization procedure.•The synergetic catalytic effect between poly(Cu-AMT) and rGO was used to construct an electrochemical sensor.•The rGO-poly(Cu-AMT)/GCE was used for ultrasensitive and selective detection of dopamine.•The prepared sensor was applied to detect DA in real sample with satisfactory results.A polymerized film of copper-2-amino-5-mercapto-1,3,4-thiadiazole (Cu(II)-AMT) complex (poly(Cu-AMT)) was successfully achieved via a simple and low-cost electrochemical methodology. Subsequently, a noncovalent nanohybrid of poly(Cu-AMT) with reduced graphene oxide (rGO) (rGO-poly(Cu-AMT)) was prepared through π–π stacking interaction as an efficient mimetic enzyme for the ultrasensitive and selective detection of dopamine (DA). The rGO-poly(Cu-AMT) nanocomposites showed considerable mimetic enzyme catalytic activity, which may be attributed to the significant promotion of the electron transfer between the substrate and graphene-based carbon materials, and also the synergistic electrocatalytic effect in mimetic enzyme between rGO sheet and poly(Cu-AMT). The electrocatalytic and sensing performances of the biomimetic sensor based on the rGO-poly(Cu-AMT) nanocomposites were evaluated in detail. The biomimetic sensor enables a reliable and sensitive determination of DA with a linear range of 0.01–40 μM and a detection limit of 3.48 nM at a signal-to-noise ratio of 3. In addition, we applied the proposed method to detect DA in real sample with satisfactory results. Accordingly, the rGO-poly(Cu-AMT) is one of the promising mimetic enzyme for electrocatalysis and biosensing.

We demonstrate that nickel–palladium hollow nanoparticles (NiPd hNPs) exhibit triple-enzyme mimetic activity: oxidase-like activity, peroxidase-like activity and catalase-like activity. As peroxidase mimetics, the catalytic activity of NiPd hNPs was investigated in detail. On this basis, a simple glucose biosensor with a wide linear range and low detection limit was developed.

Petal-like graphene–Ag (p-GR–Ag) composites with highly exposed active edge sites were designed and constructed in this work. Petal-like graphene (p-GR) was prepared using a HCl assisted hydrothermal method, which was made of basal planes and highly reactive edge planes to provide more active sites. Then the p-GR can be intentionally utilized as nucleation sites for subsequent Ag nanoparticles (NPs) deposition via modified silver mirror reaction. The composites were characterized by X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and electrochemical methods. The combination of zero-dimensional (0D) Ag NPs on a two-dimensional (2D) graphene (GR) support that came into being three-dimensional (3D) structure created a sensor for electrochemical detection of metronidazole. The designed sensor exhibited well bimodal linear behaviour in the metronidazole concentration range between 0.05 to 10 μM and 10 to 4500 μM with a detection limit of 28 nM (S/N = 3). The mechanism and the heterogeneous electron transfer kinetics constant of the metronidazole reduction were discussed in the light of the rotating disk electrode (RDE) experiments. Moreover, validation of the applicability of the prepared sensor was carried out by detecting metronidazole in human urine and local lake water samples.

•A biomimetic sensor is fabricated by combining the MIP with mimetic enzyme.•The metronidazole imprinted polymer can be used as a nitroreductase mimetic.•The synthetic mimetic enzyme exhibits good catalytic activity and selectivity.•The MIP/AuNPs/GCE can be applied to detect metronidazole in real samples.The utility of molecularly imprinted polymer (MIP) as electrochemical sensor often suffers from its limited catalytic efficiency. Here, we proposed an alternative approach by combining the concept of MIP with the use of mimetic enzyme. A metronidazole imprinted polymer with nitroreductase-like activity was successfully achieved via an electrochemical method, where melamine served two purposes: functional monomer of MIP and component of mimetic enzyme. During the imprinting process, the redox-active center, which is responsible for catalysis, was introduced into the imprinted cavities. Accordingly, the imprinted polymer, having both catalysis centers and recognition sites, exhibited enhanced electrocatalytic activity and selectivity. The sensing performances of this metronidazole imprinted biomimetic sensor were evaluated in detail. Results revealed that the response to metronidazole was linear in the concentration range of 0.5–1000 μM, and the detection limit was 0.12 μM (S/N=3). In addition, we applied the proposed sensor to detect metronidazole in an injection solution and the results implied its feasibility for practical application.

•Hole-transporting material was used as a signal amplifier in designing glucose sensor.•This metal-free electrocatalyst showed excellent catalytic activity toward glucose.•Wide linear range and low detection potential for glucose determination.•A novel signal amplification platform was established.•Electrochemical recognition, catalysis, and signal amplification were realized using the same element.Hole-transporting materials with tunable structures and properties are mainly applied in organic light-emitting diodes as transport layer. But their catalytic properties as signal amplifiers in biological assays are seldom reported. In this paper, a starburst molecule, 4,4,4″-tri(N-carbazolyl)-triphenylamine (TCT), containing a triphenylamine as the central core and three carbazoles as the peripheral functional groups was designed and synthesized. Subsequently, the hole-transporting material based on the TCT polymer, poly(TCT) (PTCT), was achieved via a low-cost electrochemical method and exploited as an efficient metal-free electrocatalyst for non-enzymatic glucose detection. Here, this hole-transporting material served three purposes: electrochemical recognition (owing to hydrogen bonding interaction and the biomimetic microenvironment created by the polymer), electrocatalysis (owing to the hole-transporting capability of triphenylamine and the catalytic property of carbazole), and signal amplification (owing to energy migration along the conductive polymer backbone). The electrocatalytic and sensing performances of the sensor based on PTCT were evaluated in detail. Results revealed that the PTCT film could efficiently catalyze the oxidation of glucose at a less-positive potential (+0.20 V) in the absence of any enzymes. The response to glucose was linear in the concentration range of 1.0–6000 μM, and the detection limit was 0.20 μM. With good stability and selectivity, the proposed sensor could be feasibly applied to detect glucose in practical samples. The encouraging sensing performances suggest that the hole-transporting material is one of the promising biomimetic catalysts for electrocatalysis and relevant fields.

•Mimetic enzyme with reductase-like activity was first used in electroanalysis.•A novel mimetic enzyme, Cu-poly(Cys), was synthesized via electrochemical method.•The Cu-poly(Cys) exhibited excellent electrocatalytic activity to nitro compound.•The Cu-poly(Cys) was used as nitroreductase mimetic for metronidazole detection.•This method was successfully applied to detect metronidazole in real samples.A copper-poly(cysteine) (Cu-poly(Cys)) film was successfully achieved via a simple electrochemical method and subsequently exploited as a nitroreductase mimetic for metronidazole detection. The electropolymerized film of cysteine provides a microenvironment for the enzymatic reaction while the metallic Cu acts as an active center with catalytic function. Electrochemical treatments lead to this mimetic enzyme with greatly enhanced catalytic performance, which make it suitable for the construction of biomimetic sensors. The electrocatalytic and sensing performances of the sensor based on the Cu-poly(Cys) film were evaluated in detail. Results revealed that the sensor exhibited linear response to metronidazole in the concentration range of 0.5-400 μM, with a detection limit of 0.37 μM. In addition, we applied the proposed method to detect metronidazole in an injection solution with satisfactory results, demonstrating that the Cu-poly(Cys) film is one of the promising biomimetic catalysts for electrocatalysis and relevant fields.

Electrochemical detection of dopamine (DA) plays an important role in medical diagnosis. In this paper, tremella-like graphene–Au (t-GN–Au) composites were synthesized by a one-step hydrothermal method for selective detection of DA. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and Fourier transform infrared (FTIR) spectroscopy were used to characterize as-prepared t-GN–Au composites. The t-GN–Au composites were directly used for the determination of DA via cyclic voltammetry (CV) and the chronoamperometry (CA) technique. CA measurement gave a wide linear range from 0.8 to 2000 μM, and the detection limit of 57 nM (S/N = 3) for DA. The mechanism and the heterogeneous electron transfer kinetics of the DA oxidation were discussed in the light of rotating disk electrode (RDE) experiments. Moreover, the modified electrode was applied to the determination of DA in human urine and serum samples.

A non-enzymatic electrochemical sensor based on polystyrene@reduced graphene oxide (RGO)–Pt core–shell microspheres was developed for sensitive detection of hydrogen peroxide (H2O2). The polystyrene@RGO–Pt microspheres were prepared by microwave-assisted reduction of graphene oxide (GO) and a Pt precursor (H2PtCl6) that were adsorbed on polystyrene microspheres. No surfactants or polyelectrolytes were used as stabilizing agents for the preparation of the nanocomposite. Alternatively, polystyrene microspheres served as the core for supporting RGO nanosheets and Pt nanoparticles, which prevented the aggregation of the electrode material and resulted in the high electrochemically active surface area. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), Ultraviolet-vis absorption spectroscopy (UV), and Brunauer–Emmett–Teller (BET) measurements characterized the nanostructure of the polystyrene@RGO–Pt microspheres. Cyclic voltammetry revealed the enhanced electrocatalytic activity of these core–shell microspheres. Such a non-enzymatic electrochemical sensor is capable of detecting H2O2 with a wide linear range (0.5 μM–8000 μM), high sensitivity (38.57 μA mM−1 cm−2), low detection limit (0.1 μM, S/N = 3), long-term stability, and good selectivity. Furthermore, this sensor was found to be suitable for the determination of H2O2 in human serum samples. Considering their simple synthetic procedure and high catalytic activity, the polystyrene@RGO–Pt microspheres hold great promise in the development of high performance electrochemical sensors.

Uniformly dispersed Ag/AgCl nanocubes (AgNC) were successfully obtained on reduced graphene oxide (rGO) through the simultaneous reduction of Ag+ and graphene oxide (GO) by chitosan in the presence of a little HCl. The AgCl acted as a seed. The obtained nanocomposite was characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM). The as-synthesized AgNC/rGO was immobilized on the surface of a poly(5-amino-1,3,4-thiadiazole-2-thiol) (p-ATT) modified carbon fiber disk ultramicroelectrode (CFME) for the simultaneous determination of hydroquinone (HQ) and catechol (CT). This sensor showed a wide linear range of 0.08–1000 μmol L−1 for HQ and 1.5–900 μmol L−1 for CT in the presence of 30 μmol L−1 of the counterpart.

A mild preparation tactic was developed for the fabrication of a reduced graphene oxide (rGO) and poly(sulfosalicylic acid) (PSA) nanocomposite film by a one-step electrochemical method. The nanocomposite film was characterized by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and electrochemical methods. The electrochemical properties of the nanocomposite were evaluated by means of cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Based on the synergistic effect of the rGO and PSA nanocomposite film, a sensitive electrochemical sensor for acetaminophen (AC) was successfully fabricated. The electrochemical reaction of AC at a glassy carbon electrode (GCE) modified with the PSA and rGO nanocomposite film (PSA/rGO/GCE) was proved to be a surface-controlled process involving the same number of protons and electrons. Under optimum experimental conditions, the anodic peak currents were linear over the AC concentrations ranging from 0.5 to 300 μM, with a limit of detection (LOD) of 0.041 μM (S/N = 3). Furthermore, the modified electrode was demonstrated to be feasible for analytical purposes in real samples. A linear calibration curve was obtained for the determination of AC in urine within the range of 1 to 300 μM with a LOD of 0.36 μM under optimized conditions. In addition, the PSA/rGO/GCE was demonstrated in human serum with satisfactory results.

•A label-free and amplified impedimetric aptasensor was developed for thrombin.•Novel graphene nanocomposites were used to fabricate a sandwich sensing platform.•The aptasensor offered high sensitivity and selectivity toward thrombin.•This approach offers a promising signal amplified model for protein detection.A label-free and amplified electrochemical impedimetric aptasensor based on functionalized graphene nanocomposites (rGO–AuNPs) was developed for the detection of thrombin, which played a vital role in thrombosis and hemostasis. The thiolated aptamer and dithiothreitol (TBA15–DTT) were firstly immobilized on the gold electrode to capture the thrombin molecules, and then aptamer functionalized graphene nanocomposites (rGO–TBA29) were used to fabricate a sandwich sensing platform for amplifying the impedimetric signals. As numerous negative charges of TBA29 on the electrode repelled to the [Fe(CN)6]4−/3− anions, resulting in an obvious amplified charge-transfer resistance (Rct) signal. The Rct increase was linearly proportional to the thrombin concentration from 0.3 to 50 nM and a detection limit of 0.01 nM thrombin was achieved. In addition, graphene could also be labeled with other probes via electrostatic or π–π stacking interactions to produce signals, therefore different detection methods expanding wide application could be used in this model.

In this contribution, the nanopore array derived from l-cysteine oxide/gold hybrids (NA-COGH) was applied to the simultaneous determination of hydroquinone (HQ) and catechol (CT). NA-COGH was prepared by a sequential electrodeposition of l-cysteine oxide and gold into the voids of polystyrene spheres template, followed by removing the template using tetrahydrofuran. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analyses validated the formation of NA-COGH on the glassy carbon electrode. The charge transfer ability of NA-COGH was studied by means of scanning electrochemical microscopy (SECM). A better charge transfer rate was obtained by optimizing the electrodeposition conditions. More interestingly, the as-prepared NA-COGH presents high electrocatalytic activity for the oxidation of HQ and CT. The sensing platform based on NA-COGH shows a wide linear response for HQ and CT in the concentration range of 4.0 × 10−7–6.0 × 10−4 M and 8.0 × 10−7–5.0 × 10−4 M, with detection limit of 1.9 × 10−8 M and 3.4 × 10−8 M (S/N = 3), respectively. Rotating disk electrode experiments revealed that the catalytic rate constants were as high as 7.99 × 10−3 cm s−1 and 7.38 × 10−3 cm s−1 at potential of 302 mV and 416 mV for HQ and CT, respectively. With good stability and reproducibility, we applied the present method to the simultaneous determination of HQ and CT in tap water and lake water. These results indicate that NA-COGH is a promising electrode material with great potential in electrocatalysis and electrochemical sensing.

Graphene-poly(styrene sulfonate)-Pt nanocomposite (GN-PSS-Pt) was synthesized with the mixture of graphene oxide and chloroplatinic acid by microwave treatment. The nanocomposite was characterized by Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy, respectively. The electrochemical behaviour of dopamine on the nanocomposite was investigated. A chronoamperometry method was developed for the determination of dopamine. The corresponding linear range was in 2.0 × 10−7–4.0 × 10−3 M and the detection limit was estimated as 4.0 × 10−8 M (S/N = 3). Moreover, the modified electrode was applied to the determination of dopamine in human urine and serum samples.

A novel composite film derived from cysteic acid and poly(diallydimethylammonium chloride)-functionalized graphene (PDDA-GN) was employed as an enhanced electrode material for ultrasensitive determination of metronidazole. The cysteic acid/PDDA-GN composite film was prepared by the electrochemical grafting of cysteic acid onto the PDDA-GN coated glassy carbon electrode (GCE). The cyclic voltammetry investigations reveal that the peak current of metronidazole reduction at the cysteic acid/PDDA-GN/GCE was remarkably enhanced compared to the bare GCE, the cysteic acid/GCE and the PDDA-GN/GCE. This result implies the synergistic electrocatalytic effect of cysteic acid and PDDA-GN. The fabricated sensor shows linear response to metronidazole in the ranges of 10 nM–1 μM and 70 μM–800 μM, with a detection limit of 2.3 nM (S/N=3). The heterogeneous electron transfer rate constant and the diffusion coefficient of metronidazole were further evaluated by rotating disk electrode experiments. Moreover, we applied the present method to the determination of metronidazole in urine and lake water with satisfactory results.Highlights► Facile microwave-heating procedure for preparing PDDA-functionalized graphene. ► Cysteic acid/PDDA-functionalized graphene composite film as a new electrode material. ► High electrocatalytic activity for metronidazole reduction. ► Wide linear range, low detection limit for the determination of metronidazole. ► Application in practical samples with satisfactory results.

We demonstrate that nickel–palladium hollow nanoparticles (NiPd hNPs) exhibit triple-enzyme mimetic activity: oxidase-like activity, peroxidase-like activity and catalase-like activity. As peroxidase mimetics, the catalytic activity of NiPd hNPs was investigated in detail. On this basis, a simple glucose biosensor with a wide linear range and low detection limit was developed.

A mild preparation tactic was developed for the fabrication of a reduced graphene oxide (rGO) and poly(sulfosalicylic acid) (PSA) nanocomposite film by a one-step electrochemical method. The nanocomposite film was characterized by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and electrochemical methods. The electrochemical properties of the nanocomposite were evaluated by means of cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Based on the synergistic effect of the rGO and PSA nanocomposite film, a sensitive electrochemical sensor for acetaminophen (AC) was successfully fabricated. The electrochemical reaction of AC at a glassy carbon electrode (GCE) modified with the PSA and rGO nanocomposite film (PSA/rGO/GCE) was proved to be a surface-controlled process involving the same number of protons and electrons. Under optimum experimental conditions, the anodic peak currents were linear over the AC concentrations ranging from 0.5 to 300 μM, with a limit of detection (LOD) of 0.041 μM (S/N = 3). Furthermore, the modified electrode was demonstrated to be feasible for analytical purposes in real samples. A linear calibration curve was obtained for the determination of AC in urine within the range of 1 to 300 μM with a LOD of 0.36 μM under optimized conditions. In addition, the PSA/rGO/GCE was demonstrated in human serum with satisfactory results.