Reduced graphene oxide-supported methylene blue nanocomposite as a glucose oxidase-mimetic for electrochemical glucose sensing

In this paper, a hybrid nanocomposite (MB-rGO) was synthesized based on the π–π stacking interactions between methylene blue (MB) and reduced graphene oxide (rGO). The as-synthesized nanocomposite was characterized by SEM, TEM, XRD, FTIR, UV-vis and XPS spectra. UV-vis spectroscopy and electrochemical tests suggested the MB-rGO modified on the electrode exhibited glucose oxidase-mimetic catalytic activity towards glucose, and displayed excellent electrocatalytic performance for electrochemical detection of glucose with a wide linear range from 1.04 to 17.44 mM, a low detection limit of 45.8 μM and a large sensitivity of 13.08 μA mM⁻¹ cm⁻². The proposed glucose sensor also showed high stability, reproducibility and good abilities of anti-interference to dopamine, ascorbic acid and uric acid. Moreover, the modified electrode was used to determine glucose concentration in human blood serum samples with satisfactory results.

A novel electrochemical glucose sensor based on methylene blue-reduced graphene oxide nanocomposite was constructed, and the sensor exhibited good glucose oxidase-mimetic electrocatalytic activity towards glucose and practical applicability.     

Construction of a non-enzymatic electrochemical sensor based on graphitic carbon nitride nanosheets for sensitive detection of procalcitonin

In this study, we established a label free and ultrasensitive electrochemical sensor based on graphitic nitride nanosheets (g-C₃N₄ NS) for procalcitonin (PCT) detection. Firstly, an easy-to-prepare and well-conducting g-C₃N₄ NS was synthesized. Next the g-C₃N₄ NS was immobilized on the electrode surface by π–π stacking, and further used to anchor the specific recognition peptide (PP). The surface morphology and structure after g-C₃N₄ NS and PP modification was characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and electrochemistry. The sensing property of this sensor was evaluated by differential pulse voltammetry (DPV) and showed a detection sensitivity with a dynamic range from 0.15 to 11.7 fg mL⁻¹ with a low limit of detection (LOD) of 0.11 fg mL⁻¹. Besides, the electrochemical biosensor was successfully used to detect PCT in human serum samples, and the results suggest its potential use in clinical application.

A simple and ultra-sensitive electrochemical biosensor based on graphitic carbon nitride nanosheets (g-C₃N₄ NS) was developed for the detection of PCT. This sensor presented excellent sensing performance and demonstrates potential for clinical application.     

Nickel oxide decorated MoS2 nanosheet-based non-enzymatic sensor for the selective detection of glucose

Understanding blood glucose levels in our body can be a key part in identifying and diagnosing prediabetes. Herein, nickel oxide (NiO) decorated molybdenum disulfide (MoS₂) nanosheets have been synthesized via a hydrothermal process to develop a non-enzymatic sensor for the detection of glucose. The surface morphology of the NiO/MoS₂ nanocomposite was comprehensively investigated by field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) analysis. The electro-catalytic activity of the as-prepared NiO/MoS₂ nanocomposite towards glucose oxidation was investigated by cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and amperometry in 0.1 M NaOH. The NiO/MoS₂ nanocomposite-based sensor showed outstanding electrocatalytic activity for the direct electro-oxidation of glucose due to it having more catalytic active sites, good conductivity, excellent electron transport and high specific surface area. Meanwhile, the NiO/MoS₂ modified glassy carbon electrode (GCE) showed a linear range of glucose detection from 0.01 to 10 mM by amperometry at 0.55 V. The effect of other common interferent molecules on the electrode response was also tested using alanine, l-cysteine, fructose, hydrogen peroxide, lactose, uric acid, dopamine and ascorbic acid. These molecules did not interfere in the detection of glucose. Moreover, this NiO/MoS₂/GCE sensor offered rapid response (2 s) and a wide linear range with a detection limit of 1.62 μM for glucose. The reproducibility, repeatability and stability of the sensor were also evaluated. The real application of the sensor was tested in a blood serum sample in the absence and presence of spiked glucose and its recovery values (96.1 to 99.8%) indicated that this method can be successfully applied to detect glucose in real samples.

This study reported that NiO/MoS₂ based nanocomposite can be used as an electrocatalytic material to detect glucose with high selectivity in a blood serum.     

A novel electrochemical sensor for glucose detection based on a Ti3C2Tx/ZIF-67 nanocomposite

A Ti₃C₂T_x/ZIF-67 nanocomposite with outstanding conductivity has been prepared by loading ZIF-67 onto a two-dimensional Ti₃C₂T_x nanosheet. Ti₃C₂T_x sheets were synthesized by etching Ti₃AlC₂, and then ZIF-67 was grown in situ on the Ti₃C₂T_x nanosheet. The Ti₃C₂T_x/ZIF-67 nanocomposite exhibits excellent detection performance for glucose, with a low LOD of 3.81 μM and wide linear detection range of 5–7500 μM. This terrific result is contributed by the synergistic effect of the high electrically conductive ability of Ti₃C₂T_x and active catalytic performance of ZIF-67. Moreover, the electrochemical sensor prepared using the Ti₃C₂T_x/ZIF-67 nanocomposite also shows excellent selectivity, stability and repeatability for glucose detection. The Ti₃C₂T_x/ZIF-67 nanocomposite with outstanding performance has potential applications for electrochemical sensors.

A Ti₃C₂T_x/ZIF-67 nanocomposite with outstanding conductivity has been prepared. The electrochemical sensor based on this nanocomposite exhibits excellent selectivity, stability and repeatability for glucose detection.     

PVDF-supported graphene foam as a robust current collector for lithium metal anodes

Lithium metal batteries have drawn much attention due to their ultrahigh energy density. However, the safety hazards and limited lifetime caused by severe lithium dendrite growth during cycling hinder their real application. To address this issue and improve the electrochemical performance of current lithium batteries, a current collector beyond the traditional copper foil for lithium anodes is highly needed. We proposed and prepared a PVDF-supported graphene foam (PSGF) structure as an effective current collector for lithium metal anodes. Because of its structural stability, large surface area, and lithiophilic surface chemistry, the corresponding lithium metal anode (PSGF@Li) shows an extended cycling life (∼1000 h), a decreased voltage hysteresis (∼80 mV) and an improved coulombic efficiency (∼99%). Furthermore, the practical application of the PSGF@Li anode in a full cell system was also demonstrated.

A robust lithium metal anode comprising of PSGF current collector showing long cycling life.     

Superficial fabrication of gold nanoparticles modified CuO nanowires electrode for non-enzymatic glucose detection

This paper describes a low-cost facile method to construct gold (Au) nanoparticles (NPs) modified copper oxide (CuO) nanowires (NWs) electrode on copper foil for the detection of glucose. Copper foil has been converted to aligned CuO NWs arrays by sequential formation of Cu(OH)₂ followed by heat treatment induced phase transformation to CuO. Au NPs are deposited on CuO NWs via simple reductive solution chemistry to impart high surface to volume ratio and enhanced catalytic activity of the resulting electrode. Structure, microstructure and morphology of Cu, Cu(OH)₂ NWs, CuO NWs, and Au NPs modified CuO NWs are investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The homogeneous distribution of Au NPs (average diameter ∼12 nm) on CuO NWs (average diameter 100 nm and aspect ratio ∼20) is confirmed by high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM) and elemental mapping. This CuO based glucose detection method gives the highest sensitivity along with the maximum linearity range. This non-enzymatic glucose sensor based on Au modified CuO NWs electrode gives broad linearity range from 0.5 μM to 5.9 mM. The sensor exhibits sensitivity of 4398.8 μA mM⁻¹ cm⁻², lower detection limit of 0.5 μM, and very fast response time of ∼5 s. Properties of the proposed glucose sensor are also investigated in human blood and it is found that the sensor is highly accurate and reliable. In addition, higher sensitivity and lower detection limit confirm that this device is suitable for invasive detection in saliva and urine.

This paper describes a low-cost facile method to construct gold (Au) nanoparticles (NPs) modified copper oxide (CuO) nanowires (NWs) electrode on copper foil for the detection of glucose.     

Ultra-small dispersed CuxO nanoparticles on graphene fibers for miniaturized electrochemical sensor applications

A graphene microfiber (GF) modified with ultrafine Cu_xO nanoparticles (Cu_xONPs/GF) has been fabricated by direct annealing of electrodeposited nano-sized copper-based metal organic frameworks (HKUST-1) and used as an electrode for nonenzymatic H₂O₂ sensing. Benefiting from the unique microfiber architecture and synergetic effects, as well as strong coupling between components with many active sites and boosted electron transport, the Cu_xONPs/GF electrode shows prominent sensitivity, selectivity and long-term operational stability for the detection of H₂O₂. Further work successfully applied this Cu_xONPs/GF electrode to detection of H₂O₂ in real samples such as milk and human serum. These results indicate that the Cu_xONPs/GF is a promising mini-sized sensor in electrochemical analysis.

A graphene microfiber modified with ultrafine Cu_xO nanoparticles (Cu_xONPs/GF) is fabricated by direct annealing of electrodeposited nano-sized copper-based metal–organic frameworks (HKUST-1) and used as an electrode for nonenzymatic H₂O₂ sensing.     

One-step potentiostatic electrodeposition of NiS–NiS2 on sludge-based biochar and its application for a non-enzymatic glucose sensor

Conventional nanomaterials are available in electrochemical glucose nonenzymatic sensing, but their broad applications are limited due to their high cost and complicated preparation procedures. In this study, NiS–NiS₂/sludge-based biochar/GCE was fabricated by one-step potentiostatic electrodeposition on biochar and used as an interface material for non-enzymatic sensing of glucose in 0.1 M NaOH. With an electrodeposition time of 300 s, the as-prepared sensors delivered the best electrochemical performance toward glucose due to the synergistic effects of NiS–NiS₂ and sludge-based biochar. The as prepared NiS–NiS₂/sludge-based biochar surface morphology, surface composition, and electrochemical properties were characterized by SEM elemental mapping, XPS and cyclic voltammetry. Under optimized conditions, the linearity between the current response and the glucose concentration has been obtained in the range of 5–1500 μM with a detection limit of 1.5 μM. More importantly, the fabricated sensor was successfully utilized to measure glucose in serum of sweetened beverages and human blood. Accordingly, NiS–NiS₂/sludge-based biochar/GCE can hopefully be applied as a normal enzyme-free glucose sensor with excellent properties of sensitivity, reproducibility, stability, as well as sustainability.

NiS–NiS₂/sludge-based biochar/GCE was fabricated by one-step potentiostatic electrodeposition on biochar and used as an interface material for non-enzymatic sensing of glucose with a lower detection limit of 1.5 μM.     

β-Cyclodextrin functionalized 3D reduced graphene oxide composite-based electrochemical sensor for the sensitive detection of dopamine

A three-dimensional reduced graphene oxide nanomaterial with β-cyclodextrin modified glassy carbon electrode (3D-rGO/β-CD/GCE) was constructed and used to detect the electrochemical behavior of dopamine (DA). The nanocomposite materials were characterized by scanning electron microscopy (SEM), infrared spectrometry (FT-IR), Raman spectrogram and thermogravimetric analysis (TGA), which showed that β-CD was well modified on 3D graphene with a porous structure. The electrochemical properties of different modified electrodes were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), proving the highest electron transfer rate of the 3D-rGO/β-CD modified electrode. The experimental conditions such as scan rate, pH, enrichment time and layer thickness were optimized. Under the best experimental conditions, DA was detected by differential pulse voltammetry (DPV) by 3D-rGO/β-CD/GCE with excellent electrocatalytic ability and satisfactory recognition ability, resulting in a wide linear range of 0.5–100 μM and a low detection limit (LOD) of 0.013 μM. The modified electrode based on 3D-rGO/β-CD nanocomposites is promising in the field of electrochemical sensors due to its high sensitivity and other excellent properties.

A 3D-rGO/β-CD nanocomposite was successfully synthesized and further modified onto the surface of GCE to construct a new biosensor for electrochemically sensing DA.     

Sensitive electrochemical sensor based on poly(l-glutamic acid)/graphene oxide composite material for simultaneous detection of heavy metal ions

Heavy metal pollution can be toxic to humans and wildlife, thus it is of great significance to develop rapid and sensitive methods to detect heavy metal ions. Here, a novel type of electrochemical sensor for the simultaneous detection of heavy metal ions has been prepared by using poly(l-glutamic acid) (PGA) and graphene oxide (GO) composite materials to modify the glassy carbon electrode (GCE). Due to the good binding properties of poly(l-glutamic acid) (PGA) for the heavy metal ions (such as Cu²⁺, Cd²⁺, and Hg²⁺) as well as good electron conductivity of graphene oxide (GO), the heavy metal ions, Cu²⁺, Cd²⁺, and Hg²⁺ in aqueous solution can be accurately detected by using differential pulse anodic stripping voltammetry method (DPASV). Under the optimized experiment conditions, the modified GCE shows excellent electrochemical performance toward Cu²⁺, Cd²⁺, and Hg²⁺, and the linear range of PG/GCE for Cu²⁺, Cd²⁺, and Hg²⁺ is 0.25–5.5 μM, and the limits of detection (LODs, S/N ≥ 3) Cu²⁺, Cd²⁺, and Hg²⁺ are estimated to be 0.024 μM, 0.015 μM and 0.032 μM, respectively. Moreover, the modified GCE is successfully applied to the determination of Cu²⁺, Cd²⁺, and Hg²⁺ in real samples. All obtained results show that the modified electrode not only has the advantages of simple preparation, high sensitivity, and good stability, but also can be applied in the field of heavy metal ion detection.

A novel electrochemical sensor with high stability and good reproducibility for the simultaneous detection of heavy metal ions was prepared by using PGA/GO to modify the GCE, showing high sensitivity of superior to most of the reported values.     

A dopamine electrochemical sensor based on a platinum–silver graphene nanocomposite modified electrode

A platinum–silver graphene (Pt–Ag/Gr) nanocomposite modified electrode was fabricated for the electrochemical detection of dopamine (DA). Electrochemical studies of the Pt–Ag/Gr nanocomposite towards DA detection were performed by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The CV analysis showed that Pt–Ag/Gr/GCE had enhanced electrocatalytic activity towards DA oxidation due to the synergistic effects between the platinum–silver nanoparticles and graphene. The DPV results showed that the modified sensor demonstrated a linear concentration range between 0.1 and 60 μM with a limit of detection of 0.012 μM. The Pt–Ag/Gr/GCE presented satisfactory results for reproducibility, stability and selectivity. The prepared sensor also showed acceptable recoveries for a real sample study.

A platinum–silver graphene nanocomposite was synthesized and characterized. A nanocomposite modified electrode was fabricated in order to investigate the electrochemical detection of dopamine.     

Three-dimensional paper-based microfluidic electrochemical integrated devices (3D-PMED) for wearable electrochemical glucose detection

Wearable electrochemical sensors have attracted tremendous attention in recent years. Here, a three-dimensional paper-based microfluidic electrochemical integrated device (3D-PMED) was demonstrated for real-time monitoring of sweat metabolites. The 3D-PMED was fabricated by wax screen-printing patterns on cellulose paper and then folding the pre-patterned paper four times to form five stacked layers: the sweat collector, vertical channel, transverse channel, electrode layer and sweat evaporator. A sweat monitoring device was realized by integrating a screen-printed glucose sensor on polyethylene terephthalate (PET) substrate with the fabricated 3D-PMED. The sweat flow process in 3D-PMED was modelled with red ink to demonstrate the capability of collecting, analyzing and evaporating sweat, due to the capillary action of filter paper and hydrophobicity of wax. The glucose sensor was designed with a high sensitivity (35.7 μA mM⁻¹ cm⁻²) and low detection limit (5 μM), considering the low concentration of glucose in sweat. An on-body experiment was carried out to validate the practicability of the three-dimensional sweat monitoring device. Such a 3D-PMED can be readily expanded for the simultaneous monitoring of alternative sweat electrolytes and metabolites.

A three-dimensional paper-based microfluidic device (3D-PMED) has been developed for dynamic sweat metabolite monitoring with continuous sweat flow avoiding accumulation.     

Enhanced electron transfer mediated detection of hydrogen peroxide using a silver nanoparticle–reduced graphene oxide–polyaniline fabricated electrochemical sensor

The current study aims at the development of an electrochemical sensor based on a silver nanoparticle–reduced graphene oxide–polyaniline (AgNPs–rGO–PANI) nanocomposite for the sensitive and selective detection of hydrogen peroxide (H₂O₂). The nanocomposite was fabricated by simple in situ synthesis of PANI at the surface of rGO sheet which was followed by stirring with AEC biosynthesized AgNPs to form a nanocomposite. The AgNPs, GO, rGO, PANI, rGO–PANI, and AgNPs–rGO–PANI nanocomposite and their interaction were studied by UV-vis, FTIR, XRD, SEM, EDX and XPS analysis. AgNPs–rGO–PANI nanocomposite was loaded (0.5 mg cm⁻²) on a glassy carbon electrode (GCE) where the active surface area was maintained at 0.2 cm² for investigation of the electrochemical properties. It was found that AgNPs–rGO–PANI–GCE had high sensitivity towards the reduction of H₂O₂ than AgNPs–rGO which occurred at −0.4 V vs. SCE due to the presence of PANI (AgNPs have direct electronic interaction with N atom of the PANI backbone) which enhanced the rate of transfer of electron during the electrochemical reduction of H₂O₂. The calibration plots of H₂O₂ electrochemical detection was established in the range of 0.01 μM to 1000 μM (R² = 0.99) with a detection limit of 50 nM, the response time of about 5 s at a signal-to-noise ratio (S/N = 3). The sensitivity was calculated as 14.7 μA mM⁻¹ cm⁻² which indicated a significant potential as a non-enzymatic H₂O₂ sensor.

The current study aims at the development of an electrochemical sensor based on a silver nanoparticle–reduced graphene oxide–polyaniline (AgNPs–rGO–PANI) nanocomposite for the sensitive and selective detection of hydrogen peroxide (H₂O₂).     

Lignosulfonate in situ-modified reduced graphene oxide biosensors for the electrochemical detection of dopamine

Lignosulfonate (LS), a biomass by-product from sulfite pulping and the paper-making industry, which has many excellent characteristics, such as renewable, environmentally friendly, amphiphilic nature, and especially the abundant content of hydrophilic functional groups in its architecture, making it highly reactive and can be used as a sensitive material in sensors to show changes in electrical signals. Herein, we report a one-step in situ method to fabricate lignosulfonate-modified reduced graphene oxide (LS–rGO) green biosensors, which can be used for the sensitive electrochemical detection of dopamine without interference from uric acid and ascorbic acid. The modified LS molecular layers act as chemical-sensing layers, while the rGO planar sheets function as electric-transmitting layers in the as-assembled dopamine biosensors. After the in situ-decoration of the LS modifier, the sensing performance of LS–rGO for the detection of dopamine was much higher than that of the pure rGO electrode, and the highest current response of the biosensor toward dopamine greatly improved from 11.2 μA to 52.07 μA. The electrochemical sensitivity of the modified biosensor was optimized to be 0.43 μA μM⁻¹, and the detection limit was as low as 0.035 μM with a wide linear range (0.12–100 μM), which is better than that of most previously reported metal- and organic-based modified graphene electrodes. The newly designed biosensor has unique advantages including rapid, stable, sensitive and selective detection of dopamine without interference, providing a facile pathway for the synthesis of green resource-derived sensing materials instead of the traditional toxic and expensive modifiers.

One-step situ method to fabricate lignosulfonate modified reduced graphene oxide sensors for sensitive, selective and highly repetitive electrochemical detection of dopamine.     

A comparative electrochemical study of non-enzymatic glucose,ascorbic acid,and albumin detection by using a ternary mesoporous metal oxide (ZrO2, SiO2 and In2O3) modified graphene composite based biosensor

In this study, we present an electrochemical investigation of a ternary mesoporous metal oxide (ZrO₂, SiO₂ and In₂O₃) modified graphene composite for non-enzymatic glucose, ascorbic acid, and albumin detection in urine at physiological pH. Synergetic property of ZrO₂–Ag–G–SiO₂ and In₂O₃–G–SiO₂ were investigated via cyclic voltammetry (CV) using FTO glass and copper-foil electrodes with no prerequisite of solid antacid expansion. The mesoporous ZrO₂–Ag–G–SiO₂ and In₂O₃–G–SiO₂ composites were synthesized and characterized using XRD, SEM, TEM, Raman spectroscopy, XPS, DRS, BET, and photocurrent measurements. Upon increasing the glucose concentration from 0 to 3 mM, CV results indicated two anodic peaks at +0.18 V and +0.42 V versus Ag/AgCl, corresponding to Zr³⁺ and Zr⁴⁺, respectively, considering the presence of glucose in urine. Moreover, the effects of high surface area In₂O₃–G–SiO₂ were observed upon the examination of ZrO₂–Ag–G–SiO₂. In₂O₃–G–SiO₂ demonstrated a decent electrochemical pattern in glucose, ascorbic acid, and albumin sensing. Nevertheless, insignificant synergistic effects were observed in In₂O₃-G, ZrO₂-G, and ZrO₂–G–SiO₂. In₂O₃–G–SiO₂ performed well under a wide range of electrolytes and urine, and showed no activity toward uric acid, suggesting potential for biodetection in urine.

In this study, we present an electrochemical investigation of a ternary mesoporous metal oxide (ZrO₂, SiO₂ and In₂O₃) modified graphene composite for non-enzymatic glucose, ascorbic acid, and albumin detection in urine at physiological pH.     

Low-fouling CNT-PEG-hydrogel coated quartz crystal microbalance sensor for saliva glucose detection

Saliva glucose detection based on a quartz crystal microbalance (QCM) sensor has emerged as a promising tool and a non-invasive diagnostic technique for diabetes. However, the low glucose concentration and strong protein interference in the saliva hinder the QCM sensors from practical applications. In this study, we present a robust and simple anti-fouling CNT-PEG-hydrogel film-coated QCM sensor for the detection of saliva glucose with high sensitivity. The CNT-PEG-hydrogel film consists of two layers; the bottom base PBA-hydrogel film is designed to recognize the glucose while the top CNT-PEG layer is used to restrict protein adsorption and improve the biocompatibility. Our results show that this CNT-PEG-hydrogel film exhibited a 10-fold enhancement on the detection limit compared to the PBA-hydrogel. Meanwhile, the adsorption of proteins on the surface of the CNT-PEG-hydrogel film, including bovine serum albumin (BSA), mucin (MUC), and fibrinogen (FIB), were reduced by 99.1%, 77.8%, and 83.7%, respectively. The CNT-PEG-hydrogel film could detect the typical saliva glucose level (0–50 mg L⁻¹) in 10% saliva with a good responsivity. To sum up, this new tool with low-fouling film featuring high stability, specificity, and selectivity holds great potential for non-invasive monitoring of saliva glucose in human physiological levels.

We successfully achieved the direct detection of saliva glucose by a CNT-PEG-hydrogel. The top CNT-PEG layer provides channels for transporting glucose molecules and filters macromolecular impurities and the bottom base PBA-hydrogel film provides the glucose binding sites.     

Simultaneous detection of acetaminophen,catechol and hydroquinone using a graphene-assisted electrochemical sensor

Simple, rapid and sensitive analysis of drug-derived pollutants is critically valuable for environmental monitoring. Here, taking acetaminophen, hydroquinone and catechol as a study example, a sensor based on an ITO/APTES/r-GO@Au electrode was developed for separate and simultaneous determination of phenolic pollutants. ITO electrodes that are modified with 3-aminopropyltriethoxysilane (APTES), graphene (GO) and Au nanoparticles (Au NPs) can significantly enhance the electronic transport of phenolic pollutants at the electrode surface. The redox mechanisms of phenolic pollutants include the electron transfer with the enhancement of r-GO@Au. The modified ITO electrode exhibits excellent electrical properties to phenolic pollutants and a good linear relationship between ECL intensity and the concentration of phenolic pollutants, with a limit of detection of 0.82, 1.41 and 1.95 μM, respectively. The separate and simultaneous determination of AP, CC and HQ is feasible with the ITO/APTES/r-GO@Au electrode. The sensor shows great promise as a low-lost, sensitive, and rapid method for simultaneous determination of drug-derived pollutants.

Simple, rapid and sensitive analysis of drug-derived pollutants is critically valuable for environmental monitoring.     

Vertically-oriented graphene nanosheet as nano-bridge for pseudocapacitive electrode with ultrahigh electrochemical stability

A hierarchical structure consisting of Ni–Co hydroxide nanosheets (NCHN) electrodeposited on vertically-oriented graphene nanosheets (GN) on carbon cloth (CC) was fabricated for high-performance pseudocapacitive electrodes. NCHN was uniformly distributed on GN, forming a sheet-on-sheet hierarchical structure. Such NCHN/GN/CC hybrid electrodes exhibit high capacitance and ultrahigh electrochemical-stability that structure and electrochemical properties of hybrid electrodes are not affected by the cyclic low-rate scanning (at 5 mV s⁻¹ even over 1000 cycles). GN vertically grown on CC is used as nano-bridge between NCHN active materials and CC current collector, which effectively facilitates ion/charge transfer between the electrolyte and electrode, consequently leading to the ultrahigh electrochemical-stability of hybrid electrodes. To assess functional behavior, two-terminal flexible asymmetric supercapacitor devices with NCHN/GN/CC as positive electrode and GN/CC as negative electrode were assembled and electrochemically treated to demonstrate the ultrahigh electrochemical stability.

Vertically-oriented graphene nanosheet as nano-bridge for pseudocapacitive electrode facilitates the ion/charge transfer efficiency, leading to ultrahigh electrochemical stability.     

Nanoporous hybrid CuO/ZnO/carbon papers used as ultrasensitive non-enzymatic electrochemical sensors

In this research, we demonstrate a facile approach for the synthesis of a graphite-analogous layer-by-layer heterostructured CuO/ZnO/carbon paper using a graphene oxide paper as a sacrificial template. Cu²⁺ and Zn²⁺ were inserted into the interlayer of graphene oxide papers via physical absorption and electrostatic effects and then, the Mⁿ⁺-graphene oxide paper was annealed in air to generate 2D nanoporous CuO/ZnO nanosheets. Due to the graphene oxide template, the structure of the obtained CuO/ZnO nanosheets with an average size of ∼50 nm was duplicated from the graphene oxide paper, which displayed a layer-by-layer structure on the microscale. The papers composed of nanosheets had an average pore size of ∼10 nm. Moreover, the as-prepared CuO–ZnO papers displayed high hybridization on the nanoscale. More importantly, the thickness of the single-layer CuO/ZnO nanosheet was about 2 nm (3–4 layer atom thickness). The as-synthesized nano-hybrid material with a high specific surface area and conjunct bimodal pores could play key roles for providing a shorter diffusion path and rapid electrolyte transport, which could further facilitate electrochemical reactions by providing more active sites. As an electrode material, it displayed high performances as a non-enzymatic sensor for the detection of glucose with a low potential (0.3 V vs. SCE), high sensitivity (3.85 mA mM⁻¹ cm⁻²), wide linear range (5 μM to 3.325 mM), and low detection limit of 0.5 μM.

In this research, we demonstrate a facile approach for the synthesis of a graphite-analogous layer-by-layer heterostructured CuO/ZnO/carbon paper using a graphene oxide paper as a sacrificial template.

CuO/ZnO-based multifunctional nanohybrid materials are widely used in various applications, such as sensors,^1–5 photo degradation,^6,7 catalysis,⁸ energy storage,⁹ memory applications,¹⁰ and especially non-enzymatic sensors for the detection of glucose.^13,14 The heterostructured ZnO–CuO composite material has been extensively used for non-enzymatic sensing applications due to the extension of its electron depletion layer through the formation of a p–n junction.^11,12 Therefore, a number of studies have been devoted to the synthesis of CuO–ZnO hybrid materials,^12–18 such as three-dimensional porous ZnO–CuO hierarchical nanocomposites fabricated by coelectrospinning¹⁹ and flower-like CuO–ZnO heterostructured nanowire arrays on a mesh substrate obtained using a solution method.²⁰ Among them, 0D nanoparticles or 1D nanowires have been dominant for a long period. Recently, ultrathin two-dimensional (2D) nanostructured materials have attracted increasing attention due to their large surface-to-volume ratios and confined thickness on the nanometer scale and tunable nanoporous structure. They are expected to exhibit high performances for achieving superior catalytic, photovoltaic and electrochemical applications. Thus, 2D metal oxide (MO) nanostructures have been widely demonstrated in adsorption, catalysis, energy conversion and storage, and optoelectronic and biological applications.²¹ However, it is still very challenging to prepare large-scale 2D ultrathin and porous structured nanofilms via a simple pathway. To date, only a few cases have been successful in the synthesis of ultrathin 2D metal²² and metal oxide nanosheets^23,24 using graphene oxide (GO) as a template. Recently, our group used a GO paper as a template to obtain an ultralight single MO paper with tunable thickness and transparency.²⁵A graphene paper or film derived from various graphene-like materials such as GO, reduced graphene oxide (rGO) and pristine graphene has captured significant interest in a wide range of areas from physics,²⁶ materials science,²⁷ and chemistry²⁸ to environmental engineering.²⁹ In particular, the GO paper is widely used since it is prepared via assembling single-layer GO nanosheets, which are functionalized by abundant oxygen-containing groups and many defects on both their planes and edges. As a result, GO paper can capture ions and nanoparticles or covalently bind with other functional groups.³⁰ GO paper also displays a well-defined layer-by-layer porous nanostructure, with a d-spacing of about 1.2 nm, which allows ions to diffuse into the nanometer-scale pores of their interlayers.³¹ Most importantly, GO paper can be easily decomposed and removed by application of high temperature in an oxygen-rich atmosphere. Consequently, GO paper is one of the most desirable sacrificial templates for the synthesis of layer-by-layer, nanoporous and ultrathin 2D metal or metal oxide nanosheets.In this work, GO paper was prepared via the vacuum filtration of GO solution and was further used as a template for the synthesis of 2D CuO/ZnO/carbon paper on a large scale. Cu²⁺ and Zn²⁺ were loaded on the GO paper via electrostatic interaction and physical absorption between the oxygen groups and the desired cation. After removal of GO by heat, the metal oxide maintained the layer-by-layer, ultrathin and nanoporous structure. More interestingly, the as-prepared ZnO–CuO hybrid films displayed significantly improved performances as a non-enzymatic sensor for the detection of glucose.GO nanosheets was synthesized via a modified Hummer''s method according to our previous work.^32,33 Subsequently, some advanced techniques were applied to characterize the obtained GO, such as UV-vis spectroscopy (Fig. S1^†), AFM (Fig. S2^†) and XPS (Fig. S3A^†). All the results indicate a significant amount of oxygen-containing groups is present on the GO sheet surface, which significantly increased the sites for attaching positively charged molecular groups and nanoparticles on the GO sheet. In our case, Cu²⁺ and Zn²⁺ were successfully attached on the GO paper via the direct immersion of GO paper into the ion source solution, which was confirmed by XPS analysis (Fig. S4^†). In this process,²⁵ the metal ions were incorporated into the GO paper. After heat treatment, the obtained MO papers (as shown in the insert picture in Fig. 2, S5 and S6^†) mimicked the GO paper structure and the sample colour changed to white, black and black, respectively for ZnO/carbon, CuO/carbon, and CuO/ZnO/carbon.Open in a separate windowFig. 2SEM image of (A) CuO/ZnO/carbon and TEM images of (B and C) CuO/ZnO with different magnifications. The inset image of (A) is the picture of the CuO/ZnO/carbon paper, and the insert image of (C) is the diffraction pattern of the CuO/ZnO paper. (D) Detailed tapping mode AFM height images of CuO/ZnO and (E) line profile. The line profile was taken from the red lines marked in (D).The as-synthesized MO films were examined first by powder X-ray diffraction (Fig. 1). In the XRD pattern of ZnO, the definite line broadening of the XRD peaks indicates that the prepared material consisted of particles in the nanoscale range. From the XRD patterns analysis, we determined the peak intensity, position and width, and full-width at half-maximum (FWHM). The diffraction peaks located at 2θ = 31.7°, 34.4°, and 36.3° correspond to the (100), (002), and (101) planes, respectively, exhibiting the characteristic diffractions of a mixed-valence compound with a face-centred cubic structure (PDF #36-1451). The XRD pattern of CuO shows peaks at 2θ = 35.4°, 38.7°, and 48.7°, corresponding to the (002), (111), and (202) planes, respectively. All the diffraction peak can be readily indexed to the standard monoclinic phase (PDF #48-1548) without characteristic peaks assigned to possible impurities such as Cu₂O or Cu(OH)₂. In the case of CuO/ZnO, it displayed the diffraction peaks of both CuO and ZnO. It should be noted, in the XRD patterns of all the MO paper, the diffraction peck located at 2θ = 19.8° is attributed to the undestroyed graphene. XPS was also used to confirm that the CuO/ZnO hybrid materials were obtained (see Fig. S5^†). The binding energies of Cu 2p_3/2, Zn 2p_3/2, and O 1 s were identified at 933.63, 1021.9, and 529.67 eV, respectively. The high-resolution Cu and Zn scans indicate that both Cu and Zn were in the +2 oxidation state. The XPS measurements verify that the structures consist of CuO and ZnO, and furthermore, the atom ratio of Cu to Zn is about 1 : 1, suggesting that each cation has equal opportunity to diffuse in the interlayer GO paper. It should be mentioned that the carbon that appeared in Fig. S5A^† comes from the undestroyed graphene paper template.Open in a separate windowFig. 1XRD patterns of 2D nanoporous CuO/carbon, ZnO/carbon and hybrid CuO/ZnO/carbon papers.In general, MO is synthesized via chemical methods, where the shapes of most products are uncontrolled, and some further ligands are needed to prevent aggregation. Due to the fast grown process and no limit in growth direction, it is difficult to the control morphology of metal oxide. In our case, MO displayed ultrathin 2D nanosheets, with the size of about 20 nm and 50 nm corresponding to ZnO and CuO, respectively, according to TEM images shown in Fig. 2 and S6–S8.^† More interestingly, the TEM images suggest that the MO nanosheets connected each other, resulting in the formation of a continuous nanoporous network structure (the porous size ranged from about 5 nm to 10 nm). There are probable two reasons for the assembled nanoporous structure, one is that the MO nanosheets connected each other during the growth process, and the other is GO has lots defects and pores, which affected the growth of the MO nanosheets. As we expected, the BET surface area of the as-obtained CuO/ZnO/carbon hybrid material was estimated to be 80 m² g⁻¹ (Fig. S8^†), which was contributed by both the porous CuO/ZnO and undestroyed graphene, and the average pore size of ∼10 nm. The high-resolution TEM image of CuO/ZnO in Fig. S8^† shows lattice fringes with an interplanar spacing of 0.25 nm and 0.28 nm corresponding to the CuO 002 lattice face and ZnO 100 lattice face, respectively. The selected area electron diffraction (SAED) pattern in the inset in Fig. 2C is in good agreement with the XRD pattern and illustrates the polycrystalline structure of CuO/ZnO. The thickness of the nanosheets was measured via tapping mode AFM to be about 3 nm and 10 nm (Fig. 2D and S10^†) for ZnO and CuO, respectively. Furthermore, from the SEM images (Fig. 2A and B) of CuO/ZnO, the layer-by-layer structure is clearly seen on the microscale, which is similar with our previous report.²⁵ Moreover, the density of the 2D nanoporous CuO/ZnO nanosheet was calculated to be about 50 mg cm⁻², which is 10 times lighter than commercial CuO nanoparticles.To investigate the elemental of Cu and Zn distribution in the 2D nanoporous CuO/ZnO/carbon hybrid nanosheets, SEM and mapping analysis were performed for a single MO sheet. The SEM mapping images of the C, O, Cu and Zn elements are shown in Fig. 3. All the atoms were distributed uniformly in the nanocomposite, which suggests that a highly hybridized CuO/ZnO film was obtained. Both the high-resolution TEM image and FFT (Fig. S7C and D,^† respectively) indicate that the single CuO and ZnO nanosheets connected with each other on the nanoscale, which will provide huge chances for the continuous formation of p–n junction interfaces on the nanoscale, which is beneficial for facile electron transfer.Open in a separate windowFig. 3EDX mapping images of a single 2D nanoporous CuO/ZnO/carbon nanosheet. (A) Superimposed EDS image, (B)–(E) images of the elemental distributions, and (F) corresponding TEM image. Scale bar: 2 μm.More importantly, this type of layer-by-layer nanoporous CuO/ZnO/carbon nanosheet has a high surface area with conjunct bimodal pores, which can play key roles in providing shorter diffusion paths and rapid electrolyte transport, while providing more active sites for electrochemical reactions. The accurate detection of glucose is important for clinical diagnostics in diabetes control, analytical applications in biotechnology, and the food industry. Enzyme-based amperometric glucose biosensors can satisfy the requirements of clinical blood glucose level measurement. Glucose oxidase (GOx)-modified electrodes are the most common class of amperometric biosensors for glucose detection because GOx enables the catalytic oxidation of glucose with high sensitivity and selectivity. However, enzyme-modified electrodes have some disadvantages. For instance, the instability of the electrode and unsatisfactory reproducibility, complicated immobilization procedures, and expensive enzymes are easily deactivated. Thus, to address these issues, the development of non-enzymatic sensors with high sensitivity and stability, and interference-free glucose determination is very important. According to the XRD and XPS results, graphene was not completely removed, and thus graphene could still exist as a film coated with metal oxide surface or edge, which will be beneficial for electron transfer. Accordingly, the 2D CuO/ZnO nanosheets may exhibit good electrochemical performances as non-enzymatic sensors for the detection of glucose.Owing to their high specific area, 2D nanoporous nanosheets are promising materials for the fabrication of non-enzymatic glucose sensors. The cyclic voltammetry profiles of the 2D nanoporous CuO, ZnO and CuO/ZnO hybrid nanosheet-modified GCE electrodes in 40 mL 0.1 M KOH solution with and without 1 mM glucose were studied (Fig. 4). Obvious reduction peaks at around 0.6 V (vs. SCE) can be observed in the blank NaOH solution, which corresponds to the Cu(ii)/Cu(iii) redox couple according to previously reported studies (eqn (1)) (except the pure ZnO nanosheet electrode). After the injection of glucose, the Cu(iii) ion obtains an electron and acts as an electron delivery system, and electrons are transferred from glucose to the electrode, which leads to an increase in the peak current, and the reaction process is demonstrated in eqn (2). Comparing the CV without and with 1 mM glucose, the oxidation current dramatically increased, and the results suggest a kinetic limitation in the reaction between the redox sites of the CuO and CuO/ZnO modified-electrodes and glucose. Initially, the reaction potential was as low as 0.3 V (vs. SCE).CuO + OH⁻ → CuO(OH) + e⁻1CuO(OH) + glucose → CuO + gluconolactone + H₂O2Open in a separate windowFig. 4CVs of the (A) ZnO–CuO/carbon and (B) CuO/carbon papers measured in 0.1 M KOH. The dashed line and solid line indicate the electrolyte without and with 1 mM glucose, respectively. Scan rate 50 mV s⁻¹.To evaluate the electrochemical performance for the analysis of glucose, the non-enzymatic amperometric detection of glucose was performed at 0.6 V (vs. SCE) with the successive addition of glucose. Fig. 5A and B show the I–t curves of the different 2D porous ZnO, CuO and CuO/ZnO nanosheet-modified GCE electrodes in 0.1 M NaOH solution upon the successive addition of different concentrations of glucose. Both CuO and CuO/ZnO showed the typical steady-step response with very low noise, and remarkable increase in the oxidation current, which show the electrocatalytic activity of the modified electrodes towards the oxide of glucose. A linear relationship between the glucose concentration and catalytic current was found in the range of 5 μM to 3.325 mM at the CuO/ZnO/carbon-modified electrode (Fig. 5C), with a detection limit of 0.5 μM. The linear regression equation with a high correlation coefficient (R) of 0.998 was obtained, and the sensitivity was estimated to be 3850 μA cm⁻² mM⁻¹, which is over 3-fold higher than that for CuO (≈1000 μA cm⁻² mM⁻¹). It is understood that the formation of a p–n junction in the CuO/ZnO heterostructures results in the extension of the space charge region, which locally narrows the conducting channel for the charge carriers in ZnO, thus making the p–n junction more sensitive to glucose-induced charge transfer. The application of CuO/ZnO as a glucose sensor involves two types of sensing behaviours: adsorption-induced surface depletion and chemical conversion with a change in the electrical potential. It is also notable that the active area of CuO increased in the heterostructures.Open in a separate windowFig. 5Quantitative detection of glucose and selectivity analysis. (A) Full-range amperometric responses to the successive addition of glucose for three types of MO papers, (B) enlarged amperometric curves in the low-concentration region, (C) calibration curves, and (D) selectivity analysis by adding possible interfering species, while the successive addition of glucose was performed. Electrolyte 0.1 M KOH, and working electrode potential fixed at 0.6 V vs. (SCE).Since the 2D nanoporous CuO/ZnO acts as glucose oxidase, the electrochemical version of the Michaelis–Menten equation (eqn (3)) was used to evaluate the Michaelis constant (K_m).3where j_cat is the catalyst current obtained using the I–t curve, S is the glucose concentration, A is the surface area, n is number of electrons transferred, F is the Faraday constant, and K_cat is the apparent catalytic turnover rate. Considering n, F, A and K_cat as constants, eqn (3) could be simplified to eqn (4).4As shown in Fig. S10,^† the I–t curve could be fitted using eqn (4) with R = 0.999 and K_m is 5.4 mM. The apparent K_m is relatively low, close to that of a GOD-based sensor,³³ which suggests that the 2D CuO/ZnO/carbon displays good biocompatibility and has a high biological affinity to glucose.To further verify its sensing performance, some electrochemical characteristic tests were performed on the 2D porous CuO/ZnO/carbon nanosheet-based electrode. First, the reproducibility of the 2D porous CuO/ZnO nanosheet for nonenzymatic glucose sensing was evaluated. Three modified electrodes were prepared, and the relative standard deviation (RSD) was no more than 8.7% for the current response, indicating satisfactory electrode-to-electrode reproducibility. Additionally, the recovery was 97% with an RSD of 3.5% (S/N = 3) for 25 μM of glucose, demonstrating good intra-electrode reproducibility. To explore the specific detection of glucose in a real sample using the proposed approach based on the 2D nanoporous CuO/ZnO nanosheet-modified electrode, the I–t curve of the 2D CuO/ZnO nanosheet-modified GCE electrode was measured in 0.1 M NaOH solution upon the successive addition of 25 μM of glucose. Furthermore, the electrochemical response of common interfering species for 2D porous CuO/ZnO nanosheet-modified GCE electrode was examined, as shown in Fig. 5D. Some common interfering species in human serum, such as 250 μM uric acid (UA), ascorbic acid (AA), dopamine acid (DA), nicotinamide adenine dinucleotide (NADH), Mg²⁺, K⁺, Na⁺ and Ca²⁺, were chosen as the interfering species. As is clearly seen in Fig. 5D, the current responses of the interfering species has little effect compared to glucose, which indicated that 10 times higher concentration of interfering species than glucose has no response for the detection 25 μM glucose. Here, the good selectivity of the 2D nanoporous CuO/ZnO nanosheet against some reducing compounds, such as AA, can be ascribed to the obvious repulsion with the negatively charged CuO and ZnO (the isoelectric point of both is about 9.5) and the negatively charged AA (deprotonated) under highly basic conditions. The good selectivity of the 2D porous CuO/ZnO/carbon nanosheet electrode for glucose and against other interfering materials can be ascribed to the synergetic action of the well-designed 2D porous structure and high surface area, which provides the maximum number of active free paths to the glucose molecules and facilitates faster electron transfer. The results obtained with the proposed method were compared with other methods for the detection of glucose (

A photoelectrochemical glucose sensor based on gold nanoparticles as a mimic enzyme of glucose oxidase

This work reports the first construction of the ternary layers of ITO/PbS/SiO₂/AuNPs nanostructure for development of photoelectrochemical (PEC) glucose sensor. Herein, the thioglycolic acid-capped PbS quantum dots was employed as a PEC active probe, which is very sensitive to oxygen. The small gold nanoparticles (AuNPs) were act as nanozyme (mimic enzyme of glucose oxidase) to catalytically oxidize glucose in the presence of oxygen, meanwhile consumed oxygen and then resulted in the decrease of cathodic photocurrent. The insertion layer of SiO₂ nanoparticles between PbS and AuNPs could reduce efficiently the base current due to its low electroconductivity, which improved the detection limit. The proposed PEC sensor exhibited high sensitivity and gold selectivity towards glucose. The linear response of glucose concentrations ranged from 1.0 μM to 1.0 mM with detection limit of 0.46 μM (S/N = 3). The results suggest the potential of design and development of numerous nanozyme-based PEC biosensors with the advantage of the simplicity, stability, and efficiency.

This work reports the first construction of the ternary layers of ITO/PbS/SiO₂/AuNPs nanostructure for development of photoelectrochemical (PEC) glucose sensor.