Morphology–dependent electrochemical sensing properties of manganese dioxide–graphene oxide hybrid for guaiacol and vanillin

Surfactant-Assisted Synthesis of Metallic-Ag/Nickel Oxide on Graphitic Carbon Nitride Composite: An Electrochemical Investigation of Synthetic Vanillin

In this study, we developed a sensor based on surfactant-assisted synthesis of metallic silver-enriched nickel oxide confined on graphitic carbon nitride (Ag/NiO/g-CN)-modified electrode to construct a sensitive and selective voltammetric sensor for detecting vanillin in confectionaries samples. The X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy analyses confirmed the crystal structure and respective functional groups of the synthesized Ag/NiO/g-CN composite. The valence states of silver, nickel, oxygen, carbon, and nitrogen were analyzed using X-ray photoelectron spectroscopy (XPS), while energy-dispersive X-ray analysis (EDX) and morphological investigations revealed the elemental distribution and nano-structured particles, respectively. The electrocatalyst-modified electrode properties and electrochemical sensing performances were evaluated using different voltammetric and spectroscopic techniques. The Ag/NiO/g-CN composite, exhibiting a large active surface area, excellent conductivity, and synergistic interaction, proved to be a suitable electrode material for electrochemical sensor applications. The sensor demonstrated a detection limit of 0.9 nM and a broad linear range of 0.004-366.8 μM. Electrochemical investigations further highlighted the sensor's excellent reproducibility, repeatability, fast response, and functional stability. The constructed sensor also exhibited outstanding selectivity against potential interferents and demonstrated its practical applicability by successfully detecting vanillin in spiked food samples.

Synthesis of nickel-sphere coated Ni-Mn layer for efficient electrochemical detection of urea

Using a trustworthy electrochemical sensor in the detection of urea in real blood samples received a great attention these days. A thin layer of nickel-coated nickel-manganese (Ni@NiMn) is electrodeposited on a glassy carbon electrode (GC) (Ni@NiMn/GC) surface and used to construct the electrochemical sensor for urea detection. Whereas, electrodeposition is considered as strong technique for the controllable synthesis of nanoparticles. Thus, X-ray diffraction (XRD), atomic force microscope (AFM), and scanning electron microscope (SEM) techniques were used to characterize the produced electrode. AFM and SEM pictures revealed additional details about the surface morphology, which revealed a homogenous and smooth coating. Furthermore, electrochemical research was carried out in alkaline medium utilizing various electrochemical methods, including cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). The electrochemical investigations showed that the electrode had good performance, high stability and effective charge transfer capabilities. The structural, morphological, and electrochemical characteristics of Ni@NiMn/GC electrodes were well understood using the analytical and electrochemical techniques. The electrode showed a limit of detection (LOD) equal to 0.0187 µM and a linear range of detection of 1.0-10 mM of urea. Furthermore, real blood samples were used to examine the efficiency of the prepared sensor. Otherwise, the anti-interfering ability of the modified catalyst was examined toward various interfering species.

Electron coupling effect-triggered monatomic copper laccase-mimicking nanozyme for the degradation and detection of guaiacol produced by Alicyclobacillus acidoterrestris

The generation of guaiacol by Alicyclobacillus acidoterrestris (A. acidoterrestris) in fruit juices negatively affects public health and causes severe environmental pollution. Therefore, the sensitive detection and efficient degradation of guaiacol in real samples are crucial. Here, we develop an electrochemical sensor utilizing a copper single-atom nanozyme (CuN4-G) to detect and degrade guaiacol at the picomolar level. Density functional theory (DFT) calculations verify that the bonding electron coupling effect in the CuN4-G facilitates rapid electron transfer, enhances electrical conductivity, and provides abundant active sites, thereby leading to exceptional catalytic performance. Moreover, CuN4-G demonstrates a Km value similar to that of natural laccase but a higher Vmax, highlighting its potential as a highly efficient biocatalyst. The CuN4-G-based electrochemical sensor achieves a detection from 5 to 50,000 pM for guaiacol, with a 1.2 pM (S/N = 3) detection limit. Additionally, CuN4-G-modified electrodes display high selectivity and excellent stability. CuN4-G nanozyme can keep its activity in conditions of pH (3-9), temperature (30-90 °C), ionic strength (0-400 mM), and organic solvent (0-50% (v/v)), overcoming the deficiencies of natural enzymes. Furthermore, our electrochemical sensor can not only accurately detect guaiacol, but also degrade it in actual fruit juice samples infected by A. acidoterrestris, demonstrating its potential applications in food and environmental monitoring.

MXene-reinforced octahedral PtCu nanocages with boosted electrocatalytic performance towards endocrine disrupting pollutants sensing

Rational tailoring of hollow and porous bimetallic structures with excellent electrocatalytic performance is appealing yet challenging. Further, combining independent bimetallic nanoparticles with flexible two-dimensional substrate by forming stable heterocomplex is still highly desired for electrocatalysis. Herein, hierarchical PtCu alloy octahedrons with hollow interiors and nanosheet-assembled nanoshells were synthesized by a facile and efficient chemical transformation strategy using Cu₂O as sacrificial templates. Such octahedral PtCu nanocages displayed significantly enhanced electrocatalytic activity owing to their unique hollow and porous architectures which provided easy access for analytes to the catalyst surface. Thereafter, introduction of Ti₃C₂T_x MXene was realized via simple incubation of Ti₃C₂T_x in solution containing the 3-aminopropyltriethoxysilane-capped PtCu, and their electrostatic interaction guaranteed the firm adsorption of PtCu nanocages on Ti₃C₂T_x nanosheets. It turned out that the sensitivity of the hybrid sensor was remarkably improved for electrochemical monitoring of endocrine disrupting pollutants in water, exhibiting ultrawide linear ranges and sub-nanomole detection limits. The eminent electrode performance is attributed to the high specific area, fast electrochemical kinetics, decent electrical catalytic ability, and the synergistic effect between Pt, Cu, and MXene.

Recent developments on graphene and its derivatives based electrochemical sensors for determinations of food contaminants

The sensing of food contaminants is essential to prevent their adverse health effects on the consumers. Electrochemical sensors are promising in the determination of electroactive analytes including food pollutants, biomolecules etc. Graphene nanomaterials offer many benefits as electrode material in a sensing device. To further improve the analytical performance, doped graphene or derivatives of graphene such as reduced graphene oxide and their nanocomposites were explored as electrode materials. Herein, the advancements in graphene and its derivatives-based electrochemical sensors for analysis of food pollutants were summarized. Determinations of both organic (food colourants, pesticides, drugs, etc.) and inorganic pollutants (metal cations and anions) were considered. The influencing factors including nature of electrode materials and food pollutants, pH, electroactive surface area etc., on the sensing performances of modified electrodes were highlighted. The results of pollutant detection in food samples by the graphene-based electrode have also been outlined. Lastly, conclusions and current challenges in effective real sample detection were presented.

A Simple but Efficient Voltammetric Sensor for Simultaneous Detection of Tartrazine and Ponceau 4R Based on TiO2/Electro-Reduced Graphene Oxide Nanocomposite

In this work, we report a simple but efficient voltammetric sensor for simultaneous detection of ponceau 4R and tartrazine based on TiO2/electro-reduced graphene oxide nanocomposites (TiO2/ErGO). TiO2/ErGO nanocomposites were prepared by ultrasonically dispersing TiO2 nanoparticles (TiO2 NPs) into graphene oxide (GO) solution followed by a green in-situ electrochemical reduction. TiO2 NPs were uniformly supported on ErGO nanoflakes, which provides a favorable interface for the adsorption and subsequent oxidation of target analytes. TiO2/ErGO showed remarkable electrocatalytic capacity for the oxidation of ponceau 4R and tartrazine, with minimized oxidation overpotentials and boosted adsorptive striping differential pulse voltammetric (AdSDPV) response peak currents. Under the optimal experimental conditions, the anodic peak currents of ponceau 4R and tartrazine increase linearly with the respective natural logarithm of concentrations from 0.01 to 5.0 μM. The detection limits (LOD = 3σ/s) for ponceau 4R and tartrazine are 4.0 and 6.0 nM, respectively. The extraordinary analytical properties of TiO2/ErGO/GCE are primarily attributed to the synergistic enhancement effect from ErGO nanoflakes and TiO2 NPs. Moreover, the proposed TiO2/ErGO/GCE achieves reliable determination of ponceau 4R and tartrazine in orange juice with excellent selectively, reproducibility and stability. Together with simplicity, rapidness, and low cost, the proposed sensor demonstrates great potential for on-site detection of azo colorants.

Facile Electrochemical Demethylation of 2-Methoxyphenol to Surface-Confined Catechol on the MWCNT and Its Efficient Electrocatalytic Hydrazine Oxidation and Sensing Applications

Owing to its biological significance, preparation of stable surface-confined catechol (CA) is a long-standing interest in electrochemistry and surface chemistry. In this connection, various chemical approaches such as covalent immobilization (using amine- and carboxylate-functionalized CA, diazotization-based coupling, and Michael addition reaction), self-assembled monolayer on gold (thiol-functionalized CA is assembled on the gold surface), CA adsorption on the ad-layer of a defect-free single-crystal Pt surface, π-π bonding, CA pendant metal complexes, and CA-functionalized polymer-modified electrodes have been reported in the literature. In general, these conventional methods are involved with a series of time-consuming synthetic procedures. Indeed, the preparation of a surface-fouling-free surface-confined system is a challenging task. Herein, we introduce a new and facile approach based on electrochemical demethylation of 2-methoxyphenol as a precursor on the graphitic surface (MWCNT) at a bias potential, 0.5 V vs Ag/AgCl in neutral pH solution. Such an electrochemical performance resulted in the development of a stable and well-defined redox peak at E ^o' = 0.15 (A2/C2) V vs Ag/AgCl within 10 min of preparation time in pH 7 phosphate buffer solution. Calculated surface excess (16.65 × 10^-9 mol cm^-2) is about 10-1000 times higher than the values reported with other preparation methods. The product (catechol) formed on the modified electrode was confirmed by collective electrochemical and physicochemical characterizations such as potential segment analysis, TEM, Raman, IR, UV-vis, GC-MS, and NMR spectroscopic techniques, and thin-layer chromatographic studies. The electrocatalytic efficiency of the surface-confined CA system was demonstrated by studying hydrazine oxidation and sensing reactions in a neutral pH solution. This new system is found to be tolerant to various interfering biochemicals such as uric acid, xanthine, hypoxanthine, glucose, nitrate, hydrogen peroxide, ascorbic acid, Cu²⁺, and Fe²⁺. Since the approach is simple, rapid, and reproducible, a variety of surface-confined CA systems can be prepared.

Morphology-dependent MnO2/nitrogen-doped graphene nanocomposites for simultaneous detection of trace dopamine and uric acid

Four nanostructured MnO₂ with various controllable morphologies, including nanowires, nanorods, nanotubes and nanoflowers were synthesized, and then further composited with nitrogen-doped graphene (NG) with the assistance of ultrasonication. The surface morphologies, phase structures, and electrochemical performances of the proposed MnO₂/NG nanohybrids were investigated by various techniques, and their catalytic activities on the electrooxidation of dopamine (DA) and uric acid (UA) were compared systematically. The sensing performances were found to be highly correlated with their morphologies. Among these morphologies, the nanoflower-like MnO₂, composited with NG, displayed the most sensitive response signals for DA and UA. The boosted electrocatalytic activity was ascribed to the unique porous structure, large electroactive area, and low charge transfer resistance (R_ct), which facilitated the electron transfer between electrode and analytes. Two linear response ranges (0.1 μM-10 μM and 10 μM-100 μM) were accompanied with very low detection limits of 34 nM and 39 nM for DA and UA, respectively. Moreover, the successful application of the MnO₂NFs/NG composites for the simultaneous detection of DA and UA in human serum was realized using second-derivative linear sweep voltammetry (SDLSV). These findings give valuable insights for understanding the morphology-dependent sensing properties of MnO₂ based nanomaterials, which is conducive to the rapid development of ubiquitous MnO₂-based electrochemical sensors.

Facile Synthesis of MnO2 Nanoflowers/N-Doped Reduced Graphene Oxide Composite and Its Application for Simultaneous Determination of Dopamine and Uric Acid

This study reports facile synthesis of MnO2 nanoflowers/N-doped reduced graphene oxide (MnO2NFs/NrGO) composite and its application on the simultaneous determination of dopamine (DA) and uric acid (UA). The microstructures, morphologies, and electrochemical performances of MnO2NFs/NrGO were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS), respectively. The electrochemical experiments showed that the MnO2NFs/NrGO composites have the largest effective electroactive area and lowest charge transfer resistance. MnO2NFs/NrGO nanocomposites displayed superior catalytic capacity toward the electro-oxidation of DA and UA due to the synergistic effect from MnO2NFs and NrGO. The anodic peak currents of DA and UA increase linearly with their concentrations varying from 0.2 μM to 6.0 μM. However, the anodic peak currents of DA and UA are highly correlated to the Napierian logarithm of their concentrations ranging from 6.0 μM to 100 μM. The detection limits are 0.036 μM and 0.029 μM for DA and UA, respectively. Furthermore, the DA and UA levels of human serum samples were accurately detected by the proposed sensor. Combining with prominent advantages such as facile preparation, good sensitivity, and high selectivity, the proposed MnO2NFs/NrGO nanocomposites have become the most promising candidates for the simultaneous determination of DA and UA from various actual samples.

Morphology-Dependent Electrochemical Sensing Properties of Iron Oxide-Graphene Oxide Nanohybrids for Dopamine and Uric Acid

Various morphologies of iron oxide nanoparticles (Fe2O3 NPs), including cubic, thorhombic and discal shapes were synthesized by a facile meta-ion mediated hydrothermal route. To further improve the electrochemical sensing properties, discal Fe2O3 NPs with the highest electrocatalytic activity were coupled with graphene oxide (GO) nanosheets. The surface morphology, microstructures and electrochemical properties of the obtained Fe2O3 NPs and Fe2O3/GO nanohybrids were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques. As expected, the electrochemical performances were found to be highly related to morphology. The discal Fe2O3 NPs coupled with GO showed remarkable electrocatalytic activity toward the oxidation of dopamine (DA) and uric acid (UA), due to their excellent synergistic effect. The electrochemical responses of both DA and UA were linear to their concentrations in the ranges of 0.02-10 μM and 10-100 μM, with very low limits of detection (LOD) of 3.2 nM and 2.5 nM for DA and UA, respectively. Moreover, the d-Fe2O3/GO nanohybrids showed good selectivity and reproducibility. The proposed d-Fe2O3/GO/GCE realized the simultaneous detection of DA and UA in human serum and urine samples with satisfactory recoveries.

A Novel Modified Electrode for Detection of the Food Colorant Sunset Yellow Based on Nanohybrid of MnO₂ Nanorods-Decorated Electrochemically Reduced Graphene Oxide

The nanohybrid of electrochemically-reduced graphene oxide (ERGO) nanosheets decorated with MnO₂ nanorods (MnO₂ NRs) was modified on the surface of a glassy carbon electrode (GCE). Controlled potential reduction was applied for the reduction of graphene oxide (GO). The characterization was performed by scanning electron microscopy, X-ray diffraction and cyclic voltammetry. Compared with the poor electrochemical response at bare GCE, a well-defined oxidation peak of sunset yellow (SY) was observed at the MnO₂ NRs-ERGO/GCE, which was attributed to the high accumulation efficiency as well as considerable electrocatalytic activity of ERGO and MnO₂ NRs on the electrode surface. The experimental parameters for SY detection were optimized in detail. Under the optimized experiment conditions, the MnO₂ NRs-ERGO/GCE showed good linear response to SY in concentration range of 0.01–2.0 μM, 2.0–10.0 μM and 10.0–100.0 μM with a detection limit of 2.0 nM. This developed method was applied for SY detection in soft drinks with satisfied detected results.

Rapid and Sensitive Determination of Vanillin Based on a Glassy Carbon Electrode Modified with Cu₂O-Electrochemically Reduced Graphene Oxide Nanocomposite Film

A facile cuprous oxide nanoparticles functionalized electro-reduced graphene oxide modified glassy carbon electrode (denoted as Cu₂O NPs-ERGO/GCE) was fabricated via a simple physical adsorption and electrochemical reduction approach. Cyclic voltammetry and second-order derivative linear scan voltammetry were used to investigate the electrocatalysis oxidation of vanillin on the Cu₂O NPs-ERGO/GCE. The compound yielded a well-defined voltammetric response in 0.1 M H₂SO₄ at 0.916 V (vs. saturated calomel electrode (SCE)). A linear calibration graph was obtained in the concentration range of 0.1 μM to 10 μM and 10 μM to 100 μM, while the detection limit (S/N = 3) is 10 nM. In addition, the Cu₂O NPs-ERGO/GCE presented well anti-interference ability, stability, and reproducibility. It was used to detect vanillin sensitively and rapidly in different commercial food products, and the results were in agreement with the values obtained by high performance liquid chromatography.

Morphologically Tunable MnO2 Nanoparticles Fabrication, Modelling and Their Influences on Electrochemical Sensing Performance toward Dopamine

The morphology or shape of nanomaterials plays an important role in functional applications, especially in the electrochemical sensing performance of nanocomposites modified electrodes. Herein, the morphology-dependent electrochemical sensing properties of MnO2-reduced graphene oxide/glass carbon electrode (MnO2-RGO/GCE) toward dopamine detection were investigated. Firstly, various morphologies of nanoscale MnO2, including MnO2 nanowires (MnO2 NWs), MnO2 nanorods (MnO2 NRs), and MnO2 nanotubes (MnO2 NTs), were synthesized under different hydrothermal conditions. Then the corresponding MnO2-RGO/GCEs were fabricated via drop-casting and the subsequent electrochemical reduction method. The oxidation peak currents increase with the electrochemical activity area following the order of MnO2 NWs-RGO/GCE, MnO2 NTs-RGO/GCE, and MnO2 NRs-RGO/GCE. The spatial models for MnO2 NWs, MnO2 NTs, and MnO2 NRs are established and accordingly compared by their specific surface area, explaining well the evident difference in electrochemical responses. Therefore, the MnO2 NWs-RGO/GCE is selected for dopamine detection due to its better electrochemical sensing performance. The response peak current is found to be linear with dopamine concentration in the range of 8.0 × 10−8 mol/L–1.0 × 10−6 mol/L and 1.0 × 10−6 mol/L–8.0 × 10−5 mol/L with a lower detection limit of 1 × 10−9 mol/L (S/N = 3). Finally, MnO2 NWs-RGO/GCE is successfully used for the determination of dopamine injection samples, with a recovery of 99.6–103%. These findings are of great significance for understanding the relationship between unlimited nanoparticle structure manipulation and performance improvement.

Facile synthesis of MnO2-embedded flower-like hierarchical porous carbon microspheres as an enhanced electrocatalyst for sensitive detection of caffeic acid

Tailored designs/fabrications of hierarchical porous advanced electrode materials are of great importance for developing high-performance electrochemical sensors. Herein, we demonstrate a simple and low-cost in situ chemical approach for the facile synthesis of MnO₂-embedded hierarchical porous carbon microspheres (MnO₂/CM). By the characterizations of scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray powder diffraction and energy dispersive spectroscopy, we evidenced that the synthesized product were flower-like carbon microspheres (CM) assembled by the bent flakes with thickness of about several nanometers and MnO₂ nanorods were highly dispersed and successfully decorated on the CM layers, resulting in a rough surface and three-dimensional microstructure. The greatest benefit from the combined porous CM with MnO₂ nanorods is that the MnO₂/CM modified electrode has the synergetic catalysis effect on the electro-oxidation of caffeic acid, leading to the remarkable increase in the electron transfer rate and significant decrease in the over-potential for the caffeic acid oxidation in contrast to the bare electrode and CM modified electrode. This implies that the prepared MnO₂/CM can be employed as an enhanced electrocatalyst for the sensitive detection of caffeic acid. Under the optimum conditions, the anodic peak current of caffeic acid is linear with its concentration in the range of 0.01-15.00 μmol L^-1, and a detection limit of 2.7 nmol L^-1 is achieved based on S/N = 3. The developed sensor shows good selectivity, sensitivity, reproducibility, and also excellent recovery in the detections of real samples, revealing the promising practicality of the sensor for the caffeic acid detection.

Controlled synthesis of dendritic gold nanostructures by graphene oxide and their morphology-dependent performance for iron detection in coastal waters

Dendritic gold nanostructures were controllably synthesized by graphene oxide for voltammetric determination of Fe(iii) in coastal waters.

Fabrication of Palladium Nanoparticles on Porous Aromatic Frameworks as a Sensing Platform to Detect Vanillin

Here, we report the fabrication of palladium nanoparticles on porous aromatic frameworks (Pd/PAF-6) using a facile chemical approach, which was characterized by various spectro- and electrochemical techniques. The differential pulse voltammetry (DPV) response of Pd/PAF-6 toward the vanillin (VA) sensor shows a linear relationship over concentrations (10-820 pM) and a low detection limit (2 pM). Pd/PAF-6 also exhibited good anti-interference performance toward 2-fold excess of ascorbic acid, nitrophenol, glutathione, glucose, uric acid, dopamine, ascorbic acid, 4-nitrophenol, glutathione, glucose, uric acid, dopamine, and 100-fold excess of Na(+), Mg(2+), and K(+) during the detection of VA. The developed electrochemical sensor based on Pd/PAF-6 had good reproducibility, as well as high selectivity and stability. The established sensor revealed that Pd/PAF-6 could be used to detect VA in biscuit and ice cream samples with satisfactory results.

Synthesis and Characterization of Mesoporous Tin Oxide-Functionalized Reduced Graphene Oxide Nanoplatelets for Ultrasensitive Detection of Guaiacol in Red Wines

This work describes for the first time the use of mesoporous tin oxide-functionalized reduced graphene oxide (SnO2-rGO) as electrode modifier in combination with differential pulse voltammetry techniques for preconcentration and detection of guaiacol in red wine samples. SnO2-rGO was prepared through in situ growth of SnO2 particles on the rGO surface using cetyltrimethylammonium bromide as the structure-directing agent. Using the best set of experimental conditions, a linear response for guaiacol in the concentration range of 0.05 to 60 μM with a limit of detection of 7.2 nM (signal-to-noise ratio = 3) was obtained. Finally, the method was successfully applied to determine guaiacol in red wine samples, and the contents closely corresponded to those obtained by the reported chromatographic method.

Radially oriented nanostrand electrodes to boost glucose sensing in mammalian blood

Architecture of nanoscale electrochemical sensors for ultra-trace detection of glucose in blood is important in real-life sampling and analysis. To broaden the application of electrochemical sensing of glucose, we fabricated, for the first time, a glucose sensor electrode based on radially oriented NiO nanostrands (NSTs) onto 3D porous Ni foam substrate for monitoring, as well as selective and sensitive sensing of glucose in mammalian blood. The simple, scalable one-pot fabrication of this NST-Ni sensor design enabled control of the pattern of radially oriented NSTs onto 3D porous Ni foam substrate. The radial orientation of NST-Ni electrode onto the interior of the 3D porous substrate with controlled crystal structure size and atomic arrangement along the axis of the strands, intrinsic surface defects, and superior surface properties, such as hydrophilicity, high surface energy, and high density led to highly exposed catalytic active sites. The hierarchical NST-Ni electrode was used to develop a sensitive and selective sensor over a wide range of glucose concentrations among actively competitive ions, chemical species and molecular agents, and multi-cyclic sensing assays. The NST-Ni electrode shows significant glucose sensing performance in terms of unimpeded diffusion pathways, a wide range of concentration detection, and lower limit of detection (0.186 µM) than NiO nanosheet (NS)-Ni foam electrode pattern, indicating the effectiveness of the shape-dependent structural architecture of NST-Ni electrode. In this study, the NST-Ni electrode is fabricated to develop a simple, selective method for detecting glucose in physiological fluids (e.g., mammalian blood).

Cajeput tree bark derived activated carbon for the practical electrochemical detection of vanillin

A facile synthetic strategy for the fabrication of Cajeput tree bark derived functional porous carbon and its applications as a vanillin sensor is reported.