A novel electrochemical sensor based on self-assembled platinum nanochains - Multi-walled carbon nanotubes-graphene nanoparticles composite for simultaneous determination of dopamine and ascorbic acid

Electrochemical sensors based on the composite of reduced graphene oxide and a multiwalled carbon nanotube-modified glassy carbon electrode for simultaneous detection of hydroquinone, dopamine, and uric acid

Using a simple drop-casting technique, we successfully fabricated a sensitive electrochemical sensor based on the composite of reduced graphene oxide (RGO) and multiwalled carbon nanotubes (MWCNT) deposited on the surface of a glassy carbon electrode (GCE) for individual and simultaneous measurements of hydroquinone (HQ), dopamine (DA), and uric acid (UA). The nanocomposite of RGO/MWCNT was further characterized in terms of its structural properties, surface morphology, and topography using Raman, FT-IR spectroscopy, SEM, HRTEM, and AFM. Then, the proposed sensor for simultaneous measurement of HQ, DA, and UA based on RGO/MWCNT-modified GCE was investigated for its electrochemical behavior and electroanalytical performances using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), and differential pulse voltammetry (DPV). In addition, the composition ratio between RGO and MWCT was 1 : 1 showing the highest electrochemical response for simultaneous detection of HQ, DA, and UA. Owing to the synergistic effect between RGO and MWCNT leading to excellent conductivity properties, the proposed sensor exhibited improved electrochemical response at pH 7 toward the oxidation processes of HQ, DA, and UA on the surface of modified electrode. The proposed sensor demonstrated three well-defined anodic peaks of these analytes with their linear concentrations ranges of 3.0-150.0 μM for HQ, 4.0-100.0 μM for DA, and 2.0-70.0 μM for UA. The limit of detection values for the simultaneous detection of HQ, DA, and UA were found as follows 0.400 ± 0.014, 0.500 ± 0.006, and 0.300 ± 0.016 μM, respectively. The additional features of this proposed sensor are high reproducibility and stability for the simultaneous detection of HQ, DA, and UA with negligible interference effect from interferents such as Mg2+, K+, Cl-, ascorbic acid, and glucose. An acceptable percentage of recovery was also shown by this sensor for simultaneous measurements of HQ, DA, and UA using 6 samples of human urine. In summary, the RGO/MWCNT nanocomposite has been shown to be a promising platform for rapid, simple, and reliable determination of simultaneous measurements of HQ, DA, and UA in practical applications.

Hydrazo Pyrazole-Pyridone Fluorescent tag for NLO, Live cell imaging, LFPs visualization, Photophysical probing, and Electrochemical sensor for Dopamine detection

The present communication reports on the synthesis of a novel methyl-pyridone azo fluorescent tag (MPAFT) were proven through 1H (NMR), FT-IR, UV-vis, and high-resolution mass spectrometry. The quantum chemical parameters of MPAFT were evaluated using density functional theory (DFT) analysis. It was further investigated for its latent fingerprint (LFPs) in various surfaces and anticounterfeiting applications. By exposing Level I-Level III, ridge features to UV light with a wavelength of 365 nm, a bioimaging investigation has also demonstrated the potential of MPAFT's emission behaviour. The cyclic voltammetry (CV) and linear sweep voltammetry (LSV) at MPAFT/MGCE (modified glassy carbon electrode) were used to explore the electrochemical sensitivity and reliable detection of dopamine (DA) in neutral PBS (pH 7) electrolyte solution, and the results show good sensitivity and detection. The lower detection limit for LSV was 0.81 μM under optimum conditions.

A novel pencil graphite electrode modified with an iron-based conductive metal-organic framework exhibited good ability in simultaneous sensing bisphenol A and bisphenol S

Bisphenol A (BPA) and its substitute bisphenol S (BPS) are desirable materials widely used in manufacturing plastic products but can pose carcinogenic risks to humans. A new conductive iron-based metal-organic framework (Fe-HHTP)-modified pencil graphite electrode (PGE) for electrochemically sensing BPA and BPS was prepared and fully characterized by SEM, TEM, FT-IR, XRD, and XPS. Results showed that the optimal conditions for preparing Fe-HHTP/PGE were a pH of 6.5, a Fe-HHTP concentration of 2 mg·mL-1, a deposition potential of 0 V, and a deposition time of 100 s. The Fe-HHTP/PGE prepared under such conditions harbored a significant electrocatalytic activity with a detection limit of 0.8 nM for BPA and 1.7 nM for BPS (S/N = 3). Correspondingly, the electrochemical response current was linearly correlated to BPA and BPS, ranging from 0.01 to 100 μM. Fe-HHTP/PGE also obtained satisfactory recoveries by 93.8-102.1% and 96.0-101.3% for detecting BPA and BPS in plastic food packaging samples. Our work has provided a novel electrochemical tool to simultaneously detect BPA and BPS in food packaging samples and environmental matrixes.

Crumpled MXene nanosheets for sensing of ascorbic acid in food, biological fluids, and erythrocytes in-vitro microenvironment

In this work, a simple and facile method was developed to achieve controlled oxidation and enhance the surface area of MXene nanosheets and their utilization in the efficient sensing of ascorbic acid (AA or vitamin C). After etching of MAX phase to MXene via the MILD technique, controlled flash oxidation was carried out in the open air environment for 1.5 h, followed by flocculation of oxidized MXene nanosheets by using H2SO4, consequently achieving crumpled MXene possessing anatase phase, porosity, and improved surface area as revealed and confirmed by SEM, TEM, Raman, and BET analysis results. The as-prepared crumpled MXene was coated over a glassy carbon electrode (GCE) and used to determine AA successfully via cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and differential pulse voltammetry (DPV) with a linear concentration range of 300 μM to 0.005 μM with a detection limit (LOD) of 2 nM (2.8 % RSD and S/N = 3). The developed electrochemical sensor was used to determine the AA in various actual samples such as juice, urine, serum, and erythrocytes spiked with AA with excellent recoveries in the 94-103 % range. The sensor also demonstrated excellent reproducibility (~1 % RSD for five repetitive assays) and a shelf life of nearly one month with a negligible decrease in response. Furthermore, it lost only 10 % of its response for the next ten days. It also showed satisfactory selectivity toward AA in the presence of other similar compounds, including uric acid (UA), dopamine (DA), and glucose.

Multiwalled Carbon Nanotubes-Modified Metallic Electrode Prepared Using Chemical Vapor Deposition as Sequential Injection Analysis Detector for Determination of Ascorbic Acid

A carbon nanotubes modified silver electrode (CNTs-Ag) was prepared via catalytic chemical vapor deposition and characterized. The morphology, crystallinity, elemental composition, and other quality parameters of the prepared electrode were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman techniques. The characterization results revealed the modification of the silver metal surface with CNTs of good characteristics. A sequential injection analysis (SIA) system was developed for studying the reaction of ascorbic acid with KIO₃ using the prepared CNTs-Ag electrode. Electrodes were polarized with both direct current (DC) and periodic square wave (SW). Various experimental conditions affecting the differential electrolytic potentiometric (DEP) peak such as current density, SW bias value, and flow rate were appraised. Under the optimum conditions, good linear responses for ascorbic acid were obtained in the range of 60.0-850.0 µM for both types of polarization with detection limits of 14.0-19.0 µM. The results obtained showed that the periodic polarization method was more sensitive than DC polarization and the electrode response was faster. Ascorbic acid in pharmaceutical tablets was determined with satisfactory results using this method. The prepared CNTs-based electrode exhibited good performance for a long period of use. The method is simple, rapid, and inexpensive for routine analysis.

Research on Temperature-Switched Dopamine Electrochemical Sensor Based on Thermosensitive Polymers and MWCNTs

A temperature-controlled electrochemical sensor was constructed based on a composite membrane composed of temperature-sensitive polymer poly (N-isopropylacrylamide) (PNIPAM) and carboxylated multi-walled carbon nanotubes (MWCNTs-COOH). The sensor has good temperature sensitivity and reversibility in detecting Dopamine (DA). At low temperatures, the polymer is stretched to bury the electrically active sites of carbon nanocomposites. Dopamine cannot exchange electrons through the polymer, representing an “OFF” state. On the contrary, in a high-temperature environment, the polymer shrinks to expose electrically active sites and increases the background current. Dopamine can normally carry out redox reactions and generate response currents, indicating the “ON” state. In addition, the sensor has a wide detection range (from 0.5 μM to 150 μM) and low LOD (193 nM). This switch-type sensor provides new avenues for the application of thermosensitive polymers.

Activated Glassy Carbon Electrode as an Electrochemical Sensing Platform for the Determination of 4-Nitrophenol and Dopamine in Real Samples

Glassy carbon electrode (GCE) was electrochemically activated using a repetitive cyclic voltammetric technique to develop an activated glassy carbon electrode (AGCE). The developed AGCE was optimized and utilized for the electrochemical assay of 4-nitrophenol (4-NP) and dopamine (DA). Cyclic voltammetry (CV) was employed to investigate the electrochemical behavior of the AGCE. Compared to the bare GCE, the developed AGCE exhibits a significant increase in redox peak currents of 4-NP and DA, which indicates that the AGCE significantly improves the electrocatalytic reduction of 4-NP and oxidation of DA. The electrochemical signature of the activation process could be directly associated with the formation of oxygen-containing surface functional groups (OxSFGs), which are the main reason for the improved electron transfer ability and the enhancement of the electrocatalytic activity of the AGCE. The effects of various parameters on the voltammetric responses of the AGCE toward 4-NP and DA were studied and optimized, including the pH, scan rate, and accumulation time. Differential pulse voltammetry (DPV) was also utilized to investigate the analytical performance of the AGCE sensing platform. The optimized AGCE exhibited linear responses over the concentration ranges of 0.04-65 μM and 65-370 μM toward 4-NP with a lower limit of detection (LOD) of 0.02 μM (S/N = 3). Additionally, the AGCE exhibited a linear responses over the concentration ranges of 0.02-1.0 and 1.0-100 μM toward DA with a lower limit of detection (LOD) of 0.01 μM (S/N = 3). Moreover, the developed AGCE-based 4-NP and DA sensors are distinguished by their high sensitivity, excellent selectivity, and repeatability. The developed sensors were successfully applied for the determination of 4-NP and DA in real samples with satisfactory recovery results.

Synthesis of Metal–Organic Frameworks Quantum Dots Composites as Sensors for Endocrine-Disrupting Chemicals

Hazardous chemical compounds such as endocrine-disrupting chemicals (EDCs) are widespread and part of the materials we use daily. Among these compounds, bisphenol A (BPA) is the most common endocrine-disrupting chemical and is prevalent due to the chemical raw materials used to manufacture thermoplastic polymers, rigid foams, and industrial coatings. General exposure to endocrine-disrupting chemicals constitutes a serious health hazard, especially to reproductive systems, and can lead to transgenerational diseases in adults due to exposure to these chemicals over several years. Thus, it is necessary to develop sensors for early detection of endocrine-disrupting chemicals. In recent years, the use of metal–organic frameworks (MOFs) as sensors for EDCs has been explored due to their distinctive characteristics, such as wide surface area, outstanding chemical fastness, structural tuneability, gas storage, molecular separation, proton conductivity, and catalyst activity, among others which can be modified to sense hazardous environmental pollutants such as EDCs. In order to improve the versatility of MOFs as sensors, semiconductor quantum dots have been introduced into the MOF pores to form metal–organic frameworks/quantum dots composites. These composites possess a large optical absorption coefficient, low toxicity, direct bandgap, formidable sensing capacity, high resistance to change under light and tunable visual qualities by varying the size and compositions, which make them useful for applications as sensors for probing of dangerous and risky environmental contaminants such as EDCs and more. In this review, we explore various synthetic strategies of (MOFs), quantum dots (QDs), and metal–organic framework quantum dots composites (MOFs@QDs) as efficient compounds for the sensing of ecological pollutants, contaminants, and toxicants such as EDCs. We also summarize various compounds or materials used in the detection of BPA as well as the sensing ability and capability of MOFs, QDs, and MOFs@QDs composites that can be used as sensors for EDCs and BPA.

Strategies, advances, and challenges associated with the use of graphene-based nanocomposites for electrochemical biosensors

Graphene is an intriguing two-dimensional honeycomb-like carbon material with a unique basal plane structure, charge carrier mobility, thermal conductivity, wide electrochemical spectrum, and unusual physicochemical properties. Therefore, it has attracted considerable scientific interest in the field of nanoscience and bionanotechnology. The high specific surface area of graphene allows it to support high biomolecule loading for good detection sensitivity. As such, graphene, graphene oxide (GO), and reduced GO are excellent materials for the fabrication of new nanocomposites and electrochemical sensors. Graphene has been widely used as a chemical building block and/or scaffold with various materials to create highly sensitive and selective electrochemical sensing microdevices. Over the past decade, significant advancements have been made by utilizing graphene and graphene-based nanocomposites to design electrochemical sensors with enhanced analytical performance. This review focus on the synthetic strategies, as well as the structure-to-function studies of graphene, electrochemistry, novel multi nanocomposites combining graphene, limit of detection, stability, sensitivity, assay time. Finally, the review describes the challenges, strategies and outlook on the future development of graphene sensors technology that would be usable for the internet of things are also highlighted.

Tellurium-based chemical sensors

The various tellurium-based chemical sensors are described. This article focuses on four types of Tellurium sensors such as CdTe quantum dots-based sensor, Te thin films-based sensor, Te nanostructures or nanoparticles-based sensor, and TeO2-based sensor.

Determination of Ascorbic Acid in Pharmaceuticals and Food Supplements with the New Potassium Ferrocyanide-Doped Polypyrrole-Modified Platinum Electrode Sensor

This paper reports the results obtained from the determination of ascorbic acid with platinum-based voltammetric sensors modified with potassium hexacyanoferrate-doped polypyrrole. The preparation of the modified electrodes was carried out by electrochemical polymerization of pyrrole from aqueous solutions, using chronoamperometry. Polypyrrole films were deposited on the surface of the platinum electrode, by applying a constant potential of 0.8 V for 30 s. The thickness of the polymer film was calculated from the chronoamperometric data, and the value was 0.163 μm. Cyclic voltammetry was the method used for the Pt/PPy-FeCN electrode electrochemical characterization in several types of solution, including KCl, potassium ferrocyanide, and ascorbic acid. The thin doped polymer layer showed excellent sensitivity for ascorbic acid detection. From the voltammetric studies carried out in solutions of different concentrations of ascorbic acid, ranging from 1 to 100 × 10−6 M, a detection limit of 2.5 × 10−7 M was obtained. Validation of the analyses was performed using pharmaceutical products with different concentrations of ascorbic acid, from different manufacturers and presented in various pharmaceutical forms, i.e., intravascular administration ampoules, chewable tablets, and powder for oral suspension.

Persimmon Tannin-Reduction Graphene Oxide-Platinum-Palladium Nanocomposite Decorated on Screen-Printed Carbon Electrode for Enhanced Electrocatalytic Reduction of Hydrogen Peroxide

According to studies, Hydrogen peroxide (H2O2) is a significant biomarker of physiological processes. Unnormal H2O2 levels in human body may result in diseases. Hence, there is an increasing demand for monitoring the H2O2 concentrations in biological specimen. Here, we construct a non-enzymatic H2O2 electrochemical biosensor based on persimmon tannin-reduced graphene oxide-platinum-palladium nanocomposite (PrG-Pt@Pd NPs) modified with screen-printed carbon electrode (SPE). Combined with suitable electrocatalytic mode for Pt@Pd NPs, high specific large specific volume and good electrical conductivity of RGO, well as the superior sorption capacity of PT for metal-based nano-ion, the PrGPt@Pd striped pleasing heterogeneous catalytic activity toward H2O2 reduction via the synergistic effect. In experimental conditions of optimal, this non-enzymatic electrochemical sensor exhibited excellent electrocatalytic performance for H2O2 with less negative potential (−0.5 V), fast response time (<3 s), it shows good linearity in the range of 5.0–100.0 μM, in addition to this LOD of this sensor was 0.059 μM as well as the excellent sensitivity of the sensor (13.696 μA·μM−1·cm−2). Due to excellent specificity, lower detection limit, and good recovery (98.70–99.96%) in the spiked measurements of human serum samples, this non-enzymatic electrochemical biosensor paves the way for H2O2 detection at ultra-low concentrations in physiology and diagnosis.

Mo and Zn-Dual doped CuxO nanocrystals confined High-Conductive Cu arrays as novel sensitive sensor for neurotransmitter detection

The development of sensitive and selective sensors using facile and low-cost methods for detecting neurotransmitter molecules is a critical factor in the health care system in regard to early diagnosis. In this research, an electrocatalyst derived from Mo,Zn dual-doped Cu_xO nanocrystals-based layer coating over one-dimensional copper nanowire arrays (Mo,Zn-Cu_xO/CuNWs) was successfully designed using a facile electrodeposition approach and used as an electrochemical sensor for non-enzymatic dopamine (DA) neurotransmitter detection. The synergistic effect caused by the dual-doping effect along with its excellent conductivity produced a large electroactive surface area and an improved hetero-charge transfer, thereby boosting DA sensing ability with a low limit detection of 0.32 µM, wide-range of detection (0.5 µM - 3.9 mM), long-term stability (5 weeks), and high selectivity in phosphate buffer solution (pH 7.4). Also, the sensor accurately determined DA in real blood serum-spiked solutions. The achieved results evidenced that the Mo,Zn-Cu_xO/CuNWs derived sensor is highly suitable for DA detection. Therefore, it also opens new windows for the development of low-cost, accurate, high-performance, and stable sensors for other neurotransmitter sensing for the purposes of better health care and early diagnosis.

A label-free electrochemical aptasensor based on platinum@palladium nanoparticles decorated with hemin-reduced graphene oxide as a signal amplifier for glypican-3 determination

An electrochemical aptasensor for highly sensitive detection of glypican-3 has been developed using the GPC3 aptamer as the biorecognition probe and H-rGO-Pt@Pd NPs as an electroactive reagent.

Ultrasensitive multiwall carbon nanotube-mesoporous MCM-41 hybrid-based platform for the electrochemical detection of ascorbic acid

An ultrasensitive multiwall carbon nanotube-MCM-41 hybrid-based ascorbic acid sensor for electro-detection in real samples is proposed. The MWCNT–MCM-41 hybrid preparation via dispersion was optimized through an experimental design based on CCD/RSM.

Graphene and Carbon Nanotube-based Electrochemical Sensing Platforms for Dopamine

Dopamine (DA) is an important neurotransmitter, which is created and released from the central nervous system. It plays a crucial role in human activities, like cognition, emotions, and response to anything. Maladjustment of DA in human blood serum results in different neural diseases, like Parkinson's and Schizophrenia. Consequently, researchers have started working on DA detection in blood serum, which is undoubtedly a hot research area. Electrochemical sensing techniques are more promising to detect DA in real samples. However, utilizing conventional electrodes for selective determination of DA encounters numerous problems due to the coexistence of other materials, such as uric acid and ascorbic acid, which have an oxidation potential close to DA. To overcome such problems, researchers have put their focus on the modification of bare electrodes. The aim of this review is to present recent advances in modifications of most used bare electrodes with carbonaceous materials, especially graphene, its derivatives, and carbon nanotubes, for electrochemical detection of DA. A brief discussion about the mechanistic phenomena at the electrode interface has also been included in this review.

Voltammetric Determination of Amoxicillin Using a Reduced Graphite Oxide Nanosheet Electrode

A reduced graphite oxide nanosheet electrode (RGOnS) was prepared as a sensor for amoxicillin (AMX) detection, an antibiotic commonly used in the livestock farm, by the square-wave adsorptive stripping voltammetry technique. Graphite oxide with nanosheet shape was produced from a graphite electrode by a chronoamperometry process at 5 V and then an electrochemical reduction process was carried out to form RGOnS with restored long-range conjugated networks and better conductivity. The electrodes were characterized by SEM, EDX, and FTIR spectroscopy. The RGOnS electrode prepared at an optimal reduction potential of -1 V for 120 s exhibits a larger electrochemical active surface area, and the obtained oxidation signal of AMX is approximately ten times higher than that of the pristine graphite electrode. The analytical conditions such as the pH of electrolyte and accumulation time were optimized. The calibration curve built under the optimal conditions provided a good linear relationship in the range of AMX concentration from 0.5-80 µM with the correlation coefficient of 0.9992. The limit of detection was calculated as 0.193 µM. Satisfactory results are obtained from the detection of the AMX in different samples using the prepared electrode.

Highly sensitive and selective determination of p-nitrophenol at an interpenetrating networks structure of self-assembled rod-like lanthanum hydroxide-oxidized multi-walled carbon nanotubes nanocomposite

In this study, a novel electrochemical sensor based on self-assembled rod-like lanthanum hydroxide-oxidized multi-walled carbon nanotubes (La(OH)₃-OxMWCNTs) nanocomposite was developed for sensitive determination of p-nitrophenol (p-NP). The La(OH)₃-OxMWCNTs nanocomposite with an interpenetrating networks structure was characterized by field emission electron microscope (FE-SEM), Fourier transform infrared (FT-IR) spectroscopy, Raman spectra and X-ray photoelectron spectroscopy (XPS). The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed to study the electrochemical behaviors of La(OH)₃-OxMWCNTs modified glassy carbon electrode (La(OH)₃-OxMWCNTs/GCE). The La(OH)₃-OxMWCNTs/GCE was used for sensitive determination of p-NP by CV and linear sweep voltammetry (LSV). Under the optimum conditions, the peak currents of LSV versus the concentrations of p-NP in the range 1.0-30.0 μmol L^-1 showed a good linear relationship (R²=0.9971), and the limit of detection (LOD) was calculated to be 0.27 μmol L^-1 (signal-to-noise ratio of 3, S/N=3). The recoveries of p-NP in real samples of industrial wastewater and Xiangjiang water at La(OH)₃-OxMWCNTs/GCE were in the range of 95.62-110.75% with relative standard deviation (RSD) in the range of 1.65-3.85%. The intra-day and inter-day precisions were estimated to be less than 2.76% (n= 5), indicating that La(OH)₃-OxMWCNTs/GCE possessed highly stability. In addition, La(OH)₃-OxMWCNTs/GCE sensor showed good anti-interference ability for determination of p-NP in aqueous mixtures containing high concentrations of inorganic and organic interferents, and a decrease of oxidation peak currents by less than 3.57% relative to the initial levels indicated it possessed excellent selectivity. Therefore, La(OH)₃-OxMWCNTs/GCE could be used as a fast, selective and sensitive electrochemical sensor platform for the selective determination and quantification of aqueous p-NP.

An Electrochemical Immunosensor for Sensitive Detection of the Tumor Marker Carcinoembryonic Antigen (CEA) Based on Three-Dimensional Porous Nanoplatinum/Graphene

Carcinoembryonic antigen (CEA) is an important broad-spectrum tumor marker. The quantitative detection of a low concentration of CEA has important medical significance. In this study, three-dimensional porous graphene-oxide-supported platinum metal nanoparticles (3DPt/HGO) composites were prepared by a wet chemical method and modified on an electrode with enhanced conductivity, a large surface area, and good adsorption of immobilizing antibodies (Ab₁). Horseradish peroxidase (HRP)-functionalized Au nanoparticles were fabricated to label the secondary antibodies (Ab₂). The proposed immunosensor showed a good linear relationship in the range of 0.001–150 ng/mL for CEA and a detection limit of 0.0006 ng/mL. The immunosensor had high sensitivity, good stability and reproducibility, and has great application prospects for the clinical diagnosis of cancer.

MPA-CdTe quantum dots as "on-off-on" sensitive fluorescence probe to detect ascorbic acid via redox reaction

Mercaptopropionic acid (MPA) capped CdTe quantum dots (MPA-CdTe QDs) were synthesized in aqueous medium by hydrothermal method, which modified by Fe³⁺ could be used as a fluorescent probe to detect ascorbic acid (AA). MPA-CdTe QDs fluorescence probe could be used as successive sensor for metal ions and AA with "on-off-on" process. The fluorescence of QDs was quenched after adding Fe³⁺ to MPA-CdTe QDs. Then, the fluorescence of the Fe³⁺@MPA-CdTe QDs can be sensitively turned on by AA to give an "on-off-on" fluorescence response according to the oxidation-reduction between Fe³⁺ and AA. There was a linear relationship between fluorescence intensity quenching value and the concentration of Fe³⁺ in the range of 2-10 μM since Fe³⁺ sensitively reacted with CdTe QDs. The linear detection range for AA was 0.1-1 μM with a limit of detection of 6.6 nM. The principle is proved by fluorescence emission spectroscopy, nuclear magnetic resonance spectroscopy. The proposed method is successfully used to detect the AA in human plasma sample.

Sensitive electrochemical sensing platform for selective determination of dopamine based on amorphous cobalt hydroxide/polyaniline nanofibers composites

In this study, amorphous cobalt hydroxide/polyaniline nanofibers (Co(OH)₂/PANINF) composites were successfully prepared. The formation of amorphous Co(OH)₂ with irregular surface structure was confirmed by x-ray diffraction, scanning electron microscopy, and selected-area electron diffraction. The non-enzymatic electrochemical sensor for the selective and sensitive determination of dopamine (DA) has been constructed by using Co(OH)₂/PANINF composites modified glassy carbon electrode (Co(OH)₂/PANINF/GCE), which exhibited excellent electrocatalytic activity toward DA, in a large part owing to the advantages of large surface area of amorphous Co(OH)₂ and the synergetic effect between Co(OH)₂ and PANINF. The electrochemical kinetics reveal that the DA oxidation involves two electrons and two protons in a quasi-reversible electrode reaction. Differential pulse voltammetry (DPV) studies show remarkable sensing performance for the determination of DA, with a low detection limit of 0.03 μM, and a wide linear range from 0.1 to 200 μM. From a broader perspective, the present study demonstrates that Co(OH)₂/PANINF composites would be promising supporting materials for novel sensing platforms.

One-step phosphite removal by an electroactive CNT filter functionalized with TiO2/CeOx nanocomposites

Compared with phosphate (+5 valence), phosphite (HPO₃^2-/H₂PO₃^-, +3 valence) possesses higher solubility, and is more resistant to biotransformation. Herein, we designed a one-step electroactive filter technology for rapid and efficient phosphite removal. The filter consists of carbon nanotubes (CNT) and functionalized with nanoscale TiCe binary oxides. The phosphite removal kinetics and capacity increased with electric field (e.g., from 54.5% at 0 V to 75.6% at 2 V) and flow rate (e.g., from 63.1% at 1.5 mL/min to 81.2% at 6 mL/min). This can be attributed to synergistic effects of the filter's electrochemical reactivity, limited pore size, more exposed active sites and flow-through design. Meanwhile, phosphite can be converted to phosphate once adsorbed under electric field. The TiO₂/CeO_x-CNT filter could work effectively across a wide pH range, and the presence of various coexisting anions posed negligible impact on phosphite removal. Electrochemical characterizations verified the essential role of CeO_x and applied electric field, which synergistically accelerated electron transfer rate and increased charge capacity. The TiO₂/CeO_x-CNT filter can be regenerated effectively by chemical washing. The system efficacy was further supported by a comparable phosphite removal efficiency of 72.8% in actual lake water conditions. Therefore, this TiO₂/CeO_x-CNT filter technology is promising for mitigating the challenging issue of phosphite contamination from water bodies.

A sensitive sensor based on MOFs derived nanoporous carbons for electrochemical detection of 4-aminophenol

A novel electrochemical sensor based on zinc oxide/nitrogen doped porous carbons (ZnO/NPC) modified electrode has been constructed for detecting 4-aminophenol (4-AP). The ZnO/NPC material was synthesized by one-step carbonization of MOF-5-NH₂. The modified glassy carbon electrode (ZnO/NPC/GCE) holds excellent electrocatalytic activity toward 4-AP, with a sensitivity of about 31.02 μA/μM/cm². Under optimal conditions, its oxidation peak current increases linearly with the increasing concentration of 4-AP (from 5 to 120 μmol/L), and the detection limits is 0.014 μmol/L (S/N = 3). Furthermore, favorable selectivity, superior reproducibility and outstanding stability have been achieved. The ZnO/NPC/GCE has been applied in detecting 4-AP in industrial waste water and achieved positive results with the recovery of 4-AP ranging from 94.02% to 107.7%, which confirms that this sensor is a reliable platform for the detection of 4-AP in waste water.

PtNPs-GNPs-MWCNTs-β-CD nanocomposite modified glassy carbon electrode for sensitive electrochemical detection of folic acid

A novel electrochemical sensor, platinum nanoparticles/graphene nanoplatelets/multi-walled carbon nanotubes/β-cyclodextrin composite (PtNPs-GNPs-MWCNTs-β-CD) modified carbon glass electrode (GCE), was fabricated and used for the sensitive detection of folic acid (FA). The PtNPs-GNPs-MWCNTs-β-CD nanocomposite was easily prepared with an ultrasound-assisted assembly method, and it was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrochemical behavior of FA at PtNPs-GNPs-MWCNTs-β-CD/GCE was investigated in detail. Some key experimental parameters such as pH, amount of PtNPs-GNPs-MWCNTs-β-CD composite, and scan rate were optimized. A good linear relationship (R² = 0.9942) between peak current of cyclic voltammetry (CV) and FA concentration in the range 0.02-0.50 mmol L^-1 was observed at PtNPs-GNPs-MWCNTs-β-CD/GCE. The detection limit was 0.48 μmol L^-1 (signal-to-noise ratio = 3). A recovery of 97.55-102.96% was obtained for the determination of FA in FA pills (containing 0.4 mg FA per pill) at PtNPs-GNPs-MWCNTs-β-CD/GCE, indicating that the modified electrode possessed relatively high sensitivity and stability for the determination of FA in real samples.

Novel electrochemical sensors from poly[N-(ferrocenyl formacyl) pyrrole]@multi-walled carbon nanotubes nanocomposites for simultaneous determination of ascorbic acid, dopamine and uric acid

Novel multi-walled carbon nanotubes coated with poly[N-(ferrocenyl formacyl) pyrrole] (MWCNTs@PFFP) nanocomposites were prepared through the in situ oxidation polymerization reaction of N-(ferrocenyl formacyl) pyrrole in the presence of MWCNTs. The MWCNTs@PFFP nanocomposites were characterized by FT-IR, Raman, TGA, XRD, XPS, SEM and TEM techniques. The MWCNTs@PFFP nanocomposites were fabricated into novel electrochemical sensors for simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA). The electrochemical behavior of the MWCNTs@PFFP/GCE sensors was examined, and the parameters that influence electrochemical signals were optimized. The experimental results showed that the fabricated modified electrode sensors exhibited good sensitivity, selectivity, specificity, repeatability and a long lifetime, remaining the initial current of at least 92.5% after 15 days storage in air. The sensors possessed a linear response concentration range over 200-400 μM for AA, 2-16 μM for both DA and UA, and a limit of detection as low as 40.0, 1.1 and 7.3 × 10-1 μM for AA, DA and UA, respectively. They are expected to be used as a potential tool for the simultaneous detection of DA, AA and UA in the human body.

Composites of Bimetallic Platinum-Cobalt Alloy Nanoparticles and Reduced Graphene Oxide for Electrochemical Determination of Ascorbic Acid, Dopamine, and Uric Acid

The ultimate aim of this study is to produce a composite of bimetallic platinum-cobalt nanoparticles and reduced graphene oxide (Pt-Co@rGO) based biosensor for the detection of ascorbic acid (AA), dopamine (DA) and uric acid (UA). Those are biologically important molecules with the key functions for the human body. Pt-Co@rGO was synthesized using a microwave-assisted technique and utilized for the production of a highly sensitive and stable electrochemical biosensor. Detailed spectral XPS and Raman analysis, XRD, and TEM/HR-TEM characterization were also studied. Due to the superior activity and excellent conductivity of rGO, well-separated oxidation peaks of these biomolecules is proven by DPV (differential pulse voltammetry) and CV (cyclic voltammetry) measurements. The prepared Pt-Co@rGO-based biosensor showed high electrochemical activity, a broad linear response, high sensitivity, and acceptable limit of detection values for individual and simultaneous determination of AA, DA, and UA, under optimized conditions. The linear range of Pt-Co@rGO was found to be 170-200; 35-1500 and 5-800 µM for AA, DA, and UA, respectively. Moreover, the detection limit of the prepared composite was calculated as 0.345; 0.051; 0.172 µM for AA, DA, and UA, respectively. In the field of electrochemical biosensors, Pt-Co@rGO based sensor is highly promising due to its superior sensitivity and good selectivity properties.

Three-dimensional porous graphene oxide-maize amylopectin composites with controllable pore-sizes and good adsorption-desorption properties: Facile fabrication and reutilization, and the adsorption mechanism

Three-dimensional (3D) porous graphene oxide-maize amylopectin (GO-MA) composites with controllable pore-sizes composites in the range of 6-40 nm were prepared by facile hydrothermal-assisted assembly approaches. The morphologies, pore sizes, specific surface area (SSA) and compositions of GO-MA_x:y composites with and different GO-to-MA mass ratios (x:y) were characterized by scanning electron microscopy (SEM), N₂ adsorption-desorption isotherms, Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). To reveal the adsorption-desorption mechanism, effects of contact time, temperature, initial adsorbate concentration, pH value of the solution on the adsorption process were studied in detail. The adsorption capacities of 3D GO-MA_20:1 composite for organic contaminants including tert-butyl hydroquinone (TBHQ), p-aminophenol (PAP), p-nitrophenol (PNP), o-nitrophenol (MNP), hydroquinone (HQ), alizarin red S (ARS) and neutral red (NR) were 22.17, 116.4, 44.78, 36.96, 16.10, 39.92 and 24.23 mg g^-1, respectively. The adsorption capacities of GO-MA_30:1 composite for inorganic substances including Pb²⁺, Mn²⁺, Cr₂O₇^2-, Cd²⁺, Cu²⁺, Nd³⁺, La³⁺, Y³⁺, Yb³⁺ and Er³⁺ were 84.76, 7.92, 13.6, 17.64, 30.56, 25.52, 12.48, 16.96, 23.32 and 30.32 mg g^-1, respectively. In addition, GO-MA_x:y composites also exhibited high mechanical properties and good reusability. Consequently, GO-MA_x:y composites could be used as reusable adsorbents for removal/enrichment inorganic/organic substances in aqueous solutions.

A ruthenium(IV) disulfide based non-enzymatic sensor for selective and sensitive amperometric determination of dopamine

An electrochemical dopamine (DA) sensor has been fabricated by modifying a glassy carbon electrode (GCE) with ruthenium disulfide (RuS₂) nanoparticles (NPs). FESEM and TEM micrographs show the NPs to have an average size of ~45 nm. XRD, Raman and EDS, in turn, confirm the successful formation of cubic phased RuS₂ NPs. The modified GCE displays has attractive features of merit that include (a) an ultra-low detection limit (73.8 nM), (b) fast response time (< 4 s), (c) a low oxidation potential (0.25 V vs. Ag|AgCl), (d) excellent reproducibility and stability, (e) an electrochemical sensitivity of 18.4 μA μM^-1 cm^-2 and 1.8 μA.μM^-1.cm^-2 in the linear ranges from 0.1-10 μM of DA (R² = 0.97) and 10-80 μM of DA (R² = 0.99), respectively. The sensor exhibits excellent specificity over potential interferents like ascorbic acid, glucose and uric acid. The superior performance of the sensor is attributed to its high electrical conductivity, large electro-active surface, and large numbers of exposed catalytically active sites resulting from the presence of unreacted sulfur atoms. Graphical abstract A ruthenium disulfide modified electrochemical sensor material was obtained by single-step hydrothermal synthesis. Sensitive and highly selective detection of dopamine is demonstrated.

Highly-sensitive and selective determination of bisphenol A in milk samples based on self-assembled graphene nanoplatelets-multiwalled carbon nanotube-chitosan nanostructure

Graphene nanoplatelets (GNPs), multiwalled carbon nanotube (MWCNTs) and chitosan (CS) were self-assembled by a facile one-step hydrothermal reaction to obtain novel MWCNTs-CS enfolded GNPs (GNPs-MWCNTs-CS) composite. Fourier transform infrared (FT-IR) spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), UV-visible (UV-vis) absorption spectroscopy and zeta potential analysis were employed to characterize the morphology, surface composition, interaction, surface charge and stability of the GNPs-MWCNTs-CS composite. The electrochemical behaviors of GNPs-MWCNTs-CS composite modified glassy carbon electrode (GNPs-MWCNTs-CS/GCE) were investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The GNPs-MWCNTs-CS/GCE was used for fast and high sensitive determination of bisphenol A (BPA) by differential pulse voltammetry (DPV). Under the optimum conditions, the calibration curve obtained is linear for the current versus the BPA concentration in the range 0.1-100 μM with a detection limit of 0.05 nM (signal-to-noise ratio of 3, S/N = 3). The between-sensor reproducibility was 1.29% (n = 6) for 0.04 mM BPA. The proposed GNPs-MWCNTs-CS/GCE based sensor showed high resistance to interference, good repeatability and excellent reproducibility. Trace BPA in milk samples could also be reliably determined.