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A conventional resistance measured by two-point technique includes bulk and contact components. The contact resistance can be excluded by a four-point technique. A simultaneous use of the two- and four-point techniques allows to separate the total resistance between two electrodes for the contact and bulk parts. This approach has been used to analyze electrical coupling between metal electrodes of interdigitated structures and chemically sensitive coating formed by polyaniline (PANI). The polymer layers were deposited by electropolymerization on bare gold and platinum electrodes as well as on gold electrodes precoated by a self-assembled monolayer of 4-aminothiophenol. An exposure of the PANI films formed on interdigitated structures to gaseous HCl displays proportional changes of resistance measured by the two- and four- point techniques for platinum electrodes and for the precoated gold electrode, while a strong deviation from this proportionality was observed for the gold electrodes without precoating. The results demonstrate a high contribution of the contact resistance between PANI and bare gold electrodes into the resistance measured by the two-point technique. This contribution is small and therefore not measurable for bare platinum electrodes or for gold electrodes coated by 4-aminothiophenol. [Copyright &y& Elsevier]

A simple and sensitive label-free electrochemical biosensor for nimodipine was fabricated by using nitrogen-doped graphene (NGE) modified electrode. The surface morphology and electron transfer behavior of the modified electrodes were studied by scanning electron microscopy and electrochemical impedance spectroscopy, respectively. The electrochemical behaviors of nimodipine at the modified electrodes were investigated by using cyclic voltammetry. On the basis of the excellent electrocatalytic activity of NGE, an obvious cathodic peak of nimodipine could be caused at the NGE modified glassy carbon electrode (GCE), moreover, the electrochemical signal was greatly enhanced compared with the undoped graphene modified GCE. Under the optimized conditions, the present sensor showed excellent performances for nimodipine detection including wide linear range of 0.02–3 μM, low detection limit (9.1 nM), good selectivity to the general coexisted interferences, etc. The proposed biosensor was also successfully applied to validate its capability for the analysis of nimodipine in tablets. The work would promote the potential application of NGE as an excellent material in fabricating electrochemical sensors for chemical or biochemical analysis. [ABSTRACT FROM AUTHOR]

Abstract: The optical properties of electrochemically polymerized N-methylaniline are changed in the presence of gaseous hydrogen chloride. This effect was used for preparation of chemical sensors with transduction based on surface plasmon resonance. The interaction of hydrogen chloride with a two nm layer of poly-(N-methylaniline) on gold leads to the shift of the surface plasmon resonance. The analysis of the resonance spectra demonstrates that the effect is caused by an increase of the imaginary component of the refractive index; a minor contribution is also provided by film condensation leading to an increase of the real component of the refractive index and a decrease in the thickness. The effect is selective and quasi-reversible. The concentration dependence of the gas effect obeys Langmuir''s adsorption isotherm with a reciprocal value of the binding constant of 850 ± 160ppm. [Copyright &y& Elsevier]

The repeated outbreaks of coronavirus disease 2019 (COVID-19) have seriously threatened human health and economic development. Therefore, it is still significant to explore an efficient method for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) detection. Herein, we designed a dual-mode colorimetric and fluorescent immunosensor based on Fe2+ doped ZIF-67 derivatives-based composites, Pt nanoparticles loaded on CoFe nanoparticles embedded porous carbon-nitrogen hexahedron (CoFe/NC@Pt), for detecting SARS-CoV-2 nucleocapsid protein. The doping of Fe2+ generated bimetallic CoFe nanoparticles and formed more defects after pyrolysis. Meanwhile, the peroxides-like activity significantly improved owning to the synergistic effect of Co and Fe nanoparticles. Furthermore, loaded Pt nanoparticles not only provided more active sites, but also could couple antibodies by modifying functional groups. CoFe/NC@Pt exhibited good peroxidase-like activity and could catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) to blue oxides in the presence of H 2 O 2 , generating visible color signals. While the oxTMB could quench the fluorescence of carbon dots through inner filter effect, thereby providing fluorescent signals. The limit of detection were 0.022 ng/mL (colorimetric) and 0.018 ng/mL (fluorescent). And the specificity of the immunosensor was verified by comparing with other interferents. Therefore, this dual-mode immunosensor with good performance provided a reliable strategy for diagnosis of COVID-19. [Display omitted] • A dual-mode colorimetric and fluorescent immunosensor based on CoFe/NC@Pt nanozyme. • CoFe/NC@Pt exhibited good peroxidase-like activity. • The quenching mechanism of CDs was inner filter effect. • The accuracy of the sensor was improved by mutual authentication of double signals. [ABSTRACT FROM AUTHOR]

Highlights • Graphene oxide (GO) hydrogel was induced by synergistic effect of pyrrole and Co2+. • S and N are doped in graphene framework with doping/reducing agent of L -Cysteine. • 3D-sturctrue are obtained via evaporation induced drying without freeze-drying. • The 3D S/N-co-doped graphene show enhanced response for hydroquinone and catechol. Abstract Herein, a three-dimensional self-assembled hydrogel of graphene oxide sheets induced by pyrrole and Co2+ is described. Controlled shrinkage and sulfur/nitrogen co-doping occurred in the subsequent hydrothermal process. The obtained 3D sulfur/nitrogen co-doped graphene was further dried through direct evaporation, without freeze-drying. SEM images demonstrated a surface 3D structure with high roughness and composed of interconnected sheets. This newly fabricated 3D sulfur/nitrogen co-doped graphene was used in the simultaneous detection of catechol and hydroquinone with low detection limits of 0.28 and 0.15 μM, respectively. [ABSTRACT FROM AUTHOR]

A new furfuran-based rhodamine B fluorescent probe ( RBFF ) has been designed and synthesized, and its sensing behavior towards various metal ions was evaluated via UV–vis and fluorescence spectroscopic techniques. RBFF exhibits a highly sensitive and selective “turn-on” fluorescent response toward Fe 3+ ion and a fluorescence “turn-off” response when added B 4 O 7 2− to the RBFF -Fe 3+ in the EtOH/H 2 O solution (1:1, v/v, HEPES, 1 mM, pH 7.20). The detection limit of RBFF for Fe 3+ was calculated to be 0.025 μM. The fluorescence microscopy experiment suggested that RBFF could also be served as a biological fluorescence probe for the detection of Fe 3+ in human cervical carcinoma cells (HeLa). [ABSTRACT FROM AUTHOR]

Poly(3-methylthiophene)/nitrogen doped graphene modified glassy carbon electrode (P3MT/NGE/GCE) was prepared through simple drop-casting of NGE, followed by the electrodeposition of P3MT film for sensitive electrochemical determination of trace amounts of pesticide phoxim. The morphology and structure of P3MT/NGE/GCE were characterized by scanning electron microscopy; the electrochemical behavior of P3MT/NGE/GCE was investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and chronocoulometry. The preparation conditions of modified electrode, the dropping amount of NGE and polymerization laps of P3MT, were optimized. Under the optimized conditions, the reduction peak current of cyclic voltammetric curve varied linearly with the concentration of phoxim over the two linear ranges of 0.02–0.2 μM and 0.2–2.0 μM, and the low detection limit was 6.4 nM according to lower linear range (S/N = 3). The sensor also exhibited excellent selectivity to phoxim over a wide range of possible interferents including ions, biomolecules, common environment pollutants and widely-used organophosphate pesticides. Moreover, the satisfactory practical feasibility of the sensor was evaluated by the environmental phoxim measurements. [ABSTRACT FROM AUTHOR]

Highlights: [•] PEDOT/NGE modified electrode was fabricated via CV method using nitrogen-doped graphene (NGE) coated GCE. [•] Highly reversible and well-separated redox couples of hydroquinone (HQ) and catechol (CT) were obtained at PEDOT/NGE/GCE. [•] A facile electrochemical sensor based on PEDOT/NGE/GCE was developed for simultaneous detection of HQ and CT. [Copyright &y& Elsevier]

Highlights: [•] A PEDOT/GO modified electrode was obtained via potentiostatic method using GO as a dopant. [•] The electrochemical behavior of acetaminophen (AP) showed highly reversibility. [•] An electrochemical sensor based on PEDOT/GO was developed for the detection of AP. [•] The sensor shows high sensitivity and selectivity to AP. [ABSTRACT FROM AUTHOR]

Highlights: [•] The stable blue fluorescent GQDs were obtained via a hydrothermal method using reoxidized GO. [•] The fluorescence of GQDs were effectively quenched by Cu(II). [•] A facile fluorescent sensor based on GQDs has been developed for the detection of Cu2+ ions. [•] The sensor shows high sensitivity and selectivity to Cu2+ ions. [ABSTRACT FROM AUTHOR]

Abstract: Polycarbazole (PCz) and PCz/chemically reduced graphene oxide (PCz/CRGO) modified glassy carbon electrodes (GCEs) were fabricated by a simple procedure and characterized. The modified electrodes based on PCz or PCz/CRGO were used as sensing electrodes for the voltammetric investigation of imidacloprid (IMI) insecticides. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used as sensing techniques. Both of modified electrodes have good electrocatalytic activity toward the reduction of IMI. The presence of CRGO enhanced the electrochemical signal of the modified glassy carbon electrode (GCE), which increased the sensitivity of the sensor. The limit of detection (LOD) and the limit of quantitation (LOQ) values for CV are 0.22μM and 0.74μM, respectively; while those values for DPV are 0.44μM and 1.52μM, respectively. [Copyright &y& Elsevier]