Electrochemical Sensors based on Gold-Silver Core-Shell Nanoparticles Combined with a Graphene/PEDOT:PSS Composite Modified Glassy Carbon Electrode for Paraoxon-ethyl Detection.

Herein, a nonenzymatic detection of paraoxon-ethyl was developed by modifying a glassy carbon electrode (GCE) with gold-silver core-shell (Au-Ag) nanoparticles combined with the composite of graphene with poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). These core-shell nanoparticles (Au-Ag) were synthesized using a seed-growth method and characterized using UV-vis spectroscopy and high-resolution transmission electron microscopy (HR-TEM) techniques. Meanwhile, the structural properties, surface morphology and topography, and electrochemical characterization of the composite of Au-Ag core-shell/graphene/PEDOT:PSS were analyzed using infrared spectroscopy, field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), and electrochemical impedance spectroscopy (EIS) techniques. Moreover, the proposed sensor for paraoxon-ethyl detection based on Au-Ag core-shell/graphene/PEDOT:PSS modified GCE demonstrates good electrochemical and electroanalytical performance when investigated with cyclic voltammetry (CV), differential pulse voltammetry (DPV), and chronoamperometry techniques. It was found that the synergistic effect between Au-Ag core-shell nanoparticles and the composite of graphene/PEDOT:PSS provides a higher conductivity and enhanced electrocatalytic activity for paraoxon-ethyl detection at an optimum pH of 7. At pH 7, the proposed sensor for paraoxon-ethyl detection shows a linear range of concentrations from 0.2 to 100 µM with a limit of detection of 10 nM and high sensitivity of 3.24 µA µM-1 cm-2. In addition, the proposed sensor for paraoxon-ethyl confirmed good reproducibility, with the possibility of being further developed as a disposable electrode. This sensor also displayed good selectivity in the presence of several interfering species such as diazinon, carbaryl, ascorbic acid, glucose, nitrite, sodium bicarbonate, and magnesium sulfate. For practical applications, this proposed sensor was employed for the determination of paraoxon-ethyl in real samples (fruits and vegetables) and showed no significant difference from the standard spectrophotometric technique. In conclusion, this proposed sensor might have a potential to be developed as a platform of electrochemical sensors for pesticide detection.

Effect of Aspect Ratio of a Gold-Nanorod-Modified Screen-Printed Carbon Electrode for Carbaryl Detection in Three Different Samples of Vegetables.

In this study, three different sizes of gold nanorods (AuNRs) were synthesized using the seed-growth method by adding various volumes of AgNO3 as 400, 600, and 800 µL into the growth solution of gold nanoparticles. Three different sizes of AuNRs were then characterized using UV-vis spectroscopy, high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) patterns, and atomic force microscopy (AFM) to investigate the surface morphology, topography, and aspect ratios of each synthesized AuNR. The aspect ratios from the histogram of size distributions of three AuNRs as 2.21, 2.53, and 2.85 can be calculated corresponding to the addition of AgNO3 volumes of 400, 600, and 800 µL. Moreover, each AuNR in three different aspect ratios was drop-cast onto the surface of a commercial screen-printed carbon electrode (SPCE) to obtain three different SPCE-modified AuNRs (SPCE-A400, SPCE-A600, and SPCE-A800, respectively). All SPCE-modified AuNRs were then evaluated for their electrochemical behavior using cyclic voltammetry and electrochemical impedance spectroscopy (EIS) techniques and the highest electrochemical performance was shown as the order of magnitude of SPCE-A400 > SPCE-A600/SPCE-A800. The reason for the highest electrocatalytic activity of SPCE-A400 might be due to the smallest particle size and uniform distribution of AuNRs ∼ 2.2, which enhanced the charge transfer, thus providing the highest electroactive surface area (0.6685 cm2) compared to other electrodes. These results also confirm that the sensing mechanism for all SPCE-modified AuNRs is controlled by diffusion phenomena. In addition, the optimum pH was obtained as 4 for carbaryl detection for all SPCE-modified AuNRs with the highest current shown by SPCE-A400. Furthermore, SPCE-A400 has the highest fundamental parameters (surface coverage, catalytic rate constant, electron transfer rate constant, and adsorption capacity) for carbaryl detection, which were investigated using cyclic voltammetry and chronoamperometric techniques. The electroanalytical performances of all SPCE-modified AuNRs for carbaryl detection were also investigated with SPCE-A400 displaying the best performance among other electrodes in terms of its linearity (0.2-100 µM), limit of detection (LOD) ∼ 0.07 µM, and limit of quantification (LOQ) ∼ 0.2 µM. All SPCE-modified AuNRs were also subsequently evaluated for their stability, reproducibility, and selectivity in the presence of interfering species such as NaNO2, NH4NO3, Zn(CH3CO2)2, FeSO4, diazinon, and glucose and show reliable results as depicted from %RSD values less than 3%. At last, all SPCE-modified AuNRs have been employed for carbaryl detection using a standard addition technique in three different samples of vegetables (cabbage, cucumber, and Chinese cabbage) with its results (%recovery ≈ 100%) within the acceptable analytical range. In conclusion, this work demonstrates the great potential of a disposable device based on an AuNR-modified SPCE for rapid detection and high sensitivity in monitoring the concentration of carbaryl as a residual pesticide in vegetable samples.

Electrochemical Sensors Based on a Composite of Electrochemically Reduced Graphene Oxide and PEDOT:PSS for Hydrazine Detection.

In this study, hydrazine sensors were developed from a composite of electrochemically reduced graphene oxide (ErGO) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), deposited onto a glassy carbon electrode (GCE). The structural properties, electrochemical characterization, and surface morphologies of this hydrazine sensor were characterized by Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM). In addition, the proposed hydrazine sensor also demonstrates good electrochemical and analytical performance when investigated using cyclic voltammetry (CV), differential pulse voltammetry (DPV), and amperometry techniques under optimal parameters. Using these investigated parameters, DPV and amperometry were chosen as techniques for hydrazine measurements and showed a linear range of concentration in the range of 0.2-100 µM. The obtained limits of detection and limits of quantitation for hydrazine measurements were 0.01 and 0.03 µM, respectively. In addition, the proposed sensor demonstrated good reproducibility and stability in hydrazine measurements in eight consecutive days. This fabricated hydrazine sensor also exhibited good selectivity against interference from Mg2+, K+, Zn2+, Fe2+, Na+, NO2 -, CH3COO-, SO4 2-, Cl-, ascorbic acid, chlorophenol, and triclosan and combined interferences, as well as it depicted %RSD values of less than 5%. In conclusion, this proposed sensor based on GCE modified with ErGO/PEDOT:PSS displays exceptional electrochemical performance for use in hydrazine measurements and have the potential to be employed in practical applications.

Selective non-enzymatic uric acid sensing in the presence of dopamine: electropolymerized poly-pyrrole modified with a reduced graphene oxide/PEDOT:PSS composite.

A highly selective electrochemical sensor based on a molecularly imprinted polymer (MIP) to be developed for uric acid detection in the presence of dopamine as an interference molecule was demonstrated in this study. This non-enzymatic uric acid sensor was developed by electropolymerizing poly-pyrrole onto a composite of electrochemically reduced graphene oxide (ErGO) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) on a glassy carbon electrode (GCE) to give MIP/ErGO/PEDOT:PSS electrodes. The structural properties, surface morphology, and electrochemical interface of this fabricated uric acid sensor was then characterized using infrared spectroscopy, scanning electron microscopy, atomic force microscopy, and electrochemical impedance spectroscopy. This fabricated non-enzymatic electrochemical sensor (MIP/ErGO/PEDOT:PSS modified GCE) also showed excellent analytical performance at the optimum ratio of monomer/template concentration and optimized electropolymerization conditions, for example in the low concentration range of 0.1-100 µM with a detection limit of 0.05 µM towards uric acid detection in the presence of dopamine. Furthermore, this fabricated uric acid sensor also exhibited good reproducibility and stability for uric acid measurements in the presence of dopamine for 5 consecutive days. In addition, this sensor demonstrates highly selective detection of uric acid in the presence of several interfering species such as dopamine, urea, glucose, magnesium ions, and ascorbic acid. This fabricated uric acid sensor ultimately showed satisfactory uric acid measurement in samples of human urine and is expected to be used in early-stage disease diagnosis.

Multifunctional SiC@SiO2 Nanofiber Aerogel with Ultrabroadband Electromagnetic Wave Absorption.

Traditional ceramic materials are generally brittle and not flexible with high production costs, which seriously hinders their practical applications. Multifunctional nanofiber ceramic aerogels are highly desirable for applications in extreme environments, however, the integration of multiple functions in their preparation is extremely challenging. To tackle these challenges, we fabricated a multifunctional SiC@SiO2 nanofiber aerogel (SiC@SiO2 NFA) with a three-dimensional (3D) porous cross-linked structure through a simple chemical vapor deposition method and subsequent heat-treatment process. The as-prepared SiC@SiO2 NFA exhibits an ultralow density (~ 11 mg cm- 3), ultra-elastic, fatigue-resistant and refractory performance, high temperature thermal stability, thermal insulation properties, and significant strain-dependent piezoresistive sensing behavior. Furthermore, the SiC@SiO2 NFA shows a superior electromagnetic wave absorption performance with a minimum refection loss (RLmin) value of - 50.36 dB and a maximum effective absorption bandwidth (EABmax) of 8.6 GHz. The successful preparation of this multifunctional aerogel material provides a promising prospect for the design and fabrication of the cutting-edge ceramic materials.

Voltammetric detection of vitamin B1 (thiamine) in neutral solution at a glassy carbon electrode via in situ pH modulation.

Voltammetric analysis is often dependent on pH and on the addition of buffer reagents to optimise the analytical procedure. This approach is not always possible for in situ analytical measurements, for example when studying biological fluids or ingredients in food. Therefore, a method is proposed herein, which employs a working electrode to do both, that is, to locally modulate the pH value and to measure the analytical response. As a model system, thiamine (vitamin B1) is detected in aqueous KCl with a pH modulation brought about with negative potentials applied to the working electrode. Interferences from ascorbic acid and uric acid are considered. Exploratory data are presented and methods for improving the detection limit are suggested. Their potential for applications in electroanalysis (and in a broader range of processes) is discussed and the detection of thiamine in rice is demonstrated.

Bacteriophage M13 Aggregation on a Microhole Poly(ethylene terephthalate) Substrate Produces an Anionic Current Rectifier: Sensitivity toward Anionic versus Cationic Guests.

Bacteriophage material (M13, wild-type) deposited as a film onto a poly(ethylene terephthalate) (PET) substrate (6 µm thick with a 20 µm diameter laser-drilled microhole) has been investigated for ion conductivity and ionic current rectification effects for potential applications in membranes. The M13 aggregate membrane forms under acidic conditions (in aqueous 10 mM acids) and behaves like a microporous anion conductor with micropores defined by the packing of cylindrical virus particles. Asymmetric deposition on the PET film substrate in conjunction with semipermeability leads to anionic diode behavior. Typical rectification ratio values are around 10 (determined at ±1 V) in aqueous 10 mM acids. Cationic guest species (aqueous Cu2+, Co2+, Ag+) consistently lead to a rectification minimum at 0.5 mM guest concentration. In contrast, anionic guest species (indigo carmine) lead to a similar rectification minimum already at 5 µM concentration. The behavior is proposed to be associated with cation exclusion effects on transport.

Voltammetric characterisation of diferrocenylborinic acid in organic solution and in aqueous media when immobilised into a titanate nanosheet film.

Diferrocenylborinic acid (Fc2BOH, 1) has been synthesized in good yield via an improved synthetic path. Characterisation by nuclear magnetic resonance (NMR), mass spectrometry (HRMS), infrared spectroscopy (FTIR), X-ray crystallography, and by electrochemical methods reveal two one-electron oxidation processes for the two electronically coupled ferrocenyl moieties. The oxidation of 1 dissolved in organic media is contrasted to the oxidation of 1 in aqueous environments (by incorporation of 1 into a lamellar film of 2D titanate nanosheets on a glassy carbon electrode). Data from cyclic voltammetry and from square wave voltammetry suggest that the bridging boron can bind to nucleophiles (hydroxide, fluoride) upon oxidation of the ferrocenyl groups. A multi-pathway ECE reaction scheme is proposed. Potential applications in sensing are discussed.

Cationic Rectifier Based on a Graphene Oxide-Covered Microhole: Theory and Experiment.

Cation transport through nanochannels in graphene oxide can be rectified to give ionic diode devices for future applications, for example, in desalination. A film of graphene oxide is applied to a 6 µm thick poly(ethylene terephthalate) substrate with a 20 µm diameter microhole and immersed in aqueous HCl solution. Strong diode effects are observed even at high ionic strength (0.5 M). Switching between open and closed states, microhole size effects, and time-dependent phenomena are explained on the basis of a simplified theoretical model focusing on the field-driven transport within the microhole region. In aqueous NaCl, competition between Na+ transport and field-driven heterolytic water splitting is observed but shown to be significant only at low ionic strength. Therefore, nanostructured graphene oxide is demonstrated to exhibit close to ideal behavior for future application in ionic diode desalination of seawater.

Extraction of hydrophobic analytes from organic solution into a titanate 2D-nanosheet host: Electroanalytical perspectives.

Titanate nanosheets (single layer, typically 200 nm lateral size) deposited from aqueous colloidal solution onto electrode surfaces form lamellar hosts that bind redox active molecular redox probes. Here, hydrophobic redox systems such as anthraquinone, 1-amino-anthraquinone, deca-methylferrocene, 5,10,15,20-tetraphenyl-21H,23H-porphine manganese (III) chloride (TPPMnCl), and α-tocopherol are shown to bind directly from cyclopentanone solution (and from other types of organic solvents) into the titanate nanosheet film. For anthraquinone derivatives, stable voltammetric responses are observed in aqueous media consistent with 2-electron 2-proton reduction, however, independent of the pH of the outside solution phase environments. For decamethylferrocene a gradual decay of the voltammetric response is observed, but for TPPMnCl a more stable voltammetric signal is seen when immersed in chloride containing (NaCl) electrolyte. α-Tocopherol exhibits chemically irreversible oxidation and is detected with 1 mM-20 mM linear range and approximately 10-3 M concentration limit of detection. All redox processes exhibit an increase in current with increasing titanate film thickness and with increasing external electrolyte concentration. This and other observations suggest that important factors are analyte concentration and mobility within the titanate host, as well as ion exchange between titanate nanosheets and the outside electrolyte phase to maintain electroneutrality during voltammetric experiments. The lamellar titanate (with embedded tetrabutyl-ammonium cations) behaves like a hydrophobic host (for hydrophobic redox systems) similar to hydrophobic organic microphase systems. Potential for analytical applications is discussed.