Picomolar-sensitive impedimetric sensor for salivary calcium analysis at POC based on SAM of Schiff base–modified gold electrode

Journal of Solid State Electrochemistry - Analysis of saliva is a potential diagnostic tool in the management of human diseases. Analysis of saliva in healthy individuals is vital to comparison in...     

An ionophore–Nafion modified bismuth electrode for the analysis of cadmium(II)

A novel ionophore–Nafion modified bismuth electrode is described for sensitive and selective anodic stripping analysis of cadmium(II). The electrode is prepared by coating the glassy carbon electrode with the cadmium ionophore N,N,N′,N′-tetrabutyl-3,6-dioxaoctanedi(thioamide) and Nafion composite. Bismuth is deposited in situ on the electrode surface by plating simultaneously with cadmium in sample solution. Numerous key variables affecting the current response of cadmium have been optimized. The electrode has a linear concentration range of 0.5–10 nM with a deposition time of 180 s. The detection limit is 1.3 × 10^?10 M and the relative standard deviations for 0.5 and 7 nM cadmium are 6.5% and 4.5%, respectively. The proposed electrode shows excellent selectivity over other heavy metals, such as copper, lead and indium. The attractive performance of such electrode offers a feasible way to monitor trace cadmium(II) rapidly and precisely in complex matrixes.     

Reversible electrochemistry of DNA on multi-walled carbon nanotube modified electrode

Calf thymus DNA was electrochemically oxidized at a multi-walled carbon nanotube modified electrode. The potentials for DNA oxidation at pH 7.0 were 0.71 and 0.81 V versus SCE, corresponding to the oxidation of guanine and adenine residues, respectively. The initial 6e-oxidation of adenine, observed in the first scan, resulted a quasi-reversible 2e-redox process of the oxidation product in the following scans.     

Direct electrochemistry of glucose oxidase on a graphite nanosheet–Nafion composite film modified electrode

The direct electrochemistry of glucose oxidase (GOD) immobilized in a modified electrode based on a composite film of exfoliated graphite nanosheets (GNSs) and Nafion has been investigated for the first time. Direct electron communication between GOD and the electrode was achieved with a fast electron transfer rate (12.6 s^?1). In addition, the bioactivity of GOD was retained after immobilization in the composite film and glucose could be determined based on the decrease of the electrocatalytic response of the reduced form of GOD to dissolved oxygen. The resulting biosensor exhibited higher sensitivity (3.4 μA mM^?1). Considering much lower cost of GNSs and ready preparation from graphite, the GNSs-based modified electrode described here is superior to the carbon nanotubes (CNTs)-based modified electrodes and should have wide applications in third-generation biosensors, bioelectronics and electrocatalysis.     

Infrared reflection–absorption properties of platinum nanoparticle films on metal electrode substrates: control of anomalous optical effects

A simple method is described for preparing ultrathin films of carbon-supported platinum nanoparticles on gold and other substrates for examination by electrochemical infrared reflection–absorption spectroscopy (EC-IRAS) in which the anomalous “anti-absorbance” properties commonly observed for metal particle arrays can be minimized as well as controlled. The tactic involves physical deposition as an aqueous suspension, followed by drying and thorough rinsing. The Pt monolayer-level films prepared by this means exhibit largely positive unipolar absorbance bands for CO adlayers, whereas an anomalous inverted band component appears for thicker films, especially those featuring high Pt nanoparticle loadings. These findings demonstrate the value of the film preparation tactic for exploring the electrocatalytic properties of C/Pt materials by using combined electrochemical and EC-IRAS measurements.     

Sensitive electrochemical determination of calcium dobesilate on the carbon–iron nanoparticle modified glassy carbon electrode

A carbon–iron nanoparticle modified glassy carbon electrode (CIN-GCE) has been developed for the determination of calcium dobesilate (CD) in pharmaceutical formulations. The CINs were characterized by Transmission electron microscopy and X-ray diffraction. It was found that the CIN has strong electrocatalytic effect for CD and leads to a greatly improved anodic detection of CD including higher sensitivity and better reproducibility. A detection limit of 2.0 × 10⁻⁷ M (S/N = 3) was obtained. The proposed CIN-GCE was applied to detect CD in pharmaceutical formulations with satisfactory results. The proposed CIN electrochemical sensing platform holds great promise for simple, rapid and accurate detection of CD in pharmaceutical formulations.     

Sensitive electrochemical sensor of tryptophan based on Ag@C core–shell nanocomposite modified glassy carbon electrode

We here reported a simple electrochemical method for the detection of tryptophan (Trp) based on the Ag@C modified glassy carbon (Ag@C/GC) electrode. The Ag@C core–shell structured nanoparticles were synthesized using one-pot hydrothermal method and characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), and Fourier transform-infrared spectroscopy (FTIR). The electrochemical behaviors of Trp on Ag@C/GC electrode were investigated and exhibited a direct electrochemical process. The favorable electrochemical properties of Ag@C/GC electrode were attributed to the synergistic effect of the Ag core and carbon shell. The carbon shell cannot only protect Ag core but also contribute to the enhanced substrate accessibility and Trp-substrate interactions, while nano-Ag core can display good electrocatalytic activity to Trp at the same time. Under the optimum experimental conditions the oxidation peak current was linearly dependent on the Trp concentration in the range of 1.0 × 10⁻⁷ to 1.0 × 10⁻⁴ M with a detection limit of 4.0 × 10⁻⁸ M (S/N = 3). In addition, the proposed electrode was applied for the determination of Trp concentration in real samples and satisfactory results were obtained. The technique offers enhanced sensitivity and may trigger the possibilities of the Ag@C nanocomposite towards diverse applications in biosensor and electroanalysis.     

Investigation of electrochemical behavior of anisomycin on gold electrode followed by HPLC–MS/MS analysis

Anisomycin is an immunosuppressant in low doses (< 0.1 μM) with possible application in treatment of some autoimmune diseases and in inhibiting transplantation rejection. Anisomycin suppresses malignant tumor cell growth and affects memory. For the first time it was the subject of the electrochemical investigations by cyclic voltammetry and square wave voltammetry on gold electrode in 0.05 M NaHCO₃ using its electrochemical activity. The cyclic voltammetry experiments at different sweep rates show that electrochemical process is irreversible and diffusion controlled. Based on square wave voltammetry measurements, the calculated values of LOD and LOQ were 1 and 4 nM (in the absence of biological fluid), as well as 2 and 6 nM (in the presence of spiked urine) indicating the high sensitivity of the proposed electroanalytical method. High performance liquid chromatography–tandem mass spectrometry was a reference method for quantification of anisomycin and served for structural identification of its hydrolysis product (deacetylanisomycin).     

Electroreduction of α-glucose on CNT/graphite electrode modified by Zn and Zn–Fe alloy

Electroreduction of -glucose to form sorbitol on Zn/CNTs and Zn alloy/CNTs electrodes has been investigated in this paper. Carbon nanotubes (CNTs) used in this paper are grown directly on graphite disks by chemical vapor deposition. Zn and Zn alloy are electrodeposited on the activated CNTs/graphite electrode by pulse galvanostatic method. The micrographs of Zn/CNTs and Zn alloy/CNTs electrodes are characterized by scanning electron microscopy. The results show that the current efficiency of -glucose reduction on CNTs electrodes is much better than that on flat Zn electrodes. The order of the current efficiency on different electrodes is as follows: Zn/CNT (0.58) Zn–Fe/CNT (0.57)>Zn–Ni/CNT (0.43) Zn/graphite (0.42)>Zn (0.40). It indicates that CNTs have good potential application in electrosynthesis. Additionally, effects of some operating parameters, such as pH, temperature and -glucose concentration, on the current efficiency of -glucose reduction are also discussed.     

Comparison of performances of bioanodes modified with graphene oxide and graphene–platinum hybrid nanoparticles

The performances of graphene oxide (GO) and graphene–platinum hybrid nanoparticles (Gr-Pt hybrid NPs) were compared for biofuel cell (BFC) systems. This is the first study that constitutes these nanomaterials in BFC systems. For this purpose, fabricated bioanodes were combined with laccase modified biocathode in a single cell membraneless BFC. Power and current densities of these systems were calculated as 2.40 μW cm^− 2 and 211.90 μA cm^− 2 for GO based BFC and 4.88 μW cm^− 2 and 246.82 μA cm^− 2, for Gr-Pt hybrid NPs based BFC. As a result, a pioneer study which demonstrates the effective performances of combination of graphene with Pt was conducted.     

Development of a stable biosensor based on a SiO2 nanosheet–Nafion–modified glassy carbon electrode for sensitive detection of pesticides

SiO₂ nanosheets (SNS) have been prepared by a chemical method using montmorillonite as raw material and were characterized by scanning electron microscopy and X-ray diffraction. SiO₂ nanosheet–Nafion nanocomposites with excellent conductivity, catalytic activity, and biocompatibility provided an extremely hydrophilic surface for biomolecule adhesion. Chitosan was used as a cross-linker to immobilize acetylcholinesterase (AChE), and Nafion was used as a protective membrane to efficiently improve the stability of the AChE biosensor. The AChE biosensor showed favorable affinity for acetylthiocholine chloride and catalyzed the hydrolysis of acetylthiocholine chloride with an apparent Michaelis–Menten constant of 134 μM to form thiocholine, which was then oxidized to produce a detectable and fast response. Based on the inhibition by pesticides of the enzymatic activity of AChE, detection of the amperometric response from thiocholine on the biosensor is a simple and effective way to biomonitor exposure to pesticides. Under optimum conditions, the biosensor detected methyl parathion, chlorpyrifos, and carbofuran at concentrations ranging from 1.0?×?10^?12 to 1?×?10^?10?M and from 1.0?×?10^?10 to 1?×?10^?8?M. The detection limits for methyl parathion, chlorpyrifos, and carbofuran were 5?×?10^?13?M. The biosensor developed exhibited good sensitivity, stability, reproducibility, and low cost, thus providing a new promising tool for analysis of enzyme inhibitors.

A survey on the effect of ionic liquid on electrochemical behavior and electrocatalytic activity of a phosphomolybdic acid-ionic liquid-MWCNT–modified glassy carbon electrode

Journal of Solid State Electrochemistry - The surface of a glassy carbon electrode modified with multi-walled carbon nanotubes (GCE/MWCNTs) was coated with a layer of phosphomolybdic acid (PMo12)...     

Electrochemical determination of quercetin by self-assembled platinum nanoparticles/poly(hydroxymethylated-3,4-ethylenedioxylthiophene) nanocomposite modified glassy carbon electrode

A simple and sensitive platinum nanoparticles/poly(hydroxymethylated-3,4-ethylenedioxylthiophene)nanocomposite(PtNPs/PEDOT-MeOH) modified glassy carbon electrode(GCE) was successfully developed for the electrochemical determination of quercetin.Scanning electron microscopy and energy dispersive X-ray spectroscopy results indicated that the PtNPs were inserted into the PEDOTMeOH layer.Compared with the bare GCE and poly(3,4-ethylenedioxythiophene)(PEDOT) electrodes,the PtNPs/PEDOT-MeOH/GCE modified electrode exhibited a higher electrocatalytic ability toward the oxidation of quercetin due to the synergic effects of the electrocatalytic activity and strong adsorption ability of PtNPs together with the good water solubility and high conductivity of PEDOT-MeOH.The electrochemical sensor can be applied to the quantification of quercetin with a linear range covering0.04-91 μmol L~(-1) and a low detection limit of 5.2 nmol L~(-1).Furthermore,the modified electrode also exhibited good reproducibility and long-term stability,as well as high selectivity.     

Electrochemistry of adenosine-5′-diphosphate on ionic liquid modified carbon electrode and its detection

A sensitive electrochemical method was proposed for the determination of adenosine-5′-diphosphate (ADP) on an ionic liquid (IL) 1-(3-chloro-2-hydroxy-propyl)-3-methylimidazole chloride modified carbon paste electrode (CPE) in a pH 4.5 Britton-Robinson (B-R) buffer solution. Compared with CPE, IL modified CPE (CILE) showed strong electrocatalytic ability to promote the electrochemical oxidation of ADP. A well-defined irreversible oxidation peak of ADP appeared at +1.381 V with an adsorption-controlled process, which was due to the presence of high conductive IL on the electrode. The experimental conditions were optimized and the electrochemical parameters of ADP were calculated with the electron transfer coefficient (α) as 0.293, the electron transfer number (n) as 1.23, the apparent heterogeneous electron transfer rate constant (k ^s) as 3.325 × 10^?6 s^?1 and the surface coverage (Γ_T) as 0.92 × 10^?8 mol/cm². Under the optimum conditions, the oxidation peak current was linear to ADP concentration in the range from 3.0 to 1000.0 μmol/L with the detection limit as 2.78 μmol/L (3σ) by differential pulse voltammetry. The CILE also eliminated the interferences of commonly coexisting substances and was successfully applied to detect the ADP artificial samples.     

Simultaneous determination of uric acid and ascorbic acid at a ferrocenium–thioglycollate modified electrode

Self-assembled monolayers (SAMS) of chemisorbed thioglycollate on a gold electrode surface have been used as a base interface for the electrostatic adsorption of ferrocenium ion. Electrochemical impedance spectra (EIS) and cyclic voltammetry (CV) were used to evaluate the electrochemical properties of the supramolecular film. The bare gold electrode failed to distinguish the oxidation peaks of ascorbic acid (AA) and uric acid (UA) in phosphate buffer solution (PBS, pH 7.0), while the ferricinium–thioglycollate modified electrode could separate them efficiently. In differiential pulse voltammetric measurements, the prepared gold electrode could separate AA and UA signals, allowing the simultaneous determination of AA and UA. Under optimal conditions and within the linear range of 1.0 × 10⁻⁶ to 5.0 × 10⁻⁴ M, the detection limits of AA and UA achieved were 2.0 × 10⁻⁷ and 1.0 × 10⁻⁷ M, respectively. The applicability of the prepared electrode was demonstrated by measuring AA and UA in human urine without any pretreatment. Figure Fabrication process for the modified electrode     

Non-enzymatic sensing of hydrogen peroxide using a glassy carbon electrode modified with a composite consisting of chitosan‐encapsulated graphite and platinum nanoparticles

We report on a non-enzymatic hydrogen peroxide (H₂O₂) sensor which makes use of a nanocomposite consisting of platinum nanoparticles (PtNPs) and chitosan-encapsulated graphite (graphite-CS). The composite was prepared by sonication of pristine graphite in chitosan (CS) in 5 % acetic acid. The PtNP decorated graphite-CS (graphite-CS/PtNPs) composite was prepared by electrodeposition of PtNPs on the graphite-CS modified glassy carbon electrode. The graphite-CS/PtNP composite was characterized by scanning electron microscopy, elemental analysis and FTIR spectroscopy. The modified electrode displays an enhanced reduction peak current for H₂O₂ when compared with electrodes modified with graphite/PtNPs and PtNPs. The modified electrode exhibits excellent electrocatalytic activity towards the reduction of H₂O₂, and the amperometric response is linear over the concentration range from 0.25 to 2890 μM. The sensitivity and the detection limit are 0.465 μA?μM￣¹ ? cm￣² and 66 nM, respectively. The sensor shows fast response (3 s) in detecting H₂O₂. It is also highly selective in the presence of potentially interfering compounds, and may therefore be used as a feasible platform for sensing H₂O₂ in real samples.

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Graphical abstract Preparation of graphite-CS/PtNP composite, and its application to electrochemical reduction of H₂O₂.

     

Density functional theory and topological analysis on the hydrogen bonds in cysteine–propanoic acid complexes

The complexes formed via hydrogen bonding interactions between cysteine and propanoic acid have been studied at the density three-parameter hybrid functional DFT-B3LYP/6-311++G(d,p) level regarding their geometries, energies, vibrational frequencies, and topological features of the electron density. The quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analysis was employed to elucidate the interaction characteristics in cysteine–propanoic acid (Cys–Prop) complexes. More than 10 kinds of hydrogen bonds (H-bonds) including intra- and inter-molecular H-bonds have been found in Cys–Prop complexes. The results show that both the strength of H-bonds and the deformation are important factors for the stability of Cys–Prop complexes. The strongest H-bonds (O2H^A···O1^B and O2H^A···O1^B) exist in the most stable Cys–Prop complex. The stronger H-bonds formed between hydroxyl and O (or N) atom usually stronger than those involve C (or S) atom. Relationships between the electron density (ρ) of BCP and H-bond length as well as the Fock matrix element (F _ij) has also been investigated and used to study the nature of H-bonds. Moreover, the results show that the change of the bond length linearly correlates with the corresponding frequency shift.     

The immobilization of Cytochrome c on MWNT–PAMAM–Chit nanocomposite incorporated with DNA biocomposite film modified glassy carbon electrode for the determination of nitrite

A novel and sensitive biosensor was developed for the determination of nitrite. Firstly, multi-walled carbon nanotubes–poly(amidoamine)–chitosan (MWNT–PAMAM–Chit) nanocomposite along with the incorporation of DNA was used to modify the glassy carbon electrode. Then the immobilization of Cyt c was accomplished using electrochemical deposition method by consecutive cyclic voltammetry (CV) scanning in a neutral Cyt c solution. CV behaviors of the modified electrodes showed that the MWNT–PAMAM–Chit nanocomposite is a good platform for the immobilization of DNA and Cyt c in order, at the same time, an excellent promoter for the electron transfer between Cyt c and the electrode. At high potential, the immobilized Cyt c could be further oxidized into highly reactive Cyt c π-cation by two-step electrochemical oxidation, which could oxidize NO₂ ⁻ into NO₃ ⁻ in the solution. Therefore, a nitrite biosensor based on the biocatalytic oxidation of the immobilized Cyt c was fabricated, which showed a fast response to nitrite (less than 5 s). The linear range of 0.2–80 μM and a detection limit of 0.03 μM was obtained. Finally, the application in food analysis using sausage as testing samples was also investigated.     

Structure and properties of Pd–Mn hexaaluminate catalysts modified with platinum for the high-temperature oxidation of methane

The effect of Pt additives on the catalytic characteristics of a Pd-containing catalyst based on manganese hexaaluminate was studied. It was found that the bimetallic PtPd-containing catalysts based on MnLaAl₁₁O₁₉ with the Pt/Pd atomic ratio smaller than 0.25 exhibited a comparable or somewhat smaller activity in the methane oxidation, but their stability at elevated temperatures and gas flow rates was higher than that of the Pd-based catalyst. The state of the active constituent of the resulting catalysts was investigated. Main correlations between the state of the active component and the catalytic activity were revealed.