Graphene Oxide Doped Poly（hydroxymethylated-3,4- ethylenedioxythiophene）： Enhanced Sensitivity for Electrochemical Determination of Rutin and Ascorbic Acid

A novel graphene oxide doped poly(hydroxymethylated-3,4-ethylenedioxythiophene)(PEDOT-MeOH/GO) composite film was synthesized and utilized as an efficient electrode material for simultaneous detection of rutin and ascorbic acid(AA). PEDOT-MeOH/GO films were synthesized on glassy carbon electrode(GCE) by a facile one-step electrochemical approach and were characterized by scanning electron microscopy, UV-Vis spectroscopy, FTIR spectra and electrochemical methods. Then the PEDOT-MeOH/GO/GCE was applied successfully in the simultaneous detection of rutin and AA. The results showed that the oxidation peak currents of rutin and AA obtained at the PEDOT-MeOH/GO/GCE were much higher than those at the traditional conducting polymer PEDOT/GO/GCE, PEDOT-MeOH/GCE, PEDOT/GCE and bare GCE. Under optimized conditions, the linear ranges for rutin and AA are 20 nmol/L-10 μmol/L and 8 μmol/L-1 mmol/L, respectively. The detection limit is 6 nmol/L for rutin and 2 μmol/L for AA(S/N = 3), which are lower than those of the reported electrochemical sensors.     

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.     

One-step electrosynthesis of poly(3,4-ethylenedioxy-thiophene)–ethylsulfate matrix for fabricating vitamin C electrochemical biosensor and its determination in commercial juices

A stable and specific electrochemical biosensor based on a poly(3,4-ethylenedioxythiophene)–ethyl sulfate (PEDOT–EtSO₄) matrix with high conductivity and stability was easily fabricated. 1-Ethyl-3-methylimidazolium ethyl sulfate ([Emim][EtSO₄]), a halogen-free and relatively hydrolysis-stable hydrophilic ionic liquid, was selected as the supporting electrolyte for the one-step electrosynthesis of the PEDOT–EtSO₄ matrix under the optimum conditions. The PEDOT–EtSO₄ matrix electrosynthesized in the [Emim][EtSO₄] aqueous solution displayed high conductivity and stability. Inspired by preceding studies, the electrochemical biosensor based on the resulting PEDOT–EtSO₄ matrix was facilely developed to determine the vitamin C (VC) level in commercial juices. Ascorbate oxidase (AO) was dip-coated on the surface of the as-prepared matrix, then Nafion was covered on the surface of AO layers for preventing the leakage of enzyme molecules. The fabricated biosensor displayed an excellent bioelectrocatalytic activity to the oxidation of VC. Under optimal conditions, the fabricated amperometric biosensor showed rapid response (less than 2?s) to VC at a low potential of 0.2?V over a wide range of concentrations from 8.0?×?10^?7 to 1?×?10^?3?M with a high sensitivity of 104.8?mA?M^?1?cm^?2, and the limit of detection and the limit of quantification of presented method was 0.147?μM and 0.487?μM, respectively. Moreover, the bioaffinity, specificity, stability, and reproducibility of the biosensor were also evaluated. Finally, the biosensor was employed to determine the content of VC in commercial juice samples by amperometric and voltammetric methods. The satisfactory results indicated that the as-prepared conducting PEDOT–EtSO₄ films as immobilization matrix of biologically active species could be a promising candidate for the design and application of biosensors.     

Electrochemical immunosensor for the carcinoembryonic antigen based on a nanocomposite consisting of reduced graphene oxide,gold nanoparticles and poly(indole-6-carboxylic acid)

We describe a label-free electrochemical immunosensor for the carcinoembryonic antigen (CEA). It is based on a nanocomposite consisting of electrochemically reduced graphene oxide, gold nanoparticles (AuNPs), and poly(indole-6-carboxylic acid). Coupled to nanoparticle-amplification techniques and modified with ionic liquid (IL), this immunoassay shows high sensitivity and good selectivity for CEA. At the best working voltage of 0.95 V (vs. Ag/AgCl), the lower detection limit is 0.02 ng·mL^?1, and the response to CEA is linear in the range from 0.02 to 90 ng·mL^?1. The method was applied to the determination of CEA in spiked serum samples and gave recoveries in the range from 98.5 % to 102 %.

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Graphical abstract A label-free electrochemical immunosensor was fabricated for the carcinoembryonic antigen (CEA) with a detection limit of 0.02 ng·mL^?1. It is based on a nanocomposite consisting of electrochemically reduced graphene oxide (erGO), gold nanoparticles (Au NP), and poly(indole-6-carboxylic acid) (PICA).

     

Use of 2-(tert-butoxy)-N-(3-carbamothioylphenyl)acetamide and graphene oxide for separation and preconcentration of Fe(Ⅲ),Ni(Ⅱ),Cu(Ⅱ) and Zn(Ⅱ) ions in different samples

A simple and selective method using a column packed with graphene oxide(GO) as a solid phase extractant has been developed for the multi-element preconcentration of Fe(Ⅲ),Ni(Ⅱ),Cu(Ⅱ) and Zn(Ⅱ)ions prior to flame atomic absorption spectrometric determinations.The method is based on the sorption of mentioned ions on synthesized GO using 2-(tert-butoxy)-N-(3-carbamothioylphenyl)acetamide as a chelating agent.Several parameters on the extraction and complex formation were optimized.Under the optimized conditions(pH 6,flow rate 9 mL/min),metal ions were retained on the column,then quantitatively eluted by HNO3solution(5 mL,3.0 mol/L).The preconcentration factor was calculated as250.The detection limits for the analyte ions of interest were found in the range of 0.11 ng/mL(Ni2+) to0.63 ng/mL(Cu2+).The column packed with GO was adequate for metal ions separation in matrixes containing alkali,alkaline earth,transition and heavy metal ions.     

Fabrication of β-cyclodextrin-coated poly (diallyldimethylammonium chloride)-functionalized graphene composite film modified glassy carbon-rotating disk electrode and its application for simultaneous electrochemical determination colorants of sunset yellow and tartrazine

We proposed a green and facile approach for the synthesis of β-cyclodextrin-coated poly(diallyldimethylammonium chloride)-functionalized graphene composite film (β-CD-PDDA-Gr) by using l-ascorbic acid (l-AA) as the reducing agent at room temperature. The β-CD-PDDA-Gr composite film modified glassy carbon-rotating disk electrode (GC-RDE) was then developed for the sensitive simultaneous determination of two synthetic food colorants: sunset yellow (SY) and tartrazine (TT). By cyclic voltammetry (CV), the peak currents of SY and TT increased obviously on the developed electrochemical sensor. The kinetic parameters, such as diffusion coefficient D and standard heterogeneous rate constant k_b, were estimated by linear sweep voltammetry (LSV). Under the optimal conditions, the differential pulse voltammetry (DPV) signals of SY and TT on the β-CD-PDDA-Gr modified GC-RDE were significantly enhanced. The enhanced anodic peak currents represented the excellent analytical performance of simultaneous detection of SY and TT in the range of 5.0 × 10⁻⁸ to 2.0 × 10⁻⁵ mol L⁻¹, with a low limit of detection (LOD) of 1.25 × 10⁻⁸ mol L⁻¹ for SY and 1.43 × 10⁻⁸ mol L⁻¹ for TT (S N⁻¹ = 3). This proposed method displayed outstanding selectivity, good stability and acceptable repeatability and reproducibility, and also has been used to simultaneously determine SY and TT in some commercial soft drinks with satisfactory results. The obtained results were compared to HPLC of analysis for those two colorants and no significant differences were found. By the treatment of the experimental data, the electrochemical reaction mechanisms of SY and TT both involved a one-electron-one-proton-transfer process.     

A disposable electrochemical sensor for the determination of indole-3-acetic acid based on poly(safranine T)-reduced graphene oxide nanocomposite

A disposable electrochemical sensor for the determination of indole-3-acetic acid (IAA) based on nanocomposites of reduced graphene oxide (rGO) and poly(safranine T) (PST) was reported. The sensor was prepared by coating a rGO film on a pre-anodized graphite electrode (AGE) through dipping-drying and electrodepositing a uniform PST layer on the rGO film. Scanning electron microscopic (SEM) and infrared spectroscopic (IR) characterizations indicated that PST-rGO formed a rough and crumpled composite film on AGE, which exhibited high sensitive response for the oxidation of IAA with 147-fold enhancement of the current signal compared with bare AGE. The voltammetric current has a good linear relationship with IAA concentration in the range 1.0 × 10⁻⁷-7.0 × 10⁻⁶ M, with a low detection limit of 5.0 × 10⁻⁸ M. This sensor has been applied to the determination of IAA in the extract samples of several plant leaves and the recoveries varied in the range of 97.71-103.43%.     

Metal complexes of indole-3-acetic acid: synthesis,crystal structures,and Pb2+ chemosensing by cation-exchange reaction

Two binuclear complexes [Cu₂(IA)₄(DMSO)₂]·CH₃OH (1), [Cd₂(IA)₂(phen)₂I₂] (2), and one 1-D {[Pb₂(IA)₄]·CH₃OH}_n (3) (IAH = indole-3-acetic acid, phen = 1,10-phenanthroline) have been prepared and characterized by single crystal X-ray diffraction. Both 1 and 2 are binuclear wherein the central Cu ions are bridged by IA in 1, while Cd ions are bridged by I^? in 2. Complex 3 has a 1-D chain structure based on secondary building units (SBUs) of [Pb₂(IA)₄]. The three complexes show strong fluorescence emissions, and chemosensor behaviors for metal cations are investigated in mixed DMF/H₂O (1?:?9 v/v) solvent. The results reveal that 2 shows effective sensing to Pb²⁺. The mechanism of the detection to Pb²⁺ can be attributed to cation-exchange reaction between cadmium and lead ions.     

Water and time dependent interaction of iron(III) with indole-3-acetic acid

The influence of water on the interaction between iron(III) and indole-3-acetic acid (IAA) was studied in different organic solutions using rapid-scan stopped-flow spectrophotometry and rapid-freeze/quench Mössbauer spectroscopy. Measurements were also performed in ethanol–water and acetone–water mixtures. The results showed that the interaction between Fe^III and IAA resulted in dimeric Fe^III–IAA complex within 1 s, followed by a slow second step to give Fe²⁺ and IAA(oxidized). No such products were formed in the absence of water. The visible and Mössbauer spectra reflect the nature of the organic solvent and that of the anion of iron(III) salts.     

Two zinc(II) complexes based on indole-3-acetic acid: crystal structures,fluorescence sensors,and ion exchange with mercury(II)

Two new coordination complexes, [Zn(IA)₂(phen)] (1) and [Zn(IA)₂(4,4′-bipy)] _n ?·?C₂H₅OH (2) (IAH?=?indole-3-acetic acid, phen?=?1,10-phenanthroline, 4,4′-bipy?=?4,4′-bipyridine), have been prepared and characterized by single-crystal X-ray diffraction. Complex 1 is mononuclear and 2 presents a 1-D zigzag chain, in which 4,4′-bipy connects the Zn(II) ions. Both complexes show fluorescence emissions and exhibit fluorescence quenching when Hg²⁺ ions are present. ICP, EDS, and SEM experiments reveal that zinc in both complexes can be exchanged by toxic mercury ions.     

Simultaneous determination of gibberellic acid, indole-3-acetic acid and abscisic acid in wheat extracts by solid-phase extraction and liquid chromatography-electrospray tandem mass spectrometry

A liquid chromatography–tandem mass spectrometry (LC–MS/MS) method was developed for simultaneous determination of three representative phytohormones in plant samples: gibberellic acid (GA₃), indole-3-acetic acid (IAA) and abscisic acid (ABA). A solid-phase extraction (SPE) pretreatment method was used to concentrate and purify the three phytohormones of different groups from plant samples. The separation was carried out on a C₁₈ reversed-phase column, using methanol/water containing 0.2% formic acid (50:50, v/v) as the isocratic mobile phase at the flow-rate of 1.0 mL min⁻¹, and the three phytohormones were eluted within 7 min. A linear ion trap mass spectrometer equipped with electrospray ionization source was operated in negative ion mode. Selective reaction monitoring (SRM) was employed for quantitative measurement. The SRM transitions monitored were as 345 → 239, 301 for GA₃, 174 → 130 for IAA and 263 → 153, 219 for ABA. Good linearities were found within the ranges of 5–200 μg mL⁻¹ for IAA and 0.005–10 μg mL⁻¹ for ABA and GA₃. Their detection limits based on a signal-to-noise ratio of three were 0.005 μg mL⁻¹, 2.2 μg mL⁻¹ and 0.003 μg mL⁻¹ for GA₃, IAA and ABA, respectively. Good recoveries from 95.5% to 102.4% for the three phytohormones were obtained. The results demonstrated that the SPE-LC–MS/MS method developed is highly effective for analyzing trace amounts of the three phytohormones in plant samples.     

Isolation of novel indole-3-acetic acid conjugates by immunoaffinity extraction

An analytical protocol for the isolation and quantification of indole-3-acetic acid (IAA) and its amino acid conjugates was developed. IAA is an important phytohormone and formation of its conjugates plays a crucial role in regulating IAA levels in plants. The developed protocol combines a highly specific immunoaffinity extraction with a sensitive and selective LC-MS/MS analysis. By using internal standards for each of the studied compounds, IAA and seven amino acid conjugates were analyzed in quantities of fresh plant material as low as 30 mg. In seeds of Helleborus niger, physiological levels of these compounds were found to range from 7.5 nmol g⁻¹ fresh weight (IAA) to 0.44 pmol g⁻¹ fresh weight (conjugate with Ala). To our knowledge, the identification of IAA conjugates with Gly, Phe and Val from higher plants is reported here for the first time.     

Use of a graphite–polyurethane composite electrode for electroanalytical determination of indole-3-acetic acid in soil samples

A new electroanalytical methodology was developed for the quantification of the phytohormone indole-3-acetic acid (IAA), using a graphite–polyurethane composite electrode (GPU) and the square wave voltammetry (SWV), in 0.1 mol L^− 1 phosphoric acid solution (pH 1.6). Analytical curves were constructed under optimized conditions (f = 100 s^− 1, a = 50 mV, E_i = 5 mV) and the reached detection and quantification limits were 26 μg L^− 1 and 0.2 mg L^− 1, respectively. The developed methodology is simple and accurate for the routine determination of IAA. In order to verify the application of the electroanalytical methodology in fortified soil samples without previous treatment, an IAA assay was performed without serious interferences of the soil constituents.     

Electrosynthesis of a multilayer film stacked alternately by poly(3,4-ethylenedioxythiophene) and reduced graphene oxide from aqueous solution

Multilayer films stacked alternately by poly(3,4-ethylenedioxythiophene) and reduced graphene oxide were synthesized electrochemically from a single aqueous solution containing 3,4-ethylenedioxythiophene (EDOT) and graphene oxide (GO) by repeatedly oxidizing EDOT and reducing GO on a conductive substrate. Using the proposed technique, the thickness of the different layers could be easily tuned by varying the electrolysis time, and the number of layers could be increased simply by repeating the sequence of potential steps.     

Covalent immobilization of glucose oxidase on films prepared by electrochemical copolymerization of 3-methylthiophene and thiophene-3-acetic acid for amperometric sensing of glucose: Effects of polymerization conditions on sensing properties

For the purpose of glucose sensing, enzyme electrodes were fabricated via covalent immobilization of glucose oxidase on the films of conducting polymer. The films were prepared electrochemically by the copolymerization of 3-methylthiophene and thiophene-3-acetic acid. The properties of the films were investigated by taking into account the polymerization conditions (the kind of supporting electrodes, the current, the amount of passed charge, and the monomer concentration) and the dedoping treatment. The glucose sensing performance of the enzyme electrode was found to be affected markedly by the following three factors of the conducting polymer film: surface morphology, conductivity and cohesion with support electrodes. It was suggested that the ideal conducting polymer used for the enzyme electrode should be a thin film having high conductivity and ordered nanostructure.