Novel lipoate-selective membrane sensor for the flow injection determination of alpha-lipoic acid in pharmaceutical preparations and urine

A potentiometric lipoate-selective sensor based on mercuric lipoate ion-pair as a membrane carrier is reported. The electrode was prepared by coating the membrane solution containing PVC, plasticizer, and carrier on the surface of graphite electrode. Influences of the membrane composition, pH, and possible interfering anions were investigated on the response properties of the electrode. The sensor exhibits significantly enhanced response toward lipoate ions over the concentration range 1 × 10⁻⁷ mol L⁻¹ to 1 × 10⁻² mol L⁻¹ with a lower detection limit of (LDL) of 9 × 10⁻⁸ mol L⁻¹ and a slope of −29.4 mV decade⁻¹, with S.D. of the slope is 0.214 mV. Fast and stable response, good reproducibility, long-term stability, applicability over a pH range of 8.0–9.5 is demonstrated. The sensor has a response time of ≤12 s and can be used for at least 6 weeks without any considerable divergence in its potential response. The proposed electrode shows good discrimination of lipoate from several inorganic and organic anions. The CGE was used in flow injection potentiometry (FIP) and resulted in well defined peaks for lipoate ions with stable baseline, excellent reproducibility and reasonable sampling rate of 30 injections per hour. The proposed sensor has been applied for the direct and FI potentiometric determination of LA in pharmaceutical preparations and urine; and has been also utilized as an indicator electrode for the potentiometric titration of LA.     

Effect of various additives on the performance of a newly developed PVC based potentiometric sensor for anionic surfactants

A new poly(vinyl chloride) PVC membrane electrode to determine monomer concentrations of dodecylbenzenesulphonate ions (DBS⁻) based on a neutral ion-pair carrier complex of dodecyltrimethylammonium–dodecylbenzenesulphonate (DTA⁺–DBS⁻), is reported here. The electrode exhibits a slope of 51.25 mV per decade for DBS⁻ ions. The DBS⁻ ion selective electrode (ISE) can determine monomer units down to concentrations as low as 3.32 × 10⁻⁴ M. The effect of three kinds of additives, i.e. alcohols, glycols and triblock polymers on the performance of the surfactant selective electrode is studied systematically. The effect of foreign anions along with primary ions on the performance of ion-selective electrode is investigated in terms of potentiometric selectivity coefficients, which were determined using the fixed interference method (FIM) at 1.0 × 10⁻² M concentration of foreign anions. The sensor responds well to the surfactant ions in the presence of additives at lower concentration. The Gibbs free energy of micelle formation (ΔG_m) of sodium dodecylbenzenesulphonate (SDBS) in the presence of various additives is calculated and found to vary differently with respect to the increase in the amount of additives. The sensor worked in the acidic pH range with a short response time of 30 s. The lifetime of the sensor is more than three months. The sensor was further used to determine the amount of DBS⁻ in local detergents. This method of determining anionic surfactants was found to be quite accurate when compared with classical methods.     

Enhanced sensitivity for Cu(II) by a salicylidine-functionalized polysiloxane carbon paste electrode

A new approach for decreasing the detection limit for a copper(II) ion-selective electrode (ISE) is presented. The ISE is designed using salicylidine-functionalized polysiloxane in carbon paste. This work describes the attempts to develop the electrode and measurements of its characteristics. The new type of renewable three-dimensional chemically modified electrode could be used in a pH range of 2.3–5.4, and its detection limit is 2.7 × 10⁻⁸ mol L⁻¹ (1.2 μg L⁻¹). This sensor exhibits a good Nernstian slope of 29.4 ± 0.5 mV/decade in a wide linear concentration range of 2.3 × 10⁻⁷ to 1.0 × 10⁻³ mol L⁻¹ of Cu(II). It has a short response time (8 s) and noticeably high selectivity over other Cu(II) selective electrodes. Finally, it was satisfactorily used as an indicator electrode in complexometric titration with EDTA and determination of copper(II) in miscellaneous samples such as urine and various water samples.     

A new potentiometric ibuprofenate ion sensor immobilized in a graphite matrix for determination of ibuprofen in tablets

The characteristics, performance, and application of an electrode, namely, Pt|Hg|Hg₂(IBP)₂|Graphite, where IBP stands for ibuprofenate ion, are described. This electrode responds to IBP with sensitivity of (58.6 ± 0.9) mV decade^− 1 over the range 5.0 × 10^− 5–1.0 × 10^− 1 mol L^− 1 at pH 6.0–9.0 and a detection limit of 3.8 × 10^− 5 mol L^− 1. The electrode is easily constructed at a relatively low cost with fast response time (within 15–30 s) and can be used for a period of 5 months without any considerable divergence in potentials. The proposed sensor displayed good selectivity for ibuprofen in the presence of several substances, especially concerning carboxylate and inorganic anions. It was used for the direct assay of ibuprofen in commercial tablets by means of the standard additions method. The analytical results obtained by using this electrode are in good agreement with those given by the United States Pharmacopeia procedure.     

Flow injection determination of ketotifen fumarate using PVC membrane selective electrodes

In this study a PVC membrane electrode for determination of ketotifen fumarate is reported, where ketotifen tetraphenylborate (Keto-TPB) was used as ion exchanger. The electrode has linear range of 5.6 × 10^− 6–1.0 × 10^− 2 and 1.0 × 10^− 5–1.0 × 10^− 2 mol/L, with detection limits 2.37 × 10^− 6and 4.60 × 10^− 6 mol/L in batch and flow injection analysis (FIA), respectively. The electrodes show a Nernstian slope value (58.40 and 61.50 mV/decade in batch and FIA, respectively), and the response time is very short (≤ 10 s). The potential is nearly stable over the pH range 2.0–8.0. Selectivity coefficient values towards different inorganic cations, sugars and amino acids reflect high selectivity of the prepared electrodes. These are used for determination of Ketotifen using potentiometric titration and standard addition methods in pure samples and its pharmaceutical preparations (Zaditen tablets and syrup). The average recovery values are 99.5 and 99.2% with RSD 1.4 and 1.2% for potentiometric titrations and standard addition methods, respectively. The electrode response at different temperatures was also studied.     

Voltammetric determination of N-acetylcysteine using a carbon paste electrode modified with copper(II) hexacyanoferrate(III)

The preparation and electrochemical characterization of a carbon paste electrode modified with copper(II) hexacyanoferrate(III) (CuHCF) as well as its behavior as electrocatalyst toward the oxidation of N-acetylcysteine were investigated. The electrochemical behavior of the modified electrode and the electrooxidation of N-acetylcysteine were explored using sweep linear voltammetry. The best voltammetric response was observed for a paste composition of 20% (w/w) copper(II) hexacyanoferrate(III) complex, acetate buffer solution at pH of 6.0 as the electrolyte and scan rate of 10 mV s^− 1. A linear voltammetric response for N-acetylcysteine was obtained in the concentration range from 1.2 × 10^− 4 to 8.3 × 10^− 4 mol L^− 1, with a detection limit of 6.3 × 10^− 5 mol L^− 1. The proposed electrode is useful for the quality control and routine analysis of N-acetylcysteine in pharmaceutical formulations.     

An electropolymerized Nile Blue sensing film-based nitrite sensor and application in food analysis

This paper reports a poly-Nile Blue (PNB) sensing film based electrochemical sensor and the application in food analysis as a possible alternative for electrochemical detection of nitrite. The PNB-modified electrode in the sensor was prepared by in situ electropolymerization of Nile Blue at a prepolarized glassy carbon (GC) electrode and then characterized by cyclic voltammetry (CV) and pulse voltammetry in phosphate buffer (pH 7.1). Several key operational parameters affecting the electrochemical response of PNB sensing film were examined and optimized, such as polarization time, PNB film thickness and electrolyte pH values. As the electroactive PNB sensing film provides plenty of active sites for anodic oxidation of nitrite, the nitrite sensor exhibited high performance including high sensitivity, low detection limit, simple operation and good stability at the optimized conditions. The nitrite sensor revealed good linear behavior in the concentration range from 5.0 × 10⁻⁷ mol L⁻¹ to 1.0 × 10⁻⁴ mol L⁻¹ for the quantitative analysis of nitrite anion with a limit of detection of 1.0 × 10⁻⁷ mol L⁻¹. Finally, the application in food analysis using sausage as testing samples was investigated and the results were consistent with those obtained by standard spectrophotometric method.     

The potentiometric performances of cyclotetrahydroxysiloxane containing ferrocenyl moiety as an anion-selective membrane electrode

A test electrode based on cyclotetrasiloxane containing ferrocenyl, hydroxide and amine moiety exhibits very good potentiometric performance toward anions. Especially, acetate ion shows non-Hofmeister behavior on selectivity. The addition of the tetraalkylammonium salt provides lipophilic cationic sites within the membrane and enhances the response to more lipophilic anions. As expected, potentiometric performances of slopes and detection limits for the most of lipophilic anions, such as ClO₄⁻ and SCN⁻, are the most enhanced. However, the response to acetate ion was significantly decreased. In our knowledge, this is the first report that cyclic siloxane containing functional groups such as ferrocenyl moiety and hydroxyl group shows possible usage as a neutral carrier for an anion sensor.     

Sub- and post-micellar catalytic and inhibitory effects of cetlytrimethylammonium bromide in the permanganate oxidation of phenylalanine

The kinetics of phenylalanine (phe) oxidation by permanganate has been investigated in absence and presence of cetlytrimethylammonium bromide (CTAB) using conventional spectrophotometric technique. The rate shows first- and fractional-order dependence on [MnO₄⁻] and [phe] in presence of CTAB. At lower values of [CTAB] (≤10.0 × 10⁻⁴ mol dm⁻³), the catalytic ability of CTAB aggregates are strong. In contrast, at higher values of [CTAB] (≥10.0 × 10⁻⁴ mol dm⁻³), the inhibitory effect was observed in absence of H₂SO₄. We find that anions (Br⁻, Cl⁻ and NO₃⁻) in the form of sodium salts are strong inhibitors for the CTAB catalyzed oxidation. Kinetic and spectrophotometric evidences for the formation of an intermediate complex and an ion-pair complex between phe and MnO₄⁻, CTAB and MnO₄⁻, respectively, are presented. A mechanism consistent with kinetic results has been discussed. Complex formation constant (K_c) and micellar binding constant (K_s) were calculated at 30 °C and found to be K_c = 319 mol⁻¹ dm⁻³ and K_s = 1127 mol⁻¹ dm⁻³, respectively.     

A new hydrogen peroxide biosensor based on gold nanoelectrode ensembles/multiwalled carbon nanotubes/chitosan film-modified electrode

Gold nanoelectrode ensembles were produced by electrodeposition using multiwalled carbon nanotubes (MWNTs) as template. A new third generation amperometric biosensor for hydrogen peroxide was developed based on adsorption of horseradish peroxidase (HRP) at the glassy carbon (GC) electrode modified with Au nanoelectrode ensembles/multiwalled carbon nanotubes/chitosan film. The resulting HRP biosensor offered an excellent detection for hydrogen peroxide at −0.11 V with a linear response range of 2.08 × 10⁻⁷ to 7.6 × 10⁻³ M with a correlation coefficient of 0.998, and response time <5 s. The detection limit was 1.02 × 10⁻⁷ M at 3σ. The biosensor displays rapid response, expanded linear response range, and excellent repeatability. The simple and fast fabrication of the sensor makes it superior to other techniques.     

The Preparation of a Decylidene Bis(4′-benzo-15-crown-5) Ether Membrane Rubidium(I) Electrode

A rubidium-selective electrode has been developed by coating a graphite rod with a PVC membrane containing decylidene bis(4′-benzo-15-crown-5) ether, which is used as a neutral carrier. This electrode exhibited a near Nernstian response to rubidium in the range 1.0 × 10⁻¹to 5.0 × 10⁻⁵mol/liter with a slope of 52 mV/pC. The detection limit was 2.0 × 10⁻⁵mol/liter. This decylidene bis(4′-benzo-15-crown-5) ether is a new compound prepared by Sichuan University and has never been used to prepare a rubidium electrode.     

Non-extraction flow injection determination of cationic surfactants using eriochrome black-T

A new, rapid, sensitive, non-extraction batch, and flow injection spectrophotometric method for the determination of cationic surfactants (CSs) such as cetyltrimethyl ammonium bromide (CTAB), tetra-n-butyl ammonium chloride (TBAC) and cetylpyridinium chloride (CPC) is proposed. The method is based on the interaction of cationic surfactants with eriochrome black-T to form an ion-association complex. This complex has strong absorbance at 708 nm. The effects of chemical parameters and FIA variables on the determination of cationic surfactants were studied in detail, especially for CTAB. Under optimum conditions, the two linear calibration ranges of the method are 3.0 × 10⁻⁶ to 5.0 × 10⁻³ mol L⁻¹ CTAB, CPB and DTAB for the batch spectrophotometric method and 2.0 × 10⁻⁶ to 2.0 × 10⁻⁴ mol L⁻¹ CTAB, CPB and TBC for the flow injection spectrophotometric method. The sample throughput was 35 ± 5 samples h⁻¹ at room temperature. The relative standard deviations for 10 replicates of analysis of (2.0, 0.6 and 0.2) × 10⁻⁴ mol L⁻¹ CTAB were 1.2, 1.3, and 0.8%, respectively. In addition, the influence of potential interfering substances on the determination of cationic surfactants was studied. The proposed method is simple and rapid, using no toxic organic solvents. It was applied to the determination of trace CS in industrial wastewater with satisfactory results.     

C60 trianion-mediated electrocatalysis and amperometric sensing of hydrogen peroxide

Heterogeneous electrocatalytic reduction of hydrogen peroxide (H₂O₂) by C₆₀ is reported for the first time. C₆₀ is embedded in tetraoctylammonium bromide (TOAB) film and is characterized by scanning electron microscopy and cyclic voltammetry. Electrocatalytic studies show that the trianion of C₆₀ mediates the electrocatalytic reduction of H₂O₂ in aqueous solution containing 0.1 M KCl. Application of such film modified electrode as an amperometric sensor for H₂O₂ determination is also examined. The sensor shows a fast response within 1 s and a linear response is obtained (R = 0.9986) in the concentration range from 3.33 × 10⁻⁵ to 2.05 × 10⁻³ mol L⁻¹ for H₂O₂, with the detection limit of 2 × 10⁻⁵ mol L⁻¹ and the sensitivity of 1.65 μA mM⁻¹. A good repeatability and stability is shown for the sensor during the experiment.     

A selective sensing membrane for the determination of tetracycline with heptakis (2,6-di-O-isobutyl)-β-cyclodextrin as the substrate

When heptakis (2,6-di-O-isobutyl)-β-cyclodextrin(DOB-β-CD) is immobilized in a plasticized poly vinyl chloride (PVC) membrane, it extracts tetracycline (TC) from the sample solution into the organic membrane phase to form a complex of DOB-β-CD and TC. Since the complex formation results in an enhancement of fluorescence intensity of TC at 506 nm, the chemical recognition process can be directly translated into an optical signal. The maximum response of the sensitive membrane for TC was obtained in 0.2 mol/l KH₂PO₄–KOH buffer solution (pH 8.01). In the optimum conditions described, the proposed sensor responds linearly in the measuring range of 2.00×10⁻⁶ mol/l to 4.00×10⁻⁴ mol/l, and has a detection limit of 8.00×10⁻⁷ mol/l. The response time of the sensor is within 2.0 min. In addition to high reproducibility and reversibility, the sensor also exhibits good selectivity over some common pharmaceutical species and some common organic and inorganic compounds.     

A ratiometric chemosensor for fluorescent determination of Hg based on a new porphyrin-quinoline dyad

Quinolin-8-ol p-[10′,15′,20′-triphenyl-5′-porphyrinyl]benzoate (1) was synthesized for the first time and developed as a ratiometric fluorescent chemosensor for recognition of Hg²⁺ ions in aqueous ethanol with high selectivity. The 1–Hg²⁺ complexation quenches the fluorescence of porphyrin at 646 nm and induces a new fluorescent enhancement at 603 nm. The fluorescent response of 1 towards Hg²⁺ seems to be caused by the binding of Hg²⁺ ion with the quinoline moiety, which was confirmed by the absorption spectra and ¹H NMR spectrum. The fluorescence response fits a Hill coefficient of 1 (1.0308), indicating the formation of a 1:1 stoichiometry for the 1–Hg²⁺ complex. The analytical performance characteristics of the chemosensor were investigated. The sensor shows a linear response toward Hg²⁺ in the concentration range of 3 × 10⁻⁷ to 2 × 10⁻⁵ M with a limit of detection of 2.2 × 10⁻⁸ M. Chemosensor 1 shows excellent selectivity to Hg²⁺ over transition metal cations except Cu²⁺, which quenches the fluorescence of 1 to some extent when it exists at equal molar concentration. Moreover, the chemosensor are pH-independent in 5.0–9.0 and show excellent selectivity for Hg²⁺ over transition metal cations.     

Fluorescence enhancement effect for the determination of DNA with calcein-cetyl trimethyl ammonium bromide system

A new spectrofluorimetric method was developed for the determination of trace amounts of DNA using the calcein as a fluorescent probe. In the presence of appropriate amounts of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB), the anionic dye calcein dimerizes. The weak fluorescence intensity of the dimer was enhanced by adding DNA at pH 6–7. The interaction between calcein–CTAB and DNA was studied on the basis of this behavior and a new method was developed for determining DNA. Under the optimal conditions, the enhanced fluorescence intensity was in proportion to the concentration of DNA in the range of 4.0 × 10⁻⁶ to 8.0 × 10⁻⁵ g L⁻¹ for fsDNA and thermally denatured ctDNA (4.5 × 10⁻⁶ to 9.0 × 10⁻⁵ g L⁻¹). The detection limits (S/N = 3) were 2.0 × 10⁻⁶ and 2.2 × 10⁻⁶ g L⁻¹, respectively. This method was used for determining the concentration of DNA in synthetic samples with satisfactory results.     

Influence of the surfactant bromide of cetyltrimetyl ammonium in the determination of chlorogenic acid in instant coffee and mate tea samples

In this study, two quantitative differential-pulse polarography (DPP) and square-wave voltammetry (SWV) methods were developed to determine total chlorogenic acid (CGA). Studies on this compound involve its reduction at a hanging mercury drop electrode in micellar media—a simple, fast, reliable, and sensitive method. The use of surfactant cationic cetyltrimethylammonium bromide (CTAB) was pivotal to the development of these methods, allowing for satisfactory changes in CGA reduction. The supporting electrolyte which provided the best-defined CGA determination was 0.04-mol L⁻¹ phosphate buffer at pH 6.0 in the presence of CTAB. Based on this use and under optimized conditions, the two new DPP and SWV methods for CGA analysis had detection limits of 2.36 × 10⁻⁷ and 1.34 × 10⁻⁹ mol L⁻¹, respectively, for a pure standard. Analysis of the standard in the presence of treated instant coffee and mate tea samples allowed for good average recovery rates, ranging from 97.06% to 105.90%.     

Electroanalysis of myoglobin and hemoglobin with a boron-doped diamond electrode

The electrochemical behavior of myoglobin (Mb) and hemoglobin (Hb) was investigated with a boron-doped diamond (BDD) electrode by cyclic voltammetry. In acetate buffer solutions, the oxygen reduction at the BDD electrode showed a very high overpotential while the reduction of Mb or Hb was observed in the more positive potential region. Owing to the electrocatalytic reaction of O₂ and the participation of H⁺ following the electrochemical reduction of ferric proteins, the voltammetric responses for Mb and Hb on the BDD electrode in the negative going scans became remarkable in acidic buffer solutions in air. The peak current was linearly proportional to the concentration of Mb in the range 1×10⁻⁶–2×10⁻⁵ M or the concentration of Hb from 1×10⁻⁶ to 1×10⁻⁵ M.     

A novel poly(taurine) modified glassy carbon electrode for the simultaneous determination of epinephrine and dopamine

A novel taurine modified glassy carbon electrode was prepared by electropolymerization method. The electrochemical behaviors of epinephrine (EP) and dopamine (DA) at the modified electrode were studied by cyclic voltammetry. The modified electrode exhibited enhanced sensitivity and excellent electrochemical discrimination to DA and EP. The cathodic peaks of the two species were well-separated with a potential difference of about 390 mV, so the poly(taurine) modified electrode was used for simultaneous voltammetric measurement of EP and DA by differential pulse voltammetry. Under the optimum conditions, the cathodic peak currents were linear to concentrations of EP and DA in the range of 2.0 × 10⁻⁶ to 6.0 × 10⁻⁴ mol L⁻¹ and 1.0 × 10⁻⁶ to 8.0 × 10⁻⁴ mol L⁻¹, respectively. The detection limits for EP and DA were 3.0 × 10⁻⁷ and 1.0 × 10⁻⁷ mol L⁻¹, respectively. Because the oxidation of ascorbic acid (AA) is an irreversible reaction at modified electrode, the interference of AA for determining EP and DA was eliminated. The modified electrode has been satisfactorily used for the simultaneous determination of EP and DA in pharmaceutical injections.     

A new Tb-selective fluorescent sensor based on 2-(5-(dimethylamino)naphthalen-1-ylsulfonyl)-N-henylhydrazinecarbothioamide

A novel fluorescent chemosensor 2-(5-(dimethylamino)naphthalen-1-ylsulfonyl)-N-phenylhydrazinecarbothioamide (L) has been synthesized, which revealed an emission of 530 nm and when excited at 360 nm. The fluorescent probe undergoes a fluorescent emission intensity quenching upon binding to terbium ions in MeCN solution. The fluorescence quenching of L is attributed to the 1:1 complex formation between L and Tb(III) which has been utilized as the basis for the selective detection of Tb(III). The linear response range covers a concentration range of Tb(III) from 4.0 × 10⁻⁷ to 1.0 × 10⁻⁵ M and the detection limit is 1.4 × 10⁻⁷ M. The association constant of the 1:1 complex formation for L–Tb⁺³ was calculated to be 6.01 × 10⁶ M⁻¹, and the fluorescent probe exhibits high selectivity over other common metal ions mono-, di-, and trivalent cations indicate good selectivity for Tb(III) ions over a large number of interfering cations.