Tyrosinase/laccase bienzyme biosensor for amperometric determination of phenolic compounds

Tyrosinase/laccase bienzyme biosensor for amperometric determination of phenolic compounds was constructed. Enzymes were immobilized in titania gel matrix. The obtained biosensor was successfully used for determination of 2,6-dimethoxyphenol, 4-tertbutylcatechol, 4-methylcatechol, 3-chlorophenol and catechol. The highest sensitivity and the widest linear range were noticed for catechol, 234 mA L mol^− 1 and 2.0 × 10^− 7–3.2 × 10^− 5 mol/L, respectively. Detection limit for catechol, at signal-to-noise ratio of 3 was 1.3 × 10^− 7 mol/L.     

Application of Voltammetric Method of Total Chromium Determination in the Presence of Cupferron for Selective Determination of Cr(VI) in Water Samples

A voltammetric method of Cr(VI) determination in a flow system based on the combination of selective accumulation of the product of Cr(VI) reduction on hanging mercury drop electrode and a very sensitive method of chromium determination in the presence of cupferron previously described is proposed. The calibration graphs were linear from 3 × 10⁻⁹ to 3 × 10⁻⁸ and from 5 × 10⁻¹⁰ to 5 × 10⁻⁹ mol L⁻¹ for accumulation times of 120 and 600 s, respectively. The detection limit for the accumulation time of 600 s was 9 × 10⁻¹¹ mol L⁻¹. The relative standard deviation was 5.1% (n = 5) for Cr(VI) concentration 1 × 10⁻⁸ mol L⁻¹ and the accumulation time of 120 s. The influence of foreign ions commonly present in water samples is presented. The validation of the method was made by studying the recovery of Cr(VI) from spiked natural water samples.     

L-Cysteine-coated CdSe/CdS core-shell quantum dots as selective fluorescence probe for copper(II) determination

Quantum dots (QDs) or semiconductor nanocrystals have been receiving great interest in the last few years. In this paper, L-cysteine-coated CdSe/CdS core-shell QDs (λ_em = 585 nm) have been prepared, which have excellent water-solubility. The full width at half maximum (FWHM) of the photoluminescence of these nanocrystals is very narrow (about 30 nm), and the quantum yield (QY) is 15% relative to Rhodamine 6G in ethanol (QY = 95%). With excess free L-cysteine in the solution, the fluorescence intensity of L-cysteine-coated CdSe/CdS QDs showed improved stability. It was found that the fluorescence of L-cysteine-capped CdSe/CdS QDs could be quenched only by copper (II) ions and was insensitive to other physiologically important cations, such as Ca²⁺, Mg²⁺, Zn²⁺, Al³⁺, Fe³⁺, Mn²⁺ and Ni²⁺ etc. Based on this finding, the quantitative analysis of Cu²⁺ with L-cysteine-capped CdSe/CdS QDs has been established. The linear range was from 1.0 × 10^− 8 to 2.0 × 10^− 7 mol L^− 1 and the limit of detection (LOD) was 3.0 × 10^− 9 mol L^− 1 (S/N = 3). The proposed method has first been applied to the determination of Cu²⁺ in vegetable samples with recoveries of 99.6–105.8%.     

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 quercetin at a multi-walled carbon nanotubes paste electrode

Quercetin can effectively accumulate at multi-walled carbon nanotubes-paraffin oil paste electrodes (CNTPE) and cause a sensitive anodic peak at around 0.32 V (vs. SCE) in a 0.10 M phosphate buffer solution (pH = 4.0). Under optimized conditions, the anodic peak current is linear to quercetin concentration in the ranges of 2.0 × 10^− 9−1.0 × 10^− 7 M and 1.0 × 10^− 7−2.0 × 10^− 5 M, and the regression equations are i_p (μA) = 0.0017 + 0.928c (μM, r = 0.999) and i_p (μA) = 0.183 + 0.0731c (μM, r = 0.995), respectively. This paste electrode can be regenerated by repetitively cycling in a blank solution for about 2 min. A 1.0 × 10^− 6 M quercetin solution is measured for 10 times using the same electrode regenerated after every determination, and the relative standard deviation of the peak current is 1.7%. The method has been applied to the determination of quercetin in hydrolysate product of rutin and the recovery is 99.2–102.6%. In comparison with graphite paste electrode, carbon nanotubes-nujol paste electrode and carbon nanotubes casting film modified glassy carbon electrode, the CNTPE gives higher ratio of signal to background current and better defined voltammetric peak.     

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.     

Kinetic determination of organosulphur ligands by inhibition: Trace determination of cysteine and maleonitriledithiolate (MNDT)

Some organosulphur ligands have been found to inhibit the mercury(II) catalyzed substitution of cyanide in hexacyanoferrate(II) by N-methylpyrazinium ion (Mpz⁺). The inhibitory effect is due to the binding tendency of catalyst Hg²⁺ with these inhibitors. This effect has been used as a basis to develop a kinetic method for the determination of trace amounts of two organosulphur ligands viz. cysteine and MNDT. The reaction was followed spectrophotometrically at 655 nm by measuring the decrease in absorbance of the product [Fe(CN)₅Mpz]²⁻. The influence of the reaction variables has also been studied. A general mechanistic scheme of the indicator reaction system including the role of inhibitor has been proposed and applied to determine the organosulphur ligands. Under the selected experimental conditions cysteine and MNDT have been determined in the range of 2–20 × 10^− 7 M and 5 × 10^− 8 M to 12 × 10^− 7 M respectively in various aqueous samples. The analytical concentration range depends upon the amount of Hg²⁺ present in the indicator reaction and also on the stability of the Hg²⁺-inhibitor complex in question. Under specified conditions, the detection limit for cysteine and MNDT are 2 × 10^− 7 M and 5 × 10^− 8 M respectively. The influences of possible interference by major amino acids, on the determination of cysteine and their limits have been investigated.     

Combined use of Pd and HF as chemical modifiers for the determination of total chromium in produced waters from petroleum exploration by ET AAS

This work reports the evaluation of the combined use of Pd and HF as chemical modifiers for the direct determination of total chromium in waters derived from petroleum exploration employing electrothermal atomic absorption spectrometry (ET AAS). Such waters, usually called as produced waters, have complex composition presenting a number of organic and inorganic substances. When obtained from offshore operations they also present high salinity. In order establish conditions for chromium measurement pyrolysis and atomization curves were built up in different media and employing Pd and HF as chemical modifiers. Also, a detailed study about calibration strategy was performed. At best conditions, pyrolysis and atomization temperatures were 1200 °C and 2600 °C, respectively, and 10 μL of a 500 mg L^− 1 Pd solution was added together with 10 μL of a 50% (v/v) HF solution on 20 μL of sample. Obtained results indicate that, in this kind of sample, chromium can be determined by standard addition method or employing external calibration with standard solutions prepared in 0.8 mol L^− 1 NaCl medium. In order to evaluate the accuracy of the procedure, a recovery test was performed with seven spiked samples of produced waters. The detection limit, quantification limit and the relative standard deviation in 0.8 mol L^− 1 NaCl were also calculated and the values found were 0.45 μg L^− 1, 1.5 μg L^− 1 and 6.0% (at 2.5 μg L^− 1 level), respectively.     

Electrochemical behavior and voltammetric determination of tryptophan based on 4-aminobenzoic acid polymer film modified glassy carbon electrode

Herein, a novel electrochemical method was developed for the determination of tryptophan based on the poly(4-aminobenzoic acid) film modified glassy carbon electrode (GCE). The electrochemical behaviors of tryptophan at the modified electrode were investigated. It was found that the oxidation peak current of tryptophan at the modified GCE was greatly improved compared with that at the bare GCE. The effects of supporting electrolyte, pH value, scan rate, accumulation potential and time were examined. The oxidation peak current of tryptophan was proportional to its concentration over the range from 1.0 × 10⁻⁶ to 1.0 × 10⁻⁴ mol L⁻¹. The limit of detection was evaluated to be 2.0 × 10⁻⁷ mol L⁻¹. The proposed method was sensitive and simple. It was successfully employed to determine tryptophan in pharmaceutical samples.     

Field-amplified sample injection and in-capillary derivatization for capillary electrophoretic analysis of metal ions in local wines

In-capillary derivatization and field-amplified sample injection (FASI) coupled to capillary zone electrophoresis (CZE) was evaluated for the analysis of metals (Co(II), Cu(II), Ni(II), and Fe(II)) using 2-(5-Nitro-2-Pyridylazo)-5-(N-Propyl-N-Sulfopropylamino)Phenol (Nitro-PAPS) as the derivatizing agent. For FASI, the optimum conditions were water as sample solvent, 1 s hydrodynamic injection (0.1 psi) of a water plug, 5 s of electrokinetic introduction (10 kV) of the sample. The in-capillary derivatization was successfully achieved with zone-passing strategy in order tandem injection of Nitro-PAPS reagent (0.5 psi, 7 s), a small water plug (0.1 psi, 1 s), and metal ion introduction (10 kV, 5 s). The solution of 45 mmol L^− 1 borate pH 9.7 and 1.0 × 10^− 5 mol L^− 1 Nitro-PAPS containing 20% acetonitrile was used as the running buffer. The limit of detection obtained by the proposed method was lower than those from pre-capillary derivatization about 3–28 times. The recovery of the method was comparable to pre-capillary derivatization method. In-capillary derivatization-FASI-CZE was applied to analysis of metals in wine samples. The results were compared with those obtained by CZE with pre-capillary derivatization method and atomic absorption spectrometry (AAS).     

Simultaneous determination of hydroquinone and catechol at PASA/MWNTs composite film modified glassy carbon electrode

A poly-amidosulfonic acid and multi-wall carbon nanotubes composite (PASA/MWNTs) modified electrode has been constructed by electropolymerization on glassy carbon electrode (GCE). The electrochemical behaviors of hydroquinone (HQ) and catechol (CC) were investigated using cyclic and differential pulse voltammetries (DPVs) at the prepared electrode. Separation of the reductive peak potentials for HQ and CC was about 120 mV in pH 6.0 phosphate buffer solution (PBS), which makes it suitable for simultaneous determination of these compounds. In the presence of 1.0 × 10⁻⁴ mol L⁻¹ isomer, the reductive peak currents of DPV are proportional to the concentration of HQ in the range of 6.0 × 10⁻⁶ to 4.0 × 10⁻⁴ mol L⁻¹, and to that of CC in the range of 6.0 × 10⁻⁶ to 7.0 × 10⁻⁴ mol L⁻¹. When simultaneously changing the concentration of both HQ and CC, the linear concentration range of HQ (or CC) is 6.0 × 10⁻⁶ to 1.0 × 10⁻⁴ mol L⁻¹ (or 6.0 × 10⁻⁶ to 1.8 × 10⁻⁴ mol L⁻¹), and the corresponding detection limits are 1.0 × 10⁻⁶ mol L⁻¹. The proposed method has been applied to simultaneous determination of HQ and catechol in water sample, and the results are satisfactory.     

Bifunctional sensor of nitric oxide and oxygen based on hematite nanotubes embedded in chitosan matrix

A novel hybrid bifunctional sensing platform for simultaneous determination of NO and O₂ has been developed, whereby hematite nanotubes are immobilized into the chitosan matrix onto a gold electrode (labeled as HeNTs-Chi/Au). The HeNTs distributed in porous-structured chitosan matrix not only offer abundant active sites for bifunctional sensing of NO and O₂, but also facilitate oxidation of NO and reduction of O₂ dramatically. Straight calibration curves are achieved in analyte concentration ranges of 5.0 × 10⁻⁸ to 1.25 × 10⁻⁶ mol L⁻¹ for NO and 2.5 × 10⁻⁷ to 6.0 × 10⁻⁶ mol L⁻¹ for O₂. Also, the detection limits are low of 8.0 × 10⁻⁹ mol L⁻¹ for NO and 5.0 × 10⁻⁸ mol L⁻¹ for O₂. Such an efficient bifunctional sensor for NO and O₂ offers great potential in quantitation of NO levels in biological and medical systems, since NO level is highly regulated by various reactive oxygen species.     

Spectrofluorimetric quantification of bromazepam using a highly selective optical probe based on Eu–bromazepam complex in pharmaceutical and serum samples

A rapid, simple and sensitive spectrofluorimetric method for determination of trace amount of bromazepam is developed. In phosphate buffer of pH 7.4. The bromazepam enhance the luminescence intensity of the Eu³⁺ ion in Eu³⁺–bromazepam complex at λ_ex = 390 nm. The produced luminescence intensity of Eu³⁺–bromazepam complex is in proportion to the concentration of bromazepam. The working range for the determination of bromazepam is 2.3 × 10⁻⁸ to 6.2 × 10⁻⁷ M with detection limit (LoD) and quantitative detection limit (LoQ) of 3 × 10⁻⁹ and 1.2 × 10⁻⁸ M, respectively. While, the working range, detection limit (LoD) and quantitative detection limit (LoQ) in case of the quantum yield calculations are 3.7 × 10⁻⁸ to 3.4 × 10⁻⁷ M with of 3.4 × 10⁻⁹ and 9.2 × 10⁻⁸ M, respectively. The enhancement mechanism of the luminescence intensity in the Eu³⁺–bromazepam system has been also explained.     

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.     

Luminescent CuS nanotubes as silver ion probes

CuS nanotubes (NTs) made up of nanoparticles were successfully prepared in large quantities in an O/W microemulsion system under low temperature. Based on the characteristics of synchronous fluorescence spectroscopy (SFS), a new method with high sensitivity and selectivity was developed for rapid determination of silver ion with functional copper sulphide (CuS) nanotubes as a fluorescence probe. Under optimal conditions, functional copper sulphide displayed a calibration response for silver ion over a wide concentration range from 1.0 × 10⁻¹⁰ to 1.0 × 10⁻⁸ mol L⁻¹. The limit of detection was 0.5 × 10⁻¹⁰ mol L⁻¹ and the relative standard deviation of eight replicate measurements for the highest concentration (1 × 10⁻⁸ mol L⁻¹) was 3%. Compared with several fluorescence methods, the proposed method had a wider linear range and improved the sensitivity. Furthermore, the concentration dependence of the synchronous fluorescence intensity is effectively described by a Langmuir-type binding isotherm.     

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.     

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%.     

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.     

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.     

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.