Construction and Application of Flow Enzymatic Biosensor Based of Silver Solid Amalgam Electrode for Determination of Sarcosine

In this paper, the flow amperometric enzymatic biosensor based on polished silver solid amalgam electrode for determination of sarcosine in model sample under flow injection analysis conditions is presented. The biosensor works on principle of electrochemical detection of oxygen decrease during enzymatic reaction which is directly proportional to the concentration of sarcosine in sample. The whole preparation process takes about 3 h. The RSD of repeatability of 10 consecutive measurements is 1.6 % (c_sarcosine=1.0×10^?4 mol dm^?3). Under optimal conditions the calibration dependence was linear in the range 7.5×10^?6–5.0×10^?4 mol dm^?3 and limit of detection was 2.0×10^?6 mol dm^?3.     

Magnetite NPs@C with Highly‐Efficient Peroxidase‐Like Catalytic Activity as an Improved Biosensing Strategy for Selective Glucose Detection

This work reports the novel application of carbon‐coated magnetite nanoparticles (mNPs@C) as catalytic nanomaterial included in a composite electrode material (mNPs@C/CPE) taking advantages of their intrinsic peroxidase‐like activity. The nanostructured electrochemical transducer reveals an enhancement of the charge transfer for redox processes involving hydrogen peroxide. Likewise, mNPs@C/CPE demonstrated to be highly selective even at elevated concentrations of ascorbic acid and uric acid, the usual interferents of blood glucose analysis. Upon these remarkable results, the composite matrix was further modified by the addition of glucose oxidase as biocatalyst, in order to obtain a biosensing strategy (GOx/mNPs@C/CPE) with enhanced properties for the electrochemical detection of glucose. GOx/mNPs@C/CPE exhibit a linear range up to 7.5×10^?3 mol L^?1 glucose, comprising the entirely physiological range and incipient pathological values. The average sensitivity obtained at ?0.100 V was (1.62±0.05)×10⁵ nA L mol^?1 (R²=0.9992), the detection limit was 2.0×10^?6 M while the quantification limit was 6.1×10^?6 mol L^?1. The nanostructured biosensor demonstrated to have an excellent performance for glucose detection in human blood serum even for pathological values.     

An amperometric uric acid biosensor based on chitosan-carbon nanotubes electrospun nanofiber on silver nanoparticles

A novel amperometric uric acid biosensor was fabricated by immobilizing uricase on an electrospun nanocomposite of chitosan-carbon nanotubes nanofiber (Chi–CNTsNF) covering an electrodeposited layer of silver nanoparticles (AgNPs) on a gold electrode (uricase/Chi–CNTsNF/AgNPs/Au). The uric acid response was determined at an optimum applied potential of ?0.35 V vs Ag/AgCl in a flow-injection system based on the change of the reduction current for dissolved oxygen during oxidation of uric acid by the immobilized uricase. The response was directly proportional to the uric acid concentration. Under the optimum conditions, the fabricated uric acid biosensor had a very wide linear range, 1.0–400 μmol L^?1, with a very low limit of detection of 1.0 μmol L^?1 (s/n?=?3). The operational stability of the uricase/Chi–CNTsNF/AgNPs/Au biosensor (up to 205 injections) was excellent and the storage life was more than six weeks. A low Michaelis–Menten constant of 0.21 mmol L^?1 indicated that the immobilized uricase had high affinity for uric acid. The presence of potential common interfering substances, for example ascorbic acid, glucose, and lactic acid, had negligible effects on the performance of the biosensor. When used for analysis of uric acid in serum samples, the results agreed well with those obtained by use of the standard enzymatic colorimetric method (P?>?0.05).

IONIC STRENGTH EFFECTS ON THE GROUND STATE COMPLEXATION and TRIPLET STATE ELECTRON TRANSFER REACTION BETWEEN ROSE BENGAL and METHYL VIOLOGEN

Abstract— The equilibrium constants, K_c, for complexation between methyl viologen dication (MV²+) and Rose Bengal, or Eosin Y, decrease with increasing ionic strength. At zero ionic strength K_c is 6500 (± 500) mol^?1 dm³ for Rose Bengal and 3200 (± 200) mol^?1 dm³ for Eosin Y, and these values decrease to 1500 (± 100) and 680 (± 40) mol^?1 dm³, respectively, at an ionic strength of 0.1 mol dm^?3. K_c is independent of pH between 4.5 and 10. ΔH is -25 (± 1) kJ mol^?1 for complexation with either dye, whereas ΔS is -15 (± 3) J K^?1 mol^?1 for Rose Bengal, and - 23 (± 3) J K^?1 mol^?1 for Eosin Y. The complexation constant for Rose Bengal and the neutral viologen, 4,4'-bipyridinium-N, N'-di(propylsulphonate), (4,4'-BPS), is 420 (± 35) mol^?1 dm³, and independent of ionic strength. No complexation could be observed for either Rose Bengal or Eosin with another neutral viologen, 2,2'-bipyridinium-N,N'-di(propylsulphonate), (2,2'-BPS). MV²+ quenches the triplet state of Rose Bengal with a rate constant of 7 × 10⁹ mol^?1 dm³ s^?1, and this rate constant decreases slightly as ionic strength increases. The cage escape yield following quenching, Φ_cc is very low (Φ_cc= 0.02 (± 0.005), and independent of ionic strength. 4,4'-BPS quenches the triplet state of Rose Bengal with a rate constant of 2.2 (± 0.1) × 10⁹ mol^?1 dm³ s^?1, and gives a cage escape yield of 0.033 (± 0.006). 2,2'-BPS quenches the Rose Bengal triplet with a rate constant of 6 (± 1) × 10⁸ mol^?1 dm³ s^?1 and gives a cage escape yield of 0.07 (± 0.01). Conductivity measurements indicate that MV²+(Cl^?)₂ is completely dissociated at concentrations below 2 × 10^?2 mol dm^?3.     

A Lactulose Bienzyme Biosensor Based on Self‐Assembled Monolayer Modified Electrodes

A bienzyme biosensor in which the enzymes β‐galactosidase (β‐Gal), fructose dehydrogenase (FDH), and the mediator tetrathiafulvalene (TTF) were coimmobilized by cross‐linking with glutaraldehyde atop a 3‐mercaptopropionic acid (MPA) self‐assembled monolayer on a gold disk electrode, is reported. The working conditions selected were E_app=+0.10 V and (25±1) °C. The useful lifetime of one single TTF‐β‐Gal‐FDH‐MPA‐AuE was surprisingly long, 81 days. A linear calibration plot was obtained for lactulose over the 3.0×10^?5–1.0×10^?3 mol L^?1 concentration range, with a limit of detection of 9.6×10^?6 mol L^?1. The effect of potential interferents (lactose, glucose, galactose, sucrose, and ascorbic acid) on the biosensor response was evaluated. The behavior of the SAM‐based biosensor in flow‐injection systems in connection with amperometric detection was tested. The analytical usefulness of the biosensor was evaluated by determining lactulose in a pharmaceutical preparation containing a high lactulose concentration, and in different types of milk. Finally, the analytical characteristics of the TTF‐β‐Gal‐FDH‐MPA‐AuE are critically compared with those reported for other recent enzymatic determinations of lactulose.     

Application of Non‐Stop‐Flow Differential Pulse Voltammetry at a Tubular Detector of Silver Solid Amalgam for Electrochemical Determination of Lomustine (CCNU)

An application of the flow differential pulse voltammetry with tubular detector based on silver solid amalgam for determination of antineoplastic drug lomustine in pharmaceutical preparations is presented. The highest sensitivity was obtained in [0.10 mol dm^?3 MES; 2.00 mol dm^?3 NaCl; pH 6.0]:EtOH (9 : 1) with flow rate 0.50 mL min^?1, and the magnitude of the modulation amplitude ?0.070 V. The calibration dependence was linear in the range 1×10^?6–1 × 10^?4 mol dm^?3 (R²=0.999). The limit of detection was 1.5×10^?7 mol dm^?3. This method was successfully used for determination of lomustine in real samples of chemotherapy drug CeeNU Lomustine 40 mg.     

A Uric Acid Biosensor Based on Langmuir-Blodgett Film as an Enzyme-Immobilizing Matrix

A uric acid biosensor was fabricated by the Langmuir–Blodgett (LB) technique to immobilize the uricase on chitosan/Prussian blue (CS/PB) prefunctionalized indium-tin oxide (ITO) electrode. The effects of ionic strengths, acidity of subphase, and uricase amount on the film were studied. The electrochemical properties of the uricase/n-nonadecanoic acid (UOx/NA) LB film proved that CS/PB was a good electro-catalyst for the reduction of hydrogen peroxide produced by enzymatic reaction of UOx, and protein molecules retained their natural electro-catalytic activity. The linear range of uric acid detection was from 5 × 10^?6 mol/L to 1.15 × 10^?3 mol/L with a detection limit of 1.8 × 10^?7 mol/L.     

Binding of Copper(II) to Pectins by Electrochemical Methods

The interaction between Cu(II) and pectin extracted from citrus fruit was studied in KNO₃ 0.10 mol dm^?3 at 25 °C and pH 5.5, using ion selective electrode potentiometry and voltammetry, namely differential pulse polarography and square‐wave voltammetry. Although many independent variables may affect Cu(II)‐polymer interactions such as charge density, polymer concentration and copper to polymer concentration ratio, a good fitting was observed for the model with ML and ML₂ complex species, when M:L total concentration (mol dm^?3) ratio varies from 0.2 to 2.7 and the ligand concentration is in the range (0.2 to 1) g dm^?3, i.e., (0.4 to 2)×10^?3 mol COO^? dm^?3. The complex parameters found in these conditions were log β_CuL=3.5±0.1 and log β_CuL2= 8.0±0.2. For lower total ligand and total metal ion concentrations, used in voltammetry, the interaction Cu(II)‐pectin is affected by a cooperative mode (increase of metal ion‐ligand affinity) when the total metal ion concentration increases and by an anti‐cooperative mode when the total ligand concentration increases, possibly due to different conformations of the polymer.     

In vitro studies of carbon fiber microbiosensor for dopamine neurotransmitter supported by copper-graphene oxide composite

A composite was prepared from copper and graphene oxide (Cu-GO) by in-situ chemical reduction of a mixture containing GO and Cu(II) ions with potassium borohydride. The morphology and structure of the composite were confirmed by various physicochemical techniques. The materials were used in a tyrosinase-based microbiosensor where the enzyme is immobilized in a biocompatible matrix consisting of poly(ortho-phenylene diamine) and Cu-GO. The composite was deposited on the surface of an 8-μm thick carbon fiber microelectrode. The role of each component in the sensing layer was systematically investigated with respect to the analytical performance of the system. In its optimal configuration, the biosensor demonstrated (a) a sensitivity of 6.1?±?3 nA mM^-1 dopamine (DA), (b) a linear response to DA (with a Michaelis-Menten constant of 0.29?±?0.03 mM), (c) good selectivity over ascorbic acid and uric acid, and (d) a high blocking capacity (112.2?±?2 mM) for ascorbic acid.

Chromium(III) Ion Selective Electrode Based on Oxalic Acid Bis(Cyclohexylidene Hydrazide)

A poly(vinyl chloride) membrane sensor based on oxalic acid bis (cyclohexylidene hydrazide) as membrane carrier was prepared and investigated as a Cr(III)‐selective electrode. The electrode reveals a Nernstian behavior (slope 19.8±0.4 mV decade^?1) over a wide Cr(III) ion concentration range 1.0×10^?7–1.0×10^?2 mol dm^?3 with a very low limit of detection (i.e., down to 6.3×10^?8 mol dm^?3). The potentiometric response of the sensor is independent of the pH of the test solution in the pH range 1.7–6.5. The electrode possesses advantage of very fast response, relatively long lifetime and especially good selectivity to wide variety of other cations. The sensor was used as an indicator electrode, in the potentiometric titration of chromium ion and in the determination of Cr(III) in waste water and alloy samples.     

A Cholesterol Biosensor Based on Entrapment of Cholesterol Oxidase in a Silicic Sol‐Gel Matrix at a Prussian Blue Modified Electrode

A cholesterol biosensors fabricated by immobilization of cholesterol oxidase (ChOx) in a layer of silicic sol‐gel matrix on the top of a Prussian Blue‐modified glassy carbon electrode was prepared. It is based on the detection of hydrogen peroxide produced by ChOx at ?0.05 V. The half‐lifetime of the biosensor is about 35 days. Cholesterol can be determined in the concentration range of 1×10^?6?8×10^?5 mol/L with a detection limit of 1.2×10^?7 mol/L. Normal interfering compounds, such as ascorbic acid and uric acid do not affect the determination. The high sensitivity and outstanding selectivity are attributed to the Prussian Blue film modified on the sensor.     

Kinetics and mechanism of ruthenium (III) catalyzed oxidation of ethanol by cerium (IV) in aqueous sulfuric acid media

The kinetics of oxidation of ethanol by cerium(IV) in presence of ruthenium(III) (in the order of 10^?7 mol dm^?3) in aqueous sulfuric acid media have been followed at different temperatures (25–40°C). The rate of disappearance of cerium(IV) in the title reaction increases sharply with increasing [C₂H₅OH] to a value independent of [C₂H₅OH] over a large range (0.2–1.0 mol dm^?3) in which the rate law conforms to: where [Ru]_T gives the total ruthenium (III) concentration. The values of 10^?3k_c and 10^?3k_d are 3.6 ± 0.1 dm³ mol^?1 s^?1 and 3.9 ± 0.2 s^?1, respectively, at 40°C, I = 3.0 mol dm^?3. The proposed mechanism involves the formation of ruthenium(III)^? substrate complex which undergoes oxidation at the rate determining step by cerium(IV) to form ruthenium(IV)^? substrate complex followed by the rapid red-ox decomposition giving rise to the catalyst and ethoxide radical which is oxidized by cerium(IV) rapidly. The mechanism is consistent with the existence of the complexes Ru^III · (C₂H₅OH) and Ru^III · (C₂H₅O^?) and both are kinetically active. The overall bisulphate dependence conforms to: k_obsd = A[Ru]_T/{1 + C[HSO₄^?]} where A = 2.2 × 10⁴ dm³ mol^?1 s^?1, C = 1.3 at 40°C, [H⁺] = 0.5 mol dm^?3, and I = 3.0 mol dm^?3. The observations are consistent with the Ce(SO₄)₂ as the kinetically active species. © 1995 John Wiley & Sons, Inc.     

Highly sensitive uric acid biosensor based on individual zinc oxide micro/nanowires

We describe the use of individual zinc oxide (ZnO) micro/nanowires in an electrochemical biosensor for uric acid. The wires were synthesized by chemical vapor deposition and possess uniform morphology and high crystallinity as revealed by scanning electron microscopy, X-ray diffraction, and photoluminescence studies. The enzyme uricase was then immobilized on the surface of the ZnO micro/nanowires by physical adsorption, and this was proven by Raman spectroscopy and fluorescence microscopy. The resulting uric acid biosensor undergoes fast electron transfer between the active site of the enzyme and the surface of the electrode. It displays high sensitivity (89.74 μA cm^?2 mM^?1) and a wide linear analytical range (between 0.1 mM and 0.59 mM concentrations of uric acid). This study also demonstrates the potential of the use of individual ZnO micro/nanowires for the construction of highly sensitive nano-sized biosensors.

Direct determination of bismuth(III) in sea water by square-wave anodic stripping voltammetry at a glassy-carbon rotating-disk electrode

A method for the determination of bismuth(III) in untreated sea water at its natural pH of 8.1 is described. A bare glassy-carbon rotating-disk electrode is preconditioned by placing in the sample at an applied potential of ?0.8 V vs. Ag/AgCl for 20 min; after stripping to ?0.4 V, bismuth is accumulated for 5 min at ?0.8 V and finally stripped in the square-wave mode. The bismuth peak appears at ca. + 0.10 V vs. Ag/AgCl; peak height is linearly related to concentration up to 2×10^?10 mol dm^?3. The method is highly selective for bismuth. The concentration of Bi(III) in the investigated sample was (6±1)×10^?11 mol dm^?3, or 12±2 ng dm^?3. The different types of response obtained are discussed.     

Mutual Separation and Determination of Th(IV) and U(VI) Using Arsenazo III as a Dye Collector Reagent by Flotation‐Spectrophotometric Method

This paper reports a simple and highly selective method for the separation, preconcentration, and determination of trace amounts of thorium and uranium in some complex samples via staircase flotation. The method is based on the initial flotation of the Th(IV)‐arsenazo III complex in the presence of U(VI) from a solution of 5 mol dm^?3 HCl, then reduction of U(VI) to U(IV) and repetition of the flotation step. In both steps, the floated complex was dissolved in a 5‐mL portion of methanol and its absorbance was measured at 655 nm, spectrophotometrically. For a 30‐mL portion of the sample, Beer's law was obeyed over the concentration ranges of 3.40 × 10^?7to 3.06 × 10^?6 mol dm^?3 for Th(IV) and3.40 × 10^?7 to 3.40 × 10^?6 mol dm^?3 for U(IV) with the apparent molar absorptivity of 4.20 × 10⁵ dm³ mol^?1 cm^?1 and 3.59 × 10⁵ dm³ mol^?1 cm^?1, respectively. The RSDs (n = 7) corresponding to 1.7 × 10^?6 mol dm^?3 of Th(IV) and U(IV) were obtained as 1.7% and 1.87%. The detection limits (7 blanks) for both the metal ions were found to be 1.7 × 10^?7 mol dm^?3. The important benefit of the method is that the determinations are free from the interference of almost all cations and anions found in the complex matrixes, such as seawater samples. The proposed method was also applied to reference materials, and the determinations were shown to have good agreement with the certified values.     

Kinetics and mechanism of oxidation of aurate(I) by peroxydisulphate in aqueous hydrochloric acid

The reaction between Au(I), generated by reaction of thallium(I) with Au(III), and peroxydisulphate was studied in 5 mol dm^?3 hydrochloric acid. The reaction proceeds with the formation of an ion‐pair between peroxydisulphate and chloride ion as the Michealis–Menten plot was linear with intercept. The ion‐pair thus formed oxidizes AuCl₂^? in a slow two‐electron transfer step without any formation of free radicals. The ion‐pair formation constant and the rate constant for the slow step were determined as 113 ± 20 dm^?3 mol^?1 and 5.0 ± 1.0 × 10^?2 dm³ mol^?1 s^?1, respectively. The reaction was retarded by hydrogen ion, and formation of unreactive protonated form of the reductant, HAuCl₂, causes the rate inhibition. From the hydrogen ion dependence of the reaction rate, the protonation constant was calculated to be as 0.6 ± 0.1 dm³ mol^?1. The activation parameters were determined and the values support the proposed mechanism. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 589–594, 2002     

Kinetics and mechanism of silver(I) catalyzed oxidation of water with bismuth(V) in HClO4?HF mixture

The kinetics of the oxidation of water with bismuth(V) in presence of silver(I) has been investigated in a mixture of HClO₄ (1.0 mol dm^?3) and HF (1.5 mol dm^?3). The reaction is second order, viz., first order with respect to bismuth(V) and silver(I), each, and the second order rate constant is (6.6 ± 0.7) × 10^?3 dm³ mol^?1 s^?1. However, rate is independent of hydrogen ion concentration. A comparative analysis of these results with the results obtained for pdp, pds, and Ce(IV), reactions with silver(I) has also been made to correlate the rate constants and the redox-potentials of the oxidant couples.     

Cerium (IV)-2-chloroethanol redox-pair initiated polymerization of acrylamide in aqueous medium

The kinetics of acrylamide polymerization has been investigated by employing cericammoniumnitrate-2-chloroethanol redox pair under nitrogen atmosphere at 30 ± 1°C. The rate of monomer disappearance is directly proportional to the concentration of 2-chloroethanol (1.0 × 10^?2 ? 10.0 × 10^?2 mol. dm^?3) and is inversely proportional to the ceric ion concentration (2.5 × 10^?3 ? 10.0 × 10^?3 mol. dm^?3) but shows square dependence to the concentration of monomer (5.0 × 10^?2 ? 25.0 × 10^?2 mol. dm^?3). The rate of ceric ion disappearance is directly proportional to the initial concentration of ceric ion and 2-chloroethanol but independent of acrylamide concentration. The viscometric average molecular weight (M _v) decreases on increasing the concentration of ceric ion and increases on increasing the concentrations of acrylamide and 2-chloroethanol. A tentative mechanism has been proposed.     

Glucose biosensor based on a platinum electrode modified with rhodium nanoparticles and with glucose oxidase immobilized on gold nanoparticles

We have developed an enzymatic glucose biosensor that is based on a flat platinum electrode which was covered with electrophoretically deposited rhodium (Rh) nanoparticles and then sintered to form a large surface area. The biosensor was obtained by depositing glucose oxidase (GOx), Nafion, and gold nanoparticles (AuNPs) on the Rh electrode. The electrical potential and the fractions of Nafion and GOx were optimized. The resulting biosensor has a very high sensitivity (68.1 μA mM^?1 cm^?2) and good linearity in the range from 0.05 to 15 mM (r?=?0.989). The limit of detection is as low as 0.03 mM (at an SNR of 3). The glucose biosensor also is quite selective and is not interfered by electroactive substances including ascorbic acid, uric acid and acetaminophen. The lifespan is up to 90 days. It was applied to the determination of glucose in blood serum, and the results compare very well with those obtained with a clinical analyzer.

Catalytic Adsorptive Stripping Voltammetry of Molybdenum: Redox Kinetic Measurements

The electrode mechanism of Mo(VI) reduction was studied under catalytic adsorptive stripping mode by means of square‐wave voltammetry (SWV). Mo(VI) creates a stable surface active complex with mandelic acid. The electrode reaction of Mo(VI)‐mandelic acid system undergoes as one‐electron reduction, exhibiting properties of a surface electrode process. In the presence of chlorate, bromate, and hydrogen peroxide, the electrode reaction is transposed into a catalytic mechanism. The experimental results are compared with the recent theory for surface catalytic reaction, enabling qualitative characterization of the electrode mechanism in the presence of different catalytic agents. Utilizing both the method of “split SW peaks” and “quasireversible maximum” the standard redox rate constant of Mo(VI)‐mandelic acid system was estimates as k_s=150±5 s^?1. By fitting the experimental and theoretical results, the following catalytic rate constants have been estimated: (8.0±0.5)×10⁴ mol^?1 dm³ s^?1, (1.0±0.1)×10⁵ mol^?1 dm³ s^?1, and (3.2±0.1)×10⁶ mol^?1 dm³ s^?1, for hydrogen peroxide, chlorate, and bromate, respectively.