PVC membrane based potentiometric sensors for uranium determination

Two novel uranyl PVC matrix membrane sensors responsive to uranyl ion are described. The first sensor incorporates tris(2-ethylhexyl)phosphate (TEHP) as both electroactive material and plasticizer and sodium tetraphenylborate (NaTPB) as an ion discriminator. The sensor displays a rapid and linear response for UO₂²⁺ ions over the concentration range 1×10⁻¹–2×10⁻⁵ mol l⁻¹ UO₂²⁺ with a cationic slope of 25.0±0.2 mV decade⁻¹. The working pH range is 2.8–3.6 and the life span is 4 weeks. The second sensor contains O-(1,2-dihydro-2-oxo-1-pyridyl)-N,N,N′,N′-bis(tetra-methylene)uronium hexafluorophosphate (TPTU) as a sensing material, sodium tetraphenylborate as an ion discriminator and dioctyl phenylphosphonate (DOPP) as a plasticizer. Linear and stable response for 1×10⁻¹–5×10⁻⁵ mol l⁻¹ UO₂²⁺ with near-Nernstian slope of 27.5±0.2 mV decade⁻¹ are obtained. The working pH range is 2.5–3.5 and the life span of the sensor is 6 weeks. Interference from many inorganic cations is negligible for both sensors. However, interference caused by some ions (e.g. Th⁴⁺, Cu²⁺, Fe³⁺) is eliminated by a prior ion exchange or solvent extraction step. Direct potentiometric determination of as little as 5 μg ml⁻¹ uranium in aqueous solutions shows an average recovery of 97.2±1.3%. Application for the determination of uranium at levels of 0.01–1 wt.% in naturally occurring and certified ores gives results with good correlation with data obtained by X-ray fluorescence.     

Electrochemical study and flow-injection amperometric detection of trace NO(2)(-) at CuPtCl(6) chemically modified electrode

A thin film of mixed-valent CuPtCl₆ is deposited on a glassy carbon electrode by continuous cyclic scanning in a solution containing 3×10⁻³ M CuCl₂+3×10⁻³ M K₂PtCl₆+1 M KCl in the potential range from 700 to −800 mV. The cyclic voltammetry is used to study the electrochemical behaviors of nitrite on CuPtCl₆/GC modified electrode and the electrode displays a good catalytic activity toward the oxidation of nitrite. The effects of the film thickness, pH, the electrode stability and precision have been evaluated. Experiments in flow-injection analysis are performed to characterize the electrode as an amperometric sensor for the detection of nitrite. The modified electrode shows a wide dynamic range, quite a low detection limit and short response time. The linear relationship between the flow-injection peak currents and the concentrations of nitrite is at a range of 1×10⁻⁷–2×10⁻³ M with a detection limit of 5×10⁻⁸ M.     

Simultaneous derivative spectrophotometric determination of thorium and uranium in a micellar medium

Derivative photometric methods for trace analysis of Th(IV) and UO₂(II), and their simultaneous determination in mixtures using 5,8-dihydroxy-1,4-naphthoquinone in a micellar medium are reported. Molar absorptivity and Sandell's sensitivity of 1:2 Th(IV) and 1:1 UO₂(II) complexes at their λ_max, 614.5 nm and 637.0 nm are, 1.19 × 10⁴ 1/mol/cm and 1.12 × 10⁴ 1/mol/cm and 1.95 × 10⁻² μg/cm² and 2.13 × 10⁻² μg/cm² μg/cm², respectively. Calibration graph is linear over the range 9.28 × 10⁻²−18.56 μg/ml of Th(IV) and 9.52 × 10⁻²−19.04 μg/ml of UO₂(II). Though presence of Th(IV) and UO₂(II) causes interference in each others determination, 9.28 × 10⁻¹−9.28 μg/ml Th(IV) and 9.52 × 10⁻¹−9.52 μg/ml UO₂(II) when present together, can be simultaneously determined using derivative spectra.     

Zn(II)-selective membrane electrode based on tetra(2-aminophenyl) porphyrin

A new PVC membrane electrode for Zn²⁺ ions based on tetra(2-aminophenyl) porphyrin (TAPP) as membrane carrier is prepared. The sensor exhibits a linear stable response over a wide concentration range (5.0×10⁻⁵ to 1.0×10⁻¹ M) with a slope of 26.5 mV/decade and a limit of detection 3.0×10⁻⁵ M (1.96 ppm). It has a response time of about l0 s and can be used for at least 8 months without any divergence in potential. The propose membrane sensor revealed good selectivities for Zn²⁺ over a wide variety of other metal ions and can be used in pH range of 3.0–6.0. It was successfully applied to the direct determination of zinc in a pharmaceutical sample and also as an indicator electrode in potentiometric titration of Zn²⁺ ions.     

Microbiosensor for acetylcholine and choline based on electropolymerization/sol–gel derived composite membrane

Amperometric enzyme biosensors for the determination of acetylcholine (ACh) and choline (Ch) have been described. For the fabrication of the biosensors, N-acetylaniline (nAN) was first electropolymerized on a Pt electrode surface to be served as a permselective layer to reject interferences. Bovine serum albumin (BSA) and choline oxidase (CHOD) were co-immobilized in a zinc oxide (ZnO) sol–gel membrane on the above modified Pt electrode for a Ch sensor, or CHOD, acetylcholinesterase (AChE) and BSA immobilized together for an ACh/Ch sensor. The poly (N-acetylaniline) (pnAN) film was the first time used for an ACh/Ch sensor and found to have excellent anti-interference ability, and the BSA in the sol–gel can improve the stability and activity of the enzymes. Amperometric detection of ACh and Ch were realized at an applied potential of +0.6 V versus SCE. The resulting sensors were characterized by fast response, expanded linear range and low interference from endogenous electroactive species. Temperature and pH dependence and stability of the sensor were investigated. The optimal ACh/Ch sensor gave a linear response range of 1.0 × 10⁻⁶ to 1.5 × 10⁻³ M to ACh with a detection limit (S/N = 3) of 6.0 × 10⁻⁷ M and a linear response range up to 1.6 × 10⁻³ M to Ch with a detection limit of 5.0 × 10⁻⁷ M. The biosensor demonstrated a 95% response within less than 10 s.     

Electrokinetic characterization of a microporous non-commercial polysulfone membrane: phenomenological coefficients and transport parameters

Electrical and electrokinetic phenomena (electrical resistance, streaming potential and membrane potential) in a porous polysulfone membrane was studied in the framework of the linear thermodynamics of irreversible processes and the phenomenological coefficients were determined for different concentrations of NaCl and MgCl₂ solutions (10⁻³M<5×10⁻²M). From experimental values, other characteristic membrane parameters such as the concentration of fixed charge in the membrane (=−3×10⁻³M), the ionic transport numbers and permeabilities through the membrane (t(Na⁺)=0.392 and t(Mg⁺²)=0.363; P(Na⁺)=3.5×l0⁻⁸m/sec and P(Mg⁺²)=2.9×10⁻⁸m/sec) were also obtained. Membrane surface-electrolyte solution interface was characterized by zeta potential values. The effect of both salt concentration and pH on zeta potential results was also studied.     

Simultaneous determination of selenium and lead in whole blood samples by differential pulse polarography

The polarographic reduction of lead in the presence of selenite gives rise to an additional peak corresponding to the reduction of lead (Pb) on adsorbed selenium (Se) on mercury at −0.33 V. The selenium and lead content can be determined using this peak by the addition of a known amount of one of these ions first and then the second ion. The linear domain range of lead is 5.0×10⁻⁷–2.0×10⁻⁵ M and for selenium 5.0×10⁻⁷–1.0×10⁻⁵ M. Using this method 4.90×10⁻⁷ M Se(IV) and 1.47×10⁻⁶ M Pb(II) in a synthetic sample could be determined with a relative error of +2.0% and 1.8%, respectively (n=4). A recovery test after acid digestion for a synthetic sample was 97% for selenium and 96.5% for lead. The method was applied to 1 ml of digested blood, and 328±23 μg l⁻¹ Se(IV) and 850±62 μg l⁻¹ Pb(II) could be determined with a 90% (n=5) confidence interval.     

Anodic characterization of mercury microelectrodes for determinations of anions in real samples

The anodization of mercury microelectrodes was investigated in synthetic samples containing several strong and weak electrolytes at different concentrations. In particular, the effects on mercury anodization due to the presence of NaOH, HClO₄, NaCl, NaI, NaF, Na₂SO₄, NaHCO₃, Na₂CO₃, tartaric and citric acids, were studied in solutions containing either each species or mixtures of them, and without addition of supporting electrolyte. Some of the electrode processes studied led to linear calibration plots e.g. 1 × 10⁻⁵ − 1 × 10⁻⁴M Cl⁻, 1 × 10⁻⁶ − 1 × 10⁻⁵M I⁻, 5 × 10⁻⁴ − 3 × 10⁻³M SO₄²⁻, 5 × 10⁻⁴ − 2 × 10⁻²M HCO₃⁻, with typical correlation coefficients of 0.998–0.999. The anodization of mercury microelectrodes was also investigated directly in wine, rain, tap and mineral water, without pretreatment and without addition of supporting electrolyte. In the real samples only the ions Cl⁻ and HCO₃⁻ could be quantified, and the values found were in agreement, within 3–5%, with the reference values obtained by using Italian standard methods for food.     

Selective enzyme amplification of NAD/NADH using coimmobilized glycerol dehydrogenase and diaphorase with amperometric detection

A flow system for substrate recycling of NAD⁺/NADH was set up with an enzyme reactor containing coimmobilized glycerol dehydrogenase (GDH) and diaphorase. The product from the diaphorase catalysis, hexacyanoferrate(II), aws detected amperometrically at a glassy carbon electrode. The amplification factor was 150 for a reactor volume of 100 μ l at a flow-rate of 0.5 ml/min. With a stopped flow of four minutes, the signal increased another 88 times, resulting in a signal amplification of 13 300 times. Equations are derived for the amplification factor and used for a discussion of the optimization of amplification systems. The K_m for GDH with glycerol as a substrate was found to be 5 × 10⁻³ M at pH 8.0. GDH from Cellulomonas sp. was purified on a gel filtration column and the purified enzyme showed a specificity toward NAD⁺, compared to NADP⁺, that was higher than 99.9%. Due to the NAD⁺ specificity of the purified GDH, the enzyme amplification system reported here could be used in detection systems for enzyme immunoassays when using alkaline phosphatase as a label and NADP⁺ as a substrate. The stability of immobilized GDH and diaphorase is several orders of magnitude better than that of alcohol dehydrogenase, which is the enzyme commonly used for NAD⁺-specific detection in these applications.     

Effect of presence of 18-crown-6 on the response of 1-pyrrolidine dicarbodithioate-based lead selective electrode

The lipophilic ammonium salt of 1-pyrrolidine dicarbodithioic acid (PCDT)[I] was introduced as a selective ionophore for a sensitive Pb-ion selective electrode. Also, the effect of immobilization of 18-crown-6 (CW), into the above membrane, on the electrode performance was discussed. The slope of the PCDT-based [I] electrode was (26–30 mV decade⁻¹), while it was (29-30 mV decade⁻¹) for (PCDT+CW)-based [II] electrode according to the doping time. The linear concentration ranges were (1×10⁻⁶–1×10⁻¹ M) and (5×10⁻⁵–1×10⁻¹ M) for electrode types [I] and [II] after one-day doping. The working pH ranges were (5.0–10.0) and (7.0–10.0) for electrode types [I] and [II], respectively. Most of the common cations were tested for the evaluation of the electrode selectivity with correlation to the ionic radii of the tested cations. Among them only Na⁺, Ag⁺ and Fe³⁺ were the real interference. Application of using the electrode for the determination of lead in lubrication oil samples was performed with RSD (0.86–1.03%). The obtained results were compared to those of an atomic absorption spectrophotometric method.     

Determination of fenoterol and salbutamol in pharmaceutical formulations by electrogenerated chemiluminescence

Fenoterol and salbutamol were determined by electrogenerated chemiluminescence (ECL) coupled with flow injection analysis (FIA), using Ru(bpy)₃²⁺ as the luminescent substance. Fenoterol and salbutamol oxidize together with the ruthenium 2,2-bipyridyl at a platinum electrode, which leads to an increase in the luminescent intensity, and this increase is proportional to the analyte concentration. For fenoterol a linear calibration curve within the range from 1.0 × 10⁻⁵ to 1.0 × 10⁻⁴ mol l⁻¹ was obtained with a correlation coefficient of 0.998 (n = 5) and for salbutamol the linear analytical curve was also obtained in this range with a correlation coefficient of 0.995 (n = 5). The relative standard deviation was estimated as ≤2.5% for 3 × 10⁻⁵ mol l⁻¹ for fenoterol solution and as ≤1.3% for 5.0 × 10⁻⁵ mol l⁻¹ salbutamol solution for 15 successive injections. The limit of detection for fenoterol was 2.4 × 10⁻⁷ mol l⁻¹ and for salbutamol was 4.0 × 10⁻⁷ mol l⁻¹. Fenoterol and salbutamol were successfully determined in drug tablets and the soluble components of the matrix did not interfere in the luminescent emission. The results obtained using the luminescent methodology were not statistically different from those obtained by UV-spectrophotometry at 95% confidence level.     

Cathodic adsorptive stripping voltammetric behaviour of guanine in the presence of copper at the static mercury drop electrode

The cathodic adsorptive electrochemical behavior of guanine in the presence of some metal ions at the static mercury drop electrode was investigated. A 1.0×10⁻³ mol l⁻¹ NaOH or a 2.0×10⁻² mol l⁻¹ Hepes buffer at pH 8.0 solutions were used as supporting electrolytes. The reduction peak potential for guanine was found to be around −0.15 V, which is very close to the mercury reduction wave. A new peak appears at −0.60 V in the presence of copper or at −1.05 V in the presence of zinc. A square wave voltammetric procedure for electroanalytical determination of guanine in 2.0×10⁻² mol l⁻¹ Hepes buffer at pH 8.0 containing 1.6×10⁻⁵ mol l⁻¹of copper ions, was developed. An accumulation potential of −0.15 V during 270 s for the prior adsorption of guanine at the electrode surface was used. The response of the system was found to be linear in the range of guanine concentration from 6.62×10⁻⁸ to 1.32×10⁻⁷ mol l⁻¹ and the detection limit was 7.0×10⁻⁹ mol l⁻¹. The influence of DNA bases such as adenine, cytosine and thymine was also examined. Cyclic voltammetry was used to characterize the interfacial and redox mechanism.     

Electrocatalytic response of norepinephrine at a thiolactic acid self-assembled gold electrode

The thiolactic acid (TLA) self-assembled monolayer modified gold electrode (TLA/Au) is demonstrated to catalyze the electrochemical response of norepinephrine (NE) by cyclic voltammetry. A pair of well-defined redox waves were obtained and the calculated standard rate constant (k_s) is 5.11×10⁻³ cm s⁻¹ at the self-assembled electrode. The electrode reaction is a pseudo-reversible process. The peak current and the concentration of NE are a linear relationship in the range of 4.0×10⁻⁵–2.0×10⁻³ mol l⁻¹. The detection limit is 2.0×10⁻⁶ mol l⁻¹. By ac impedance spectroscopy the apparent electron transfer rate constant (k_app) of Fe(CN)³⁻/Fe(CN)⁴⁻ at the TLA/Au electrode was obtained as 2.5×10⁻⁵ cm s⁻¹.     

An iron(III) ion-selective sensor based on a μ-bis(tridentate) ligand

A μ-bis(tridentate) ligand named 2-phenyl-1,3-bis[3′-aza-4′-(2′-hydroxyphenyl)-prop-4-en-1′-yl]-1,3-imidazolidine (I) has been synthesized and scrutinized to develop iron(III)-selective sensors. The addition of sodium tetraphenyl borate and various plasticizers, viz., chloronaphthalene, dioctylphthalate, o-nitrophenyl octyl ether and dibutylphthalate has been used to substantially improve the performance of the sensors. The membranes of various compositions of the ligand were investigated and it was found that the best performance was obtained for the membrane of composition (I) (10 mg):PVC (150 mg):chloronaphthalene (200 mg):sodium tetraphenyl borate (9 mg). The sensor showed a linear potential response to iron(III) over wide concentration range 6.3 × 10⁻⁶ to 1.0 × 10⁻¹ M (detection limit 5.0 × 10⁻⁶ M) with Nernstian slope (20.0 mV/decade of activity) between pH 3.5 and 5.5 with a quick response time of 15 s. The potentiometric selectivity coefficient values as determined by match potential method (MPM) indicate excellent selectivity for Fe³⁺ ions over interfering cations. The sensor exhibits adequate life of 2 months with good reproducibility. The sensor could be used in direct potentiometry.     

Amperometric biosensors based on the immobilization of oxidases in a Prussian blue film by electrochemical codeposition

Prussian blue has been formed by cyclic voltammetry onto the basal pyrolytic graphite surface to prepare a chemically modified electrode which provides excellent electrocatalysis for both oxidation and reduction of hydrogen peroxide. It is found for the first time that glucose oxidase or -amino oxidase can be incorporated into a Prussian blue film during its electrochemical growth process. Two amperometric biosensors were fabricated by electrochemical codeposition, and the resulting sensors were protected by coverage with a thin film of Nafion. The influence of various experimental conditions was examined for optimum analytical performance. The glucose sensor responds rapidly to substrates with a detection limit of 2 × 10⁻⁶ M and a linear concentration range of 0.01–3 mM. There was no interference from 2 mM ascorbic acid or uric acid. Another ( -amino acid) sensor gave a detection limit of 3 × 10⁻⁵ M -alanine, injected with a linear concentration range of 7.0 × 10⁻⁵-1.4 × 10⁻² M. Glucose and -amino acid sensors remain relatively stable for 20 and 15 days, respectively. There is no obvious interference from anion electroactive species due to a low operating potential and excellent permselectivity of Nafion.     

Voltammetric study of the interaction of ciprofloxacin-copper with nucleic acids and the determination of nucleic acids

The behavior of the ciprofloxacin (CPFX) complex with copper, Cu(II)L₂, at a mercury electrode has been investigated in borax–boric acid buffer. The adsorption phenomena were observed by linear sweep voltammetry. The mechanism of the electrode reaction was found to be reduction of Cu(II)L₂ adsorbed on the surface of the electrode by an irreversible charge transfer to metal amalgam, Cu(0)(Hg). In the presence of DNA, the formation of the electrochemically non-active complexes Cu(II)L₂–DNA results in the decrease of the equilibrium concentration of Cu(II)L₂ and its peak current. Under the optimum conditions, the decrease of the peak current is proportional to DNA concentration. The linear ranges are 6.67×10⁻⁸ to 1.20×10⁻⁵ g ml⁻¹ for calf thymus DNA (ctDNA), 3.30×10⁻⁸ to 2.33×10⁻⁶ g ml⁻¹ for fish sperm DNA (fsDNA) and 1.0×10⁻⁸ to 1.2×10⁻⁶ g ml⁻¹ for yeast RNA. The detection limits are 5.00×10⁻⁹, 3.00×10⁻⁹ and 2.50×10⁻⁹ g ml⁻¹, respectively. This method exhibits good recovery and high sensitivity.     

Flow-injection chemiluminescence determination of tetracyclines with in situ electrogenerated bromine as the oxidant

By designing a novel flow-through electrolytic cell (FEC), bromine was produced near to the surface of the platinum electrode by electrochemical oxidation of acidic KBr. The fast and weak chemiluminescence signal produced by the chemical reaction of the electrogenerated bromine with H₂O₂ was greatly enhanced by tetracyclines Based on these observations, a new, sensitive and simple electrogenerated chemiluminescence (ECL) method for the determination of tetracyclines was developed. Under the optimum experimental conditions, the calibration graphs are linear over the range 3.0×10⁻⁸ to 5.0×10⁻⁵ g ml⁻¹ for tetracycline, 2.0×10⁻⁷ to 2.4×10⁻⁵ g ml⁻¹ for oxytetracycline and 1.0×10⁻⁷ to 5.0×10⁻⁵ g ml⁻¹ for chlortetracycline. The limits of detection (S/N=3) are 1.0×10⁻⁸ g ml⁻¹ for tetracycline, 7.0×10⁻⁸ g ml⁻¹ for oxytetracycline and 1.5×10⁻⁷ g ml⁻¹ for chlortetracycline. For the determination 5.0×10⁻⁷ g ml⁻¹ tetracycline, the relative standard deviation was <5%. The proposed method was used to determine tetracyclines in pharmaceutical formulations.     

Fluorescence reaction and complexation equilibria between norfloxacin and aluminium (III) ion in chloride medium

Norfloxacin, 1-ethyl-6-fluoro-1,4-dihydro-4-oxo-7-(1-piperazinyl)-3-quinoline carboxylic acid (NORH), reacts with aluminium(III) ion forming the strongly fluorescent complex [Al(HNOR)]³⁺, in slightly acidic medium. The complex shows maximum emission at 440 nm with excitation at 320 nm. The fluorescence intensity is enhanced upon addition of 0.5% sodium dodecylsulphate. Fluorescence properties of the Al-NOR complex were used for the direct determination of trace amounts of NOR in serum. The linear dependence of fluorescence intensity on NOR concentration, at a NOR to Al concentration ratio of 1:10, was found in the concentration range 0.001–2 μg/ml NOR with a detection limit of 0.1 ng/ml. The ability of aluminium (III) ion to form complexes with NOR was investigated by titrations in 0.1 M LiCl medium, using a glass electrode, at 298 K, in the concentration range: 2 × 10⁻⁴ ≤ [Al] ≤ 8 × 10⁻⁴; 5 × 10⁻⁴ ≤ [NOR] ≤ 9 × 10⁻⁴ mol/dm³; 2.8 ≤ pH ≤ 8.3. The experimental data were explained by the following complexes and their respective stability constants, log(β ± σ): [Al(HNOR)], (14.60 ± 0.05); [Al(NOR)], (8.83 ± 0.08); [A1(OH)₃(NOR)], (−14.9 ± 0.1), as well as several pure hydrolytic complexes of A1³⁺. The structure of the [Al(HNOR)] complex is discussed, with respect to its fluorescence properties.     

Highly selective and sensitive copper(II) membrane coated graphite electrode based on a recently synthesized Schiff’s base

A PVC membrane electrode based on bis-2-thiophenal propanediamine (TPDA) coated directly on graphite is described. The electrode exhibits a Nernstian response for Cu²⁺ over a very wide concentration range (1.0×10⁻¹ to 6.0×10⁻⁸ M) with a detection limit of 3.0×10⁻⁸ M (2.56 ng ml⁻¹). It has a fast response time and can be used for at least 2 months without any major deviation. The proposed sensor revealed very good selectivities for Cu²⁺ over a wide variety of other metal ions and could be used in the pH range of 3.0–7.0. It was successfully used for direct determination of copper in black tea and as an indicator electrode in potentiometric titration of copper ion.     

Selective potentiometric determination of nitrite ion using a novel (4-sulphophenylazo-)1-naphthylamine membrane sensor

A novel polymeric membrane sensor sensitive to (4-sulphophenylazo-)1-naphthylamine (SPAN) based on the use of tris(bathophenanthroline) Ni(II)–SPAN ion pair as an ion exchanger in plasticised PVC membrane is described. The sensor exhibits a linear calibration plot with near-Nernstian anionic slope of −55.0±0.3 mV log[SPAN]⁻¹ over the concentration range 10⁻⁶–10⁻² mol l⁻¹ at pH 7. The sensor shows working range over the pH 6–8, response time of 20 s for 10⁻⁵ mol l⁻¹ and operational lifetime of 8 weeks. The sensor is used for quantification of micro quantities of nitrite ion by a prior conversion into the more lipophilic SPAN ion, which is measured with adequate sensitivity, and high selectivity using SPAN sensor. Validation of the method according to the quality assurance standards shows good performance characteristics. The sensor is satisfactory utilised for potentiometric determination of nitrite ion in wastewater samples and meat products. The results are favourably compared with data obtained using the standard spectrophotometric procedure involving the same reaction.