A pH responsive electrochemical switch sensor based on Fe(notpH3) [notpH6 = 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylene-phosphonic acid)

The electrochemistry of a macrocyclic metal complex Fe(notpH₃) [notpH₆ = 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylene-phosphonic acid)] reveals that the protonation/deprotonation of the non-coordinated P-OH groups in Fe(notpH₃) affects its formal potential value (E⁰′) considerably. Plotting E⁰′ as function of solution pH gives a straight line with a slope of −585 mV pH⁻¹ in the pH range of 3.4-4.0, which is about ten times larger than the theoretical value of −58 mV pH⁻¹ for a reversible proton-coupled single-electron transfer at 20 °C. A sensitive pH responsive electrochemical switch sensor is thus developed based on Fe(notpH₃) which shows an “on/off” switching at pH ∼ 4.0.     

pHstatvs. single extraction tests to evaluate heavy metals and arsenic leachability in environmental samples

Here we compared the pH_stat test, which examines the leachability of major elements (Ca, Mg, Al, Fe, and Mn), dissolved organic carbon, and trace elements (Cd, Zn, Cu, Pb, and As) in a wide pH range, with single extraction tests based on the use of mild extractants (calcium chloride, acetic acid or EDTA). For this purpose, we examined samples from a variety of environmental conditions (sludges, mineral soils, organic soils, and soils with particulate and/or soluble contamination). Extraction yields obtained with CaCl₂ (0.01 mol L⁻¹) and CH₃COOH (0.43 mol L⁻¹) correlated well with those from the pH_stat at the same pH (r = 0.98 and 0.95, respectively), while the use of EDTA (0.05 mol L⁻¹) led to systematically higher extraction yields than those quantified with the pH_stat at the same pH. However, the pH_stat test had three distinct advantages: (1) it revealed the relationship between the solubility of the main soil phases and pH; (2) it showed the variation in pollutant leachability due to changes in pH; and (3) it better predicted the maximum contaminant availability. Thus we propose that the pH_stat is the best laboratory tests to evaluate the contaminant leachability over a wide range of sample types (soil, sludge, and sediment).     

Electrocatalytic sensor devices: (I) cyclopentadienylnickel(II) thiolato Schiff base monolayer self-assembled on gold

The fabrication of a self-assembled monolayer (SAM) of a cyclopentadienylnickel(II) thiolato Schiff base compound, [Ni(SC₆H₄NC(H)C₆H₄OCH₂CH₂SMe)(η⁵-C₅H₅)]₂ on a gold electrode is described. Effective electronic communication between the Ni(II) centres and the gold surface was established by electrochemically cycling the Schiff base-doped Au electrode in 0.1 M NaOH from −200 mV to +600 mV. The SAM-modified electrode exhibited quasi-reversible electrochemistry. The integrity of this electrocatalytic SAM, with respect to its ability to block and electro-catalyse certain Faradaic processes, was interrogated using cyclic voltammetric experiments. The formal potential, E°′, varied with pH to give a slope of about −30 mV pH⁻¹. The surface concentration, G, of the nickel redox centres was found to be 1.548×10⁻¹¹ mol cm⁻². By electrostatically doping the SAM using an applied potential of +700 mV versus Ag/AgCl, in the presence of horseradish peroxidase (HRP), it was fine-tuned for amperometric determination of H₂O₂. The electrocatalytic-type biosensor displayed typical Michaelis-Menten kinetics and the limit of detection was found to be 6.25 mM.     

High dielectric constant PrYxOy sensing films electrolyte-insulator-semiconductor pH-sensor for the detection of urea

In this paper, we describe the structural and sensing properties of high-k PrY_xO_y sensing films deposited on Si substrates through reactive co-sputtering. Secondary ion mass spectrometry and atomic force microscopy were employed to analyze the compositional and morphological features of these films after annealing at various temperatures. The electrolyte-insulator-semiconductor (EIS) device incorporating a PrY_xO_y sensing membrane that had been annealed at 800 °C exhibited good sensing characteristics, including a high sensitivity (59.07 mV pH⁻¹ in solutions from pH 2 to 12), a low hysteresis voltage (2.4 mV in the pH loop 7 → 4 → 7 → 10 → 7), and a small drift rate (0.62 mV h⁻¹ in the buffer solution at pH 7). The PrY_xO_y EIS device also showed a high selective response towards H⁺. This improvement can be attributed to the small number of crystal defects and the large surface roughness. In addition, the enzymatic EIS-based urea biosensor incorporating a high-k PrY_xO_y sensing film annealed at 800 °C allowed the potentiometric analysis of urea, at concentrations ranging from 1 to 16 mM, with a sensitivity of 9.59 mV mM⁻¹.     

Direct electrochemistry and electrocatalysis of hemoglobin in gelatine film modified glassy carbon electrode

Direct electrochemical and electrocatalytic behavior of hemoglobin (Hb) immobilized on glass carbon electrode (GCE) containing gelatine (Gel) films was investigated. The characteristics of Hb/Gel film modified GC electrode were performed by using SEM microscopy, UV-vis spectroscopy and electrochemical methods. The immobilized Hb showed a couple of quasi-reversible redox peak with a formal potential of −0.38 V (versus SCE) in 0.1 M pH 7.0 PBS. The formal potential changed linearly from pH 4.03 to 8.41 with a slope value of −52.0 mV pH⁻¹, which suggested that a proton transfer was accompanied with each electron transfer (ET) in the electrochemical reaction. The Hb/gelatine/GCE displayed a rapid amperometric response to the reduction of H₂O₂ and nitrite.     

Simultaneous voltammetric detection of ascorbic acid, dopamine and uric acid using a pyrolytic graphite electrode modified into dopamine solution

Pyrolytic graphite electrodes (PGE) were modified into dopamine solutions using phosphate buffer solutions, pH 10 and 6.5, as supporting electrolyte. The modification process involved a previous anodization of the working electrode at +1.5 V into 0.1 mol L⁻¹ NaOH followed by other anodization step, in the same experimental conditions, into dopamine (DA) solutions. pH of the supporting electrolyte performed an important role in the production of a superficial melanin polymeric film, which permitted the simultaneous detection of ascorbic acid (AA), (DA) and uric acid (UA), ΔE_AA-DA = 222 mV; ΔE_AA-UA = 360 mV and ΔE_DA-UA = 138 mV, avoiding the superficial poisoning effects. The calculated detection limits were: 1.4 × 10⁻⁶ mol L⁻¹ for uric acid, 1.3 × 10⁻⁵ mol L⁻¹ for ascorbic acid and 1.1 × 10⁻⁷ mol L⁻¹ for dopamine, with sensitivities of (7.7 ± 0.5), (0.061 ± 0.001) and (9.5 ± 0.05) A mol⁻¹ cm⁻², respectively, with no mutual interference. Uric acid was determined in urine, blood and serum human samples after dilution in phosphate buffer and no additional sample pre-treatment was necessary. The concentration of uric acid in urine was higher than the values found in blood and serum and the recovery tests (92-102%) indicated that no matrix effects were observed.     

Determination of Imidacloprid and metabolites by liquid chromatography with an electrochemical detector and post column photochemical reactor

A procedure for the determination of Imidacloprid and its main metabolites was set up by means of liquid chromatography with an electrochemical detector and post-column photochemical reactor (LC-hν-ED). Sample clean-up was developed for bees, filter paper and maize leaves. Chromatographic conditions were based on a reversed-phase C-18 column operated by phosphate buffer 50 mM/CH₃CN (80/20, v/v) at pH 2.9. Detection of Imidacloprid and its metabolites was performed at a potential of 800 mV after photoactivation at 254 nm. Compared to conventional techniques such as gas chromatography/mass spectrometry (GC/MS) or LC coupled to other detectors, the present method allows simultaneous trace-level determination of both Imidacloprid (0.6 ng ml⁻¹) and its main metabolites (2.4 ng ml⁻¹).     

Batch and hydrodynamic monitoring of vitamin C using novel periodate selective sensors based on a newly synthesized Ni(II)-Schiff bases complexes as a neutral receptors

A highly selective membrane electrodes based on a two newly synthesized nickel (II) Schiff bases, [NiL¹] and [NiL²] where L¹ and L² are N,N/bis(salicylaldehyde)4,5-dimethyl-1,2-phenylenediamine (H₂L¹) and N,N/bis(salicylaldehyde)4,5-dichloro-1,2-phenylenediamine (H₂L²) were used as a neutral carrier ionophores for static and hydrodynamic potentiometric mode of operations for the determination of periodate. Under static mode of operation, the sensors displayed a near-Nernstian slope of −66.1 ± 0.8 and −59.9 ± 1.1 mV decade⁻¹ of activity and detection limits to 5.2 × 10⁻⁶ and 7.3 × 10⁻⁶ mol L⁻¹ for the sensors based on [NiL¹] and [NiL²], respectively. Under hydrodynamic mode of operation (FIA), the slope of the calibration plot, limit of detection, and working linear range were −71.1 mVdecade⁻¹ of activity, 7.3 × 10⁻⁶ and 1.0 × 10⁻⁵ to 1.0 × 10⁻³ mol L⁻¹, respectively. The response time of the sensors in whole concentration ranges was very short (<10 s). The response of the sensors was independent on the pH range of 3-8. A tubular version was further developed and coupled to a flow injection system for ascorbic acid (AA) determination in beverages and pharmaceutical preparations. This approach was achieved by selecting a 50-cm reactor and an overall flow of 3 mL min⁻¹, and injecting volume 100 μL of AA standards in a 1.0 × 10⁻⁴ mol L⁻¹ IO₄⁻ solution. Under these conditions, a linearity range of 2-13 μg mL⁻¹, with a slope of 4.97 mV (mg/L)⁻¹ (r² = 0.9995), detection limit 0.9 mg L⁻¹ and a reproducibility of ±1.1 mV (n = 5) was recorded. This simple and inexpensive flow injection analysis manifold, with a good potentiometric detector, enabled the analysis of ∼50 samples h⁻¹ without requiring pretreatment procedures. An average recovery of 98.8% and a mean standard deviation of 1.3% were obtained.     

p-Aminobenzoate ion determination in pharmaceutical formulations by using a potentiometric sensor immobilized in a graphite matrix

The characteristics, performance, and application of an electrode, namely, Pt|Hg|Hg₂(PABzt)₂| graphite, where PABzt stands for p-aminobenzoate ion, are described. This electrode responds to PABzt with sensivity of (58.1±1.0) mV per decade over the range 1.0×10⁻⁴ to 1.0×10⁻¹ mol l⁻¹ at pH 6.5-8.0 and a detection limit of 3.2×10⁻⁵ mol l⁻¹. The electrode shows easy construction, fast response time (within 10-30 s), low-cost, and excellent response stability (lifetime greater than 6 months, in continuous use). The proposed sensor displayed good selectivity for p-aminobenzoate in the presence of several substances, especially, concerning carboxylate and inorganic anions. It was used to determine p-aminobenzoate in pharmaceutical formulations by means of the standard additions method. The results obtained by using this electrode compared very favorably with those given by an HPLC procedure.     

Application of a new potentiometric method for determination of phosphate based on a surfactant-modified zeolite carbon-paste electrode (SMZ-CPE)

A phosphate-selective electrode based on surfactant-modified zeolite (SMZ) particles into carbon-paste has been proposed (SMZ-CPE). The electrode was fully characterized in terms of composition, response time, ionic strength, thermal stability and usable pH range. The electrode containing 20% SMZ exhibited linear response range to phosphate species in the range of 1.58 × 10⁻⁵ to 1.00 × 10⁻² M with a detection limit of 1.28 × 10⁻⁵ M and a Nernstian slope of 29.9 ± 0.9 mV per decade of phosphate concentration. The electrode response to phosphate remains constant in the pH range of 4-12 and in the presence of 1 × 10⁻⁴ to 4 × 10⁻³ M NaNO₃. The response of the electrode reaches equilibrium within several seconds after immersing the electrode in phosphate solution. Common anions such as Cl⁻, Br⁻, I⁻, NO₃⁻, SO₄²⁻ and Cr₂O₇²⁻ have little effect on the determination of phosphate but AsO₄³⁻ shows some interference. A successful application of the electrode for determination of phosphate in a fertilizer, using direct potentiometry, is presented. The electrode was also used for the potentiometric titration of phosphate. The validation of the obtained results in each case was proved by statistical methods.     

A newly synthesized macrocyclic dithioxamide receptor for phosphate sensing

Polyvinyl chloride (PVC) based membranes using macrocyclic dithioxamide receptor (I) derived from isophthaloyl dichloride and dithioxamide have been prepared and explored as HPO₄²⁻-selective sensors. Effect of various plasticizers viz., bis(2-ethylhexyl) sebacate (DOS), dibutylphosphate (DBP), tri-n-butylphosphate (TBP), O-nitrophenyl octyl ether (NPOE), tris(2-ethylhexyl)phosphate (TEHP) and a cation excluder, tridodecylmethylammonium chloride (TDDMACl) was studied in detail and improved performance was observed at several instances. Optimum performance was observed with the membrane having (I)-PVC-TDDMACl-NPOE in the ratio 2:33:1.5:63.5 (w/w). The sensor works satisfactorily in the concentration range 1.7 × 10⁻⁶ to 1.0 × 10⁻² M (detection limit 0.2 ppm) with Nernstian compliance (29.6 mV/decade of activity) at pH 8.0 with a fast response time of about 8 s. The potentiometric selectivity coefficient values as determined by the matched potential method (MPM) and the fixed interference method (FIM) indicate selective response for HPO₄²⁻ in presence of interfering ions. The sensor exhibits adequate shelf life (∼2 months) with good reproducibility (S.D. ± 0.4 mV). The sensor was also used successfully in the potentiometric titration of HPO₄²⁻ with Ba²⁺.     

A lead(II)-selective PVC membrane based on a Schiff base complex of N,N'-bis(salicylidene)-2,6-pyridinediamine

PVC membrane electrodes for lead ion based on N,N’-bis(salicylidene)-2,6-pyridinediamine as membrane carrier were prepared. Among their membranes, a membrane electrode (m-3) containing o-NPOE as a plasticizer and 50 mol% additive displays an excellent Nernstian response (29.4 mV/decade) and the limit of detection of −log a (M) = 6.04 to Pb²⁺ in Pb(NO₃)₂ solutions at room temperature. It has a rapid response time within 10 s over the entire concentration range. The proposed electrode revealed good selectivity and response for Pb²⁺ over a wide variety of other metal ions in a pH 5.0 buffer solutions, and good reproducibility of base line in subsequent measurements.     

Enthalpy change and mechanism of oxidation of o-phenylenediamine by hydrogen peroxide catalyzed by horseradish peroxidase

The hydrogen peroxide-oxidation of o-phenylenediamine (OPD) catalyzed by horseradish peroxidase (HRP) at 37 °C in 50 mM phosphate buffer (pH 7.0) was studied by calorimetry. The apparent molar reaction enthalpy with respect to OPD and hydrogen peroxide were −447 ± 8 kJ mol⁻¹ and −298 ± 9 kJ mol⁻¹, respectively. Oxidation of OPD by H₂O₂ catalyzed by HRP (1.25 nM) at pH 7.0 and 37 °C follows a ping-pong mechanism. The maximum rate V_max (0.91 ± 0.05 μM s⁻¹), Michaelis constant for OPD K_m,S (51 ± 3 μM), Michaelis constant for hydrogen peroxide Km,H₂O₂ (136 ± 8 μM), the catalytic constant k_cat (364 ± 18 s⁻¹) and the second-order rate constants k₊₁ = (2.7 ± 0.3) × 10⁶ M⁻¹ s⁻¹ and k₊₅ = (7.1 ± 0.8) × 10⁶ M⁻¹ s⁻¹ were obtained by the initial rate method.     

Rosmarinic acid determination using biomimetic sensor based on purple acid phosphatase mimetic

A new heterodinuclear Fe(III)Zn(II) complex which mimics the active site of the hydrolytic enzyme red kidney bean purple acid phosphatase was synthesized and characterized by IR, CHN and X-ray crystallographic analyses. This complex, [Fe^IIIZn^II(μ-OH)bpbpmp-CH₃](ClO₄)₂, containing the ligand (H₂bpbpmp-CH₃ = {2-[bis(2-pyridylmethyl)aminomethyl]-6-[(2-hydroxy-5-methylbenzyl) (2-pyridyl-methyl) aminomethyl]-4-methyl-phenol}) was employed in the construction of a biomimetic sensor and used in the determination of rosmarinic acid in plant extract samples. The response parameters and optimization of the biomimetic sensor design were evaluated. The best performance of this sensor was obtained for 75:15:10% (w/w/w) of the graphite powder:nujol:Fe(III)Zn(II) complex, 0.1 mol L⁻¹ phosphate buffer solution (pH 7.5), 1.19 × 10⁻⁴ mol L⁻¹ hydrogen peroxide with frequency, pulse amplitude, and scan increment at 30 Hz, 100 mV, and 0.6 mV, respectively. The rosmarinic acid concentration was linear in the range of 2.98 × 10⁻⁵ to 3.83 × 10⁻⁴ mol L⁻¹ (r = 0.9991) with a detection limit of 2.30 × 10⁻⁶ mol L⁻¹. This biomimetic sensor demonstrated long-term stability (300 days; 900 determinations) and reproducibility, with a relative standard deviation of 12.0%. The recovery study of rosmarinic acid in plant extract samples gave values from 90.3 to 98.3% and the concentrations determined showed agreement when compared with those obtained using capillary electrophoresis at the 95% confidence level.     

A hydrogen peroxide biosensor based on Langmuir-Blodgett technique: Direct electron transfer of hemoglobin in octadecylamine layer

The present paper describes the modification of hemoglobin (Hb)-octadecylamine (ODA) Langmuir-Blodgett (LB) film on a gold electrode surface to develop a novel electrochemical biosensor for the detection of hydrogen peroxide. Atomic force microscopy (AFM) image of Hb-ODA LB film indicated Hb molecules existed in ODA layer in a well-ordered and compact form. The immobilized Hb displayed a couple of stable and well-defined redox peaks with an electron transfer rate constant of 4.58 ± 0.95 s⁻¹ and a formal potential of −185 mV (versus Ag/AgCl) in phosphate buffer (1.0 mM, pH 5.0) contain 0.1 M KCl at a scan rate of 200 mV s⁻¹, characteristic of Hb heme Fe(III)/Fe(II) redox couple. The formal potential of Hb heme Fe(III)/Fe(II) redox couple in ODA film shifted linearly between pH 5 and 8 with a slope of −23.8 mV pH⁻¹, suggesting that proton took part in electrochemical reaction. The ODA could accelerate the electron transfer between Hb and the electrode. This modified electrode showed an electrochemical activity to the reduction of hydrogen peroxide (H₂O₂) without the aid of any electron mediator.     

Organic tin compounds combined with anionic additives-an ionophore system leading to a phosphate ion-selective electrode?

The potentiometric response characteristics of electrodes based on organic trialkyl/aryl-tin compounds combined with various amounts of anionic additive (NaTFPB) were investigated in 0.1 M bis-Tris-H₂SO₄ buffer solution at pH 7.0. The best result for phosphate sensing was obtained for the electrode membrane containing tributyltin chloride and 25 mol% NaTFPB, where the electrode exhibits high selectivity towards phosphate anions with a slope of −60 mV per decade. It was demonstrated that the interference from more lipophilic anions is drastically suppressed (, i=H₂PO₄⁻: salicylate, 0.5; SCN⁻, −0.8; ClO₄⁻, −2.3) under this optimized measurement conditions.     

Stabilization of the trivalent oxidation state of copper by tridentate imine–oxime–amine ligands

Two tridentate imine–oxime–amine ligands have been synthesized and their corresponding copper(II) complexes have been isolated. These copper(II) complexes are readily oxidized both chemically and electrochemically to give relatively stable copper(III) complexes. In the pH range 1.5–3.0 the electron transfer process is electrochemically reversible with ΔE_p = 60 mV and i_pa/i_pc ∼ 1. Plots of E_1/2 versus pH are linear with a slope = −60 indicating the involvement of one proton in the electron transfer process. Aqueous solutions of copper(III) complexes have high molar absorption at λ_max with ε > 10⁴ M⁻¹ cm⁻¹. Solid samples of the complexes are diamagnetic consistent with a d⁸ square planar geometry. It seems that only imine–oxime nitrogens are coordinated to copper(II) with the NH₂ group being free as indicated by i.r. spectra. Substitution of a –CH₃ group on the carbon atom adjacent to the oxime group by the more electron donating group –CH(CH₃)₂ lowers electrode potential by more than 90 mV. This is consistent with an earlier observation that electron-donating substituents on the carbon atom adjacent to the oxime group lower the potential of Cu^III/Cu^II couples and stabilize the higher oxidation state.     

The petentiometric determination of peroxide hydrogen and glucose on the glassy electrode modified by the calix[4]arene

A new petentiometric method to determine peroxide hydrogen and glucose had been studied. This method had been applied on the petentiometric determination of peroxide hydrogen and glucose in the total ionic strength adjustment buffer (TISAB) (pH 7.5) solution with the glassy electrode modified by the calix[4]arene. The glassy carbon electrode covered with the calix[4]arene depended on the H₂O₂ concentration in the range of log[H₂O₂] from −3.3 to −1.2 in the solution of TISAB (pH 7.5) with nearly Nernstian slope of about 65.6 ± 3 mV and the detection limit of peroxide hydrogen was 4.0 × 10⁻⁵ mol L⁻¹. The glassy carbon electrode covered with the calix[4]arene depended on the glucose concentration in the range of log[glucose] from −3.6 to −2.8 in the solution of TISAB (pH 7.5) with nearly Nernstian slope of about 50.2 ± 2 mV and the detection limit of glucose was 2.0 × 10⁻⁵ mol L⁻¹. The electrode had the good selectivity, sensitivity, stability and repeatability.     

Electrochemical determination of 2,4-D at a mercury electrode

An indirect electrochemical determination of 2,4-dichlorophenoxyacetic acid (2,4-D), has been presented. The method is based on the adsorption and desorption of 2,4-D on mercury electrode. Also, the electrochemical behavior of 2,4-D in aqueous solutions at different pH values and different 2,4-D concentrations were studied. A simple and rapid method has been developed for its extraction from water and soil. The subsequent determination was carried out by a tensammetric method. Three calibration curves could be obtained from different parts of voltammogram. Under the optimum conditions (pH = 2.3; E_acc = −1100 mV; t_acc = 60 s; alternative current mode; ν = 40 mV s⁻¹; pulse height = 20 mV; modulation frequency = 60 Hz; phase angle = 90°) the limit of detection was 50 μg L⁻¹. The proposed method was applied to the determination of 2,4-D in real samples such as soil and water.     

Determination of chlorogenic acid in coffee using a biomimetic sensor based on a new tetranuclear copper(II) complex

A new tetranuclear copper(II) complex which mimics the active site of catechol oxidase was synthesized and characterized by IR, CHN, electronic spectroscopic and ¹H NMR methods. The title complex [Cu₂(μ-OH)(bpbpmp-NO₂)]₂[ClO₄]₂ was employed in the construction of a novel biomimetic sensor and used in the determination of chlorogenic acid by square wave voltammetry. The performance and optimization of the resulting biomimetic sensor were studied in detail. The best response of this sensor was obtained for 75:15:10% (w/w/w) ratio of the graphite powder:nujol:Cu(II) complex, 0.1 mol L⁻¹ phosphate buffer solution (pH 7.0), with frequency, pulse amplitude, and scan increment at 30 Hz, 100 mV, and 3.0 mV, respectively. The chlorogenic acid concentration was linear in the range of 5.0 × 10⁻⁶ to 1.45 × 10⁻⁴ mol L⁻¹ (r = 0.9985) with a detection limit of 8.0 × 10⁻⁷ mol L⁻¹. This biomimetic sensor demonstrated long-term stability (250 days; 640 determinations) and reproducibility, with a relative standard deviation of 10.0%. The recovery study of chlorogenic acid in coffee samples gave values from 93.2% to 106.1% and the concentrations determined showed good agreement when compared with those obtained using capillary electrophoresis at the 95% confidence level.