Electrochemical characteristics of a platinum electrode modified with a matrix of polyvinyl butyral and colloidal Ag containing immobilized horseradish peroxidase

A new hydrogen peroxide biosensor was constructed, which consisted of a platinum electrode modified by a matrix of polyvinyl butyral (PVB) and nanometer-sized Ag colloid containing immobilized horseradish peroxidase (HRP), and using Co(bpy)₃³⁺ as mediator in the hydrogen peroxide solution. The electrochemical characteristics of the biosensor were studied by cyclic voltammetry and chronoamperometry. The modified process was characterized by electrochemical impedance spectroscopy and cyclic voltammetry. The HRP immobilized on colloidal Ag was stable and retained its biological activity. The sensor displays excellent electrocatalytic response to the reduction of H₂O₂. Analytical parameters such as pH and temperature were also studied. Linear calibration for H₂O₂ was obtained in the range of 1×10^–5 to 1×10^–2 M under optimized conditions. The sensor was highly sensitive to H₂O₂, with a detection limit of 2×10^–6 M, and the sensor achieved 95% of steady-state current within 10 s. The sensor exhibited high sensitivity, selectivity and stability.     

Development of a Potentiometric Chemical Sensor for the Rapid Detection of Carbofuran Based on Air Stable Lipid Films with Incorporated Calix[4]arene Phosphoryl Receptor Using Graphene Electrodes

The present article describes a miniaturized potentiometric carbofuran chemical sensor on graphene nanosheets with incorporated lipid films. The graphene electrode was used for the development of a very selective and sensitive chemical sensor for the detection of carbofuran by immobilizing an artificial selective receptor on stabilized lipid films. The artificial receptor was synthesized by transformation of the hydroxyl groups of resorcin[4]arene receptor into phosphoryl groups. This chemical sensor responded for the wide range of carbofuran concentrations with fast response time of ca. 20 s. The presented potentiometric carbofuran chemical sensor is easy to construct and exhibits good reproducibility, reusability, selectivity, rapid response times, long shelf life and high sensitivity of ca. 59 mV/decade over the carbofuran logarithmic concentration range from 10^?6 to 10^?3 M.     

Potentiometric Membrane Sensor for Determination of Saccharin

A potentiometric poly(vinyl chloride) membrane sensor for determination of saccharin is described. It is based on the use of Aliquat 336S-saccharinateion-pair as an electroactive material in plasticized PVC membranes with o-nitrophenyloctylether or dioctylphthalate. The sensor is conditioned for at least two days in 0.1 mol L⁻¹ sodium saccharinate before use. It exhibits fast, stable and Nernstian response for saccharinate ions over the concentration range of 1.0 × 10⁻¹–5.0 × 10⁻⁵ mol L⁻¹ and pH range of 4.5–11. The sensor is used for determination of saccharin in some dosage forms. Results with an average recovery of 101% and a mean standard deviation of 0.2% are obtained which is compared favourably with data obtained using the British pharmacopoeia method. The sensor shows reasonable selectivity towards saccharin in presence of many anions and natural sweeteners.     

Novel Nitrite Membrane Sensor Based on Cobalt(II) Salophen for Selective Monitoring of Nitrite Ions in Biological Samples

A novel PVC-based membrane sensor based on the Co(II)-salophen complex (CSC) is described. The electrode revealed a Nernstian response over a wide nitrite ion concentration range (1.0×10^–6–1.0×10^–1M). The detection limit of the sensor is 8.0×10^–7M. The best performance was obtained with a membrane composition of 33% PVC, 61% ortho-nitrophenyloctyl ether, 3% cobalt(II)-salophen, and 3% hexadecyltrimethylammonium bromide. The potentiometric response of the sensor is independent of the pH of solution in the pH range 4.5–11.9. The electrode exhibits a very fast response time and good selectivity over a variety of common inorganic and organic anions, including fluoride, bromide, iodide, sulfite, nitrate, thiocyanate, thriiodide and perchlorate. The selectivity behavior of the proposed sensor shows substantial improvements compared to the previously reported electrodes for nitrite ion. The membrane sensor can be used for at least 2 months without any divergence in potential. The electrode was successfully applied to the monitoring of nitrite ion in water, sausage, flour, wheat, cheese and milk.     

Enzyme sensor for L-lactate using differential pulse amperometric detection

An enzyme sensor using differential pulse (DP) amperometric detection has been developed based on the measurement of the reduction current of the oxidized form of -nicotinamide adenine dinucleotide (NAD⁺) consumed by an enzyme reaction. This biosensor has the definite advantage to prevent interference caused by electrooxidative species such as ascorbate and uric acid and exhibits higher sensitivity and selectivity in comparison to the classical DC amperometric detector. The linear detection range of this biosensor was 5.0×10^–5 — 5.0×10^–4 mol/l and the relative standard deviation (R.S.D.) at 2.5×10^–4 mol/l was 5.0%.     

Enzyme sensor for L-lactate with a chitosan-mercury film electrode

Summary An amperometric enzyme sensor composed of a mercury film electrode and an enzyme-immobilized chitosan membrane is developed. This biosensor is based on both a mercury film electrode detecting the consumption of dissolved dioxygen following enzymatic reaction, and a chitosan membrane. The latter provides an excellent permselectivity and excludes electroactive interferents. The detection range of this biosensor was 1.0×10^–5–3.0×10^–4 mol/l and the relative standard deviation, R.S.D. at 5.0×10^–5 mol/l was 1.4% (n=3). This biosensor was applied to the direct determination of L-lactate in human serum without pretreatment.     

Potentiometric and conductometric determination of ammonium by gas-diffusion flow injection analysis

Summary Considerable enhancement of selectivity in the potentiometric and conductometric determination of ammonium is provided by gas-diffusion separation in flow injection analysis. Ammonium and potassium selective liquid membrane electrodes can be used for determinations in the concentration range 10^–7–10^–2 mol/l with high precision and fast sample throughput. No interferences are encountered in the presence of ionic species and molecules that likely adsorb when the sensors are in direct contact with the sample. The selectivity over volatile amines is enhanced due to kinetic discrimination. Conductometric detection is shown to be as sensitive as the potentiometric detection. A major advantage, however, is the linear rather than logarithmic relationship between concentration and conductivity.     

Potentiometric sensor for phenylephrine based on phenylephrine-tetraphenylborate lipophilic salt

Phenylephrine (PE) ion-selective electrode based on incorporation of phenylephrine-tetraphenylborate lipophilic salt in plasticized PVC matrices is constructed. The electrode shows near Nernstian response over the concentration range 1.5 × 10^–4-10^–1 M in solutions of pH 2.9–8.0 at 25°C. The electrode resists the effect of heat in the temperature range 25–55°C exhibiting an isothermal temperature coefficient of 0.0007 V/°C. The selectivity of the electrode for PE towards large number of inorganic cations, sugars and amino acids is investigated. Determination of PE in an eye-drops pharmaceutical preparation is carried out using the proposed electrode as potentiometric sensor.     

Potentiometric study using chemically modified silica gel with pyridinium as membrane for ClO 4 − ions

A potentiometric sensor for the perchlorate anion was developed by mixing chemically modified silicagel with pyridinium perchlorate, with an epoxy polymer and graphite. The electrode showed Nernstian response between 1.0 × 10^–2 and 1.0 × 10^–3 M perchlorate concentrations. The electrode showed high selectivity to this ion at solutions pH between 5.5 and 8.0. The presence of IO ₄ ^– , NO ₃ ^– ,Br^–, IO ₃ ^– , Cl^– and SO ₄ ^2– ions in the solutions, had only small interference in the electrode response in the range mentioned.     

Novel Potentiometric Sensor for Determination of Cysteine Based on Substituted Poly(diphenylporphyrins and Metalloporphyrins

This paper describes the application of an electropolymerized film of poly(bis(2-aminophenyl)-2,8,12,18-tetraethyl-3,7,13, 17-tetramethylporphyrin) and its metalloderivative for determination of amino acids. A remarkable selectivity for cysteine (log K _{Cys/Amino acids} ^pot = 10^–2–10^–3) has been found. The potentiometric response to sulfur-containing amino acids is discussed and compared with selectivity data for PVC-membranes based on metallotetraphenylporphyrins.     

Glucose biosensor based on glucose oxidase immobilized in sol–gel chitosan/silica hybrid composite film on Prussian blue modified glass carbon electrode

An improved amperometric glucose biosensor based on glucose oxidase immobilized in sol–gel chitosan/silica hybrid composite film, which was prepared from chitosan (CS) and methyltrimethoxysilane (MTOS), on the surface of Prussian blue (PB)-modified glass carbon electrode was developed. The film was characterized by FT-IR. Effects of some experimental variables such as ratio of CS to silica, buffer pH, temperature, and applied potential on the current response of the biosensor were investigated. The biosensor fabricated under optimal conditions had a linear response to glucose over the range 5.0×10^–5 to 2.6×10^–2 M with a correlation coefficient of 0.9948 and a detection limit of 8.0×10^–6 M based on S/N =3. The biosensor had a fast response time of less than 10 s, a high sensitivity of 420 nA mM^–1, a long-term stability of over 60 days, and a good selectivity. The apparent Michaelis–Menten constant K_m was found to be 3.2×10^–3 M. The activation energy for enzymatic reaction was calculated to be 21.9 kJ mol^–1. This method has been used to determine the glucose concentration in real human blood samples.     

Transport properties and electroanalytical response characteristics of drotaverine ion-selective sensors

The construction and electroanalytical response characteristics of poly(vinyl chloride) matrix ion-selective sensors (ISSs) for drotaverine hydrochloride are described. The membranes incorporate ion-association complexes of drotaverine with tetraphenylborate, picrate, tetraiodomercurate, tetraiodobismuthate, Reinecke salt, and heteropolycompounds of Keggin structure—molybdophosphoric acid, tungstophosphoric acid, molybdosiliconic acid and tungstosiliconic acid as electroactive materials for ionometric sensor controls. These ISSs have a linear response to drotaverine hydrochloride over the range 8×10^–6 to 5×10^–2 mol L^–1 with cationic slopes from 51 to 58 mV per concentration decade. These ISSs have a fast response time (up to 1 min), a low determination limit (down to 4.3×10^–6 mol L^–1), good stability (3–5 weeks), and reasonable selectivity. Permeabilities and ion fluxes through a membrane were calculated for major and interfering ions. Dependences of the transport properties of the membranes on the concentrations of the ion exchanger and near-membrane solution and their electrochemical characteristics are presented. The ISSs were used for direct potentiometry and potentiometric titration (sodium tetraphenylborate) of drotaverine hydrochloride. Results with mean accuracy of 99.1±1.0% of nominal were obtained which corresponded well to data obtained by use of high-performance liquid chromatography.     

Primary amine drug sensitive electrodes based on macrocyclic polyethers with 2,2′-dinaphthyl subunits

Dinaphthyl macrocyclic polyethers were synthesized and used as neutral carriers for preparing primary amine drug sensitive PVC membrane electrodes. Contrary to the ion-associate based electrodes, which show an excellent potentiometric response to quaternary ammonium ions and the like, but a very poor response to primary amines, the macrocyclic polyether-based electrodes showed potentiometric response characteristics with primary amines preferred. Dinaphthyl macrocyclic polyether-based electrodes are superior to those based on common macrocyclic polyethers for their potentiometric selectivity coefficients much lower than those of the latter. The main characteristics of a dinaphthyl-20-crown-6-based benzyl amine sensitive electrode are as follows: linear response range, 4.2 × 10^–5 – 1.0 ×10^–1 M; slope, 51.3 mV/decade; and detection limit, 4.6 × 10^–6 M. A mexiletine sensitive electrode was prepared using dinaphthyl-23-crown-7 with following performance features: linear response range, 2.0 × 10^–5 – 1.0 ×10^–1 M; slope, 52.1mV/decade; and detection limit, 5.0 × 10^–6 M.     

Flow and batch systems for copper(II) potentiometric sensing

A new carbon paste electrode modified with tetramethyl thiuram disulfide is prepared to use as copper potentiometric sensor in batch and flow analysis. The influence of pH and carbon paste composition on the potentiometric response is studied. The principal parameters of the flow system are optimized and the detection limits and the selectivity coefficients of the potentiometric sensor are calculated for static and flow mode. In both cases, the sensor shows high selectivity to copper ions but in flow analysis this selectivity is higher. The obtained detection limits are 4.6 × 10⁻⁸ M for batch measurements and 2.0 × 10⁻⁷ M for on-line analysis. The potentiometric sensor is applied to copper(II) determination in real samples in static and flow measurements. In both analysis modes, successful results are obtained.     

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.     

Amperometric choline biosensors prepared by layer-by-layer deposition of choline oxidase on the Prussian blue-modified platinum electrode

An amperometric choline biosensor was developed by immobilizing choline oxidase (ChOx) in a layer-by-layer (LBL) multilayer film on a platinum (Pt) electrode modified with Prussian blue (PB). 6-O-Ethoxytrimethylammoniochitosan chloride (EACC) was used to prepare the ChOx LBL films. The choline biosensor was used at 0.0 V versus Ag/AgCl to detect choline and exhibited good characteristics such as relative low detection limit (5 × 10⁻⁷ M), short response time (within 10 s), high sensitivity (88.6 μA mM⁻¹ cm⁻²) and a good selectivity. The results were explained based on the ultrathin nature of the LBL films and the low operating potential that could be due to the efficient catalytic reduction of H₂O₂ by PB. In addition, the effects of pH, temperature and applied potential on the amperometric response of choline biosensor were evaluated. The apparent Michaelis-Menten constant was found to be (0.083 ± 0.001) ×10⁻³ M. The biosensor showed excellent long-term storage stability, which originates from a strong adsorption of ChOx in the EACC multilayer film. When the present choline biosensor was applied to the analysis of phosphatidylcholine in serum samples, the measurement values agreed satisfactorily with those by a hospital method.     

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

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

Development of a One-Shot Optical Citrate Sensor Based on a Guanidinium Synthetic Receptor

A new optical absorption-based one-shot sensor for the determination of citrate is described. The citrate-sensitive element is a plasticized polymeric membrane containing a synthetic guanidinium receptor and a pH-sensitive lipophilic dye. The rigid amide-based receptor with C₃ symmetry controls the anion selectivity pattern of the optical element. The optical selectivity coefficients obtained for citrate over a variety of common carboxylate anions meet the requirements for the determination of citrate in different types of samples. The one-shot sensor responds rapidly – in 40 s – and reversibly to citrate over a dynamic range of 0.779 to 25.0 mM with a sensor-to-sensor reproducibility of 5.7%, as log a_cit, in the medium level of the range. The performance of the optical one-shot sensor has been tested in the analysis of citrate in soft drinks and pharmaceuticals, validating the results against a reference procedure. The proposed method is quick, inexpensive, selective and sensitive, and uses conventional instrumentation only.     

Electrochemical Biosensor for Naphthalene Acetic Acid in Fruits and Vegetables Based on Lipid Films with Incorporated Auxin‐binding Protein Receptor Using Graphene Electrodes

The present work describes a miniaturized potentiometric naphthalene acetic acid (NAA) sensor on graphene nanosheets with incorporated lipid films. Auxin‐binding protein 1 receptor immobilized on the stabilized lipid films provided adequate selectivity for detection over a wide range of hormone concentrations, fast response time of ca. 5 min, and detection limit of 10 nM. The proposed sensor is easy to construct and exhibits good reproducibility, reusability, selectivity, long shelf life and high sensitivity of ca. 56 mV/decade of hormone concentration. The reliability of the biosensor was successfully evaluated using a wide range of NAA‐spiked fruits and vegetables.     

A highly sensitive PVC membrane iodide electrode based on complexes of mercury(II) as neutral carrier

A novel solvent polymeric membrane electrode based on bis(1,3,4-thiadiazole) complexes of Hg(II) is described which has excellent selectivity and sensitivity toward iodide ion. The electrode, containing 1,4-bis(5-methyl-1,3,4-thiadiazole-2-yl-thio)butanemercury(II) [Hg(II)BMTB(NO₃)₄], has a Nernstian potentiometric response from 2.0×10^–8 to 2.0×10^–2 mol L^–1 with a detection limit of 8.0×10^–9 mol L^–1 and a slope of –59.0±0.5 mV/decade in 0.01 mol L^–1 phosphate buffer solution (pH 3.0, 20°C). The selectivity sequence observed is iodide>bromide>thiocyanate>nitrite>nitrate>chloride>perchlorate>acetate>sulfate. The selectivity behavior is discussed in terms of the UV–Vis spectrum, and the process of transfer of iodide across the membrane interface is investigated by use of the AC impedance technique. The electrode was successfully applied to the determination of iodide in Jialing River and Spring in Jinyun Mountains, with satisfactory results.