Determination of SCN- in urine and saliva of smokers and non-smokers by SCN(-)-selective polymeric membrane containing a nickel(II)-azamacrocycle complex coated on a graphite electrode

Potentiometric Solid-Contact Ion-Selective Electrode for Determination of Thiocyanate in Human Saliva

A new solid-contact potentiometric ion-selective electrode for the determination of SCN- (SCN-ISE) has been described. Synthesized phosphonium derivative of calix[4]arene was used as a charged ionophore. The research included selection of the ion-selective membrane composition, determination of the ISEs metrological parameters and SCN-ISE application for thiocyanate determination in human saliva. Preparation of the ISEs included selection of a plasticizer for the ion-selective membrane composition and type of the electrode material. The study was carried out using ISE with liquid internal electrolyte (LE-ISE) and solid-contact electrodes made of glassy carbon (GC-ISE) and gold rods (Au-ISE). The best parameters were found for GC sensors for which the ion-selective membrane contained chloroparaffin as a plasticizer (S = 59.9 mV/dec, LOD = 1.6 ´ 10-6 M). The study of potentiometric selectivity coefficients has shown that the thiocyanate-selective sensor could be applied in biomedical research for determination of SCN- concentration in human saliva. The accuracy of the SCN- determination was verified by testing 59 samples of volunteers' saliva by potentiometric sensors and UV-Vis spectrophotometry as a reference technique. Moreover, SCN- concentrations in the smokers' and non-smokers' saliva were compared. In order to investigate the influence of various factors (sex, health status, taken medications) on the thiocyanate level in the saliva, more extensive studies on a group of 100 volunteers were carried out. Additionally, for a group of 18 volunteers, individual profiles of SCN- concentration in saliva measured on a daily basis for over a month were collected.

New selenite ion-selective electrodes based on 5,10,15,20-tetrakis-(4-methoxyphenyl)-21H,23H-porphyrin-Co(II)

New polymeric membrane (PME), modified carbon paste (MCPE), and coated wire (CWE) selenite ion-selective electrodes based on 5,10,15,20-tetrakis-(4-methoxyphenyl)-21H,23H-porphyrin-Co(II) (CoTMeOPP) are reported. The best composition was the electrode containing 2% CoTMeOPP as the active material and 49% TCP as plasticizer. The electrodes reveal a Nernstian behavior over a concentration range of 5.5x10(-5) to 1.1x10(-2) M for PME, 5.2x10(-5) to 1.2x10(-2) M for MCPE and 1.2x10(-4) to 4.4x10(-3) M for CWE. The potentiometric response is pH dependent, since selenous acid is a diprotic acid. The slope of the selenite PVC electrode was -57.0 mV for the monovalent anion at pH 6.47, and -26.0 mV for the divalent anion at pH 11.00. The detection limits were 3.4x10(-5) and 4.7x10(-5) M at pH values 6.47 and 11.00, respectively. The electrodes manifest advantages of low resistance, very short response time (15 s), and most importantly good selectivities relative to a wide variety of other anions. In fact, the proposed selenite ion-selective electrodes show a great improvement compared to previously reported electrodes for selenite ion. The electrode was used for the determination of selenite in selenite/selenate mixture, in sodium selenite raw material powder, and in VitaFit Selenium ACE antioxidant tablets with recovery ranges of 90.0-103.3%.

Kinetic spectrophotometric method for trace determination of thiocyanate based on its inhibitory effect

A kinetic spectrophotometric method for the determination of thiocyanate, based on its inhibitory effect on silver(I) catalyzed substitution of cyanide ion, by phenylhydrazine in hexacyanoferrate(II) is described. Thiocyanate ions form strong complexes with silver(I) catalyst which is used as the basis for its determination at trace level. The progress of reaction was monitored, spectrophotometrically, at 488 nm (lambda(max) of [Fe(CN)(5)PhNHNH(2)](3-), complex) under the optimum reaction conditions at: 2.5x10(-3)M [Fe(CN)(6)](4-), 1.0x10(-3)M [PhNHNH(2)], 8.0x10(-7)M [Ag(+)], pH 2.8+/-0.02, ionic strength (mu) 0.02 M (KNO(3)) and temperature 30+/-0.1 degrees C. A linear relationship obtained between absorbance (measured at 488 nm at different times) and inhibitor concentration, under specified conditions, has been used for the determination of [thiocyanate] in the range of 0.8-8.0x10(-8)M with a detection limit of 2x10(-9)M. The standard deviation and percentage error have been calculated and reported with each datum. A most plausible mechanistic scheme has been proposed for the reaction. The values of equilibrium constants for complex formation between catalyst-inhibitor (K(CI)), catalyst-substrate (K(s)) and Michaelis-Menten constant (K(m)) have been computed from the kinetic data. The influence of possible interference by major cations and anions on the determination of thiocyanate and their limits has been investigated.

Developments in the Field of Conducting and Non-conducting Polymer Based Potentiometric Membrane Sensors for Ions Over the Past Decade

Many research studies have been conducted on the use of conjugated polymers in the construction of chemical sensors including potentiometric, conductometric and amperometric sensors or biosensors over the last decade. The induction of conductivity on conjugated polymers by treating them with suitable oxidizing agents won Heeger, MacDiarmid and Shirakawa the 2000 Nobel Prize in Chemistry. Common conjugated polymers are poly(acetylene)s, poly(pyrrole)s, poly(thiophene)s, poly(terthiophene)s, poly(aniline)s, poly(fluorine)s, poly(3-alkylthiophene)s, polytetrathiafulvalenes, polynapthalenes, poly(p-phenylene sulfide), poly(p-phenylenevinylene)s, poly(3,4-ethylenedioxythiophene), polyparaphenylene, polyazulene, polyparaphenylene sulfide, polycarbazole and polydiaminonaphthalene. More than 60 sensors for inorganic cations and anions with different characteristics based on conducting polymers have been reported. There have also been reports on the application of non-conducting polymers (nCPs), i.e. PVC, in the construction of potentiometric membrane sensors for determination of more than 60 inorganic cations and anions. However, the leakage of ionophores from the membranes based on these polymers leads to relatively lower life times. In this article, we try to give an overview of Solid-Contact ISE (SCISE), Single-Piece ISE (SPISE), Conducting Polymer (CP)-Based, and also non-conducting polymer PVC-based ISEs for various ions which their difference is in the way of the polymer used with selective membrane. In SCISEs and SPISEs, the plasticized PVC containing the ionophore and ionic additives govern the selectivity behavior of the electrode and the conducting polymer is responsible of ion-to-electron transducer. However, in CPISEs, the conducting polymer layer is doped with a suitable ionophore which enhances the ion selectivity of the CP while its redox response has to be suppressed.

Schiff's Bases and Crown Ethers as Supramolecular Sensing Materials in the Construction of Potentiometric Membrane Sensors

Ionophore incorporated PVC membrane sensors are well-established analyticaltools routinely used for the selective and direct measurement of a wide variety of differentions in complex biological and environmental samples. Potentiometric sensors have someoutstanding advantages including simple design and operation, wide linear dynamic range,relatively fast response and rational selectivity. The vital component of such plasticizedPVC members is the ionophore involved, defining the selectivity of the electrodes' complexformation. Molecular recognition causes the formation of many different supramolecules.Different types of supramolecules, like calixarenes, cyclodextrins and podands, have beenused as a sensing material in the construction of ion selective sensors. Schiff's bases andcrown ethers, which feature prominently in supramolecular chemistry, can be used assensing materials in the construction of potentiometric ion selective electrodes. Up to now,more than 200 potentiometric membrane sensors for cations and anions based on Schiff's bases and crown ethers have been reported. In this review cation binding and anioncomplexes will be described. Liquid membrane sensors based on Schiff's bases and crownethers will then be discussed.

Determination of trace amounts of thiocyanate by a new kinetic procedure based on an induction period

A new, simple and sensitive kinetic spectrophotometric method with no need for removing of interfering substances is proposed for the determination of thiocyanate ion in biological and water samples. The procedure is based on the inhibiting effect of thiocyanate on the sodium periodate-potassium bromide-meta cresol purple (MCP) system in acidic media. The induction period of the reaction is proportional to the SCN- concentration. The decolorization of meta cresol purple by the reaction products was used to monitor the reaction spectrophotometrically at 525 nm. Under optimum conditions, thiocyanate can be determined in the range of 0.02-0.8 microg ml(-1) with a 3sigma detection limit of 5 ng ml(-1). The relative standard deviations for 10 replicate determinations of 0.060, 0.10 and 0.50 microg ml(-1) thiocyanate are 3.7, 2.4 and 1.0%, respectively. This method has been successfully used to the determination of thiocyanate content in smokers and non-smokers saliva and spiked water sample.

Extractive liquid–liquid spectrophotometric procedure for the determination of thiocyanate ions employing the ion pair reagent amiloride monohydrochloride

An accurate, inexpensive and less laborious liquid-liquid extractive spectrophotometric procedure for the determination of thiocyanate ions in aqueous media has been developed. The method has been based upon the formation of a yellow colored complex ion associate of the ion-pairing reagent 1-(3, 5-diamino-6-chloropyrazinecarboxyl) guanidine hydrochloride monohydrate, namely amiloride hydrochloride, DPG+.Cl- and the thiocyanate ions in aqueous media containing HNO3 (0.5 mol L(-1)) and subsequent extraction with 4-methyl-2-pentanone. The absorption electronic spectrum of the ion associate showed one well-defined peak at lambda(max) 366 nm. The stoichiometric mole ratio of DPG+.Cl- to the thiocyanate ions is 1:1. The effective molar absorptivity (epsilon) of the ion associate at lambda(max) 366 nm is 1.1+/-0.1x10(4) L mol(-1) cm(-1). The extraction constants (K(d), K(ex), and beta) enabled a simple and convenient use of the developed binary ion associate for the extractive spectrophotometric determination of traces of thiocyanate ions in the aqueous media. Beer's law and Ringbom's plots are obeyed in the concentration range 0.05-10 and 0.1-7 microg mL(-1) of the thiocyanate ions, respectively with a relative standard deviation of +/-2.3%. The calculated lower limits of detection (LOD) and quantitation (LOQ) of the developed procedure for the thiocyanate ions were found equal to 0.02 and 0.066 microg mL(-1), respectively. The developed method has been applied for the determination of trace amounts of thiocyanate ions in tap-, waste- and natural water samples and compared successfully with the reported methods at the 95% confidence level. The proposed method was also applied successfully for the determination of thiocyanate ions in saliva samples.

Determination of trace thiocyanate with nano-silver coated multi-walled carbon nanotubes modified glassy carbon electrode

Novel nano-silver coated multi-walled carbon nanotube composites were prepared and used to fabricate a modified electrode. The application of the nano-silver coated multi-walled carbon nanotube composites modified electrode for determination of trace thiocyanate is demonstrated for the first time. The influence of substrate, pH and interference of coexisting substances was investigated for response properties of the electrode. There was a linear relationship at the range 2.5 x 10(-9) to 5 x 10(-8) mol L(-1) and 5 x 10(-8) to 1 x 10(-6) mol L(-1) of thiocyanate with the decrement of anodic DPV peak currents. The limit of detection was 1 x 10(-9) mol L(-1)(S/N=3). The constructed electrode showed excellent reproducibility and stability. Actual urine and saliva samples of smoker and non-smoker were analyzed and satisfactory results were obtained. This method provides a new way to construct any electrode for biological and environmental analysis.

Potentiometric anion selectivity of polymer-membrane electrodes based on cobalt, chromium, and aluminum salens

Metallo-salens of cobalt(II) (Co-Sal), chromium(III) (Cr-Sal), and aluminum(III) (Al-Sal) are used as the active ionophores within plasticized poly(vinyl chloride) membranes. It is shown that central metal-ion plays a critical role in directing the ionophore selectivity. Polymer-membrane electrodes based on Co-Sal, Cr-Sal, and Al-Sal are demonstrated to exhibit enhanced responses and selectivity toward nitrite/thiocyanate, thiocyanate, and fluoride anions, respectively. The improved anion selectivity of the three ionophore systems is shown to deviate significantly from the classical Hofmeister pattern that is based only on ion lipophilicity. For example, optimized membrane electrodes for nitrite ion based on Co-Sal exhibit logK(Nitrite,Anion)(pot) values of -5.22, -4.66, -4.48, -2.5 towards bromide, perchlorate, nitrate, and iodide anions, respectively. Optimized membrane electrodes based on Co-Sal and Cr-Sal show near-Nernstian responses towards nitrite (-57.9+/-0.9 mV/decade) and thiocyanate (-56.9+/-0.8 mV/decade), respectively, with fast response and recovery times. In contrast, Al-Sal based membrane electrodes respond to fluoride ion in a super-Nernstian (-70+/-3 mV/decade) and nearly an irreversible mode. The operative response mechanism of Co-Sal, Cr-Sal, and Al-Sal membrane electrodes is examined using the effect of added ionic sites on the potentiometric response characteristics. It is demonstrated that addition of lipophilic anionic sites to membrane electrodes based on the utilized metallo-salens enhances the selectivity towards the primary ion, while addition of cationic sites resulted in Hofmeister selectivity patterns suggesting that the operative response mechanism is of the charged carrier type. Electron spin resonance (ESR) data indicates that Co(II) metal-ion center of Co-Sal ionophore undergoes oxidation to Co(III). This process leads to formation of a charged anion-carrier that is consistent with the response behavior obtained for Co-Sal based membrane electrodes.

Highly Selective Iodide Electrode Based on the Copper(II)-N,N′-bis(salicylidene)-1,2-bis(p-aminophenoxy)ethane Tetradentate Complex

In this paper, a new PVC-based liquid-membrane anion-selective electrode based on a copper(II) of N,N'-bis(salicylidene)-1,2-bis(p-aminophenoxy)ethane tetradentate complex (Cu(II)BBAP) is described, which displays a preferential potentiometric response to iodide ion at pH 2.0 and an anti-Hofmeister selectivity sequence: I->SCN->ClO4->NO2->H2PO4->NO3->SO4(2-)>Br->Cl-. The electrode exhibits a near-Nernstian potential linear range of 8.2x10(-7)-1.0x10(-1) M with a detection limit of 5.3x10(-7) M and a slope of -58.8 mV per decade. The A.C. impedance technique and the UV/Vis spectroscopy technique were used to analyze the response mechanism. The electrode could be applied to determine iodide in medicine analysis, and the obtained results were fairly satisfactory.

Thiocyanate Ion-selective PVC Membrane Electrode Based on N,N'-Ethylene-bis(4-methylsalicylidineiminato)nickel(II)

A highly selective poly(vinyl chloride) (PVC) membrane electrode based on an N,N'-ethylene-bis(4-methyl-salicylidineiminato) nickel(II) [Ni(EBMSI)] complex as a carrier for a thiocyanate-selective electrode is reported. The influences of the membrane composition, pH and possible interfering anions were investigated based on the response properties of the electrode. The electrode exhibited a good Nernstian slope of -58.9 +/- 0.7 mV decade(-1), over a wide pH range of 3.5 - 8.5 and a linear range of 1.0 x 10(-6) - 1.0 x 10(-1) M for thiocyanate. The detection limit of electrode was 3.1 x 10(-7) M SCN(-). The selectivity coefficients determined by a fixed interference method (FIM) indicate that a good discriminating ability towards the SCN- ion compared to other anions. The proposed sensor had a fast response time of about 5 - 15 s and could be used for at least 3 months without any considerable divergence in the potential. It was applied as an indicator electrode in the titration of thiocyanate with Ag+ and in the potentiometric determination of thiocyanate in saliva and urine samples.

Potentiometric flow injection analysis of mebeverine hydrochloride in serum and urine

Four PVC membrane electrodes for the determination of mebeverine hydrochloride (MvCl) were fabricated and fully characterized in terms of composition, life span, usable pH range, working concentration range and temperature. The membranes of these electrodes consist of mebeverinium-silicotungstate (Mv-ST), silicomolybdate (Mv-SM), phosphotungstate (Mv-PT), or phosphomolybdate (Mv-PM) ion-associations dispersed in PVC matrix with dibutyl phthalate plasticizer. The electrodes showed near-Nernstian response over the concentration range of 4.0 x 10(-6) to 1.0 x 10(-2)M MvCl and were applied to the potentiometric determination of mebeverinium ion in pharmaceutical preparations, serum and urine in steady state and flow injection (FI) conditions with average recoveries of 96.4-102 % and relative standard deviations of 0.132-1.86%. The electrodes exhibit good selectivity for MvCl with respect to a large number of inorganic cations, organic cations, sugars and amino acids. The sensitivities of these electrodes are high enough to measure as low as 1.86 microg/ml of MvCl which permit the determination of the Ksp values of the ion-associates used. The proposed potentiometric methods offer the advantages of simplicity, accuracy, automation feasibility and applicability to turbid and colored sample solutions.

Potentiometric Membrane Electrode for Salicylate Based on an Organotin Complex with a Salicylal Schiff Base of Amino Acid

A novel salicylate-selective electrode based on an organotin complex with a salicylal Schiff base of amino acid salicylaldehydeaminoacid-di-n-butyl-Sn(IV) [Sn(IV)-SAADB] as ionophore is described, which exhibits high selectivity for salicylate over many other common anions with an anti-Hofmeister selectivity sequence: Sal- >> PhCOO- > SCN- > Cl04- > I- > NO3- > NO2- > Br- > Cl- > CH3COO-. The electrode, based on Sn(IV)-SAADB, with a 30.44 wt% PVC, a 65.45 wt% plasticizer (dioctyl phthalate, DOP), a 3.81 wt% ionophore and a 0.3 wt% anionic additive is linear in 6.0 x 10(-6) - 1.0 x 10(-1) mol l(-1) with a detection limit of 2.0 x 10(-6) mol l(-1) and a slope of 62.0 +/- 1.2 mV/decade of salicylate concentration in a phosphate buffer solution of pH 5.5 at 25 degrees C. The influence on the electrode performances by lipophilic charged additives was studied, and the possible response mechanism was investigated by UV spectra. The electrode was applied to medicine analysis and the result obtained has been satisfactory.

Highly selective PVC-membrane electrodes based on Co(II)-Salen for determination of nitrite ion

A cobalt(II) derivative was used as a suitable ionophore for the preparation of a polymeric membrane nitrite-selective electrode. The electrode reveals a Nemstian behavior over a very wide NO2- ion concentration range (1.0 x 10(-6)-1.0 x 10(-1) M) and a very low detection limit (5.0 x 10(-7) M). The potentiometric response is independent of the pH of solution in the pH range 4.0-9.5. The electrode shows advantages such as low resistance, fast response and, most importantly, good selectivity relative to a wide variety of inorganic and organic anions. In fact, the selectivity behavior of the proposed NO2- ion-selective electrode shows great improvements compared to the previously reported electrodes for nitrite ion. The proposed electrodes could be used for at least 2 months without any significant changes in potentials. The electrode was successfully applied to the determination of nitrate ion concentrations in sausage and milk samples.

Novel Imidazole PVC-Based Sensor Based on a Thiopyrilium Compound

A novel imidazole membrane sensor based on 2,4,6-triphenyl thiopyrilium perchlorate (TTP) as an excellent charged carrier, poly(vinyl chloride) (PVC), the plasticizer dibutyl phthalate (DBP) and oleic acid as an additive is described. The sensor responds to imidazole over a wide concentration range from 1.0 x 10(-5) to 1.0 x 10(-1) M with a slope of +33.5 +/- 0.5 mV per decade. The detection limit of the electrode is 3.0 x 10(-6) M and it can be used for at least four weeks without any measurable change in sensitivity. The sensitivity of the electrode is high enough to permit the detection of as little as 0.2 microg/ml of imidazole without any significant interference from high levels of other components and especially, amino acids. The potentiometric selectivity coefficient data revealed negligible interference from common cations, anions and amino acids. The electrode has a relatively fast response time (<30 s), and good slope stability. The proposed sensor was successfully applied to the determination of imidazole in synthetic serum samples.