Characteristics of new composite- and classical potentiometric sensors for the determination of pharmaceutical drugs

The characteristics of new types of coated wire potentiometric sensors based on composite coatings were compared with classical potentiometric sensors. The composite sensors contained respectively the clay montmorillonite (MM) and the zeolite NaY as the ionically conducting components, embedded in PVC based rubber phase membranes. The behavior of 20 basic medicinal drugs and 5 biogenic amines was studied on 9 potentiometric sensors of different composition. The behavior of 3 composite sensors, and 6 more classical PVC based sensors either of the “inner solution” or “coated wire” type were studied. The analytes were chosen to cover a wide log P range of, e.g. −1.54 (noradrenaline) up to +5.55 (promethazine). All sensors were tested using a high-throughput FIA-based method. The results were interpreted via statistical data analysis. The responses of all electrodes had a very high correlation to the log P of the analytes. This was also the case for the ion-pair based electrodes containing a specific cationic drug as the counterion. Classical ion-pair based sensors containing tetrakis (p-chlorophenyl) borate (TCPB) and a counterion with a high log P value (e.g. promazine) were the least sensitive. The composite-based sensors were the most sensitive. Coated wire electrodes were statistically shown to behave in the same way (selectivity and sensitivity) as inner solution electrodes. The results are discussed using a physico-chemical model. Practical applications of the most performant (composite) sensors are shown in HPLC detection of the pharmaceutical drugs and the biogenic amines. Detection limits in the 10⁻⁷ M regio (injected concentrations) are obtained for lipophilic drugs (log P > 2).     

PVC membrane and coated graphite potentiometric sensors based on 1-phenyl-3-pyridin-2-yl-thiourea for selective determination of iron(III)

The construction and performance characteristics of PVC membrane (PME) and coated graphite (CGE) Fe³⁺ ion selective electrodes based on 1-phenyl-3-pyridin-2-yl-thiourea (PPT) are described. The electrodes exhibit a Nernstian slope of 20.2 ± 0.8 (CGE) and 19.9 ± 0.4 (PME) mV decade⁻¹ of activity in Fe³⁺ ion over a wide concentration range from 3.0 × 10⁻⁷ to 1.0 × 10⁻² M for CGE and 6.9 × 10⁻⁷ to 1.0 × 10⁻² M Fe³⁺ for PME. The lower detection limits by CGE and PME are 2.0 × 10⁻⁷ and 3.9 × 10⁻⁷ M, respectively, in the pH range of 1.8-5.6 for PME and 1.8-5.8 for CGE with a fast response time (<10 s). The standard electrode potentials were determined at different temperatures and used to calculate the isothermal coefficient of the PME. The PME showed the working temperature range of 22-55 °C with isothermal temperature coefficient of 1.33 × 10⁻³ V/°C and it was also used in non-aqueous solvents. The electrodes were successfully applied to determine iron(III) in water samples.     

A new cerium (III) ion selective electrode based on 2,9-dihydroxy-1,10-diphenoxy-4,7-dithia decane, a novel synthetic ligand

In the present study, a novel electrode based on 2,9-dihydroxy-1,10-diphenoxy-4,7-dithiadecane (DHDPDTD) that is selective to cerium (III) cations was evaluated electrochemically, and a Nerenstian slope (19.3 ± 1 mV decade⁻¹) over a concentration range of 1.0 × 10⁻⁸–1.0 × 10⁻¹ M and a detection limit of 2.1 × 10⁻⁹ M were observed. The proposed electrochemical sensor displayed a rapid response time of 10 s, improved selectivity towards Ce (III) cations in the presence of alkali, alkaline earth, transition and heavy metal cations, and could be used in a pH range of 5.0–8.0. Additionally, the proposed sensor was used as an indicator in the potentiometric titration of fluoride and the determination of F⁻ ions in real samples.     

Influence of ionic and nonionic surfactants on analytical parameters of ion-selective electrodes based on chelating active substances

The effects of the cationic (tetrabutylammonium chloride, TBC), anionic (sodium dodecyl sulfate, SDS) and nonionic (TRITON X-100) surfactants on the potentiometric properties of zinc- and cadmium-selective electrodes (ISEs) were investigated. The studies were carried out with plasticized PVC membranes doped with several new acidic chelating ionophores. The electrode basic analytical parameters, such as measurement range, slope characteristics, detection limit, response time and selectivity coefficients in relation to some inorganic cations in the presence and absence of surfactants, were investigated. As follows from the studies, the presence of surfactants in the sample is responsible first of all for the increase in response time and in detection limit, and a decrease in the characteristic slope as well as reduction of electrode selectivity.     

Novel coated graphite electrode for the selective determination of Gd(III) in rocks and waste water samples

A novel gadolinium selective coated graphite electrode based on 2,6-bis-[1-{N-cyanopropyl,N-(2-methylpridyl)}aminoethyl]pyridine [P] is described. The best performance was exhibited by the electrode having membrane composition P:NaTPB:PVC:NPOE as 8:4:30:58 (%, w/w). The electrode demonstrates excellent potentiometric characteristics towards gadolinium ion over several interfering ions. The electrode exhibited a Nernstian response to Gd³⁺ ion over a wide concentration range 2.8 × 10⁻⁷ to 5.0 × 10⁻² M with a detection limit (6.3 ± 0.1) × 10⁻⁸ M and slope 19.6 ± 0.1 mV decade⁻¹ of a_Gd³⁺. Furthermore, it showed a fast response time (12 s) and can be used for 2.5 months without significant divergence in its characteristics. Noticeably, the electrode can tolerate the concentration of different surfactants up to 1.0 × 10⁻⁴ M and can be used successfully in 30% (v/v) ethanol media and 10% (v/v) methanol and acetonitrile water mixture. The useful pH range of this sensor is 2.0 to 8.0. It is sufficiently selective and can be used for the determination of Gd³⁺ ions in waste water and rock samples. It also serves as a good indicator in the potentiometric titration of GdCl₃ with EDTA.     

Determination of neodymium(III) ions in soil and sediment samples by a novel neodymium(III) sensor based on benzyl bisthiosemicarbazone

The PVC membrane, containing benzyl bisthiosemicarbazone (BTC) as a suitable ionophore, exhibited a Nernstian response for the Nd³⁺ ions over a wide concentration range between 1.0 × 10⁻² and 1 × 10⁻⁶ M, with a detection limit of 6.2 × 10⁻⁷ M in the pH range of 3.7-8.3. It demonstrated a fast response time (<10 s) and it could be used for at least 7 weeks without any major potential deviation. Furthermore, the electrode revealed high selectivity with respect to all the common alkali, alkaline earth, transition and heavy metal ions, including the members of the lanthanide family other than Nd³⁺. Concerning its applications, it was effectively employed for the determination of neodymium ions in soil and sediment samples and its validation with CRM.     

Design and construction of two new polymers featuring macrocyclic subunits based on a rigid clamp-like ligand

Reaction of a rigid conjugated clamp-like ligand N,N′-bis(pyridin-3-yl)-2,6-pyridinedicarboxamide (bppdca) with ZnSO₄·7H₂O or CoSO₄·7H₂O resulted in the formation of two new polymers, namely, {[Zn₂(bppdca)(SO₄)₂(DMF)₂]·(DMF)}_n (1) and {[Co(bppdca)(SO₄)(CH₃OH)(DMF)]·H₂O}_n (2). Both 1 and 2 feature one-dimensional double-chain structures with macrocyclic subunits, and the chains further self-assemble into a higher-dimensional framework via the hydrogen-bonding and π–π stacking interactions. Fluorescence studies show that the free ligand displays a strong fluorescence emission in solid state at room temperature, but in 1 and 2, the complexation of the ligand with metal ions and weak intermolecular interactions make the fluorescence emission partial quenching.     

Zinc ion-imprinted polymer based on silica particles modified carbon paste electrodes for highly selective electrochemical determination of zinc ions

ABSTRACT

In this study, Zn(II) ion-imprinted polymer was prepared on the surface of vinyl silica particles and applied for detection of Zn(II) ions using differential pulse voltametry. The ion- imprinted polymer particles were prepared by free radical polymerization. The prepared particles were characterized by different morphological and elemental techniques. The ion-imprinted particles were used to fabricate the carbon paste electrode as a zinc ions sensor. The modified zinc sensor showed linear response in the concentration range 6.12 × 10^?9 to 4.59 × 10^?8 mol L^?1. The limit of detection and limit of quantification of the electrode were 1.351 × 10^?8 and 4.094 × 10^?8 mol L^?1, respectively.     

A highly selective amperometric sensor for ascorbic acid based on mesopore-rich active carbon-modified pyrolytic graphite electrode

A kind of new mesopore-rich active carbon (MRAC) was firstly prepared by carbonizing bamboo micro-particles, which was utilized for constructing MRAC-modified pyrolytic graphite electrode (MRAC/PGE). The electrochemical behavior of ascorbic acid (AA) was in detail investigated at the MRAC/PGE. The proposed MRAC/PGE showed an excellent electrocatalytic response towards AA oxidation in neutral buffer solution. The oxidation potential of AA significantly decreased at MRAC/PGE. The oxidation current of AA obtained at the MRAC/PGE is 4.6-fold higher than that of the bare PGE. Using amperometric method, the anodic current is linear with AA concentration in the range of 0.5-2000 μM, with a detection limit about 0.3 μM. Furthermore, the proposed electrode can also avoid major interferences such as dopamine, uric acid, urea, and so on. This new method has been successfully applied for AA assay in urine and pharmaceutical preparations.     

Fabrication of novel coated graphite electrodes for the selective nano-level determination of Cd ions in biological and environmental samples

Novel cadmium selective coated graphite electrodes were prepared using three different ionophores N¹, N²-dicyanoethyl-N¹, N²-bis(pyridin-2-ylmethyl)benzene-1, 2-diamine [L₁], N¹, N²-dicyanoethyl-N¹, N²-bis(thiophen-2-ylmethyl) benzene-1, 2-diamine [L₂] and N¹, N²-dicyanoethyl-N¹, N²-bis(furan-2-ylmethyl)benzene-1, 2-diamine [L₃], and their potentiometric characteristics were determined. Membranes having different compositions of poly(vinylchloride) (PVC), the plasticizer o-nitrophenyloctylether (o-NPOE), sodium tetraphenylborate (NaTPB) as an anionic additive and ionophores were coated onto the graphite surface. The potential response measurements showed that the best performance was exhibited by the electrodes with membranes having the composition L₁: o-NPOE:NaTPB:PVC as 4:51:2.5:42.5 (wt.%), L₂: o-NPOE:NaTPB:PVC as 3:52.5:1.5:43 (wt.%) and L₃: o-NPOE:NaTPB:PVC as 7:49:3.5:40.5 (wt.%). These electrodes had the widest working concentration range, Nernstian slope and fast response times of 12 s, 7 s and 17 s for L₁, L₂ and L₃, respectively. The selectivity studies showed that these electrodes have higher selectivity towards Cd²⁺ over a large number of cations and could tolerate up to 20 vol.% non-aqueous impurities. Furthermore, the electrodes generated constant potentials in the pH range 2.0–8.0, with a shelf life of approximately four to six weeks. The high selectivity of these electrodes permits their use in the detection of the Cd²⁺ content in some medicinal plants, soil and industrial wastewater samples. The electrodes could also be used as an indicator electrode in the potentiometric titration of Cd²⁺ with EDTA.     

Electrochemical sensor for sulfite determination based on a nanostructured copper-salen film modified electrode

The electrochemical preparation described herein involved the electrocatalytic oxidation of sulfite on a platinum electrode modified with nanostructured copper salen (salen = N,N′-ethylenebis(salicylideneiminato)) polymer films. The complex was prepared and electropolymerized at a platinum electrode in a 0.1 mol L⁻¹ solution of tetrabutylammonium perchlorate in acetonitrile by cyclic voltammetry between 0 and 1.4 V vs. SCE. After cycling the modified electrode in a 0.50 mol L⁻¹ KCl solution, the estimated surface concentration was found to be equal to 2.2 × 10⁻⁹ mol cm⁻². This is a typical behavior of an electrode surface immobilized with a redox couple that can usually be considered as a reversible single-electron reduction/oxidation of the copper(II)/copper(III) couple. The potential peaks of the modified electrode in the electrolyte solution (aqueous) containing the different anions increase with the decrease of the ionic radius, demonstrating that the counter-ions influence the voltammetric behavior of the sensor. The potential peak was found to be linearly dependent upon the ratio [ionic charge]/[ionic radius]. The oxidation of the sulfite anion was performed at the platinum electrode at +0.9 V vs. SCE. However, a significant decrease in the overpotential (+0.45 V) was obtained while using the sensor, which minimized the effect of oxidizable interferences. A plot of the anodic current vs. the sulfite concentration for chronoamperometry (potential fixed = +0.45 V) at the sensor was linear in the 4.0 × 10⁻⁶ to 6.9 × 10⁻⁵ mol L⁻¹ concentration range and the concentration limit was 1.2 × 10⁻⁶ mol L⁻¹. The reaction order with respect to sulfite was determined by the slope of the logarithm of the current vs. the logarithm of the sulfite concentration.     

基于PANI/EG层间复合材料的Cd2 电化学传感器

以电化学氧化法制备的膨胀石墨(EG)为碳骨架，采用真空插层辅助原位氧化聚合法，使多孔塔尖状的聚苯胺(PANI)有序包覆生长在EG的石墨片层表面，构建了PANI/EG层间复合材料。采用SEM、TEM、XRD、FTIR、Raman、XPS和BET对PANI/EG复合材料的结构和组成进行表征。以PANI/EG复合材料修饰玻碳电极，采用SWASV法对Cd2 进行检测，通过CV和EIS测试修饰电极的电化学行为。结果表明：PANI/EG复合材料呈层状分级空间结构。EG质量分数为12%的PANI/EG修饰电极对痕量Cd2 检测的敏感度为7.814 μA/μM，检测极限为3.24 nM，检测范围为0.25~6 μM，重复性及抗干扰性良好。     

NiO-MWCNT修饰热解石墨电极的电化学行为研究

采用滴涂法将适量碳纳米管修饰到热解石墨电极上,后电沉积纳米氧化镍得到MWCNT/NiO/PG复合修饰电极。研究了它的电化学行为,并用于抗坏血酸的测定。试验表明,在pH=6的磷酸盐缓冲溶液中,抗坏血酸在修饰电极上产生一灵敏的氧化峰。当抗坏血酸的浓度在1.0×10-5～5.0×10-4 mol/L时,氧化峰电流与浓度呈线性关系,线性方程为:I(uA)=-0.4458-0.5922C(mmol/L),相关系数为R=-0.9989。检出限低至5.5×10-7 mol/L。该传感器重现性、稳定性、抗干扰性良好。     

Electrochemical determination of cysteine based on conducting polymers/gold nanoparticles hybrid nanocomposites

In this study, a hybrid nanocomposite consisting of a conducting polymer and gold nanoparticles (AuNPs) is fabricated onto a screen-printed carbon electrode (SPCE). A thin layer of poly(3,4-ethylenedioxythiophene) (PEDOT) is coated electrochemically on a bare SPCE; then, the nano-sized AuNPs are embedded by electrochemical deposition. The resultant SPCE/PEDOT/AuNPs-modified electrode is characterized by electrochemical methods, field emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS). The SPCE/PEDOT/AuNPs-modified electrode possesses great catalytic activity for the oxidation of cysteine in various pH buffer solutions (pH 2.0–8.0). The selectivity of the method is demonstrated by the separation of the oxidation peaks at up to 240 mV for cysteine and glutathione in pH 6.0 buffer solutions. The effects of the oxidizable interferences are also investigated. Flow-injection amperometry is performed for 0.5–200 μM of cysteine in pH 4.0 buffer solutions, and a linear calibration plot with a slope of 0.115 μA/μM is obtained. The detection limit (S/N = 3) is 0.05 μM. Additionally, the proposed methods obtain satisfactory results in the detection of cysteine-containing medicine samples.     

Electrochemical sensors based on platinum electrodes modified with hybrid inorganic-organic coatings for determination of 4-nitrophenol and dopamine

New electrochemical sensors for dopamine (DA) and phenol derivative, based on hybrid inorganic redox material-organic conducting polymers, were developed. Hybrid inorganic-organic coatings based on Prussian blue, polyazulene, poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3-[(E)-2-azulene-1-yl)vinyl]thiophene) have been prepared by electrochemical methods in various configurations onto Pt substrate. Dopamine and 4-nitrophenol (4-NP) have been determined by square wave voltammetry using both mono and bilayer modified electrodes. The modified electrodes exhibited a linear response over wide range of 4-nitrophenol concentrations from 30 to 90 μM, with a detection limit of 8.23 μM (s/n = 3). A linear trend for the responses for dopamine concentrations ranging from 2 to 100 μM, with a sensitivity of 0.116 μA/μM, has been also obtained.     

Effects of conductive ceramic on the electrochemical performance of ZnO for Ni/Zn rechargeable battery

A novel ZnO/conductive-ceramic nanocomposite was prepared by a homogeneous precipitation method. The conductive ceramic with the nominal chemical composition of (ZnO)_0.92(Bi₂O₃)_0.054(Co₂O₃)_0.025(Nb₂O₅)_0.00075(Y₂O₃)_0.00025, as the nucleation sites of ZnO, was prepared by ball milling and surface modification process and its effects on electrochemical performance of ZnO were investigated by charge/discharge cycling, slow rate cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Compared with pure ZnO, the ZnO/conductive-ceramic electrode exhibited improved electrochemical properties, such as superior cycle stability, higher discharge capacity and utilization ratio. When the conductive ceramic content reached 14 wt.%, the discharge capacity of the ZnO/conductive-ceramic nanocomposite hardly declined over 50 cycling test, the average utilization ratio could reach 99.5%, and the electrodes had no obvious weight loss after cycling tests.     

Electrochemical detection of acetaminophen on the functionalized MWCNTs modified electrode using layer-by-layer technique

A selective electrochemical method is fabricated via layer-by-layer (LBL) method using both positively and negatively charged multi walled carbon nanotubes (MWCNTs) on poly (diallyldimetheylammonium chloride) (PDDA)/poly styrene sulfonate (PSS) modified graphite electrode, for the determination of acetaminophen (ACT) in the presence of dopamine (DA) and high concentration of ascorbic acid (AA). The modified electrode was characterized by cyclic voltammetry (CV) electrochemical impedance spectroscopy (EIS), atomic force microscopy (AFM) and scanning electron microscopy (SEM). Experimental conditions such as pH, accumulation potential and time, effect of potential sweep rates and interferents were studied. In CV well defined peaks for AA, ACT and DA are obtained at 24, 186 and 374 mV, respectively. The separations of peaks were 210, 188 and 398 mV between AA and DA, DA and ACT and AA and ACT, respectively. The diffusion coefficient was calculated by chronocoulometric. Chronoamperometric studies showed the linear relationship between oxidation peak current and concentration of ACT in the range 25–400 μM (R = 0.9991). The detection limit was 5 × 10⁻⁷ mol/L. The proposed method gave satisfactory results in the determination of ACT in pharmaceutical and human serum samples.     

The effect of cytosine on the two-step electroreduction of Zn ions in acetic buffers

Cytosine plays an important role in many biological processes since it constitutes the buildings blocks of DNA and RNA. A two-step reduction of Zn²⁺ ions at the dropping mercury electrode in acetic buffers at pH 4 and 5 in the presence of cytosine was examined. The measurements were performed using an impedance method in a wide potential and frequency ranges.The values of the standard rate constants k_s in the both studied system decrease from 3.8 × 10⁻³ to 2 × 10⁻³ cm s⁻¹ at pH 4 and from 5.1 × 10⁻³ to 2.5 × 10⁻³ cm s⁻¹ at pH 5. The values of the standard rate constants k_s1 characterizing the stage of the first electron transfer decrease similarly. However, the values of the standard rate constants k_s2 characterizing the stage of the second electron exchange decrease more markedly in the buffer at pH 4 than in the buffer at pH 5.     

Theoretical and practical investigations of copper ion selective electrode with polymeric membrane based on N,N′-(2,2-dimethylpropane-1,3-diyl)-bis(dihydroxyacetophenone)

A novel ion-selective poly(vinyl chloride) (PVC) membrane sensor for Cu²⁺ ions based on N,N′-(2,2-dimethylpropane-1,3-diyl)-bis(dihydroxyacetophenone) (NDHA) as a new ionophore was prepared and studied. The best performance was observed for the membrane composition, including 30:65:1:4 (wt%) = PVC:DBP:KTpClPB:NDHA. The electrode showed a good Nernstian slope of 30.0 ± 0.5 mV/decade in a wide linear range activity of 3.0 × 10⁻⁷ to 1.0 × 10⁻² mol dm⁻³ Cu(NO₃)₂ with limit of detection 2.5 × 10⁻⁷. Sensor exhibited a fast response time (t_95% < 10 s) and could be used for about 4 months in the pH range of 3.0–7.4. The proposed potentiometric sensor was found to work satisfactorily in partially non-aqueous media up to 30 (vol%) content of methanol, ethanol and acetone. Applications of this electrode for the determination of copper in real samples, and as an indicator electrode for potentiometric titration of Cu²⁺ ion using EDTA, were reported. In order to predict the extraction ability of NDHA for different metallic ions, the complexes [M(NDHA)] and [M(H₂O)₆] (where M = Cu²⁺, Co²⁺, Hg²⁺, Pb²⁺, Ag⁺, Mg²⁺, Ca²⁺, Mn²⁺, Zn²⁺, Cd²⁺, K⁺ and Al³⁺) were investigated using ab initio theoretical calculations. The metal binding capability was evaluated using the binding energy. Results of our study could be useful for prediction of the extraction power of this Schiff base and could play a guiding role in planning experiments.