A New Planar Potentiometric Sensor for In Situ Measurements

A new construction of a potentiometric sensor was introduced for the first time. It relies on the use of two membranes instead of one, as in the well-known coated-disc electrode. For this purpose, a new electrode body was constructed, including not one, but two glassy carbon discs covered with an ion-selective membrane. This solution allows for the sensor properties to be enhanced without using additional materials (layers or additives) on the membrane. The new construction is particularly useful for in situ measurements in environmental samples. Two ion-selective polymeric membranes were used, namely H+ and K+-selective membranes, to confirm the universality of the idea. The tests conducted included chronopotentiometric tests, electrochemical impedance spectroscopy, and potentiometric measurements. The electrical and analytical parameters of the sensors were evaluated and compared for all tested electrodes to evaluate the properties of the planar electrode versus previously known constructions. Research has shown that the application of two membranes instead of one allows for the resistance of an electrode to be lowered and for the electrical capacitance to be elevated. Improving the electrical properties of an electrode resulted in the enhancement of its analytical properties. The pH measurement range of the planar electrode is 2-11, which is much wider in contrast to that of the single-membrane electrode. The linear range of the K+-selective planar electrode is wider than that of the coated-disc electrode and equals 10-6 to 10-1 M. The response time turned out to be a few seconds shorter, and the potential drift was smaller due to the application of an additional membrane in the electrode construction. This research creates a new opportunity to design robust potentiometric sensors, as the presented construction is universal and can be used to obtain electrodes selective to various ions.

A Novel Composite Voltammetric Sensor Based on Yttria-Stabilized Zirconia Doped with Neodymium-Carbon Black-Nafion Glassy Carbon Electrode for Metoprolol Determination

For the first time, a new composite voltammetric sensor based on yttria-stabilized zirconia doped with neodymium-carbon black-Nafion glassy carbon electrode (YSZNd-CB-Nafion/GCE) for the determination of metoprolol (MET) has been developed. The instrumental parameters and supporting electrolyte were optimized. For 105 s accumulation time, linearity was achieved in the range of 0.01 to 0.2 µM. The limit of detection (for 105 s accumulation time) was equal to 2.9 nM (2 µg/L), and was the best result in comparison to other voltametric sensors. The reproducibility of the metoprolol signal presented as relative standard deviation (RSD) was equal to 1.9% (n = 7). Additionally, our electrode is characterized by high stability, is easy to use, and has a short preparation time. The proposed sensor was found useful for MET determination in plasma and urine, as well as for pharmaceutical samples, with a good recovery parameter (96-108%). Flow injection analysis (FIA) with amperometric detection was also performed for MET determination. The recovery was calculated and was in the range 101-103%, suggesting that the proposed material may be applied in flow injection analysis.

Application of Metal Oxide Nanoparticles in the Field of Potentiometric Sensors: A Review

Recently, there has been rapid development of electrochemical sensors, and there have been numerous reports in the literature that describe new constructions with improved performance parameters. Undoubtedly, this is due to the fact that those sensors are characterized by very good analytical parameters, and at the same time, they are cheap and easy to use, which distinguishes them from other analytical tools. One of the trends observed in their development is the search for new functional materials. This review focuses on potentiometric sensors designed with the use of various metal oxides. Metal oxides, because of their remarkable properties including high electrical capacity and mixed ion-electron conductivity, have found applications as both sensing layers (e.g., of screen-printing pH sensors) or solid-contact layers and paste components in solid-contact and paste-ion-selective electrodes. All the mentioned applications of metal oxides are described in the scope of the paper. This paper presents a survey on the use of metal oxides in the field of the potentiometry method as both single-component layers and as a component of hybrid materials. Metal oxides are allowed to obtain potentiometric sensors of all-solid-state construction characterized by remarkable analytical parameters. These new types of sensors exhibit properties that are competitive with those of the commonly used conventional electrodes. Different construction solutions and various metal oxides were compared in the scope of this review based on their analytical parameters.

Electrochemical determination of thiethylperazine using semi-graphitized carbon nanofibers-MnO nanocomposite

A new voltammetric method is proposed for high sensitive thiethylperazine (THP) determination, using a glassy carbon electrode modified with semi-graphitized carbon nanofibers/MnO nanocomposite (eCNF/MnO/GC). To the best of our knowledge, this is the first electrochemical assay of THP  determination, and the first use of the eCNF/MnO as the electrode modifier. The proposed method using eCNF/MnO/GC is characterized by high repeatability and sensitivity of measurements, with the linearity of THP in the range from 0.05 to 2.2 µmol L-1. The lowest detection limit achieved on the eCNF/MnO/GC electrode for 30 s of preconcentration was 6.3 nmol L-1 THP in 0.05 mol L-1 acetate buffer of  pH 5.6. The proposed method was successfully applied to highly sensitive THP determination in complex matrices, such as tablets and plasma with good recovery (98-103%). The RSD value obtained for THP measurement at a concentration of 0.1 µmol L-1 was 1.3%. Amperometric measurements of THP under the flow injection conditions were also performed to indicate the possibility of its fast and accurate determination (103% and 95% for unmodified and modified electrode, respectively), with the duration of single analysis of approx. 30 s.

Electrochemical Sensor Based on the Hierarchical Carbon Nanocomposite for Highly Sensitive Ciprofloxacin Determination

A new voltammetry method for the highly sensitive antibacterial drug ciprofloxacin (CIP) is presented using glassy carbon electrodes modified with hierarchical electrospun carbon nanofibers with NiCo nanoparticles (eCNF/CNT/NiCo-GCE). The use of a modified glassy carbon electrode in the form of hierarchical electrospun carbon nanofibers with NiCo nanoparticles (eCNF/CNT/NiCo) led to an LOD value as low as 6.0 µmol L-1 with a measurement sensitivity of 3.33 µA µmol L-1. The described procedure was successfully applied for CIP determination in samples with complex matrices, such as urine or plasma, and also in pharmaceutical products and antibiotic discs with satisfactory recovery values ranging between 94-104%. The proposed electrode was characterised by great stability, with the possibility of use for about 4 weeks without any significant change in the CIP peak current. The repeatability of the CIP response on the eCNF/CNT/NiCo/GC is also very good; its value measured and expressed as RSD is equal to 2.4% for a CIP concentration of 0.025 µmol L-1 (for 7 consecutive CIP voltammogram registrations). The procedure for electrode preparation is quick and simple and does not involve the use of expensive apparatus.

Highly Sensitive Voltammetric Method for Quinoline Yellow Determination on Renewable Amalgam Film Electrode

A novel electrochemical method for the determination of quinoline yellow (QY) was developed using the renewable amalgam film electrode (Hg(Ag)FE). The sensors used can be characterized by good stability and long lifespan. Irreversible QY reduction peaks were recorded in 0.05 mol L^-1 HCl with a potential of about -630 mV. The use of the Hg(Ag)FE electrode with a regulated working surface allowed the QY limit of detection to be as low as 0.48 nmol L^-1. The obtained result is the lowest in comparison to other voltammetric methods described in the literature. The effects of parameters such as the size of the working electrode surface, influence of the pH value, accumulation time, and potential were investigated to provide precision and high sensitivity of the performed measurements. This new procedure was applied for the highly sensitive determination of quinoline yellow in different beverages, pre-workout supplements, and throat lozenges. The process of sample preparation was relatively simple. Calculated recoveries (96-107%) suggest that the method can be considered accurate.

Glassy Carbon Electrode Modified with CB/TiO2 Layer for Sensitive Determination of Sumatriptan by Means of Voltammetry and Flow Injection Analysis

Sumatriptan is an organic chemical compound from the tryptamine group. It is used as a medicine for migraine attacks and in the treatment of cluster headaches. In this work, a new voltammetric method is proposed for highly sensitive SUM determination, using glassy carbon electrodes modified with carbon black and titanium dioxide suspension. The novelty of the presented work is the usage of the mixture of carbon black and TiO₂ as glassy carbon electrode modifier for the first time for SUM determination. The mentioned sensor was characterized by great repeatability and sensitivity of measurements, which resulted in the obtention of a wide range of linearity and a low detection limit. The electrochemical properties of the CB-TiO₂/GC sensor was characterized using the LSV and EIS method. The effect of different factors on the SUM peak, such as supporting electrolyte type, preconcentration time and potential, or influence of interferents, were tested using the square wave voltammetry technique. The linear voltammetric response for the analyte was obtained in the concentration range of 5 nmol L^-1 to 150 μmol L^-1 with a detection limit of 2.9 nmol L^-1 for a preconcentration time of 150 s in the 0.1 mol L^-1 phosphate buffer pH 6.0. The proposed method was successfully applied for highly sensitive sumatriptan determination in complex matrices, such as tablets, urine, and plasma, with a good recovery parameter (94-105%). The presented CB-TiO₂/GC electrode is characterized by great stability, it was used for 6 weeks without significant changes in the SUM peak current. Amperometric and voltammetric measurements of SUM under the flow injection conditions were also performed to indicate the possibility of its fast and accurate determination with a time of single analysis of approx. 30 s.

All-Solid-State Carbon Black Paste Electrodes Modified by Poly(3-octylthiophene-2,5-diyl) and Transition Metal Oxides for Determination of Nitrate Ions

This paper presents new paste ion-selective electrodes for the determination of nitrate ions in soil. The pastes used in the construction of the electrodes are based on carbon black doped with transition metal oxides: ruthenium, iridium, and polymer-poly(3-octylthiophene-2,5-diyl). The proposed pastes were electrically characterized by chronopotentiometry and broadly characterized potentiometrically. The tests showed that the metal admixtures used increased the electric capacitance of the pastes to 470 μF for the ruthenium-doped paste. The polymer additive used positively affects the stability of the electrode response. All tested electrodes were characterized by a sensitivity close to that of the Nernst equation. In addition, the proposed electrodes have a measurement range of 10^-5 to 10^-1 M NO₃^- ions. They are impervious to light conditions and pH changes in the range of 2-10. The utility of the electrodes presented in this work was demonstrated during measurements directly in soil samples. The electrodes presented in this paper show satisfactory metrological parameters and can be successfully used for determinations in real samples.

Improved Lead Sensing Using a Solid-Contact Ion-Selective Electrode with Polymeric Membrane Modified with Carbon Nanofibers and Ionic Liquid Nanocomposite

A new solid-contact ion-selective electrode (ISE) sensitive to lead (II) ions, obtained by modifying a polymer membrane with a nanocomposite of carbon nanofibers and an ionic liquid 1-hexyl-3-methylimidazolium hexafluorophosphate, is presented. Electrodes with a different amount of nanocomposite in the membrane (0-9% w/w), in which a platinum wire or a glassy carbon electrode was used as an internal electrode, were tested. Potentiometric and electrochemical impedance spectroscopy measurements were carried out to determine the effect of the ion-sensitive membrane modification on the analytical and electrical parameters of the ion-selective electrode. It was found that the addition of the nanocomposite causes beneficial changes in the properties of the membrane, i.e., a decrease in resistance and an increase in capacitance and hydrophobicity. As a result, the electrodes with the modified membrane were characterized by a lower limit of detection, a wider measuring range and better selectivity compared to the unmodified electrode. Moreover, a significant improvement in the stability and reversibility of the electrode potential was observed, and additionally, they were resistant to changes in the redox potential of the sample. The best parameters were shown by the electrode obtained with the use of a platinum wire as the inner electrode with a membrane containing 6% of the nanocomposite. The electrode exhibited a Nernstian response to lead ions over a wide concentration range, 1.0 × 10^-8-1.0 × 10^-2 mol L^-1, with a slope of 31.5 mV/decade and detection limit of 6.0 × 10^-9 mol L^-1. In addition, the proposed sensor showed very good long term stability and worked properly 4 months after its preparation without essential changes in the E⁰ or slope values. It was used to analyze a real sample and correct results of lead content determination were obtained.

The New Reliable pH Sensor Based on Hydrous Iridium Dioxide and Its Composites

The new reliable sensor for pH determination was designed with the use of hydrous iridium dioxide and its composites. Three different hIrO₂-based materials were prepared and applied as solid-contact layers in pH-selective electrodes with polymeric membrane. The material choice included standalone hydrous iridium oxide; composite material of hydrous iridium oxide, carbon nanotubes, and triple composite material composed of hydrous iridium oxide; carbon nanotubes; and poly(3-octylthiophene-2,5-diyl). The paper depicts that the addition of functional material to standalone metal oxide is beneficial for the performance of solid-state ion-selective electrodes and presents the universal approach to designing this type of sensors. Each component contributed differently to the sensors' performance-the addition of carbon nanotubes increased the electrical capacitance of sensor (up to 400 µF) while the addition of conducting polymer allowed it to increase the contact angle of material changing its wetting properties and enhancing the stability of potentiometric response. Hydrous iridium oxide contacted electrodes exhibit linear response in wide linear range of pH (2-11) and stable potentiometric response (the lowest potential drift of 0.036 mV/h is attributed to the electrode with triple composite material).

Modification of Carbon Nanomaterials by Association with Poly(3-octylthiophene-2,5-diyl) as a Method of Improving the Solid-Contact Layer in Ion-Selective Electrodes

A new group of carbon nanomaterials modified with poly(3-octylthiophene-2,5-diyl) for solid-contact layers in ion-selective electrodes was obtained. The materials were characterized by scanning electron microscopy and measurement of the contact angle. The modification greatly improved the hydrophobicity of the materials, and the highest contact angle (175°) was obtained for a polymer-modified carbon nanofibers/nanotube nanocomposite. The electrical parameters of the electrodes were determined using the methods of chronopotentiometry and electrochemical impedance spectroscopy. The highest electrical charge capacity was obtained for polymer-modified carbon nanofibers (7.87 mF/cm²). For this material, the lowest detection limit (10^-6.2 M) and the best potential reversibility (SD = 0.2 mV) were also obtained in potentiometric measurements.

Advances on Hormones and Steroids Determination: A Review of Voltammetric Methods since 2000

This article presents advances in the electrochemical determination of hormones and steroids since 2000. A wide spectrum of techniques and working electrodes have been involved in the reported measurements in order to obtain the lowest possible limits of detection. The voltammetric and polarographic techniques, due to their sensitivity and easiness, could be used as alternatives to other, more complicated, analytical assays. Still, growing interest in designing a new construction of the working electrodes enables us to prepare new measurement procedures and obtain lower limits of detection. A brief description of the measured compounds has been presented, along with a comparison of the obtained results.

Development of a New Voltammetric Method for Aceclofenac Determination on Glassy Carbon Electrode Modified with Hierarchical Nanocomposite

Aceclofenac (ACL) is an anti-inflammatory drug, which is taken by patients who mainly suffer from rheumatoid conditions. In this work, we propose a new voltammetric method that allows the determination of ACL in pharmaceutics, urine, and plasma. As a working electrode, a glassy carbon electrode (GCE) modified with carbon nanofibers, carbon nanotubes, and NiCo nanoparticles (eCNF/CNT/NiCo-GCE) was used. The mentioned sensors are characterized by good repeatability and sensitivity, and their process of preparation is simple, fast, and cost-effective. Instrumental and method parameters were optimized, and the influence of interferences was investigated. To validate the analytical performance of the method, calibration was conducted. Good linearity was obtained (0.05-1.4 µM, r = 0.998), as well as excellent limit of detection (LOD) and limit of quantification (LOQ) values (0.7 nM and 2.1 nM, respectively). Calculated recoveries that were in the range of 98%-105% indicate that this method is accurate and might be used in routine laboratory practice.

Hierarchical Nanocomposites Electrospun Carbon NanoFibers/Carbon Nanotubes as a Structural Element of Potentiometric Sensors

This work proposes new carbon materials for intermediate layers in solid-contact electrodes sensitive for potassium ions. The group of tested materials includes electrospun carbon nanofibers, electrospun carbon nanofibers with incorporated cobalt nanoparticles and hierarchical nanocomposites composed of carbon nanotubes deposited on nanofibers with different metal nanoparticles (cobalt or nickel) and nanotube density (high or low). Materials were characterized using scanning electron microscopy and contact angle microscopy. Electrical parameters of ready-to-use electrodes were characterized using chronopotentiometry and electrochemical impedance spectroscopy. The best results were obtained for potassium electrodes with carbon nanofibers with nickel-cobalt nanoparticles and high density of nanotubes layer: the highest capacity value (330 µF), the lowest detection limit (10^−6.3 M), the widest linear range (10⁻⁶–10⁻¹) and the best reproducibility of normal potential (0.9 mV). On the other hand the best potential reversibility, the lowest potential drift (20 μV·h⁻¹) in the long-term test and the best hydrophobicity (contact angle 168°) were obtained for electrode with carbon nanofibers with cobalt nanoparticles and high density of carbon nanotubes. The proposed electrodes can be used successfully in potassium analysis of real samples as shown in the example of tomato juices.

Hydrous Cerium Dioxide-Based Materials as Solid-Contact Layers in Potassium-Selective Electrodes

This paper introduces hydrous cerium dioxide applied for the first time as a solid-contact layer in ion-selective electrodes. Cerium dioxide belongs to the group of metal oxides that exhibit both redox activity and a large surface area and therefore was considered to be an appropriate material for the solid-contact layer in potentiometric sensors. The material was examined both standalone and as a component of composite materials (with the addition of carbon nanomaterial or conducting polymer). Three cerium dioxide-based materials were tested as solid-contact layers in potentiometric sensors in the context of their microstructure, wettability, and electrical properties. The addition of hydrous cerium dioxide was shown to enhance the properties of carbon nanotubes and poly(3-octylthiophene-2,5-diyl) by increasing the value of electrical capacitance (798 μF and 112 μF for hCeO₂-NTs and hCeO₂-POT material, respectively) and the value of contact angle (100° and 120° for hCeO₂-NTs and hCeO₂-POT material, respectively). The proposed sensor preparation method is easy, without the need to use an advanced apparatus or specific conditions, and fast; sensors can be prepared within an hour. Designed hCeO₂-based electrodes exhibit competitive linear range and potential stability within the wide range of pH values (2.0–11.5). Designed electrodes are dedicated to potassium determination in environmental and clinical samples.

Highly sensitive voltammetric determination of captopril on renewable amalgam film electrode

New highly sensitive voltammetric method for captopril (CPT) determination was developed. The main novelty of the work was the application of a renewable amalgam film electrode (Hg(Ag)FE) for this purpose. During the research instrumental parameters of the developed method were optimized and were as follows: t_w = t_s = 5 ms, E_s = 5 mV, ΔE = 100 mV. Preconcentration potential and time were equal to 100 mV and 20 s, respectively. All measurements were conducted in electrolyte consisted of 0.1 M HClO₄. Limit of detection was calculated and was equal to 1.9 nM (0.39 ng mL^-1) for 20 s preconcentration time and Hg(Ag)FE surface area approximately 11.2 mm². Linearity was achieved in the concentration range 0.05-1 μM. Repeatability of the method expressed as variation coefficient was estimated at 3.5% (0.15 μM CPT, n = 10). Applicability of the method was confirmed by analysis of tablets containing CPT and urine. Recoveries were in the range from 95 to 109% suggesting that the method might be assumed as accurate. Obtained results were also in good agreement with the producer declaration.

Graphene Flakes Decorated with Dispersed Gold Nanoparticles as Nanomaterial Layer for ISEs

This paper proposes a new type of solid-contact layer based on graphene/gold nanoparticles for ion-selective electrodes. A novel approach to preparing the material for intermediate layer by modifying the graphene flakes by gold nanoparticles is presented. With this approach, we observed a large surface area of material and in consequence high electrical capacitance of electrodes. We have obtained satisfactory results demonstrating that the modification of graphene with gold allows for enhancing electrical and wetting properties of carbon nanomaterial. Electrical capacitance of designed nanocomposite-contacted electrode equals to approximately 280 µF, which in consequence ensures great long-term potential stability defined by the potential drift of 36 μV/h. The modification of graphene with nanoparticles completely changed its wetting properties, as the designed material turned out to be hydrophobic with a water contact angle of 115°. Graphene/gold nanoparticles-contacted electrodes are insensitive to the changing light conditions, exhibiting near-Nernstian response in the potassium concentration range between 10-5.9 M and 10-1 M of K+ ions and may be applied in the pH range between 2 and 10.5.

Potentiometric Sensor with High Capacity Composite Composed of Ruthenium Dioxide and Poly(3,4-ethylenedioxythiophene) Polystyrene Sulfonate

This work presents the first-time application of the ruthenium dioxide-poly(3,4-ethylenedioxythiophene) polystyrene sulfonate high-capacity composite material as a mediation layer in potassium selective electrodes, which turned out to significantly enhance the electrical and analytical parameters of the electrodes. The idea was to combine the properties of two different types of materials: a conducting polymer, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, and a metal oxide, ruthenium dioxide, in order to obtain the material for a solid-contact layer of great electrical and physicochemical parameters. The preparation method for composite material proposed in this work is fast and easy. The mediation layer material was examined using a scanning electron microscope and chronopotentiometry in order to confirm that all requirements for mediation layers materials were fulfilled. Ruthenium dioxide-poly(3,4-ethylenedioxythiophene) polystyrene sulfonate nancomposite material turned out to exhibit remarkably high electrical capacitance (of approximately 17.5 mF), which ensured great performance of designed K+-selective sensors. Electrodes of electrical capacity equal to 7.2 mF turned out to exhibit fast and stable (with only 0.077 mV potential change per hour) potentiometric responses in the wide range of potassium ion concentrations (10-6 M to 10-1 M). The electrical capacity of ruthenium dioxide-poly(3,4-ethylenedioxythiophene) polystyrene sulfonate-contacted electrodes characterized by electrical capacitance parameters was the highest reported so far for this type of sensor.

Potassium-Selective Solid-Contact Electrode with High-Capacitance Hydrous Iridium Dioxide in the Transduction Layer

This work presents new material for solid-contact layers-hydrous iridium dioxide IrO₂·2H₂O, characterized by high electrical capacitance value, evaluated using chronopotentiometry (1.22 mF) and electrochemical impedance spectroscopy (1.57 mF). The remarkable electrical parameters of layers resulted in great analytical parameters of IrO₂·2H₂O-contacted potassium-selective electrodes. Various parameters of ion-selective electrodes were examined in the scope of this work using a potentiometry method including: linear range, repeatability, stability of potentiometric response and sensitivity to varying measurement conditions. The analytical parameters obtained for solid-contact electrodes were compared with the ones obtained for coated disc electrodes to evaluate the influence of the iridium dioxide layer. The linear range of the IrO₂·2H₂O-contacted K⁺-selective electrodes covered concentrations of K⁺ ions from 10^-6 to 10^-1 M and the potential stability was estimated at 0.097 mV/h. The IrO₂·2H₂O-contacted electrodes turned out to be insensitive to varying light exposure and changes in the pH values of measured solutions (in the pH range of 2 to 10.5). A water layer test proved that, contrary to the coated disc electrode, the substantial water film is not formed between the ion-selective membrane and iridium dioxide layer.

Highly Sensitive Levodopa Determination by Means of Adsorptive Stripping Voltammetry on Ruthenium Dioxide-Carbon Black-Nafion Modified Glassy Carbon Electrode

A new, highly sensitive Adsorptive Stripping Voltammetric method for levodopa determination was developed. As a working electrode, the glassy carbon electrode (GCE) modified with carbon black (CB), RuO2·xH2O (RuO2) and Nafion was used (CB-RuO2-Nafion GCE). Levodopa signal obtained on the modified electrode was 12 times higher compared to GCE. During research, instrumental parameters were optimized: sampling time ts = 10 ms, waiting time tw = 10 ms, step potential Es = 5 mV and pulse amplitude ΔE = 50 mV. Preconcentration potential Eprec was equal to 0 mV. The best results were obtained in 0.025 M perchloric acid (approx. pH 1.4). Signal repeatability measured on the CB-RuO2-Nafion modified electrode for 0.2 µM of levodopa was equal to 2.1% (levodopa concentration 1 µM, n = 5). Linearity of the method was achieved in the concentration range from 1 to 8 µM. Limit of detection was equal to 17 nM. Recoveries calculated for pharmaceutical products and tap water measurements were in the range 102-105%, which confirms the accuracy of the developed. The applicability of the method was confirmed by analysis of pharmaceutical products and tap water samples. Based on obtained results, it might be concluded that the developed voltammetric method could be a useful tool in routine drug analysis.

Ruthenium dioxide nanoparticles as a high-capacity transducer in solid-contact polymer membrane-based pH-selective electrodes

A new approach is presented for the design of ion selective electrodes. Ruthenium dioxide nanoparticles were incorporated into solid-contact electrodes, and their properties were studied for the case of pH-selective electrodes. The use of the RuO2 is shown to significantly improve the potentiometric response, while no redox response is observed. The use of RuO2 results in a Nernstian slope (59 mV/decade) towards hydrogen ions over a wide linear range (pH 2 to 12). The results obtained by chronopotentiometry reveal small resistance, and the capacitance is as high as 1.12 mF. This results in a good stability of the response and in a low potential drift (0.89 μV∙s-1). The electrodes exhibit properties nearly as excellent as those of a glass electrode, but they are much smaller, less fragile, and easy to use. Graphical abstractSchematic representation of the construction of the new kind of electrodes along with calibration and chronopotentiometric plots compared to non-modified GCD/H+-ISM and modified GCD/RuO2/H+-ISM electrodes, respectively. The use of ruthenium dioxide results in a wide analytical pH range (2-12) and in high electrical capacitance (1.12 mF).

Spironolactone voltammetric determination on renewable amalgam film electrode

In this study a new high sensitive method of spironolactone (SP) determination is proposed using voltammetric technique. As the working electrode amalgam film silver based electrode (Hg(Ag)FE) was applied, that resulted in obtaining low limit of spironolactone detection, ex. for the preconcentration time 45 s obtained detection limit was 4.7·10^-9 mol L^-1 (1.96·10^-6 g L^-1). The linear response of spironolactone signal on mercury film electrode was obtained in the range of concentration from 15·10^-9 to 3.0·10^-6 mol L^-1 and the obtained repeatability measured as RSD was very good, with the value of 2.0% (n = 5; concentration 0.5·10^-6 mol L^-1). The proposed method was successfully applied and validated by studying the recovery in commercially available pharmaceutical formulations containing known amount of spironolactone.

High selective potentiometric sensor for determination of nanomolar con-centration of Cu(II) using a polymeric electrode modified by a graphene/7,7,8,8-tetracyanoquinodimethane nanoparticles

In this paper we present several possibilities for modification of the all-solid-state copper(II)-selective electrodes in order to obtain high stability and selectivity potentiometric sensor signal. The copper(II)-selective electrodes were modified in a simple way by introducing solid contact layer based on graphene or graphene oxide, 7,7,8,8-tetracyanoquinodimethane or its copper salt. Each of electrodes shown a Nernstian response, wide linear range of Cu²⁺ concentration and a low detection limit. Nevertheless, some differences between the groups of electrodes have been noticed and resulted from application of intermediate layers of various compositions. The electrode modified with graphene and 7,7,8,8-tetracyanoquinodimethane nanocomposite exhibited the best potential reproducibility and detection limit. Moreover, prepared sensors were characterized by an improved selectivity for copper(II) in relation to non-modified electrodes. The developed electrodes were successfully used for the Cu²⁺ determination in different samples and results were comparable with those obtained using voltammetry.

Voltammetric Determination of Codeine on Glassy Carbon Electrode Modified with Nafion/MWCNTs

A glassy carbon electrode modified with a Nafion/MWCNTs composite is shown to enable the determination of codeine using differential pulse voltammetry in phosphate buffer of pH 3.0. At a preconcentration time of 15 s, the calibration graph is linear in the 0.5 µM (0.15 mg·L(-1)) to 15 µM (4.5 mg·L(-1)) concentration range with a correlation coefficient of 0.998. The detection limit at a preconcentration time of 120 s is as low as 4.5 μg·L(-1). The repeatability of the method at a 0.6 μg·L(-1) concentration level, expressed as the RSD, is 3.7% (for n = 5). The method was successfully applied and validated by analyzing codeine in drug, human plasma, and urine samples.

Application of nanostructured TCNQ to potentiometric ion-selective K(+) and Na(+) electrodes

A new type of potentiometric solid-state ion-selective electrode (SS-ISE) has been fabricated with an intermediate layer made of 7,7,8,8-tetracyanoquinodimethane (TCNQ) or its ion-radical salts and an ionophore-based ion-selective membrane. To show the influence of the TCNQ layer on electrode selectivity, sodium- and potassium-sensitive membranes were applied. A good Nernstian response with a slope of 59.24 mV/dec in the range from 10(-6.5) to 10(-1) M KCl and 58.68 mV/dec (10(-6) to 10(-1) M NaCl) was observed. The influence of an interfacial water film was assessed by an aqueous-layer test performed during potentiometric measurements. The stability of the electrical potential of the new solid-contact electrodes was tested by performing current-reversal chronopotentiometry, and the capacitance of the electrodes is 132 μF or 154 μF for K(+) and Na(+) solid-contact electrodes. These properties confirmed the analytical applicability of TCNQ-based SC-ISEs and should allow the development of a new solid-state ion sensor group.

Biomimetic study of the Ca(2+)-Mg2+ and K(+)-Li+ antagonism on biologically active sites: new methodology to study potential dependent ion exchange

Competitive divalent (magnesium and calcium) or monovalent (potassium, lithium and sodium) ion exchange and its influence on a membrane potential formation was studied at biological ligands (BL) such as adenosine triphosphate (ATP), asparagine (Asn) and glutamine (Gln) sites. The sites are dispersed electrochemically in membranes made of the conducting polymers (CPs)--poly(N-methylpyrrole) (PMPy) and poly(pyrrole) (PPy). The membranes are made sensitive to calcium and magnesium or to potassium, sodium and lithium by optimized electrodeposition and soaking procedures supported by the study of membrane topography and morphology. Distinctively different electrochemical responses, i.e. electrical potential transients or currents, are observed in the case of "antagonistic" calcium and magnesium or potassium and sodium/lithium ion pairs. Dissimilarity in the responses is ascribed to a difference between on site vs. bulk concentrations of ions, and is dictated by different transport properties of the ions, as shown by using the Nernst-Planck-Poisson (NPP) model and the diffusion-layer model (DLM). The method described allows inspecting potential-dependent competitive ion-exchange processes at the biologically active sites. It is suggested that this approach could be used as an auxiliary tool in study of potential dependent block in realistic membrane channels, such as Mg block in the N-methyl D-aspartate receptor channel (NMDA).

Conducting polymers in modelling transient potential of biological membranes

The possibility of using conducting polymer (CP) films doped with biological ligands as artificial biological membranes to study potential formation mechanisms is presented. Calcium and magnesium ion-binding anionic sites--asparagine, glutamine, adenosinotriphosphate and heparin are incorporated into the poly(pyrrole) film during electrochemical polymerization. This approach allows the competitive calcium-magnesium ion-exchange to be inspected by open circuit measurements. After a close-to-Nernstian sensitivity of the CP membranes was induced by soaking in alkaline solutions of calcium or magnesium, dynamic experiments were performed by a change in the bulk concentration of magnesium or calcium ions. A characteristic transitory potential response, though distinctively different for the calcium and magnesium ions, was observed and explained using the diffusion layer model (DML).