Potentiometric monitoring DNA hybridization

The usual procedure to monitor the ion exchange of small ions utilizes a potentiometer with a selective membrane as part of the working electrode. As the next step, we have applied polyaniline electrodes to the monitoring the activity macromolecular ions during DNA hybridization. Single-strand oligonucleotide (ssODN) probes were immobilized using a nucleophilic substitution reaction of the thiolated ssODN molecules with polyaniline. The anionic phosphate groups of the probe molecules also interacted with the cationic-doped polyaniline surface. Three useful findings were observed with the potentiometric experiments. First, the binding of the complimentary target molecules with the immobilized probes revealed a substantial potential change. Further, potential change was observed neither with the non-complimentary targets nor with the samples with a mutation in the sequence. The last two experiments were important for the future evaluation of the impact of medium and potential interfering compounds: anionic groups and hydrogen bonding groups in the non-complimentary samples did not cause any interactions.

Elimination of undesirable water layers in solid-contact polymeric ion-selective electrodes

This study aimed to develop a novel approach for the production of analytically robust and miniaturized polymeric ion sensors that are vitally important in modern analytical chemistry (e.g., clinical chemistry using single blood droplets, modern biosensors measuring clouds of ions released from nanoparticle-tagged biomolecules, laboratory-on-a-chip applications, etc.). This research has shown that the use of a water-repellent poly(methyl methacrylate)/poly(decyl methacrylate) (PMMA/PDMA) copolymer as the ion-sensing membrane, along with a hydrophobic poly(3-octylthiophene 2,5-diyl) (POT) solid contact as the ion-to-electron transducer, is an excellent strategy for avoiding the detrimental water layer formed at the buried interface of solid-contact ion-selective electrodes (ISEs). Accordingly, it has been necessary to implement a rigorous surface analysis scheme employing electrochemical impedance spectroscopy (EIS), in situ neutron reflectometry/EIS (NR/EIS), secondary ion mass spectrometry (SIMS), and small-angle neutron scattering (SANS) to probe structurally the solid-contact/membrane interface, so as to identify the conditions that eliminate the undesirable water layer in all solid-state polymeric ion sensors. In this work, we provide the first experimental evidence that the PMMA/PDMA copolymer system is susceptible to water "pooling" at the interface in areas surrounding physical imperfections in the solid contact, with the exposure time for such an event in a PMMA/PDMA copolymer ISE taking nearly 20 times longer than that for a plasticized poly(vinyl chloride) (PVC) ISE, and the simultaneous use of a hydrophobic POT solid contact with a PMMA/PDMA membrane can eliminate totally this water layer problem.

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.

Conducting polymers in chemical sensors and arrays

The review covers main applications of conducting polymers in chemical sensors and biosensors. The first part is focused on intrinsic and induced receptor properties of conducting polymers, such as pH sensitivity, sensitivity to inorganic ions and organic molecules as well as sensitivity to gases. Induced receptor properties can be also formed by molecularly imprinted polymerization or by immobilization of biological receptors. Immobilization strategies are reviewed in the second part. The third part is focused on applications of conducting polymers as transducers and includes usual optical (fluorescence, SPR, etc.) and electrical (conductometric, amperometric, potentiometric, etc.) transducing techniques as well as organic chemosensitive semiconductor devices. An assembly of stable sensing structures requires strong binding of conducting polymers to solid supports. These aspects are discussed in the next part. Finally, an application of combinatorial synthesis and high-throughput analysis to the development and optimization of sensing materials is described.

Optimalization of Poly(neutral red) Coated-wire Electrode for Determination of Citrate in Soft Drinks

This report presents an optimization of potentiometric measurements with citrate-selective electropolymerized poly(neutral red) electrodes. The optimal background electrolyte for these measurements is a TRIS buffer with nitrate at pH 8.5. The electrodes described here exhibit stable and reproducible near-Nernstian response to citrates with a low detection limit of 6 × 10^-6 M. Electrodes polymerized from sulfuric acid and acetonitrile are compared in detail. Simple and sensitive method for quantification of citrate in real-life samples by potentiometry with poly(neutral red) electrodes are presented. Data from potentiometric measurements of citrate are compared with capillary electrophoresis.

Evidence of a water layer in solid-contact polymeric ion sensors

This paper presents the very first direct structural evidence for the formation of a 100 +/- 10 A water layer in coated-wire polymeric-membrane ion-selective electrodes (ISEs).

Electroanalysis of NADH Using Conducting and Redox Active Polymer/Carbon Nanotubes Modified Electrodes-A Review

Past few decades, conducting and redox active polymers play a critical role in the development of transducers for biosensing. It has been evidenced by increasing numerous reports on conducting and redox active polymers incorporated electrodes for assay of biomolcules. This review highlights the potential uses of electrogenerated polymer modified electrodes and polymer/carbon nanotubes composite modified electrodes for electroanalysis of reduced form of nicotinamide adenine dinuceltoide (NADH). In addition, carbon electrodes modified with organic and inorganic materials as modifier have been discussed in detail for the quantification of NADH based on mediator or mediator-less methods.

Galvanic cell without liquid junction for potentiometric determination of copper

This paper describes potentiometric measurements in an integrated galvanic cell with both indicator and reference electrodes. Both electrodes are conducting polymer-based. The copper-sensitive indicator electrode is made by using poly(3,4-ethylenedioxythiophene) (PEDOT) doped with 2-(o-arsenophenylazo)-1,8-dihydroxynaphthalene-3,6-disulphonic sodium salt (Arsenazo-I) as the electroactive substance in the film, while the reference electrode is based on PEDOT doped by 2-morpholineoethanesulfonic acid (MES). It is shown that the galvanic cell can be used for determination of copper both in non-aqueous media (where all PVC-based membranes failed) and in the presence of chloride ions, which disturb the signal of conventional copper ion-selective electrodes with solid-state membranes. It is further shown that the titration of copper ions can be successfully monitored using the described electrochemical cell.

Overoxidized Polypyrrole Doped with 4,5-Dihydroxy-3-(p-sulfophenylazo)-2,7-naphthalene Disulfonic Acid as a Selective and Regenerable Film for the Stripping Detection of Copper(II)

A conducting polymer modified electrode based on the incorporation of 4,5-dihydroxy-3-(p-sulfophenylazo)-2,7-naphthalene disulfonic acid, SPADNS, as an anionic complexing ligand into polypyrrole film during electropolymerization was prepared. The electroanalysis of copper(II) in this modified electrode was achieved by medium exchange and differential pulse voltammetry. Copper ions were accumulated from ammonia buffer on the electrode surface by the formation of a chemical complex at open circuit. The resulting electrode with complexed Cu(2+) was then transferred to an acetate buffer and subjected to anodic stripping voltammetry. The analytical performance was evaluated and, finally, linear calibration graphs were obtained in the concentration range of 2 - 250 ng ml(-1) for Cu(II). The detection limit was found to be 1.1 ng ml(-1) and RSD was obtained at 3.1 and 1.9% for two different concentrations. Many coexisting metal ions had little or no effect on the determination of copper. The developed method was applied to Cu(II) determination in natural water and human hair samples. Also, the rapid and convenient regeneration of electrode allows the use of a single modified electrode in multiple analyses.

Voltammetric Heparin-Selective Electrode Based on Thin Liquid Membrane with Conducting Polymer-Modified Solid Support

A novel, solid-supported voltammetric ion-selective electrode to detect anticoagulant/antithrombotic heparin at polarizable poly(vinyl chloride) (PVC) membrane/water interfaces was developed. An approximately 3-4.5-microm-thick PVC membrane plasticized with 2-nitrophenyl octyl ether was supported on a gold electrode modified with a poly(3-octylthiophene) (POT) film as an ion-to-electron transducer. Charge transport through the PVC-covered POT film is electrochemically reversible, as demonstrated by cyclic voltammetry with nonpolarizable membrane/water interfaces. In addition to the fast charge transport, adequate redox capacity of the POT film and a small ohmic potential drop in the thin PVC membrane enable ion transfer voltammetry at polarizable macroscopic membrane/water interfaces in a standard three-electrode cell. Reversible ClO4- transfer at the interfaces coupled with oxidation of a neutral POT film was examined by cyclic voltammetry to determine the distribution of the applied potential to the two polarizable interfaces by convolution technique. Interfacial adsorption and desorption of heparin facilitated by octadecyltrimethylammonium were studied also by cyclic voltammetry and convolution technique to demonstrate that the processes are electrochemically irreversible. Stripping voltammetry based on the interfacial processes gives a low detection limit of 0.005 unit/mL heparin in a saline solution, which is slightly lower than the detection limit of most sensitive heparin sensors reported so far (0.01 unit/mL).

Factors affecting the potentiometric response of all-solid-state solvent polymeric membrane calcium-selective electrode for low-level measurements

An all-solid-state calcium-selective electrode was constructed with poly(pyrrole) solid-contact doped with calcium complexing ligand Tiron. The potentiometric response of this sensor can have a linear range down to 10(-)(9) M with a slope close to Nernstian and detection limit equal to 10(-)(9.6). The effects of pH and the activity of the interfering ion in the conditioning solution on the potentiometric behavior of the constructed sensors were examined. Potential stability, reproducibility, and impedance studies were performed. The selectivity of the constructed electrode is better than that of the conventional calcium-selective electrode with internal filling solution of 10(-)(2) M CaCl(2) and comparable to that of the best liquid-contact electrodes.

Polymeric Sensor Materials: Toward an Alliance of Combinatorial and Rational Design Tools?

Increased selectivity, response speed, and sensitivity in the chemical and biological determinations of gases and liquids are of great interest. Particular attention is paid to polymeric sensor materials, which are applicable to sensors exploiting various energy transduction principles, such as radiant, electrical, mechanical, and thermal energy. Ideally, numerous functional parameters of sensor materials can be tailored to meet specific needs using rational design approaches. However, increasing the structural and functional complexity of polymeric sensor materials makes it more difficult to predict the desired properties. Combinatorial and high-throughput methods have had an impact on all areas of research on polymer-based sensor materials including homo- and copolymers, formulated materials, polymeric structures with engineered morphology, and molecular shape-recognition materials. Herein we report on the state-of-the-art, the development trends, and the remaining knowledge gaps in the area of combinatorial polymeric sensor materials design.

Optimizing the analytical performance and construction of ion-selective electrodes with conducting polymer-based ion-to-electron transducers

All-solid-state ion-selective electrodes that use a conducting polymer as the ion-to-electron transducer have emerged as one of the most promising classes of all-solid-state potentiometric sensors in recent years. This is largely because it has many analytical advantages, including high response stability, which is unique in the field of internal-solution-free ion-selective electrodes. This paper reviews the considerable progress that has been made in this area of sensing in recent years, in terms of detection limits, selectivity coefficients and novel construction methods.