Near-Infrared Laser Irradiation-Modulated High-Temperature Solid-Contact Ion-Selective Electrodes: Potentiometric Detection of Ca2+ in Seawater.

The high-temperature potentiometry operated by nonisothermal heating is a promising way to break through the traditional potentiometric responses of ion-selective electrodes (ISEs) at room temperature. Herein, a locally heated strategy through near-infrared region (NIR) laser irradiation upon the photothermal mesoporous carbon material placed between the ion-selective membrane and the glassy carbon substrate is introduced to obtain the high-temperature potentiometric performance of a solid-contact Ca2+-ISE for detection of Ca2+ in seawater. Based on the light-to-heat conversion of the mesoporous carbon-based solid contact, the temperature of the solid-contact Ca2+-ISE upon continuous NIR laser irradiation can be increased from room temperature to 60-70 °C, and the slope of the electrode is promoted up to about 30% according to the thermodynamic steady-state potentiometric response. The pulsed potentiometric response of the solid-contact Ca2+-ISE upon a pulsed NIR laser irradiation of 5 s also shows a linear change as a function of Ca2+ activities, and the improved slope from 27.1 ± 0.6 to 38.1 ± 0.9 mV/dec can be obtained under dual control of the temperature of the electrode and the transient current induced by the pulsed NIR laser irradiation. As compared to the traditional potentiometric measurement under zero-current conditions at room temperature, the NIR laser-modulated high-temperature potentiometric response provides an alternative way for measurement of the solid-contact ISEs.

Redox probe-based amperometric sensing for solid-contact ion-selective electrodes.

The transformation from the traditional potentiometric response of an ion-selective electrode (ISE) to other signal readout is promising to realize the potential signal amplification. In this work, the redox probes, including ferrocyanide/ferricyanide (Fe(CN)63-/4-), hexaammineruthenium (Ru(NH3)63+) and ferrocene derivatives, were introduced to read out the potentiometric response for the solid-contact Ca2+-ISE. The mechanism is that the oxidation current of the redox probe on a glassy carbon electrode is modulated by the potential of the ISE through changing the concentrations/activities of Ca2+ under the control of the constant applied potential. Results show that the linear range and the slope sensitivity for detection Ca2+ by using the amperometric signal based on Fe(CN)64-/3- redox probe are adjustable through changing the applied potentials. Moreover, the redox probe-based amperometric signal for the solid-contact Ca2+-ISE is found to be related to both of the types of the redox probes and the electrode areas. This work provides a convenient and general method for translating the potential response at mV grade to the amperometric signal at the µA level, and is promising for detection of ions with high sensitivity by using the ISEs.

Translating potentiometric detection into non-enzymatic amperometric measurement of H2O2.

The developments of alternative signal readout strategies for the ion-selective electrodes (ISEs) are necessary in order to break through the limitation of the Nernst equation. In this work, a simple, convenient and easily operated strategy based on the non-enzymatic amperometric measurement of H2O2 is proposed to read out the potentiometric responses for the ISEs. The proposed amperometric signal readout based on H2O2 is carried out in a two compartment electrochemical cell configuration containing a detection cell and a sample cell, physically connected by a salt bridge. A glassy carbon (GC) electrode is placed in the detection cell to monitor the oxidation current of H2O2, and an ISE is placed in the sample cell to act as both the reference electrode and the potentiometric sensor for obtaining the ion activities. The oxidation of H2O2 is induced by a constant potential applied between the GC electrode and the ISE, and subsequently modulated by the potential change of the ISE in the presence of the primary ion. The proposed amperometric signal readout based on H2O2 shows the satisfied slope sensitivity and detection limit, which are better than or compared to those for the potentiometric responses for the ISEs. This work provides a general strategy for transforming the potential response of the ISEs into the amperometric readout, and is promising for detection of cations (eg., Ca2+) and anions (eg., NO3-) with high sensitivity and excellent selectivity.

Alternative coulometric signal readout based on a solid-contact ion-selective electrode for detection of nitrate.

Traditional potentiometric NO3--selective electrodes suffer from a fundamental limitation of the Nernst slope (59.1 mV/dec at 25 °C) due to the relationship between the potential and the logarithmic of ionic activity. Herein, a coulometric signal readout is proposed instead of the potentiometric response for detection of NO3- based on an ordered mesoporous carbon (OMC)-based solid-contact ion-selective electrode (ISE). The mechanism for obtaining the coulometric signal is based on the electrical double layer capacitance of OMC compensating the potential change at the ion-selective membrane/solution interface during the measurements under the control of a constant applied potential. Under the optimized conditions, the coulometric signal for the OMC-based solid-contact NO3--ISE shows two linear responses in the activity range of 1.0 × 10-6-8.0 × 10-6 M and 8.0 × 10-6-8.0 × 10-4 M, and the detection limit is 4.0 × 10-7 M (3σ/s). The proposed coulometric response also shows excellent reproducibility and stability in the presence of O2 and CO2 and light on/off. Additionally, the coulometric response shows acceptable and reliable results for detection of NO3- in mineral water as compared to the traditional potentiometric response and the ion chromatography. This work provides a promising alternative signal readout for detection of ions by using solid-contact ion-selective electrodes.

Real-time monitoring of the dissolution of silver nanoparticles by using a solid-contact Ag+-selective electrode.

The dissolution kinetics of silver nanoparticles (Ag NPs) to Ag+ ions is a critical factor determining the toxicity of silver nanoparticles. In this work, a solid-contact Ag+-selective electrode (Ag+-ISE) is fabricated and used to monitor the dissolution of Ag NPs. Ordered mesoporous carbon is compared with disordered mesoporous carbon as the solid-contact material for the Ag+-ISE. The ordered mesoporous carbon based solid-contact Ag+-ISE shows a linear potential response in the range of 1.0 × 10-6-1.0 × 10-3 M AgNO3 with the slope of 55.6 ± 0.8 mV/dec (n = 7) and the detection limit of 10-6.8 M. The solid-contact Ag+-ISE is used to monitor the concentration changes of Ag+ during spontaneous dissolution of Ag NPs in deionized water, and the dissolution kinetics of Ag NPs is consistent with that obtained by inductively coupled plasma-mass spectrometry (ICP-MS). Stimulated dissolution of Ag NPs induced by addition of H2O2 to the Ag NP solution is also investigated by the proposed solid-contact Ag+-ISE. This work provides a fast tool for charactering the dissolution of Ag NPs to Ag+ in real time, which is important for studying the toxicology of nanoparticles.

A magnetic field-directed self-assembly solid contact for construction of an all-solid-state polymeric membrane Ca2+-selective electrode.

A magnetic field-directed self-assembly solid contact has been proposed for developing an all-solid-state polymeric membrane Ca2+-selective electrode. The solid contact is prepared by physically adsorbing magnetic graphene powder on a magnetic gold electrode under the direction of the magnetic field. The proposed method for preparing solid contact avoids using the aqueous solutions and is simple, fast and general as compared to the multilayer drop-casting and electrodeposition methods. The all-solid-state Ca2+-selective electrode based on magnetic graphene as solid contact shows a stable potential response in the linear range of 1.0 × 10-6-1.0 × 10-3 M with a slope of 28.2 mV/decade, and the detection limit is about 4.0 × 10-7 M. Additionally, the magnetic graphene-based electrode shows a comparable potential stability performance to other graphene-based all-solid-state ion-selective electrodes, such as reduced undesirable water layer and insensitive to the interferences of O2, CO2 and light. This work provides a favorable way to prepare solid contact for use in the field of all-solid-state ion-selective electrodes.

Potentiometric Aptasensing of Vibrio alginolyticus Based on DNA Nanostructure-Modified Magnetic Beads.

A potentiometric aptasensing assay that couples the DNA nanostructure-modified magnetic beads with a solid-contact polycation-sensitive membrane electrode for the detection of Vibrio alginolyticus is herein described. The DNA nanostructure-modified magnetic beads are used for amplification of the potential response and elimination of the interfering effect from a complex sample matrix. The solid-contact polycation-sensitive membrane electrode using protamine as an indicator is employed to chronopotentiometrically detect the change in the charge or DNA concentration on the magnetic beads, which is induced by the interaction between Vibrio alginolyticus and the aptamer on the DNA nanostructures. The present potentiometric aptasensing method shows a linear range of 10-100 CFU mL-1 with a detection limit of 10 CFU mL-1, and a good specificity for the detection of Vibrio alginolyticus. This proposed strategy can be used for the detection of other microorganisms by changing the aptamers in the DNA nanostructures.

An all-solid-state polymeric membrane Pb²âº-selective electrode with bimodal pore C60 as solid contact.

An all-solid-state polymeric membrane Pb(2+) ion-selective electrode (Pb(2+)-ISE) based on bimodal pore C60 (BP-C60) as solid contact has been developed. A BP-C60 film can be readily formed on the surface of a glassy carbon electrode by electrochemical deposition. Cyclic voltammetry and electrochemical impedance spectroscopy have been employed to characterize the BP-C60 film. The large double layer capacitance and fast charge-transfer capability make BP-C60 favorable to be used as solid contact for developing all-solid-state ISEs. The all-solid-state BP-C60-based Pb(2+)-ISE shows a Nernstian response in the range from 1.0×10(-9) to 1.0×10(-3)M with a detection limit of 5.0×10(-10)M. The membrane electrode not only displays an excellent potential stability with the absence of a water layer between the ion-selective membrane and the underlying BP-C60 solid contact, but also is insensitive to interferences from O2, CO2 and light. The proposed solid-contact Pb(2+)-ISE has been applied to determine Pb(2+) in real water samples and the results agree well with those obtained by anodic stripping voltammetry.

A simple approach for fabricating solid-contact ion-selective electrodes using nanomaterials as transducers.

A simple and robust approach for the development of solid-state ion-selective electrodes (ISEs) using nanomaterials as solid contacts is described. The electrodes are fabricated by using the mixture of an ionic liquid (IL) and a nanomaterial as intermediate layer, formed by melting the IL. Tetradodecylammonium tetrakis(4-chlorophenyl)borate (ETH 500) is chosen as an model of IL to provide strong adhesion between the inner glassy carbon electrode and the intermediate layer. Nanomaterials including single-walled carbon nanotubes (SWCNTs) and graphene were used as active ion-to-electron transducers between the glassy carbon electrode and the ionophore-doped ISE membrane. By using the proposed approach, the solid-contact Cu(2+)- and Pb(2+)-selective electrodes based on ETH 500/SWCNTs and ETH 500/graphene as transducers, respectively, have been fabricated. The proposed electrodes show detection limits in the nanomolar range and exhibit a good response time and excellent stability.

All-solid-state polymeric membrane ion-selective miniaturized electrodes based on a nanoporous gold film as solid contact.

A new type of all-solid-state polymeric membrane ion-selective electrodes (ISEs) is developed by using a nanoporous gold (NPG) film as solid contact. The NPG film is in situ formed on the surface of a gold wire electrode by the multicyclic electrochemical alloying/dealloying method. The characteristics of the NPG film, such as the large surface area, high double layer capacitance, and good conductivity, have been demonstrated by cyclic voltammetry and electrochemical impedance spectroscopy. The NPG film offers a well-defined interface between the electronic conductor and the ion-selective membrane. The NPG film-based all-solid-state K(+) ISE shows a stable Nernstian response within the concentration range from 10(-6) to 10(-2) M, and the detection limit is 4.0 × 10(-7) M. The proposed electrode exhibits an improved potential stability with a reduced water layer in comparison with the coated-wire K(+)-ISE, which is due to the bicontinuous electron- and ion-conducting properties of the ionophore-doped polymeric membrane/NPG film interlayer. Unlike the additionally coated intermediate layers as single-use solid contacts, the in situ formed NPG film as solid contact is reusable. This work provides a versatile method for fabricating the robust, reliable, and low-maintenance miniaturized ISEs.

Trace-level potentiometric detection in the presence of a high electrolyte background.

Polymeric membrane ion-selective electrodes (ISEs) have become attractive tools for trace-level environmental and biological measurements. However, applications of such ISEs are often limited to measurements with low levels of electrolyte background. This paper describes an asymmetric membrane rotating ISE configuration for trace-level potentiometric detection with a high-interfering background. The membrane electrode is conditioned in a solution of interfering ions (e.g., Na(+)) so that no primary ions exist in the ISE membrane, thus avoiding the ion-exchange effect induced by high levels of interfering ones in the sample. When the electrode is in contact with the primary ions, the interfering ions in the membrane surface can be partially displaced by the primary ions due to the favorable ion-ligand interaction with the ionophore in the membrane, thus causing a steady-state potential response. By using the asymmetric membrane with an ion exchanger loaded on the membrane surface, the diffusion of the primary ions from the organic boundary layer into the bulk of the membrane can be effectively blocked; on the other hand, rotation of the membrane electrode dramatically reduces the diffusion layer thickness of the aqueous phase and significantly promotes the mass transfer of the primary ions to the sample-membrane interface. The induced accumulation of the primary ions in the membrane boundary layer largely enhances the nonequilibrium potential response. By using copper as a model, the new concept offers a subnanomolar detection limit for potentiometric measurements of heavy metals with a high electrolyte background of 0.5 M NaCl.

A solid-contact Pb(2+)-selective electrode using poly(2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene) as ion-to-electron transducer.

In this work, a novel all-solid-state polymeric membrane Pb(2+)-selective electrode was developed by using for the first time poly(2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) as solid contact. To demonstrate the ion-to-electron transducing ability of MEH-PPV, chronopotentiometry and electrochemical impedance spectroscopy measurements were carried out. The proposed electrodes showed a Nernstian response of 29.1 mV decade(-1) and a lower detection limit of subnanomolar level. No water film was observed with the conventional plasticized PVC membrane. This work demonstrates a new strategy for the fabrication of robust potentiometric ion sensors.

Synthesis and characterization of monoazathiacrown ethers as ionophores for polymeric membrane silver-selective electrodes.

Nine monoazathiacrown ethers have been synthesized and explored as ionophores for polymeric membrane Ag(+)-selective electrodes. Potentiometric responses reveal that the ion-selective electrodes (ISEs) based on 2,2'-thiodiethanethiol derivatives can exhibit excellent selectivities toward Ag(+). The plasticized poly(vinyl chloride) membrane electrode using 22-membered N(2)S(5)-ligand as ionophore has been characterized and its logarithmic selectivity coefficients for Ag(+) over most of the interfering cations have been determined as <-8.0. Under optimal conditions, a lower detection limit of 2.2x10(-10)M can be obtained for the membrane Ag(+)-ISE.

Potentiometric measurement of ascorbate by using a solvent polymeric membrane electrode.

A novel potentiometric method for the determination of ascorbate is described in this communication. It is based on ascorbate oxidation with permanganate which is continuously released from the inner reference solution of a ligand-free tridodecylmethylammonium chloride (TDMAC)-based polymeric membrane ion selective electrode (ISE). The ISE potential determined by the activity of permanganate ions released at the sample-membrane phase boundary is increased with the consumption of permanganate. The proposed membrane electrode is useful for continuous and reversible detection of ascorbate at concentrations in 0.1M NaCl ranging from 1.0 x 10(-6) to 1.0 x 10(-3)M with a detection limit of 2.2 x 10(-7)M.

Kinetics parameter estimation for the binding process of salicylic acid to human serum albumin (HSA) with capacitive sensing technique.

A novel method for real-time investigating the binding interaction between human serum albumin (HSA) and salicylic acid with capacitive sensing technique was successfully proposed. HSA was immobilized on the surface of a gold electrode modified with an insulating poly (o-phenylenediamine) (o-PD) film and colloid Au nanoparticles layers. The bioactivity of HSA was remained and major binding sites were available because of the excellent biocompatibility of gold nanoparticles. The capacitance and interfacial electron resistance of the sensor were altered, owing to the binding of HSA to salicylic acid. The time courses of the capacitance change were acquired with capacitive sensing technique during the binding process. Based on the capacitance response curves with time, the response model for the binding was derived in theory and the corresponding regression parameters were determined by fitting the real-time experimental data to the model. The binding and the dissociation rate constants (k(1) and k(-1)) were estimated to be 54.8 (mol l(-1))(-1) s(-1) and 2.9 x 10(-3) s(-1), respectively. And the binding equilibrium constant (K(a)) was calculated to be 1.89 x 10(4) (mol l(-1))(-1).

Selective detection of dopamine in the presence of ascorbic acid by use of glassy-carbon electrodes modified with both polyaniline film and multi-walled carbon nanotubes with incorporated beta-cyclodextrin.

A simple, sensitive, and reliable method based on a combination of multi-walled carbon nanotubes with incorporated beta-cyclodextrin (beta-CD-MWNTs) and a polyaniline (PANI) film-modified glassy-carbon (GC) electrode has been successfully developed for determination of dopamine (DA) in the presence of ascorbic acid (AA). The PANI film had good anti-interference properties and long-term stability, because of the permselective and protective properties of the conducting redox polymer film. The acid-treated MWNTs with carboxylic acid functional groups promoted the electron-transfer reaction of DA and inhibited the voltammetric response of AA. Sensitive detection of DA was further improved by the preconcentration effect of formation of a supramolecular complex between beta-CD and DA. The analytical response of the beta-CD-MWNTs/PANI film to the electrochemical behavior of DA was, therefore, better than that of a MWNTs/PANI film, a PANI film, or a bare glassy-carbon (GC) electrode. Under the conditions chosen a linear calibration plot was obtained in the range 1.0 x 10(-7)-1.0 x 10(-3) mol L(-1) and the detection limit was 1.2 x 10(-8) mol L(-1). Interference from AA was effectively eliminated and the sensitivity, selectivity, stability, and reproducibility of the electrodes was excellent for determination of DA.