Keywords:Aptasensor Biosensor Carbon-based nanomaterial Chemosensor Ionophore Ion-selective electrode Ion-selective microelectrode Nanomaterial Neutral species Potentiometric sensor a b s t r a c t Nanomaterials play an important role in the fabrication of chemosensors and biosensors, due to their unique physical and chemical properties, such as large surface area/volume ratio, good conductivity, excellent electrocatalytic activity and high mechanical strength. We review recent advances in the applications of these nanomaterials in potentiometric sensors. We highlight the development of stable solidstate polymeric membrane ion-selective electrodes (ISEs). We describe ISEs based on ionophore-modified nanomaterials. Also, we present highly-sensitive potentiometric biosensors based on nanomaterials.

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.

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. S ince the discovery of lowering the detection limit of ionselective electrodes (ISEs) in 1997, 1 spectacular progress in the development and application of potentiometric sensors has been made, and the new wave of ISEs has already arrived. 2−4The lower detection limit of polymeric membrane ISEs has been improved by a factor of up to 10 6 , while the discrimination of interfering ions has even improved by a factor up to 10 10 . 5Nowadays, ISEs have evolved to be a promising technique for trace-level environmental and biological measurements. 6−9However, it should be noted that most of the applications of polymeric membrane-based ISEs developed so far are limited to measurements with low levels of electrolyte background, typically for drinking water 10,11 or well-controlled solutions. 8,9Very few polymeric membrane ISEs have been reported for potentiometric detection with a high-interfering background. The ISE with an optimal internal solution buffered for primary ions has been used for determination of low levels of calcium with an intracellular fluid background containing high concentrations of K + , Na + , and Mg 2+ . 12 To avoid super-or sub-Nernstian behavior, tedious optimization of the inner solution is required for each sample. By coupling the microliter potentiometric detection to an efficient online electrochemical accumulation step, solid-contact ion-selective microele...

ABSTRACT: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 filmbased 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.

A general method for fabricating nanomaterials based solid-contact ISEs is developed. The mixture of an ionic liquid and a nanomaterial is used as intermediate layer.The detection limits of the proposed sensors are in the nanomolar range. The developed electrodes exhibit a good response time and excellent stability. 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.

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A novel chemically modified electrode for stripping determination of cadmium is presented in this paper, based on carbon nanotube-hydroxyapatite (CNT-HAP) nanocomposite, which can be prepared by an easy and effective onestep sonication. The newly synthesized nanocomposite was characterized with FTIR, TEM, and electrochemical methods. Due to the combination of the strong absorption ability of HAP and excellent electroanalytical properties of CNTs, the GC/CNT-HAP electrode has been successfully used for determination of Cd 2þ by anodic stripping voltammetry with a linear range of 20 nM -3 mM. The sensitivity and detection limit are 25.6 mA/mM and 4 nM, respectively. The practical application of the proposed electrode has been carried out for the determination of trace levels of Cd 2þ in real water samples.