An overview of potentiometric sensors that are capable of detecting toxic heavy metal ions in environmental samples is presented and discussed. Notwithstanding the tremendous work performed so far, it is obvious that still several limitations do exist in terms of selectivity, limits of detection, dynamic ranges, applicability to specific problems, and reversibility. A survey on important advances in potentiometric sensors with regard to high selectivity, lower detection limit, fast response time, and on-line environmental analysis is presented in this review article. [Supplemental materials are available for this article. Go to the publisher's online edition of Critical Reviews in Analytical Chemistry to view the free supplemental file.].

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Ionophore incorporated PVC membrane sensors are well-established analytical tools routinely used for the selective and direct measurement of a wide variety of different ions in complex biological and environmental samples. Potentiometric sensors have some outstanding advantages including simple design and operation, wide linear dynamic range, relatively fast response and rational selectivity. The vital component of such plasticized PVC members is the ionophore involved, defining the selectivity of the electrodes' complex formation. Molecular recognition causes the formation of many different supramolecules. Different types of supramolecules, like calixarenes, cyclodextrins and podands, have been used as a sensing material in the construction of ion selective sensors. Schiff's bases and crown ethers, which feature prominently in supramolecular chemistry, can be used as sensing materials in the construction of potentiometric ion selective electrodes. Up to now, more than 200 potentiometric membrane sensors for cations and anions based on Schiff's bases and crown ethers have been reported. In this review cation binding and anion complexes will be described. Liquid membrane sensors based on Schiff's bases and crown ethers will then be discussed.

Supramolecular chemistry can be defined as a field of chemistry, which studies the complex multi-molecular species formed from molecular components that have relatively simpler structures. This field has been subject to extensive research over the past four decades. This review discusses classification of supramolecules and their application in design and construction of ion selective sensors.

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In this work, we have developed a novel nanocomposite material of ceria (CeO 2 )-reduced graphene oxide (RGO) by sonochemical route for the application as symmetric supercapacitors. CeO 2 nanoparticles have been anchored on RGO sheets in order to maximize the specific capacitances of these materials. Nanostructure studies and electrochemical performances of the CeO 2 nanoparticles on the RGO sheets were systematically investigated. The morphology and crystalline structure of nanocomposites were examined by field emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FT-IR), and X-ray diffraction (XRD). Electrochemical properties of the nanocomposite electrodes were examined by cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) measurements. CeO 2 -RGO nanocomposite electrodes exhibited excellent supercapacitive behavior with high specific capacitance of (211 F g -1 at 2 mV s -1 and 185 F g -1 at 2.0 A g -1 ), high rate capability and well reversibility. Cycling stability of electrode was measured by continues cyclic voltammetry (CCV) technique. Upon 4000 cycles, the specific capacitance of the electrode increases and reaches a maximum value of 105.6 % of the initial value. ARTICLE Journal NameWhere SC is the specific capacitance (F g -1 ), ν is the potential scan rate (mV s -1 ), V c -V a is the potential range and I denotes the response current (mA g -1 ) based on the mass of electroactive material. Fig. 6b and 6c show the CV curves of

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 Sensors 2008, 8 2332 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.

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