which are highly robust, much reliable (easily calibrated with little potential drift),
easy handling and can be used for wider range of species. One of the most conventional membrane electrodes are Glass membrane electrodes (which are highly
selective, low detection limit, very stable and free from redox interferences. The
selectivity of these membranes for various ions of interest largely depends upon
their composition. They are not only used for measuring protons concentration but
can also be applied for sensing metal ions such as alkali metals (Lithium, sodium,
Potassium) or transition metals ions (copper, Lead, Cadmium, Silver).
Chalcogenides based Glass membranes have been developed to detect metal ions.
They can be used in combination with microactuators or microelectronics for
monitoring heavy metals in waste water [15].
Solid state membranes are made up of inorganic salts. For example F
−
ions can
be detected by using single crystal LaF 3 doped with Europium (to enhance ionic
activity) based membranes [16]. Other doped atoms used are Nd, Sm, Ca etc. which
are used as dopant for enhancing ionic activity [17]. Silver salts of iodide, chloride,
bromide and sulphides are being used for the detection of metal ions such as Cu
2+ ,
Pd
2+ , Br
− , I
− , etc. these salts are dispersed into inert matrices such as PVC,
methacrylate, epoxy resins or polycrystalline membranes [18–20]. The robustness
of these solid state membranes is increased by subtracting conventional internal
electrolytes and hence reducing its temperature and pressure sensitivity and
increasing the chances of its miniaturization [20].
Liquid ion selective electrodes are mainly synthesized by inducing ionophores
(macrocyclic ion carriers, lipid-soluble entities either natural or synthetic chemicals
that transport ions across a membrane) into viscous ionic liquid membranes [21].
Ionophores can also be incorporated in PVC or other polymers to function as liquid
membranes and these polymer based membranes are much selective in comparison
to liquid membranes [21]. Conventional liquid electrolytes can easily be replaced
by attaching conducting polymers to liquid membranes. This will lead to an
enhanced ion-to-electron transduction between the electronic conductor and liquid
membranes [22, 23]. Conducting polymers may also be applied as ion selective
membranes by creating ion recognition sites in the polymer matrix [24]. Ca
2+
selective electrodes have been studied widely and many organophosphoric acid
based liquid membranes have been developed in past few years [25, 26].
Neutral carriers doped plasticized PVC membranes have been developed for a
wide range of metal ions [21, 27]. Polymer membranes incorporated with crown
ethers and bis-crown ether ionophores are used as site specific binding of metal ions
with ligands. Host cavity can be tuned for recognizing of pollutant materials.
Calixerenes (phenol-formaldehyde condensates) are widely used for metal ion
binding recognition. Electropolymerization techniques of monomers to conducting
polymer from a counter ion solution to tailor the cavity size in polymer film from
the desired ion for detection. Such efforts play an important role in enhancing the
selectivity of sensors with respect to interfering species using chemical recognition
techniques.
Materials in Electrochemical Detection of Water Pollutants
165
easy handling and can be used for wider range of species. One of the most conventional membrane electrodes are Glass membrane electrodes (which are highly
selective, low detection limit, very stable and free from redox interferences. The
selectivity of these membranes for various ions of interest largely depends upon
their composition. They are not only used for measuring protons concentration but
can also be applied for sensing metal ions such as alkali metals (Lithium, sodium,
Potassium) or transition metals ions (copper, Lead, Cadmium, Silver).
Chalcogenides based Glass membranes have been developed to detect metal ions.
They can be used in combination with microactuators or microelectronics for
monitoring heavy metals in waste water [15].
Solid state membranes are made up of inorganic salts. For example F
−
ions can
be detected by using single crystal LaF 3 doped with Europium (to enhance ionic
activity) based membranes [16]. Other doped atoms used are Nd, Sm, Ca etc. which
are used as dopant for enhancing ionic activity [17]. Silver salts of iodide, chloride,
bromide and sulphides are being used for the detection of metal ions such as Cu
2+ ,
Pd
2+ , Br
− , I
− , etc. these salts are dispersed into inert matrices such as PVC,
methacrylate, epoxy resins or polycrystalline membranes [18–20]. The robustness
of these solid state membranes is increased by subtracting conventional internal
electrolytes and hence reducing its temperature and pressure sensitivity and
increasing the chances of its miniaturization [20].
Liquid ion selective electrodes are mainly synthesized by inducing ionophores
(macrocyclic ion carriers, lipid-soluble entities either natural or synthetic chemicals
that transport ions across a membrane) into viscous ionic liquid membranes [21].
Ionophores can also be incorporated in PVC or other polymers to function as liquid
membranes and these polymer based membranes are much selective in comparison
to liquid membranes [21]. Conventional liquid electrolytes can easily be replaced
by attaching conducting polymers to liquid membranes. This will lead to an
enhanced ion-to-electron transduction between the electronic conductor and liquid
membranes [22, 23]. Conducting polymers may also be applied as ion selective
membranes by creating ion recognition sites in the polymer matrix [24]. Ca
2+
selective electrodes have been studied widely and many organophosphoric acid
based liquid membranes have been developed in past few years [25, 26].
Neutral carriers doped plasticized PVC membranes have been developed for a
wide range of metal ions [21, 27]. Polymer membranes incorporated with crown
ethers and bis-crown ether ionophores are used as site specific binding of metal ions
with ligands. Host cavity can be tuned for recognizing of pollutant materials.
Calixerenes (phenol-formaldehyde condensates) are widely used for metal ion
binding recognition. Electropolymerization techniques of monomers to conducting
polymer from a counter ion solution to tailor the cavity size in polymer film from
the desired ion for detection. Such efforts play an important role in enhancing the
selectivity of sensors with respect to interfering species using chemical recognition
techniques.
Materials in Electrochemical Detection of Water Pollutants
165