Rt

where E is the potential in response to an ion, i, of activity ah and charge z\ k¡- is the selectivity coefficient; and j is any interfering ion of charge y and activity ar

Glasses exist that function as selective electrodes for many different monovalent and some divalent cations. Alternatively, a hydrophobic membrane can be made semipermeable if a hydrophobic molecule that selectively binds an ion (an iono-phore) is dissolved in it. The selectivity of the membrane is determined by the structure of the ionophore. One can detect K+, Mg2+, Ca2 Cd2+, Cu2+, Ag+, and NH4 by using specific ionophores. Some ionophores are natural products, such as gramicidin, which is highly specific for K+, whereas others such as crown ethers and cryptands are synthetic. S2I~, Br CI" , CN", SCN~, F", NO3-, CIO4 , and BF" can be detected by using quaternary ammonium cationic surfactants as a lipid-soluble counterion. ISEs are generally sensitive in the 10_1 to 10M range, but none is perfectly selective, so to unambiguously determine ionic concentrations it is necessary to use two or more ISEs with different selectivities. Also, ISEs require a reference electrode like that used in pH measurements. One can immobilize ionophore-containing membranes over planar potential-sensitive devices such as FETs, to create ion-sensitive FETs (ISFETs). As the potential does not depend on the area of the membrane, these work as well as larger bench-scale electrodes. An advantage of this approach is that a dense array of different ISFETs can be manufactured in a small area by using microfabrication techniques. Early probems with adhesion of the membranes to silicon have been largely solved by modifications of the design of the FETs themselves (Blackburn, 1987). The most typical membrane material used in ISEs is polyvinyl choride plasticized with dialkylsebacate or other hydrophobic chemicals. This membrane must be protected from fouling if an accurate measurement is to be made.

Elect rochemlcally Active Molecules

If a chemical can be oxidized or reduced, there is a good chance that this process can occur at the surface of an electrode. Selectivity can be achieved because each compound has a unique potential below which it is not converted, so under favorable conditions a sweep of potential can allow identification and quantification of different species with a single elec-

Inorganic species

Single electron transfers:

Solvated metal ions such as Fe2t/Fe3 +

All M°/Mnf pairs

Many species undergo multielectron transfer reactions: The oxygen-water series 02/H20;/H;0/0H H" Organic species

Most aromatics, generally nitrogen-containing aromatic heterocy-cles (reactions usually involve changes in number of atoms attached to molecule, and therefore are multistep, multielectron processes)

Metallo-organics

Ferrocene/ferrocinium

Biochemical species Hemes, chlorophylls, qui nones NAD7NADH NADP+/NADPH FAD/FADH FMN/FMNH

(not affected by O,) (not affected by Oz)

trode. This process is the basis for detection of a number of important biochemicals such as catecholamines (Table 3). Some species are determined directly at electrodes and others indirectly by interactions with mediator chemicals that are more easily detected as particular electrode surfaces.

Because detection involves conversion of one species to another, this is a consuming sensor, with all of the attendant problems. At least two electrodes are needed, and current must flow through the sample for a measurement to be made, although a precise reference electrode is not as necessary as in a potentiometric sensor. Near the electrode surface the concentration of either the oxidized or reduced species may differ greatly from the bulk concentration. This is partially because of depletion of the analyte near the surface (the diffusion layer), as well as attraction or repulsion of charged species from the charged electrode surface in the diffuse double layer (Fig. 5).

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