Electrochemical sensors sites

The concept of thin films of a molecularly imprinted sol-gel polymer with specific binding sites for a target analyte is general and can be applied also to electrochemical sensors. For example, a sensor to detect parathion in aqueous solutions is based on films cast on glass substrates and on glassy carbon electrodes (Figure 6.14).12... 

Domenech, A., and Alarcon, J. 2003. Vanadium-doped zircon and zirconia materials prepared from gel precursors as site-selective electrochemical sensors. Instrumentation, Science Technology 31, 121-139. 

In the other detection technology the chemical/electrical response of a small sensor located at the desired site is monitored. One usually distinguishes here between solid state sensors and electrochemical sensor cells. Since the sensor output is transmitted electrically, the distance between it and the monitoring system is usually not a limitation. 

Nitrite Nitrite is an important indicator of fecal pollution in natural waters as well as a potential precursor of carcinogenic species. A rush of flow and sequential injection spectrophotometric method based on Griess-type reactions has been proposed, also coupled to online sorbent enrichment schemes. The catalytic effect of nitrite on the oxidation of various organic species constitutes the basis of fairly sensitive spectrophotometric methods. Fluorometric methods based on the formation of aromatic azoic acid salts, quenching of Rhodamine 6G fluorescence, and direct reaction with substituted tetramine or naphthalene species have been also reported. Indirect CL methods usually involve conversion into nitric oxide and gas-phase detection as mentioned in the foregoing section. The redox reaction between nitrite and iodide in acidic media is the fundamental of a plethora of flow injection methodologies with spectrophotometric, CL, or biamperometric detection. New electrochemical sensors with chemically modified carbon paste electrodes containing ruthenium sites, or platinum electrodes with cellulose or naphthalene films, have recently attracted special attention for amperometric detection. 

The main drawback of redox polymer electrodes, both in fundamental studies and in chemical analysis, stems from the difficulty in controlhng the concentration and spatial organization of redox sites within the film (4). The main use of redox polymer modified electrodes has been in fundamental studies of electron transfer mechanisms. These electrodes are also used as electrochemical sensor devices, but on a much more limited scale (117). Many of these limitations are overcome by the use of ion-exchange polymers, as outlined below. 

---