Chemically modified electrodes exhibit unique behavior that can greatly benefit electrochemical sensing. In this article we review the development and applications of modified electrodes for chemical sensing. After a general introduction, different strategies for surface modification, as employed in chemical sensing, are discussed in detail. Special attention is given to permselective coatings, which offer controlled access at the sensor surface or the surface immobilization of biological entities that specifically recognize the analyte. Better understanding of these modified electrodes is achieved by using high resolution surface characterization techniques. Future prospects are evaluated.
mTRODUCTIOiVThe development of electrochemical sensors continues to be a rapidly growing area of electrochemistry. Improvements in the stability, selectivity, and scope of such sensors are highly desirable to meet new challenges posed by clinical and environmental samples. The utility of solidelectrode-based sensors is often hampered by a gradual fouling of the surface due to adsorption of large organic surfactants or of reaction products. Amperometric sensors lack the ability to discriminate between solutes possessing similar redox characteristics. Finally, the sensing of many important solutes is often hindered by their slow electron transfer kinetics at the commonly used electrodes materials.One field that offers great potential for alleviating the problems above, hence for enhancing the power and scope of electrochemical sensors, is that comprising chemically modified electrodes (CMEs) [ 1, 21. The ability to deliberately control and manipulate the surface properties can lead to a variety of attractive effects. Such tailoring of the surface can meet the needs of many sensing problems. The field of CMEs, which has experienced a period of rapid growth over the past decade, has now reached a level of maturity that allows the use of these electrodes for routine sensing applications.
SURFACE-MODIFID AMPEROMETHIC SENSORSThe most common approaches for incorporating a modifier into surfaces of amperometric sensors have been coverage with an appropriate polymer film and doping of a carbon paste. The modifier should be firmly attached to the surface, and it should not dissolve in the surrounding solution. The long-term stability of the attached moieties is a key factor for the performance of reliable sensors. Stability of days or weeks is essential to achieve widespread acceptance.The preparation of structure interfaces of deliberately designed character offers many promising prospects with regard to the development of chemical sensors. There are different directions by which the resulting CMEs can benefit sensing applications. These include acceleration of electron transfer reactions, prevention of fouling, permselective transport, preferential accumulation, and biocatalysis. Chemical and biological surface modifiers can thus enhance the selectivity, sensitivity, and scope of electrochemical sensors. The following sections...