Despite the tremendous variety of methods suitable for sensing applications, we face the fact that chemical sensors displaying sensitivity, selectivity and reversibility are still scarce and are mostly confined to low-molecular-weight species. Obviously, it is not the lack of optical (or other) transduction methods that limit the performance of present day sensor designs, but rather the insufficient selectivity of the recognition process, particularly in the field of sensors for organic and bioorganic species. The use of enzymes, ion carriers and natural or synthetic receptor/carriers which can undergo specific interactions with the species to be recognized (such as through hydrogen bonding or charge-transfer interaction) can result in specific recognition and, consequently, sensing. Examples for optical sensing schemes for clinically or biologically important species including enzyme substrates, metabolites, drugs, alkali and ammonium ions and others will be given. In enzyme-based sensors various options exist: depending on which species is immobilized, assays for substrates (such as glucose, ethanol, lactate or creatinine), enzymes (such as esterases) or inhibitors (such as organophosphates) can be designed. In addition, the intrinsic optical properties of certain enzymes, coenzymes or metabolites can be utilized for sensing purposes, a fact that presents an interesting alternative to enzyme sensors with chemical transducers. Notwithstanding the selectivity of biocatalytic sensors, their stability and sensitivity is moderate. Bioorganic synthetic molecules which can recognize and reversibly bind other species offer an attractive alternative, particularly in terms of stability. However, quite a different situation is found in such cases because such receptors, in contrast to enzymes, do not "digest" their substrates. Hence, while the steady-state response in enzyme-based sensors is a result of kinetic equilibration, substrate binding in non-metabolizing receptors results in thermodynamic equilibration. However, most existing receptors (except antibodies) lack the unique specificity of enzymes. On the other side, new bioorganic molecules and stable receptor/carriers along with polymer materials of proper permeation selectivity can help to overcome current limitations of protein-based systems. Neutral ion carriers, which may be considered as ion receptors, are a useful example of sometimes highly specific recognition/carrier molecules with excellent stability. Unfortunately, no receptor/carrier molecules of similar specificity do exist yet for most other organic and clinical parameters of interest. There is an obvious need for new and stable molecules suitable for specific recognition of low-molecular weight organic species. We will report on the use of such new receptor/carrier molecules, the respective sensor materials, and how the process of recognition can be coupled to optical transduction. Such receptors/carriers also allow other kinds of discriminations: if, for instance. it is enantio-selective (i.e. preferably binds one species out of a pair of optical isomers), a fairly specific recognition of enantiomers of biogenic amines (such as some drugs and biogenic amines) will become possible. Specific examples will also be given of new types of sensors based on recognition by charge-transfer interaction, through-space interaction and hydrogen bonding, with fair specificity for thiamine, penicilline, nitrate, salicylate and cholic acids. Finally, current problems and the significant challenges for sensors research in the 1990s will be discussed.
