Chemical Molecular Selectivity
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ne of the most important themes in analytical chemistry is selectivity in detection and determination. Given the enormous but often subtle diversity of molecular structures found in almost any environmental, biological, chemical manufacturing, or human work place sample matrix you care to name, achieving molecular selectivity is a great and continuing challenge. Historically,the needed selectivitywas sought by bringing samples of the complex matrix into the analytical laboratory, where combinations of GC and LC with IR spectroscopy and MS were used. These have risen to heights of sophistication and analytical power. Today, many factors are urging the analytical laboratory into the field-for chemical process control; for chemical weapons treaty verification; for work place safety; for chemical waste and environmental air monitoring; for automotive emission control; and for agricultural testing, to name a few among many. The means by which this is done is commonly called the chemical sensor, and these gadgets represent one of analytical chemistry's marquee frontiers. When the word chemical sensor appeared in the analytical chemistry literature, I at first regarded it as one of those word fads-some new analytical chernistry in sheep's clothes. That was wrong, because the technical requirements for a useful chemical sensor are both distinctive and uniquely demanding. Chemical sensors require "ruggedness," a manifestation of resilience of accurate performance to field use. They require fast response; a sensor in a manufacturing stream needs a response time shorter than the process control time constant. The per-unit cost can be crucial, as in personal monitors for the work place. Remote sensors in deepocean monitoring require some means of selfcalibration. And there are other features. But most of all, chemical sensors exposed to a natural, unfiltered sample matrix require excellent selectivity, or molecular recognition, for the chemical species whose detection and quantification are desired. There is a substantial body of current research
aimed at sensor platforms that detect selective binding to or reaction with a molecular layer of some sort. These include optical fibers and waveguides, surface acoustic wave devices and related piezoelectric formats, and ion-selectiveand chemically modified electrodes. There seems, however, to be more progress in designing such schemes than there is in achieving the selectivity in binding and reaction chemistry needed for the new generation of analytical tools that the chemical sensor field represents. Impressive selectivities have been described, but the number of target analytes for which sensors are needed is climbing even faster than the accomplishments. In the chemical sensor community, several lines of thinking about this problem merit consideration. One is that the substantial research effort required for each new example of a selective sensor is daunting, and it reasonably demands a prioritization of future target analytes on which scarce resources are expended. Another line of thought places reliance on mathematical approaches that extract selectivity from the combined responses of arrays of imperfectly selective sensors. Another apt suggestion favors a heavier involvement of synthetic chemists in the molecular design of selective coatings. Finally, other workers think about pathways that are generic in application yet selective at the end. The rearing and design of biological materials such as monoclonal antibodies is an important and rather successful generic approach. An even newer example is the idea of using a fast chemical separation system as a chemical sensor. I see exciting possibilities in the fast separations-based chemical sensor; I will return to that topic in another editorial. Chemical sensors offer a great challenge in analytical chemistry, and the sensor community's research efforts and accomplishments deserve the pride and praise of our discipline.
Analytical Chemistry, Vol. 66, No. 9, May 1, 1994 505 A
