Editorial
Molecular Recognition
W
e are in an era in which analysis with chemical sensors and the development of their required selectivities are major research agendas. Selectivity (molecular recognition) in chemical analysis is one of those vast frontiers in analytical chemistry. Sensors are composed of a chemical matrix with which an analyte interacts, and they are mounted on a platform that yields a response to the interaction. The quest for selectivity is directed at highly selective ideally, reversible binding interactions between analytes and the chemical matrix of the sensor, and at using arrays of sensors in which individual analytes are measured based on differences in or patterns of responses to different members of the arrav A third approach in which a rapid miniaturized separation step and a relatively less-selective detector are combined on a common sensor platform is also a promisinp' tool
The degree of molecular recognition required for highly selective analyte-sensor binding is a daunting task, but the progress in the past several years has been impressive. Several different approaches are being pursued by the molecular recognition research community. One approach is to mount natural recognition systems, such as antigen-antibody pairs, membrane receptors, and membrane channel-formers, onto sensor platforms. However, their fragility can be a problem. A second theme is to design molecular cavities baskets, and other nests (hosts) in which molecules (guests) fit and bind selectively and reversibly. The ideal cavity is similar in size and shape to the guest contains a complementary spatial distribution of interaction points (dipole-dipole hydrogen bonding) and still allows fast entrance and egress dynamics Multidentate metal chelons and crown ethers are the forerunners of these tvnes of selective hosts; current versions offer fiirther sinifhiral definition as rlemnn strated bv the well-defined cavities of cavitands cyclodextrans calixarenes and cyclophanes T h e mechani V t lockino-of moWirles that can orri r d ri If
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sign motif relies on incorporating binding sites or channels into ordered molecular layers (vesicles or selfassembled monolayers) and even switching the binding interactions, as is done in ion- and electron-transfer gates. These fascinating areas of research are far from fully exploited in regards to their applications for chemical sensors. So-called molecularly imprinted polymers (MIPs) and their sol-gel and cation/anion polymer pair relatives are another recent approach for preparing host cavities. The general idea is to create the cavity in the presence of the guest. The guest organizes and promotes an energy-minimized interaction with the polymer forming around it. Thus, after washing the guest out, the polymer retains a templated cavity of the guest size, shape, and dipole distribution, which subsequently displays binding selectivity toward the guest The success of the templating idea obviously relies on generating a relatively rigid host structure, such as those found in highly cross-linked polymer structures and sol-gels. At the same time a reasonably porous structure is needed to allow for the transport of the guest through it Although the MIP approach is still synthetically unsophisticated, interesting and promising observations have appeared in the literature. .n looking through the recent literature, however, I was struck by a frequent absence of quantitative partitioning measurements to support claims of strong and/or selective binding. Strong binding of a guest by a MIP is not surprising if the alternative guest is only a dissimilar solvent molecule. In using MIPs as chromatographic phases, only small differences in partition constants are needed for separation, and these should be available from the chromatographic data. Progress in designing template reactions will surely be facilitated by a more quantitative reporting and I wish to encourage that The world of molecular recognition has a huge role to play in chemical sensors and I look forward to seeing it further unfold
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self-assembly reaction (rotoxanes) is another, albeit quite special, route to binding selectivity. A third de-
Analytical Chemistry News & Features, June 1, 1999 3 5 9 A
