Three approaches confer chemical selectivity upon a sensor-based system.1 The “traditional” approach uses a single sensor functionalized with a chemically sensitive material selective for the analyte. This works well when interferences are few, or chemically unlike the analyte, or when highly specific materialsse.g., suitable biological or molecular-recognition complexessare available. Even with exquisite selectivity, the transducer platform often must include a physical selectivity mechanism as well, so that nonspecific adsorption does not defeat a carefully crafted interface. An example is wavelength-specific optical detection that can differentiate between analytes physisorbed on the surfaces of a molecular cage and those residing inside. In contrast, a “nonspecific” transducer such as the mass-sensitive surface acoustic wave (SAW) device cannot differentiate between physisorbed and molecularly complexed mass on the surface of a device, unless a secondary perturbation such as a change in mechanical or electrical properties accompanies the mass change.2 Recent advances in both miniaturized and “micro” versions of traditional analytical techniques drive a second approach to chemical selectivity, that of separating analytes in time, space, or the spectral domain.3-5 Typified by electrophoretic separations in microfabricated channels on glass chips, the micro-total-analytical system (μ-TAS) approach can diminish the role of the sensor to that of a nonspecific detector that responds to any impinging compound. The elapsed time from sample introduction to detection, rather than detector selectivity, identifies the analyte. We focus in this Account on chemical sensor arrays as a means to obviate the difficult, costly process of developing a new material with high chemical specificity for each analyte: one array can provide distinct responses for tens of chemicals and mixtures. We emphasize that many of the subsections of this Account are, to varying degree, “platform independent”, so that arrays of optical fibers, electrochemical sensors, chemiresistors, metal oxides, or thermal devices can be conceptually substituted for the SAW platforms that we discuss in detail. The moderate selectivity requirements for arrays allow consideration of a much wider range of materialsseverything from common organic polymers to porous ceramicssthan for molecularly specific sensors. Furthermore, arrays retain some of the “passive sampling” features of discrete sensors that μ-TAS must relinquish due to its reliance upon addition of reagents, pumping, mixing, and the like.
A. Ricco, R. Crooks, G. Osbourn