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Colorimetric Sensor Arrays:  An Adventure in Molecular Recognition

Kenneth S. Suslick

We have developed an entirely new class of lightweight chemical identification systems based on  colorimetric sensor arrays:  essentially a digital, multidimensional extension of litmus paper.  For the detection of volatile organic compounds (VOC), we have demonstrated high sensitivity (below PEL levels) for the detection of toxic industrial chemicals (TICs).  In addition, highly selective discrimination of pure analytes and of complex mixtures has been demonstrated. The technology is also particularly suitable for detecting many of the most odiferous compounds produced by bacteria. We are able to distinguish bacterial growth even at very low levels of detection (e.g., <200 cells in 20 min.) and can easily distinguish one pathogenic bacteria from another.  Finally, the arrays are highly effective at discrimination between closely related odors and classification of complex odors from beers, coffees, teas, etc.

Array based vapor sensing has emerged as a powerful approach toward the detection of chemically diverse analytes. We have developed and patented a unique chemical detection technology in which colorimetric changes in an array of dyes constitute a signal much like that generated by the mammalian olfaction system; each dye is a cross-responsive sensor.  This technology uses a disposable array of chemoresponsive dyes printed on an inert membrane as the primary sensor elements.  Striking visual identifications of a wide range of VOCs are easily made at ppb levels, for example to hydrogen sulfide, methylsulfide, formic acid, acetic acid, ammonia, and hexylamine (i.e., sensitivities comparable to GC-MS detection). 

The design of the colorimetric sensor array is based on two fundamental requirements:  (1) the chemo-responsive dye must contain a center to interact strongly with analytes, and (2) this interaction center must be strongly coupled to an intense chromophore.  The first requirement implies that the interaction must not be simple physical adsorption, but rather must involve other, stronger chemical interactions.  Chemoresponsive dyes are those dyes that change color, in either reflected or absorbed light, upon changes in their chemical environment.  The consequent dye classes from these requirements are (1) Lewis acid/base dyes (metal ion containing dyes), (2) Bronsted acidic or basic dyes (pH indicators), and (3) dyes with large permanent dipoles (zwitterionic solvatochromic dyes).

[1] Rakow, N. A.; Suslick, K. S. "A Colorimetric Sensor Array for Odor Visualization" Nature, 2000, 406, 710-714.

[2] Suslick, K. S.; Rakow, N. A. "Colorimetric Artificial Nose Having an Array of Dyes & Method for Artificial Olfaction" U.S. Patent 6,368,558; April 9, 2002.; Suslick, K. S.; Rakow, N. A.; Sen, A. "Colorimetric Artificial Nose Having an Array of Dyes and Method for Artificial Olfaction: Shape Selective Sensors" U.S. Patent 6,495,102; Dec. 17, 2002.

[3] Wang, J.; Luthey-Schulten, Z. A.; Suslick, K. S.  “Is the Olfactory Receptor a Metalloprotein?” Proc. Natl. Acad. Sci. U.S.A., 2003, 100, 3035-3039.

[4] Suslick, K. S.; Rakow, N. A. “A Colorimetric Nose: ‘Smell-Seeing’” Artificial Chemical Sensing:  Olfaction and the Electronic Nose, Stetter, J.R.; Pensrose, W.R., eds.  Electrochem. Soc.:  Pennington, NJ, 2001; pp. 8-14.

[5] Suslick, K. S. "An Optoelectronic Nose: Colorimetric Sensor Arrays" MRS Bulletin, 2004, 29, 720-725.

[6] Suslick, K. S.; Rakow, N. A.; Sen, A. "Colorimetric Sensor Arrays For Molecular Recognition" Tetrahedron 2004, 60, 11133-11138.

[7] Rakow, N. A.; Sen, A., Janzen, M.C.; Ponder, J. B.; Suslick, K. S. “Molecular Recognition and Discrimination of Amines with a Colorimetric Array” Angew. Chem. Int. Ed. 2005, 44, 4528-4532. (also Angew. Chem. 2005, 117, 4604-4608.)

[8] Zhang, C.; Suslick, K. S. “A Colorimetric Sensor Array for Organics in Water”, J. Am. Chem. Soc. 2005, 127, 11548-11549.