A comprehensive review on the development and state of the art of colorimetric and fluorometric sensor arrays is presented. Chemical sensing aims to detect subtle changes in the chemical environment by transforming relevant chemical or physical properties of molecular or ionic species (i.e., analytes) into an analytically useful output. Optical arrays based on chemoresponsive colorants (dyes and nanoporous pigments) probe the chemical reactivity of analytes, rather than their physical properties (e.g., mass). The chemical specificity of the olfactory system does not come from specific receptors for specific analytes (e.g., the traditional lock-and-key model of substrate-enzyme interactions), but rather olfaction makes use of pattern recognition of the combined response of several hundred olfactory receptors. In a similar fashion, arrays of chemoresponsive colorants provide high-dimensional data from the color or fluorescence changes of the dyes in these arrays as they are exposed to analytes. This provides chemical sensing with high sensitivity (often down to parts per billion levels), impressive discrimination among very similar analytes, and exquisite fingerprinting of extremely similar mixtures over a wide range of analyte types, in both the gas and liquid phases. Design of both sensor arrays and instrumentation for their analysis are discussed. In addition, the various chemometric and statistical analyses of high-dimensional data (including hierarchical cluster analysis (HCA), principal component analysis (PCA), linear discriminant analysis (LDA), support vector machines (SVMs), and artificial neural networks (ANNs)) are presented and critiqued in reference to their use in chemical sensing. A variety of applications are also discussed, including personal dosimetry of toxic industrial chemical, detection of explosives or accelerants, quality control of foods and beverages, biosensing intracellularly, identification of bacteria and fungi, and detection of cancer and disease biomarkers.

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The analysis of complex mixtures presents a difficult challenge even for modern analytical techniques, and the ability to discriminate among closely similar such mixtures often remains problematic. Coffee provides a readily available archetype of such highly multicomponent systems. The use of a low-cost, sensitive colorimetric sensor array for the detection and identification of coffee aromas is reported. The color changes of the sensor array were used as a digital representation of the array response and analyzed with standard statistical methods, including principal component analysis (PCA) and hierarchical clustering analysis (HCA). PCA revealed that the sensor array has exceptionally high dimensionality with 18 dimensions required to define 90% of the total variance. In quintuplicate runs of 10 commercial coffees and controls, no confusions or errors in classification by HCA were observed in 55 trials. In addition, the effects of temperature and time in the roasting of green coffee beans were readily observed and distinguishable with a resolution better than 10 °C and 5 min, respectively. Colorimetric sensor arrays demonstrate excellent potential for complex systems analysis in real-world applications and provide a novel method for discrimination among closely similar complex mixtures.The evaluation and discrimination of complex mixtures remains an important challenge to chemical analysis. The most common strategy for analysis of mixtures is a complete component-by-component approach, i.e., fractionation of the mixture and characterization of the individual components. This generally implies the use of hyphenated techniques, i.e., the sequential combination of a separation technique (e.g., a chromatography) with single or multiple spectroscopic techniques (e.g., mass spectrometry). 1,2 While gas chromatography/ mass spectrometry (GC/MS) is the most popular of all hyphenated techniques, it often proves cumbersome for accurate discrimination among similar complex mixtures. 2,3 Moreover, even for high-performance separation techniques, the number of compounds that can be differentiated is disappointingly small relative to the extremely large number of components in truly complex mixtures. 3,4 For complex mixtures with hundreds of components, there are often multiple analytical goals: in addition to the occasional requirement for a full component-by-component analysis, more common needs involve comparisons against a standard, discrimination of subtle differences © 2010 American Chemical Society * Corresponding author. Tel: 1-217-333-2794. Fax: 1-217-333-2685. ksuslick@uiuc.edu. In recent years, we have developed a rather different approach using a colorimetric sensor array. The design of such an array 17-24 is based on strong dye-analyte interactions, which is quite different from other electronic nose technologies that generally rely on weak, nonspecific intermolecular interactions. Optical arrays have also found other applications for sensing in aqueous solutions of anions, organic compounds, amino aci...

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The origin of “sonochemistry” is acoustic cavitation: the formation, expansion, and implosive collapse of bubbles in liquids irradiated with ultrasound. The compression of such bubbles generates intense local heating, which has been quantified recently from both chemical kinetic thermometry and from high-resolution sonoluminescence spectra. The temperatures reached during cavitation are ≈5000 K, but have an effective lifetime of only a few microseconds. Consistent with this, the sonoluminescence that accompanies sonochemistry closely resembles flame emission! The chemistry generated by these hot spots is different than either ordinary thermal or photochemical processes and sonochemistry represents a fundamentally unique interaction of energy and matter. [For recent reviews see K. S. Suslick, Sci. Am. 260, 80 (Feb. 1989) and Science 247, 1439 (1990).] Recently, the use of ultrasound in liquid-powder slurries to enhance dramatically their chemical reactivity has been explored. For example, heterogeneous catalysis can be induced in normally nonreactive metals and the catalytic activity of Ni has been enhanced by 105. Using a variety of surface science techniques, it was shown that ultrasound removed the passivating oxide coating normally found on Ni and other metal surfaces, thus increasing their activity. The origin of these effects comes from extremely high-speed interparticle collisions which occur during ultrasonic irradiation of liquid-solid slurries. Turbulent flow and shockwaves produced by acoustic cavitation can drive metal particles together at sufficiently high velocities to induce melting upon collision. A series of transition metal powders have been used to probe the maximum temperatures and speeds reached during such interparticle collisions. Metal particles that are irradiated in hydrocarbon liquids with ultrasound undergo collisions at roughly half the speed of sound and generate localized effective temperatures between 2600°C and 3400°C at the point of impact. [Work supported by NSF and the UIUC Materials Res. Lab.]

Low concentrations of short chain aliphatic alcohols and organic acids and bases suppress single-bubble sonoluminescence (SBSL) in water. The degree of SL quenching increases with the length of the aliphatic end of the alcohol, and is related to the concentration of the alcohol at the bubble/water interface. The light is preferentially quenched in the shorter wavelength region of the spectrum. Radius−time measurements of the bubble are not dramatically affected by the low levels of alcohol used. Butyric acid and propylamine behave in the same manner, but only in their neutral forms, indicating that the SBSL suppression is due to processes occurring within the bubble.