|Abstract:||Developing a sensitive, rapid and inexpensive sensor for in-situ identification of hazardous airborne volatiles has become an urgent need for human welfare and has important applications in various areas, including chemical toxin detection, security screening, food quality inspection, and health monitoring of general population. The use of colorimetric sensor arrays has proven to be a fast, convenient and effective method for liquid and gas analysis of relevant analytes where the specificity derives from the pattern of response from cross-reactive and chemoresponsive sensor sets. Colorimetric sensor arrays have seen successful applications in differentiating among diverse families of analytes, ranging from single compounds to composite mixtures, including toxic industrial chemicals, food and beverage, bacteria and fungi. Colorimetric sensor arrays have generally been limited to work under laboratory conditions due to the bulk of complicated instrumentation. Therefore, we have recently developed a portable gas analyzer that allows for the real-time analysis of colorimetric data. The hand-held device is equipped with a color contact image sensor (CCIS) for optical transduction, a disposable sealed cartridge for sensor array loading, and a diaphragm micropump for analyte flow control.
First, a colorimetric sensor array comprising multiple types of sensor elements were employed for the detection and discrimination among home made explosives, variations of peroxide-based explosives, as well as fuels and pre- or post-combustion residues. In addition to those acid/base-sensitive dyes that we commonly used for targeting industrial chemicals, new sensors include ones that use Fenton reagent chemistry (Fe(II) catalyzed production of strong radical oxidants) to cause color changes, other generic redox-sensitive dyes (tolidine, odianisidine, etc.), and hydrazines for ketone detection. Hierarchical cluster analysis, principal component analysis, and support vector machine analysis show excellent discrimination among explosive analytes from a variety of sources. Detection limits are calculated to be as low as ~50 μg for head gas sampling above solids and sub-ppm level for vapors. Our colorimetric sensing method therefore has significant implications in explosives identification and may prove to be a useful supplement to other available detection technologies used in security checks and forensic evaluation of improvised explosives.
Second, the disposable colorimetric sensor array was modified and combined with a handheld gas analyzer for rapid sensing of the freshness of common meat products and the ethanol concentration of liquor samples. Take the meat freshness monitoring as an example, the preliminary measurement on the representative sulfides and biogenic amines from the meat samples shows great sensitivity was achieved by introducing metal ion chromogens into the sensor array, with the detection limits at low ppb levels. The hand-held analyzer permits accurate discrimination with very high accuracy among the headspace of samples of five different meats as a function of storage periods from freshly purchased to 4 days. We have demonstrated excellent repeatability from both separate batches of sensor array printings and from multiple purchases over a period of a month of the same meat product. This study reveals the promising application of our colorimetric sensor arrays in the determination of meat freshness and liquor adulteration, which can serve as a useful supplement to other methods of food safety inspection. Furthermore, the possible application of the colorimetric sensor arrays in medical diagnosis was explored. We have made attempts to target trimethylamine as a biomarker from vapor or aqueous solution with relevance to point-of-care evaluation of trimethylaminuria (TMAU), using the more accessible cell phone camera as the imaging device. We made use of highly porous sol-gel/plasticized formulations to obtain a better responsiveness to gaseous analytes and the ideal hydrophobicity of the matrices to minimize the dissolution of the dyes during liquid sensing. Apparent color differences shown by the sensor arrays allow for a quick identification of high and low concentrations of trimethylamine. LODs for trimethylamine are ~4 ppb in gaseous phase and ~2 μM in aqueous solution, both of which are well below the diagnostically significant concentrations. The optoelectronic nose promises to be a useful point of care device for rapid, quantitative monitoring of trimethylamine levels for patients.