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An exact knowledge of environmental particulate levels at their sources of production, geographic sites of dissemination and/or anatomic sites of deposition after they have been inhaled or ingested by the body is considered a main priority in the environmental health field. One must therefore recognize the need for rapid, accurate and sensitive means of sampling and analyzing these materials, often found in trace amounts, with little alteration from their original physical and chemical structure. A summary of the various theoretical approaches and methodologies investigated in our laboratory towards that goal are discussed with emphasis on X-ray techniques used, correction factors involved, and areas requiring further research.

Carbon dioxide (CO2) has been shown to contribute to human health consequences indoors, such as shortness of breath, nasal and optic irritation, dizziness, and nausea. Given these precarious conditions for human health, various efforts have emerged toward development of CO2 monitors for indoor deployment. The chief problem in gas sensing is the design of a sensor that is highly-sensitive and selective to targeted species, energy-efficient, inexpensive to fabricate, stable, and convenient for users. One of the most popular CO2 sensors in commercial operations is the nondispersive infrared (NDIR) CO2 sensor. While boasting high accuracy and longevity, the CO2 NDIR sensor has traditionally been challenged by power consumption, cost, and bulky dimensions. In contrast to the NDIR sensing technology, colorimetric sensing retains several advantages, including easy preparation, user convenience, and obvious responses detectable by the human eye.In this work, we situate metal–organic frameworks (MOFs) as highly-porous, crystalline sorbents for sensitive colorimetric CO2 detection. In particular, a zeolitic imidazolate framework (ZIF) is chosen as the sorptive material due to its chemical stability and tunable CO2 affinity. The colorimetric gas sensor is developed by incorporating a CO2-affinative basic function and a pH indicator into the ZIF. The developed colorimetric CO2 sensor exhibits an obvious response to a wide range of CO2 levels of interest to indoor air detection (with a lower limit of detection below 1,000 ppm CO2). Powder X-ray diffraction confirms its chemical stability over one month in ambient conditions. Ultraviolet-visible spectroscopy reveals quantitative differences in colorimetric responses across a span of CO2 concentrations (700 – 7,500 ppm CO2). The color response is attributed to a two-step reaction mechanism whereby the pH indicator deprotonates the reaction intermediate between the adsorbed CO2 reacting with the basic function, shifting the pH and inducing a color change. Given its simple fabrication, rapid and obvious response, and stability in ambient environment, the ZIF-based colorimetric sensor provides a promising route for an improved indoor air quality monitoring. Acknowledgements We would like to thank the National Science Foundation for support in the form of a Graduate Research Fellowship (GRF) and Grant # 1903188, as well as the Bakar Fellows Program.