Metal-Organic Framework-Based Colorimetric Gas Sensors toward an Improved Indoor Air Quality Monitoring

As respiratory illness infections and related environmental antagonisms continue to beleaguer our contemporary moment, the construction of low-cost, scalable, highly sensitive, remarkably selective, ultralow power, and user-friendly technologies for detecting hazardous chemical species indoors remains imperative. Namely, the accumulation of carbon dioxide (CO2) and select volatile organic compounds (VOCs) in indoor settings is associated with deleterious human health conditions, such as fatigue, headaches, and irritation of the throat. While commercialized indoor gas detectors exhibit desirable analyte sensitivity and long-term sensing endurance, these devices characteristically suffer from cost, bulk, and power requirements. Toward addressing these limitations, this work introduces amine-functionalized, dye-loaded metal-organic framework (MOF)-based chemical sensors whose color change upon exposure to indoor analytes produces a more passive, smaller, cheaper, and simpler alternative to existing technologies. In this dissertation, the iterative synthesis and spectroscopic characterization of color-based, MOF-based indoor analyte sensors are accomplished toward the realization of an ideal sensor for an improved indoor air quality monitoring. Chapter 1 situates the relationship among indoor CO2 and indoor VOCs in the ongoing Coronavirus disease 2019 (COVID-19) pandemic, illustrating the range of detrimental realities for human and environmental health. To best articulate the stakes of structured public health violence, the chapter engages an interdisciplinary analysis of power in which antiblack worldbuilding is linked to previous, present, and emerging environmental violence and human unwellness. Once certain predatory formations are more concretely assigned answerability for air-based violence, the chapter closes with attendant scientific interventions and provides the rationale for the development of color-based chemical sensors to preempt adverse exposures to indoor analytes. Chapter 2 introduces a first-generation colorimetric gas sensor composed of a MOF, primary amine, dye, and methanol blended, drop-cast on cellulose filter paper, and exposed to indoor levels of CO2 (700 parts per million, ppm, and up). Here, the pristine MOF—the zeolitic imidazolate framework-8 (ZIF-8) consisting of zinc (Zn2+) cations tetrahedrally coordinated by 2-methylimidazolate (Hmim¬-) organic linkers—serves as the highly-porous adsorbent with known physisorptive affinity to CO¬2. Following its unstirred, room temperature synthesis, ZIF-8 is blended with the primary amine, ethylenediamine (ED), and the dye, phenolsulfonpthalein (PSP, or phenol red). The capacity of the resulting sensor, termed PSP-ED/ZIF-8, to effectively function in plausible indoor air conditions is probed via several characterization techniques. Powder X-ray diffraction (PXRD) is used to demonstrate the long-term chemical stability of ZIF-8 in the basic environment created from the addition of ethylenediamine. Moreover, scanning electron microscopy (SEM) is used to define the morphological properties of PSP-ED/ZIF-8 in relation to the molar ratio of the ZIF-8 metal : ZIF-8 linker : methanolic solvent precursors, as well as the post-synthetically incorporated colorimetric ingredients. A LabView-enabled gas dosing apparatus (coupled with a nondispersive infrared, NDIR, gas sensor to substantiate gas levels, humidity, and temperature) is implemented to deliver a range of CO2 levels (700-7,500 ppm) under various humidity (0-80% RH) at room temperature to PSP-ED/ZIF-8 drop-cast on cellulose filter paper. Through smartphone video recording, qualitative assays of gas-exposed PSP-ED/ZIF-8 are collected, with an increasing intensity of the fuchsia-to-yellow color change observed with increased concentrations of CO2. In realizing that the perceived color change is only permissible in the presence of the MOF, Brunaeur-Emmett-Teller (BET) surface area analysis is performed to evaluate the role of high surface area on ZIF-8’s ability to accommodate both ethylenediamine and phenol red, as well as provide sorption sites for indoor CO2. Finally, an ex-situ ultraviolet-visible (UV-Vis) diffuse reflectance spectroscopic technique is achieved to quantify how the Kubelka-Munk, F(R), values at 443 and 570 nm resonant with phenol red change relative to each other as the concentration of CO2 and humidity levels are modified. Despite immediate and increasing responses of PSP-ED/ZIF-8 to 700 ppm CO2 (and up) in dry environment, qualitative color assays and quantitative UV-Vis measurements exhibit a largely suppressed color change in the presence of humidity. To improve the colorimetric gas response across humidity, a revised sensor recipe is accomplished. In Chapter 3, an enhanced colorimetric indoor CO2 sensor is attained through the direct incorporation of phenol red into the ZIF-8 metal and linker precursor broth. The orange crystals formed, PSP:ZIF-8, are then blended with ethylenediamine to form a second-generation sensor, ED/PSP:ZIF-8. Collected PXRD patterns, as well as Fourier transform infrared (FTIR) spectroscopic transmittance scans, confirm the structural integrity of ZIF-8 in both PSP:ZIF-8 and ED/PSP:ZIF-8. In addition, SEM and transmission electron microscopy (TEM) demonstrate the fourfold increase in size of ZIF-8 crystals upon growth in a phenol red-loaded methanolic precursor mixture (compared to the PSP-ED/ZIF-8 first generation chemical sensor). Reimplementation of the LabView-based gas apparatus demonstrate a significantly more intense fuchsia-to-yellow color change of the ED/PSP:ZIF-8 sensor than the PSP-ED/ZIF-8 sensor upon exposure to indoor CO2 levels (600 ppm and up) across humidity (0-80% RH). To better elucidate these assays, the smartphone-gathered images are read into a MATLAB script and decomposed into their respective red-, green-, and blue (RGB) distributions, which both confirms a stronger color change in the second-generation sensor and enables a reasonable approach with which to index color change data for the optimal indoor gas sensor. In addition to this approach, an in-situ UV-Vis diffuse reflectance spectroscopic technique is developed in which room-temperature Kubelka-Munk spectra at 443 and 570 nm are directly collected as indoor levels of CO2 are exposed to both first- and second-generation sensors across humidity, as well as in the presence of VOCs (acetone). Consistent with the qualitative assays, ED/PSP:ZIF-8 exhibits a stronger color change than the first-generation sensor in dry CO2, CO2 and VOC environment, and humid CO2. However, compared to its dry CO2 and CO2+VOC colorimetric response, ED/PSP:ZIF-8 still displays a noticeably suppressed color change at heightened humidity. To initially determine differences in the sensing performance of the first- and second-generation sensors, washing studies are conducted in which both sensors are successively washed with methanol. Upon collecting FTIR spectroscopic scans with each wash, the nitrogen-hydrogen (N-H) stretching vibrations indicative of ethylenediamine disappear in both sensors, which suggests the localization of ethylenediamine to the external surface of ZIF-8 in both sensors. Similarly, in a second washing experiment, both sensors are washed with methanol and replenished with (i) fresh methanol, (ii) fresh methanol and ethylenediamine, and (iii) fresh methanol, ethylenediamine, and phenol red. Upon being replenished in these three ways, both sensors are exposed to dry CO2. Neither sensor responds to CO2 upon sole replenishment in methanol, which reaffirms the probable location of ethylenediamine to the external surface area of ZIF-8. However, when both sensors are washed and replenished with an ethylenediamine methanolic solution, only the ED/PSP:ZIF-8 sensor recovers its original color change (whereas the PSP-ED/ZIF-8 sensor does not). Once both phenol red and ethylenediamine replenish the sensor (in methanol), the PSP-ED/ZIF-8 partially recovers its colorimetric gas response from its fresh state. These observations imply that phenol red is located on the external surface of ZIF-8 in the first-generation sensor (but inside the internal pore cavities in the second-generation sensor). Given the unresolved hypothesis of phenol red location between both sensors (as well as how phenol red binds to ZIF-8 in general and the underlying chemical sensing mechanism informing the color change), mechanistic studies are pursued. Chapter 4 details an exhaustive set of spectroscopic analyses used to (i) distinguish the first- and second-generation colorimetric gas sensors and (ii) examine the role of surface basicity (or inclusion of ethylenediamine) on the thermodynamics of phenol red adsorption to ZIF-8 for colorimetric indoor analyte monitoring. The first half of this chapter involves the use of carbon, hydrogen, nitrogen, and sulfur (CHNS) elemental analysis to discern how much phenol red adsorbs to ZIF-8 between the first- and second-generation sensors. A gas-dosed FTIR spectroscopy is later implemented to investigate the physisorptive and chemisorptive nature of ZIF-8 and ethylenediamine-modified ZIF-8 exposure to dry CO2. The second half of this chapter uses a liquid-phase UV-Vis spectroscopy to distinguish the adsorption isotherms of room temperature phenol red adsorption onto ZIF-8 (with and without ethylenediamine). In addition, FTIR spectroscopy is implemented to discern whether the presence of ethylenediamine controls how phenol red binds to the ZIF-8 adsorbent. This chapter concludes with an introduction of second-harmonic spectroscopic techniques to evaluate the Gibbs free energy of adsorption (ΔGADS) of phenol red to ZIF-8 (with and without ethylenediamine) to illustrate how the components of the “full” colorimetric gas sensor interact to bind indoor CO2. In addition, X-ray photoelectron spectroscopy (XPS) is briefly discussed toward evaluating MOF/dye/amine interactions. Once the internal workings of the previous sensors are elaborated, Chapter 5 describes a third-generation sensor prepared from the admixture of ZIF-8 species crystallized from precursor solutions blended with different dyes. Specifically, a universal pH indicator (mixed with phenol red) produces an improved sensor with a strong colorimetric CO2 response in low and intermediate humidity. To simulate indoor air conditions more closely, this sensor is exposed to CO2, humidity, and acetone, and its response is quantified via an in-situ UV-Vis diffuse reflectance spectroscopy. Chapter 6 completes the dissertation with a summary of key results and recommends future studies involving multi-color, multi-transduction MOF-based analyte sensing. This work presents a novel, proof-of-concept colorimetric sensor whose optimization and characterization are accomplished through coupled spectroscopies toward the eventual creation of an ideal indoor analyte sensor for widespread use.

Advisor: McCloskey, Bryan BM
Landry, Markita ML