Water Quality Monitoring using Quartz Sensors modified with Metal-Organic Frameworks: Detection of dissolved gases and volatile organic compounds in aqueous environments

Methane emissions present an urgent climate challenge, with aquatic sources like wetlands and leaks from abandoned oil wells contributing significantly to the global methane budget but remaining poorly assessed. Wetlands, a major natural methane source, are seeing increased emissions due to permafrost thawing. Understanding the complex interactions between methane (CH₄) and carbon dioxide (CO₂) that drive these emissions underscores the need for high-resolution, in situ monitoring of dissolved methane. Similarly, minor leaks from abandoned offshore oil wells pose a risk of localized marine pollution, potentially harming ecosystems by saturating nearby waters with methane and other hydrocarbons. Current methane sensors in aquatic environments largely depend on gas extraction membranes, focusing on gas-phase methane quantification. In contrast, next-generation in situ sensors with selective methane-binding materials, such as the one developed in this work, offer improvements by enabling direct, rapid and continuous monitoring of dissolved methane.

Metal-organic frameworks (MOFs) are porous materials composed of metal clusters and organic linkers, with tunable porosities that can be engineered to selectively adsorb specific molecules. This makes them highly promising for detecting analytes like methane, even within complex mixtures containing potential interferents. Quartz crystal microbalance (QCM) is a gravimetric sensor that can detect minute mass changes (as small as 1-10 ng) by monitoring shifts in its resonance frequency. Integrating MOFs with QCM enables the development of methane sensors that achieve both high sensitivity and selectivity.

This thesis presents a MOF-based QCM sensor specifically designed for methane detection, using Cu-MOF-2 as the active sensing material. Cu-MOF-2, constructed from copper paddlewheel units and hydrophobic bent linkers, achieves effective methane adsorption through a small pore size (5 Å) while maintaining water-exclusion. This MOF combines a high methane adsorption capacity (5-6 mg g^-1) with hydrolytic stability in both saline and freshwater environments and resilience across a broad pH range (4 to 11). A layer-by-layer (LBL) deposition technique was employed to functionalize quartz resonators with a Cu-MOF-2, resulting in stable, hydrophobic, and crystalline MOF films. The randomly oriented anisotropic crystallites in the films create interstitial voids that result in an uneven mass distribution. This structure enables unique distribution patterns for different analytes within the framework, a feature crucial for analyte discrimination.

In gas-phase, the Cu-MOF-2 sensor demonstrated a rapid response time (t₉₀ = 69 seconds) towards CH₄ detection, with a limit of detection (LOD) of 0.4% CH4 by volume and a limit of quantification (LOQ) of 1% CH₄ by volume. The sensor exhibited high stability, reversibility, and minimal interference from oxygen and humidity. Enhanced sensitivity to CO₂ (approximately 25 times greater than to CH₄) expanded the sensor's potential as a dual gas detector. To address cross-sensitivity, a gas discrimination method was established. The unique distribution patterns of each analyte within the framework produce distinct frequency shifts in the higher harmonic oscillations of the quartz crystal. This allows for selective detection of both CH₄ and CO₂, despite the cross-sensitivity challenges.

Aqueous-phase testing demonstrated Cu-MOF-2 sensor’s potential for detection of dissolved CH₄ in water, with an LOD of 0.11 mg L^-1 CH₄ and a rapid response time (t90 = 39 seconds), representing over 25 times the sensitivity observed in the gas phase. The sensor exhibited robustness against common interferences, including CO₂ and varying salinity, and effectively detected dissolved CH₄ in spiked lake water, containing multitude of potential interferents.

A prototype system was developed, integrating micropumps and a 3D-printed housing for real-time CH₄ monitoring in aquatic environments. This prototype confirmed the practical applicability of Cu-MOF-2 sensors for continuous monitoring of water sources for dissolved CH₄.

Expanding beyond CH₄ detection, the MOF-QCM approach was adapted for detection of BTEX (benzene, toluene, ethylbenzene, and xylenes) using UHMOF-100, another hydrophobic MOF. Preliminary results with UHMOF-100 sensors showed promise for BTEX detection at 20 mg L^-1 concentrations in water, where higher harmonic frequency deviations provided a potential fingerprint response for each analyte, supporting the feasibility of selective BTEX sensing with further optimization.

This work advances the development of MOF-based QCM sensors, offering novel solutions for selective methane and BTEX detection and supporting high-resolution, in situ monitoring of aquatic pollutants. The findings highlight the potential of MOF-functionalized QCM sensors for environmental monitoring, contributing critical tools for addressing climate impact and pollution in aquatic environments.