Gas sensors are crucial for various applications, including environmental monitoring, industrial safety, and healthcare. The increasing need for sensitive, selective, and cost-effective gas detection has driven the search for novel sensing materials and mechanisms. MXenes, a class of two-dimensional transition metal carbides and nitrides, have emerged as promising candidates due to their unique properties: high surface area, excellent electrical conductivity, and tunable surface chemistry. Traditional MXene-based gas sensors primarily utilize changes in electrical conductivity as the sensing mechanism. However, this approach has limitations, especially when chemical modifications are needed to enhance selectivity. This study explores an alternative signal transduction mechanism based on mass change, leveraging the high sensitivity and stability of micro-quartz tuning forks (MQTFs) as transducers. By employing MQTFs, the sensor directly measures the mass change resulting from gas adsorption on the MXene surface, overcoming limitations associated with conductivity-based sensing. This allows for greater flexibility in surface functionalization of MXenes to tailor selectivity without compromising the sensor's performance. The focus is on developing a high-performance, cost-effective, and tunable MXene-based gas sensor for detecting important gaseous pollutants, such as carbon monoxide (CO), sulfur dioxide (SO2), and ammonia (NH3).

Extensive research has explored the use of MXenes in various applications, including energy storage, catalysis, and sensing. Their metallic conductivity and large surface area have made them attractive for chemiresistive gas sensors, where changes in resistance upon gas adsorption are measured. However, the dependence on conductivity limits the scope of surface modifications that can be used to improve selectivity. Other transduction mechanisms, like capacitive and electrochemical sensing, have also been explored with MXenes. The use of MQTFs as transducers in gas sensing has shown promise due to their high quality factor, leading to high sensitivity. Previous work has demonstrated MQTF-based sensors for various gases using different sensing materials. The combination of MXenes’ unique surface chemistry and MQTF’s mass sensitivity offers a potential breakthrough for creating highly sensitive and selective gas sensors.

The study involved the synthesis of Ti3C2Tx MXene through selective etching of Ti3AlC2 using a LiF/HCl solution. The resulting MXene was then surface-functionalized with different chemical groups using 1H,1H,2H,2H-perfluorooctyltrimethoxysilane (FOTS) to introduce fluorine (-F) groups and [3-(2-aminoethylamino)propyl]trimethoxysilane (AEAPTMS) to introduce amine (-NH2) groups. The surface modification processes were conducted at varying temperatures (25°C, 35°C, and 60°C) to investigate the effect of temperature on sensor sensitivity. The surface-modified Ti3C2Tx MXenes were then coated onto the prongs of MQTFs using a simple dip-coating method. The resulting MXene-MQTF sensors were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) to confirm the successful synthesis and surface functionalization of MXenes. Gas sensing performance was evaluated using a custom-built gas-sensing system. The sensors were exposed to different concentrations of CO, SO2, and NH3 gases, and the frequency shifts were measured to determine sensitivity and selectivity. Response and recovery times were also determined. The impact of humidity was also investigated and discussed.

The study demonstrated the successful fabrication of a highly sensitive and reversible MXene-MQTF gas sensor. The sensor exhibited a significant frequency shift upon exposure to target gases, demonstrating its ability to detect gases based on mass change. Surface functionalization of Ti3C2Tx with different groups significantly improved sensing performance and enabled tuning of selectivity. Specifically, Ti3C2Tx-NH2 sensors showed high selectivity toward SO2, while Ti3C2Tx-F sensors demonstrated higher sensitivity to CO. The response and recovery times were relatively fast, making the sensor suitable for real-time monitoring. Increasing the temperature during surface modification enhanced sensor sensitivity, particularly for SO2 detection with Ti3C2Tx-NH2. The sensitivity of the Ti3C2Tx-NH2-MQTF sensor for SO2 detection doubled when the surface modification temperature was increased from 25 to 60 °C. This demonstrated that surface chemistry engineering of Ti3C2Tx effectively tunes the sensitivity and selectivity of the MXene-MQTF gas sensors. The sensor exhibited good repeatability and long-term stability over three weeks. Quantitative data on sensitivity (in mHz/ppm), response times (in seconds), and recovery times (in seconds) for different gases and surface modifications were provided in figures and tables within the original paper.

The results demonstrate that the mass-transduction mechanism using MXenes and MQTFs provides a viable alternative to conductivity-based sensing for MXene-based gas sensors. The ability to tune selectivity through surface modification is a significant advantage. The high sensitivity, fast response and recovery times, and good stability of the sensor suggest its potential for practical applications. The observed selectivity aligns with the expected interactions between the functional groups (-NH2 and -F) and the target gases, confirming the success of the surface modification strategy. The improvement in sensitivity with increasing temperature is attributed to increased surface functionalization, indicating an optimal temperature for achieving high sensitivity without compromising MXene integrity. While the sensor exhibits some sensitivity to humidity, the linear relationship between humidity and sensor response suggests that humidity compensation algorithms could mitigate this effect. Further research could focus on optimizing surface modification strategies, exploring other MXene compositions, and integrating the sensor into portable and wearable devices.

This study successfully developed a highly sensitive and reversible MXene-based gas sensor using a mass-transduction mechanism with MQTF as the transducer. The tunable selectivity achieved through surface functionalization demonstrates the potential of this approach for designing high-performance chemical sensors. The sensor shows promise for a variety of applications, particularly in environmental monitoring, wearable technology, and the IoT. Future work could focus on further optimizing the sensor design, exploring alternative MXene compositions, and integrating the sensor into complete sensing systems.

The sensor's sensitivity to humidity is a limitation, although this can be addressed through humidity compensation techniques. The study focused on three specific gases; further research is needed to explore the sensor's performance with a broader range of gases. The long-term stability of the sensor under various environmental conditions requires further investigation. While the sensor showed good stability over three weeks, more extensive testing is necessary to fully determine its lifespan and reliability.