Chitosan Polymer Incorporated Carbon Nanotubes Composites as Functional Material for Formaldehyde and Isopropyl Alcohol Detection

Abstract

Volatile organic compounds (VOCs), such as formaldehyde, are compounds that readily evaporate at room temperature, posing significant risks to human health and the environment upon prolonged exposure. To mitigate these risks, VOCs must be continuously detected and monitored using highly sensitive and selective techniques, such as quartz crystal microbalance (QCM) sensors, which offer real-time, label-free detection based on mass-sensitive measurements. However, the effectiveness of QCM sensors is highly dependent on the properties of the sensing layer, which often faces limitations such as low selectivity, poor sensitivity at low VOC concentrations, slow response and recovery times, and instability under prolonged exposure to harsh environments. Hence, a composite of multi-walled carbon nanotubes (MWCNT) and chitosan (CS) is introduced, leveraging the high surface area along with the biocompatibility and functional groups of chitosan to enhance VOC adsorption, selectivity, and sensing performance. Three synthesis methods were investigated: Method I involved simple mixing with glutaraldehyde crosslinking; Method II included sonication of MWCNT and CS followed by extended stirring and crosslinking; and Method III, proposed in this study, utilized carboxyl-functionalized MWCNT followed by sonication and crosslinking. Composite formulations with MWCNT-COOH to CS ratios of 25:50 mg, 50:50 mg, and 75:50 mg were synthesized and deposited on QCM sensors via drop casting. The materials were characterized using FTIR, SEM/FESEM, and TEM. Sensor performance was evaluated through frequency shift, response time, sensitivity, repeatability, and linearity to identify the optimal composition for formaldehyde and isopropyl alcohol (IPA) detection. Method III showed the highest performance, with the 50:50 mg ratio achieving the greatest frequency shift of approximately 200 Hz for formaldehyde. Additionally, dynamic measurements revealed a robust detection capability, with a maximum frequency shift of 179.0 Hz at 6.0 ppm formaldehyde and a sensitivity of 23.48 Hz/ppm. The response and recovery analysis of composite towards formaldehyde showed rapid adsorption within 61.5 seconds and partial desorption at 158.5 seconds. For IPA, the same composite showed a frequency shift of 95.12 Hz and a sensitivity of 0.2669 Hz/ppm. The enhanced performance is attributed to the combined effects of MWCNT-COOH�s high surface area and CS�s hydrophilic and reactive functional groups, improving VOC adsorption and selectivity. These findings demonstrate the potential of MWCNT-COOH/CS composites for effective detection of VOCs, especially formaldehyde and IPA, paving the way for improved environmental monitoring and health protection.