Polyaniline/Poly(vinyl alcohol) Conductive Hydrogel for a Chemiresistive Ultrasensitive Ammonia Gas Sensor

Printing technology is an emerging microfabrication technique that is being explored as a potential alternative to traditional semiconductor technology. This innovative approach allows for high precision, ease of fabrication, integration of multiple incompatible materials, and scalability, leading to the production of low-cost and high-performance devices. Herein, we present the fabrication and investigation of a chemiresistive sensor utilizing micro girder (μG) printing technology employing a repetitive freeze–thaw (FT)-cycled polyaniline/poly(vinyl alcohol) (PANI/PVA) hydrogel as the sensing material for the detection of ammonia (NH3) gas down to 100 ppm at room temperature. The sensor shows an improved sensitivity of 94.7% for detecting 100 ppm of NH3 with a limit of detection of 1 ppm. The repetitive FT cycles promote the formation of well-defined microstructures of the hydrogels, which has been shown by detailed structural and morphological analyses. This hydrogel-based chemiresistive sensor demonstrated a good response to NH3 gas by monitoring changes in resistance, offering a cost-effective and practical alternative for NH3 detection. The sensor exhibited high sensitivity to NH3 with rapid response and recovery times of 25 and 10 s, respectively, making it suitable for real-time applications. Additionally, the study explored the hydrogel sensor's cross-sensitivity to NH3, nitrogen dioxide (NO2), and carbon dioxide (CO2). The sensitivity of the sensor toward some of the breath volatile organic compounds is also analyzed. The sensor's behavior is found to adhere to the Langmuir adsorption/desorption model, further confirming its efficacy in gas detection. The excellent sensing ability with a high cross-sensitivity of the developed sensor is attributed to the formation of well-developed cross-linked network structures during FT cycles. The stability of hydrogel-based sensors over a 3-month period further supports their potential for practical deployment in various environmental and healthcare monitoring scenarios, aiming to provide a straightforward strategy for detecting exhaled breath ammonia.