Ultralow-Cost and Selective Water-Based Colorimetric Ink for Indoor CO₂ Monitoring

This work introduces a novel water-based colorimetric ink for CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> monitoring, offering a significant advancement in indoor air quality assessment. The ink uses a highly specific and reversible reaction between CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and an amine, enabling precise detection within a broad operational range of 150-1500 ppm, encompassing typical indoor CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentrations. Optimized rheological properties allow for seamless application on paper substrates, facilitating scalable production and widespread adoption. The resulting colorimetric labels exhibit exceptional resistance to common interfering gases, ensuring accurate and reliable CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> readings even in challenging indoor environments with fluctuating humidity and temperature and potential cross-contamination. Rigorous characterization of the sensor showcases outstanding performance in terms of specificity, repeatability, and reproducibility, validating its robustness and suitability for real-world applications. To further enhance accuracy, a calibration methodology incorporating a signal compensation algorithm is proposed, effectively mitigating humidity-induced effects. The sensor response can be effortlessly captured using readily available and cost-effective electronic components, paving the way for an accessible and versatile solution for real-time CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> monitoring in both residential and commercial settings. Moreover, the inherent versatility of this technology allows for integration with other colorimetric inks, opening doors to a multi-parametric sensing platform capable of monitoring a wider array of indoor air quality pollutants.