Drone- and Paper-Based Analytical Devices: A Powerful Combination for the Colorimetric Detection of Tropospheric Ozone

Ozone is a harmful atmospheric pollutant whose elevated concentrations (i.e., higher than 0.16 mg m^-3) and prolonged exposure cause severe damage to the human respiratory system and negatively affect flora and fauna. Vertical ozone monitoring remains challenging due to the limitations of traditional sensors, which are bulky, expensive, and slow to provide results at the site of interest. To address this problem, there is a critical need for portable technologies that allow for rapid and efficient in situ detection. This study presents, for the first time, the integration of paper-based analytical devices (PADs) with a commercial drone to combine them for remote sampling and ozone colorimetric detection. The PADs were manufactured using a stencil-printing technique on chromatographic paper, with circular vinyl stencil masks (Ø = 5 mm) applied to define the detection areas on the paper. The hydrophobic barrier was created by depositing varnish resin onto the stencil/paper surface, with the masks removed after drying, resulting in PADs ready for use. As proof of concept, the paper detection zone surfaces were impregnated with potassium indigotrisulfonate (ITS) and polyethylene glycol (PEG), aiming to sample and detect gaseous ozone. The colorimetric method was performed using a desktop scanner to capture images, which were analyzed by graphical software to evaluate the resulting color intensity that varied from blue to colorless. A commercial ozone generator was used to optimize the method parameters. Parameters such as reaction time, reagent volume, and PEG concentration were optimized, resulting in a linear response range for ozone between 0.9 and 7.6 mg, with an <i>R</i>² of 0.996, and a limit of detection of approximately 0.25 mg. A customized holder was fabricated by 3D printing to ensure the attachment of PADs on the aerial drone platform. The system successfully monitored tropospheric ozone levels, recording 6.8 ± 0.7 mg during the dry season and 0.9 ± 0.1 mg during wet periods, with an in-flight sampling time of just 120 s. This innovative system has great potential to advance environmental monitoring, offering a portable, low-cost solution for remote and real-time ozone detection.