A low-cost colorimetric HKUST-1 sensor for ppm-level humidity detection

Accurate detection of trace moisture is critical in various industries, from microelectronics to petrochemicals. While electrical humidity sensors are prevalent, optical methods offer superior sensitivity for low relative humidity (RH) detection, particularly in demanding applications requiring ppb sensitivity in harsh environments. This work explores the potential of HKUST-1, a metal-organic framework (MOF), as a colorimetric humidity sensor for ppm level detection across a broad range (0–80% RH). By spraying HKUST-1 onto polyester substrates and coupling it with a low-cost, commercially available LED-photodiode module, we developed a simple, portable sensing platform. The observed color transitions, from dark blue to turquoise, correlate with the MOF's moisture adsorption mechanism, achieving a detection limit of 18 ppm H 2 O (<0.05% RH). Comprehensive characterization, including interferent gas studies, revealed significant color changes upon exposure to NH 3 , CO, and ethanol, while CO 2 , CH 4 , and N 2 O showed minimal interference. One-week stability tests demonstrated consistent sensor performance and full reversibility upon exposure to dry air at room temperature. This low-cost, sensitive, and easily fabricated HKUST-1-based sensor offers a promising alternative for humidity monitoring in diverse industrial applications. • A novel HKUST-1 metal-organic framework-based humidity sensor is presented. • Low-cost, scalable fabrication of the colorimetric sensor is demonstrated. • The work establishes a direct correlation between color and humidity. • Selectivity of the sensor to humidity over common gases is established. • Stable and reversible sensor response is demonstrated over a one-week period.