Development of a colorimetric strip sensor for ammonia gas detection using butterfly pea ( Clitoria ternatea ) extract

Abstract Ammonia is a critical gaseous compound widely recognized as an indicator of both environmental pollution and food spoilage. Conventional detection techniques, while highly sensitive, often rely on expensive instrumentation and are impractical for on-site or small-scale monitoring. Here, we report the development of an eco-friendly, strip-based colorimetric sensor utilizing butterfly pea flower ( Clitoria ternatea ) extract as a natural indicator. The sensor design leverages the pH-sensitive anthocyanins present in the extract, which undergo a distinct color transition from red to blue upon exposure to ammonia. Extraction conditions were systematically varied by altering the solvent-to-sample ratio, and their effects were examined through viscosity measurements, FTIR analysis, and sensor performance evaluations. Higher solvent ratios (15 mL/g) produced extracts with lower viscosity and enhanced anthocyanin solubilization, resulting in strips with stronger and more linear colorimetric responses. FTIR spectra confirmed the presence of hydroxyl, carbonyl, and aromatic groups associated with anthocyanins and phenolic compounds, validating their role as active sensing agents. To enable quantitative evaluation, digital image processing was applied by converting RGB data into HSV color space. Among the HSV parameters, Hue (H) demonstrated a consistent linear decrease with increasing ammonia concentration up to 250 ppm, while Saturation (S) and Value (V) showed irregular trends. These results establish Hue as a reliable quantitative indicator of ammonia concentration, reinforcing the potential of integrating colorimetric sensing with digital image analysis. Overall, this work highlights the feasibility of employing natural pigment–based materials to fabricate low-cost, biodegradable, and portable ammonia sensors.