Development of a paper-based analytical device integrated with a custom-built smartphone application for on-site...

This study presents the design and development of a user-friendly paper-based analytical device (PAD) integrated with smartphone-based analysis for the rapid and simultaneous detection of five key chemical parameters in water, namely total hardness, chloride (Cl^-), nitrate (NO₃^-), nitrite (NO₂^-), and fluoride (F^-). The PAD was fabricated using a simple and cost-effective punch-and-cut method. Colorimetric detection was employed, where specific reagents impregnated on the paper reacted with water samples to produce visible color changes corresponding to each analyte. A custom-developed smartphone application was employed to analyze the color intensity, converting it into quantitative concentration values using image processing and pre-established calibration algorithms. The system achieved limits of detection (LOD) of 4.85 mg L^-1 for total hardness, 2.63 mg L^-1 for chloride, 0.25 mg L^-1 for nitrate, 0.045 mg L^-1 for nitrite, and 0.004 mg L^-1 for fluoride. All five parameters were simultaneously determined within 2 min. This PAD-smartphone platform offers a portable, affordable, and efficient solution for on-site water quality monitoring, particularly in resource-limited settings. The integration of accessible materials and digital technology highlights the potential for empowering communities to assess environmental health and ensure the safety of drinking water, tap water, and surface water. This approach could be adapted to low-resource regions and contribute directly to achieving the United Nations Sustainable Development Goals (SDGs) on clean water and sanitation.