Eco-friendly pH detecting paper-based analytical device: Towards process intensification

This work describes the development of a miniaturized paper-based pH detection platform using natural dye extracted from red cabbage (Brassica oleracea). The easily available paper was used as a substrate and the requisite patterned zone was created with the aid of a punching machine. Experimental parameters were optimized to obtain the best signal readout. The performance of the device at different pH values was quantitatively assessed using digital image analysis with various color space models. Regression analysis suggested that a∗ parameter in CIEL∗a∗b∗ color space model, which captures the variations on the red-green scale, exhibited the best fit with experimental data (R² = 0.9754). This parameter was used for the quantitative estimation of pH variations in a wide range of pH (1-12). A series of real test samples were examined using the paper-based device and results validated with a standard pH meter. The use of paper and natural dye makes the device eco-friendly. The simplicity of fabrication, ease of usage and low reagent and sample volume requirements render the methodology suitable for in situ measurements of pH. The approach demonstrated here would pave the way for the development of clean, sustainable and intensified chemical sensor technologies.