A leaching free nanopigment for extended linear colorimetric pH sensing over 10 pH units.

The precise measurement of pH variations is pivotal across scientific and industrial domains, with colorimetric pH sensors gaining prominence for their simplicity and advantages over electrochemical alternatives. However, their widespread adoption has been hindered by challenges such as dye leaching, limited long-term stability, and a narrow dynamic range (typically ~3 pH units). To address these constraints, we engineered nanopigments by covalently bonding sulfonephthalein dyes to raspberry-like silica nanoparticles (RSNs), which were subsequently embedded within an agarose/polyethylene oxide (PEO) matrix to create stable, non-leaching pH-sensing films. To further expand the detection range, we integrated two distinct sulfonephthalein nanopigments-Bromocresol Green and Phenol Red into the matrix, leveraging their complementary pH sensitivities. CIELAB color space analysis revealed a synergistic interplay within the RSN-agarose-PEO microenvironment, driving multiple protonation and deprotonation events that extend the sensor's operational range to pH 1-10 with a uniform linear response. The versatility of the nanopigments was demonstrated by coating them onto various substrates, where they maintained robust pH responsiveness. This innovative strategy yields a durable, colorimetric pH sensor that overcomes the limitations of conventional systems, offering a practical, wide-ranging tool for applications in research, industry, and beyond.