Halochromism of Oxazolidine in Carboxylated Terpolymer Nanoparticles as Optical Sensors: Effects of π-Conjugation Length and Macromolecular Architecture on Colorimetric and Fluorimetric Responsivities

Dual-responsive colorimetric and fluorimetric chemosensors are a significant and interesting category of intelligent functional materials because of their advantages for visual detection and sensing of triggers or analytes by the naked eye across any time and space in a fast manner. Here, carboxylic acid-functionalized terpolymer nanoparticles based on methyl methacrylate (MMA), butyl acrylate (BA), and methacrylic acid (MAA) were synthesized by emulsion copolymerization, where different copolymer compositions were prepared by varying the concentrations of MMA and BA comonomers. To develop halochromic terpolymer nanoparticles, all samples were physically modified with two oxazolidine derivatives (OXOH₁ and OXOH₂) with different optical properties. The results obtained from investigating the solvatochromism and halochromism indicated the successful dual-mode colorimetric and fluorimetric detection of pH by halochromic terpolymer nanoparticles containing both OXOH₁ and OXOH₂ in both the visual mode and spectroscopy methods (ultraviolet-visible (UV-vis) and fluorescence). Afterward, the halochromic papers were prepared by coating halochromic terpolymer latex nanoparticles onto cellulose by the layer-by-layer strategy. The halochromism and pH-responsivity of halochromic latex nanoparticles are stable after incorporation into a cellulose matrix and have remarkable intensity and sensitivity because of the protective role of terpolymer nanoparticles as the carrier for oxazolidine molecules. Investigation of the pH-responsivity of halochromic papers impregnated with aqueous solutions having pHs in the range of 1-14 indicates their potential applications for developing dual-responsive colorimetric and fluorimetric paper-based pH sensors. When impregnated by solutions with pHs of 1, 7, and 14, respectively, halochromic papers displayed distinctive colors, including orange, red, and yellow, under visible light and distinctive fluorescence emission in different colors, including orange, red, and cyan-blue, under UV light irradiation (365 nm). Observed colorimetric and fluorimetric pH-responsivities for halochromic papers illustrated the successful development of paper-based pH sensors for the fast visual detection of pH across any space and time by the naked eye.