Development of low-cost colourimetric and pH sensors based on PMMA@Cyanine polymers

Cyanine derivatives 1–3 were successfully characterized and their sensorial ability studied against Zn²⁺, Co²⁺, Cd²⁺, Ni²⁺, Cu²⁺, Hg²⁺ and Ag⁺ metal ions. All dyes revealed a colourimetric selectivity towards Cu²⁺ metal ions, with the formation of mononuclear species. Cyanine 2 can detect and quantify the lowest concentrations of Cu²⁺ in aqueous media, 0.3 and 0.6 μM. With the aim of exploring the optical properties of these dyes, solid-supported devices were designed by the synthesis of polymethylmethacrylate (PMMA) polymers doped with dyes 1, 2, and 3 and their copper complexes. All PMMA_doped polymers were tested as temperature and low-pH sensors. PMMA_1 and PMMA_1@Cu²⁺ showed to be reversible fluorimetric thermoresponsive polymers, while PMMA_2, PMMA_3, PMMA_2@Cu²⁺ and PMMA_3@Cu²⁺ irreversible colourimetric and fluorimetric thermoresponsive polymers. From all acids tested, HCl, HBr, HClO₄, H₃PO₄, HCOOH and CH₃SO₃H, the best results were obtained for PMMA_3 and PMMA_3@Cu²⁺ with HCl. After activation in 12 M HCl, both polymers were capable of sensing 8 M, 10 M, 11 M and 12 M HCl: their color changed from green to yellow and they showed an increase in their emission maxima at 805 nm in seconds. The capability to reversibly sense higher acid concentrations in seconds, after activation, for more than 10 cycles, make PMMA_3 and PMMA_3@Cu²⁺ good candidates for superacid sensing, allowing for a real-time follow up of contaminated affluents, or their incorporation in quality control in industrial processes.