Stimuli-directed selective detection of Cu²⁺ and Cr₂O₇^2- ions using a pH-responsive chitosan-poly(aminoamide)...

In this work, the preparation of a pH-responsive fluorescent microgel, (NANO-PAMAM-CHT), is presented for the selective detection of Cu²⁺ and Cr₂O₇^2- ions. The NANO-PAMAM-CHT (nanosized polyaminoamide-chitosan microgel) is synthesized via aza-Michael addition reactions in a controlled and stepwise manner in water, using easily affordable starting materials like 1,4-diaminobutane, N,N'-methylene-bis-acrylamide, NIPAM and chitosan. NANO-PAMAM-CHT shows pH-responsive fluorescent properties, whereas the fluorescence intensity shows a pH-responsive change. Due to the selective fluorescence quenching, the microgel can detect both Cu²⁺ ions and Cr₂O₇^2- ions selectively at ambient pH in aqueous medium. Moreover, it can selectively differentiate between Cu²⁺ ion and Cr₂O₇^2- ions at pH ∼3 in water. The limits of detection for Cu²⁺ ions and Cr₂O₇^2- ions are reported as 16.9 μM and 2.62 μM, respectively (lower than the minimum allowed level in drinking water) at pH ∼7. Mechanistic study further reveals the dynamic quenching phenomenon in the presence of Cu²⁺ ions and static quenching in the presence of Cr₂O₇^2- ions.