Aggregation-induced emission (AIE) from poly(1,4-dihydropyridine)s synthesized by Hantzsch polymerization and their specific detection of Fe2+ ions

Abstract

As an important metal element widely existing in nature and the human body, the simple and specific detection of Fe²⁺ ions has always been of interest. In this work, poly(1,4-dihydropyridine) derivatives (PDPEs) with significant AIE properties were prepared via three-component Hantzsch polymerization as fluorescent probes for Fe²⁺ ions through introducing dihydropyridine rings into the main chain of the polymer. A series of comparative experiments shows that the rigid connection of the benzene ring, an appropriate CH₂ spacer length, the dihydropyridine heterocycle, and intramolecular aggregation all play important roles in determining the AIE performance of PDPEs. Also, the fluorescence of PDPEs is significantly influenced by some external factors, including the polymer concentration, excitation wavelength, and precipitant used. To our surprise, PDPEs can specifically recognize and detect Fe²⁺ ions via a facile fluorescence-based method, even in the presence of fifteen other metal ions. Furthermore, multiple reversible detection can be realized using AIE-active PDPEs with the assistance of EDTA, with a limit of detection (LOD) for Fe²⁺ of 1.2 μM. In addition, the revealed static quenching process illustrates that Fe²⁺ interacts with the ground state of PDPEs, and the stoichiometric ratio of 1,4-dihydropyridine in PDPEs to Fe²⁺ ions is measured to be 2 : 1 according to a Job plot. From an application perspective, AIE-active PDPEs can be applied in the form of simple, portable, and reusable fluorescent test paper for the specific detection of Fe²⁺.