Recoverable fluorescence chemosensors for Ni2+ ions based on hydrogen-bonded side-chain copolymers presenting pendent benzoic acid and pyridyl receptor units

Abstract

In this study we synthesized the novel fluorescent conjugated homopolymers P1 and P4 and the copolymers P2 and P3 containing proton-acceptor (pyridyl receptor) M1 monomer units and proton-donor (benzoic acid) M2 monomer units and investigated them as recoverable chemosensors for Ni²⁺ ions (based on adjusting the fluorescence energy transfer between the M1 and M2 moieties through varying the degree of hydrogen bonding). The levels of available (i.e., non-hydrogen-bonded) pyridyl receptors in the copolymers P2 and P3 were modified using different molar ratios of the pyridyl monomer M1 and the benzoic acid monomer M2. Significant chelation of the Ni²⁺ ions by the fluorescent homopolymers P1 and P4 and the copolymers P2 and P3 was evident by their distinct fluorescence quenching behaviors; the specific quenching constants (K_SV) decreased in the order P1 > P2 > P3 > P4, following the trend of the molar ratios of pyridyl receptors. When we added a third component, cyclodextrin (CD), to cap the benzoic acid moieties and, thereby, disrupt the hydrogen bonding between the M1 and M2 moieties in the copolymers P2 and P3, the levels of available pyridyl receptors in the recoverable chemosensor copolymers increased, resulting in relatively higher quenching constants for the P2 + CD, P3 + CD and blend (P1/P4) + CD systems. Fluorescence decay experiments revealed that the addition of CD also increased the fluorescence quenching efficiencies of the copolymers P2 and P3 towards Ni²⁺ ions. Such supramolecular (hydrogen-bonded) side-chain fluorescent copolymers might have practical applicability as recoverable and/or adjustable chemosensors.