Synthesis of a post-modified covalent organic framework as a “turn-on” fluorescent chemosensor for highly selective and sensitive quantitative detection of common trivalent metal ions

Abstract

The selective and ultrasensitive detection of common trivalent cations (Al³⁺, Ga³⁺, In³⁺, Cr³⁺, and Fe³⁺) using a single fluorophore is an attractive prospect. For the first time, herein, we have successfully synthesized a novel Ac–Py–Np COF via post-modification of an imine-linked covalent organic framework (COF) comprising 4,4′,4″,4‴-(pyrene-1,3,6,8-tetrayl)tetraaniline (Py) and naphthalene-2,6-dicarbaldehyde (Np) with L-ascorbic acid (Ac). FT-IR, ¹³C CP/MAS NMR and XPS analyses confirmed the synthesis of the Ac–Py–Np COF, while PXRD and N₂ adsorption/desorption illustrated the crystallinity and porosity, respectively. The Ac–Py–Np COF demonstrated a highly selective and ultrasensitive “turn on” fluorescence response for the detection of common trivalent metal ions in acetonitrile in the presence of other competitive metal ions. The Ac–Py–Np COF exhibited the lowest limit of detection (LOD) of 0.67 µM for Fe³⁺, 1.57 µM for Al³⁺, 0.58 µM for Ga³⁺, 0.63 µM for In³⁺, and 1.40 µM for Cr³⁺. Moreover, the Ac–Py–Np COF exhibited good recovery in the practical samples of water, chicken, mutton and kale with high accuracy. The mechanism of complex formation of the Ac–Py–NP COF with trivalent cations was elucidated by DFT calculations, while photoinduced electron transfer (PET) accompanied by chelation-enhanced fluorescence (CHEF) explained the sensing mechanism. The present study reveals that post-modified COFs have potential as fluorescence chemosensors for the detection of metal ions and application to practical samples.