Cyclized conjugated microporous polymer-coated silica nanospheres as fluorescent sensors for iron (III) and chromium (III)

Schiff-base condensation reaction was often used to prepare conjugated microporous polymers (CMPs). However, due to the relatively unstable imine bond in the product, the application of CMPs in some strong acid-base environments was limited. Therefore, it was necessary to further strengthen the stability of CMPs by bottom up strategy. For this purpose, three kinds of cyclized CMPs (x-CMP x = m, p, bi) were successfully prepared by Schiff-base reaction followed by an acid-catalyzed in situ cyclization with 1,5,9-triamine-triphenylene (TATP) and m-phthalaldehyde (m-PA), p-phthalaldehyde (p-PA) or 4,4'-biphenyldicarboxaldehyde (bi-PA) as precursors. The in situ cyclization at the imine bond was demonstrated by X-ray photoelectron spectroscopy and Raman spectroscopy. Because cyclized CMPs were amorphous powder, their dispersity were not ideal, which affects them further applications. Therefore, through a one-pot method, these CMPs were also coated onto silica nanospheres, assigned as x-CMP@SiO2 (x = m, p, bi), exhibiting core-shell structure and excellent monodispersity. Due to the difference dispersity of three x-CMP@SiO2 (x = m, p, bi) in aqueous solution, the fluorescence quantum yields were different, which were 15.3%, 14.4% and 11.2%, respectively. As fluorescent sensors, three cyclized x-CMP@SiO2 (x = m, p, bi) could selectively generated fluorescence quenching response to Fe3+ and Cr3+ in aqueous solution. Compared with amorphous CMPs, the resulting CMP@SiO2 nanomaterials not only improved the dispersity and mechanical strength, but also offered the convenience for subsequent applications. It is expected this feasible method would facilitate CMPs to be widely explored in fluorescence detection and other research fields.

Automated summary

Three cyclized conjugated microporous polymers (CMPs) were successfully prepared by Schiff-base condensation reaction followed by acid-catalyzed in situ cyclization. These cyclized CMPs were then coated onto silica nanospheres to form core-shell structures with excellent monodispersity. The resulting CMP@SiO2 nanomaterials exhibited improved dispersity and mechanical strength, and could be used as fluorescent sensors.