Controllable FRET Behaviors of Supramolecular Host–Guest Systems as Ratiometric Aluminum Ion Sensors Manipulated by Tetraphenylethylene-Functionalized Macrocyclic Host Donor and Multistimuli-Responsive Fluorescein-Based Guest Acceptor

Abstract

The novel multistimuli-responsive monofluorophoric supramolecular polymer <b>Poly(TPE-DBC)/FL-DBA</b> and pseudo[3]rotaxane <b>TPE-DBC/FL-DBA</b> consisted of the closed form of nonemissive fluorescein guest <b>FL-DBA</b> along with TPE-based main-chain macrocyclic polymer <b>Poly(TPE-DBC)</b> and TPE-functionalized macrocycle <b>TPE-DBC</b> hosts, respectively. By the combination of various external stimuli, these fluorescent supramolecular host-guest systems could reveal interesting photoluminescence (PL) properties in DMF/H₂O (1:1, v/v) solutions, including bifluorophoric host-guest systems after the complexation of Al³⁺ ion, i.e., <b>TPE-DBC/FL-DBA-Al</b>^<b>3+</b> and <b>Poly(TPE-DBC)/FL-DBA-Al</b>^<b>3+</b> with their corresponding open form of fluorescein guest <b>FL-DBA-Al</b>^<b>3+</b>. Importantly, the Förster resonance energy transfer (FRET) processes occurred in both bifluorophoric host-guest systems between blue-emissive TPE donors (λ_em = 470 nm) and green-emissive fluorescein acceptors (λ_em = 527 nm) after aluminum detection, which were further verified by time-resolved photoluminescence (TRPL) measurements to acquire their FRET efficiencies of 40.4 and 31.1%, respectively. Both supramolecular host-guest systems exhibited stronger green fluorescein emissions as well as appealing ratiometric PL behaviors within the desirable donor-acceptor distances of FRET processes in comparison with their detached analogous mixtures. Regarding the pH effects, the optimum green fluorescein emissions with effective FRET processes of all compounds and host-guest systems were sustained in the range pH = 7-10. Interestingly, both host-guest systems <b>TPE-DBC/FL-DBA</b> and <b>Poly(TPE-DBC)/FL-DBA</b> possessed high sensitivities and selectivities toward aluminum ion to display their strong green emissions via FRET-ON behaviors due to the chelation-induced ring opening of spirolactam moieties to become green-emissive guest acceptor <b>FL-DBA-Al</b>^<b>3+</b>, which offered excellent limit of detection (LOD) values of 50.61 and 38.59 nM, respectively, to be further applied for the fabrication of facile test strips toward aluminum detection. Accordingly, the inventive ratiometric PL and FRET sensor approaches of supramolecular host-guest systems toward aluminum ion with prominent sensitivities and selectivities were well-established in this study.