Construction of a controllable self-assembled dual-mode Al3+ sensor based on substituent effects of flavonoids

Abstract

Hydroxyl substituents direct the intrinsic excited-state intramolecular proton transfer (ESIPT) tautomerism of flavonoids, while substituent hydrophilicity-hydrophobicity and π–π interactions regulate their self-assembly, highlighting substituents’ decisive role in flavonoid properties. Herein, an innovative dual-mode self-assembled fluorescent sensor for Al³⁺ was constructed by precisely introducing a strongly electron-withdrawing amphiphilic recognition group at flavonoid hydroxyl sites, addressing the disordered self-assembly and unregulated ESIPT of natural flavonoids. This substituent modulated intermolecular non-covalent interactions to enable ordered self-assembly and selective Al³⁺ binding, yielding probe 3-[2-(methylamino) benzoyloxy]flavone (BCN). Mechanistically, the substituent blocked flavonoid ESIPT via steric–electronic effects while preserving excited-state charge transfer (ESCT) potential; Al³⁺ binding triggered photoinduced electron transfer (PET) shutdown and intramolecular charge transfer (ICT) activation, endowing BCN with an ultrahigh signal-to-noise ratio for absorbance-fluorescence dual-mode detection. BCN exhibited excellent Al³⁺ sensing performance (detection limit: 0.147 µM in water) and applicability in tap water and weak acid-alkaline samples. Br-functionalized 6-bromo-3-[2-(methylamino)benzoyloxy]flavone (BBCN) retained substituent-controlled self-assembly for portable semi-quantitative Al³⁺ detection through stable color/absorbance changes. Binding studies confirmed 2 : 1 stoichiometry between the probes and Al³⁺, and TD-DFT calculations verified the high oscillator strength of BCN–Al³⁺ transitions, supporting its superior fluorescence. This work provides a rational strategy for developing high-performance flavonoid-based sensors via substituent engineering.