Construction of a Controllable Self-Assembled Dual-Mode Al 3+ 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 Al 3+ 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 Al 3+ 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; Al 3+ 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 Al 3+ 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 Al 3+ detection through stable color/absorbance changes. Binding studies confirmed 2:1 stoichiometry between the probes and Al 3+, and TD-DFT calculations verified the high oscillator strength of BCN-Al 3+ transitions, supporting its superior fluorescence. This work provides a rational strategy for developing high-performance flavonoid-based sensors via substituent engineering.