Supramolecular ICT Modulation in π-Extended 2D-Azeno Nanosheets for Reversible Fluorescence Sensing of Spermine at the Sub-10 nM Level

Abstract

Biogenic amine surveillance is central to food safety and clinical diagnostics, yet real-time, selective detection of spermine (SPM) remains challenging due to molecular similarity among amines and probe instability in complex matrices. Here, we report a reusable, wavelength-shift-responsive fluorescent probe based on two-dimensional azene nanosheets (2D-Azeno NSs), synthesized through a Schiff condensation route followed by ultrasonic exfoliation. The robust, π-extended framework exhibits intense green photoluminescence (λem = 501 nm) and achieves an ultralow detection limit of 4.35 nM via a unique ratiometric fluorescence response accompanied by a distinct hypsochromic shift. Mechanistic interrogation, supported by DFT calculations, reveals that selective sensing arises not from conventional Inner Filter Effects but from site-specific, cooperative hydrogen bonding between the polyamine functional groups of SPM and the N–H/C═N sites of the 2D-Azeno scaffold. This supramolecular recognition selectively perturbs the Intramolecular Charge Transfer (ICT) pathway, leading to a measurable widening of the frontier orbital energy gap creating a Hydrogen-bond induced charge transfer modulation (HICTM) sensor. Furthermore, this sensing pathway is inherently reversible; acid fuming protonates the amine sites, disrupting the H-bond network and fully regenerating the initial emission state over nine operational cycles. This work establishes a paradigm for supramolecular control over a solid-state electronic structure in rigid 2D organic materials, offering a generalizable design principle for developing robust, regenerable, and high-fidelity optical sensors applicable to environmental monitoring and precision biomedicine.