Rational Design of pH-Tunable Magnetic Microspheres for Dual-Selective Dye Extraction

The development of adsorbents with precise recognition and reversible switching remains a long-standing challenge in solid-phase extraction (SPE), particularly for the sensitive monitoring of illegal additives in complex food matrices. Herein, a pH-tunable amphoteric magnetic microsphere, M-PEI@HESP, was rationally fabricated and integrated with HPLC for high-selectivity dye detection. Unlike pristine M-HESP, which exhibits inherent single-selectivity toward cationic dyes (e.g., Rhodamine B), M-PEI@HESP employs protonatable/deprotonatable amine groups from PEI to achieve pH-dependent charge inversion: efficiently capturing anionic dyes (e.g., amaranth, acid Red 73) under acidic conditions, while specifically retaining banned cationic dyes (e.g., Rhodamine B) under alkaline conditions. A combination of pH-dependent adsorption, zeta potential, FT-IR, XPS, and DFT simulations reveals that electrostatic interaction is the dominant driving force, synergistically reinforced by hydrogen bonding, π–π stacking, and H+/OH– interactions. Notably, the adsorbent enables reversible selectivity switching and accurate quantification of anionic (amaranth) and cationic (rhodamine B) dyes within the same complex matrix simply by pH modulation. The resultant MSPE–HPLC method delivers a low LOD (0.1–1.0 μg L–1), a broad linearity (1.0–1500.0 μg L–1), high recoveries (90.58–105.64%), and satisfactory resistance to matrix effects. This work not only provides a versatile platform for the precise screening of illegal food additives but also establishes a new paradigm for designing intelligent, stimuli-responsive adsorbents.