Rhodamine B-encapsulated MOF@boric acid-modified COF for adsorption and ratiometric fluorescence detection of baicalin.

Efficient adsorption and sensitive detection of baicalin (BAI) are essential for maximizing its medical value. In this work, a rhodamine B (Rb)-encapsulated metal-organic framework (MOF)@boric acid-modified covalent organic framework (COF) composite material (Rb-MOF@COF-BA) was designed for high-capacity adsorption and high-sensitivity detection of BAI. Specifically, the Zr-based MOF of UiO-66 was prepared with Zr4+ and 2-aminoterephthalic acid, during which Rb was encapsulated, resulting in strong red fluorescence. The -NH2 group on the Rb-MOF surface then reacted with 2,5-dimethoxyterephthalaldehyde and coupled with the ligand 1,3,5-tris(4-aminophenyl)benzene to integrate the COF. Subsequently, to enhance the recognition of cis-diol-containing BAI through boron affinity, Rb-MOF@COF was functionalized with (4-ethynylphenyl)boronic acid via aza-Diels-Alder cyclization, obtaining the Rb-MOF@COF-BA composite with blue fluorescence. For BAI, Rb-MOF@COF-BA demonstrated a high adsorption capacity of 114.2 mg g-1 and reached equilibrium within 200 min. Additionally, the Rb-MOF@COF-BA composite displayed dual emissions at 440 and 580 nm, enabling its use as a ratiometric fluorescence sensor for BAI detection, accompanied by a visible blue-to-red spectral transition. The resulting radiometric fluorescence method demonstrated linear ranges of 0.2-3.0 and 3.0-20.0 μmol L-1 with a low detection limit of 0.0173 μmol L-1 for BAI. This analytical method was employed to identify and quantify BAI in Yinhuang oral solution, yielding favorable results that confirmed the potential of this ratiometric detection for practical applications.