Integration of nanohydrogel with covalent organic framework: Tailoring interfacial amphiphilicity for advanced chromatographic separation.

Covalent organic frameworks (COFs), featuring high surface reactivity and robust stability, are promising candidates for chromatographic stationary phases. However, the abundant aromatic groups in COF may lead to overly strong interactions with hydrophobic analytes, causing chromatographic peak tailing and low column efficiency. Moreover, the limited hydrophilicity of the conventional COF material results in insufficient separation ability of COF@SiO2 composites for polar substances, thereby restricting their wide applicability. With the aim of improving the flexibility, selectivity and separation efficiency of COF@SiO2, a hydrophilic nanohydrogel (NHG) was modified on the surface of COF. Firstly, COF@SiO2 was synthesized via a mild one-pot approach using the eco-friendly deep eutectic solvent as the reaction medium. Subsequently, NHG-COF@SiO2 composite material was fabricated through the surface modification of COF@SiO2 with poly(N-isopropylacrylamide)-based NHG. Hydrophilic/hydrophobic compounds (nucleoside bases, sulfonamides, B vitamins, polycyclic aromatic hydrocarbons, alkylbenzenes and phenyl ketones), aromatic positional isomers, and ionizable benzoic acids were successfully separated by the NHG-COF@SiO2 in different chromatographic modes. The NHG functionalization significantly enhances chromatographic performance of COF@SiO2 by increasing the flexibility of COF material, while COF reciprocally modulates the strong hydrophilicity of NHG to improve separation selectivity. Leveraging its multifunctional interaction sites, the NHG-COF composite demonstrates high-resolution separation performance. The synergistic integration of NHG and COF has achieved a triple enhancement in hydrophilicity, selectivity and separation efficiency for the NHG-COF@SiO2. This achievement facilitates the realization of multi-mode separation in a single-column system, serving as a valuable reference for the ongoing research on COF-based silica columns.