Monolithic column with multiple COF surface layers via an in-situ surface self-assembly strategy for efficient in-tube solid-phase microextraction of benzodiazepines

Covalent organic framework (COF)-based monolithic columns have been extensively used for sample preparation. In order to maximize the COF extraction properties, it is attractive to increase the COF surface coverage of the COF-based monolithic column. Herein, a novel monolithic column with multiple COF surface layers was prepared via an in-situ surface self-assembly strategy to increase the COF surface coverage. For this preparation, the poly (p-phenylenediamine) (PPDA) fibers were first grafted onto the parent monolith. Then, due to the abundant NH2-anchor sites of the PPDA fiber, a large number of COF microspheres were self-assembled on PPDA fibers, and further interlinked with each other to form multiple COF surface layers on the monolith, which further improve the COF surface coverage. Various characterization techniques were employed to confirm this successful preparation. Benefiting from the enhanced COF surface coverage, the developed monolith exhibited superior extraction ability towards benzodiazepines (BZDs), which are central nervous system depressants that are tightly controlled by governments. The hydrophobic and pi-pi stacking interactions should be the main adsorption mechanism for in-tube SPME of BZDs. Under the optimal SPME conditions, a simple in-tube SPMEHPLC-DAD method was established for the detection of trace BZDs in saliva, with a wide linear range (5 - 300 ng/mL), excellent sensitivity (LODs <= 2 ng/mL) and satisfactory recoveries (81.4% - 98.0%). This research provided a versatile route to fabricate the COF-based monolith with high surface coverage, demonstrating its great potential in efficient sample preparation.