Cationic guanidine covalent organic frameworks with abundant hydrogen-bonding sites for efficient enrichment and sensitive detection of nitrobenzenes in tea.

Tea (Camellia sinensis) is deeply loved by consumers worldwide, with hundreds of millions of people maintaining daily tea-drinking habits, making its food safety a significant concern. Among various potential contaminants, hydrophilic nitrobenzene compounds (NBs) are particularly problematic due to their high solubility in tea infusion, strong bioaccumulation potential, and high toxicity. Even trace residues can migrate into the tea infusion during brewing, posing long-term risks to human health. Therefore, Monitoring trace NBs in tea is crucial. However, due to matrix interference from tea leaves and the extremely low concentrations of NBs, existing standard methods cannot accurately quantify trace NBs in tea. To address this challenge in enrichment and analysis, this study, based on the physicochemical properties of NBs, designed and prepared a cationic guanidine covalent organic framework (COF), named TG-DHTP. This material contains abundant hydrogen bond donors (N-H, O-H) and cationic sites (=+NH-), enabling efficient and selective enrichment of NBs through synergistic interactions including O-H···O hydrogen bonds, N-H···O hydrogen bonds, electrostatic attraction, π-π interactions, and van der Waals forces. By combining TG-DHTP with SPME-GC-MS, an analytical method was established for trace NBs in tea. The established method exhibited a broad linear range (80.0-10000.0 ng L-1), low detection limit (20.0 ng L-1), and good recoveries (83.6-117.6%). This study not only provides a reliable analytical method for monitoring trace NBs in tea, but also offers a rational design strategy for developing new adsorbent materials targeting polar and hydrophilic contaminants in food safety.