High Na-Adducted Ion Selectivity in Laser Desorption/Ionization Mass Spectrometry with a Steric-Tuned COF

Laser desorption-ionization mass spectrometry (LDI-MS) is prized for its rapidity, simplicity, low sample consumption, and high throughput in metabolic analysis, with great potential as a diagnostic tool. Herein, an LDI-MS method with sulfone-containing covalent organic frameworks (S-COFs) as the substrate was developed. By engineering the electron cloud density and steric hindrance of the LDI substrate, selective binding sites with higher affinity for Na⁺ than K⁺ were obtained. DFT calculations confirmed that the sulfone group (O-O distance: 2.14 Å; O═S═O angle: 117.43°) favors Na⁺ adsorption over K⁺. For glucose, the intensity of [M + Na]⁺ was 75-fold higher than [M+K]⁺, enhancing detection sensitivity while simplifying serum metabolic fingerprints. S-COF-assisted LDI-MS exhibited robust responses to carbohydrates, amino acids, nucleosides, and other small-molecule metabolites. It achieved an excellent limit of detection (LOD) of 10 pg for glucose, 1,926 times more sensitive than the conventional organic matrix DHB. The method was successfully applied to detect serum metabolites in hepatitis C virus (HCV) patients and healthy controls. Support vector machine (SVM) and neural network (NNW) machine learning algorithms enabled high-precision HCV diagnosis, both yielding an AUC of 0.990. 36 metabolites were identified, and 5 of them were strongly enriched in the cysteine/methionine pathway (-log P = 5.70, Impact = 0.347), implicating perturbed redox homeostasis. This study not only offers novel insights into the modular design of functionalized COFs as LDI substrates but also provides a versatile tool for large-scale clinical diagnostics.