ZnO-poly(methyl methacrylate) nanobeads for enriching and desalting low-abundant proteins followed by directly MALDI-TOF MS analysis.

A fast solid-phase microextraction method using core-shell ZnO-poly (methyl methacrylate) nanobeads (ZnO-PMMA) as adsorbent was established. This fast method with high enriching efficiency and salt tolerance capability depends on the structure of the core-shell nanobeads. First, the large surface area of the PMMA shell makes the dispersive nanobeads capture samples quickly, by virtue of multi-interactions between ZnO-PMMA and samples except for the interaction with salts. Second, the small nanosize of the ZnO-core (2.1 nm) and the flexible hydrophobic PMMA shell, which can prevent the cores from aggregation, make the nanobeads form a homogeneous layer on the matrix-assisted laser desorption/ionization (MALDI) plate and do not hinder the cocrystallization of the matrix and samples. Third, the ZnO core also prevents PMMA from fragmentation and ionization in mass spectrometer. In this article, approximately 80% bovine serum albumin digests were enriched by ZnO-PMMA from 100 amol/muL solution within 10-min incubation, and the solid phase can be directly analyzed by MALDI mass spectrometry. Mass intensity can be increased 5-10-fold (ZnO-PMMA enrichment vs lyophilization). High-quality mass spectra can be obtained, even with the presence of saturated NaCl (6.2 M), saturated NH 4HCO 3 (2.6 M), or 1 M urea. This method has been successfully applied to human colorectal cancer proteome research, and eight new proteins have been found.