Out of site: A protein expression system was combined with CuI‐catalyzed 1,2,3‐triazole formation to modify a target protein at its C terminus. The immobilized core protein can be modified by diverse small molecules in a site‐specific manner (see picture). Protein conjugation with a diazido linker gives a homodimeric protein, and microarrays can be fabricated by site‐specific covalent binding to functionalized surfaces.

Functionalized magnetic nanoparticles (MNPs) were synthesized to serve as laser desorption/ionization elements as well as solid-phase extraction probes for simultaneous enrichment and detection of small molecules in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) analysis. Two laser-absorbing matrices were each conjugated onto MNP to give MNP@matrix which provided high ionization efficiency and background-free detection in MS leading to unambiguous identification of target small molecules in a complex mixture. MNP@matrix was demonstrated to serve as a general matrix-free additive in MALDI-TOF MS analysis of structurally distinct small molecules. Also, MNP@matrix provides a simple, rapid, and reliable quantitative assay for small molecules by mass without either the use of an internal standard or an isotopic labeling tag. Furthermore, the affinity extraction of small molecules from complex biofluid was achieved by probe protein-conjugated MNP@matrix without laborious purification. We demonstrated that a nanoprobe-based assay is a cost-effective, rapid, and accurate platform for robotic screening of small molecules.

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