Microchamber-Free Digital Flow Cytometric Analysis of T4 Polynucleotide Kinase Phosphatase Based on Single-Enzyme-to-Single-Bead Space-Confined Reaction.

Digital bioassays have attracted extensive attention in biomedical applications due to their ultrahigh sensitivity. However, traditional digital bioassays require numerous microchambers such as droplets or microwells, which restricts their application scope. Herein, we propose a microchamber-free flow cytometric method for the digital quantification of T4 polynucleotide kinase phosphatase (T4 PNKP) based on an unprecedented phenomenon that each T4 PNKP molecule-catalyzed reaction can be spatially self-confined on a single microbead, which ultimately enables the one-target-to-one-fluorescence-positive microbead digital signal transduction. The digital signal-readout mode can clearly detect T4 PNKP concentrations as low as 1.28 × 10 U/μL, making it most sensitive method to date. Significantly, T4 PNKP can be specifically distinguished from other phosphatases and nucleases in complex samples by digitally counting the fluorescence-positive microbeads, which cannot be realized by traditional bulk measurement-based methods. Taking advantage of the novel space-confined enzymatic feature of T4 PNKP, this digital mechanism can use T4 PNKP as the enzyme label to fabricate digital sensing systems toward various biomolecules such as digital enzyme-linked immunosorbent assay (ELISA). Therefore, this work not only enlarges the toolbox for high-sensitivity biomolecule detection but also opens new gates to fabricate next-generation digital assays.