Radical-Mediated In Situ Fluorescence Dye Deposition: A Simple Interfacial Signal Amplification Reaction for Ultrasensitive Immunoassay on Barcode Beads.

Ultrasensitive detection of low-abundance protein biomarkers is crucial for early disease diagnosis but remains challenging for the conventional barcode beads-based suspension chip platform due to the limited detection sensitivity. Here, we report a conceptually novel reaction termed "radical-mediated in situ fluorescence dye deposition" (RIFD) as a simple interfacial signal amplification strategy to overcome the limitation. This first discovered RIFD follows a universal "three-element principle", where the coexistence of beads, free radicals, and dyes suffices for fluorescence labeling, eliminating the pre-conjugation of dyes to detection probes required in traditional methods. Mechanistic studies reveal that this radical-triggered RIFD possibly follows three distinct pathways, including amide condensation on lysine residues, covalent biphenyl formation on tyrosine residues, or radical-dye co-deposition. It facilitates local and ultrafast (within 5 min) dye-trapping specifically on target-positive barcode beads. Consequently, the established RIFD-based immunoassay achieves a limit of detection of 12 fg/mL for IL-10, a 100-fold improvement over the conventional suspension chip method, and also successfully differentiates Alzheimer's disease patients from healthy controls by quantifying low-abundance plasma p-Tau217. Multiplexed detection is further validated with a three-plex cytokines panel. Our reported RIFD represents a powerful in situ fluorescence labeling tool, advancing protein biomarker detection toward the sub-pg/mL level with broad implications for clinical diagnostics.