Plasmon-Enhanced Fluorescence Lateral Flow Assay Using Silica-Spaced Ag@SiO2@RuBpy Nanoprobes for Ultrasensitive and Dual-Mode Procalcitonin Detection

Abstract Ultrasensitive and quantitative detection of procalcitonin is crucial for the early diagnosis of bacterial sepsis, yet conventional gold-nanoparticle-based lateral flow assays (AuNP-LFAs) are often limited by insufficient sensitivity. Herein, we report a dual-mode colorimetric-fluorescence LFA platform employing Ag@SiO2@RuBpy core-shell NPs that harnesses plasmon-enhanced fluorescence (PEF) to achieve markedly improved analytical performance. The nanoprobes consist of an Ag NP as the plasmonic core, a tunable SiO2 spacer layer to precisely control the fluorophore–metal separation, and an outer RuBpy-doped silica shell serving as the fluorescent emitter. Systematic distance-dependent optimization reveals a maximum fluorescence enhancement factor of 16.2 at an optimal spacer thickness of ∼7 nm, attributed to the synergistic amplification of excitation rate and radiative decay induced by localized surface plasmon resonance. Theoretical simulations further support the experimentally observed optimal spacer thickness. The resulting PEF nanoprobes exhibit robust stability against pH extremes, high ionic strength, and prolonged illumination, enabling reliable signal generation in complex biological matrices. The PEF-LFA achieves a detection limit of 0.196 ng/mL, representing a 255-fold improvement over a conventional AuNP-LFA. The assay further demonstrates good specificity, reproducibility, and reliable performance in clinical samples. This work provides a generalizable strategy for the rational design of PEF-enabled nanoprobes and establishes a high-performance PEF-LFA platform for early diagnosis.