CRISPR-Cas12a integrated with nanomaterials has formulated powerful biosensors for viral protein detection, addressing the urgent need for point-of-care diagnostics. However, existing platforms are hindered by either multi-step separation procedures or insufficient signal amplification, limiting their sensitivity and practicality. Here, we report a one-pot "on-off" biosensor that combines metal-enhanced fluorescence (MEF) and nanoscale spatial confinement by co-localizing both reporter substrates and the CRISPR-Cas12a system on gold-silica core-shell nanoparticles (Au@SiONPs), enabling rapid and ultrasensitive protein detection. Using SARS-CoV-2 nucleocapsid (N) protein as a model analyte, Au@SiONPs are co-functionalized with (i) ssDNA activators blocked by N protein-specific aptamers, (ii) light-up hairpin DNA (DAP) complexed with auramine O (AO) as reporters, and (iii) short polyethylene glycol (PEG) spacers to mitigate steric hindrance. The nanoplatform displays an ultrabright "on-state" fluorescence, with an intensity >860-fold higher than that of free AO, enabled by the interaction with DAP and optimized fluorophore-metal spacing (∼20 nm). Upon target binding, aptamer displacement exposes the activator to locally initiate Cas12a trans-cleavage, disrupting proximal DAP structure and its interaction with AO, thereby producing a distinct "off-state" signal. Within the linear detection range, the system demonstrates up to ∼85% signal reduction relative to the initial signal and a signal-to-noise ratio (SNR) of 83.89, corresponding to a ∼2.5-fold higher SNR than the solution-phase system. The platform attains a limit of detection at 67.2 fg/mL within 30 min, with excellent sensitivity, selectivity, stability, and recovery in bronchoalveolar lavage fluid. By combining MEF-driven signal amplification with surface-confined CRISPR-Cas12a trans-cleavage, this platform establishes an efficient strategy for sensitive N protein detection. 2 2
