In extinction-based immunosorbent assays, enzymes are commonly used as signal transducers; however, their intrinsic 

instability has prompted the search for more stable alternatives, such as inorganic nanomaterials. In this study, we developed

a robust signal transduction strategy utilizing the localized surface plasmon resonance (LSPR) of gold nanoclusters

(AuNCs) to generate distinct and easily interpretable extinction signals, which were applied to C-reactive protein 

(CRP) quantification. To implement this, AuNCs and anti-CRP antibodies were co-immobilized on magnetic nanoparticles 

(MNPs), preparing “MagANC immunocomplexes” that can be rapidly purified via magnetic separation. By exploiting the 

seed-mediated growth of AuNCs, these immunocomplexes generated CRP-dependent extinction signals, enabling sensitive 

quantification. This approach effectively eliminates the need for unstable enzymes while leveraging the superparamagnetic 

properties of MNPs to simplify the purification process. The assay demonstrated a wide dynamic range of 0.001–1 µg/

mL with two complementary readouts: maximum LSPR wavelength (0.03–1 µg/mL) and the absorbance at 600 nm 

(0.001–0.03 µg/mL). The limit of detection (LOD) was 1.58 ng/mL. Furthermore, the method showed excellent reliability 

in CRP-spiked serum, with a mean recovery of 100.47% and a coefficient of variation of 4.79%. Overall, the proposed 

MagANC strategy offers a practical, enzyme-free alternative for clinical CRP diagnostics, delivering a wide dynamic range 

and competitive sensitivity with a simplified workflow.