WS₂–NH₂ Enhanced Fiber SPR Biosensor for Ultralow-Level β-Actin Detection via Cu–Glycine Functionalization

Beta-actin (ACTB), a highly conserved cytoskeletal protein, is frequently dysregulated in early-stage malignancies, making its sensitive and specific detection crucial for cancer screening. Here, we develop a reflective fiber-optic SPR biosensor functionalized with copper–glycine engineered WS₂ nanosheets for label-free ACTB detection. The Cu–glycine modification not only enables WS₂ surface amination for efficient antibody conjugation, but also introduces Cu²⁺-mediated near-infrared absorption to amplify the plasmonic resonance response. Combined with the intrinsically large surface area and high refractive index sensitivity of WS₂, this synergistic design markedly enhances both molecular binding and signal transduction. The resulting WS₂-NH₂-based sensor achieves a refractive index sensitivity of 1467.5 nm/RIU, representing a 1.67-fold enhancement over unmodified fiber probes. For ACTB detection, it delivers an ultralow detection limit down to 9.766 pM, a broadened linear range of 2.4–120 pM (5×expansion), and a biosensing sensitivity of 84.477 nm/nM (1.4× improvement). Furthermore, at 1 ng/mL, a rapid response time of 20 min is achieved, demonstrating applicability for trace-level biomarker screening. Integrating high sensitivity, broad dynamic range, amplified spectral response, and strong anti-interference capability, this plasmonic WS₂-NH₂-modified fiber platform enables highly selective, label-free ACTB quantification. These findings highlight its strong potential for early cancer diagnosis, biomarker monitoring, and precision medical applications.