Bifunctionalized Core–Shell Magnetic Nanoparticles Enabling Catalytically Amplified Chemiluminescence for Ultrasensitive Detection of Pyrophosphate ions

In this work, luminol and Cu ²⁺ bifunctionalized magnetic core-shell Fe ₃ O ₄ @Au nanoparticles (BFCS-Fe ₃ O ₄ @Au NPs) with high chemiluminescence (CL) efficiency were synthesized via an improved approach for pyrophosphate ions (PPi) sensing in complicated samples. First, a uniform Au shell was formed in situ on the Fe ₃ O ₄ core through the controlled deposition enabled by luminol's reducing property. This process also resulted in the immobilization of luminol molecules on the Au shell via Au-N coordination, where they functioned as the CL signal units. Subsequently, Cysteine (Cys) was modified onto the Fe ₃ O ₄ @Au surface via Au-S bonds, which introduced specific coordination sites for metal ions. Then, copper ions (Cu ²⁺ ) were captured by these sites and fixed onto the Fe ₃ O ₄ @Au surface, which efficiently catalyze the luminol-H ₂ O ₂ reaction, resulting in a strong “signal-on” state. Finally, the presence of PPi could quantitatively quench this signal by competing with Cys for Cu ²⁺ binding, thereby switching the system “off”. Due to this “signal-on-off” mechanism, an ultrasensitive sensor for PPi was developed, exhibiting a broad linear detection range from 0.1 nM to 10 µM and a low detection limit of 8.37 pM. This sensing strategy provides a reliable and promising pathway for PPi monitoring in biological analysis and food safety.