Label-free upconversion nanoparticles-based fluorescent probes for sequential sensing of Cu²⁺, pyrophosphate and alkaline phosphatase activity

An efficient near-infrared fluorescence probe has been developed for the sequential detection of Cu²⁺, pyrophosphate (P₂O₇⁴⁻, PPi), and alkaline phosphatase (ALP), which is based on the “off−on−off” fluorescence switch of branched polyethyleneimine (PEI)-capped NaGdF₄:Yb/Tm upconversion nanoparticles (UCNPs). The fluorescence is quenched via energy transfer from UCNPs to Cu²⁺ for the coordination of PEI with Cu²⁺. The strong affinity between Cu²⁺ and PPi leads to the formation of Cu²⁺-PPi complex and results in the detachment of Cu²⁺ from the surface of UCNPs, thus the fluorescence is triggered on. ALP-directed hydrolysis of PPi causes the disassembly of Cu²⁺-PPi complex and re-conjugation between Cu²⁺ with PEI, which leads to the switch-off fluorescence of UCNPs. The system allows sequential analysis of Cu²⁺, PPi, and ALP by modulating the switch of the fluorescence of UCNPs with detection limits of 57.8 nM, 184 nM, and 0.019 U/mL for Cu²⁺, PPi, and ALP, respectively. By virtue of the NIR feature and excellent biocompatibility, the UCNPs-based probes are suitable for bioimaging. Taking Cu²⁺ visualization as a model, the nanoprobes have been successfully applied for intracellular imaging of Cu²⁺ in living cells.