Fluorescent Probe Based on Lanthanide Coordination Polymer Nanoparticles for Selective Cascade Detection of Caffeic Acid and Cu²⁺

In this study, we developed lanthanide coordination polymer nanoparticles (PDA-Eu) using 2,6-pyridinedicarboxylic acid (PDA) and Eu ions (Eu3+), and employed them as fluorescent probes to detect caffeic acid (CA) and copper ions (Cu2+). This fluorescent sensor enabled the cascade detection of CA and Cu2+ through changes in the ratio of fluorescence response and fluorescence quenching, respectively. CA exhibited a strong absorption band that significantly overlapped with the excitation spectra of PDA-Eu, enabling it to filter out the excitation energy of PDA-Eu via the internal filtering effect. In addition, CA emitted strong blue fluorescence. Consequently, as the concentration of CA increased, the red fluorescence of PDA-Eu was gradually quenched, while the blue fluorescence of CA was significantly enhanced. The fluorescence intensity ratios at 465 and 615 nm exhibited a linear relationship with CA concentrations in the range of 0–16 μM, with a limit of detection (LOD) for CA as low as 0.12 μM. Furthermore, upon the subsequent introduction of Cu2+, the blue fluorescence of CA at 465 nm was suppressed through static quenching, thereby enabling the quantitative detection of Cu2+ within a linear range of 0–2 μM (LOD = 37 nM). This strategy was successfully used to detect CA and Cu2+ in real samples, yielding satisfactory recoveries ranging from 95.2% to 106.5%.