Lanthanide-doped nanoparticles encountering porphyrin hydrate: Boosting a dual-mode optical nanokit for Cu²⁺ sensing

A facile dual-mode optical nanokit for Cu²⁺ detection has been developed based on a distinctive synergy between ligand-free lanthanide-doped upconversion nanoparticles (UCNPs) and porphyrin hydrate. Oleic acid (OA)-capped β-NaYF₄: 20%Yb, 0.5%Tm UCNPs were synthesized and then experienced an acid treatment for the preparation water-dispersible ligand-free UCNPs. The resulting UCNPs can readily combine a commercially available porphyrin hydrate (TPPS) to form UCNPs/TPPS complex, showing a green color from TPPS and low upconversion fluorescence of UCNPs quenched by TPPS due to fluorescence resonance energy transfer (FRET). The as-prepared UCNPs/TPPS can further combine Cu²⁺ into its cavity through the Cu²⁺-TPPS bonding. Then, a distinctive synergy between UCNPs and TPPS would be generated including the Cu²⁺-regulated color change of TPPS and its concomitant inhibiting effect on the preceding FRET, thus giving rise to an obvious color change from green to pink and a simultaneous enhancement of the upconversion fluorescence of UCNPs. Because of such distinctive synergy, UCNPs/TPPS has been demonstrated to be a novel dual-mode colorimetric and fluorometric nanokit for Cu²⁺ monitoring with high sensitivity and selectivity. Moreover, coupled with our proposed colorimetric nanokit, a realtime quantitative sensing has been also presented by using a smartphone with easy-to-access color-scanning APP as the detection platform.