Modulated upconversion luminescence of water-dispersed NaErF4@NaYF4 nanoparticles by introducing Tm3+/Ho3+ as energy capture centers under multi-wavelength NIR excitation

Water-dispersed red upconversion nanoparticles (UCNPs) have great application prospects in the biomedical field. Thus, a series of Er 3+ -based upconversion nanoparticles (Er-UCNPs) with excellent red upconversion luminescence under three-wavelength NIR excitation (808/980/1550 nm) were prepared by a simple thermal decomposition method, and then further functionalized by PAA to achieve good water dispersibility. Compared with NaErF 4 bare core, the emission intensity of Er-UCNPs samples doping of 0.5%Tm 3+ and 0.5%Ho 3+ as energy capture centers was significantly enhanced, and further improved after coating with NaYF 4 inert shell. Meanwhile, among all Er-UCNPs@NaYF 4 samples, NaErF 4 @NaYF 4 exhibited the highest emission intensity and longest luminescence lifetime due to the efficient suppression of luminescence quenching effect, while NaErF 4 :0.5%Tm 3+ @NaYF 4 showed highest red luminescence purity owing to the energy transfer process between Tm 3+ ( 3 H 5 ) and Er 3+ ( 4 I 13/2 ). The hydrophilic Er-UCNPs@NaYF 4 @PAA samples still maintained good red luminescence under multi-wavelength excitation. The luminescence enhancement mechanism in Er-UCNPs@NaYF 4 was discussed in detail based on spectral characteristics. The as-prepared water-dispersed Er-UCNPs@NaYF 4 @PAA with bright red luminescence under multi-wavelength NIR excitation will have potential applications in targeted therapy and deep tissue imaging.