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 Er³⁺-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 NaErF₄ bare core, the emission intensity of Er-UCNPs samples doping of 0.5%Tm³⁺ and 0.5%Ho³⁺ as energy capture centers was significantly enhanced, and further improved after coating with NaYF₄ inert shell. Meanwhile, among all Er-UCNPs@NaYF₄ samples, NaErF₄@NaYF₄ exhibited the highest emission intensity and longest luminescence lifetime due to the efficient suppression of luminescence quenching effect, while NaErF₄:0.5%Tm³⁺@NaYF₄ showed highest red luminescence purity owing to the energy transfer process between Tm³⁺(³H₅) and Er³⁺(⁴I_13/2). The hydrophilic Er-UCNPs@NaYF₄@PAA samples still maintained good red luminescence under multi-wavelength excitation. The luminescence enhancement mechanism in Er-UCNPs@NaYF₄ was discussed in detail based on spectral characteristics. The as-prepared water-dispersed Er-UCNPs@NaYF₄@PAA with bright red luminescence under multi-wavelength NIR excitation will have potential applications in targeted therapy and deep tissue imaging.