Engineering Red-Enhanced and Biocompatible Upconversion Nanoparticles

Abstract

The exceptional optical properties of lanthanide-doped upconversion nanoparticles (UCNPs) make them among the best fluorescent markers for many promising bioapplications. To fully utilize the unique advantages of the UCNPs for bioapplications, recent significant efforts have been put into improving the brightness of small UCNPs crystals by optimizing dopant concentrations and utilizing the addition of inert shells to avoid surface quenching effects. In this work, we engineered bright and small size upconversion nanoparticles in a core-shell-shell (CSS) structure. The emission of the synthesized CSS UCNPs is enhanced in the biological transparency window under biocompatible excitation wavelength by optimizing dopant ion concentrations. We also investigated the biosafety of the synthesized CSS UCNP particles in living cell models to ensure bright and non-toxic fluorescent probes for promising bioapplications.

Keywords: bioapplication; bioimaging; red-enhanced fluorescent markers; upconversion nanoparticles.

Figure 1

(a) Illustration of a superimposed photoluminescence of red enhanced emission of the synthesized CSS UCNPs under 808 nm continuous laser excitation, the illumination and absorption spectra of distilled water (DI water), and biological tissue taken over the visible and the near-infrared (NIR) range. (Data presented in Figure 1a were experimentally collected for this present study). (b) Electronics structure and energy transfer processes among Nd⁺³, Yb⁺³, and Er⁺³ of red enhanced CSS UCNPs (photon upconversion) under 808 nm laser excitation. (c) An overview of the synthesized UCNP architectures and doping ratio at each stage.

Figure 2

(a–c) Structural characterization of the core–shell–shell NaGdF₄:Yb (20%), Er (8%)@ NaGdF₄:Yb (20%), and Er (8%). (a) TEM images of the C particles NaGdF₄:Yb (20%), Er (8%). (b) TEM images of the CS particles NaGdF₄:Yb (20%), Er (8%)@ NaYb_0.90F₄:Yb (10%), and Er (10%). (c) TEM images of the CSS particles NaGdF₄:Yb (20%), Er (8%)@ NaYb_0.90F₄:Yb (10%), and Er (10%)@ NaYF4. (d) Corresponding DLS size distributions of the core–shell–shell UCNPs at different synthesis stages. (e, inset) A high resolution TEM image of the CSS UCNPs. (e) EDX spectrum recorded from the synthesized CSS UCNPs.

Figure 3

Upconversion luminescence spectra (UCL) of (a) core particles NaGdF₄:Yb (20%) and Er(8%). (b) Core–shell particles NaGdF₄:Yb (20%) and Er (8%)@ NaYb_0.90F₄:Nd (10%). (c) Core–shell–shell particles NaGdF₄:Yb (20%) and Er (8%)@ NaYb_0.90F₄:Nd (10%) @ NaYF₄ UCNP particles under 980 nm laser excitation at different laser intensities. UCL of (d) core–shell particles NaGdF₄:Yb (20%) and Er (8%)@ NaYb_0.90F₄:Nd (10%). (e) Core–shell–shell particles NaGdF₄:Yb (20%) and Er (8%)@NaYb_0.90F₄:Nd (10%)@ NaYF₄ UCNP particles under 808 nm laser excitation at different laser intensities. (f) Comparison of temperature rise of 1 mg/mL CSS-PAA UCNPs dispersed in 2 mL of distilled water under 808 and 980 nm laser irradiation at 50 W ·cm⁻² for 20 min.

Figure 4

Cell viability of CSS-PAA UCNPs after incubation for 4 h with A549 and B16-F10 cells. These data are the results of (a) MTS and (b) LDH assays, which are expressed as cell viability (%) and presented as the mean ± SD (n = 3).