Dye Sensitization for Ultraviolet Upconversion Enhancement

Abstract

Upconversion nanocrystals that converted near-infrared radiation into emission in the ultraviolet spectral region offer many exciting opportunities for drug release, photocatalysis, photodynamic therapy, and solid-state lasing. However, a key challenge is the development of lanthanide-doped nanocrystals with efficient ultraviolet emission, due to low conversion efficiency. Here, we develop a dye-sensitized, heterogeneous core-multishelled lanthanide nanoparticle for ultraviolet upconversion enhancement. We systematically study the main influencing factors on ultraviolet upconversion emission, including dye concentration, excitation wavelength, and dye-sensitizer distance. Interestingly, our experimental results demonstrate a largely promoted multiphoton upconversion. The underlying mechanism and detailed energy transfer pathway are illustrated. These findings offer insights into future developments of highly ultraviolet-emissive nanohybrids and provide more opportunities for applications in photo-catalysis, biomedicine, and environmental science.

Keywords: dye sensitization; energy transfer; heterogeneous nanoparticles; lanthanide nanoparticles; luminescence enhancement; ultraviolet upconversion.

Scheme 1

Schematic illustration of the key factors that influence UV enhancement in IR-806-loaded upconversion nanoparticles, including dye concentration, excitation wavelength, and dye–sensitizer distance.

Figure 1

Schematic illustration and characterization of Gd-CS_YS₂S₃ heterogeneous nanoparticles. (a) Diagrammatic representation of Gd-CS_YS₂S₃ nanostructure. (b) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of Gd-CS_YS₂S₃ nanoparticles. Inset: high-resolution TEM of as-prepared Gd-CS_YS₂S₃ nanoparticle. (c) HAADF-STEM image and elemental mapping image of Gd-CS_YS₂S₃ nanoparticles, revealing the spatial distribution of the Y, Nd, Gd, and Yb elements in the heterogeneous nanoparticles.

Figure 2

Preparation and characterization of Gd-CS_YS₂S₃@IR-806. (a) IR-806 emission spectrum and Gd-CS_YS₂S₃ nanoparticles absorption spectrum. (b) FTIR of Gd-CS_YS₂S₃@IR-806 and IR-806. (c) The absorption spectra of Gd-CS_YS₂S₃, IR-806, and Gd-CS_YS₂S₃@IR-806. (d) Emission spectra of Gd-CS_YS₂S₃ with and without IR-806 loading under 808 nm CW diode laser at a power density of 10 W/cm². (e) The enhancement factors of upconversion emission were obtained by comparing the results for samples with and without IR-806 loading. The emission intensities were calculated by integrating the spectral intensities in the UVC (240–280 nm), UVB (280–320 nm), UVA (320–400 nm), and visible (400–650 nm) ranges.

Figure 3

Optimizing the weight ratio of Gd-CS_YS₂S₃ to IR-806 and calculating the energy transfer efficiency. (a) The emission spectrum of Gd-CS_YS₂S₃ (4 mL in CHCl₃, 0.375 mg/mL) after adding various masses of IR-806 dye under 808 nm excitation. (b) The absorption spectrum of Gd-CS_YS₂S₃ (4 mL in CHCl₃, 0.375 mg/mL) with various masses IR-806 dye. (c) The decay curves of Gd-CS_YS₂S₃, Gd-CS_YS_{2(90%, 10%Yb)}S₃@IR-806, and Gd-CS_YS₂S₃@IR-806. (d,e) The emission spectra of Gd-CS_YS₂S₃ (4 mL in CHCl₃, 0.375 mg/mL) after adding various masses of IR-806 dye under 793 nm and 980 nm excitation, respectively.

Figure 4

The effect of the distance between IR-806 and sensitizer Nd³⁺ on upconversion emission. (a) Schematic illustration of the nanostructural design to study the distance effect on upconversion emission. (b) The emission spectra of Gd-CS_YS₂S₃, Gd-CS_YS₂S₃@IR-806, Gd-CS_YS₂, Gd-CS_YS₂@IR-806 under 808 nm excitation.

Scheme 2

Schematic illustration of the mechanism for cascade energy transfer in Gd-CS_YS₂S₃@IR-806. Upon 808 nm laser excitation, IR-806 first absorbs excitation energy and transfers it to Nd³⁺. Next, Yb³⁺ accepts the energy from Nd³⁺, contributing to populating photons in the ³P₂ state of Tm³⁺ through a continuous five-photon energy transfer process and then relaxing to the ¹I₆ state of Tm³⁺. Trapping the energy from both five-photon upconversion from Tm³⁺ and one-photon upconversion from Yb³⁺, six-photon and five-photon upconversion luminescence from ⁶D_J, ⁶I_J, and ⁶P_J state of Gd³⁺ is observed.

Figure 5

The decreased lifetime of Tm³⁺ and Gd³⁺ for Gd-CS_YS₂S₃@IR-806. (a–f) The Tm³⁺ and Gd³⁺ lifetime decay curves of Gd-CS_YS₂S₃ and Gd-CS_YS₂S₃@IR-806 at 253, 276, 290, 310, 360, and 475 nm under 808 nm excitation, respectively.