Harnessing lanthanides for blue-to-UVB upconversion and its dye-sensitization

Abstract

Narrowband UVB light (λ ∼ 310 nm) is broadly applied in topical phototherapies. However, conventional UVB phototherapies have the risk of overexposing healthy skin to UV toxicity. This paper demonstrates the scientific principles in support of an alternative phototherapy protocol, where the UVB radiation is on-site generated by an organic–inorganic hybrid upconverter with a blue light source. Leveraging the intrinsic non-linear nature of photo-upconversion, the process is mechanistically triggered by the two-photon excitation of Ho³⁺ and the subsequent resonant energy transfer to nearby Gd³⁺ ions in the NaYF₄ host lattice. A sharp upconverted UVB emission peaking at 310 nm is detected from the Gd^{3+ 6}P_7/2 excited state. However, the upconversion output is inherently limited by the parity-forbidden intra-4f¹⁰ electronic transitions of Ho³⁺. To boost the blue-to-UVB upconversion, a perylene derivative (PPA2) with a small Stokes shift is deployed as an organic sensitizer. The sensitization effect is realized through Förster resonant energy transfer from PPA2 to the NaYF₄:Ho³⁺,Gd³⁺ nanoparticles. Under optimized conditions, UVB upconversion of the hybrid nanomaterials outperforms the unsensitized case by a factor of 27. Such sensitized blue-to-UVB upconversion fundamentally consolidates the concept of blue light-stimulated on-site phototherapy, paving the way towards safer therapies in dermatology.