Functionalized β-NaGdF4:YbIII luminescent nanothermometer based on the direct population of triplet states and NIR emission

Abstract

Lanthanide (LnIII)-doped NaGdF4 nanoparticles functionalized with organic ligands have been investigated for cell imaging and luminescence thermometry for instance due to their versatility regarding radiation conversion, e.g, near-infrared (NIR)-to-NIR, NIR-to-visible, or UV-to-NIR. Yet, in the later case, the high energy of radiation into the human tissue is an issue, aiming cell imaging application. This drawback could be surpassed by shifting the absorption bands of the organic luminescent sensitizers attached on the particle to longer wavelengths, which can be achieved by singlet (S0) → triplet (Tn) transitions instead of the classic S0 → Sn processes. Herein, this possibility is discussed for hybrid systems based on spheroidal β-NaGdF4:YbIIII nanoparticles (average diameter of 7.8 nm) functionalized on the surface with 3,5-dibromobenzoate (3,5-bbza-) coordinated to LnIII ions. The Tn states of 3,5-bbza- are directly populated upon excitation at 368 nm or 430 nm, followed by energy transfer to YbIII that emits within the NIR spectral region. On the other hand, the ligand singlet states are populated at 300 nm. Taking advantage of the intensity ratio of the emission spectrum collected upon S0 → Tn (368 nm or 430 nm) or S0 → Sn (300 nm) excitation, luminescence thermometry was performed for the hybrid system, which displays maximum relative thermal sensitivity of 0.58% K−1 at 77 K. Therefore, the S0 → Tn transitions observed in the NaGdF4:YbIIII nanoparticles functionalized with 3,5-bbza- enable visible-shifted excitation bands and luminescence thermometry, which are interesting features aiming application in cell imaging and temperature probing, respectively.