Temperature-dependent Förster resonance energy transfer from upconversion nanoparticles to quantum dots

The Förster resonance energy transfer (FRET) from the β-NaYF₄:Yb³⁺, Er³⁺/NaYF₄ upconversion nanoparticles (UCNPs) to ZnCdSe/ZnS quantum dots (QDs) as a function of temperature (77-427 K) is demonstrated. With an increasing of temperature, both the intensity and peak position of QDs emission variated, which is attributed to the combining of the FRET and thermal quenching effect. By analyzing the dependence of the photoluminescence (PL) spectra on temperature, the UCNP + QD sample can be considered as dual thermal probes with high sensitivity based on either the UCL or the spectral shift of QD emission under 980 nm excitation. The lifetime of the UCNP and UCNP + QD samples are collected to investigate the dynamics of the FRET at various temperatures, showing a decrease and then an increase of the FRET efficiency from UCNPs to QDs with temperature from 77 to 427 K. This result is mainly attributed to the variation of the Förster distance R₀ with the increasing of temperature. The work will be significant to detect the nanoscale interaction and it can be widely applied in biomedical, sensing and imaging.