In vitro temperature sensing with up-conversion NaYF₄:Yb³⁺/Tm³⁺-based nanocomposites: Peculiarities and pitfalls

The luminescence intensity ratio method, exploiting the temperature-dependent luminescence of the thermally coupled energy levels, is regarded as a very promising approach for optical temperature measurement at the cellular level. In this study, it was found that bare NaYF₄:Yb³⁺/Tm³⁺ nanoparticles cannot be used as a cellular thermosensor in principle because of their tendency to aggregate, which significantly affects the luminescent properties of the complex, introducing uncertainty in the intensity ratio measurement. NaYF₄:Yb³⁺/Tm³⁺ up-conversion nanoparticles, coated with polyethylene glycol (PEG) and carboxyl groups (COOH), on the other hand, proved to be promising candidates for the role of thermosensors. For the first time the temperature sensitivity of the NaYF₄:Yb³⁺/Tm³⁺@PEG@COOH thermosensor was calculated in water and in biotissues. It was found that the sensitivity of the thermosensor increased by 1.3 times during the transition from water to egg white and urine - from 1.17% × K^-1 to 1.58% × K^-1. This effect is associated with the chemical composition of the studied media. The results obtained suggest that using upconversion nanocomplexes as primary thermosensors is still difficult.