Thermal properties of nanofluids using hydrophilic and hydrophobic LiYF₄:Yb/Er upconverting nanoparticles

Luminescent nanoparticles have shown great potential for thermal sensing in bio-applications. Nonetheless, these materials lack water dispersibility that can be overcome by modifying their surface properties with water dispersible molecules such as cysteine. Herein, we employ LiYF₄:Er³⁺/Yb³⁺ upconverting nanoparticles (UCNPs) capped with oleate or modified with cysteine dispersed in cyclohexane or in water, respectively, as thermal probes. Upconversion emission was used to sense temperature with a relative thermal sensitivity of ∼1.24% K^-1 (at 300 K) and a temperature uncertainty of 0.8 K for the oleate capped and of 0.5 K for cysteine modified NPs. To study the effect of the cysteine modification in the heat transfer processes, the thermal conductivity of the nanofluids was determined, yielding 0.123(6) W m^-1 K^-1 for the oleate capped UCNPs dispersed in cyclohexane and 0.50(7) W m^-1 K^-1 for the cysteine modified UCNPs dispersed in water. Moreover, through the heating curves, the nanofluids' thermal resistances were estimated, showing that the cysteine modification partially prevents heat transfer.