Effect of surface grafting coefficient and chain length of fatty acids on the luminescence of neodymium³⁺-doped LaF₃ nanoparticles

The challenge in synthesising lanthanide-doped nanoparticles (NPs) is the shielding of lanthanide luminescence from the quenching ions around the NPs. In this work, the effects of chain length and surface grafting coefficient (average number of molecules per nm² (ξ)) of functionalising fatty acids on the luminescence intensity of LaF₃:Nd³⁺ NPs were investigated. Nearly 14-fold enhancements in luminescence intensity were observed as the oleic acid surface grafting coefficient increased from 0 to 5.7. This increase in luminescence intensity is attributed to the shielding of the NP surfaces from surrounding quenching ions such as OH-, by the fatty acid molecules. In view of the changing ligand length, different fatty acids (decanoic acid, hexanoic acid, oleic acid, octanoic acid, and tetradecanoic acid) were used to functionalise the NPs. A 1 nm increase in fatty acid chain length resulted in a three-fold increase in luminescence intensity.