Mitigating Aqueous-Phase Quenching in Upconversion Nanoparticles with a Gradient-Methylated Silica Coating

The nonlinear luminescence properties of rare earth (RE) based upconversion nanoparticles (UCNPs) hold significant value for biomedical imaging and sensing applications. UCNPs synthesized in high-boiling-point solvents have achieved excellent qualities such as high crystallinity, low defect rate, and controllable morphology. However, these UCNPs suffer from severe luminescence quenching after being surface-modified and transferred into aqueous media. While UCNPs passivated by optically inert epitaxial lattices show appreciable resistance to quenchers, thoroughly eliminating aqueous-phase quenching remains a key challenge due to the passivation thickness limits dictated by UCNP size. Herein, we investigate how surface modification affects UCNP luminescence and develop a methylated SiO2 coating method that mitigates aqueous-phase luminescence quenching. This method tunes the hydrophobic microenvironment inside the SiO2 shell of water-dispersible UCNPs, thereby reducing the presence of hydrophilic quenchers (e.g., Si–OH and H2O). The optimized SiO2 coating combines a methylated SiO2 inner layer and a pristine SiO2 outer layer, endowing UCNPs simultaneously with quencher-repelling property and water dispersibility. Compared with the conventional SiO2 modification, the gradient-methylated SiO2 coating method improves the luminescence intensity of NaREF4@SiO2 UCNPs in water by up to 111% and prolongs the luminescence lifetime by 22%, with both metrics outperforming those of UCNPs modified by ligand exchange or DSPE–PEG encapsulation. This work further investigates the influence of SiO2 coating on the spectral thermometry performance of Yb3+/Er3+ co-doped UCNPs. In summary, the gradient-methylated SiO2 coating method provides a considerable reinforcement for water-dispersible UCNPs.