Fast coating of hydrophobic upconversion nanoparticles by NaIO₄-induced polymerization of dopamine: Positively charged surfaces and in situ deposition of Au nanoparticles

In this work, hydrophobic upconversion nanoparticles (UCNPs) were transformed into hydrophilic NPs via the rapid oxidative polymerization of dopamine (DA) induced by NaIO4 in a water-in-oil microemulsion system. The polydopamine (PDA)-coated UCNPs synthesized using NaIO4 as the oxidant exhibited a positive charge (+26.4 mV) at pH 7.0. Those prepared using ambient O-2 as the oxidant had a negative charge (-15.4 mV). The difference in the composition between PDA-coated UCNPs using NaIO4 (UCNPs@PDA-NaIO4) and those using ambient O-2 (UCNPs@PDA-O-2) as oxidants was supported by the XPS and FTIR results. The difference in the surface charges may be due to variation in the composition, which probably attributed to the different oxidants used. A series of controlled experiments were carried out to infer the origination of the positive charges of UCNPs@PDA-NaIO4. Furthermore, Au nanoparticles (Au NPs) were deposited on UCNPs@PDA-NaIO4 where PDA served as both the reductant and the stabilizer so that additional reagents and thermal treatments were not necessary. The effect of pH on the deposition of Au NPs was also studied. In addition, the load density of Au NPs on the surface of the PDA-coated UCNPs could be adjusted by changing the dosage of HAuCl4, which could effectively enhance the upconversion luminescence (UCL) intensity. This work provides a method to regulate the surface charge of the PDA coating, and the nanocomposites with unique properties synthesized by this method may be employed in biological imaging, diagnostic analysis, and other biological fields.