Evaluation of Luminescence Properties of Single Hydrophilic Upconversion Nanoparticles by Optical Trapping

By using multifunctional optical tweezers (OT) equipped with a 980 nm continuous-wave single-mode diode laser and multiple detectors, we are able to trap single upconversion nanoparticles stably as well as synchronously analyze their luminescence properties. Successive trapping of individual octylamine-modified poly(acrylic acid) encapsulated UCNPs (OPA-UCNPs) is proved by real-time monitoring of forward scattering (FSC), luminescence intensity and spectra, and the results verify that these nanoparticles possess excellent colloidal stability and uniform luminescence properties. Besides, ligand/solvent-dependent surface quenching effect of single UCNPs is investigated, and the results show that OPA-UCNPs by hydrophobic encapsulation strategy possess outstanding luminescence properties and the property of OPA to reduce the quenching effect of ligands and water molecules are well identified. Upconversion luminescence decay lifetime also explains the mechanism that the presence of OPA molecules reduces surface quenching effect, thus OPA-UCNPs exhibit longer luminescence decay lifetime. Therefore, we not only provide a new method to evaluate the luminescence properties of single nanoparticles by using multifunctional OT but also prove that encapsulating hydrophobic UCNPs with amphiphilic molecules is an alternative strategy to prepare monodisperse hydrophilic UCNPs while significantly maintaining their luminescence properties.