Synthesis of highly luminescent β-NaYF4:Yb3+, Er3+ upconversion nanoparticles regulated by sodium hexametaphosphate: For customs supervision

Abstract

Smuggling has caused serious adverse effects on the global market and national security, among which drug smuggling is increasingly rampant. Thus, the development of high-tech security strategies has become an urgent challenge for customs supervision. Recently, lanthanide ions (Ln3+) doped upconversion nanoparticles (UCNPs) have opened up new ways for security and anti-fake applications due to their unique optical properties under near-infrared excitation. Here, we propose a novel upconversion microcrystal composed of hexagonal phase NaYF4:Ln3+ capped by sodium hexametaphosphate (SHMP), successfully used in customs supervision. The addition of SHMP can not only regulate the morphology of microcrystals, but also significantly improve the luminescence properties of UC. The absolute quantum yield is up to 7% at the low power density of 30 W∙cm−2, above most currently reported levels. Without adding any organic additives, the precise control of particle phase and morphology (e.g., microrod, hexagonal prism and hollow sphere) would be achieved by adjusting the doping ratio of SHMP/Ln3+. As the reaction time extends, the morphology of NaYF4:Ln3+ changes from ellipsoid to microrod, while cubic phase transforms into hexagonal phase. Under the excitation of 980 nm laser, the green upconversion emission of hexagonal NaYF4:Ln3+ microrods is significantly strengthened, which is more powerful than other observed particles with other shapes. This strong upconversion emission that depends on morphology and phase has huge application potential in security anti-counterfeiting and photonic devices.