Ascorbic acid mediated fluorescence emission of MnO modified upconversion nanoparticles for anti-counterfeiting.

Anti-counterfeiting ink can prevent important documents from being forged or tampered with. We reported a strategy to improve upconversion luminescence intensity of NaYF:18%Yb,0.5%Tm core nanoparticles (NPs) by coating the NaYF shell. We synthesized NaYF:18%Yb,0.5%Tm core NPs and NaYF:18%Yb,0.5%Tm/NaYF core-shell NPs by high temperature thermal decomposition method. In comparison with the core NPs, the upconversion luminescence intensity of the core-shell NPs was enhanced by 2.3 times in the wavelength range of 445 nm to 495 nm. We designed composite nanomaterials based on NaYF:Yb,Tm/NaYF core-shell NPs and MnO, and synthesized NaYF:Yb,Tm/NaYF@MnO composite NPs by physical doping method. Here, MnO acts as a quencher to quench the upconversion fluorescence of Tm ions of the core-shell NPs. Afterwards, we used the prepared product for document anti-counterfeiting. And then reducing agent (AA) can destroy the structure of MnO to restore the upconversion luminescence of Tm ions. We use NaYF:18%Yb,0.5%Tm/NaYF@MnO composite NPs as anti-counterfeiting ink to write the letter "L". Under ambient conditions or the irradiation of 980 nm continuous light, "L" does not emit light. After AA is evenly applied on the letter "L", "L" can emit blue fluorescence under the irradiation of 980 nm continuous light. These results showed that NaYF:Yb,Tm/NaYF@MnO composite NPs can be used in important document anti-counterfeiting tasks to enhance information security.