Ascorbic acid mediated fluorescence emission of MnO2 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 NaYF4:18%Yb3+,0.5%Tm3+ core nanoparticles (NPs) by coating the NaYF4 shell. We synthesized NaYF4:18%Yb3+,0.5%Tm3+ core NPs and NaYF4:18%Yb3+,0.5%Tm3+/NaYF4 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 NaYF4:Yb3+,Tm3+/NaYF4 core–shell NPs and MnO2, and synthesized NaYF4:Yb3+,Tm3+/NaYF4@MnO2 composite NPs by physical doping method. Here, MnO2 acts as a quencher to quench the upconversion fluorescence of Tm3+ 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 MnO2 to restore the upconversion luminescence of Tm3+ ions. We use NaYF4:18%Yb3+,0.5%Tm3+/NaYF4@MnO2 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 NaYF4:Yb3+,Tm3+/NaYF4@MnO2 composite NPs can be used in important document anti-counterfeiting tasks to enhance information security.