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.