Preparation of NaYF4:Yb3+,Tm3+@NaGdF4:Ce3+,Eu3+ double-jacket microtubes for dual-mode fluorescent anti-counterfeiting

Novel hydrophilic NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺ double-jacket microtubes (DJMTs) with upconversion/ downconversion dual-mode luminescence were designed and prepared through epitaxial growth of NaGdF₄:Ce³⁺, Eu³⁺ shell onto the NaYF₄:Yb³⁺,Tm³⁺ microtube via poly(acrylic acid) (PAA) mediated hydrothermal method. It is demonstrated that PAA ligand played an important role in guiding the direct growth of NaGdF₄:Ce³⁺,Eu³⁺ shell onto the surface of NaYF₄:Yb³⁺,Tm³⁺ parent microtubes. The growth of NaGdF₄:Ce³⁺,Eu³⁺ shell experienced a crystal phase transition from β-NaGdF₄ and β-NaYF₄ mixture to β-NaYF₄@NaGdF₄ composite crystal, and morphology evolution from mixture of β-NaGdF₄:Ce³⁺,Eu³⁺ nanorods and β-NaYF₄:Yb³⁺,Tm³⁺ microtubes to NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄: Ce³⁺,Eu³⁺ DJMTs. The formation mechanism of DJMTs was the dissolution?renucleation of β-NaGdF₄:Ce³⁺,Eu³⁺ nanorods and the growth of β-NaGdF₄:Ce³⁺,Eu³⁺ shell via the classical Ostwald ripening mechanism. The as-prepared DJMTs could exhibit blue upconversion and red downconversion luminescence, which was further made into environmentally benign luminescent inks for creating highly secured and fluorescent-based anti-counterfeiting patterns via inkjet printing.