Dual-mode photoluminescent microspheres with silica core, separator layer and envelope for anti-counterfeiting ink

The microsphere is concentrically constructed on the basis of SiO2 nanocore on which Gd2O3:Yb,Er upconversion luminescent layer, SiO2 separator, Gd2O3:Ln@SiO2 (Ln3+ = Tb3+, Eu3+) downconversion luminescent layer, and SiO2 envelope are successively deposited by means of wet chemistry method. Under 980 nm near-infrared laser excitation, the microspheres emit bright upconverted green light due to Gd2O3:Yb,Er luminescent layer. While, under 254 nm portable ultraviolet lamp irradiation, they produce red and green downconversion fluorescence because of Gd2O3:Ln (Ln3+ = Tb3+ or Eu3+) luminescent layer. The silica core fixes the luminescence center, reduces non radiative transitions caused by molecular vibration, enhances fluorescence intensity, and saves rare earth resources. Impressively, the silica separator layer isolates the upconversion luminescence of Er3+ and downconversion of Eu3+, preventing cross relaxation and quenching of Er3+ fluorescence. The silica envelope plays a crucial role in the photoluminescence of the prepared microspheres. The photoluminescence quantum yield of the microspheres initially increases from 90.4 % to 95.9 % and then decreases to 82.9 % as the thickness of SiO2 layer on the ∼3 nm-thick Gd2O3:Ln shell rises from 3 nm to 16 nm. The absorption of photons reaches its maximum when the SiO2 layer is 8 nm thick. This thickness corresponds to the lowest rate constant for non-radiative transitions. The proto inks prepared from the dual-mode microspheres exhibit good performance including good luminescence and high dispersibility for practical anti-counterfeiting applications.