Dual-emitting ZIF-8⊃4-MU@Zn2GeO4:Mn2+ nanocomposites for ratiomertic luminescent thermometer and information encryption

Abstract

Dual-emitting ratiometric optical thermometers with the advantages of excellent self-calibration and high-sensitivity have become a research hot spot. Herein, a dual-emissive 4-methylumbelliferone encapsulated ZIF-8 (ZIF-8⊃4-MU) modified Zn2GeO4:Mn2+ nanocomposite was simply prepared by adding Zn2GeO4:Mn2+ nanorods into a methanol solution containing 4-methylumbelliferone (4-MU), zinc (II) nitrate hexahydrate, and 2-methylimidazole. Accompanying with the self-assembly of ZIF-8⊃4-MU, the binding affinity of Zn2+ ions to the surface of Zn2GeO4:Mn2+ nanorods enables the formation of ZIF-8⊃4-MU@Zn2GeO4:Mn2+ nanocomposite. The photoluminescence violet-blue (420 nm) and green emissions (531 nm) of the nanocomposite can be ascribed to the two independent luminescent centers of 4-MU dyes confined in ZIF-8 and the characteristic transition of 4T1-6A1 for Mn2+ ions doped in Zn2GeO4, respectively. The fluorescent intensity ratios (FIRs) of 4-MU to Mn2+ (I420 nm/I531 nm) showed excellent self-calibrating temperature monitoring functions with a maximum relative sensitivity (Sr) of 1.82%·oC−1 and stable reusability in the temperature range of 20–100 °C. Also, the ZIF-8⊃4-MU@Zn2GeO4:Mn2+ hybridized polyvinyl alcohol (PVA) transparent film showed a similar multi-emitting behavior, as well as good processability and tensile properties. Benefiting from the excitation-dependent emitting property of ZIF-8⊃4-MU@Zn2GeO4:Mn2+, a multiple optical encryption of digital information was successfully realized based on a two-dimensional code, achieving an enhanced security level for preventing counterfeiting. This proof-of-concept study offers a feasible assembled way in fabricating multi-mode luminescent nanocomposites for ratiometric temperature sensing and information encryption.