Synthesis of Mn2+ and Pr3+ co-doped Zn2GeO4-graphene quantum dot nanorod bundles as an optical probe for sensing pH and temperature and for information encryption

Abstract

This paper reports the synthesis of a Mn²⁺ and Pr³⁺ co-doped Zn₂GeO₄ optical probe by introducing boron-doped and histidine- and serine-functionalized graphene quantum dots (BHS-GQD) to Mn²⁺/Pr³⁺-Zn₂GeO₄. The resulting Mn²⁺/Pr³⁺-Zn₂GeO₄-BHS-GQD have a rod-like nanostructure, with rods that have an average length of 378 ± 1.8 nm and a width of 11 ± 0.8 nm, where the rods self-assemble into nanorod bundles. The Mn²⁺/Pr³⁺-Zn₂GeO₄-BHS-GQD can produce a strong green fluorescence emission under the excitation of 254 nm ultraviolet light. The fluorescence emission strongly depends on the pH and ambient temperature. Increases of pH in the ranges 3.0–5.8 and 6.8–8.0 bring a slow increase in the fluorescence intensity. However, a sudden leap in the fluorescence intensity appears at pH values between 5.8 and 6.8, indicating good pH-switching properties. These properties should be attributed to the acid–base substitution reaction that occurs at the interface between the Mn²⁺/Pr³⁺-Zn₂GeO₄-BHS-GQD and the aqueous solution. At a relatively low pH value, the H⁺ ions in the aqueous solution can replace the GeO₄⁴⁻ ions in the Mn²⁺/Pr³⁺-Zn₂GeO₄-BHS-GQD to form weakly acidic species, H₄GeO₄, leading to the reduction in the peak fluorescence intensity. In addition, the study reveals that an increase of the temperature between 20 and 80 °C causes a linear decrease in the peak fluorescence intensity and that this fluorescence intensity change exhibits an excellent reversibility when the temperature is varied. The Mn²⁺/Pr³⁺-Zn₂GeO₄-BHS-GQD have been satisfactorily used as an optical probe for sensing pH and temperature and for encrypting information.