Effects of surface modification and SiO2 thickness on the optical and superparamagnetic properties of the water-soluble ZnS:Mn2+nanowires/Fe3O4quantum dots/SiO2 heterostructures

Abstract

In this paper, one-dimensional ZnS:Mn²⁺ nanowires (NWs)/Fe₃O₄ quantum dots (QDs)/SiO₂ heterostructures were successfully synthesized by the Stöber method to form the water-soluble fluorescent/superparamagnetic nanocomposites. The average diameter of the ZnS:Mn²⁺ NWs, Fe₃O₄ QDs and ZnS:Mn²⁺ NWs/Fe₃O₄ QDs/SiO₂ heterostructures was about 6–8 nm, 4–5 nm and 18 nm, respectively. The Fe₃O₄ QDs were covalently linked to the ZnS:Mn²⁺ NWs by the conjugation of the hydroxyl groups on the surface of the QDs and the carboxyl groups modified on the surface of the NWs. It was found that the covalent bonds between the NWs and QDs could effectively suppress the energy transfer from the ZnS:Mn²⁺ NWs to the Fe₃O₄ QDs. As the SiO₂ shell thickness increased, the fluorescence intensity reached the highest value when the hydrolysis time of tetraethyl orthosilicate was 5 hours, which was comparable to that of the ZnS:Mn²⁺ NWs. The superparamagnetic properties of the heterostructures were observed at room temperature, which decreased as the SiO₂ thickness increased.