PDDA-Assisted\nSynthesis of Magnetic Fluorescent Fe<sub>3</sub>O<sub>4</sub>@SiO<sub>2</sub>–CQD Composites

Magnetic fluorescent nanomaterials have broad application\nprospects\nas taggants in fields such as anticounterfeiting identification, suspicious\nobject tracking, and potential fingerprint recognition in forensic\nmedicine. It is a common method to synthesize magnetic fluorescent\ncomposite nanoparticles by preparing a shell on the surface of magnetic\nparticles to load fluorescent materials. In this work, a magnetic\nfluorescence nanohybrid was synthesized by in situ encapsulation of\ncarbon quantum dots (CQDs) during the preparation of a SiO<sub>2</sub> shell on the surface of Fe<sub>3</sub>O<sub>4</sub> nanoparticles.\nThe traditional Stöber method was employed to synthesize the\nSiO<sub>2</sub> shell. Meanwhile, CQDs were introduced into the hydrolysis\nprocess of tetraethyl orthosilicate. To address the issue of charge\nrepulsion between the negatively charged hydrolysis intermediate of\ntetraethyl orthosilicate and the negatively charged CQDs, poly(diallyldimethylammonium\nchloride) with positive charge was introduced in the synthesis process.\nThe repulsive charges in the reaction system were balanced by poly(diallyldimethylammonium\nchloride), allowing for the successful encapsulation of CQDs in the\nSiO<sub>2</sub> shell. The structure, morphology, and magnetic and\nfluorescence properties of the prepared Fe<sub>3</sub>O<sub>4</sub>@SiO<sub>2</sub>–CQDs were characterized by X-ray diffraction,\nscanning electron microscopy, transmission electron microscopy, a\nvibrating sample magnetometer, and a fluorescence spectrophotometer.\nThe Fe<sub>3</sub>O<sub>4</sub>@SiO<sub>2</sub>–CQDs exhibited\nexcellent magnetic and fluorescence properties, making them suitable\nfor fluorescence labeling on various substrates and showing great\npotential in labeling, tracing, and fluorescence sensors.