Magnetic fluorescent nanofluids obtained by a colloidal approach

Abstract

Magneto-photoluminescent nanomaterials have emerged as highly appealing systems. However, achieving colloidal stability is challenging, as most strategies produce rather large particles. Therefore, aqueous dispersions are generally prone to sedimentation or phase separations induced by an external magnetic field. We propose a novel approach involving the colloidal mixture of carbon dots (CDs) and magnetic nanoparticles (MNPs), achieved by adjusting the physicochemical parameters of the dispersions individually to produce a stable fluorescent magnetic liquid (FML). Long-term colloidal stability (even under an external magnetic field) was achieved in water by fine-tuning pH, ionic strength, and surface functionalization of magnetic fluids based on core@shell nanoparticles and fluorescent dispersions of nitrogen-rich carbon nanodots. The pH-dependent electrostatic interparticle interactions were elucidated through zetametry. A multiscale analysis based on optical microscopy and small-angle X-ray scattering confirmed the colloidal stability of the FML. These results demonstrated low interparticle interactions and the absence of irreversible aggregation. The magnetic properties were studied by SQUID magnetometry. The saturation magnetization and dipolar interactions between MNPs remained practically unaffected by the presence of CDs. On the other hand, fluorescence spectroscopy and optical absorption data showed a considerable quenching effect in the CDs emission intensity due to MNPs. Nevertheless, the fluorescence with nanosecond decay lifetimes, tunability, and emission spectrum were maintained. Additionally, we assessed the capability of the FML for magnetohyperthermia. The heat generation efficiency was preserved, showing possible use in such therapeutic applications. Further, the proposed approach can be extended to other types of nanomaterials, enabling synergistic effects and enhancing the overall performance of the resulting liquid for targeted technological applications.