Facile preparation of fluorescent carbon quantum dots from denatured sour milk and its multifunctional applications in the fluorometric determination of gold ions, in vitro bioimaging and fluorescent polymer film

In the present study, the multifunctional applications of food waste derived carbon quantum dots (CQD) have been demonstrated. The results obtained from the absorption and fluorescence spectroscopic analysis affirmed the superior photophysical property of the prepared CQD. X-ray diffraction (XRD), Fourier transformed infra-red spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) studies further confirmed the crystallographic phase and surface functional groups present in the CQD. The ultrafine size of the CQD is established from the high-resolution transmission electron microscope (HR-TEM) micrographs with the grain size of 2−4 nm. The excellent fluorescence property of the CQD is utilised to detect trace levels of Au3+ ions by simply mixing aqueous dispersion of CQD with ascorbic acid (AA). The sensing mechanism is based on the reduction of Au3+ to gold nanoparticles (AuNPs) by AA and the subsequent AuNPs induced fluorescence quenching of CQD by synergistic static quenching and inner filter effect. The CQD/AA probe could selectively detect Au3+ ion as low as 0.95 μM. The cytotoxicity of the CQD toward U-251 MG glioblastoma cell revealed that the CQD did not possess any significant cytotoxicity. The application of CQD in bioimaging of cells is ascertained by fluorescence microscopic analysis. Furthermore, fluorescent polymer films were prepared by incorporating CQD within the polymer matrix (poly-vinyl-alcohol).