Synthesis of carbon dots from yogurt and their sensing applications

Carbon dots are fascinating nanomaterials due to their straightforward synthesis, unique optical properties, sensing capability, and biocompatibility. In this work, biocompatible carbon dots were prepared from yogurt using a two-step pyrolysis/hydrothermal method. They were spherical in shape with an average size of 4.7 nm. They showed blue emission under UV illumination with a quantum yield of 1.5%. Their photoluminescence was stable over 3 months and in strong buffer solutions and highly concentrated salt solutions. As optically unique nanomaterials, both optical absorption and photoluminescence properties of carbon dots were employed as vapor and metal ion sensors, respectively. For the first time, the carbon dots were integrated in a home-made optical electronic nose for the detection of formic acid vapor at room temperature. The sensing principle was based on monitoring optical transmission through the carbon dot film upon exposure to vapor, also confirmed by UV-visible spectroscopy. The carbon dot-integrated electronic nose was able to distinguish vapor from different formic acid/water solutions at various concentrations with a detection limit of 7.3%. The sensitivity of carbon dots towards metal ions was also tested by measuring photoluminescence emission intensity at various excitation wavelengths. The principal component analysis was used to differentiate the metal ions and suggested that interaction between carbon dots and metals ions occurred through various binding sites. The biocompability of carbon dots was also excellent. Based on these results, carbon dots from yogurt are multipurpose fluorescent nanomaterials for sensing and biomedical applications.