In this work, we present the fabrication of highly luminescent carbon dots (CDs) by a double passivation method with the assistance of Ca(OH)2. In the reaction process, Ca2+ protects the active functional groups from overconsumption during dehydration and carbonization, and the electron-withdrawing groups on the CD surface are converted to electron-donating groups by the hydroxyl ions. As a result, the fluorescence quantum yield of the CDs was found to increase with increasing Ca(OH)2 content in the reaction process. A blue-shift optical spectrum of the CDs was also found with increasing Ca(OH)2 content, which could be attributed to the increasing of the energy gaps for the CDs. The highly photoluminescent CDs obtained (quantum yield: 86%) were used to cultivate fluorescent carnations by a water culture method, while the results of fluorescence microscopy analysis indicated that the CDs had entered the plant tissue structure.

Novel fluorescent carbon dots (CDs) were fabricated using the hydrothermal carbonisation method by peat, which is a very abundant and low-cost natural material in the world. The transmission electron microscopy images revealed that the peat-derived CDs (PCDs) are well-crystalline with an average size of 4 nm. Fourier transform infrared and X-ray photoelectron spectra suggested large amounts of oxygenous functionality which made the synthesised PCDs water-soluble. The optical characterisations turned out that PCDs showed a stable and excellent excitation-dependent photoluminescence (PL), while the quantum yield was calculated to be 18.3%. As the PL of the PCDs could be efficiently quenched by ferric ions with a low-detection limit of 20 nM, the as-prepared PCDs could be employed as a highly fluorescent probe for ferric ions. Furthermore, the obtained PCDs have been successfully used for fluorescent imaging and Fe 3+ detecting at cell level.

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