Full-colors by luminescence induced by UV excitation in carbon dots synthesized by graphite laser ablation and oxidation in liquids

A Graphite laser ablation in biocompatible phosphate buffer solution (PBS) has been produced at different laser fluences, synthesizing carbon dots (CDs) with tunable photoluminescence (PL). The used laser has been a repetitive ns pulsed Nd:YAG operating at 1064 nm. By exciting with a single-wavelength UV light at 365 nm, CDs emit stable luminescence in gradient colors, from blue to red. The plasma generated by the laser at the carbon-liquid interface was investigated, with the objective of giving more information on the type of CDs synthesized and on their optical properties. Optical spectroscopy was employed to analyze the CDs PL between 250 nm and 800 nm. The dispersions were submitted to UV–Vis transmittance and absorbance spectroscopies. The CDs deposition on a silicon substrate, after drying, has been analyzed with attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Results indicated that the emission band depends on the laser fluence and the different oxidation processes induced in the PBS solution. The quantum yield (QY) has been evaluated in the order of 15% using a neutral pH solution and about 30% using an acidic pH solution. The observed luminescence band emission shift, from about 400 nm to about 670 nm, due to a reduction of the laser fluence, has been ascribed to a gradual reduction in the CDs band gaps with the increasing incorporation of oxygen species into their surface structures. Due to the high biocompatibility, stability, low cytotoxicity, and excellent PL properties, these CDs could be used in bio-medicine and in other fields, as will be presented and discussed.