Microwave preparation of carbon quantum dots with different surface modification

Carbon quantum dots (CQDs) have great potential to be utilized as an optical sensing probe due to its unique photoluminescence and less toxic properties. This work reports a simple and novel synthesis method of carbon dots via direct acid hydrolysis in presence of polyethylene glycol (PEG), polyvinylpyrrolidone (PVP) and bovine serum albumin protein (BSA). In this study, fluorescent CQDs were synthesized by using citric acid and ascorbic acid as the source of carbon precursors, which was covered with polyethylene glycol (PEG), polyvinylpyrrolidone (PVP) and with bovine serum albumin (BSA), by microwave irradiation. Furthermore, the synthesis parameters as power, reaction time and temperature were studied and quality of prepared CQDs were investigated by spectral methods. Short reaction time (20 min) and temperature from 120 ºC to 140 ºC under microwave irradiation are sufficient to prepare luminescence carbon quantum dots. Absorption spectra and photoluminescence spectra were measured to characterize prepared dots in water solution. The photoluminescence spectra of CQDs doped with different protection compound show the different luminescent and excitation wavelengths starting from 330 nm to 430 nm. Importantly, these CQDs are demonstrated to be excellent bioimaging agents and fluorescent ink due to their stable emission, well dispersibility, low toxicity, long emission life time, and good compatibility with different macromolecules.

Fig. 1.: Diagram for the synthesis of PEG, PVP and BSA-functionalized CQDs. (a) Synthesis of CQDs using citric acid and ascorbic as carbon source. (b) Capping of CQDs with PEG. (c), Capping of CQDs with PVP. (d), Capping of CQDs with BSA.

Fig. 2.: Measuring the CQDs fluorescence with excitation wavelength at 280, 330, 340, 360, 380, 400, 420 and 430 nm. Blue curve-120 ºC, red curve-130 ºC and green curve 140 ºC. Maxima of fluorescence intensity, (a) Citric acid capped with PVP, (b) Ascorbic acid capped with PVP, (c) Citric acid capped with PEG, (d) Ascorbic acid capped with PEG, (e) Citric acid capped with BSA, (f) Ascorbic acid capped with BSA.

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