Preparation Of Multicolor Carbon Dots For Cell Imaging

Due to the advantages of small size, light stability, good biocompatibility and lowtoxicity, tunable excitation and emission spectra, carbon dots attracted widespread attention inbiomedical imaging. However, compared with quantum dots and organic dyes, carbon dotshas lower fluorescence quantum yield. Therefore, it is significant to prepare carbon dots withhigh fluorescence quantum yield and good optical properties.First of all, two types of water-soluble multicolor carbon dots have been prepared by thereactions of egg white or milk with glucose by hydrothermal method. The CDs derived fromegg white/glucose (E-CDs) and milk/glucose (M-CDs) were purified by filtering membraneand column chromatography. The results show that the two types of multicolor CDs preparedwith hydrothermal methods were highly water-soluble, and have particles size of2.5nm and4.9nm, respectively, which can emit bright multicolor fluorescence under the excitation ofUV-vis light. Their maximum excitation wavelength is250nm, whereas the maximumfluorescence emission wavelengths were located at400nm and420nm for E-CDs andM-CDs, respectively. Meanwhile, the obtained CDs exhibited upconversion fluorescenceproperties at the excitation wavelength of660~800nm. Moreover, the obtained CDs havegood stability in the pH range0f2~11and good light stability under405nm laser light. Theemission peak position from both CDs shifted to longer wavelengths and the intensitygradually decreased with increased excitation wavelengths. FTIR spectra analysis revealed thepresence of-COOH,-NH2and–OH on both of CDs’ surface. Cell imaging resultsdemonstrated that fluorescence in the cells cultured with CDs was clearly visible under405nm and488nm laser irradiation, respectively, and both of the CDs showed very lowcytotoxicity when the CDs’ concentration is less than2.5mg/mL.Next, carbon dots have been prepared by the reactions of BSA with formic acid, aceticacid, formaldehyde and BSA itself by hydrothermal method. The obtained have been purifiedby filtering membrane and column chromatography. The quantum yield of CDs derived fromBSA reacted with formic acid is17.1%, which is the highest one. The average particle size ofobtained CDs is2.15nm, whereas the maximum fluorescence excitation and emissionwavelengths were located at320nm and407nm respectively. Meanwhile, the obtained CDsexhibited upconversion fluorescence properties at the excitation wavelength of660~800nm.Moreover, the obtained CDs have good stability in the pH range0f2~11and good lightstability under405nm laser light. Cell imaging results demonstrated that fluorescence in thecells cultured with CDs was clearly visible under405nm and800nm laser irradiation,respectively, which can be applied to the two-photon imaging, and the CDs showed very lowcytotoxicity when the CDs’ concentration is less than5mg/mL. Finally, carbon dots have been prepared by the reactions of eighteen kinds of aminoacids reacted with formic acid by hydrothermal method. The quantum yield of CDs derivedfrom tryptophan reacted with formic acid is58.4%, which is the highest one. The averageparticle size of obtained CDs is1.7nm, whereas the maximum fluorescence excitation andemission wavelengths were located at360nm and445nm respectively. The obtained CDsexhibited upconversion fluorescence properties at the excitation wavelength of660~800nm.The fluorescence intensity of Try/formic-CDs is gradient descent in the pH range of2~11andremains stable under405nm laser light. The Try/formic-CDs has been grafted with TATpeptide, which is a kind of nucleus targeted peptides. Cell imaging results show thatTry/formic-CDs can be imaging in in the nucleus.