One-pot synthesis of full-color carbon dots with N, O surface-state energy-gap-controlled emission

An easy and effective route for preparing full-color (blue, green, yellow and red) carbon dots (CDs) with brilliant and stable photoluminescent (PL) was developed by using gallic acid and o-Phenylenediamine as precursors. The resulting CDs were then separated by silica-gel column chromatography. The prepared four types of CDs owned high quantum yields (up to 11.2%) and exhibited attractive features such as excellent solubility, high stability, excitation wavelength-independent PL, mono-exponential PL lifetimes, low cell toxicity and homogeneous optical properties. Then, the mechanism of full-color CDs PL was explored. The results suggested that the surface-states of prepared CDs, especially the pyridine N (C = N) and C = O functional groups, were primarily responsible for the full-color PL emission. The blue CDs had the largest content of C = O while the CDs turned to be green as the C = O decreased and the C = N content increased. The surface state regulation of C = N bond was dominant in yellow CDs while red CDs possessed highest content of C = N and new -COOH appeared. The gradual substitution of C = N to C = O made the band gap of CDs narrowed, which is the main reason for the redshift of CDs PL emission peak. Importantly, the easy preparation and unique optical features of these CDs made them potentially useful in numerous applications such as full-color cell labeling, drug delivery and optoelectronic technologies.