Figure 2

(a) Fluorescence spectra of the quantum dots as a function of the excitation wavelength. The corresponding absorbance spectrum including an image of the GQD solution under excitation with blue light is shown in the inset. (b) Integrated PL intensity vs. absorbance of fluorescein and GQDs to determine the GQD quantum yield of 23% at 360 nm excitation. (c) Comparison of the fluorescence spectrum of freshly prepared GQDs and GQDs stored for 96 h in the incubator in a humidified atmosphere at 5% CO2 and 37 °C dissolved in cell culture medium at 360 nm excitation.