Morphology and microstructure of O, N-CDs were characterized by means of transmission electron microscopy (TEM, Jeol JEM-2100F, Tokyo, Japan, 200 kV) and atomic force microscopy used in tapping mode (AFM, Integra Prima Basic, NT-MDT LCC, Moscow, Russia). UV-visible absorption spectra were recorded with an UV-visible spectrophotometer (UV-2700, Shimadzu, Kyoto, Japan). Raman spectrum of O, N-CDs deposited on a previously cleaned Si wafer was collected with an integrated confocal micro-Raman system (LabRAM ARAMIS μ-Raman spectrometer, Horiba-Jobin Yvon, Inc., Edison, NJ, USA) equipped with a diode-pumped 633 nm solid-state laser [27 (link)]. Fourier-transform infrared attenuation reflection (FTIR ATR) spectrum was recorded with an IR spectrometer (Prestige-21, Shimadzu, Kyoto, Japan) equipped with an ATR accessory (MIRacle, PIKE Technologies, Fitchburg, WI, USA). Zeta potential (ZP) of a colloid solution of O, N-CDs was measured at 21 °C with a particle size and zeta potential analyzer (Zetasizer Nano analyzer, Malvern Pananalytical, Malvern, UK).
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