3.1 Synthesis and characterizations of the CDs
The water-soluble CDs were prepared by a simple one-step hydrothermal method using CA and PEI as the raw materials, as shown in Figure 2a . CA as an organic acid is widely distributed in nature, and can also be synthesized in large quantities, occupying an important position in the food industry.[24] PEI is rich in amino groups, which may render the product a positive charge.[25] After the hydrothermal reaction, a brown solution was obtained. The TEM image of the synthetic product reveals that the small particles are in good mono-dispersity and possess an amorphous structure. Statistical analysis indicates an average diameter of 3.3 nm for the small particles, with a relatively narrow size distribution (Figure 2b ). As shown in Figure 2c , the small particles appear to be light yellow in color under ambient light irradiation at a lower concentration, consistent with the occurence of the absorption band centered at 351 nm and a shoulder peak at 235 nm. The peak at 235 nm may be attributed to theπ -π * leap of C=C in the particle core.[26] Under the excitation of UV light, interestingly, a strong blue fluorescence could be seen from the solution, a consequence of the emission of the small particles. With the excitation wavelength increasing from 305 to 425 nm, the emission wavelength shifts from 435 nm to 475 nm, with the maximum intensity occurring under the excitation at 365 nm. The QY under excitation at 365 nm was calculated to be 18.6%. The excitation wavelength-dependent fluorescence emission property of the small particles can be attributed to the presence of different surface states, including functional groups and surface defects.[27]
The functional group and chemical composition of the particles were further analyzed by FTIR and XPS. As shown in the FTIR spectra (Figure 2d ), stretching vibrations of O–H/N–H (3450-3100 cm-1), –CH3/–CH2– (3000-2700 cm-1), C=O/C=N/C=C (1755-1670,1690-1640 and 1680-1620 cm-1), C–N (1420-1350 cm-1), and O–C–O (1130-1060 cm-1) can be observed. These FTIR peaks are in line with those of the previously reported CDs.[23] The FTIR characterization also rationalized the occurrence of the absorption peak at 351 nm, which can be ascribed to the n-π* leap of C=O in the surface defects.[28] The XPS spectra pointing to a carbon atomic percentage of 68.81% validate the dominant formation of the CDs (Figure 2e ). The other major elements involve N (19.57%) and O (11.62%). The high-resolution C 1s spectrum that is centered at 283.9 eV displays peaks at 284.7, 286.3 and 287.2 eV, attributable to C=C/C–C, C–N/C–O and C=N bonds,[29]respectively (Figure S1 ). The high-resolution N1s curve centered at 397.8 eV exhibits peaks at 398.7, 399.8 and 401.1eV, which can be assigned to C–N–C, N–(C)3 and N–H bonds,[30] individually (Figure S2 ). Collectively, the FTIR and XPS results indicate that the synthetic products have various bonds and functional groups. To test the surface charge property of the CDs, the zeta potential of the synthetic products was analyzed. A zeta potential of +13.4 mV (Figure S3 ) may be ascribed to the presence of abundant amino group on the surface.