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.