## 3 Results and Discussion  
### Characterizations of CDs/g-C\({}_{3}\)N\({}_{4}\) Composites  
The microtopography, structural compositions, chemical groups, specific surface area, and pore size distribution of materials can reveal the formation of microstructures and the active adsorption sites of composites. Therefore, various characterizations were carried out to investigate the properties of CDs/g-C\({}_{3}\)N\({}_{4}\) composites, including SEM, TEM analysis for microtopography, XRD, FTIR, and XPS analysis for crystal structures and chemical group compositions, and N\({}_{2}\) adsorption-desorption and pore size distribution isotherms for surface area and porosity. The SEM and TEM were used for investigating the microtopographies of CDs/g-C\({}_{3}\)N\({}_{4}\) composites (Figure 1). As shown in Figure 1a, as-prepared CDs/g-C\({}_{3}\)N\({}_{4}\) composites present the irregular stacked structure composed of lamellae. TEM images (Figure 1b,c) show that as-prepared CDs display a circular structure with diameters of approximately 2 nm (Figure 1c (inset)), and the CDs are uniformly distributed on the surface of the g-C\({}_{3}\)N\({}_{4}\) sheets without visible aggregation. The interactions of CDs and g-C\({}_{3}\)N\({}_{4}\) can be ascribed to \(\pi\)-\(\pi\) stacking [32]. The optical microscope image (Figure 1d) shows that the uniform coating is formed on the surface of the stainless-steel wire, and the coating thickness is calculated to be approximately 35 \(\mu\)m.  
To investigate the crystal structures of CDs/g-C\({}_{3}\)N\({}_{4}\) composites, XRD patterns were recorded for pristine g-C\({}_{3}\)N\({}_{4}\) and CDs/g-C\({}_{3}\)N\({}_{4}\)-3 (Figure 2a). It can be seen that the XRD patterns display two peaks at 13.1\({}^{\circ}\) and 27.6\({}^{\circ}\), which can be ascribed to the planes of (100) and (002), respectively, referring to g-C\({}_{3}\)N\({}_{4}\) (JCPDS 87-1526) [33]. The stronger peak, at 27.6\({}^{\circ}\), is derived from inter-planar stacking of the aromatic system, and the diffraction peak at 13.1\({}^{\circ}\) describes the formation of in-plane repeated tri-s-triazine units [34; 35]. There is no diffraction peak recorded that corresponds to CDs, due to their low contents on the composites, and the doping of CDs does not change the crystal structure of g-C\({}_{3}\)N\({}_{4}\). Figure 1: SEM (**a**) and TEM (**b**,**c**) images of CDs/g-C\({}_{3}\)N\({}_{4}\) composites, magnified TEM image (**c** (inset)) of CDs anchored on composites, and optical microscope image of as-prepared SPME fiber (**d**). Figure 2b exhibits the FTIR spectra of pristine g-C\({}_{3}\)N\({}_{4}\) and CDs/g-C\({}_{3}\)N\({}_{4}\) composites. Multiple peaks from 1200 to 1700 cm\({}^{-1}\), which formed both on g-C\({}_{3}\)N\({}_{4}\) and CDs/g-C\({}_{3}\)N\({}_{4}\) composites, correspond to the stretching vibration of heptazine-derived repeating units and C-N or C-NH-C groups. The peaks at 806.1 cm\({}^{-1}\) reflect the flexural vibrations of triazine units, and the wide peak that emerges at 3000-3500 cm\({}^{-1}\) is ascribed to NH\({}_{2}\) or NH groups [33; 36]. In comparison with g-C\({}_{3}\)N\({}_{4}\) and CDs/g-C\({}_{3}\)N\({}_{4}\) composites, the obtained FTIR spectra are similar in shape, which indicates that the decoration of CDs does not change the chemical structures of g-C\({}_{3}\)N\({}_{4}\). It should be noted that the adsorption peaks are enhanced after the CD-decoration, with the result that more abundant chemical groups are formed. XPS analysis was performed to determine the chemical composition and surface groups of CDs/g-C\({}_{3}\)N\({}_{4}\) composites. As shown in the survey scan spectrum (Figure 2c), three peaks are observed at 288.3, 398.8, and 532.2 eV for CDs/g-C\({}_{3}\)N\({}_{4}\), which are attributed to C 1 s, N 1 s, and O 1 s, with atom contents of 47.17%, 46.62%, and 6.21%, respectively.