## 3 Results and Discussion
### Optimization of SPME Conditions  
To further improve the extraction efficiencies towards CBs, the SPME conditions, including extraction temperature, extraction time, salt contents, and pH, were optimized with spiked aqueous solutions with a concentration of 5 ng mL\({}^{-1}\).  
Figure 4: Extraction efficiencies of pristine g-C\({}_{3}\)N\({}_{4}\) and CDs/g-C\({}_{3}\)N\({}_{4}\) composites with different doping amounts of CDs (**a**); GC-MS chromatograms of CBs by CDs/g-C\({}_{3}\)N\({}_{4}\)-3 and commercial SPME coatings (**b**). Error bars represent the standard deviation of the mean (\(n=3\)).  
Figure 3: N\({}_{2}\) adsorption-desorption isotherms (**a**) and pore diameter distributions (**b**) of g-C\({}_{3}\)N\({}_{4}\) and CDs/g-C\({}_{3}\)N\({}_{4}\), TG curve of CDs/g-C\({}_{3}\)N\({}_{4}\) (**c**).  
Extraction temperature plays an important role on the extraction efficiency for SPME. In general, increased temperature can facilitate the diffusion rate of analytes, and thus, enables the extraction process to reach equilibrium in a short time. However, excessive temperature will decrease the distribution coefficient between the sample matrix and the fiber coating for analytes, which lead to a severe diminishment of the extraction. Therefore, the extraction efficiencies towards CBs for the CDs/g-C\({}_{3}\)N\({}_{4}\) coating were evaluated in the temperature range of 30-70 \({}^{\circ}\)C. It can be seen from Figure 5a that the extraction efficiencies towards eight CBs exhibit a trend of increasing before decreasing with elevating temperature. The maximum extraction efficiencies are observed at an extraction temperature of 50 \({}^{\circ}\)C.  
Enough extraction time is indispensable to achieve the optimum extraction efficiency, due to the fact that SPME is an equilibrium-based process. Herein, the effect of extraction times ranging from 10 to 50 min on the extraction efficiencies for eight CBs was investigated. As shown in Figure 5b, the extraction efficiencies towards CBs increase, following the increase of extraction time from 10 to 30 min. With further prolonged extraction time, the extraction efficiencies maintain a constant value, which indicates that the SPME process reaches equilibrium.  
A certain salt concentration in aqueous solution can effectively decrease the analyte solubility, which leads to an increase of extraction efficiency for non-polar and weak polar compounds [41]. In this work, NaCl was used for adjusting the salt concentration in aqueous solution. The extraction efficiencies of as-prepared SPME coating towards eight CBs were studied in aqueous solution with NaCl contents varying from 0 to 30% (Figure 5c). Results show that the extraction efficiencies display a distinct increase with an increase of NaCl content from 0 to 20%. However, further increased NaCl content leads to a certain reduction in extraction efficiencies, which can be ascribed to excessive NaCl adhesion on the surface of the coating during the extraction process, resulting in the inhibition of active adsorption sites [29; 42].  
The extraction efficiencies can also be affected by solution pH due to changes in the surface charge of coating materials and analytes [43; 44]. The effect of solution pH ranging from 3 to 11 on the extraction efficiencies of as-prepared SPME coatings are evaluated  
Figure 5: Effect of extraction temperature (**a**), extraction time (**b**), salt concentration (**c**), and pH (**d**) on the extraction efficiencies of the CDs/g-C\({}_{3}\)N\({}_{4}\) SPME coating for CBs. Error bars represent the standard deviation of the mean (\(n=3\)).  
herein (Figure 5d). It can be observed that the highest extraction efficiencies were obtained with solution pH of 7. Therefore, the solution pH was set to 7 for further experiments.