*3.5. 3D Response Surface*

For the response surface test, the three-dimensional surface of the interaction term of two factors is shown in Figure 9. The steeper the slope change of the 3D surface, the denser the contour line, and more like an oval or saddle shape, the more significant the interaction between the two factors. As shown in Figure 9, the contour lines of the AB, AC, AD, BC, BD, and CD interaction terms were basically saddle-shaped or elliptic, and the contour map of the AD interaction terms was typical elliptic. The contour map of the AC, AD, BC, BD, and CD interaction terms was elliptic, but the slope change of the 3D surface of the interaction terms in group BD was relatively gentle compared to those in groups AB, AC, AD, and BC. This is consistent with the results of the analysis of variance in Table 5.

**Figure 9.** 3D response surface. Notes: (**a**) the reciprocal response of A and B, (**b**) the reciprocal response of A and C, (**c**) the reciprocal response of A and D, (**d**) the reciprocal response of B and C, (**e**) the reciprocal response of B and D, (**f**) the reciprocal response of C and D.

#### *3.6. Microstructure of Plant Material*

The residues of different extraction methods were observed by scanning electron microscopy (SEM). As shown in Figure 10, it can be seen that the cells of *A. dahurica* medicinal materials obtained by DES ultrasonic assisted treatment were the most seriously broken, indicating that DES had the highest efficiency in the treatment of *A. dahurica*. This result is the same as that shown in Section 3.1. The degree of cell fragmentation was proportional to the extraction rate. By contrast, the outer surface of the sample powder was significantly cracked by pulsed ultrasonic treatment, and loose and broken structures could be observed. This is similar to what Sukor et al. observed when extracting phenolic acids using ultrasonic processing of an ionic liquid probe [28], and Chen et al. used ultrasoundassisted extraction methods for imperatorin and isoimperatorin from *A. dahurica* roots [22].

**Figure 10.** The field emission scanning electron microscope images: (**a**) unextracted *A. dahurica* powder, (**b**) 75% ethanol ultrasound, (**c**) methanol ultrasonic, and (**d**) the selected DES ultrasound.

Organic solvent extraction, microwave-assisted extraction, and ultrasonic extraction have been widely used in the extraction of single medicinal materials [29]. In addition, Feng et al. combined ultrasonic-assisted extraction and microwave-assisted extraction technology to rapidly determine the essential oil of *A. dahurica* [30]. Ultrasonic-assisted extraction is a technique based on cavitation phenomena or mechanical waves generated by highfrequency impulses [31]. This force can break down plant cell walls and reduce particle size, promote swelling, and extract solvents to penetrate cells and release bioactive compounds from vacuoles [32,33]. Therefore, ultrasound-assisted extraction of coumarins compounds from *A. dahuricae* with DESs can replace the traditional organic solvent extraction method, becoming a green and efficient extraction method.

### *3.7. Antioxidant Activity of Extracts*

DPPH has been widely used to determine the antioxidant capacity of biological samples, pure compounds, and extracts in vitro. This method is simple and can be used to evaluate the antioxidant activity of the samples [28]. With VC as the control, the DPPH radical scavenging activities of the two different treated solutions are shown in Figure 11.

**Figure 11.** Comparison of DPPH radical scavenging activity in different solutions. (**a**) VC reference solution; (**b**) DES solution; (**c**) 75% ethanol solution.

As shown in Figure 11, with the increase in the concentration of different sample solutions, the scavenging ability of antioxidants in different solutions on DPPH free radical gradually increased, and gradually leveled off after reaching a certain concentration. The clearance rate of VC to DPPH tended to be stable when the concentration was 50 μg/mL, and reached the maximum at 100 μg/mL, with a maximum clearance rate was 99.54%. The DPPH clearance rate of DES extract tended to be stable at 20 mg/mL, and the maximum clearance rate was 97.22%. When reflux extraction with a concentration of 75% ethanol reached the specified concentration, its clearance rate was lower than that of DES solution (79.83%). According to SPSS software analysis, the IC50 values of VC extract, DES extract, and 75% ethanol heated reflux extract were 0.289 μg/mL, 0.477 mg/mL, and 0.772 mg/mL, respectively. In conclusion, VC control solution has the strongest DPPH radical scavenging ability. The scavenging ability of DES extract was better than that of 75% ethanol extract. The antioxidant capacity of 75% ethanol extract was not as good as that of DES solution, but it still had a significant effect.

#### **4. Conclusions**

This study aimed to provide a more green, efficient, economic, and environmentally protective target ingredient extraction method, which could better promote the in-depth development of Chinese herbal *A. dahurica* resources and provide data support for the development of green chemistry and the study of natural products in China. DESs were used to extract coumarin compounds from *A. dahurica* with ultrasonic assistance. A rapid and green extraction method for coumarins such as xanthotoxol, psoralen, byakangelicin, bergapten, oxypeucedanin, imperatorin, and isoimperatorin from *A. dahuricae* was developed. Experiments showed that the ultrasonic-assisted DES system is an effective system for extracting coumarin compounds from *A. dahurica*, which solves the problems of environmental pollution and solvent residue in extracting natural active ingredients of traditional Chinese medicine with traditional organic solvents.

**Author Contributions:** Conceptualization, T.W. and Q.L.; data curation, T.W.; formal analysis, T.W.; investigation, T.W. and Q.L.; methodology, T.W. and Q.L.; project administration, Q.L.; supervision, T.W. and Q.L.; validation, T.W.; writing—original draft, T.W.; writing—review and editing, T.W. and Q.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Discipline Construction Fund Project of Gansu Agricultural University (GSAU-XKJS-2018-086), the National Science Foundation of China (31860102), Young Talents introduction projects (GSU-RCZX201704), the research program sponsored by Gansu Provincial Key Laboratory of Aridland Crop Science, Gansu Agricultural University (No.GSCS-2018-3), and the Outstanding Graduate Student Innovation Star Project in Gansu Province (2021CXZX-399).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** There are no conflict of interest to declare.

**Sample Availability:** Samples of the compounds are available from the authors.

#### **References**

