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. 2022 Jun 17;27(12):3905. doi: 10.3390/molecules27123905

Characterization of Volatile Flavor Compounds in Supercritical Fluid Separated and Identified in Gurum (Citrulluslanatus Var. colocynthoide) Seed Oil Using HSME and GC–MS

Emad Karrar 1, Isam A Mohamed Ahmed 2, Wei Wei 1, Frederick Sarpong 3, Charalampos Proestos 4,*, Ryszard Amarowicz 5, Emel Oz 6, Aly Farag El Sheikha 7,8,9, Ayman Y Allam 9, Fatih Oz 6, Xingguo Wang 1,*
Editor: Nurhayat Tabanca
PMCID: PMC9230783  PMID: 35745026

Abstract

In this study, the volatile compound profiles of gurum seed oil were determined using two methods: supercritical CO2 extraction (SFE) and the screw press process (SPP). For volatile compounds extraction and identification, headspace solid-phase micro-extraction (HS-SPME) and GC–MS were used, respectively. A total number of 56 volatile compounds were revealed and identified in oil extracted by SFE, while only 40 compounds were detected in extracted oil by SPP. Acids, aldehydes, esters, ketones, furans, and other components were present in the highest ratio in oil extracted by SFE. In contrast, alcohols and alkenes were found in the highest proportion in oil extracted by SPP. In this study, it was observed that SFE showed an increase in the amounts of volatile compounds and favorably impacted the aroma of gurum seed oil. The results reveal that different extraction methods significantly impact the volatile components of gurum seed oil, and this study can help evaluate the quality of the oil extracted from gurum seeds.

Keywords: gurum seed oil, supercritical CO2 extraction, screw press process, volatile compounds, HS-SPME, GC–MS

1. Introduction

With the globally growing demand for vegetable oils, researchers’ primary aim is to explore the utility and functional characteristics of by-products from plant sources. Gurum (Citrulluslanatus var. colocynthoide) is a wild kind of watermelon belonging to the Cucurbitaceae family and is abundantly available in African countries [1,2]. Gurum has historically been utilized in traditional medicine, especially in Sudan, while gurum seeds are plant by-products that are barely used for [1,3]. Gurum seeds play a major role in human nutrition and health due to their high oil content (27–35.5%). Several studies illustrate that gurum seed oil is a rich source of unsaturated fatty acids (UFA), including ω-6, ω-9, and ω-3, which positively impact human health [3,4,5,6,7].

Numerous conventional methods exist for oil extraction. Among these, solvent (SE) or mechanical extraction (pressing) is considered the main system used for commercial and industrial plant oil production [8]. Supercritical fluid extraction (SFE) has several advantages over the traditional method/systems due to its efficiency, eco-friendliness, and greenness [6]. Supercritical CO2 is low in surface tension and viscosity, and has a higher diffusion factor that enhances mass transfer [9,10]. Researchers have successfully extracted oils using the SFE method [6,11,12]. Supercritical CO2 is used in oil extraction due to the complete dissolving of triacylglycerols (TGA), fatty acids, and cholesterol.

Moreover, CO2 is cheap, non-toxic, and has a high purity [6]. In comparison to Soxhlet extraction, the SFE method prevents the problem of pollution of the oil by residual solvents and reduces the cost of production. For yield results, the solvent extraction method is higher than the screw press process and the SFE method. For supercritical CO2, pressure and temperature are crucial factors that may impact the oil’s yield. Again, CO2 reaches a crucial point under mild (31 °C, 74 bar) conditions in the absence of O2, and it is effective in preserving the bioactive compounds inside [13].

Plant oils are significant to human life owing to their contribution to providing energy, nutritional components, and exhibiting an appealing range of flavors [14,15]. Flavor is a specific parameter and significant quality standard for plant oils. Intuitively, plant oils possess a distinctive aroma [14]. Volatile compounds possess low molecular weights, generating a unique smell, and easily evaporate at room temperature [14]. The flavors of these compounds are believed to be connected with the quantitative and qualitative composition of the volatile components present in the oil [8]. Flavor is considered a significant factor that impactsconsumers’ choices. The flavor of plant oils depends on their diversity, maturity grade, environmental conditions, growing area, storing conditions, and processing techniques [8,16,17].

Processing techniques also significantly impact the concentrations of volatiles and are therefore responsible for causing an alteration in plant oil flavors when different techniques are employed [8,18]. Many research methods have been suggested to isolate and identify the volatile compounds contributing to the distinctive aroma of oils. Among those methods, solid-phase micro-extractions are a simple and fast method for extracting volatile compounds without a solvent. Headspace is a rapid, sensitive technique that can be used without solvents, and is also an economical technique for isolating volatiles’ analytes from the complex matrix. Headspace’s solid-phase micro-extraction gas chromatography technique was used to determine aldehydes in plant oils.

To the best of our knowledge, no published reports mention the volatile compounds of gurum seed oil. Therefore, the present study is the first to provide the profiles of volatile compounds of gurum seed oil. Thus, the aim of this study is to determine the volatile components in extracted gurum seed oil using headspace solid-phase micro-extraction (HS-SPME), later separated and identified using GC–MS (gas chromatography–mass spectrometry). The present study used the oil extracted by supercritical CO2 extraction (SFE) and the screw press process (SPP) for comparison purposes.

2. Results and Discussion

The SFE and SPP techniques were used for the oil extraction from gurum seeds. The identification and quantification of volatile compounds were performed using GC–MS, and data from these analyses are depicted in Figure 1.

Figure 1.

Figure 1

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Schematic representation.

2.1. Acid Compounds

The presence of acids in food is responsible for taste perceptions, such as cheesy and sour notes, which mainly depend on acetic acid (strong, pungent, and sour), octanoic acid (rancid, oily, fatty), and pentanoic acid (cheesy, sweaty, rancid) [18,19,20,21]. In this study, acids were predominantly present in volatile components, accounting for 34.13% of volatile compounds in the oil obtained by SFE, mainly acetic acid (24.81%) and hexanoic acid (5.12%). In comparison, acid components accounted for 12.89% of total volatiles in the oil obtained by SPP, and were mainly hexanoic acid (7.55%), nonionic acid (1.88%), and tetradecanoic acid (1.18%) (Table 1).

Table 1.

Relative content of volatile compounds identified in gurum seeds oil obtained by SFE and SPP using GC–MS.

RT a Compounds b Molecular
Formula		 CAS, No c Relative Peak Area (%)
SFE d SPP e
Acids
12.35 Acetic acid C2H4O2 64-19-7 24.81 ND
14.22 Propanoic acid C3H6O2 79-09-4 0.59 ND
18.05 Pentanoic acid C5H10O2 109-52-4 0.44 ND
18.06 Pentanoic acid C5H10O2 109-52-4 ND 0.58
19.99 Hexanoic acid C6H12O2 142-62-1 5.12 7.55
21.05 8-Phenyloctanoic acid C14H20O2 26547-51-3 0.10 ND
21.84 Heptanoic acid C7H14O2 111-14-8 0.55 0.55
23.60 Octanoic acid C8H16O2 124-07-2 0.80 0.74
25.25 Nonanoic acid C9H18O2 112-05-0 ND 1.88
25.49 2-Octenoic acid, (E)- C8H14O2 1871-67-6 0.13 ND
26.05 17-Octadecynoic acid C18H32O2 34450-18-5 0.13 ND
26.63 n-Decanoic acid C10H20O2 334-48-5 0.70 0.41
31.09 Tetradecanoic acid C14H28O2 544-63-8 0.55 1.18
32.64 Octadecanoic acid C18H36O2 57-11-4 0.21 ND
Alcohols
7.91 3-Decyn-2-ol C10H18O 69668-93-5 0.07 ND
8.04 1-Pentanol C5H12O 71-41-0 ND 1.19
9.57 2-Hexanol, 5-methyl- C7H16O 627-59-8 ND 0.47
14.36 Linalool C10H18O 78-70-6 0.14 ND
10.27 1-Hexanol C6H14O 111-27-3 ND 7.02
12.36 1-Octen-3-ol C8H16O 3391-86-4 ND 0.93
12.63 6-Hepten-1-ol, 2-methyl- C8H16O - ND 0.17
13.60 2-Heptenol C7H14O - ND 1.29
15.64 2-Octen-1-ol, (E)- C8H16O 18409-17-1 ND 0.26
15.74 Ethanol, 2-(2-ethoxyethoxy)- C6H14O3 111-90-0 0.19 ND
16.56 1-Nonanol C9H20O 143-08-8 0.49 1.89
16.69 2-Butyl-2,7-octadien-1-ol C12H22O - ND 0.19
17.28 endo-Borneol C10H18O 507-70-0 2.71 ND
17.58 2-Nonen-1-ol C9H18O 22104-79-6 ND 0.20
18.49 1-Undecanol C11H24O 112-42-5 ND 0.17
19.47 2-Decen-1-ol C10H20O 22104-80-9 ND 0.06
20.46 Benzyl alcohol C7H8O 100-51-6 ND 0.57
21.06 Phenylethyl Alcohol C8H10O 60-12-8 ND 0.28
22.09 1-Undecanol C11H24O 112-42-5 ND 0.20
22.51 2,4-Decadien-1-ol C10H18O 14507-02-9 ND 0.05
22.67 Phenol C6H6O 108-95-2 0.47 0.18
22.80 Methyleugenol C11H14O2 93-15-2 1.95 ND
26.27 Ethanol, 2-(dodecyloxy)- C14H30O2 4536-30-5 ND 0.66
Aldehydes
4.66 Hexanal C6H12O 66-25-1 0.51 ND
8.99 Octanal C8H16O 124-13-0 0.70 ND
9.60 2-Heptenal, (Z)- C7H12O 57266-86-1 0.65 ND
11.14 Nonanal C9H18O 124-19-6 2.28 0.44
11.86 2-Octenal, (E)- C8H14O 2548-87-0 0.57 0.09
13.20 2,4-Heptadienal, (E,E)- C7H10O 4313-03-5 3.44 ND
13.35 Decanal C10H20O 112-31-2 0.13 0.12
13.77 Benzaldehyde C7H6O 100-52-7 0.53 0.80
14.06 trans-2-Nonenal C9H16O - ND 0.16
14.07 2-Nonenal, (Z)- C9H16O 60784-31-8 0.38 ND
18.25 2-Undecenal C11H20O 2463-77-6 1.73 ND
19.30 2,4-Decadienal C10H16O 2363-88-4 0.87 ND
30.00 cis,cis,cis-7,10,13-Hexadecatrienal C16H26O 56797-43-4 0.38 ND
Esters
25.95 Pentadecanoic acid, 14-methyl-, methyl ester C17H34O2 5129-60-2 0.24 ND
26.44 Hexadecanoic acid, ethyl ester C18H36O2 628-97-7 2.22 ND
28.55 6-Octadecenoic acid, methyl ester, (Z)- C19H36O2 2777-58-4 0.56 ND
28.71 Pentadecanoic acid, ethyl ester C17H34O2 41114-00-5 0.43 ND
28.90 (E)-9-Octadecenoic acid ethyl ester C20H38O2 6114-18-7 1.95 ND
29.02 11,14-Eicosadienoic acid, methyl ester C21H38O2 2463-02-7 0.23 ND
29.35 9(E),11(E)-Conjugated linoleic acid, ethyl ester C20H36O2 - 3.07 0.06
29.45 Phthalic acid, hex-3-yl isobutyl ester C18H26O4 - 0.46 ND
Alkanes
7.02 Dodecane C12H26 112-40-3 0.79 ND
7.04 Undecane, 2,6-dimethyl- C13H28 17301-23-4 ND 0.68
7.71 Hexane, 1,1-diethoxy- C10H22O2 3658-93-3 0.58 ND
10.26 Cyclopropane, propyl- C6H12 2415-72-7 0.40 ND
12.29 Undecane, 4,7-dimethyl- C13H28 17301-32-5 ND 0.04
13.43 Dodecane, 2,7,10-trimethyl- C15H32 74645-98-0 0.09 0.10
14.55 Cyclopropane, pentyl- C8H16 2511-91-3 0.52 0.84
15.45 Hexadecane C16H34 544-76-3 0.24 ND
29.12 Dodecane, 2,6,11-trimethyl- C15H32 31295-56-4 0.14 ND
Alkenes
12.62 3,5-Dimethylcyclopentene C7H12 7459-71-4 1.34 ND
13.14 α-Cubebene C15H24 17699-14-8 ND 2.61
16.64 Humulene C15H24 6753-98-6 0.27 ND
30.95 Squalene C30H50 111-02-4 0.60 ND
Terpenes
6.89 D-Limonene C10H16 5989-27-5 ND 0.15
Ketones and hydroxyketones
15.84 Butyrolactone C4H6O2 96-48-0 0.21 ND
21.11 δ -Dodecalactone C12H22O2 2305-05-7 0.08 ND
22.03 2-Methyl-6-methyleneoct-7-en-4-one C10H16O 19860-68-5 0.50 ND
Furans
7.65 Furan, 2-pentyl- C9H14O 3777-69-3 ND 0.40
17.26 2(3H)-Furanone, 5-ethyldihydro- C6H10O2 695-06-7 ND 0.20
22.99 2(3H)-Furanone, dihydro-5-pentyl- C9H16O2 104-61-0 0.54 0.18
32.97 2(3H)-Furanone, 5-dodecyldihydro- C16H30O2 730-46-1 0.31 ND
Other compounds
29.63 Vanillin lactoside C20H28O13 - 0.28 ND
30.25 l-Gala-l-ido-octose C8H16O8 - 0.13 ND
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All values given are means of two determinations. a RT, retention time. b Volatiles identified by the SPME-GC–MS. c CAS no: Chemical Abstracts Service Registry Number. d SFE, supercritical CO2 extraction. e SPP, screw press process ND, not detected. ‘-’means there is no CAS no. for this compound.

The presence of acetic acid was expected since it is the common volatile compound in most vegetable oil and is formed in the process of the treating the seeds [22,23]. However, acetic acid was present in oil extracted by SFE but not detected in oil obtained by SPP, signifying that extraction methods impacted the concentrations of volatiles, and thus caused an alteration in plant oil flavors. Hexanoic acid, on the other hand, is produced through an enzymatic reaction produced from polyunsaturated fatty acids (PUFA) in oils through the LOX (lipoxygenase) pathway [16,18].

2.2. Alcohol Compounds

The presence of alcohol in food products generally produces alcoholic, fruity, sweet, balmy, and green aroma and sensations. However, these aromas depend on the molecular structure of the alcohol produced [21,24]. In this study, alcohols accounted for 6.02% of volatiles in the oil obtained by SFE, mainly endo-borneol (2.71%) and methyl eugenol (1.95%), while alcohol compounds accounted for 15.78% of volatiles in the oil obtained by SPP, mainly 1-hexanol (7.02%), 1-nonanol (1.89%), 2-heptenol (1.29%), and 1-pentanol (1.19%).

These components played a greater role than other alcohol components in the oil obtained by SPP, especially 1-hexanol. The 1-hexanol compound is an important flavor compound in some vegetables; it produces a green odor, is woody and herbaceous. It is obtained from the bioremediation of UFA (unsaturated fatty acids) and serves as the basis for forming long-chain esters [18,25]. At the same time, endo-borneol and methyl eugenol were found to have the highest ratios in the oil obtained by SFE.

Borneol elevates the numerous medicinal properties of essential oils in the Dipterocarpaceae family [26]. In traditional Chinese and Japanese medicine, borneol is often used in incense formulas for its uplifting effects on the mind [27]. Methyleugenols are applied as a flavoring in cookies, ice cream, pies, candy, puddings, chewing gums, and cola-based soft drinks [26]. These results demonstrate that different methods of extraction exhibit a significant impact on the flavor components of gurum seed oil.

2.3. Aldehyde Compounds

Aldehydes in vegetable oils are fundamentally produced by two pathways: the first is the fat oxygenase pathway (which occurs through the cell fragmentation in the oilseeds), and the second is the automatic oxidation pathway (which occurs in the production and storage process) [18]. In this study, aldehydes were observed to be 12.17% of volatiles in the oil obtained by SFE, mainly 2,4-heptadienal, (E, E) (3.44%), nonanal (2.28%), and 2-undecenal (1.73%), while aldehyde compounds were observed to account for 1.61% of volatile compounds in the oil obtained by SPP, mainly benzaldehyde (0.80%). Nonanal and 2-undecenal are the two main products of linoleic acid oxidation, while 2, 4-heptadienal is the main product of linoleic acid oxidation [18].

2.4. Esters Compounds

Ester compounds cover a wider spectrum of flavoring and odor effects; thus, they are widely distributed as the main component in fruit and essential oils. Esters provide a floral, fruity, honey, sweet, and flowery perception in food and other products [21]. In addition to the natural occurrence of ester, the compound can also be produced from the chemical reactions of alcohols and acids [17,28]. Ester components accounted for 9.16% of the total volatiles in oil obtained by SFE and mainly comprised 9(E),11(E)-conjugated linoleic acid, ethyl ester (3.07%), and (E)-9-octadecenoic acid ethyl ester (1.95%), while 9(E),11(E)-conjugated linoleic acid was only found in oil obtained by SPP with a low ratio (0.06%), and it does not have a significantly contribute to the aroma of the oil obtained by SPP (Table 1).

2.5. Alkane and Alkene Compounds

Lower amounts of alkene and alkane compounds are always found in oils [23]. In this study, alkane components accounted for 2.76% of the total volatiles in oil obtained by SFE and mainly comprised Dodecanese (0.79%). In comparison, alkene components accounted for 2.21% of the total volatiles, and mainly comprised 3, 5-dimethylcyclopentene (1.34%), squalene (0.60%), and humulene (0.27%) (Table 1). Squalenes are classified as bioactive substances and are known to exhibit beneficial effects on human health [29]; they are also widely available and occur naturally in vegetable oils. Squalenes also act as a precursor to sterols and belong to the terpenoid family [30].

In our study, alkane components accounted for 1.66% of the total volatiles in oil obtained by SPP. They were mainly cyclopropane, pentyl (0.84%), and undecane, 2, 6-dimethyl (0.68%), while α-cubebene (2.61%) was the only alkene detected in oil obtained by SPP. This study observed that one terpene (D-Limonene, 0.15%) was also detected in oil obtained by SPP. Terpenes are naturally-occurring compounds in plants and form part of the essential oils [23].

2.6. Ketones and Furan Compounds

Ketones are reportedly formed by the β-oxidation of fatty acids (FAs), which produce a few important aromatic compounds [18,31]. In this study, ketone components amounted to 0.79% of the total volatiles in oil obtained by SFE, mainly 2-methyl-6-methyleneoct-7-en-4-one (0.50%), while ketones were not detected in the oil obtained by SPP (Table 1). Various seed oils reported furan compounds [23,31], as previously reported. Furan compounds are either produced by fat oxidation or carbohydrate degradation [23]. In this study, furan components amounted to 0.85% of the total volatiles in oil obtained by SFE, mainly 2(3H)-furanone, dihydro-5-pentyl-(0.54%). In comparison, furan components amounted to 0.78% of the total amount of volatiles in the oil obtained by SPP, mainly furan, 2-pentyl (0.40%).

2.7. Other Compounds

Through the other compounds found in oil from gurum seeds, two compounds were classified in this group (Table 1). Vanillin lactoside and l-Gala-l-ido-octose were detected in the oil obtained by SFE, while other compounds were not present in the oil obtained by SPP. Low amounts of vanillin lactoside (0.28%) and l-Gala-l-ido-octose (0.13%) in oil obtained by SFE were also observed. The SFE caused an increase in the amounts of volatile compounds of oils, and this perhaps has a greater impact on the aroma of gurum seed oils.

Volatiles that are studied by humans have a considerably greater effect on the flavor of gurum seed oil. A significant difference was found in the volatile constituent’s oil from gurum seeds obtained by SFE and SPP. For the total volatile compounds studied in this work, a total of 56 volatile components were noted in the oil from gurum seeds by SFE, including 12 acids, 7 alcohols, 12 aldehydes, 8 esters, 7 alkanes, 3 alkenes, 3 ketones and hydroxy ketones, 2 furans, and 2 other compounds. Meanwhile, 40 volatile components were identified in oil from gurum seeds obtained by the SPP, including 7 acids, 18 alcohols, 5 aldehydes, 1 ester, 4 alkanes, 1 alkene, 1 terpene, 1 ketone and hydroxy ketones, and 3 furans, as shown in Figure 2a.

Figure 2.

Figure 2

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Comparison of volatile components obtained by SPP, SFE and SE of gurum seeds: (a) compounds number; (b) relative amount of volatile compounds (%).

The results reveal that the oil obtained by SFE could result in the isolation of some characteristic volatile substances. This may explain the great odor differences between the oil obtained by SFE and SPP. Acids, aldehydes, esters, alkanes, ketones, furans, and other components were found to be present in the highest ratios in the oil obtained by SFE compared to SPP (Figure 2b). These results demonstrate that different methods of extraction resulted in a significant impact on the flavor components of gurum seed oil. Alcohols and alkenes were found in the highest yield in oil obtained by SPP compared to SFE (Figure 2b).

Thus, oils obtained by different methods result in rich bioactive metabolites, ensuring a potential application as an antiproliferative and antioxidant agent in the pharmaceutical and food industries. Furthermore, additional studies on the differences in extraction methods and conditions are required, which could present a clearer view of the metabolic pathways connected with flavor formation.

3. Materials and Methods

3.1. Raw Materials and Chemicals

Gurum seeds were purchased from Dongola city in Sudan and carried to the National Engineering Research Center at Jiangnan University, China. All chemicals used in the experiments were of high purity. High-purity CO2 was employed as a solvent carrier.

3.2. Oil Extraction

3.2.1. Supercritical CO2 Extraction (SFE)

The oil extracted through the supercritical CO2 (SFE) system was obtained through a detailed protocol, as laid out in previous reports [6]. Briefly, gurum (Citrulluslanatus var. Colocynthoideis) seed oil was extracted by the SFE system equipped with Process Suite software (Waters, Milford, MA, USA) and carefully washed with ethanol. The gurum seed powders were then placed in an extraction vessel and stored for 120 min at 40 °C. The system pressure of 35 MPa was applied with the flow rate of 0.5 L min−1 of solvent (CO2, 100%). The oil was then collected and put into a vacuum drying oven (SLN 75 POL-EKO-APARATURA, Śląski, Poland) under 50 ± 1 °C for 2 h to remove any traces of ethanol. The prepared oils were then stocked at −20 °C until analysis.

3.2.2. Screw Press Process (SPP)

According to the method described in detail in our previous study [6], 1000 g of gurum seeds were used to extract the oil via a screw-press machine. The obtained oil was then stored at 4 °C for further analysis.

3.3. Headspace Solid-Phase Micro-Extraction Gas Chromatography–Mass Spectrometry (HS-SPME-GCMS)

The volatile flavor components in gurum seed oil were extracted using headspace solid-phase micro-extraction (HS-SPME), then separated and identified using volatile compounds of gurum seed oil. The gurum seed oil was separated by GC–MS (Scion SQ 456-GC, Bruker, Billerica, MA, USA) using a DBWAX column (30 m × 0.25 mm × 0.25 µm) while maintaining the injector at 250 °C for 3 min. To determine these compounds, 5 g of gurum seed oil obtained by different methods was measured and analyzed following a fully described protocol in our previous report [4]. Each volatile compound profile of gurum seed oil obtained by different extraction methods was separated and identified by comparing mass spectra and RI, as previously demonstrated [4].

3.4. Statistical Analysis

Data collection was performed in triplicate and presented as means ± SD through the processing of Origin-Pro 9.2 (Origin Lab Corporation, Northampton, MA, USA). Data comparison was performed using one-way analysis of variance (ANOVA) and Fisher’s multiple comparison tests.

4. Conclusions

The present study first reported the characterization of volatile compounds obtained from gurum seed oil by SFE and SPP that were separated and analyzed using GC–MS. A total of 56 volatile compounds were analyzed and identified in gurum seed oil obtained by SFE, compared to 40 compounds obtained by SPP. Acids, aldehydes, esters, alkanes, ketones, furans, and other components were found to be the highest ratio of compounds in the oil obtained by SFE compared to SPP. Alcohols and alkenes had the highest yield in oil obtained by SPP compared to SFE. These results demonstrate that different extraction methods have a significant impact on the volatile flavor composition and overall quality of gurum seed oil.

Author Contributions

E.K.: Conceptualization, methodology, validation, formal analysis, investigation.; writing—original draft preparation; I.A.M.A.: writing—review and editing; W.W.: resources, writing—review and editing; F.S.: writing—review and editing; C.P.: writing—review and editing; R.A.: writing—review and editing; E.O.: writing—review and editing; A.F.E.S.: writing—review and editing; A.Y.A.: writing—review and editing; F.O.: writing—review and editing; X.W.: project administration; supervision. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Sample Availability

Not applicable.

Funding Statement

This work was supported by the National Natural Science Foundation of China (32150410365).

Footnotes

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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