1. Introduction
Atmospheric free radicals are molecules, ions, or groups with unmatched electrons which have strong oxidizing capacity. They are the most important chemical intermediates or media in the atmosphere and play a crucial role in the atmospheric photo-chemical reactions. They determine the oxidation capacity of polluted air, in turn endangering human health and leading to a series of environmental problems [
1,
2,
3]. Environmental toxicology research shows that free radicals in the urban atmosphere are a potential source which make humans more susceptible to developing cardiovascular, respiratory, and other diseases [
4].
The natural active molecules found in the roots and leaves of plants and fruits, such as flavonoids, are a kind of polyphenol compounds in nature. Furthermore, they have important use value in pharmaceutical, industrial, and other industries [
5,
6]. The chemical structure of flavonoids consists of two benzene rings (ring A and ring B) which are connected by three carbon atoms. Most of them contain phenolic hydroxyl groups in their molecular structure; the phenolic hydroxyl group of flavonoids is the site of its antioxidant activity [
7]. The phenolic hydroxyl group can provide hydrogen for the atmosphere free radical and then itself transform into a phenolic radical. This occurs under the resonance of the aromatic conjugated system forming semiquinone radical [
8,
9,
10]. Therefore, they have a wide range of pharmacological activities, including anti-oxidation, anti-cancer, and anti-inflammatory properties, as well as helping with the recovery of injured tissues [
11,
12,
13].For example, quercetin [
14,
15]; glycyrrhizic acid [
16], mangosteen peel extract [
17], etc., have been proven to have strong antioxidant capacity, free radical scavenging ability, and various pharmacological activities.
In recent years, there have been several methods developed to be applied for atmospheric free radicals capture, such as salicylic acid impregnated membrane capturing atmospheric OH radicals, spin trapping technology, and flavonoid trap of electrospinning technology. In comparison, electrospinning technology has been viewed as the most versatile and effective technique for the preparation of continuous nanofibers with controlled morphology, structure and functional composition. The preparation of electrospinning active molecular membrane and applying it to capture atmospheric free radicals has rarely been reported. Gao et al. studied a low filtration resistance three-dimensional composite membrane via free surface electrospinning for effective PM
2.5 capture [
18]. Lin et al. studied electro spinning polyacrylonitrile–ionic nanofibers with superior PM
2.5 capture capacity [
19]. The electrospinning technique involves the application of a high-voltage electrostatic field to overcome the surface tension of droplets. The polymer charged solution is subjected to high-voltage static electricity. As the solvent evaporates while electrospinning, the jet will be elongated by electrostatic repulsion, and finally it is deposited on the collector to form a fiber mat similar to non-woven fabric [
20,
21]. Morphologies of the electrospun fibers could be affected by the following parameters: (a) the physical properties of spinning solution (e.g., concentration [
22], viscosity, rheological [
23] and conductivity [
24]); (b) the operating parameters of the electrospinning instrument (e.g., voltage, flow rate [
25], metal nozzle diameter and the distance between tip of the needle and collector [
26]); (c) ambient conditions (e.g., temperature, relative humidity, etc.) [
27,
28,
29].Through a series of screening experiments, we can obtain electrospun micro/nanofibers with ideal morphology, diameter, and minimal string-up bead formation.
In this paper, three kinds of natural active molecules (quercetin, glycyrrhizic acid and mangosteen peel extract) were prepared into nanofiber membrane by using electrospinning technology. The active molecules were uniformly distributed and loaded into the nanofiber membrane in an array state by electrostatic action. The microstructure of nanofiber membrane was characterized by using scanning electron microscopy (SEM). The nanofiber membrane was used for atmospheric free radicals scavenging, and then evaluated by ultraviolet spectrophotometer, fluorescence spectrophotometer, and electrochemistry to select the molecular membrane with the best trapping ability. The nanofiber membrane with large specific surface area, high porosity and active molecule array distribution can be used for the capture, monitoring and scavenging of free radicals under heavy pollution conditions.
3. Experimental Section
3.1. Three Natural Active Molecules
Quercetin is the widely distributed flavonoid in the plant kingdom. Glycyrrhizic is a high-sweet sweetener and detoxification food and its extract (glycyrrhizic acid) has the ability of scavenging hydroxyl radical. A variety of molecules extractable from mangosteen peel, including mangostin and procyanidins etc., have been proved to have strong antioxidant capacity, free radical scavenging ability and various pharmacological activities. Their chemical structural formulas are shown in
Table 9 and
Figure 10.
3.2. Chemicals and Equipment
Polyvinylpyrrolidone (PVP) was purchased from Shanghai McLean Biochemical Technology Corp. (Shanghai, China). Absolute methanol (analytic reagent) was obtained from Shanghai Xingke high Purity Solvent Corp. (Shanghai, China).
Intelligent medium flow air total suspended particulate sampler was purchased from Wuhan TianHong Instrument Corp. (TH-150A, Wuhan, China). Electrostatic spinning equipment was purchased from Beijing Yongkang Leye Technology Development Corp. (NL18-20, Beijing, China). UV-VIS spectrophotometer was purchased from Thermo Fisher scientific. (GEN10S UV-Vis, Waltham, MA, USA). A fluorospectro photometer was purchased from PerkinElmer Corp. (LS55, Boston, MA, USA). A multichannel Electrochemical Tester was purchased from Princeton Technology. (Parsat MC, Berwyn, PA, USA).
3.3. Extraction of Active Material
The fresh oak bark, glycyrrhiza, and mangosteen peel were dried and pulverized, and a certain amount of three kinds of powder after dry were separately put into flask with round bottom with hot reflux extraction. The separation conditions are shown in
Table 10.
3.4. Electrospinning
Electrospinning was carried out at room temperature, First of all, PVP and active moleculars including quercetin of 0.04% (
w/v), glycyrrhizic acid of 0.06% (
w/v) and α-mangostin of 0.03% (
w/v) was dissolved into 5 mL methanol, respectively; and then the spinning solution was stirred with a magnetic stirrer for 1 h and standing for 5 min. The prepared three kinds of spinning solution were placed in a 5 mL syringe. The metallic needle was connected to the positively-charged, and connected to the aluminum foil paper with glued three glass fiber membranes to grounded collector. In addition to adjusting the operating parameters of the electrospinning apparatus, the receiving speed was set at 40 r/min. Spinning parameters of the three active substances are shown in
Table 11. Flow diagram of the preparation of electrospinning active molecular membrane are shown in
Figure 11.
3.5. Free Radical Trapping
The sampling site was selected as the rooftop on the 15th floor of tall building of Lanzhou University of Technology. The prepared nanofiber membrane was placed in the intelligent medium-flow air total particulate sampler (TH-150A, Wuhan, China) for sampling for 3 h, and the flow rate was 100 L/min, while recording environmental parameters. And then the after sampling nanofiber membrane was dissolved in 10ml methanol, and transferred to a sample bottle and analyzed by the detection systems.
3.6. Detection Method
Ultraviolet absorption spectrum detection parameters: the PVP blank membrane dissolved in methanol was used as the background, the scanning range was from 190 to 1100 nm, and the experiment repeated 3 times. The characteristic absorption peak of quercetin was 370 nm [
46], the characteristic absorption peak of glycyrrhizic acid was 252 nm [
47], and the characteristic absorption peak of α-mangostin was 317 nm [
48].
Fluorescence emission spectra detection parameters: with the change of excitation wavelength, we obtained the emission spectra of fluorescence substances varying from 200 to 800 nm. The wavelength of quercetin excitation was 360 nm, the wavelength of glycyrrhizin excitation was 350 nm, and the wavelength of alpha-mangostin excitation was 391 nm.
Electrochemical experimental parameters: The experiment used a three-electrode system with a calomel electrode as a reference electrode, a platinum wire electrode as an auxiliary electrode, and a bare glassy carbon electrode as a working electrode. In the condition of Na2HPO4-NaH2PO4 buffer system, 10 mL of Na2HPO4-NaH2PO4 buffer solution was decanted into the electrolytic cell. The quercetin solution, glycyrrhizic acid solution and α-mangostin solution were separately added to the buffer by a pipette and were allowed to stand. The material to be detected was enriched to the surface of the working electrode and was detected by cyclic voltammetry (CV). The scanning range was −2~2 V, the scanning cycle was 3, the quercetin sweep rate was 50 mv/s, and the glycyrrhizic acid sweep rate was 70 mv/s, α-mangostin sweep rate was 100 mv/s.
The clearance rate (reactivity) and free radical concentration calculation formula was:
Fx—Sample clearance (%)
K—correction factor (1 × 10−8)
N—The amount of substance in the sample (mol)
NA—Avogadro’s number (6.02 × 1023)
Fg—The sampler sets the intake flow (cm3/min)
t—Sampling time (min)
α—The product loss rate (5%)
4. Conclusion
In this paper, active glycyrrhizic acid electrospinning membranes, active quercetin electrospinning membranes and active quercetin electrospinning membranes were prepared in our lab. The reaction capacity of three different membranes for free radicals are shown as follows: glycyrrhizic acid membrane > quercetin membrane > α-mangostin membrane. The free radicals detection were carried out by fluorescence emission spectroscopy, UV absorption spectroscopy, and the electrochemistry method, respectively. The prepared active molecular electrospinning membranes have large specific surface area and high porosity, which could increase the interaction area between active molecules and free radicals. The active molecular electrospinning membranes also have improved anti-airflow impact strength, anti-contaminant air molecular interference ability, and good ability to capture free radicals. Additionally, the active molecular electrospinning membranes are also suitable for carrying sampling by drones, which provides a new technical method for the study of free radicals and atmospheric oxidative spatial variation.