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Abstract

As a new preparation process of nanofibers, electrospinning technology developed by bionic spider spinning can further improve the molecular structure and mechanical properties of nanofibers, and obtain nanofibers and base membrane products with better mechanical properties. The spider spinning needs minimal energy consumption though it has a very long liquid transport tube. Because spider silk has high mechanical properties and wide industrial applications, so a kind of new biomimetic electrospinning technology which harnessed a very long spiral-needle was proposed. Then poly(lactic acid)PLA/graphite oxide(GO)/dopamine (DA) nanofiber membrane was fabricated by the new biomimetic electrospinning technology. The results showed that compared with the pure PLA nanofiber membranes, the average pore size of PLA/GO porous nanofiber membranes decreased, the total number of pores increased, and the fracture strength and elongation at break increased. At the same time, the spun nanofibers were also more uniform. The pore size distribution of nanofiber was more dense after adding DA, and the fracture strength and elongation at break of nanofiber increased, while the nanofiber was also more even. After adding DA, the fracture strength was greatly increased and the elongation at break was similar to that of pure PLA nanofiber. The nanofiber membrane was obtained after oxidative polymerization of DA into poly(dopamine) (PDA), a layer of nanofiber membrane was attached to the surface of nanofiber. The performance of PLA/GO/PDA nanofiber membranes prepared based on PLA/GO/DA nanofiber membranes was superior to that of PLA/GO/PDA nanofiber membranes prepared based on PLA/GO nanofiber membranes, and its adsorption rate after 24 h was as high as 98.81%.

Keywords

Biomimetic spider electrospinning nanofiber membrane oxidative polymerization adsorption

Introduction

Traditional electrospinning (such as single needle electrospinning and multi needle electrospinning) uses high voltage electric field to draw the spinning solution to obtain nanofibers. These methods do not pay attention to the controllability of the internal molecular structure of nanofibers, so the physical and mechanical properties of nanofibers prepared by traditional electrospinning are not outstanding. Spider silk has been studied by many domestic surgical researchers, mainly because its many advantages fully meet the high-performance requirements of various industry materials, such as lightweight, good elasticity, UV resistance, biodegradability, and excellent biocompatibility.

Moreover, the outstanding mechanical performance of spider silk is incomparable to any material, and this performance is closely related to the process of spider spinning. The straight and orderly arrangement of macromolecular chains after spider spinning is closely related to laminar flow, and a large number of theoretical and experimental verifications have been conducted. In laminar flow state, the mutual resistance between macromolecular chains decreases, and the macromolecular chains are straightened and move forward in an orderly manner, thereby changing the structural state of macromolecular arrangement. The spider spinning process is extremely captivating for many scientists, because the properties of the spider silk is inextricably linked with its spinning process.^1,2 The bubble electrospinning^3,4 was invented by inspiration of the spider spinning, and 3-D printing technology^5–9 can be optimized by spider’s web design. It was reported that the spider spinning has the attractive property of minimal energy consumption,² the energy saving^10,11 and energy harvesting^12,13 are two key factors for a green earth. Spider silk has many advantages, such as high elasticity, high toughness, high water absorption, light weight and biodegradability, which make it in many fields, such as aerospace (structural materials and composite materials of artificial satellites and space suits), military (tanks, armor, aircraft, bulletproof vests and parachutes), industrial (high-strength materials, wheel casings, etc.) Medical fields (artificial tissues or organs, biodegradable surgical sutures and other physiological tissues and biological materials) and textile fields (clothing, scarves, etc.) have great application prospects. The traditional electrospinning process is a energy-cost process, and the mechanical properties of the nanofibers (membranes) prepared by spinning are relatively low. so a biomimetic one is much needed. Now the biomimetic nanofiber membrane can be used for antibacterial applications and nano/microelectromechanical (N/MEMS) systems^14,15 and environmental science.¹⁶

Since the 1990s, China has been a major producer and user of dyestuffs, especially in the present period of industrial boom, and the dyeing industry in China is producing more dyestuffs.¹⁷ In 2017, China produced 990,000 t of dyestuffs, with the consequent production of about 740 million cubic meters of printing and dyeing industrial wastewater, which is a particularly huge amount of printing and dyeing wastewater and poses a great threat to the ecological environment and human health in China.¹⁸ The volume of printing and dyeing wastewater is particularly huge, posing a great threat to the ecological environment and human health in China. The printing and dyeing industry has many kinds of products, complicated production process, and many dyestuff additives used, which will cause serious water pollution once discharged into the water, and the dyeing wastewater has high chromaticity, high organic pollutant content, and complex organic composition, which is difficult to degrade in the natural environment, so it is more difficult to treat.¹⁹ At present, the commonly used treatment methods of dye wastewater mainly include physical adsorption^20,21 photocatalytic degradation, membrane separation^22,23 ion exchange^24,25 chemical deposition, microbial treatment, etc. Among them, physical adsorption has become a hot research topic in the field of water purification because of its low cost, high operability and low energy consumption.^26–28 Among them, physical adsorption has become a hot spot in the field of water purification because of its low cost, high operability and low energy consumption.²⁹

The electrospinning process is the main method for the preparation of nanofibers in laboratory research. The technique involves the application of a high-voltage electrostatic field on a polymer solution to form a charged jet and to stretch and refine it in the high-voltage electrostatic field by electric field forces to form a fine filament, accompanied by the rapid evaporation of the solvent from the polymer spinning solution, which is finally cured on a collection device to obtain nanofibers. The existing electrospinning technology focuses on the controllability of the internal molecular structure of nanofibers, so it is necessary to further enhance the molecular structure of nanofibers and their mechanical properties to obtain nanofibers and their base film products with better mechanical properties by bionic spider silk.

PLA is a bio-absorbable polymer that, due to this unique property, can completely absorb heavy metals and dyes within a pre-designed time frame. In addition the biodegradable material PLA as an environmentally friendly material, decomposition products are only CO₂ and H₂O and it is a good choice to use it as a substrate.^29,30 Combined with the hydrophilicity and abundant functional groups of graphite oxide, and the biocompatibility and long-lasting stability of poly(dopamine) (PDA) coating, it will have far-reaching applications in the field of aqueous adsorption, etc.^31–33

Tian et al. proposed that bionicmetic spider electrospinning technology can use laminar flow theory to control the internal structure of nanofibers, that is, using long needle spinning technology for nanospinning, it was found that long needle spinning can effectively control the orientation arrangement of macromolecules inside nanofibers, and the longer the spinning needle length (up to 150 mm), the more orderly the internal structure of nanofibers. The series of experimental studies (PVA/PAN) and its related energy characterization proved the versatility of long needle spinning in electrospinning in low viscosity spinning fluid.^34–36 However, from the schematic diagram of spider spinning can also be carefully observed that the spider spinning process of the long tube produced two bends, indicating that the long tube in the spider is close to infinite length, while the study of Tian et al. only achieved a linear needle length of 150 mm, so this study plans to further increase the length of the spinning needle, infinitely close to the spider spinning, taking into account the limitations of the spinning space after the needle is too long and the voltage instability, we will The front part of the needle is spirally arranged, and at the same time, in order to compensate for the spinning fluid energy loss during the spiral movement, we apply DC power at both ends of the spiral to ensure that the spiral part generates a certain spiral centripetal force.

In this paper, poly(lactic acid) (PLA), graphite oxide (GO) and dopamine (DA) were co-blended and spun to prepare PLA/GO/DA porous nanofiber membrane by using new biomimetic spider electrospinning technology, and then DA oxidative polymerization into PDA, which was loaded on the surface of the nanofiber. The porous structure of the nanofiber was also occupied to prepare PLA/GO/PDA nanofiber membrane, and finally the adsorption effect of this composite nanofiber membrane on the dye MB was discussed, and then its application in dye wastewater treatment was investigated.

As shown in Figure 1, spiral spinning technology is a macromolecular spinning control technology extended on the basis of long needle spinning technology, in the case of polymer macromolecules are straightened, the use of spiral needles under the action of the spiral magnetic field to achieve the twisting of large polymer molecular chains, enhance the lateral pressure between macromolecules, improve the slip resistance between macromolecules, so as to improve the single nanofiber filament physical and mechanical properties of single nanofiber filaments. In order to verify the theoretical feasibility of spiral spinning, a self-assembled spiral spinning set was built based on the spiral spinning principle, as shown in Figure 2(a), firstly, a single spiral needle was configured, the first 150 mm of the long needle was flat (this part can be defined as orientation spinning or long needle spinning during spinning), and the last 150 mm could be bent into different spiral distance (spiral number) structure form, in spinning is to make polymer macromolecules produce spiral centripetal force, in the long needle spiral front and rear end and AC power connected to the line installed high power adjustable resistor and switch, high power adjustable resistor can prevent the circuit from short circuit, when the long needle spiral part with resistor and power supply form a closed loop circuit, as shown in Figure 2(b), according to the principle of magnetic field generation, the long needle spiral part will produce spiral centripetal force acting on the polymer macromolecule chain inside the long needle, so that the polymer macromolecule chain produces spiral twisting effect.^15–18 In the spiral spinning preparation process, a single spiral needle is installed at the front of the syringe, the syringe is mounted on the booster, the injection speed (spinning fluid flow rate) is controlled by a micro-controller, the front of the spiral needle is connected to a high-voltage electrostatic generator, and the collection device is grounded, when the spinning fluid is spirally twisted and further drawn into nanofibers under the action of high-voltage electrostatic.

Figure 1.

Spiral paper technical principle diagram.

Figure 2.

The real object of the spiral spinning device and the screw control principle: (a) the real object of the spiral spinning device and (b) screw control principle.

The self-assembly equipment includes a single-push syringe pump, a high-voltage electrostatic generator with a maximum adjustable 50 kv, a 10 ml syringe, a stainless steel bending flex needle with a length of 300 mm, a 9 V battery, an adjustable resistance of 0–20 Ω, a stable conductive wire, and a syringe tube. In spiral spinning, the syringe is pumped into the pre-matched spinning solution, the syringe tube is fixed on the single-push syringe pump, set the feed rate, the stainless steel bending flex needle pin is closely connected to the nipple of the syringe, the stainless steel bending flex needle before 150 mm (by the nipple) is straight state, the stainless steel bending flex needle after 150 mm (near the needle) is wound into a different number of spirals, the battery, conductive wire. When a smaller current is generated in the line, the polymer spinning fluid from the stainless steel bending flex needle spiral part through the magnetic field under the spiral centripetal force on the polymer macromolecule chain, so that the polymer macromolecule chain between the twisting effect, then the polymer spinning fluid finally sprayed out of the stainless steel bending flex needle, the needle and high-voltage electrostatic. When the high-voltage electrostatic generator generates high-voltage electrostatic, the polymer spinning liquid is stretched into nano-fibers and collected evenly on the aluminum foil by the high-voltage electrostatic generator.

Materials and methods

Experimental materials and apparatus

Polylactic acid (PLA), Shenzhen Guanghua Weiye Co., Ltd; Graphite oxide (GO), Suzhou Gerry Pharmaceutical Technology Co. Ltd.; tris(hydroxymethyl)aminomethane (Tris-HCL), Suzhou Gerui Pharmaceutical Technology Co., Ltd.; methylene blue (MB), Sinopharm Group Chemical Reagent Co. Hydrochloric acid (HCl), Jiangsu Qiangsheng Functional Chemical Co., Ltd; Sodium hydroxide (NaOH), Jiangsu Qiangsheng Functional Chemistry Co., Ltd; Deionized water, self-made.

SL-5200DT ultrasonic cleaning machine (Nanjing Shunliu Instrument Co., Ltd.); S-4800 cold field emission scanning electron microscope (SEM) (HITACNT, Japan); INSTRON-3365 Universal Material Testing Machine (American INSTRON Company); POROMETER 3 G full-automatic specific surface pore tester (Quantachrome Company, USA); Kruss DSA 100 contact angle tester (CA) (Kruss, Germany); Q600 simultaneous thermal analyzer (TGA/DSC) (TA, USA); FLY-100/200 thermostatic shaker (Shanghai Shenxian Thermostatic Equipment Factory, China); Cary 5000 UV spectrophotometer (Agilent Instruments, China).

Preparation of PLA nanofiber membrane

A certain amount of PLA particles were dissolved in DCM and DMF with a mass ratio of 9:1, placed in a brown wide-mouth flask for strict sealing, stirred at room temperature for 6 h until the solution was clarified and transparent, and configured to obtain a spinning solution with a solution concentration of 8% by mass. Then the spinning was carried out by biomimetic spider spinning with the following spinning parameters: spinning flow rate of 1 ml/h, spinning voltage of 20 kv, spinning receiving distance of 18 cm, ambient temperature of 25°C ± 1°C and relative humidity of 55% ± 1%. The spinning process was subjected to certain shading treatment to prevent DCM from photocatalysis during the spinning process.

Preparation of PLA/GO porous nanofiber membrane

Based on the solution preparation and spinning parameters of PLA nanofiber membrane, GO with a mass fraction of 0.5% was added to DMF and shaken in ultrasound for 2 h to make GO dispersed in DMF. After that, PLA particles and DCM were added, where the concentration of PLA was 8% by mass and the solvent ratio between DCM and DMF is 9:1. The configured spinning solutions were placed in brown wide-mouth flasks and stirred at room temperature for 6 h. Before spinning, they were shaken by ultrasonic waves for half an hour. The spinning process and spinning process parameters were the same as the preparation of PLA nanofiber membrane.

Preparation of PLA/GO/DA porous nanofiber membrane

According to the principle of PLA/GO solution preparation, GO with a mass fraction of 0.5% was added to DMF and shaken in ultrasonic waves for 2 h to make GO dispersed in DMF. After that, PLA particles and DCM were added, where the concentration of PLA was 8% by mass and the solvent ratio between DCM and DMF is 9:1, and then PLA/GO/DA solution was prepared by adding dopamine hydrochloride (DA) with a mass fraction of 1.5% by mass and stirring for 1 h at room temperature. The spinning solution was ultrasonically shaken for 0.5 h before spinning to make the graphite oxide uniformly dispersed, and the configured spinning solution was placed in a brown wide-mouth bottle to prevent DCM photocatalysis. The spinning process and spinning process parameters were the same as the preparation of PLA nanofiber membrane.

Preparation of PLA/GO/PDA nanofiber membrane

The PLA/GO and PLA/GO/DA porous nanofiber membranes prepared above were used and prepared by infiltration with anhydrous ethanol. 0.121 g of Tris-HCL was added to 100 ml of deionized water to prepare a 10 mM Tris-HCL solution. The pH was adjusted to 8.5 by adding HCL solution dropwise, and then a certain amount of dopamine hydrochloride was added. Meanwhile, the alcoholized PLA/GO and PLA/GO/DA porous nanofiber membranes were added respectively, placed on a magnetic stirrer, and the solution would change from colorless and transparent to tan after stirring for 1 min at room temperature, and then slowly change from tan to black after stirring for 1 h at room temperature. Then, the nanofiber was placed in a shaker and the temperature and time were set at 80 RPM, and finally the PLA/GO/PDA nanofiber membrane was rinsed several times with deionized water to remove the excess PDA on the surface. Then the nanofiber membrane were placed in an oven at 50°C and dried for 6 h to obtain brown PLA/GO/PDA nanofiber membrane.

Testing and characterization

The morphological observation was performed by scanning electron microscopy (SEM) to observe the morphology of nanofiber, and a sample size of 100 nanofibers was randomly selected in the SEM image for measurement and statistics, and the nanofiber diameter was measured by ImageJ software, and the probability distribution of nanofiber diameter was analyzed by Origin software. nanofiber membrane pore structure characterization and analysis The nanofiber membrane samples were cut into square circles with a diameter of 25 mm, and the nanofiber membrane samples to be tested were first completely wetted with Porofil in a glass dish, and then the samples were laid flat in the test apparatus to determine the pore size distribution of the nanofiber membrane samples in the wet and dry states, respectively.

Characterization and analysis of the thermal properties of nanofiber membrane The nanofiber membrane was dried and cut into powder. The experimental tests were carried out with a synchronous thermal analyzer under an inert gas N2 environment with a flow rate of 50 ml/min, a set temperature range of 30°C–600°C, and a heating rate of 10°C/min. The slow heating rate can make the thermal decomposition more adequate, and the mass of nanofiber membrane should be added between 5 and 10 mg. The mechanical properties of nanofiber membrane were characterized and analyzed Firstly, the nanofiber membrane was cut into rectangular specimens of 40 mm × 10 mm size, and the thickness was measured at three different positions of the nanofiber membrane with a spiral micrometer, and the average value was obtained. Nanofiber membrane wettability characterization and analysis (CA) Deionized water was taken in a syringe, the volume of each drop was about 6 μL, and each sample was tested five times in different positions, and the average value of contact angle was obtained. The flat edge of nanofiber membrane was chosen to make the measurement results more accurate. The adsorption performance was characterized and analyzed by first weighing 10 mg of the sample and placing it in 30 ml of MB solution with a certain concentration for MB adsorption test. A small amount of the solution was taken with a pipette gun for each sample and then diluted for UV measurement to reduce the volume change of the solution brought by sampling and to facilitate the determination of MB UV spectra, and the calculation formula for the adsorption rate ŋ was calculated as follows.

η = \frac{c_{0} - c_{t}}{c_{0}} \times 100 %

(1)

In the above equation: c₀ – the original dye concentration; c_t – the dye concentration after t moments of adsorption.

Results and discussion

Oxidative polymerization mechanism of PDA in PLA/GO/DA fibers

Place the fiber membrane in Tris HCL buffer, and when dopamine in the fiber and surrounding liquid is oxidized and polymerized into polydopamine under the action of Tris HCL solution, the chemical reaction of dopamine based on PLA/GO/DA porous nanofiber membrane may be as shown in Figure 3. Finally, polydopamine occupies the original porous structure inside the porous fiber and is uniformly loaded on the outer surface of the fiber, Make the porous structure on the fiber surface completely disappear, as shown in Figure 4.

Figure 3.

Oxidative polymerization equation of dopamine.

Figure 4.

Schematic diagram of dopamine oxidative polymerization.

Surface morphology analysis

The nanofiber membrane was prepared by spinning using PLA/GO/DA hybrid spinning solution, and the nanofiber membrane morphology was observed by scanning electron microscopy, and the results are shown in Figure 5. From Figure 5(a), it can be seen that PLA nanofiber is coarse and the diameter of nanofiber is not uniform. The average diameter of PLA/GO porous nanofiber is reduced by 27.1% compared with PLA porous nanofiber, and the diameter of nanofiber is uniform, as shown in Figure 5(b). The addition of GO makes the conductivity of the spinning solution increase, and the electric field force makes the spinning solution easier to be drawn and elongated, so the diameter of nanofiber decreases and the diameter is more uniform. After adding DA, the diameter of nanofiber increases and the diameter is not uniform, while the pores on nanofiber start to become dense, and some granular substances appears on the surface of nanofiber, as in Figure 5(c).

Figure 5.

SEM (×2000, ×20,000) of three different porous nanofibers and their diameter distribution: (a) PLA nanofibers, (b) PLA/GO nanofibers, and (c) PLA/GO/DA nanofibers.

Analysis of nanofiber membrane pore structure

As shown in Figure 6, the pore size distribution image of pure PLA nanofibers and PLA/GO porous nanofibers are similar. There are two peaks in both plots, and the small peaks are to the right of the large peak with small pore size, as shown in Figure 6(a) and (b). This shows that there are two main concentration areas in the distribution of pore size, and the distribution of pore size is relatively wide. In contrast, there is only one peak in PLA/GO/DA porous nanofibers, which is reached at a pore size of 3.388 μm and accounts for 18.79% of the total number of pores with an increased percentage, as shown in Figure 5(c), which indicates that the addition of dopamine hydrochloride makes the pore distribution get narrower and narrower. From the comparison of the three peaks and pore size distributions in Figure 6(a) to (c), it can be observed that the pore size distribution of PLA/GO/DA nanofiber membranes is more concentrated (around 3.388 microns), while the pore size distribution of the other two nanofiber membranes is much wider.

Figure 6.

Pore size distribution of three different porous nanofiber membranes:(a) PLA nanofiber membrane, (b) PLA/GO nanofiber membrane, and (c) PLA/GO/DA nanofiber membrane.

From Table 1, it is found that the total number of pores in the nanofibers increased significantly after the addition of graphite oxide, but the average pore size decreased. The addition of graphite oxide turned the large pores in PLA nanofibers into small pores, which greatly increased the total number of pores while making the pore size distribution more concentrated and increasing the pore density. the addition of DA had no significant effect on the pore size, but the total number of pores per cm2 was reduced compared to PLA/GO nanofibers, and about 8.67 × 106 pore structures were occupied by dopamine hydrochloride.

Table 1.

Pore size related data of pure PLA, PLA/GO, and PLA/GO/DA nanofiber membranes.

Sample Type	Pore size/μm	Average pore size/μm	Pore density/(per/cm² )	Total number of pore/(pcs/cm² )
Pure PLA	6.74–10.1	8.42	2.023 × 10⁵	2.231 × 10⁶
PLA/GO	2.73–4.1	3.33	4.202 × 10⁶	2.562 × 10⁷
PLA/GO/DA	3.13–4.13	3.37	3.722 × 10⁶	1.695 × 10⁷

Analysis of the thermal properties of nanofiber membrane

As shown in Figure 7, only 0.002% of pure PLA remains after thermal decomposition, and there is basically no residual material; while when graphite oxide is added to the pure PLA porous nanofibers, the residual material after thermal decomposition increases significantly, and the residual mass fraction increases significantly to 2.625%. During the heat treatment of PLA/GO/DA nanofibers, there is an obvious DTG curve near 319 °C The peak of DTG curve of PLA/GO/PDA nanofibers is located around 366 °C, which indicates that the quality of nanofibers decreases steeply at this time; after thermal re-treatment, the residual impurities of nanofibers become more than that of PLA/GO nanofibers, increasing to 8.5%, which is 2.27 times of the residual mass of PLA/GO nanofibers. The DTG of PLA/GO/PDA nanofibers reaches the maximum rate of change, and the dehydrogenation reaction occurs in PLA/GO/PDA nanofibers, that is, the carbonization reaction, and the nanofiber mass starts to lose rapidly; meanwhile, there is also a small peak of DTG at around 85°C, which may be due to the fact that there are still a small amount of residual water and DCM solvent inside the nanofibers being evaporated, making the nanofiber mass decrease suddenly. Figure 5 shows the TG curves of PLA, PLA/GO and PLA/GO/PDA nanofibers, from which it is obvious that the addition of graphite oxide increases the residual mass by 2.625%, and then the addition of PDA increases the residual mass to 16.159%, and the mass residual increases significantly.

Figure 7.

Thermogram of the porous nanofibers: (a) TG-DTG curve of PLA/GO/PDA and (b) thermogravimetric curves of different nanofibers.

Analysis of mechanical properties of nanofiber membrane

As shown in Figure 8, the breaking strength and elongation at break of pure PLA nanofibers are significantly smaller than those of PLA/GO porous nanofibers. This is because graphite oxide has higher strength, while PLA itself has certain brittleness, so the fracture strength and elongation at break increase when GO is added, thus it is clear that the addition of graphite oxide optimizes the fracture strength and elongation at break of PLA/GO porous nanofibers. The fracture strength of PLA/GO/DA nanofibers in Figure 6 is much higher than that of pure PLA and PLA/GO nanofibers, and the area under the stress-strain curve in the figure is nearly twice as much as that of pure PLA and PLA/GO nanofibers. It is obvious from the data curve that the fracture strength of nanofibers is significantly enhanced after adding DA, which becomes 3.78 MPa, 2.52 times that of pure PLA nanofibers and 1.91 times that of PLA/GO nanofibers. However, the elongation at break was reduced to 76.2%, which was close to that of the pure PLA nanofiber.

Figure 8.

Mechanical diagram of three different porous nanofiber membranes: (a) stress-strain curve and (b) comparison of breaking strength and breaking elongation.

Nanofiber membrane wettability analysis (CA)

As shown in Figure 9, the pure PLA nanofiber itself is a kind of hydrophobic material, and graphite oxide and dopamine are hydrophilic materials. With the addition of graphite oxide and dopamine, the contact angle of the nanofiber membrane decreases by about 10°, respectively. However, by observing the electron micrographs of nanofiber, it is easy to find that the surface of nanofiber is still porous, and the proportion of graphite oxide and dopamine in the spinning solute is much smaller than that of poly(lactic acid). So both the nature of the material itself and the fiber structure make the PLA/GO and PLA/GO/DA nanofibers present hydrophobicity. When dopamine is self-polymerized by oxidation, the surface of nanofiber is wrapped by polydopamine, which changes the porous structure of nanofiber, while the polydopamine attached to the surface of nanofiber is hydrophilic material, making PLA/GO/PDA nanofiber become hydrophilic material and the contact angle is reduced to 0°.

Figure 9.

Contact angle and its visual diagram of PLA, PLA/GO, PLA/GO/DA, PLA/GO/PDA nanofiber membrane.

Adsorption performance analysis

Weigh 10 mg of the above three samples and place them in a 30 ml concentration of 30 mg/l MB solution for MB adsorption testing. In the experiment, samples were taken every 1 h, 3, 6, 12, and 24 h, and the samples were diluted to an appropriate multiple for UV testing. The samples at the last 24 h of each test may not be diluted, and the data were processed through the standard curve of MB concentration and absorbance (Formula 2), multiplied by the corresponding dilution multiple, Calculate the concentration of MB at this time, as shown in Table 2. Calculate the change in adsorption rate according to the adsorption rate formula (1).

y = 0.165 x + 0.085

(2)

Table 2.

The concentration change of MB adsorbed by three different nanofibers.

Time/h		1	3	6	12	24
Concentration of MB(mg/L)	X1	26.958	26.43	25.908	22.695	21.564
	X2	21.081	13.914	7.617	6.495	4.998
	X3	20.316	13.923	7.008	3.918	0.357

Three samples were tested for adsorption performance, and the three samples were: X1 for PLA/GO/DAnanofiber membrane in Tris-HCL solution with reaction temperature 45°C, reaction time 30 h, DA concentration 0 mg/ml for dopamine polymerization; X2 for PLA/GOnanofiber membrane in Tris-HCL solution with reaction temperature 45°C, reaction time 30 h, DA concentration 2 mg/ml in Tris-HCL solution for dopamine polymerization; X3 is PLA/GO/DAnanofiber membrane in reaction temperature 45°C, reaction time 30 h, DA concentration 2 mg/ml in Tris-HCL solution for dopamine polymerization. When DA is self-polymerized into PDA in Tris-HCL solution, the nanofiber surface inside the membrane is not only covered by it, but also the PDA can enter the pores inside the nanofiber.

As shown in Figure 10, the adsorption rate of sample X1 was only 28.12%, in which the adsorption effect of the fiber was obvious within 1 h, and then leveled off, and then climbed again within 6 h-12 h. The first adsorption may be due to MB adsorption by polydopamine on the fiber surface after oxidative polymerization of Tris-HCL solution, and the second may be secondary MB adsorption due to the polydopamine and graphite oxide contained inside the fiber. Comparing samples X2 and X3, the change curves of the adsorption amount-time relationship in the first period basically overlapped and the growth trend was obvious, while the growth trend of sample X2 leveled off in the later period, and the 24 h adsorption rate was 83.34%, while X3 still had a climbing trend and the final adsorption rate was 98.81, which may be due to the fact that the PLA/GO/PDA fibers prepared from PLA/GO/DA fibers also contained a large amount of polydopamine internally. contains a large amount of polydopamine, which allows the fiber to still adsorb MB after 6 h.

Figure 10.

Folding graph of adsorption rate of three different fiber membranes.

Different concentrations of MB solutions were prepared, 20 mg/l, 30 mg/l, 40 mg/l, 50 mg/l. The samples were weighed at a mass of about 10 mg, with an error of less than 0.1 mg, and placed in 30 ml of different MB solutions. Samples were taken at intervals of 1, 3, 6, 12, and 24 h, and the samples were diluted by appropriate multiples for UV testing, and the samples at the last 24 h in each test could be processed without dilution, and the test values of adsorption rate changes are shown in Figure 11. From Figure 11, it can be seen that when the initial concentration of MB is 20 mg/l, the fiber has leveled off by 12 h, and the 24 h adsorption rate is 100%. When the initial concentration of MB was 30 mg/l and 40 mg/l, the adsorption rate curves of both were similar, and the 24 h adsorption rate was 98.81% and 96.71% respectively, while when the concentration added value of 50 mg/l, the principal of fiber adsorption of MB obviously declined, and the 24 h adsorption rate was 81.03%.

Figure 11.

Folding line graph of adsorption rate of fiber membrane at different MB concentration.

Conclusion

The PLA/GO porous nanofiber membrane was prepared by synthetic spider spinning technique, and the PLA/GO/DA porous nanofiber membrane was prepared on the basis of this technique, and finally the PLA/GO/ PDAnanofiber membrane by dopamine oxidative polymerization to study its adsorption properties, and the main conclusions are as follows.

(1) PLAnanofiber membrane was prepared using electrospinning technology, and graphite oxide was added to the pure PLA nanofiber spinning process for characterization and analysis of related experiments, and its average diameter was 617 nm, with uniform nanofiber thickness; the average pore size was 3.33 μm, and the total number of pores was 2.562 × 106/cm², which is 11.48 times of pure PLA; the fracture strength is 1.97 MPa and the elongation at break is 93.2 %, and the total number of pores and mechanical properties of nanofibers are improved.

(2) PLA/GO/DA porous nanofiber membrane was prepared on the basis of electrostatic PLA/GO porous nanofiber membrane, and the average diameter was increased to 737 nm. the average pore size remained unchanged, and the total number of pores declined. However, the fracture strength was increased to 3.78 MPa. Meanwhile, thermogravimetric analysis showed that the DTG peak was located at 319°C, when the carbonization reaction was the most intense, and the final residual mass was 8.5%, mostly DA mass residual.

(3) DA is oxidative polymerized into PDA to obtain PLA/GO/PDAnanofiber membrane with an average diameter of 996 nm and a layer of polydopamine with a thickness of about 129 nm attached to the surface with a dense and uniform distribution and durable stability, thus making the contact angle drop to 0° and becoming hydrophilic The nanofiber membrane is a hydrophilic material. The residual mass of nanofiber membrane under thermogravimetric analysis is 16.159 %, which is mostly PDA mass residue. Finally, the adsorption application of PLA/GO/PDA nanofiber membrane was investigated. By adsorbing 30 mg/l of MB solution, the adsorption rate was 98.81% at 24 h, which was significantly better than that of PLA/GO/PDA nanofiber prepared under PLA/GO nanofiber, and the adsorption of MB might have a secondary climbing phenomenon. And comparing the adsorption performance of nanofiber membrane under different MB concentrations, all of them showed good performance, and the best MB concentration was 40 mg/l, and the adsorption rate at 24 h was 96.71%.

Footnotes

Acknowledgements

Ting Zhu and Lei Zhao are co-first authors of the article.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research is funded by Jiangsu Higher Vocational College Teachers’ Professional Leaders’ High and Training(Team Visit) Project(2022TDFX008). The work is also funded by Qing Lan Project of Jiangsu Colleges and Universities for Excellent Teaching Team in 2023, the Integration Platform of Industry and Education of Jiangsu Higher Vocational Education(Grant number: Jiangsu Vocational Education 2019. No 26), Jiangsu Province Higher Vocational Education High-level Major Group Construction Project-Modern Textile Technology Major Group(Grant number: Jiangsu Vocational Education 2020. No 31). Brand Major Construction Project of International Talent Training in Colleges and Universities-Modern Textile Technology Major(Grant number: Jiangsu Foreign Cooperation Exchange Education 2022. No 8) also supports the research of this subject. Key technology innovation platform for flame retardant fiber and functional textiles in Jiangsu Province (2022JMRH-003) also suppoets this research.

Institutional review board statement

This research did not require ethical approval.

Informed consent statement

Not applicable.

Data availability statement

Not applicable.

ORCID iD

Lei Zhao

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Biomimetic electrospinning for fabrication of poly(lactic acid)/graphite oxide/poly(dopamine) nanofiber membrane

Abstract

Keywords

Introduction

Materials and methods

Experimental materials and apparatus

Preparation of PLA nanofiber membrane

Preparation of PLA/GO porous nanofiber membrane

Preparation of PLA/GO/DA porous nanofiber membrane

Preparation of PLA/GO/PDA nanofiber membrane

Testing and characterization

Results and discussion

Oxidative polymerization mechanism of PDA in PLA/GO/DA fibers

Surface morphology analysis

Analysis of nanofiber membrane pore structure

Analysis of the thermal properties of nanofiber membrane

Analysis of mechanical properties of nanofiber membrane

Nanofiber membrane wettability analysis (CA)

Adsorption performance analysis

Conclusion

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

Institutional review board statement

Informed consent statement

Data availability statement

ORCID iD

References