Preparation and adsorption application of PLA/GO/PDA nanofiber membrane (2024)

PLA/GO/PDA porous nanofiber membranes were prepared by electrospinning, and then soaked in anhydrous ethanol for later use. 0.121 g of Tris-HCL was added to 100 ml of deionized water to prepare Tris-HCL solution. The pH value was adjusted to 8.5 by adding HCL solution, and a certain amount of hydrochloric dopamine was added. The alcoholized PLA/GO/PDA porous nanofiber membranes were added and placed on a magnetic stirrer. After stirring at room temperature for 1 min, the solution changed from colorless and transparent to brown. After stirring at room temperature for 1 h, the solution gradually turned black. Then, it was placed in a shaker with a certain temperature and time, and the speed was set to 80RPM. Finally, the prepared PLA/GO/PDA fiber membrane was rinsed several times with deionized water to remove excess PDA on the surface, and then dried in an oven at 50 °C for 6 h to obtain a brown PLA/GO/PDA nanofiber membrane. The specific operation process is shown in figure 1.

The fiber membrane was placed in Tris-HCL buffer solution, and dopamine inside and around the fiber underwent oxidative polymerization to form polydopamine under the action of Tris-HCL solution. The chemical reaction of dopamine on the porous PLA/GO/DA nanofiber membrane may be as shown in figure 2, and finally, polydopamine occupies the original porous structure inside the porous fiber and is uniformly loaded on the outer surface of the fiber, completely eliminating the porous structure on the fiber surface.

In the preparation process of PLA/GO/PDA nanofibers, the main influencing process factors considered are DA concentration (mg/ml), reaction time (h), and reaction temperature (°C). The approximate ranges for each factor are taken into account, and the specific factors and levels designed are shown in table 2.

DA concentration (mg/ml), reaction time (h), and reaction temperature (°C) are all set at four levels, represented by A, B, and C, with the number of levels denoted by a, b, and c, respectively. Hence, f_A = f_B = f_C = 4-1 = 3, f = f_A+f_B+f_C = 3+3+3 = 9. Since the total number n > 9, the L₁₆(4³) orthogonal experimental design is selected for the experiments, and the specific experimental parameters are shown in table 3. The experimental sequence follows the random principle.

In order to further analyze and explore the influence of factors A, B, and C on the adsorption performance of the prepared fibers, variance analysis is conducted on the orthogonal experimental results. And calculate the relevant data based on 1, 2, 3, and 4.

Note: a- the number of levels of factor A, B and C factors SB and SC are the same as formula 3; n is the total number of experiments; f = a-1.

The adsorption of MB on PLA/GO/PDA fiber membranes prepared by orthogonal experiment F1∼F16 will be tested, and the absorbance of the dye will be accurately measured using UV-visible absorption spectroscopy to determine the adsorption effect of the prepared fiber membranes.

2.3.4.1.Determination of the standard curve of MB

2.3.4.2.Determination of adsorption performance of PLA/GO/PDA fiber membrane

The breaking strength and elongation at break of the fiber membranes were determined using a universal material testing machine. First, rectangular samples with dimensions of 40 mm × 10 mm were prepared. The ends of the samples were wrapped with aluminum foil, leaving a 10 mm × 10 mm section exposed. The thickness of the fiber membrane was measured at three different positions using a screw micrometer, and the average value was calculated. The stretching test was performed on a section of the sample with uniform thickness. After placing the samples in a constant temperature and humidity chamber for 24 h, they were individually subjected to the stretching test. The test distance was set at 20 mm and the stretching speed was 20 mm min⁻¹. Each sample was measured 5 times to obtain the average value. The data was organized, and the breaking strength and elongation at break of the fiber membrane were calculated using formulas 7 and 8.

Low magnification electron microscopy images of PLA/GO/PDA fibers prepared under different process parameters are shown in figure 3 (high magnification image in the upper right corner). From figure 3, it can be seen that there is only a small amount of PDA on the surface and between the fibers of F1, F5, and F9, which may be due to the low amount of PDA attached to the fibers themselves. After several deionized water washes, the remaining PDA is even less, and there are still a few very fine fibers in F1 and F5, indicating that F9 may not be completely dried. More PDA starts to appear between the fibers of F7, F10, and F14, but there is uneven distribution, which may be due to the lack of anchoring points between the fibers, making it easier for the PDA to be washed away by deionized water. In F2, F3, and F4, there is a large amount of PDA not only between the fibers but also wrapped around the fibers. The amount of PDA between the fibers in F6, F12, F13, and F15 is significantly reduced compared to F2, F3, and F4, and there is rarely a large aggregation of PDA. At the same time, the amount of PDA attached to the fibers increases. There is almost no PDA between the fibers of F8, F11, and F16. PDA is mainly attached to the surface of each fiber, which may be because PDA undergoes oxidation polymerization inside the fibers and is not easily washed away even after several deionized water washes.

From the high magnification electron microscope image in figure 3, it can be seen that the surfaces of fibers F1, F5, and F9 still have obvious porous structures, with only a few fibers having less noticeable porous structures, and the fibers are basically not attached to PDA. Fibers F7, F10, and F14 do not have the same obvious porous structures as F1 and F5, and a small amount of PDA begins to adhere to the fibers, and PDA also starts to aggregate between the fibers. The porous structures on the surfaces of fibers F2, F3, and F4 have basically disappeared, with only a few fibers showing localized porous structures. The amount of PDA adhered to the fibers begins to increase, which may be the result of the rapid oxidation of DA inside the fibers and in the solution to form PDA. Fibers F6, F12, F13, and F15 have completely lost their porous structures, and there is a large amount of PDA present between and on the fibers. However, it can be clearly seen that the diameter of the PDA between the fibers is larger than that on the fibers, which may be due to the former being formed by the oxidation polymerization of dopamine in the solution, while the latter is formed by the oxidation polymerization of dopamine inside the fibers, and the latter has stronger adhesion to the fibers. Fibers F8, F11, and F16 start to have a large amount of PDA adhered to the fibers, while the PDA between the fibers begins to decrease, possibly transferred to the fibers. At the same time, the unattached PDA on the fibers is removed by deionized water washing. Meanwhile, the diameter of the fibers in F11 also starts to increase. To some extent, this increase in fiber diameter may be due to the swelling of the PLA matrix caused by the absorption of water during the degradation process [41, 42].

Before conducting adsorption performance testing, we selected 10 different locations on the selected characterization membrane for thickness testing. We found that the thickness of the nanofiber membrane prepared under all process schemes was controlled within ± 5%, indicating that the thickness of the nanofiber membrane was relatively uniform.

From figure 4(a), it can be seen that the UV absorption peak of MB is located at 664 nm, and the standard curve of MB concentration and absorbance is shown in figure 4(b). The results of the adsorption performance measurement of the fiber membrane under orthogonal experiment are shown in table 5. The intuitive graph of the influence of factors A, B, and C on the adsorption rate is shown in figure 5.

Through the analysis of experimental data and the combination of figure 5 and table 4, it is found that in figure 5(a), k_A1 < k_A3 < k_A2 < k_A4, and the data values of kA2, kA3, and kA4 are similar, indicating that the effect of oxidative polymerization of DA in the fiber is small when the temperature reaches 40 °C. In figure 5(a), k_B1 < k_B2 < k_B3 < k_B4, and the curve trend in the figure is close to a linear function, indicating that the oxidative polymerization effect of DA improves with increasing time. It is also found that the trend gradually slows down, and as the reaction time continues to increase, the degree of oxidation of DA tends to level off. In figure 5(c), it can be seen that k_C4 < k_C1 < k_C3 < k_C2. When the DA content is 1 mg ml⁻¹, the content of DA in the solution is obviously insufficient, resulting in a lower oxidation polymerization content of PDA and a decrease in adsorption rate. At the same time, with the rapid increase in DA content, reaching 3 mg ml⁻¹ or even higher concentrations, it is possible that excessive loading of PDA on the fiber surface causes agglomeration, resulting in a large amount of PDA falling off during the deionized water washing process of the fiber surface.

The variance analysis results of the adsorption performance of PLA/GO/PDA fibers are shown in table 5. By referring to the table, it can be concluded that F_0.01(3,6) = 9.78, F_0.05(3,6) = 4.76, F_0.1(3,6) = 3.29. In table 5, it can be found that the variance of factor A is significantly smaller than the error column, and F_A < F_0.1(3,6), indicating that factor A is not significant. Since F_0.05(3,6) < F_B < _0.01F(3,6), factor B is significant. F_0.1(3,6) < F_C < _0.05F(3,6), indicating that factor C is relatively significant.

Through intuitive analysis, variance analysis, and F-test of fiber adsorption data, the primary and secondary relationships of factors are determined to be: B > C > A, with the optimal scheme being B₄C₂A₄ considering energy saving. The impact of factor A is not significant. Therefore, the optimal scheme is chosen: B₄C₂A₂, which means a reaction temperature of 45 °C, a reaction time of 30 h, and a DA concentration of 2 mg ml⁻¹.

In order to verify the stability of the nanofiber membrane, the PLA/GO/PDA nanofiber membrane prepared by the optimal orthogonal experimental process was reused 5 times, and after 24 h, the adsorption rate of MB dye still reached over 80%.

As shown in figure 6, the PLA/GO/PDA nanofiber membrane was prepared under the conditions of a temperature of 45 °C, a time of 30 h, and a DA concentration of 2 mg ml⁻¹. It can be clearly seen from the figure that polydopamine is uniformly loaded on the outer surface of the fibers. At the same time, the original porous structure on the fiber surface is occupied by polydopamine, allowing the fibers to load polydopamine to the maximum extent. From the fiber diameter distribution chart, it can be observed that the majority of fiber diameters are distributed around 1000 nm. The average fiber diameter is calculated to be 996.16 nm, with a standard deviation of 232.86 ± 45.64 nm. This indicates that the loading of polydopamine has increased the fiber diameter to some extent and caused uneven thickness in some parts of the fibers.

The FTIR spectra of A-pure PLA, B-PLA/GO, and C-PLA/GO/PDA nanofibers are shown in figure 7. It can be observed that with the addition of PDA, the characteristic peaks corresponding to PLA are weakened, such as the C=O stretching vibration peak at 1746 cm⁻¹ and the C-H stretching vibration peak at 2940 cm⁻¹. At the same time, the curve shows the appearance of the C=C stretching vibration of PDA at 1594 cm⁻¹, and a broad absorption band appears at 3327 cm⁻¹, which is the N-H stretching vibration peak of PDA.

The thermogravimetric analysis curves of the above three types of nanofibers are shown in figure 8. In figure 8(a), the peak of DTG is around 366 °C, indicating that the mass change rate reaches its maximum, and the PLA/GO/PDA fiber undergoes a dehydrogenation reaction, which is the carbonization reaction, and the fiber mass begins to rapidly decrease. At the same time, there is a small peak in DTG around 85 °C, which may be due to the residual moisture and DCM solvent evaporating from the fiber, causing a sudden decrease in fiber mass. Figure 8(b) shows the TG curves of PLA, PLA/GO, and PLA/GO/PDA fibers. It can be clearly seen that the addition of graphite oxide increases the residual mass by 2.625%, and the addition of PDA further increases the residual mass to 16.159%, significantly increasing the mass residue.

The wettability test results of the fiber membrane are shown in figure 9. Pure PLA fiber itself is a superhydrophobic material, while graphite oxide and dopamine are hydrophilic materials. With the addition of graphite oxide and dopamine, the contact angle of the fiber membrane decreases by about 10°. By observing the fiber electron microscope images, it is not difficult to find that the fiber surface still has a porous structure. Both the material properties and the fiber structure make PLA/GO and PLA/GO/PDA fibers hydrophobic. When dopamine undergoes oxidative self-polymerization, the fiber surface is wrapped by polydopamine, changing the porous structure of the fiber. At the same time, the polydopamine attached to the fiber surface is a hydrophilic material, making the PLA/GO/PDA fiber hydrophilic with a contact angle of 0°.

Figure 10 shows the distribution of C, N, and O elements in the PLA/GO/PDA fiber. The samples were placed above a conductive adhesive. If the conductivity was poor, a 90 s gold spraying treatment could be performed. In this sample, no gold spraying treatment was conducted. Since the fibers were spun on aluminum foil during the electrospinning process of PLA and PLA/GO fibers, there is some Al in the samples. However, PLA/GO/PDA fibers have already separated from the aluminum foil during the preparation process, so there is no Al present. Based on figure 11, it is evident that the C element has the highest content, followed by O, which are both widely and evenly distributed. On the other hand, N has a relatively low content, with only a few scattered in the fibers, while the majority of the surrounding area is black.

The energy spectra of PLA, PLA/GO, and PLA/GO/PDA fibers are shown in figure 11. Due to the molecular structure of polylactic acid (C₃H₄O₂)_n, it contains elements C, H, and O. As shown in figures 11, it is obvious that the content of C is much higher than that of O. Graphite oxide undergoes graphite oxidation–reduction reaction, which increases the element O in addition to C and H. As shown in figures 11, the content of C increases while the content of O decreases. The molecular structure of dopamine is C₈H₁₁O₂N, so the elements contained in polydopamine are: C, H, O, and N. Figures 11 indicates that the mass of N is 8.752, accounting for 8.26% of the total atomic mass, while the mass of C is 59.568%, which is 6.8 times that of N, and the mass of O is 31.68%, which is 3.6 times that of N. Although the content of N is small, it indirectly indicates that a large amount of polydopamine is loaded on the fiber, which can be shown in the energy spectrum with 8.752% of N.

Using MB as the adsorption object, the adsorption performance of three samples was tested. The three samples are X1-PLA/GO/DA fibers, which were subjected to dopamine polymerization in a Tris-HCL solution with a reaction temperature of 45 °C, a reaction time of 30 h, and a DA concentration of 0 mg ml⁻¹. X2-PLA/GO fibers were subjected to dopamine polymerization in a Tris-HCL solution with a reaction temperature of 45 °C, a reaction time of 30 h, and a DA concentration of 2 mg ml⁻¹. X3-PLA/GO/DA fibers were subjected to dopamine polymerization in a Tris-HCL solution with a reaction temperature of 45 °C, a reaction time of 30 h, and a DA concentration of 2 mg ml⁻¹.

Weighing 10 mg of each sample, they were placed in a 30 ml solution of MB with a concentration of 30 mg l⁻¹ for MB adsorption testing. A small amount of solution was taken using a pipette for each sampling, and then diluted for UV measurement to reduce the volume change caused by sampling and facilitate MB UV spectrum measurement. This is because a high MB concentration can easily result in a low correlation coefficient R² of the MB standard curve and is not applicable.

In the experiment, samples were taken at intervals of 1 h, 3 h, 6 h, 12 h, and 24 h, and after appropriate dilution, UV testing was performed. The adsorption rate change was calculated based on the adsorption rate formula, as shown in figure 12. From the graph, it can be seen that the adsorption rate of sample X1 is only 28.12%. The fiber adsorption effect is obvious within 1 h, then becomes flat, and then rises again between 6 h and 12 h. The first adsorption may be due to the adsorption of MB by the polydopamine formed by the oxidation polymerization of the fiber surface in the Tris-HCL solution, and the second adsorption may be due to the secondary MB adsorption by the polydopamine and oxidized graphite inside the fiber. Comparing samples X2 and X3, the initial adsorption amount-time relationship curves of the two samples overlap, showing a significant growth trend. However, the growth trend of sample X2 becomes flat in the later stage, with an adsorption rate of 83.34% at 24 h, while sample X3 still shows an upward trend, with a final adsorption rate of 98.81%. This may be because PLA/GO/DA fibers prepared from PLA/GO/PDA fibers also contain a large amount of polydopamine, allowing the fibers to continue adsorbing MB after 6 h.

Prepare different concentrations of MB solutions, including 20 mg L⁻¹, 30 mg L⁻¹, 40 mg L⁻¹, and 50 mg l⁻¹. Weigh samples with a mass of about 10 mg, with an error less than 0.1 mg, and place them in 30 ml of different MB solutions. Take samples every 1 h, 3 h, 6 h, 12 h, and 24 h, dilute the samples appropriately, and perform UV testing. The samples at the last 24 h of each test can be tested without dilution. Calculate the MB concentration using formula 5, multiplied by the corresponding dilution factor, and record it in table 5. Then, calculate the adsorption rate change using formula 6, as shown in figure 13. From figure 13, it can be seen that when the initial concentration of MB is 2 0 mg L⁻¹, the adsorption rate of the fiber becomes stable at 12 h, and the adsorption rate at 24 h is 100%. When the initial concentration of MB is 30 mg L⁻¹ and 40 mg L⁻¹, the adsorption rate curves of both are similar, with adsorption rates of 98.81% and 96.71% at 24 h, respectively. However, when the concentration increases to 50 mg L⁻¹, the ability of the fiber to adsorb MB decreases significantly, with an adsorption rate of 81.03% at 24 h.