Characterization of 4-DMPY
The FTIR spectra of 4-DMPY are shown in Fig. 1(a). Two peaks near 3436 and 3123 cm-1 are attributed to N-H stretching vibrations in the haines. Two weak peaks at 687 and 616 cm-1 are due to the out-of-plane swinging vibrations of N-H and O = C-N in the haines. Absorption peaks at 1193 and 1178 cm-1 are due to C-N bond stretching vibrations in the haines ring. Two absorption peaks at 1782 and 1749 cm-1 are attributed to symmetric coupling vibrations and asymmetric coupling vibrations of C = O bonds in the haines(Wen et al. 2020). The absorption peaks at 3065, 3035, 2958, 2923, 2894 and 2845 cm-1 are C-H absorption peaks, inferring that the compound contains both saturated and unsaturated C-H bonds. C = C out-of-plane deformation vibration was generated at 1730 cm-1. C = N stretching vibration was generated at 1639 cm-1. Pyridine ring skeleton vibrations were at 1583, 1548 and 1518 cm-1. Based on the above peak analysis, it can be tentatively proved that the target product 4-DMPY was successfully synthesized.
1HNMR was used to further determine the structure of the compounds. The 1HNMR of the intermediate product is shown in Fig. 1(b, left), 1HNMR (DMSO,400MHz,TMS) δH: 11.46(s,1H), 8.19(d,J = 5.96Hz,1H), 7.84(s, 1H), 8.19(d,J = 5.96Hz,1H), 4.43(s,2H). The 1HNMR of compound 4-DMPY is shown in Fig. 1(b, right), 1HNMR (DMSO, 400 MHz, TMS) δH: 8.57(d,J = 6.48Hz,1H), 8.57(d,J = 6.48Hz,1H), 7.95(s,1H), 6.14(m,1H), 5.52(m,2H), 5.20(d ,2H), 4.54 (s,2H). The numerical labels of the chemical formula in Fig. 1 corresponded to labeled numbers of the peaks in the 1HNMR spectra, and the structure of the synthesized product 4-DMPY could be determined to be consistent with that of the target product by the above analysis.
Characterization Of Fabrics
The EDX spectrum of Cotton- 4DMPY-Cl is shown in Fig. 2(a). The additional appearance of Si, S, N and Cl elements on the cotton fabric is related to the MPTES, 4-DMPY and sodium hypochlorite used in the finishing process. In addition, the atomic ratios of N, S and Si elements were about 4:1:1,(Tan and Obendorf. 2007), which was consistent with the atomic ratio of 4-DMPY synthesized in Scheme 1. The above analysis tentatively confirmed that 4-DMPY was successfully grafted onto the cotton fabric
The XPS test was used to further confirm the grafting of the finishing fabric, and the results are shown in Fig. 2(b). Only two elements, C and O, were presented on the cotton fabric. Two additional elements, Si and S, were presented on Cotton-SH, indicating the presence of MPTES on the cotton fabric after finishing. New N and Cl elements were presented on Cotton-4DMPY-Cl, where N elements were associated with 4 DMPY, and the Cl element was associated with the conversion of N-H bonds to N-Cl bonds in 4-DMPY after chlorination treatment. The N1s at 401 eV and the Cl2p signal peaks at 198 eV were the most reliable evidence for the formation of N-haloamines on the finished fabrics.(Jiang et al. 2014)
The efficacy of the modification was analyzed by observing whether new functional groups appeared on the modified fabric by FTIR spectroscopy. The results are shown in Fig. 2(c). Compared with the Cotton-SH specimen, the Cotton-4DMPY-Cl specimen has an absorption peak at 3345 cm-1, which is due to the stretching vibration of the N-H bond in 4-DMPY. The absorption peaks of C = N bond stretching vibrations in 4-DMPY were at 1660 cm-1. Pyridine ring backbone vibrations in 4-DMPY were distributed at 1556, 1541 and 1517 cm-1. The above analysis indicates that 4-DMPY was successfully grafted onto the fabric.
The binding mode of 4-DMPY with cotton fabric was further analyzed by Raman spectra, and the results are shown in Fig. 2(d). Compared with the Cotton specimen, the Cotton-SH specimen showed a vibrational absorption peak of -SH at 2569 cm− 1, indicating that the hydrolytic condensation reaction occurred between MPTES and the hydroxyl groups of the cotton fabric. Compared with the Cotton-SH specimen, the vibrational absorption peak of -SH disappeared in the Raman spectrum of the Cotton-4DMPY specimen, indicating that the terminal olefin on the 4-DMPY underwent a mercapto-olefin click reaction with the sulfhydryl group on the Cotton-SH specimen. In addition, the Raman spectrum of the Cotton-4DMPY specimen showed C = O stretching vibrational peaks at 1751 and 1775 cm− 1. C = N bond vibrational absorption peaks at 1653 cm− 1, indicating that 4-DMPY was successfully grafted onto the cotton fabric by chemical bonding.
SEM images were used to observe the microscopic morphological changes of cotton fabrics before and after finishing, and the results are shown in Fig. 2(e). The surface of desizing cotton fabric is smooth. After a pretreatment by MPTES, the surface is attached with a little flaky thin layer. While 4-DMPY finishing and chlorination, the surface flaky thin layer increases with a bumpiness. Based on the analysis of SEM images, the finishing process made the surface of cotton fabric rough, but this change is relatively weak that will not cause serious impact on the subsequent usage.
Antibacterial Properties
The results of the 4-DMPY antibacterial activity test are shown in Fig. 3(a). The concentration of 4-DMPY gradually decreased in the 13 test tubes from left to right, and the solution in the test tubes became turbid as the concentration of 4-DMPY decreased. The solution was clear at a concentration of 20 g/L and translucent at a concentration of 17.5 g/L. The above indicates that 4-DMPY has antibacterial activity against both E. coli and S. aureus, and its minimum inhibitory concentration is about 20g/L.
The absorption method was used to detect the antibacterial activity of finishing cotton fabrics, Cotton-4DMPY and Cotton-4DMPY-Cl, and the results are shown in Fig. 3(b)(c)(d). From Fig. 3 (b), the antibacterial activity of Cotton-4DMPY increased with an increasing compound 4DMPY concentration. Its inhibition rate against E. coli and S. aureus was 99% and 98% when the concentration was 7%. From Fig. 3 (c) (d), the antibacterial activity of Cotton-4DMPY-Cl increased with an increasing concentration of compound 4-DMPY under constant chlorination treatment conditions. When the concentration of 4-DMPY was 0.5%, the inhibition rates of Cotton-4DMPY-Cl against E. coli and S. aureus were 98.9% and 98%, respectively, and both of them reached to 100% when the concentration of 4-DMPY was 1%. The above results showed that the antibacterial performance of Cotton-4DMPY was significantly improved after a chlorination treatment. It indicates that the combination of quaternary ammonium salt and N-halamine could effectively improve the antibacterial activity.
Antibacterial Mechanism
The results of Cotton-4DMPY-Cl bactericidal pathway test are shown in Fig. 4(b). he sample soaking solution was unable to inhibit bacterial growth. The finishing cotton fabric placed in the upper layer of the transwell did not kill the bacteria in the lower layer of the culture. When the sample was in direct contact with the bacteria, it could kill them. The above results show that the antimicrobial agent on Cotton-4DMPY-Cl is not released into the culture solution to kill the bacteria, but by contacting with the bacteria and then killing them.The results of the inhibition circle test results are shown in Fig. 4 (c). No inhibition circle appeared around the Cotton-4DMPY-Cl sample, but it was confirmed to have good antibacterial activity in the previous experiments, indicating that the antibacterial substance on Cotton-4DMPY-Cl is non-leaching. SEM images of bacteria on Cotton and Cotton-4DMPY-Cl samples are shown in Fig. 4 ( d), the bacteria on the surface of the original cotton fabric had a clear outline. While the bacteria on the surface of Cotton-4DMPY-Cl were depressed in the middle and the contents flowed out, indicating that the death of the bacteria was related to their disrupted structure. Figure 4(e) shows the bactericidal rate of Cotton-4DMPY-Cl in contact with bacteria. Cotton-4DMPY-Cl can kill 98.4% of Escherichia coli and 98.3% of Staphylococcus aureus within one minute, and its bactericidal rate can reach 100% within ten minutes.
The above analysis shows that the antimicrobial substance on the finishing cotton fabric is non-soluble and requires direct contact with bacteria to kill them. This result can be attributed to two reasons. Firstly, the antimicrobial substance is bonded to the cotton fabric in the form of a chemical bond so that the antimicrobial agent is not easily soluble. Secondly, the N-haloamine structure of the antimicrobial compound is relatively stable so that the active halogen is not easily dissociated into the solution during service. The antimicrobial substance is inactivated by disrupting the cell membrane which causes a leakage of intracellular components. So it is reasonable to speculate that the mechanism of action on bacteria could be the electrostatic attraction of pyridine quaternary ammonium salt, which attracted the anionic bacteria by the N-halamine site, where then kill them (Li et al. 2015; Luo et al. 2006; Xue and Xiao. 2015). as shown in Fig. 4(a).
Stability And Rechargeability
Table 1
Washing stability of Cotton-4DMPY-Cl
Wash cycles | Bacteria reduction(%) | Bacteria reduction(%) | Remained Chlorine(%) | Recovered chlorine after Rechlorination(%) |
E.coli | S.aureus |
0 | 100 | 100 | 0.86 | — |
5 | 97.51 | 98.10 | 0.81 | 1.11 |
15 | 95.02 | 93.33 | 0.80 | 1.06 |
30 | 91.59 | 93.05 | 0.70 | 1.17 |
The stability of Cotton-4DMPY-Cl is closely related to its antibacterial activity. Table 1 and Fig. 5 shows the results of washing cycle stability, storage stability and stability under UV irradiation.
From Table 1 Cotton-4DMPY-Cl retained 81% of the original active chlorine content after 30 washes, with 91.59% and 93.05% inhibition of E. coli and S. aureus, respectively. The chlorine content was restored by re-chlorination treatment. The storage stability test results are shown in Fig. 5(a). The active chlorine content on the specimen decreased slightly with fluctuation as the storage time was extended and retained 63% after 30 days of storage. The above results indicate that Cotton-4DMPY-Cl has good washing fastness and storage stability, which may be related to the stability of the cyclic N-haloamine structure on 4-DMPY in aqueous solution and a strong bonding of 4-DMPY with cotton fabric.
Cotton-4DMPY-Cl UV stability test results are shown in Fig. 5 (b), irradiation time of 24h active chlorine content decreased to 35% of the original, which may be related to the destruction of the N-Cl bond under UV light. The active chlorine content after chlorination again did not return to the original content, which may be related to the decomposition mechanism of the silicone coating, where the loss of the Hain ring from the material surface causes the N-H bond to be converted to a reduced number of N-Cl bonds(Kocer et al. 2010; Liu et al. 2016).
The N-Cl bond on the finishing fabric is consumed during sevice and can be restored again after chlorination treatment., and the results of Cotton-4DMPY-Cl rechargeability are shown in Fig. 5(c). After 8 consumption-rechlorination cycles, the active chlorine content remained at a relatively stable level, indicating that the antibacterial activity of Cotton-4DMPY-Cl has good rechargeability.
Whiteness, Strength And Hydrophilicity
The whiteness, strength and hydrophobicity of the fabric have important effects on its application. Figure 6 (a) (b) (c) shows the test results of whiteness, breaking strength and contact angle of the fabric before and after finishing, respectively. The increased whiteness of Cotton-4DMPY-Cl compared to the original cotton fabric is due to the bleaching effect of sodium hypochlorite used in the chlorination process. The breaking strength of cotton fabric increased to some extent after 4-DMPY finishing, but decreased after chlorination, indicating that the chlorination treatment would cause some damage to the fabric. The hydrophilicity of cotton fabric decreases after MPTES treatment, but has good hydrophilicity after 4DMPY and chlorination treatment, which is related to the pyridine ring in 4-DMPY. In conclusion, a series of finishing processes did not seriously affect its appearance, strength and hydrophilicity, and did not affect its subsequent application.