In this study, a hydrophobic antibacterial cotton fabric was achieved via a simple in-situ deposition method, as shown in Scheme 1. First, the cotton fabric was vertically immersed in a precursor solution containing 2-MI. Upon the addition of ZnCl2 or ZnCl2/CuCl2, simultaneous nucleation and crystal growth took place on the cotton fabrics. Alternatively, Cu2+ could form coordination with dimethylimidazole; however, its binding affinity was comparatively weaker than that of Zn2+ (Yang et al. 2023). During this process, ZIF-L or Cu@ZIF-L formed and deposited on the fabric surface, which could be attributed to electrostatic and coordination interaction between the hydroxyl groups of cellulose and Zn2+/Cu2+ ions (Yang et al. 2020). The Cu@ZIF-L@Cotton fabric was subsequently subjected to chemical vapor deposition of MTMS for surface modification, aiming to achieve a low-energy state and thereby obtain a hydrophobic cotton fabric.
The surface morphology structure of the pristine and modified cotton fabrics was characterized by SEM. As shown in Figure S1, the pristine cotton fabric has a smooth surface. When ZIF-L or Cu@ZIF-L were formed and in-suit grew on the surface of cotton fabric, the surface morphology of cotton fibers was changed obviously. As shown in Figs. 1a and b, the coated cotton fabric was covered by a thick layer of ZIF-L or Cu@ZIF-L nano-dagger. Each nano-dagger resembled a vertically extending sharp-tipped leaf, intricately arranged on the cotton fabric surface and fully covered the substrates. These structures correspond to the characteristic appearance of the ZIF-L nanostructure. The observed vertical alignment could primarily be attributed to the confined growth of ZIF-L, wherein the complete coverage of cotton fiber by small ZIF-L crystals leads to the subsequent appearance of larger ZIF-L crystals during continuous growth. As indicated by the red arrows, the ZIF scattered on the fabric surface reveals nano-dagger with dimensions of ~ 2 µm in width and ~ 6 µm in height. By adjusting the molar ratio of 2-MI/Zn2+ and reaction time, the amount and morphology of the ZIF-L could be changed (Figure S2). When the amount of 2-MI was low, the surface coverage of the fabric appeared incomplete or the structure of the surface layer was indistinct. With an increase in the content of 2-MI and reaction time, both the size and density of ZIF-L or Cu@ZIF-L exhibit a corresponding increase. Notably, as shown in Figure S3, for Cu@ZIF-L, at a molar ratio of 2-MI to Zn2+ equal to 5, a relatively sparse distribution of Cu@ZIF-L was observed on the fabric surface. When the ratio of 2-MI to Zn2+ reached 7:1, the dense and uniform layer of Cu@ZIF-L be achieved. After MTMS modification, it could be observed that the morphological structure is unchanged compared with Cu@ZIF-L@Cotton fabric, and it was still coated with a tight nano-dagger structure (Fig. 1c). The chemical composition of samples was characterized using EDS mapping. In the case of ZIF@Cotton, C, O, N, and Zn elements were detected (Figure S4a). Additionally, Cu element was observed in Cu@ZIF@Cotton along with the aforementioned elements found in ZIF@Cotton (Figure S4b). These results further confirmed the successful formation of ZIF-L or Cu@ZIF-L on the surface of cotton fibers. Furthermore, EDS analysis revealed that MTMS/Cu@ZIF-L@Cotton exhibited surface elements including C, O, N, Zn, Cu, and Si (from MTMS) (Fig. 1d). Notably, these elements were uniformly distributed on the surface of fabric fibers, so it could be proved that MTMS was well attached to the surface of the Cu@ZIF-L@Cotton.
The chemical composition of different samples was characterized by FT-IR, which was depicted in Figs. 2a and b. For ZIF-L and Cu@ZIF-L, the absorption peak at 3100 cm− 1 and 2920 cm− 1 belonged to the aromatic and aliphatic C-H stretch of imidazole, respectively. The peak observed at 1147 cm− 1 corresponded to the C-N stretching vibration of imidazole linkers in ZIF-L. The strong absorption bands detected at 757 cm− 1 and 690 cm− 1 could be attributed to the out-of-plane bending of the Hmim ring, while the band observed at 1308 cm− 1 was associated with the in-plane bending (Zhang et al. 2018). The band observed at 1567 cm− 1 corresponded to the C = N double band stretching vibration. Furthermore, the absorption peak around 420 cm− 1 signified the Zn-N stretching mode of the formed ZIF-L, indicating the coordination of Zn2+ with nitrogen atoms in 2-MI to form imidazolate(Bustamante et al. 2014). The absorption peak at about 550–620 cm− 1 belonged to the Cu-N bond, which was observed on the spectrum of Cu@ZIF-L (Yang et al. 2023). This suggested the successful embedding of Cu 2+ on ZIF-L. As depicted in Fig. 2b, the pristine cotton fabric exhibited only two main characteristic peaks at 2901 cm− 1 and 3320 cm− 1, corresponding to the stretching vibrations of C-H and -OH groups, respectively. After the deposition of ZIF-L and Cu@ZIF-L, distinct characteristic peaks attributed to ZIF-L and Cu@ZIF-L emerged, indicating the successful growth of these materials on the cotton substrate. After the silane modification, the MTMS/Cu@ZIF-L@Cotton exhibited two new characteristic peaks at 1262 cm− 1 and 801 cm− 1, attributed to Si-CH3 and Si-O-Si, indicating the formation of a siloxane network (Qin et al. 2021; Fan et al. 2023).
To further determine the chemical composition changes of fabrics at each modification step, the XPS testing was performed to further study the elemental composition and valence of the related samples. In the wide-scan XPS spectrum of pristine cotton fabric, as shown in Fig. 2c, only peaks corresponding to C1s and O1s were observed. Upon deposition of ZIF-L, three additional peaks emerged at 1044.58 eV, 1021.62 eV, and 400.09 eV, which could be attributed to Zn 2p 1/2, Zn 2p 3/2, and N1s states respectively (Fig. 2d) (Ananth et al. 2017; Wang et al. 2023). After depositing Cu@ZIF-L, resulted in the appearance of Cu 2p peaks in addition to the aforementioned ones. The high-resolution spectrum of Cu 2p was divided into three parts (Fig. 2e), in which the peaks centered at approximately 953.18 and 933.35 eV correspond to Cu 2p 1/2 and Cu 2p 3/2, respectively (Li et al. 2019). Additionally, it is noteworthy that two satellite peaks were observed at 944.48 and 940.82 eV, indicating the predominant presence of the Cu2+ valence state (Elfeky et al. 2019; Sun et al. 2020). In addition, the deconvolution of N 1s revealed three distinct peaks at 407.50, 400.08, and 398.75 eV (Fig. 2f), corresponding to the N = C, N − Zn & N − Cu, and N − C bonds respectively (Bhattacharyya et al. 2016; Yang et al. 2023). Furthermore, after coating with MTMS, two new peaks around 101.0 (Si 2p) and 154.4 (Si 2s), indicating that Cu@ZIF-L@Cotton fabric was covered by MTMS (Yang et al. 2021).
The wettability of a material's surface is comprehensively determined by its chemical composition, surface roughness, and surface energy(Campoccia et al. 2013). Measurement of the water contact angle (WCA) is widely employed to characterize the surface wettability of materials. The WCA of the original cotton fabric was 0°, indicating a good hydrophilic (Fig. 3a). After surface modification by in-situ grown ZIF-L or Cu@ZIF-L, the WCA of the ZIF-L@Cotton and Cu@ZIF-L@Cotton fabric was still 0° on the basis of the increased surface roughness (Figs. 3b and c). The subsequent modification of MTMS on the Cu@ZIF-L@Cotton enhanced the hydrophobicity with a WCA of 132° ± 0.58 (Fig. 3d). The synergy between nanostructures and low surface energy contributes to the good hydrophobic performance of this material. In addition, water droplets repel the fabric surface when the contact angle is tested, which also illustrates the hydrophobicity of the sample surface (Fig. 3e and Movie S1). Furthermore, as shown in Fig. 3f, four kinds of droplets including dyed red water, milk, coffee, and juice steadily stood on the surface of Cu@ZIF-L@Cotton fabric displaying good anticontamination and water-resistant properties. To investigate the self-cleaning capability of the as-prepared fabric, the surface of Cu@ZIF-L@Cotton was contaminated with Congo red dye powders. As shown in Fig. 3g, water droplets could roll easily away from the surface carrying away the Congo red dye from the fabric surface. Therefore, the as-prepared Cu@ZIF-L@Cotton fabric showed good self-cleaning performance.
Furthermore, Gram-positive S. aureus was selected as the representative microorganism to explore the bacterial repellency properties. The adherence of bacteria on cotton, Cu@ZIF-L@Cotton, and MTMS/Cu@ZIF-L@Cotton fabrics was observed by SEM as shown in Fig. 3h. For cotton fabric, large amounts of bacteria adhere to the raw cotton fabric surface. Cu@ZIF-L@Cotton fabric also exhibited a significant adherence of S. aureus to the Cu@ZIF-L nanostructures. On one hand, the positive electrical property of Cu@ZIF-L enabled it to attract negatively charged bacteria. On the other hand, owing to its metal ion core and unsaturated nitrogen from the organic linker, Cu@ZIF-L exhibited an affinity for amino acid residues (Tanum et al. 2022), which are abundant in peptidoglycan-the main component of S. aureus cell wall (Romaniuk and Cegelski 2015) (Nikolic and Mudgil 2023)-thus facilitating its attraction towards bacteria. While in the MTMS/Cu@ZIF-L@Cotton fabric, the adhered bacteria were reduced. The hydrophobicity surface demonstrated the repelling effect against bacteria, those tenacious bacteria that managed to be in touch with the surface were killed by Cu@ZIF-L itself.
The antibacterial performance of as-prepared Cu@ZIF-L@Cotton fabric was evaluated using the agar diffusion plate method and shake flask method against E. coli and S. aureus. As displayed in Fig. 4a and Table 1, without inhibition zone around the pristine cotton was observed for E. coli and S. aureus. For ZIF-L@Cotton and Cu@ZIF-L@Cotton fabric, obvious inhibition zones were seen against all of the strains compared to the uncoated sample fabric. However, after MTMS coating, the distinct inhibition zone was still observed except for a slight decrease against E. coli and S. aureus, which might be attributed to the prohibition of the diffusion of Zn2+ and Cu2+ by hydrophobic treatment.
The shaking flask method was also employed to assess their antibacterial activities. As depicted in Fig. 4b, the ZIF-L@Cotton, and Cu@ZIF-L@Cotton fabrics exhibited pronounced antibacterial properties compared to the pristine cotton fabric. The antibacterial efficacy of the ZIF-L@Cotton fabric against S. aureus was significantly superior to that against E. coli, owing to the absence of an outer membrane in Gram-positive bacteria (S. aureus), rendering them more susceptible to destruction as opposed to Gram-negative bacteria (E. coli) with their dual-membrane structure separated by peptidoglycan (Han et al. 2018). Remarkably, the Cu@ZIF-L@Cotton fabric demonstrated exceptional antibacterial activities, achieving a 5 log CFU reduction of both E. coli and S. aureus, indicating a promising killing efficacy of 99.999%. Consequently, doping Cu2+ into ZIF-L enhanced the antibacterial properties of the fabric considerably. Furthermore, it was worth noting that the antibacterial properties of the sample remained unaffected by modification through MTMS, as evidenced by the absence of bacterial growth on the agar plate. The MTMS/Cu@ZIF-L@Cotton fabric exhibited remarkable antibacterial properties and self-cleaning capabilities, effectively killing tenaciously adhered bacteria on its surface due to the exceptional antibacterial efficacy of Cu@ZIF-L.
Table 1
Zone of inhibition (mm) of samples.
Microorganism | Pristine Cotton | ZIF-L@Cotton | Cu@ZIF-L@Cotton | MTMS/Cu@ZIF-L@Cotton |
E. coli/mm | 0 | 10.2 | 10.3 | 5.8 |
S. aureus/mm | 0 | 11.2 | 12.9 | 8.8 |
The functional stability of the as-prepared cotton fabric against washing was evaluated by testing the changes in the inhibition zone and WCA. As shown in Fig. 5a, the Cu@ZIF-L@Cotton fabric still displayed an obvious inhibition zone after undergoing washing, indicating its good antibacterial activity. In addition, the WCA of the Cu@ZIF-L@Cotton fabric was still above 120° after undergoing five laundering cycles. These results indicated that the cotton fabric prepared in this study exhibited good functional stability.