Silver nitrate (AgNO3, 0.0242 mol/L), Tetraethyl orthosilicate (TEOS, ≥99%), trimethoxy (octadecafluorodecyl) silane (FAS, 97%), Poly(vinylpyrrolidone) (PVP, Mw≈30k, powder), PDMS (prepolymer named as PDMS-A and the thermal curing agent named as PDMS-B) was purchased from Dow Corning Corp. Diethanolamine (DEA，≥99.5%), methanol, ethanol, tetrahydrofuran (THF, ≥99.5%), hexadecyltrimethylammonium chloride (CTAC, 25 wt.%), and hydrochloric acid (37%, analytical grade), were purchased from Sigma-Aldrich. Gram-positive bacteria Staphylococcus aureus (S. aureus, ATCC 6538) and Gram-negative bacteria Escherichia coli (E. coli, ATCC 25922) were incubated at 37ºC for 24 h before use. The cotton fabrics (200 g/m2) were purchased from a local store. All these above-mentioned reagents were used as received.
2.2. Preparation of two nanoparticles (MSNs and F-MSNs)
MSNs were synthesized with the use of the Stöber method according to the previous paper (Wu et al. 2019). Firstly, the solution of ethanol (9 mL) and water (64 mL) with 0.10 g of CTAC was magnetically stirred for 3 min. After that, the DEA (0.02 g) was added to control the pH of the solution. And then, 7 mL of TEOS was added dropwise while the above solution was still stirred. The reaction mixture gradually became milky white due to the formation of silica particles at 60ºC. After 3 h, the silica particles were obtained by centrifugation at 8500 rpm for 15 min. To remove the unreacted materials, the above silica particles were washed three times with ethanol and water respectively. In addition, F-MSNs were synthesized by the same process except that 1.0 g of FAS was also slowly added to the solution. To remove the CTAC which was the template in the above silica particles, two kinds of particles were all dispersed in 500 mL of the methanol solution with hydrochloric acid (30 mL), and the solution was refluxed for 24 h at 40ºC, respectively. To ensure that there was no residual hydrochloric acid, the particles after centrifuged were washed with distilled water and ethanol three times, respectively. Finally, two kinds of mesoporous nanoparticles were obtained after drying in a vacuum oven for 24 h.
2.3. Preparation of Ag-MSNs
Ag-MSNs were synthesized according to a previous report with appropriate modifications (Nie et al. 2018). Firstly, 1.0 g of MSNs was ultrasonically dispersed in 100 mL of ethanol, and then the mixture was stirred for 7 h at 45ºC. Whereafter, 87.5 mg of AgNO3 was slowly added into the above mixture under vigorous stirring and refluxed in the dark. And then, 2 mL ethanol mixed with 0.02 g NaBH4 was added dropwise and stirred for 3 h. Finally, Ag-MSNs were separated by using a centrifuge and washed with ethanol and water to remove the superfluous Ag+ if any, and the precipitates were dried at 25ºC.
2.4. Preparation of functionalized fabrics
To obtain functionalized fabrics, the 0.15 g of Ag-MSNs and 0.12 g F-MSNs were immersed into 30 mL THF containing 0.15 g of PDMS -A and 0.015 g PDMS-B by ultrasonic for 45 min. After that, clear cotton fabrics (4 cm × 10 cm) were put into the above solution under vortex shocking for 15 min. Finally, the fabrics were placed into the beaker and cured at 70ºC for 3 h.
2.5. Characterization of F-MSNs, Ag-MSNs and functionalized fabrics
Surface microstructures and morphology of F-MSNs, Ag-MSNs, and functionalized fabrics were obtained by TEM and SEM, and element mapping images were obtained from an energy-dispersive spectroscope which was attached to the SEM. The chemical functional groups of the nanoparticles were analyzed by a Fourier transform infrared spectroscopy (FTIR, Thermo Fisher Scientific Inc.) which covers a 250-4000 cm−1 infrared range with a resolution of 2 cm−1. A Kruss Easy Drop goniometer (Kruss Germany) was used to measure the water contact angles (CAs) and sliding angles (SAs) of each sample. Distilled water droplets with a volume of 8 μL were deposited on the samples at 25ºC. The functionalized fabrics were glued to the glass plate by double-sided tape before measured to ensure the accuracy of CAs and SAs, and average CAs and SAs had been measured on 5 different positions.
2.6. Wetting evaluations of functionalized fabrics
The non-wetting properties of the functionalized fabrics were evaluated by the droplets’ static contact and rolling on the fabrics according to the previous report (Su et al. 2017; Pan et al. 2018). Besides, the floating experiment of glass glued with functionalized fabrics on the water was also obtained.
2.7. Antibacterial activity of Ag-MSNs and functionalized fabrics
The antibacterial activity of Ag-MSNs was determined by the colony count method, and the detailed steps were described in our previous report (Ye et al. 2020). E. coli and S. aureus were cultivated at 37ºC for 2 h. After cultivation, the bacterial solution (E. coli and S. aureus, respectively) were diluted by PSB buffer to 106 CFU mL-1. And then, 1 mL of the diluted solution was mixed with 1 mL of PBS buffer containing Ag-MSNs with a concentration of 40 μg mL-1, the newly mixed solution was incubated at a constant temperature and shaking table (37ºC, 150 rpm min-1) for 24 h. Finally, 100 μL resulting bacterial solution was quickly dispersed onto the LB agar plate by bio-clean SS-Spreader. Bacterial colonies on the cultural plates were counted after incubation at 37ºC for 24 h.
To evaluate the antibacterial activity of functionalized fabrics, the fabrics were cut into 1 cm × 2 cm, and three samples were tested in parallel. The functionalized fabrics and the pristine fabrics were all immersed in 5 mL of bacterial suspension (S. aureus or E. coli, 105 CFU mL-1) and subjected to Vortex oscillation for 3 min to make the bacteria solution contacted with the fabrics well. Then, the samples were taken out and held vertically to allow the droplets remaining on the surface to slide away. Additionally, the samples were transferred into tubes at 37ºC for 24 h. After the incubation, the fabrics were put into test tubes containing 5 mL of PBS and experienced a water bath ultrasonication for 10 min, respectively. Finally, 100 μL of above-detached bacteria in PBS was spread onto the LB agar plates to incubate at 37ºC for 24 h. The antibacterial activity of functionalized fabrics was calculated by counting the number of colonies on plates.
To further test the antibacterial activity, inhibition zone test of functionalized fabrics against S. aureus, which is regarded as a quantitative technology against S. aureus, was conducted according to the AATCC100-2004 standard. As for the inhibition zone test, 100 μL of bacterial solution (S. aureus, 106 CFU mL-1) was spread onto LB agar plate, and then the sample (1.5 cm × 1.5 cm circles of functionalized fabrics) was put on it to incubate at 37ºC for 24 h.
2.8. Mechanical robustness of functionalized fabrics
We further tested the stability of the antibacterial properties of the fabrics by ultrasonic washing. And the fabrics (5 cm × 5 cm) were submerged in 50 mL distilled water for 4 h under ultrasonication (40 kHz, 180 W). After that, the samples were collected at certain intervals and dried at 105ºC for 2 h, followed by the measurements of CAs and antibacterial properties, respectively.
Abrasion resistance of fabrics is particularly critical in daily use. Herein, the wear resistance of functionalized fabrics was tested by sandpaper abrasion. A 200 g weight was put on the fabric while it was placed facedown to the sandpaper (standard sandpaper, grit no. 360). Then, the fabric was moved back and forth at about 2 cm/s. Each iteration was counted as a cycle, and CAs and antibacterial properties of the fabric were tested after every 100 abrasion cycles.