3.1. Characterization of Ag/GO
The functional groups of GO and Ag/GO were identified through FTIR spectra as shown in
Fig. 2(a). Particularly, an adsorption band was distributed at 3427 cm–1 due to the stretching of the carboxylic acid of absorbed water. The representation of other bands at 2913 and 2848, 2354, 1730, and 1372 cm–1, corresponding to symmetric and asymmetric –CH2, O=C=O, C=O, and C–O–C stretching vibrations, respectively. In addition, the 1637 cm–1 peak was attributed to aromatic bending vibration of unsaturated C=C of the non–oxidized graphitic domain [19]. As for Ag/GO, it had a similar pattern of FTIR spectrum to GO but the intensities of diffraction peaks declined due to the linkage between AgNPs and GO sheets [20].
Fig. 2(b) shows the XRD pattern of the Ag/GO. Accordingly, four 2θ values of Ag/GO were observed at 38, 44, 64, and 77o, representing characteristic diffraction peaks of AgNPs and well-matched with standard peaks in JCPDS file No. 04–0783 [21,22]. The diffraction peak at 2θ = 9.5o was recorded, which reflects (002) plane of GO [23]. In Ag/GO, this peak disappeared due to the AgNPs formulated on the GO surface, preventing the restacking of GO sheets.
As shown in Fig. 2(c), Raman spectra of GO and Ag/GO are characterized by two bands: D and G, which were observed at approximately 1330 and 1595 cm‒1, respectively. The D band is assigned to the breathing mode of K–point phonons with A1g symmetry, and the G band is ascribed to the tangential stretching mode of E2g phonon of sp2 carbon atoms corresponding to the defects in the graphitic structure and sp2 bonding carbon in GO structure [24]. After anchoring AgNPs onto the GO surface, the D and G band values increase as a result of chemical bonds formation and intercalation of GO surface and AgNPs [25].
The TEM image were used to analyze the morphologies of Ag/GO as shown in Fig 3(a). The results demonstrated that spherical-like AgNPs were uniformly distributed on the GO nanosheets-dark thin films with an average size of 10–15 nm. The results suggested that the addition of GO as a stability agent, was vitally crucial as it played an important role in stabilizing the formation of AgNPs onto GO, preventing the agglomeration of those particles.
According to the EDS spectrum in Fig. 3(b), the results indicated that C and O of the material were 23.06 and 23.40 w.t%, respectively, while the percentage of Ag was 54.54 w.t.%. This result did not change too much compared to the precursor ratio (1:1 AgNO3:GO mass ratio). There were peaks of Ag at 2 to 4 keV indicating the presence of Ag, and two small impurities peaks at 1 and 2.1 keV possibly corresponded to Na and Au in analyzed equipment, respectively [26]. Besides, the SEM images were used to investigate the morphological surface of Ag/GO as shown in Fig. 3(c-d). It is indicated that, on the surface of Ag/GO, there are many folds with rougher surfaces due to AgNPs evenly distributed on the GO sheets. The images revealed the presence of an ultrafine layer, which forms a wavelike structure when they are stacked together [27]. Consequently, the results affirm the uniform presence of Ag on the GO structure.
3.2. Characterization and antibacterial activity of Ag/GO/cotton
The influence of concentration and number of dips on the mass % of Ag/GO is shown in Fig. 4(a). The test results showed that when the Ag/GO concentration increased, the amount of material attached to the cotton increased. After 10 dip coatings, the amount of material adhering to the cotton still tended to increase in the concentration range of 160 to 400 mg/L. Meanwhile, the mass of cotton fabric virtually remained the same after 6 times of dipping coating at a concentration of 520 mg/L, indicating that raising the concentration of Ag/GO suspension would reduce the number of dips, thereby decreasing the period of fabrication time. When the Ag/GO concentration was raised to 640 mg/L, the quantity of Ag/GO clinging to the fabric began to develop slowly by the third dip coating, and by the fifth dip, the amount of Ag/GO clinging to the cotton had increased insignificantly. Based on the foregoing findings, 640 mg/L was the appropriate concentration for the Ag/GO dip-coating procedure on cloth. To determine the best number of dip-coating, Fig. 4 (b, c) presents the comparison of antibacterial activity on Ag/GO/cotton samples fabricated after dip-coated 10, 5, and 3 times. Test results revealed that the antibacterial capacity of all samples after different dip coatings times was identical. This could be explained by the fact that the quantity of Ag/GO in samples after a dip-coated 10, 5, and 3 times was not substantially different. Therefore, the number of suitable dip coating was determined to be 3 times at an Ag/GO suspension concentration of 640 mg/L.
The surface morphology of the Ag/GO/cotton samples was analyzed by SEM images. Before Ag/GO dip-coating, the surface of the fabric had an interlocking fiber structure (Fig. 5 (a, b)) and the surface of the fibers was smooth (Fig. 5(c)). When dipped with Ag/GO material, the surface of the fabrics still maintained the original fiber structure, suggesting dip-coating with Ag/GO did not affect the fabric structure (Fig. 5(d, e)). However, it is interesting to observe that the surface of each fabric changed significantly. The fiber surface became rougher due to being covered by Ag/GO sheets. More importantly, the presence of small white spots is uniformly distributed on the surface of the fabric fibers, indicating the presence of AgNPs, as shown in Fig. 5(f).
The effect of reducing conditions on the antibacterial activity of Ag/rGO/cotton is depicted in Fig. 6. When increasing the reaction temperature from 100 to 200 ºC, the % mass loss gradually increased. The cotton samples after being fabricated at 100 and 120 ºC, with the % mass loss increased by 5.20 and 5.89 %, respectively. It could be seen that 140 ºC was the appropriate temperature for GO to be reduced to rGO with the % mass loss when performing the reaction at this temperature of 7.08 %. However, the cotton samples after reduction at a temperature of 160 ºC onwards appeared material peeling on the cotton surface with the % mass loss also increasing, corresponding to the values of 7.32, 9.47, and 12.77 %, respectively. When the reaction temperature was higher than 200 ºC, the cotton sample began to show signs of decomposition. It is worth noticing that not only GO was reduced to rGO, but the cotton was also decomposed leading to an increase in the % mass loss of the cotton at high temperatures. Besides, the antibacterial results against two types of bacteria on the Ag/rGO/cotton samples at the reaction temperature investigation in Fig. 6(a) showed that in both agar plates, samples reduced at 140 ºC exhibited the best antibacterial ability. Although the inhibitory zone of the samples reduced at 160, 180, and 200 ºC did not change significantly, there was material peeling on the cotton surface of these samples. Therefore, the cotton fabric reduced to 140 ºC was suitable and had higher antibacterial ability than the remaining samples.
The effect of the VC:Ag/GO ratio on the % mass loss in the reduction process was investigated and illustrated in Fig. 6(b). The results revealed that the % mass loss went up rapidly when the ratio between Ag/GO on the fabric surface and reducing agent increased. When increasing the mass ratio of VC:Ag/GO from 1:0 to 1:1, the % mass loss increased sharply with values of 5.49 and 6.76 %, respectively. When the mass ratio of VC:Ag/GO increased from 1:2 to 1:5, the % of mass loss increased insignificantly with values of 6.81, 6.99, 6.92, and 7.06 %, respectively. According to the results of the above study, the ratio 1:1 was exactly adequate for reducing GO to rGO; however, if the VC:Ag/GO mass ratio continued to rise, the % mass loss would not change significantly and the amount of excess residual chemical, at the same time, would increase the cost of the synthesis process. Therefore, the mass ratio of VC:Ag/GO 1:1 was chosen to conduct the following experiments. The antibacterial results against two bacteria strains of the Ag/rGO/cotton samples for reducing agent ratio investigation in Fig. 6(b) showed that in both agar plates, the samples were reduced in the condition of reducing agent ratio 1:1 to 1:5 exhibited a minor antibacterial zone difference and were larger than the sample reduced at 1:0 ratio.
The results of investigating the influence of reaction time on the % mass lost in the reduction process are shown in Fig. 6(c). When the reaction time increased from 20 to 40 min, the % mass loss increased with values of 6.23 and 7.08 %, respectively. When the reaction time increased from 60 to 100 min, % mass loss reached 7.62, 7.80, and 7.93 %, respectively, these values increased but did not differ significantly. However, when the reaction time was 120 min, the % mass loss peaked at 8.45%. As a result, even after 100 min, it was inadequate to convert all GO to rGO, thus the reaction time had to be increased. The results of antibacterial activity against two strains of bacteria of Ag/rGO/cotton samples with different reducing times in Fig. 6(c) showed that in all two agar plates, the sample, which was reduced for 120 min, has the largest inhibition zone out of others. Therefore, this sample had the highest antibacterial ability of the other samples.
After evaluating the antibacterial capacity of Ag/rGO/cotton at different reduction conditions, the sample with the highest antibacterial ability was reduced at a temperature of 140 ºC, VC:Ag/GO mass ratio of 1:1, and reaction time of 120 min. Besides, Ag/rGO/cotton showed somewhat greater bactericidal activity than Ag/GO/cotton, albeit the difference was not significant. This could be explained by the fact that when in direct cell contact, highly conductive rGO mediated stronger oxidative stress than lower conductive GO [28,29]. Although rGO was not as well dispersed in water as GO, in this study Ag/rGO/cotton was only partially reduced, so Ag/rGO clinging to the fabric surface did not completely lose the water absorption properties of cotton fabrics, thereby allowing to contact with bacteria to cause physical damage to cell membranes and oxidative stress. The results of this antibacterial evaluation were also in good agreement with the results of a previous study published on the antibacterial effectiveness of fabric dipped in the following order: rGO – Ag > Ag > GO > Ag – GO [30]. The SEM images of Ag/GO/cotton and Ag/rGO/cotton are illustrated in Fig. 7. The results showed that there were no significant differences between the two samples of Ag/GO/cotton and Ag/rGO/cotton, indicating over the reduction process did not affect the fabric structure or surface.
The distribution of C, O, and Ag elements of the Ag/rGO/cotton sample after surface modification was presented in Fig. 8 (a-d). The results showed that the material was still evenly distributed on the surface of the cotton fabric and did not affect by the reduction process. After reduction, the % mass of elements altered, with the % O decreasing from 37.65 to 35.22 %, proving the oxygen-containing functional groups in the structure of GO had been successfully reduced as shown in Fig. 8(e). While the % Ag did not change significantly, which could be explained by the fact that the Ag/rGO has bonded strongly with the fabric surface, hence there was less Ag loss during the reduction process.
The result of measuring the contact angle of the best Ag/rGO/cotton sample was shown in Fig. 9 with the left and right contact angles being: 103.1 and 104.6 º, respectively. The cotton sample after being reduced has a contact angle of less than 150 º showing a strong hydrophobicity of the synthesized cotton. It can be seen that the modification of the Ag/GO/cotton surface to become hydrophobic did not significantly affect the initial antibacterial effect of Ag/GO/cotton.
3.4. Durability evaluation
The adhesion of Ag/GO and Ag/rGO sheets to the cotton was evaluated by determining the colourfastness of the cotton under different conditions, as shown in Table 1. The analysis results showed that under acidic, alkaline, and high-temperature conditions, the adhesion of Ag/GO and Ag/rGO to the cotton was quite high. When hot washing with soap, the adhesion of Ag/GO and Ag/rGO to the cotton decreased, but still managed to reach the level of 2–3 on the scale of 1–5. However, the hydrophobic character of Ag/rGO/cotton, which reduces interaction with water molecules and so helps to prevent washout throughout the process, may account for the much greater strength of Ag/rGO/cotton. These findings demonstrate that increasing the hydrophobicity of the outer cotton also supports enhancing the adhesion strength of Ag/rGO to the cloth.
Table 1: Survey results on the adhesion of Ag/GO and Ag/rGO on cotton fabric
No.
|
Target
|
Standard
|
Condition
|
Test results (*)
|
Ag/GO/cotton
|
Ag/rGO/cotton
|
Color staining/cotton
|
1
|
Colour fastness to washing at 40 oC
|
ISO 105‒C06 A1S‒2010
|
30 min machine wash at 40 oC in 4 g ECE phosphate reference detergent per litre of water with 10 steel balls
|
2‒3
|
3
|
4‒5
|
2
|
Colour fastness to acid perspiration
|
ISO 105 E04‒2013
|
|
4
|
4
|
3
|
3
|
Colour fastness to alkaline perspiration
|
ISO 105 E04‒2013
|
|
4
|
4
|
2‒3
|
4
|
Colour fastness to hot pressing at 150 oC
|
ISO 105 X04‒1994
|
Dry
|
|
|
|
Immediately
|
4
|
4
|
|
After 4 hours
|
4
|
4
|
|
Damp
|
|
|
|
Immediately
|
4
|
4
|
3
|
After 4 hours
|
4
|
4
|
3
|
Wet
|
|
|
|
Immediately
|
4
|
4
|
1‒2
|
After 4 hours
|
4
|
4
|
1‒2
|
5
|
Colour fastness to dry heat
|
AATCC 117‒2009
|
|
4
|
4
|
4‒5
|
(*) Greyscale rating is based on the 5 step scale of 1‒5; where 1 is bad and 5 is good.
3.5. Fabrication and antibacterial mechanism of Ag/rGO/cotton
Ag/rGO/cotton was produced by dipping cotton with Ag/GO suspension. Then, the oxygen-containing functional groups on the GO surface were reduced to rGO by chemical means. In particular, VC was used as a non-toxic and environmentally friendly reducing agent. This method gave the product a relatively high-water resistance and the cotton after modification retained its antibacterial effect. In GO there was a conjugate system p–p, p–p from the O of the –OH group and –C=O, making the H of the OH group attached to the double-bonded C very mobile. The mechanism of Ag/GO reduction by VC is shown in Fig. 10. First, the five-ring electron recovery chain of VC increased the acidity of the β and γ-hydroxyl groups, leading to the dissociation of the two protons to form an anion (HOAO–). Then, the anion underwent reactivation on the epoxy and diol groups of GO, yielding the intermediate product. The intermediate was reduced and the VC was oxidized to dehydroascorbic acid [31].
The Ag/rGO nanocomposite material was evenly coated on the stable cotton fabric, increasing the chance of bonding between the bacterial cell and the rGO sheet with silver attached to it. The sharp rGO plate cut the cell wall by physical interaction. At the same time, AgNPs release Ag+ ions, which attached strongly with thiol groups (−SH) of enzymes and proteins on the cell surface by electrostatic attraction, destroying the function of cell membranes and cell walls in selective permeation of intracellular and extracellular substances. Inside the bacterial cell, AgNPs and Ag+ ions played a role in the formation of reactive oxygen species (ROS) due to oxygen imbalance. In addition, Ag+ bound to the DNA of microbial cells and inhibited the replication function, stopped and prevented the growth of bacteria. To get further insights into the bactericidal mechanism of Ag/rGO nanocomposite material, the interaction process is illustrated in Fig. 11 and divided into four stages: (1) rGO plates with the large specific surface area helped to capture bacteria; (2) Formation of ROS free radicals, inhibiting and destroying bacterial cell membranes; (3) The sharp edges of rGO penetrated the membrane, damaging and cutting the cell membrane for Ag+ to enter the cell; (4) Ag+ ions bound to DNA, the enzyme inhibited replication, disrupted metabolism inside cells, killed bacteria.