Results & discussion
Effect of Concentration of silver colloidal solution on antibacterial efficacy
The durability of the nanosilver antimicrobial finish on textile lasts longer than any other antimicrobial finishes.
The effect of silver colloidal solution concentration on antibacterial efficacy was studied by determining the bacteria growth on untreated fabric as control fabric against treated fabrics (3, 6, 9 & 12 gm/l) using JIS L 1902:2008 test method. The preliminary tests indicated that silver colloidal solution concentration varying from 3 to 9 gm/l showed poor washing durability towardscantibacterial characteristics. Even after 10 washes, antibacterial efficacy dropped down significantly. Therefore, the entire study was conducted using 12 gm/l silver colloidal solution.
The results are given below:
Calculation of percent reduction of bacteria:
100 (C – A) / C = R
where,
R = % reduction
A = the number of bacteria recovered from the inoculated treated test specimen swatches in the jar incubated over the 18 hr contact period
C 18 = the number of bacteria recovered from the inoculated untreated control specimen swatches in the jar after 18 hr contact period.
It can be seen from Table 1 that cotton fabric sample treated with 12 gm/l silver colloidal solution showed 99.02% antibacterial efficacy even after 20 washes against Staphylococcus aureus (gram positive) bacteria. It indicates excellent antibacterial characteristics of silver colloidal solution against gram positive bacteria along with very good wash durability.
It can be seen from Table 2 that cotton fabric sample treated with 12 gm/l silver colloidal solution showed 98.99% antibacterial efficacy even after 20 washes against Klebsiella Pneumonia (gram negative) bacteria. It indicates excellent antibacterial characteristics of silver colloidal solution against gram negative bacteria along with very good wash durability.
Determination of particle size of silver in colloidal solution
The particle size of silver particles was determined using Malvern Zetasizer Nano ZS instrument. The instrument works on the principle of Dynamic Light Scattering (DLS) which measures particle size in the submicron region. The results are shown in Fig 1.
Fig 1 Silver particle size distribution in collidal solution
It can be seen in Fig 1 that three peaks were observed. The particle size of silver 427.9 nm was found in highest 85.9% intensity. However, the particle size of silver 51.68 nm was 11.2%, and the particle size of silver 4570 nm was 2.9%. It shows that the maximum intensity of silver particle size was 427.9 nm which is relatively higher than the particle size reported in the literature around 100 nm. The higher particle size is probably one of the reasons that controlled release of silver takes place from the fibre surface in the presence of moisture, and antibacterial effect of silver is durable to more than 20 washings.
Determination of zeta potential of silver colloidal solution
The zeta potential of silver colloidal solution was determined using Malvern Zetasizer Nano ZS instrument. Zeta potential determines the colloidal solution stability. If zeta potential becomes zero, the colloidal particles will attract each other because of Vander Waals forces and agglomeration of particles into bigger particle size; consequently settling down of particles will take place. The results are shown in Fig 2
.
Fig 2 Zeta Potential of Silver Collidal Solution
It can be seen that zeta potential of silver colloidal solution was found as – 4.58 mV which is quite good for maintaining dispersion stability of colloidal solution. We have also seen that no
settling down of particle has place even after one year of storage of sliver colloidal solution.
Determination of silver content in treated cotton fabric after 10 & 20 washes
The silver content in treated cotton fabric (with 12 gm/l silver colloidal solution) before and after 10 and 20 washes, respectively, was determined using Environmental Scanning Electron Microscope model FEI Quanta 200F with Oxford – EDX system IE 250 X Max 80. It is a type of electron microscope that images the sample surface by scanning it with high energy beam of electrons. The EDX consists of latest 80 mm 2 silicon drift detector (SDD), which enables detection of elements under high resolution. The results are given in Figs 3, 4, 5 & 6.
Fig 3 : Electron image showing silver content in untreated cotton fabric
Fig 4 : Electron image showing silver content in treated cotton fabric
Fig 5 : Electron image showing silver content in treated cotton fabric after 10 washes
Fig 6 : Electron image showing silver content in treated cotton fabric after 20 washes
It can be seen in Fig 3 that the untreated cotton fabric showed atomic weight percentage of carbon 53.05%, oxygen 46.95% and Ag (silver) 0.0% because no silver colloidal solution treatment is given.
In Fig 4, treated cotton fabric with 12 gm/l solution showed atomic weight percentage of carbon 51.26%, oxygen 44.26% and Ag (silver) 4.48%.
In Fig 5, treated cotton fabric (with 12 gm/l solution) after 10 washes showed atomic weight percentage of carbon 61.77%, oxygen 36.27% and Ag (silver) 1.96%. Although the silver
content is reduced in comparison to treated cotton from 4.48% to 1.96% because of leaching out of silver particles during washing, but still the reactivity of silver against gram positive and gram negative bacteria is very high and shows as above 99% antibacterial efficacy.
In Fig 6, treated cotton fabric (with 12 gm/l solution) after 20 washes showed atomic weight percentage of carbon 66.45%, oxygen 31.89% and Ag (silver) 1.66%. Although the silver content is reduced in comparison to treated cotton from 4.48% to 1.66% because of leaching out of silver particles during washing, but still the reactivity of silver against gram positive and gram negative bacteria is very high and shows as above 98.99% antibacterial efficacy.
Conclusion
- It is possible to produce antibacterial cotton fabric using silver colloidal solution with antibacterial efficacy above 98.99%.
- The maximum intensity (85.9%), of 427.9 nm particle size of silver showed that because of somewhat bigger particle size, controlled release of silver from cotton surface in presence of moisture has taken place
- This is indicated in excellent antibacterial efficacy (above 98.99%), even after 20 washes.
- The zeta potential of -4.58 mV of colloidal solution shows that repulsive forces exist between silver particles and agglomeration of silver particles, to make bigger particles is prevented thereby resulting in stable colloidal solution. Even after storage for more than one year, no settling down of particles was observed.
- The SEM micrographs attached with EDX elemental analysis, indicated the presence of silver even after 20 washes. It confirms very good antibacterial efficacy even after repeated washes.
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Article was originally published in Asian Dyer Magazine (Issue Aug-Sept-2020)