Fabrication of superhydrophobic and antibacterial surface on cotton fabric by doped silica- based sols with nanoparticles of copper
© Berendjchi et al; licensee Springer. 2011
Received: 13 May 2011
Accepted: 15 November 2011
Published: 15 November 2011
The study discussed the synthesis of silica sol using the sol-gel method, doped with two different amounts of Cu nanoparticles. Cotton fabric samples were impregnated by the prepared sols and then dried and cured. To block hydroxyl groups, some samples were also treated with hexadecyltrimethoxysilane. The average particle size of colloidal silica nanoparticles were measured by the particle size analyzer. The morphology, roughness, and hydrophobic properties of the surface fabricated on cotton samples were analyzed and compared via the scanning electron microscopy, the transmission electron microscopy, the scanning probe microscopy, with static water contact angle (SWC), and water shedding angle measurements. Furthermore, the antibacterial efficiency of samples was quantitatively evaluated using AATCC 100 method. The addition of 0.5% (wt/wt) Cu into silica sol caused the silica nanoparticles to agglomerate in more grape-like clusters on cotton fabrics. Such fabricated surface revealed the highest value of SWC (155° for a 10-μl droplet) due to air trapping capability of its inclined structure. However, the presence of higher amounts of Cu nanoparticles (2% wt/wt) in silica sol resulted in the most slippery smooth surface on cotton fabrics. All fabricated surfaces containing Cu nanoparticles showed the perfect antibacterial activity against both of gram-negative and gram-positive bacteria.
Studying over 200 species of water repellent plants, Neinhuis and Barthlott  found an ideally wonderful superhydrophobic effect on lotus (Nelumbo nucifera) leaves which leads to supreme self-cleaning properties, so-called lotus effect [2, 3]. The rough structure of lotus leaves (hills and valleys template) causes a reduced contact area with water. The presence of the hydrophobic nanoparticles, however, will prevent water from penetrating hills .
To simulate or produce such superhydrophobic surface on substrates, among different methods ( such as chemical vapor deposition , phase inversion , electrospinning , electrowetting , lithography , and etching ), the sol-gel method seems more conventional to be used on textile materials, due to easy processing and acceptable treatment conditions (e.g., low temperature) [11–17]. In this method, hydrolysis and condensation reactions of the precursor material are carried out to form a nano-colloidal solution, and a network of nanoparticles will be formed on the substrate through the gradual evaporation of the solvent. The precursors are often based on metal organic compounds such as acetylacetonate, or metal alkoxides like tetraethoxysilane Si(OC2H5)4 (TEOS), titanium(IV) isopropoxide Ti(OC3H7)4, and Al(OC4H9)3.
According to its natural properties, cotton fabric is among the very popular textiles. Producing superhydrophobic surface on cotton fabric will guarantee its dryness and cleanness which are considered as desired features, in particular on its outside facet [11–17, 19–21]. Furthermore, cotton fabric is an ideal place for settling and growing pathogenic bacteria because of its porous and hydrophilic structure. So, antibacterial finishing is also of importance, especially in some specific applications like medical usage. There are many antibacterial agents used in this field, including metal nanoparticles like silver and copper [22–29]. The latter is the most familiar antibacterial agent used for centuries. Like many other particles, the desired properties of copper may be improved by reducing its size to nano-scale. Hence, these nanoparticles can be developed and applied in various new fields, such as water purification, medical science, human tissue, antifouling and antibacterial agent, etc. .
Few researches have been focused on developing two abovementioned properties on cellulosic substrates like cotton fabric, simultaneously [30–32]. On the other hand, nanoparticles of copper and core shell SiO2/Cu have been less developed for textile finishing [33–36]. The current aimed to fabricate an antibacterial and superhydrophobic surface on the cotton fabric, by introducing Cu nanoparticles into the silica sols. It was expected that due to their chemical activities, such nanoparticles would change the morphology and arrangement of silica nanostructure, and in addition, promote antibacterial activity on cotton fabrics.
Bleached and desized cotton fabric was provided by Polpine Co (Iran, Rasht). Tetraethylorthosilicate (TEOS), hydroxide ammonium (NH4OH 25%) and ethanol (C2H5OH 98%) were purchased from Merck Company. Nano-Cu (average particle size, 40 ±5 nm) was obtained from Plasma Company (PlasmaChem GmbH, Berlin, Germany), and hexadecyltrimethoxysilane (HDTMS) was purchased from Fluka Company (Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany). All chemicals were used as received without any further purification.
Colloidal SiO2 nanoparticle solutions were prepared considering the principles of Stöber method: 25 ml of ethanol, 1 ml of ammonium hydroxide, 3.6 ml of distilled water, and 11.5 ml of TEOS were mixed for 2 h at room temperature .
The prepared silica sols doped with two different amounts (0.5% and 2.0% wt/wt) of Cu nanoparticles, and were then sonicated for 30 min. Cotton fabric samples were immersed in the sols at 30°C for 5 min, dried for 24 h at ambient temperature and cured at 160°C for 5 min, respectively. Some samples were immersed in hydrolyzed and diluted HDTMS (with 4% wt/wt ethanol) for 4 h at room temperature. Again, these samples were cured at 120°C for 1 h .
Particle sizes of the silica sols prepared were measured with a particle size analyzer (Malvern Instruments, Malvern, Worcestershire, UK). The surface morphology was investigated by the scanning electron microscope (SEM) (XL30, Philips, Royal Philips Electronics, Amsterdam, Netherlands), while the surface roughness was analyzed via the scanning probe microscope (SPM) (DualScope™ C26, DME, Herlev, Denmark) using AC mode. The SiO2/Cu hybrid structure was observed with the transmission electron microscope (TEM) (EM 10C, Zeiss, Oberkochen, Germany). The static water contact angle (SWC) was determined by using a contact angle measurement device (Krüss G10, KRÜSS GmbH, Hamburg, Germany). At 23 ± 5°C, a 10-μl droplet of deionized water was placed into five different positions on the sample surfaces, and the angles of drops on the fabrics were determined. The static contact angle values for the sample reported were the average of five measurements.
Water shedding angle (WSA) of various samples was measured by the method of Zimmermann et al. . After releasing a drop of water (15 μl) in a height of 1 cm, the minimum angle of inclination at which the drop completely rolls off the surface was determined.
The antibacterial activity of samples was quantitatively evaluated using AATCC 100 method. Two non-spore-forming bacteria, one Gram-positive Staphylococcus aureus (ATCC = 25923) and one Gram-negative Escherichia coli (ATCC = 25922), were used for antibacterial testing.
For determining the number of bacteria after 0 contact time, autoclaved swatches were placed in wide mouth glass jars and 100 μl of inoculums (containing 106 colony-forming units (CFU) was poured on each of them. Immediately after inoculation ("0" contact time), 100 ml of neutralizing solution (phosphate-buffered saline (PBS)) was added to each jar. After vigorous stirring (2,500 rpm for 1 min), the solution in each jar was poured on a nutrient agar plate.
where R is the percent reduction, A is the number of bacteria recovered from the inoculated treated test specimen swatches in the jar incubated over 24 h, and B is the number of bacteria recovered from the inoculated treated test specimen swatches in the jar immediately after inoculation (at "0" contact time).
Results and discussions
Static water and water shedding angles of fabricated surface on cotton fabric samples
Kind of surface
Mean of static contact angle "SWC" (°)
Standard error of "SWC"
Mean of waster shedding angle "WSA" (°)
Standard error of "WSA"
The addition of 0.5% wt/wt Cu into silica sol caused the flocculation of colloidal silica nanoparticles (Figure 5b). The emersion of two peaks and the broadening of silica peaks in a size distribution graph just 5 min after introducing Cu nanoparticles may be attributed to the gradual agglomeration of silica and Cu particles (Figure 1b).
Such agglomeration would produce more grape-like clusters on the final fabricated surface. Compared with ordinary SiO2 nanostructured surface, this morphology showed higher air trapping capability and SWC(Table 1).
where R f denotes the roughness factor, and θ and θ 0 are the contact angles of liquid droplets on rough and flat surfaces, respectively. The WSA value for such sample was decreased and reached to 24°, and also, the slippery of treated surface was increased.
Post post hoc test (Duncan's multiple range test) for three samples
Subset for alpha = 0.05
Percent reduction of bacteria on the fabricated control and doped silica surfaces
Undoped silica sol
Copper, especially in its nano scale, has noticeable antibacterial activity with a more low cost compared with other similar antibacterial metals. In addition, the sol-gel method is a conventional process to coat thermo-sensitive substrates like cotton fabrics by nanoparticles. Introducing Cu nanoparticles into silica sol will fabricate a surface with higher air trapping capability on cotton fabrics. Therefore, it can imply superior properties of superhydrophobicity on the substrate and eliminate the need for post-treatment of silica surfaces with alkylsilane. Besides the intrinsic antibacterial properties, disintegration of Cu nanoparticle through the settling on SiO2 particles will simultaneously lead to an efficient antibacterial activity of the surface fabricated. Further study can also be conducted on more interesting properties such as self-cleaning capability of fabricated hierarchical surfaces.
The work is supported by Textile Research Center at Tehran South Branch, Islamic Azad University. We appreciate Mr. Rezaei in Tarbiyat Modarres University for preparing the micrographs of scanning electron microscope and energy dispersive X-ray spectroscopy graphs, and also Mr. Mojtaba Hoseinpour, a member of Advanced Materials & Nanotechnology Research Lab of KNTU University for preparing the micrographs of transmission electron microscope.
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