The Effects Of Ag Nanoparticles

Published Date: 02 Nov 2017

Silver

The effects of Ag nanoparticles (Ag NP) on microbes have been investigated by many researchers and found that Ag+ interacts with thiol groups in proteins resulting in inactivation of respiratory enzymes that are leading to the production of ROS (Reactive Oxygen Species) for further attacks [24, 25]. Also another researcher has shown that Ag+ ions can prevent DNA replication and affect the structure and the permeability of the cell membrane [26]. Silver species are also photoactive in the presence of UV radiation, leading to enhanced inactivation of bacteria and viruses [27, 28].

Most popular Ag NP synthetic method is reduction of Ag+ with sodium citrate, which is given below.

?4 Ag?_((aq))^++C_6 H_5 O_7 ?Na?_(3(aq))+?2 H?_2 O_((l))??4 Ag?_((s))^0+C_6 H_5 O_7 H_(3(aq))+?3 Na?_((aq))^++H_((aq))^++O_(2 (g))

The pulse radiolysis technique has shown that, the citrate ion functiones as a reductant, a complexant, and a stabilizer in the Ag NP growing process [29]. Formation kinetics of this reaction was studied by Sileikaite et al and have prepared spherical nanoparticles ranging from 45 nm-80 nm [30].

Preparation of silver nanoparticles

Colloidal silver is of particular interest because of distinctive properties, such as good conductivity, chemical stability, catalytic and antibacterial activity [31]. Chemical reduction is the most frequently applied method for the preparation of silver nanoparticles as stable, colloidal dispersions in water or organic solvents [32]. Commonly used reducing agents are borohydride, citrate, ascorbate, and elemental hydrogen [33-39]. The reduction of silver ions (Ag+) in aqueous solution generally yields colloidal silver with particle diameters of several nanometers. Initially, the reduction of various complexes with Ag+ ions leads to the formation of silver atoms (Ag�), which is followed by agglomeration into oligomeric clusters [40]. These clusters eventually lead to the formation of colloidal Ag NPs [40]. Previous studies showed that use of a strong reducing agent such as borohydride, resulted in small particles that were somewhat monodisperse, but the generation of larger particles was difficult to control. Use of a weaker reducing agent such as citrate, resulted in a slower reduction rate, but the size distribution is observed [33, 41]. Controlled synthesis of Ag NPs is based on a two-step reduction process. In this technique a strong reducing agent is used to produce small Ag NPs, which are enlarged in a secondary step by further reduction with a weaker reducing agent [42]. Different studies reported the enlargement of particles in the secondary step from about 20�45 nm to 120�170 nm [33, 39, 43].

Green synthesis of silver nanoparticles

Researchers have moved towards the green synthesis of Ag NPs to minimize the environmental trouble. Raveendran et al proposes a gentle heating and preparation of Ag nanoparticles by green pathway [44]. This method includes selection of solvent medium, selection of environmentally benign reducing agent, and selection of nontoxic substances for the Ag NPs stability.

Polyoxometalates method

Polyoxometalate (POM) is a polyatomic anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form a large, closed 3-dimensional framework [45]. They are able to undergo stepwise multielectron redox reactions when soluabalized in water and form Ag NPs without altering their own 3-D structure [46-48]. Troupis et al has synthesised metal nanoparticles of Ag, Au, Pd, and Pt at room temperature, by simple mixing of the corresponding metal ions with reduced polyoxometalates which serve both as photocatalysts and stabilizers in presence of UV illumination [48].

Polysaccharide method

Starch-protected nanoparticles have been used for the applications in biological and pharmaceutical fields. Tai et al has used spinning disk reactor to synthesise Ag NPs using glucose as the reducing agent and starch as the capping agent [49]. This method has avoided the use of harmful solvents and allowed the environmental benign water and starch in the process to make particles with less time. Additionally, the binding interactions between starch and Ag NPs are weak and can be reversible at higher temperatures, allowing separation of the synthesized particles at higher temperatures. Vigneshwaren et al has synthesised Ag NPs of 10-34 nm by autoclaving starch and silver nitrate solution [50].

Biological method

As per literature Ag NPs can be synthesised by the natural extracts as well. Xie et al has used extract of green alga Chlorella vulgaris to produce silver nanoparticles. Some researchers have used leaf extracts of Geranium plants [51], mushroom substrate secreted proteins [52], Capsicum annuum vegetable [53] and vitamin E [54] for the purpose. These extracts contain amino acids polysaccharides and proteins. They suggest that these bio-extracts react as both reducing agent and capping agent in the synthesis process. Researchers propose that aspartic (Asp) acid, Tyrosine (Tyr) and glutamine (Glu) residues do the reducing of Ag+ and this is further confirmed by the carrying out reduction of Ag+ by Asp-Asp-Tyr-OMe bifunctional try peptide which yielded a high concentration of smaller Ag NPs [55]. This kind of process may have less adverse effects on the environment, on the other hand extracting these chemicals is another process and the reaction times lasts for 24 h which is not suitable for an industrial process.

Though it is considered that silver is a common anti-microbial agent, it is interesting that several microbes has been utilised to synthesise Ag NPs in intracellular and extracellular paths. Several microorganisms have been utilized to grow Ag NPs intracellularly or extracellularly. Shahverdi et al has synthesised Ag NPs using the culture supernatants of Klebsiella pneumonia, Escherichia coli, and Enterobacter cloacae quite efficiently than the previous biological methods [56]. Again using these bacteria is not secure for the human health as they can cause serious illness for human, especially Klebsiella pneumonia.

Tollens method

Another method of preparation of Ag NPs is the reduction of Ag+ ions by the Tollens reagent. This results a controlled size particles in a single step as a film on a substrate [57, 58]. In the Tollens reaction, Ag+ is reduced by an aldehyde as follows.

???2 OH?_((aq))^-+[?Ag(NH?_3 )_2 ]?^+?_((aq))+ ?RCHO?_((aq))??Ag?_((s))+?RCOOH?_((aq)+)+H_2 O_((l))+?4 NH?_(3 (g))

Kvitek et al has used different reducing sugars instead of aldehyde to reduce [?Ag(NH?_3 )_2 ]^+ complex and obtained Ag NPs from 45 nm- 380 nm [59].

Irradiation method

Ag NPs has been successfully synthesized using ionising radiation, laser radiation, and microwave irradiation methods. Several successive attempts were reported using ionisation irradiation to produce Ag NPs [60, 61]. Dimitrijevic et al has used solvated electrons and produced Ag NP less than 10 nm using supercritical ethane at 80 �C [61].

Another successful method is laser irradiation which does not require the reducing agent and particles are generated within a short time. Abid et al has used this method to produce Ag NP in the presence of the stabilizer SDS [62].

Chen et al has used carboxymethyl cellulose sodium as both a reducing and a stabilizing reagent in the Ag NP formation reaction. Further they have reported that the hydrolysis of carboxymethyl cellulose sodium in aqueous solution is nearly impossible without the help of catalyst using conventional heating method, but microwave irradiation has achieved it without catalysts [63]. Hu et al has reported the synthesis of microwave assisted Ag NP in 80 mL vessel in the presence of amino acids as reducing agents and soluble starch as a capping agent, which showed the practical potential among other methods [64].

Titanium dioxide

TiO2 nanoparticles have been using to disinfect drinking water. TiO2 nanoparticles have been produced by many methods such as chemical solution decomposition (CSD) [65], ultrasonic irradiation [66], chemical vapor decomposition [67], wet chemical method [68], sol�gel methods [69, 70]. Zhu et al has reported that phase transitions from the titanate nanostructures to TiO2 polymorphs take place readily in simple wet-chemical processes at temperatures close to ambient temperature [71]. In sol-gel method, the nanocrystalline titanium dioxide sol�gel formulations were prepared by hydrolysis and condensation reaction of 5% titanium tetra-isopropoxide in acidic aqueous solution containing 5% acetic acid and 1.4% hydrochloric acid. The solutions were heated at 60 �C under vigorous stirring for 2 and 16 h. It is suggested that, TiO2 nanoparticles show antimicrobial activity under illumination due to photocatalytic effect, which is discussed in the section 1.8.1.

Zinc oxide

ZnO is another antimicrobial nanoparticles used in the commercial products. Common synthetic methods of ZnO are given in the Table 1.2 below. Use of ZnO as an antimicrobial agent was increased in the end of 1990�s, and extensive studies of anti-microbial activity was carried out during that period [72]. Anti-bacterial activity of ZnO has been investigated by many researchers, but requires further understanding to get an exact mechanism. Kasemets et al propose that the release of Zn2+ ions causes the toxicity to bacteria [73]. Zhang et al proposes that the reaction of nanoparticles with cells result the activity [74], while Jalal et al and Gordon et al propose formation of Reactive Oxygen Species causes the activity [75, 76]. This is similar to ROS formation from TiO2, described in section 1.8.1. Brayner et al has observed disrupted cell walls, altered morphology and intracellular content leakage of E.coli after treating with ZnO nanoparticles [77]. These observations suggest inactivation of bacteria by ZnO has direct interaction with ZnO and the surface of cells other than the irradiation of light.

Table 1.2: Synthetic methods of ZnO nanoparticles

Method Precursor Solvent Size/ nm Shape Reference

Co precipitation technique Zinc acetate Double distilled water 80 (length), 30-60 (diameter) Nano rod Bhadra et al [78]

Microwave decomposition Zinc acetate dehydrate 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide 37-47 (diameter) Sphere Jalal et al. [75]

Hydrothermal process Zinc acetate dihydrate Polyvinylpyrrolidone 5 �m (length) 50-200 (diameter) Nano rod Lepot et al. [79]

Wet chemical method Zinc acetate hexahydrate Sodium hydroxide as precursor and soluble starch as stabilizing agent 20-30 (diameter) Acicular Premanathan et al. [80]

Sol-gel method in gelatine media Zinc nitrate Distilled water and gelatine as substrate 30-60 (diameter) Circular, hexagonal Zak et al [81]

(Table adapted from: Espitia, P. et al., Zinc Oxide Nanoparticles: Synthesis, Antimicrobial Activity and Food Packaging Applications. Food and Bioprocess Technology, 2012. 5(5): p. 1447-1464. [82])

Further, Adams et al have observed activity in both under light irradiation and in dark [83]. Hirota in 2010 investigated that ZnO NPs show activity in dark. They suggests that the activity under dark conditions may due to the superoxide ion formation in the media [84]. These observations suggest that the further studies are needed to determine the antibacterial mechanism of ZnO.

Common sterilization methods involved in microbial work

Bacteriocidal methods include heat, filtration, radiation, and the exposure to chemicals to control contamination in laboratory. The use of heat is a very popular method of sterilization in a microbiology laboratory. The dry heat of an open flame incinerates microorganisms like bacteria, fungi and yeast. The moist heat of a device like an autoclave can cause deformation of the protein constituents of the microbe, as well as causing the microbial membranes to liquefy. The effect of heat depends on the time of exposure in addition to form of heat that is supplied. For example, in an autoclave that supplies a temperature of 121 �C an exposure time of 15 minutes is sufficient to kill vegetative form of bacteria.

A specialized form of bacteriocidal heat treatment is called pasteurization after Louis Pasteur, the inventor of the process. Pasteurization achieves total killing of the bacterial population in fluids such as milk and fruit juices without changing the taste or visual appearance of the product. Another bacteriocidal process, although an indirect one, is filtration. Filtration is the physical removal of bacteria from a fluid by the passage of the fluid through the filter. The filter contains holes of a certain diameter. If the diameter is less than the smallest dimension of a bacterium, the bacterium will be retained on the surface of the filter it contacts. The filtered fluid is sterile with respect to bacteria. Filtration is indirectly bactericidal since the bacteria that are retained on the filter will, for a time, be alive. However, because they are also removed from their source of nutrients, the bacteria will eventually die.

Exposure to electromagnetic radiation such as ultraviolet radiation is a direct means of killing bacteria. The energy of the radiation severs the strands of DNA in many locations throughout the bacterial genome. With only one exception Deinococcus, the damage is so severe that repair is impossible. This genus has the ability to piece together the fragments of DNA in their original order and enzymatic stitch the pieces into a functional whole.

Exposure to chemicals can be bacteriocidal. For example, the gas ethylene oxide can sterilize objects. Solutions containing alcohol can also kill bacteria by dissolving the membranes that surround the contents of the cell. Laboratory benches are routinely �swabbed� with an ethanol solution to kill bacteria that might be adhering to the bench top. Other chemical means of achieving bacterial death involve the alteration of the pH, salt or sugar concentrations, and oxygen level.

Anti-microbial testing methods

Anti microbial activity of samples are determined by using anti-microbial tests. There are several standard methods involved to measure the activity of different microbes. Each method is varied with the type of micro-organism used. For convention, some standard methods are defined for the anti-microbial tests.

Agar diffusion method is used for the test against bacteria and fungi. Aqueous suspension and cell suspension methods are used for bacteria alone and humidity chamber method is used for the tests of fungi alone. Soil burial test is used for measure resistance of material against bacteria and fungi [23].

Agar diffusion tests

AATCC 147 standard test method

This is used to check the susceptibility to bacteria. The treated material is placed onto the surface of an agar plate that has been inoculated with bacteria and incubated for 24 hours. For a successful result, no bacterial growth should be observed under the test sample. This test is very easy to assess with antimicrobials that leach, as a zone of inhibition is often noted as well. With antimicrobials that are fixed to the fabric, there is no zone of inhibition and growth can be seen under the test piece. Especially if the fabric is ribbed � there is no actual growth where the material touches the agar. Some of the more recent antimicrobials do not diffuse very well in agar so even if they were not fixed to the fabric they would not give a zone of inhibition in this test [23].

AATCC 30 standard

Similar to AATCC 147, but fungi is used instead of bacteria.

Cell suspension tests

AATCC 100 method involves the direct application of the test bacteria to the material under test. After a contact time of 24 hours the bacteria are rinsed from the fabric and their numbers enumerated. However reduction in numbers will mean that the applied antimicrobial will prevent growth in practice whilst an increase in numbers will show no effect.

There are many standards available in antimicrobial susceptibility testing. Some tests are given in the Table 1.3.

Table 1.3: Various standards for assessing antimicrobial function of textiles.

Test Title Description Examples of textiles tested

AATCC-147-1998

(USA) Qualitative - Antibacterial assessment of diffusible antibacterial agents Socks, T-shirts etc.

SNV-195 920,1994

(Swiss) Qualitative � Agar diffusion Test Assessment of antibacterial effect of agents and impregnated textiles Socks, T-shirts etc.

SNV-195 921, 1994

(Swiss) Qualitative � Agar diffusion Test Assessment of antifungal effect of agents and impregnated textiles Swimwear, clothing liable to get wet

AATCC-100-1998

(USA) Quantitative assessment of antibacterial finishes on textiles � measures the degree of anti-bacterial activity Socks, T-shirts, underwear

BS EN ISO 11721, 2001 Soil Burial Test

Severe test conditions Cellulose containing products in contact with soil- sand bags, shoe liners, textile based sports equipment

BS 6085 Part 4, 1992 Resistance of Textiles to bacterial degradation Clothing: woollen articles

BS 6085 Part 5, 1992 Mildew Fungi Growth Analysis Swim wear, clothing in contact with water

Source: Antimicrobial Testing � an overview September 2003. Dr. T. Ramachandran et al Just-style.com March 2004 �Antimicrobial fibres help fight war against germs� American Journal of Infection Control (April 2001)

Self-Cleaning

Self-cleaning surfaces are being achieved by two methods. One is photo-catalytic activity or photodegradation activity, and the other is super hydrophobicity or water-repellence which is shown by lotus leaf and many insects like water striders. As the structure of fibrous materials gets dirt from the environment, they have to be cleaned on regular basis which causes pilling and hence reduce the value of the cloth. Researchers have already reported that titanium dioxide withhold these due to its strong oxidation activity, which can remove shoe odour, vehicle smoke, oil spills which cannot be effectively cleaned by traditional laundry method [85-87].

Photodegradation

Photodegradation is a light-assisted catalytic process which utilizes the substrate under irradiation of light and to initiate chemical reactions. Photo sensitive materials absorb the light and excite an electron to the higher energy level if the provided energy is sufficient. The observation was first reported by Goodeve and Kitchener in 1938 with the degradation of blue colour organic substance with TiO2 at a wavelength of 365 nm [88]. Though activity was improved and reported by Kato and Mashio in 1964, breakthrough was initiated by the discovery of water splitting during photosynthesis, further Honda and Fujishima [89] had discovered that water can be decomposed into oxygen and hydrogen through photochemical reaction in the presence of UV light using TiO2 [86].

Once the light with necessary energy to excite an electron form valence band to the conduction band fallen on the material, photocatalysis triggers [85, 86, 90, 91]. Once it is excited, positive holes are formed in the valence band and negatively charged electrons are in the conduction band as follows.

TiO_2+h??e�+h^+

This electron-hole pair can under go different reaction paths as shown in Figure 1.4 [85, 86, 89, 91].

Once the electron-hole pair has formed, they can combine, at the same time on the surface of photocatalytic material, which is known as recombination and is given by the next equation. This reduces the efficiency of the process.

e^-+h^+??TiO?_2

Highly reactive hydroxyl radicals can be formed once the electron holes react by oxidising the water molecules adsorbed on the surface as follows. This is known as photooxidation.

h^++H_2 O??HO?^�+H^+

Other form of production of highly oxidative reactive species is due to the excited electrons. Oxygen molecules are adsorbed onto the surface of the photocatalyst, and these collide with the excited electrons, hence reactive oxygen radicals are formed. These oxygen radicals react with H+ and holes to make hydroxyl radicals and oxygen radicals respectively as follows. This is known as photoreduction.

e^-+O_2?O_2^-

O_2^-+H^+??HO?_2^�

O_2^-+h^+?2O^�

Accordingly, free radicals formed can oxidize foreign dirt on cotton textile.

Figure 1.4: Formation of advanced oxidants in self-cleaning activity of TiO2

Progress on fabricating self-cleaning cotton

As reported by Hashimoto et al TiO2 has the most efficient photoactivity, with the highest stability and the lowest cost for industrial use [85]. TiO2 was used as an environmental friendly photocatalyst for solar-energy conservation in 1972 [89]. Frank and Bard investigated the detoxification of cyanide in water using TiO2 and reported in 1977 [92].This was done without platinization at ambient conditions and took the attention of scientists in a greater extent. TiO2 was mainly used as a self-purification material for water and air purification. Fugishima et al and Heller initially used TiO2 to engineer self-cleaning property in early 1990s[93]. Since then, many researchers have used TiO2 into many surfaces including ceramic, glass, tents, etc to engineer many surfaces[86]. So far application of TiO2 as a self cleaning material was limited to thermo stable materials due to the requirement of high temperature treatments during the process [94, 95]. Another mile stone was placed by Daoud and Xin in 2004 by fabricating self-cleaning cotton by the concept of bottom-up approach in nanotechnology with the aid of sol-gel process through in situ nucleation and growth of anatase titanium particles on cotton at low temperature and ambient pressure using pad-dry-cure method [96]. This has opened the doors to fabricate numerous low thermally resistant materials [97-101]. Schematic diagram of pad-dry-cure method is illustrated in the Figure 1.5 below.

Figure 1.6: Schematic diagram of dip-pad-cure method

Dong et al reported that dosage of TiO2 dispersion affects the efficiency of ammonia decomposition, in the study of ability to decompose air bone pollutants by TiO2 particles grown on cotton using polyglycol based P25 [100]. Over the last few years, various sol-gel synthesis conditions were adopted and modified to prepare titanium dioxide colloids for the formation of photocatalytic textile fibres. They involved modification in temperature, reaction time, change of precursors, and stabilizers. These methods are summarized below in Table 1.4.

Table 1.4: Summary of TiO2 sol-gel synthesis

Starting material Conditions Medium Stabilizer Phase Reference

Degaussa P25 5 min ultrasonication followed by addition addition of polyethyleneglycol and stir for 30 min Water - Anatase, Rutile [100]

TIP Stirred at 60 �C for 16 h Ethanol and water Acetic acid Anatase [102]

TIP Stirred at 80 �C for 30 min, autoclaved at 250 �C for 12 h Water HNO3 Anatase [103]

TIP Stirred for 30 min under Ar gas flow Isopropanol HCl Anatase [104]

TIP Stirred at 25, 40 or 60 �C for 16 h Ethanol and water HNO3 Anatase [99]

TIP Cooled in an ice bath and stirred for 1 h 2-Propanol HNO3 Rutile [105]

TIP Stirred for 6 h for peptization, autoclaved for 3 h at 130 �C, pH=0.7, 3, 7 Water HNO3 and acetic acid Anatase, rutile, brookit [106]

TiCl4 Cooled at 0 �C in an ice bath HCl - Rutile [98]

Though TiO2 was formed by researchers using different methods as above, it is not useful, if they are not attached to the cotton properly for long lasting activity. Many researchers have adapted the pad-dry-cure method to slip in particles between the cotton fibre [99, 100, 107-110].

On the other hand, Zhang et al have fabricated highly anatase titanium dioxide on cotton surface via microwave-assisted liquid phase deposition with hexafluorotitanate ammonium as a precursor. This liquid phase deposition took the attention as of its simplicity in forming titanium dioxide at low temperature [101]. They have shown the self-cleaning performance by degradation of Methylene blue within 3 h. Kiwi group has reported the possibility of producing self-cleaning textile by loading P25 particles via non toxic chemical spacers in 2005 [97]. Kiwi et al have contributed in a different manner using dip coating method to produce self-cleaning cotton [98, 103, 111]. Different pre-treatments like radio frequency (RF) plasma (vacuum/atmospheric), microwave (MV) plasma (vacuum/atmospheric) UV irradiation surface activation, negatively charged functional groups were introduced to cotton through formation of active oxygen species to anchor titanium dioxide [103, 105, 106, 111]. RF/MV-plasma and UV irradiation pre-treatment are considered as the more environmentally friendly methods with compared to high temperature and heat treatment or solvent based sol-gel methods. In contrast, they need quick placement of titanium dioxide after pre-treatment as active species has a shorter life time otherwise no reaction will occur between titanium and cotton.

Water-repellence

Wettability is a property of a surface, and occurs due to the surface tension, which cannot be measured easily for solid surfaces. It is easy to measure contact angle of liquid rather than the surface tension. The morphology of the solid surface has a huge impact on the wettability. Once surface do not show affinity for water, it tends to repel water. The aspects of water repellence are discussed in the following section.

Contact angle and wettability

Surface wettability of a flat surface is determined by the surface chemical composition. According to Young�s equation, the contact angle is a well-defined property that depends on the surface tension coefficients of solid, liquid and gas.

Liquid/solid contact angle (?) on a flat surface is correlated by three interfacial surface tensions, solid-vapour (?_SV), solid-liquid (?_SL) and liquid-vapour (?_LV), as per the Young�s equation below.

cos???_flat ?=(?_SV-?_SL)/?_LV , (1)

Figure 1.7: Young�s representation of liquid on surface. a) Hydrophilic b) hydrophobic

Highest water contact angle that can be obtained for a flat surface is 115� [112, 113], which was reported by using the lowest surface free energy possessed by trifluoromethyl group terminated surface. They suggest that topological modification of surface is required to increase the water contact angle further.

As Young�s equation is valid only for partial wetting of solid having a smooth surface, it cannot be applied to real solids as they behave differently. The effect of surface roughness on wettability was first discussed by Wenzel in 1936 [114] and then by Cassie and Baxter in 1944 [115]. Wenzel suggested that the effective surface area increases as the surface becomes rough, and hence water will tend to spread more on a rough hydrophilic substrate to develop more solid-liquid contact, while spread less on a rough hydrophobic substrate to decrease the contact area to solid, both of which are thermodynamically more favourable. A key assumption of this conclusion is that water is in complete contact with the solid surface, i.e water fills the grooves on the rough surface, which is called Wenzel state (Figure 1.7a). The relationship between the apparent contact angle on a rough surface (?_rough) and its intrinsic contact angle (?_flat) has been described by the Wenzel equation:

cos???_rough ?=r cos???_flat ? (2)

Where, r is roughness ratio, which is defined as the ratio of true area of the solid surface to the apparent area.

Figure 1.8 : Water droplet on rough surface a) Wenzel model; Water fills the grooves of rough surface b) Cassie and Baxter model; Water do not fills the grooves of surface, air, water and material make a new composite material.

However in 1944 Cassie and Baxter came up with a different mathematical model. In their model, a liquid sits on a composite surface made of a solid and air (Figure 1.7b). Therefore, the liquid does not fill the grooves of a rough solid, and air is trapped in between liquid and solid which makes the new composite. They have found that, micro sized liquid droplet placed on a micron-textured hydrophobic surface finds it favourable to follow the contours of the surface, sinking down and spreading out compared to the same drop on a smooth surface. So the increasing of roughness of a hydrophilic surface reduces the contact angle of water droplet and on a hydrophobic surface vice versa. According to this model, both structural and chemical properties of a surface determine the contact angle.

Contact angle hysteresis

When the volume of a liquid drop placed on the surface by a syringe is steadily increased until the contact line advances, the contact line begins to move. The contact angle observed when it just begins to move is the advancing contact angle (?A). On the other hand, when the liquid droplet is retracted steadily until the contact line recedes, the contact line begins to move again. The contact angle observed when the contact line is just set in motion by this process is defined as the receding contact angle (?R). The difference between the advancing angle and the receding angle is the contact angle hysteresis defined as ??H.

Lotus effect

Superhydrophobic surfaces are often found in nature, such as lotus leaf, butterfly wings and legs of water striders [116]. SEM images showing surface topology of such different species are shown in the Figure 1.8.

Figure 1.9: SEM images showing morphology of different species in nature. a) Wing of Orthoptera Acrida cinerea cinerea (Thunberg) CA of 150 � [116] b) Wing of Lepidoptera Papilio xuthus (Linnaeus) CA of 168� [116] c)Upper leaf surface of Nelumbo nucifera (Lotus) CA of 162� [117] d) Wing of Neuroptera Grocus bore (Tjeder) CA of 159� [116]

(Images were adapted from Byun, D., et al., Wetting Characteristics of Insect Wing Surfaces. Journal of Bionic Engineering, 2009. 6(1): p. 63-70 And Ensikat, H.J., et al., Superhydrophobicity in perfection: the outstanding properties of the lotus leaf. Beilstein Journal of Nanotechnology, 2011. 2: p. 152-161 with written permission)

Patent was issued to Neinhaus and Barthlott in 1998 describing the �Lotus-Effect�. Cleaning of the surface due to the lotus effect arise with the higher water contact angle. Once the contact angle is high, water will form almost spherical shape and trend to roll-off with the dust particles on the surface. They suggested that the advancing contact angle of a water droplet easily reaches the receding contact angle on a self-cleaning rough surface when the surface is slightly tilted and the rolling motion more efficiently cleans the surface. Murmur describes the rolling motion as a complex physical phenomenon, the lotus-effect [113]. According to his findings, when a droplet rolls, a hysteresis is developed in the both advancing and receding contact angles in three phase interface between solid, liquid and gas. When this hysteresis is zero or equal to zero, the drops are to be rolled easily. The requirement for a self cleaning surface is considered as ?_s>150� and the roll off angle is below 15� [113].

Engineered water-repellent surfaces

Though the fabrication of superhydrophobic cotton surface was emerged with a patent in 1940�s it was not success. In contrast, researchers have made impressive efforts to make superhydrophobic surfaces in variety of solid surfaces. In order to fabricate superhydrophobic surfaces, micro-scale patterns have to be made on the surface. This has been done by physical processing methods like ion etching and compression of polymer beads [118]; by chemical methods such as plasma-chemical roughening on various surfaces [119]. These methods are accompanied with the drawbacks, like costly batch production and time consuming, which limit the wide use of the technique. Although it is a relatively simple and one-step process to make superhydrophobic surfaces by using intrinsically hydrophobic materials, unfortunately, many materials do not possess a low enough surface free energy to be intrinsically hydrophobic. Conventional fabrication is involved with two approaches, one is the creation of a surface with necessary roughness and the next is to modify the surface with hydrophobic coating with chemicals, such as alkanethiols, organic silanes [120], and fatty acids, which can offer a low surface free energy after linked to the modified surface [121]. In 1996, Onda et al. prepared a super-water-repellent fractal surface by solidifying the melted alkylketene dimmer (AKD, a kind of wax) [122]. It has been demonstrated that the contact angle of a liquid droplet placed on a fractal surface can be expressed as a function of the fractal dimension, the range of fractal behaviour, and the contacting ratio of the surface. Jin et al. reported a laser etching method to make superhydrophobic polydimethylsiloxane (PDMS) surface, which contains micro, submicro and nano-composite structures [123]. The water contact angle for the etched PDMS surface is higher than 160� and sliding angle is lower than 5�. Ma et al. made a superhydrophobic membrane in the form of a nonwoven fiber mat by electrospinning a PS-PDMS block polymer blended with PS homopolymer [124]. The superhydrophobicity is attributed to the combined effect of surface enrichment in siloxane and surface roughness of the electrospun mat itself. Phase separation has also been utilized to make superhydrophobic surfaces. Erbil et al. prepared a superhydrophobic gel-like porous polypropylene coating by casting the polymer solution, where nonsolvents were used in conjunction with p-xylene solvent as a polymer precipitator to increase the extent of polymer phase separation [125]. Zhang et al. demonstrated the fabrication of 2-D arrays of nanopillars made from perfluoropolyether derivatives using a porous anodic aluminum oxide membrane as a template. Both nanopillars on a flat surface and on a lotus-leaf-like topology exhibit superhydrophobcity, low contact angle hysteresis, and self-cleaning properties [126]. Yan et al. fabricated superhydrophobic poly(alkylpyrrole) films by an electrochemical synthesis method.

Furthermore, certain inorganic materials have also been employed in the fabrication of superhydrophobic surfaces. Feng et al synthesized aligned ZnO nanorods via a two-step solution approach. The ZnO nanorods films exhibit superhydrophobicity which is due to the surface roughness and the low surface energy of the (001) plane of the nanorods exposed on the film surface. More interestingly, reversible superhydrophobicity to superhydrophilicity transition was observed and well controlled on the ZnO nanorod films by alternation of UV illumination and dark storage [127]. A similar result was also obtained on the TiO2 nanorod films from their following work [128].

Because of the well established micro and nano-fabrication technologies on silicon substrate, silicon has been widely used for making superhydrophobic surfaces through fabrication of a variety of surface structures. Oner and McCarthy investigated the wettability of patterned silicon surfaces, which were prepared by photolithography, followed by surface modification using silanization chemistry [129]. Water droplets moved very rapidly on these surfaces and rolled off when the substrate is slightly tilted. Baldacchini et al reported a way to create micro/nanoscale roughness on silicon wafers by using a femtosecond laser to etch the silicon wafers [130]. The resultant surfaces were coated with a layer of fluoroalkylsilane molecules to yield contact angles of higher than 160� and negligible hysteresis. Verplanck et al. made silicon nanowires on Si/SiO2 substrates through a vapor-liquid-solid (VLS) mechanism [131]. The resulting rough surfaces were modified with a fluoropolymer C4F8, and exhibited superhydrophobicity.

Metal surfaces can be made superhydrophobic by first etching the surfaces to obtain roughness in both micrometer and nanometer scales and then modifying the roughened surfaces with a hydrophobic coating. Qian et al. have made superhydrophobic surfaces on aluminum, copper, and zinc polycrystalline substrates by first using chemical etching methods and then coating the substrates with a fluoroalkysilane [132]. The key to the etching technique is the use of a dislocation etchant that preferentially dissolves the dislocation sites in the grains. The etched metallic surfaces, after modified with the fluoroalkylsilane, exhibited superhydrophobic properties with water contact angles of larger than 150�, as well as sliding angles of less than 10�.

Electrochemical methods have also been employed to make superhydrophobic surfaces on metal substrates. Zhang et al described the use of polyelectrolyte multilayers in electrochemical deposition to adjust the morphology of gold clusters for fabrication of superhydrophobic surfaces [133]. They also fabricated a branch-like structure of silver aggregates by electrodeposition techniques. Surface was suyperhydrophobic after modification with a self-assembled monolayer of n-dodecanethiol [134].

With these knowledge, in recent years, many approaches have been adopted to prepare superhydrophobic textiles [9, 10, 96, 135-141]. For example, Gao and McCarthy in 2006, grafted a silicone coating to microfiber polyester fabric to render the fabric superhydrophobic according to 1945 patent, but this could applicable to only tightly woven polyester fabric not for cotton [135]. Wang et al incorporated gold particles into cotton fabrics to induce a dual size surface topology, but there were no chemical bond between cotton and gold particles, and the method was expensive to be applied to cotton [136]. Liu et al has used a different approach and artificial lotus leaf structures were fabricated on cotton substrates via the controlled assembly of carbon nanotubes to obtain superhydrophobic cotton substrates [137]. They have used both pristine carbon nanotubes (CNTs) and surface modified carbon nanotubes (PBA-g-CNTs) as building blocks to biomimic the surface microstructures of lotus leaves at the nanoscale. Michielson and Lee have adopted a combination treatment of mechanical and chemical surface modifications, followed by further grafting of 1H,1H-perfluorooctylamine or octadecylamine to poly(acrylic acid) chains on the pre-treated nylon 6,6 fabric, to prepare superhydrophobic textile, but this method relies on use of expensive coupling agent, 4-(4,6-dimethoxy-1,3,5-triazin-2yl)-4-methylmorpholinium chloride. Hoefnagels et al have chemically bonded the silica particles with amine groups at surface to the cotton fibres; then the amino groups were utilized to hydrophobize the surface via the reaction with mono-epoxy-functionalized poly(polydimethylsiloxane) [10]. Though this was a two step reaction to in situ generate silica particles, 6 h mechanical stirring can not be evaded, which causes damage to the textile due to the entanglement problem and therefore, not suitable for industrial production. Li et al have prepared superhydrophobic surfaces by adapting sol-gel coating method from sodium metasilicate and nonfluorinated alkylsilane on cotton substrates [9].

Typical superhydrophobic surfaces discovered in the nature have shown multiscaled roughness consisting of nanometer sized flakes on top of micrometer sized protrusions. This type of morphology for superhydrophobic surfaces can be achieved using lithographic methods, template-based techniques, plasma treatment, self-assembly and self-organization, chemical deposition, layer-by-layer (LBL) deposition, colloidal assembly, phase separation, and electrospinning.

Superhydrophobic surfaces can also be produced by chemically depositing a thin film on the selected substrates. Typical chemical deposition methods include chemical vapor deposition (CVD), electrochemical deposition, and LBL deposition.

The sol�gel methods have been utilized to create superhydrophobic surfaces in accordance with Wenzel or Cassie�Baxter's theories since the very early stage of mimicking lotus leaves' surface structure [120, 121, 142, 143]