Polyoxometalate Pom Is A Polyatomic Anion Biology Essay

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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.

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.

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)

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