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Nanostructured semiconductors are potentially used in technological applications due to their versatile fabrication techniques and chemical and physical properties. Particularly Zinc Oxide semiconductors are widely investigated, because of its wide technological application from catalytic activity, cosmetics to room temperature blue UV laser. ZnO nanoparticles are regarded as useful material in light emitting diodes. However ZnO nanoparticles can possibly induce some adverse effects when comes in contact with the animals such as cytotoxicity, genotoxicity, pulmonary toxicity and etc. By attempting to reduce the toxicity of ZnO nanoparticles, it can be potentially used in many applications without the scare.
Nanotechnology is the field come out from different fields like physical, chemical, biological and engineering sciences where novel techniques are being developed to control single atoms and molecules. Nanoparticles are particles in which minimum of two dimensions will be less than hundred nanometers. Nanoparticles are viewed as fundamental building blocks of nanotechnology. The most important property is nanoparticles can exhibit high surface to volume ratio. Nanoparticles properties differ greatly from those atoms and bulk materials. This review consists of literature summary of current knowledge on zinc oxide nanoparticles and its toxicity. Zinc oxide is an inorganic compound with the formula ZnO. It usually appears as a white powder, nearly insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber, lubricants, paints, ointments, adhesives, sealants, pigments, foods, batteries, ferrites, fire retardants, etc. Molecular weight of ZnO is about 81.39 AMU.
ZnO is often called a II-VI semiconductor because zinc and oxygen belong to the 2nd and 6th groups of the periodic table, respectively. This semiconductor has several favourable properties: good transparency, high electron mobility, wide bandgap, strong room temperature luminescence, etc. Those properties are already used in emerging applications for transparent electrodes in liquid crystal displays and in energy-saving or heat-protecting windows, and electronic applications of ZnO as thin-film transistor and light emitting diode. ZnO nanoparticles crystallizes in three forms, such as hexagonal wurtzite, cubic zinc blende and rarely observed cubic rock salt. Wursite is the most common form observed. Zinc blende can be stabilized by growing ZnO on substrates with cubic lattice structure .
Among infinite number of nanoparticles, ZnO nanoparticles have specific properties like high specific surface area, optical transparency, chemical and photochemical stability, ease of fabrication and so on.
Optical property of a semiconductor is associated with intrinsic and extrinsic effects. Intrinsic optical transition takes place between electrons in the conduction band and holes in the valence band. Group velocity of electron and hole should be equal to form an exciton. Excitons are of 2 type free excitons, bound excitons. Extrinsic property of semiconductor is related to defects and dopants parameters. Which create discrete electronic states in the band gap. ZnO film appears transparent till o.3-2.5µm. depending on the carrier concentration plasma range lies between 2-4µm. Optical properties can be various methods like optical obsorption, transmission, reflection, elipsometry, photoluminescence, spectroscopic methods photoluminescence spectrum of ZnO bands composed of near UV emission band (375 nm), green band emission (510 nm). In some cases yellow-orange band (610 nm) was also observed .
As ZnO is a direct and wide band gap semiconductor. It is used in optoelectronic and electronic devices in wide range because wide band gap materials may have high break down voltage, low noise generation and operate at high temperature. At low electric field electrons present in ZnO will not affected due to low energy distribution. So mobility will be low. But in high electric field the energy of electron will be equal to thermal energy of electron. So electron distribution function changes significantly from equilibrium value and become as hot electrons whose temperature is higher than lattice temperature. So there is no energy loss during critical time .
ZnO is a soft material with harness about4.5 on the Mohs scale. It has high heat capacity, high heat conductivity, low thermal expansion, and high melting point. ZnO is the highest piezoelectric tensor. This property makes in very important material in piezoelectric application.
ZnO nanoparticles can be prepared by different synthesis methods including sol-gel technique, microemulsion method, spray pyrolisis, mechanochemical processing, drying thermal decomposition of organic precursor, RF plasma synthesis, hydrothermal and solvo thermal synthesis and etc .
Hydrothermal is a promising synthesis method because of low process temperature and easy to control the particle size. This method has other advantages like uniform growth of nanoparticles, catalyst free growth, low cost, less hazardous, when compared to other techniques. The particle properties like shape and size can be controlled by adjusting the reaction temperature, time and concentration of the precursors .
To synthesize the ZnO nanoparticles, stock solutions of Zn(CH3COO)2.2H2O (0.1 M) was prepared in 50ml methanol under stirring. To this stock solution 25ml of NaOH (0.5 M) solution prepared in methanol was added under continuous stirring in order to get the pH value of reactants between 8 and 11. These solutions was transferred into teflon lined sealed stainless steel autoclaves and maintained at various temperature in the range of 100 - 200 ÌŠC for 6 and 12 h under autogenous pressure. It was then allowed to cool naturally to room temperature. After the reaction was complete, the resulting white solid products were washed with methanol, filtered and then dried in air in a laboratory oven at 60 ÌŠC.
The aqueous phase was prepared by dissolving zinc acetate in de-ionized water to obtain a zinc cation equal to 0.2 M. Normal heptane was used for preparing the oil-phase. For stabilizing the water-in oil (w/o) emulsions, surfactant Span-SO was added into n-heptane. The added amount of Span-80 was fixed to be five volume percent of the total volume of the aqueous and oil phases. After both aqueous and the oil phases were prepared, these two phases were mixed at different ratios. After continuous mixing by a mixer at 1000 rpm for 1 h, homogeneous emulsions were obtained. Then NH,OH was addedinto the emulsion solution to precipitate zinc cations. The aging time for precipitation was 2.5 h. The precipitates were dried in a rotary evaporator and then calcined at 7OO ÌŠC-1000°C for 2 h. After calcination, white powders were obtained.
ZnO powder prepared by this method possesses nearly spherical morphology. The particle size was much smaller than that obtained in the conventional precipitation process. The ratio of the volume of the oil phase to that of the aqueous phase significantly affected the
stability of emulsion, and influenced the mean particle size of ZnO powder .
(iii) SOLVOTHERMAL SYNTHESIS
Synthesis of ZnO nanoparticles done by solvothermal process at 80°C. Poly(vinyl pyrrolidone) PVP 30K was dissolved in pure ethanol under stirring at room temperature, then zinc acetate dihydrate was slowly added to the solution. Consequently, the solid NaOH was put into the reaction mixture. The resulting solution was stirred for several minutes. The solution was then transferred to polypropylene vessel, then sealed and heated in temperature-controlled autoclave at 80°C for 24 h. After cooling to room temperature, the white powder was precipitated and then washed with absolute ethanol several times to eliminate other impurities. Finally, the powder was dried under vacuum at 60°C overnight.
By this method using absolute ethanol as solvent, zinc rods of diameter and length of 8.2±2 nm and 54.3±11 nm, respectively can be obtained. The absence of PVP with the solvent gave the ZnO semi-sphere shape with its diameter and length of 68.1±9 nm and
108.4±9 nm respectively .
Anhydrous ZnCl2 powder, ammonia and an appropriate additive were used to produce the precursors. ZnCl2 powder was dried in air at 150 _C overnight prior to use and milled simply in a ceramic mortar. It was then added to a beaker containing NH4OH aqueous solution to produce the complex. Concentrated NH4OH was gradually poured into the beaker until a white precipitate of zinc hydroxide was formed. Further addition of NH4OH resulted in dissolution of the precipitate indicating the time for addition of the additives. The solution, which was transparent, was diluted with de-ionized water. It was then dropped into the second solution containing the additive at 100 _C to produce zinc oxide nanopowder.
Nanopowders were then washed with ethanol and saved into a glass case. The powder was subsequently dried at 60 _C in an oven holding the sample for several hours.
Mean particle size ZnO prepared by this method was approximately 45 nm. Particles will be in elongated morphology with a noticeable amount of porosity. Solochemical synthesis method is suitable for a large scale production of ZnO .
To prepare 10 g of ZnO-NPs, 135 mL pure water was mixed with 65 mL isopropanol and the mixed solution was stirred for 5 min at 30 ÌŠC. Then the temperature was increased to 45 ÌŠC and zinc acetate was gradually added to the solution. After 30 min, 50 mL acetic acid was added to the clear solution and it was stirred again for 30 min at 40 C. The prepared sol was refluxed for 4 h at 110 _C. The container of the solution was placed in the water bath and the temperature was kept at 80 _C for 16 h to prepare a viscous gel from the refluxed sol. Finally, a xerogel was obtained by treating the gel with nitric acid. The xerogel was then calcined for 2 h at 650 _C or 750 _C to obtain a white ZnO-NP powder.
Particle size prepared by this method was about 20 nm. A high quality nanocrystaline film of ZnO with wurtzite over a fluorine dopped tin oxide glass substrate can be prepared by sol-gel synthesis along with spin coating .
(i)X-RAY DIFFRACTION METHOD
XRD is a non destructive analytical method for identification and qualitative determination of various crystalline phases present in a solid sample. Diffraction occurs when waves interact with crystalline samples, for this the wavelength of the wave and inter atomic distance of the sample should be same. Wavelength of X-rays and inter atomic distance of crystals will be in the order of few angstroms so x\X-rays can be diffracted by the minerals of crystalline samples.
nÎ» =2dsinÎ¸ where
d=inter atomic distance
Î»=wavelength of incident x-rays (1.54AÌŠ)
Î¸= the angle of diffraction.
Crystallite size can also be determined by XRD method using scherrer's formula
Î»=wave length of x-rays
Î²=full width at the half maxima
(ii)SCANNING ELECTRON MICROSCOPE ANALYSIS
SEM can give information like morphology, chemical composition, crystalline structure, and orientation of materials of a sample. Sample of width about 1 cm -5 microns can be analysed by SEM. Accelerated electrons in an SEM carry significant amounts of kinetic energy, and this energy is dissipated as a variety of signals produced by electron-sample interactions when the incident electrons are decelerated in the solid sample. These signals include secondary electrons, backscattered electrons, diffracted backscattered electrons, photons, visible light, and heat. Secondary electrons and backscattered electrons are commonly used for imaging samples. Secondary electrons are used for showing morphology and topography on samples. By using specific detectors secondary electrons are collected, for the sample analysis.
(iii)ENERGY DISPERSIVE X-RAY SPECTROSCOPY
EDX is an analytical technique used for elemental analysis and chemical characterisation of a sample. As a type of spectroscopy, it relies on the investigation of a sample through interactions between electromagnetic radiation and matter, analyzing x-rays emitted by the matter in response to being hit with charged particles.
To stimulate the emission characteristic X-ray from a specimen, a high energy beam of charged particles or a beam of X-ray is focused in the sample being studied. The incident beam excite an electron in the inner shell of an atom and create a hole, these inner shell vacancies are refilled by free electrons of the outer shell, difference in the two energy levels are emitted in the form of X-rays. Its characterization capabilities are due to the fundamental principle that each element has a unique atomic structure allowing x-rays that are characteristic of an element's atomic structure to be identified uniquely from each other.
(iii)DIFFERENTIAL SCANNING CALORIMETRY
DSC is a thermal analysis method where differences in heat flow into a substance and a reference are measured as a function of sample temperature, while both are subjected to a controlled temperature program. The main application of DSC is in studying phase transitions, such as melting, glass transitions, or exothermic decompositions. These transitions involve energy changes or heat capacity changes that can be detected by DSC with great sensitivity .
Due to unique properties exhibited by ZnO nanoparticles they are used in various preparing cosmetic creams, gas sensors, solar cells, chemical absorbant, chemical and optical devices, electrostatic decipative coatings, catalyst for liquid phase hydrogenation and catalyst for photocatalytic degradation. For example larger band gap (3.3 e V) is suitable for UV/blue emittors and long excitation binding energy affords stable excitation states at room temperature for optical applications.
(i)GAS SENSOR APPLICATION
Gas sensing application of ZnO nanoparticles were detected by dispersing ZnO nanoparticles in ethanol. Then drop coated on quartz sheat with 2 Pt wires connected on both sides using Ag paste to form sensor phototype. Using this sensor gas phase concentration of formaldehyde was detected.
Gas sensor of ZnO nanoparticles are explored by manipulating their defects. Anealing of ZnO in oxygen decreases the concentration of donor and increases acceptor concentration which annealing in H2/N2 gives rise to the reverse. Thus ZnO was taken as an example to be annealed in O2, H2, N2 at 600 ÌŠC for 2 hrs respectively. Photo luminescence measurement confirms that with sample 1 ZnO O2 emits in more yellow luminescence at 600 nm, While ZnO H2 & N2 emit green luminescence at 500 nm. ZnO has been successfully employed to detect various gases, such as H2, NO2, O2, H2S, CH3CH2OH and NH3 .
(ii)APPLICATION IN FABRICS
100 % cotton woven fabric of 30 *30 cm was used for this application. ZnO particles was applied on cotton using pad-dry-cure method. Fabrics were immersed in ZnO solution (2%) and acrylic binder for 5 minutes and passed through a padding mangle. 100% wet pick up was maintained. Padding the fabric was air-drop and cured for 3 minutes at 140_c. To remove the unbound ZnO nanoparticles the fabric was immerses in sodium laurel sulphate of 2g/l for 5 minutes. Fabric was washed at least 10 times in soup solution and air dried.
Specimens were taken in known concentration of bacterial suspension and the reduction in bacterial activity was measured. The efficiency of the antibacterial treatment was determined by comparing the reduction in bacterial concentration by modified Hohenstain method evaluation method.
ZnO nanoparticles treated fabric showed antibacterial activity about 94.16% against S.aureus and 86.5% against E.Coli species.
(iii)APPLICATIONS IN COSMETICS
ZnO particles are important the components of sun screens which can as the UV blockers. It is capable of blocking both UVA (320-400 NM) and UVB (280-320 nm) so that it prevent the skin from sun burns, cancer, premature aging. As the size of the particle is in nanometers the surface area is very high so more amount of light can be reflected so that it will be transparent and appears good rather than giving an opaque white mesh appearance on the face .
Ultrasensitive cholesterol sensor was prepared by modifying gold electrode with well crystallised ZnO nanoparticles. This electrode can convert the cholesterol to cholestenone and hydrogen peroxide. This conversion was done by Nafion/CHOx/ZnO/Au electrode.
Cholesterol + O2 Í¢ Cholestenone +H2O2. The sensitivity of this sensor was about 23.7 µAm/M/ cm2. Response time is less than 5 seconds and detectable limit is 0.37 n M. This much low sensitivity is aided by the ZnO nanoparticles modification .
Though ZnO nanoparticles are used in wide range of applications, it will create adverse reactions and threatens the human society. ZnO reaches the human in two ways intentionally (cosmetics) and unintentionally. The effects of ZnO nanoparticles are given below.
Cells exposed to ZnO nanoparticles shows a morphological changes. 6 h/8µg/ml exposure leads to loss of morphology. On high exposure to ZnO cells reformed in to spherical shape and formed cluster in media after detachment from the surface. Report says that rabidoblastoma cells shrinks and detaches from the medium when the dos is greater than 100 µg/ml and also showed high mitochondrial activity. LDH levels was in the range of 50-100µg after 24-48 hrs of exposure .
Oral exposure of ZnO nanoparticles was investigated in rodent models. Depending on the particle size, coating and chemical composition acute, subacute and subchronic toxicities were occurred. Acute toxicity occurs at high doses.
(iii)LONG TERM TOXICITY
Long term exposure of nanoparticles was measured by other types of exposure. Information from the studies showed the effects of nanoparticles on different organ systems, including immune, inflammatory and cardio-vascular system. Pro-thrombotic effects can be caused by the nanoparticles that affects the cardiac function .
Exposure to zinc oxide ultra fine powders was recorded in guinea pig models. In this research zinc oxide nanoparticles exposed to models through inhalation to measure the effects on lungs and its activity .
Dose (in mg)
Days(3 hours exposure/day)
Changes in neutrophils and activities of lactate dehydrogenase and alkaline phosphatase in the pulmonary fluid.
Increased protein concentration, neutrophils, and enzyme activities in lung lavage fluids were seen, together with significant centriacinar inflammation of the pulmonary tissue.
A gradual decrease in total lung capacity, vital capacity and reduction of carbon monoxide diffusing capacity were seen in combination with inflammatory changes and edema.
The reactive oxygen species production is the diverse range of nanomaterials which is one of the primary mechanisms of nanoparticles toxicity. Due to photocatalytic activation ZnO create free radicals when exposed to UV radiation.
The ROS production leads to increase oxidative stress, inflammation, consequent damage to proteins, and DNA. OH- radicals create high lipid peroxidation which can cause membrane damage. Oxidative stress will disturb the cell processes and release proapoptotic signals and cause apoptosis .
This can be done by surface modification of zinc oxide nanoparticles along with structural modifications like doping ZnO in crystalline lattice. Biogenic production of ZnO nanoparticles from natural precursor can also reduce the adverse effects. Synthesis methods like sol-gel synthesis can significantly reduce the toxicity of ZnO nanoparticles. When metal oxides are in contact cells only the free radicals can cause the membrane damage the membrane and induce cytotoxicity. When it is entrapped by a gel the possibility of free radicals production is reduced and also reduces the cell uptake and it can become a safe material.
In summary, over the past decade the ability to engineer and produce materials Zinc Oxide nanoparticles has triggered rapidly due to their interesting properties. Industrial applications using nanoparticles resulted in growing demand for ZnO nanoparticles. Humans are increasingly exposed to nanomaterials beyond exposure during the exposure of production, since ZnO nanoparticles are applied in series of consumer products. Classical chemical compounds are routinely subjected to toxicity test prior to release to the consumer. Similarly complete toxicity studies need to be done for ZnO nanoparticles and invent methods to overcome the toxicity before they are incorporated in various applications.