Zinc Oxide Nanoparticles - History, Synthesis And Characterization

History of Zinc Oxide Quantum Dots

Various fields of science and engineering have adopted the utilization of nanotechnology. This generally deals with the scaling down of materials to a Nanoscale. This implies that the size of the material must be greatly reduced, and this influences the mechanical, chemical and physical properties of the materials. This research paper will generally focus on the zinc oxide nanoparticle. Its history and the role that it technological plays in the technology industry have also been underlined (Bishop, 2013).

Zinc Oxide Quantum Dots encompass of zinc oxide particles which are covered with -COOH and –OH groups. They are hydrophilic Nanocrystals that have an emission wavelength estimated at 550nm. These dots does not comprise any pollutant -.their elements are accessible in large volumes, for instance, Cobalt and Lanthanum-doped ZnO (Chopra, 2012).

Besides, the zinc oxide quantum dots are categorized by their nature of X-ray powder diffraction, energy dispersive X-ray analysis and the high-resolution transmission electron microscopy. They possess desirable optical properties and have a reduced size and an improved fluorescence more so for the Cobalt and Lanthanum-doped ZnO. These doped elements exhibit an electromagnetic behavior at room temperature. In addition, these quantum dots have the possibility of making the BGC 803 cells effectively within a very short duration with no elements of toxicity or adversarial effects on the growth of cell even at high concentrations. The zinc oxide nanoparticles have a very wide application in the fields of nanotechnology i.e. bioimaging, gene therapy, drug delivery and cancer therapy (Ebbesen, 2010).

There exists different structure of zinc oxide nanoparticles and they are synthesized using different techniques. These techniques can either group as either physical, biological or chemical synthesis .however the structures can be grouped depending on the dimension of the nanoparticles. They can either be a one-dimensional nanoparticle, two-dimensional nanoparticle or even three-dimensional nanoparticle.  Below are some of the pictures of the structures of the nanoparticles.

These type of synthesis is chemically achieved and can further be divided into either liquid or gas synthesis (Etching, 2008).

These include methods such as synthesis, colloidal aspects, hydrothermal synthesis, and co- synthesis among others.  In this technique, a liquid agent is always allowed to react with the zinc oxide nanoparticles.  Thereafter the resulting particle which is either soluble or insoluble is washed and heated at different temperatures to come up with the differently desired nanoparticles. 

Also known as vacuum synthesis techniques is either achieved through gas condensation method or through pyrolysis. Pyrolysis is whereby the flame heating is used to generate aerosol droplets from the zinc salt and then dispersed into gas thereby reducing their size. After which it is exposed to condensation and sintering (Franssila, 2010).

Structure and Synthesis of Zinc Oxide Quantum Dots

The gas condensation techniques are achieved either physically or chemically but the main principle that they use is the condensation of a zinc source within a chamber by use of a resistive heat.  

There are also different techniques entailed in this aspect including physical vapour deposition, squatter deposition, ion implantation, ball milling among other methods. The production rates of the zinc nanoparticles are always increased thereby appropriate for industrial applications. The ball milling process has been proved to be simple and effective and efficient since they only involve the placing of a powdered material inside a ball mill and then passing balls containing the high energy of collisions (Liu, 2013).

Also known as the green synthesis of the nanoparticles, produces the least amount of pollutants. This technique is cheap and efficient enough and uses a variety of microorganisms during the synthesis i.e.  Fungi, virus and algae are used in the synthesis process. The methods used in the biological synthesis includes

Plant-mediated synthesis – this method involve the use of plant extracts to either act as reducing or capping agents.  This technique is very simple and applies the vivo and vitro mechanisms, but they are only limited to some materials (Fan, 2014).

Microorganism mediated synthesis – usually in this techniques, the eco-friendly microbes are used and they can either be extracellular or intracellular synthesis. It is one of the recommendation techniques due to its non-pollutant nature and thus is widely applied in the fields of bioleaching, Biomineralization and also bioremediation. The intracellular synthesis will involve the incubation of fungal biomass in darkness with a solution of zinc salt for a particular time period while the extracellular synthesis involves treatment of the fungal filtrates with a precursor solution.

Other mediated techniques include distillation method by use of methanol proceeded by boiling thereby producing aggregates of the nanoparticles (Tilli, 2009).

 Zinc oxide nanoparticles can either be characterized as a solar cell, photoluminescence or absorption. The photoluminescence characterization involves the application of an excitation with a certain dimension of the light wavelength to determine the luminesce characteristics. The absorption characterization involves the use of a spectrometer device to determine the absorption extent of the nanoparticles. 

Finally, solar cell characterization involves the use of a solar simulator to determine the performance of the solar cells. An open circuit voltage is used and then the zinc quantum dotes are deployed (Tilli, 2009).

Mechanical Properties of Zinc Oxide Quantum Dots

• Tensile strength – zinc oxide particles have good tensile strength properties at a lower concentration, but at higher concentration the tensile strength reduces.
• They have a high ultraviolet light absorption properties due to the wide band gap
• Besides, the nanoparticles exhibit a high optical transparency and thus used as transparent conductors in various sensors and applications.
• The zinc oxide nanoparticles also have a high ultraviolet light resistance.
• The addition of zinc oxide particles also increases the storage modulus due to increased aggregation.
• Zinc oxide particles also have a good dispersion stability(Mattox, 2014). 

These zinc oxide nanoparticles have many advantages in terms of health aspects to human. Below are some of the advantages

• Due to its sunscreen efficiency, these nanoparticles are useful during situations of knee or ligament injury as they will be able to adequately scatter light and encourage the desired whitening.
• The zinc oxide dots also have a good texture and desirable spreadability thus they are advantageous in promoting cosmetics
• Also, the zinc oxide nanoparticles have a property of green luminescence thus significant agents during bioimaging(Mattox, 2016). 

Characterization

Disadvantages

• When the zinc oxide nanoparticles are used at a higher concentration, it can be toxic to the human body.
• During the chemical synthesis of the nanoparticles, there are environmental pollutants that are always produced which are harmful to the plant and human health.
• During the synthesis of zinc oxide, some of the reducing agents if not properly taken care of may cause serious skin burns or any other form of injury an examples hydrazine.

The use of zinc oxide nanoparticles have a lot of environmental implications and these are tracked down from the time of synthesis. Hence below is a discussion of the various environmental impacts from different agents used during the synthesis process (Nanomaterials, 2011).

 Stabilizers, surfactants and Capping agents- studies show that chemical synthesis involves the wide application of these agents. Prior to their selection, there are factors that should be keenly considered. These factors include the choice of the solvent, nontoxicity and the use of an environmentally friendly agent.  This is because if not properly chosen, they may result in pollution of the environment

Ph. and Reducing agents-the use of reducing agents is always accompanied by a condition of either high acidity or basicity. This means that either way there is an aspect that is jeopardized. For instance, the use sodium hydroxide as a reducing agent in precipitation highly degrades the amount of protein (Nicholson, et al., 2008). 

Sonication –this process generally uses sound energy or ultrasonic means for the production of nanoparticles.  Usually, they employ up to 20 kHz frequency and this may cause permanent hearing impairment if not properly taken off.

Temperature – most of the synthesis techniques that involve the use of a high amount of heat may seriously cause an imbalance of the earth’s overall temperature. Besides, these high temperatures may result in heat stroke, heat rashes or even heat syncope.

The widespread of nanotechnology has sparkled the use of zinc oxide quantum dots in very many fields of applications.  This has been as a result of the low toxicity and the biodegradability nature of the materials. Below are some areas where the zinc oxide quantum dots have been applied.

Bioimaging; as a result of the efficient epitomic blue and near ultraviolet emission from the zinc nanoparticles, they are appropriate for the bioimaging application. Moreover, the intrinsic fluorescence and the desired optical properties of the nanoparticles makes it possible for the penetration of the human skin (Nicholson, et al., 2008). 

Drug delivery -The versatile nature, phototoxic effect and the large surface area of the zinc oxide quantum particles allow for east delivery of the drug.

Gene delivery -A number of research has been conducted concerning the use of zinc oxide nanoparticles in the safe gene delivery. Usually, the protection of gene from any form of degradation has been a problem and due to their properties, the three-dimensional zinc oxide nanoparticle has been round to be ideal for the application of gene delivery.

Advantages of Zinc Oxide Quantum Dots

Biosensors -The biosensors including piezoelectric, calorimetric and others have been broadly applied in the food industry, environmental monitoring and also health care. The development of biosensors that are effective, efficient and have high performance is thus significant in these applications. Research shows that the involvement of these zinc oxide nanoparticles significantly helps to bring some desirable characteristics that help in achieving the above requirement

Cancer therapy –apart from other applications, the zinc oxide nanoparticles due to their optical properties and other characteristics find their application in the cancer therapy (Ostrikov, 2007).

Other applications include the use of the zinc oxide nanoparticles as light emitters, sensors, components of high-power electronics in transparent conductor solar cells

The composites of ZnO and PMMA can be potentially applied  in protective layers, UV protecting films, as flame retardant materials transparent barrier, and as and consequently, antireflection coatings

Conclusion

The research report was to describe one of the nanomaterial in terms of some aspects such as the characterization, synthesis techniques, mechanical properties, history and then the various fields of applications and its advantages and the limitations.  Zinc oxide quantum dots was chosen as the topic of research and the various applications have been described above. There are very many areas in the nanotechnology sector where the zinc oxide particles are used. For instance bioimaging, gene therapy, drug delivery and cancer therapy. These applications are directly linked to the chemical, physical and mechanical properties of the zinc oxide quantum dots. As seen above zinc oxide poses desirable mechanical properties such as good tensile strength properties at a lower concentration, high ultraviolet light absorption properties, high optical transparency, and high ultraviolet light resistance.  Increased modulus and good dispersion stability. These properties make it suitable for the applications in respective fields (Nanomaterials, 2011).  

References

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