Friday, October 11, 2019

Nanomaterials - Types and Applications

An overview of Nanotechnology

Over the last few decades nanotechnology has emerged as a promising research area for emerging scientific and technological revolution. Materials in the nanometer range have attracted much research interest because of high surface area to volume ratio and the presence of more loosely bound surface atoms compared to that of bulk materials. Nanoparticle is defined as a small object in the order of 1 to 100 nm. The first concept of nanotechnology was given by physicist Richard P. Feynman, in his famous lecture “There's Plenty of Room at the Bottom” at the California Institute of Technology, 29th December, 1959. It was suggested that devices and materials could someday be fabricated to atomic specifications. After that in 1970 Norio Taniguchi, a professor of Tokyo Science University, defined the term nanotechnology. He defined the term nanotechnology as “Nanotechnology mainly consists of the processing, separation, consolidation, and deformation of materials by one atom or by one molecule”. Then in 1980 Dr. K. Eric Drexler, Chief Technical Advisor to Nanorex, further promoted the technological significance of nano-scale phenomena and devices through speeches and books.

            The prefix of the term “nanotechnology”, which is “nano”, originates from the Greek word “dwarf” that means something small. A nanomaterial can be defined as a material which has at least one dimension less than 100 nm out of three dimensions. Where as a nanoparticle is one which has three dimensions less than 100 nm.

Types of Nanomaterials

The progress of nanoparticles research passed through several advancements such as simple spherical nanoparticles to different shapes (anisotropic), hollow, core/shell, doped, movable core/shell or yolk shell etc because of more advanced properties. A details of the types of nanomaterials are sown in Figure 1. When the nanoparticles are made of multi-materials, they not only show improved the properties than the main materials but also developed multi-functionality.

 

types of nanomaterials
Figure 1. Types of nanomaterials.

Applications of Nanomaterials

Nanomaterials have noble properties over the bulk materials, such as high surface area to volume ratio, low density, high surface energy, more reactive, etc. Because of these important properties nanomaterials have wide range of applications in catalysis, biomedical, electronics, solar cells, fuel cell, light emitting diodes, laser, rechargeable batteries, cosmetics, and so on. Some specific applications of nanomaterials are discussed here with details

 

Photocatalytic Degradation of Organic Compounds

Environmental pollution because of organic pollutants is a major environmental problem today. Different classes of organic compounds, such as dyes, pharmaceutical products, aromatics, volatile organic compounds, etc are responsible for the water, soil, and air pollution. Presence of these organics in water causes serious problems to aquatic life as well as human beings. Therefore removal of these organics from the environment is very necessary. The sources of these organic compounds, their toxic effect to the environment, and the conventional methods available for the degradation of these compounds are discussed here in details.

Degradation of Pharmaceutical Products

Pharmaceutical compound is one important class of toxic organic pollutants, which causes mainly soil and water pollution. Some specific classes of pharmaceutical compounds, such as antibiotics, antipyretic, etc are used in huge quantity and the excess quantity are discharged to the environment. Some classes of antibiotics, such as nitroimidazole (metronidazole, ronidazole, tinidazole), fluoroquinolones (ciprofloxacin), are widely used in human and veterinary medicine. Like different pharmaceutical compounds these antibiotics are not completely metabolized in the body and released to the environment. The major sources of the pharmaceutical contaminants are the sewage treatment plant of pharmaceutical industries and the municipal waste water. The presence of pharmaceutical products, for instance antibiotics in the environment, even at low concentrations, causes the growth of antibiotic-resistant bacteria and creates microbial population, which may cause of ineffectiveness of the present forms of treatment and major epidemics. For this reason complete removal of this kinds of organics from the environment is very necessary. Some conventional method, such as advanced oxidation processes (AOP), namely electrochemical oxidation, ultra sound radiation, ozonation, are already been used for the degradation of these pharmaceutical compounds. Complete removal of antibiotics from the environment by conventional method is still difficult because of the low degradability efficiency of the methods and high solubility of the antibiotics in water. In this approach nanomaterials can be applied as the photocatalyst for the degradation of such kinds of organic compounds.

Degradation of Dyes

Dye contamination is another critical environmental problem and addressed by several researchers till now. The sources of synthetic dyes in wastewater are from different industries such as, textile, dye and dye intermediates, paper and pulp, printing, colour photography, petroleum industries, and so on. In the textiles dyeing processes a wide variety of synthetic dyes such as, azo, polymeric, anthraquinone, triphenyl-methane, and heterocyclic dyes are used. In most of the textile dying process, almost 15% of synthetic dyes are unutilized, which are released into the waste water stream. Continuous discharge of dye-bearing effluents from these industries into natural stream and rivers poses severe environmental problems as toxic to useful microorganisms, aquatic life, and human beings. So, suitable and efficient techniques are highly essential for the treatment of these industrial effluents. Different physical, chemical, and biological methods have been developed for the removal of dyes from the waste water. However complete removal of the dyes from the water by these conventional methods is difficult because some drawbacks, such as low degradation efficiency, high cost method, and complex structure of some organic compounds. Nanoparticles based dye degradation are attracted over the last few years because of their complete removal efficiency of organics from the contaminated water. These methods are easy, low cost, less time consuming, and environmentally friendly.

Degradation of Aromatic Compounds

Different classes of aromatics, such as benzo compounds, phenolic compounds, naphthalene, trinitrotoluene, etc are responsible for the environmental pollution. The benzo compounds such as nitrobenzene are highly toxic organic compound mainly used for the production of aniline, paper and pulp, pesticides, dyes, explosives, cosmetics, pharmaceuticals, and so on. The long term exposure of nitrobenzene to the environment, even at low concentration, causes risks to human, such as liver or kidney damage, lung irritation, increase heart rate, skin problem, vomiting, etc. Therefore, removal of nitrobenzene from the environment is a major concern. Degradation of nitrobenzene in effluent water is difficult by conventional chemical method because of the nitro group which has strong electron withdrawing property and inhibits its oxidation, or by biological method because of its toxic and mutagenic effect on the biological systems.

     Over the last few decades semiconductor nanomaterials have attracted a lot as the photocatalyst for the degradation of organic pollutants in the contaminated water. The main objective of the process is to convert the toxic compounds to the non-toxic or usefull products. Efficient photocatalyst should have some specific characteristics, such as high surface area, visible light active, more electron and hole generator, low recombination rate, reusability, and so on. Among different semiconductor nanomaterials, TiO2, CdS, ZnS are generally recognized as the photocatalyst because of their strong oxidizing power, low cost, chemical stability, and low cost. Pure semiconductor nanomaterials have the electron hole recombination problem and are active mainly under UV-light irradiation because of high band gap. Since only a small fraction (3-5%) of solar light corresponds to UV region, so, it is expected that merely 3-5% of whole radiant solar energy is use full for the photocatalysis purposes. For this reason for a particular photocatalyst dopant helps to gain those specific properties, for instances doping can change the band gap of the host material and as a result the photo catalytic activity of the host material will enhance due to the presence of more photo generated electrons and holes. Doping of noble metal, such as silver to TiO2 improves the anatase crystallinity, surface area, lowering the band gap and makes the TiO2 as an environmentally sustainable efficient photocatalyst for the degradation of persistent organic pollutants. Doping of samarium ions to TiO2 improves the specific surface area, separation efficiency of electron-hole pairs and prevents the recombination tendency of the photoinduced electron and holes, as results the overall photocatalytic activity for methylene blue degradation of TiO2 improves significantly. Fe doping to TiO2 helps to obtained high specific surface areas, small crystal sizes, mesoporous structure, as well as a large amount of surface adsorbed water and hydroxyl groups, which contribute to their high photocatalytic activity for the degradation of XRG dye. Fluorine doping to TiO2 powders enhances the surface acidity, creation of oxygen vacancies, an increase of active sites, and finally the Vis-light photocatalytic activity of TiO2 powders has been achieved.  Doping of other materials, such as Sn, Si, and F–B–S tri-doping to TiO2 nanoparticles promote the UV-induced photodecomposition activities of TiO2, visible light photocatalytic activity, much larger speciļ¬c surface area, relatively shorter duration of photocatalytic cleaning reactions, inhibiting the recombination of photogenerated electrons and holes, and so on. Apart from TiO2, other photocatalyst such as ZnO, CdS and ZnS, Nb2O3 and others have been studied in presence of different doping element for improving their photocatalytic performances.

Antifungal Agent

Wastage of crops because of fungal infection and diseases is a major global problem in agricultural farming. The fungal phytopathogens affect on the plants and crops both and rotting the crops, as a results farmers do not get proper value. For agricultural based countries this is a big problem. Some potent fungal phytopathogens, such as Fusarium solani is responsible for wilt disease to potato, tomato etc; Venturia inaquaelis is responsible for apple scab disease. 362 Antimicrobial agents kill or inhibit the growth of the microorganisms. To protect crops from such type of phytopathhogens different types of Pesticides embrace insecticides, herbicides, and fungicides chemical base pesticides are used. But these types of chemical pesticides are highly toxic and cause not only health problems, but also responsible for air, water and soil pollution. Antimicrobial agents are not only used in farming, but also used in different industrial applications, such as food packaging, synthetic textiles, health care, cosmetics, sunscreen, and medicine production. Over the last few years researchers have tried to use nanoparticles as the antimicrobial agent against different types of potent phytopathogens because of their suitable physical, chemical and biological properties over the chemical based pesticides. This approach is not only economic, but also environmentally friendly and green because of its nontoxic in nature.

Molecular Sensor

Contamination of water because of different molecules, such as metal ions (As(III)/As(V), Hg2+, Cr3+), anions (F-, Cl-, Br-, I-), explosives (2,4,6-trinitrotoluene (TNT)), drugs (paraceutamol, albendazole), aromatics (nitrobenzene, phenol), etc is major environmental problem. There is a permissible limit of each ion in water as per the World Health Organization (WHO). If the molecules are present in water above this permissible limit it will effect on the aquatic life as well as human beings. Water contamination because of some metal ions (As(III/V)) and anions (F-) are becoming a serious environmental issue in recent years because of their dangerous effect on the environment above the permissible limit in water. For this reason detection of these kinds of molecules in water before use is very necessary.

Detection of Metal Ions

Detection of metal ions, such as As(III/V), Cr(VI), Hg2+, Cu2+, Fe2+, Fe3+, Zn2+, Al3+, etc in drinking water are important because of their serious environmental and health concern. Concentration of these ions in drinking water above the permissible limit can cause several environmental and health problems. For this reason detection of these metal ions in drinking water is essential. Amoung different metal ion in water arsenic is considered as the most unsafe elements because of the high level toxicity of arsenic to human health. According to WHO the permissible limit of arsenic in drinking water should be 0.01 – 0.05 mg/L. Contamination of arsenic in drinking water above the permissible limit causes skin diseases, respiratory problem, lungs, and even it can also lead to cancer in skin, lungs, liver, and kidney. For this reason very low level detection of arsenic in drinking water is very essential. Various analytical methods have been developed for this purpose, such as atomic absorption spectroscopy (AAS), hydride generation atomic absorption spectrometry (HG-AAS), polarographic technique, inductively coupled plasma (ICP), ICP atomic emission spectrometry (ICP-AES), ICP mass spectrometry (ICP-MS), high performance liquid chromatography (HPLC) with optical spectrometric detection, and voltammetry study. But these methods are costly, complicated in nature, selectivity problem, and lagging of low level and accuracy of the detection. As an alternative nanoparticle based fluorometric detection of arsenic becomes in research interest recently. Nanoparticles based fluorescence sensing of arsenic is easy, less time consuming, and low cost method.

Detection of Anions

Different anions species, such as F-, cyanide, Cl-, I-, SO42-, NO3-, CO32- play a major role in a broad range of applications in chemical, biological, and medical processes. Amoung different anions fluoride is considered as one important anion species because of its physical and chemical important. Fluoride is the smallest anions and having some unique properties, such as high charge density, high electronegativity, and high polarizing ability. Because of this fluoride ions have important role in biological systems. Fluoride ions are widely used as additives in toothpaste for protecting the dental health and as pharmaceutical agents to treat dental cavities, osteoporosis. It is added in the drinking water to maintain the optimum level of the fluoride ion in the human body. Fluoride is also used for manufacturing nuclear weapons. In addition, the increasing number of industries on fluorine products is causing the fluorine contamination in the environment. However, high concentration of fluoride may cause fluorosis, urolithiasis, kidney failure, cancer, and even leading to death. The maximum permissible limit of fluoride ion in drinking water is 1.5 mg/L according to World Health Organization (WHO). For this reason qualitative and quantitative detection of fluoride ions in drinking water is essential. Some available methods are there to detect the fluoride ions in water, such as desilylation, fluoride-hydrogen bonding, boron-fluoride complexation, colorimetric changes, fluorescence quenching, and fluorescence enhancing. However, fluorescence based fluoride anion sensing method is important because of its low cost, simplicity, sensitivity, and high selectivity towards the anion species.

 

References:

1. Boxi, S.S., PhD Thesis: Synthesis and Characterization of Ag Doped TiO2, CdS, ZnS Nanoparticles for Photocatalytic, Toxic Ions Detection, and Antimicrobial Applications, Department of Chemical Engineering, National Institute of Technology Rourkela.

2. Elmolla, E. S.; Chaudhuri, M. Degradation of the Antibiotics Amoxicillin, Ampicillin and Cloxacillin in Aqueous Solution by the Photo-Fenton Process. J. Hazard. Mater. 2009, 172, 1476–1481.

3. Muruganandham, M.; Shobana, N.; Swaminathan, M. Optimization of Solar Photocatalytic Degradation Conditions of Reactive Yellow 14 Azo Dye in Aqueous TiO2. J. Mol. Catal. A 2006, 246, 154–161.

4. O'connor, O. A.; Young, L. Y. Toxicity and Anaerobic Biodegradability of Substituted Phenols under Methanogenic Conditions. Environ. Toxicol. Chem. 1989, 8, 853-862.

5.  Suyana, P.; Kumar, S. N.; Kumar, B. S. D.; Nair, B. N.; Pillai, S. C.; Mohamed, A. P.; Warrier, K. G. K.; Hareesh, U. S. Antifungal Properties of Nanosized ZnS Particles Synthesised by Sonochemical Precipitation. RSC Adv. 2014, 4,8439 –8445.


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