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.
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