25 Application of Nanoparticles

S.S. islam

1. Introduction

The particles which are from 1 to 100 nm in size, are termed as nanoparticles. In nanotechnology, the particle is an object which acts as a complete unit with regard to its properties. Individual molecules are not regarded as nanoparticles, rather these are the particles which demonstrate size dependent properties. The properties of a nanoparticle substantially differ from those of bulk forms of the same materials.

Nanoclusters are the groups of atoms or molecules which have at least one dimension in 1-10 nm range. The size distribution within a nanocluster is very narrow, i.e., almost all the particles present in the cluster are of approximately same size. The agglomerates of ultra fine particles, nanoparticles, or nanoclusters are called nanopowders. Single crystals of nanometer dimensions or single domain ultra-fine particles are called nanocyrstals

Classification

Particles can be further classified into following types, depending upon their diameters:

1. Ultrafine particles are similar to nanoparticles and have dimensions in the range of 1-100 nm.

2. Fine particles with dimensions between 100-2500nm.

3. Coarse particles with dimensions between 2500-10000nm.

Nanoparticles are highly significant for scientific research owing to their potential use in medicine, physics, electronics, optoelectronics, etc.

Background

Even though, the nanoparticles have attracted immense attention in the recent times, they have previously been used since long time. Artisans in Rome as early as fourth century used nanoparticles in the most famous Lycurgus cup produced from dichroic glass. Nanoparticles were also used in the ninth century Mesopotamia to create glittering patterns on the pots. In modern history, gold and copper based patterns have been used in potteries during Middle ages as well as Renaissance. The luster is usually from a metal film applied to the transparent surface of a glazing. If film is resistant towards oxidising agents such as oxygen, the luster is still visible.

The optical behaviour of nano sized metals was first described by Michael Faraday in his 1857 paper. Later, Turner pointed out that the when thin films of metals such as gold and silver are coated over the glass and annealed at around 500°C, the properties of the metal film change, and the film is no longer continuous. These changes lead to the enhanced transmission of white light, reduced reflection, and enormous increase in the resistance of the films.

Some astonishing facts about Nanoparticles

Nanoparticles are essentially a bridge between atoms/molecules and bulk form of the material. The bulk materials demonstrate constant physical behaviours regardless of the size of the object. However, nanoscaled materials exhibit size dependent properties, and in most cases these properties are remarkably different from the bulk counterparts. Therefore, as: (a) size of a material approaches nanometer scale, and (b) the surface to volume ratio increases, the materials undergo changes in their properties. On the other hand, in bulk materials (usually larger than one micrometer in size), the ratio of surface are to the volume of the material is negligible. Thus it may be concluded that the astonishing properties of nanoscaled materials are caused by the larger surface area of the nanoparticles, and the surface dominates the behaviour of such particles.

Since nanoparticles are very small, they confine the electrons thereby causing quantum effects, thus the optical properties of nanoparticles are highly unexpected. For instance, gold nanoparticles can have deep red to black colour depending upon their size. The gold nanoparticles have very low melting points (~300°C for the nanoparticles of 2.5nm dimensions) than the bulk gold (1064 °C). Nanoparticles have high absorption coefficients towards solar radiation than thin films of bulk materials. By adjusting the size, shape and composition of the particles, the solar absorption of the material can be controlled. Recent experiments have shown a zero backward scattering and improved forward scattering in core (metal)-shell (dielectric) nanoparticles. Both electric as well as magnetic resonances can be supported by core-shell nanoparticles, thus exhibiting novel properties in comparison to pristine metallic nanoparticles.

By controlling the size of the nanoparticles, the following property changes can be observed:

– Quantum confinement in nanoparticles made from semiconductor materials

– Surface plasmon resonance in certain metallic nanoparticles

– Evolution of super paramagnetism in magnetic materials

In addition to these desirable changes, some undesirable changes can also occur at the nanoscale. For example, nanoparticles (< 10 nm) made from ferromagnetic materials change the direction of their magnetisation using room temperature thermal energy, thereby making them unsuitable for information storage applications.

Owing to their large surface areas, the interaction of nanoparticles with solvent can overcome the destiny differences, thus suspensions of nanoparticles can be formed. Without this property, the materials either sink or float in a solvent.

The incorporation of clay nanoparticles in polymers increases the strength of the resulting plastics. This has been supported by higher glass transition temperatures observed in these plastics. Additionally, mechanical tests have also demonstrated high stiffness of such materials. This happens because the hard nanoparticles transfer their favourable properties to the polymer host matrices. The applications of nanoparticles in textiles are important to prepare smart and functional clothes.

Nanoparticles have been prepared from various materials such as metals, dielectrics, semiconductors, in addition to this, hybrid structures such as core-shell, nanoparticles have also been prepared. Semiconductor nanoparticles are also termed as quantum dots (QDs) when their size is small enough (usually <10 nm) to demonstrate quantisation of energy levels. These nanoparticles have diverse applications in medicine as drug carriers, or imaging agents.

Figure 1 Semiconductor quantum dot (~5nm) of lead sulfide passivated by oleic acid, oleic amine and hydroxyl ligands.

Semi-solid and soft nanoparticles can also be fabricated, an example of semi-solid nanoparticle is liposome. Liposome nanoparticles can be used as targeted delivery agents for anticancer drugs as well as vaccines.

The surface of nanoparticles can be modified to work as hydrophile as well as hydrophobic, i.e., one half is hydrophobic and other half is hydrophilic. Such particles are called as Janus particles and are very effective in stabilising emulsions. They can self-assemble at water/oil interfaces and act as solid surfactants.

2. Some Specific Applications:

Surface coatings in biological applications

The surface of the nanoparticles should be polar to provide good aqueous solubility and prevent nanoparticle coagulation. Highly charged surfaces lead to non-specific interactions, while the polyethylene glycol terminated cells avoid non-specific bindings. Biomolecules can be attached to the nanoparticles to direct them to specific sites in the body, even specific organelles in a cell, or to monitor individual protein or RNA molecules. Most commonly used tags to mark the nanoparticles include monoclonal antibodies, aptamers, streptavidin or peptides. These tags must be attached with the nanoparticles covalently, and also, their quantity per nanoparticles must be controlled for efficient operation. Multivalent nanoparticles have several tags attached to them which may cause their clustering, thereby activating the cell signaling paths, giving stronger anchoring. On the other hand, monovalent nanoparticles bear single binding sites, thus prevent cluster formation. These nanoparticles are suitable to track the behaviour of individual protein molecules.

Health and Safety

There are various speculations both medically as well as environmentally that nanoparticles are hazardous. Owing to their huge surface areas, these particles are highly reactive or catalytic. As they are extremely small, they can pass through the cell membrane and may interact with the cell organelles. Presently, this interaction is not very well understood. Nonetheless, the nanoparticles are unlikely to enter the cell nucleus, Golgi complex, or other cell organelles mainly because of the particle size and intercellular aggregation.

These days, a lager variety of cosmetics and sunscreens are produced which contain nanoparticles for effective working. It is still unknown whether the presence of nanoparticles in these products pose any health hazards or not. The investigations on the use of zinc nanoparticles have revealed that they do not get absorbed in the bloodstream in vivo.

The following sections discuss the possible health risks posed by various frequently used nanoparticles.

a. Carbon Nanotubes (CNTs): Carbon nanomaterials are widely used in production of composites for vehicles, sports equipments, etc., as well as integrated circuits in electronics industry. The interactions of carbon nanomaterials, e.g., CNTs with natural organic matter greatly affect their coagulations as well as deposition, thereby affecting their transport, transformation, and exposure in aquatic environments. CNTs have shown some toxic impacts in various environments, and a complete understanding of these harmful effects requires intense investigations.

b. Cerium Oxide: Cerium oxide (CeO2) nanoparticles are widely utilised in electronics, biomedical tools, energy, fuel additives, etc. Due to such diverse range of applicants, the nanoparticles of cerium oxide usually disperse in environment, thereby exposing to their risks. CeO2 nanoparticles are being continuously released into the environment due to their use in fuel additives.

c. Titanium Dioxide: Titanium dioxide nanoparticles are used in multiple products. Different sized nanoparticles made of titanium dioxide can be found sunscreens, cosmetics, and paints and coatings, etc. Recent application area for these particles is to remove contamination from drinking water.

d. Nano Silver: Silver nanoparticles are used in textiles, food packaging, etc., due to their antibacterial properties. The possibility of these particles to enter the food chain and thereby their impact needs extensive investigations.

e. Iron: Apart from its applications as smart fluids for optics polishing as well as nutrient supplements, nanoscale iron is being investigated for water treatment as well.

Nanomedicine

Nanomedicine implies the application of nanotechnology for medical uses. It includes medical uses of nanomaterials and biological devices, nanoscaled biosensors, etc. Future generation applications include biological nanoscaled machines. However, the possibilities of toxicity and environment impact of nanomaterials is an important concern.

Nanomaterials can be manipulated to perform various specialised functions. This can be achieved by interfacing the nanomaterials with various biomolecules or structures. Since the nanomaterials are similar in size to various biomolecules and structures, they can be used for in vivo as well as in vitro biomedical research and applications. Till now, various diagnostic devices, contrast agents, analytical tools, physical therapy applications, and drug delivery vehicles, etc., have been developed by integrating nanomaterials with biology.

Drug Delivery

One of the most celebrated application of nanotechnology is in targeted drug delivery to specific cells. Due to the possibility of transport of medicine directly to the affected area, the drug consumption can be minimised. This also lowers the side effects caused by the drugs. Targeted drug delivery is extremely significant in reducing the side effects of the drug as well as decreasing the treatment costs by lowering the drug consumption. Drug delivery focuses to maximise bioavailability at the specific site in the body and also maximises the availability of the drug over a certain period of time. Nanoengineered devices can be used to achieve this due to their ability to target individual molecules. Nanoscaled devices offer various advantages such as less invasiveness, fast biochemical reactions, and possibility to implant within the body. These devices are much faster and effective than the conventional drug delivery practices. The efficiency of nanomedicine depends on: (a) efficient drug encapsulation, (b) successful transport of the drug to specific site, and (c) efficient release of drug.

(a) (b) (c)

Figure 2 Examples of few nanomaterials being studied for their potential use in nanomedicine: (a) nanoparticles, (b) liposomes, and (c) dendrimers.

Cancer Treatment

Owing to the large surface area to volume ratios, nanoparticles can attach multiple functional groups to it, which can locate and bind to specific tumor cells. Furthermore, the small size of nanoparticles (10-100 nm) allows them to preferentially accumulate at tumor sites (as tumors lack an effective lymphatic drainage system). The limitations to conventional cancer chemotherapy such as drug resistance, lack of selectivity as well as solubility, can be overcome by using nanoparticles.

Imaging

Nanoparticles have great potential as in vivo imaging tools and devices. Nanoparticle based contrast agents, images (e.g, ultrasound) can have favourable distribution and enhanced contrast. Nanoparticles can aid visualisation of various stages in cardiovascular problems such as blood pooling, angiogenesis, atherosclerosis, etc. Due to their small size, nanoparticles can be very useful in oncology, especially in imaging. QDs when used for MRI (magnetic resonance imaging), create exceptional images of tumor sites. Cadmium selenide nanoparticles glow on exposure to UV. When injected, they enter the cancer cells, thereby highlighted them. Nanoparticles are much brighter than organic dyes and can be excited only with single light source. Thus, use of fluorescent QDs can create much higher contrast images at lower costs in comparison to organic dyes. The disadvantage is that QDs are made from toxic materials.

Sensing

Nanotechnology-on-a-chip is analogous to the lab-on-a-chip technology. Sensor test chips containing thousands of nanowires, able to detect proteins and other biomarkers left behind by cancer cells, could enable the detection and diagnosis of cancer in the early stages from a few drops of a patient’s blood.

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