Nanoparticles : A Comprehensive Guide For 2021
Generally speaking, nanoparticles refer to ultra-fine particles that range in size between 1 to 100 nanometers in diameter. The term can also apply to objects that fall within the scope range in only two directions: fibers and tubes.
Nanoparticles can either be naturally occurring or produced from manufacturing processes.
The size of nanoparticles makes them distinct from both bulk materials and particles on the molecular scale. Due to their unique properties, nanoparticles can exhibit surprising characteristics physically, chemically, and optically. Objects in the nanoscale can also display quantum mechanics effects.
Nanoparticles have unique properties that have made them valuable in various fields, including medicine, engineering, optics, and environmental science.
With the wealth of applications of nanoparticles, it has become one of the most studied areas of science and research. It has also created a branch of manufacturing concerned with the production of nanoparticles, aptly called nanotechnology.
History of nanoparticles
Although the study of nanoparticles came about because of modern science, there is evidence of the use of nanoparticles by ancient civilizations. Since some nanoparticles are naturally occurring, they have used by artisans to adorn pottery back in ninth-century Mesopotamia. This practice carried on during the Middle Ages using silver and copper nanoparticles to give a characteristic luster to ceramic pottery.
Richard Feynman is considered the father of modern nanotechnology. In 1959, during a meeting of the American Physical Society, Feynman proposed using machines to construct progressively smaller devices until they reached the molecular scale. This was the premise of the famous lecture “There’s Plenty of Room at the Bottom” and caught the interest of many scientists in a potentially new field of research.
The term “nanotechnology” only came about 15 years later when Norio Taniguchi used it in a paper published at a 1974 conference. In his study, Taniguchi defined nanotechnology as manipulating a material via various methods at the nanometer scale.
Unaware of Taniguchi’s work, Eric Drexler independently developed his concept of nanotechnology and published it in a book called “Engines of Creations: The Coming Era of Nanotechnology” in 1986. In his book, Drexler described the idea of “molecular manufacturing” or machines in the nanoscale creating copies of themselves. He then expounded on the concept in his Ph.D. thesis, “Molecular Machinery and Manufacturing with Applications to Computation.”
In 1981, physicists Gerd Binnig and Heinrich Rohrer of the IBM Zurich Research Laboratory developed the Scanning Tunneling Microscope (STM). A new type of microscope, the STM allowed the scientists to capture the first image taken at the atomic level. The same technology was also used to manipulate atoms and molecules by “tunneling” current through the microscope’s tip to break or create chemical bonds.
Almost simultaneously, scientists Robert Curl, Richard Smalley, and Harold Kroto discovered the foundations of what we now know as carbon-based nanomaterials. By evaporating graphite in an inert atmosphere, the scientists found that carbon formed very stable spheres of C60 or C70 compounds. These were called fullerenes and buckyballs. A few years later, another group of scientists manipulated these compounds to form carbon nanotubes which can then used as composite fibers.
Nanoscience has branched out several applications, such as biomedicine, molecular biology, engineering, and computer science. Emphasizing how vital nanoscience as a field of research is, President George W. Bush signed into law the 21st Century Nanotechnology Research and Development Act in 2003. This launched the National Nanotechnology Initiative (NNI), which aimed to advance world-class nanotechnology research thru a coordinated effort by federal agencies and departments in the US.
Between 2001 and 2004, over sixty countries undertook government-sponsored research programs focusing on nanotechnology. This initiative was joined by large corporations such as Samsung, Canon, Toshiba, and IBM. There has also been a significant uptick of research publications on the topic.
Properties of nanoparticles
Any matter only needs to have at least one external dimension in the range of 1 to 100 nanometers to be considered a nanoparticle. All other characteristics and unique properties of nanoparticles come as a consequence of this particle size.
Very high specific surface area
Due to their size, nanoparticles can provide a vast surface area normalized to volume compared to any bulk material. This has made nanoparticles a very effective delivery mechanism for medicine, as they can be absorbed by the body rapidly.
The high specific surface area of nanoparticles has also proven revolutionary in the design of photovoltaic cells for solar energy production.
Compared to their component bulk material, nanoparticles are significantly more rigid because of the high internal stress caused by surface stresses in the nanoparticles. The lack of void spaces in the nanoparticles also reduces the probability of vacancy migration. The effect is a lower degree of plastic deformation and a corresponding reduction in flexibility and malleability
Low melting temperature
Another consequence of the increased specific surface area of nanoparticles is that they have a much lower melting temperature than their equivalent bulk material. For instance, silver nanoparticles can melt at 112 degrees C compared to the standard melting point of silver at 962 degrees C.
The highly mobile nature of nanoparticles in their free state makes them especially useful in applications that involve diffusion.
Nanoparticles can achieve homogenous equilibrium very quickly concerning heat or other molecules and ions. Elevated temperatures can further enhance the diffusion capability of nanoparticles.
Difficult to apply as coating
There are several practical applications of nanoparticles as the coating. Common examples include the application of graphene in electronic components and carbon nanotubes in photovoltaic receptors. Due to the low surface energy, applying nanoparticles as a coating is difficult. It means that special techniques are needed to coat any object in nanoparticles.
A common approach is the Langmuir-Blodgett method. The nanoparticles are compressed within air-water interphase in this method, where their packing density significantly increased. The substrate is then dipped (under controlled speed and temperate) into the nanoparticles’ solution to create the coating.
Nanoparticles are small enough to exhibit the effects of quantum mechanics. This means that the properties of nanoparticles are restricted to quantized or discrete values and may not be described or predicted based on our knowledge of classical physics.
Quantum mechanics can explain the drastic changes in the behavior of nanoparticles compared to their bulk material counterparts. For instance, natural insulators can have conductive nanoparticles. Opaque substances like copper also become transparent when they make as nanoparticles. The quantum effects can alter by changes in the sub-atomic energy levels of electrons.
How to make nanoparticles
There are two primary strategies in producing nanoparticles – the so-called ‘top down’ and ‘bottom up’ methods. The top-down approach implies the breakdown of bulk material to smaller and smaller particles until the nanoscale qualification is satisfied. In contrast, the bottom-up method chemical reaction binds atoms and molecules together.
The choice of which method to use depends on the desired properties of the nanoparticles.
The ‘top-down’ process
Most top-down processes for nanoparticle production are physical and involve friable materials. It is the typical process for producing ceramic or metallic nanoparticles. They use high-energy ball mills with steel or carbide-based grinding media, then crush bulk material to create nanoparticles.
The disadvantage of the physical method is that it is very energy-intensive and has poor particle size control. For instance, nanoparticles produced thru milling often have a broad range of particle sizes. However, this method is suitable for preserving the nanostructure in applications that require good interconnection between the particles, as in electronic circuitry.
Some chemical methods take the top-down approach. In breaking down cellulosic biopolymers, mechanical milling is combined with the enzymatic breakdown or acid-based hydrolysis. The result of this process is the production of fibrous nanoparticles with anisotropic properties.
Carbon nanoparticles also can be produced from the combustion or pyrolysis of fuel. While this reaction typically has carbon agglomerates, you can control the particle size by forcing the power through a narrow orifice at high pressure before combustion or pyrolysis occurs.
Erosion by wind and water provides the driving force of the naturally occurring top-down production of nanoparticles. Through attrition, rocks are broken down into particles small enough to form clays. They also provide the mechanism for the distribution of plant and microorganism matter.
The ‘bottom-up’ process
The ‘bottom-up’ approach physicochemical processes that take advantage of the self-assembly behavior of atoms and molecules. These methods provide better particle size and shape control but usually require highly controlled conditions.
The wet chemistry describes how do compounds react together to form insoluble precipitates. It is a standard method to produce metallic nanoparticles from metal salts. By adding a precipitating agent to the solution, the nanoparticles can induce precipitation in the process.
The advantage of this method is that it allows for precise control of particle size and the particle’s crystal structure and size distribution. Several variables, such as the solution’s temperature and pH, can be altered based on the kinetics of the reaction. Other metals and organic dyes can also be introduced to dope the nanoparticles after they have already formed.
Once the nanoparticles have formed, they can recover through filtration, evaporation, centrifugation, and other separation methods. They can deposit to a surface through dipping, coating, or electrolysis.
It’s also possible for chemical reactions that produce nanoparticles to take place in the gas phase. You can do this by simply vaporizing the reactants and allowing the products from the chemical reaction to deposit as nanoparticles. There are also special flame reactors, laser reactors, and plasma reactors to provide the energy needed for a chemical reaction to proceed.
The sol-gel process is a particular type of method that produces porous nanostructures. The base material here is a dispersion of nanoparticles called the ‘sol.’ The reaction proceeds after adding catalysts and with pH and temperature control. Through hydrolysis, the nanoparticles from three-dimensional cross-links, creating a gel. This porous material can then be processed to produce coatings, fibers, or highly dense ceramic products.
What are the uses and applications of nanoparticles?
Nanoparticle applications in materials
The use of nanoparticles in material science has rapidly grown during the 21st century. Nanoparticles are often added to usual materials like plastics and rubber to enhance their properties. These so-called nanocomposite materials are now quite common in food packaging, construction, and batteries, among other applications.
One of the most common examples of nanocomposite materials is in vulcanized rubber. By combining natural rubber with sulfur and silica or carbon black nanoparticles, rubber gains enhanced mechanical and electrical properties.
A similar approach is taken in designing nanocomposite-based polymers that are not derived from petroleum sources. These biopolymers come from natural sources of cellulose or starch and are biodegradable.
Nanoparticle technology can use to enhance the antimicrobial properties of standard plastics for food packaging. These nanoparticles are typically made of silver or copper, either of which inhibits microbial growth. You can apply the same treatment to paints for areas that need antimicrobial protection, such as industrial kitchens and hospital operating rooms.
The discovery of graphene via cross-linking of carbon nanoparticles has proven to be revolutionary for electronics. Graphene provides a method for storing electrical charge and transmission of current without the drawbacks of using metallic materials. The large specific surface area of nanostructures has proven to be beneficial for this particular purpose.
Nanotechnology has paved the way for a new class of ceramics. Instead of the brittle old-fashioned ceramics, we now have ceramics infused with nanoparticles. Ceramic nanoparticles can have carbides, carbonates, and metals such as titanium. Quite surprisingly, the most common use of ceramic nanoparticles is as a drug delivery agent.
The quantum effect of nanoparticles is greatly utilized in LED lights and coatings for greenhouses. Nanoparticles of inorganic phosphor can absorb sunlight and convert it into different wavelengths. We now have LED lights of different colors and greenhouse coatings that convert light to the optimal wavelength for plant growth through this mechanism.
Nanoparticle applications in medicine
Nanoparticles have been well-studied as alternative means for drug delivery. The high surface area of nanoparticles allows for rapid absorption of the medicine into the body’s cell membranes. This benefit also extends to topical treatment, including ultraviolet-blocking sunblock.
The size of nanoparticles has also made them a valuable diagnostic tool. In the field of medical imaging, iodine-based liposomal nanoparticles can help in identifying cancer-related lesions. A relatively new technique is photoacoustic imaging which uses gold nanoparticles as contrast agents of stem cells.
Targeted medication is another area that is heavily explored as a field of application for nanoparticles. The idea is to deliver medicine using a nanostructure that can anchor at targeted sites, allowing for slow and sustained drug delivery. There is massive potential in using nanostructure-embedded medicine that patients can inhale to treat conditions associated with the brain, such as Alzheimer’s Disease.
Nanoparticles can also be used to deliver agents such as heat and light to specific types of cells. For instance, the nanoparticles can be engineered to target cancer cells only. This potential treatment will avoid damaging healthy cells while providing effective treatment. Stem cells can also be delivered to help in the tissue repair of specific internal organs.
The uptake of nanoparticles in endothelial cells is a significant research area. Because of the crucial role that endothelial cells play as permeable barriers to blood vessels. Penetrating endothelial cells allows for better drug delivery through blood flow. Promising research on this topic makes use of a polymer nanoparticle that contains bovine serum albumin as a model protein for biocompatibility. With good uptake, the study showed how nanoparticles could be used for localized delivery of therapeutic agents.
Other more novel applications of nanoparticles are being researched at the Chase Western Reserve University and the University of Wisconsin. Polymer nanoparticles applied to an open wound can act as synthetic platelets, allowing for near-instant closing of injuries and reduction of blood loss. Nanogenerators have also been explored for delivering electrical pulses to an open wound to achieve the same purpose.
Tissue engineering refers to the field of the growth of new tissues and organs. It typically starts from an implanted manifold or cell base. Nanoparticles come in handy in the design of these cell manifolds. Biocompatible nanoparticles, such as calcium hydroxyapatite or titanium dioxide, promote cell proliferation and easily diffuse across membranes. Nanoparticles have also been shown to provide mechanical reinforcement and improve the electrical properties of these tissue scaffolds.
Nanoparticle applications in biology
Aside from medical treatment, nanoparticles have also become valuable tools for the study of biology. Nanoparticles are about the size of proteins. It makes them ideal for tagging or labeling. Nanoparticles with unique magnetic or optical properties are typically used for these applications.
Examples of the use of nanoparticles for biological studies include fluorescent signaling, contrast enhancement of MRI, detection of proteins, and analysis of DNA structures. For instance, the Mirkin group has developed a bio-barcode method for DNA detection using gold nanoparticles. This can help identify the susceptibility of the specimen to diseases such as certain cancers and Alzheimer’s disease.
In biological research, nanoparticles can be used to separate and isolate cells from complex mixtures. A concrete example of this is in the study of circulating tumor cells (CTCs) through the use of magnetic nanoparticles. An understanding of CTCs can be beneficial in the prognosis of breast and prostate cancers. This technique has been developed as the CellSearch device – the only FDA-approved test for studying CTCs.
Nanoparticle technology has been critical in helping researchers understand complex cell functions and molecular processes. Although lots of work has already been done, it is still a relatively young research field, and there is still a lot of room for more innovative applications.
Potentially harmful effects of nanoparticles
Not all nanoparticles are beneficial. At this point, we know that standard processes such as combustion can produce nanoparticles. That means that nanoparticles can be created from daily manufacturing processes such as smoking, combustion of fuel in cars, and even cooking. When inhaled, these nanoparticles can cause lung damage, respiratory conditions, genetic mutation, and other health effects.
Some nanoparticles, such as titanium dioxide (found in some food additives), have also been shown to disrupt gut flora when ingested. This can cause bowel inflammation and disruption of normal digestive function.
Nanoparticles can be particularly problematic because of how easily they can be absorbed by cells. At least one clinical trial has shown that inhaled nanoparticles can be absorbed by cell membranes and blood cells and eventually find their way to internal organs such as the liver or the heart. Animals are also susceptible to this, thus posing the danger of bioaccumulation in higher-ordered predators.
Knowledge of the adverse effects of nanoparticles on human health has made it imperative for workers to wear respiratory protection when working near or with them. There have also been campaigns to inform the general public about the effects of nanoparticles. However, there is no detailed study yet on the actual levels of regular exposure of humans to nanoparticles. This is a field of study that will undoubtedly receive more attention as nanoparticle use becomes more widespread.