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There is no accepted international definition of a nanoparticle, but one given in the new PAS71 document developed in the UK is: "A particle having one or more dimensions of the order of 100nm or less".
There is a note associated with this definition: "Novel properties that differentiate nanoparticles from the bulk material typically develop at a critical length scale of under 100nm".
The "novel properties" mentioned are entirely dependent on the fact that at the nano-scale, the physics of nanoparticles mean that their properties are different from the properties of the bulk material.
This makes the size of particles or the scale of its features the most important attribute of nanoparticles.
What is different about a nanoparticle?
There is no strict dividing line between nanoparticles and non-nanoparticles. The size at which materials display different properties to the bulk material is material dependant and can certainly be claimed for many materials much larger in size than 100nm.
Definitions certainly become more difficult for materials that are a very long way from being a sphere, such as carbon nanotubes for example. One of the aims for these materials is to grow them into long tubes, certainly not 'nano' in length, but as they have a diameter in the order of 3nm for a single walled tube, they have properties that distinguish them from other allotropes of carbon, and hence can be described as 'nanomaterials'.
This sort of nanomaterial has led to the extension of the idea of nanomaterials being considered as such if any one of their structural features are on a scale of less than 100nm, that cause their properties to be different from that of the bulk material.
Manufacturing methods for nanoparticles
Many of these nanomaterials are made directly as dry powders, and it is a common myth that these powders will stay in the same state when stored. In fact, they will rapidly aggregate through a solid bridging mechanism in as little as a few seconds. Whether these aggregates are detrimental will depend entirely on the application of the nanomaterial.
If the nanoparticles need to be kept separate, then they must be prepared and stored in a liquid medium designed to facilitate sufficient interparticle repulsion forces to prevent aggregation.
There are four fundamental routes to making nano materials. * Form in place
These techniques incorporate lithography, vacuum coating and spray coating.
This is a 'top-down' method that reduces the size of particles by attrition, for example, ball milling or planetary grinding.
* Gas phase synthesis
These include plasma vaporization, chemical vapour synthesis and laser ablation.
* Wet chemistry
This is the range of techniques that are most applicable for characterization by light scattering techniques. These are fundamentally 'bottom-up' techniques, i.e. they start with ions or molecules and build these up into larger structures.
These nanoparticle manufacturing techniques historically come under the title of 'colloid chemistry', and involve classical 'sol-gel' processes, or other aggregation processes.
These wet chemistry techniques currently offer the best quality nanoparticles from a number of points of view.
* They produce nanoparticles that are already in the form of a dispersion, hence high inter-particle forces can be designed in to prevent agglomeration.
* The formation of aggregates can be reduced or eliminated.
* The nanoparticles can be made to be very monodisperse, i.e. all the same size to within small tolerances.
* The chemical composition and morphology can be closely controlled. This is especially important for research purposes where the quality of the material must be very high to ensure repeatable and meaningful results.
One of the most recent developments is the production in liquid carbon dioxide. This offers the promise of the controlled conditions of the 'bottom-up' wet chemistry approach, as well as the benefit of being able to remove the dispersant by simply reducing the pressure of the reaction container. This technique is currently used for removing the caffeine from tea and coffee, so the mechanics of handling the materials are well understood.
Nanoparticles and their Applications
A survey of nanoparticle applications under development:
Nanoparticles are particles that have one dimension that is 100 nanometers or less in size. The properties of many conventional materials change when formed from nanoparticles. This is typically because nanoparticles have a greater surface area per weight than larger particles; this causes them to be more reactive to certain other molecules.
Nanoparticles are used, or being evaluated for use, in many fields. The list below introduces many of the uses under development. You can use the links in each paragraph to go to a detailed explanation. Iron oxide nanoparticles can used to improve MRI images of cancer tumours. The nanoparticle is coated with a peptide that binds to a cancer tumour; once the nanoparticles are attached to the tumour the magnetic property of the iron oxide enhances the images from the Magnetic Resonance Imagining scan.
In nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties. It is further classified according to size: In terms of diameter, fine particles cover a range between 100 and 2500 nanometers, while ultrafine particles, on the other hand, are sized between 1 and 100 nanometers. Similarly to ultrafine particles, nanoparticles are sized between 1 and 100 nanometers, though the size limitation can be restricted to two dimensions. Nanoparticles may or may not exhibit size-related properties that differ significantly from those observed in fine particles or bulk materials .
Nanoclusters have at least one dimension between 1 and 10 nanometers and a narrow size distribution. Nanopowders are agglomerates of ultrafine particles, nanoparticles, or Nanoclusters. Nanometre sized single crystals, or single-domain ultrafine particles, are often referred to as nanocrystals. Nanoparticle research is currently an area of intense scientific research, due to a wide variety of potential applications in biomedical, optical, and electronic fields. The National Nanotechnology Initiative has led to generous public funding for nanoparticle research in the United States
Diagram of application of nanoparticles:-
Nanoparticle research is currently an area of intense scientific research, due to a wide variety of potential applications in biomedical, optical, and electronic fields. Nanoparticles are of great scientific interest as they are effectively a bridge between bulk materials and atomic or molecular structures.
A bulk material should have constant physical properties regardless of its size, but at the nano-scale this is often not the case.
Size-dependent properties are observed such as quantum confinement in semiconductor particles, surface plasmon resonance in some metal particles and superparamagnetism in magnetic materials. The properties of materials change as their size approaches the nanoscale and as the percentage of atoms at the surface of a material becomes significant.
For bulk materials larger than one micrometre the percentage of atoms at the surface is minuscule relative to the total number of atoms of the material.
The interesting and sometimes unexpected properties of nanoparticles are not partly due to the aspects of the surface of the material dominating the properties in lieu of the bulk properties. Nanoparticles exhibit a number of special properties relative to bulk material.
For example, the bending of bulk copper (wire, ribbon, etc.) occurs with movement of copper atoms/clusters at about the 50 nm scale.
Copper nanoparticles smaller than 50 nm are considered super hard materials that do not exhibit the same malleability and ductility as bulk copper.
The change in properties is not always desirable.
Ferroelectric materials smaller than 10 nm can switch their magnetisation direction using room temperature thermal energy, thus making them useless for memory storage.
Suspensions of nanoparticles are possible because the interaction of the particle surface with the solvent is strong enough to overcome differences in density, which usually result in a material either sinking or floating in a liquid.Nanoparticles often have unexpected visible properties because they are small enough to confine their electrons and produce quantum effects.For example gold nanoparticles appear deep red to black in solution. Nanoparticles have a very high surface area to volume ratio.
This provides a tremendous driving force for diffusion, especially at elevated temperatures.Sintering can take place at lower temperatures, over shorter time scales than for larger particles.This theoretically does not affect the density of the final product, though flow difficulties and the tendency of nanoparticles to agglomerate complicates matters.The large surface area to volume ratio also reduces the incipient melting temperature of nanoparticles.
Nanoparticles make biofuel production more efficient
Biofuel production currently involves a complex mixture of hydrophilic and hydrophobic liquids, along with one or more catalysts. Getting them all together and separating out the fuel can be a time-consuming challenge. Researchers have now used carbon nanotubes and oxidized metals to create a solid that is both hydrophilic and hydrophobic and sits between oil and alcohol layers, mediating their interactions.
Making biofuel using current methods can be a bit tedious. Recipes generally involve mixing some kind of bio-oil, often vegetable oil, with an alcohol, usually methanol, along with a catalyst such as lye. Once these have all been combined, they react to form the desired biofuel, glycerine, and some excess soap, water, and alcohol. All of these will, for the most part, separate into layers like with a vinaigrette dressing if allowed to sit for a long enough time.
The glycerine can be drained off easily enough, and most of the impurities will settle between the glycerine and biofuel, but the biofuel must be "washed" a few times to extract any errant soap particles and other impurities that are suspended in it, and boiled to remove the water. All told, the process can take between a couple of days and a week, depending on how much you're making. There are machines that will carry out the mixing and washing, but the process can't be shortened much because of the impurities that are introduced due to the use of lye as a catalyst.
Researchers set out to solve this problem by finding a catalyst that would not introduce any impurities that would be difficult to remove. They also wanted to find one that would that could stabilize an oil and water emulsion, which would help the reaction components form a stable mix, in the same way that egg yolks stabilize mayonnaise. A stabilized emulsion would significantly increase the surface area where the two substances can react-typically, this function is performed by the solid catalysts. Ideally, the newly engineered catalysts would also be reusable.
The researchers' solution involved a combination of hydrophilic and hydrophobic materials that would both emulsify the oil/water mixture by sitting at the interface of the two substances, and facilitate their reaction to form biofuels. To accomplish this, they grew hydrophobic carbon nanotubes on small pellets of hydrophilic oxidized metals that contained enough palladium catalyst to speed up the reaction.
They found this combination helped the aqueous and organic phases emulsify, and would remain at the boundary between the two substances; the palladium facilitated the hydrogenation, hydrogenolysis, and decarbonylation reactions. Hydrogenation was the dominant reaction at around 100°C, hydrogenolysis at 200°C, and decarbonylation at 250°C. Each of these reactions is useful for the conversion of different combinations of alcohols and oils, and because of the increased surface area. Thanks to the inclusion of palladium, these reactions happen at a much faster rate than when performed using lye.
Once the reactions had occurred, the authors found that all of the desired products had moved into the organic phase, or what was once just bio-oil, leaving any waste and water in the aqueous phase, where it was still bound by the catalytic nanoparticles.Â
To separate the catalyst and waste, they strained the liquid through a regular paper filter, which managed to catch most of the catalyst. They then passed the organic liquid through a polytetrafluoroethylene filter to catch the nanoparticles that had gotten through the paper filter, leaving them with purified biofuel.
These solid nanohybrid particles seem to be a strong candidate for fuel production, given the greater amount of precision and control they provide fuel makers and the speedier reaction times they enable. But they do still require a filtration process, an aspect of the experiment that was not extensively studied. Since reducing production time and increasing purity would be beneficial to the future of biofuel, streamlining the waste-removal step in this process will be critical. The paper also made no mention of whether their chosen nanoparticles were reusable after their initial reaction. Still, the basic principles seem solid, provided that these aspects of the catalysts can be optimized.
New nanoparticles could improve cancer treatment
Particles can deliver a combination of chemotherapy drugs directly to prostate-cancer cells.
In recent years, studies have shown that for many types of cancer, combination drug therapy is more effective than single drugs. However, it is usually difficult to get the right amount of each drug to the tumor. Now researchers at MIT and Brigham and Women's Hospital have developed a nanoparticle that can deliver precise doses of two or more drugs to prostate cancer cells.
In a study appearing online this week in the Proceedings of the National Academy of Sciences, the researchers tailored their particles to deliver cisplatin and docetaxel, two drugs commonly used to treat many different types of cancer.
Drug-carrying nanoparticles designed by MIT and Brigham and Women's Hospital researchers are decorated with tags that bind to molecules found on the surface of tumor cells.
Such particles could improve the effectiveness of chemotherapy while minimizing the side effects normally seen with these drugs, according to the researchers. They could also be adapted to target cancers other than prostate cancer, or even to deliver drugs for other diseases that require combination therapy.
To build their nanoparticles, the researchers developed a new strategy that allowed them to incorporate drugs with very different physical properties, which had been impossible with previous drug-delivering nanoparticles. In earlier generations of nanoparticles, drug molecules were encapsulated in a polymer coating. Using those particles, hydrophobic (water-repelling) drugs, such as docetaxel, and hydrophilic (water-attracting) drugs, such as cisplatin, can't be carried together, nor can drugs with different charges.
"With the old way, you can only do it if the two drugs are physically and chemically similar," said Omid Farokhzad, director of the Laboratory of Nanomedicine and Biomaterials at Brigham and Women's Hospital and a senior author of the paper. "With this way, you can put in drugs that are relatively different from each other."
MIT Institute Professor Robert Langer and Stephen Lippard, the Arthur Amos Noyes Professor of Chemistry at MIT, are also senior authors of the paper. Former Brigham and Women's postdoctoral associate Nagesh Kolishetti is the lead author. The research was funded by the National Cancer Institute, National Institute of Biomedical Imaging and Bioengineering, and the David Koch Prostate Cancer Foundation.
With the researchers' new technique, called "drug-polymer blending," drug molecules are hung like pendants from individual units of the polymer, before the units assemble into a polymer nanoparticle. That allows the researchers to precisely control the ratio of drugs loaded into the particle. They can also control the rate at which each drug will be released once it enters a tumor cell.
The new particles offer a much-needed ability to fine-tune drug combinations and personalize treatment for individual patients, said Michael Pishko, professor of chemical engineering at Texas A&M University, who was not involved in this study. "They're right on the money in terms of what these systems should look like," he said.
Once the drugs are loaded into the nanoparticle, the researchers add a tag that binds to a molecule called PSMA, which is located on the surfaces of most prostate tumor cells. This tag allows the nanoparticles to go directly to their target, bypassing healthy tissues and potentially reducing the side effects caused by most chemotherapy drugs. That could permit doctors to give much higher doses to a larger number of patients.
The researchers have filed for a patent on the polymer-blending fabrication technique, and are now testing the drug-delivering particles in animals. Once they gather enough animal data, which could take a few years, they hope to begin clinical trials.
Risk Assessment of Nanoparticles
Nanotechnology is expected to be the basis of many of the main technological innovations of the 21st century. Research and development in this field is growing rapidly throughout the world. A major output of this activity is the development of new materials in the nanometre scale, including nanoparticles. These are usually defined as particulate materials with at least one dimension of less than 100 nanometres (nm). One nanometre is 10-9 m. By comparison, a human hair is approximately 70,000 nm in diameter, a red blood cell is approximately 5,000 nm wide and simple organic molecules have sizes ranging from 0.5 to 5 nm.
Nanoparticles include carbon nanotubes, metal nanowires, semiconductor quantum dots and other nanoparticles produced from a huge variety of substances. Responsible development of any new materials requires that risks to health and the general environment associated with the development, production, use and disposal of these materials are addressed. This is necessary to protect workers involved in production and use of these materials, the public and the ecosystem. However, it also helps inform the public debate about the development of these new, potentially beneficial, materials.
Epithelial cell with intracellular nanoparticles
Assessment of health risks arising from exposure to chemicals or other substances, requires understanding of the intrinsic toxicity of the substance, the levels of exposure (by inhalation, by ingestion or through the skin) that may occur and any relationship between exposure and health effects. Concerns about the lack of knowledge and possible risks arising from exposure to nanoparticles led the UK Government to request advice from the Royal Society and Royal Academy of Engineering and to the formation of their Nanoscience and Nanotechnology Working Group. Their report, published in July 2004 makes wide ranging recommendations about the need for more and better information and for a coherent approach to these concerns.
The IOM has unique and extensive experience concerning the potential risks from particles and fibres. Our approach combines expertise and state-of-art techniques to identify and characterise exposure scenarios, conduct toxicity evaluations and undertake risk assessments to identify, characterise and estimate the relative safety of nanoparticles. This approach allows us to extend current knowledge and facilitate comparisons between existing substances and new nanoparticles.
Gold nanoparticles turn light into electrical current
Material scientists at the Nano/Bio Interface Centre of the University of Pennsylvania have demonstrated the transduction of optical radiation to electrical current in a molecular circuit
Turning sunlight into electrical power is all but a new problem, but recent advancements made by researchers at the University of Pennsylvania have given a new twist to the subject. While not currently aimed at solar panel technology, their research has uncovered a way to turn optical radiation into electrical current that could lead to self-powering molecular circuits and efficient data storage.
Professor of materials science Dawn Bonnell and colleagues placed light-sensitive gold nanoparticles on a glass substrate, minimizing the distance between the nanoparticles. The team then stimulated conductive electrons with optical radiation to ride the surface of the gold nanoparticles, creating so-called "surface plasmons" that induce electrical current across molecules.
Under these conditions, surface plasmons were found to increase the efficiency of current production by a factor of four to 20. The size, shape and separation of the array of golden nanoparticles can be customized independently of the optical characteristics of the molecule, and optimization of these parameters could, the researchers say, produce enhancement factors of thousands, and the resulting electrical current could be easily transported to the outside world.
"If the efficiency of the system could be scaled up without any additional, unforeseen limitations, we could conceivably manufacture a 1A, 1V sample the diameter of a human hair and an inch long," Prof Bonnell explained.
The results may lead to better nano-sized circuits that can power themselves, potentially through sunlight. Another interesting application suggested by the researchers could be for data storage, where a photovoltaic circuit could encode bits using wavelengths of light rather than electrical charge.