Nanoparticle

Nanoparticles - What are Nanoparticles?

A nanoparticle is a small object that behaves as a whole unit in terms of its transport and properties.

Size of nanoparticles

In terms of diameter, fine particles cover a range between 100 and 2500 nanometers, while ultrafine particles are sized between 1 and 100 nanometers. Nanoparticles may or may not exhibit size-related properties that are seen in fine particles. Despite being the size of the ultrafine particles individual molecules are usually not referred to as nanoparticles.

Nanoclusters have at least one dimension between 1 and 10 nanometers and a narrow size distribution. Nano powders on the other hand are agglomerates of ultrafine particles, nanoparticles, or nanoclusters. Nano particle sized crystals are called nanocrystals.

Colorized transmission electron micrograph showing chains of cobalt nanoparticles. Image credit: G. Cheng, A.R. Hight Walker/NIST

Nanoparticle research and uses

Nanoparticle research is currently the most studied branch of science with the number of uses of nanoparticles in various fields. The particles have wide variety of potential applications in biomedical, optical and electronic fields.

History of nanoparticle research

The history of nanoparticle research is long and the use of these particles dates back to the 9th century in Mesopotamia when artisans used these to generate a glittering effect on the surface of pots.

This lustre or glitter over pottery from the Middle Ages and Renaissance is due to a metallic film that was applied to the transparent surface of a glazing. The lustre can still be visible if the film has resisted atmospheric oxidation and other weathering.

The lustre is within the film itself which contained silver and copper nanoparticles dispersed homogeneously in the glassy matrix of the ceramic glaze. Artisans created the nanoparticles by adding copper and silver salts and oxides together with vinegar, ochre and clay, on the surface of previously-glazed pottery. Then the pots were placed into a kiln and heated to about 600 °C in a reducing atmosphere. With the heat the glaze would soften, causing the copper and silver ions to migrate into the outer layers of the glaze.

Michael Faraday provided the first description, in scientific terms, of the optical properties of nanometer-scale metals in his 1857 paper.

Uses and advantages of nanoparticles in medicine

Some of the uses of nanoparticles in biology and medicine include:

  • Creating fluorescent biological labels for important biological markers and molecules in research and diagnosis of diseases
  • Drug delivery systems
  • Gene delivery systems in gene therapy
  • For biological detection of disease causing organisms and diagnosis
  • Detection of proteins
  • Isolation and purification of biological molecules and cells in research
  • Probing of DNA structure
  • Genetic and tissue engineering
  • Destruction of tumours with drugs or heat
  • In MRI studies
  • In pharmacokinetic studies.

Nanoparticles are being increasingly used in drug delivery systems.  The advantages of using nanoparticles as a drug delivery system include:

  • The size and surface characteristics of nanoparticles can be easily manipulated. This could be used for both passive and active drug targeting
  • Nanoparticles can be made to control and sustain release of the drug during the transportation as well as the location of the release. Since distribution and subsequent clearance of the drug from the body can be altered, an increase in drug therapeutic efficacy and reduction in side effects can be achieved.
  • Choosing an appropriate matrix also helps in increasing the efficacy and reducing side effects
  • Targeted drugs may be developed
  • Various routes of administration including oral, nasal, injection, intra-ocular (within the eyes) etc. can be used.

Nanoparticle Uniformity

When nanoparticles are synthesized, high level of purity and uniformity of structure is necessary for putting these particles to use in private, industrial and military sectors. There must be high purity ceramics, polymers, glass-ceramics and material composites in creation of these particles. Nanoparticles are generally classified based on their dimensionality, morphology, composition, uniformity, and agglomeration.

Synthesis of nanoparticles

During synthesis, the process of condensation leads to fine powders with irregular particle sizes and shapes in a typical powder. This may lead to non-uniformity of structure in the packaged nanoparticles. That may result in packing density variations in the powder compact. In addition, uncontrolled agglomeration of powders due to attractive van der Waals forces may also result in non-homogenous formation nanoclusters and nanoparticle packages.

There are variations in stresses that can result in non-uniform drying shrinkage and this is directly proportional to the rate at which the solvent can be removed. Porosity and its distribution thus determines the process to a large extent. Such stresses have been associated with a plastic-to-brittle transition in consolidated bodies and lead to propagation of cracks.

Homogenisity may further be compromised where there are fluctuations in packing density in the compact as it is prepared for the kiln and during the sintering process. Some pores and other structural defects associated with density variations have been shown to play a detrimental role in the sintering process.

Thus the aim should be to produce nanomaterials of uniform size, shape and most importantly the distribution of components and porosity. This will maximize the green density. The containment of a uniformly dispersed assembly of strongly interacting particles in suspension requires total control over interparticle forces.

Nanoparticles with colloids

Nanoparticles with colloids can provide this feature and produce increased uniformity. Monodisperse powders of colloidal silica, for example, may be stabilized sufficiently to ensure a high degree of order in the colloidal crystal or polycrystalline colloidal solid which results from aggregation.

Such defective polycrystalline colloidal structures would appear to be the basic elements of submicrometer colloidal materials science. These may form the first step in developing a more rigorous understanding of the mechanisms involved in production of more uniform nanoparticles that can be used in various fields.

Properties of Nanoparticles

Nanoparticles are important scientific tools that have been and are being explored in various biotechnological, pharmacological and pure technological uses. They are a link between bulk materials and atomic or molecular structures.

While bulk materials have constant physical properties regardless of its size, among nanoparticles the size often dictates the physical and chemical properties. Thus, 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, those larger than one micrometer (or micron), the percentage of atoms at the surface is insignificant in relation to the number of atoms in the bulk of the material.

Physical properties of nanoparticles

Nanoparticles are unique because of their large surface area and this dominates the contributions made by the small bulk of the material. Zinc oxide particles have been found to have superior UV blocking properties compared to its bulk substitute. This is one of the reasons why it is often used in the preparation of sunscreen lotions.

Other examples of the physical properties of nanoparticles:

  • Color – Nanoparticles of yellow gold and gray silicon are red in color
  • Gold nanoparticles melt at much lower temperatures (~300 °C for 2.5 nm size) than the gold slabs (1064 °C)
  • Absorption of solar radiation in photovoltaic cells is much higher in nanoparticles than it is in thin films of continuous sheets of bulk material - since the particles are smaller, they absorb greater amount of solar radiation

Optical properties of nanoparticles

Nanoparticles also often possess unexpected optical properties as they are small enough to confine their electrons and produce quantum effects. One example of this is that gold nanoparticles appear deep red to black in solution.

Formation of suspensions

An important physical property of nanoparticles is their ability to form suspensions. This is possible since the interaction of the particle surface with the solvent is strong enough to overcome density differences. In bulk materials this interactions usually result in a material either sinking or floating in a liquid.

Magnetization and other properties of nanoparticles

Other properties unique among nanoparticles are quantum confinement in semiconductor particles, surface plasmon resonance in some metal particles and superparamagnetism in magnetic materials.

For example, ferroelectric materials smaller than 10 nm can switch their magnetisation direction using room temperature thermal energy, thus making them unsuitable for memory storage. Thus this property is not always desired in nanoparticles.

Diffusion properties of nanoparticles

At elevated temperatures especially, nanoparticles possess the property of diffusion. Sintering can take place at lower temperatures, over shorter time scales than for larger particles. Although this does not affect the density of the final product but there is a chance of agglomeration.

Hard nanoparticles

Clay nanoparticles, when incorporated into polymer matrices, increase reinforcement, leading to stronger plastics. These nanoparticles are hard, and impart their properties to the polymer (plastic). Nanoparticles have also been attached to textile fibers in order to create smart and functional clothing.

Semisolid or soft nanoparticles

Semi-solid and soft nanoparticles have been manufactured. Of these notable is the liposome. Various types of liposome nanoparticles are currently used clinically as delivery systems for anticancer drugs, antibiotics and antifungal drugs and vaccines.

Dimensionality

Nanoparticles are generally classified based on their dimensionality, morphology, composition, uniformity, and agglomeration.

  • 1D nanomaterials

These are one dimensional in the nanometer scale are typically thin films or surface coatings, and include the circuitry of computer chips and the antireflection and hard coatings on eyeglasses. These have been used in electronics, chemistry, and engineering.

  • 2D nanomaterials

Two-dimensional nanomaterials have two dimensions in the nanometer scale. These include 2D nanostructured films, with nanostructures firmly attached to a substrate, or nanopore filters used for small particle separation and filtration. Asbestos fibers are an example of 2D nanoparticles.

  • 3D nanomaterials

Materials that are nanoscaled in all three dimensions are considered 3D nanomaterials. These include thin films deposited under conditions that generate atomic-scale porosity, colloids, and free nanoparticles with various morphologies

Synthesis of Nanoparticles

Nanoparticles may be created using several methods. Some of them may occur in nature as well. The methods of creation include attrition and pyrolysis. While some methods are bottoms up, some are called top down. Top down methods involve braking the larger materials into nanoparticles.

Nanoparticle Synthesis

Top-Down via Bottom-Up via
Attrition / Milling Pyrolysis
  Inert gas condensation
  Solvothermal reaction
  Sol-Gel fabrication
  Structured media

Attrition

Attrition methods include methods by which macro or micro scale particles are ground in a ball mill, a planetary ball mill, or other size reducing mechanism. The resulting particles are air classified to recover nanoparticles.

  • Involves mechanical thermal cycles
  • Yields
    • broad size distribution (10-1000 nm)
    • varied particle shape or geometry
    • impurities
  • Application
    • Nanocomposites
    • Nano-grained bulk materials

Bottoms up methods

These are further classified according to phases:

  • Gas (Vapor) Phase Fabrication: Pyrolysis, Inert Gas Condensation
  • Liquid Phase Fabrication: Solvothermal Reaction, Sol-gel, Micellar Structured Media

Pyrolysis

In pyrolysis, a vaporous precursor (liquid or gas) is forced through a hole or opening at high pressure and burned. The resulting solid is air classified to recover oxide particles from by-product gases. Pyrolysis often results in aggregates and agglomerates rather than singleton primary particles.

Instead of gas, thermal plasma can also deliver the energy necessary to cause evaporation of small micrometer size particles. The thermal plasma temperatures are in the order of 10,000 K, so that solid powder easily evaporates. Nanoparticles are formed upon cooling while exiting the plasma region. Examples of plasma used include dc plasma jet, dc arc plasma and radio frequency (RF) induction plasmas.

For example, silica sand can be vaporized with an arc plasma at atmospheric pressure. The resulting mixture of plasma gas and silica vapour can be rapidly cooled by quenching with oxygen, thus ensuring the quality of the fumed silica produced.

The advantages of vapor phase pyrolysis include it being a simple process, cost effective, a continuous operation with high yield.

Liquid phase synthesis methods

The liquid phase fabrication entails a wet chemistry route.

Methods are:

  • Solvothermal Methods (e.g. hydrothermal)
  • Sol-Gel Methods
  • Synthesis in Structure Media (e.g., microemulsion)

Effectiveness of Solvothermal Methods and Sol-gel methods demands a simple process, low cost, continuous operation and high yield.

Solvothermal process

Precursors are dissolved in hot solvents (e.g., n-butyl alcohol) and solvent other than water can provide milder and friendlier reaction conditions If the solvent is water then the process is referred to as  hydrothermal method.

Sol-gel process

The sol-gel process is a wet-chemical technique (also known as chemical solution deposition) widely used recently in the fields of materials science and ceramic engineering.

Steps include:

  • Formation of stable sol solution
  • Gelation via a polycondensation or polyesterification reaction
  • Gel aging into a solid mass. This causes contraction of the gel network, also phase transformations and Ostwald ripening.
  • Drying of the gel to remove liquid phases. This can lead to fundamental changes in the structure of the gel.
  • Dehydration at temperatures as high as 8000 degree C, used to remove M-OH groups for stabilizing the gel, i.e., to protect it from rehydration.
  • Densification and decomposition of the gels at high temperatures (T > 8000 degree C), i.e., to collapse the pores in the gel network and to drive out remaining organic contaminants

The ultimate microstructure of the final component will clearly be strongly influenced by changes implemented during this phase of processing. The precursor sol can be either deposited on a substrate to form a film (e.g. by dip-coating or spin-coating), cast into a suitable container with the desired shape (e.g. to obtain a monolithic ceramics, glasses, fibers, membranes, aerogels), or used to synthesize powders (e.g. microspheres, nanospheres).

Advantages of the sol-gel process

Advantages of the sol-gel process are that it is a cheap and low-temperature technique that allows for the fine control of the product’s chemical composition. Even small quantities of dopants, such as organic dyes and rare earth metals, can be introduced in the sol and end up uniformly dispersed in the final product.