thumb|400px|[[Transmission electron microscopy|TEM (a, b, and c) images of prepared mesoporous silica nanoparticles with mean outer diameter: (a) 20nm, (b) 45nm, and (c) 80nm. SEM (d) image corresponding to (b). The insets are a high magnification of mesoporous silica particle.]]

A nanoparticle or ultrafine particle is a particle of matter 1 to 100 nanometres (nm) in diameter. or electric properties.

Being more subject to the Brownian motion, they usually do not sediment, like colloidal particles that conversely are usually understood to range from 1 to 1000 nm.

Being much smaller than the wavelengths of visible light (400–700 nm), nanoparticles cannot be seen with ordinary optical microscopes, requiring the use of electron microscopes or microscopes with laser. For the same reason, dispersions of nanoparticles in transparent media can be transparent, but they do support a variety of dislocations that can be visualized using high-resolution electron microscopes. However, nanoparticles exhibit different dislocation mechanics, which, together with their unique surface structures, results in mechanical properties that are different from the bulk material.

Non-spherical nanoparticles (e.g., prisms, cubes, rods etc.) exhibit shape-dependent and size-dependent (both chemical and physical) properties (anisotropy). Non-spherical nanoparticles of gold (Au), silver (Ag), and platinum (Pt) due to their fascinating optical properties are finding diverse applications. Non-spherical geometries of nanoprisms give rise to high effective cross-sections and deeper colors of the colloidal solutions. Other examples are nanolignin, nanochitin, or nanostarches.

Nanoparticles with one half hydrophilic and the other half hydrophobic are termed Janus particles and are particularly effective for stabilizing emulsions. They can self-assemble at water/oil interfaces and act as pickering stabilizers.

Hydrogel nanoparticles made of N-isopropylacrylamide hydrogel core shell can be dyed with affinity baits, internally. Homogeneous nucleation occurs when nuclei form uniformly throughout the parent phase and is less common. Heterogeneous nucleation, however, forms on areas such as container surfaces, impurities, and other defects. Crystals can form simultaneously when nucleation occurs rapidly, resulting in a more uniform (monodisperse) product. In contrast, slow nucleation rates often lead to a diverse (polydisperse) population of crystals with varying sizes. This phenomenon is exemplified in the formation of CaCO<sub>3</sub> crystals. Controlling nucleation allows for the control of size, dispersity, and phase of nanoparticles.

The process of nucleation and growth within nanoparticles can be described by nucleation, Ostwald ripening or the two-step mechanism-autocatalysis model.

Nucleation

The original theory from 1927 of nucleation in nanoparticle formation was Classical Nucleation Theory (CNT). It was believed that the changes in particle size could be described by burst nucleation alone. In 1950, Viktor LaMer used CNT as the nucleation basis for his model of nanoparticle growth. There are three portions to the LaMer model: 1. Rapid increase in the concentration of free monomers in solution, 2. fast nucleation of the monomer characterized by explosive growth of particles, 3. Growth of particles controlled by diffusion of the monomer. This model describes that the growth on the nucleus is spontaneous but limited by diffusion of the precursor to the nuclei surface. The LaMer model has not been able to explain the kinetics of nucleation in any modern system.

Ostwald ripening

Ostwald ripening is a process in which large particles grow at the expense of the smaller particles as a result of dissolution of small particles and deposition of the dissolved molecules on the surfaces of the larger particles. It occurs because smaller particles have a higher surface energy than larger particles. This process is typically undesirable in nanoparticle synthesis as it negatively impacts the functionality of nanoparticles.

Two-step mechanism – autocatalysis model

In 1997, Finke and Watzky proposed a new kinetic model for the nucleation and growth of nanoparticles. This 2-step model suggested that constant slow nucleation (occurring far from supersaturation) is followed by autocatalytic growth where dispersity of nanoparticles is largely determined. This F-W (Finke-Watzky) 2-step model provides a firmer mechanistic basis for the design of nanoparticles with a focus on size, shape, and dispersity control. Next, a fourth step (another autocatalytic step) was added to account for a small particle agglomerating with a larger particle. Finally in 2014, an alternative fourth step was considered that accounted for a atomistic surface growth on a large particle.

Measuring the rate of nucleation

As of 2014, the classical nucleation theory explained that the nucleation rate will correspond to the driving force. One method for measuring the nucleation rate is through the induction time method. This process uses the stochastic nature of nucleation and determines the rate of nucleation by analysis of the time between constant supersaturation and when crystals are first detected. Another method includes the probability distribution model, analogous to the methods used to study supercooled liquids, where the probability of finding at least one nucleus at a given time is derived.

As of 2019, the early stages of nucleation and the rates associated with nucleation were modelled through multiscale computational modeling. This included exploration into an improved kinetic rate equation model and density function studies using the phase-field crystal model.

Properties

The properties of a material in nanoparticle form are unusually different from those of the bulk one even when divided into micrometer-size particles. The final shape of a nanoparticle is also controlled by nucleation. Possible final morphologies created by nucleation can include spherical, cubic, needle-like, worm-like, and more particles.

Large surface-area-to-volume ratio

thumb|right|280px|1 kg of particles of 1 mm<sup>3</sup> has the same surface area as 1 mg of particles of 1 nm<sup>3</sup>

Bulk materials (>100&nbsp;nm in size) are expected to have constant physical properties (such as thermal and electrical conductivity, stiffness, density, and viscosity) regardless of their size, for nanoparticles, however, this is different: the volume of the surface layer (a few atomic diameters-wide) becomes a significant fraction of the particle's volume; whereas that fraction is insignificant for particles with a diameter of one micrometer or more. In other words, the surface area/volume ratio impacts certain properties of the nanoparticles more prominently than in bulk particles. This causes a lattice strain that is inversely proportional to the size of the particle, also well known to impede dislocation motion, in the same way as it does in the work hardening of materials. For example, gold nanoparticles are significantly harder than the bulk material. Furthermore, the high surface-to-volume ratio in nanoparticles makes dislocations more likely to interact with the particle surface. In particular, this affects the nature of the dislocation source and allows the dislocations to escape the particle before they can multiply, reducing the dislocation density and thus the extent of plastic deformation.

There are unique challenges associated with the measurement of mechanical properties on the nanoscale, as conventional means such as the universal testing machine cannot be employed. As a result, new techniques such as nanoindentation have been developed that complement existing electron microscope and scanning probe methods. Atomic force microscopy (AFM) can be used to perform nanoindentation to measure hardness, elastic modulus, and adhesion between nanoparticle and substrate. The particle deformation can be measured by the deflection of the cantilever tip over the sample. The resulting force-displacement curves can be used to calculate elastic modulus. However, it is unclear whether particle size and indentation depth affect the measured elastic modulus of nanoparticles by AFM. The adhesion and friction force can be obtained from the cantilever deflection if the AFM tip is regarded as a nanoparticle. However, this method is limited by tip material and geometric shape. The colloidal probe technique overcomes these issues by attaching a nanoparticle to the AFM tip, allowing control oversize, shape, and material. While the colloidal probe technique is an effective method for measuring adhesion force, it remains difficult to attach a single nanoparticle smaller than 1 micron onto the AFM force sensor. In general, the measurement of the mechanical properties of nanoparticles is influenced by many factors including uniform dispersion of nanoparticles, precise application of load, minimum particle deformation, calibration, and calculation model.

There are several methods for creating nanoparticles, including gas condensation, attrition, chemical precipitation,

Mechanical

Friable macro- or micro-scale solid particles can be ground in a ball mill, a planetary ball mill, or other size-reducing mechanism until enough of them are in the nanoscale size range. The resulting powder can be air classified to extract the nanoparticles.

Breakdown of biopolymers

Biopolymers like cellulose, lignin, chitin, or starch may be broken down into their individual nanoscale building blocks, obtaining anisotropic fiber- or needle-like nanoparticles. The biopolymers are disintegrated mechanically in combination with chemical oxidation or enzymatic treatment to promote breakup, or hydrolysed using acid.

Pyrolysis

Another method to create nanoparticles is to turn a suitable precursor substance, such as a gas (e.g. methane) or aerosol, into solid particles by combustion or pyrolysis. This is a generalization of the burning of hydrocarbons or other organic vapors to generate soot. Traditional pyrolysis often results in aggregates and agglomerates rather than single primary particles. This inconvenience can be avoided by ultrasonic nozzle spray pyrolysis, in which the precursor liquid is forced through an orifice at high pressure.

Condensation from plasma

Nanoparticles of pure metals, oxides, carbides, and nitrides, can be created by vaporizing a solid precursor with a thermal plasma and then condensing the vapor by expansion or quenching in a suitable gas or liquid. The plasma can be produced by dc jet, electric arc, or radio frequency (RF) induction. The thermal plasma can reach temperatures of 10.000 K and can thus also synthesize nanopowders with very high boiling points. Metal wires can be vaporized by the exploding wire method.

In RF induction plasma torches, energy coupling to the plasma is accomplished through the electromagnetic field generated by the induction coil. The plasma gas does not come in contact with electrodes, thus eliminating possible sources of contamination and allowing the operation of such plasma torches with a wide range of gases including inert, reducing, oxidizing, and other corrosive atmospheres. The working frequency is typically between 200&nbsp;kHz and 40&nbsp;MHz. Laboratory units run at power levels in the order of 30–50&nbsp;kW, whereas the large-scale industrial units have been tested at power levels up to 1 MW. As the residence time of the injected feed droplets in the plasma is very short, it is important that the droplet sizes are small enough in order to obtain complete evaporation.

Inert gas condensation

Inert-gas condensation is frequently used to produce metallic nanoparticles. The metal is evaporated in a vacuum chamber containing a reduced atmosphere of an inert gas.

Nanoparticles can be linked to biological molecules that can act as address tags, directing them to specific sites within the body

Coatings that mimic those of red blood cells can help nanoparticles evade the immune system. and pass through cell membranes in organisms, and their interactions with biological systems are relatively unknown. As of 2013 the U.S. Environmental Protection Agency was investigating the safety of the following nanoparticles: and clinical medicine, physics,

Biomedical

Nanoscale particles are used in biomedical applications as drug carriers or imaging contrast agents in microscopy. Anisotropic nanoparticles are a good candidate in biomolecular detection.

Using nanoparticles in cancer treatment is being extensively researched. Certain characteristics of the tumor microenvironment, including leaky vasculature and poor lymphatic drainage, lead to the accumulation of NPs in the tumor. This is known as the enhanced permeability and retention (EPR) effect, and is a type of passive targeting. Additionally, ligands that bind to certain expressed or over-expressed receptors in the tumor microenvironment can be conjugated to the surface of nanoparticles to actively target the tumor. The accumulation of nanoparticles in the tumor can reduce adverse side effects, which is a major drawback of chemotherapy.

In drug delivery, the acidic pH of the tumor microenvironment is often exploited to increase the release of the drug from pH-sensitive materials. Additionally, some NPs can generate heat under laser irradiation (photothermal therapy) or alternating magnetic field (magnetic hyperthermia), which can both kill cancer cells, and release drugs loaded in the nanoparticle.

Some high-Z metal NPs are currently being investigated as radiosensitizers to enhance the effects of radiation therapy in cancer treatment.

Sunscreens

Titanium dioxide nanoparticles imparts what is known as the self-cleaning effect, which lend useful water-repellant and antibacterial properties to paints and other products. Zinc oxide nanoparticles have been found to have superior UV blocking properties and are widely used in the preparation of sunscreen lotions,

Further reading

  • Nanoparticles Used in Solar Energy Conversion (ScienceDaily).
  • "Nanoparticles: An occupational hygiene review" by RJ Aitken and others. Health and Safety Executive Research Report 274/2004
  • "EMERGNANO: A review of completed and near completed environment, health and safety research on nanomaterials and nanotechnology" by RJ Aitken and others.
  • High transmission Tandem DMA for nanoparticle studies by SEADM, 2014.
  • Nanohedron.com images of nanoparticles
  • Lectures on All Phases of Nanoparticle Science and Technology
  • EC FP7 Project led by the Institute of Occupational Medicine