{{short description|Particle with size between 1 and 100 nm with an outer layer}}
{{Use dmy dates|date=January 2017}}
[[File:Mesoporous Silica Nanoparticle.jpg|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. [[Scanning electron microscopy|SEM]] (d) image corresponding to (b). The insets are a high magnification of mesoporous silica particle.]]
{{Nanotechnology}}
{{Nanomaterials}}

A '''nanoparticle''' or '''ultrafine particle''' is usually defined as a particle of [[mater]] that is between 1 and 100 [[nanometre]]s (nm) in [[diameter]].<ref name=epaXXXXa/> <ref name=iupac2012a/>  The term is sometimes used for larger particles, up to 500 nm,{{cn|date=February 2020}} or fibers and tubes that are less than 100 nm in only two directions.<ref name=iupac2012b/>  Nanoparticles are usually distinguished from "fine particles", sized between 100 and 2500 nm, and "coarse particles", ranging from 2500 to 10,000 nm. 

The properties of nanoparticles often differ markedly from those of larger particles of the same material.  Since a larger fraction of the material is close to the surface, the properties of the latter may dominate those of the bulk material ([[thermal conductivity|thermal]] and [[electrical conductivity]], [[stiffness]], [[density]], [[viscosity]], etc.).  This effect is particularly strong for nanoparticles dispersed in a different material, since the interactions between the two materials at their interface also becomes significant.<ref name=bati2015/>

Nanoparticles occur widely in nature and are objects of study in many sciences such as [[chemistry]], [[physics]], [[geology]] and [[biology]].  Being at the transition between bulk materials and [[atom]]ic or [[molecular]] structures, they often exhibit phenomena that are not observed at either scale. They are an important component of [[atmospheric pollution]], and key ingredients in many industrialized products such as [[paint]]s, [[plastic]]s, [[metal]]s, [[ceramic]]s, and [[magnetism|magnetic]] articles.  The production of nanoparticles with specific properties is an important branch of [[nanotechnology]].

== Definitions ==

===IUPAC===
In its 2012 proposed terminology for biologically related [[polymer]]s, the [[International Union of Pure and Applied Chemistry|IUPAC]] defined a nanoparticle as "a particle of any shape with dimensions in the 1 × 10<sup>−9</sup> and 1 × 10<sup>−7</sup> m range".<ref name=iupac2012a/>  This definition evolved from one given by IUPAC in 1997.<ref name=iupac1997/><ref name=alem2007/>

In another 2012 publication, the IUPAC extends the term to include tubes and fibers with only two dimensions below 100 nm.<ref name=iupac2012b/>

===ISO===
According to the [[International Standards Organization]] (ISO) technical specification [[ISO/TS 80004|80004]], a nanoparticle is an object with all three external dimensions in the nanoscale, whose longest and shortest axes do not differ significantly, with a significant difference typically being a factor of at least 3.<ref name=iso80004-2/>

===Common usage===
"Nanoscale" is usually understood to be the range from 1 to 100 nm because the novel properties that differentiate particles from the bulk material typically develop at that range of sizes. 

For some properties, like [[transparency]] or [[turbidity]], [[ultrafiltration]], stable dispersion, etc., substantial changes characteristic of nanoparticles are observed for particles as large as 500 nm.  Therefore, the term is sometimes extended to that size range.{{cn|date=February 2020}}

===Related concepts===
[[Nanocluster]]s are agglomerates of nanoparticles with at least one dimension between 1 and 10 nanometers and a narrow size distribution. [[Nanopowder]]s<ref name=fahlm2007/> are agglomerates of ultrafine particles, nanoparticles, or nanoclusters. Nanometer-sized [[single crystal]]s, or [[single domain (magnetic)|single-domain]] ultrafine particles, are often referred to as [[nanocrystal]]s.

The terms [[colloid]] and nanoparticle are not interchangeable.  A colloid is a mixture which has solid particles dispersed in a liquid medium. The term applies only if the particles are larger than atomic dimensions but small enough to exhibit [[Brownian motion]], with the critical size range (or particle diameter) typically ranging from nanometers (10<sup>−9</sup> m) to micrometers (10<sup>−6</sup> m).<ref name=pais2005/>  Colloids can contain particles too large to be nanoparticles, and nanoparticles can exist in non-colloidal form, for examples as a powder or in a solid matrix.

==History==
===Natural occurrence===
Nanoparticles are naturally produced by many [[cosmology|cosmological]],<ref name=sima2018/> geological,<ref name=sima2018/><ref name=sima2015/> [[meteorology|meteorological]], and biological processes.  A significant fraction (by number, if not by mass) of [[interplanetary dust]], that is still falling on the [[Earth]] at the rate of thousands of tons per year, is in the nanoparticle range;<ref name=plan2012/><ref name=zook2001/>  and the same is true of [[atmospheric dust]] particles.  Many [[virus]]es have diameters in the nanoparticle range.

===Pre-industrial technology===
Nanoparticles were used by [[artisan]]s since prehistory, albeit witout knowledge of their nature. They were used by [[glassmaking|glassmakers]] and [[pottery|potters]] in [[Classical Antiquity]], as exemplified by the [[Ancient Rome|Roman]] [[Lycurgus Cup|Lycurgus cup]] of [[dichroism|dichroic]] glass (4th century CE) and the [[lusterware]] pottery of  [[Mesopotamia]] (9th century CE).<ref name=time2016/><ref name=reiss2010/><ref name=khan2012/>  The latter is characterized by [[silver]] and [[copper]] nanoparticles dispersed in the glassy [[glaze (pottert)|glaze]].

===19th century===

[[Michael Faraday]] provided the first description, in scientific terms, of the optical properties of nanometer-scale metals in his classic 1857 paper. In a subsequent paper, the author (Turner) points out that: "It is well known that when thin leaves of gold or silver are mounted upon glass and heated to a temperature that is well below a red heat (~500 °C), a remarkable change of properties takes place, whereby the continuity of the metallic film is destroyed. The result is that white light is now freely transmitted, reflection is correspondingly diminished, while the electrical resistivity is enormously increased."<ref name=fara1857/><ref name=beil1903/><ref name=turn1908/>

===20th century===

During the 1970s and 80s, when the first thorough fundamental studies with nanoparticles were underway in the United States (by [[Claes-Göran Granqvist|Granqvist]] and Buhrman)<ref name=gran1976c/> and Japan (within an ERATO Project),<ref name=haya1997/> researchers used the term ultrafine particles. However, during the 1990s, before the [[National Nanotechnology Initiative]] was launched in the United States, the term nanoparticle had become more common (for example, see the same senior author's paper 20 years later addressing the same issue, lognormal distribution of sizes<ref name=kiss1999/>).
==Properties==
[[File:Nano Si 640x480.jpg|thumb|280px|Silicon nanopowder]]
[[File:Vergelijk nanodeeltje.jpg|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>]]

The properties of a material in nanoparticle form are usually very different from those of the bulk material even when divided into micrometer-size particles.<ref name=buze2007/><ref name=vale2016a/><ref name=vale2016e/> A number of causes contribute to that effect.

===Large area/volume ratio===
A bulk material should have constant physical properties regardless of its size, but at the nano-scale, size-dependent properties are often observed. Thus, the properties of materials change as their size approaches the nanoscale and as the percentage of the surface in relation to the percentage of the volume of a material becomes significant. For bulk materials larger than one micrometer (or micron), the percentage of the surface is insignificant in relation to the volume in the bulk of the material. The interesting and sometimes unexpected properties of nanoparticles are therefore largely due to the large surface area of the material, which dominates the contributions made by the small bulk of the material.

===Interfacial layer===
For nanoparticles dispersed in a medium of different composition, the interfacial layer — formed by ions and molecules from the medium that are within a few atomic diameters of the surface of each particle —  can mask or change its chemical and physical properties. Indeed, that layer can be considered an integral part of each nanoparticle.<ref name=bati2015/> 

===Quantum mechanics effects===
Nanoparticles often possess unexpected physical and optical properties, as they are small enough to confine their electrons and produce quantum effects.<ref name=hewa2013/> For example, [[gold]] nanoparticles appear deep-red to black in solution. Nanoparticles of yellow gold and grey silicon are red in color. Gold nanoparticles melt at much lower temperatures (~300&nbsp;°C for 2.5&nbsp;nm size) than bulk gold (1064&nbsp;°C);.<ref name=buff1976/>
Absorption of solar radiation is much higher in materials composed of nanoparticles than in thin films of continuous sheets of material. In both solar [[Photovoltaics|PV]] and [[Solar thermal energy|solar thermal]] applications, by controlling the size, shape, and material of the particles, it is possible to control solar absorption.<ref name=wu2015/><ref name=tayl2012a/><ref name=tayl2013c/><ref name=tayl2011/>

Other size-dependent property changes include [[quantum confinement]] in [[semiconductor]] particles, [[localized surface plasmon]]<ref name=r2/> in some metal particles and [[superparamagnetism]] in [[magnetic]] materials. Some changes in physical properties are not always desirable; for example, [[ferromagnetic material]]s smaller than 10&nbsp;nm can switch their magnetisation direction using room temperature thermal energy, thus making them unsuitable for memory storage.<ref name=gubi2009/>

Nanoparticles made of semiconducting material may also be labeled [[quantum dots]] if they are small enough (typically sub 10&nbsp;nm) that quantization of electronic [[energy level]]s occurs. Such nanoscale particles are used in biomedical applications as [[drug carrier]]s or [[Contrast medium|imaging agent]]s with work being done to try to understand the fluid dynamic properties (e.g. drag forces) in nanoscale applications.<ref name=evan2018/><ref name=hafe2017/>  This has shown the relationship between the fluid forces on nanoparticles and the fluid [[Reynolds number|Reynolds]] and [[Knudsen number|Knudsen]] numbers.
[[File:Colloidal nanoparticle of lead sulfide (selenide) with complete passivation.png|thumbnail|right|Semiconductor nanoparticle ([[quantum dot]]) of lead sulfide with complete passivation by oleic acid, oleyl amine and hydroxyl ligands (size ~5nm)]]

===Solvent affinity===
[[Suspension (chemistry)|Suspension]]s of nanoparticles are possible since the interaction of the particle surface with the [[solvent]] is strong enough to overcome [[density]] differences, which otherwise usually result in a material either sinking or floating in a liquid.

===Diffusion across the surface===
The high surface area to volume ratio of nanoparticles 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. In theory, this does not affect the density of the final product, though flow difficulties and the tendency of nanoparticles to agglomerate complicates matters. Moreover, nanoparticles have been found to impart some extra properties to various day to day products. For example, the presence of 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,<ref name=fdasun2016/> being completely photostable<ref name=mitch1999/> though toxic.<ref name=heim2015/>
<ref name=zhang2014/> 
<ref name=gosen2015/> 
<ref name=hana2015/> 
<ref name=kim2015/> 
<ref name=mori2015/>

[[Metal]], [[dielectric]], and [[semiconductor]] nanoparticles have been formed, as well as [[resonance (chemistry)|hybrid structure]]s (e.g., core–shell nanoparticles).,<ref name=vale2018a/><ref name=tayl2013b/><ref name=chau2012/> The core-shell nanoparticles can support simultaneously both electric and magnetic resonances, demoonstrating entirely new properties when compared with bare metallic nanoparticles if the resonances are properly engineered. The formation of the core-shell structure from two different metals enables an energy exchange between the core and the shell, typically found in upconverting nanoparticles and downconverting nanoparticles, and causes a shift in the emission wavelength spectrum.<ref name=jacky2019/> By introducing a dielectric layer, plasmonic core (metal)-shell (dielectric) nanoparticles enhance light absorption by increasing scattering. Recently, the metal core-dielectric shell nanoparticle has demonstrated a zero backward scattering with enhanced forward scattering on a silicon substrate when surface plasmon is located in front of a solar cell.<ref name=yu2017/>

===Regular packing===
Nanoparticles of sufficiently uniform size may spontaneously arrange themselves into regular arrangements, forming a [[colloidal crystal]].  These arrangements may exhibit original physical properties, such as observed in [[photonic crystal]]s<ref name=white1991/><ref name=dabbs2000/>

==Production==
There are several methods for creating nanoparticles, including [[condensation|gas condensation]], [[wear|attrition]], [[Precipitation (chemistry)|chemical precipitation]],<ref name=anan2019/> [[ion implantation#Ion implantation-induced nanoparticle formation|ion implantation]], [[pyrolysis]] and [[hydrothermal synthesis]]. In attrition, macro- or micro-scale particles are ground in a [[ball mill]], a planetary [[ball mill]], or other size-reducing mechanism. The resulting particles are [[elutriation|air classified]] to recover nanoparticles. In pyrolysis, a vaporous precursor (liquid or gas) is forced through an orifice at high pressure and burned. The resulting solid (a version of soot) is air classified to recover oxide particles from by-product gases. Traditional pyrolysis often results in aggregates and agglomerates rather than single primary particles. [[Ultrasonic nozzle]] spray pyrolysis (USP) on the other hand aids in preventing agglomerates from forming.

A [[plasma (physics)|thermal plasma]] can deliver the energy to vaporize 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. The main types of the thermal plasma torches used to produce nanoparticles are dc plasma jet, dc arc plasma, and radio frequency (RF) induction plasmas. In the arc plasma reactors, the energy necessary for evaporation and reaction is provided by an electric arc formed between the anode and the cathode. For example, silica sand can be vaporized with an arc plasma at atmospheric pressure, or thin aluminum wires can be vaporized by [[Exploding Wire Method|exploding wire method]]. 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.

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. The RF plasma method has been used to synthesize different nanoparticle materials, for example synthesis of various ceramic nanoparticles such as oxides, carbours/carbides, and nitrides of Ti and Si (see [[Induction plasma technology]]).

=== Inert gas condensation ===
[[Inert gas|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.<ref name=gran1976/> Condensation of the supersaturated metal vapor results in creation of nanometer-size particles, which can be entrained in the inert gas stream and deposited on a substrate or studied in situ. Early studies were based on thermal evaporation.<ref name=gran1976/> Using magnetron sputtering to create the metal vapor allows to achieve higher yields.<ref name=hahn1990/> The method can easily be generalized to alloy nanoparticles by choosing appropriate metallic targets. The use of sequential growth schemes, where the particles travel through a second metallic vapor, results in growth of core-shell (CS) structures.<ref name=wang2005/><ref name=henn2014/><ref name=llam2014/> 

=== Radiolysis method ===
[[File:Nanoparticles grown via inert gas condensation.png|thumb|a) [[Transmission electron microscopy]] (TEM) image of Hf nanoparticles grown by magnetron-sputtering inert-gas condensation (inset: size distribution)<ref name=mich2018/> and b) [[Energy-dispersive X-ray spectroscopy|energy dispersive x-ray]] (EDX) mapping of Ni and Ni@Cu core@shell nanoparticles.<ref name=henn2014/>]]
Nanoparticles can also be formed using [[radiation chemistry]]. Radiolysis from gamma rays can create strongly active [[free radicals]] in solution. This relatively simple technique uses a minimum number of chemicals. These including water, a soluble metallic salt, a radical scavenger (often a secondary alcohol), and a surfactant (organic capping agent). High gamma doses on the order of 10<sup>4</sup> [[Gray (unit)|Gray]] are required. In this process, reducing radicals will drop metallic ions down to the zero-valence state. A scavenger chemical will preferentially interact with oxidizing radicals to prevent the re-oxidation of the metal. Once in the zero-valence state, metal atoms begin to coalesce into particles. A chemical surfactant surrounds the particle during formation and regulates its growth. In sufficient concentrations, the surfactant molecules stay attached to the particle. This prevents it from dissociating or forming clusters with other particles. Formation of nanoparticles using the radiolysis method allows for tailoring of particle size and shape by adjusting precursor concentrations and gamma dose.<ref name=bello1998/>

===Sol–gel===
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]]. Such methods are used primarily for the [[manufacturing|fabrication]] of materials (typically a [[metal oxide]]) starting from a chemical [[solution]] (''sol'', short for solution), which acts as the precursor for an integrated network (or ''gel'') of either discrete particles or network [[polymers]].
<ref name=brin1990/>

Typical [[precursor (chemistry)|precursor]]s are metal [[alkoxides]] and metal [[chloride]]s, which undergo [[hydrolysis]] and polycondensation reactions to form either a network "elastic solid" or a [[colloid]]al [[suspension (chemistry)|suspension]] (or [[dispersion (chemistry)|dispersion]])&nbsp;– a system composed of discrete (often [[amorphous]]) submicrometer particles dispersed to various degrees in a host fluid. Formation of a metal oxide involves connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or metal-hydroxo polymers in solution. Thus, the sol evolves toward the formation of a gel-like diphasic system containing both a [[liquid]] phase and [[solid]] phase whose morphologies range from discrete particles to continuous polymer networks.<ref name=hench1990/>

In the case of the colloid, the volume fraction of particles (or particle density) may be so low that a significant amount of fluid may need to be removed initially for the gel-like [[properties]] to be recognized. This can be accomplished in a number of ways. The most simple method is to allow time for [[sedimentation]] to occur, and then pour off the remaining liquid. [[Centrifugation]] can also be used to accelerate the process of [[phase separation]].

Removal of the remaining liquid (solvent) phase requires a drying process, which typically causes [[shrinkage (casting)|shrinkage]] and densification. The rate at which the solvent can be removed is ultimately determined by the distribution of [[porosity]] in the gel. The ultimate microstructure of the final component will clearly be strongly influenced by changes implemented during this phase of processing. Afterward, a thermal treatment, or firing process, is often necessary in order to favor further polycondensation and enhance mechanical properties and structural stability via final sintering, densification, and grain growth. One of the distinct advantages of using this methodology as opposed to the more traditional processing techniques is that densification is often achieved at a much lower temperature.

The [[precursor (chemistry)|precursor]] sol can be either [[deposition (chemistry)|deposited]] on a [[substrate (materials science)|substrate]] to form a film (e.g., by dip-coating or spin-coating), [[casting|cast]] into a suitable container with the desired shape (e.g., to obtain a monolithic [[ceramic]]s, [[glass]]es, [[fiber]]s, [[membrane (selective barrier)|membrane]]s, [[aerogel]]s), or used to [[wikt:synthesize|synthesize]] [[powder (substance)|powder]]s (e.g., [[microsphere]]s, [[nanosphere]]s). The sol–gel approach 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. It can be used in [[ceramics processing]] and manufacturing as an [[investment casting]] material, or as a means of producing very [[thin film]]s of metal [[oxide]]s for various purposes. Sol–gel derived materials have diverse applications in [[optics]], [[electronics]], [[energy]], [[space]], (bio)[[sensors]], [[medicine]] (e.g., controlled drug release) and separation (e.g., [[chromatography]]) technology.<ref name=klein1994/><ref name=corr2009/>

===Ion implantation===
Ion implantation may be used to treat the surfaces of dielectric materials such as sapphire and silica to make composites with near-surface dispersions of metal or oxide nanoparticles. See [[ion implantation#Ion implantation-induced nanoparticle formation]]
===Variation in properties===
The chemical processing and synthesis of high-performance technological components for the private, industrial, and military sectors requires the use of high-purity [[ceramic materials|ceramics]] ([[oxide ceramics]], such as [[aluminium oxide]] or [[copper(II) oxide]]), [[polymers]], [[glass-ceramic]]s, and [[composite material]]s, as [[Silicon carbide|metal carbide]]s ([[SiC]]), [[nitride]]s ([[Aluminum nitride]]s, [[Silicon nitride]]), [[metal]]s ([[Aluminium|Al]], [[Copper|Cu]]), non-metals ([[graphite]], [[carbon nanotube]]s) and layered ([[Aluminium|Al]] + [[Aluminium carbonate]], Cu + C). In condensed bodies formed from fine powders, the irregular particle sizes and shapes in a typical powder often lead to non-uniform packing morphologies that result in packing density variations in the powder compact.

Uncontrolled [[Flocculation|agglomeration]] of powders due to [[force|attractive]] [[van der Waals forces]] can also give rise to microstructural heterogeneity. Differential stresses that develop as a result of non-uniform drying shrinkage are directly related to the rate at which the [[solvent]] can be removed, and thus highly dependent upon the distribution of [[porosity]]. Such stresses have been associated with a plastic-to-brittle transition in consolidated bodies, and can yield to [[crack propagation]] in the unfired body if not relieved.<ref name=onoda1979/><ref name=aksay1983/><ref name=fran1996/>

In addition, any fluctuations in packing density in the compact as it is prepared for the kiln are often amplified during the [[sintering]] process, yielding inhomogeneous densification. Some pores and other structural defects associated with density variations have been shown to play a detrimental role in the sintering process by growing and thus limiting end-point densities. Differential stresses arising from inhomogeneous densification have also been shown to result in the propagation of internal cracks, thus becoming the strength-controlling flaws.<ref name=evan1969b/><ref name=evan1970c/><ref name=lange1983/>

Inert gas evaporation and inert gas deposition<ref name=gran1976c/><ref name=haya1997/> are free many of these defects due to the distillation (cf. purification) nature of the process and having enough time to form single crystal particles, however even their non-aggreated deposits have [[lognormal]] size distribution, which is typical with nanoparticles.<ref name=haya1997/> The reason why modern gas evaporation techniques can produce a relatively narrow size distribution is that aggregation can be avoided.<ref name=haya1997/> However, even in this case, random residence times in the growth zone, due to the combination of drift and diffusion, result in a size distribution appearing lognormal.<ref name=kiss1999/>

It would, therefore, appear desirable to process a material in such a way that it is physically uniform with regard to the distribution of components and porosity, rather than using particle size distributions that will maximize the green density. The containment of a uniformly dispersed assembly of strongly interacting particles in suspension requires total control over interparticle forces. [[Monodisperse]] nanoparticles and colloids provide this potential.
<ref name=evan1987d/>
==Morphology==
[[File:Nanostars-it1302.jpg|thumb|right|300px|Nanostars of [[vanadium(IV) oxide]]]]
Scientists have taken to naming their particles after the real-world shapes that they might represent. Nanospheres,<ref name=agam2007/> [[nanorod]]s, [[Magnetic nanochains|nanochains]],<ref name=kral2015/> nanoreefs,<ref name=choy2004/> nanoboxes<ref name=sun2002/> and more have appeared in the literature. These morphologies sometimes arise spontaneously as an effect of a templating or directing agent present in the synthesis such as miscellar [[emulsion]]s or anodized alumina pores, or from the innate crystallographic growth patterns of the materials themselves.<ref name=murp2002/> Some of these morphologies may serve a purpose, such as long [[carbon nanotube]]s used to bridge an electrical junction, or just a scientific curiosity like the stars shown at right.

Amorphous particles usually adopt a spherical shape (due to their microstructural isotropy), whereas the shape of anisotropic microcrystalline whiskers corresponds to their particular crystal habit. At the small end of the size range, nanoparticles are often referred to as [[cluster (physics)|cluster]]s. [[Sphere (geometry)|Sphere]]s, rods, [[fiber]]s, and cups are just a few of the shapes that have been grown. The study of fine particles is called [[micromeritics]].

===Functionalization===
Many properties of nanoparticles, notably stability, solubility, and chemical or biological activity, can be radically altered by [[coating]] them with various substances — a process called '''functionalization'''. Functionalized [[nanomaterial-based catalyst]]s can be used for catalysis of many known organic reactions.

For example, suspensions of [[graphene]] particles can be stablized by functionalization with [[gallic acid]] groups.<ref name=sadr2017/>

For biological applications, the surface coating should be polar to give high aqueous solubility and prevent nanoparticle aggregation. In serum or on the cell surface, highly charged coatings promote non-specific binding, whereas [[polyethylene glycol]] linked to terminal hydroxyl or methoxy groups repel non-specific interactions.<ref name=prim1991/><ref name=liu2010/> 

Nanoparticles can be [[nanoparticle–biomolecule conjugate|linked to biological molecules]] that can act as address tags, directing them to specific sites within the body<ref name=aker2002/> specific organelles within the cell,<ref name=hosh2004/> or causing them to follow specifically the movement of individual protein or RNA molecules in living cells.<ref name=suzu2007/> Common address tags are [[monoclonal antibodies]], [[aptamer]]s, [[streptavidin]] or [[peptide]]s. These targeting agents should ideally be covalently linked to the nanoparticle and should be present in a controlled number per nanoparticle. Multivalent nanoparticles, bearing multiple targeting groups, can cluster receptors, which can activate cellular signaling pathways, and give stronger anchoring. Monovalent nanoparticles, bearing a single binding site,<ref name=sung2004/><ref name=fu2004/><ref name=howa2008/> avoid clustering and so are preferable for tracking the behavior of individual proteins.

Coatings that mimic those of red blood cells can help nanoparticles evade the immune system.<ref name=physwo2011/>

===Variations===

Semi-solid and soft nanoparticles have been produced. A prototype nanoparticle of semi-solid nature is the [[liposome]]. Various types of liposome nanoparticles are currently used clinically as delivery systems for anticancer drugs and vaccines.

Nanoparticles with one half hydrophilic and the other half hydrophobic are termed [[Janus particles]] and are particularly effective for stabilizing emulsions. They can [[self-assembly|self-assemble]] at water/oil interfaces and act as solid surfactants.

Hydrogel nanoparticles made of N-isopropylacrylamide hydrogel core shell can be dyed with affinity baits, internally.<ref name=luch2008/> These affinity baits allow the nanoparticles to isolate and remove undesirable proteins while enhancing the target analytes.<ref name=luch2008/>

==Characterization==
{{Main|Characterization of nanoparticles}}
Nanoparticles have different analytical requirements than conventional chemicals, for which chemical composition and concentration are sufficient metrics.  Nanoparticles have other physical properties that must be measured for a complete description, such as [[Particle size|size]], [[shape]], [[Surface science|surface properties]], [[crystallinity]], and [[Dispersion (chemistry)|dispersion state]].  Additionally, sampling and laboratory procedures can perturb their dispersion state or bias the distribution of other properties.<ref name=hass2008/><ref name=powe2007/>  In environmental contexts, an additional challenge is that many methods cannot detect low concentrations of nanoparticles that may still have an adverse effect.<ref name=hass2008/>   For some applications, nanoparticles may be characterized in complex matrices such as water, soil, food, polymers, inks, complex mixtures of organic liquids such as in cosmetics, or blood.<ref name=boxa2008/><ref name=lins2011/>

There are several overall categories of methods used to characterize nanoparticles.  [[Microscopy]] methods generate images of individual nanoparticles to characterize their shape, size, and location.  [[Electron microscopy]] and [[scanning probe microscopy]] are the dominant methods.  Because nanoparticles have a size below the [[Diffraction-limited system|diffraction limit]] of [[visible light]], conventional [[Optical microscope|optical microscopy]] is not useful.  Electron microscopes can be coupled to spectroscopic methods that can perform [[elemental analysis]].  Microscopy methods are destructive, and can be prone to undesirable [[Artifact (error)|artifacts]] from sample preparation, or from probe tip geometry in the case of scanning probe microscopy.  Additionally, microscopy is based on [[Single-molecule experiment|single-particle measurements]], meaning that large numbers of individual particles must be characterized to estimate their bulk properties.<ref name=hass2008/><ref name=boxa2008/>

[[Spectroscopy]], which measures the particles' interaction with [[electromagnetic radiation]] as a function of [[wavelength]], is useful for some classes of nanoparticles to characterize concentration, size, and shape.  [[X-ray spectroscopy|X-ray]], [[Ultraviolet–visible spectroscopy|ultraviolet–visible,]] [[Infrared spectroscopy|infrared]], and [[nuclear magnetic resonance spectroscopy]] can be used with nanoparticles.<ref name=hass2008/><ref name=boxa2008/>  [[Light scattering by particles|Light scattering]] methods using [[laser]] light, [[X-ray scattering techniques|X-rays]], or [[neutron scattering]] are used to determine particle size, with each method suitable for different size ranges and particle compositions.<ref name=hass2008/><ref name=boxa2008/>  Some miscellaneous methods are [[electrophoresis]] for surface charge, the [[BET theory|Brunauer–Emmett–Teller method]] for surface area, and [[X-ray diffraction]] for crystal structure,<ref name=hass2008/> as well as [[mass spectrometry]] for particle mass, and [[particle counter]]s for particle number.<ref name=boxa2008/>  [[Chromatography]], [[centrifugation]], and [[filtration]] techniques can be used to separate nanoparticles by size or other physical properties before or during characterization.<ref name=hass2008/>

==Health and safety==
{{See also|Health and safety hazards of nanomaterials|Particulates|Nanotoxicology}}

Nanoparticles present possible dangers, both medically and environmentally.<ref name=zoro2014/>
<ref name=cris2017/>
<ref name=mnyu2003/><ref name=mu2013/> Most of these are due to the high surface to volume ratio, which can make the particles very reactive or [[catalytic]].<ref name=ying2016/> They are also able to pass through [[cell membrane]]s in organisms, and their interactions with biological systems are relatively unknown.<ref name=euheXXXX/> 
<ref name=thak2013/> However, it is unlikely the particles would enter the cell nucleus, Golgi complex, endoplasmic reticulum or other internal cellular components due to the particle size and intercellular agglomeration.<ref name=greu2011/> A recent study looking at the effects of [[ZnO]] nanoparticles on human immune cells has found varying levels of susceptibility to [[cytotoxicity]].<ref name=hanle2009/> There are concerns that pharmaceutical companies, seeking regulatory approval for nano-reformulations of existing medicines, are relying on safety data produced during clinical studies of the earlier, pre-reformulation version of the medicine. This could result in regulatory bodies, such as the FDA, missing new side effects that are specific to the nano-reformulation.<ref name=vines2009/> However considerable research has demonstrated that zinc nanoparticles are not absorbed into the bloodstream in vivo.<ref name=bens2005/>

Concern has also been raised over the health effects of respirable nanoparticles from certain combustion processes.<ref name=howa2009/><ref name=piet2015/> As of 2013 the [[U.S. Environmental Protection Agency]] was investigating the safety of the following nanoparticles:<ref name=epa2013a/>
*[[Carbon Nanotubes]]: Carbon materials have a wide range of uses, ranging from composites for use in vehicles and sports equipment to integrated circuits for electronic components. The interactions between nanomaterials such as carbon nanotubes and natural organic matter strongly influence both their aggregation and deposition, which strongly affects their transport, transformation, and exposure in aquatic environments. In past research, carbon nanotubes exhibited some toxicological impacts that will be evaluated in various environmental settings in current EPA chemical safety research. EPA research will provide data, models, test methods, and best practices to discover the acute health effects of carbon nanotubes and identify methods to predict them.<ref name=epa2013a/>
*[[Cerium(IV) oxide|Cerium oxide]]: Nanoscale cerium oxide is used in electronics, biomedical supplies, energy, and fuel additives. Many applications of engineered cerium oxide nanoparticles naturally disperse themselves into the environment, which increases the risk of exposure. There is ongoing exposure to new diesel emissions using fuel additives containing CeO<sub>2</sub> nanoparticles, and the environmental and public health impacts of this new technology are unknown. EPA's chemical safety research is assessing the environmental, ecological, and health implications of nanotechnology-enabled diesel fuel additives.<ref name=epa2013a/>
*[[Titanium dioxide]]: Nano titanium dioxide is currently used in many products. Depending on the type of particle, it may be found in sunscreens, cosmetics, and paints and coatings. It is also being investigated for use in removing contaminants from drinking water.<ref name=epa2013a/>
*[[Nano Silver]]: Nano silver is being incorporated into textiles, clothing, food packaging, and other materials to eliminate bacteria. EPA and the [[U.S. Consumer Product Safety Commission]] are studying certain products to see whether they transfer nano-size silver particles in real-world scenarios. EPA is researching this topic to better understand how much nano-silver children come in contact with in their environments.<ref name=epa2013a/>
*Iron: While [[nano-scale iron]] is being investigated for many uses, including “smart fluids” for uses such as [[optics polishing]] and as a better-absorbed [[iron nutrient supplement]], one of its more prominent current uses is to remove contamination from groundwater. This use, supported by EPA research, is being piloted at a number of sites across the United States.<ref name=epa2013a/>

== Regulation ==
As of 2016, the U.S. Environmental Protection Agency had conditionally registered, for a period of four years, only two nanomaterial pesticides as ingredients. The EPA differentiates nanoscale ingredients from non-nanoscale forms of the ingredient, but there is little scientific data about potential variation in toxicity. Testing protocols still need to be developed.<ref name=wayla2016/>

==Applications==
As the most prevalent morphology of nanomaterials used in consumer products, nanoparticles have an enormous range of potential and actual applications. Table below summarizes the most common nanoparticles used in various product types available on the global markets.
 
Clay nanoparticles when incorporated into polymer matrices increase reinforcement, leading to stronger plastics, verifiable by a higher [[glass transition temperature]] and other mechanical property tests. 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 functionWal clothing.<ref name=corn2016/>
 
{| class="wikitable"
|+Various nanoparticles which are commonly used in the consumer products by industrial sectors
! No.
! Industrial sectors
! Nanoparticles
|-
|1
|agriculture
| {{pagelist|silver|silicon  dioxide|potassium|calcium|iron|zinc|phosphorus|boron|zinc  oxide|molybdenum  }}
|-
|2
|automotive
| {{pagelist|tungsten|  disulfidesilicon  dioxide|clay|titanium  dioxide|diamond|copper|cobalt  oxide|zinc  oxide|boron  nitride|zirconium  dioxide|tungsten
 |γ-aluminium  oxide|boron|palladium|platinum|cerium(IV)  oxide|carnauba|aluminium  oxide|silver|calcium  carbonate|calcium  sulfonate}}
|-
|3
|construction
| {{pagelist|titanium|  dioxidesilicon  dioxide|silver|clay|aluminium  oxide|calcium  carbonate calcium silicate hydrate|carbon|aluminium  phosphate cerium(IV) oxide|calcium  hydroxide }}
|-
|4
|cosmetics
| {{pagelist|silver|titanium  dioxide|gold|carbon|zinc  oxide|silicon  dioxide|clay|sodium  silicate|kojic  acid|hydroxy  acid }}
|-
|5
|electronics
| {{pagelist|silver|aluminum|silicon  dioxide|palladium}}
|-
|6
|environment
| {{pagelist|silver|titanium  dioxide|carbonmanganese  oxide|clay|gold|selenium}}
|-
|7
|food
| {{pagelist|silver|clay|titanium  dioxide|gold|zinc  oxide|silicon  dioxide|calcium|copper|zinc|platinum|manganese|palladium|carbon  }}
|-
|8
|home appliance
| {{pagelist|silver|zinc  oxide|silicon  dioxide|diamond|titanium  dioxide }}
|-
|9
|medicine<ref name=sala2004/>
| {{pagelist|silver|gold|hydroxyapatite|clay|titanium  dioxide|silicon  dioxide|zirconium  dioxide|carbon|diamond|aluminium  oxide|ytterbium  trifluoride }}
|-
|10
|petroleum
| {{pagelist|tungsten|  disulfidezinc  oxide|silicon  dioxide|diamond|clay|boron|boron  nitride|silver|titanium  dioxide|tungsten
 |γ-aluminium  oxide|carbon|molybdenum  disulfide
 |γ-aluminium  oxide}}
|-
|11
|printing
| [[toner]], deposited by a [[Laser printer|printer]] onto paper or other substrate
|-
|12
|renewable energies
| {{pagelist|titanium|palladium|tungsten  disulfide|silicon  dioxide|clay|graphite|zirconium(IV)  oxide-yttria stabilized|carbon|gd-doped-cerium(IV)  oxide|nickel  cobalt oxide|nickel(II)  oxide|rhodium|sm-doped-cerium(IV)  oxide|barium  strontium titanate|silver }} 
|-
|13
|sports and fitness
| {{pagelist|silver|titanium  dioxide|gold|clay|carbon  }}
|-
|14
|textile
| {{pagelist|silver|carbon|titanium  dioxide|copper  sulfide|clay|gold|polyethylene  terephthalate|silicon  dioxide }}
|}
Scientific research on nanoparticles is intense as they have many potential applications in medicine, physics,<ref name=hubler2010/><ref name=steph2015/><ref name=hubler2013/> optics,<ref name=omid2019/><ref name=rash2017/><ref name=omid2018/> and electronics.<ref name=tayl2013b/><ref name=tayl2012a/><ref name=hewa2013/><ref name=tayl2013c/> The U.S. [[National Nanotechnology Initiative]] offers government funding focused on nanoparticle research.|The use of nanoparticles in laser dye-doped [[poly(methyl methacrylate)]] (PMMA) laser [[gain media]] was demonstrated in 2003 and it has been shown to improve conversion efficiencies and to decrease laser beam divergence.<ref name=duar2003/> Researchers attribute the reduction in beam divergence to improved dn/dT characteristics of the organic-inorganic dye-doped nanocomposite. The optimum composition reported by these researchers is 30% w/w of SiO<sub>2</sub> (~ 12&nbsp;nm) in dye-doped PMMA.|Nanoparticles are being investigated as potential drug delivery system.<ref name=singh2017/> Drugs, [[growth factor]]s or other biomolecules can be conjugated to nano particles to aid targeted delivery.<ref name=wang2017/> This nanoparticle-assisted delivery allows for spatial and temporal controls of the loaded drugs to achieve the most desirable biological outcome. Nanoparticles are also studied for possible applications as [[dietary supplement]]s for delivery of biologically active substances, for example [[Mineral (nutrient)|mineral elements]].<ref name=josw2018/>

==See also==
{{Portal|Science|Technology|Biology}}
{{div col|colwidth=22em}}
*[[Ceramic engineering]]
*[[Coating]]
*[[Colloid]]
*[[Colloid-facilitated transport]]
*[[Colloidal crystal]]
*[[Colloidal gold]]
*[[Eigencolloid]]
*[[Fullerenes]]
*[[Fungal-derived nanoparticles]]
*[[Gallium(II) selenide]]
*[[Icosahedral twins]]
*[[Indium selenide]]
*[[Liposome]]
*[[Magnetic immunoassay]]
*[[Magnetic nanoparticles]]
*[[Magnetic nanochains]]
*[[Micromeritics]]
*[[Nanobiotechnology]]
*[[Nanocrystalline silicon]]
*[[Nanofluid]]
*[[Nanogeoscience]]
*[[Nanomaterials]]
*[[Nanomedicine]]
*[[Nanoparticle deposition]]
*[[Nanoparticle Tracking Analysis]]
*[[Nanotechnology]]
*[[Patchy particles]]
*[[Photonic crystal]]
*[[Plasmon]]
*[[Platinum nanoparticles]]
*[[Quantum dot]]
*[[Self-assembly of nanoparticles]]
*[[Silicon]]
*[[Silver Nano]]
*[[Sol-gel]]
*[[Transparent material]]
*[[Upconverting nanoparticles]]
{{div col end}}

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==Further reading==
* {{cite book|author=Jackie Y. Ying|title=Nanostructured Materials|url=https://books.google.com/books?id=_pbtbJwkj5YC&pg=PA5|year=2001|publisher=Academic Press|isbn=978-0-12-744451-2|pages=5–}}
* [https://www.sciencedaily.com/releases/2002/08/020809071535.htm Nanoparticles Used in Solar Energy Conversion] (''[[ScienceDaily]]'').
* [http://www.hse.gov.uk/research/rrpdf/rr274.pdf "Nanoparticles: An occupational hygiene review"] by RJ Aitken and others. [[Health and Safety Executive]] Research Report 274/2004
* [https://web.archive.org/web/20110726130221/http://www.iom-world.org/pubs/IOM_TM0901.pdf "EMERGNANO: A review of completed and near completed environment, health and safety research on nanomaterials and nanotechnology"] by RJ Aitken and others.
* [http://seadm.com/descargas/SEADM%20DMA%20nanoparticle%20half%20mini.pdf High transmission Tandem DMA for nanoparticle studies] by SEADM, 2014.

==External links==
{{wikibooks|Nanotechnology}}
* [https://web.archive.org/web/20071130030606/http://nanohedron.com:80/ Nanohedron.com] images of nanoparticles
* [http://nanoparticles.org/primers/ Lectures on All Phases of Nanoparticle Science and Technology]
* [http://www.enpra.eu/ ENPRA – Risk Assessment of Engineered NanoParticles] EC FP7 Project led by the [[Institute of Occupational Medicine]]

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