A nanofluid is a fluid containing nanometer-sized particles, called nanoparticles. These fluids are engineered colloidal suspensions of nanoparticles in a base fluid. The nanoparticles used in nanofluids are typically made of metals, oxides, carbides, or carbon nanotubes. Common base fluids include water, ethylene glycol and oil.
Nanofluids have novel properties that make them potentially useful in many applications in heat transfer, including microelectronics, fuel cells, pharmaceutical processes, and hybrid-powered engines,
engine cooling/vehicle thermal management, domestic refrigerator,
chiller, heat exchanger, in grinding, machining and in boiler flue gas
temperature reduction. They exhibit enhanced thermal conductivity and the convective heat transfer coefficient compared to the base fluid. Knowledge of the rheological behaviour of nanofluids is found to be critical in deciding their suitability for convective heat transfer applications.
Nanofluids also have special acoustical properties and in ultrasonic
fields display additional shear-wave reconversion of an incident
compressional wave; the effect becomes more pronounced as concentration
increases.
In analysis such as computational fluid dynamics (CFD), nanofluids can be assumed to be single phase fluids;
however, almost all new academic papers use a two-phase assumption.
Classical theory of single phase fluids can be applied, where physical
properties of nanofluid is taken as a function of properties of both
constituents and their concentrations. An alternative approach simulates nanofluids using a two-component model.
The spreading of a nanofluid droplet is enhanced by the
solid-like ordering structure of nanoparticles assembled near the
contact line by diffusion, which gives rise to a structural disjoining pressure in the vicinity of the contact line.
However, such enhancement is not observed for small droplets with
diameter of nanometer scale, because the wetting time scale is much
smaller than the diffusion time scale.
Synthesis
Nanofluids are produced by several techniques:
Direct Evaporation (1 step)
Gas condensation/dispersion (2 step)
Chemical vapour condensation (1 step)
Chemical precipitation (1 step)
Bio-based (2 step)
Several liquids including water, ethylene glycol, and oils have been
used as base fluids. Although stabilization can be a challenge, on-going
research indicates that it is possible. Nano-materials used so far in
nanofluid synthesis include metallic particles, oxide particles, carbon nanotubes, graphene nano-flakes and ceramic particles.
A bio-based, environmentally friendly approach for the covalent
functionalization of multi-walled carbon nanotubes (MWCNTs) using clove
buds was developed.
There are no any toxic and hazardous acids which are typically used in
common carbon nanomaterial functionalization procedures, employed in
this synthesis. The MWCNTs are functionalized in one pot using a free
radical grafting reaction. The clove-functionalized MWCNTs are then
dispersed in distilled water (DI water), producing a highly stable MWCNT
aqueous suspension (MWCNTs Nanofluid).
Smart cooling nanofluids
Realizing
the modest thermal conductivity enhancement in conventional nanofluids,
a team of researchers at Indira Gandhi Centre for Atomic Research
Centre, Kalpakkam developed a new class of magnetically polarizable
nanofluids where the thermal conductivity enhancement up to 300% of
basefluids is demonstrated. Fatty-acid-capped magnetite nanoparticles
of different sizes (3-10 nm) have been synthesized for this purpose. It
has been shown that both the thermal and rheological properties of such
magnetic nanofluids are tunable by varying the magnetic field strength
and orientation with respect to the direction of heat flow.
Such response stimuli fluids are reversibly switchable and have
applications in miniature devices such as micro- and
nano-electromechanical systems.
In 2013, Azizian et al. considered the effect of an external magnetic
field on the convective heat transfer coefficient of water-based
magnetite nanofluid experimentally under laminar flow regime. Up to 300%
enhancement obtained at Re=745 and magnetic field gradient of 32.5
mT/mm. The effect of the magnetic field on the pressure drop was not as
significant.
Response stimuli nanofluids for sensing applications
Researchers
have invented a nanofluid-based ultrasensitive optical sensor that
changes its colour on exposure to extremely low concentrations of toxic
cations.
The sensor is useful in detecting minute traces of cations in
industrial and environmental samples. Existing techniques for
monitoring cations levels in industrial and environmental samples are
expensive, complex and time-consuming. The sensor is designed with a
magnetic nanofluid that consists of nano-droplets with magnetic grains
suspended in water. At a fixed magnetic field, a light source
illuminates the nanofluid where the colour of the nanofluid changes
depending on the cation concentration. This color change occurs within a
second after exposure to cations, much faster than other existing
cation sensing methods.
Such response stimulus nanofluids are also used to detect and
image defects in ferromagnetic components. The photonic eye, as it has
been called, is based on a magnetically polarizable nano-emulsion that
changes colour when it comes into contact with a defective region in a
sample. The device might be used to monitor structures such as rail
tracks and pipelines.
Magnetic
nanoparticle clusters or magnetic nanobeads with the size 80–150
nanometers form ordered structures along the direction of the external
magnetic field with a regular interparticle spacing on the order of
hundreds of nanometers resulting in strong diffraction of visible light
in suspension.
Nanolubricants
Another word used to describe nanoparticle based suspensions is Nanolubricants.
They are mainly prepared using oils used for engine and machine
lubrication. So far several materials including metals, oxides and
allotropes of carbon have been used to formulate nanolubricants. The
addition of nanomaterials mainly enhances the thermal conductivity and
anti-wear property of base oils. Although MoS2, graphene, Cu based
fluids have been studied extensively, the fundamental understanding of
underlying mechanisms is still needed.
Molybdenum disulfide (MoS2) and graphene work as third body
lubricants, essentially becoming tiny microscopic ball bearings, which
reduce the friction between two contacting surfaces.
This mechanism is beneficial if a sufficient supply of these particles
are present at the contact interface. The beneficial effects are
diminished as the rubbing mechanism pushes out the third body
lubricants. Changing the lubricant, like-wise, will null the effects of
the nanolubricants drained with the oil.
Other nanolubricant approaches, such as Magnesium Silicate
Hydroxides (MSH) rely on nanoparticle coatings by synthesizing
nanomaterials with adhesive and lubricating functionalities. Research
into nanolubricant coatings has been conducted in both the academic and
industrial spaces.Nanoborate additives as well as mechanical model descriptions of
diamond-like carbon (DLC) coating formations have been developed by Ali
Erdemir at Argonne National Labs.
Companies such as TriboTEX provide consumer formulations of synthesized
MSH nanomaterial coatings for vehicle engines and industrial
applications.
Nanofluids in petroleum refining process
Many researches claim that nanoparticles can be used to enhance crude oil recovery. It is evident that development of nanofluids for oil and gas industry has a great practical aspects.
Applications
Nanofluids are primarily used for their enhanced thermal properties as coolants in heat transfer equipment such as heat exchangers, electronic cooling system(such as flat plate) and radiators. Heat transfer over flat plate has been analyzed by many researchers. However, they are also useful for their controlled optical properties. Graphene based nanofluid has been found to enhance Polymerase chain reaction efficiency. Nanofluids in solar collectors is another application where nanofluids are employed for their tunable optical properties. Nanofluids have also been explored to enhance thermal desalination technologies, by altering thermal conductivity and absorbing sunlight, but surface fouling of the nanofluids poses a major risk to those approaches. Researchers proposed nanofluids for electronics cooling. Nanofluids also can be used in machining.
Thermophysical properties of nanofluids
Thermal
conductivity, viscosity, density, specific heat, and surface tension
are considered some main thermophysical properties of nanofluids.
Various parameters like nanoparticle type, size, and shape, volume
concentration, fluid temperature, and nanofluid preparation method have
effect on thermophysical properties of nanofluids.
The
early studies indicating anomalous increases in nanofluid thermal
properties over those of the base fluid, particularly the heat transfer
coefficient, have been largely discredited. One of the main conclusions
taken from a study involving over thirty labs throughout the world
was that "no anomalous enhancement of thermal conductivity was observed
in the limited set of nanofluids tested in this exercise". The COST
funded research programme, Nanouptake (COST Action CA15119)
was founded with the intention "to develop and foster the use of
nanofluids as advanced heat transfer/thermal storage materials to
increase the efficiency of heat exchange and storage systems". One of
the final outcomes, involving an experimental study in five different
labs, concluded that "there are no anomalous or unexplainable effects".
Despite these apparently conclusive experimental investigations
theoretical papers continue to follow the claim of anomalous
enhancement, see, particularly via Brownian and thermophoretic mechanisms, as suggested by Buongiorno.
Brownian diffusion is due to the random drifting of suspended
nanoparticles in the base fluid which originates from collisions between
the nanoparticles and liquid molecules. Thermophoresis induces
nanoparticle migration from warmer to colder regions, again due to
collisions with liquid molecules. The mismatch between experimental and
theoretical results is explained in Myers et al.
In particular it is shown that Brownian motion and thermophoresis
effects are too small to have any significant effect: their role is
often amplified in theoretical studies due to the use of incorrect
parameter values. Experimental validation of the assertions of are provided in Alkasmoul et al. Brownian diffusion as a cause for enhanced heat transfer is dismissed in the discussion of the use of nanofluids in solar collectors.
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.The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 100 nm in only two directions. At the lowest range, metal particles smaller than 1 nm are usually called atom clusters instead.
Nanoparticles are distinguished from microparticles
(1-1000 µm), "fine particles" (sized between 100 and 2500 nm), and
"coarse particles" (ranging from 2500 to 10,000 nm), because their
smaller size drives very different physical or chemical properties, like
colloidal properties and ultrafast optical effects 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, whereas suspensions of larger particles usually scatter some or all visible light incident on them. Nanoparticles also easily pass through common filters, such as common ceramic candles, so that separation from liquids requires special nanofiltration techniques.
The properties of nanoparticles often differ markedly from those of larger particles of the same substance. Since the typical diameter of an atom
is between 0.15 and 0.6 nm, a large fraction of the nanoparticle's
material lies within a few atomic diameters of its surface. Therefore,
the properties of that surface layer may dominate over those of the bulk
material. This effect is particularly strong for nanoparticles
dispersed in a medium of different composition since the interactions
between the two materials at their interface also becomes significant.
Idealized model of a crystalline nanoparticle of platinum, about 2 nm in diameter, showing individual atoms.
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 atomic 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 paints, plastics, metals, ceramics, and magnetic products. The production of nanoparticles with specific properties is a branch of nanotechnology.
In general, the small size of nanoparticles leads to a lower concentration of point defects compared to their bulk counterparts, 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. The possibility of shifting the resonance
wavelengths by tuning the particle geometry allows using them in the
fields of molecular labeling, biomolecular assays, trace metal
detection, or nanotechnical applications. Anisotropic nanoparticles
display a specific absorption behavior and stochastic particle
orientation under unpolarized light, showing a distinct resonance mode
for each excitable axis.
Definitions
International Union of Pure and Applied Chemistry (IUPAC)
In its 2012 proposed terminology for biologically related polymers, the IUPAC defined a nanoparticle as "a particle of any shape with dimensions in the 1 × 10−9 and 1 × 10−7 m range". This definition evolved from one given by IUPAC in 1997.
In another 2012 publication, the IUPAC extends the term to include tubes and fibers with only two dimensions below 100 nm.[3]
International Standards Organization (ISO)
According to the International Standards Organization (ISO) technical specification 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.
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.
Related concepts
Nanoclusters
are agglomerates of nanoparticles with at least one dimension between 1
and 10 nanometers and a narrow size distribution. Nanopowders are agglomerates of ultrafine particles, nanoparticles, or nanoclusters. Nanometer-sized single crystals, or single-domain ultrafine particles, are often referred to as nanocrystals.
The terms colloid
and nanoparticle are not interchangeable. A colloid is a mixture which
has particles of one phase dispersed or suspended within an other phase.
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 (10−9 m) to micrometers (10−6 m).
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 cosmological, geological, 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; and the same is true of atmospheric dust particles. Many viruses have diameters in the nanoparticle range.
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."
20th century
During the 1970s and 80s, when the first thorough fundamental studies with nanoparticles were underway in the United States by Granqvist and Buhrman and Japan within an ERATO Project, 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.
The shapes of nanoparticles may be determined by the intrinsic crystal habit of the material, or by the influence of the environment around their creation, such as the inhibition of crystal growth on certain faces by coating additives, the shape of emulsion droplets and micelles in the precursor preparation, or the shape of pores in a surrounding solid matrix. Some applications of nanoparticles may require specific shapes, as well as specific sizes or size ranges.
Amorphous particles typically adopt a spherical shape (due to their microstructural isotropy).
The study of fine particles is called micromeritics.
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.
Hydrogel nanoparticles made of N-isopropylacrylamide hydrogel core shell can be dyed with affinity baits, internally. These affinity baits allow the nanoparticles to isolate and remove undesirable proteins while enhancing the target analytes.
Nucleation and growth
Impact of nucleation
Nucleation
lays the foundation for the nanoparticle synthesis. Initial nuclei play
a vital role on the size and shape of the nanoparticles that will
ultimately form by acting as templating nuclei for the nanoparticle
itself. Long-term stability is also determined by the initial nucleation
procedures.
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 may form simultaneously if nucleation is fast, creating a more
monodisperse product. However, slow nucleation rates can cause
formation of a polydisperse population of crystals with various sizes.
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.
The model was later expanded to a 3-step and two 4-step models between
2004-2008. Here, an additional step was included to account for small
particle aggregation, where two smaller particles could aggregate to
form a larger particle.
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. Many of them arise from spatial confinement of sub-atomic particles
(i.e. electrons, protons, photons) and electric fields around these
particles. The large surface to volume ratio is also significant factor
at this scale.
Controlling properties
The initial nucleation stages of the synthesis process heavily
influence the properties of a nanoparticle. Nucleation, for example, is
vital to the size of the nanoparticle. A critical radius must be met in
the initial stages of solid formation, or the particles will redissolve
into the liquid phase.
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.
Nucleation can be controlled predominately by time and temperature as
well as the supersaturation of the liquid phase and the environment of
the synthesis overall.
Large surface-area-to-volume ratio
1 kg of particles of 1 mm3 has the same surface area as 1 mg of particles of 1 nm3
Bulk materials (>100 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.
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.
Solvent affinity
Suspensions 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.
Coatings
Semiconductor nanoparticle (quantum dot) of lead sulfide with complete passivation by oleic acid, oleyl amine and hydroxyl ligands (size ~5nm)
Nanoparticles often develop or receive coatings
of other substances, distinct from both the particle's material and of
the surrounding medium. Even when only a single molecule thick, these
coatings can radically change the particles' properties, such as and
chemical reactivity, catalytic activity, and stability in suspension.
Diffusion across the surface
The high surface area of a material in nanoparticle form allows heat, molecules, and ions to diffuse
into or out of the particles at very large rates. The small particle
diameter, on the other hand, allows the whole material to reach
homogeneous equilibrium with respect to diffusion in a very short time.
Thus many processes that depend on diffusion, such as sintering can take place at lower temperatures and over shorter time scales inducing catalysis.
Ferromagnetic and ferroelectric effects
The small size of nanoparticles affects their magnetic and electric properties. The ferromagnetic materials in the micrometer range is a good example: widely used in magnetic recording
media, for the stability of their magnetization state, those particles
smaller than 10 nm are unstable and can change their state (flip) as the
result of thermal energy at ordinary temperatures, thus making them
unsuitable for that application.
Mechanical properties
The reduced vacancy concentration in nanocrystals can negatively affect the motion of dislocations, since dislocation climb requires vacancy migration. In addition, there exists a very high internal pressure due to the surface stress present in small nanoparticles with high radii of curvature. This causes a latticestrain 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.
Adhesion and friction forces are important considerations in nanofabrication, lubrication, device design, colloidal stabilization, and drug delivery. The capillary force is the main contributor to the adhesive force under ambient conditions.
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.
Another technique is in situTEM, which provides real-time, high resolution imaging of nanostructure response to a stimulus. For example, an in situ force probe holder in TEM was used to compress twinned nanoparticles and characterize yield strength.
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.
Like bulk materials, the properties of nanoparticles are materials dependent. For spherical polymer nanoparticles, glass transition temperature and crystallinity may affect deformation and change the elastic modulus when compared to the bulk material. However, size-dependent behavior of elastic moduli could not be generalized across polymers. As for crystalline metal nanoparticles, dislocations
were found to influence the mechanical properties of nanoparticles,
contradicting the conventional view that dislocations are absent in
crystalline nanoparticles.
Melting point depression
A
material may have lower melting point in nanoparticle form than in the
bulk form. For example, 2.5 nm gold nanoparticles melt at about 300 °C,
whereas bulk gold melts at 1064 °C.
Quantum effects are responsible for the deep-red to black color of gold or silicon nanopowders and nanoparticle suspensions.
Absorption of solar radiation is much higher in materials composed of
nanoparticles than in thin films of continuous sheets of material. In
both solar PV and solar thermal applications, by controlling the size, shape, and material of the particles, it is possible to control solar absorption.
Core-shell nanoparticles can support simultaneously both electric
and magnetic resonances, demonstrating 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.
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.
Regular packing
Nanoparticles of sufficiently uniform size may spontaneously settle into regular arrangements, forming a colloidal crystal. These arrangements may exhibit original physical properties, such as observed in photonic crystals.
Production
Artificial nanoparticles can be created from any solid or liquid material, including metals, dielectrics, and semiconductors. They may be internally homogeneous or heterogenous, e.g. with a core–shell structure.
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 kHz and 40 MHz. Laboratory units run
at power levels in the order of 30–50 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-gascondensation
is frequently used to produce metallic nanoparticles. The metal is
evaporated in a vacuum chamber containing a reduced atmosphere of an
inert gas.
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. Using magnetron sputtering to create the metal vapor allows to achieve higher yields.
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.
Radiolysis method
a) Transmission electron microscopy (TEM) image of Hf nanoparticles grown by magnetron-sputtering inert-gas condensation (inset: size distribution) and b) energy dispersive X-ray (EDX) mapping of Ni and Ni@Cu core@shell nanoparticles.
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 104gray
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.
Wet chemistry
Nanoparticles of certain materials can be created by "wet" chemical processes, in which solutions of suitable compounds are mixed or otherwise treated to form an insoluble precipitate
of the desired material. The size of the particles of the latter is
adjusted by choosing the concentration of the reagents and the
temperature of the solutions, and through the addition of suitable inert
agents that affect the viscosity and diffusion rate of the liquid. With
different parameters, the same general process may yield other
nanoscale structures of the same material, such as aerogels and other porous networks.
The nanoparticles formed by this method are then separated from
the solvent and soluble byproducts of the reaction by a combination of evaporation, sedimentation, centrifugation, washing, and filtration.Alternatively,
if the particles are meant to be deposited on the surface of some solid
substrate, the starting solutions can be by coated on that surface by
dipping or spin-coating, and the reaction can be carried out in place.
Electroless deposition
provides a unique opportunity for growing nanoparticles onto surface
without the need for costly spin coating, electrodeposition, or physical
vapor deposition. Electroless deposition processes can form colloid
suspensions catalytic metal or metal oxide deposition. The suspension of
nanoparticles that result from this process is an example of colloid. Typical instances of this method are the production of metal oxide or hydroxide nanoparticles by hydrolysis of metal alkoxides and chlorides.
Besides being cheap and convenient, the wet chemical approach
allows fine control of the particle's chemical composition. Even small
quantities of dopants, such as organic dyes and rare earth metals, can
be introduced in the reagent solutions end up uniformly dispersed in the
final product.
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.
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 catalysts can be used for catalysis of many known organic reactions.
For example, suspensions of graphene particles can be stabilized by functionalization with gallic acid groups.
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. By the immobilization of thiol groups on the surface of nanoparticles or by coating them with thiomers high (muco)adhesive and cellular uptake enhancing properties can be introduced.
Nanoparticles can be linked to biological molecules that can act as address tags, directing them to specific sites within the body specific organelles within the cell, or causing them to follow specifically the movement of individual protein or RNA molecules in living cells. Common address tags are monoclonal antibodies, aptamers, streptavidin, or peptides.
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, avoid clustering and so are preferable for tracking the behavior of individual proteins.
It has been shown that catalytic activity and sintering rates of a
functionalized nanoparticle catalyst is correlated to nanoparticles'
number density
Coatings that mimic those of red blood cells can help nanoparticles evade the immune system.
Uncontrolled agglomeration of powders due to attractivevan 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.
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.
Inert gas evaporation and inert gas deposition
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.
The reason why modern gas evaporation techniques can produce a
relatively narrow size distribution is that aggregation can be avoided.
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.
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.
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 size, shape, surface properties, crystallinity, and dispersion state.
Additionally, sampling and laboratory procedures can perturb their
dispersion state or bias the distribution of other properties.
In environmental contexts, an additional challenge is that many methods
cannot detect low concentrations of nanoparticles that may still have
an adverse effect.
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.
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 limit of visible light, conventional 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 artifacts
from sample preparation, or from probe tip geometry in the case of
scanning probe microscopy. Additionally, microscopy is based on single-particle measurements, meaning that large numbers of individual particles must be characterized to estimate their bulk properties.
Nanoparticles present possible dangers, both medically and environmentally. Most of these are due to the high surface to volume ratio, which can make the particles very reactive or catalytic. They are also thought to aggregate on phospholipid bilayers and pass through cell membranes in organisms, and their interactions with biological systems are relatively unknown.
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. A recent study looking at the effects of ZnO nanoparticles on human immune cells has found varying levels of susceptibility to cytotoxicity.
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. However considerable research has demonstrated that zinc nanoparticles are not absorbed into the bloodstream in vivo.
Concern has also been raised over the health effects of respirable nanoparticles from certain combustion processes.
Preclinical investigations have demonstrated that some inhaled or
injected noble metal nano-architectures avoid persistence in organisms. As of 2013 the U.S. Environmental Protection Agency was investigating the safety of the following nanoparticles:
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.
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 CeO2
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.
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.
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.
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.
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.
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.
Scientific research on nanoparticles is intense as they have many potential applications in pre-clinicaland clinical medicine, physics, optics, and electronics. 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.
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 SiO2 (~ 12 nm) in dye-doped PMMA. Nanoparticles are being investigated as potential drug delivery system. Drugs, growth factors or other biomolecules can be conjugated to nano particles to aid targeted delivery.
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 supplements for delivery of biologically active substances, for example mineral elements.
Polymer reinforcement
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
functional clothing.
Liquid properties tuner
The
inclusion of nanoparticles in a solid or liquid medium can
substantially change its mechanical properties, such as elasticity,
plasticity, viscosity, compressibility.
Photocatalysis
Being
smaller than the wavelengths of visible light, nanoparticles can be
dispersed in transparent media without affecting its transparency at
those wavelengths. This property is exploited in many applications, such
as photocatalysis.
Road paving
Asphalt
modification through nanoparticles can be considered as an interesting
low-cost technique in asphalt pavement engineering providing novel
perspectives in making asphalt materials more durable.
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.
Moreover, nanoparticles for nucleic acid delivery offer an
unprecedented opportunity to overcome some drawbacks related to the
delivery, owing to their tunability with diverse physico-chemical
properties, they can readily be functionalized with any type of
biomolecules/moieties for selective targeting.
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, being completely photostable though toxic.
Platinum nanoparticles are usually in the form of a suspension or colloid of nanoparticles of platinum in a fluid, usually water. A colloid is technically defined as a stable dispersion of particles in a fluid medium (liquid or gas).
Spherical platinum nanoparticles can be made with sizes between about 2 and 100 nanometres (nm), depending on reaction conditions.[1][2] Platinum nanoparticles are suspended in the colloidal solution of brownish-red or black color. Nanoparticles come in wide variety of shapes including spheres, rods, cubes,[3] and tetrahedra.[4]
Platinum nanoparticles are the subject of substantial research,[5][6][7] with potential applications in a wide variety of areas. These include catalysis,[7] medicine,[5] and the synthesis of novel materials with unique properties.[2][6][7]
Synthesis
Platinum nanoparticles are typically synthesized either by the reduction of platinum ion precursors in solution with a stabilizing or capping agent to form colloidal nanoparticles, or by the impregnation and reduction of platinum ion precursors in a micro-porous support such as alumina.
As the platinum metal precursor is reduced to neutral platinum metal (Pt0), the reaction mixture becomes supersaturated with platinum metal and the Pt0 begins to precipitate in the form of nanoscale particles. A capping agent or stabilizing agent such as sodium polyacrylic acid or sodium citrate is often used to stabilize the nanoparticle surfaces, and prevents the aggregation and coalescence of the nanoparticles.
The size of nanoparticles synthesized colloidally may be
controlled by changing the platinum precursor, the ratio of capping
agent to precursor, and/or the reaction temperature.
The size of the nanoparticles can also be controlled with small
deviation by using a stepwise seed-mediated growth procedure as outlined
by Bigall et al. (2008).
The size of nanoparticles synthesized onto a substrate such as alumina
depends on various parameters such as the pore size of the support.
Platinum nanoparticles can also be synthesized by decomposing Pt2(dba)3 (dba = dibenzylideneacetone) under a CO or H2 atmosphere, in the presence of a capping agent. The size and shape distributions of the resulting nanoparticles depend on the solvent,
the reaction atmosphere, the types of capping agents and their relative
concentrations, the specific platinum ion precursor, as well at the
temperature of the system and reaction time.
Shape and size control
Electron micrographs of Ostwald ripening in Pd nanoparticles dissolved in formaldehyde at 6 (a), 24 (b), 48 (c) and 72 hours (d). The small Pd particles are being consumed as the larger ones grow bigger.
Ramirez et al. reported the influence of ligand and solvent effects on the size and shape of platinum nanoparticles. Platinum nanoparticle seeds were prepared by the decomposition of Pt2(dba)3 in tetrahydrofuran (THF) under carbon monoxide
(CO). These conditions produced Pt nanoparticles with weakly bound THF
and CO ligands and an approximate diameter on 1.2 nm. Hexadecylamine
(HDA) was added to the purified reaction mixture and allowed to displace
the THF and CO ligands over the course of approximately seven days,
producing monodispersed spherical crystalline Pt nanoparticles with an
average diameter of 2.1 nm. After the seven-day period, an elongation of
the Pt nanoparticles occurred. When the same procedure was followed
using a stronger capping agent such as triphenyl phosphine or octanethiol, the nanoparticles remained spherical, suggesting that the HDA ligand affects particle shape.
Oleylamine, oleic acid and platinum(II) acetylacetonate (Pt(acac)2)
are also used in the synthesis of size/shape controlled platinum
nanoparticles. Research showed that alkylamine can coordinate with Pt2+ ion and form tetrakis(amine)platinate precursor and replace the original acac− ligand in Pt(acac)2, and oleic acid can further exchange with acac− and tune the formation kinetics of platinum nanoparticles.
When Pt2(dba)3 was decomposed in THF under hydrogen gas in the presence HDA, the reaction took much longer, and formed nanowires with diameters between 1.5 and 2 nm. Decomposition of Pt2(dba)3 under hydrogen gas in toluene
yielded the formation of nanowires with 2–3 nm diameter independent of
HDA concentration. The length of these nanowires was found to be
inversely proportional to the concentration of HDA present in solution.
When these nanowire syntheses were repeated using reduced concentrations
of Pt2(dba)3, there was little effect on the size, length or distribution of the nanowires formed.
Platinum nanoparticles of controlled shape and size have also
been accessed through varying the ratio of polymer capping agent
concentration to precursor concentration. Reductive colloidal syntheses
as such have yielded tetrahedral, cubic, irregular-prismatic, icosahedral, and cubo-octahedral
nanoparticles, whose dispersity is also dependent on the concentration
ratio of capping agent to precursor, and which may be applicable to
catalysis.
The precise mechanism of shape-controlled colloidal synthesis is not
yet known; however, it is known that the relative growth rate of crystal facets within the growing nanostructure determines its final shape. Polyol syntheses of platinum nanoparticles, in which chloroplatinic acid is reduced to PtCl42− and Pt0 by ethylene glycol, have also been a means to shape-controlled fabrication. Addition of varying amounts of sodium nitrate
to these reactions was shown to yield tetrahedra and octahedra at high
concentration ratios of sodium nitrate to chloroplatinic acid. Spectroscopic studies suggest that nitrate is reduced to nitrite by PtCl42−
early in this reaction, and that the nitrite may then coordinate both
Pt(II) and Pt(IV), greatly slowing the polyol reduction and altering the
growth rates of distinct crystal facets within the nanoparticles,
ultimately yielding morphological differentiation.
Green synthesis
An
ecologically-friendly synthesis of platinum nanoparticles from
chloroplatinic acid was achieved through the use of a leaf extract of Diospyros kaki
as a reducing agent. Nanoparticles synthesized as such were spherical
with an average diameter ranging from 212 nm depending on reaction
temperature and concentration of leaf extract used. Spectroscopic analysis suggests that this reaction is not enzyme-mediated and proceeds instead through plant-derived reductive small molecules. Another eco-friendly synthesis from chloroplatinic acid was reported using leaf extract from Ocimum sanctum and tulsi as reducing agents. Spectroscopic analysis suggested that ascorbic acid, gallic acid, various terpenes, and certain amino acids were active in the reduction. Particles synthesized as such were shown through scanning electron microscopy to consist in aggregates with irregular shape. It has been shown that tea extracts with high polyphenol content may be used both as reducing agents and capping agents for platinum nanoparticle synthesis.
Properties
The
chemical and physical properties of platinum nanoparticles (NP) make
them applicable for a wide variety of research applications. Extensive
experimentation has been done to create new species of platinum NPs, and
study their properties. Platinum NP applications include electronics,
optics, catalysts, and enzyme immobilization.
Catalytic properties
Platinum NPs are used as catalysts for proton exchange membrane fuel cell (PEMFC), for industrial synthesis of nitric acid, reduction of exhaust gases from vehicles and as catalytic nucleating agents for synthesis of magnetic NPs. can act as catalysts in homogeneous colloidal solution or as gas-phase catalysts while supported on solid state material. The catalytic reactivity of the NP is dependent on the shape, size and morphology of the particle
One type of platinum NPs that have been researched on are colloidal platinum NPs. Monometallic and bimetallic
colloids have been used as catalysts in a wide range of organic
chemistry, including, oxidation of carbon monoxide in aqueous solutions,
hydrogenation of alkenes in organic or biphasic solutions and hydrosilylation of olefins in organic solutions. Colloidal platinum NPs protected by Poly(N-isopropylacrylamide)
were synthesised and their catalytic properties measured. It was
determined that they were more active in solution and inactive when
phase separated due to its solubility being inversely proportional to
temperature.
Optical properties
Platinum
NPs exhibit fascinating optical properties. Being a free electron metal
NP like silver and gold, its linear optical response is mainly
controlled by the surface plasmon resonance (SPR). Surface plasmon resonance occurs when the electrons in the metal surface are subject to an electromagnetic field that exerts a force on the electrons and cause them to displace from their original positions. The nuclei then exert a restoring force
that results in oscillation of the electrons, which increase in
strength when frequency of oscillations is in resonance with the
incident electromagnetic wave.
The SPR of platinum nanoparticles is found in the ultraviolet
range (215 nm), unlike the other noble metal nanoparticles which display
SPR in the visible range Experiments were done and the spectra obtained
are similar for most platinum particles regardless of size. However,
there is an exception. Platinum NPs synthesized via citrate reduction do
not have a surface plasmon resonance peak around 215 nm. Through
experimentation, the resonance peak only showed slight variations with
the change of size and synthetic method (while maintaining the same
shape), with the exception of those nanoparticles synthesized by citrate
reduction, which did not exhibit and SPR peak in this region..
Through the control of percent composition of 2–5 nm platinum nanoparticles on SiO2,
Zhang et al. modeled distinct absorption peaks attributed to platinum
in the visible range, distinct from the conventional SPR absorption.
This research attributed these absorption features to the generation and
transfer of hot electrons from the platinum nanoparticles to the
semiconductive material. The addition of small platinum nanoparticles on semiconductors such as TiO2 increases the photocatalytic oxidation activity under visible light irradiation.
These concepts suggest the possible role of platinum nanoparticles in
the development of solar energy conversion using metal nanoparticles. By
changing the size, shape and environment of metal nanoparticles, their
optical properties can be used for electrontic, catalytic, sensing, and
photovoltaic applications.
Applications
Fuel cells application
Hydrogen fuel cells
Among
the precious metals, platinum is the most active toward the hydrogen
oxidation reaction that occurs at the anode in hydrogen fuel cells. In
order to meet cost reductions of this magnitude, the Pt catalyst loading
must be decreased. Two strategies have been investigated for reducing
the Pt loading: the binary and ternary Pt-based alloyed nanomaterials
and the dispersion of Pt-based nanomaterials onto high surface area
substrates.
Methanol fuel cells
The methanol oxidation reaction occurs at the anode in direct methanol fuel cells
(DMFCs). Platinum is the most promising candidate among pure metals for
application in DMFCs. Platinum has the highest activity toward the
dissociative adsorption of methanol. However, pure Pt surfaces are
poisoned by carbon monoxide,
a byproduct of methanol oxidation. Researchers have focused on
dispersing nanostructured catalysts on high surface area supporting
materials and the development of Pt-based nanomaterials with high
electrocatalytic activity toward MOR to overcome the poisoning effect of
CO.
Electrochemical oxidation of formic acid
Formic acid
is another attractive fuel for use in PEM-based fuel cells. The
dehydration pathway produces adsorbed carbon monoxide. A number of
binary Pt-based nanomaterial electrocatalysts have been investigated for
enhanced electrocatalytic activity toward formic acid oxidation.
Glucose detection applications
Enzymatic glucose sensors have drawbacks that originate from the nature of the enzyme.
Nonenzymatic glucose sensors with Pt-based electrocatalysts offer
several advantages, including high stability and ease of fabrication.
Many novel Pt and binary Pt-based nanomaterials have been developed to
overcome the challenges of glucose oxidation on Pt surfaces, such as low
selectivity, poor sensitivity, and poisoning from interfering species.
Other applications
Platinum catalysts are alternatives of automotive catalytic converters, carbon monoxide gas sensors, petroleum refining,
hydrogen production, and anticancer drugs. These applications utilize
platinum nanomaterials due to their catalytic ability to oxidize CO and
NOx, dehydrogenate hydrocarbons, and electrolyze water and their ability
to inhibit the division of living cells.
Biological interactions
The
increased reactivity of nanoparticles is one of their most useful
properties and is leveraged in fields such as catalysis, consumer
products, and energy storage. However, this high reactivity also means
that a nanoparticle in a biological environment may have unintended
impacts. For example, many nanoparticles such as silver, copper, and
ceria interact with cells to produce reactive oxygen species or ROS which can cause premature cell death through apoptosis.
Determining the toxicity of a specific nanoparticle requires knowledge
of the particle’s chemical composition, shape, size and is a field that
is growing alongside advances in nanoparticle research.
Determining the impact of a nanoparticle on a living system is not straightforward. A multitude of in vivo and in vitro studies must be done to fully characterize reactivity. In vivo studies often use whole organisms such as mice or zebrafish
to infer the interaction of the nanoparticle on a healthy human body.
In vitro studies look at how nanoparticles interact with specific cell
colonies, typically of human origin. Both types of experiments are
needed for a complete understanding of nanoparticle toxicity, especially
human toxicity, since no one model has complete human relevance. Few
studies have investigated the ADMET of platinum nanoparticles, and the results demonstrated that they are the ones who persist most in organisms respect to silver and gold nanoparticles.
Drug delivery
A topic of research within the field of nanoparticles is how to use these small particles for drug delivery.
Depending on particle properties, nanoparticle may move throughout the
human body are promising as site-specific vehicles for the transport of
medicine. Current research using platinum nanoparticles in drug delivery
uses platinum-based carries to move antitumor medicine. In one study,
platinum nanoparticles of diameter 58.3 nm were used to transport an
anticancer drug to human colon carcinoma cells, HT-29. Uptake of the nanoparticles by the cell involves compartmentalization of the nanoparticles within lysosomes. The high acidity environment enables leaching
of platinum ions from the nanoparticle, which the researchers
identified as causing the increased effectiveness of the drug. In
another study, a Pt nanoparticle of diameter 140 nm was encapsulated
within a PEG nanoparticle to move an antitumor drug, Cisplatin, within a prostate cancer cell (LNCaP/PC3) population.
Use of platinum in drug delivery hinges on its ability to not interact
in a harmful manner in healthy portions of the body while also being
able to release its contents when in the correct environment.
Toxicity stemming from platinum nanoparticles can take multiple forms. One possible interaction is cytotoxicity or the ability of the nanoparticle to cause cell death. A nanoparticle can also interact with the cell’s DNA or genome to cause genotoxicity.
These effects are seen in different levels of gene expression measured
through protein levels. Last is the developmental toxicity that can
occur as an organism grows. Developmental toxicity looks at the impact
the nanoparticle has on the growth of an organism from an embryonic
stage to a later set point. Most nanotoxicology research is done on
cyto- and genotoxicity as both can easily be done in a cell culture lab.
Platinum nanoparticles have the potential to be toxic to living
cells. In one case, 2 nm platinum nanoparticles were exposed to two
different types of algae in order to understand how these nanoparticles interact with a living system.
In both species of algae tested, the platinum nanoparticles inhibited
growth, induced small amounts of membrane damage, and created a large
amount of oxidative stress. In another study, researcher tested the effects of differently sized platinum nanoparticles on primary human keratinocytes.
The authors tested 5.8 and 57.0 nm Pt nanoparticles. The 57 nm
nanoparticles had some hazardous effects including decreased cell
metabolism, but the effect of the smaller nanoparticles was much more
damaging. The 5.8 nm nanoparticles exhibited a more deleterious effect
on the DNA stability of the primary keratincoytes than did the larger
nanoparticles. The damage to the DNA was measured for individual cells
using single-gel electrophoresis via the comet assay.
Researchers have also compared the toxicity of Pt nanoparticles
to other commonly used metallic nanoparticles. In one study, the authors
compared the impact of different nanoparticle compositions on the red blood cells
found in the human bloodstream. The study showed that 5–10 nm platinum
nanoparticles and 20–35 nm gold nanoparticles have very little effect on
the red blood cells. In the same study it was found that 5–30 nm silver
nanoparticles caused membrane damage, detrimental morphological
variation, and haemagglutination to the red blood cells.
In a recent paper published in Nanotoxicology, the authors found
that between silver (Ag-NP, d = 5–35 nm), gold (Au-NP, d = 15–35 nm),
and Pt (Pt-NP, d = 3–10 nm) nanoparticles, the Pt nanoparticles were the
second most toxic in developing zebrafish embryos, behind only the Ag-NPs.
However, this work did not examine the size dependence of the
nanoparticles on their toxicity or biocompatibility. Size-dependent
toxicity was determined by researchers at the National Sun Yat – Sen
University in Kaohsiung, Taiwan. This group’s work showed that the
toxicity of platinum nanoparticles in bacterial cells is strongly
dependent on nanoparticle size and shape/morphology.
Their conclusions were based on two major observations. First, the
authors found that platinum nanoparticles with spherical morphologies
and sizes less than 3 nm showed biologically toxic properties; measured
in terms of mortality, hatching delay, phenotypic defects and metal
accumulation.
While those nanoparticles with alternative shapes—such as cuboidal,
oval, or floral—and sizes of 5–18 nm showed biocompatibility and no
biologically toxic properties.
Secondly, out of the three varieties of platinum nanoparticles which
exhibited biocompatibility, two showed an increase in bacterial cell
growth.
The paper introduces many hypotheses for why these observations
were made, but based on other works and basic knowledge of bacterial
cell membranes, the reasoning behind the size dependent toxicity
observation seems to be twofold. One: The smaller, spherically shaped
nanoparticles are able to pass through cell membranes simply due to
their reduced size, as well as their shape-compatibility with the
typically spherical pores of most cell membranes.
Although this hypothesis needs to be further supported by future work,
the authors did cite another paper which tracked the respiratory intake
of platinum nanoparticles. This group found that 10 µm platinum
nanoparticles are absorbed by the mucus of the bronchi and trachea, and
can travel no further through the respiratory tract. However, 2.5 µm particles showed an ability to pass through this mucus layer, and reach much deeper into the respiratory tract.
Also the larger, uniquely shaped nanoparticles are too large to pass
through the pores of the cell membrane, and/or have shapes which are
incompatible with the more spherically shaped pores of the cellular
membrane.
In regards to the observation that the two largest platinum
nanoparticles (6–8 nm oval, and 16–18 nm floral) actually increase
bacterial cell growth, the explanation could originate in the findings
of other works which have shown that platinum nanoparticles have
demonstrated significant antioxidative capacity.
However, in order for these antioxidative properties to be exploited,
the platinum nanoparticles must first enter the cells, so perhaps there
is another explanation for this observation of increased bacterial cell
growth.
Most studies so far have been size based using an in vivo mouse
model. In one study, researchers compared the effects of sun 1 nm and
15 nm platinum nanoparticles on mice.
The 15 mg/kg dose of sub 1 nm platinum nanoparticles were found to
cause liver damage while the larger particles had no effect. A similar
study using a singular injection as an exposure source of platinum
nanoparticles into the mouse model found necrosis of tubular epithelial
cells for particles under 1 nm, but no effect with those particles of
8 nm.
These in vivo studies show a trend that the toxicity of the platinum
nanoparticles is size dependent, most likely due to the ability of the
nanoparticle to get into a high impactful region within the body. A
complete study analyzing the effect of varying sized platinum
nanoparticles used both in vivo and in vitro models is used to gain a
better understanding what impact these nanoparticles could have.
Using mice as a model, they found retention of the platinum
nanoparticles by the respiratory tract of the mouse. This was
accompanied by a minor to mild inflammation of the surrounding lung
tissue. However, their in vitro tests using human and lung epithelial cells found no cytotoxic or oxidative stress effects caused by the platinum nanoparticles despite clear evidence of cellular uptake.
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