Environmental Engineering Reference
In-Depth Information
used in many nanodevices where the size of the particle plays a major role [44-47]. There are various techniques used to purify
NPs; a few of them are described below:
1.
Centrifugation
: This is the most widely adopted technique used for the separation and purification of NPs. This involves
the separation of particles based on their size and shape by the action of a centrifugal force. A particle in a centrifugal
field experiences three main types of forces: centrifugal force, buoyant force, and frictional force. The centrifugal force
and buoyant force act in one direction, while the frictional force acts in the opposite direction. The particles are acceler-
ated under the centrifugal field, until all three forces are balanced, after which they settle under constant velocity. The
main basis for the separation of NPs by centrifugation is that particles of different sizes and shapes have different settling
velocities. For examples, gold NPs and gold nanorods have been separated by centrifugation at 5600 rpm for half an hour
[48]. A drawback that persists in the normal centrifugation process is that particles that are similar would have almost
similar settling velocity, and hence result in reduced quality of separation. This drawback can be overcome by the use of
superior centrifugation techniques like density gradient centrifugation, isopycnic centrifugation, rate zonal centrifuga-
tion. Lighter NPs like carbon nanotubes have been efficiently purified using isopycnic centrifugation [49, 50]. Heavier
NPs like metallic and inorganic NPs have been effectively separated using rate zonal centrifugation [51].
2.
Electrophoresis
: This technique implies the separation of charge particles using an electric field. When these particles are
placed in an electric field, they drift toward the oppositely charged electrode, making this an effective tool for the sepa-
ration of NPs based on their charge, size, and shape. The most frequently employed electrophoresis techniques for sepa-
ration of charged NPs include gel electrophoresis [52] and free-flow electrophoresis [53]. Isoelectric focusing is also an
important electrophoresis technique used to separate proteins based on their isoelectric points. Gel electrophoresis has
been used effectively to separate both gold and silver NPs [54]. Ho and his coworkers showed that free-flow electrophoresis
can be used as an effective tool to separate semiconductor NPs. [55]
3.
Magnetic field
: This can be an effective tool to separate the NPs that have magnetic receptiveness, based on their sizes.
For example, iron oxide NPs can be separated by this method. In this case it has been stated theoretically that particles of
sizes up to 50 nm can be efficiently separated and that smaller particles show hindrance due to thermal diffusion and
Brownian movement [56]. Practically, the separation of the smaller NPs was possible due to certain unique properties
exhibited by the NPs compared to the bulk material and also due to the overlook of dipole-dipole interactions among the
magnetic moment of particles, which can be present even in the absence of an external magnetic field. Moeser and
coworkers visualized aggregates of magnetic NPs after high gradient magnetic separation (HGMS) [57]; these were
mainly due to the dipole-dipole interaction, which causes aggregation of NPs. This explained the ability of a magnetic
field to separate particles smaller than predicted by theory [57, 58]. The magnetic NPs can also be separated based on
their material composition with the help of magnetic field flow fractionation (MFFF) This separation takes place mainly
due to the competition between magnetic forces and hydrodynamic forces [59]. Thus the magnetic field is one of the
major purification techniques that is used to separate and purify magnetic NPs; practically these NPs can also be purified
by other techniques like chromatography, centrifugation [60]
4.
Chromatography
: Chromatography is one of the techniques used to separate nonmagnetic NPs. The driving force is
mainly the variation in their partition coefficient. The NPs that need to be purified are taken in the mobile phase and
passed through the stationary phase. Among the various chromatography techniques available, size exclusion chromatog-
raphy (SEC) is the most preferred and commonly used technique for purification of NPs. For example, SEC is used to
separate gold [61] and silica [62] NPs. A drawback associated with SEC is the irreversible binding of the NPs to the
stationary phase, and this difficulty was overcome with the addition of sodium dodecyl sulfate (SdS) to the mobile phase.
The idea behind this was the simple concept that like charges repel each other and the negatively charged SdS surfactant
would electrostatically repel the negatively charged particles [63, 64]. Other chromatographic techniques like High-
performance liquid chromatography (HPLC) have also been reported. Other chromatographic techniques like HPLC have
also been reported to purify nanoparticles [65, 66].
5.
Precipitation
: Selective precipitation is a technique that separates the NPs based mainly on their size by precipitating them
on the basis of their physical and chemical properties. Gold NPs have been successfully purified by this method [67].
6.
Filtration
: Membrane filtration remains one of the most widely preferred methods for the separation of NPs. This tech-
nique separates the particles based mainly upon the pore size of the membrane. Polymeric membrane filters have been
shown to be an efficient means for the purification of gold NPs [68]. Sweeney and his fellow workers showed that this
filtration technique could be an effective tool for the separation of soluble NPs. Their experiment involved the purification
of soluble gold NPs. Their experimental setup showed a continuous filtration system that contained a reservoir, a filter
membrane, and a pump. The sample addition rate and the elution rate were kept constant. Based on the size of the filter
membrane, the smallest NPs were elected while the larger particles were retained [69] (Fig. 19.4).
