by Professor Shlomo Magdassi
Utilization of nanomaterials very often requires their dispersion in various
liquids, in order to enable embedding them homogenously in a device or in a
final liquid product. For example, application of metallic nanoparticles or
carbon nanotubes in printed electronics is usually based on placing a dispersion
of the nanomaterials on various substrates, while the material is kept in its
Since most dispersions of nanomaterials are not thermodynamically stable and
represent a metastable state as compared to the bulk material, agglomeration and
coagulation of these materials tend to occur spontaneously. The driving force of
the aggregation is the interaction between the particles or nanotubes. For
example, while dispersing carbon nanotubes (CNT) in water, the van der Waals
attraction is so strong that it prevents the dispersion of individual CNTs and,
therefore, only bundles are present in the liquid. As shown in figure 1a, these
bundles eventually sediment, obviously rendering the dispersion useless in
various applications, such as those based on coatings.
In general, obtaining dispersions of powder of nanomaterials requires the use
of colloids chemistry tools, and can be divided into three stages:
1. wetting of the powder with liquid,
the agglomerates of the nanomaterials by applying high shear forces, and
3. stabilizing by proper dispersing agents.
If the synthesis of the nanomaterials results in a liquid dispersion of the
material, without going through the drying stage, only the latter stage is of
Wetting of powders can be achieved by a proper selection of the dispersion
liquid, or by the addition of a wetting agent. High shear forces can be obtained
by proper instrumentation, such as sonicators, high pressure homogenizers, and
bead mills. Stabilization of nanomaterials in dispersions is achieved by adding
dispersing agents, which increase the energy barrier for aggregation, thus
providing their kinetic stability1.
Since the stability of nanomaterials is governed by the balance of various
interactions, such as van der Waals attraction and electrical and steric
repulsion, the optimal approach to obtain stable dispersions is by using
stabilizers which have groups with affinity to the surface of the particles, and
groups that provides electro-steric stabilization. The use of proper dispersion
agent can lead to the formation of stable dispersions, such as that of CNT
presented in Fig 1b.
Figure 1. Unstable (a)
and stable (b) dispersion of multi wall
Evaluation of dispersion quality also presents challenges, especially for
nanomaterials which are not simple spherical nanoparticles. We recently
reported2 on a rapid and simple process for
producing dispersions MWCNTs by using a high pressure homogenization process
(HPH)4, and on a simple valuation method for CNT
dispersions by centrifugal sedimentation analysis.
Many nanomaterials are produced by the "wet chemistry" processes. In this
case the stabilizing agent can be present during the nanoparticles synthesis, or
even be one of the reactants, as in formation of gold nanoparticles, while the
reducing agent citric acid, also provides electrostatic stabilization. However,
as we found in many research projects, such stabilization is not sufficient for
stabilizing dispersions containing metallic nanoparticles at high concentration,
and in order to achieve this, a steric or electrosteric stabilizer is
required3. Such a stabilizer is polyacrilic acid
sodium salt, which we used in obtaining dispersions of silver, copper and Cu@Ag
Having stable dispersions of these metallic nanoparticles, enabled us to use
them in inkjet printing of conductive patterns composed (Fig 2a), in RFID tags
(Fig2b) and in several electroluminescent devices.
Figure 2. A printed
layer composed of closely packed silver nanoparticles and inkjet printed RFID
Another field in which dispersion of nanomaterials is of high importance is
drug delivery systems. Proper use of dispersion agents in dispersions of organic
nanoparticles can lead to improved dissolution and, thus, to improved
bioavailability. It can even prevent crystallization of nanomaterials, as we
have recently demonstrated for several active materials8,9.
In conclusion, understanding stabilization mechanisms of colloidal systems is
of utmost importance in utilizing nanomaterials in material science, as well as
in many applications.
1. Kamyshny, A.; Magdassi, S. In Structure and Functional
Properties of Colloidal Systems (Surf. Sci. Ser., v. 147); Starov, V., Ed.; CRC
Press: Boca Raton-London-New York, 2010 (in press).
S.; Magdassi, S. Carbon 48, in press (2010).
3. Kamyshny, A.; Ben-Moshe, M.; Aviezer, S.; Magdassi, S. Macromol.
Rapid. Communn, 26, 281. ( 2005).
Grouchko, M.; Kamyshny, A.; Magdassi, S. J. Mater. Chem. , 19, 3057
5. Magdassi, S.; Grouchko,M.;
Berezin, O.; and Kamyshny ,A.; ACS Nano, 4 , 1943-1948 (2010).
6. Layani, M.. ,Grouchko M.., Millo O., Azulay
D.;Balberg I..; Magdassi S., ACS NANO, 11,3537-3542 (2009).
7. Grouchko, M..; Kamyshny, A..; Ben-Ami,K.; Magdassi, S.,
J. Nanopart. Res. 11, 713-716 (2009).
8. Margulis-Goshen, K.;
Magdassi, S.; Nanomedicine,5,274-281 (2009).
Margulis-Goshen,K.; Donio (Netivi) H.; Major, D. T.; Gradzielski,M.; Raviv,U.;
Magdassi,S.; J. Colloid Interface Sci., 342,283-292 (2010).
Copyright AZoNano.com, Professor Shlomo Magdassi (The Hebrew
University of Jerusalem)