Wolfgang J. Parak &
Liberato Manna talk with ScienceWatch.com and
answer a few questions about this month's Emerging Research
Front Paper in the field of Materials Science. The authors
have also sent along an image of their
Article: Hydrophobic nanocrystals coated with an
amphiphilic polymer shell: A general route to water soluble
L;Kudera, S;Liedl, T;Koktysh, D;Rogach, AL;Keller,
Journal: NANO LETT, 4 (4): 703-707 APR 2004
Addresses: Univ Munich, Dept Phys, Munich, Germany.
Univ Munich, Dept Phys, Munich, Germany.
Univ Munich, Ctr Nanosci, Munich, Germany.
(addresses have been truncated.)
Would you summarize the significance of your paper
in layman's terms?
Colloidal nanoparticles are emerging new materials which have the potential
for use in many different disciplines ranging from electronics
(solar cells, LEDs) to the life sciences
(fluorescence labels). The wet chemical synthesis of nanoparticles has
experienced a great upsurge during the past decade.
Today, colloidal nanoparticles can be made out of a huge variety of
materials and the current synthetic routes allow for the tailoring of their
size, shape, and also for combining different particles to form hybrids.
stabilized in organic
solvents by hydrophobic
present on their
surface. Addition of an
side-chains and a
renders the particles
For most materials, the synthesis in organic solvents is preferred, as a
wider range of temperatures and surfactants is available. However, the
resulting particles are hydrophobic and thus cannot be used for most
applications in the life sciences.
Early approaches tried to circumvent this problem by subsequent ligand
exchange. However, for many materials the resulting particles possess only
limited colloidal stability, particularly when salts are present, as is the
case in all body fluids. Naturally, medical use of unstable particles is
The paper describes an important step towards the solution of this problem.
Instead of performing a ligand exchange, which depends on the chemical
nature of the surface of these particles, this approach is based on a more
Addition of an amphiphilic polymer leads to the water-solubility of the
particles. The hydrophobic tails of the polymer intercalate the hydrophobic
surfactant molecules originally present on the particles' surface, whereas
the hydrophilic backbone of the polymer is exposed to the environment and
thus renders the particles water-soluble.
The considerable advantage of the procedure stands in its generality. As
the whole coating procedure is only based on hydrophobic interaction, it
can be applied to virtually all nanoparticles possessing a hydrophobic
surfactant shell. The resulting particles have a significantly higher
colloidal stability and thus resist aggregation to a much better extent.
Even though the polymer contributes to a slight enlargement in the size of
the particles, the resulting particles still are quite small and many
commercially available particles are nowadays distributed with similar
The basic idea of this work originated in the group of Paul Alivisatos, who
is currently the Larry and Diane Bock Professor of Nanotechnology at the
University of California, Berkeley, where Liberato Manna and Wolfgang Parak
were both working almost a decade ago as postdocs. In the Alivisatos lab,
they were first introduced to the beauty of colloidal nanoparticles.
While Liberato Manna focused on growing particles in organic solvents,
Wolfgang Parak was dedicated to their transfer to an aqueous solution and
their use in biology. Both discussed the problems of limited colloidal
stability of these particles.
Back in Europe, both researchers started working towards a solution of this
problem. Together with Teresa Pellegrino, who also originated from the
Alivisatos group, they developed the idea of adding an additional layer to
the particle surface instead of exchanging the existing one.
They were all working at the Ludwig Maximilians Universität at the
Center for Nanoscience at the time when they put their ideas into practice.
Their development ran somehow parallel to the research of the
Quantum Dot Corporation in Hayward, California,
which, at the same time, introduced a similar polymer-coating procedure.
As colloidal stability is paramount for life science
applications—i.e., who wants to work with aggregates?—the
polymer coating procedure soon was accepted in the field. So far, many
groups have reproduced and actually improved upon the procedure in a
variety of ways, which has led to a large number of citations of our
As the procedure is general, it is nowadays used for several types of
nanocrystals, ranging from fluorescence quantum dots up to magnetic
nanoparticles. As applications in the life sciences are starting to emerge,
the method developed by Manna and Parak surely will have an increasing
impact into the future.
Prof. Dr. Wolfgang J. Parak
Philipps Universität Marburg
Marburg, Germany Web
Liberato Manna, Ph.D.
Research Scientist, Leader of the Nanochemistry Division
National Nanotechnology Lab of CNR-INFM
Istituto Italiano di Tecnologia (IIT)
Genova, Italy Web