Cross-section through two chip-sized glass plates in which a nanoparticle is trapped in an energy hole (or “potential well” to use the scientific term). The colored fields show the different charges in the electrostatic field. The red zone signifies a very low charge, while the blue edges have a strong charge. |
Prof.
Madhavi Krishnan, a biophysicist at the University of Zurich, has
developed a new method that measures not only the size of the particles
but also their electrostatic charge. Up until now it has not been
possible to determine the charge of the particles directly. This unique
method, which is the first of its kind in the world, is just as
important for the manufacture of drugs as in basic research. The process
has now been introduced for the first time in Nature Nanotechnology.
In
order to observe the individual particles in a solution, Prof. Madhavi
Krishnan and her co-workers “entice” each particle into an
“electrostatic trap”. It works like this: between two glass plates the
size of a chip, the researchers create thousands of round energy holes.
The trick is that these holes have just a weak electrostatic charge. The
scientists than add a drop of the solution to the plates, whereupon
each particle falls into an energy hole and remains trapped there. But
the particles do not remain motionless in their trap. Instead, molecules
in the solution collide with them continuously, causing the particles
to move in a circular motion. “We measure these movements, and are then
able to determine the charge of each individual particle,” explains
Prof. Madhavi Krishnan.
Put
simply, particles with just a small charge make large circular
movements in their traps, while those with a high charge move in small
circles. This phenomenon can be compared to that of a light-weight ball
which, when thrown, travels further than a heavy one. The U.S. physicist
Robert A. Millikan used a similar method 100 years ago in his oil drop
experiment to determine the velocity of electrically charged oil drops.
In 1923, he received the Nobel Prize in physics in recognition of his
achievements.
“But he examined the drops in a vacuum,” Prof. Krishnan
explains. “We on the other hand are examining nano particles in a
solution which itself influences the properties of the particles”.
Electrostatic charge of “nano drugs packages”
For
all solutions manufactured industrially, the electrical charge of the
nanoparticles contained therein is also of primary interest, because it
is the electrical charge that allows a fluid solution to remain stable
and not to develop a lumpy consistency.
“With our new method, we get a
picture of the entire suspension along with all of the particles
contained in it,” emphasizes Prof. Madhavi Krishnan. A suspension is a
fluid in which miniscule particles or drops are finely distributed, for
example in milk, blood, various paints, cosmetics, vaccines and numerous
pharmaceuticals. “The charge of the particles plays a major role in
this,” the Zurich-based scientist tells us.
One
example is the manufacture of medicines that have to be administered in
precise doses over a longer period using drug-delivery systems. In this
context, nano particles act as “packages” that transport the drugs to
where they need to take effect. Very often, it is their electrical
charge that allows them to pass through tissue and cell membranes in the
body unobstructed and so to take effect.
“That’s why it is so important
to be able to measure their charge. So far most of the results obtained
have been imprecise,” the researcher tells us.
“The
new method allows us to even measure in real-time a change in the
charge of a single entity,” adds Prof. Madhavi Krishnan. “This is
particularly exciting for basic research and has never before been
possible.”
This is because changes in charge play a role in all bodily
reactions, whether in proteins, large molecules such as the DNA double
helix, where genetic make-up is encoded, or cell organelles.
Source: University of Zurich
