Author: Tommaso Mazza
E-mail: tommaso.mazza@mi.infn.it
Movie: Nano, the Next Dimension
Movie Clip: From 23:07 To 24:42 (DVD)
Director: Pierre Oscar Lévy
Film Studio: Ex-Nihilo on behalf of the European Commission
Scientific level
Reactions, such as crystallization, at the nanoscale can only be detected with indirect
observation. One possible way is to observe the changing of colour of a solution.
When a solution containing water, iron chloride and caustic soda changes its colour
from orange to black it means that small iron oxide crystals with a size of few
nanometers have formed. This reaction can be exploited on an industrial level
controlling the size of the iron oxide nanocrystals at the nanometer scale. Dimension
can be controlled by calculating the ingredients and the reaction time, though the
recipe is an industrial secret.
In general, at the nanoscale different crystal sizes bring to different solution colours,
due to quantum effects in light absorption and/or fluorescence.
Fluorescence induced by exposure to ultraviolet light in vials containing various sized Cadmium Selenide
(CdSe) quantum dots.
Courtesy of D. Talapin & A. Rogach, taken from
http://en.wikipedia.org/wiki/Image:Fluorescence_in_various_sized_CdSe_quantum_dots.png
This effect was empirically known also in the middle ages, when the glassmakers
masters were able to obtain glasses of any colour and with a very high brilliance
mixing gold or other metals powders, actually nanometre-sized, with the glass silica.
A window in the Tours Cathedral, France. http://www.paui.it/
1
Iron oxide nanoparticles can be used for HIV virus detection in blood with a very high
accuracy. Due to their positively charged surface, the nanoparticles repel each other
remaining isolated when dissolved in human blood.
The tail of a HIV virus antibody is negatively charged, so the particle and the antibody
bind together because of their opposite electrostatic charge. If a HIV virus is present,
the antibody sticks to it. So, the iron oxide particle together with the antibodies and the
HIV viruses forms a micelle.
Schematic representation of a micelle.
http://it.wikipedia.org/wiki/Micella
A micelle is in general an aggregate of surfactant molecules dispersed in a liquid
colloid. Micellae are roughly spherical or globular aggregate because of energetical
equilibrium between the electrostatic forces of the molecules and the properties of the
solvent, which lead to the autoaggregation of such ordered systems. The shape of a
micelle is a function of the molecular geometry of its surfactant molecules and solution
conditions such as surfactant concentration, temperature, pH, and ionic strength.
A most common example of micelle formation is the working principle of soap. The
soap molecule is a long chain, commonly a carboxylic acid, which has an hydrophilic
positively charged head, and an oleophilic tale.
Example for the structure of the soap molecule.
http://it.wikipedia.org/wiki/Sapone
The heads of different soap molecules repel with each other, because they are
positively charged, whereas the tales are dissolved in fat particles, making a micelle
around them. The micelle can be thereafter suspended in water and washed away.
In the case HIV detection, instead of a fat particle the nucleus of the micelle is the iron
oxide nanoparticles, to which, as discussed, the HIV virus is bounded through the
antibody. As iron oxide is ferromagnetic, it can be drifted by a magnetic field, taking
the HIV virus for an extremely sensitive screening method. In fact, there are drug
cocktails that are used to treat AIDS patients that can lower the HIV virus presence in
the blood below normal detection levels, though not healing the patient completely.
This system could be used to monitor the presence of the virus also when it is reduced
down to undetectable concentrations.
This is only one example from the wide spectrum of applications of autoassembling
with nanoparticles and biological material. The surface of magnetic nanoparticles can
be properly functionalised, so, for instance, chemotherapeutic drugs can be bound to
them in order to address drug delivery to tumors efficiently, making use of radiofrequency pulsed magnetic fields.
Potentially, any kind of drug can be bound to a proper magnetic nanoparticle making
use of a proper functionalizer. In this way, any disease can be attacked at the same
time significantly reducing the unpleasant side effects of the drug. Moreover, the
concept of “targeting treatment” opens possibilities to much more efficient cures.