Biomedical Engineering Reference
In-Depth Information
8.2.2.1 Latex Beads
Latex particles, which were mentioned earlier as potential fluorescent tracers, are
commonly used because of their variety: they can be found commercially with dif-
ferent surface chemistries, in different sizes, and with other distinctive properties.
For instance, one can easily get fluorescent or magnetic beads.
These particles are synthesized by emulsion polymerization in which the hydro-
phobic organic monomer is encapsulated by surfactant molecules in a micelle (see
Section 8.2.2.3) and then polymerized in the water phase. This produces nice mono-
disperse suspensions that unfortunately need the surfactant to stay stable. However,
it is possible to add at the polymerization step a monomer that can play the role
of the surfactant and that stays at the bead-water interface after copolymerization,
stabilizing the particles by electrostatic interactions. Moreover, these chemical func-
tions that are now at the surface of the particles can be used to initiate the coupling
of biomolecules on the beads.
For instance, the widely used latex agglutination tests consist in adsorbing an-
tibodies to micron-sized latex particles. When the corresponding antigen is present,
these beads interact with it and clump together. Because of their size, the particles
and the clumps scatter light differently and a simple visual observation gives the
answer on the presence of the antigen.
8.2.2.2 Gold Nanoparticles
The main use of gold nanoparticles is electron microcopy. They can be coupled to
antibodies by electrostatic nonspecific adsorption. When the target protein is pres-
ent, the nanoparticles couple specifically to it and appear as distinctive tiny black
spots in transmission electron microscopy.
The particular optical properties of these nanoparticles can also be used to im-
prove agglutination tests classically performed with latex particles (see above). For
instance, they are used in some commercial pregnancy tests. Because of their small
size, these particles have a characteristic red color caused by a phenomenon called
plasmon resonance that we will review in more detail in Section 8.3.3.1. The urine
of pregnant women contains a particular hormone whose corresponding antibody
is adsorbed both on the nanoparticles and on micron-sized latex particles. When the
hormone is present the two types of particles coagglutinate and yield the formation
of red clumps.
In optical microscopy, gold (and other metals) nanoparticles are also used al-
though not as frequently. In particular, relatively large metallic nanoparticles (a few
tens of nanometers) can be easily detected by the apparition of a surface plasmon
described above that enhances their diffusion by several orders of magnitude [15].
For smaller nanoparticles, photothermal heating by the laser illumination modifies
the index of refraction very locally and allows their detection in live cells down to
diameters of a few nanometers [16].
8.2.2.3 Surfactants and Micelles
Surfactants (also called amphiphiles) are molecules composed of two antagonistic
parts: a hydrophilic polar head and a hydrophobic nonpolar tail. Phospholipids
