Biomedical Engineering Reference
In-Depth Information
Physiological ionic
strength
Repulsion
Electrostatic force
5nm
Primary attraction
Attraction
van der Waal's force
Figure 10.11
Schematic representation of the interactions in the DLVO theory.
of nanoparticles, particularly when narrow size distribution is required.
h is process occurs when the metal particle is covered with a layer of a
voluminous material (polymer). Polymeric layer absorbed on the surface
of nanoparticles serves as a dif usion barrier, resulting in a dif usion-
limited growth in the subsequent growth of nuclei, and keeps the metal
centers separated from each other, preventing possible agglomeration. In
this process N, P, and S donor ligands are used. Another type of stabiliza-
tion also used within the steric category is by trapping of nanoparticles
in a polymeric material PVP poly(vinyl pyrrolidone), PVA poly(vinyl
alcohol) and PMVE poly(methyl vinyl ether). Metals can be consid-
ered as coni ned plasma of positive ions (consisting of nuclei and core
electrons which are i xed) and conduction electrons (free and mobile)
which are equal in number. In a neutral case, the positive charge cloud of
ions and negative charge cloud of electrons overlap each other [74]. By
some external disturbance, i.e., interaction with electrons or electromag-
netic radiations, the charge cloud is disturbed and electrons are moved
away from equilibrium position as shown in Figure 10.12. If the density
of electrons in one region increases, they repel each other and tend to
return to their original equilibrium position. As electrons move towards
their original positions, they pick up kinetic energy and instead of com-
ing to rest in equilibrium coni guration, they overshoot the mark. h ey
oscillate back and forth and this collective oscillation of conduction elec-
trons in metals is known as plasmon [75]. h ese oscillations give rise to
a strong absorption band in the visible range of electromagnetic spec-
tra. Since net charge dif erence occurs at the nanoparticle boundaries or
