Biomedical Engineering Reference
In-Depth Information
15 min period of exposure to a magnetic fi eld; that is, the particles were precipi-
tated at only one side of the vial wall. Magnetization versus fi eld measurements
recorded at 300 K on Mnp and Mnp/Si/Au nanoparticles, respectively, showed a
lack of hysteretic behavior which indicated that, in both samples, the nanoparticles
were in a superparamagnetic state at room temperature. A strong decrease in satu-
ration magnetization was observed in Mnp/Si/Au particles with respect to the
saturation magnetization of Mnp particles. Si/Au nanoparticles without Mnp
cores exhibited a diamagnetic behavior as expected. This led to the belief that
the reduction of Ms in Mnp, in respect to that of Mnp/Si/Au nanoparticles,
was the result of the high fraction of silica/Au in the coated nanoparticles,
and that the gold and silica layers did not affect the saturation magnetization of
Mnp.
When Ji and coworkers [111] conducted magnetic resonance relaxation- and
photothermal studies on Mnp/Si/Au nanoparticles, the samples exhibited a strong
absorbance in the near-infrared (NIR) region of the electromagnetic spectrum, and
an effi cient photothermal effect, both of which should open new perspectives in
MRI imaging and photothermal therapy.
12.4.2.2
Effect of Particle Size and Particle Size Distribution on the Magnetic
Properties of Magnetite/PDMS Nanoparticles
For “
in vivo
” biomedical applications (e.g., drug delivery, dosage and cell uptake),
it is important to control not only particle size but also particle size distribution,
and it is for this reason that sterically stabilizing polymers on the particle surface
are often required. In addition, in order to fi nely control particle size, magnetic
separation is often used as a method of particle size selection, because it offers
simplicity and a low - cost throughput.
In a recent report, Riffl e and coworkers [114] described the synthesis of magne-
tite nanoparticles with polydimethyl siloxane (PDMS) chains bound to their sur-
faces. The size and size distribution of the particles were adjusted by the magnetic
removal of any larger particles, via magnetic separation. An accurate characteriza-
tion was performed in order to evaluate the effect of the magnetic separation on
the morphological and magnetic features of the nanoparticles. The report also
included some useful elements concerning the general methodology for character-
izing metal oxide/polymer nanoparticles.
Magnetite (Fe
3
O
4
) particles were synthesized by the reaction of a stoichiometric
ratio of iron (II) and iron (III) chloride salts (1 : 2) with ammonium hydroxide
(NH
4
OH). The base was added in the aqueous solution of salts, until it turned
black and reached a pH of 9-10 (see Section 12.3.1.1). The surfaces of magnetite
nanoparticles were then coated by adsorbing a PDMS oligomer that had three
carboxylate groups on one end of the molecule, and a nonfunctional trimethylsilyl
group at the other end. To this purpose, a PDMS dispersion, dissolved in dichlo-
romethane (DCM), was added to the basic magnetite dispersion. After stirring the
solution for 30 min, aqueous hydrochloric acid was added slowly until the solution
became slightly acidic (pH 5-6). Since the isoelectric point of magnetite in water
is pH
∼
6.8, there would be a net positive charge on the metal oxide surface at pH
