Biomedical Engineering Reference
In-Depth Information
Aside from the extended blood half-life that it can provide, one of the great
advantages of PEG coating is that it can also be easily conjugated to antibodies or
other biomolecules so as to achieve a specifi c targeted delivery. For example, in a
recent report, biocompatible water-soluble magnetite nanocrystals were fabricated
via the thermal decomposition of ferric triacetylacetonate in 2-pyrrolidone in the
presence of monocarboxyl- terminated PEG (MPEG - COOH) [93] . The carboxylic
acid groups on the surface of the particles were conjugated with a cancer-targeting
anti-carcinoembryonic antigen (CEA) monoclonal antibody, via a carbodiimide
coupling reaction. The resultant materials were assessed for their ability to label
cancer tissues
in vivo
, for subsequent MRI detection [100]. PEG-coated iron oxide
nanoparticles may also be conjugated to specifi c targeting peptides and receptors
such as chlorotoxin [101] , transactivator protein (Tat) of HIV - 1 [102 - 104] , and
integrins [105, 106] .
The coating of magnetic nanoparticles with PEG-modifi ed phospholipids, which
often are introduced as micelles during the synthesis, produces highly biocompat-
ible and water- stable “ magnetoliposomes ” [107 - 109] . Such liposome encapsula-
tion delays the natural dilution of the contrast agents, and limits their interactions
with biological media. In addition, this approach may enable the simultaneous
combination of diagnosis and therapeutic action by encapsulating a MRI contrast
agent and a drug together [110].
Other polymers and copolymers, which have been used to coat
magnetic nanoparticles, include PVP) [111 - 113] , polyethylenimine ( PEI ) [114] ,
polyvinyl alcohol ( PVA ) [115 - 117] , polysodium - 4 - styrene sulfonate [118] ,
poly(trimethylammonium ethylacrylate methyl sulfate)- poly - (acrylamide) [119] ,
polyvinylbenzyl-
O
- beta -
D
- galactopyranosyl -
D
- gluconamide (PVLA) [120] , polycap-
rolactone [121], and gummic acid [122]. In addition, several stable and biocompat-
ible magnetic fl uids have been prepared by coating magnetic nanoparticles with
proteins, such as human serum albumin ( HSA ) [123] , avidin [124] , and Annexin
A5 (anxA5)- VSOP [125] .
Finally, both double- and single-stranded DNA have been shown to be very good
stabilizers for magnetic iron oxide nanoparticles, allowing the preparation of
highly stable magnetic fl uids that have exhibited unprecedented high relaxivities
and also show a good potential for MRI [126].
4.3.3
Modifi cation Using Inorganic Coatings
Inorganic coatings for magnetic nanoparticles include silica, carbon, precious
metals (e.g., Ag and Au), or metal oxides [61].
Silica coating
represents one of the most frequently used inorganic coatings, for
several reasons. The silica coating signifi cantly improves the stability of magnetic
nanoparticles, protecting them from oxidation, and it may also reduce any poten-
tial toxic effects of the nanoparticles [127]. Such coating also helps to prevent
particle aggregation and to increase particle stability in solution. As the isoelectric
point of magnetite is pH 7, it is necessary to further coat the particles in order to
