Biomedical Engineering Reference
In-Depth Information
functional particles. The effectiveness of this approach is dictated by the surface
charge or zeta potential of the nanoparticles, and the strength of the surface- ligand
interaction.
Many of these methods make use of particles that have been produced by copre-
cipitation, and are most common for metal oxides such as iron oxide. As men-
tioned above, particles produced by this method are often synthesized in the
presence of hydroxide, leaving the native surface terminated with hydroxyl groups
and an associated counterion of tetramethylammonium in solution. Lucas
et al.
have reported detailed investigations into the nature of surface charge density
under acidic, neutral, and alkaline conditions for iron oxide nanoparticles; from
zetometry and potentioconductometric titrations, these authors concluded that the
charge density under basic conditions was larger than those under acidic condi-
tions [49] .
Recently, surface adsorption has been applied to magnetic nanomaterials in a
variety of forms. Stabilization by simple organic compounds such as sodium
oleate, citrate with dimercaptosuccinic acid (DMSA) [50], biotin with phosphates
[51], have all been used. In the case of citrate and DMSA with citrate, Wilhelm
and Gazeau showed that iron oxide particles coated with these anionic “mono-
mers” experienced a more rapid uptake into mammalian cells compared to par-
ticles passivated with dextran or albumin [51]. Wan and coworkers showed that
the adsorption of block copolymers of methyl-terminated polyetheylene glycol
(MPEG; molecular weight
10
6
g mol
- 1
) and oligo-aspartic acid could stabilize iron
oxide nanoparticles over a broad pH range [52]. Furthermore, their investigations
revealed that the MPEG copolymer produced a signifi cantly higher relative cell
viability compared to that of polyacrylic acid and MPEG- polyacrylic acid.
Surface adsorption is also used as a method of biomolecule immobilization.
Rotello and coworkers have used the surface adsorption of bovine serum albumin
(BSA) under ultrasonication conditions to effectively heat the surrounding aqueous
solution using alternating applied magnetic fi elds [53]. Particles such as these have
potential applications in the localized heating of cells for hypothermia- induced cell
death [53] . Ivanisevic
et al.
showed that DNA adsorbed onto cobalt ferrite nanopar-
ticles could still be effectively cleaved using restriction enzymes, even in the pres-
ence of excess CoFe
2
O
4
[54]. Deng and Zhang demonstrated through progressive
surface adsorption, imine formation, and then reduction, that trypsin could be
attached effectively to iron oxide particle surfaces while still maintaining its ability
to digest proteins [55]. The same group also used these magnetically separable
enzymatic nanoreactors for rapid mass spectral analysis sample preparation,
showing that the particle-protein conjugates maintained statistically equivalent
activity for the fi rst eight digestions, with activity rapidly diminishing thereafter
[55] .
Surface adsorption has also seen use in the biological materials with nanorods.
Keating
et al.
have shown that the adsorption of thiolated single-stranded (ss) DNA
to segmented nanowires containing magnetic Ni segments provided an effective
barcode-based visual assay for fl uorescently tagged complementary oligonucle-
otide strands [56]. The same team also incorporated ssDNA hairpin molecular
∼
