Biomedical Engineering Reference
In-Depth Information
the ratio of the iron precursor and the gold source. For example, Xu
et al.
described
the use of HAuCl
4
· 3H
2
O [67] to coat oleic acid-stabilized Fe
3
O
4
nanoparticles by
stirring the gold salt in the presence of the nanoparticles and oleylamine for 20 h
at room temperature. The gold shell thickness prepared using this method was
∼
1 nm, with subsequent gold layers being grown up to 3.5 nm. The resultant
nanoparticles could be further functionalized with an organic ligand, or the Ag
could be nucleated and grown over the gold. An alternative method describing the
growth of a gold shell on an iron oxide core utilizes Au(OOCCH
3
)
3
[68] as the gold
source, with the coating being achieved by heating the reactants at a high tem-
perature for 1.5 h. In this case, the resultant gold shell thickness was
0.7 nm.
Although less ubiquitous than silica or gold coating, a silver shell on the Fe
3
O
4
core also offers the combination of a magnetic core with a surface plasmon reso-
nance at
∼
400 nm. Several methods have involved silver encasement, including
microemulsion [88], sintering [89], and various solution-based growths on iron
oxide surfaces [69, 70, 90]. Tang
et al.
proposed the use of core-shell materials for
a biosensing application, where the Fe
3
O
4
@Ag were prepared using the heating
and sonication method originally described by Mandal
et al.
to generate core-shell
materials with varying shell thicknesses. The nanoparticles prepared in this
manner were further functionalized with carcinoembryonic antigen ( CEA ), a
common cancer protein marker, by agitation in the presence of the nanoparticles
at 4 °C. The nanoparticles produced showed absorption shifts at different stages
of their modifi cation, which simplifi ed their characterization; electrochemical
sensing could then be used to detect the specifi c presence of cancer cells.
∼
9.2.2.4
Less Common Methods of Passivation
More recently, several groups have described the formation of a QD shell grown
over a magnetic core seed. Core-shell structures of Co/CdS [71], FePt/CdS [72]
and/or FePt/CdSe [91] have each shown potential for generating bifunctional
materials for sensing applications. When a Co core was used, there was a complete
loss in saturation magnetization, yet the FePt core appeared to retain its magnetic
character following coating with CdS or CdSe [72, 91]. Since this method is still
in its infancy, there remain many synthetic challenges to creating functional
materials of this core-shell composition; however, the promise of a magnetic/fl uo-
rescent hybrid material with tunable magnetic and optical properties represents
an intriguing topic for many sensing applications.
9.3
Magnetic Nanoparticles for the Separation and Detection of Analytes
9.3.1
Chemical Separations with Functionalized Magnetic Nanoparticles
The subject here is the use of functionalized magnetic nanoparticles as sorbent
materials for chemical, biological, and heavy-metal contaminants, that enable
