Biomedical Engineering Reference
In-Depth Information
microspheres are swollen in a solution of the dyes in a nonpolar solvent. The dyes
are able to partition into the microspheres which, on being transferred to a polar
solvent, will shrink such that the dyes become entrapped. This method is used to
encode microspheres for use in the commercial suspension arrays, although the
number of codes that can be prepared is limited by the properties of the fl uorescent
dyes.
Many of the limitations imposed by fl uorescent dyes can be eliminated, however,
by using semiconductor quantum dot s ( QD s) [42 - 44] . These photoluminescent
nanoparticles have narrow, size-tunable emission spectra, and many colors can be
excited at a single wavelength far removed (
100 nm) from their emission wave-
lengths. In theory, up to eight different colors of QDs can be resolved in the
visible/near-infra-red region of the spectrum, with acceptable spectral overlap. Nie
and colleagues conferred both a spectral code and magnetic properties on porous
silica microspheres by doping them with CdSe@ZnS QDs and iron oxide nanopar-
ticles capped with hydrophobic ligands [45]. The microspheres had a diameter of
3 - 5
>
m and a mean pore size of 30 nm, and all surfaces - both internal and exter-
nal - were functionalized with hydrophobic octadecyl silane. For encoding pur-
poses, the microspheres were fi rst mixed with the nanoparticles in butanol. Then,
after allowing time for the particles to partition into the pores, the microspheres
were washed with ethanol and then rendered hydrophilic by coating them with
amphiphilic poly(acrylic acid) functionalized with octylamine. One potential
problem with this method is that iron oxide nanoparticles signifi cantly reduce the
photoluminescence intensities of the QDs because of their broad absorbance
spectrum, which interferes with both excitation and emission. Some commercial
microspheres have structures which comprise a core containing iron oxide
nanoparticles that does not swell in nonpolar solvents, but is surrounded by a shell
that does. Whitman and colleagues used solvent swelling to incorporate either
fl uorescent dyes or QDs into the shells of microspheres with this structure, and
then used these in immunoassays for
Bacillus anthracis
spores [46]. In these immu-
noassays, an applied magnetic fi eld was used to remove any microspheres bound
to the substrate by weak nonspecifi c interactions, thereby reducing the background
and increasing sensitivity.
One problem with solvent swelling as a means of encoding microspheres is that
all dyes must be incorporated at the same time, but this becomes increasingly
imprecise as the number of dyes and their concentrations are increased. Layer-by-
layer (LBL) self-assembly is a technique in which materials are assembled in
sequence by virtue of their mutual attraction [47]. As a method of encoding micro-
spheres, this imposes a high degree of control over the amount of photolumines-
cent polymers or nanoparticles that can be deposited on the microsphere cores.
Provided that the encoding elements are in excess, the amount assembled will
depend only on the surface area of the microspheres, and therefore precise control
over their concentrations is unnecessary. A number of reports have been produced
describing the LBL assembly of QDs on nonmagnetic microspheres [48 - 51] , most
of which have involved iterative cycles of assembly and washing, with the latter
generally being carried out by either centrifugal precipitation [48-51] or fi ltration
μ
