Biology Reference
In-Depth Information
due to the adsorbed citrate. The particles were attached in a
manneranalogousto polyelectrolytes: adispersion ofnanoparticles
was added to the latex suspension and then incubated at room
temperature for 30 min. The modified latex was then isolated by
filtration with a membrane (pore size 0.2
μ
m) which would admit
theunattachedgoldparticles(meandiameter
=
15.5nm
±
1.6nm),
but not the latex (mean diameter
0.338
μ
m and 0.493
μ
m). We
found that the 0.493-
μ
m latex had a higher gold coverage. Latex
particles have also been modified by coating with streptavidin and
then attached to biotin-coated gold nanoparticles via the strong
avidin-biotin bond [86]. However, the method gave a nanoparticle
coverage of 1 order of magnitude less than l-b-l deposition. This
correspondedto2ordersofmagnitudelessmetalionsreleased,due
to usinggold nanoparticles of asmaller size.
=
(b) Electrochemical labels from hollow capsules
Capsule Formation:
The l-b-l based construction of hollow capsules
was developed mainly as a technique for achieving localized drug
delivery, since the capsule can protect the drug from degradation
by the body. Recent thorough reviews of capsule formation and use
are available [87-91]. As shown in Fig. 8.9, there are three general
methods of constructing the capsules: (A) loading a preformed
capsule, (B) encapsulating crystals by l-b-l assembly, and (C)
incorporation into a porous sphere which is then coated by an l-b-l
process. The construction of nanoparticle electrochemical labels
from latex is based on method (A) and so only that will be
discussed further. The preformed capsule used in (A) is made by
l-b-l deposition onto an organic core which is then dissolved, as
shown in Fig. 8.10. Typical latexes which have been used for the
core are PSS and PSA, both of which have a negative surface charge,
as explained earlier, that can be utilized for l-b-l modification. Both
cores can be dissolved by THF. Other organic cores which have
been used for this method are melamine formaldehyde (MF), which
dissolves at low pH, and polylactic acid (PLA) or polylactic-
co
-
glycolic acid (PLGA), which can be dissolved in acetone/
N
-methyl-
2-pyrrolidinone mixtures. It should be noted that in this type
of capsule formation, the incomplete removal of core material is
