Biology Reference
In-Depth Information
only approximately 3% was dissociated. The rate constant for
PAH dissociation on the latex decreased with increasing pH, as
expected, and was one order of magnitude lower than the value for
dissociation of PAH in solution at the same pH. This was attributed
to the hindering effect of neighboring
−
COO
−
groups on the latex
surface.
(a) Electrochemical labels by adsorption
Thus far, most of the studies of l-b-l material loading onto latex
spheres has focused on the layered deposition of biological macro-
molecules such as hemoglobin [82], DNA [75, 76], immunoglobulin
G [83], or enzymes such as glucose oxidase [84], horseradish
peroxidase [84], urease [85], and tyrosinase [55]. The layers were
depositedtypicallyonPSlatexstabilizedbynegativesurfacegroups.
The available charge of the biological molecules, at any pH other
than the isoelectric point, meant that the deposited species could
replace one of the polyelectrolytes. To modify latex colloids the l-
b-l process is performed by adding the polyelectrolyte or biological
molecule to a colloidal suspension of the charge excess latex. After
20 min to 1 h (often under stirring), the suspension is centrifuged
down to a pellet and the solution decanted off to be replaced by
water. The colloids are then redispersed into the water by vortex
shaking. The centrifugation/water redispersion is performed twice
more. This provides the rinsing step noted previously to remove
weakly bound material. An oppositely charged material can then be
incubated with the latex in the same manner. Overall, the process is
simple and almost as reproducible as the modification of a planar
surface(allowingforapossiblesizedistributionofthelatex).Where
zeta potentials have been measured [83-85], the oscillating values
characteristic of l-b-ldeposition have been found.
To the best of our knowledge, the previous l-b-l nanoparticle
modifications of latex have all been directed at incorporation into
the walls of hollow capsules, as described below. However, if metal
nanoparticles are stabilized by a surface charge then they can
be adsorbed to appropriately modified l-b-l latex by electrostatic
attraction,asshowninFig.8.1b.Inthisfigure,thegoldnanoparticles
were produced by citrate reduction and so had a negative charge
