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 Absorption spectroscopy was utilized to investigate
the conjugation of amyloid b protein solution (Ab1-40) and chicken egg
albumin (ovalbumin) with various sizes of gold
colloidal nanoparticles under various pHs, ranging
from pH 2 to pH 10. The pH value that
indicates the colour change, pHo,
exhibited colloidal size dependence for both Ab1-40 and ovalbumin-coated
particles. In particular, Ab1-40-coated gold
colloidal particles exhibited non-continuous size dependence peaking at 40
and 80 nm, implying that their corresponding cage-like structures provide
efficient net-charge cancellation at these core sizes.
Remarkably, only the pHo value for ovalbumin-coated
80 nm gold colloid was pH>7, and a specific cage-like structure is
speculated to have a positive net charge facing outward when ovalbumin self-assembles over this particular gold
colloid. The previously reported
reversible colour change between pH  4 and 10 took
place only with Ab1-40–coated 20 nm
gold colloids; this was also explored with ovalbumin-coated
gold colloids. Interestingly, all
tested sizes of gold colloidal nanoparticles showed
a quasi-reversible colour change when they were coated with ovalbumin. The ovalbumin coated gold colloid was found to maintain
reversible properties longer than Ab1-40–coated gold colloid.
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