Further in formation about the structure of protein on the
AuNPs surface can be extracted from the above resolution results.
The actually average projected area occupied on the AuNPs surface
by an individual HSA molecule,referred to as the molecular
footprint, can be calculated from the division of the total surface
area per AuNP by the saturated number of HSA molecules per
AuNP.Derived from the diameter (13.4nm) of the bare AuNPs
measured from TEM,the radius of AuNPs is calculated to be
6.7 nm. And the total surfacearea per AuNP can be estimated to
be about 564 nm 2 (4π6.72 nm2). Asdiscussed above,the saturated number of HSA molecules per AuNP is 11.Therefore,the
actually saturated surface area per HSA molecules on AuNP can be
calculated to be 51.3nm2. Following the equilateral triangular
prism model with sides of 8.4nm and a height of 3.15 nm for
serum albumin in solution,the theoretica surface area per HSA
molecule attached to the AuNP surface in a “flat-on” and “end-on”
conformation approximates to 30.6 nm2 (1/28.42sin(π/3) nm2)
and 26.5nm2 (3.158.4 nm2), respectively.Both theoreticalsurface area values are less than the actually saturated surface area
value,suggesting that HSA is most likely inclined to lie flat on the
AuNPs with a full monolayer coverage.In addition,according to
the method previously proposed by Nienhaus's group [50], the
theoretically saturated number, N, of the bound protein molecules
for a monolayer coverage on a spherical nanoparticle can be
estimated as follows: