3.2 Treatment for removing surfactant/impurity from AuNP
solution
Figure 2(B) & (E) illustrated the decrease in both
AuNP diameter and number corresponding to the reduction of
surfactant/impurity by centrifugal treatment. A decreasing trend of
diameter referring to more centrifugal cycles could be seen and 2-cycle
of centrifugation (C2) was examined with the greatest reduction of AuNP
diameters, where Au-10 and Au-5 were 12.2 nm and 8.2 nm and the
difference from nominal diameter were 22 % and 64 % (Table
S1 ). No apparent change in detected diameter between C2 and C4 while C4
displayed > 10 % loss in AuNP number compared to C2
(Figure S1 ). The similar result of C2 and C4 processes were
thought as the equilibrium of surfactant in solution, which was the
basic amount for the nanoparticle dispersion.
To completely remove the surfactant, tube furnace was installed between
the aerosol generator and ES-SMPS to burn out the surfactant/impurity
from the original AuNP solution. The thermal treatment showed the
decrease AuNP size and number compared to the original AuNP at 20 ˚ C
(Figure 2 (C) & (F) ). The diameter of Au-10 was
measured as 12.2 nm at 200 ˚ C and remained constant at 500 ˚ C. In the
case of Au-5, its diameter was 10.1 nm and 9.0 nm under temperature of
200 ˚ C and 500 ˚ C, respectively. At 800 ˚C both Au-10 and Au-5 showed
a reverse growth, where Au-10 had a diameter of 13.6 nm and Au-5 was 9.3
nm (Table S1 ). The other method applying higher temperature to
burnout the surfactant in the aerosol of gold nanoparticle was found to
remove the organic components of surfactant, which resulted in the size
reduction, but the inorganic surfactant remained on the nanoparticles.
The expansion in AuNPs might be a result of reshape or recharge of AuNPs
under high-temperature treatment and the phenomenon was not discussed in
this paper.