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.