2.1 AuNP suspension preparation
The commercially available AuNPs with diameter of 10 nm (NIST-8011) and 5 nm (nanoComposix) were combined with electrolyte, which was 50 mM ammonium acetate (AmAc) in deionized water, at a volume ratio of 3:2 and had the final AmAc concentration of 20 mM.
Centrifugal sedimentation was applied on AuNP solution to reduce the amount of surfactant in original solution. 400 μl of the original AuNP solution was centrifuged at 14,000 rpm for 15 minutes and AuNPs were separated from the solvent by protein concentrator. 400 μl deionized water were then added and pipetted for AuNP resuspension. To perform two or more centrifugal cycles, AuNP suspensions were experienced another centrifugal process and solvent replacement. The centrifugal treated AuNP (in 400 μl deionized water) were mixed with 266 μl 50 mM AmAc solution and the final AuNP suspensions with 20 mM AmAc were prepared for ES-SMPS size scanning.
The thermal treatment was performed using a tube furnace installed between aerosol generator and ES-SMPS for the burnout of the surfactant in AuNP aerosols. 300 μl of the original AuNP solution was mixed with 200 μl 50 mM AmAc solution to form the AuNP suspensions with 20 mM AmAc. Four temperatures 20 ˚C, 200 ˚C, 500 ˚C, 800 ˚C were performed in gas condition.