1.1.2 Synthesis
Concentrated HCl and concentrated HNO3 were combined in a 3:1 volume ratio to create aqua regia, which was used to clean all glass flasks before the synthesis.
A stock aqueous solution of 0.5 mL 1% HAuCl4 was added into a flask containing 49.5 mL Milli-Q water. The mixture was vigorously stirred (600 rpm) while being heated to 250 °C on a hotplate. Aluminium foil was utilised to cover the flask during the synthesis to prevent contamination and solvent evaporation. 0.45 mL of 1% Na₃C₆H₅O₇ was rapidly added into the HAuCl4 solution flask after it had boiled under atmospheric pressure. The mixture was continuously heated and stirred for 20 min. After the heat was turned off, it was stirred for an additional 10 min. The main variable controlled to achieve the required particle size is the molar ratio (MR) of Na₃C₆H₅O₇ to HAuCl4. The sample then was cooled naturally to room temperature and stored in the fridge at 4 °C.
Gold nanostars preparation (AuNS)
1.2.1 Reagents
1% HAuCl4, silver nitrate (AgNO3), and L-Ascorbic acid (C6H8O6), HCl and HNO3 solutions were purchased from Sigma-Aldrich (Sydney, Australia). Milli-Q water (18.2 MΩ ⋅ cm at 25 °C) was used for all aqueous solutions.
1.2.2 Synthesis
Concentrated HCl and concentrated HNO3 were combined in a 3:1 volume ratio to create aqua regia, which was used to clean all glass beakers before the synthesis.
AgNO3 and C6H8O6 solutions were freshly prepared before synthesis. Briefly, 400 µL of 1% HAuCl4 were mixed with 600 µl of Milli-Q water to make 1 mL of 10 mM HAuCl4. 10 mL of 10 mM AgNO3solution and 1 mL of 100 mM C6H8O6 solution were prepared by dissolving 17.2 mg of solid AgNO3 in 10 mL of Milli-Q water and 17.6 mg of solid C6H8O6 in 1 mL of Milli-Q water, respectively.
In a glass beaker 10 mM AgNO3 was mixed with 10 mM HAuCl4 in 1:18 ratio for 30 sec. The solution colour was slightly yellow, due to presence of yellow coloured gold salt ions. This was followed by the slow dropwise addition of 100 mM C6H8O6 with a 1:6 ratio (AgNO3: C6H8O6) and then stirring for another 30 sec, during which the colour of the solution turned blue. The synthesized particles were stored in the refrigerator at 4 °C until further use.
Gold nanoparticle characterization
1.3.1 Morphology: Transmission electron microscopy (TEM)
TEM (Philips CM10 TEM, Eindhoven, The Netherlands) was used to estimate the size and morphology of nanoparticles. TEM samples were prepared by the following method: a 10 µL drop of nanoparticle suspension was placed on top of the carbon side of the carbon-coated copper grid (Zhongjingkeyi Film Technology, Beijing, China), the drop was then removed with filter paper after it had settled for 3 minutes. This was then repeated three times, after which sample was then allowed to dry overnight.