An aqueous solution (1 mL) of HAuCl4 A total of 100 µL of HAuCl4•3H2O (1% W/V) was added to a scintillation vial containing 9.488 mL of Milli-Q® water, before the addition of 20 nm AuNPs (290 µL) which was then set to stir at room temperature for 10 minutes. 22 µL of Sodium citrate tribasic dihydrate (1% W/V) was then added to the stirring solution and immediately followed by the addition of 100 µL of hydroquinone (30 mM). The solution rapidly changed colour and was mixed for a further 10 minutes. The obtained coloured solution was filtered using 0.22 µm filter and stored at 4 °C until further use.