A 1 mL solution of aqueous HS-PEG-COOH (2000 Da, 1 mM) was added to 20 mL of cit-AuNPs under stirring and left overnight. The following day the solution was washed twice using centrifugation at 13,400 rpm for 30 minutes to form a pellet of AuNP functionalized with PEG-COOH (AuNP-PEG). The pellet was then resuspended in ultrapure water and stored at 4°C until further use.