A 1 mL solution of aqueous thiol-PEG-carboxyl (2000 Da, 1 mM) was added to 20 mL of 20 nm or 50 nm 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 functionalised with PEG-COOH (AuNP-PEG). The pellet was then resuspended in ultrapure water and stored at The solution was centrifuged at 13,400 rpm for 3 cycles using Milli-Q® water to remove any unbound Cyt c and Zn Porph. The obtained pellet was resuspended in 1 mL of Milli-Q® water to yield AuNP-PEG-Cyt c/Zn Porph.