We have prepared 14 nm GNPs by using classic citrate synthesis method where trisodium citrate is being used as reducing and stabilising agent. Then we put 6-thioguanine (purine analogue with an S in it (C5H5N5S) at a final concentration of 0.3 mM in these particles by assuming a self assembled monolayer formation of drug onto the surface of GNPs but they immediately turned blue (aggregated).
we are wondering what happened and also what to do to make it work.