Unlike AuNP-3 that shows no or limited SP shift under all conditions, AuNP-4 only show comparably retained stability in H2O, 1×PBS, at pH 2/12, H2O2, and GHS tests, but decreased stability in NaCl (0.6 M), FBS and against PEG-SH, as indicated by the red-shifts and broadening of the SP band (for UV-Vis spectra see ESI; Figure -31). To further confirm the stability of AuNP-3 and azide-terminated AuNP-4 under certain conditions over the period of 7 days, TEM micrographs were taken after the exposure to 1×PBS, H2O2, and PEG-SH. TEM images confirm the retention of size and shape in the case of 1×PBS for AuNP-3 and AuNP-4 (d, ~6.0 and 6.1 nm, respectively). In the case of oxidative conditions or the presence of excess PEG-SH, AuNP-3 and AuNP-4 show ripening and morphology changes as indicated by previous UV-Vis studies. The size increase observed by TEM correlates well with the observed red shift of the respective SP bands (for TEM micrographs see ESI; Figure and). To verify that during stability studies no ligand exchange occurs, both AuNP-3 and AuNP-4 were exposed to H2O2 (1 M) and GSH (3 mM) and monitored by 1 H-NMR, showing, the resilience of surface bond PEG-NHCs despite observed shifts of the SP band and morphology changes by TEM (see ESI for 1 H-NMR, Figure -29, S32-33).