The obtained red dispersions of AuNPs coordinated with 1 NHC and 2 NHC prepared by procedure P3 show a surface plasmon (SP) band at 515 nm in deionized water (H2O), and 510 and 515 nm for AuNP-3 and AuNP-4 in THF, respectively (for spectra see ESI, Figure ). Comparing the SP band of obtained AuNPs with OAm@AuNPs (Figure , B; SP band, 515 nm) in THF, no significant change in the SP band is visible despite the observed size increase (by TEM) of the gold core of NHC@AuNPs. This is attributed to the changed refractive index of the AuNP system. Due to the anchoring of a thick PEG shell, slight peak broadening is observed which is a common observation upon PEGylation of AuNPs. TEM micrographs of AuNP-3 and AuNP-4 reveal spherical particles with an average diameter (d) of ~6 nm showing ripening compared to precursor OAm@AuNPs (d, ~5 nm; Figure ). Despite the observed 20% diameter increase, obtained PEGylated AuNPs show no significant shape and dispersity changes. DLS measurements of PEG-NHC@AuNP in H2O result in a hydrodynamic diameter of ~15 nm. Comparing DLS results against TEM micrographs allows the estimation of the PEG shell thickness (~8-9 nm). The observed thickness of the PEG layer indicates almost full extension of utilized PEG chains (calcd. PEG2000 length ~12-13 nm in H2O) and a densely packed PEG brush conformation on the AuNP surface. The dense PEG coverage was further confirmed by thermogravimetric analysis showing an organic contribution of 85 wt% for AuNP-4. Measured zeta (ζ) potentials of PEG-NHC@AuNPs in the neutral range (± 10 mV) indicates successful PEG-NHC installation (for values see Table ).