Initially, a routine TD approach using potassium bis(trimethylsilyl)amide (KHMDS) as a strong base to generate the free NHC was used. Activation of 1 was followed by the addition of the respective free NHC to OAm@AuNPs dispersed in THF, facilitating ligand exchange (procedure P1; Figure ). Using 1 as NHC precursor yielded AuNP-1 upon purification. Characterization by NMR, FT-IR, and XPS of obtained AuNPs reveals successful coordination of PEG-NHCs on the NP surface but also base-induced damage of the PEG chain. Such AuNP-1 display partial -terminal vinyl groups, suggesting base-triggered elimination of the -methoxy PEG terminus. Due to the observed polymer damage, we concluded that procedure P1 is not suitable for clickable ligand 2 containing AuNPs due to the potential loss of the -terminal azide unit. The observed incompatibility of the PEG chains to the strong base used in the exchange reactions led us to employ a milder TD approach using corresponding masked NHC precursor 1' (procedure P2; Figure ). The addition of 1' to dispersed OAm@AuNPs at room temperature (RT) yielded AuNP-2. Such AuNPs show reduced stability of the attached ligand shell, suggesting detachment of loosely bound PEG species and causing subsequent partial aggregation of AuNPs in dispersion. Detailed NMR investigations of AuNP-2 in deuterium oxide (D2O) obtained via P2 show the presence of IMZ•HCO3 1'-HCO3, NHC-CO2 1'-CO2 as well as the desired NHC 1 NHC on the AuNP surfaces (Figure ) indicating an incomplete transformation of 1' to 1 NHC during the reaction. As 1'-HCO3 is less easily activated than 1'-CO2, this transformation is dependent upon two processes: 1) the initial conversion of 1'-HCO3 to 1'-CO2, and 2) CO2 loss from 1'-CO2 to yield the unmasked 1 NHC . As the equilibrium of the IMZ•HCO3/NHC-CO2 pairs are affected by the presence of H2O, we investigated the equilibrium of masked NHCs via 1 H-NMR spectroscopy of 1' in different deuterated solvents. While dry aprotic d6-DMSO stabilizes the equilibrium between IMZ•HCO3/NHC-CO2 (ratio ~3:2 by integration of the -C6H3 group), while 1 H-NMR spectra recorded in D2O show the equilibrium pushed to IMZ•HCO3 with minimal contributions of NHC-CO2 (based on the identification of respective imidazole backbone protons C 4/5 H). Furthermore, the basicity of HCO3 -counterion in D2O leads to a rapid H/D exchange of the imidazolium proton (C 2 -H) and the corresponding disappearance of the signal in the 1 H-NMR spectra (for spectra see ESI, Figure ). These shifts in equilibrium between the NHC-CO2 and IMZ•HCO3 demonstrates why the masked NHC is not activated to form only the desired NHC species on AuNPs when used in protic or non-absolute solvents. Ligand exchange reactions in such conditions allow only the exchange of pre-installed ligands with IMZ•HCO3 and small contributions of NHC-CO2, resulting in AuNPs which do not possess the same properties as AuNPs stabilized solely by NHCs. Furthermore, this also points out the importance of carefully controlling any moisture level during the top-down fabrication of NHC@AuNPs via masked NHCs.