Through an established bottom-up protocol , NHC gold complexes 1-Au (with terminal-Ome) and 2-Au (with terminal-N3) were reduced to the corresponding AuNP-3 BU and AuNP-4 BU (Figure , A) allowing direct comparison with NPs obtained by the TD approach. Characterization of AuNP-3 BU and AuNP-4 BU by 1 H-NMR and XPS reveals a comparable composition when compared to AuNP-3 and AuNP-4, respectively (see ESI for 1 H-comparison, Figure ). Despite these comparable chemical compositions, AuNPs obtained from the BU synthesis showed rapid ripening during mild heat exposure (40 °C) as well as after drying and redispersion. Similar to reports by Crudden and coworkers , AuNP-3 BU and AuNP-4 BU obtained directly after purification by dialysis only showed a very weak and broad SP band indicating particles with a diameter < 3 nm (for UV-Vis see ESI, figure ). Subsequently, TEM micrographs obtained by drying of AuNP-3 BU and AuNP-4 BU on the TEM grid show a broad size distribution with distinct populations at ~1.5 and ~5 nm. The observed bigger NPs are attributed to digestive ripening during concentrating and mild drying (40 °C) for TEM grid preparation. Identical behaviour is observed during careful drying in vacuo followed by redispersion which can be followed by UV-Vis spectroscopy through the appearance of a corresponding SP band at ~510 nm. Exposing aq. dispersions of AuNP-3 BU and AuNP-4 BU obtained after dialysis to 40 °C for 4 h, causes further ripening indicated by a red shift of SP band to ~520 nm. In contrast, AuNP-3 and AuNP-4 obtained by the TD route P3 treated under identical conditions show no changes of the SP band.