Ultrasmall Gold Nanoclusters Inhibit Colon Cancer Cell Growth

Ultrasmall gold nanoclusters (AuNCs) have attracted considerable attention in biomedical applications owing to their precisely controlled size and favorable biological permeability. In this study, ultrasmall AuNCs Au 25 (p‐MBA) 18 and larger gold nanoparticles (AuNPs) with diameters of 5 and 12 nm, all functionalized with the same ligand, were selected to investigate size‐dependent cytotoxicity. Their biological effects were evaluated in multiple colorectal cancer (CRC) cell lines, including HCT116, SW480, and SW620. The results demonstrated that Au 25 (p‐MBA) 18 exhibited significant cytotoxicity against multiple CRC cell lines, whereas the larger AuNPs showed no comparable cytotoxic effects. To investigate the origin of this size‐dependent behavior, scanning transmission electron microscopy (STEM) was employed to examine the cellular uptake and subcellular localization of these nanomaterials in HCT116 cells. STEM analysis revealed that, after internalization by HCT116 cells, the larger AuNPs showed limited ability to further penetrate into the nucleus, whereas Au 25 (p‐MBA) 18 was capable of entering the nuclear region. These findings demonstrate the high bioactivity of ultrasmall AuNCs, highlighting their unique potential for biomedical and anticancer applications compared with larger AuNPs. The results further reveal distinct nuclear penetration capabilities associated with different‐sized gold nanomaterials, providing mechanistic insight into their size‐dependent interactions with CRC cells. Finally, this work demonstrates the value of STEM as a powerful high‐resolution technique for visualizing and characterizing ultrasmall AuNCs in complex biological environments.