[Au24Pt(TBBT)12(PDT)3] 0 (4). The exchange of thiol (RSH) ligands on Au NCs with aliphatic dithiol ligands (such as HS(CH2)xSH, where x is an integer) can lead to polymerization. This occurs because the carbon backbone of the latter is flexible and these ligands readily form strong inter-cluster Au-S or S-S bonds (such as Au-S(CH2)xS-Au or Au-S(CH2)xS-S(CH2)xS-Au). Hence, it was necessary to optimize the ratio of 1 to the reacting thiols in addition to the thiol concentration. In the optimized synthesis, 5.0 mg of 1 was dissolved in 3.4 mL of toluene, to which 486 µL of TBBTH (98%) and 14.5 µL of PDTH2 were added at 60 °C, and the mixture was stirred for 1.5 h to allow the ligand exchange reaction to proceed. The resulting product was washed with ultrapure water and a mixture of ultrapure water and methanol sequentially and extracted with dichloromethane. Finally, the products were separated by PTLC (dichloromethane:hexane = 7:13), and the top layer was scraped off and extracted with dichloromethane to obtain NC 4 (Scheme 1 and S5). 4.3. Preparation of Au24Pt NC/CB (1.0 wt% metal) An impregnation method was employed to make Au24Pt NC/CB (1.0 wt% metal). A dichloromethane solution of Au24Pt NCs was dropped on the CB and then dried. For the electrochemical measurements using Au24Pt NC/CB(1.0 wt% metal), we prepared a catalyst slurry. First, a dried powder of the catalyst (10.1 mg of Au24Pt NC/CB; 1.0 wt% metal) was dispersed in a solution of H2O (19.1 mL) and 2-propanol (6.0 mL), to which a Nafion solution (100 µL) was added. Finally, the mixture was sonicated for 60 mins in an ice bath while sealed in a reaction vial.