Dominating Role of 2-Mercaptobenzothiazole as a Capping Ligand in Silver Nanocluster Synthesis

This work highlights the dominating role of the capping ligand 2-mercaptobenzothiazole (2-MBT) in the synthesis of a silver nanocluster (AgNC). AgNC was synthesized through ligand exchange (LE) in a two-phase reaction system by reacting a preformed water-soluble glutathione (GSH)-stabilized silver nanocluster (Ag-SGNC) with 2-MBT dissolved in dichloromethane. We observed the formation of a fluorescent AgNC in the organic phase from the nonfluorescent Ag-SGNC. The composition of the nanocluster was determined with the help of UV–vis spectroscopy, Fourier transform infrared spectroscopy, fluorescence spectroscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy, MALDI mass spectrometry, and thermogravimetric analysis. We recently synthesized Ag₁₆(2-MBT)₁₂ clusters by the direct reduction (DR) of AgNO₃ using sodium borohydride in the presence of 2-MBT in a one-phase reaction. Interestingly, the composition of the nanocluster synthesized through the LE reaction was determined to be exactly the same as that of Ag₁₆(2-MBT)₁₂, which was synthesized by the DR method. To our knowledge, there is no such report on the synthesis of luminescent AgNCs where the same nanocluster is obtained from two drastically different synthetic methods. In general, syntheses of metal nanoclusters are highly susceptible to changes in reaction parameters and often result in nanoclusters of variable compositions. Moreover, we justified the LE reaction and tried to find a possible mechanism through time-dependent studies. We proposed that Ag-SGNCs were etched into smaller fragments on reacting with 2-MBT. Some of these fragments where glutathione molecules were replaced by 2-MBT transferred into the organic phase from the aqueous phase. These fragments then possibly combined together to yield a thermodynamically stable Ag₁₆(2-MBT)₁₂ cluster.