Sayaka Hashimoto received her BSc in Applied Chemistry at Tokyo University of Science, Japan, in 2018. Her main research interest and work includes “Establishment of high resolution-separation technique of reversed-phase high-performance liquid chromatography (RP-HPLC)”. She has recently succeeded in separating gold-silver alloy cluster by improving the resolution of RP-HPLC.
By doping heteroatoms to thiolate-protected gold clusters, we can add more functionality to the cluster depending on the number of doped heteroatoms and the difference in doping positions. However, in the synthesis of alloy clusters, the mixture of clusters with a different number of heteroatoms is produced. Therefore, a precise separation of each cluster is required to understand their properties. In this study, we attempted to separate gold-silver alloy clusters precisely, furthermore investigated isomer change by reversed-phase high-performance liquid chromatography (RP-HPLC). Au38-nAgn(SC4H9)24 was used as a sample. This cluster was synthesized by two methods, (1) by adding [Ag(SC4H9)] complex to Au38(SC4H9)24 (metal exchange) and gold and silver ions were reduced in the presence of butanethiol in solution (co-reduction). The mixture of alloy clusters was separated by RP-HPLC using gradient program for controlling mobile phase, and core-shell type column. The peaks obtained from the chromatogram were evaluated by electrospray ionization (ESI) mass spectrometry connected to RP-HPLC directly. Figure.1 (a) shows the chromatogram of Au38-nAgn(SC4H9)24 obtained by the metal exchange. Some clear peaks were observed in chromatogram. Each peak was attributed to the cluster having a precise number of silver atoms (Figure. 1(b)). These results indicate that the mixture of Au38-nAgn(SC4H9)24 was precisely separated according to the number of silver atom. Furthermore, the shape of chromatogram of Au38-nAgn(SC4H9)24 prepared by the metal exchange changed by leaving this cluster in toluene for 6 days (Figure. 1(c)). Interestingly, the shape was similar to that of Au38-nAgn(SC4H9)24 prepared by the co-reduction (Figure. 1 (d)). These results suggest that Au38-nAgn(SC4H9)24 prepared by the metal exchange contains metastable clusters, these are transformed to the stable clusters by leaving in toluene. In conclusion, we have succeeded in the high-resolution separation of alloy clusters according to each chemical composition, and observation of isomer transformation.
Kana Yoshida has completed her BSc in 2017 in Applied Chemistry at Tokyo University of Science, Japan. Her main research interests are high resolution separation of noble metal clusters protected by hydrophilic thiolate ligands. She has presented her work many times at conferences. Furthermore, her research was published to Nanoscale (IF=7.367) which is one of the high impact factor journal in nanoscience field.
Small thiolate-protected gold clusters show size-specific physical and chemical properties, which are not observed in bulk gold. Especially, hydrophilic thiolate-protected gold clusters exhibit a good affinity for biomaterials. Therefore, much research has been conducted in this field. However, it is difficult to selectively synthesize the clusters with specific chemical composition. Therefore, to evaluate properties of hydrophilic thiolate-protected gold clusters accurately, we need to separate single cluster from the mixture of these clusters with high resolution. In this work, we report on precise separation of various hydrophilic thiolate-protected gold clusters (Aun(SR)m, SR=SG (glutathionate), NALC (N-acetyl-L-cysteine), p-MBA (p-mercaptobenzoic acid)) by hydrophilic interaction liquid chromatography (HILIC). Furthermore, we attempted to evaluate the chemical composition of each cluster by introducing LC/MS which was directly connected the chromatograph with the mass spectrometer. The mixture of clusters used in this work was synthesized by reducing gold ions in the presence of thiols. Figure 1(a) shows the UV chromatogram of Aun(SG)m.¬ Multiple peaks were observed in the chromatogram. Figure 1(b) shows the ESI-mass spectrum of each peak observed in the chromatogram. It was found that almost only one cluster was contained in each peak. These results indicate that mixture was separated with high resolution according to the chemical composition of clusters. Similar separation has also been achieved for Aun(NALC)m and Aun(p-MBA)m. Therefore, it was revealed that the use of HILIC columns is powerful tool for separating of gold clusters protected by many kinds of hydrophilic thiolates. Furthermore, Although chemical compositions observed for Aun(SG)m and Aun(NALC)m were similar each other, that for Aun(p-MBA)m was less compared with these clusters. These results indicate that chemical compositions of the Aun(SR)m vary depending on the ligand structure. In this manner, we have succeeded in revealing how difference of ligand affects synthesis of gold clusters protected by hydrophilic thiolate ligands.