Controlled synthesis of pure Au₂₅(2-Nap)₁₈ and Au₃₆(2-Nap)₂₄ nanoclusters from 2-(diphenylphosphino)pyridine protected Au nanoclusters

Abstract

The controlled synthesis of pure Au₂₅(2-Nap)₁₈ and Au₃₆(2-Nap)₂₄ nanoclusters were achieved via etching 2-(diphenylphosphino)-pyridine protected polydispersed Au nanoclusters with a mass of 1 to 3 kDa. Au₂₅(2-Nap)₁₈ was obtained at the etching temperature of 80 °C, while Au₃₆(2-Nap)₂₄ was synthesized at the etching temperature of 50 °C. This demonstrates that the simple adjustment of the etching temperatures can realize the controlled synthesis of different atomically precise Au nanoclusters.

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In recent years Au nanoclusters have attracted much attention as new nanomaterials, which are usually protected by organic ligands and contain up to two hundred Au atoms. The structures and properties of Au nanoclusters are usually sensitive to the particle size. When the size is smaller than 2 nm, Au nanoclusters usually adopt non-FCC structure and the valence electrons become non-consecutive. These characteristics endow Au nanoclusters with the distinct physical and chemical properties. So far Au nanoclusters have been applied to different fields, such as optical properties,^1–4 photovoltaics,^5,6 catalysis^7–10 and biology.^11,12 Until now Au nanoclusters protected by thiols as a family have attracted extensive research interests, and the various thiolated sized Au nanoclusters have been reported, such as Au₁₀₂(SR)₄₄ (ref. 13) Au₂₄(SR)₂₀,¹⁴ Au₂₅(SR)₁₈,^15,16 Au₃₈(SR)₂₄,¹⁷ Au₁₃₀(SR)₅₀,¹⁸ etc.

With respect to the synthetic method of atomically precise Au nanoclusters,^19,20 “size focusing” and “ligand exchange” are the common methods. “Size focusing” process usually consists two steps:²¹ (1) synthesis of polydispersed Au nanoclusters with the proper size distribution; (2) etching of polydispersed Au nanoclusters to atomically precise Au nanoclusters under harsh conditions (excess thiols or high temperature). So far different Au nanoclusters have been successfully prepared by “Size focusing”, such as Au₂₅(SR)₁₈,^16,22 Au₃₈(SR)₂₄,¹⁷ and Au₉₉(SR)₄₂.²³ “Ligands exchange” method also usually requires two steps: (1) Au nanoclusters are prepared by virtue of suitable organic ligands; (2) a new organic ligand is added to replace the ligands adsorbed on the parent Au nanoclusters, and then parent Au nanoclusters are converted or decomposed to form new atomically precise Au nanoclusters. Chen et al. reported the synthesis of Au₃₆(SR)₂₄ nanoclusters using “ligand exchange” to etch Au₃₈(SR)₂₄ in the present of excess thiols.²⁴ Interestingly, some Au nanoclusters such as Au₁₃₃(SR)₅₀ could be synthesized from Au₁₄₄(SR)₆₀ via both “size focusing” method and “ligand exchange” method.²⁵ Previous research reported that Au₂₅(SR)₁₈ and Au₃₈(SR)₂₄ could be obtained at room temperature and 80 °C, respectively, from thiols protected polydispersed clusters via “size focusing”.²⁶ Although the significant advances have been achieved regarding the synthesis of Au nanoclusters, the synthesis of atomically precise Au nanoclusters with high efficiency and high yield is still a challenge.

As reported previously, the ligands played an important role in synthesizing gold nanoclusters, and the bulky ligands tended toward the synthesis of gold nanoclusters of different sizes and different crystal structures.²⁷ In this work, we tried to use 2-Nap as ligand to synthesize pure Au₂₅(2-Nap)₁₈ (2-Nap = 2-naphthalenethiol) and Au₃₆(2-Nap)₂₄ from 2-(diphenylphosphino)pyridine (PPh₂Py) protected crude Au nanoclusters via “size focusing” and “ligand exchange” methods. Firstly, polydispersed Au nanoclusters with the mass of 1 kDa to 3 kDa was prepared as the precursors. Then, the precursors was etched at different temperatures to obtain Au₂₅(2-Nap)₁₈ and Au₃₆(2-Nap)₂₄, respectively. In addition, Au nanoclusters protected by different phosphine ligands were synthesized as the precursors to investigate the effect of precursors on the size of Au nanoclusters.

As shown in Scheme 1, Au³⁺ salt was firstly reduced to [Au(I)-PPh₂Py] complex via the reaction of HAuCl₄ with PPh₂Py in acetone solvent.²⁸ Then, [Au(I)-PPh₂Py] complex was reduced to Au(0) polydispersed nanoclusters with excess NaBH₄. Fig. 1a showed that the polydispersed Au nanoclusters ranged from 1 kDa to 3 kDa without peaks > 3 kDa. The UV-Vis absorption in Fig. 1b showed the spectrum was the decay curve from 350 nm to 800 nm, and the plasma peak around 520 nm was not observed, which was usually attributed to the characteristic adsorption peak of Au nanoparticles larger than 2 nm. Therefore, the prepared product was polydispersed Au nanoclusters, which were smaller than 2 nm and had a relative wide size distribution.

Scheme 1 The synthesis procedure of atomically precise Au nanoclusters.

Fig. 1 The MALDI-mass (a) and UV-Vis absorption spectrum (b) of PPh₂Py protected polydispersed Au nanoclusters.

In this work, both “size focusing” and “ligand exchange” methods were used to synthesize pure Au nanoclusters. To prepare Au₂₅(2-Nap)₁₈ nanoclusters, the polydispersed nanoclusters with the mass of 1 kDa to 3 kDa were etched at 80 °C for 24 hours with excess 2-naphthalenethiol. MALDI-MS characterization indicated that after etching at 80 °C for 4 hours the polydispersed Au nanoclusters was transformed to bigger clusters with the wide distribution of mass of 2 kDa to 10 kDa (Fig. 2a). With the extension of reaction time to 10 hours and 19 hours, the mass distribution of Au nanoclusters became much narrower and mainly followed in the range of 6 kDa to 10 kDa. Interestingly, after reaction for 24 hours, only a single peak appearing at 7750 Da was observed, which should be attributed to Au₂₅(2-Nap)₁₈ clusters. This means that PPh₂Py protected Au nanoclusters have been successfully converted to Au₂₅(2-Nap)₁₈ clusters during “size focusing” process at 80 °C.

Fig. 2 The time-dependent MALDI-MS (a) and UV-Vis absorption spectra (b) of PPh₂Py protected Au nanoclusters during the etching process at 80 °C.

To further investigate the transformation process of PPh₂Py protected Au nanoclusters to Au₂₅(2-Nap)₁₈, the solution was monitored with UV-Vis spectrometry at different reaction time (Fig. 2b). After reaction for 4 hours at 80 °C, UV-Vis spectrum was still featureless, which indicated the product was still comprised of different sized Au nanoclusters. With the extension of reaction time to 10 hours and 19 hours, two weak peaks at 460 nm and ∼710 nm appeared, which should be attributed to Au₂₅(2-Nap)₁₈ nanoclusters.¹⁶ This indicated that some of crude Au nanoclusters have been converted to Au₂₅(2-Nap)₁₈ nanoclusters. When the reaction time was further prolonged to 24 hours, both peaks at 460 nm and ∼710 nm became very clear. Based on MALDI-MS and UV-Vis spectroscopy characterizations, it could be concluded that after etching at 80 °C for 24 hours, polydispersed Au nanoclusters have been successfully converted to Au₂₅(2-Nap)₁₈ nanoclusters.

The crude Au₂₅(2-Nap)₁₈ in Fig. 2a (reaction time was 24 hours) was further purified by the extraction with dichloromethane for twice. The MALDI-MS spectrum of purified Au₂₅(2-Nap)₁₈ in Fig. 3a showed only one peak at ∼7775 Da, which should be assigned to Au₂₅(2-Nap)₁₈ (theoretical value 7790.3 Da). UV-Vis spectra of purified Au₂₅(2-Nap)₁₈ in Fig. 3b showed two stepwise shoulder peaks at 460 nm and ∼710 nm, respectively. It should be noted that the peak at ∼710 nm of Au₂₅(2-Nap)₁₈ clusters occurred the red shift in comparison to that of Au₂₅(SC₂H₄Ph)₁₈. This red shift might be caused by the use of 2-naphthalenethiol as the ligand instead of PhC₂H₄SH.²⁹ Based on MALDI-MS and UV-Vis spectroscopy characterization in Fig. 3, it could be induced that pure Au₂₅(2-Nap)₁₈ nanoclusters have been obtained by the purification of crude products with dichloromethane extraction.

Fig. 3 MALDI-MS (a) and UV-Vis adsorption spectrum (b) of the purified Au₂₅(2-Nap)₁₈ nanoclusters.

It has been reported that Au₃₆(SR)₂₄ could be prepared from Au₃₈(SC₂H₄Ph)₂₄ via “size focusing” and “ligand exchange”.^29,30 In this work, we tried to synthesize Au₃₆(2-Nap)₂₄ nanoclusters through “size focusing” method with polydispersed Au nanoclusters as the precursors instead of Au₃₈(SC₂H₄Ph)₂₄. During the experiments, it was found that the etching temperature played an important role for the synthesis of atomically precise Au nanoclusters. As described above, at the etching temperature of 80 °C, Au₂₅(2-Nap)₁₈ nanoclusters were synthesized. When the etching temperature was lowered to 60 °C, the product was still Au₂₅(2-Nap)₁₈ (Fig. S1†). However, if the etching temperature was further lowered to 50 °C, Au₃₆(2-Nap)₂₄ nanoclusters instead of Au₂₅(2-Nap)₁₈ nanoclusters were successfully synthesized. In this case, the reaction time was also very important to the preparation of Au₃₆(2-Nap)₂₄ nanoclusters. To investigate the effect of reaction time on the formation process of Au₃₆(2-Nap)₂₄ nanoclusters, MALDI-MS characterization was used to test the mass of Au nanoclusters formed after different reaction time (Fig. 4a). The mass of Au nanoclusters precursors ranged from 1 kDa to 3 kDa in mass spectra (Fig. 1a). After reaction for 4 hours and even 10 hours the mass spectrum still only displayed a broad peak from 4 kDa to 8 kDa, which demonstrated that the polydispersed Au nanoclusters grew larger with the reaction time. After reaction for 24 hours the mass of polydispersed nanoclusters continued to increase to 8–12 kD and 14–19 kDa, respectively. Interestingly, when the reaction time was further prolonged to 72 hours, the mass spectra of Au nanoclusters displayed two major peaks at ∼10 453 Da and ∼9342 Da, respectively.

Fig. 4 The time-dependent of MALDI-MS (a) and UV-Vis absorption spectra (b) of PPh₂Py protected Au nanoclusters during the etching process at 50 °C.

To further detect the evolution of PPh₂Py protected Au nanoclusters during the etching process at 50 °C, the solution was monitored with UV-Vis spectrometry after different reaction time (Fig. 4b). UV-Vis spectrum of Au nanoclusters precursors displayed a decay curve, which demonstrated the mixture contained different sized Au nanoclusters (Fig. 1b). After reaction for 4 hours, UV-Vis spectrum showed a weak peak at 450 nm. After reaction 24 hour or 72 hour, the sample showed distinct peaks at 440 nm and 580 nm, respectively.

The crude product of Au nanoclusters etched at 50 °C for 72 hours were further purified by extraction with CH₂Cl₂ and then characterized by MALDI-MS and UV-Vis spectroscopy (Fig. 5). MALDI-MS spectrum in Fig. 5a displayed two major peaks at 10 453 Da and 9342 Da. The mass of 10 453 Da was attributed to Au₃₅(2-Nap)₂₂S₂ nanoclusters. The mass of 9342 Da was attributed to the fragment of Au₃₆(2-Nap)₂₄, losing Au₃SR₆ caused by the destructive MALDI treatment. The mass of 10 890 Da was Au₃₆(2-Nap)₂₄ nanoclusters (theoretical value 10 911.9 Da). UV-Vis spectrum in Fig. 5b gave two distinct adsorption peaks at ∼450 nm and 580 nm which were characteristic of Au₃₆(2-Nap)₂₄ clusters.²⁹ Therefore, both MALDI-MS and UV-Vis spectroscopy characterizations indicated that Au₃₆(2-Nap)₂₄ nanoclusters were pure.

Fig. 5 MALDI-MS (a) and UV-Vis adsorption spectrum (b) of pure Au₃₆(2-Nap)₂₄ nanoclusters.

Taken together, the reaction temperature played an important role for the synthesis of Au₂₅ and Au₃₆ nanocluster by affecting the mass distribution of Au nanocluster precursors during the etching process. In the case of Au₂₅, the mass of Au nanoclusters precursors was 3 kDa to 10 kDa after reaction at 80 °C for 4 hours. As for Au₃₆, the mass of Au nanoclusters precursors was 8–12 kDa and 14–19 kDa after reaction at 50 °C for 24 hours. These Au nanoclusters precursors would be converted to much more stable Au₂₅ and Au₃₆ after etching at 80 °C for 24 hours and at 50 °C for 72 hours, respectively.

We also investigated the effect of different phosphine capped Au nanoclusters on the synthesis of atomically precise Au nanoclusters. Triphenylphosphine (PPh₃) was used to replace 2-(diphenylphosphino)pyridine as the ligands to synthesize Au nanoclusters precursors. In previous study, Au nanoclusters precursors with PPh₃ ligands were used to prepare Au₂₅(SPhC₂H₄)₈(PPh₃)₁₀ nanorod at room temperature.³¹ In this work, UV-Vis spectra in Fig. 6 showed that when the PPh₃ protected Au nanoclusters were used as the precursors, no Au₂₅(2-Nap)₁₈ was formed at the etching temperature of 50 °C and 80 °C, respectively. However, it was interesting that Au₃₆(2-Nap)₂₄ could be formed as major products in the “size focusing” process if the reaction was carried out at 50 °C and lasted for 8 hours or shorter (Fig. 6a). If the reaction time was extended to 20 hours, Au₃₆(2-Nap)₂₄ would decompose to form Au complex (Fig. 6a). This indicated Au₃₆(2-Nap)₂₄ were not stable during the etching process. Thus, it was clear that the phosphine ligands used to synthesize Au nanoclusters precursors would remarkably influence the size and stability of atomically precise Au nanoclusters during the etching process.

Fig. 6 The time-dependent UV-Vis adsorption spectra of PPh₃ capped Au nanoclusters etched at 50 °C (a) and 80 °C (b).

Conclusions

In summary, this work demonstrated the process of synthesis of Au₂₅(2-Nap)₁₈ and Au₃₆(2-Nap)₂₄ clusters from the same 2-(diphenylphosphino)pyridine protected Au nanoclusters precursors by “size focusing” and “ligand exchange” methods. On one hand, the Au nanoclusters precursors could be converted to Au₂₅(2-Nap)₁₈ when the etching temperature reached up to 80 °C. On the other hand, Au₃₆(2-Nap)₂₄ was obtained from the Au nanoclusters when the etching process was performed at relatively lower temperature of 50 °C. As well as the etching temperature, the reaction time also displayed an important role for the synthesis of atomically precise Au nanoclusters. The extension of reaction time was beneficial to the formation of Au₂₅(2-Nap)₁₈ and Au₃₆(2-Nap)₂₄.

Acknowledgements

This work was financially supported by the Young Thousand Talents Program of China, the “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDA09030103), and National Natural Science Foundation of China (No. 21473186 and No. 21601178).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22216a

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