Decoupling the CO-Reduction Protocol to Generate Luminescent

Jan 9, 2015 - parameters that may affect the synthesis of Au22(SR)18 were not understood .... Au22(SR)18 was formed during the CO-reduction process st...
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Decoupling the CO-Reduction Protocol to Generate Luminescent Au22(SR)18 Nanocluster Yong Yu,† Jingguo Li,† Tiankai Chen,† Yen Nee Tan,*,‡ and Jianping Xie*,† †

Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 ‡ Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 S Supporting Information *

ABSTRACT: Development of efficient synthetic strategies for highly luminescent gold nanoclusters (Au NCs) requires a good understanding of their synthesis process and environmental factors involved which could affect their tailorability. In a recent study, we reported a new type of luminescent Au NCs featuring with the aggregation-induced emission (AIE) characteristic. This AIE-type luminescent Au NC has a molecular formula of Au22(SR)18 (SR denotes thiolate ligand), and it shows strong red emission at ∼665 nm with a high quantum yield of ∼8%. However, the formation process and reaction parameters that may affect the synthesis of Au22(SR)18 were not understood because of the lack of experimental evidence. Here, we revisit the synthetic protocol, a two-step carbon monoxide (CO) reduction method, to further understand the formation process of Au22(SR)18. First, we systematically investigate several reaction conditions (e.g., the solution pH and the duration of each step) that could affect the yield of red-emitting Au22(SR)18 in the protocol. Second, we use the time-course measurements of the optical properties (UV−vis absorption and photoemission) of the reaction solution to study the formation process of Au22(SR)18, which allows us to identify several key NC intermediates and makes possible the reconstruction of the formation process of redemitting Au22(SR)18. Upon the basis of our experimental data, we propose a two-stage process for the growth of Au22(SR)18: (1) the reduction of Au(I)-thiolate complexes to form Au NCs with a narrow size distribution, which are subsequently focused to Au18(SR)14; and (2) a pH-induced aggregation of short Au(I)-thiolate complexes on Au18(SR)14, which are finally converted to Au22(SR)18. Our study suggests that the pH-induced aggregation of Au(I)-thiolate complexes on the in situ formed thiolated Au NCs could be an effective way to generate luminescent Au NCs with the AIE characteristic. This principle can also be used to synthesize other AIE-type metal NCs with strong luminescence.

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imaging and sensing applications in biological systems.28−33 Such promising applications in turn have also stimulated the recent development of efficient synthetic strategies for luminescent Au NCs in aqueous phase.34−37 For example, we recently reported the synthesis of a new type of luminescent Au NCs in aqueous solution, which showed intense orange emission at ∼610 nm with a high quantum yield (QY) of ∼15%.38 We rationalized that the luminescence of such Au NCs was originated from the Au(I)thiolate complexes depositing on the NC surface via a pathway of aggregation-induced emission (AIE). This property was also shown in the bimetallic NC system. For example, we reported an AIE-type Au@Ag NCs showing intense red emission at ∼667 nm with a QY of ∼6.8%. The red-emitting Au@Ag NCs were formed by using Ag(I) ions as a linker to bridge Au(I)-

INTRODUCTION Thiolate-protected gold nanoclusters (or thiolated Au NCs for short) are a new class of supramolecules, typically consisting of a number of ( 0.5 h) were not suitable for the formation of Au NC intermediates featuring with a desirable size distribution which could be subsequently focused to Au18(SG)14. In the second stage, the conversion of Au18(SG)14 to Au22(SG)18 was mainly assisted by the controlled aggregation of Au(I)-thiolate complexes on the Au18(SG)14 surface, which is made possible in a highly acidic reaction environment. Because GSH contains two carboxylic groups and one amine group with an isoelectric point of ∼2.5, the Au(I)-thiolate complexes would be charge neutral when pH2 is close to 2.5. Therefore, at a highly acidic pH the strong aurophilic interaction between the Au(I)-thiolate complexes may induce the aggregation of Au(I)thiolate complexes on the surface of the preformed and smaller sized Au NCs (e.g., Au18(SG)14). A slow and controlled deposition of these Au(I)-thiolate complexes finally converted Au18(SG)14 into Au22(SG)18, which was accompanied by ∼21fold increase of QY. The conversion of Au18(SG)14 into Au22(SG)18 was not only supported by the experimental evidence (e.g., time-course spectroscopic studies) but also suggested by some recent theoretical studies. In particular, the recent studies suggest that both Au18(SG)14 and Au22(SG)18 adopted a 4e− configuration according to a superatom model.49 The transformation of Au18(SG)14 to Au22(SG)18 may follow an isoelectric addition process, as suggested in a recent study.50 In addition, the recent DFT calculations have determined the most stable structure of Au18(SG)14 and Au22(SG)18, where Au18(SG)14 adopts a core-staple structure with a Au8 core and two [RS-(Au-SR)2] and two [RS-(Au-SR)3] staples,51 and Au22(SG)18 adopts a core−shell structure with a Au8 core and two [RS-(Au-SR)3] and two [RS-(Au-SR)4] staples. Their cluster structures are very similar and such similarity may further facilitate the transformation of Au18 (SG) 14 to Au22(SG)18. In addition, the DFT calculations also showed that Au22(SG)18 was more stable than Au18(SG)14,40 which

suggests that the conversion of Au18(SG)14 to Au22(SG)18 is thermodynamically favorable.

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CONCLUSIONS In summary, we have systematically studied the formation process of red-emitting Au15−22 NCs in a two-step COreduction protocol. The solution pH and the duration of each step were optimized to produce a Au NC product with the largest proportion of Au22(SG)18, which also showed the most intense red emission in aqueous solution. In addition, the timecourse UV−vis absorption and luminescence spectroscopic measurements were used to monitor the formation of redemitting Au22(SG)18 and helped indentify Au18(SG)14 as an important intermediate NC species during the formation of Au22(SG)18. A two-stage growth process was therefore proposed for Au22(SG)18. The first stage is the reduction of Au(I)-thiolate complexes to form thiolated Au NCs with a narrow size distribution that could be further focused into Au18(SG)14. The second stage is the pH-induced aggregationassisted conversion of Au18(SG)14 to Au22(SG)18 at a highly acidic reaction environment. This study is of interest not only because it provides a detailed understanding for the formation of a AIE-type luminescent Au NC, but also because it exemplifies an efficient strategy (pH-induced aggregation) to convert a nonluminescent NC into a highly luminescent one. By utilizing this strategy, it is possible to synthesize more AIEtype metal NCs with good luminescence properties (e.g., high QYs and tunable emission wavelengths), which could further advance such luminescent metal NCs for biomedical applications.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1: List of photoabsorption and photoemission peaks of the four species in Au15−22 NCs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (J.X.) [email protected]. *E-mail: (Y.N.T.) [email protected]. Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Ministry of Education, Singapore, under the Grant R-279-000-409-112. REFERENCES

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DOI: 10.1021/jp510829d J. Phys. Chem. C XXXX, XXX, XXX−XXX