Optically Active Au-Cu Bimetallic Nanoclusters: Insights Into the

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C: Physical Processes in Nanomaterials and Nanostructures

Optically Active Au-Cu Bimetallic Nanoclusters: Insights Into the Alloying Effect on the Chiroptical Behaviour Tengfei Long, Hongmei He, Jizhou Li, Mengke Yuan, Jianjia Wei, and Zhongde Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02554 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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The Journal of Physical Chemistry

Optically Active Au-Cu Bimetallic Nanoclusters: Insights Into the Alloying Effect on the Chiroptical Behaviour Tengfei Long, Hongmei He, Jizhou Li, Mengke Yuan, Jianjia Wei, and Zhongde Liu* †

Key Laboratory of Luminescent and Real-Time Analytical Chemistry (Southwest University), Ministry of Education, College of Pharmaceutical Sciences, Southwest University, Chongqing 400716, China.

ABSTRACT: Inspired by the knowledge that the chiroptical activity of Cu, Ag nanoclusters (NCs) is generally about several-fold larger than that of the corresponding Au NCs but poor stability, and alloy metal particles often demonstrate unique stability, in this contribution the chiral Au-Cu alloy NCs were attempted to synthesize via reduction of varying mole fractions of metal precursors by sodium borohydride in the presence of D/L-penicillamine (D/L-Pen), to gain insights into the alloying effect on the chiroptical activity. As a result, the g factors of ∼0.57×10−3, obviously larger than that of the pure Au NCs, is found for the bimetallic AuCu NCs, suggesting the incorporation of Cu heteroatoms plays a dramatic role for the creation of the strong optically active metal nanostructures. More importantly, compared with those of monometallic Cu counterparts, the optical active stability of bimetallic Au-Cu NCs is obviously improved due to the alloy formation. The location of the copper dopant, observed from the TEM results and deduced by the XPS measurements, can be speculated to be incorporated both in the “core” region and the “staple” region. Additionally, it was found that the different thiolated chiral molecule ligands exert influence on the chiroptical behaviour of the Au-Cu NCs. Thus, the alloy formation strategy could produce novel nanostructures with the characteristic of large and stable chiroptical activity.

1.

INTRODUCTION

Since Schaaff and Whetten did the first observation of the intense chiroptical activity in giant glutathione-gold nanoclusters (GSH-AuNCs) in 20001, chirality at the nanoscale has been extensively studied. Up to now the bulk of existing work has focused on exploration of synthesis methods and understanding of the origin of their optical activity. In recent years, significant progress was made in determining the total structure of these noble metal clusters, especially various types of thiolated chiral molecule protected AuNCs from experiment and theory. In summary, after the breakthroughs of crystallization of Au102(SR)44 and Au25(SR)18, the structures of Au38(SR)24, Au36(SR)24, and Au28(SR)20 were consecutively resolved via single-crystal X-ray diffraction2-6. Based on the resolved structure of thiolated chiral molecules capping AuNCs, a heuristic structural rule that the Aum(SR)n NCs system are composed of a metal core surrounded by oligomeric –RS-(Au-SR)x- motifs often called “staples” or “semi-rings” is now widely accepted and responsible for the observed circular dichroism (CD) activity7-9. However, compared with the explorations of the origin of the optical activity, how to obtain large and stable chiroptical metal nanoparticles or clusters nearly remains in burgeon stage and a worthwhile yet challenging undertaking, considering that they bear the intriguing applications in the fields of heterogeneous enantioselective catalysis10-12, chiral separation13-14, chiral recognition and sensing1416 , and nonlinear optics17. Thus in this study, the main objective is to obtain large and stable chiroptical nanostructure. Generally, chiroptical activity of Cu, Ag nanoclusters (NCs) is about several-fold larger than

that of the corresponding Au NCs with comparable size18. Nevertheless, relative to the Au, to obtain stable Ag NCs, especially Cu NCs remain still a significant challenge, due to the susceptibility to oxidation upon exposure to air. On the other hand, alloy metal nanostructures often demonstrate unique characteristics compared with single-component systems. However, so far most of efforts towards the development of optically active nanostructures have been focused on singlecomponent systems. Although small mixed-metal clusters have been synthesized by adulterating Pt, Pd, Ag, or Cu heteroatoms into atomically precise Au NCs to obtain enhanced chemical, catalytic and optical properties, the chiroptical behaviour of bimetallic NCs protected by chiral ligands are barely studied19-21. Considering the factors described above, as a test case for chiral bimetallic NCs synthesis, here the chiral Au-Cu alloy NCs were attempted to synthesize via reduction of varying mole fractions of chloroauric acid (HAuCl4) and cupric chloride (CuCl2) by sodium borohydride (NaBH4) in the presence of D/L-penicillamine (D/L-Pen) as shown in Scheme 1, to obtain the large and stable chiroptical alloy NCs. To gain insight into the alloying effect on the chiroptical activity, the chiroptical behaviour of bimetallic Au-Cu NCs was compared with those of monometallic Au and Cu NCs with comparable size. Interestingly, optical spectroscopy shows that chiral responses for the bimetallic NCs exhibit quite different cotton effects from both Au and Cu counterparts in metal-based electronic transition regions. The maximum anisotropy factor of ∼0.57×10−3 is typically found for the bimetallic Au-Cu NCs, obviously larger than that of the pure Au NCs, suggesting the incorporation of Cu heteroatoms plays a dramatic role for the creation of the

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strong optical activity within the alloy Au-Cu NCs. Further, the chemical stability of bimetallic Au-Cu NCs, monometallic Au and Cu NCs were investigated, proving that the as-synthesized Cu NCs are indeed poor stability, but the AuCu NCs display good stability due to the alloy formation. Additionally, it is worth mentioning that the location of the copper dopant, observed from TEM results and deduced by the XPS measurements, can be speculated to be incorporated both in the “core” region and the “staple” region. Also, with the different thiolated chiral molecules (D-Cys, L-Cys, L-GSH), the Au-Cu NCs were synthesized in similar average diameter distribution, and similar results were obtained but it was found that the different chiral ligands exert influence on the chiroptical response of the Au-Cu NCs. We believe that the alloy formation strategy described in this study would open new opportunities for producing more novel nanostructures with the characteristic of large and stable chiroptical activity.

dichroism (CD) spectrometer J-810 (JASCO, Japan) respectively. 2.3 Preparation of Au-Cu NCs Similar to the synthetic method previously reported22-24, 514 µl of 1% HAuCl4, 83 µl of 150 mM CuCl2, and 4.2 ml of 11.8 mM D/L-Pen were at first mixed in deionized water (0.4 ml), followed by the addition of a freshly prepared 0.2 M aqueous NaBH4 solution (1.25 mL) under vigorous stirring. After further stirring for 2 h in ice bath, the solution was stored overnight. Addition of ethanol into the stored solution gave a brownish black crude precipitate. The precipitate was then thoroughly washed with ethanol for 3 times. Finally, the Au-Cu NCs powder was obtained by a freeze-drying procedure. The Pen enantiomer protected monometallic Au or Cu NCs were also prepared in a similar manner. For the purpose of confirming that the orientation of CD signals is from the bimetallic NCs instead of the corresponding metal-ligand complexes, chiral coordination complexes were also prepared in a similar manner, but the procedure of the addition of a freshly prepared 0.2 M aqueous NaBH4 solution was omitted. Briefly, for D/L-Pen-AuCu complexes, 514 µl of 1% HAuCl4, 83 µl of 150 mM CuCl2, and 4.2 ml of 11.8 mM D/L-Pen were mixed in deionized water (1.49 ml), for D/LPen-Au complexes, 1028 µl of 1% HAuCl4 and 4.2 ml of 11.8 mM D/L-Pen were mixed in deionized water (1.06 ml), and for D/L-Pen-Cu complexes, 166 µl of 150 mM CuCl2 and 4.2 ml of 11.8 mM D/L-Pen were mixed in deionized water (1.96 ml). After stirring for 2 h in ice bath, the solution was stored overnight. The supernatant obtained through centrifugation is the corresponding metal-ligand complexes solution.

3. Scheme 1 Schematics for displaying a pair of optically active Au-Cu bimetallic alloy NCs and an almost perfect mirror-image CD spectrum.

2.

EXPERIMENTAL SECTION

2.1 Materials Chemicals and solvents were reagent grade and commercially available. Chloroauric acid tetrahydrate (HAuCl4.4H2O) was purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) and dissolved in water for further use. Cupric chloride dihydrate (CuCl2.2H2O) and NaBH4 were both obtained from Kelong Chemical (Chengdu, China). D-Pen was bought from Aladdin (Shanghai, China). L-Pen, D-Cys, L-Cys, and LGSH were all provided by Sigma-Aldrich (Shanghai, China). Mili-Q puriﬁed water (18.2 MΩ cm) was used throughout the experiments. All the chemicals were used without further purification. 2.2 Apparatus The powder of Au-Cu NCs was gained by CoolSafe Bench Top Freeze Dryers (Labogene, Denmark). The morphology and size distribution of Au-Cu NCs samples were observed on a high resolution transmission electron microscope (HRTEM) Tecnai G2 F20S-TWIN (FEI Company, USA). Fourier transform infrared spectroscopy (FTIR) 8400S (Shimadzu, Japan) was utilized to know about the functional groups on the surface of Au-Cu NCs. The information about the valence of the corresponding NCs was carried with X-ray photoelectron spectroscopy (XPS) Escalab 250Xi (Thermo Scientific, America). The optical properties were measured by spectrophotometer UV-3100 (Hitachi, Japan), and circular

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RESULTS AND DISCUSSION

Substituting some Au atoms with foreign atoms of other metal elements should provide exciting opportunities to modulate the electronic structure of Au nanostructures, thus tuning the optical properties of them. For example, the electronic structures of Au-Ag NPs and NCs are significantly modulated by incorporation of Ag atoms23-25. Here as another test case for Cu doping, we will focus on the chiroptical response of the Au-Cu NCs, originating from that the heteroatom doping results in either metal core or staple layer modification. As shown in Figure 1, all ligand-protected NCs have the molecule-like optical transitions and the absorbance onset is determined by linearly extrapolating the low-energy absorbance tail to the spectral baseline26. From Figure 1a, c and e, we can find the Cu NCs, pale yellow solution, are rather featureless in absorption spectra, and in comparison, between Au-Cu and Au NCs, doping of Cu heteroatoms makes the first absorption peak shift to lower energy. The products of Au-Cu NCs are brown, and their characteristic broad absorption locates at ∼361 nm, a slightly red shift compared with the ones of brownish red Au NCs (∼339 nm). To obtain information on the chiroptical activity of the Pen-protected Au-Cu, Au and Cu NCs, we measured the CD spectra of the present NCs. Then we can examine the effect of Cu doping on the chiroptical response with comparable sizes. The information for the morphology and size distribution of the Pen-protected pure Au and Cu NCs is provided by TEM measurements (Figure S1). The CD spectra for the sequential sample of the Pen-protected Au-Cu, Au and Cu NCs, are displayed in Figure 1b, d and f, respectively. Interestingly, chiroptical response for the bimetallic NCs exhibited quite different cotton effects from both the Au and the Cu counterparts in metalbased electronic transition regions: the almost symmetrical


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measurable cotton effects at ∼292, ∼334 and ∼385 nm were obviously observed between L-Pen- and D-Pen-protected Au-Cu NCs, inconformity with that of Au NCs (∼268, ∼296m and ∼363 nm) and Cu NCs (∼322 nm). These CD spectra are also considerably different from the one of free D- and L-Pen ligands, which shows CD signals at 225 nm (Figure S2a). For a better comparison, the anisotropy factors (or g factor, defined as g = ∆ε/ε, where the intensity of the molar dichroic absorption ∆ε is normalized to the extinction coefficient ε) are calculated. As shown in Figure 2a, the maximum g factor of ∼0.57×10−3 is typically found for the bimetallic Au-Cu NCs, although it is still smaller than that of the monometallic Cu NCs (Figure 2c). However, compared with the pure Au NCs (Figure 2b), the bimetallic Au-Cu alloy NCs exhibit the stronger CD activities, suggesting that the incorporation of Cu heteroatoms plays a dramatic role for the creation of the strong optical activity within the alloy Au-Cu NCs.

optically active CuNCs and Pen-Cu complex could be obtained, in which enantiomers of Pen serve as both a reducing agent and a stabilizing ligand (Figure S4a). On the contrary, when the copper (II) was present in equivalence or in excess, no fluorescence but pale-yellow solution was observed (Figure S4b), suggesting that no CuNCs were produced and only mixed valence complexes are formed28. For the purpose of confirming that the orientation of CD signals in our studies is from the bimetallic NCs instead of the metal-ligand complexes, we also measure the absorption and CD spectra of the corresponding complexes (Figure S5). Compared with the Au-Cu NCs, the corresponding precursors, metal complexes have no characteristic absorption peaks at ~361 nm (Figure S5a) and obviously display different cotton effects (~243 nm, ~273 nm, ~300 nm, ~340 nm) (Figure S5b). Both the L-Penand D-Pen-Cu complexes display the characteristic broad absorption located at ~560 nm (Figure S5e) and in the CD spectra an almost perfect mirror-image relationship was observed, in the visible region obviously exhibiting a pair of measurable cotton effect at 488 nm (Figure S5f), complete inconformity with CD activities of Au-Cu NCs. Similar phenomena also occurs to the Pen-Au complexes (Figure S5c, d). Thus, it is deemed that the observed optical activities are from the Au-Cu NCs rather than the metal-ligand complexes.

Figure 1. The absorption and CD spectra of the Pen enantiomer protected Au-Cu NCs (a, b), Au NCs (c, d), and Cu NCs (e, f). The insets (a, c, e) display the corresponding photographs under the daylight, respectively.

Figure 2. Comparison of the chiroptical behaviour among the Au-Cu (a), Au (b) and Cu NCs (c) and the dependence of the g factors on the mole ratio of the two metal precursors (d).

To maximize the chiroptical activity, the bimetallic Au-Cu NCs were synthesized with the varying initial Au/Cu feeding molar ratios. As shown in Figure 2d, upon increasing the Au/Cu feed mole ratios, it has been found that the ellipticities reach the maximum when the mole ratio between the two precursors is 1:1, and the g factors at 385 nm are found to be ∼0.57×10−3 and ∼-0.52×10−3 for both the D-Pen- and L-Penprotected Au-Cu NCs (Table S1), respectively. While the larger Cu/Au feed mole ratios (Figure S3) do not produce the bimetallic Au-Cu NCs with the stronger chiroptical activity, indicating that the composition of the Au-Cu alloy, controlled through the ratio of the precursors, was a fairly important step in gaining large chiroptical response. Note that there exist numerous ways of generating chiral coordination complexes27, for example, in the presence of excess of Pen enantiomers, the complexation and redox reaction between copper (Ⅱ) and Pen concurrently occur and intensely red-emitting

More intuitive information about the morphology, size and electron density of the alloy NCs could be provided by TEM studies. As shown in Fig. 3a and b, the Au-Cu NCs synthesized with the D-Pen ligands shows well-dispersed nature and the average diameter of them is 2.1 ± 0.3 nm (the inset in Figure 3a). For the Au and Cu nanocrystals, the interplanar spacing of the {1 1 1} diffraction plane is 0.235 and 0.207 nm29-30, respectively, and the crystal lattice pattern of the asprepared Au-Cu NCs is approximately 0.223 nm (the inset in Figure 3b), which is in good agreement with that of the AuCu NC alloys31-32. Further, observed from the TEM results, the electron density within each particle is in fact uniform, also supporting that the Au-Cu alloy NCs rather than coreshell nanostructure are actually responsible for the observed CD activity. Additionally, in order to gain insights into the influence of the chiral ligands on chiroptical responses, with the different thiolated chiral molecules (D-Cys, L-Cys, L-GSH),



the Au-Cu NCs were synthesized in similar average diameter distribution. They also display the well-dispersed nature and the average diameter is 2.1 ± 0.3, 2.2 ± 0.2, 2.2 ± 0.3 and 1.9 ± 0.3 nm for L-Pen-, D-Cys-, L-Cys- and L-GSHprotected Au-Cu NCs (Figure S6), respectively. From Fig. 3c and d, a further observation is that the CD spectra of the LGSH-Au-Cu NCs compared with that of the L-Cys-protected NCs, besides of a slight blue shift, both of them exhibit great similarity. However, all of them obviously show different cotton effects from that of the Pen-protected Au-Cu alloy NCs, which also differ from the CD scans of free D/L-Cys and L-GSH ligands (Figure S2b and c), suggesting that the different chiral ligands exert influence on the chiroptical behaviour of the as-prepared alloy NCs.

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atomic ratio of 60.9/39.1 (=1.56) for the Au-Cu NCs. Considering that X-ray can normally penetrate a depth of about several nanometers into the sample surface, however, in the present case, the sizes of the Au-Cu NCs including the Pen ligand shells are still smaller than ~3 nm, thus the obtained Au/Cu compositional ratio can be that of the whole NCs.23 The results mean that the actual Au/Cu atomic ratios of the NCs are different from that of the 1:1 initial Au/Cu feeding solution molar ratio.

Figure 4. XPS spectra of Au 4f (a), Cu 2p (b) and S 2p (c, d, e) regions for the alone D-Pen-protected Au, Cu as well as Au-Cu alloy NCs.

Figure 3. TEM images of the D-Pen-protected Au-Cu NCs (a, b) and the CD spectra of Au-Cu NCs synthesized with the different thiolated chiral molecules (c, d). The insets (in Fig. 3a, b) are the size distribution and the representative HRTEM images showing lattice fringes of the as-prepared Au-Cu NCs.

It is now well-established that the heuristic structural rule for the thiolate-protected metal NCs, composed of a metal core surrounded by the oligomeric motifs, producing a staple-like layer, is now widely accepted and responsible for the observed CD activity. Therefore, to further investigate the composition distribution or heteroatom location, and chemical information on the valence state of metals in the NCs, FTIR spectra and XPS measurements were carried out. The disappearance of the S-H stretch mode at 2511 cm-1, the existence of characteristic peaks for COO- stretch mode at 1392 and 1500 cm-1, and for N-H stretch mode at 3441 cm-1 and N-H bending mode at 1627 cm-1 suggest that the Pen molecules anchor on the surface of Au-Cu NCs through strong metal-ligand bonds28, 33 (Figure S7). The XPS wide-scan of the D-Pen-protected NCs reveals the obvious photoelectron peaks for C 1s, N 1s, O 1s, S 2p, Au 4f and Cu 2p (Figure S8), supporting the formation of the Au-Cu alloy nanostructures. The XPS spectra of Au 4f and Cu 2p regions for the DPen-protected Au-Cu NCs together with those of the D-PenAu and the D-Pen-Cu NCs are shown in Figure 4a and b, respectively. We first made the compositional ratio analysis of the Au-Cu NCs. Normalization of XPS peaks by their relative elemental sensitivity factors yields an average Au/Cu

Further, the location of the copper dopant can also be deduced by XPS. The D-Pen-Au NCs exhibit two peaks at binding energies of 88.1 and 84.3 eV (the red curve in Figure 4a) that were close to peak positions of bulk Au metal (for example, 87.6 eV for Au 4f5/234 and 84.0 eV for Au 4f7/235), but slightly shift to blue regions, which can be assigned to Au 4f5/2 and Au 4f7/2 spin states of zerovalent gold, respectively. Similar phenomenon has been observed for the DPen-Au-Cu NCs (the black curve in Figure 4a), and the positive Au 4f binding energies can be attributed to the Pen ligands withdrawing electronic effect. In the D-Pen-Cu NCs (the red curve in Figure 4b), the binding energies of 952.0 and 932.3 eV correspond to the Cu 2p3/2 and the Cu 2p1/2, respectively. It was found that there exist a very slight negative shift compared with the bulk Cu (for example, 952.5 eV for Cu 2p3/2 and 932.6 eV for Cu 2p1/236). For copper, oxidation-based changes to the lattice potential or extra-atomic relaxation energies are thought to negatively shift the Cu 2d peaks, thus the negative shift supports an inclusion of the state of Cu (I)-thiolate. Analogously, the bimetallic Au-Cu NCs display the similar Cu 2d binding energies to those of the monometallic Cu counterparts (the black curve in Figure 4b). These spectral similarities give the following implications for the NCs’ structures: (i) A positive binding energy shift for the Au 4f peaks suggests the presence of Au-thiolate species in the NCs. (ii) A negative binding energy shift for the Cu 2d peaks can be due either to the valence state of Cu atoms such as Cuthiolate formation or to electron donation from the less electronegative Cu heteroatoms (χ = 1.9) to the more electronegative Au


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atoms (χ = 2.4)23. Note that only from the Au 4f and Cu 2d XPS spectra, we cannot determine which one is the essential origin. Generally, significant information on the core/ligand-shell binding properties could be further obtained by an analysis of the S 2p binding energy (161 to 163 eV for S 2p3/2 and 163 to 164 eV for S 2p1/223) for the bimetallic NCs and their counterparts and a set of XPS spectra in the S 2p region are shown in Figure 4c, d and e. From the spectra of monometallic counterparts, the lower binding characteristics of S 2p3/2 in the D-Pen-Au NCs are evident, whereas the D-Pen-Cu NCs exhibit higher binding energies. Of importance here is the striking similarity between the spectra of D-Pen-Au-Cu NCs and D-Pen-Cu NCs rather than that of D-Pen-Au NCs. A previous work has revealed that two sulfur species with different binding energies could provide a way to determine the relative amount of two different metals on the surface of the NCs37. Hence a higher binding energy component for S 2p detected in the present bimetallic Au-Cu NCs can be attributed to the possibility of Cu-S sites or staples on the outermost surface of the NCs. In other words, we can speculate that some Cu atoms are selectively incorporated in the “staple” region. Thus, we can conclude that the surface staple region is composed of–RS-(Au-SR)x- and –RS-(Cu-SR)x- oligomer units in the bimetallic Au-Cu NCs, and considering that the average Cu doping level of Au-Cu alloy is 0.391, the center atom in the “core” region is mainly Au occupancy, while the staples are jointly occupied by Au and Cu.

Figure 5. Investigation of the stability of the chiroptical response of present NCs. (a) D-Pen-protected Au-Cu NCs; (b) D-Pen-protected Au NCs; (c) D-Pen-protected Cu NCs; (d) the variation of g factors within 8 h. The insets (in Fig.5a, b, c) display the corresponding g factors for the bimetallic Au-Cu NCs and monometallic Au and Cu counterparts.

Another focus on the as-prepared chiral bimetallic NCs is about the alloy formation exerting influence on the stability of the chiroptical response of alloy NCs. Therefore, the chemical stability of Au-Cu NCs, monometallic Au and Cu counterparts was investigated. As shown in Figure 5, although the monometallic D-Pen-Cu NCs have unique and large chiroptical activities, it is obvious that the optical activities rapidly decrease within 8 h (Figure 5c), proving the poor stability of the Cu NCs because of being easily oxidized. As for the monometallic D-Pen-Au NCs, compared with the D-Pen-protected Cu NCs, exhibiting the fairly weaker CD activities, however, the CD spectra remain nearly unchanged within 8 h and they display good stability (Figure 5b). But from Figure 5a, we can find that both the CD intensity and the stability of bimetallic Au-Cu NCs are obviously improved due

to the alloy formation. The g factors of D-Pen-Au-Cu NCs were enhanced nearly 4 times compared with those of monometallic DPen-Au NCs, and also keep constant as time goes on within 8 h. The more intuitive results, the g factors, calculated at 385, 363 and 322 nm for the bimetallic D-Pen-Au-Cu NCs, monometallic D-Pen-Au and D-Pen-Cu counterparts, respectively, reflecting the optically active stability of them, are shown in Figure 5d. The g factors of ∼-0.57 × 10−3 and ∼-0.16 × 10−3 are found for the bimetallic D-Pen-Au-Cu NCs and the D-Pen-Au NCs, respectively, and they nearly remain constant within 8 h. However, the g factors of D-Pen-Cu NCs reduce from ∼1.4 × 10−3 to ∼-0.02 × 10−3 within 8 h. In addition, similar results also occur to the bimetallic L-Pen-Au-Cu NCs, monometallic L-Pen-Au and L-Pen-Cu counterparts, respectively (Figure S9). From these results, we can safely conclude that the stable optically active nanostructures could be obtained by the alloy formation strategy.

4.

CONCLUSIONS

Inspired by the knowledge that the chiroptical activity of Cu, Ag NCs is generally about several-fold larger than that of the corresponding Au NCs but poor stability, and alloy metal particles often demonstrate unique stability, here the chiral Au-Cu alloy NCs were attempted to synthesize via reduction of varying mole fractions of chloroauric acid and cupric chloride by sodium borohydride in the presence of D/L-Pen, to gain insights into the alloying effect on the chiroptical activity. As a result, the chiroptical behaviour of bimetallic Au-Cu NCs, compared with those of monometallic Au and Cu NCs with comparable size, exhibit quite different cotton effects from both Au and Cu counterparts in metal-based electronic transition regions. The g factors of ∼0.57×10−3, obviously larger than that of the pure Au NCs, is found for the bimetallic Au-Cu NCs, suggesting the incorporation of Cu heteroatoms plays a dramatic role for the creation of the strong optical activity within the alloy Au-Cu NCs. Moreover, the optical active stability of bimetallic Au-Cu NCs is obviously improved due to the alloy formation. The location of the copper dopant, observed from the TEM and deduced by the XPS results, can be speculated to be incorporated both in the “core” region and the “staple” region. Additionally, with the different thiolated chiral molecules, the Au-Cu NCs were synthesized in similar average diameter distribution and the results show that the different chiral ligands can exert influence on the chiroptical behaviour of the alloy NCs. Thus, the alloy formation strategy could produce novel nanostructures with the characteristic of large and stable chiroptical activity.

ASSOCIATED CONTENT Supporting Information Additional information including Figure S1-S9 and Table S1, providing the morphology and size information of the Penprotected monometallic clusters and the Au-Cu NCs synthesized with different thiolated chiral molecules, the CD spectra of pure ligands, the absorption and CD spectra of mental-ligand complexes, the optical activity responses of the Au-Cu NCs to the Au/Cu feed mole ratios, the stability of the chiroptical response of the LPen-protected Au-Cu NCs and L-Pen-protected corresponding monometallic counterparts etc. as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Tel.: (+86) 23-68251910. Fax: (+86) 23-68367257. E-mail: [email protected].

Notes



The authors declare no competing financial interest.

ACKNOWLEDGMENT

17.

All authors herein greatly appreciate the financial support from the National Natural Science Foundation of China (NSFC, 21205096), the Fundamental Research for the Central Universities (XDJK2014B023).

18.

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TOC Graphic


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Scheme 1 308x244mm (96 x 96 DPI)



Figure 1 169x192mm (300 x 300 DPI)


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Figure 2 199x163mm (95 x 95 DPI)





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Optically Active Au-Cu Bimetallic Nanoclusters: Insights Into the

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