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Ultrastable Gold Nanorod Assembly@Silica Core-shell Nanostructures with Enhanced Chiroptical Properties Bing Han, Lin Shi, Xiaoqing Gao, Jun Guo, Ke Hou, Yonglong Zheng, Zhiyong Tang( ) Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-015-0926-4 http://www.thenanoresearch.com on Oct. 24, 2015 © Tsinghua University Press 2015

Just Accepted

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Nano Research DOI 10.1007/s12274-015-0926-4

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TABLE OF CONTENTS (TOC)

Ultrastable Gold Nanorod Assembly@Silica

Core-shell Nanostructures with Enhanced

Chiroptical Properties

Bing Han1, Lin Shi1, Xiaoqing Gao1, Jun Guo1, Ke

Hou1, Yonglong Zheng1, Zhiyong Tang1*

1 CAS Key Laboratory of Nanosystem and Hierarchical

Fabrication, National Center for Nanoscience and

Technology, Beijing 100190, P. R. China

By combining the advantage of optical active side-by-side gold

nanorod assembly and mesoporous silica, chiral core-shell

nanostructures exhibit highly tunable and ultrastable plasmonic

circular dichroism response.

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Ultrastable Gold Nanorod Assembly@Silica Core-shell Nanostructures with Enhanced Chiroptical Properties

Bing Han1, Lin Shi1, Xiaoqing Gao1, Jun Guo1, Ke Hou1, Yonglong Zheng1, Zhiyong Tang1( )

1 Laboratory of Nanomaterials, National Center for Nanoscience and Technology, Beijing 100190, P. R. China Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011

ABSTRACT Chiral nano-assemblies with amplified optical activity have attracted particular interest for their potential application in photonics, sensing and catalysis. Yet it still remains a great challenge to realize their real applications due to the instability of these assembled nanostructures. Herein, we demonstrate a facile and efficient method to fabricate ultrastable chiral nanostructures with strong chiroptical properties. In these novel chiral nanostructures, side-by-side assembly of chiral cysteine modified gold nanorods serves as core while mesoporous silica acts as shell. The chiral core-shell nanostructures exhibit evident plasmonic circular dichroism (CD) response originating from the chiral core. Impressively, such plasmonic CD signals can be easily manipulated by changing the number as well as the aspect ratio of Au nanorods in the assemblies located at the core. Also thanks to the stabilization effect of silica shells, the chiroptical performance of these core-shell nanostructures is significantly improved in different chemical environment.

KEYWORDS

chiral core-shell nanostructure, plasmonic circular dichroism, gold nanorods, self-assembly, high stability

Nano Res DOI (automatically inserted by the publisher) Research Article

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Optical activity from molecular level to nanometer scale is of great interest due to its considerable scientific and technological importance [1-4]. Tremendous progress in synthetic knowledge and skill provides the opportunity to achieve well-defined nanomaterials with unique optical properties [5-8]. As an example, conjugation of noble metal nanoparticles (NPs) with chiral molecules has become the simplest way to achieve chiroptical activity in nanostructures, which are explored by circular dichroism (CD) spectroscopy

[9-11]. The new CD signal located at the frequency of surface plasmonic resonance (SPR) band of the noble metal NPs is termed as plasmonic CD, which arises from coupling between nonchiral dipoles of the noble metal NPs and the chiral dipoles of organic molecules [12, 13]. Due to the large viability of the SPR feature of metal NPs, plasmonic CD response has been manipulated in the whole visible range [14]. Unfortunately, the optical activity of individual noble metal NPs capped by chiral molecules is generally very weak [10, 13]. In order to realize its practical applications, much effort has been made to achieve advanced nanostructures with amplified optical activity [14-16], and two strategies have been successfully adopted. The first method is template assisted organization of noble metal NPs with chiral arrangement and the resultant plasmonic CD response may be largely enhanced through long-range plasmon-plasmon interaction between NPs [16, 18-21]. Another way to obtain the improved chiroptical properties is fabrication of nano-assemblies with strong near-field coupling by taking advantage of the amplified electromagnetic field in the hot spot between the noble metal NPs [22, 23]. Note that in both strategies, the plasmonic CD enhancement can be tuned by modulating the conformation of anisotropic building blocks like gold nanorods (GNRs) [24, 25].

However, in the above two methods, there exist obvious disadvantages that significantly restrict the further application of chiral metal nanostructures. As for the noble metal NPs with chiral arrangement, DNA origami is often used as the template to

realize precise control of the structure [15, 16, 18]. The heavy workload and high cost related to the nanostructure fabrication make it difficult for real application. On the contrary, though being convenient and cost effective, the structure of NP assemblies is rather dynamic and uncontrolled, making it hard to get precise and steady chiroptical property [14, 17]. What’s more, precipitation of the chiral nanostructures results in complete loss of the plasmonic CD response. Chiral core-shell nanostructures may be an efficient way to improve

the stability of the chiropitical properties. However, the previously-reported chiral core-shell nanostructures exhibit very weak plasmonic CD response due to the single particle core structure [26-28]. Therefore, it is imperative to develop simple but efficient methods for construction of chiral metal nanostructures with enhanced and stable chiroptical properties.

Herein, we report a facile method to fabricate ultrastable chiral metal nanostructures, which consist of side-by-side (SS) GNR assembly core and mesoporous silica shell. Chiral cysteine (CYS) is used to modify the GNRs and induce the plasmonic CD response of the nanostructure. Notably, by controlling the geometry of GNR building blocks as well as their assembly extent, the plasmonic CD wavelength and intensity of the L-CYS-GNR assembly@silica core-shell nanostructures are manipulated in the visible range. It deserves to be noticed that compared with uncoated GNR assemblies that precipitate in a few days, the stability of L-CYS-GNR assembly@silica core-shell nanostructures may be prolonged to several months. Thus, this novel type of core−shell nanostructures, in combination of enhanced plasmonic CD response of GNR assembly core and the protection effect of silica shell, hold great promise for various applications in photonics, sensing, enantioselective separation and biological diagnosis.

The GNRs were prepared followed by the previously-reported seed-mediated growth method [29]. After purification, the GNRs with aspect ratio (AR) of ~2.4 were dispersed in 1 mM cetyltrimethylammonium bromide (CTAB) solution,

———————————— Address correspondence to Zhiyong Tang, zytang@nanoctr.cn

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which endowed the positive charge nature of GNRs in solution. L-CYS was then added to be grafted onto GNR surfaces through Au-S bond. Afterward, negative sodium citrate (Cit) molecules were used to induce the SS assembly of GNRs due to maximization of the electrostatic attraction. Finally, the SS GNR assemblies were coated with mesoporous silica via the surfactant-templating sol-gel reaction of tetraethoxysilane (TEOS) (see detailed information in the Electronic Supplementary Material (ESM)) [30]. In our system, the cationic CTAB molecules act as templates and the silica shell forms along with the assembly process of GNRs. Figure S1 manifests the growth process of the silica shell on GNR assembly in mixed ethanol/water solution. After 1 h, thin silica layer can be discerned. The subsequent growth of silica shell helps stop the assembly process of GNRs. The accurate control over the concentration of CTAB and Cit would balance the assembly rate of GNRs and the hydrolysis speed of TEOS. When the concentration of CTAB is lowered than 500 μM, the speed of GNR assembly is much faster than that of TEOS hydrolysis, so the formed silica shell is not thick enough to stop the assembly process of GNRs, which easily results in precipitation of the product. The core-shell nanostructures of high yield are obtained when the concentration of CTAB is fixed at 1 mM. The morphologies of as-prepared chiral core-shell nanostructures are observed by transmission electron microscopy (TEM) imaging (Fig. 1). As shown in Fig. 1(a)-(d), the obtained core-shell nanostructures are highly dispersed. The silica shell is very uniform and its thickness is about 48 nm. The number of GNRs in each silica shell may be further controlled by changing the concentration of Cit, revealing that silica is able to freeze the GNRs of varied assembly patterns very well. The corresponding statistic analysis (Fig. 1(e)) shows that the number of GNRs per single core-shell nanostructure increases with Cit concentration. It should be pointed out that when the extent of GNR assembly reaches large enough (Fig. 1(d)), the length distribution of the string-like GNR assembly becomes very broad, so the detailed statistics data can’t be accurately given. In addition, the shell thickness can be also tuned by changing the concentration of CTAB and TEOS (see Fig. S2 and

S3 in the ESM), which largely benefits the application of the GNR assembly@silica core-shell nanostructures.

Besides the TEM imaging, UV-Vis spectra are used to identify the structure change of the GNR assembly@silica core-shell nanostructures. As shown in Fig. 2(a), the separated GNRs display a longitudinal SPR (LSPR) band at 673 nm and a transverse SPR (TSPR) band at 518 nm. Addition of Cit induces the blue shift and intensity reduction of the longitudinal band along with the red shift of the transverse band, which is the typical feature corresponding to SS GNR assembly [31]. With increase of the concentration of Cit (from 55 μM to 100 μM), the LSPR peaks exhibit continuous blue shift, indicating the larger extent of GNR assembly in the silica shell. This spectroscopy result is consistent with TEM observation (Figure 1).

After successful preparation of the chiral GNR assembly@silica core-shell nanostructures, the next question that we should answer is whether the coating of silica shell affects the optical activity of L-CYS-GNR assembly. Hence, CD spectroscopy was used to monitor the chiroptical properties of the L-CYS-GNR assembly@silica core-shell nanostructures. As shown in Fig. 2(a), the dispersed achiral GNRs have negligible CD signal (black curve), while only very weak plasmonic CD response is observed in the L-CYS modified single GNR@silica nanostructures (see Fig. S4 and S5 in the ESM, L-CYS modified single GNR@silica nanostructures are synthesized without addition of Cit). As comparison, the L-CYS-GNR assembly@silica core-shell nanostructures exhibit obvious plasmonic CD signals near the frequency of SPR bands, which consists of one peak at 480−540 nm corresponding to TSPR absorption and a bisignate peak in the region of 550−900 nm arising from LSPR absorption. The strong bisignated CD peaks corresponding to LSPR absorption are related to splitting of the excited-state levels via assembly, which has been clarified in previous studies [14, 23]. These plasmonic CD peaks are generated by coupling between the chiral dipole of L-CYS and dipole of the GNRs upon formation of the L-CYS-GNR assemblies [24]. When the assembly extent of GNRs increases with Cit concentration

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(from 55 μM to 100 μM), the plasmonic CD response of the core-shell nanostructures becomes stronger. The anisotropic factor (g factor) of the negative CD peak near the LSPR absorption is used for comparison of chiroptical response (see Table S1 and followings in the ESM for calculation details of g factor). As displayed in Table S1, the g factor of the obtained product under 100 μM (~5.37×10-4) is ~9 times larger than that of the chiral core-shell nanostructures obtained under 55 μM (~0.56×10-4). Such assembly dependent plasmonic CD enhancement is expected, which have been observed in several previous studies [17, 24, 25]. The above experimental results demonstrate that the chiroptical properties of L-CYS-GNR assemblies are well preserved after coated with silica shell.

Another question is how to manipulate and further enhance the chiroptical properties of the chiral GNR assembly@silica core-shell nanostructures. Since the plasmonic CD signals of the product arise from the CYS-GNR assembly core, changing the building blocks (CYS and GNR) would be the most efficient way. When D-CYS molecules are used to modify GNR surfaces, D-CYS–GNR assembly@silica core-shell nanostructures exhibit characteristic plasmonic CD response, which is in mirror symmetry with that of the L-CYS modified product (Fig. S6(a) in the ESM). Additionally, similar Cit concentration dependent core structure manipulation and plasmonic CD modulation are implemented by using GNRs with AR of ~2.8 and 3.3 as building blocks (Fig. 3, Fig. S7-S8). Evidently, under 100 μM of Cit, when the AR of GNRs increases from 2.4 to 2.8 and 3.3, the g factor of the core-shell nanostructures is improved ~3 and ~5 times, respectively (see Table S1 in the ESM). The above experimental evidences reveal that GNR assembly with both higher assembly extent and larger AR is desired to achieve chiral core-shell nanostructures with stronger chiroptical performance. Such CD response enhancement might be explained by two reasons. The first one is formation of chiral arrangement in SS GNR assemblies, which causes additional structural chirality [16, 32]. However, this possibility can be excluded by several experimental evidences. When racemic DL-CYS and achiral 3-mercaptopropionic

acid (MPA) molecules are used as modifiers (see Fig. S9(a) and (b) in the ESM), no plasmonic CD signal is observed in the product. Furthermore, GNR assemblies in the cores possess morphologies including ladders or bundles, but no fixed chiral configurations are distinguished (Fig. S6(c) and (d), S9(c) and (d) in the ESM). These experimental results indicate that the plasmonic CD enhancement with increasing AR of GNRs does not come from the structural chirality. Another likely reason is improvement of electromagnetic field from adjacent noble metal NPs. As proven by our previous study [24], the plasmonic CD of GNR assembly is inversely proportional to the cube of dipole−dipole distance between neighboring building blocks. In this system, GNRs are assembled into SS mode, so the dipole-dipole distance is determined by the transverse dimension of a GNR. As shown in Table S2 in the ESM, the diameter of GNRs decreases when the AR increases. Therefore, as AR of GNRs increases, the dipole-dipole distance in the SS GNR assembly decreases, giving rise to the amplified plasmonic CD signal.

The final question is whether coating with silica would effectively improve the optical performance of the product. Accordingly, the plasmonic CD response of the L-CYS GNR assembly@silica core-shell nanostructures is monitored in different chemical environment at different time after freshly prepared. The bare L-CYS GNR assembly is also tested for comparison. As displayed in Fig. 4(a), after addition of Cit, the plasmonic CD signal of GNR assembly firstly increases with time due to formation of larger GNR oligomers. Such plasmonic CD response reaches maximum at 1~2 h after addition of Cit. In the following 19 h, the plasmonic CD signal decreases rapidly and macroscopic precipitation is found at the bottom of the container. After about 70 h, GNR assembly shows negligible CD response and the sediment is irreversible. In contrast, thick silica shell forms in 1~2 h after addition of TEOS, and simultaneously the plasmonic CD response of the core-shell nanostructures reaches a maximum value (Fig. S1 in the ESM). Remarkably, silica shell perfectly freezes the GNR assemblies with the strongest optical activity. Furthermore, by tuning the concentration

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of Cit, GNR assemblies with different assembly extent can be fixed very well. As demonstrated in Fig. 4(b), the plasmonic CD signal of the final product shows negligible change in ethanol after 3 months. In addition, the stability of the L-CYS GNR assembly@silica core-shell nanostructures in PBS solutions with different pH value was also tested for potential biological application. As shown in Fig. S10, the chiral core-shell nanostructures exhibit high stability in acidic PBS solution. In comparison, the uncoated chiral GNR assemblies lose their chiroptical activity within 20 h. From these experimental evidences, it is clear that coating of silica shell on the chiral GNR assembly significantly prolongs the chiroptical stability of the nano-assemblies, for the best, to several months.

In conclusion, we have developed a facile method to fabricate chiral nano-assembly@silica core-shell nanostructures with ultrastable and enhanced plasmonic CD response, which is crucial for practical application of chiral nanomaterials. The novel core-shell nanostructures combine the advantage of chiral SS GNR assembly and silica shell. Through changing the geometry of GNRs as well as their assembly extent, manipulation of the chiroptical property of the core-shell nanostructure product is easily realized. Interestingly enough, the silica shell not only freezes the assemblies efficiently but also significantly improves their chiroptical performance in different chemical environment. These well-defined nanocomposites with excellent optical active cores and promising functionalized shells will open up many applications in plasmonic devices, biosensors and chiral recognition.

Acknowledgements This work was supported by National Basic Research Program of China (2014CB931801, Z.Y.T.), National Natural Science Foundation of China (21475029, Z.Y.T.), the Instrument Developing Project of the Chinese Academy of Sciences (Grant No.YZ201311) and the CAS-CSIRO Cooperative Research Program (Grant No. GJHZ1503).

Electronic Supplementary Material: Supplementary material (experimental methods, characterization of the chiral GNR assembly@silica core-shell nanostructures, and detailed CD and UV-Vis spectra) is available in the online version of this article at http://dx.doi.org/10.1007/s12274-***-****-*. References [1] Pendry, J. B. A chiral route to negative refraction. Science

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Figure 1 TEM images of L-CYS-GNR assembly@silica core-shell nanostructures fabricated under different concentration of sodium

citrate (Cit): (a) 55 μM, (b) 70 μM, (c) 85 μM, (d) 100 μM; scale bar = 100 nm, aspect ratio of GNRs is ~2.4. (e) Statistic distribution

of GNRs number inside each silica shell under different concentration of Cit. The numbers in the labels stand for the GNR aspect

ratio and the Cit concentration in the system, respectively. For instance, GNR 2.4-55 Cit represents that the L-CYS-GNRs with aspect

ratio of 2.4 are assembled in 55 μM citrate solution.

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Figure 2 (a) UV−Vis and (b) CD spectra of original GNRs (black curves) with aspect ratio of ∼2.4 and L-CYS-GNR

assembly@silica core-shell nanostructures under different concentration of Cit. The numbers in the labels correspond to the Cit

concentration in the system. For instance, 55 Cit represents that the L-CYS-GNRs are assembled in 55 μM citrate solution.

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Figure 3 CD spectra (upper curves) and UV-Vis spectra (bottom curves) of original GNRs (black curves) and L-CYS-GNR

assembly@silica core-shell nanostructures under different Cit concentrations. The aspect ratio of GNRs is (a) ~2.8 and (b) ~3.3. The

numbers in the labels correspond to the Cit concentration in the system. For instance, 55 Cit represents that the L-CYS-GNRs are

assembled in 55 μM citrate solution.

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Figure 4 Plasmonic CD intensity of the LSPR negative CD peak of (a) GNR assemblies and (b) GNR assembly@silica core-shell

nanostructures at different time after addition of Cit. The Cit concentration in the solution is 100 μM and aspect ratio of GNRs is

~2.8.

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Electronic Supplementary Material

Ultrastable Gold Nanorod Assembly@Silica Core-shell Nanostructures with Enhanced Chiroptical Properties

Bing Han1, Lin Shi1, Xiaoqing Gao1, Jun Guo1, Ke Hou1, Yonglong Zheng1, Zhiyong Tang1( )

1 Laboratory of Nanomaterials, National Center for Nanoscience and Technology, Beijing 100190, P. R. China Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

1. Experimental Methods

Chemical reagents

Tetrachloroauric acid (HAuCl4·3H2O), cetyltrimethylammonium bromide (CTAB) and anhydrous ethanol were purchased from Sinopharm. Silver nitrate (AgNO3), sodium hydroxide (NaOH), ascorbic acid, tetraethoxysilane (TEOS), sodium citrate dehydrate (Cit) and 3-mercaptopropionic acid (MPA) were bought from Alfa Aesar. Sodium borohydride (NaBH4), L-cysteine (L-CYS), D-cysteine (D-CYS), DL-cysteine (DL-CYS) and phosphate buffer saline (PBS) were obtained from Sigma Aldrich. All the chemicals were used as received without further purification.

Synthesis of GNRs

The gold nanorods (GNRs) were synthesized followed by the previously-reported seed-mediated growth method.[S1] Firstly, a seed solution was prepared in an aqueous solution of 0.075 M CTAB by reduction of 10 mL 0.25 mM HAuCl4 with NaBH4 as a reducing agent. The CTAB capped Au seed solution was stored at 30°C for 2 h. Secondly, 12 μL of seed solution was added to 10 mL growth solution containing 0.1 M CTAB, 0.5 mM HAuCl4, 0.6 mM ascorbic acid and a defined amount of AgNO3. Finally, the GNR solution was left undisturbed and aged for 12 h. GNRs with different aspect ratio (AR) were prepared using the above method by changing the concentration of AgNO3 in the growth solution from 0.06 mM, to 0.05 mM and to 0.04 mM.

Preparation of CYS-GNR assembly@silica core-shell nanostructures

As-synthesized GNRs were centrifuged (12000 rpm, 10 min) twice followed by removal of the supernatant. The precipitates were redispersed in 1 mM CTAB. The concentration of GNRs was estimated to be around 0.65 nM. [S2] GNR assembly@silica core-shell nanostructures were prepared according to a reported protocol with some modifications. [S3] 1.4 mL ethanol was added into 5.6 mL GNRs solution, followed by addition of 80 μL L-CYS (1 mM), 45 μL NaOH (0.1 M) and 420 μL Cit (1 mM). In order to obtain core with

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different GNR assembly extent, the volume of Cit added in the solution would be varied. The above solution was stirred thoroughly. At ~5 min after addition of Cit, 90 μL TEOS (15%wt in ethanol) was added for 3 times with 30 μL each time at 30 min interval. The resultant solution was stirred for 5 h. D-CYS and DL-CYS assembled GNR@silica core-shell nanostructures were fabricated following the above method except for changing the L-CYS to D-CYS or DL-CYS. GNR assembly@silica core-shell nanostructures with CTAB and TEOS of different concentration were also synthesized following the above method. The obtained products were centrifuged (8000 rpm, 10 min) twice and stored in ethanol for further characterization. To test the stability of the obtained products in PBS solutions, the chiral core-shell nanostructures were centrifuged (8000 rpm, 10 min) twice and dispersed in PBS solutions with different pH value.

Characterization

TEM imaging was carried out on Tecnai G2 F20 U-TWIN under the accelerating voltage of 200 kV. CD spectra were recorded by a Jasco J-1500 spectropolarimeter in aqueous solution. A 3 mL portion of each sample was infused into a 1 cm quartz cell and measured at the scan speed of 500 nm/min with a bandwidth of 10 nm. Accumulation was conducted three times continuously for each sample. UV-Vis absorption measurements were carried out using a Lambda 950 UV-Vis spectrometer, and the method for preparing the each sample was the same as the CD measurement.

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2. Preparation of GNR assembly@silica core-shell nanostructures

Figure S1 TEM images of the L-CYS assembled GNR@silica core-shell nanostructures obtained at different time after addition of

TEOS: (a) 30 min, (b) 60 min, (c) 120 min, (d) 240 min; scale bar = 20 nm.

The hydrolysis of TEOS happens along with the assembly of GNRs. After 1 h of addition of TEOS, very thin silica shells are observed (Fig. S1(b)). In the following 1 h, silica shells become much thicker, freezing GNR assemblies in them (Fig. S1(c)). With prolonging the reaction time, chiral GNR assemblies with dense and mesoporous silica shells are obtained (Fig. S1(d)). 3. Manipulation of the thickness of silica shells

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By tuning the concentration of CTAB:

Figure S2 TEM images of L-CYS assembled GNR@silica core-shell nanostructures synthesized with different CTAB

concentration: (a) 1 mM, (b) 1.5 mM and (c) 2.0 mM; scale bar = 100 nm.

When the concentration of CTAB increases from 1 mM to 1.5 mM and 2.0 mM, the thickness of the silica shells are tuned from ~49 nm (Fig. S2(a)) to ~27 nm (Fig. S2(b)) and ~13 nm (Fig. S2(c)). This result is in agreement with previous studies. [S4]

By tuning the concentration of TEOS

Figure S3 TEM images of L-CYS-GNR assembly@silica core-shell nanostructures synthesized with different TEOS

concentrations: (a) 3.5 mM, (b) 5 mM and (c) 7.5 mM; scale bar = 100 nm.

The thickness of silica can also be tuned by changing the concentration of TEOS. With the concentration of TEOS increases from 3.5 mM (Fig. S3(a)) to 5 mM (Fig. S3(b)) and 7.5 mM (Fig. S3(c)), the thickness of the shell can be tuned from ~14 nm to ~ 29 nm and ~46 nm.

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4. Manipulation of the building blocks in the core to tune the plasmonic CD response of CYS-GNR@silica nanostructures

Figure S4 TEM images of single L-CYS-GNR@silica nanostructures with GNRs AR of (a) ~2.4, (b) ~2.8, (c) ~3.3; scale bar =

100 nm.

Figure S5 (a) CD spectra and (b) UV−Vis of single L-CYS-GNR@silica nanostructures with GNR’s AR of (a) ~2.4, (b) ~2.8, (c)

~3.3.

The structures of the samples used for optical measurement in Figure S5 are demonstrated in Figure S4.

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Figure S6 CD spectra (a) and UV-Vis spectra (b) of GNR assembly@silica nanostructures with GNRs modified with D-CYS (red

curve) and L-CYS (blue curve). TEM images of (c) D-CYS and (d) L-CYS modified core-shell nanostructures.

As shown in Fig. S6(a), GNR assembly@silica core-shell nanostructures with D-CYS as modifier exhibit characteristic plasmonic CD response that is in mirror symmetry with that of the L-CYS modified product. No structural enantiomers are observed from TEM images, indicating that the plasmonic CD response of the product is not caused by the chiral arrangement of GNRs.

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Figure S7 TEM images of L-CYS-GNR assembly@silica core-shell nanostructures prepared under different concentration of

sodium citrate (Cit): (a) 55 μM, (b) 70 μM, (c) 85 μM, (d) 100 μM; scale bar = 100 nm, aspect ratio of GNRs is ~2.8. (e) Statistic

distribution of GNRs number inside each silica shell under different concentration of Cit. The numbers in the labels stand for the

GNR aspect ratio and the Cit concentration in the system, respectively. For instance, GNR 2.8-55 Cit represents that the

L-CYS-GNRs with aspect ratio of 2.8 are assembled in 55 μM citrate solution.

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Figure S8 TEM images of L-CYS-GNR assembly@silica core-shell nanostructures prepared under different concentration of

sodium citrate (Cit): (a) 55 μM, (b) 70 μM, (c) 85 μM, (d) 100 μM; scale bar = 100 nm, aspect ratio of GNRs is ~3.3. (e) Statistic

distribution of GNRs number inside each silica shell under different concentration of Cit. The numbers in the labels stand for the

GNR aspect ratio and the Cit concentration in the system, respectively. For instance, GNR 3.3-55 Cit represents that the

L-CYS-GNRs with aspect ratio of 3.3 are assembled in 55 μM citrate solution.

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Figure S9 CD spectra (a) and UV-Vis spectra (b) of GNRs assembly@silica nanostructure with GNRs modified with DL-CYS

(red curve) and MPA (blue curve). TEM images of (c) DL-CYS and (d) MPA modified core-shell nanostructures.

As shown in Fig. S9(a) and (b), when racemic DL-CYS and achiral 3-mercaptopropionic acid (MPA) molecules are used as modifiers, no plasmonic CD signal can be discerned in the products. Furthermore, TEM images (Fig. S9 (c) and (d)) indicate that GNR assemblies in the core possess the morphologies including ladders or bundles, and no fixed chiral configurations can be observed. These results support the deduction that the strong plasmonic CD response in our system doesn’t come from structural chirality. Table S1 Anisotropic factors (g factor) of CD peaks corresponding to LSPR of GNRs assembly@silica nanostructures prepared

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with different AR of GNRs as building blocks under different concentration of citrate.

Aspect

ratio

g factor of LSPR negative CD Peak ×10-4

Cit Concentration (μM)

55 70 85 100

2.4 0.56 1.35 2.77 5.37

2.8 5.45 7.53 11.91 16.75

3.3 1.41 10.22 16.33 27.42

Anisotropic factor g is defined as follows:

g εε

Δ= , where L Rε ε εΔ = − and L Rε ε ε= + . Lε and Rε are the molar extinction coefficients for

left circularly polarized (LCP) and right circularly polarized (RCP) light, respectively. Meanwhile, according to Beer–Lambert law, A c lε= × × , where c (mol/L) is the concentration of the sample, l (cm) is the length of the cell, so the difference between absorbance of LCP and RCP light can be described as: A c lεΔ = Δ × × , where 33A θΔ = × , and θ is the molar ellipticity. Therefore, g factor can be calculated by

33

gA

θ=×

As observed from Table S1, for each GNR of the certain aspect ratio, chiral GNR assembly@silica core-shell nanostructures exhibit enhanced plasmonic CD response with the increasing number of GNRs per shell, which is tuned by changing the concentration of Cit. At a specific Cit concentration, g factor of the chiral core-shell nanostructures containing the GNRs with increasing AR shows an overall increasing trend, but the enhancement factor is not the same for different Cit concentration. For instance, g factor in 55 μM Cit solution with AR 2.8 is even larger than that with AR 3.3. The possible reason is that at a specific concentration of Cit (especially in the solution of low concentration Cit), the assembly extent of GNRs may have some deviations for GNRs of different AR. As shown in Fig. 3, in 55 μM Cit solution, the longitudinal SPR adsorption band of GNR (AR = 2.8) assembly core shows larger blue shift than that of GNR (AR = 3.3) assembly core, suggesting that the former GNR assembly core has larger assembly extent. This abnormal optical property can be explained by the fact that in 55 μM Cit solution, the GNR assembly extent is very low only with 1~3 GNRs in each silica shell (Fig. 1(a) and 1(e)). At such a low assembly extent, the plasmonic CD signal of the chiral core-shell nanostructures is very weak. Little change in assembly extent will result in large change in CD signal. Therefore, manipulation of plasmonic CD signals of the core-shell nanostructures in low concentration Cit solution is relatively hard, and accurate control of the assembly extent may eliminate this problem. Nevertheless, fabricating chiral GNR assembly with higher assembly extent and larger AR is essential to achieve chiral core-shell nanostructures with stronger chiroptical performance.

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Table S2 Statistics of the dimension of GNRs with different aspect ratios.

AR Length (nm) Diameter (nm)

2.4 47.8 20.2

2.8 48.6 17.3

3.3 49.7 15.0

As shown in Table S2, the longitudinal and transverse dimensions of GNRs have anisotropic change with

AR increasing. The diameter decreases with AR of NRs increasing.

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Figure S10. Plasmonic CD intensity of LSPR negative CD peak of (a) GNR assemblies and (b) GNR assembly@silica core-shell

nanostructures in PBS solutions with different pH value at different time. The aspect ratio of GNRs is ~2.8.The numbers in the

labels represent the solution pH value.

As shown in Fig. S10, the chiral GNR assembly@silica core-shell nanostructures exhibit high stability in PBS solution with pH = 4.5. But when pH value increases to 6 and 7.4, the nanostructures become less stable. Especially in basic solutions (pH = 9.0 and 10.5), the CD intensity of the nanostructures decreases quickly in the first 5 h. The experimental result can be easily explained by the fact that amorphous silica dissolves quickly in salt water and basic solution.[S5, S6] In comparison, bare chiral GNR assemblies show very low stability in PBS solutions regardless of the pH value, and the CD intensity of all the samples almost disappear within 20 h. Altogether, our products exhibit rather high stability in acidic PBS solutions, suggesting that this novel core-shell composite would be a very promising platform in cancer cell imaging or therapy.

References

[S1] Nikoobakht, B.; El-Sayed, M. A. Preparation and growth mechanism of gold nanorods (NRs) using seed-mediated growth method. Chem. Mater. 2003, 15, 1957-1962.

[S2] Orendorff, C. J.; Murphy, C. J. Quantitation of metal content in the silver-assisted growth of gold nanorods. J. Phys. Chem. B. 2006, 110, 3990-3994.

[S3] Guo, X.; Deng, Y.; Gu, D.; Che, R.; Zhao, D. Synthesis and microwave absorption of uniform hematite nanoparticles and their core-shell mesoporous silica nanocomposites. J. Mater. Chem. 2009, 19, 6706-6712.

[S4] Zhan, Q. Q.; Qian, J.; Li, X.; He, S. L. A study of mesoporous silica-encapsulated gold nanorods as enhanced light scattering probes for cancer cell imaging. Nanotechnology 2010, 21.

[S5] Kato, K.; Kitano, Y. Solubility and dissolution rate of amorphous silica in distilled and seawater at 20 ºC. J. Oceanogr. Soc Japan. 1968, 24, 147-152.

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[S6] Zhang, Q.; Zhang, T.; Ge, J.; Yin, Y. Permeable silica shell through surface-protected etching. Nano Lett 2008, 8, 2867-2871.