Reversible Plasmonic Circular Dichroism via Hybrid Supramolecular

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Reversible Plasmonic Circular Dichroism via Hybrid Supramolecular Gelation of Achiral Gold Nanorods Xue Jin, Jian Jiang, and Minghua Liu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b06233 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016

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Reversible Plasmonic Circular Dichroism via Hybrid Supramolecular Gelation of Achiral Gold Nanorods Xue Jin†, Jian Jiang‡ and Minghua Liu*,†,‡,§, ǁ

†

ǁ

Beijing National Laboratory for Molecular Science, CAS Key Laboratory of

Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. ‡

National Center for Nanoscience and Technology, CAS Center for Excellence in

Nanoscience, Beijing 100190, P. R. China. §

A Collaborative Innovation Center of Chemical Science and Engineering, Tianjin

300072 (China). ǁ

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

* Address correspondence to [email protected]

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ABSTRACT

The fabrication of chiroptical plasmonic nanomaterials such as chiral plasmonic gold nanorod (GNR) has been attracting great interest. Generally, in order to realize the plasmonic circular dichroism (PCD) from achiral GNR, it is necessary to partially replace the surface coated cetyltrimethyl ammonium bromide with chiral molecules. Here, we present a supramolecular approach to generate and modulate the PCD of GNR through the hybrid gelation of GNRs with an amphiphilic chiral dendron gelator. Upon gelation, the PCD could be produced and further regulated depending on the ratio of the dendron to GNR. It was revealed that the wrapping of the self-assembled nanofibers around the GNRs is crucial for generating the PCD. Furthermore, the hybrid gel underwent a thermo-triggered gel-sol and sol-gel transformation, during which the PCD can disappear (solution) and reappear (gel) respectively, and such process can be repeated many times. In addition, the hybrid gel could also undergo shrinkage upon addition of slight amount of Mg2+ ions, during which the PCD disappeared also. Thus, through the gel formation and subsequent metal ion or temperature triggered phase transition, PCD can be reversibly modulated. The results not only clarified the generation mechanism of PCD from the achiral GNRs without the chiral modification on the surface but also offered a simple and efficient way to modulate the PCD.

KEYWORDS ：

plasmonic circular dichroism, gold nanorod, self-assembly,

hydrogels, reversible modulation 2 ACS Paragon Plus Environment

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Currently, there is an increasing interest in the fabrication of the chiral plasmonic materials since their new applications in biosensing,1, 2 detection,3, 4 enantioselective separation,5 and chiroptical nanomaterials.6 Among various chiral plasmonic materials, the gold nanorod (GNR) is attracting great interest due to its biocompatibility, easy treatment, large g-values and near-infrared absorption. Generally, GNR is synthesized by the seed-growth method with cetyltrimethyl ammonium bromide (CTAB) as stabilizer and the fabricated GNR is achiral. In order to produce the plasmonic circular dichroism (PCD) from the GNR, two strategies have been developed. One is to modify the surface of GNR by partial replacement of CTAB with chiral molecules. Those thiolated chiral molecules such as L- or D-cysteine,7 thiolated DNA8 and thiolated peptide9, 10 have been widely used. In these cases, not only the coating of the chiral molecules on the surface of GNR is important, but also the further assembly of the GNR is necessary to obtain enhanced PCD signals.11-14 On the other hand, PCD can also be produced by incorporating achiral GNR into chiral matrix, such as liquid crystals,15 cellulose nanocrystals,16-18 mesoporous silica thin films19 and the polymer nanofibers.20-22 In these cases, the alignment of the GNR played an important role in producing the PCD. As for supramolecular hydrogels, in which small molecules self-assembled into nanostructures to immobilize water, have been extensively investigated since these hydrogels have many advantages to be applied.23-26 In addition, it is an easy way to construct chiral nanostructures by using the chiral small molecular units through gelation.27 It is supposed that the gel chiral nanostructures can be used as a template 3 ACS Paragon Plus Environment

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or matrix to incorporate the nanorod. With such incorporation, the nanorod can be possibly aligned on the template nanostructure or embedded in the gel matrix. Furthermore, the stimuli-responsiveness of the gel could provide the possibility to regulate the alignment of the nanorod. Unfortunately, there are only a few reports on the hybrid gels between small molecules and nanomaterials.28-31 Recently, we have found that a thermal and metal ion-responsive supramolecular hydrogel could be formed by an amphiphilic L-glutamic acid peptide dendron, N-octadecanoyl-1, 5- bis (L-glutamic acid)-L-glutamic diamide (OGAc).32 It was further found that the hydrogel containing magnesium ion exhibited shrinkage performance and the gelation process is thermo-reversible. These properties made us to produce and regulate the hybrid gel of OGAc and GNRs to modulate the possible PCD. Although most of the investigations on the plasmonic CD have been focused on the origin33-37 and amplification of PCD,38-42 several reports showed the dynamic evolution and reversibility of the PCD. For example, Tang and his co-workers have used the DNA linked GNR43 or gold nanobipyramid44 assemblies to fabricate the reversible plasmonic CD through DNA hybridization. Liu’s group has got the photosensitive reversible plasmonic chiroptical response of GNR by using the azobenzene modified DNA linkers.45 All these studies exhibited excellent light- or thermo-responsive plasmonic CD. However, in all these studies, the covalent connecting of the chiral species such as thiol-modified DNA or azobenzene modified DNA origami on the surface of GNRs is necessary. Herein, we use a simple supramolecular approach to regulate the PCD of GNR through gelation. We have found that upon co-assembly of 4 ACS Paragon Plus Environment

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the GNRs with a chiral amphiphilic peptide dendron OGAc, the PCD of achiral GNR coated with CTAB can be generated. Furthermore, the PCD can be regulated by the temperature and the mixing ratio of the GNRs and the peptide dendron gelator. On one hand, the result clarified why PCD can be produced without the covalent chiral modification on the GNRs surface. On the other hand, it provided a simple way to regulate the PCD from achiral GNRs.

Figure 1. A) Molecular structure of OGAc gelator and GNR synthesized by seed-growth method. B) Schematic illustration of the phase transition in the co-assembled processes under multiple stimuli (photographs show the solution, hydrogel and shrunken gel, respectively). The concentration of the gelator OGAc is 2 mM, the molar ratio of Mg2+ and OGAc is 1: 5.

RESULTS

Hybrid supramolecular gelation based on GNR and OGAc 5 ACS Paragon Plus Environment

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The molecular structure of the gelator used in this work is shown in Figure 1A, which is synthesized by a convergent method and found to hierarchically self-assemble into nanotubes through H-bond. 32 Since the four carboxylic groups of OGAc can partial protonate in aqueous solution, it is an ideal molecule to co-assemble with the positively charged GNRs. The process of hydrogel formation is simple and convenient. Experimentally, the aqueous solution of GNR with aspect ratio of 2.7 (Figure S1) was added into an aqueous dispersion of OGAc and the mixture was heated to 70 oC. After cooling down to the room temperature, the red brown transparent hydrogel was obtained. The gels could be formed in a wide range of the GNR/OGAc ratio. The gel was stable and could be kept at room temperature for several days without any change. Previously, we have found that addition of some metal ions can cause the shrinkage of the gel. In the present gel, a slight amount of Mg2+ was also added into the gel. Upon resting for several hours, a similar shrinkage of the gel was observed. Interestingly, during the shrinkage, only water molecule was expelled, which is similar to our previous paper.32 Such shrinkage of the gel will greatly influence the PCD of the GNRs, which will be discussed later.


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Figure 2. TEM images of the xerogels from the hybrid assemblies of GNR and OGAc with varied GNR concentrations. (A) GNRs (0.04 nM) embed in the OGAc nanofibers. (B) GNRs (0.16 nM) incorporated and aligned with the longitudinal direction of OGAc superstructure. (C) GNRs (0.3 nM) arranged along the superhelical nanofibers like chains. (D) Alignment of the nanorods (0.3 nM) in high magnification TEM image. All the OGAc concentration is 2 mM. The gels were composed of the nanostructures and water media. In order to observe the nanostructures of the hybrid gel, the xerogels of various ratios of GNR/OGAc were investigated by TEM. As previously reported, a right-handed 1D helical nanostructure was observed for the OGAc hydrogels.32 The 1D nanostructures exhibit lengths well over tens of micrometers. Besides the thinner helical nanofiber, coiled superhelix structures are also observed, which could be due to the hierarchical self-assembly of OGAc (Figure S2). These superhelixes could pack into more giant supercoiled structures as shown in Figure S3. When 2 mM OGAc assembled with a lower concentration GNR (below 0.16 nM), it is clearly seen that OGAc formed fiber network, while the GNRs were discretely distributed in the fibers, as shown in Figure 7 ACS Paragon Plus Environment

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2A. When the concentration of GNRs increased, distances between GNR individuals were reduced and GNRs began to assemble. TEM images show that GNRs co-assembled onto OGAc nanofibers surface. And the GNRs are preferentially aligned along the longitudinal direction of the nanofibers, as shown in Figure 2B. Continual increment of the GNR concentration in hydrogel caused the further assembly of the nanorod. The image of Figure 2C shows that the assembly is well ordered over long distance in the whole nanofibers, which in general form chains. The high-magnification image (Figure 2D) shows the alignment is essentially uniform with the end to end form, though defects (rods with different orientation) are also observed. It is interesting to note that there is a main tendency for the GNRs to align along the OGAc nanofibers. This is because the surface of the OGAc nanofiber is negatively charged and the GNR was positively charged in the gel. The electrostatic interactions lead to the alignment. Plasmonic circular dichroism of the hybrid gels

The circular dichroism (CD) spectra were measured for the hybrid gels composed of GNR and OGAc with different molar ratios, where the concentration of OGAc was fixed at 2 mM. When there is a small amount of GNR in the gel, a positive Cotton effect at 688 nm and negative Cotton effect at 515 nm are observed, as shown in Figure 3A. Upon increasing the concentration of GNR, this Cotton effect increased its intensity but without any band shift until the concentration of 0.08 nM. With further increment of the concentration, the plasmonic coupling is observed, with the positive Cotton effect at 718nm, negative Cotton effect at 664 nm and a zero crossover at 690 8 ACS Paragon Plus Environment

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nm, shown in Figure 3B e-i. This clearly indicated that the PCD generated and transferred to a coupling depending on the concentration of GNR.

Figure 3. CD (A and B) and UV−Vis absorption (C) spectra of GNR co-assembled with OGAc. The concentration of OGAc is fixed at 2 mM, which the concentration of GNR changed from 0 (a), 0.02 nM (b), 0.04 nM (c), 0.08 nM (d), 0.16 nM (e), 0.24 nM (f), 0.3 nM (g), 0.32 nM (h) to 0.4 nM (i). (D) Plot of the anisotropy factor (g-ratio) of the hybrid gels against the GNR concentrations. On the other hand, in the UV-Vis spectra, as shown in Figure 3C, the well-dispersed GNRs solution has the transverse and longitudinal plasmon band at 515 nm and 688 nm, respectively. After co-assembling with the chiral supramolecular nanofibers OGAc, the longitudinal absorption peak of GNRs exhibited slight redshifts, but the transverse plasmonic bands were almost unchanged. Compared to the pure GNRs solution, the longitudinal peaks of hybrids shifted from 688 nm to 691 nm, and all the bands had small broadening. These changes reveal gradually shorten of interparticle 9 ACS Paragon Plus Environment

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distances in the crowded condition of hydrogel, even some extent assembling of GNRs. In order to evaluate the chiral interactions more clearly, the anisotropic g-ratio of the system was estimated.38 The g-ratio is defined as:

g =

∆Α CD(m deg) = Α 32980 * A

Where ∆A is the difference of the sample’s absorbance between left and right circularly polarized light, and A is its absorbance of unpolarized light. Due to the acquired PCD signals are all positive but with different band profiles, the g-ratios were evaluated by two different methods. For the PCD showing only positive Cotton effect shown in Figure 3A, the intensity of the CD band at around 690 nm was taken, while for the PCD showing exciton couplet as shown in Figure 3B, the difference of PCD intensity between positive peak at 718nm and the negative peak at 664 nm was taken. The calculated g-ratios of the gel with different concentrations were plotted against the concentration of GNRs, as shown in Figure 3D. It is interesting to note that the g-ratio has two maxima with the change of GNR concentration. The first peak is obtained at 0.08 nM and the g ratio is 0.0175, which is higher than most plasmonic optical response made by GNRs without thiol modified. The second peak is shown at 0.3 nM with the g-ratio of 0.01. The turning point is at 0.16 nM which is corresponding to the change point of PCD spectrum profiles. In many PCD systems, once the GNR are synthesized or organized, the g-ratio stays unchanged. Here, we


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showed an interesting change of the g-ratios and implied that the self-assembly of the GNR with the OGAc played important roles in producing the PCD.

Modulation of the PCD through the gel phase transition

Because the OGAc supramolecular hydrogel is thermo-responsive and the phase transition from sol to gel is reversibly controlled by the heating to cooling cycle and

vice versa. We have further investigated the PCD changes during such transitions. The hybrid gel of 2 mM OGAc and 0.3 nM GNR was chosen to study by means of variable-temperature CD spectroscopy. In Figure 4A, split Cotton effects which belonged to the GNRs in plasmonic hydrogel were varied by the temperature. From 5 °C to 60 °C, PCD intensity was gradually vanished and the zero crossover was also blue shifted. Such a thermo-responsive process can be repeated many times and the CD signals changed subsequently. Figure 4B shows a plot of the intensity change of the CD signal for the co-assembly as a function of the heating/cooling cycles. The hydrogel was regulated by heating/cooling to obtain the transition from CD-silent to the plasmon-split positive CD signal at 718 nm. Thus, compared to the chiroptical switch manifesting reversible supramolecular chirality between the gel and sol, a thermo-driven plasmonic chiroptical switch was realized based on the assembling to dissembling process in the gel. Furthermore, the hydrogel showed shrinkage triggered by the divalent metal ions. So the Mg2+ was chosen to investigate the influence of the gel shrinkage on the PCD of GNR. As we can see in Figure 4C, after the solution mixture of GNR, OGAc and


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Mg2+ co-assembled to hydrogel, a shrinkage process started. By monitoring the UV-Vis and CD spectral changes as a function of shrinking time, it is possible to monitor the effect of Mg2+ on the self-assembly as well as the PCD. From solution to hydrogel, the longitudinal absorption peak of GNR decreased with slight redshift and broadening. However, when the shrinkage process began, the absorption peak increased continuously, broadened and redshift until the shrinkage have reached the equilibrium. When we look at the PCD changes, it is astonishing to find that PCD disappeared when the gel shrunk. This is because Mg2+ has a strong interaction with carboxylic acid than CTAB with OGAc. The added Mg2+ will react first with carboxylic acid and self-assembly into bundled nanofibers. During such process, the OGAc wrapped around the GNRs were disassembled and PCD disappeared. It is worth mention that the shrinkage process with plasmonic chiroptical variation is a continuous, reversible macroscopic volume phase transition as well. If the shrunk gel was gently heated to form a transparent solution and cooled down, the hybrid gel will formed initially and then undergo shrinkage again. When the gel was formed, PCD would be reproduced and gradually disappeared accompanying with the gel shrinkage. (Figure S4)


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Figure 4. (A) Plasmonic CD of GNR-OGAc co-assembled hydrogel from 5 °C to 60 °C, the interval of temperature is 5 °C. (B) CD intensity at 718 nm as a function of the temperature cycle. (C) Plasmonic CD modulated by the sol-gel-shrunken gel transition process with the addition of Mg2+ ion. sol (a), gel (b), shrunk gel in 3 h (c) and 8 h (d). (D) The corresponding UV-Vis absorption spectrum of the process of shrunken gel formation. The concentration of GNR is 0.3 nM, OGAc is kept at 2 mM, the concentration of Mg2+ 0.4 mM. PCD via the adsorption of the GNR on OGAc nanostructures

There are two ways to fabricate the hybrid of OGAc and GNR. The first way is the self-assembly of molecular OGAc and GNR. That is, OGAc is heated to solution in a molecular state first and then mixed with GNR at a higher temperature. Let the transparent solution cool down, the hybrid gels could be formed, which we have discussed above. The second way is the self-assembly of the OGAc nanostructures with GNRs. The OGAc nanostructures were formed first and then aqueous GNRs solution was added into the preformed OGAc hydrogel and mixed. The CD spectra of


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this mixture were measured as shown in Figure 5A. To our surprise, the mixture had no plasmonic optical responses at the longitudinal absorption wavelength. This indicated that simple mixing of the chiral nanostructure with the GNR cannot produce the PCD of GNRs. Interestingly, if the mixture was heated and cooled down to form the hydrogel, it can generate significant plasmonic CD at 688 nm, which agreed with the longitudinal absorption peak of GNRs. The corresponding UV-Vis spectrum of Figure 5B also shows the variation between mixture and co-assembled hybrid gel. While the transverse and longitudinal absorption peak of GNR showed in the same positions, their corresponding CD spectra showed significant differences. In order to clarify this, the morphology of GNR-OGAc hydrogel mixture was observed by TEM, as shown in Figure 5C. GNRs were arranged on the nanofibers randomly by the electrostatic interaction through simple mixing at room temperature. However, after heating-cooling process, GNRs were co-assembled with OGAc nanofibers. The TEM image (Figure 5D) shows that GNRs were covered by the hydrogel, and all the GNRs were discretely distributed in the hydrogel. These results revealed that the physical absorption between GNRs and OGAc hydrogel is not the main reason for plasmonic CD. The co-assembled supramolecular gelation is indispensable for the fabrication of plasmonic chiroptical activities.


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Figure 5. CD spectra of (A) The mixture of GNRs and OGAc hydrogel without plasmonic CD (black line); after heating/cooling cycle, the formed hydrogel showed plasmonic CD (red line). (B) UV-Vis absorption spectrum of mixture and co-assembly, the co-assembled hydrogel had lower absorbance intensity because the coagulation of a few GNRs in the mixture. TEM images of mixture (C) and co-assembled hydrogel (D), the mixture with disordered absorbent on the nanofibers of hydrogel; the reheating and cooling process let the GNRs recoating by the hydrogel. The concentration of GNR is 0.24 nM. All the OGAc concentration is kept at 2 mM. DISCUSSION

The above results clearly illustrated the generation and modulation of the PCD from the CTAB-coated achiral GNR. A strong correlation between the CD and UV-Vis peaks of the hybrid hydrogels, in terms of both their spectral position and intensity suggested that the CD signal was caused by GNR plasmon. It is obviously that two


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distinct line shaped CD were in the plasmonic hydrogel, with the core made from either “discrete” or “aligned” GNRs surrounded by OGAc helical nanofibers depending on the GNR concentration, which can be illustrated in Scheme 1. Since the GNR surface is covered by a bilayer of CTAB and is positively charged, it is easy to react with the carboxylic acids of OGAc when mixed them together. Upon gel formation, OGAc formed a nanofiber structures composed of multi-bilayer structure, and the GNR was wrapped by the OGAc nanofibers. When lower concentration of GNR co-assembled with OGAc, GNR was surrounded by OGAc nanofibers, forming a mode like Scheme 1A. In this case, the OGAc produce a chiral environment and the PCD of GNR was induced. Thus, the plasmonic CD had two separated peaks correspond to the transverse and longitudinal bands of GNR, respectively. Since the dipole moment of the transverse and longitudinal bands is perpendicular, their PCD signals are opposite. This means that the chirality of GNR was induced from the OGAc nanofiber. When the concentration of GNR was increased, the GNRs get closed and aggregated in a shoulder to shoulder manner, as illustrated in Scheme 1B. In this case, the exciton type CD spectra are observed. In addition, with the increment of concentration, the CD signals also increased due to the formation of more such kind of nanoaggregates. It should be mentioned that if increasing the GNR concentration further, the PCD signal decreased. This might be because there is no sufficient OGAc nanofiber to encapsulate the discrete GNR.


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On the other hand, when GNR was further increased, the alignment between the GNRs started, which is confirmed by the TEM observation. At a lower concentration, discrete GNR was observed, while GNR assembled on the fibers when increasing the concentration. From the TEM, it was observed that GNR mainly aligned in a head to tail way, but some with the shoulder to shoulder way, which can be illustrated in Scheme 1B and C, respectively. These nanorods were aligned on the nanofiber due to electrostatic interactions and extend over a very long distance. It has been reported that the head to tail alignment of the GNRs would cause the change of absorption spectra as well as the PCD.7 However, only a slight shift was observed for the present hybrid gel. This is because in the present case, although GNR was aligned, they are not directly connected to form the dimer or oligomer. The GNRs were separated by the carboxylic acid in the surface of OGAc. Thus, their plasmonic coupling is not strong enough to induce the shift of the absorption. An interesting feature of the present PCD is its thermo-responsiveness. At a higher temperature where OGAc is in a molecular state no PCD was observed. Upon forming the nanostructures in gels, the PCD appeared. This means that the PCD was produced by the supramolecular chirality rather than molecular chirality.46 It is interesting to note that the PCD signals changed with the temperature, suggesting the dynamic formation of the PCD. In addition, since the gel can experience gel-sol transition, the PCD can be modulated repeatedly. Another interesting feature is the gel shrinkage caused by Mg2+, which caused the disappearance of the PCD of the GNRs. This is because Mg2+ react with the 17 ACS Paragon Plus Environment

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carboxylic acid stronger that CTAB, which inhibited the self-assembly of GNRs with OGAc nanofibers.

Scheme 1. Schematic illustration of the interaction modes between GNR and OGAc nanofibers in hybrid hydrogel. (A) The individual GNR wrapped by the nanofibers, which showed positively plasmonic CD. (B) The shoulder to shoulder assembly of the GNR, which showed split PCD. (C) Head to tail alignment of the GNRs. All the GNRs were wrapped by the OGAc nanostructures. (D) The mixture of GNR and OGAc nanofiber in hydrogel, GNRs absorbed on the OGAc nanofibers through strong electrostatic interactions but were not wrapped by the OGAc. In this case no PCD appeared. On the other hand, GNR could adsorb on the nanostructures of OGAc at a room temperature. In this case, although the GNRs adsorbed on the surface of OGAc nanofiber, no PCD was induced. This is because that the GNRs just adsorbed on the surface of OGAc nanofibers but not wrapped by the fibers, as illustrated in Scheme 1D. It seemed that the OGAc nanostrucutres wrapping around the GNRs is crucial for generating the PCD. This hypothesis is verified by the following experiment. If the adsorbed gel was heated to a molecular solution and subsequently cool down, PCD would appear again, as shown in Figure 5A. 18 ACS Paragon Plus Environment

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Thus, through the fabrication method, gelation, gel-sol phase transition by the temperature and metal ions, the PCD of GNRs can be modulated.

CONCLUSIONS

In summary, we have demonstrated that the hybrid gelation of OGAc dendron and achiral GNRs produce remarkable plasmonic CD signals at the visible light region. The intensity and line shape of plasmonic CD can be tailored by the GNR concentration. The separated and split CD profiles reveal two modes of GNRs in hydrogel, one is discrete, the other is aligned, which is depended on the GNR concentration. A mechanism for the generation of the plasmonic CD is discussed, which indicated that the OGAc nanofibers wrapping around the GNRs is crucial for generating the PCD. Most interestingly, the plasmonic CD is reversibly switched through the sol-gel phase transition triggered by the temperature. The plasmonic CD responses obtained by the OGAc and GNRs hydrogels will shed light on creating intelligent materials with multiple chiroptical responses as well as the potential application in chiroptical sensor and chiroptical materials.

METHODS

Synthesis of Gold Nanorods: The gold nanorods were synthesized by using a seed-growth method. The CTAB-capped gold seeds were synthesized by chemical reduction of HAuCl4 with NaBH4: a freshly prepared, ice-cold NaBH4 solution (0.01 M, 0.6 mL) was added into a mixture of CTAB (0.2 M, 5.0 mL), HAuCl4 (1 mM, 2.5 mL), and water (2.5 mL). The mixture was kept stirring for 3 min and kept 19 ACS Paragon Plus Environment

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undisturbed at 30 °C prior to any further experiment. The seeds can be used within 2– 5 h after preparation. After that, the growth solution was made, which consisted of CTAB (0.1 M, 100 mL), HAuCl4 (24 mM, 2.04 mL), AgNO3 (0.1 M, 105 µL), and L-ascorbic acid (0.1 M, 552 µL) solutions. Then 120 µL of seed solution was added into the growth solution to initiate the growth of GNRs. The resulting reaction solution was gently mixed by inversion and then left undisturbed. After 4 h, L-ascorbic acid (0.1 M, 55.2 µL) was added twice with a 40 min interval. The reaction mixture was reacted for 12 h. The GNRs were purified by centrifugation (12000 rpm, 10 min) to remove the superfluous CTAB. The precipitates were collected and redispersed in deionized water. Sample Preparation of OGAc - GNRs Hydrogels: OGAc solutions with different concentration of GNRs aqueous solution were mixed in a sealed tube, the total volume is 1 mL and the concentration of OGAc is kept at 2 mM. The mixtures were slightly heated to 70 °C. Then the tube was allowed to cool down to room temperature and stored in 4 °C refrigerator. After keeping rest for 12 h, the co-assembled hydrogels were obtained. The shrunken gel was obtained by adding Mg2+ into the GNRs and OGAc mixture solution, the Mg2+/OGAc molar ratio is 1:5. ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Distribution of GNR dimensions, determination of OGAc nanofibers by AFM and 20 ACS Paragon Plus Environment

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TEM measurements. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Basic Research Development Program (2013CB834504) the National Natural Science Foundation of China (Nos. 91427302 and 21227802), “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDB12020200), and the Fund of the Chinese Academy of Sciences. Specially, we gratefully acknowledge Dr. S. Hou for valuable discussion on the synthesis of gold nanorods.

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Reversible Plasmonic Circular Dichroism via Hybrid Supramolecular

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