Chiroptical Study of PlasmonâMolecule Interaction: The Case of

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Chiroptical Study of Plasmon-Molecule Interaction: the Case of Glutathione Interaction with Silver Nanocubes Maria Chiara di Gregorio, Assaf Ben Moshe, Einat Tirosh, Luciano Galantini, and Gil Markovich J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03272 • Publication Date (Web): 06 Jul 2015 Downloaded from http://pubs.acs.org on July 13, 2015

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The Journal of Physical Chemistry

Chiroptical Study of Plasmon-Molecule Interaction: The Case of Glutathione Interaction with Silver Nanocubes Maria C. di Gregorio, † Assaf Ben Moshe, ‡ Einat Tirosh, ‡ Luciano Galantini, † Gil Markovich‡* †

Department of Chemistry, “Sapienza” University of Rome, P.le Aldo Moro 5, 00185 Rome,

Italy. ‡ School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel.

ABSTRACT. Inorganic nanoparticle - chiral molecule coupling may produce chiroptical effects at the inorganic particles' excitation wavelengths. In this work we demonstrate that low concentrations of glutathione molecules adsorbed to cube shaped silver nanoparticles lead to the appearance of new circular dichroism signals in the plasmon resonance absorption range. Moreover, for pH values below 4.5, the nanoparticle surface promotes reversible thiol group oxidation, leading to the formation of diglutathione and consequent reversible modification of both the molecular and plasmonic circular dichroism and absorption spectra.

This study

demonstrates the importance of chiroptical effects as probes of molecule-inorganic nanocrystal interaction.

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INTRODUCTION Chirality is an important theme in biomolecular and organic chemistry due to its key role played in biological processes and in determining pharmacological, pharmacokinetic and toxic features of various molecules. The main spectroscopic technique for chirality detection is circular dichroism (CD)1 which measures the difference in absorption between left and right handed circularly polarized light passing through a sample. Special information, primarily conformational, can be inferred from CD spectra, depending on the incident radiation wavelength and sample properties. Biomolecules show CD bands in the UV region at the energy range typical of electronic transitions in amino or nucleic acids. Several contributions that are related to the chirality of the system at different levels, from the stereogenic center conformation to the supramolecular organization, can co-participate in determining CD band shape whereas their intensity is connected to intrinsic features of the molecules, concentration and intermolecular interaction strength. When aggregation phenomena are negligible, CD profiles work as fingerprints for chiral molecules, allows differentiating enantiomers, deducing conformations, and performing quantitative analyses. These characteristics make CD spectroscopy a useful tool, for example in biomedical and pharmacological fields where enantioselective and non-disruptive detection is often required for synthesis and separation products and for various applications. However, CD signals are weak for most chiral molecules, reducing the limit of detection and sensitivity of CD spectroscopy and limiting its use. Therefore new strategies for the increase of the sensitivity of CD spectroscopy are needed in order to overcome this problem, such as chiral molecule-metal nanoparticle based platforms, which exhibit enhanced chiroptical effects. Symmetric noble metal nanoparticles present strong absorption bands (extinction, to be more precise) in the visible wavelength range, corresponding


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to localized surface plasmon resonances (LSPR), but these bands do not exhibit any CD lines because of lack of chirality. Nevertheless, it has been both theoretically predicted2-6 and experimentally shown that such nanoparticles capped with chiral molecules can have optically active plasmon resonances due to several modes of mutual interactions between the two partners which can doubly modify the CD spectroscopic response: i) the CD bands related to the molecule can be enhanced by the local plasmonic fields and/or ii) new CD signals can appear in the plasmonic absorption range (visible range for gold and silver), providing a chirality related signal in a different region of the electromagnetic spectrum and often with stronger CD intensity than the molecular lines. Several reports on such hybrid systems deal with different combinations of organic molecule-metallic nanostructure in which the organic part can be either low molecular weight molecules7-10 or macromolecules11-14 whereas the inorganic part can range in size,10, 15-16 materials and shapes.12 In particular, regarding the latter point, by using shapes different than the spherical one, such as nanocubes,17 nanotriangles,18 nanostars,19 nanoprisms20 etc., theoretical calculations have predicted stronger LSPR modes (especially at edges and vertices) with various symmetries. Advancements in controlled synthesis of symmetric metallic nanostructures have allowed in some cases to experimentally reproduce theoretical data,12,

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confirming the

efficiency of detection and setting in motion challenging experiments aimed to explore new interactions between plasmonic nanoparticles of various shapes and different kinds of chiral molecules. Following intriguing results on CD induction at particular surface plasmon resonance modes of silver cube-shaped nanoparticles (CNPs) by Gang and coworkers,12 but with partial understanding of the involved mechanism, we chose a different chiral molecule to combine with


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silver nanocubes, and we report on a CD study of glutathione (GSH) molecules (Figure 1a) interacting with silver CNPs.

Figure 1. a) Molecular structure of GSH; b) CD (up, left axis) and absorption (bottom, right axis) spectra of 30 µM GSH and 0.04 nM bare CNPs; c) TEM image of bare nanocubes. GSH is the most important non protein thiol source in living systems. It is an endogenous antioxidant which is involved in many biological functions such as the maintenance in the reduced state of protein thiol groups and the retention of cells from oxidative stress through the


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neutralization of free radicals. Many systems in the body can be influenced by the GSH state because of its participation in pivotal biochemical reactions directed to DNA and protein synthesis and its unbalanced level in tissues can be connected to several diseases, like Alzheimer and Parkinson.22 Due to its biological importance, in the last decades different detection methods have been developed by exploiting electrochemical systems,23 fluorescence spectrometry24 and chromatography/mass spectrometry.25 Capping noble metal atomic clusters and nanoparticles with GSH has been explored.10, 26-30 In the present work GSH capping of silver CNPs was chosen for several reasons: i) previous reports in the literature about GSH-silver interaction29,31, ii) in general, silver nanostructures' plasmonic resonances span the visible range, and in particular for cubic shape they range from about 300 to 700 nm depending on the CNP size, iii) the extinction coefficient of silver LSPR bands is larger than the gold counterparts and the local field strengths are also stronger.12

EXPERIMENTAL METHODS Chemicals. All reagents used for the CNPs synthesis, including silver nitrate (99%), polyvinylpyrrolidone (Mw= 55000), sodium sulphide nonahydrate (99.9%) and ethylene glycol (99.8%) were purchased from Sigma-Aldrich and used without any further purification. All water used was ultrapure (18 MΩ·cm), obtained from a USF ELGA UHQ system. Dialysis tubing in cellulose membrane (Sigma-Aldrich), having molecular weight cut off 12000 u, was used.

Synthesis and sample preparation. Silver CNPs were synthesized on the basis of Xia’s poly(vinyl pyrrolidone)-assisted polyol synthesis,32-36 following a modification proposed by


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Panfilova et al.37 The CNP solution obtained from the synthesis was centrifuged at 10000 rpm for 30 min and the precipitate was re-dispersed in 30 mL of pure ethanol. This solution (solution A) was stored at 4°C, stable for months. 0.5 mL of solution A was centrifuged for 30 min at 10000 rpm for 3 times, washing each time with water. At the final stage the collected precipitate was dispersed in 15 mL of water (solution B). GSH-CNP solutions were prepared by adding GSH water solution (0.1M) to 3 mL of solution B.

Characterization. CD spectra were recorded on a Jasco spectrometer model 715. Absorption spectra were measured with a Cary/1E spectrometer. The measurements were performed at a wavelength range 200−700 nm, by using 1 cm path length cuvette. The spectral resolution was 1 nm. The reported spectra are results of 20 scans. TEM images were recorded with a FEI Tecnai F20 FEG-TEM. 30 µL of solution were placed on carbon-coated 300-mesh Cu grids and left to dry. RESULTS AND DISCUSSION The absorption spectrum of 30 µM GSH in water presents a single band in the 200-230 nm region (Figure 1b, red curve). According to the literature this band is the result of the convolution of three absorption lines, related to n→π*, π→π* transitions of carbonyl groups and 3p→4s transition of -CH2SH with maxima at 220, 200 and 195 nm respectively.38 In the same wavelength range, the CD spectrum shows a negative profile characterized by a negative peak at 220 nm and an additional negative intensity increase towards the lowest recorded wavelengths. The aqueous solution of bare CNPs, having a side length of about 45 nm (Figure 1c) and rounded edges/vertices, shows two plasmonic absorption bands at 355 and 425 nm (Figure 1b, black curve). These peaks fit the theoretical plasmonic spectrum calculated by the discrete dipole


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approximation method.12 The absorption around 350-400 nm could be associated with several LSPR modes. In our case only a single peak was observed and additional resonance peaks could not be observed, probably owing to the rounded morphology of the cube edges and vertices.39 No CD lines were evident for bare CNPs (Figure 1b). Upon addition of GSH to the CNP solution (pH=5.0), noticeable changes in the CD and absorption spectra of both components started to appear after about 2 hours (Figure 2a, red curve).

Figure 2. a) CD (up, left axis) and absorption (bottom, right axis) spectra of CNPs in 30 µM GSH as a function of time from the sample preparation (mixing of CNPs and GSH solution); b) a


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four hours aged CNP-GSH sample was dialyzed and CD (up, left axis) and absorption (bottom, right axis) spectra shown as a function of dialysis time.

At the UV region, below 325 nm, the absorption spectrum seems to be a superposition of the pure CNP and GSH contributions, whereas a positive signal appears in the GSH CD profile around 200 nm without a significant change throughout the incubation period. On the other hand at the silver plasmon resonance region, the bare CNP absorbance peak at 425 nm have progressively underwent a blue shift, with the plasmonic band initially incremented in intensity and later decreased, upon incubation with GSH. Remarkably, new CD signals were revealed at the plasmonic resonance wavelength range (~350-500 nm). They consist of a positive band at the main CNP absorption peak, corresponding to a plasmonic resonance mode of primarily dipolar nature, and a bisignate Cotton effect having peaks at 360 (positive) and 375 nm (negative). The plasmonic Cotton effect time evolution mostly occurred within 3 hours, leading to the formation of a roughly symmetric bisignate band with an absolute intensity of around 1 mdeg (Figure 2a, blue curve). A slight line-shape modification, consisting of a reduction of Cotton effect symmetry, was detected after additional 23 hours of incubation of CNPs with GSH molecules (Figure 2a, green curve). Regarding the main peak's intensity variation, it is well known that the peak intensity (as well as shift) depends on the optical dielectric constant of the surrounding medium and the increase of a plasmon absorption peak is generally due to the replacement of the surrounding water molecules with a material with a larger refractive index. For spherical NPs the intensity is proportional to the dielectric constant to the power of 3/2 which is basically the refractive index to the power of 3.(40) This fact could be invoked to justify, at least from a qualitative point of


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view, the initial intensity increase of plasmonic absorption in the GSH-CNP complex where the water molecules close to the CNPs (refractive index of 1.33) could be partially replaced by GSH (peptides refractive index is about 1.5). However this phenomenon is generally accompanied by a red-shift of the main plasmon peak, while in our case the blue shift of the main CNP LSPR, which was observed in parallel to the intensity change, is probably dominated by a different effect. It is reasonable to assume that the CNP vertices underwent etching by the thiol groups. The free volume between the capping surfactant molecules (PVP) is usually larger at the vertices and edges, where the surface curvature is the largest. This etching has been demonstrated to lead to a significant blue -shift of the main LSPR mode.(39) It is also possible that removal of silver atoms from the corner did not occur and that the observed result was a consequence of the simple thiolate-silver polar bond formation. This would affect the corner's surface silver atoms to lose their valence electron and stop behaving as metallic atoms, leading to an effective corner "rounding" for the plasmon resonance. Focusing on the time evolution of the absorption spectra in Fig. 2a, at the wavelength range of ~340-370 nm, it is possible to observe an isosbestic point, i.e., a specific wavelength at which the absorbance level remains constant with time. This implies equilibrium between the absorption of two different states (LSPR modes) located to the left and right of the isosbestic point wavelength. The broad absorption tail between the two observed plasmonic peaks hides additional LSPR modes which are not resolvable in the present CNP samples,39 probably due to non-uniformities in CNP vertex and edge sharpness. In addition, it should be noted that the bisignate plasmonic CD signal, centered at 367 nm, does not coincide with the plasmonic peak appearing at 355 nm. Consequently, it can be concluded that the 355 nm plasmon resonance mode does not exhibit induced CD, while another plasmon mode, with an unresolvable absorption peak, centered at


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~367 nm does exhibit induced CD and slightly blue-shifts with increased GSH adsorption. The apparent "red-shift" of the 355 nm peak is primarily not a real shift, but a change in the absorption profile where the 355 nm absorption diminishes and an absorption band around 367 nm increases. Our results can be compared to the results of Gang and coworkers, who combined chiral DNA molecules with a different type of Ag CNPs.12 They observed an additional plasmon resonance mode (since their CNPs probably had sharper vertices) between the two observed modes of the present work, which gave rise to a strong induced plasmonic CD. This specific mode involved oscillation of the plasmonic fields at the vertices of the cubes, where the local field intensity seems to be the highest of all plasmon modes. The mode that is observed both in our and ref. 12's CNPs around 350 nm corresponds to field oscillation at the cube edges, with weaker peak field intensities.12, 39 In order to further analyze the significance of the observed plasmonic and molecular CD lines, a four-hour aged GSH-CNP sample presenting the Cotton effect in the plasmonic region was dialyzed against distilled water to purify it from unbound GSH. This might allow observation of the adsorbed molecular UV CD lines without interference of unbound GSH CD. CD and absorption spectra were collected throughout the dialysis process (Figure 2b) which lasted 26 hours, providing an efficient removal of the unbound GSH as evidenced by the strong GSH absorption drop at the molecular absorption range (200-250 nm). After 3 hours of dialysis, a completely positive CD signal instead of the free GSH negative band around 220 nm was detected in the 200-250 nm region. The CD sign change can be attributed not only to the bound GSH having a different conformation from the free one but also to the mutual interaction of the plasmonic mode associated with the CNP vertex and the GSH molecules.6


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After 26 hours of dialysis the UV absorbance (Figure 2b, magenta curve) became very similar to the curve of bare CNPs (Figure 2b, black curve), reflecting the relatively small amount of bound GSH, while the positive molecular UV CD after 1 and 3 hours of dialysis (blue and green CD curves, 200-230 nm) was quite large, indicating an enhancement of the adsorbed molecular CD lines by the plasmon resonance. The decline of the molecular CD line after 26 hours of dialysis could be due to (morphological or effective) etching of the vertices and accompanying decrease in local field intensity, as evidenced in the diminishing 355 nm plasmon absorption peak. The sensitivity of the spectroscopic response of the GSH-CNP complex to the solution's pH value was also studied. On increasing the pH from a value of 5 up to extremely basic values, the absorption and CD spectra did not change. Conversely, a slight decrease of pH (0.5 unit) induced drastic spectral variations (Figure 3, red curve).

Figure 3. CD (up, left axis) and absorption (bottom, right axis) spectra of dialyzed GSH-CNP complex at different pH values.


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The formation of a new absorption band at 280 nm together with a further increase and red shift of the small 355 nm CNP absorption peak were observed. Meanwhile, a very large CD signal enhancement occurred in the 200-450 nm region. A new bisignate Cotton effect was detected at 280 nm in alignment with the new UV absorption peak along with a positive intense CD profile with two clear maxima at 248 and 205 nm on the short wavelength side. Moreover, the intensity of the CNP vertices-related plasmon resonance CD was significantly larger and showed a different line-shape (all positive), and an additional CD band seemed to appear in the form of a shoulder to the left of this resonance. The latter feature might correspond to an even weaker high-order plasmonic resonance.39 Also the induced CD at the dipolar plasmon resonance mode was enhanced with a strong line-shape change. The UV absorption band at 280 nm is unrelated to the original CNP and GSH species, suggesting the formation of new species. Relevant absorptions at 280 nm were already detected in other thiols interacting with PVP coated silver nanoparticles and attributed to disulfide bonds formed because of surface catalyzed oxidative coupling of thiols.41 A similar transformation probably occurred in our system, leading to the formation of diglutathione (GSSG). In order to prove the GSSG presence, NaBH4 solution was added to the system for disulfide bond reduction,42 demonstrating an almost perfect reversibility of the CD profile and the absorption peaks at 280 nm and 360 nm (Figure 4, magenta curve). Hence, it is clear that the whole change in the CD spectrum was related to the formation of the GSSG, whose disulfide group was probably attached to the silver surface.


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Figure 4. CD (top, left axis) and absorption (bottom, right axis) spectra of dialyzed GSH-CNP complex at pH=5.0 (black) and GSH-CNP complex at pH=4.5 after addition of NaBH4 (magenta).

The interaction of GSH with a silver nanoparticle as function of the pH has been recently studied by Huang et al. by the surface-enhanced Raman scattering technique.43 From their analysis it emerges that the main interaction with the silver surface occurs through the sulfur of the cysteine residue along with the participation of carbonyl groups and nitrogen atom of the terminal glutamic acid amine group. Upon the decrease of pH from 5 to 4, the proximity of these groups to the surface suddenly increases, suggesting that at this pH new conditions enhance the reciprocal interaction between the molecule and the surface, probably forcing the molecule to a different orientation with respect to the surface, and perhaps improving the uniformity of molecular conformation at the surface due to the disulfide bond formation. The reported pH range of the Raman enhancement factor variation perfectly matches the pH interval where we observed the different spectroscopic behavior of GSH-CNP.43 However, considering the reported GSH pKa values (2.05, 3.40, 8.72, 9.49), the pH induced effect does not seem attributable to a


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mere molecular conformation change related to molecular group deprotonation/protonation phenomena. Since the pKa of the silver oxide surface is expected to be significantly higher than 7 than it is left to assume that it is the combination of the GSH and silver (oxide) surface that was sensitive to the pH transition from 5 to 4.5 and enabled the catalysis of GSSG formation. It was shown that around this pH range an increase in reactive oxygen species is detected for oxidized silver surfaces.44 By analyzing the CD and absorption curve shape of the disulfide bond it is possible to get information about the conformation of the GSSG molecule with respect to the surface as the peak wavelengths and line shapes are strongly dependent on the S-S bond dihedral angle.45 The GSSG absorption spectrum presents, in addition to most of the spectral elements observed for GSH, an absorption at higher wavelength due to the n→π* transition of the disulphide bond lone pair.38 For dihedral angle values different from 90°, which is the angle of the minimum energy conformation of the unconstrained disulphide, the n ground state becomes split , thus generating two distinct transitions. It is therefore possible, that in our spectra a second S-S weak absorption peak is hidden to the red side of the clearer 280 nm peak, around the CD shoulder at ~330-350 nm. As the disulfide was probably still adsorbed to the silver (oxide) surface, the two S-C bonds were forced to a nearly parallel configuration, i.e. dihedral angle close to 0°. This should move the higher absorbing state of the S-S system to around 350 nm,45 where accordingly we detected a clear enhancement of the absorption curve upon pH lowering (Figure 3). The last interesting spectral feature of the low pH CNP-GSH system is the relatively sharper small plasmon resonance absorption peak associated with the cube edges, relative to the higher pH. This might be related to the oxidation of the thiol groups, which could result in reduced silver atoms at the


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vertices and increased free electron density at these locations, reversing the abovementioned silver oxidation effect associated with the GSH adsorption to the CNP vertices. A previous work on the interaction of GSH with spherical PVP-capped Ag nanoparticles in acid environment reports a CD profile similar to that shown in Figure 3 (without the vertex-related plasmonic feature),31 which makes the disulfide related CD contribution visible in the broad wavelength range of 250-370 nm. In that paper it is also clearly visible that in the case of pure GSSG, there is only a 280 nm –SS- related CD band, while on top of the Ag particles the extra band to the red side also appears and the line-shape of the 280 nm band completely changes. This further shows that the GSSG bound to the silver surface assumes a very different conformation relative to the free dimer, together with apparent strong electronic interaction of the disulfide group with the silver surface. We believe that this change in alignment is primarily responsible for the strong increase in the intensity and for the change in lineshape of the induced plasmonic CD with the reduction of pH value to 4.5. A major question that remains open is why does GSH interaction with high order plasmon resonance modes produce a strong induced CD, while the main dipolar resonance exhibits much smaller CD induction? This phenomenon was first observed by Gang and coworkers for DNA molecules adsorbed on CNPs.12 One possible explanation is that the molecules adsorbed to the cube vertices are better oriented with respect to the surface relative to molecules adsorbed to the cube facets. The dipolar resonance is probing the facet area, while the high order resonances are concentrated at the vertices and edges. The more disordered molecules are not expected to contribute to induced plasmonic CD, while the well-oriented ones would contribute to this effect.

CONCLUSION


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In conclusion, new CD response and modifications of bare CNP absorption were detected at the plasmonic region (λ> 350 nm) by GSH functionalized CNPs. The observed CD signals were correlated to the CNP higher order plasmonic modes and related to the CNP vetices and therefore probably attributable to the interaction between the giant electric field at the CNP vertices and the chiral molecules. Moreover, the system exhibited extreme pH sensitivity, combining a chemical effect, of catalyzing the formation of GSSG and consequently drastically modifying the absorption and CD responses both at the molecular and plasmonic excitations when the pH value was lowered from 5 to 4.5. This chemical reaction and the related CD and absorption changes were reversible. This work is a demonstration of the potential usefulness of chiroptical spectroscopy in understanding molecule-inorganic surface interactions, and in particular surface-plasmonmolecule interactions. A theory covering all the details of the interaction of complex plasmon resonance modes with adsorbed (chiral) molecules is not available so far and would be very valuable in the interpretation of the above-mentioned effects.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT This research was supported by The Israel Science Foundation grant no. 507/14. ABM was supported by an Adams fellowship.

ABBREVIATIONS CD, circular dichroism; LSPR, localized surface plasmon resonances; GSH, glutathione; CNP, cube-shaped nanoparticles; GSSG, diglutathione. REFERENCES (1) Circular Dichroism: Principles and Applications; Nakanishi, K., Berova, N., Woody, R. W., Eds.; Wiley-VCH: New York, 2000. (2) Fan, Z.; Govorov, A. O. Helical Metal Nanoparticle Assemblies with Defects: Plasmonic Chirality and Circular Dichroism. J. Phys. Chem. C 2011, 115, 13254-13261. (3) Droulias, S.; Yannopapas, V. Broad-Band Giant Circular Dichroism in Metamaterials of Twisted Chains of Metallic Nanoparticles. J. Phys. Chem. C 2013, 117, 1130-1135. (4) Fan, Z.; Govorov, A. O. Plasmonic Circular Dichroism of Chiral Metal Nanoparticle Assemblies. Nano Lett. 2010, 10, 2580-2587. (5) Govorov, A. O.; Gun'ko, Y. K.; Slocik, J. M.; Gérard, V. A.; Fan, Z.; Naik, R. R. Chiral Nanoparticle Assemblies: Circular Dichroism, Plasmonic Interactions, and Exciton Effects. J. Mater. Chem. 2011, 21, 16806-16818.


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(6) Govorov, A. O.; Fan, Z.; Hernandez, P.; Slocik, J. M.; Naik, R. R. Theory of Circular Dichroism of Nanomaterials Comprising Chiral Molecules and Nanocrystals: Plasmon Enhancement, Dipole Interactions, and Dielectric Effects. Nano Lett. 2010, 10, 13741382. (7) Layani, M. E.; Ben Moshe, A.; Varenik, M.; Regev, O.; Zhang, H.; Govorov, A. O.; Markovich, G. Chiroptical Activity in Silver Cholate Nanostructures Induced by the Formation of Nanoparticle Assemblies. J. Phys. Chem. C 2013, 117, 22240-22244. (8) Nishida, N.; Yao, H.; Kimura, K. Chiral Functionalization of Optically Inactive Monolayer-Protected Silver Nanoclusters by Chiral Ligand-Exchange Reactions. Langmuir 2008, 24, 2759-2766. (9) Yao, H.; Saeki, M.; Kimura, K. Induced Optical Activity in Boronic-Acid-Protected Silver Nanoclusters by Complexation with Chiral Fructose. J. Phys. Chem. C 2010, 114, 15909–15915. (10) Schaaff, T. G.; Whetten, R. L. Giant Gold-Glutathione Cluster Compounds: Intense Optical Activity in Metal-Based Transitions. J. Phys. Chem. B 2000, 104, 2630-2641. (11) Slocik, J. M.; Govorov, A. O.; Naik, R. R. Plasmonic Circular Dichroism of PeptideFunctionalized Gold Nanoparticles. Nano Lett. 2011, 11, 701-705. (12) Lu, F.; Tian, Y.; Liu, M.; Su, D.; Zhang, H.; Govorov, A. O.; Gang, O. Discrete Nanocubes as Plasmonic Reporters of Molecular Chirality. Nano Lett. 2013, 13, 31453151.


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TOC Figure


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Figure 1 83x176mm (600 x 600 DPI)



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