Strong Enhancement of Circular Dichroism in a Hybrid Material

Article pubs.acs.org/JPCC

Strong Enhancement of Circular Dichroism in a Hybrid Material Consisting of J‑Aggregates and Silver Nanoparticles Dzmitry Melnikau,*,†,⊥ Diana Savateeva,† Yurii K. Gun’ko,‡ and Yury P. Rakovich†,§ †

Centro de Física de Materiales (MPC, CSIC-UPV/EHU), Donostia International Physics Center (DIPC), Po Manuel de Lardizabal 5, Donostia-San Sebastian 20018, Spain ‡ School of Chemistry and CRANN, University of Dublin, Trinity College, Dublin 2, Ireland § IKERBASQUE, Basque Foundation for Science, Bilbao, 48011, Spain ⊥ CIC nanoGune Consolider, Tolosa Hiribidea, 76 Donostia-San-Sebastian, 20018, Spain ABSTRACT: We report on the chiroptical (circular dichroism) properties of hybrid organic/inorganic nano- and microstructures consisting of dye molecules in J-aggregate states (arranged in direct and reverse micelles) and silver nanoparticles. The main results are formation of micelles in a system of interacting J-aggregates and silver nanoparticles and strong (up to 8 times) enhancement in the optical activity of complexes of J-aggregates and silver nanoparticles as compared to the CD signal from the original J-aggregates. These experimental results might have potential applications in chiral species sensing and can provide some insights on mechanisms of self-assembly of supermolecular structures and the induction of their optical activity.

1. INTRODUCTION Circular dichroism spectroscopy (CD) is a very important technique widely used in molecular biology,1 chemistry,2 and nanoscience3 to examine the chirality of organic and bioorganic molecules, biological systems, and inorganic nanostructures. Chirality is one of the most fascinating occurrences in the natural world. A chiral molecule has two mirror-image forms, which are not three-dimensionally superimposable. These mirror-image forms of the chiral molecule are known as enantiomers. As a phenomenon chirality plays an important role in the fields of chemistry, pharmacology, biology, and medicine. Chirality is also one of the key factors in molecular recognition, which has many practical uses in chemistry and biology. Therefore the enhancement of the sensitivity of circular dichroism spectroscopy is one of the emerging trends in areas of high scientific and technological importance. Currently, an increasing number of theoretical and experimental papers report on various aspects of CD activity enhancement in hybrid organic/inorganic nanostructures consisting of chiral organic molecules and achiral nanocrystals and nanoparticles.4−6 The field of hybrid chiral organic/ inorganic nanostructures is rapidly developing and very promising for biosensing applications.7 As most biological molecules are naturally chiral, their interaction with nanocrystals leads to significant changes in the optical activity of these hybrid nanostructures.8,3 Among other supramolecular systems, chiral J-aggregates of cyanine dyes are particularly interesting because of their ability to delocalize and migrate excitonic energy over a large number © XXXX American Chemical Society

of aggregated dye molecules which is essential for potential applications of J-aggregates as optoelectronic materials, in photosynthesis, and for the development of advanced photonic technologies.9 Usually the chirality of J-aggregates can be generated by using an appropriate template.10,11 Also it is well-known that enhancement of CD activity of J-aggregates can be induced by a variety of chiral molecules-additives (DNA, D- and L-phenylalanine and others).10,12 By contrast, in this work we report chiroptical properties of hybrid organic/inorganic nanostructures consisting of Jaggregates of cyanine dye and silver nanoparticles which were formed without any chiral agent. We demonstrate that efficient polyelectrolyte-induced formation of micelles in a system of interacting J-aggregates and silver nanoparticles results in strong enhancement in the optical activity of complexes of Jaggregates and silver nanoparticles as compared to the CD signal from pure J-aggregates.

2. EXPERIMENTAL SECTION Colloidal silver nanoparticles (NPs) of 2−30 nm average size were synthesized by the conventional citrate reduction method by adding 0.8 mL of 10 mM AgNO3 to 1.4 mL of water. After adjusting the pH to 10, this solution was stirred at 0 °C. Finally, 0.8 mL of 10 mM NaBH4 was added. Doubly purified Received: April 16, 2013 Revised: June 5, 2013

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dx.doi.org/10.1021/jp4037777 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. Schematic illustration of the formation of the J-aggregates/silver nanoparticles hybrid system as a result of the anion exchange mechanism between the JC1 molecule and PDDA and electrostatic interaction between the positively charged PDDA and the negatively charged surface of Ag NPs (not to scale).

deionized water from an 18 M Millipore system was used for all dilutions. 5,5′,6,6′-Tetrachloro-1,1′,3,3′-tetraethylimidacarbocyanine iodide (JC1) carrying net positive charge and polydiallyldimethylammonium chloride (PDDA) were purchased from SigmaAldrich and used without further purification. To produce hybrid organic/inorganic nanostructures of J-aggregates/silver nanoparticles, J-aggregates (spontaneously formed when cyanine dye JC1 was dissolved in water at pH 8) were mixed with silver nanoparticles covered by polyelectrolyte PDDA in different concentration ratios. Here take advantage of electrostatic interaction between positively charged PDDA and negative charged surface of Ag NPs. PDDA covers the whole surface of silver nanoparticle that protects the particles from aggregation and makes the colloidal solution of NPs more stable. The molecular J-aggregate of JC1 dye interacts with PDDA through an anion exchange mechanism, where the I− anion of JC1 is exchanged for the Cl− of PDDA. In this case polyelectrolyte PDDA works like a linker between silver NPs and J-aggregates (Figure 1). Jasco V-630Bio and FP6600 (Jasco) spectrometers were used to measure the absorption and PL spectra, respectively. CD spectra were recored with a Jasco J-815 CD spectrometer. Scanning electron microscope (SEM) images were taken with Environmental Scanning Electron Microscope (ESEM) Quanta 250 FEG.

Figure 2 also displays that J-aggregates of JC1 show very weak CD activity both at the maximum of absorption of J-

Figure 2. Absorption and circular dichroism spectra of J-aggregates in the presence of PDDA (red) and without PDDA (blue curves) at pH 8. Inset: Scanning electron microscope images of J-aggregates in the presence of PDDA.

aggregates and monomer molecules of dye regardless of the presence of PDDA. Generally, spontaneously formed J-aggregates of JC1 show positive Cotton effect, which is typical for left-handed helix structures (Figure 2). Indeed the inset in Figure 2 presents an SEM image where one can clearly see the left-handed helix structure of individual rod-like J-aggregates. However, although the solution contains these chiral structures, the CD signal from the sample is very weak. Anisotropy g-factors (calculated

3. RESULTS AND DISCUSSION J-aggregates of JC1 show a narrow absorption band (J-band) at 595 nm with a full width at half-maximum of 7 nm, alongside a broader absorption band, positioned at the lower wavelength side from the J-band (at 500 nm), which we assign to the absorption of JC1 monomers.13 B



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the plasmon peak observed in Figure 4 is the result of an interaction of Ag NPs with PDDA and with J-aggregates of JC1 due to the increase in the refractive index near the nanoparticle surface. Along with the changes in the absorption spectrum in the presence of Ag NPs we also observed a significant quenching of the luminescence of J-aggregates (Figure 4b) which provided further evidence for direct interaction of molecular excitons and surface plasmons in hybrid Jaggregates/silver nanoparticles complexes. Analysis of SEM images of hybrid structures shows that the interaction of J-aggregates and Ag NPs covered by polyelectrolyte PDDA induces the formation of spherical micelles and, in a smaller amount, rod shaped J-aggregates of JC1. Figure 5a demonstrates that indeed Ag NPs are integrated with micelles of J-aggregates forming the J-aggregates/Ag NPs hybrid nanostructures. Ag NPs covered by PDDA stick to the surface of micelles of J-aggregates and stimulate their further aggregation, resulting in the formation of various chiral-like super structures. For these hybrid structures (Figure 6) we observed a strong (8 times) increase in optical activity of J-aggregates/Ag NPs complexes as compared to the CD signal of J-aggregates with PDDA and without AG NPs. In contrast, while mixing Jaggregates and silver NPs solution without PDDA reverse micelles of J-aggregates are formed. Figure 4b shows that in this case Ag NPs are located inside the reversed micelles of Jaggregates and almost do not interface with both monomeric and aggregated dye molecules. As a result for these hybrid structures we did not detect any enhancement of optical activity of either J-aggregates or Ag NPs. While Ag NPs induce the formation of spherical micelles and rod-shaped J-aggregates of JC1 (shown in Figure 5), the intensity of the absorption of the J-band does not change much after the injection of Ag NPs in J-aggregate solutions (Figure 6). However, in Figure 6 one can clearly see a decrease of the intensity of the monomer absorption band of JC1 at 500 nm and at the same time broadening and 5 nm blue shift of the Jaggregate band originally centered at 594 nm (Figure 2), probably due to enhanced scattering of light as a result of the formation of spherical micelles and rod-shaped J-aggregates of JC1. Also we observe equal increases in optical activity of

according to ref 14) for J-aggregates and monomer were estimated to be 0.6 × 10−3 and 0.9 × 10−4, respectively. Recently, CD activity has been reported in ligand-protected metal clusters and nanoparticles when chiral ligands were used.15 Although not in our case (we did not use any chiral agents in our experiments), it is noteworthy that as-synthesized silver nanoparticles also show a very feeble, but detectable CD signal at the plasmon maximum (Figure 3) with an anisotropy g-factor 0.5 × 10−4.

Figure 3. Absorption and circular dichroism spectra of silver nanoparticles without (blue) and with PDDA (red).

A suggestion of very weak CD activity in our plasmonic nanocrystals is the formation of chiral states on the surface of the metal nanoparticles due to the chiral arrangement of the stabilizing molecules at the surface.16 Spectral analysis (Figure 4) and SEM imaging (Figure 5) confirmed incorporation of silver nanoparticles into the hybrid nanostructures. The red shift of the plasmon peak at 400 nm, clearly seen in Figure 4a, proves that the interaction of Ag NPs with both PDDA and dye molecules takes place. It is well-known that silver nanoparticle optical properties depend on the refractive index near the nanoparticle surface.17 Therefore the red shift of

Figure 4. (a) Absorption spectra of Ag NPs alone (black), Ag NPs with PDDA (red), and hybrid nanostructures of J-aggregates/silver nanoparticles (green). (b) Absorption and PL (inset) spectra of hybrid nanostructures of J-aggregates/silver nanoparticles (red curves) and J-aggregates in the presence of PDDA (blue curves). C



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Figure 5. Scanning electron microscope images: (a) hybrid nanostructures of J-aggregate micelles and silver NPs covered by PDDA and (b) reversed micelles of J-aggregates and silver NPs without PDDA.

Amplification of optical activity of J-aggregates/Ag NPs nanostructures in the spectral region of the plasmon peak (around 400 nm) can be explained by the plasmon-induced resonant circular light absorption enhancement arising from the larger particles and particle aggregates. In this scenario the mechanism involves orbital interactions between weak chiral agents (J-aggregates) and metal nanoparticles, and dipolar Coulomb interactions between chiral optical dipoles of these two systems.18 It is important to emphasize that plasmonic enhancement of CD signals in the developed hybrid system takes place only after aggregation which enhances the molecule−plasmon interactions. Therefore the observed enhancement of CD signal in the region of plasmon peak (400 nm) can be reasonably explained by the mechanism based on the Coulomb and electromagnetic interactions between chiral molecular strucures and plasmons. Unfortunately not much is known about chirality effects in micellar aggregates of geometrically defined structures. To explain the reason for the increase in the intensity of the CD signal in J-aggregates/Ag NPs nanostructures at the position of the J-band (at 595 nm) and the band of dye monomers (at 502 nm), it is appropriate to reconsider some of the data reported on chiral properties of cyanine dye J-aggregates and plasmonic nanoparticles. It was recently reported that under certain conditions J-aggregates possessing optical activity can be observed from initially achiral cyanine dyes presumably due to specific orientations of the chemical groups of the dye monomers upon aggregation.19 This mechanism can explain the spontaneous formation of rod-like structures of J-aggregates with weak CD activity (Figure 2). Another and more common way to induce the chirality during the formation of J-aggregates is to use chiral molecules−additives or templates even with weak chirality.10 In this respect an injection of Ag NPs possessing some degree of chirality in dye solution can also result in the formation of chiral rod-like J-aggregate structures (Figure 5a). It has also been demonstrated that chiral molecules adsorbed on the surface of nanocrystals can induce chirality of the nanocrystalś core. It was further suggested that the atomic origin of chiral sites in nanocrystals is topologically similar to that in organic compounds.20 Therefore, the supramolecular

Figure 6. Absorption and circular dichroism spectra of J-aggregates with PDDA (red curves), Ag NPs alone covered by PDDA (black), and hybrid nanostructures of J-aggregates/Ag NPs (green). The inset shows an SEM image of J-aggregate micelles interconnected by Ag NPs.

hybrid J-aggregates/Ag NPs nanostructure in the spectral region of J-bands and the monomer of cyanine dye JC1 (Figure. 6). The formation of hybrid organic/inorganic nanostructures of J-aggregates/Ag NPs leads to CD activity enhancement of not only J-aggregates of JC1 but also plasmonic silver nanoparticles at 400 nm (Figure 6). Optical activity of J-aggregates/Ag NPs nanostructures in the region of plasmon peak is 1.5 times higher than that for Ag NPs alone or covered by polyelectrolyte PDDA (Figure 6, black curve). In Figure 6 one can see an enhancement of CD signal around 400 nm together with the drop in intensity and red shift of plasmon absorption peak as a result of the formation of a hybrid nanostructures of Jaggregates/silver nanoparticles. These changes in absorption spectra of Ag NPs together with SEM images confirm both interaction of NPs with J-aggregates and aggregation of Ag NPs. D



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Chain Reaction: Collective Interactions of Nanoparticles and a New Principle for Chiral Materials. Nano Lett. 2009, 9, 2153−2159. (7) Fan, Z.; Govorov, A. O. Chiral Nanocrystals: Plasmonic Spectra and Circular Dichroism. Nano Lett. 2012, 12, 3283−3289. (8) Román-Velázquez, C. E.; Noguez, C.; Garzón, I. L. Circular Dichroism Simulated Spectra of Chiral Gold Nanoclusters: A Dipole Approximation. J. Phys. Chem. B 2003, 107, 12035−12038. (9) Steiger, R.; Pugin, R.; Heier, J. J-Aggregation of Cyanine Dyes by Self-Assembly. Colloids Surf. B 2009, 74, 484−491. (10) Slavnova, T. D.; Görner, H.; Chibisov, A. K. Cyanine-Based JAggregates as a Chirality-Sensing Supramolecular System. J. Phys. Chem. B 2011, 115, 3379−3384. (11) Zhang, Y.; Xiang, J.; Tang, Y.; Xu, G.; Yan, W. Chiral Transformation of Achiral J-Aggregates of a Cyanine Dye Templated by Human Serum Albumin. ChemPhysChem. 2007, 8, 224−226. (12) Zeng, L. X.; He, Y. J.; Dai, A. Z. F.; Wang, J.; Wang, C. Q.; Yang, Y. G. Chiral Assembly of Achiral Pseudoisocyanine with D- and LPhenylalanine. Sci. China, Ser. B: Chem. 2009, 52, 1227−1234. (13) Melnikau, D.; Savateeva, D.; Susha, A. S.; Rogach, A. L.; Rakovich, Y. P. Strong Plasmon-Exciton Coupling in a Hybrid System of Gold Nanostars and J-Aggregates. Nanoscale Res. Lett. 2013, 8, 1−6. (14) Kuhn, H.; Försterling, H. D.; Waldeck, D. H. Principles of Physical Chemistry; John Wiley & Sons: Hoboken, NJ, 2009. (15) 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. (16) Dolamic, I.; Knoppe, S.; Dass, A.; Bü r gi, T. First Enantioseparation and Circular Dichroism Spectra of Au38 Clusters Protected by Achiral Ligands. Nat. Commun. 2012, 3, 798. (17) Mock, J. J.; Smith, D. R.; Schultz, S. Local Refractive Index Dependence of Plasmon Resonance Spectra from Individual Nanoparticles. Nano Lett. 2003, 3, 485−491. (18) 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, 1374−1382. (19) Kirstein, S.; von Berlepsch, H.; Böttcher, C.; Burger, C.; Ouart, A.; Reck, G.; Dähne, S. Chiral J-Aggregates Formed by Achiral Cyanine Dyes. ChemPhysChem 2000, 1, 146−150. (20) Zhou, Y.; Yang, M.; Sun, K.; Tang, Z.; Kotov, N. A. Similar Topological Origin of Chiral Centers in Organic and Nanoscale Inorganic Structures: Effect of Stabilizer Chirality on Optical Isomerism and Growth of CdTe Nanocrystals. J. Am. Chem. Soc. 2010, 132, 6006−6013. (21) Fan, Z.; Govorov, A. O. Plasmonic Circular Dichroism of Chiral Metal Nanoparticle Assemblies. Nano Lett. 2010, 10, 2580−2587. (22) Govan, J. E.; Jan, E.; Querejeta, A.; Kotov, N. A.; Gun’ko, Y. K. Chiral Luminescent CdS Nano-Tetrapods. Chem. Commun. 2010, 46, 6072−6074. (23) Bovet, N.; McMillan, N.; Gadegaard, N.; Kadodwala, M. Supramolecular Assembly Facilitating Adsorbate-Induced Chiral Electronic States in a Metal Surface. J. Phys. Chem. B 2007, 111, 10005−10011.

arrangement of optically active plasmonic nanoparticles interacting in assemblies may not only enhance their chirality,21 but also induce helical arrangement22 of micells in chiral superstructures with strongly enhanced optical activity (Figures 5 and 6). Indeed, SEM imaging revealed the formation of similar superstructures (Figure 6, inset). Thus, we believe that there are several concurrent processes in the J-aggregates/silver nanoparticle system which result in the significant enhancement of CD response.

4. CONCLUSIONS We have shown that the chiral supramolecular micellar Jaggregates of JC1 dye can be formed in the presence of Ag NPs in aqueous solution. Using CD spectroscopy we recorded strong (up to 8 times) enhancement in the optical activity of complexes of J-aggregates and silver nanoparticles as compared to the CD signal from pure J-aggregates. Our experimental findings may give some insight into the mechanisms of selfassembly of supermolecular structures and the induction of their optical activity. We also belive that these chiral hybrid aggregates might have potential applications in devices for sensing of chiral species,23 chiral information transfer, and other uses.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +34-943-01-8817. Fax: +34-943-01-5800. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was in part financially supported by the ETORTEK 2011−2013 project “nanoIKER” from the Department of Industry of the Basque Government; D.S., Y.G., and Y.R. acknowledge the support received from the European Science Foundation (activity PLASMON-BIONANOSENSE) and Science Foundation Ireland (grant SFI 07/IN.1/I1862). The authors are thankful to Professor A. Chuvilin for assistance with SEM imaging.

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REFERENCES

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Strong Enhancement of Circular Dichroism in a Hybrid Material

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