Plasmonic Core–Satellites Nanostructures with High Chirality and

Letter pubs.acs.org/JPCL

Plasmonic Core−Satellites Nanostructures with High Chirality and Bioproperty Liguang Xu,# Changlong Hao,# Honghong Yin, Liqiang Liu, Wei Ma, Libing Wang, Hua Kuang,* and Chuanlai Xu State Key Lab of Food Science and Technology, School of Food Science and Technology, Jiangnan University, Wuxi, Jiangsu 214122, China S Supporting Information *

ABSTRACT: In this Letter, gold nanorods (Au NRs) and gold nanoparticles (Au NPs) were assembled into core−satellites (Au NR−NPs) nanostructures using DNA as the linkers. Circular dichroism (CD) measurements of the nanoassemblies displayed two plasmonic CD (PCD) peaks in the vicinity of the surface plasmon resonance (SPR). Interestingly, the number of Au NPs in the assemblies had a significant influence on the shape and intensity of the CD line. The assemblies were enzymatically disassembled by deoxyribonuclease I (DNase I), and the CD responses were simultaneously reversed. With the proof-of-concept design, the PCD response was no change by addding enzyme inhibitor. These experiments suggested that the chirality depended upon the structure of core−satellites nanoassemblies, and the results clarify that the possible origin of the optical activity comes from chiral arrangement of building blocks and Coulomb dipole−dipole interactions. This research also illuminated that the assemblies can be used to develop a new sensor for the sensitive screening of the enzyme inhibitors. SECTION: Plasmonics, Optical Materials, and Hard Matter

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chirality in the hybrid of Au NP and Au NR has never been investigated. Moreover, some reports34−36 have proved that enzymes can be used to control (selective ligation and cleavage) nanostructure assembly and disassembly of Au NPs. For example, Alivisatos and his co-workers33 created Au NP dimer and trimer nanostructures through enzymatic ligation. In this study, core−satellites (a Au NR core surrounded by Au NPs) nanostructures were fabricated to study the chirality. With the aid of endonuclease and its inhibitor, the circular dichroism (CD) changes of the nanostructures were carried out in detail to further illustrate the origin of chirality, and the inhibitor could be sensitively screened using the chirality of core− satellites superstructures. Three-Dimensional Self-Assembly and CD Response of the Core−Satellites Assemblies. Following incubation of Au NR− DNA-1 and Au NP−DNA-2 for 12 h, the core−satellites architecture (Figure 1f and Figure S1, Supporting Information) was formed, as defined in Scheme 1. The transmission electron microscopy (TEM) images in Figure 1a−f clearly demonstrate that the number of Au NPs (satellites) per Au NR (core) increases with reaction time. More obvious and direct results can be seen from the statistical analysis (Figure S2, Supporting Information) of the assemblies from 100 TEM images at different hybridization times. Long hybridization times allowed

hirality is an important aspect of molecular biology and can provide significant structural properties of many biomolecules. Numerous studies have confirmed that chirality and metals can be placed in the same context.1 Although the chiral induction mechanism in many examples is unclear, the chiral effect can be classified into two distinct origins, (i) the intrinsically chiral nanoclusters with individual chirality and (ii) collective chirality based on collective plasmon interactions in the chiral structures.2 Since Schaaff and Whetten3 first demonstrated the new optical activity in glutathione-protected gold nanoclusters in 2000, many studies on chiral-ligandprotected metal (especially Au4−9 and Ag10−18) nanoclusters followed. Compared with these individual chiral nanoparticles, chiral superstructures have been attracting more and more attention because of their higher anisotropy coefficients, and they are relatively easy to prepare with the help of many templates including protein,13 virus,19 lipid,20 peptide,21 and chiral fiber.22 A great number of artificial chiral nanostructures and theoretical models such as the helical Ag nanochain,13 Au pyramid,23,24 and asymmetric assemblies of gold nanoparticles (Au NPs)25−28 have been constructed due to their potentially wide application. For example, Xu and co-workers26 applied asymmetric plasmonic Au NP dimers to fabricate a universal chirality-based detection platform. More recently, Zhu and coworkers29 reported the detection of mercury ions using gold nanorod (Au NR) ladder assemblies as the chiroptical sensor. Many of these studies were carried out with Au NPs19,21,30,31 or Au NRs,20,22,32,33 individually; however, the induction of © 2013 American Chemical Society

Received: May 16, 2013 Accepted: July 4, 2013 Published: July 4, 2013 2379

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from −50 to +50° with 2° tilt intervals were collected. These tilted projections were aligned and were used for a threedimensional (3D) reconstruction of the assemblies. As illustrated in Figure S3 (Supporting Information), the Au NPs were spatially arranged around the Au NR in 3D space, which confirmed successful controlled formation of the core− satellites assemblies. Dynamic light scattering measurements showed that the average hydrodynamic diameter (DH) of core−satellites assemblies in solution was 105 nm (Figure S4, Supporting Information). No population was observed at DH greater than 200 nm, which indicated that there were no large aggregations in the nanoassemblies. It is worth noting that the aqueous solution of assembled core−satellites nanostructures was highly stable for 1 month in the absence of DNA cleaving enzymes. Figure 2 shows the optical properties of the assembled core− satellites nanoassemblies at different hybridization times. As shown in Figure 2a, the longitudinal absorption peak of Au NR gradually shifted from 700 to 714 nm with time, and the superposition peak (the superposition of the plasmon band of the satellites and the transverse band of the core) bathochromically shifted from 515 to 523 nm, which confirmed the stepwise growth of the nanoassemblies and was consistent with the TEM data and other published studies.37,38 These spectral position changes were typical for multiple plasmon resonance modes to split the energy level of charge oscillations, which arose by both symmetric and antisymmetric hybridization between the Au NR and the surrounding Au NPs in core−satellites assemblies.38−42 In the visible range from 400 to 800 nm, two new peaks at 520 and 715 nm appeared in the CD spectrum of the nanoassemblies (Figure 2b), while the Au NR−DNA-1 and Au NP−DNA-2 samples showed no chiral effect in this region. The CD response around 520 nm may be due to the coupling between transverse localized surface plasmon resonance (LSPR) of the Au NR and the surrounding Au NPs, and the close proximity of satellites (Au NPs) can induce coupling of their plasmon oscillations, which also results in a red shift in the absorbance band to 523 nm. The CD peak at 715 nm may be due to the longitudinal LSPR of Au NR. The CD signal strength at 200−300 nm decreased in the order core−satellites assemblies > nanocrystals (NCs)−DNA conjugation (Au NR−DNA-1 and Au NP−DNA-2) > ssDNA (DNA-1 and DNA-2). These CD bands in these regions should be attributed to the chiral DNAs, which typically exhibit their chiroptical activities in this region. Note that other CD bands, for instance, dark plasmon modes of the core−satellites

Figure 1. Representative TEM images of the assemblies for different hybridization times; the assembled times were (a) 5 and (b) 30 min and (c) 1, (d) 3, (e) 6, and (f) 12 h.

Scheme 1. Schematic Illustration of the Assembly and Disassembly of Core−Satellites Au NR−Au NPs Superstructures

more satellites to be absorbed onto the core. The organization of Au NPs in the assemblies was further investigated by electron tomographic analysis (FEI 300-kV Titan Krios cryo electron microscope equipped with a Gatan Ultra Scan 4000 (model 895) 16-megapixel CCD). A series of tilted images

Figure 2. UV−vis (a) and the corresponding CD spectra (b) of Au NP, Au NR, and the core−satellites assemblies for different hybridization times. 2380

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superstructures, could also appear in this region.43 According to a recent theoretical study,44 a large electromagnetic field led to a significant increase in CD intensity of DNAs in the assemblies. Furthermore, the coupling of the plasmon between Au NR and Au NPs is much stronger than that of individual Au NRs and Au NPs; therefore, the CD signal intensity of Au NCs modified with DNAs on their surfaces is lower than that of core−satellites assemblies. The spectral changes as a function of hybridization time were also determined and were identical to the trend in UV−vis spectral changes. During the period from 5 min to 12 h, the shape of the plasmonic CD (PCD) spectra was almost unchanged, while the CD signal intensity progressively increased. We can conclude that the position and intensity of the new resonance band depend mainly on the number of Au NPs around the core Au NR. The more Au NPs absorbed onto the surface of the Au NR, the stronger the CD signal intensity. Therefore, the new CD states emerge, and the signal gradually increases as the hybridization time increases. Where did the CD signal observed at the plasmonic frequency of Au originate from? According to Govorov and other researchers reports,45 chirality can be introduced when an achiral object is in close contact with a chiral chromophore and is attributed to the structural perturbation of the chiral component. In the case of core−satellites nanoassemblies, which consist of nanosized building blocks (Au NPs and Au NR) and chiral DNAs, the chirality of the assemblies might originate from five different aspects, (1) chiral NCs, (2) chiral arrangement of the DNAs on the surface of metal nanoparticles, (3) chirality transferred from the chiral DNAs to the achiral metal nanoclusters, (4) plasmon-enhanced CD of chiral DNAs located in the region of hot spots, and (5) collective plasmon interactions between the building blocks that were arranged into chiral 3D architectures. The first mechanism can be ruled out due to aspects of experiment and theory. The sizes of the reported chiral Au NPs are mainly subnanometer clusters,46 their kernels are achiral, and their chirality originated from ligand induction and/or the chiral arrangement of surface atoms. In general, the larger the Au NPs, the less prominent the CD response. The diameter of Au NPs used in our experiment was about 15 nm, and the size of the Au NR was much larger; thus, the chiral feature in this scale range could be ignored. Moreover, our experimental results prove that the second and third mechnisms can also be ruled out as the control experiments of Au NR−DNA-1 and Au NP−DNA-2 (Figure S5, Supporting Information) did not show the chiral effect in the gold plasmonic frequency. Govorov and Naik47 obtained PCD response in peptide-functionalized Au NPs and adjusted the CD line using metal ions, which led to aggregation of Au NPs and altered the peptide secondary structure. This may have been due to the peptides that transferred more energy to Au NPs than DNA. Moreover, from the CD spectra, the peptides show much stronger CD intensity in the UV region than DNA. These findings also show that the conformational change in the peptides binding to the Au NPs surface was not the dominant mechanism in the CD effect. Therefore, the two remaining mechanisms are the key to the origin of the chiral NP assemblies. In previous studies, it was shown experimentally that for the asymmetric Au NP assemblies (including dimers,26 tetramers,48 and other multimers,25 3D helix,49 and chains50), using the polymerase chain reaction (PCR), that the side-by-side arrangement of Au NRs through DNAs can introduce intense CD signals in the

plasmonic region of the Au NP and Au NR, respectively. In the present core−satellites (Au NR−Au NPs) assemblies, we obtained the CD signals at both the region of the Au NP and Au NR. By monitoring the entire process of dynamic selfassembly, the increase in Au NPs absorbed on the Au NR may increase the dissymmetry of the system. This strengthened the chirality of the system, indicating a close relationship between the structure and chirality. Enzymatic Disassembly of the Core−Satellites Superstructures. To further confirm the hypothesis that the CD effect originated from the chiral arrangement of Au NPs around the Au NR, deoxyribonuclease I (DNase I) was used to control disassembly of the nanoassemblies. From the TEM images of core−satellites assemblies after the addition of DNase I (Figure 3) and the

Figure 3. Representative TEM images of the core−satellites after being cleaved by DNase I for different hybridization times, (a,b) 10, (c,d) 20, (e,f) 30, (g) 40, and (h) 50 min.

statistical analysis shown in Figure S6 (Supporting Information), it was observed that as the digestion time increased, the number of Au NPs around the Au NR decreased. The organization of Au NPs was also investigated using 3D tomography (Figure S7, Supporting Information). The results clearly show that the Au NPs and Au NR were not in the same plane. Kinetic studies of the spectral changes in the presence of DNase I were carried out using UV−vis and CD spectroscopy. It was found that these changes were directly related to the enzyme cleaving time. After enzymatic digestion, the UV−vis spectra (Figure 4a) displayed two extinctions at 515 and 700 nm, which were also found in the control experiment (mixtures of Au NP and Au NR, no assemblies), blue-shifted from the bands of the assembled structures at 523 and 724 nm, respectively. This phenomenon further showed the dissociation of the superstructure after enzymatic digestion. The CD spectral changes were also investigated. As seen in Figure 4b, the intensity of the two newly formed CD bands located at about 520 and 715 nm gradually decreased with time and finally diappeared within 6−12 h. Simultaneously, the position of the peak blue shifted with increasing enzyme digestion time. As expected, the shape of the CD line was related to the different transitions of the absorption spectra. Both the CD and UV−vis spectral changes showed that the enzymatic digestion procedure was more like the reverse process of self-assembly. From these resuls, it could be concluded that the chirality was related to the structure of the assemblies. In addition, the dynamic light scattering (DLS) analysis (Figure S8, Supporting Information) also showed that the DH of the assemblies progressively decreased with cleaving time, which was consistent with the fact that the number of Au 2381

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Figure 4. UV−vis (a) and the corresponding CD spectra (b) of Au NP, Au NR, and the core−satellites assemblies after being cleaved for different times.

Figure 5. CD spectra of the core−satellites assemblies protected with DAPI (a) and 9-AA (b) before being cleaved for different times.

NPs in the assembly was reducing. The difference in DH between assembly and disassembly was negligible, suggesting a reversible process. Assay of DNase I Activity and Inhibition. As previous experiments have shown, the PCD response is very sensitive to the conformational transmutation of assembled superstructures. Therefore, we employed such nanoassemblies to develop a chiral sensor for screening the inhibitors of DNase I, which was accomplished using UV−vis detection. As a proof-of-principle experiment, 4′,6-diamidino-2-phenylindole (DAPI), which is known to inhibit DNase I through DNA binding, was added to the solution of core−satellites assemblies to examine the inhibition of DNase I activity. The PCD response was monitored as a function of digestion time in the presence of DNase I (30 units/mL). As displayed in Figure 5a, the CD line was almost unchanged with increasing time, indicating maintenance of the structure. Furthermore, another inhibitor, 9-aminoacridine (9-AA), was used to test the validity and versatility of this method. For comparison, all of the experimental parameters were the same as those for DAPI. Figure 5b shows the CD spectra of the nanoassemblies after initializing the reaction by adding DNase I. In contrast, the CD response gradually decreased with increased digestion time, which was attributed to enzymatic disassembly of the assemblies. As the DNA−duplex interconnects were degraded by endonuclease, Au NPs were desorbed from the Au NR. From the CD results, it was displayed that the trend in inhibition efficiency was DAPI ≫ 9-AA, which is consistent with a previously published report.51 These results explained that the chirality of the assemblies could be adjusted by the enzyme and its inhibitors, and if the activity of the enzyme was

inhibited by inhibitors, the structure would remain unchanged, which led to maintaining of the optical activity. This could be applied to develop biosensors for nuclease enzyme inhibitor. In summary, we report that the specially designed Au NR− Au NPs core−satellites nanoassemblies with DNA produce remarkable PCD responses at their SPR frequency. With the help of DNase I, the PCD effects could be tuned at different enzymatic cleaving times. This enzyme-responsive nanostructure can be used to screen inhibitors of DNase I in solution. The core−satellites assemblies will provide an ideal model for simulating PCD signals of plasmonic NCs in a 3D chiral organization. Moreover, such chiral assemblies with the high efficiency of PCD response may have potential applications such as in optical devices and biosensors.

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METHODS

Materials. All chemicals in this study were of analytical grade purity and used without further purification. The trisodium citrate (C6H5Na3O7·2H2O, ≥99.0%), tetrachloroauric acid trihydrate (HAuCl4·3H2O, 99.9%), cetyltrimethylammonisum bromide (CTAB, >98.0%), silver nitrate (AgNO3, >99.0%), and sodium borohydride (NaBH4, 99.0%) were obtained from Sigma Aldrich Chemical Co. (Atlanta, GA, U.S.A.). DNA oligonucleotides were purchased from Sangon (Shanghai, China). DNase I was obtained from Aladdin Industrial Corporation (Shanghai, China). Ultrapure water was obtained from a Millipore filtration system and used in all experiments. All glassware was steeped in freshly prepared aqua regia (HCl/ HNO3 in a 3:1 ratio by volume) for 12 h and then followed by rinsing with large quantities of ultrapure water. 2382

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(2) Guerrero-Martínez, A.; Alonso-Gómez, J. L.; Auguié, B.; Cid, M. M.; Liz-Marzán, L. M. From Individual to Collective Chirality in Metal Nanoparticles. Nano Today 2011, 6, 381−400. (3) 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. (4) Gautier, C.; Bürgi, T. Chiral N-Isobutyryl-cysteine Protected Gold Nanoparticles: Preparation, Size Selection, and Optical Activity in the UV−Vis and Infrared. J. Am. Chem. Soc. 2006, 128, 11079− 11087. (5) Gautier, C.; Bürgi, T. Chiral Inversion of Gold Nanoparticles. J. Am. Chem. Soc. 2008, 130, 7077−7084. (6) Shukla, N.; Bartel, M. A.; Gellman, A. J. Enantioselective Separation on Chiral Au Nanoparticles. J. Am. Chem. Soc. 2010, 132, 8575−8580. (7) Zhu, M.; Qian, H.; Meng, X.; Jin, S.; Wu, Z.; Jin, R. Chiral Au25 Nanospheres and Nanorods: Synthesis and Insight into the Origin of Chirality. Nano Lett. 2011, 11, 3963−3969. (8) Dolamic, I.; Knoppe, S.; Dass, A.; Burgi, T. First Enantioseparation and Circular Dichroism Spectra of Au38 Clusters Protected by Achiral Ligands. Nat. Commun. 2012, 3, 798−804. (9) Knoppe, S.; Dolamic, I.; Dass, A.; Burgi, T. Separation of Enantiomers and CD Spectra of Au40(SCH2CH2Ph)24: Spectroscopic Evidence for Intrinsic Chirality. Angew. Chem., Int. Ed. 2012, 51, 7589−7591. (10) Li, T.; Park, H. G.; Lee, H.-S.; Choi, S.-H. Circular Dichroism Study of Chiral Biomolecules Conjugated with Silver Nanoparticles. Nanotechnology 2004, 15, S660−S663. (11) Nishida, N.; Yao, H.; Ueda, T.; Sasaki, A.; Kimura, K. Synthesis and Chiroptical Study of D/L-Penicillamine-Capped Silver Nanoclusters. Chem. Mater. 2007, 19, 2831−2841. (12) Lieberman, I.; Shemer, G.; Fried, T.; Kosower, E. M.; Markovich, G. Plasmon-Resonance-Enhanced Absorption and Circular Dichroism. Angew. Chem., Int. Ed. 2008, 47, 4855−4857. (13) Leroux, F.; Gysemans, M.; Bals, S.; Batenburg, K. J.; Snauwaert, J.; Verbiest, T.; Van Haesendonck, C.; Van Tendeloo, G. ThreeDimensional Characterization of Helical Silver Nanochains Mediated by Protein Assemblies. Adv. Mater. 2010, 22, 2193−2197. (14) Pandoli, O.; Massi, A.; Cavazzini, A.; Spada, G. P.; Cui, D. Circular Dichroism and UV−Vis Absorption Spectroscopic Monitoring of Production of Chiral Silver Nanoparticles Templated by Guanosine 5′-Monophosphate. Analyst 2011, 136, 3713−3719. (15) Qi, H.; Shopsowitz, K. E.; Hamad, W. Y.; MacLachlan, M. J. Chiral Nematic Assemblies of Silver Nanoparticles in Mesoporous Silica Thin Films. J. Am. Chem. Soc. 2011, 133, 3728−3731. (16) Liu, H.; Ye, Y.; Chen, J.; Lin, D.; Jiang, Z.; Liu, Z.; Sun, B.; Yang, L.; Liu, J. In Situ Photoreduced Silver Nanoparticles on Cysteine: An Insight into the Origin of Chirality. Chem.Eur. J. 2012, 18, 8037− 8041. (17) Maoz, B. M.; van der Weegen, R.; Fan, Z.; Govorov, A. O.; Ellestad, G.; Berova, N.; Meijer, E. W.; Markovich, G. Plasmonic Chiroptical Response of Silver Nanoparticles Interacting with Chiral Supramolecular Assemblies. J. Am. Chem. Soc. 2012, 134, 17807− 17813. (18) Xie, J.; Duan, Y.; Che, S. Chirality of Metal Nanoparticles in Chiral Mesoporous Silica. Adv. Funct. Mater. 2012, 22, 3784−3792. (19) Kobayashi, M.; Tomita, S.; Sawada, K.; Shiba, K.; Yanagi, H.; Yamashita, I.; Uraoka, Y.; Kuo, Y.-Z.; Wu, J.-P.; Wu, T.-H. Chiral Meta-Molecules Consisting of Gold Nanoparticles and Genetically Engineered Tobacco Mosaic Virus. Opt. Express 2012, 20, 24856− 24863. (20) Wang, R.-Y.; Wang, H.; Wu, X.; Ji, Y.; Wang, P.; Qu, Y.; Chung, T.-S. Chiral Assembly of Gold Nanorods with Collective Plasmonic Circular Dichroism Response. Soft Matter 2011, 7, 8370−8376. (21) George, J.; Thomas, K. G. Surface Plasmon Coupled Circular Dichroism of Au Nanoparticles on Peptide Nanotubes. J. Am. Chem. Soc. 2010, 132, 2502−2503. (22) Guerrero-Martinez, A.; Auguie, B.; Alonso-Gomez, J. L.; Dzolic, Z.; Gomez-Grana, S.; Zinic, M.; Cid, M. M.; Liz-Marzan, L. M. Intense

Preparation of Core−Satellites (Au NR−Au NPs) Nanoassemblies. Au NRs with an aspect ratio of 3.5 (14 nm × 48 nm) were synthesized using the well-known seed-mediated growth method52 with some modifications. DNA-1 with a sequence of TAGGAATAGTTATAA-A10C(6)SH was added to the solution of Au NRs to prepare Au NR−DNA-1, and Au NP−DNA-2 was prepared by functionalizing citrate-modified Au NPs (15 nm diameter) with the complementary oligonucleotides DNA-2 (TTATAACTATTCCTAA10C(6)SH) using our previously reported methods. After combining the two probes, Au NR−DNA-1 and Au NP−DNA2, in hybridization buffer (1×TBE, 100 mM NaCl, 0.01% SDS, 5 mM MgCl2, pH = 8.0), the mixture (the ratio of Au NRs to Au NPs was 1:20) was incubated at room temperature for 12 h. The detailed process of the formation of the core−satellites assemblies can be found in our recently pubished paper.38 Enzymatic Disassembly of the Core−Satellites Superstructures. After hybridization, the solution was then centrifuged at 3500 rpm for 30 min, and the residue of the Au NR−Au NPs assemblies was carefully collected and resuspended in DNase I buffer (10 mM PBS, 1 mM MgCl2, pH = 7.0). DNase I (100 units) was then added to cleave the core−satellites assemblies. The entire cleavage process was monitored using DLS, TEM, UV−vis, and CD spectroscopy. Assay of DNase I Activity and Inhibition. After careful centrifugation, the prepared core−satellites superstructures were resuspended in 0.5 mL of DNase I buffer. An aliquot of endonuclease inhibitor (0.02 mM) was added to the solution, and the mixture was thoroughly homogenized by vortexing. After incubating for 10 min, DNase I was added to initiate the assay. UV−vis and CD spectroscopy were used to monitor the disassembly.

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ASSOCIATED CONTENT

S Supporting Information *

Supplementary TEM images, statistical analysis data, tomography 3D reconstruction, and DLS data of the nanostructures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions #

L.X. and C.H. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (21071066, 91027038, 21101079, 21175034), the Key Programs from MOST (2012BAC01B07, 2012AA06A303, 2012BAD29B04, 2012 BAK11B01, 2011BAK10B07, 2011BAK10B01, 2010AA06Z302, 2010DFB3047, 2013ZX08012-001, 2012BAK17B10, 2012BAK08B01, 2012YQ090194), and grants from Jiangsu Province, MOF and MOE (NCET-12-0879, BE2011626, 201210036, 201310135, 311002).

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