Origin of the Plasmonic Chirality of Gold Nanorod Trimers Templated

Aug 3, 2018 - Exploring the Addressability of DNA Decorated Multifunctional Gold Nanoparticles with DNA Origami Template. Bioconjugate Chemistry. Nixo...
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Origin of the Plasmonic Chirality of Gold Nanorod Trimers Templated by DNA Origami Zhong Chen, Chun Kit Choi, and Qiangbin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11167 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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Origin of the Plasmonic Chirality of Gold Nanorod Trimers Templated by DNA Origami Zhong Chen,*,†,# Chun Kit K. Choi,‡,# and Qiangbin Wang*,§ †Key

Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Guangdong Engineering Technology Research Center for High-Performance Organic and Polymer Photoelectric Functional Films, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China ‡Department of Biomedical Engineering, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China §CAS Key Laboratory of Nano-Bio Interface, Division of Nanobiomedicine and i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China ABSTRACT: Templated by DNA origami, plasmonic gold nanorods (AuNRs) could be assembled into complex nanostructures with strong chiroptical activities. However, it is still not clear how the plasmonic chirality of a complex nanostructure matters with its daughter structural components. Here, we rationally design and fabricate a series of AuNR trimers and their daughter AuNR dimers. Strikingly, we corroborate by circular dichroism spectroscopy that the plasmonic chirality of asymmetrical AuNR trimers are a nearly perfect summation of the chiroptical response of all their constituent dimeric components. Our results provide fundamental insights into the origin of the plasmonic chirality of complex nanostructures. KEYWORDS: DNA origami, plasmonic nanostructures, plasmonic chirality, AuNR dimers, AuNR trimers

Assembly of plasmonic nanoparticles (NPs) into threedimensional (3D) structures has gained appreciable interests in the field of nanotechnology; yet, realizing complex nanoconstructs in a programmable manner remains technically challenging.12 Recently, DNA origami-templated approaches have shown great promise in the fabrication of nanostructures with precisely controlled architectures.3 Particularly, number and inter-particle distance of constituent gold (Au) and silver (Ag) NPs,4-5 position and orientation of Au nanorods (AuNRs),6 and geometry of nanoassemblies have been rationally designed and experimentally attained on DNA origami with high accuracy and yield.711 Such modular bottom-up strategies allow for systematic investigations of fundamental physical phenomena, including distance-dependent interaction between a metallic NP and a fluorescence dye as well as plasmon-exciton coupling between two closely associated plasmonic NPs.12-13 Most importantly, the highly controllable NP patterning on one or even multiple DNA origami scaffolds empowers the engineered nanoarchitectures with a newly emerging optical property known as plasmonic chirality, analogous to molecular chirality naturally existing in biomacromolecules such as DNA and proteins.14-16 Specifically, righthanded or left-handed nanostructures can be created not only in the form of DNA-mimicking double helices,14,17-18 but also in planar NR nanoassemblies with structural asymmetry.19-20 These plasmonic chiral nanostructures generally display exceptionally strong optical activity and

circular dichroism (CD) that lead to a multitude of applications, including negative refractive index media,21 broadband circular polarizers,22-23 and ultrasensitive sensing devices.24-25 While precedent literatures mainly focused on developing new synthetic methods or practical applications for such chiroplasmonic nanoassemblies,26 the detailed studies on the origin of their chiroptical response remain elusive. Taking asymmetrical plasmonic AuNR trimers patterned on a flexible DNA origami scaffold as an example, it is known that their chiral optical activities originate from the plasmon-plasmon interaction associated with the surface plasmon resonances (λspr) of NRs and the left-handed (LH) or right-handed (RH) circularly polarized excitations.19,27 However, how the plasmonic chirality of the AuNRs matters with its daughter structural components, i.e., AuNR dimers, is yet to be studied in details. In this work, we rationally designed and fabricated a pair of asymmetrical Tshaped plasmonic AuNR trimers together with all their dimeric building blocks, with the aid of DNA origami as templates. Furthermore, we additionally constructed a symmetrical AuNR trimer and all its daughter AuNR dimers for comparison to make a thorough investigation on the origin of their chiroptical responses. Through CD spectroscopy, we corroborated that the chiroptical signature of the T-shaped AuNR trimers is a collective behaviour combined from all their daughter dimers, independent of the handedness and the amplitude of CD signals. Our results

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not only provide fundamental insights into the origin of the chiral properties of geometrically complex nanoassemblies, but also establish design rules to engineer nanostructures with predictable and tunable chiroptical activities.

response of the resultant nanostructures to aid subsequent analysis on plasmonic chirality. Also, we prepared a symmetrical, T-shaped AuNR trimer using the same two types of AuNRs for simultaneously studying the origin of the plasmonic chirality of complex nanostructures with weak chiroptical response (Scheme 1). To attain the desired nanostructures, we firstly selfassembled rectangular DNA origami (90 nm x 60 nm x 2 nm) from a long M13mp18 viral genomic DNA and a pool of ~200 short staple strands through sequence-specific hybridization and formation of multiple crossovers (Table S1).19,30 Three sets of capturing ss-DNA strands were rationally positioned on three designated locations of the DNA origami template by extending 15 nucleotides from the selected staple strands (Table S2). Representative atomic force microscopy (AFM) images taken at low magnification depicted rectangular DNA origami templates with uniform shape and size (Figure 1a). Subsequently, we modified AuNRs of two distinct dimensions (Figure 1b), i.e., 11 nm × 36 nm and 17 nm × 46 nm, with DNA strands that are perfectly complementary to the free-hanging capturing strands already stretched out from the DNA origami via a salt-aging process (Table S3). It is worth noting that the AuNRs remain stable and do not exhibit any significant change in the extinction spectra upon DNA conjugation.

Scheme 1. Schematic illustration of DNA origamitemplated synthesis of AuNR dimers and trimers. To investigate the origin of the chiroptical response of plasmonic nanoassemblies, three distinct sets of AuNR dimers and trimers templated by rectangular DNA origami were fabricated. Dimer 1, 2, and 3 constitute the RH– Trimer, whereas Dimer 4, 5, and 6 constitute the LH– Trimer. Dimer 7, 8, and 9 constitute the QS–Trimer. Previously, we reported an efficient, DNA origamiassisted method towards the fabrication of T-shaped AuNR assemblies consisting of three geometrically identical anisotropic NPs.28 However, due to structural symmetry, the chiral optical activities measured for ideal, T-shaped nanoassemblies are relatively low as compared to their asymmetrical counterparts.29 Although the intrinsic flexibility of soft DNA origami scaffolds can slightly induce detectable CD response of symmetrical nanostructures due to the structural distortion,27 the signals are often inadequate to allow meaningful investigation into the origin of their resultant plasmonic chirality. To endow T-shaped AuNR assemblies with stronger chiroptical activity, we attempted to extend our previous method to construct two different kinds of asymmetrical, T-shaped AuNR trimers, in which two smaller AuNRs and one larger AuNR are precisely arranged onto single-layered DNA origami with designated patterns (Scheme 1). By simply breaking the structural symmetry, we anticipate observing enhanced chiroptical

Figure 1. (a) Representative AFM images of the rectangular DNA origami template employed as the template for the assembly of AuNR dimers and trimers. Scale bar is 500 nm. (b) Representative TEM images of the synthesized AuNRs with the sizes of (i) 11 nm × 36 nm and (ii) 17 nm × 46 nm, respectively. Facilitated by the hybridization between the capturing strands of DNA origami substrates and AuNRs, we successfully generated in total twelve different types of 3D AuNR assemblies, including nine dimers and three trimers. We visualized their nanoarchitectures by transmission electron microscopy (TEM) after purification from excess AuNRs with gel electrophoresis (Figure S1). By this, the structurally uniform and pure AuNR nanoassemblies with the desired architectures were obtained with a yield of 90%, as also reflected in low-magnification TEM images (Figure S2). Regarding the two types of asymmetrical trimers, the eight assemblies can be divided into two groups, where each individual sample possesses its “mirror image” with a vertical symmetrical axis (Figure 2a). In theory, Dimer 1 and 4, Dimer 2 and 5, Dimer 3 and 6, and RH– Trimer and LH–Trimer are enantiomer pairs with opposite handedness. Notably, all the dimers (Dimer 1–3 & Dimer 4–6) are particularly included to cover all the possible plasmonically chiral daughter structures of the respective trimers (RH–Trimer and LH–Trimer). Similarly, for the quasi-symmetrical (QS) AuNR trimer (QS–Trimer), we2 also obtained its all dimeric building blocks (Figure 2b).

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ACS Applied Materials & Interfaces To the best of our knowledge, this is the first systematic experimental study to investigate the origin of the plasmonic chirality of complex AuNR trimers templated by DNA origami.

ensemble plasmonic property possibly arisen from the weak plasmon-plasmon coupling, in reasonable agreement with our previous report.27 In contrast, when they are arranged side-by-side, there is a slight red-shift of the λlong (~9 nm), indicating that the interparticle coupling is stronger in this orientation. When a larger AuNR is placed beside the smaller AuNR either in L shape or linear orientation, we observed that the plasmonic properties of both types of AuNRs are disturbed (Figure 3b), i.e., the longitudinal plasmon peaks of the heterogenous AuNR dimers appear in between the corresponding peaks of the two distinct AuNR monomers upon the assembly. Regarding the plasmonic response of our AuNR trimers, the insertion of one larger AuNR slightly blue-shifts the λlong of the heterogenous AuNR dimers, including Dimer 1 and 4. In contrast, the introduction of one smaller AuNR further redshifts the λlong of the consistent dimers, as reflected by the measurements for Dimer 2/5, Dimer 3/6, and Dimer 7/8 (Figure 3c). Interestingly, the handedness does not show any noticeable effect on the extinction spectra of the fabricated AuNR trimers.

Figure 2. Representative TEM images of the fabricated AuNR dimers and trimers templated on DNA origami. (a) Regarding the asymmetrical AuNR trimer cases, we successfully assembled three pairs of AuNR dimer enantiomers (Dimer 1 and 4, Dimer 2 and 5, and Dimer 3 and 6) and one pair of AuNR trimer enantiomers (RH– Trimer and LH–Trimer) for the subsequent investigation of their chiroptical activities. (b) For comparison, we also prepared a quasi-symmetrical AuNR trimer together with its daughter AuNR dimers. A schematic drawing is presented at the bottom right corner on each micrograph to illustrate the morphology of the rationally designed AuNR nanoassembly. Scale bar is 100 nm. As expected, AuNRs exhibit two extinction peaks corresponding to the transverse and longitudinal plasmon resonance modes (λlong) in the higher and lower energy regions, respectively (Figure 3a). The only difference is that the longitudinal mode of the smaller AuNR locates at a longer wavelength (λlong= 745 nm vs 684 nm). Upon the selfassembly onto DNA origami, all the dimers and trimers still display the characteristic optical signature of AuNRs in solution, as measured by UV-vis-NIR spectroscopy. Specifically, Dimer 1, Dimer 4, and Dimer 9, with only smaller AuNRs as components, resemble the extinction spectra of their monomers. Here, we observed negligible influence of the L-shaped arrangement of two small AuNRs on their

Figure 3. Normalized extinction spectra of the ensemble 3 solutions of (a) AuNRs, (b) DNA origami-templated AuNR

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dimers, and (c) DNA origami-templated AuNR trimers. For clarity, the surface plasmon resonance peak of the longitudinal mode (λlong) of each nanoassembly is indicated by the text in each plot. Following characterization of the plasmonic properties of the assembled nanostructures, we next carefully studied their chiral optical behaviours by using CD spectroscopy. As we anticipated, all the structurally asymmetrical dimers (Dimer 1–8) AuNR dimers exhibit pronounced bisignate characteristic in their CD spectra (Figure 4a, c, and e). Since Dimer 9 consists of two smaller AuNRs positioned side-by-side, it shows a much weaker CD response compared to other dimeric structures because of the absence of asymmetry. Yet, we still observed detectable CD signals from Dimer 9, echoing our notion of the quasi-symmetrical nature of the flexible DNA origami template.27 By comparing the data obtained from the four distinct sets of dimers, it could be ascertained that each AuNR dimer possesses its own “mirror image” (Dimer 1/4, Dimer 2/5, and Dimer 3/6, and Dimer 7/8), behaving as a pair of enantiomers with nearly opposite CD response (Figure 4a, c). Their chiroptical activities, particularly handedness, can be modulated by simply engineering the geometry (T-shaped or linear) of the nanoassemblies. Most importantly, we revealed that a simple summation of the separate CD signals acquired from all daughter dimeric constituents (e.g., Dimer 1 + 2 + 3) gives rise to a plot that is almost perfectly overlapping with the experimentally measured CD spectrum of the corresponding trimer, i.e., RH–Trimer. Notably, the same observation can be made when we examined the spectral data for the LH–Trimer with left-hand optical activity, indicating that this predictable nature should generally hold for any asymmetrical, optically active nanoassemblies, independent of the handedness. In addition, we observed that there is an unusual distortion of the welldefined bisignate line shape of chiral spectra among all the asymmetrical AuNR trimers (Figure 4b,d). The chiral optical signals of RH–Trimer and LH–Trimer become obviously stronger (∆> 5 mdeg) at wavelengths beyond 700 nm, i.e., near the λlong of the trimers. We believe that the significant enhancement of optical activity at wavelengths close to the longitudinal plasmon peak of AuNRs may be attributed to the differential adsorption of circularly polarized light in different light regions by the complex AuNR trimers. Eventually, we sought to find out whether the same observation holds for any quasi-symmetrical AuNR trimer. In case of QS–Trimer, it is worth pointing out that two of its dimeric components (Dimer 7/8) are highly chiral while the remaining one (Dimer 9) is weakly responsive to circularly polarized light (Figure 4e). We also separately acquired the CD spectra for all its dimeric building blocks as well as its own chiroptical signature. Interestingly, we revealed that the simple combination of the CD signals of all the constituent dimers again results in a plot appreciably overlapping well with the experimentally measured CD spectrum of the QS–Trimer (Figure 4f). These data indicate that cancellation of the CD signals among the three dimeric building blocks occurs, leading to the merely observable CD response of the QS–Trimer.

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Figure 4. CD spectra of the ensemble solutions of DNA origami-templated AuNR (a, c, and e) dimers and (b, d, and f) trimers. Regardless of handedness, we observed that the experimentally acquired CD response of asymmetrical AuNR trimers is a nearly perfect summation of that of all their daughter AuNR dimers. In addition, regardless of the intensity of chiroptical response, we revealed that the experimental CD spectrum of the quasi-symmetrical AuNR trimer is also a nearly perfect combination of that of all their daughter AuNR dimers. In conclusion, we experimentally investigated the origin of the plasmonic chirality of AuNR trimers by resolving them into structurally simpler dimeric components. Templated by DNA origami, we rationally designed and fabricated a total of nine AuNR dimers and three AuNR trimers, followed by thorough characterization of their optical behaviours. We corroborated by CD spectroscopy that the chiral optical response of a AuNR trimer is essentially the summation of the chiroptical response of all its daughter AuNR dimers, regardless of the handedness and the amplitude of CD signal. Our findings not only reveal the predictable nature of chiroptical activity of geometrically complex metallic nanostructures, but also establish valuable design rules for the engineering of next-generation DNA origamitemplated nanoassemblies with tailorable optical chirality.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website https://pubs.acs.org. Experimental details on the synthesis of rectangular DNA origami templates, gold nanorods, DNA-functionalized gold nanorods, and gold nanorod dimers and trimers (together with their additional transmission electron microscopic images)

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AUTHOR INFORMATION Corresponding Authors *Email: [email protected]. *Email: [email protected].

ORCID Zhong Chen: 0000-0001-6641-751X Chun Kit K. Choi: 0000-0003-1994-1719 Qiangbin Wang: 0000-0001-6589-6328

Author Contributions #Z.C. and C.K.K.C. contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Grant No. 21425103, 21673280 and 51703255).

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