Article pubs.acs.org/JPCC
Plasmonic Chirogenesis from Gold Nanoparticles Superstructures Wenjing Yan,† Wei Ma,† Hua Kuang,† Liqiang Liu, Libing Wang, Liguang Xu,* and Chuanlai Xu State Key Laboratory of Food Science and Technology, School of Food Science and Technology, Jiangnan University, Wuxi, Jiangsu 214122, People’s Republic China S Supporting Information *
ABSTRACT: In this Article, heterodimers with different-sized gold nanoparticles (Au NPs) were fabricated by DNA or sodium chloride (NaCl), which exhibited distinct optical activities. The circular dichroism (CD) band has a magnitude of (−)20 or (+)10 mdeg near the surface plasmonic frequency of Au NPs for DNA or NaCl triggered heterodimers, respectively. Importantly, we provided a new insight into the origins of NPs heterodimers and further investigated the influence factors in NPs dimers. Taking into account that the shapes of the NPs are nonspherical, chiral properties of heterodimers were attributed to the cooperative effect of the small dihedral angle between two adjacent NPs and the plasmonic coupling of NPs, which break the symmetric nature of two uniform spheres; this conclusion was confirmed by 4π full space simulation. Impressively, the chiroptical response was tunable and reversible, and it was achieved by changing the size of building blocks and the temperature. This approach could lead to a further better understanding on the origin of chirality in NPs assemblies and offer rich opportunities for creating new approaches to chiral nanostructures engineering.
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assemblies represented by helical and tetrahedral.23,28,33,42 These chiral nanostructures could induce purely plasmonic CD signals at the surface plasmon frequency of metal NPs, which is different from the above-mentioned “share” chirality of chiral molecules with metal NPs.30 Govorov et al.43 illuminated the origins of the chirality in chiral complexs of metal NPs from a theoretical point of view. In that model, the chirality originates from the NP−NP Coulomb coupling in chiral NPs assemblies, such as the helix or tetramer comprised of four different elements. In the field of experimental study, since 2009, our group demonstrated a preparation method for selfassembly of NPs superstructures using the polymerase chain reaction (PCR) technique.23,24,44 By varying the density of the primer (DNA) on the surface of gold NPs and the number of PCR cycles, a mixture of discrete assemblies such as dimers, trimers, tetramers, etc., could be generated. These discrete NPs assemblies expressed strong chirality near the plasmonic bands of NPs. Recently, five types of chiral pyramids consisting of multiple NPs were fabricated based on DNA scaffold.42 This seemingly achiral 3D framework exhibited striking chiroptical activities in the range of 350−550 nm when introducing different NPs on each top. For these cases mentioned above, it is difficult to explain the origins of chirality either from the chiral geometry theory43 or from the chiral molecule induced chirality.30 Up to now, there is no experimentally established strategy to thoroughly investigate the chirality of plasmonic NPs assemblies. NPs
INTRODUCTION Plasmonic nanoparticles (NPs) assemblies with their unique and exquisitely tunable optical properties have enabled a vast array of applications.1−3 This interest stems from metal NPs acting as nanoantennas for visible light, and the electromagnetic field undergoes a resonant interaction with the conduction electrons of NPs.4,5 Au NPs as one of the most attractive noble metals have been widely investigated in the field of plasmonic applications,6−8 especially for circular dichroism (CD) recently.9−13 CD is the broadest measure of chiroptical response, aiming to capture the differences in absorbance of the left- and right-circularly polarized light (CPR).14 The shape, intensity, and wavelength position of cotton effects15 in the CD spectrum carry a wealth of valuable structural information of chiral molecules and biomolecules. 16−18 Markedly, the appearance of nanotechnology extends the optical response of chiral molecules from the ultraviolet range to the vis−nearinfrared domain and provides great potential in the application of chiral materials at nanoscale.19,20 There have been long-lasting efforts to construct hybrid frameworks with chiral features consisting of plasmonic NPs and chiral molecules.21−33 The electric dipole of the chiral molecules induces the chiral currents inside the metal nanocrystal and makes the chirality transformation from the chiral molecules to the vicinal achiral plasmonic NPs30,34,35 or to the self-assembled chiral nanostructures.36,37 An analogous situation was recently discovered on achiral thiolates protected gold clusters with a noticeable CD signal.38 The emergence of DNA nanotechnology offers great opportunities for building chiral superstructures of metal NPs;39−41 this makes it possible to create new chiral NPs © 2013 American Chemical Society
Received: June 15, 2013 Revised: July 31, 2013 Published: July 31, 2013 17757
dx.doi.org/10.1021/jp405925q | J. Phys. Chem. C 2013, 117, 17757−17765
The Journal of Physical Chemistry C
Article
With the increase in citric acid concentration, the shape of the NPs is difficult to control. In this work, the biggest NPs with an average size of 40 ± 5 nm (Au3) were synthesized using the silver-assisted growth procedure developed by a previous report with some modification.47 We observed that NPs made in this way could not be stable when it was resuspended in water after centrifugation. To overcome this problem, citric acid used as a thin layer was coated on the surface of gold NPs. Briefly, gold NPs with the diameter of 15 nm as a seed were prepared by Frens’s method. Second, a 170 mL aqueous solution consisting of 4 mL of 20 mM HAuCl4 and 0.4 mL of 10 mM AgNO3 was prepared in a conical flask, and then 15 mL of seed and 2 mL of 1% citric acid were added to the mixture under vigorous stirring. After several minutes, 3 mL of 5.3 mM ascorbic acid (Vc) was injected into the solution at the speed of 0.6 mL/min, and the color of the solution changed from pink to wine red. The optical properties and the dimensions of these gold NPs were confirmed by TEM images and UV−vis spectrum (Figure S1, Supporting Information). Functionalization of Gold Nanoparticles. The prepared gold NPs were stabilized with bis(p-sulfonatophenyl)phenylphosphine dihydrate, dipotassium salt (BPS) according to Loweth’s protocol with some modification.48 So the colloidal NPs could be a stable dispersion in high ionic strength with less number of DNA. Aqueous solutions of 40 ± 5, 25 ± 3, or 10 ± 2 nm Au NPs were concentrated 10-fold before use. Next, 10 mL of gold NPs was stirred with an excess of BPS (5, 10, or 20 mg/mL) at room temperature for more than 10 h. The solutions were then centrifuged at different speeds (3000, 7000, or 13 000 r/min) and resuspended in 0.5 × TBE buffer. DNA-functionalized gold NPs were prepared by methods described in the previous publication.49 To avoid forming more polymers in solution, a 1:2 ratio of ssDNA to Au NPs was designed. Au NPs were mixed with ssDNA and incubated for 4 h in 0.5 × TBE, 50 mM NaCl. The sample was centrifuged, and the pellet was resuspended in water. Self-Assembly of Dimers. The dimers were prepared by mixing two DNA-NPs conjugates bearing partial complementary ssDNA that were mixed in 1 × TBE buffer containing 50 mM NaCl. Hybridization mixtures were incubated for 12 h at room temperature to form gold NPs dimers. Besides, by mixing two partial complementary ssDNA modified heterogeneous NPs, asymmetric dimers were prepared. Sodium Chloride Induced Heterodimers Assembly. Au1 (10 nm) and Au2 (25 nm) were synthesized by the above method, and then mixed in the ratio of 1:5 in water. Next, 2 M NaCl was added to a final concentration of 5 mM. The sample was incubated for 1 h at room temperature to obtain the designed nanoassemblies. Theoretical Modeling of Plasmonic Dimers. Calculation of optical spectra of chiral dimers was performed on the basis of our previous report.50 The anisotropic gold nanoparticle of nonspherical shapes was used in the simulation based on the observed particle diameter statistics of typical TEM images. The optical properties of NPs assemblies were calculated on the basis of different elements of a standard scattering matrix, also known as S-matrix, corresponding to amplitudes of different scattering modes. The circular dichroism (CD) spectra were calculated from the different matrix elements of TRCP and TLCP, which have the same polarization in the incoming beam and represent the intensities of transmission of right- and left-circularly polarized light, respectively. All of these
dimers as the basic element of such structures determine their chiral responses and make it easy for investigation of chiral recognition mechanism in nanoassemblies. Besides, the difficulties in achieving both geometric and chiral control in the assembly of nanostructures limit the type, yield, and thus the application of such chiral nanomaterials. Better understanding of chirogenesis in nanoscale assemblies, the dominant role played by metal NPs, and the method of tuning CD bands poses new challenges in this area. Hence, the principal aims of this study were 3-fold: first, to develop a synthesis method for chiral material with fine controlled morphology and CD response; second, to investigate experimentally the origin of chirality in NPs assemblies and determine which factors influence the chiroptical in the system; and third, to put forward a mechanism for the chiral nanostructures and its predictive potential in promoting the applications of chiral materials. Here, a high yield of chiral heterodimers fabricated with different-sized Au NPs was constructed by chiral or achiral triggers. In the assembly process of heterodimers, a spontaneous transition from nonchiral systems to (+) or (−) enantiomers was observed, and this transformation was monitored by the time-dependent CD and UV/vis spectra and the typical TEM images. Besides achieving fundamental milestones for elucidating chirogenesis of plasmonic discrete assemblies, the chiroptical response of the heterodimers could be tuned by controlling the size of the building blocks and external factors. (i.e., temperature and distance). Because of the elliptic geometry of NPs, the cooperative effects, the plasmonic coupling of NPs pairs, and the small dihedral angle between two adjacent NPs were explored to illustrate the chirogenesis of NPs assemblies. This new perspective in chirality opens unique opportunities for further research and instills stronger confidence in the viability of chirality-based optical devices.
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EXPERIMENTAL SECTION Synthesis of Gold Nanoparticles. Three different sizes of gold NPs (nanoparticle diameters 10, 25, and 40 nm) were used in this work. Gold NPs with the diameter of 10 ± 2 nm (Au1) were synthesized by the previous method developed by Slot.45 Tannic acid and citric acid were used as reducing agent to synthesize small NPs. First, 1 mL of 1% HAuCl4 solution was added into 79 mL of Millipore-Q water under stirring. The mixture was prepared first in the flask. Next, 4 mL of 1% sodium citrate, 0.1 mL of 1% tannic acid, and 0.1 mL of 25 mM K2CO3 were added into 15.8 mL of Millipore-Q water, and the sample was mixed under stirring in another flask. Third, the two prepared solutions as we mentioned above were both, respectively, heated to 60 °C for 30 min, and then mixed together under high-speed stirring. The solution was kept at 60 °C for 30 min until the color turned to reddish orange and did not change, and then cooled to room temperature. The solutions were subsequently stored at 4 °C. The most common approach involves citrate reduction of gold salt ion to produce 25 nm size NPs with a relatively narrow size distribution developed by Frens.46 Briefly, 1% trisodium citrate solution (1.6 mL, freshly prepared) was quickly added to a boiling aqueous solution of HAuCl4 (100 mL, 0.25 mM) under vigorous stirring and reflux. After being boiled for 15−30 min, the color of the solution changed from blue to wine red. The heat source was removed to allow the reaction solution to cool to room temperature. Yet this method is unsuitable for synthesis of the larger spherical NPs (>30 nm). 17758
dx.doi.org/10.1021/jp405925q | J. Phys. Chem. C 2013, 117, 17757−17765
The Journal of Physical Chemistry C
Article
parameters were calculated using Txx, Tyy, Txy, and Tyx obtained from the simulations according to eqs 1 and 2: TRCP =
1 × ((Txx + Tyy) + i(Txx − Tyx)) 2
(1)
TLCP =
1 × ((Txx + Tyy) − i(Txy − Tyx)) 2
(2)
Scheme 1. Schematics of Synthetic Method for Chiral NPs Heterodimers
Here, Txx describes the transmission of light with an electric field polarized along the x-axis (also known as the x mode) entering at Zmin and exiting at Zmax along the z-axis. Tyy is the transmission of light with an electric field polarized along the yaxis (also known as the y mode). Txy describes the transmission of light entering at Zmin as x mode and exiting at Zmax as y mode light. Tyx corresponds to the opposite combination of entrance and exit modes of polarization. The ellipticity, that is, the point value for CD spectra for a specific wavelength of the incident light beam, was calculated according to eq 3: CD =
⎛ |T | − |TLCP| ⎞ 1 arctan⎜ RCP ⎟ 2 ⎝ |TRCP| + |TLCP| ⎠
DNA functionalized NPs at high ionic strength, phosphine ligand (bis(p-sulfonatophenyl),-phenylphosphine dihydrate), dipotassium salt (BPS) was used to replace the citrate capping layer onto the surface of NPs to avoid complex formation (see the Supporting Information).48 The number of ssDNA grafted on the surface of per NPs was calculated according to the fluorescence method,42 and a small number of ssDNA (
