Jiaji
Cheng
a,
Eric H.
Hill
b,
Yuebing
Zheng
*b,
Tingchao
He
*a and
Yanjun
Liu
*c
aCollege of Physics and Energy, Shenzhen University, Shenzhen 518060, People's Republic of China. E-mail: tche@szu.edu.cn
bDepartment of Mechanical Engineering and Materials Science and Engineering Program, The University of Texas at Austin, Texas, USA. E-mail: zheng@austin.utexas.edu
cDepartment of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen, 518055, China. E-mail: yjliu@sustc.edu.cn
First published on 26th February 2018
The rapid development of self-assembled plasmonic chiral nanostructures has proven its promising potential in many applications. The underlying mechanism lies in the localized surface plasmon resonance properties of plasmonic colloids with respect to induced chirality via chiral molecules or chiral geometries. In this review, we summarize recent advances in both experimental phenomena and theoretical modellings, in particular to plasmonic chirality based on geometric motives, to elucidate the underlying origin of chirogenesis so as to provide insights into this fascinating field.
Governed by Maxwell's equations, the characteristics of localized surface plasmon resonance (LSPR) are strongly related to the size, shape, and material of the nanoparticle, as well as its surrounding environment. In general, asymmetric shapes and high aspect ratios result in lower restoring forces and thus lower plasmon resonance frequencies. Closely packed nanoparticles can couple electric fields when their interparticle distance is less than their diameter, leading to lower resonance frequencies and providing opportunities for electromagnetic energy transfer at a length scale below the diffraction limit.18 The coupling effects of nanoparticles can regulate both linear and nonlinear optical properties resulting in novel physical properties for assembled nanostructures.19–23 It is therefore highly appealing to merge the properties of assembled hybrid nanomaterials with the electronic and optical applications in devices.11,24–31
Chirality is often referred to a system that loses mirror and inversion symmetries, like most of chiral organic molecules in which chiral centers are surrounded by four different bonding groups. While in terms of self-assembled systems, this lack of symmetry involves not only the difference in elements, but also often the architecture of overall structures in long range. This allows simple and easy bench top chemical synthesis of chiral structures at both micro- and nano-scale, thus providing alternative approaches to achieving chiral materials with novel properties. In recent years, interests in optically active hybrid nanostructures have been rising rapidly. Optical activity is the ability of a chiral molecule to rotate the plane of plane-polarized light.32 When linearly polarized light, considered as the superposition of left (LCP) and right (RCP) circularly polarized light, propagates through an optically active absorbing system, the absorption of LCP and RCP light will be different due to the difference on speed of light passing the chiral system. This results in turning linearly polarized light to elliptically or circularly polarized light, a phenomenon called circular dichroism (CD). Depending on the absorption properties, electronic circular dichroism (ECD) and vibrational circular dichroism (VCD), and sometimes scattering techniques are employed for measuring the adsorption difference in LCP and RCP light to detect chirality of a measured sample at optical frequencies. However, most chiral materials that have been studied are intrinsically chiral molecules, biomolecules, or inorganic salts at the molecular level. In these cases, the chiroptical effects arise from relatively small dipole moments caused by weak coupling with an external electromagnetic field.33 Recently, chirality has been introduced into plasmonic materials at the nanoscale due to the following merits: (I) optical activity can be greatly enhanced. In some cases, the resonance effects of plasmonic nanostructures creates opportunities to achieve novel materials with interesting optical effects, such as metamaterials with negative refractive indices; (II) generally, colloidal synthesis results in highly symmetrical nanostructures, such as spheres, ellipsoids, and spindles. The next challenge is breaking symmetry by inducing artificial chirality; (III) it is of interest to study the fundamental interactions between chiral molecules and plasmonic nanoparticles or clusters, and particle–particle interactions in chiral configurations.34–37
Generally, chiral responses in self-assembled plasmonic nanostructures arise from two cases: superstructures with chiral configurations in which the particle–particle interactions between plasmonic building blocks are the driving force for chiroptical activity (see Fig. 1B–H), and the plasmon–exciton interactions between a hybrid complex of chiral molecules and achiral plasmonic NPs (Fig. 1A). (Herein we exclude chiral ligand protected metal nanoclusters (<5 nm) because they possess discrete energy bands giving weak or even no obvious localized surface plasmon property. For interested readers, extensive reviews38,39 are published in this field concerning the syntheses, mechanisms, and applications.) Between assembled chiral plasmonic superstructures and complexes of chiral molecules with achiral plasmonic NPs, the latter has been extensively studied during the last ten years with a number of well-summarized reviews34,40–42 published recently. In contrast, cutting-edge research concerning assembled plasmonic nanostructures with defined chiral particle–particle interactions is less frequently reviewed due to the complexity of systematic control over the synthesis and a lack of a priori theoretical analysis. Therefore, understanding the origin of chiroptical effects and manipulating chiral plasmonic interactions in these systems would open the floodgates to a host of new experiments on innovative chiral materials, pushing researchers to re-evaluate the origin of electromagnetic response between light and optically-active matter toward discovering novel avenues in chiral materials’ design.
Fig. 1 “Stairway” of chiral plasmonic nanostructures in terms of the number of plasmonic building blocks. From downstairs to upstairs, chiral molecules interact with a plasmonic particle (A),40 chiral dimers (B and C),43,44 tetramer (D),45 pyramids (E and F),46,47 short helix (G)48 and helical nematic assemblies (H).49 Adapted with permission from the American Chemical Society, Nature Publishing Group and Wiley. |
Herein, we emphasize recent progress on self-assembled chiral nanostructures with two or more plasmonic building blocks in terms of their syntheses, circular dichroism effects at relative optical frequencies, mechanisms of induced optical chirality, and their potential for optical applications. In Section 2, the classical theories for understanding LSPR effects are presented, followed by a more advanced model describing the coupling phenomena of closely packed plasmonic nanoparticles, to explore the optical chirality from a fundamental point of view. Section 3 briefly introduces the origin of induced chirality in plasmonic nanostructures and discusses how CD activities are associated with the species and geometries of the plasmonic building blocks. In Section 4, recent progress exploring mechanisms of induced CD response from metal particle assembles or hybrids of metal and semiconducting nanoparticles will be discussed with respect to the number of building blocks: (i) plasmonic dimers, (ii) tetramers or pyramids, and (iii) higher order plasmonic assemblies, such as short helices and helical nematic structures (Fig. 1). Finally, Section 5 will briefly summarize the applications and potential perspectives of this research area with challenges that remain unsolved to date.
Plasmons are the incompressible electron cloud around the surface of the small particle, and the coherent oscillations from the collective electron cloud. Like a general oscillator, a plasmon also has its own frequency, ωp, which can be used to determine their dielectric constant. If the particle's dielectric constant and environment are known, the absorption spectra of spherical particles can be precisely estimated according to Mie theory, an analytical solution to Maxwell's equations for spheres. Typically, the expression of scattering and extinction for a spherical cross-section are shown in the following equations from Mie theory:
The analytical solution of Mie theory focusing on a spherical nanoparticle is dependent on the number of multipolar modes.51
The extinction can be summed by two cross-sections, absorption and scattering. Plasmons can also re-radiate energy to determine whether absorption or scattering dominates the entire process. In small nanoparticles, electron–electron scattering can quickly convert the energy of LSPR into heat, showing strong absorption. However, in large particles, electron–electron scattering can be significantly confined, and the energy can convert into a strong scattering cross-section as a result of the radiative damping effect.52
For small NPs 10 nm in size, the lowest dipolar l = 1, the absorption cross-section would be described by the expression:
As nanoparticle size increases, the most apparent effect is a red-shift of the plasmon resonance peak to longer wavelengths. This arises due to retardation effects from the incident light no longer polarizing the larger nanoparticle homogeneously, which can result in excitation of higher order modes.
Assuming only dipole contributions in the quasi-static limit l = 1, the linewidth of LSPR can be estimated from the following relation for noble metals:
Fig. 2 Extinction spectra of different sized gold nanoparticles (AuNPs) in aqueous solution (n = 1.33) reported by G. V. Hartland et al.53 Besides the obvious red-shift resulting from the retardation effect, it clearly demonstrates a broadening of linewidth (apart from 15 nm) for these ensemble nanoparticle observations. Reproduced from ref. 53 by permission of the American Chemical Society. Copyright (2011). |
When we consider a more complicated situation, the spherical shape elongated along the z-axis, the optical properties of ellipsoidal NPs has been changed following Gans theory.30,31 This is an analytical solution of Maxwell's equations describing the LSPR of nanoparticles of any aspect ratio including those of metallic nanorods. An approximate solution to the equation is shown:
Aspect ratio (AR) can describe the shape of an ellipsoid. For a sphere the factor weighting εm is 2 in order to meet the SPR condition, but with increasing aspect ratio the weighting factor [(1 − Pj)/Pj] can be much greater than 2 in a nanorod. This outcome leads to an apparent red-shift of plasmon response with increasing AR. For example, the plasmon resonance of gold nanorods can be tuned from the visible region around 640 nm to the near-infrared region >1000 nm when AR increases from 2.4 to 6.6,54 as shown in Fig. 3.
Fig. 3 Variation of the nanoparticle shape (in this case, aspect ratio) allows tuning of the plasmon resonance of an Au nanorod from the visible to the near infrared. Different colors of aqueous solutions (A), and corresponding absorption spectra (B) as a function of aspect ratio.54 Reproduced from ref. 54 by permission of Elsevier B.V. Copyright (2010). |
In order to predict the plasmonic properties of arbitrary geometries of nanostructures, a number of numerical methods have been well developed, namely the finite-difference time domain (FDTD) method,55 the discrete dipole approximation (DDA),56 Greens-function approach,57 the multiple multipole (MMP) method,58 multiple scattering techniques, transfer-matrix approaches,59 plane wave expansions,60 and the boundary element method (BEM).61 In brief, with the support of proper modeling and theory, the enhancement of the electric field as the intensity of the surface plasmon resonance absorption increases can be exploited for many potential applications.39
On the other hand, although theoretical modeling of the optical response of metallic nanoparticles is well established,62 to better understand the electromagnetic properties of complex plasmonic nanostructures, an analytical theoretical method was suggested by Halas and Nordlander.63 This theory – a plasmon hybridization approach analogous to molecular orbital hybridization theory – suggests that the plasmon modes of a complex nanostructure are expressed in terms of interactions between the plasmon resonances of its elementary components, offering an intuitive description on how the plasmon resonances in assembled hybrid nanomaterials arise from the plasmon modes of their individual components.63,64 As shown in Fig. 4A, an individual plasmonic nanoparticle would couple plasmonic fields with an adjacent nanoparticle under polarized light illumination along the interparticle axis, which is directly analogous to the molecular orbital diagram. With one particle situated next to another, these two electron clouds hybridize according to quantum mechanics and form bonding and anti-bonding orbitals. When coupled, the hybridization between plasmonic pairs leads to a decrease of plasmon energy and corresponding red-shifted absorbance wavelength.
Fig. 4 Schematic of plasmon–plasmon coupling of two plasmonic nanoparticles (A) and extinction spectra of dimers formed by spherical gold particles 20 nm in diameter (B) reported by F. J. García de Abajo et al.65 Local (dashed curves) and nonlocal (solid curves) calculations are compared for several separations between the particle surfaces. Reproduced from ref. 65 by permission of the American Chemical Society. Copyright (2008). |
The plasmon mode can be split into two collective modes, including a lower energy bonding mode which is aligned along longitudinal dipoles and a higher energy antibonding mode which is perpendicular. The bonding mode can strongly couple with the far field, but the antibonding one cannot and gives no net dipole moment and negligible induced dipole. Fig. 4B shows the scattering spectra of dimers formed by spherical gold particles of 20 nm in diameter.65 Decreased distance between two closely coupled gold particles gives rise to a red-shift of the plasmon peak, which can be explained by the occurrence of strong hybridization between plasmon dimer modes. In addition, it is also of note that when the interparticle distance is small enough (<1 nm), the non-local effect rises as indicated in by the solid lines.
Fig. 5 (A) Scheme of a complex composed of a metal nanoparticle and dye molecule. (B) Illustration of exciton–plasmon interaction in a chiral molecule–plasmonic NP complex. (C) Calculated CD signals for a molecule and two complexes from top to bottom, respectively. Insets show the geometry of the complexes.68 Reprinted from ref. 68 by permission of the American Chemical Society. Copyright (2010). |
If the absorption of the chiral molecule is located in the UV region, the electromagnetic interaction with a metal nanoparticle is off-resonance, resulting in an active CD response in both the plasmon band and molecular band (Fig. 5C). For this general case, from the definition of CD, one can refer to CD effects of a randomly oriented system as:68
CD = 〈Q+ − Q−〉Ω, |
Yet when a chiral structure is built up by achiral plasmonic nanoparticles with well-defined particle number or superstructures like short helices, the CD response close to the plasmon wavelength is often due to dipole–dipole or plasmon–plasmon interactions. For these interactions, as illustrated previously, it is evident the CD signal is strongly related to the size of nanoparticles and the interparticle distance:41,69
Fig. 6 (A) illustration of extrinsically chiral arrangement of a gold nanorod dimer. The green arrow indicates the incident CPL with θ = 60° in the yz-plane, while the gold nanorod dimer lies in the xy-plane. Observation direction for far field scattering is along the z-axis. Interaction between the incident CPL and the dimer in the view of the propagating path is shown as (B). Dipole orientations of the symmetric mode match well with the spatial evolution of electric field vectors of LCP, while they are unfavorable for RCP. For the anti-symmetric mode, a favorable match is found with RCP (C).73 Reproduced from ref. 73 with permission from the Royal Society of Chemistry, Copyright (2014). (D) Chiral optical study of dumbbell-like gold nanorod dimer. Left: CD measurement of ensembled dimers with no chiroptical activity; right: CDS measurement of single particles showing strong chiral signals depending on the chiral geometry.74 Reproduced from ref. 74 with permission from the American Chemical Society, Copyright (2016). (E) Energy diagram describing the plasmonic hybridization of the two modes. The plus and minus signs denote the charge of the plasmon oscillation and the arrows represent the directions of the overall dipole moment. (F) Experimental and calculated scattering and CDS spectra of two AuNR dimers: homodimer and heterodimer.44 Reproduced from ref. 44 with permission from the American Chemical Society, Copyright (2015). |
Very recently, Liz-Marzán and Link74 demonstrated that dumbbell-like gold nanorod dimer in a racemic mixture can express strong optical activities in single-particle CDS measurements. Due to the electromagnetic coupling between AuNRs, the twisted AuNR dimer exhibits a characteristic bisignate CDS signal in the vicinity of the plasmon frequency of the AuNRs. Additionally, an interesting finding was that gold nanodumbbell dimers were optically inactive for ensemble measurement in a conventional CD spectrometer while expressing enantioselective CDS activities as a pure single dimer enantiomer, indicating the twist angle between the two constituent AuNRs played a significant role on CDS response (Fig. 6D). Furthermore, Link et al.44 reported chiroptical activity of a twisted side-by-side Au nanorod dimer. The effects of structural symmetry-breaking parameters, such as size difference between the two Au nanorods and twist angles, were studied respectively via CDS spectroscopy and modeling based on simulations. The mechanism of observed chiroptical response was illustrated by the plasmon hybridization theory in which opposite handedness of low-energy “bonding” and high-energy “antibonding” plasmonic modes, defined by the dihedral angle between the nanorods, were considered as the origin of chirality (Fig. 6E and F). It is, however, worth noting that the antibonding mode has the same handedness as the AuNR dimer structure rendering anti-aligned dipoles on each individual AuNR, while the bonding mode has the opposite handedness of aligned longitudinal dipoles in AuNRs, resulting in the two modes possessing CD responses of opposite sign. Additionally, this hybridization theory for induced plasmonic chirogenesis is suitable for some dimers made by NP–NP combinations with strong chiral activity, despite that ellipsoidally shaped NPs can only have a small dihedral angle.75,76
Other than plasmonic dimer systems, more sophisticated nanostructures are also suggested as chiral plasmonic units. The first instructive example is chiral plasmonic pyramids reported by Alivisatos et al.47 where the pyramidal nanostructures were built up by AuNPs conjugated with DNA. The DNA molecules on the AuNP surface served as the scaffold for the pyramid shape with four AuNPs of different sizes located on the vertices presenting a tetrahedral framework, like a chiral carbon atom, which expressed chirality as defined from a geometrical point of view (Fig. 7A). However, no clear CD signals were shown in these nanostructures, probably due to the weak plasmonic coupling between NPs and structural instability caused by single-stranded DNA scaffold. Such chiral assemblies were further studied by Kotov and co-workers,77 in which they obtained AuNPs and DNA pyramids by using the polymerase chain reaction (PCR) technique. The surface density of DNA molecules on the AuNPs and the number of PCR cycles determined the morphologies of the assemblies, revealing the process for the formation of different assemblies. The assemblies showed active CD spectra far from the absorption of DNA molecules but close to the plasmon frequencies of the AuNPs, which are highly sensitive to the morphology of the assemblies. It is therefore suggested that chirality originates from the plasmon–plasmon coupling of the AuNPs (Fig. 7B). However, one should always keep in mind that molecular chirality can cause morphological chirality. AuNPs conjugated with chiral ligands such as DNA or peptides as building blocks sometimes possess chiroptical properties themselves, due to the exciton–plasmon coupling between the chiral molecules and plasmonic nanoparticles. Admittedly, it may be difficult to ascribe the observed CD activities to one of these two mechanisms.
Fig. 7 (A) Schematic and TEM images of a DNA–nanocrystal chiral pyramid and its two enantiomers.47 Reproduced from ref. 47 with permission from the American Chemical Society, Copyright (2009). (B) Schematics of chiral NP superstructures synthesized by PCR, and circular dichroism measurements of products of PCR with increasing number of cycles.77 Reprinted from ref. 77 with permission from the American Chemical Society, Copyright (2009). (C) Schematics of DNA-modified AuNPs tetrameters with left- (red), right-handed (blue), and achiral (green) geometry and their corresponding measured and calculated CD spectra.45 Reproduced from ref. 45 with permission from the American Chemical Society, Copyright (2013). |
Fig. 8 Template-directed synthesis of plasmonic helices. (A and B) Small molecules: (A) phospholipids,80 (B) anthraquinone-based oxalamide fibers.81 (C) Peptides.82–84 (D–G) DNA.48,85–87 (H) Synthetic polymer.89 (I and J) Silica template91,92 and (K) AAO.93 Adapted from the data of the cited papers by permission from the Royal Society of Chemistry, American Chemical Society, American Association for the Advancement of Science, Nature Publishing Group and Wiley. |
Peptides, as complex organic molecules with well-defined configurational chirality, are also suitable candidates for chiral deposition of nanoparticles. Rosi and colleagues,82–84 for example, reported the synthesis of highly ordered AuNPs grown on double helices by templating attachment via the peptide sequence (Fig. 8C). The AuNP–peptide conjugates are able to assemble into twisted nanofibers with a regular pitch of approximately 84 nm. However, their CD measurements did not show typical optical activity at the wavelength of AuNPs plasmon absorption. Currently, the chiroptical response of such nanostructures has been obtained by carefully tuning the geometrical parameters, namely pitch length, particle size, interparticle distance, and interhelical distance, which agrees well with simulations based on the DDA method. Moreover, Rosi et al. found that the CD intensity, peak position, and nature of the chiroptical activity can be carefully adjusted via silver overgrowth on as-synthesized gold double helices. When the thickness of the silver layer is grown above 1 nm, the CD signal starts to change sign due to the large blueshift in the plasmon resonances along with an increase in intensity.
Similarly, DNA molecules are effective platforms for chiral plasmonic assemblies with helical shape. In fact, DNA has already been shown to be an effective constituent for chiral co-assemblies, as stated earlier. DNA has also exhibited its convenience and practicality as a chiral template for self-assembly of plasmonic nanoparticles. In 2009, assemblies consisting of single-stranded DNA and AuNPs (Fig. 8D) were demonstrated as a powerful means to fabricate nanotubes of various 3D architectures ranging in shape from stacked rings, single spirals, double spirals, and nested spirals.85 Nanotube conformations can be tuned through the size-dependent steric repulsion effect by varying the AuNP size. The morphological evolution was monitored and revealed left-handed chirality in spiral tubes. Though it is admittedly hard to achieve intrinsically high morphology yield in one specific type of chiral spiral, these complex self-assembled 3D plasmonic nanostructures emphasized the possibility of fabricating chiral plasmonic helical structures at nanometer-scale precision. Shortly after, it was reported that by rolling and stapling a 2D DNA template, AuNPs that were assembled along two linear chains on a rectangular DNA origami sheet could be organized into a 3D helical configuration (Fig. 8E).86 Due to well-defined geometry and easily customized rectangular DNA origami, the deposition of AuNPs can be rationally controlled. The particle size-dependent peak-dip CD signal close to the vicinity of the plasmon resonances of AuNPs were shown to be CD signals arising from plasmon–plasmon interactions of closely packed AuNPs. Not limited to rolling up and stapling, 2D DNA origami templates also provide alternative approaches for the synthesis of chiral plasmonic helices. Wang87 reported that by designing an ‘X’ pattern of complementary DNA strands on both sides of a DNA origami template, AuNRs could be assembled into hierarchical architectures showing left-handed and right-handed helical morphologies (Fig. 8F). Such mirrored structures are finely tuned by adjusting the composition of the DNA molecules. With fixed thickness (14 nm) and inter-rod angle (45°), the helical framework is precisely controlled. The average number of AuNRs in the helix can be manipulated by modulating the ratio of AuNRs to DNA origami in a range from 2 to 4 and 9, which drastically affects the chiroptical activities of the assemblies. The observed intense CD signals show a maximum g-factor close to ∼0.02 when 9 AuNRs (AuNRs:DNA origami = 1:1) are included in one helix. As compared with other similar systems,81 they found less AuNRs are needed to generate the same g-factor, indicating well-controlled asymmetric helical configurations dominate the chiroptical activity, rather than contribution of weak plasmonic coupling among poorly-aligned AuNRs. Their simulations also demonstrated that the chiroptical response of helical AuNR superstructures is derived from the collective oscillation modes of neighboring AuNRs. Therefore, the oscillation modes, parallel or antiparallel, decide characteristic dip-peak bisignate CD line shape.88
Meanwhile, work by Kuzyk et al.,48 showed experimentally and theoretically the potential of DNA templates for programmable design of helical plasmonic nanostructures at nanoscale precision (Fig. 8G). Collective plasmon–plasmon interactions of the AuNPs were suggested to be the origin of chirality, from which they envisaged the splitting of the incident light between the longitudinal and transverse modes caused by the plasmonic dipole interactions of AuNPs within the helix, resulting in a peak-dip line shape in the CD spectrum. Plating the gold helix with silver was an effective means to modulate CD peak position and intensity, which provided a feasible way to produce negative refractive index materials without requiring either permittivity or permeability being negative.
Despite silver being less chemically and physically stable than gold, one can hardly deny its potential as a competitive candidate for the synthesis of chiral plasmonic superstructures. Silver possesses the lowest plasmon dissipation property and thus the strongest plasmon resonance of all known metals. A recent study showed that by reducing silver nitrate in presence of blue-phase (BP) polymer template, chiral plasmonic hybrid nanostructures can be obtained (Fig. 8H).89 Like chiral gold nanostructures, such hybrid architectures with silver nanoparticles exhibit optical activity close to the plasmon resonance of silver nanoparticles. Additionally, the hybrid nanostructure shows strong sensitivity to the dielectric environment, such as solvents, indicating that chirality originates from the plasmonic resonance of silver nanoparticles within the helicoidal periodicity of the BP polymer network.
Apart from organic templates, inorganic materials such as silica and metal oxides are also promising template materials. As is widely known, sol–gel chemistry has fashioned the synthesis of nanosized SiO2 or other metal oxide particles with fine controlled morphologies. Among them, chiral mesoporous silica nanostructures for instance, have been synthesized by cooperative self-assembly of chiral or achiral amphiphiles and silica precursors, and many related synthetic works have been well reviewed by Che.78,90 With the capability for robust templating, facile surface functionality and low absorption of visible light, chiral mesoporous silica templates are strong candidates for the synthesis of chiral plasmonic nanostructures. Recent work reported by Cheng et al.,91 exhibited that gold helix hybrid nanostructures could be synthesized via templating of AuNPs on silica nanohelices. The induced chirality of the nanostructures is finely tuned by the chiral geometry of the silica templates, size and concentration of the AuNPs, and the pH of the system. Simulations based on coupled dipole method (CDM) showed that the degree of disorder for the AuNPs deposited on the surface of silica helices plays a critical role for the intensity of the CD signals (Fig. 8I). Similarly, studies from Che's group92 showed that highly ordered achiral Ag nanoparticles assembled on a chiral mesoporous silica template can exhibit strong chiroptical properties. Three possible types of chirality were suggested: (i) the helical hexagonal surface, (ii) the helical pore orientation, and (iii) the helical arrangement of aminopropyl groups on the surface of the mesopores. The synthetic strategy to study the three types of chiral architectures individually is illustrated in Fig. 8J. Although all three types of chirality dramatically impacted the plasmonic CD signals, their further investigation showed the helical pore orientation was the dominant factor for optical response due to asymmetric plasmon interactions of silver nanoparticles within the chiral pores. Also, compared to helical pitch, the total length of the helical channel was dominant in determining the intensity of plasmonic CD effects.
In light of the silica template, metal oxides, such as anodic aluminium oxide (AAO) and titania, provides another means to connect chiral plasmonic materials to the field of light sensitive semiconducting nanomaterials. A recent study by Wang et al.93 illustrated the fabrication of confined assemblies of polystyrene-tethered AuNRs in AAO channels with the assistance of an electric field. By adjusting the electric field direction, pore size in AAO membranes, and molecular weight of polystyrene on the surface of the AuNRs, the morphology of the assemblies could be fine-tuned into highly ordered hierarchical structures such as chiral single-, double-, triple-, or quadruple-helices, achiral linear, and hexagonally packed structures. Among them, electric field orientation was demonstrated to be crucial for affecting the confinement strength, defined as the ratio of the pore size to NRs size, which eventually determined the packing mode of the assembly. Although no study of chiroptical activity was reported in this work, the facile manipulation of plasmonic coupling between the NPs by adjusting interparticle distance via polystyrene chain length unveiled its promising potential for chiral photoelectric device fabrication (Fig. 8K).
Fig. 9 (A) Scheme of chiral nematic assemblies of AgNPs in mesoporous silica film and typical CD spectra of AgNP loaded silica film before (blue) and after (red) soaking with water.49 Reproduced from ref. 49 with permission from the American Chemical Society, Copyright (2011). (B) Illustration of nematic assemblies of AuNRs and cellulose nanocrystals and typical CD spectra of such chiral nematic structures. Red: the single peak was the result of combined CD signal of the CNC matrix and the longitudinal LSPR mode of the NRs; green: the double peaks refer to the chiral activity of CNC host and plasmonic chiroptical activity of the AuNRs at relatively high NaCl concentration.94 Reproduced from ref. 94 with permission from the American Chemical Society, Copyright (2014). |
Fig. 10 (A) CD spectra of the chiral supra-particles assemblies with (left) low and (right) large molar excess of CdTe quantum dots. Red and blue lines represent D- and L-geometry; the inset is a schematic representation of the proposed structure of the corresponding chiral supra-particles assemblies.99 Reproduced from ref. 99 with permission from the American Chemical Society, Copyright (2014) (B) scheme of chiral NP pyramid made by plasmonic nanoparticles (gold and silver, red and grey) and semiconducting quantum dots (yellow), and CD spectra of self-assembled pyramids made from four 10 nm AuNPs (type 1) and three 15 nm AuNPs + one 25 nm AuNPs (type 2); (B) two 15 nm AuNPs + two CdSe@ZnSQDs (type 3), and one 15 nm AuNPs + one 25 nm AuNPs + two CdSe@ZnS QDs (type 4). Inset: Corresponding illustrations of the 4 types of chiral pyramids.46 Reproduced from ref. 46 with permission from the American Chemical Society, Copyright (2012) (C) tilted SEM images of (left) L-CPN and (right) R-CPN viewed from different angles. The relative direction of the views is given in the corner of each image.100 Reproduced from ref. 100 with permission from the American Chemical Society, Copyright (2013). |
A conventional direction involves chiral plasmonic sensing. With tailorable SPR and CD activity, a variety of sensors have been fabricated based on chiral plasmonic assembles. In this area, lots of advanced work is reported by Xu and his partners who have successfully utilized chiral plasmonic units for the detection of inorganic ions such as Ag+ and Hg2+, and organic molecules such as prostate-specific antigen (PSA),76,101 peptides/proteins,75 RNA102,103 and DNA.104 Generally, by using chiral plasmonic units, namely AuNP–AgNP dimers, AuNP/AgNP-QD dimers, AuNR–AuNRs/AuNP dimers, AgNPs based pyramids, or even chiral AuNRs assemblies ranging from dimers to pentamers, CD-based sensing methods can be developed to detect the concentration of bridge molecules that are involved in the synthesis of the plasmonic chiral structures. Since chiroplasmonic CD activity is highly sensitive to the geometry of the nanostructures, which can be largely affected by the concentration of the bridge molecules, this sensing method can greatly extend the limit of detection (LOD). This would provide a feasible approach for the detection of biological analytes larger than 2–5 nm, which are often difficult for conventional plasmonic sensing methods (Fig. 11A–C).104 Moreover, with rapid development of chiral sensing, chiroplasmonic therapy105,106 and manipulation of biological activities of cells through chiral plasmonic materials becomes possible.107 Xu's recent work, for example, reports DNA-based self-assembly of plasmonic shell-satellite nanostructures which can be used as chiral photosensitizers. Under circularly polarized light illumination, they can have high reactive oxygen species generating efficiency. This property makes them a useful photodynamic therapy (PDT) agent with remarkable efficiency in tumors targeting and therapy.105
Fig. 11 Applications of chiral plasmonic nanostructures. (A–C) Plasmonic sensing via chiral NP dimers. (A and B) The NP dimer was assembled from AuNPs and AgNPs which were functionalized with complementary biomacromolecules (A). The detection of small peptides, exemplified by microcystin-LR. (B) Detection of the fairly large proteins, exemplified by prostate-specific antigen; (C) scheme of the NP dimers bridged by immunocomplexes used in competitive and sandwich immunoassays.104 Reproduced from ref. 104 with permission from Nature Publishing Group, Copyright (2013). (D–G) Chiral gold helical metamaterials made by CVD; (D and E) schematics and SEM images of the gold helices film; (F) two helix pitches (left-handed) and snapshots (G) of the electric current distribution along the metal wire for the three wavelengths [open dots in (F)]. The absolute value of the current is encoded by the curve thickness, the sign by red and blue (see arrows).108 Reproduced from ref. 108 with permission from American Association for the Advancement of Science, Copyright (2009). (H–L) DNA detection by up-conversion luminescent and chiroplasmonic techniques with the NR-UCNP tetramer assembly. (H) Scheme of the DNA biosensing. (I) The CD and (J) up-conversion luminescence curves with increasing concentrations of DNA solution. (K) The CD and (L) up-conversion luminescent calibration curves for DNA detection. The longitudinal absorption peak of NR using for assembly was 750 nm.109 Reproduced from ref. 109 with permission from Wiley, Copyright (2016). |
Manipulation of polarized optical effects such as asymmetric transmission110 and polarization conversion111,112 can also be achieved via chiroplasmonic nanostructures. N. I. Zheludev's group proves that in visible to near-IR frequencies, normal incidence transmission of circularly polarized light is asymmetric in the opposite directions when anisotropic planar chiral nanostructures are used, suggesting a novel type of enantiomerically sensitive plasmon excitation.110 P. Biagioni et al.,113 for example, showed that a cross resonant optical antenna, consisting of two perpendicular nanosized gold dipoles, can be used to finely tune the propagating fields of any polarization state into correspondingly polarized, localized, and enhanced fields.
Furthermore, the emerging area of assembled plasmonics114,115 is closely related to refractive index manipulation, where the mechanism and designs of negative effective refractive index media are related to the excitation of plasmons in the nano-metallic particles. However, at a minimum, it is required that either the permittivity or permeability of the materials should be negative to achieve metamaterials at a certain frequency range. The rising interest in chirality offers another alternative without such limits.116,117 Similar to metamaterials designed for linear polarized waves, chiral metamaterials are composed of periodic arrangements of artificial building blocks with chirality.118,119 With the existence of chirality, electromagnetic coupling between the magnetic and electric fields occurs at the plasmon resonance frequency. In order to describe such coupling, chirality parameter κ is introduced to the constitutive relations as:
D = ε0εE + iκ/c0H |
B = μ0μH − iκ/c0E |
Another practical direction relates to studies of energy conversion-based applications. Photon upconversion120 is an anti-Stokes process in which the sequential absorption of two or more low energy photons leads to high energy emission of light. The conventional upconversion nanomaterials often suffer from low energy conversion efficiency, for example lanthanide-doped luminescence upconversion nanomaterials generally have luminescence efficiencies less than 1%. An efficient remedy to improve the situation is to use the SPR effect of plasmonic nanoparticles. Recently, Xu's group109 reported that propeller-like chiral assemblies of AuNRs and lanthanide-doped luminescence upconversion nanoparticles (UCNPs) can lead to an enhancement of upconversion luminescence as great as 21.3 fold via plasmonic resonance coupling. As illustrated in Fig. 11H, such chiral plasmonic tetramers are fabricated via co-assembly of DNA-modified AuNRs with DNA-modified UCNPs. By tuning parameters such as the length of DNA molecules, diameter of the UCNP, and the aspect ratios of AuNRs to the UCNPs, intense chiroptical activity can be achieved in the visible plasmonic region due to the unique propeller-like geometry of the assemblies. In addition, up-conversion luminescence (UCL) of Yb/Er doped UCNP was enhanced for emission peaks at 529, 546, and 662 nm. This phenomenon matches well with the DNA hybridization process for tetramer formation, which is largely affected by plasmon resonance interactions i.e. interparticle distances between AuNRs and UCNPs and the size of AuNRs and UCNPs. Furthermore, such materials with both chiroptical activity and UCL have been successfully used for biosensing of oligonucleotides, confirming the versatility of the method (Fig. 11H–L).
In addition, if we extended the plasmonic response of inorganic nanocrystals into metal oxides or doped semiconductors, the LSPR properties of doped semiconductor nanocrystals can also provide a versatile toolbox for obtaining chiral plasmonic nanostructures with induced chirality. Different from metallic nanomaterials whose extinction peak is generally associated to their size, shape and chemical compositions, the extinction peak of doped semiconductor nanocrystals can be easily modulated via changing carrier concentration through chemical doping. To date, a variety of substoichiometry semiconductor nanocrystals such as Cu2−xS,121,122 indium oxide,123 germanium telluride,124 tungsten oxide,125 zinc oxide,126,127 and molybdenum oxide128 have been extensively studied for their LSPR behaviors. Tunable control of their LSPR response was often achieved via defect engineering129 and liquid exfoliation approaches.130 In light of the merits of doped semiconductors nanocrystals, Kotov and colleagues131 synthesized chiral WO3−x nanoparticles via a bio-to-nano chirality transfer approach. The difference in CD response of WO3−x induced by proline (Pro) and aspartic acid (Asp) respectively are related to the degree of distortion of the inorganic crystal lattice, which is confirmed by atomistic molecular dynamics simulations. Although enantioselective preparation of chiral plasmonic nanostructures with doped semiconductors is difficult, the development of this area is still open for more systematic studies on chiral synthesis of higher-order structures and mechanisms for understanding the plasmonic chirality in these materials.
In summary, the skyrocketing development on the design and fabrication of plasmonic chiral nanomaterials presents great potential for this field, giving researchers adequate foundations for obtaining novel advances and practical merits. However, it is worth noting that the study on plasmonic optical chirality is still in its infancy. Progressive investigations on the CD generation and incisive explanations on the origin of chirality as well as theoretical simulations are still expected to improve the field. Detailed studies on the plasmonic NP–chiral molecule and NP–NP interactions supported by interdisciplinary areas such as plasmonic biosensors, chiral metamaterials, and time-resolved nonlinear optics will inevitably trigger further development of this area, providing us new horizons together with brand-new challenges.
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