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Chiral Plasmons: Au Nanoparticle Assemblies on Thermoresponsive Organic Templates Jino George, Sabnam Kar, Edappalil Satheesan Anupriya, Sanoop Mambully Somasundaran, Anjali Devi Das, Cristina Sissa, Anna Painelli, and K George Thomas ACS Nano, Just Accepted Manuscript • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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Chiral Plasmons: Au Nanoparticle Assemblies on Thermoresponsive Organic Templates Jino George,ab Sabnam Kar,a Edappalil Satheesan Anupriya,a Sanoop Mambully Somasundaran, a

a

Anjali Devi Das,b Cristina Sissa,c Anna Painelli,*c and K. George Thomas*a

School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram

(IISER-TVM), Vithura, Thiruvananthapuram, 695 551, India. b

CSIR –National Institute for Interdisciplinary Science and Technology, Trivandrum, 695 019,

Kerala, India. c

Dip. di Scienze Chimiche, della Vita e della Sostenibilità Ambientale, University of Parma,

43124 Parma, Italy.

ACS Paragon Plus Environment

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ABSTRACT

Template-assisted strategies are widely used to fabricate nanostructured materials. By taking these strategies a step forward, herein we report the design of two chiral plasmonic nanostructures based on Au nanoparticle (NP) assemblies organized in clockwise and anticlockwise directions, having opposite response to circularly polarized light. The chiral plasmonic nanostructures are obtained by growing Au nanoparticles on chiral templates based on D- and L-forms of alanine functionalized phenyleneethynylenes. Interestingly, Au nanoparticle assemblies show mirror symmetrical electronic circular dichroism (ECD) bands at their surface plasmon frequency originating through their asymmetric organization. Upon increasing the temperature, the chiral templates dissociate as evident from the disappearance of their ECD signal. The profound advantage of the thermoresponsive nature of the templates is employed to obtain free-standing chiral plasmonic nanostructures. The tilt angle high-resolution transmission electron microscopic measurements indicate that the nanoparticle assemblies, grown on template based on D-isomer organize in clockwise direction (P-form) and on L-isomer in anticlockwise direction (M-form). The inherent chirality prevailing on the surface of the template drives the helical growth of Au nanoparticles in opposite directions. Experimental results are rationalized by a modified exciton model which accounts for the large polarizability of Au nanoparticles. The large polarizability leads to large oscillating dipole moments whose effects become prominent when interparticle distances are comparable to the particle size.

KEYWORDS: chirality, electronic circular dichroism, Au nanoparticle assembly, surface plasmon resonance, thermoresponsive template, plasmon coupling


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The ability of chiral molecules and objects to differently sense right ( +) and left (-) circularly polarized light has fascinated mankind in general and scientists in particular.1-10 Various mechanisms have been reported to explain the origin of chirality in inorganic clusters, metals, and semiconductors.11-14 It is well-established that the asymmetric organization of achiral molecules,15-17 clusters,11,

12

and nanomaterials18-22 on chiral templates can induce chiroptical

properties to the former ones. This strategy has been utilized for the design of chiral nanoobjects, particularly based on noble metal nanoparticles, transition metal oxides and silica.18,

20-24

Arrangements of noble metal nanostructures on various chiral templates, such as peptide nanotubes,24, 25 proteins,26 and DNA,27-31 result in the emergence of surface plasmon electronic circular dichroism (SP-ECD). We have earlier demonstrated that Au nanoparticles grown on Dand L-isomers of diphenylalanine peptide nanotubes showed bisignated electronic circular dichroism (ECD) signals at their surface plasmon frequency with positive and negative couplets, respectively.25 Theoretical insight on the emergence of chiroplasmonic properties in noble metal nanoparticles has been provided by Govorov and coworkers28,

32-35

based on the asymmetric

organization of plasmonic nanostructures. Studies on chiroptical properties of metal nanostructures have stimulated several investigations on the design of chiroplasmonic systems for applications in various branches of science.35-42 By assembling Au nanorods on chiral supramolecular fibers, Liz-Marzán and coworkers have demonstrated the formation of SP-ECD with high levels of anisotropy factor.41 More recently, the group has demonstrated the use of intense chiroptical activity of helically organized Au nanorods for the detection of amyloid fibril formation, down to nanomolar concentrations, having practical applications in the early detection of various neurodegenerative diseases.40 Kotov and coworkers have demonstrated the strategies for the design of Au nanorod


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based chiral assemblies and their applications in biomedical research. Various studies include (i) attomolar sensing of DNA through the twist in laterally organized Au nanorods and propellerlike nanoscale tetramers,39 (ii) ultra-sensitive chiroptical readout of intra-/extracellular localization of the DNA-bridged Au nanorod dimers, exemplified in photodynamic therapy of malignancies.38 Design of molecular switches and machines using DNA origami based Au nanorod assemblies by Liu and coworkers35 is yet another important development in the field of chiral plasmonics. More recently, Nam and coworkers have developed an elegant strategy for the design of chiral plasmonic helicoids based on Au nanoparticles, by involving amino acids and peptides, which show interesting chiroptical properties.37 Chiroptical response of metamaterials based on Au nanoparticles grown on various dielectric templates have been reported by Ghosh and coworkers, and these results are supported by theoretical analysis.36 One of the convenient methods adopted for the synthesis of nanostructured materials is by growing them on an organic template. Calcination is often used for the removal of template, which can result in the oxidation of nanostructured material, thereby affecting their functional properties. Herein, we report the design of free-standing chiral nanoparticle assemblies by growing them on a thermoresponsive soft template based on an organic molecule and further removing them by varying the temperature. In this regard, two chiral molecular systems having D- and L-isomers of alanine functionalized onto the terminal ends of phenyleneethynylene (Chart 1; designated as D- and LPE-A), which form thermoresponsive templates, are synthesized. The helical growth of Au nanoparticle assemblies on these templates showed mirror symmetrical ECD bands at their surface plasmon frequency and these results are rationalized based on the plasmonic dipolar model originally proposed by Gorovov and coworkers.32,

43, 44

In the present case, chiral

plasmonic Au nanostructures are separated from the template by slightly increasing the


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temperature of the solution to 60 °C which is indeed a nondestructive method and helps in retaining their functional properties. The organization of Au nanoparticle assemblies in the clockwise and anticlockwise directions is established by tilt angle high-resolution transmission electron microscopic (HR-TEM) studies.

Chart 1. Phenyleneethynylenes possessing D- and L-alanine derivatives designated as D-PE-A and L-PE-Aa

a

The molecular systems are labeled with alphabetic extension as D, L, PE and A representing D-

isomer, L-isomer, phenyleeneethynylene unit and Boc protected alanine (Boc stands for tertButoxycarbonyl group).


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RESULTS AND DISCUSSION Design of Thermoresponsive Templates Oligo(p-phenyleneethynylene)s form an interesting class of π-conjugated rigid rod molecular systems which self-assemble through π-π stacking and are widely used for the design of functional molecular materials.45 1,4-Diethynylbenzene (1), and D- and L-isomers of phenyleneethynylene derivatives (4 and 5) are prepared by adopting Pd-catalyzed Heck-CassarSonagashira-Hagihara cross-coupling reaction (Scheme 1).46 Details of the procedure adopted for the synthesis of various intermediate compounds and final products, along with their characterization, are provided as Supporting Information. The carboxylic and amino groups of the D- and L-isomers of 4-iodophenylalanine are first protected before carrying out the crosscoupling reaction (labeled as 2 and 3, Scheme 1). The compounds 4 and 5 are first purified by column chromatography and subsequently by recycling preparative HPLC, with two columns connected in series. Typically, phenyleneethynylene derivatives (~20 mg), dissolved in chloroform (3 mL) is injected to the HPLC and eluted using the same solvent. Effective removal of impurities is achieved by recycling the samples for about 10-15 cycles. The ester groups of phenyleneethynylene derivatives 4 and 5 are deprotected using LiOH, a weak base, to yield Dand L-PE-A in quantitative yields.


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Scheme 1. Synthesis of D- and L-PE-Aa

a

(i) Pd(PPh3)4, CuI, (i-Pr)2NH; (ii) LiOH.H2O, THF, 4 h. The details on the synthesis of final

compounds are presented in Supporting Information. Boc stands for tert-Butyloxycarbonyl group. D- and L-PE-A exist in monomeric form in chloroform and their spectral properties are summarized in Table S1. Both compounds show two absorption bands in the UV-vis spectral region. Fluorescence spectrum (excited at 370 nm) showed a band at 404 nm with a shoulder at 423 nm. Both the isomers possess a high quantum yield of fluorescence of ~0.93 in their monomeric form. We have recently demonstrated solvent dependent self-assembly of phenylalanine functionalized phenyleneethynylene derivatives by increasing water content in methanol.47 The aggregates formed on increasing the water content in methanol above 70%, are stable up to 60 °C and morphological studies indicate the formation of supramolecular nanostructures. In the present study, we have prepared nanotubes through the self-assembly of


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D-/L-PE-A in methanol/water mixture (Figure 1a). Typically, a freshly prepared stock solution of D-/L-PE-A (80 μM) in methanol (1 mL) is diluted to a final concentration of 8 μM, using a mixture (1:1) of methanol and water. Preparation of templates need to be carried out with extreme care, since they are highly sensitive to temperature and details are presented in the Supporting Information. The solution is kept at 5 °C for one day to ensure the completion of selfassembly, and directly used for photophysical and microscopic studies. AFM analysis is carried out by drop-casting the solution on a freshly cleaved mica sheet. Formation of well-organized nanotubes with average height of 20 nm is observed (Figure 1b and Figure S1 and S2), and the section analysis of the nanotubes revealed a width of approximately 80 nm for the smallest fiber (Figure S1b). It is obvious from the TEM images that these nanotubes flatten due to drying effects (Figure 1d and Figure S3 and S4). The thermoresponsive nature of nanotubes is investigated by measuring absorption, emission, and ECD spectra as a function of temperature (Figures 1c, e). Both the compounds exist in their monomeric forms in methanol. However, the absorption spectrum of both the derivatives in a mixture (1:1) of methanol and water at 25 °C showed distinctly different spectral features with the formation of a red-shifted band at 385 nm (Figure 1c). A stepwise increase of temperature to 55 °C resulted in the breakage of nanotubes to their monomeric form as shown in the absorption spectrum. Temperature dependent emission and ECD investigations further confirm the breakage


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Figure 1. (a) Schematic representation of the assembly and disassembly of chiral nanotubes of D-/L-PE-A by cooling and heating. Microscopic and spectroscopic characterization of D-/L-PEA: (b) AFM and (d) TEM (100 kV) images of nanofibers of L-PE-A, (c) absorption and emission (inset) of D-isomer in a mixture (1:1) of MeOH/H2O and (e) ECD spectral changes of D- and L-isomers (blue and red traces, respectively) by varying the temperature. All spectroscopic studies are carried out by dissolving D-/L-PE-A (8.0 M) in a mixture (1:1) of MeOH/H2O. The above solution (100 μL) is drop-casted onto mica sheet, and carbon-coated Cu grid for AFM and TEM studies, respectively and dried overnight. Additional AFM and TEM images are provided as Figures S1-S4. of the template. Specifically, upon increasing the temperature from 25 °C to 55 °C, a significant enhancement in the fluorescence quantum yield is observed, in line with the formation of the monomeric form of D-/L-PE-A (inset of Figure 1c). ECD studies provided conclusive evidence of the formation and breakage of nanotubes on varying the temperature. Both isomers are found


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to be ECD silent on increasing the temperature above 55 °C, confirming the breakage of nanotubes to monomers. Supramolecular assemblies of D- and L-PE-A in a mixture (1:1) of methanol and water showed distinct ECD signals with opposite sign in the spectral region of 245-450 nm (Figure 1e). The ECD spectral profile of D- and L-PE-A possess a bisignated signal in the short wavelength region and a nonbisignated signal in the longer wavelength region. We have observed similar spectral features in phenyleneethynylene derivatives having two closely lying transitions.47 These aspects can be explained based on a standard exciton model accounting for degenerate as well as nondegenerate excitonic interactions. The g factor (anisotropy factor) of the D- and L-PE-A templates at two different wavelengths are presented as Table S2.

Growth of Au Chiral Nanostructures We have independently investigated the formation of Au nanoparticle assemblies on thermoresponsive nanotubes obtained from D- and L-PE-A by following various spectroscopic and microscopic methods (Figure 2). All experiments are carried out at 25 °C in a mixture (1:1) of methanol and water at pH 6 (sodium phosphate buffer; 50 mM) where the molecules form nanotubes. It is well-established that the surface ligands play a major role in the self- assembly of Au nanoparticles (NPs).48 The nanotubes (D- and L-PE-A; 4 μM) are first seeded with Au nanoparticles functionalized with triethylene glycol thiol (EG3-S-Au; Supporting Information for details).25 It is clear from the TEM (100 kV) images that the EG3-S-Au nanoparticles are adsorbed on the surface of nanotubes (Figure 2c). Due to the strong adsorption of EG3-S-Au nanoparticles on nanotubes, the surface plasmon resonance (SPR) band is red shifted to 520 nm (green trace; inset of Figure 2b). Nevertheless, the ECD profile of D-/L-PE-A nanotubes did not


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Figure 2. (a) Schematic representation of the formation and removal of chiral Au nanoparticle assemblies from the surface of L-PE-A nanotubes. (b) ECD (both D-/L-PE-A) and absorption spectrum (D-PE-A; inset) of nanofibers in a mixture (1:1) of MeOH/H2O (black traces) and the changes observed upon addition of EG3-S-Au nanoparticles (green traces) and HAuCl4 (blue traces). The red traces in Figure 2b illustrate the spectral changes of the resultant solution after 2 h indicating the formation of chiral Au nanoparticle assemblies. TEM (100 kV) images of L-PEA nanotubes (c) seeded with EG3-S-Au and (d) having chiral nanoparticles formed on addition of HAuCl4 and keeping them for 2 h. (e) Temperature-dependent ECD spectral changes of chiral Au nanoparticle assemblies on D-/L-PE-A nanotubes in a mixture (1:1) of MeOH/H2O. The ECD bands corresponding to D-/L-PE-A become silent at 65 °C (PE region) while the bisignated ECD signals corresponding to chiral Au nanoparticle assemblies remain unaffected (CP region). (f,g) HR-TEM (300 kV) images of formed chiral Au nanoparticle assemblies on LPE-A after removing the nanotubes by heating to 65 °C. Inset of (f) and (g) represent the FFT pattern of the corresponding high-resolution images. (i) The concentration of D-/L-PE-A, EG3-


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S-Au nanoparticles and HAuCl4 in the resultant solution are 4.0 M, 1.6 M and 53.0 M, respectively. (ii) The pH of the solution is maintained at 6 using phosphate buffer. show any appreciable change upon seeding with nanoparticles, ruling out the possibility of chirality transfer. An aqueous solution of HAuCl4 (53 µM) is added to the above solution, and the ECD and absorption spectral changes are followed at various stages of growth (Figure 2b and inset of Figure 2b). The basis of above-mentioned sequence of addition is that the reduction of Au3+ ions to elemental gold (Au(0)) is efficient near to the surface of seed nanoparticles.49 This is essentially due to the low potential requirement for the reduction process, which successively creates additional nucleation sites near to the seed nanoparticles in a controlled way. Immediately after the addition of HAuCl4, a broadening in the SPR band occurred due to the complexation of Au3+ ions onto the seed nanoparticles (blue trace; inset of Figure 2b). Correspondingly, a slight dampening in the ECD intensity of D-/L-PE-A nanotubes is observed. After 2 h, distinct SPR band centered at 545 nm (red trace; inset of Figure 2b) is observed in the extinction spectrum signifying the reduction of Au3+ ions to nanoparticles. Interestingly, Au nanoparticles grown on the surface of D-/L-PE-A nanotubes showed mirror symmetrical ECD bands at their surface plasmon frequency (red traces of Figure 2b). Au nanoparticle assemblies formed on the D-isomer of nanotubes showed positive followed by negative signal (positive couplet). In contrast, an inversion of ECD signal is observed with negative followed by positive signal (negative couplet) for L-isomer.15,

16, 25

The SP-ECD signals originate from the

asymmetric organization of Au nanoparticle assemblies25, 28, 32, 34 on the surface of D-/L-PE-A nanotubes, directed by chiral templates. The g factor (anisotropy factor) of the chiral Au nanoparticle assemblies grown on the templates at different experimental conditions are presented as Table S2.


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To further understand the formation of chiral nanoparticles, TEM images of Au nanoparticle assemblies grown on the surface of nanotubes are acquired (Figure 2d). Well-separated nanoparticle assemblies having 5-7 nanoparticles in each bunch, with individual nanoparticles having a diameter of ~15 nm, are observed on D-/L-PE-A nanotubes. The individual nanoparticles in the assembly are merged in many locations as evident from the HR-TEM (300 kV; Figures 2f,g) images. However, the organic templates tend to burn off at high acceleration voltage, which limit us from obtaining high resolution images of assembled nanoparticles bound on D-/L-PE-A nanotubes. Hence, Au nanoparticle assemblies are separated from the template before characterizing them (vide infra). The Au nanoparticle assemblies formed after 2 h of controlled reduction of Au(III) at 25 °C are stable which is obvious from the high negative zeta potential values50,

51

(ζ = -28 mV; Figure S7). Chiral organic templates are essential for the

preparation of Au nanoparticle assemblies and these aspects are investigated by carrying out control experiments (Figure S8). The chiral nanoparticle assemblies on the surface of D-/L-PEA showed negligible linear dichroism (LD)52 (Figures S9 and S10). Au nanoparticle assemblies are separated by increasing the temperature, by exploiting the thermoresponsive nature of D-/L-PE-A templates. The ECD spectrum of these hybrid nanostructures, presented in Figures 2b, e are divided into two parts: (i) ECD signals below 450 nm corresponds to D-/L-PE-A nanotubes denoted as PE region and (ii) above 450 nm belongs to SP-ECD of the Au nanoparticle assemblies denoted as chiral plasmon (CP) region. The ECD signal in the higher energy region becomes silent on increasing the temperature indicating the breakage of D-/L-PE-A nanotubes to monomers in the PE region. Interestingly, the intensity as well as the structure of the bisignated ECD signal of both the isomers, remains unaffected in the CP region on increasing the temperature to 65 °C. The removal of chiral Au nanoparticle


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assemblies from the surface of D-/L-PE-A nanotubes is further confirmed through HR-TEM studies. It is clear from the HR-TEM images that the lattices of the assembled nanoparticles merge each other which results in the formation of anisotropic nanostructures (Figure 3 and Figures S11, S12). TEM grids are prepared with extreme care (Supporting Information) and tilt angle HR-TEM measurements are carried out to understand the organization of Au nanoparticle assemblies grown on the surface of D- and L-PE-A nanotubes. In the case of nanostructures obtained from D-PE-A, it is observed that one nanoparticle is clearly visible with high contrast at +25° tilt angle with respect to the incident electron beam. The sample is kept at the eucentric height and eucentric focus for the measurements. This nanoparticle in the assembly is represented as NP1 in Figure 3a(i). Reducing the tilt angle to +20° resulted in the disappearance of lattice planes of NP1 and the appearance of distinct lattice planes of another nanoparticle denoted as NP2 (Figure 3a(ii)). By systematically varying the tilt angle measurements, we have demonstrated the successive appearance of NP1 to NP5. This essentially indicates the organization of nanoparticles in the clockwise direction or plus-form (P-form) as represented in Figure 3a(i)-(vi) and illustrated as Figure 3b. Impressively, the appearance of nanoparticles NP1 to NP5 follows anti-clockwise directionality or minus-form (M-form) for the Au nanoparticle assemblies obtained from L-PE-A (Figure 3c(i)-(vi)). Mirror image relationship in the organization of nanoparticles observed in the HR-TEM images clearly indicates that the chiral molecules on the D- and L-PE-A nanotubes drive the organization in two different ways. This is clear from the direction of arrows shown in Figures 3a(vi) and 3b for P-form and Figure 3c(vi) and 3d for M-form (corresponding enlarged images in Figures S11, S12). Bisignated ECD signal at the CP region, with mirror image symmetry, thus originates from the asymmetric plasmon coupling in the left-handed and right-handed assemblies of Au nanoparticles.


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Figure 3. HR-TEM characterization (scale bar 5 nm) of Au nanoparticle assemblies by varying the tilt angles as mentioned in the left-top corner of each frame: (a) P-form having clockwise organization (c) M-form having anticlockwise organization (after removal from the surface of Dand L-PE-A nanotubes, respectively). When the tilt angle is 0° (normal incident angle), the arrangement of Au nanoparticles in the frame (vi) of (a) is in the clockwise direction and frame (vi) of (c) in the anti-clockwise direction representing the P-and M-forms, respectively as indicated by the direction of arrows (the numbering and direction of arrows are guide to the readers eye; the enlarged view of the HR-TEM are presented as Figures S11, S12 in Supporting Information). (b,d) Schematic representations of P- and M- forms of Au nanoparticle assemblies.


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In a(vi) and c(vi) the arrow joins the topmost nanoparticle (NP1) to the bottommost nanoparticle (NP5). In general, organic templates are soft in nature compared to inorganic templates. We observe a large divergance in organization of Au nanostructures grown on template due to the structural inhomogenity of the soft organic nanotubes. As a result we observe fully oriented as well as randomly arranged nanostructures in the TEM images (Figure S5, S6). We have evaluated the size and orientation of fully oriented nanostructures as model for the calculations. We have carefully analyzed the various geometrical parameters of the Au nanoparticle assemblies by choosing several fully oriented Au nanoparticle assemblies. It is found that each assembly consists of 5-7 nanoparticles, having pitch varying between 25-45 nm and the helix diameter in the range of 20-35 nm. The chiral organic template directs the growth of the nanoparticle and it is found that a turn is complete on arrangement of 5-7 nanoparticles, preventing further growth. This may be the reason for having ~5-7 nanoparticles in each assembly. These aspects are considered while modeling the SP-ECD response of Au nanoparticle assemblies.

Modeling the SP-ECD response The simplest model to describe SP-ECD signals in the CP region of aggregates of NP was proposed by Fan and Govorov.32 It is based on the dipolar approximation, so that each NP responds to an applied field generating an induced dipole: ⃗⃗𝜇𝑖 (𝜔) = 𝛼𝑖 (𝜔)𝐸⃗𝑖 (𝜔)

(1)


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where i counts the NP inside the aggregates, 𝐸⃗𝑖 (𝜔) is the electric field at the location of the i-th NP and 𝛼𝑖 (𝜔) is the i-th NP polarizability. Of course, all the quantities in eq 1 are frequencydependent. The dipolar approximation converges to the exact limit when the NP dimensions are negligible with respect to intermolecular distances, and offers a reliable result as long as we consider intermolecular distances at least 1.5 times larger than the NP dimensions.32 Our systems are located inside the safety zone, or very close to it. Accordingly, we stick to the dipolar approximation due to its simpler approach to rationalize observed signals with the additional advantage of building upon a model that shares many similarities with the exciton model, usually adopted to describe ECD signals of molecular aggregate.47, 53 The frequency dependent polarizability of the i-th NP is expressed as follows: 𝑑𝑖 3 𝜀𝑖 (𝜔) − 𝜀0 𝛼𝑖 (𝜔) = ( ) , 2 𝜀𝑖 (𝜔) + 2𝜀0

(2)

where 𝜀0 is the medium dielectric constant (medium is taken as water and 𝜀0 = 1.8), 𝜀𝑖 (𝜔) is the frequency dependent dielectric constant of gold54, 55 and di is the diameter of the i-th NP. For the gold dielectric function we use an empirical equation54,

55

that precisely fits the experimental

dielectric function of bulk gold in a wide spectral range (200-1000 nm). Experimental data on the dielectric constant of Au NPs, having a similar dimension as our NPs56 demonstrate marginal deviation from the bulk dielectric constant in the range 510-600 nm which covers most of the spectral region of interest for this study. The local field 𝐸⃗𝑖 (𝜔) is calculated as the sum of the applied field and the contribution due to the dipole moments of all other NPs, using a self-consistent procedure.32 The induced dipoles finally enter in the expression for the extinction coefficient for the right and left circularly


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polarized light, whose difference gives the required SP-ECD spectrum.32,

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43

Since we are

interested in simulating ECD spectra measured in solution, we average calculated spectra assuming a fully random orientation of the aggregates. Guided by the information collected from HR-TEM images (Figures 3 and 4a), we consider a helical arrangement of NPs, with the helix diameter D, helix pitch P, and the angle between adjacent NPs , as defined in Figures 4g, h. The CP region of SP-ECD spectra are calculated for right-handed helices composed of n = 3-8 NPs with diameter d = 15 nm, D = 40 nm,  = 72° at different pitches (20 nm, 30 nm and 40 nm). The results are presented in Figures 4d-f. ECD spectrum with equal and opposite features are obtained for left-handed helices as shown in Figure S13. The calculated ECD spectra of helical assembly match well with the experimental results (Figures 2b, e), confirming that the model captures the essential chemistry and physics of the chiral plasmonic interactions. We notice that the calculated crossover point of the ECD spectrum is slightly blue-shifted to 550 nm, with respect to the experimental result observed at 560 nm. However, the precise location of the crossover point changes with the medium dielectric constant 𝜀0 , as shown in Figure S14.


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Figure 4. Theoretical calculation of right-handed helical assembly of Au nanoparticles: (a) HRTEM image (scale bar 10 nm) of the P-form of nanoparticle assembly and the schematic illustration of its (b) side view and (c) top view. (d, e, f) Calculated ECD spectra of right-handed nanoparticle assemblies by varying the (i) pitch (P = 20 nm, 30 nm and 40 nm) and (ii) number of particles in each pitch (n = 3-8), keeping the particle diameter d = 15 nm, angle  = 72° and helix diameter D = 40 nm. (g, h) Illustration of various geometrical parameters (d, D, P and ) of the right-handed helical assembly. Calculated ECD spectra for left-handed nanoparticle assemblies with similar geometrical parameters are shown in Figure S13. Calculated ECD spectra of right-handed nanoparticle assemblies for different helix diameter, pitch and angle are reported as Figure S14-S16.


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The calculated SP-ECD spectra of Au nanoparticle assembly having larger pitches (P = 30 and 40 nm; Figures 4e, f and Figures S13d, e, S14 and S15) obey the same chirality rule as defined in the framework of the exciton model for helicoidal molecular aggregates. It can be seen in Figures 4e, f that the right-handed helices clearly show a bisignated SP-ECD spectrum with a positive signal in the long-wavelength region followed by a negative signal in the short-wavelength region (termed as positive couplet or positive chirality). On contrary, the sign of SP-ECD spectrum changes with the number of NP in the helical assembly for P = 20 nm (Figures 4d and S9d). This result contrasts sharply with the exciton model, where the sign of the ECD spectrum is not expected to vary with the dimension of the aggregate. The exciton model, as applied to describe the ECD spectra of molecular aggregates, fully neglects the molecular polarizability57 and hence neglects the variation of the molecular dipole moments due to the applied field. This phenomenon marginally affects the ECD spectra of molecular aggregates where the molecular polarizability is comparatively small and the transition dipole moments have a fixed orientation with respect to the molecular frame. However, the plasmonic response of NP aggregates leads to large oscillating dipole moments whose size and orientation are defined by the local electric field of each NP. Since, the plasmonic oscillating dipoles are large, their contribution to the local field felt by the NPs is non-negligible.58 Therefore, one ends up with a complex interplay where each NP responds to the local field which is sum of the applied field and the contributions from the oscillating dipole moments of the surrounding NPs. This self-consistent interaction affects the amplitude of the oscillating dipoles as well as their relative orientation which contrasts sharply with the exciton picture where the molecular dipole moments have small amplitude and fixed orientation. The deviations in the SP-ECD spectra of chiral NP aggregates from the exciton model are expected to be large when the oscillating dipoles on surrounding NPs give a sizable


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contribution to the electric field at the position of the i-th NP. The oscillating dipole on each NP is proportional to the NP polarizability, and hence to the cube of the NP diameter (eqs 1 and 2). This oscillating dipole generates an electric field on a nearby NP which is proportional to the dipole itself and hence proportional to d3 divided by the cube of the interparticle distance rij, with an overall effect scaling with (d/r)3. Accordingly, we observe large deviation of the SP-ECD response from the exciton model when the distance between NPs is large with respect to the NP diameter. Indeed, the chirality rule is obeyed for smaller NPs with d = 7 nm, at P = 20 nm, but deviated for smaller pitches (P = 15 nm, Figure S16). SP-ECD spectra calculated for NP aggregates with different sizes are similar to those obtained imposing uniform sizes (Figure S17). CONCLUSIONS An elegant protocol is presented for the design of chiral nanostructured systems. Soft organic templates based on phenyleneethynylene bearing alanine moieties (D- and L-isomers) which possess bifunctional features of chirality and thermoresponsive nature are used for this purpose. The inherent chirality prevailing on their surface drives the helical growth of Au nanoparticles as established by circular dichroism signal in the plasmonic resonance region. The thermoresponsive property of the templates is exploited for the removal of organic templates from chiral plasmonic structures. The plasmon coupling in these assembled nanoparticles is in a way similar to the mechanism of exciton-coupled circular dichroism which operates in various multichromophoric systems.47, 53 A simple model for the ECD response of chiral nanoplasmonic aggregates28, 32, 43, 44 rationalizes the observed ECD spectra and allows us to differentiate between intermolecular interactions in chiral molecular aggregates and plasmonic interactions in chiral metal NP aggregates. In the first case, the electric field generated by each molecule on the surrounding molecules is negligible, while the electric field is large in plasmonic aggregates.


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This phenomenon has significant effects when the distance between adjacent NPs becomes comparable to the NP diameter. The procedure presented herein can help in developing strategies for the bulk synthesis of nanostructured systems having desired chirality. Such plasmonically powered chiroptical systems are useful for the design of materials having a negative refractive index (metamaterials),36, 59, 60 and chiral sensing and separation. 61, 62 MATERIALS AND METHODS Tetrachloroauric(III) acid (HAuCl4) and tri(ethylene glycol) monomethyl ether and all the precursors for the synthesis of compounds D- and L-PE-A are purchased from Sigma-Aldrich. All melting points are uncorrected and were determined on Mel-Temp-II melting point apparatus. 1H NMR and 13C NMR spectra are recorded on Bruker DPX-500 MHz spectrometer. FAB and high-resolution mass spectra are recorded on a JEOL JM AX 5505 mass spectrometer. Electronic absorption spectra are recorded on Agilent diode array UV-visible spectrophotometer (model 8453), infrared spectra on Shimadzu FTIR spectrophotometer (IR Prestige-21), circular dichroism and linear dichroism spectra on JASCO spectropolarimeter (J-810) and zeta potential on Malvern Zetasizer Nano Zs instrument. AFM analysis is carried out on Multimode SPM (Veeco Nanoscope V). Samples are prepared by drop-casting 100 µL solution on a freshly cleaved mica surface and dried overnight. Imaging is done under ambient conditions in tapping mode. TEM analysis is carried out on FEI HR-TEM (Tecnai 30G2, S-twin; 300 kV) or Hitachi LR-TEM (H-7650; 100 kV). For TEM analysis, samples are prepared by drop-casting the solution (100 μL) on a carbon-coated Cu grid and dried overnight, keeping at ambient conditions.


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ASSOCIATED CONTENT The authors declare no competing financial interest.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs nano. Synthesis of D- and L- PE-A, synthesis of Au nanoparticles, preparation of D- and L- PE-A nanotubes, TEM images of Au nanoparticle assemblies and their tilt angle studies, linear dichroism of D- and L- PE-A nanotubes, theoretical calculations of right- and left-handed helical Au nanoparticle assemblies by varying the particle size, pitch and helix diameter (PDF)

AUTHOR INFORMATION Corresponding Authors e-mail: [email protected] (A.P.), [email protected] (K.G.T.) ORCID iD Jino George: 0000-0002-3558-6553 Sabnam Kar: 0000-0003-1944-0236 Edappalil Satheesan Anupriya: 0000-0002-5177-8280 Sanoop Mambully Somasundaran: 0000-0001-5107-995X Anjali Devi Das: 0000-0002-5509-4302 Cristina Sissa: 0000-0003-1972-1281 Anna Painelli: 0000-0002-3500-3848 K. George Thomas: 0000-0003-1279-308X


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Present Addresses J.G.: Department of Chemical Sciences, Indian Institute of Science Education and Research Mohali, Mohali, Punjab, India (currently working as a faculty member); S.K.: Department of Chemistry, Indian Institute of Technology Bombay, Mumbai, India (currently working as a post-doctoral research associate). Author Contributions The experiments are designed by K.G.T and carried out by J.G., S.K., E.S.A and S.M.S. The calculations are designed by A.P. and carried out by A.P., C.S. and A.D.D. worked in the project under the Joint Science Academies' Summer Research Fellowship Programme. J.G., S.K., C.S., A.P. and K.G.T. analyzed the results and wrote the manuscript. Funding K.G.T. thanks the Department of Science and Technology (DST Nanomission Project (SR/NM/NS-23/2016) and J.C. Bose National Fellowship of SERB-DST, Government of India for financial support. S.K. and J.G. acknowledge the University Grants Commission, Government of India for the senior research fellowship. K.G.T., C.S. and A.P. thank the Indo−Italian Executive Program 2017−2019 of Cooperation in Scientific & Technological Cooperation (Department of Science and Technology; No. INT/Italy/P-9/2016(ER) for travel support. C.S. and A.P. thank CINECA for support through IsC58_iiCT-MM project. C.S. thanks the University of Parma for financial support (Project Cybamat, FIL 2016 “Quota incentivante”). E.S.A. thanks Department of Science and Technology, Government of India for the Innovation


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in Science Pursuit for Inspired Research (INSPIRE) fellowship. S.M.S. thanks Indian Institute of Science Education and Research Thiruvananthapuram for the senior research fellowship.

ACKNOWLEDGMENTS Authors thank Mr. P. P. Rafeeque for graphical support.

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