Article pubs.acs.org/Langmuir

Coassembly of Gold Nanoparticles and Cellulose Nanocrystals in Composite Films Ariella Lukach,† Héloïse Thérien-Aubin,† Ana Querejeta-Fernández,† Natalie Pitch,† Grégory Chauve,‡ Myriam Méthot,‡ Jean Bouchard,‡ and Eugenia Kumacheva*,† †

Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, Ontario M5S3H6, Canada FPInnovations, 570 Saint Jean Boulevard, Pointe-Claire, Québec H9R 3J9, Canada

‡

S Supporting Information *

ABSTRACT: Coassembly of nanoparticles with different size-, shape-, and compositiondependent properties is a promising approach to the design and fabrication of functional materials and devices. This paper reports the results of a detailed investigation of the formation and properties of free-stranding composite films formed by the coassembly of cellulose nanocrystals and shape-isotropic plasmonic gold nanoparticles. The effect of gold nanoparticle size, surface charge, and concentration on the structural and optical properties of the composite films has been studied. The composite films retained photonic crystal and chiroptical activity properties. The size and surface charge of gold nanoparticles had a minor effect on the structure and properties of the composite films, while the concentration of gold nanoparticles in the composite material played a more significant role and can be used to fine-tune the optical properties of materials derived from cellulose nanocrystals. These findings significantly broaden the range of nanoparticles that can be used for producing nanocomposite materials based on cellulose nanocrystals. The simplicity of film preparation, the abundance of cellulose nanocrystals, and the robust, free-standing nature of the composite films offer highly advantageous features and pave the way for the generation of functional materials with coupled optical properties. strength of the suspension,17,18 the presence of additives,19 or the evaporation rate of water from the suspension,20 consequently broadening the range of potential applications of the films and signifying the importance of control of the film structure. Incorporation of plasmonic NPs in the CNC matrix is of particular interest, owing to their localized surface plasmon resonances (SPR) that can be affected by ambient conditions21 and electromagnetic coupling (dependent on interparticle distance).22,23 Nanocomposites of CNCs and plasmonic NPs have potential applications as circular polarizers,24 detectors for circularly polarized light,25 or sensors for biomolecules,26,27 thereby paving the way for cost-effective technologies, as well as a fertile ground for new fundamental science. Although the self-assembly of CNCs in N* films has been extensively studied,17−20,28−34 the effect of NPs on the structure and optical properties of resulting composite materials remains largely unexplored.35 Initial reports focused on in situ synthesis of noble-metal and magnetic NPs36−38 or mesoporous silica,39 using CNCs as a reducing agent and NP stabilizer. Coassembly offers a new path for the formation of composite materials. On the one hand, coassembly of CNCs with NPs could provide better control over NP localization and resulting properties of the composite. On the other hand, NPs with different shapes, dimensions, and charges can affect the organization of CNCs in

1. INTRODUCTION Self-assembly of nanometer-scale building blocks with distinct sizes, shapes, and compositions is a promising approach to the design and fabrication of functional nanocomposite materials with new structures and properties.1,2 An auspicious strategy for the organization of nanoparticles (NPs) in specific, well-defined structures utilizes self-assembled template structures, e.g., block copolymer films,3−5 DNA,6 colloidal crystals,7 or templates of a biological origin, such as bacterial pilus,8 virus,9 and fungi.10 Among other templates, liquid crystals are particularly interesting, because their structure can be tuned by electric11 and magnetic fields12 or irradiation.13 Liquid crystals have been used to pattern helical configuration of NPs14 and align shapeanisotropic NPs with their local director, thus creating polarization-dependent optical response.15 In particular, liquid crystals formed by cellulose nanocrystals (CNCs) have recently attracted great attention. Highly crystalline, negatively charged, high-aspect ratio CNC nanoparticles organize into a chiral nematic (N*) phase in aqueous suspensions when their concentration is sufficiently high.16 Evaporation of water from these suspensions results in the formation of free-standing transparent CNC films that partially retain the N* structure and exhibit birefringence, iridescence, and exclusive reflection of left-handed circularly polarized light. The reflection wavelength (and thereby color) of the CNC films depends on the N* pitch (the distance over which the helical orientation of the CNCs undergoes a complete turn). The value of the pitch depends on many factors, for example, the ionic © XXXX American Chemical Society

Received: February 26, 2015 Revised: April 8, 2015

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Langmuir an N* phase. So far, the incorporation of preformed plasmonic NPs in N* CNC films has been reported for shape-anisotropic NPs, that is, positively charged gold nanorods (NRs) with dimensions significantly larger than those of CNCs.40,41 It has been found that in the presence of the NRs, the fraction of the N*-ordered domains order in the composite films was greatly reduced, and the NRs were mostly excluded from the N* domains. The objective of the present work was to explore the effect of dimensions, surface charge, and concentration of shape-isotropic plasmonic NPs on the structure and optical properties of composite CNC-based films. The CNC films were formed with gold NPs (AuNPs) that were smaller than individual CNC length and either larger or smaller that the diameter of individual CNCs. We show that in the CNC matrix, positively and negatively charged AuNPs with different dimensions were well-dispersed. The composite materials displayed the photonic band gap and chiroptical activity of the CNC matrix, as well as the SPR band of plasmonic AuNPs. All types of AuNPs partitioned in both anisotropic and disordered regions of the composite material. In comparison with the size and surface charge of AuNPs, the concentration of AuNPs in the composite films had the strongest effect on the ability of the CNC to organize in N* ordered regions. Nevertheless, all the composite films retained photonic crystal properties and strong chiroptical activity. The simplicity of film preparation, the abundance of CNCs, and the robust, free-standing nature of the composite films offer highly advantageous features and pave the way for the generation of functional materials with coupled optical properties.

Figure 1. Properties of individual CNCs and CNC films. (a) TEM image of CNCs. The inset shows a high-magnification image of an individual CNC, in which the arrows show representative sizing positions. Scale bars are 100 nm in (a) and 50 nm in the inset. (b) Distributions of the CNC lengths (right) and diameters (left) obtained from the analysis of the dimensions of 150 individual CNCs. The lines are curves fitted to the data. (c) Cross-sectional SEM image of the AuNP-free CNC film. Scale bar is 5 μm. The inset shows a highmagnification image of the same film. Scale bar is 500 nm. (d) Polarized optical microscopy image of the CNC film. Scale bar is 20 μm. (e) Extinction and (f) circular dichroism spectra of a CNC film. The casting suspension had 5 mL of CNCs at a concentration of 2.67 wt % with 1 mL of Millipore water. The mixed films were prepared by casting the suspensions into a polystyrene Petri dish and drying at 23 °C for 5 days at ∼80% relative humidity.

2. RESULTS Figure 1a shows a transmission electron microscopy (TEM) image of CNCs drop-cast from their aqueous suspension on a TEM grid. The CNCs had a rice grain-like morphology, that is, elongated rods with tapered ends.42 The average diameter and length of the CNCs were 13 and 216 nm, respectively (Figure 1a), with a polydispersity of 21 and 51% for the CNC diameter and length, respectively (Figure 1b). Due to the presence of sulfate half-ester groups on their surface, the CNCs were negatively charged with an electrokinetic potential (ζ-potential) of −43 mV. Despite the substantial CNC polydispersity, solid films formed by drying the CNC suspension exhibited an N* order, in agreement with previous reports.43,44 A representative scanning electron microscopy (SEM) image of a film crosssection (freeze-fractured) displays the periodicity of the N* structure in the direction perpendicular to the CNC layers (Figure 1c). Polarized optical microscopy (POM) images (Figure 1d) shows a marble-like texture with fingerprint interference patterns typical of N* order.45 Moreover, the films exhibited a photonic band gap characteristic of photonic crystals.46 The photonic band gap (Figure 1e) appeared in the 500−1100 nm spectral range, with the maximum centered at λPB = 780 nm, in agreement with the equation λPB = nP sin θ, where n is the effective refractive index of the CNC film, θ is the angle of incidence of light, and P is the pitch of the N* structure.46 For λPB = 780 nm, n = 1.54,47 and θ = 90°, the calculated value of P for the CNC film was 506 nm, which was in agreement with the pitch of 500 ± 57 nm obtained by SEM imaging of the film cross section. Furthermore, circular dichroism (CD) spectra of the CNC film exhibited a positive CD peak typical of a left-handed N* order,48 with a maximum located beyond 800 nm (Figure 1f). The dimensions of AuNPs used in the present work were selected to be smaller or larger than the average diameter of

CNCs of 13 nm. We synthesized two populations of positively charged AuNPs with average diameters of 9 and 43 nm (Figure 2a,b), denoted as (+)AuNP-9 and (+)AuNP-43, respectively. Both populations of AuNPs had a narrow size distribution (polydispersity of 7−10%) with very few shape impurities, and the sphericity of the AuNPs of 0.94 (Supporting Information, section S2.1). Due to the narrow size distribution, there was no overlap in the dimensions of the two AuNP populations (Figure 2c). The extinction spectra in Figure 2d exhibited a SPR bands centered at λSPR of 532 and 545 nm for (+)AuNP-9 and (+)AuNP-43, respectively.49 The red shift of λSPR with increasing AuNP dimensions was in agreement with earlier works.50,51 The SPR bands of the AuNPs did not spectrally overlap with the photonic band gap of the CNC film (Figures 2d and 1e). The AuNPs had a positive value of ζ-potential, due to their stabilization with the cationic surfactant benzyldimethylhexadecylammonium chloride (BDAC) (Table 1). In order to generate negatively charged AuNPs, ligand exchange was performed on the (+)AuNPs for the replacement of BDAC with 1-mercapto-3,6,9,12-tetraoxapentadecan-15-oic B

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individual AuNPs and CNCs, and the properties of the composite films are summarized in Table 1. 2.1. Composite Films of CNCs and Positively Charged Gold Nanoparticles. Parts a and b of Figure 3 show extinction spectra of the composite films loaded with (+)AuNP-9 and (+)AuNP-43, respectively. At the lowest CNP, the spectra of both films exhibited the photonic band gap of the CNC matrix, centered at 713 and 711 nm, respectively (Figure 3a,b, black traces). Films loaded with (+)AuNP-9 at an intermediate CNP value had two distinct peaks at 541 nm (corresponding to λSPR) and 704 nm (corresponding to λPB) (Figure 3a, red trace). Similarly, films carrying (+)AuNP-43 at the medium CNP, displayed λSPR and λPB at 557 and 706 nm, respectively (Figure 3b, red trace). At high CNP, the extinction spectra of both (+)AuNP-9 and (+)AuNP-43 exhibited only the SPR bands of the AuNPs at 540 and 555 nm, respectively (Figure 3a,b, blue traces). Parts c and d of Figure 3 show the variation in CD of the composite films, plotted for the films loaded with (+)AuNP-9 and (+)AuNP-43, respectively. At low (Figure 3c,d, dashed black traces) and medium CNP (Figure 3a,b, dashed red traces), the spectra exhibited strong, positive CD peaks in the 420−760 nm spectral range. For the films carrying (+)AuNP-43 at the medium CNP, weak splitting of the CD peak was observed with a minimum at λ = 556 nm (Figure 3d, dashed red trace). At high CNP, the spectra of films loaded with (+)AuNP-9 and (+)AuNP-43 had pronounced splitting of the CD peaks, with the minima at 536 and 556 nm, respectively (Figure 3c,d, dashed blue traces). The intensity of the CD peaks decreased with increasing CNP, due to reduction in the chiral order of the films (vide infra).40 Further insight into the effect of AuNP size and concentration on the properties of composite films was obtained by analyzing the structure of the composite films. Figure 4a,b,d,e shows representative cross-sectional SEM images of the films prepared from the CNC suspensions mixed with (+)AuNP-9 and (+)AuNP-43 at varying CNP. The cross-sections of all the films retained a periodic structure, characteristic of the N* arrangement of CNCs in AuNP-free films (Figure S7, Supporting Information),52 although the fraction of disordered regions in the films increased with increasing CNP. This effect was also evident in POM images (Figure 4c,f), where both films showed a marblelike texture with a fingerprint interference pattern typical of N* order.53 The area of the fingerprint pattern decreased to a tactoid size at the highest CNP, which indicated the reduction in the size of the N* domains. Cross-sectional SEM images of the films recorded with a backscattered electron detector revealed that (+)AuNPs were

Figure 2. Properties of positively charged AuNPs. TEM images of AuNPs with mean average diameters of (a) 9 nm and (b) 43 nm. The sphericity of the AuNPs was 0.94. Scale bars are 50 nm. (c) Population distribution of the AuNP diameters determined by analysis of TEM images using Matlab, with >500 AuNPs analyzed for each sample. (d) Extinction spectra of AuNPs. The black and red traces correspond to 9and 43-nm diameter AuNPs, respectively.

acid [HS−PEG4−(CH2)2−COOH], which rendered the AuNP surface negatively charged (ζ-potential of −41.5 mV). The anionic AuNPs with average diameters of 9 and 43 nm, denoted as (−)AuNP-9 and (−)AuNP-43, respectively, were dispersed in basic water to ensure deprotonation of the carboxylic groups. Their extinction properties were similar to those of (+)AuNPs (Figure S1, Supporting Information). Composite films were prepared by mixing a suspension of CNCs (5 mL, CNC concentration of 2.67 wt %), with 1 mL of AuNP solution. The final concentration (CNP) of AuNPs in the films varied from 1012 to 1016 AuNPs·cm−3. The mixed films were prepared by casting the suspensions into a polystyrene Petri dish and evaporating water at 23 °C for ∼5 days at a relative humidity of ∼80%. As a control system, we used AuNP-free CNC films prepared under the same conditions (evaporation rate, CNC concentration, pH) as the composite films. All the composite films had an average thickness ∼18% larger than the NP-free CNC film and exhibited iridescence; however, with increasing CNP the films had a stronger red color, due to increasing extinction of AuNPs. The notations, the characteristics of

Table 1. Characteristics of Individual AuNPs and CNCs and Composite Films notation

av NP diameter (nm)

ζ-potential (mV) −43

CNC-1 (+)AuNP-9

9

+31

(+)AuNP-43

43

+42

−43

CNC-2 (−)AuNP-43

43

−41.5

concn in the film, CNP (AuNPs·cm−3)

vol fraction of AuNPs in the film (%)

av film thickness (μm)

0 1013 (low) 1015 (medium) 1016 (high) 1012 (low) 1013 (medium) 1014 (high) 0 1012 (low) 1013 (medium) 1014 (high)

0 0.01 0.66 3.22 0.02 0.21 6.54 0 0.02 0.21 6.54

28 ± 1 33 ± 2 32 ± 3 32 ± 2 31 ± 2 32 ± 1 33 ± 1 30 ± 1 32 ± 3 34 ± 3 38 ± 3

C

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Figure 3. Optical characteristics of composite films of CNCs with positively charged AuNPs. Extinction (a, b) and CD (c, d) spectra. (a, c) The average AuNPs diameter is 9 nm and CNP is 1013 (black traces), 1015 (red traces), and 1016 (blue traces) AuNPs·cm−3. (b, d) The average AuNPs diameter is 43 nm and CNP is 1012 (black traces), 1013 (red traces), and 1014 (blue traces) AuNPs·cm−3.

Figure 4. (a, b, d, e) Cross-sectional SEM and (c, h) polarized optical microscopy images of composite films with (+)AuNPs. The diameter of the (+)AuNPs is 9 nm (a−c) and 43 nm (d−f). CNP was 1013 (a), 1016 (b, c), 1012 (d), and 1014 (e, f) AuNPs·cm−3. Scale bars are 5 μm. In part c, the circles and arrows indicate the position and direction of tactoids. Scale bars are 20 μm.

together with the uniform red color of the films (Figures S5 and S6, Supporting Information) indicate that, in contrast with earlier

present in both the N* regions and disordered domains of the composite film (Figure 5). The cross-sectional SEM images, D

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In the entire range of CNP, the extinction spectra of the composite AuNP−CNC films exhibited the SPR band of (−)AuNP-43 centered at 565 nm with its intensity increasing at higher CNP (Figure 6a). The photonic band gap of the CNC matrix at λPB = 590 nm (Table 1) largely overlapped with the extinction peak of (−)AuNP-43. At high CNP, a shoulder appeared at 750 nm (Figure 6a, blue trace), presumably due to a small fraction of aggregated (−)AuNP-43, caused by partial protonation of the carboxylic groups in the acidic medium and consequent loss of colloidal stability. The CD spectra of the films at medium and high CNP values exhibited splitting (Figure 6b, red and blue traces, respectively), similar to the films with (+)AuNP43, with the minima at 565 nm. Inspection of the SEM images of the cross-section of the composite films (Figure 7a,b) showed that the fraction of N* domains decreased with increasing CNP. Nevertheless, a fraction of N* order was preserved in the films at CNP = 1014 AuNPs· cm−3, as revealed by high-magnification SEM images (Figure 7c). Moreover, POM images (Figure 7d) showed the marblelike texture with a fingerprint pattern, confirming the preservation of N* order in the composite film with high CNP. Similar to their positively charged counterparts, the (−)AuNP-43 were present in both N* regions and disordered domains (Figure 7e,f). Figure 5. Cross-sectional SEM images of composite films with positively charged AuNPs. The diameter of the (+)AuNPs is 9 nm (a, b) and 43 nm (c, d). CNP (AuNPs·cm−3) was 1016 (a, b) and 1014 (c, d). The inset in part a shows a low-magnification image. The square in the inset shows the location where the high-magnification image was recorded. Parts a, b and c, d are images of the same position recorded with secondary electron detector (a, c) and backscattered-electron detector (b, d). Scale bars are 200 nm. The scale bar in the insets are 1 μm (a) and 100 nm (b, d).

3. DISCUSSION The results of the present work show that AuNPs with different dimensions and surface charge introduced in CNC films at different concentrations change film appearance, microstructure, and optical properties, including birefringence, extinction, and CD. The main findings are that the composite films exhibit the properties of both components and that AuNP size and surface charge do not have a significant effect on the properties of CNC matrix, while AuNP concentration in the films influences the extent of N* order and extinction and CD properties of the films. 3.1. Film Formation and Structure. All of the composite films reported here retained the N* order of the CNC matrix and corresponding optical properties, such as birefringence, iridescence, reflection, and chiroptical activity, regardless of AuNP size, surface charge, and concentration. Incorporation of AuNPs in the CNC composites in relatively high weight fractions (3.2 wt % for AuNP-9 and 6.5 wt % for AuNP-43) resulted in films that exhibited a uniform, strong extinction of AuNPs and CD. Cross-sectional SEM imaging revealed that AuNPs partitioned in the periodic N* regions and in the disordered domains of the films. The good dispersibility of AuNPs in the CNC−AuNP films agreed with previous studies on mixtures of NPs and N* liquid crystals, in which NPs that were larger than

studies of CNC composites with gold NRs,40,41 the AuNPs were dispersed significantly more uniformly in the N* matrix than large AuNRs. 2.2. Composite Films of CNCs and Negatively Charged Gold Nanoparticles. We examined the properties of composite films loaded with (−)AuNP-43, in comparison with films loaded with counterpart (+)AuNP-43. The characteristics of individual (−)AuNPs and composite films are summarized in Table 1. The blue shift of λPB to 590 nm and the reduction of average pitch to 440 nm in the CNC-2 films (Table 1) occurred due to the increased pH of the CNC suspension (Supporting Information, section 6), which affected both the charge of the CNCs and the ionic strength of the mixed suspension and resulted in a broader distribution of the pitch values in CNC-2 films, in comparison with CNC-1 films (Figure S4, Supporting Information).

Figure 6. Extinction (a) and CD (b) spectra of composite films loaded with (−)AuNP-43. CNP of (−)AuNP-43 in the films was 1012 (black traces), 1013 (red traces), and 1014 (blue traces) AuNPs·cm−3. E

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Figure 7. Cross-sectional SEM and polarized optical microscopy (POM) images of composite films loaded with (−)AuNP-43. CNP was 1012 (a) and 1014 (b−f) AuNPs·cm−3. In parts a and b the scale bars are 2 μm, and in part c the scale bar is 500 nm. (d) POM image. Scale bar is 20 μm. Parts e and f are images of the same position recorded with a secondary electron detector (e) and backscattered-electron detector (f). The insets show a higher magnification of the ordered CNC domains. Scale bars in the insets are 500 nm.

fraction of disordered regions in the SEM images of the composite films and as a decrease of the CD signal (Figures 3 and 5). This effect was stronger at higher CNP. 3.2. Extinction Properties of the Composite Films. The extinction of the composite films carrying (+)AuNPs evolved from a single photonic band gap of the CNC matrix at low CNP, to a coexistence of the CNC matrix photonic band gap and the SPR band of AuNPs at medium CNP, to a single SPR band measured at

the mesogens underwent self-assembly in the liquid crystal matrix due to entropic effects driven by hard core interactions between the mesogens and the NPs.14 Overall, regardless of the AuNPs size or surface charge, increasing CNP in the composite films resulted in a reduction of the global N* order, as well as a blue shift of the photonic band gap of the films in comparison with that of AuNP-free CNC films. The loss of the N* order manifested itself as an increasing F

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Langmuir high CNP. The red shift of the λSPR in the films, in comparison with that measured in a colloidal solution, resulted from the change in dielectric properties of the surrounding medium.49 Plasmonic extinction was noticeable even at the low concentration of AuNPs in the films, due to a significantly higher molar extinction coefficient of AuNPs compared to CNCs (≥108 M−1 cm−1 compared to 14.5 M−1 cm−1, respectively).54,55 The extinction of the composite films carrying (−)AuNPs only showed a single band, due to the spectral overlap of the SPR band with the photonic band gap of the CNC matrix. At high CNP, a weak, second SPR band at longer wavelength was visible and presumably originated from a small fraction of aggregated (−)AuNPs. We note that due the absence of significant AuNP assembly or aggregation, evident both from extinction spectra and cross-sectional SEM imaging, there was no plasmonic coupling in the films. The λPB of the CNC matrix in all the AuNP−CNC composite films blue-shifted in comparison with that of the AuNP-free CNC films. For composite films with (+)AuNPs, a blue shift of the λPB was likely caused by the increased ionic strength of the system, leading to partial screening of the surface charge of CNCs and a reduced electrostatic repulsion between them. For composites with (−)AuNP-43, the addition of the basic AuNP solution56−58 to the CNC suspension resulted in a neutralization reaction, leading to an increase of the pH of the mixed suspension from 2.4 to 2.7 and an increase of the ionic strength of the medium, also resulting in a blue shift of λPB. In both cases, the blue-shift in λPB originated from the reduction in the pitch in N* phase of the films. The second possible reason for the shift in λPB could be the change in the effective refractive index, which is generally taken as 1.54 for pure cellulose.47 In the composite films loaded with either (+)AuNPs or (−)AuNPs, the CNC packing density was reduced (and thus the free volume was increased), thereby resulting in a lower effective refractive index of the films. An indirect indication of the lower packing density of the hybrid films was provided by the increased thickness of the films, which could not be explained by the additional volume occupied by AuNPs. 3.3. Chiroptical Activity of the Composite Films. The CD spectra at high CNP exhibited splitting of the CD signal. The spectral position of the minima in CD coincided with the extinction maxima of AuNPs incorporated in the composite film. By analogy with the results obtained in earlier studies of induced CD (ICD) response of achiral shape-isotropic dye molecules loaded in N*-ordered polymer or CNC matrices,59,60 this effect may be ascribed to the combined CD properties of the CNC matrix and ICD of the AuNPs, that is, to the coupling between the electromagnetic transition moments of AuNPs and the CNC matrix. In earlier works,61−63 the shape of ICD spectra was determined by the relationship between λPB and the absorption wavelength of the dye, λabs. For λPB > λabs, the ICD spectrum of an achiral solute was of opposite sign to that of the photonic band gap; e.g., a positive CD signal from the N* matrix would generate a negative ICD signal for the achiral solute. The opposite effect, that is, a retention of the sign of the CD signal, was observed for λPB < λAbs. The composite AuNP−CNC films studied in the present work satisfy the former condition, where addition of AuNPs in the CNC films generated a negative ICD signal. The superposition of the negative ICD signal of the AuNPs with the positive CD signal of CNC matrix led to the appearance of minima (CD peak splitting) centered at the λSPR of the AuNPs (Figures 3 and 6). This effect appeared at sufficiently high CNP for AuNPs with different dimensions and surface charges. We note

that the exact spectral match of the SPR modes of AuNPs and the dips in the CD spectra of the composite films needs further investigation, possibly, by CD measurements using transmission spectroscopic ellipsometry.

4. CONCLUSION In summary, we have shown the coassembly of CNCs and shapeisotropic AuNPs, with different dimensions and surface charges and at different number densities of AuNPs, into macroscopic free-standing nanocomposite films. All the composite films retained a N* structure and preserved the birefringence, iridescence, and chiroptical activity of the CNC matrix. The AuNPs were well-dispersed in the composite films, and the films had combined optical and chiroptical properties of the CNC matrix and the NPs. The concentration of AuNPs had the strongest effect on the properties of composite films, in comparison with AuNP size and surface charge. At sufficiently high AuNP concentration in the films, the CD signal of the CNC matrix exhibited splitting, with the minima coinciding with the spectral position of AuNP extinction. We propose that this effect originates from an induced CD activity of AuNPs, but further fundamental studies of this phenomenon are needed.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of AuNPs, preparation of CNC-based films, and additional sample characterization. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b00728.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank FPInnovations for providing the samples of CNCs. The authors thank Prof. Ron Kluger for the use of CD spectropolarimeter. The authors are grateful to Ilya Gourevich and Prof. Neil Coombs of the Center of Nanostructure Imaging for assistance in electron microscopy experiments and fruitful discussions. The authors thank Prof. Amr Helmy for fruitful discussion and the use of quarter wave plates, and Bernd Kopera for fruitful discussions of the analysis of the structure of CNC films. Funding was provided by the National Science and Engineering Research Council of Canada.

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b00728 Langmuir XXXX, XXX, XXX−XXX