Exclusion from Hexagonal Mesophase Surfactant Domains Drives

Article pubs.acs.org/JPCB

Exclusion from Hexagonal Mesophase Surfactant Domains Drives End-to-End Enchainment of Rod-Like Particles Kamendra P. Sharma,†,⊥ Anal Kumar Ganai,‡ Debasis Sen,§ B. L. V. Prasad,‡ and Guruswamy Kumaraswamy*,† †

Complex Fluids and Polymer Engineering Group and ‡Physical Chemistry Division, National Chemical Laboratory, Dr. Homi Bhabha Road, 411008 Pune, India § Solid State Physics Division, Bhabha Atomic Research Centre, V. N. Purav Marg, Trombay, 400085 Mumbai, India S Supporting Information *

ABSTRACT: Anisotropic rod-like particles assemble end-to-end when the surfactant/water matrix in which they are dispersed is cooled from the isotropic to the lyotropic hexagonal phase. We demonstrate the formation of such end-to-end assemblies for gold nanorods, which are tens of nanometers in size, as well as for micrometer-sized ellipsoidal polystyrene particles. In both cases, the particles are well-dispersed in the low-viscosity surfactant/ water phase above the isotropic-H1 transition temperature. On cooling into the H1 phase, mesophase domains form and the particles are expelled to the isotropic phase. As the H1 domains grow and finally impinge, the particles are localized at the domain boundaries where they reorient and assemble end-to-end. Remarkably, we observe the formation of end-to-end assemblies of gold nanorods even for volume fractions as low as 2 × 10−6 in the initially dispersed state. The extent of particle “enchainment” increases with the particle concentration and with the aspect ratio of the particles.

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INTRODUCTION Interactions between inclusions in a mesophase depend on both the matrix and the structure of the inclusion. In small molecule nematics, colloidal inclusions organize into a variety of geometries,1−8 determined by the particle size and matrix chemistry. When the matrix is a surfactant mesophase, particulate inclusions impose an energy penalty due to the elastic distortion of the matrix.9−12 Therefore, in a hexagonal (H1) mesophase, for example, spherical particles larger than the characteristic length scale for molecular ordering in the mesophase are excluded from the mesophase domains,9−11 independent of the particle chemistry.10 In previous work,10 we have demonstrated that a variety of inorganic nanoparticles, such as silica, iron oxide, and hydroxyapatite particles, as well as polymer latices and self-assembled bionanoparticles, such as the protein ferritin, all of which are larger than 10 nm in size, are expelled from the H1 phase domains of a nonionic surfactant (C12E9)/water system. Previous reports demonstrate that particles that are smaller than the size of the hexagonal micelles are incorporated into the micellar structure. For example, the group of Eiser has demonstrated the synthesis of small (∼few nm) size metal nanoparticles that are incorporated within H1 phase micelles.12 Previously, the group of Ponsinet had demonstrated that incorporation of magnetic nanoparticles allows orientation of surfactant hexagonal mesophases on imposition of relatively small magnetic fields.13 Recently, Mezzenga and coworkers have reported that larger magnetic particles are expelled from hexagonal phase domains and that © 2013 American Chemical Society

the H1 phase in composites cannot be aligned unless the magnetic field is applied during the isotropic-H1 transition.14 Thus, localization of spherical particulate inclusions in a surfactant H1 phase is governed only by their size, relative to the characteristic matrix length scale. Unlike particulate inclusions, the localization of polymeric inclusions is determined by the interplay of polymer conformational entropy with polymer−matrix interactions in the polymer−mesophase composite.15−21 Rod-like particulate inclusions are characterized by their size and aspect ratio. Thus, in composites of rod-like particles with surfactant mesophases, it is possible to investigate the localization and orientation of the particulate inclusions, a possibility that did not exist for the case of spherical particles. Recent work from the group of Smalyukh22 showed the formation of nematic-like and helicoidal structures of gold nanorods (Au NRs) in a lyotropic discotic liquid crystal comprising SDS/pentanol/water. Au NRs with aspect ratio (A.R.) ≈ 2.5, assembled perpendicular to the director field driven by matrix-mediated long-range interparticle interactions, due to elastic energy penalty from distorting the director field. In a previous report from the same group, Au NRs were dispersed in the hexagonal phase of an ionic surfactant (CTAB) and were shown to align with the director orientation of the Received: July 25, 2013 Revised: September 19, 2013 Published: September 19, 2013 12661

dx.doi.org/10.1021/jp407403a | J. Phys. Chem. B 2013, 117, 12661−12668

The Journal of Physical Chemistry B

Article

the seed solution to the growth solution at 27−30 °C. The Au NRs were imaged using transmission electron microscopy, and the size of the rods was calculated by ImageJ analysis on 150 nanorods in five different electron micrographs. The Au NRs had an average diameter and length of 16.4 ± 4.8 and 51 ± 9.2 nm, respectively. (ii). Preparation of Polystyrene Ellipsoids. Micron-sized PS ellipsoids were prepared by uniaxial stretching of PS microspheres using a modification of the procedure mentioned in the literature.72,73 Monodisperse fluorescent iso-thiocyanate tagged PS spherical particles (diameter = 2.8 μm) were dispersed in 7−10 wt % PVA solution in water. The overall concentration of PS particles in the PVA solution was 0.5 wt %. The homogenized PVA solution containing PS particles was poured in a flat-bottomed Petri dish (diameter 12 cm) and then dried in an oven at 60 °C for 8 h. The dry film was removed and cut into rectangular strips of dimensions 1 cm × 3 cm. These strips were clamped between metal plates and stretched to a desired draw ratio (either 3 or 7) at a temperature of 115 °C, that is, above the Tg of PVA and PS. A schematic of this procedure is shown in the Supporting Information, Figure S1. Temperature control during stretching was achieved using a feedback-controlled convective oven. After stretching, the temperature was reduced to 60 °C before the strips were unloaded from the metal plates. The edges of the strips that remained unstretched were cut and discarded, and only the center portion of the strips was used for obtaining deformed particles. The stretched films were dissolved in water by continuous stirring at 40 °C for 24 h. The deformed PS particle dispersion was then centrifuged at 8000 r.p.m for 30 min, and the supernatant solution was discarded and fresh water was added. This procedure was repeated three times to ensure maximum removal of PVA. (iii). Preparation of Samples for Optical and Confocal Microscopy. A concentrated dispersion of PSe was prepared in a 1:1 water/C12E9 system at 50 °C. This composite sample was then placed on the hydrophobic glass slides and pressed with a coverslip to obtain a sample thickness of ∼20 μm. The sample was heated to 50 °C in a convective oven and allowed to cool to 35 °C at 1 °C/min. The glass slides for confocal microscopy were then sealed on the edges to prevent any water loss from the sample.

mesophase.23 Smalyukh’s group also observed that for a CTAB mesophase containing high A.R. (≈ 20) Au NR inclusions, the rods segregated at the mesophase domain boundaries such that Au NR−matrix interactions were minimized.24 There are also a few reports on the assembly of other rod-like nanoparticles, particularly carbon nanotubes, in liquid crystalline mesophases.25−31 However, no systematic understanding of the influence of nanoparticle size, concentration, and anisotropy on the assembly process exists. Here we examine the influence of anisotropy of rod-like particulate inclusions and their interplay with the structure of the matrix mesophase in determining particle organization in the nonionic surfactant/water H1 phase. The rod-like particles used in our work include Au NRs and ellipsoidal polystyrene (PS) latex particles. Au NRs represent an interesting model system for our investigations because we can benefit from the extensive literature on their synthesis and controlled surface modification.32−40 In particular, Au NRs and their assemblies can be easily characterized using UV−vis spectroscopy and, their anisotropic optical properties render them useful for applications.41−48 Typically, “programmable” assembly of Au NRs has been controlled using surface-tethered molecular species.49−55 For example, assembly can be controlled using electrostatic interactions56−59 or specific interactions (such as DNA base pairing, hydrogen bonding, etc.)32,40,60−68 or by controlling the rigidity of groups69 tethered specifically40,70,71 to the sides or ends of Au NRs. In this work, we use UV−vis spectroscopy and ultra-SANS to track the relative orientation of Au NRs as they assemble when cooled from an isotropic C12E9/water dispersion into the H1 phase. We complement this study with optical microscopic visualization of the assembly process using ellipsoidal PS particles as a model system.

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METHODS AND MATERIALS

Materials. The precursor salt for Au NR synthesis, HAuCl4, was purchased from SRL (India). Cetyltrimethylammonium bromide (CTAB), ascorbic acid, octadecyltrimethoxysilane, and hydrogen peroxide (H2O2, 30% w/v) were purchased from Sigma-Aldrich. AgNO3 and poly(vinyl alcohol) (PVA), having mol wt 1 25 000, were both obtained from Merck India. Fluorescent PS latices of diameter 2.8 μm with polydispersity of 0.1% in size were purchased as 2.5% (w/v) suspension from Microparticles, Germany. All chemicals were used as obtained from the manufacturer.

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CHARACTERIZATION UV−vis spectroscopy was performed using a Shimadzu UV1601PC spectrophotometer. The spectrum of absorption was obtained in a wavelength range of 300−1000 nm. Peaks were fitted using PeakFit v4.12 software. Optical microscopy was performed using an Olympus-BX 50 equipped with a crossed polarizer setup and images were obtained using a Lookman CCTV camera. LSM 710 Carl Zeiss laser scanning confocal microscope (LSCM) with an inverted stage was used to image the fluorescent samples. We used an argon-ion laser (488 and 514 nm) for our experiments. Scattering experiments were performed on two different machines that provided information on different range of length scales. Small-angle X-ray scattering (SAXS) experiments were performed on Bruker Nanostar equipped with rotating copper anode operating at 45 kV and 100 mA and generating X-rays of wavelength 1.54 Å. Ultrasmall-angle neutron scattering (USANS) was performed on the medium-resolution double-crystal instrument at the guide laboratory of Dhruva reactor, Bhabha Atomic Research Centre Mumbai, India. The neutron wavelength (λ) used was 0.312

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EXPERIMENTAL PROCEDURES (i). Synthesis of Au Nanorods. Au NRs were synthesized using a slight variation of a reported procedure for seedmediated growth.33 In a typical procedure, the growth solution was first prepared as follows. CTAB (25 mL, 0.20 M) was added to 0.75 mL of 0.0040 M AgNO3 solution at 25 °C. To this solution, 25.0 mL of 1 mM HAuCl4 was added, and after gentle mixing of the solution, 350 mL of 0.08 M ascorbic acid was added. Ascorbic acid is a mild reducing agent and changes the growth solution from dark yellow to colorless. To prepare the seed solution, a CTAB solution (5 mL, 0.20 M) was mixed with 5.0 mL of 0.50 mM HAuCl4. To this stirred solution, 0.60 mL of ice-cold 0.010 M NaBH4 was added, which resulted in the formation of a brownish yellow solution. Vigorous stirring of the seed solution was continued for 2 min; then, the solution was kept at 25 °C. The final step was the addition of 65 mL of 12662



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Figure 1. (a) Transmission electron micrograph (TEM) of Au NRs. The alignment observed here is an artifact of drying during sample preparation. (b) UV−vis spectra for Au NRs (aspect ratio ∼2) at concentrations of (i) 1010 per mL in water and (ii) 5.9 × 1010, (iii) 1.5 × 1011, and (iv) 2.9 × 1011 per mL in the H1 phase. The data have been vertically shifted for clarity. (c) SAXS and USANS data of 2.9 × 1011 Au NR particles/mL in the H1 phase. The peak in the USANS is at q ≈ 0.093 nm−1, and peaks characteristic of the H1 phase are observed in the SAXS data at q ≈ 1.1 and 1.8 nm−1.

nm with Δλ/λ ≈ 1%. The neutron beam was circular and had a diameter of 1.5 cm. The accessible range of scattering wave vector transfer for the present case was q = 0.05 to 0.173 nm−1, which corresponds to a range of resolvable real-space dimension of 125−40 nm.74,75 Experimental data were corrected for sample transmission and instrument resolution effect.

2.44. Thus, we believe that the aspect ratio that we obtain from the average dimensions of the Au NRs from TEM measurements (averaged over 150 particles) does not accurately characterize the anisotropy of our samples and that our Au NRs have an average aspect ratio around 2. B. Assembly of Gold Nanorods in the H1 Phase. We prepare Au NR/H1 phase-composite samples for UV−vis measurements by dispersing the Au NRs in a 1:1 C12E9 and H2O mixture at 50 °C and subsequently cooling to room temperature. For 1:1 C12E9/H2O, the H1 phase forms below about 40 °C and exists as a gel. UV−vis on H1 phase gels containing 5.9 × 1010, 1.5 × 1011, and 2.9 × 1011Au NRs/mL (Figure 1b) shows that the absorption corresponding to the transverse plasmon resonance occurs at the same position for all of the gel samples, viz. at 527.1 nm (compared with 521.4 nm for the Au NR sol). The full width at half-maximum (FWHM) for the gel samples is ∼36 nm, higher than that for the Au NR sol (27.8 nm). In contrast, the absorption corresponding to the longitudinal plasmon is strongly dependent on the Au NR concentration and increases from 612.5 nm for the Au NR sol to 657, 684, and 728.1 nm for Au NR concentrations of 5.9 × 1010, 1.5 × 1011, and 2.9 × 1011 particles/mL, respectively (Figure 1b). We note that this shift in the peak position to higher wavelengths is accompanied by an increase in the FWHM. A combination of SAXS and ultra-small-angle neutron scattering (USANS) provides information about the spatial organization of the Au NRs in the Au NR/H1 composite gel (Figure 1c, representing the sample containing 2.9 × 1011 Au NR/mL). In the SAXS data, we observe peaks at q ≈ 1.1 and 1.8 nm−1 (ratio of peak positions aproximately 1:√3), corresponding to the hexagonal ordering of cylindrical surfactant assemblies in the H1 phase. In the ultra-small-angle SANS, we observe a peak that we interpret as arising from correlations between the Au NRs. Assuming an isotropic distribution of the orientation of Au NRs in the sample, we obtain a correlation length of 61.4 nm (= 2π/qP) from the peak position (qP) of the Lorentz-corrected79 intensity (qI). C. Evidence of End-to-End Assembly of Gold Nanorods. We now discuss the implications of the UV−vis and scattering data and show that it indicates an end-to-end assembly of the Au NRs in the Au NR/H1 composites. The neat H1 phase gel is visibly transparent and shows no UV−vis absorption. Therefore, we attribute the UV−vis absorption from the composite to the Au NRs. We begin by noting that

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RESULTS AND DISCUSSION We first characterize the assembly of Au NRs using UV−vis spectroscopy and small-angle scattering. Subsequently, we use optical microscopy to describe the assembly of micrometersized PS ellipsoids. In each system, we focus on the influence of particle number density and aspect ratio. A. Characterization of the Gold Nanorods. Au NRs (diameter = 16.4 ± 4.8 nm; length = 51 ± 9.2 nm, averaged over ∼150 NRs) were imaged using transmission electron microscopy (representative micrograph shown in Figure 1a). UV−vis spectroscopy from a dilute aqueous dispersion (≈ 1010 Au nanorods/mL, Figure 1b, bottom) shows two surface plasmon resonances at 521.4 and 623.5 nm, corresponding to transverse and longitudinal dimensions of the Au NRs, respectively. The position of the longitudinal resonance in the UV−vis spectra indicates an aspect ratio close to 2 when compared with literature reports.33,76 This was also confirmed by models based on the Mie−Gans formalism77,78 used by Link and coworkers77 to model the UV−vis absorption of randomly oriented Au NRs. They use an empirical linear fit of the real part of the dielectric function for gold as a function of wavelength and arrive at a simplified equation relating the longitudinal peak in the UV−vis spectra from Au NRs, λm, to the aspect ratio of the rods, R, and to the dielectric constant of the medium, ε. This equation was modified to account for a numerical error by Yan et al.78 to give the following form: λm = (52.95R − 41.68)ε + 466.38

(1)

We use this to model the UV−vis spectra of the Au NRs. Our TEM data indicate that the Au NRs have an average diameter of 16.4 ± 4.8 nm and average length of 51 ± 9.2 nm. Thus, the average aspect ratio (L/D) is 3.1. For the measured λm = 623.5 nm, this yields a medium dielectric constant as ε ≈ 1.3, which is unreasonably low. Comparison of our data with the literature suggests that λm = 623.5 nm is indicative of a more modest aspect ratio, ∼2. Using R = 2, we obtain a much more reasonable value for the medium dielectric constant, ε ≈ 12663



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aspect ratio Au NRs, a dilute dispersion (1.4 × 1010 particles/ mL) of these Au NRs shows a transverse plasmon at 521 nm, while the longitudinal plasmon is observed at ∼750 nm, in accord with TEM micrographs that indicate a higher aspect ratio (Figure S2, Supporting Information). The UV−Vis data indicate that the Au NRs have an aspect ratio around 3 (calculated, using a dielectric constant of 2.4 for the surrounding matrix), lower than that calculated from the TEM, similar to our observation for the lower aspect ratio Au NRs. In the Au NR/H1 phase composites using the higher aspect ratio Au NRs too, the transverse peak did not show any significant change from its position of ∼521 nm. However, the longitudinal plasmon peak was broadened relative to that for the dilute dispersion and showed a Au NR concentrationdependent shift to higher wavelengths, similar to our observations for Au NRs of aspect ratio ∼2 (Figure 2). For

there is no significant change in the position of the transverse plasmon peak with increase in Au NR concentration in the composites. The small (5.7 nm) increase in the peak maximum relative to the Au NR aqueous sol may be attributed to a variation of the local dielectric in the presence of C12E9/H2O. However, there is a significant shift of the longitudinal plasmon peak position to higher wavelengths, with increase in Au NR particle concentration in the composite gel. These data indicate that in the Au NR/H1 gel there is an end-to-end assembly of Au NRs and a coupling of the longitudinal plasmon resonance of adjacent Au NRs, resulting in the observed shift in absorption to higher wavelengths. With increasing concentration of Au NR in the composite, there is an increase in the extent of the assembly, reflected in the strong Au NR concentration dependence of the shift in the absorption maximum. Because the position of the transverse plasmon absorption is not dependent on the concentration of Au NR in the composite, we infer that there is no significant population of side-by-side aggregates.57 Our interpretation of the UV−vis data is in accord with the literature on Au NR assemblies.32,46,57,63,67−69 Furthermore, we note that the correlation peak in ultra-SANS at 61.4 nm is comparable to the length of the Au NR from TEM (51 ± 9.2 nm), especially when we recognize that the ultra-SANS data are averaged over a macroscopic sample, while the TEM data represent an average over only ∼150 particles and are obtained after completely drying the sample. Previously, we have demonstrated9,10 that spherical particles that are larger than ∼10 nm are expelled from growing H1 phase domains that form when the particle dispersion in C12E9/ H2O is cooled from the isotropic phase to below the H1 transition temperature. This behavior is observed for particles, independent of their chemistry. In the H1 phase, the particles are localized to the H1 domain boundaries and form percolated particle networks above ∼0.5% (by volume) of the particles. The Au NRs that we use in this work are characterized by dimensions that are large enough that we believe that they will be expelled to the H1 domain boundaries. However, even at the highest Au NR concentrations used in our work, the volume fraction of the Au NRs is only ∼0.001%, over two orders of magnitude below the threshold for percolation of spherical nanoparticles. We are unable to prepare stable dispersions of Au NRs at higher concentrations. It is remarkable that even at concentrations of ∼0.0002% (by volume, corresponding to 5.9 × 1010Au NR/mL), we can identify the formation of end-toend Au NR assemblies using UV−vis spectroscopy. It is difficult to imagine that such end-to-end assemblies could form in the bulk of the mesophase at such low particle concentrations, as in previous reports23 (Note that the shift in the longitudinal plasmon peak position is not evident with changing concentration in an isotropic matrix). Therefore, we believe that in accord with our previous results on the spherical nanoparticles the Au NRs are expelled to the boundaries of the H1 domains. Our data suggest that these assemblies are polydisperse, evidenced by the increase in the FWHM of the longitudinal plasmon absorbance for the Au NR composites relative to the sol. It is intuitive that an increase in Au NR concentration in the composite should lead to an increase in the extent of assembly, as observed. D. Assembly of Higher Aspect Ratio Gold Nanorods. We have also synthesized Au NRs with a higher aspect ratio (∼4, based on TEM results, Supporting Information, Figure S2) and prepared Au NR/C12E9/water composites in the same manner as for the lower aspect ratio Au NRs. As with the lower

Figure 2. UV−vis spectra for Au NRs (aspect ratio ∼3) at concentrations of (a) 1.4 × 1010 per mL in water and (b) 3.5 × 109, (c) 7 × 109, (d) 1.05 × 1010, (e) 1.4 × 1010, (f) 3.5 × 1010, and (g) 7 × 1010 per mL in the H1 phase. The data have been vertically shifted for ease of comparison.

an Au NR concentration of 3.5 × 109 particles/mL, a shoulder in the longitudinal plasmon is observed at 950 nm, which becomes increasingly prominent on increasing the Au NR concentration to 7.0 × 109 particles/mL. At a concentration of 1.05 × 1010 per mL, the longitudinal plasmon is even more prominent, and, additionally, two broad peaks are observed at ∼890 and ∼975 nm. For an Au NR concentration = 1.4 × 1010 particles/mL, the longitudinal peak becomes even broader, and we observe a plateau in the absorption for higher concentrations of Au NRs ((3.5 and 7) × 1010 particles/mL). Therefore, we observe qualitatively similar behavior for the higher aspect ratio Au NRs: the transverse plasmon absorbance is independent of the Au NR concentration in the composite, while the longitudinal plasmon shifts to higher wavelengths with increased Au NR concentration and broadens significantly (Figure 2). Interestingly, we note that for these higher aspect ratio Au NRs we observe a shift in the longitudinal plasmon resonance in the gel, indicating end-to-end coupling of the Au NRs for concentrations as low as ∼0.00015% by volume (corresponding to 3.5 × 109Au NR/mL). Polarized optical microscopy was performed on H1 phase/Au NRs composites (Figure 3). Micrographs of an Au NR/C12E9/ H2O composite at room temperature, for a sample containing Au NRs with an aspect ratio ∼2, at a concentration of ∼2.9 × 1011/mL reveal no specific structures between parallel polarizers (Figure 3a), while the surfactant hexagonal domains could be clearly observed between crossed polarizers (Figure 3b). In 12664



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Figure 3. Optical micrographs of an Au NR/C12E9/water composite sample (Au NR aspect ratio ∼2, concentration ∼2.9 × 1011 per mL) seen between (a) parallel polarizers and (b) crossed polarizers. Between crossed polarizers, birefringence due to H1 phase is observed, but we cannot observe aggregation of Au NRs at the domain boundaries. The scale bar in both of the images corresponds to 50 μm. Figure 4. (a,c,d) Optical micrographs of PSe particles in the H1 phase between parallel polarizers. (a) PSe3 at a concentration of 8 × 106/ mL. (b) 2D slice from laser scanning confocal microscopy of PSe3 in the H1 phase at a concentration of 8 × 106/mL. PSe7 at a concentration of (c) 2 × 106/mL and (d) 8 × 106/mL. The particles are mainly aggregated in an end-to-end fashion in the H1 phase. For PSe7, note that even a four-fold lower concentration of particles [(c) 2 × 106/mL, compared with 8 × 106/mL of PSe3 in (a)] gives a larger number of end-to-end assembled dimers and oligomers, underlining the influence of particle anisotropy.

previous work,9,10 we have demonstrated that aggregation of 16 nm silica nanoparticles at the H1 domain boundaries can be clearly observed using optical microscopy. However, we note that the Au NR concentrations in the experiments reported in this work are significantly lower than the lowest concentration of silica nanoparticles (∼0.5%, by volume) in our previous experiments. These low Au NR concentrations preclude the use of optical microscopy to obtain insights into nanoparticle assembly in these experiments. E. Assembly of Polystyrene Ellipsoidal Latex Particles. Therefore, we chose to examine the assembly of micrometersized, fluorescent ellipsoidal PS latex particles (that we term PSe) using optical and confocal microscopy. We have previously demonstrated that spherical inclusions ranging in size from tens of nanometers to micrometers are expelled by growing H1 mesophase domains. The assembly of these particles is qualitatively similar, independent of the particle size (above a critical size for expulsion from the H1 domains) and independent of the particle chemistry. Therefore, we anticipate that our microscopic observations of PSe assembly in the H1 phase will provide insights that complement the UV−vis and scattering data on the Au NRs. As with the Au NR, PSe/H1 phase composites were prepared by cooling dispersions of PSe in 1:1 C12E9/H2O from the isotropic phase to below the H1 transition temperature. PSe with aspect ratios of 3 (PSe3) and 7 (PSe7; Supporting Information, Figure S3) were examined at concentrations between 104 and 108 particles/mL. Because both PSe3 and PSe7 are prepared by stretching the same 2.8 μm spherical PS latex, we obtain equal particle volume fractions when particle number densities are equal. We note that even the highest particle concentration used here (108/mL) is significantly lower than the lyotropic nematic concentration for the highest aspect ratio particles.80,81 Therefore, we neither anticipate nor observe orientational order even at 108/mL. At very low particle concentrations (∼5 × 104 PSe3 particles/mL in the H1 phase, corresponding to a very low volume fraction ∼6 × 10−5%), most of the particles were observed as isolated unimers and only a few multiparticle assemblies could be observed, where the particles are lined up end-to-end (Supporting Information, Figure S4). For higher particle density (8 × 106 PSe3/mL, ∼0.01%, Figure 4a), there is an increase in the fraction of particle assemblies, and we note that the particles are predominantly assembled end-to-end. An analysis of over 1000 particles in over five micrographs indicates that 34.2, 21.1, and 24.2% of the particles exist as unimers, dimers, and oligomers consisting of assemblies of three or more

particles, respectively. (The remaining ca. 20% of the particles were either irregular, viz. not end-to-end or “branched”, or out of the plane of focus; see also Supporting Information, Figure S5.) Note that for a dispersion of PSe3 (at approximately similar particle density) in the isotropic phase of C12E9/H2O, on an average ∼4.2% particles observed using optical microscopy existed as dimers or oligomers (representative image of isotropic phase dispersion in Supporting Information, Figure S6). All of the data presented in this manuscript refer to particles assembled in the H1 phase. A few “branched” assemblies were observed using optical (Figure 4a) and confocal microscopy (Figure 4b), where end-to-end assemblies of the PSe3 formed a three-armed branch. For the higher aspect ratio particles (PSe7), we observe higher incidence of end-to-end “enchainment” even at lower particle concentrations. For example, at a particle concentration of 2 × 106/mL (∼0.003%, by volume, Figure 4c and Supporting Information Figure S7b), analysis of ∼350 particles in multiple optical micrographs suggests that the concentration of particle assemblies is already greater than that for a four-fold higher concentration of PSe3 (27.1, 32.5, 34.9, and 5.5% particles existed as unimers, dimers, oligomers, and irregular/out of focus, respectively). With increase in concentration of the PSe7 (8 × 106/mL; 0.01%, by volume, Figure 4d), the fraction of unimers decreased to 17.7%, while 22.3 and 42.1% of the particles existed as dimers and higher oligomers (based on an analysis of ∼750 particles). When the PSe/H1 composite is viewed between crossed polarizers, the PSe particles are not visible due to the high contrast from the fan-shaped domain structure of the H1 phase (Figure 5a and Supporting Information, Figure S7a). To locate the PSe particles relative to the H1 domains, we imaged the composite with the polarizer axis at 15° relative to the analyzer, and thus both the H1 domain structure and the PSe particles were visible. For PSe7 at a concentration of 2 × 106/mL PSe (Figure 5b and Supporting Information Figure S7c), we can 12665



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demonstrated10 that mild shearing is able to orient the particulate scaffold generated by exclusion from the H1 domains. Therefore, it might be possible to obtain oriented assemblies of rod-like colloids that demonstrate interesting optical effects, such as giant birefringence, as demonstrated recently86 for shear-oriented thixotropic dispersions of LaPO4 nanorods.

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CONCLUSIONS We demonstrate that end-to-end assembly of anisotropic rodlike particles results when their dispersion in a C12E9/water mixture is cooled from the isotropic to the H1 phase. As H1 domains nucleate and grow on cooling, they expel Au NRs (size on the order of tens of nanometers) and ellipsoidal PS latices (micrometers in size) to the isotropic regions. As the H1 domains approach impingement, the rod-like particles are rotated so as to align with their axes parallel to the domain boundary. Thus, particles are concentrated at the domain boundaries and aligned to line up along the domain boundaries, and this results in the end-to-end assembly. We observe that such end-to-end assembly of the anisotropic particles is observed even at concentrations as low as 0.0002% (by volume). Our work clearly establishes particle anisotropy as an important control parameter for rod-like particle assembly in the H1 phase: particle enchainment is greatly enhanced with increase in aspect ratio of the rod-like particles. At constant particle concentration, we observe a greater tendency for particle alignment with increasing particle anisotropy. We believe that our work has implications for the design of high porosity macroporous supports and for engineering the assembly of metal nanorods for plasmonic applications.

Figure 5. Optical micrographs of 2 × 106/mL of PSe7 in the H1 phase observed under (a) crossed polarizers and (b) at an angle of 15° between the polarizer and analyzer. The particles either isolated or linearly aggregated are clearly seen to follow the boundaries of the H1 phase.

clearly observe that the PSe assemblies template the domain structure and are localized at the domain boundaries. Individual particles are also aligned with their axes along the domain boundaries of the H1 phase (also see Supporting Information, Figure S8). In some instances, at the border where multiple domains meet, we can observe PSe assemblies that “intersect” to form the “branches” previously described. F. Mechanism of the Assembly of Anisotropic RodLike Particles in the H1 Phase. Thus, both the Au NRs and PSe appear to form end-to-end assemblies, which template the domain structure of the nonionic H1 phase when their dispersions are cooled through the isotropic−H1 transition. As both the Au NRs and PSe are characterized by dimensions larger than ∼10 nm, their expulsion from the growing H1 domains is in accord with our previous studies.9,10 Colloidal particles characterized by weak anchoring in small molecule nematics have been shown to be swept by the moving isotropic−nematic interface so that they are localized at the boundaries of the nematic domains. 82 Our particle/H 1 composites systems are characterized by strong anchoring; however, in these, too, we have demonstrated9,10 that we observe similar particle expulsion from mesophase domains as for weakly anchored systems.83 The alignment of the rod-like particles and their assembly along the H1 domain boundaries is in accord with hydrodynamic theories for reorientation of slender bodies in creeping flows near walls.84,85 As the H1 domains grow at the expense of the isotropic regions, the isotropic/H1 domain boundaries advance toward the rod-like particles. As the isotropic/H1 interface is impenetrable to the particles, the resulting hydrodynamic flow lines up the particle axes with the H1 domain boundary.84,85 Thus, the rod-like particles are concentrated at the domain boundaries and assemble end-toend. For higher particle concentrations, it is intuitive that there is a greater extent of assembly. We also note that there is a strong effect of particle anisotropy, with higher aspect ratio particles showing a much greater extent of formation of end-toend assemblies. It is remarkable that similar end-to-end assemblies form for both the Au NRs and the PSe, whose sizes differ by almost 100 times. In both cases, at particle volume concentrations of significantly less than 1%, the particles “enchain” to form end-to-end assemblies. We believe that our results have implications for novel photonic materials. For example, as the surfactant/water system can solubilize both hydrophilic and hydrophobic materials, it might be possible to use our Au NR dispersions to obtain enhanced Raman scattering signals due to the high fields at the junctions between the Au NRs. Finally, we note that we have previously

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures and characterization of higher aspect ratio Au NR and fluorescent PSe latex particles and their assemblies in the H1 phase. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ⊥

School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TS, United Kingdom.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Amaraja Taur for helping with the preparation of the ellipsoidal polystyrene particles, Virginia D’Britto for helping with the synthesis of the gold nanorods, and Tina George and E. Venugopal for helping with the confocal experiments. We are grateful to J. C. Loudet (CRPP, Bordeaux) for introducing us to the method for preparation of ellipsoidal polystyrene particles. We are grateful to Sayam Sen Gupta for his careful reading of this manuscript and for detailed discussions. A.K.G. acknowledges receipt of a CSIR Senior Research Fellowship. 12666



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