Shape-Specific Patterning of PolymerFunctionalized Nanoparticles Elizabeth Galati,† Moritz Tebbe,† Ana Querejeta-Fernández,†,◆ Huolin L. Xin,‡ Oleg Gang,‡,§,∥ Ekaterina B. Zhulina,⊥,# and Eugenia Kumacheva*,†,∇,○ †
Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States § Department of Chemical Engineering and ∥Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States ⊥ Institute of Macromolecular Compounds of the Russian Academy of Sciences, Saint Petersburg, 199004, Russia # Saint Petersburg National University of Informational Technologies, Mechanics and Optics, Saint Petersburg 197101, Russia ∇ Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, Ontario M5S 3G9, Canada ○ Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario M5S 3E5, Canada ‡
S Supporting Information *
ABSTRACT: Chemically and topographically patterned nanoparticles (NPs) with dimensions on the order of tens of nanometers have a diverse range of applications and are a valuable system for fundamental research. Recently, thermodynamically controlled segregation of a smooth layer of polymer ligands into pinned micelles (patches) offered an approach to nanopatterning of polymer-functionalized NPs. Control of the patch number, size, and spatial distribution on the surface of spherical NPs has been achieved, however, the role of NP shape remained elusive. In the present work, we report the role of NP shape, namely, the effect of the local surface curvature, on polymer segregation into surface patches. For polymer-functionalized metal nanocubes, we show experimentally and theoretically that the patches form preferentially on the high-curvature regions such as vertices and edges. An in situ transformation of the nanocubes into nanospheres leads to the change in the number and distribution of patches; a process that is dominated by the balance between the surface energy and the stretching energy of the polymer ligands. The experimental and theoretical results presented in this work are applicable to surface patterning of polymer-capped NPs with different shapes, thus enabling the exploration of patch-directed self-assembly, as colloidal surfactants, and as templates for the synthesis of hybrid nanomaterials. KEYWORDS: nanoparticles, nanocubes, surface patterning, surface curvature, pinned micelles, polymer patches
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patterning of inorganic nanoparticles is accomplished by phase separation of two different ligands13−16 or site-specific surface functionalization via partial particle masking.17−19 These methods are limited to the generation of NPs with two distinct surface regions, surface ripples or a “raspberry” surface morphology. Recently, our group has proposed an approach to NP patterning with polymer patches.20 The strategy utilizes thermodynamically controlled segregation of polymer ligands end-grafted to the NP surface. Following transfer of the polymer-capped NPs from a good solvent to a poor solvent, an
anoparticles (NPs) patterned with chemically or topographically distinct surface regions (the so-called “patchy” NPs) have promising applications as building blocks for the next generation of functional nanomaterials.1 Carefully designed surface patches can render directionality in NP interactions, thus allowing for their self-assembly into nanostructures with desired orientation-dependent properties and functions.2,3 Other applications of surface-patterned NPs include their use as colloidal surfactants4 and templates for the synthesis of multicomponent particles.5 Substantial progress has been achieved in the generation of micrometer-size patchy particles,6−9 however, surface patterning of NPs with dimensions on the order of tens of nanometers is less common in the colloidal domain and is best developed for multicompartment copolymer micelles.10−12 Generally, surface © 2017 American Chemical Society
Received: March 8, 2017 Accepted: May 1, 2017 Published: May 1, 2017 4995
DOI: 10.1021/acsnano.7b01669 ACS Nano 2017, 11, 4995−5002
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RESULTS AND DISCUSSION Figure 1, top row, shows the approach to the surface patterning of NCs with polymer patches. The bottom row in Figure 1 shows the corresponding transmission electron microscopy (TEM) images of the NCs (two-dimensional (2D) projection or top view). Gold NCs with edge length of 59 ± 2 nm were synthesized via the procedure reported elsewhere20,40 using cetyltrimethylammonium bromide (CTAB) as a surfactant (Figure 1a and a′). The CTAB ligands were replaced with thiolterminated polystyrene molecules with a molecular weight of 50,000 g/mol (later in the text referred to as PS-50K) in tetrahydrofuran. Subsequently, the polymer-functionalized NCs were transferred to N,N-dimethylformamide (DMF), a good solvent for polystyrene (with a second virial coefficient, A2, of 3.5 × 10−4 mol cm3 g−2, equivalent to a Flory−Huggins interaction parameter, χ, of 0.46).41 In the DMF solution, the NCs were capped with a uniform PS-50K layer (Figure 1b and b′). Polymer surface segregation into patches was achieved by reducing the quality of the solvent for the polymer ligands by adding water to the NC solution in DMF to a final water concentration of Cw = 1 vol %. To avoid kinetic trapping of the polymer surface structures, the water was introduced to the NC solution in a DMF/water mixture.42 Importantly, to suppress NC self-assembly in a poor solvent, due to the association of polymer ligands of different NCs, a dilute ( 1, the NCs with facet micelles were predicted (the top part of the figure). Notably, for σ1 = σ2 = σ3, the facet patches had the lowest free energy per molecule and thus were thermodynamically most favorable, as predicted earlier by Olvera de la Cruz et al.33 For the NSs with the radius L/2 and polymer grafting density, σs, the approximation σs ≈ σ1 enabled the estimation of the equilibrium aggregation number, Ps, of polymer molecules in the NS patches as well as the free energy Fs(Ps) per molecule in the patches (pinned micelles) with the core radius Rs ≪ L/2, since Ps ≈ P1 and Fs(Ps) ≈ F1. The variation in the reduced free energy Fs(P)/F1(P1) on the normalized aggregation number for the NS patch is shown in Figure 6 with a dotted black line. The theoretical model captured the experimental trends observed for the number of patches and their spatial distribution on the NCs. It also predicted the aggregation number for the NS patches and thus the number of NS patches governed by the balance between the interfacial polymer energy in a poor solvent and the energy of polymer chain stretching.20 Interestingly, at the threshold grafting density ratios, the vertex patches were predicted to be larger (taller) than the edge micelles by ∼20% (Supporting Information), which was consistent with experimental observations (Figure 2). Thus, we conclude that the NP shape governed the polymer pattern on its surface, as is shown for the NC transforming into a NS in Figure 7. More specifically, with elimination of the high
the ratio between the NS diameter and the size of polymer ligands.20 We note that upon etching, the average polymer grafting density may change, however, as we showed in our earlier work,20 an approximately 10-fold variation in polymer grafting density did not change the number or the distribution of PS50K patches on the surface of 60 nm-diameter NSs. The location of the NC surface patches as well as the change in the patch number with the NC-to-NS transformation were modeled using an asymptotic scaling theory (see Supporting Information for details). The model predicted the relationship between the free energies of patches (or pinned micelles) formed on the NC faces, edges, and vertices, respectively. Figure 6 shows the variation in the reduced free energies per
Figure 6. Reduced free energies Fi(P)/F1(P1) per molecule in the facet (red color), edge (blue color), and vertex (green color) micelles (patches), plotted as a function of the relative aggregation number P/P1 of pinned micelles (patches) for the NC size larger or comparable with the patch radius and the ratios of polymer grafting densities on the faces, edges, and vertices exceeding the threshold values of (σ3/σ1) = 7/3 and (σ2/σ1) = 3/2. The dotted black line shows the variation in the reduced free energy per molecule, Fs(P)/ F1(P1) for patches formed on the NS. Arrows show the reduced free energies corresponding to the equilibrium aggregation numbers. F1 is the equilibrium free energy per molecule in a facet micelle with the optimized aggregation number P1.
Figure 7. Schematic of the NC-to-NS transformation with the corresponding change in the polymer surface pattern.
curvature, i.e., high polymer grafting density regions of the NCs, the number of patches on the NSs is reduced to three and was governed by the balance between the interfacial energy and the stretching energy of the polymer chains that formed patches.
CONCLUSIONS In summary, the experimental and theoretical results presented in the present work show the capability to surface pattern NPs with high surface curvature regions. The nanopatterning approach utilizes the segregation of a smooth layer of endgrafted polymer ligands into pinned micelles (polymer patches) when the solvent quality for the polymer is reduced.20 For metal NCs, we show that the patches preferentially form on the regions with a high surface curvature, that is, on the 12 edges and 8 vertices. This localization was predicted for NCs with dimensions larger or comparable with the patch size and for polymer grafting densities being higher on the edges and vertices than on the NC facets. The second important implication of our work is the governing role of NP shape in patch number and distribution. Using gradual NC-to-NS transformation, we show that when the surface curvature is uniform, as in the case of NSs, the location and distribution of the patches is a result of the balance between the reduction of surface free energy due to polymer− polymer interaction in a poor solvent and the increase in the stretching energy of polymer ligands upon patch formation. For
polymer molecule, Fi/F1, in the facet, edge, and vertex patches, plotted as a function of the reduced number of polymer molecules per patch P/P1. Here, Fi(P) is the free energy per molecule in a patch with P molecules, specific to the patch location on the facet (i = 1), edge (i = 2), and vertex (i = 3), and F1 is the free energy per molecule in a facet micelle with the aggregation number P1 (corresponding to the lowest F1). Figure 6 shows that the vertex and edge patches had a lower reduced free energy than the facet patches (and were thus thermodynamically preferred) upon two conditions: (i) the NC size L was larger or comparable with the patch radius and (ii) the ratios of polymer grafting densities on the faces, edges, and vertices exceeded the threshold values of (σ3/σ1) = 7/3 and (σ2/σ1) = 3/2. Here σ1, σ2, and σ3 are the average polymer grafting densities in the NC facet, edge, and vertex micelles, respectively. The first condition was satisfied in the experiments. The second condition, that is, a higher grafting density on the NC vertices and edges of NCs was expected, due to the reduction in steric repulsion between polymer chains on high curvature regions.30−32 Figure 6 can be considered as a state diagram, in which at Fi(P)/F1(P1) < 1 (the bottom part of the 4999
DOI: 10.1021/acsnano.7b01669 ACS Nano 2017, 11, 4995−5002
Article
ACS Nano
Functionalization of NPs with Polymer. The solution of assynthesized NPs (NSs and NCs) was concentrated from 1.5 mL to ∼20 μL using 5 min centrifugation at 5000 g and a temperature of 27 °C, followed by the removal of the supernatant. The concentrated NP solution was sonicated for ∼5 s and added to 1.5 mL of the dilute solution of PS-50K in THF (0.024 mg/mL for NCs and 0.005 mg/mL or lower for NSs). The resulting solution was maintained undisturbed at room temperature overnight. Subsequently, the NCs or NSs were separated from free (nonattached) PS via 10−14 cycles of centrifugation of the solution (5000 g, 5 min, 20 °C), removal of the supernatant, and dilution of the solution with THF. Polymer Segregation on the Nanoparticle Surface. Surface segregation was adapted from the previously reported procedure.20 The solvent was evaporated from the solution of PS-50K capped gold NPs in THF. The NPs were redispersed in DMF. Subsequently, a DMF/water mixture (2 vol % of water for NCs and 8 vol % for NSs) was added to the NP solution in DMF in a 1:1 volume ratio to reach the final concentration of water in the solution, Cw, of 1 and 4 vol %, respectively. The samples where incubated overnight at room temperature for NCs and at 40 °C for NSs. Sample Preparation for TEM Imaging. To prepare samples of NCs and NSs for TEM imaging, a droplet of the NC or NS solution was deposited onto a 300 mesh carbon-coated copper grid and left for 90 s. The remaining solution was removed with a Kimwipe tissue. TEM images were obtained using a Hitachi HT7700 microscope at 85 kV. Images were taken in different areas of the grid with approximately 100 individual species imaged. Etching of NCs in the DMF Solution. The solvent was evaporated from the solution of polymer-capped gold NCs in THF (∼0.160 mL). The NCs were redispersed in DMF (∼0.3 mL), to reach a final NC concentration of 1 × 10−11 M, and sonicated for 5 s. This NC solution was diluted with an equal volume of CTAB in DMF (3.9 × 10−3 M). Then, an aqueous solution of HAuCl4 (15 mM, typically
