Size-Defined Cracked Vesicle Formation via Self-Assembly of Gold

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Interface-Rich Materials and Assemblies

Size-defined Cracked Vesicle Formation via Self-assembly of Gold Nanoparticles Covered with Carboxylic acid-Terminated Surface Ligands JinJian Wei, Hideyuki Mitomo, Takeharu Tani, Yasutaka Matsuo, Kenichi Niikura, Masayuki Naya, and Kuniharu Ijiro Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02966 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 21, 2018

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Size-defined Cracked Vesicle Formation via Selfassembly of Gold Nanoparticles Covered with Carboxylic acid-Terminated Surface Ligands Jinjian Wei,†∥# Hideyuki Mitomo,*‡¶# Takeharu Tani,§ Yasutaka Matsuo,‡ Kenichi Niikura,‡¶⊥ Masayuki Naya,§ Kuniharu Ijiro*‡¶ †Graduate School of Chemical Sciences and Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-Ku, Sapporo 060-8628, Japan ‡Research Institute for Electronic Science, Hokkaido University, Kita 21, Nishi 10, Kita-Ku, Sapporo 001-0021, Japan ¶Global Station for Soft Matter, Global Institution for Collaborative Research and Education, Hokkaido University, Kita 21, Nishi 11, Kita-Ku, Sapporo 001-0021, Japan §FUJIFILM Corporation, Ushijima, Kaisei-Machi, Ashigarakami-gun, Kanagawa 258-8577, Japan

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ABSTRACT: The self-assembly of gold nanoparticles (GNPs) into a defined structure, particularly hollow capsule structures, provides great potential for applications in materials science and medicine. However, the complexity of the parameters for the preparation of those structures through self-assembly have limited access to critical mechanistic questions. With this in mind, we have studied gold nanoparticle vesicle (GNV) formation through self-assembly by the surface modification of GNPs with low molecular weight ligands. Here, we successfully prepared GNVs composed of GNPs with a diameter of 30 nm by surface modification with carboxylic acidterminated fluorinated oligo(ethylene glycol) ligands (CFLs). As the carboxylic acid has two states (protonated and deprotonated), the balance of the interaction and repulsion between GNPs covered with CFLs is tunable. Sodium carboxylate-terminated fluorinated oligo(ethylene glycol) ligands (SCFLs) provided smaller GNVs than did CFLs at 0.8×1011 NPs/mL. Time-course study revealed that CFL-covered GNPs quickly form small aggregates and gradually grow to larger GNVs (ca. 200 nm), but no gradual growth was observed for SCFL-covered GNPs. This result indicated the electrostatic repulsion inhibits fusion of the small GNVs. The size of the GNVs formed with the aid of CFLs was independent of the initial GNP concentration, but the extinction spectra were concentration-dependent. Electron microscopy imaging and simulations supported the defect formation in the assemblies. These results provided new insights into the vesicle formation mechanism.


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Introduction Self-assembly affords highly functional materials, particularly in living things. For example, lipids provide cell membranes and proteins provide hierarchical structures through self-assemblies. Recently, the self-assembly of conductive nanoparticles has received much attention from researchers due to the interesting optical phenomenon, such as localized surface plasmon resonance (LSPR), which appears when light interacts with sub-wavelength conductive nanoparticles (NPs), particularly when NPs are arranged in a well-defined structure.1–7 Comprehensive studies have demonstrated that plasmon coupling among spherical NPs is influenced by NP size and interparticle distance, and the number of NPs in the assemblies.8–11 The self-assembly of NPs into 1D NP chains,12,13 2D NP lattices,4,14–17 or 3D integrated or hierarchical structures11,18–26 provides another degree of freedom for the engineering of the plasmon coupling of adjacent NPs with uniform assembled structures with the confined gap distances.27,28 Recently, gold nanoparticle vesicles (GNVs) with a hollow interior have been prepared by selfassembly without any template, affording a structure similar to a liposome composed of lipids or a virus-like capsule composed of capsid proteins, and these GNVs have attracted particular attention due to their potential applications.29–34 Nie’s group showed that the plasmon hybridization of nanoparticle vesicles, which were covered with polymers, afforded two peaks in the extinction spectra for 40 nm gold nanoparticles and applied their GNVs to photo-thermal therapy.30 A fundamental understanding of GNV formation via self-assembly can enhance our ability to extend their potential applications. However, the formation mechanism of self-assembled GNVs remains unclear. With regard to the self-assembly process, the surface chemistry is crucial. From the view point of a short, controllable interparticle distance, GNVs composed of gold nanoparticles (GNPs)


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covered with small surface ligands have a potential advantage in terms of plasmonic characteristics. Thus, our group prepared GNVs through the self-assembly of GNPs, which were coated with a small surface ligand; a semi-fluorinated oligo(ethylene glycol), based on the solvophobic effect.21 To improve the stability of the GNVs, we recently synthesized sugar-terminated fluorinated oligo(ethylene glycol) ligands (SFLs) with enhanced attractive interaction and prepared more stable GNVs composed of GNPs with a larger diameter.35 Further, the preparation conditions, such as solvents, initial concentration of GNPs, and chemical structure at the terminus of the surface ligand, affected the size as well as the extinction spectra of the prepared GNVs.36 These results suggest that our system is suited to the study of vesicle formation mechanisms.21,35,36 Kegel et al. reported on the basis of simulations that the nanoparticle shell could be stabilized by the competition between repulsion and attraction.37 Thus, in this study, we synthesized carboxylic acid-terminated fluorinated oligo(ethylene glycol) ligands (CFLs) as new surface ligands with which to tune the attractive and repulsive interactions by the protonation and deprotonation of the terminal substrate, and investigated the effects of surface interactions on vesicle formation (Scheme 1).


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Scheme 1. Chemical structures of synthesized surface ligands (a) and the self-assembly with citric acid-coated GNPs into GNVs in the presence of CFLs or SCFLs (b).

(Scheme Footnote) SCFLs provided small vesicles compared to those obtained using CFLs. CFLs provided GNVs of the same size independent of initial concentration, although the assembled structures differ according to the initial particle concentration; that is, the absence or presence of cracks is dependent on the initial particle concentration. Cracks are regarded as defects larger than one particle in size. Gaps are regarded as the distance between gold nanoparticles, the size of which is less than one particle (30 nm).

Experimental Materials.

Gold nanoparticles (GNPs) coated with citric acid in aqueous solutions were

purchased from British Biocell International (BBI), Ltd. (Britain). Dioxane was purchased from Wako Pure Chemical Industries, Ltd. (Japan).

Carboxylic acid- and sodium carboxylate-

terminated fluorinated oligo(ethylene glycol) ligands were newly synthesized based on our previous reports.21,35 All commercially available reagents were used without further purification.


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GNV formation through the self-assembly of GNPs.

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The aqueous dispersion of citric

acid-coated GNPs of 30 nm in diameter (referred to as Au-30, 200 μL) was concentrated by centrifugation (8,600 g for 6 min). After removing the supernatant, concentrated Au-30 NPs (20 μL) were then quickly added to a dioxane solution of CFLs or SCFLs (480 μL, 0.22 mM) in a glass tube with a final water content of 4% in dioxane. Subsequently, the solution was mixed under a vortex for 1s to ensure homogeneous mixing. The mixture was then gently stirred for 2 h. To allow a clear comparison, samples for STEM measurements were prepared by directly casting a solution of CFL-Au-30 assembly (3 μL) without any purification onto the surface of an elastic carbon-coated copper grid and dried at room temperature under N2 in a glove box overnight. Electron microscopic images were obtained using STEM HD-2000 (Hitachi High-Tech Manufacturing & Service Co., Ltd., Japan) in secondary electron mode for scanning transmission electron microscopy (SE-SEM) or transmission electron mode for scanning transmission electron microscopy (TE-STEM) with an accelerating voltage of 200 kV and JSM-6700FT (JEOL, Japan) for field-emission scanning electron microscopy (FE-SEM) with an accelerating voltage of 5 kV. Extinction spectra were measured with a UV-vis spectrophotometer (UV-2600/2700; Shimadzu Corporation, Japan). Dynamic light scattering (DLS) analyses were performed with ELSZ-2000 (Otsuka Electronics Co., Ltd., Japan). Calculation of extinction cross-section spectra by FDTD simulation. The optical properties of a gold nanoparticle vesicle (GNV) were calculated by employing an electromagnetic optical simulation based on the finite difference time domain (FDTD) method. The vesicle structure consisted of spherical gold nanoparticles arranged in a spherical shell. The diameter of the particles was 30 nm and the minimum interparticle distance varied from 1 nm to 20 nm. The initial positions of the particles were determined on the shell using random numbers, and any particles nearer than


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the minimum interparticle distance were separated by moving the particles away from each other along the shell to a predetermined constant distance. This procedure was repeated until there were no particles nearer than the minimum interparticle distance. The number of particles was set to the maximum number while maintaining the minimum interparticle distance. The refractive index of the surrounding medium was 1.42 and the refractive index of gold was based on previously published data.38 The extinction cross-section spectra were then calculated.

Results and discussion GNV formations with the aid of CFLs or SCFLs.

First,

we

newly

synthesized

carboxylic acid-terminated fluorinated oligo(ethylene glycol) ligands (CFLs)

and sodium

carboxylate-terminated fluorinated oligo(ethylene glycol) ligands (SCFLs) based on our previous reports (Supporting Information).21,35 It was projected that carboxylic acid at the terminus allowed the competitive interactions to be balanced by the formation of inter-hydrogen bonds and electrostatic repulsion according to the degree of protonation/deprotonation. The self-assembly of GNPs was triggered by the direct addition of concentrated GNPs with a diameter of 30 nm (referred to as Au-30, 20 μL) into CFLs in dioxane (480 μL). The surface-modified GNPs were denoted as CFL-Au-30. Inductively coupled plasma optical emission spectroscopy (ICP-OES) indicated that the surface ligand density of the CFLs on the Au-30 NPs after self-assembly for 2h was ca. 8 ligands/nm2, which is comparable to previous studies (Table S1).35,39,40 The electron microscopic images of GNP assemblies were captured by scanning transmission electron microscopy in secondary electron mode (SE-SEM) or scanning transmission electron microscopy in transmission electron mode (TE-STEM). SE-SEM images showed spherical assemblies of Au-30 NPs of ca. 200 nm in diameter (Figure 1a). TE-STEM images also showed GNVs of 200 nm in size and


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showed an obvious contrast in the assembly as small interstices, indicating the formation of singlelayered GNVs composed of Au-30 NPs (Figure 1b, Figure S1a).21,35,36 When SCFLs were used as a surface ligand, smaller GNVs were observed by TE-STEM (Figure 1c, Figure S1b). The size of the assemblies measured by dynamic light scattering (DLS) is summarized in Figure 1d. DLS results clearly show that hydrodynamic diameter of SCFL-Au-30 assemblies (ca. 120 nm) was significantly smaller than that of CFL-based GNVs (ca. 200 nm). The extinction spectra of CFLbased GNVs and SCFL-based GNVs were also significantly different (Figure 1e). CFL-based GNVs showed two plasmonic peaks (at around 550 and 650 nm), while SCFL-based GNVs showed only one broad peak at around 580 nm due to plasmon coupling effects. This assembly size-dependent spectral change was consistent with previous reports.20,36,41 Time-course study of CFL-based GNV formation by extinction spectra showed a gradual plasmonic spectral shift to 650 nm over 1-2 h, while SCFL provided a small spectral shift to ca. 580 nm within 10-15 min (Figure 2a, b). A close-examination shows that the spectra of CFL- and SCFL-based GNV formation for the initial 10 min are similar. Time-course study of CFL-based GNV formation by DLS showed that there are at least two stages in this vesicle formation; one is a quick aggregation formation step that lasts only a few minutes, providing 100-150 nm assemblies, and the other one is a growth step including a fusion process that proceeds for 2 h (Figure 2c). The same time dependence were also observed under the different solution conditions (5% water content), except for the final size (Figure S2, S3). The GNV formation kinetics with CFLs are almost the same as our previous results using glucose-terminated fluorinated oligo(ethylene glycol) ligands (GFLs).35,36 On the other hand, SCFL-based GNV formation did not show a slow growth step after 10 mins. This difference in growth process during vesicle formation using CFLs and SCFLs indicates that the electrostatic repulsion from the sodium carboxylate head of the


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SCFLs led to the suppression of the growth (fusion) process of the quickly formed aggregates, which is necessary for lager GNV formation, generating small GNVs. It is worth mentioning that CFLs work as a good surface ligand for expedient vesicle formation of ca. 200 nm in size under these solvent conditions (dioxane containing 4% H2O). Thus, we focused on the CFL-based GNV formation for our further studies.

Figure 1. SE-SEM (a) and TE-STEM (b) images of CFL-Au-30 assemblies in dioxane. (c) TESTEM image of a SCFL-Au-30 assembly. (d) Hydrodynamic diameters determined by DLS of volume distributions. (e) Normalized extinction spectra of CFL-Au-30 assemblies (blue), SCFLAu-30 assemblies in dioxane (green), and Au-30 NPs in water (red). The concentration of Au-30 NPs for self-assembly is 0.8×1011 NPs/mL. The incubation time for self-assembly was 2 h. Scale bars represent 200 nm (a-c). Error bars represent standard deviations in hydrodynamic diameters (d).


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Figure 2. Time-course study of GNV formation. Time-dependent extinction spectra of Au-30 NPs in the presence of CFLs (a) and SCFLs (b); soon after mixing (red), at 5 min (black), 10 min (green), 15 min (light green), 30 min (purple), 60 min (dark grey), 90 min (brown), 120 min (light blue), 150 min (yellow), and 180 min (blue). (c) Time-dependent hydrodynamic diameter change in Au-30 NPs with CFLs shown as mean diameter of volume distribution ± standard deviation determined by DLS. The concentration of Au-30 NPs was 0.8×1011 NPs/mL. The first spectra and DLS size were measured soon after mixing (the time was taken to be 1 min due to the sample setting time for the measurements).

Effects of the initial concentration on GNV formation. Next, we investigated the effect of the initial GNP concentration on GNV formation, as previous reports on the preparation of GNVs through self-assembly mentioned that vesicle size showed a significant dependence on the initial concentration of GNPs.36,41 We varied the initial concentration of Au-30 NPs (CNAu-30) from


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0.4×1011 to 2.4×1011 NPs/mL. GNV size as determined by DLS and extinction spectra are shown in Figure 3. When the CNAu-30 was 0.4×1011 NPs/mL, GNVs with an average size of 200 nm formed in solution (Figure 3a). Even when the CNAu-30 was increased to 2.4×1011 NP/mL, the hydrodynamic diameter of GNVs unexpectedly remained constant at ca. 200 nm. These data demonstrated that the size of the GNVs composed of CFL-Au-30 is independent of CNAu-30, at least within this GNP concentration range. As our previous results using glucose-terminated ligands (GFLs) showed significant size changes in this concentration range, such as ca. 100 nm for 0.4×1011 NP/mL and ca. 200 nm for 2.8×1011 NP/mL, this result is worth mentioning.36 Further, the extinction spectra showed a red-shift of the second plasmon peak, which is located at a longer wavelength, with increasing CNAu-30, even though the GNVs were of similar size (Figure 3b). FE-SEM images showed spherical GNVs with localized surface defects, as cracks, under CNAu-30 = 0.4×1011 NPs/mL, although the surface of the GNVs showed a well-packed structure probably due to the increased incorporation of Au-30 NPs into the GNV via adequate vesicle fusion when they were prepared with CNAu-30 = 1.6×1011 NPs/mL (Figure 3c, d, Figure S4). These data indicated the apparent average nanogap distances between Au-30 NPs in a GNV prepared with a low concentration of Au-30 NPs is longer than that prepared with a high concentration, or when the number of coupling GNPs is smaller, due to the influence of the surface defects on the GNVs, resulting in a large difference in extinction spectra. However, it is unclear whether these defects on the GNVs were formed in solution or during the drying process, as the SEM images were obtained after drying. Thus, the above spectral and dynamic light scattering analyses in solution should be compared with the FDTD simulations shown in the next section. Although it is difficult to draw conclusions based only on these results, it is speculated that the carboxylic acid at the terminus provides


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stronger interactions than do the glucose-terminated ligands based on the hydrogen bonding and, therefore, supports stable vesicle structures with a defined size or curvature.

Figure 3. Effect of the initial concentration of Au-30 NPs on GNV size (a) and extinction spectra (b). Schematic and FE-SEM images of GNVs prepared at Au-30 NP concentrations of 0.4×1011 NPs/mL (c) and 1.6×1011 NPs/mL (d). Scale bars represent 200 nm (c)(d).

Defect formation in the self-assembly process. To investigate defect formation in the GNVs, FDTD simulation was systematically carried out (Figure 4). An ideal 3D model was constructed based on a single-layered GNV with a size of 120 or 200 nm with an ideal apparent nanogap distance between Au-30 ranging from 1 nm to 20 nm (Figure 4a). Simulation data supported our experimental spectra and provided an estimation of the average gap distance. The extinction spectra of the GNVs prepared with CNAu-30 = 1.6×1011 NPs/mL in the presence of CFLs showed


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good agreement with the simulated spectra of 200 nm GNVs with 5 nm gaps between GNPs (Figure 3b light green line and Figure 4c red line). Further, the extinction spectra of the GNVs prepared in the presence of SCFLs also showed good agreement with the simulated spectra of 120 nm GNVs with 5 nm gaps (Figure 1e green line and Figure 4b red line). These data indicated that the gap distances for GNVs covered with CFLs or SCFLs are ca. 5 nm in a well-packed structure. As the length of ligand molecules (CFLs and SCFLs) is estimated to be ca. 3.5 nm in a linear form, this gap distance seems to be reasonable. On the other hand, the extinction spectra of GNVs prepared with CNAu-30 = 0.4×1011 NPs/mL in the presence of CFLs showed an estimated gap distance of between 7.5-10 nm based on a comparison with the simulated spectra (Figure 3b red line and Figure 4c light green and purple line). It is unlikely that GNPs can maintain vesicular form with gap distances between the components of over 5 nm, which means that the distances between surface ligands on the each particle forming gap structures would be a few nm. Based on the above, we concluded that these GNVs had some defects of more than one particle in size in solution; that is, crack-like defects were formed under the self-organization process. The fact that these GNVs prepared with the aid of carboxylic acid-terminated ligands showed a fixed diameter regardless of the initial concentration of the components, and defect formation was depended on the preparation conditions is of great interest. This seems to be similar to virus-like capsules composed of capsid proteins rather than liposomes, even they showed fusion in the later stage of the vesicle formation process, as many viruses show icosahedral symmetric structures with quantized sizes defined by triangular number (T-number).42–45 It is speculated that defective CFL-based GNVs would be formed during hollow structure formation by self-organization, as the balanced interactions between GNPs lead to fixed curvature, which leads to a certain diameter, with an energetically favorable structure and well-packed stable structures with no fluidity,


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preventing further fusion of GNVs. Defective GNVs observed at a low NP concentration might be a kinetically trapped intermediate; in other words, an immature structure in the closed-packed GNV formation. Therefore, the results from this study support the importance of kinetics on GNV formation. However, the formation mechanism of GNVs remains unclear and is expected to be different from that of virus-like capsules to some extent. More detailed studies, in particular kinetic studies, are necessary to further clarify the mechanism.

Figure 4. Extinction cross-section spectra of GNVs with various gap distances calculated by FDTD simulation. (a) 3D images of GNP positions for the simulations, (b, c) simulated extinction spectra of GNVs with 120 nm (b) and 200 nm (c) in diameter with 1 nm (green), 2 nm (blue), 5 nm (red), 7.5 nm (light green), 10 nm (purple), and 20 nm (black) gaps between the nearest GNPs.


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Conclusion. We synthesized carboxylic acid-terminated fluorinated oligo(ethylene glycol) ligands (CFLs) as a new surface ligand and prepared GNVs composed of CFL-Au-30. The size of the CFL-based GNVs was independent of the initial concentration of GNPs in the self-assembly. However, the concentration of GNPs affected the packing structure of the assembly. An adequate concentration (CNAu-30 was 1.6×1011 NPs/mL) provided ca. 200 nm GNVs with a highly packed structure, which showed the two plasmonic absorption peaks around 550 nm and 700 nm. On the other hand, in the case of GNVs prepared with a low concentration of GNPs (CNAu-30 = 0.4×1011 NPs/mL), slightly red-shifted peaks around 550 and 630 nm were found in the extinction spectra and some defects were found in SEM images. FDTD simulation showed that these GNVs prepared with high (1.6×1011 NPs/mL) and low (0.4×1011 NPs/mL) concentration of GNPs had 5 nm and 7.5-10 nm gaps, respectively, on average. These results also suggested that the defective gold nanoparticle vesicles with a defined size were formed under these conditions. Defective structures provided some useful information on the vesicle formation mechanism.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.xxxxxxx. Figures S1-S4, Tables S1, Synthetic procedures and characterizations of the thiol ligands (including Scheme S1 and Figure S5-S7), and the calculation procedure of CFL-surface coverage on GNPs are described in the text. (PDF)

AUTHOR INFORMATION Corresponding Author *H.M.: E-mail: [email protected] *K.I.: E-mail: [email protected] Present Addresses ∥J.

W.: College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal

University, Jinan 250014, China ⊥K.

N.: Department of Applied Chemistry, and Graduate School of Environmental Symbiotic

System Major, Nippon Institute of Technology, Miyashiro, Saitama 345-8501, Japan Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. # These authors contributed equally.


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Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS We are grateful for the financial aid from JSPS KAKENHI 25286001. We thank the Instrumental Analysis Division, Equipment Management Center, Creative Research Institution of Hokkaido University for the high-resolution electrospray ionization mass spectra (HR-ESI MS) measurements. This work was performed under the Cooperative Research Program of the “Network Joint Research Center for Materials and Devices”. This work was supported in part by “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). A part of this work was conducted at Hokkaido University, supported by the "Nanotechnology Platform" Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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Table of Contents


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Size-Defined Cracked Vesicle Formation via Self-Assembly of Gold

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