Synthesis and Characterization of a Calix[4]arene ... - ACS Publications

Department of Chemistry and Biochemistry, University of Kitakyushu, 1-1 Hibikino, Kitakyushu, Fukuoka 808-0135, Japan. ‡ Department of Chemical Scie...
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Synthesis and Characterization of a Calix[4]arene Amphiphilie Bearing Cysteine and Uniform Au Nanoparticle Formation Templated by its Four Cysteine Moieties Shota Fujii,∥ Kazuo Sakurai,*,† Tadashi Okobira,‡ Noboru Ohta,§ and Atsushi Takahara∥ †

Department of Chemistry and Biochemistry, University of Kitakyushu, 1-1 Hibikino, Kitakyushu, Fukuoka 808-0135, Japan Department of Chemical Science and Engineering, Ariake National College of Technology, 150 Higashihagio, Omuta, Fukuoka 836-8585, Japan § Japan Synchrotron Radiation Research Institute (JASRI/SPring-8), 1-1-1 Kouto, Sayo, Sayo, Hyogo 679-5198, Japan ∥ Graduate School of Engineering and Institute of Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ‡

S Supporting Information *

ABSTRACT: A novel calix[4]arene amphiphilic molecule, denoted by CCaL3, was synthesized and found to form a spherical micelle consisting of 12 molecules at low pH in aqueous solution. Furthermore, uniform Au nanoparticles with 2.0 nm in diameter were synthesized in aqueous solution on the template consisting of the four cysteines of the upper rim of CCaL3. Asymmetric field flow fractionation coupled with light scattering showed that there was no dispersity in the CCaL3 micellar aggregation number. When AuCl4− ions were added into the CCaL3 micelle solution, induced circular dichroism (ICD) appeared, indicating appearance of the structural chirality of the CCaL3/AuCl4− complex. A combination of electron microscopy and small-angle X-ray scattering showed that helically coiled bilayer sheets were formed upon addition of AuCl4−. Subsequent reduction with the amine of cysteine moieties led to uniform Au nanoparticles formation with 2.0 nm in diameter on the micellar plate surface. The nanoparticle size was almost equal to the size of cavity constructed by the four cysteines on the calix[4]arene upper rim, indicating that the growth of Au nanoparticles was spatially controlled by the host−guest interaction between the cysteines and Au.

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INTRODUCTION Macrocyclic ring molecules such as crown ethers, porphyrins, and cyclodextrins encapsulate guest ligands inside the rings. Since this interaction is governed by combination of the conformational and size preferences and the electrostatic interactions, it is possible to precisely recognize the guest molecules by size and shape. Among others, calixarenes are a particularly attractive building block for supramolecular chemistry. Shinkai and co-workers1 showed that a cone shaped p-sulfonatocalixarene bearing appropriate alkyl groups forms spherical micelles in aqueous solutions and also forms a complex with metal ions such as alkali metals or silver with its lower rim oxygen atoms. The size of the lower rim cavity is about 0.43 nm, which is ideal for capturing single metal ions, but too small for metal nanoparticles such as Au nanosize clusters. Recently, we synthesized a new cataionic lipid from calix[4]arene, denoted by CaL[4]C3, that has propyl tails and primary amines and found that six CaL[4]C3 molecules can self-assemble into a cubiclike micelle at low pH.2 Calix[4]arene has four equivalent functional groups on its upper rim, and if these functional groups are properly designed, then they can © 2013 American Chemical Society

provide a larger host than that in the lower rim. To capture a Au ion, subsequently reduce to Au atom, and finally grasp the resultant Au nanoparticle, the functional group attached to the upper rim must have these three roles. For this purpose, we attach a new headgroup consisting of cysteine to the calix[4]arene (denoted by CCaL3), expecting that its amine and thiol groups can bind to the ionic and atomic forms, respectively. This paper presents the new molecular design and its supramolecular structures and then examines whether Au nanoparticles can be formed. Au particles less than 100 nm in diameter have several characteristic electrical properties determined by the quantum size effects due to the spatial confinement of electron waves. In particular, particles smaller than 4.0 nm show inherent sizedependent properties.3 Quantized energy is related to the number of wave-nodes that can exist within the particle, and therefore a small difference in size is strongly reflected in the Received: March 20, 2013 Revised: October 7, 2013 Published: October 10, 2013 13666

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Scheme 1. Synthetic Scheme of CCaL3 (IV) with the Amphiphilic Calix[4]arene Derivativea

a

Compound II was synthesized according to the reported method.9

propyl attached to the lower rim prevent interconversion between conformers. The method we used yields only the cone conformation:2 this form has still the flexibility in the methylene bonds between the aromatic rings, which allows for structures with C2v and C4v symmetries.1a The C2v structure has two opposite aromatic rings almost parallel to each other, while the planes of the other two opposite rings are almost at a right angle. In contrast, in the C4v structure, the four aromatic rings are tilted equally along the methylene bonds. Although C4v is underrepresented in the crystal state (it is less stable than C2v), both structures undergo rapid interconversion in solution. However, when alkaline metals such as sodium and potassium are trapped among the four lower rim oxygen atoms, C4v becomes a more favorable structure owing to the shielding of the electrostatic repulsion among the partially negatively charged oxygen atoms. Among the alkaline metals, the sodium cation has the highest binding affinity toward calix[4]arene derivatives.2,10 We thus carried out all the experiments under an excess [Na+] condition. Characterization of CCaL3Micelle with SAXS, AFM, and FFF-MALS. Figure 1A shows the small-angle X-ray scattering (SAXS) profile of the CCaL3 micelle in 50 mM NaCl solution at pH = 3.0. The SAXS intensity I(q) at low q values indicates asymptotic behavior of I(q) ∼ q0 at the limit q = 0, suggesting that neither aggregation nor larger structures are present. This q dependence is evidence that the CCaL3 micelle can be represented as an isolated scattering object. From the Guinier analysis, the radius of gyration (Rg) was determined to be 2.17 nm. There was a sharp minimum at q = 1.7 nm−1, indicating that the size distribution was very narrow, and the micellar shape has a high symmetry.2,11 We used the commonly employed core−shell spherical model to fit the SAXS profile:

energy levels. Precise size control and size distribution are of essential importance in practical applications.4 Au nanoparticles are generally synthesized by reducing Au ions; these Au atoms immediately congregate to form crystals, and these crystals then tend to aggregate to form nanoparticles.5 It has been shown that spatial confinement is effective in controlling the nanoparticle size, and Au ion reduction within a chemical template constructed from a supramolecular self-assembly such as micelles, DNA, and peptides is a quite efficient method.6 However, being able to control the size to be smaller than 4.0 nm is still a challenging issue confronting nanotechnology.7 In the latter part of this paper, we made an attempt to use the CCaL3 micelles as such a template for small Au particles.

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RESULTS AND DISCUSSION Molecular Design and Synthesis. Cysteine bears both amine and thiol groups, and its protonated amine can bind with an AuCl4− ion through the electrostatic interaction.8 The subsequent reduction of AuCl4− can produce Au atoms that may be immediately captured by the adjacent thiol. If multiple cysteines are covalently attached to a molecular substrate, then these cysteines may form a confined space similar to a cage. Au nanoparticle could then be grown within this space and the cage would thus control the particle size. This template synthesis is the basic idea of our molecular design. As the building blocks to provide such a space, we choose calix[4]arene. Scheme 1 presents the synthetic scheme to obtain the final product compound IV that has (L)-cysteine on the upper rim of the calix[4]arene. We also synthesized the enantiomer of IV: a (D)-cysteine derivative in the same manner. 1H NMR was used to confirm the chemical structure at each step. Compound IV is denoted here by CCaL3, where the prefix C indicates the cysteine attached to the headgroup as a hydrophilic region and the suffix 3 indicates the propyl tails. The 1H NMR spectrum for the purified IV is shown in the Supporting Information as well as the assignment of the peaks (Figure S1). Calix[4]arene can exist in four possible conformations: cone, partial cone, 1,2-, and 1,3-alternates.1a Large tail groups such as

⎛ Vc(ρ − ρ ) sin(qR c) − qR c cos(qRc) c s I(q) ∼ ⎜ (qR c)3 ⎝ Vs(ρs − ρsol ) sin(qR s) − qR s cos(qR s) ⎞ ⎟ + (qR s)3 ⎠ 13667

2

(1)

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Figure 1. (A) SAXS profile and (B) a typical AFM image of 1.0 mM CCaL3 in 50 mM NaCl solution at pH = 3.0. The solid line in (A) was calculated using the core−shell sphere model in eq 1; the fitting parameters are listed in Table S1 in the Supporting Information. The lower panel in (B) shows the height profile along the white line in the AFM image. The AFM image was obtained in tapping mode at room temperature using a silicon tip (SI-DF20(AL)). (C) FFF-MALS fractogram of CCaL3 micelles measured with LS at 90° and UV at 270 nm. The inset shows the time dependence of the molecular concentration and molar mass of CCaL3 micelles. The CCaL3 was dissolved in the eluent, 50 mM NaCl solution at pH = 3.0, to obtain a concentration of 10 mg/mL, and the solution was optically purified using an ultracentrifuge.

the solution in an ultracentrifuge at (4.0 × 105)g for 5 h just before supplying the FFF. Figure 1C shows the fractogram for the CCaL3 micelles measured with LS and UV. The UV and LS peaks at 8.2 min are completely overlapped. It is well-known that the LS intensity is proportional to the product of the molecular weight and concentration, while the UV absorbance is proportional only to the concentration. Therefore, this overlapping in the figure means that the molecular weight was constant over the whole peak. Because of the absence of optical impurities and the small angular dependence of the intensity, the weight-averaged molar mass (Mw) could be accurately determined for each fraction (Figure S2, Supporting Information). The inset in Figure 1C shows the time dependence of Mw of the micelles, and at the LS peak position, Mw = 1.74 × 104 g/mol, which corresponds to an aggregation number of 12. The molecular weight distribution determined by Mw/Mn, where Mn is the number-averaged molar mass, was equal to almost 1.00, indicating monodispersity for the CCaL3 micelles. The monodispersity in turn indicates that the present micelles have a shape persistency similar to CaL[4]C3 micelles.2 To confirm the FFF-MALS results, we determined the micellar aggregation number from the SAXS profiles (see the

where Rc and Rs are the thicknesses of the core and shell, respectively, and ρc, ρs, and ρsol are the electron densities of the core, shell, and solvent, respectively. The density ρsol was calculated to be 334 e nm−3, and ρc was assumed to be 270 e nm−3 in accordance with our previous paper.2 The best fit is shown as the solid line in Figure 1A, and the fitting parameters are listed in Table S1 in the Supporting Information. The thickness Rs of the CCaL3 micelle was larger than that of CaL[4]C3 one,2 presumably reflecting the larger headgroup or lager aggregation number. Figure 1B shows a typical atomic force microscopy (AFM) image of the CCaL3 micelle. We observed dotlike spherical objects in all the images, with a size that seemed quite uniform. The height of these objects was in the range of 4.0−4.5 nm, and as CCaL3 micelle was slightly higher than CaL[4]C3 one,2 this is consistent with the SAXS results. We carried out light scattering (LS) measurements for each fraction eluted from a field flow fractionation (FFF) instrument. We expected the micellar size to be quite small (approximately a few nanometers) for the LS, and aqueous solutions normally contain much larger dusts or impurities whose signals often overwhelm those from the nanometer-size small micelles. To remove such optical impurities, we purified 13668

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Supporting Information, Figure S3). The value of M w determined from SAXS was 1.74 × 104 g mol−1. We compare the results obtained with these different methods in Table 1; good agreement can be seen. The aggregation number of 12 suggests that the constituting molecules are arranged in a dodecahedron manner in the micelle.

the molecular shapes of CCaL3 before and after the coordination of AuCl4− in Figure 3. Here, the four amino

Table 1. Molar Masses Determined with Different Methods and the Aggregation Numbera SAXS sample CCaL3

FFF-MALS

Mw/103 g mol−1 Mw/103 g mol−1 Mw/Mn 1.74 ± 0.0200

1.74 ± 0.0500

1.00

aggregation number 12

The molar masses of CCaL3 is 1.45 × 103 g mol−1. SAXS: synchrotron small-angle X-ray scattering. FFF-MALS: static light scattering combined with field flow fractionation a

Structural Transition of CCaL3Micelle Induced by Adding AuCl4− Ions. Figure 2 shows how the SAXS profile

Figure 3. Molecular models of CCaL3 and CCaL3/AuCl4− calculated with the SCIGRESS program (Fujitsu Ltd. Japan) with MO-G and MM3, respectively. Before starting the MO-G calculation for CCaL3, the model was preoptimized using the molecular mechanics (MM) method using MM3. For the obtained structure, we added AuCl4− and applied the MO-G calculation.

groups are fully protonated because we carried out all experiments at pH = 3.0. The cone form is fixed in C4v symmetry with sodium coordinating the four oxygen atoms. We simulated a stoichiometric state in which the four amines interact with one AuCl4− ion because we found that the micellar morphology changed at this composition. Before adding the AuCl4− ion, the four cysteine arms are located as far away as possible owing to electrostatic repulsion between the positively charged amino groups. After the coordination of AuCl4−, the Au ion interacts with the four amino groups, while the thiol groups tend to remain separated. Comparing between the shapes before and after the addition of AuCl4−, we see that there is a drastic change in the volume occupied by the head. Normally, a larger head shows a higher tendency to form spherical micelles. Therefore, the morphological change from a sphere to a plate can be interpreted as this conformational change in the head. Fourier transform infrared (FT-IR) spectra of the CCaL3 before (black) and after (red) the addition of AuCl4− are shown in Figure 4. The peak intensity at 3250 cm−1 decreased slightly upon the Au addition, and this peak was assigned to the stretching mode of the NH bond. After the amine group coordinates to the metal ion, NH stretching usually becomes weaker,7c and thus its decrease indicates the amine coordination to AuCl4−. Additionally, the amide I peak at around 1640 cm−1 appeared after the addition. According to previous studies,12 the emergence of this peak suggests that a β-sheet

Figure 2. SAXS profiles for different AuCl4− ion concentrations by fixing [CCaL3] = 1.0 mM in 50 mM NaCl solution at pH = 3.0. The profiles are vertically shifted, and the straight line has a gradient of −2.

changed when AuCl4− ions were added to the CCaL3 solution. At [AuCl4−] < 0.3 mM, the profiles were almost same as that of CCaL3 itself, that is, an isolated micelle consisting of 12 mers, indicating that the AuCl4− ions induced little structural change in this range. The relation I(q) ≈ q−α, with α = 0, held over the range of q = 0.5−0.1 nm−1, indicating the absence of secondary aggregations. At [AuCl4−] > 0.3 mM, the exponent α became nonzero, and at [AuCl4−] > 1.0 mM, α = 2 over the range of 0.3 < q < 0.6 nm−1. The upturn observed at q < 0.2 nm−1 corresponds to the emergence of secondary aggregations. The α = 2 exponent signifies that the shape of the scattering object changed from a sphere to a plate.12 According to the packing parameter theory proposed by Israelachvili,13 the volume occupied by the lipid head area is essential in determining the micelle shape. Hence, we compare 13669

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attached to the cysteine head on account of the absorbance bands. This CD profile was identical to that of a CCaL3methanol solution (Figure S4, Supporting Information), where we confirmed that the CCaL3 was completely dissolved. Therefore, we can conclude that the 220 nm band is related the chiral environment of the headgroup induced from the molecular chirality of cysteine. Upon addition of AuCl4−, the intensity of 220 nm band increased and a new band at 245 nm appeared. At [AuCl4−] = 0.67 and 1.0 mM, a band appeared at around 330 nm, and the 245 nm band seemed to disappear or was overwhelmed by the increase in the intensity of the 220 nm band. For [AuCl4−] = 1.0 to 2.0 mM, a broad band appeared at around 245−270 nm, while the intensity of the 220 nm band decreased. Interestingly, the cotton effect of the 315 nm band changed from positive to negative. The broad 245−270 nm and the 315 nm bands can be assigned to the calix[4]arene moiety and AuCl4−, respectively (Figure S5, Supporting Information). Figure 5B compares the CD spectra of the enantiomers and their mixtures with AuCl4−, reveling perfect line symmetry. This is evidence that the CD spectra of the complex was produced by the chiral cysteine headgroup. However, the calixarene is far from the cysteine, and there are several free rotating bonds between them, presumably enough for the calixarene not to be under the chiral influence of the cysteine. When we mixed AuCl4− ions and CCaL3 in methanol (again, no micelle formation in methanol), no CD was observed, indicating that capturing Au ions in the cysteine head of CCaL3 did not provide sufficient spatial confinement to generate CD from AuCl4−. These two facts suggest that the CD observed for the complex is ascribed to induced circular dichroism (ICD) due to supermolecular assembly. While SAXS showed that platelike structures were present between [AuCl4−] = 1.0 and 2.0 mM, the scale of the observation was presumably too small to see the supermolecular assembly responsible for ICD. A transmission electron microscopy (TEM) image of the complex with [CCaL3]/[AuCl4−] = 1.0 in 50 mM NaCl solution (pH =

Figure 4. FT-IR spectra of the CCaL3 micelle and the complex composed of [CCaL3]/[AuCl4−] = 1.0 prepared in 50 mM NaCl solution at pH = 3.0. The amine (3250 cm−1) and amide I (1640 cm−1) bands are present in the spectra, suggesting the coordination of amine to AuCl4− ions and the presence of a β sheet structure in the complex, respectively.

structure is formed through hydrogen bonding, that is, a C O···H−N bond between adjacent amide groups. Considering the open-head conformation in Figure 3A before coordination, it might be difficult to form hydrogen bonds between different cysteine groups in either the inter- or intramolecular interaction, because two cysteine groups cannot come close enough to form a hydrogen bond owing to the steric hindrance of the open conformation. After coordination, however, the closed-head conformation shown in Figure 3B makes it possible to form hydrogen bonds between intra- and intermolecular cysteines. Therefore, we can presume that the amide-I-type hydrogen bonding between the molecules is the driving force of the sheet structure formation. Figure 5A shows how the circular dichroism (CD) changed upon addition of AuCl4−; these data were measured under the same conditions as those for SAXS. CCaL3 itself showed a CD band at around 220 nm, which can be ascribed to the triazole

Figure 5. (A) CD spectra for different AuCl4− ion concentrations by fixing [CCaL3] = 1.0 mM in 50 mM NaCl solution at pH = 3.0. The inset shows a magnified figure around the calix[4]arene moiety and AuCl4− ion absorbance area. (B) Relationship of the CD spectra among enantiomeric CCaL3/AuCl4− complexes prepared under the same conditions as in (A). 13670

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Supporting Information), the particle size was drastically increased from 60 nm to 300- 400 nm, consisting with the result of SAXS. Based on these results, we can assume that the following morphological changes occur upon the addition of AuCl4−; the changes are shown schematically in Figure 8. The empty CCaL3 takes a dodecahedral-shaped micelle because of the open-head conformation. Upon the addition of AuCl4− ions, one AuCl4− ion binds to the four amine heads and reduces the electrostatic repulsion of these heads leading to the closed-head conformation. Once the molecular shape has changed, the packing parameter also changes as does the micellar structure. Since the closed-head conformation is more favorable for a plate than for a sphere, a bilayer plate structure is formed. Plates twist easily in response to a small perturbation, and the chirality of the cysteine head may play such a perturbation role. Ziserman et al.12 and Li et al.14 have showed that coiled sheet structures eventually form nanotubes, and the chirality is a key requirement for this formation. Our model is consistent with their results. Synthesis of Au Nanoparticles. Au nanoparticles were synthesized on the CCaL3/AuCl4− micelle surface by adding the reducing agent NaBH4. This changed the solution from a clear fluid to a light yellow color (Figure S8, Supporting Information). No surface plasmon resonance appeared at around 500 nm, which means that the synthesized Au nanoparticle was smaller than 2.0 nm.3a,8b Figure 7A shows TEM images of the surface after the addition of NaBH4 and shows that small Au particles, more or less 2.0 nm in diameter, were formed. We could not find any evidence of twisted ribbons after the NaBH4 agent was added, which is consistent with the disappearance of the induced CD (Figure S8, Supporting Information). As a control experiment, we reduced the AuCl4− solution in the presence of cysteine derivative (Lcysteine ethyl ester hydrochloride) in a similar manner and found that the particle size was much larger than that seen in Figure 7A and had a much broader distribution. This confirms that the cysteine groups of CCaL3 are essential for producing small Au nanoparticles. It should be emphasized that the formation of such nanoparticles is related to the spatial confinement of the four cysteine molecules of the upper rim of CCaL3. Although the Au particle diameter was well controlled by CCaL3, the reduction with NaBH4 destroyed the preceding

3.0) is presented in Figure 6. Here, staining was not necessary because sufficient contrast was provided by the AuCl4−. The

Figure 6. TEM image of the complex composed of [CCaL3]/ [AuCl4−] = 1.0 in 50 mM NaCl solution at pH = 3.0. The white arrows indicate twist points of the sheet.

TEM image shows cylindrical objects with a diameter of about 100 nm, and we confirmed that these could be reproduced (Figure S6, Supporting Information). When we examined the images, some parts of the cylinder, indicated by the white arrows, appeared to be uncoiled; the structure was similar to twisted ribbons. Although the images are not clear, the twisting direction was the same in the cylinder. We believe that these twisted ribbons are the origin of the ICD of the complex. Note that the SAXS q-range in Figure 2 corresponded to 60−2 nm, and thus, cylindrical objects larger than this range would not be observed by SAXS. To rationalize the cylindrical image and uncoiled ribbon structure from the TEM images and the platelike structure from the SAXS profiles, we presume that CCaL3 takes a bilayer hollow cylinder or nanotube structure after the addition of AuCl4−. Additionally, when we measured the change in the hydrodynamic size with dynamic light scattering (DLS) upon the addition of AuCl4− (Figure S7,

Figure 7. TEM images of the synthesized Au nanoparticles. (A) NaBH4 was added to [CCaL3]/[AuCl4−] = 1.0. (B) A moderate reduction with amine for [CCaL3]/[AuCl4−] = 0.5. The insets show the particle size distribution. 13671

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Figure 8. Schematic representation of the morphological transitions observed for CCaL3 with the addition of AuCl4−. The morphological transition can be controlled by changing the cysteine head volume.

dimensional structures. However, there is a drawback in the lipid template method, generally being difficult to control the particle size and thus the resultant particle becomes polydisperse in size.22 There are many methods to obtain metal nanoparticles with narrow size distribution. The most common approach is to use strong capping action of ligands such as p-mercaptobenzoic acid23 or poly(N-vinyl-2-pyrrolidone)24 as well as other methods.25 The strong affinity to metal and complete coating on the particle provide a relatively small size and its narrow distribution. However, it seemed that the spatial fabrication of the obtained particle, such as tubes or plates, may be difficult for these methods because the particles are completely coated by the ligand molecules and dispersed randomly in solution. Since lipids form a variety of micellar structures, if we can introduce a strong metal binding site on the lipid which can act as a capping agent, we can arrange nanoparticles on the micellar surface as well as control their size and distribution. As presented in this paper, the method to use multiple cysteines spatially arranged on one lipid molecules may be one of the candidates for such a template synthesis.

micelle structures, and some the Au particles coagulated. A more moderate condition for the AuCl4− reduction is thus needed. Yang et al.8a have reported that the lone electron pair of the amine group can reduce Au ions without the addition of any reducing agent, and although the reaction rate is slow, the reduction takes place quite moderately. When we left the complex with [CCaL3]/[AuCl4−] = 0.5 in 50 mM NaCl solution at pH = 3.0 ([AuCl4−] = 2.0 mM) for 24 h and observed the resultant product in the TEM, we observed many cylindrical objects similar to those in Figure 6 (immediately after the addition of AuCl4−), and more importantly, the surface of the cylinders was covered with small Au nanoparticles, more or less 2.0 nm in diameter. The size of these particles was more uniform than those in Figure 7A, as seen in the inset in Figure 7B. Comparing the two reductions, NaBH4 and amine, the later takes a longer time but it is moderate enough not to destroy the preceding micellar structures. When we estimated the CCaL3 headgroup size in the open-head state, the distance between the diagonal thiol groups was 1.7 nm. This is comparable with the Au diameter; therefore, we can presume that the head cavity created by the four cysteines attached to CCaL3 plays a template role in trapping the Au particle and preventing further coagulation or growth as presented in Figure 8. As shown in the Supporting Information (Figure S9), when we carried out a control experiment: Au ion reduction with amine by use of a calix[4]arene lipid (denoted by CaL[4]C3) that has four primary amines as the headgroup,2 there was no small Au particle formation. This experiment indicates the importance of cysteine to capture Au particle and to prevent further aggregation of the particles. According to previous studies, nanotubular structures are particularly important for the synthesis of nanoparticles because they provide one-dimensional templates. So far, DNAs,15 viruses,16 peptides,6c,17 synthetic polymers,18 and low molecular weight gels19 have been demonstrated to fabricate metal nanoparticles in a one-dimensional manner. Among others, lipid nanotubes are a good candidate, because their selfassembly can be easily controlled and the materials can be generally produced at low cost. In addition, the chemical modification of lipids is relatively easier than polymers and biopolymers. These advantages have been demonstrated by many groups.20 Recently, Shimizu et al. synthesized a peptide lipid and demonstrated that the complexation between the Ag+ ion and the lipid leads to nanotubes consisting of Ag embedded bilayer membranes of the lipids.21 They showed that the lipid nanotubes can serve as a nanoscale template to form one-

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CONCLUSIONS We have synthesized an amphiphilic calix[4]arene derivative, CCaL3, and found that it self-assembles in acidic solutions into a monodispersed micelle consisting of 12 molecules. This is similar to the CaL[4]C3 micelles with a cubic hexamer. Additionally, the micellar morphology was found to change into that of a hollow tube and/or twisted sheet with the addition of AuCl4−. The change was demonstrated to be due to the change in the packing parameter of the CCaL3 molecule with AuCl4− trapped in the cavity of the CCaL3 head. Subsequent reduction of the CCaL3/AuCl4− complex leads to the capture an Au particle of more or less 2.0 nm in diameter. The present work has proposed a new method to synthesize uniform Au nanoparticles on a molecule with multiple amine heads.

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MATERIALS AND METHODS

Synthesis and Materials. Synthesis of (R)-tert-Butyl(1-oxo-1(prop-2-yn-1-ylamino)-3-(tritylthio)propan-2-yl)carbamate (I). A solution of I (1.00 g, 2.16 mmol), propargyl amine (0.178 g, 3.24 mmol), 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMT-MM) (0.895 g, 3.24 mmol), and anhydrous methanol (15 mL) was stirred at room temperature for 2 h under nitrogen atmosphere. Water was then added, and the reactant was extracted with EtOAc. The organic layer was washed three times with saturated NaCl solution and dried over MgSO4. The solution was evaporated to dryness, and the residue was purified by flash chromatography (1:2 EtOAc/hexane), which afforded I as a white solid (1.02 g, 2.05 mmol, 13672

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95%). 1H NMR (500 MHz, CDCl3): δ = 7.28 (m, 15H), 6.19 (br, 1H), 4.76 (br, 1H), 3.96 (br, 2H), 3.82 (br, 1H), 2.71−2.54 (m, 2H), 2.20 (t, J = 5.0 Hz, 1H), 1.27 (s, 9H). Synthesis of 5,11,17,23-Tetrakis[(R)-tert-butyl(1-(((1H-1,2,3-triazol-4-yl)methyl)amino)-1-oxo-3-(tritylthio) propan-2-yl)carbamate]-25,26,27,28-tetrapropoxy-calix[4]arene (III). A solution of compound II (0.200 g, 0.246 mmol), compound I (0.616 g, 1.23 mmol), copper sulfate pentahydrate (5.00 mg, 2.02 × 10−5 mol), sodium ascorbate (40.0 mg, 2.02 × 10−4 mol), and anhydrous N,Ndimethylformamide (6 mL) was stirred at 90 °C for 36h under nitrogen atmosphere. Water was then added, and the reactant was extracted with EtOAc. The organic layer was washed three times with saturated NaCl solution and dried over MgSO4. The solution was evaporated to dryness, and the residue was purified by flash chromatography (1:1 DCM/EtOAc → 20:1 EtOAc/methanol), which afforded a brown solid (0.397 g, 0.141 mmol, 57%). 1H NMR (500 MHz, CDCl3): δ = 7.44 (s, 4H), 7.36−7.15 (m, 60H), 6.44 (s, 8H), 5.30 (s, 12H), 4.43 (s, 8H), 4.35 (d, J = 13.5 Hz, 4H), 3.98 (br, 4H), 3.78 (t, J = 7.25 Hz, 8H), 3.02 (d, J = 13.5 Hz, 4H), 2.66− 2.53 (m, 8H), 1.87 (m, J = 7.0 Hz, 8H), 1.37 (s, 36H), 0.962 (t, J = 7.75 Hz, 12H). Synthesis of 5,11,17,23-Tetrakis[(1H-1,2,3-triazol-4-yl)-(L-cysteine)]-25,26,27,28-tetrapropoxy-calix[4]arene (IV). A solution of compound III (0.146 g, 5.19 × 10−5 mol) was treated with TFA for 1 h, and triethylsilane was then added to the solution and stirred for 1 h. The solvent was evaporated, and the residue was washed with CH2Cl2 and EtOAc. Finally, the compound was washed with 4 N HCl/EtOAc solution to change the counterions from TFA to HCl, and IV was obtained as a light yellow solid (80.0 mg, 97%). 1H NMR (400 MHz, methanol-d4): δ = 7.89 (s, 4H), 6.64 (s, 8H), 5.30 (m, 8H), 4.55 (m, 8H), 4.42 (d, J = 13.2 Hz, 4H), 4.07 (m, 4H), 3.83 (t, J = 7.25 Hz, 8H), 3.13 (d, J = 13.2 Hz, 4H), 2.99 (m, 8H), 1.91 (m, 8H), 1.00 (t, J = 7.48 Hz, 12H). Compound II was synthesized according to the reported method.9 The calix[4]arene derivative bearing (D)-cysteine on the upper rim was synthesized via the same route. Calix[4]arene, synthesis chemicals, and solvents were purchased from Tokyo Chemical Industry Co., SigmaAldrich Co., Wako Chemical Industries, and Watanabe Chemical Industry Co. and used without further purification. Water was purified with a Millipore Milli-Q water purification system. Sample Preparation. The concentration of the stock solutions of CCaL3, CaL3, cysteine derivative and chlorauric acid were adjusted to 5.0 mM in a solution of 50 mM NaCl at pH = 3.0. In all the experiments, we fixed the concentration of CCaL3 to 1.0 mM and the chlorauric acid solution was titrated. Au nanoparticles formation was carried out at the fixed concentrations of stabilizers (CCaL3, CaL3, or cysteine derivative) of 1.0 mM and [AuCl4−] = 1.0 mM. Since CCaL3 has four cysteine groups, we also carried out the control experiment with 4.0 mM of cysteine derivative. When we reduced Au ions, sodium borohydride (2 mM, iced water solution) was added to the solution containing of the stabilizer and Au ions, and the sample was left for 1 day to equilibrate at room temperature. Small-Angle X-ray Scattering (SAXS) Measurements. SAXS measurements were carried out on the BL-40B2 beamline at SPring-8, Japan. We used a 30 cm × 30 cm imaging plate (Rigaku R-AXIS VII) detector placed 1.8 m away from the sample, and we adjusted the wavelength of the incident beam (λ) to 0.10 nm. This setup provided a q range of 0.1−4.0 nm−1, where q is the magnitude of the scattering vector defined by q = 4π sin θ/λ with a scattering angle of 2θ. We used a bespoke SAXS vacuum sample chamber for this measurement to remove the effects of the background from the window material and air. The X-ray transmittance of the samples was determined with an ion chamber located in front of the sample and a Si photodiode (Hamamatsu Photonics S8193) placed after the sample. Detailed experimental procedures are reported elsewhere.26 Atomic Force Microscopy (AFM) Observations. A 50 μL drop of the sample solution was placed on a mica surface, wiped with filter paper, and dried in air. AFM experiments were carried out immediately after preparing the sample. AFM images were acquired using an SPA 400 (SII NanoTechnology Inc.) in tapping mode at room temperature

using a silicon tip (SI-DF20(AL)). The frequency of the tapping mode was 118 kHz, and the radius of the tip apex was around 10 nm. Field Flow Fractionation (FFF) Coupled with Light Scattering (LS). We prepared a CCaL3 micellar solution of 10 mg/mL in 50 mM NaCl at pH = 3.0. After the solution was optically purified using a ultracentrifuge, the 60 μL sample solution was immediately injected into an Eclipse 3+ separation system (Wyatt Technology Europe, Dembach, Germany) for FFF at 22−28 °C. The FFF was connected to a Dawn Heleos II multiangle LS detector (Wyatt Technology) and an Optilab rEX DSP differential refractive index (RI) detector (Wyatt Technology) operating at a wavelength of 658 nm, in that order from upstream. We used a Wyatt channel (Eclipse 3 channel LC) attached to a membrane (polyether sulfone membrane 1 kDa LC) at the bottom of the channel. The cross- and channel-flow rates were fixed at 4 and 1 mL/min, respectively. Detailed experimental procedures are reported elsewhere.27 Circular Dichroism (CD), UV, and FT-IR Measurements. CD and UV measurements were performed with J-720WI and Jasco V-630 instruments, respectively. The thickness of the quartz cell was 0.2 m, and the measurement temperature was 25 °C. After the CCaL3 micellar or CCaL3/AuCl4− complex solutions were freeze-dried, KBr pellets were prepared for the FT-IR measurements. The spectra were acquired using a Perkin-Elmer spectrum 100 FT-IR spectrometer with a 4 cm−1 resolution. Transmission Electron Microscopy (TEM) Observation. A 10 μL drop of the sample solution was placed on a copper grid coated with an elastic carbon film, and after 1 min the solution was wiped with filter paper and then dried in vacuo. The grid was then loaded into a JEOL JEM-3010 electron microscope operated at 200 kV.

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ASSOCIATED CONTENT

S Supporting Information *

Additional results as described in the text. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the JST CREST program. All SAXS measurements were carried out at SPring-8 40B2 (Proposal Numbers: 2011A1668, 2011B1735, and 2012B1690).

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REFERENCES

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