Fabrication of Water-Soluble Nanocrystals using Amphiphilic Block

Nov 1, 2010 - South Korea, and §School of Chemical and Biological Engineering, ... Received July 22, 2010; Revised Manuscript Received October 11, 20...
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DOI: 10.1021/cg1009708

Fabrication of Water-Soluble Nanocrystals using Amphiphilic Block Copolymer Patterned Surfaces

2010, Vol. 10 5187–5192

Min Kyung Lee,† Joona Bang,‡ Kyusoon Shin,§ and Jonghwi Lee*,† †

Department of Chemical Engineering and Material Science, Chung-Ang University, Seoul, 156-756, South Korea, ‡Department of Chemical and Biological Engineering, Korea University, Seoul, 136-701, South Korea, and §School of Chemical and Biological Engineering, Seoul National University, Seoul, 151-742, South Korea Received July 22, 2010; Revised Manuscript Received October 11, 2010

ABSTRACT: We present a simple and generalized fabrication method for monodispersed stabilizer-free water-soluble nanocrystals by confined nanocrystallization. Nanocrystallization inside well-ordered three-dimensional nanostructures involves surface-induced nucleation and confined growth of crystals. A question remains, however, as to whether the surface of concave-pit patterned substrates can induce the same confined nanocrystallization. Herein, poly(styrene-b-ethylene oxide) (PS-b-PEO) patterned surfaces having 30 nm ordered concave PEO domains were utilized as templates for confined nanocrystallization and in situ patterning. The hydrophilic characteristics of the patterned PEO nanodomains successfully provided the driving force to selectively confine hydrophilic nanocrystals within the surfaces of the domains. Nanocrystals of NaCl, fructose, 50 -guanosine monophosphate, and glycine were successfully formed and confined in the PEO domains by a simple dipping and annealing process. The dipping time, solution concentration, annealing time after dipping, and environmental conditions were the principal parameters. It was found that partially crystalline states or unique polymorphic forms resulted from the nanoconfined crystallization. The patterned individual nanocrystals could be detached from the substrate surfaces by sonication.

*To whom correspondence should be addressed. E-mail: [email protected]. Tel.: þ82 2 8165269.

can further regulate nanocrystallization. Also, confined nanocrystallization affects the thermal characteristics of crystals formed as well as crystal size, crystal habit, and polymorphism.6,7 A self-organizing amphiphilic block copolymer was used as the surface for confined nanocrystallization. Block copolymers spontaneously self-assemble into well-defined nanoscale structures with a domain spacing that depends on the relative lengths of the polymer blocks.8 Many potential uses of block copolymers for nanotechnologies have been suggested. Inorganic nanoparticles have been successfully prepared in block copolymer micelles, and the cooperative self-assembly of nanomaterials and block copolymers may be possible.9 The spatial arrangement of nanomaterials inside the phases of block copolymer thin films has been investigated intensively due to their potential applications in various fields.10 When the two chain segments are in a strong phase separation, which occurs much below their order-disorder transition temperature (TODT), the phase structure can be controlled to be lamellae, double gyroids, hexagonal cylinders, or face-centered cubic spheres based on volume fractions.11 Among the diverse structures, the cylindrical phase structure is particularly interesting in terms of constructing a confined environment (concave pit).12 Herein, self-organizing poly(styrene-b-ethylene oxide) (PS-bPEO) was employed to generate a nanoconfined environment. An array of ordered cylindrical nanodomains with a high degree of orientation was obtained by solvent annealing under controlled conditions.13 The cylindrical nanodomains perpendicular to the substrate surface were formed mainly due to the preferential dissolution of the minor components by solvents. A simple dipping and subsequent annealing process was attempted to induce selective nanocrystallization of hydrophilic

r 2010 American Chemical Society

Published on Web 11/01/2010

Introduction The nanoscale control of materials is an important issue in chemistry, physics, material science, and pharmaceutical fields. The fabrication of inorganic nanocrystals has been developed for device applications such as electronics and sensors.1 However, the fabrication process for inorganic nanoparticles could not be generally applied to water-soluble compounds due to the thermal instability and weak mechanical properties of water-soluble materials. Although little attention has been paid to the fabrication of water-soluble nanocrystals, a number of studies have been reported on the fabrication of similar organic nanocrystals using various methods.2 Several methods have some drawbacks; for example, the mechanical milling method3 requires applying a high mechanical energy leading to structural changes of the crystal, and the emulsification-evaporation method4 depending on the evaporation of solvents is unsuitable for unstable organic compounds. In this work, the confined nanocrystallization method for water-soluble nanocrystal preparation without using stabilizers will be discussed. We present a simple and generalized “bottom-up” method using hydrophilic interactions for the formation and in situ patterning of hydrophilic nanocrystals. This approach relies on the success of confined wetting and nanocrystallization on the patterned surfaces of hydrophilic domains surrounded by hydrophobic matrices. Crystallization can be confined by the surfaces of threedimensional (3D) nanostructures. The confined nanocrystallization involves surface-induced nucleation and confined growth.5 The large internal surface area of nanostructures

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materials within the hydrophilic PEO nanodomains (concave pits) of the PS-b-PEO thin film surface. PS-b-PEO thin films were immersed into aqueous solutions of various materials while processing conditions were adjusted. The hydrophilic surface characteristics of PEO domains can provide the driving force to position hydrophilic particles selectively within the nanopatterned domains. Various hydrophilic materials, that is, NaCl, fructose, 50 -guanosine monophosphate, and glycine, whose nanocrystals have seldom been reported up to now, were employed on the surface of hydrophilic PEO domains. This study provides insight into the selective wetting and confined crystallization on patterned nanodomains. Experimental Section Materials. Diblock copolymer PS-b-PEO with a molecular weight of 25 400 g/mol, a polydispersity of 1.05, and a PEO volume fraction of 0.25 were purchased from Polymer Source (Montreal, Canada) and used as received. D-Fructose (180.16 g/mol, anhydrous basis), glycine (75.07 g/mol), and NaCl were obtained from Sigma Aldrich (St. Louis, MO, USA). 50 -Guanosine monophosphate (533.26 g/mol) was supplied by CJ (Seoul, Korea). HPLC-grade water was purchased from J. T. Baker (Phillipsburg, NJ, USA). Thin Film Preparation. A solution of the copolymer (1 wt %) in benzene was spin-coated (SPIN-1200D, 3000 rpm, 33s, e-flex, KyungKi, Korea) onto a silicon substrate. The film thickness was controlled to be approximately 30 nm by adjusting the solution concentration and the spinning speed. Prior to the spin-coating process, the silicon substrate was sequentially cleaned in ultrasonic baths of acetone, ethanol, and deionized water. The spin-coated films were annealed under saturated benzene vapor in a humidity controller at room temperature. The samples were placed in a chamber, that is, an upside down glass container having saturated benzene vapor. To maintain the condition of saturated benzene vapor, the chamber was covered by a larger glass container. The humidity controller continually maintained a relative humidity (RH) of 90%. The samples were annealed for at least 12 h, and the upside-down glass container was then simply removed, thus allowing the benzene in the swollen PS-b-PEO films to evaporate under the given RH conditions. This controlled annealing process dramatically improved the ordering of the PEO domains,13 and highly ordered 30-nm cylindrical domains of PS-b-PEO nanopatterned substrates were obtained. The surface of the 30 nm PEO domains had 8 nm concave depth. Nanocrystal Formation. The annealed PS-b-PEO substrates were dipped into aqueous solutions of various materials. After treatment with various solutions, the substrates having particles formed on their surfaces were annealed under 90%RH. The annealing process was performed in the same way as the solvent annealing of PS-bPEO films. That is, samples were placed in the humidity controller, and the whole humidity controller was continually maintained under 90%R.H. For all the materials, the effects of initial dipping time, solution concentration, annealing time after dipping, and environmental conditions were investigated. Detaching Nanocrystals. To detach the nanocrystals formed on the nanopatterned substrates, two methods were tried. One involved immersing the substrate in water, wherein nanocrystals formed on block copolymer patterns were reimmersed into distilled water for cleaning, but this simple dipping method did not completely remove nanocrystals. The nanocrystals were successfully detached from the patterned surfaces by sonication for 3-6 min in a nonsolvent (methanol for NaCl and ethanol for D-fructose). To confirm the existence of detached crystals, a freshly cleaved mica surface or a TEM sample grid was used to capture them. Characterization. Topography images of the samples were obtained by atomic force microscopy (AFM, XE-100, Park Systems, Korea) in tapping mode at ambient conditions. Etched silicon tips on cantilevers (NSC15, Park Systems) with a force constant 40 N m-1 (as specified by the manufacturer) were used. Patterned surfaces with nanocrystals were characterized using the grazing incidence X-ray diffraction (GIXRD) of the 4C2 X-ray diffraction beamline at Pohang Light Source (PLS, Pohang, Korea).

Lee et al. X-ray from a bending magnet of the PLS storage ring was focused by a fixed-exit double crystal monochromator and two mirrors, which were coated by rhodium. The beam size at the focal point is typically less than 1 mm2. The grazing incident angle theta was fixed at 0.15°. Using a HF solution, the nanopatterned PS-b-PEO thin films were detached from the silicon substrate (200 nm silicon oxide) and transferred to copper grids for Raman scattering analysis. Raman spectra were taken on a NRS-3100 Confocal Micro-Raman spectrometer (Jasco, Easton, MD, USA) with a charge-coupled device (CCD detector) and 532 nm laser excitation. This provides high lateral and depth sample resolution of the laser spot. The diameter of the measured sample area was smaller than 2 μm when the optimum confocal instrumental setup was achieved using a 100 microscope objective. The morphology of nanoparticles detached by sonication in a nonsolvent, and their associated electron diffraction patterns, were examined using a JEOL JEM 2000EX transmission electron microscope operating at a 200 kV accelerating voltage. TEM samples were prepared by (1) dripping a drop of ethanol (nonsolvent) in between a PS-b-PEO substrate having nanoparticle arrays and a carbon-film-coated copper grid, (2) applying sonication, and (3) natural drying of the grid in air.

Results and Discussion The concave-pit patterned surfaces of the PS-b-PEO block copolymer were first prepared according to literature procedures.11,12 The prepared PS-b-PEO thin films showed highly ordered hexagonal arrays of cylindrical nanodomains oriented perpendicular to the substrate (Figures 1 and 2a). During the solvent annealing of spin-coated PS-b-PEO thin films, the high mobility of both the PS and the PEO blocks, along with the strong repulsion between the two blocks, leads to a high degree of lateral ordering. The darker areas of the AFM image correspond to the hydrophilic PEO nanodomains, and the average diameter and concave depth of the domains were about 30 and 8 nm, respectively. The concave structure of the hydrophilic PEO domain (Figure 2a) can additionally help confined crystal nucleation and growth. The resulting PS-b-PEO patterns were utilized as templates for confined nanocrystal formation of hydrophilic materials, such as NaCl, fructose, 50 -guanosine monophosphate, and glycine. Note that these materials are crystalline in the bulk states. To induce nanocrystallization of the materials selectively within the hydrophilic concave PEO nanodomains, PS-b-PEO thin films were immersed into aqueous solutions of various materials, while adjusting various processing conditions. A NaCl aqueous solution was the simplest case that resulted in successful confined (and patterned) nanocrystallization by a simple dipping process without postannealing. The patterned PS-b-PEO substrate was dipped into a NaCl solution and air-dried for several seconds. Drying appeared to occur instantaneously after removal of the patterned surface from the solution. The solution concentration and dipping time were controlled to allow for full contact between NaCl ions and PEO chains, as well as selective swelling of PEO chains. In our experiments, contact for longer than 30 min and a concentration higher than 7 wt % was sufficient for NaCl nanocrystals to form on the surface. The patterned concave structures of PEO domains were covered by 30-nm NaCl nanocrystals after preparation, and NaCl nanoparticles followed the regularity of the patterned substrate (Figure 2b). Subsequent annealing did not alter this phenomenon significantly. The stable, patterned nanocrystal layer presumably forms because of hydrophilic interactions between the PEO chains in patterned domains and the Naþ and Cl- ions.

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Figure 1. Scheme of nanocrystal formation on poly(styrene-b-ethylene oxide) nanopatterned substrates.

Figure 2. (a) AFM topography image (500  500 nm) of bare PS-b-PEO thin film pattern obtained by spin-coating and annealing for 12 h in benzene vapor, (b) AFM topography image (500  500 nm) of NaCl nanocrystals formed on the patterned substrate after immersion of the substrate in 7 wt % NaCl solution for 30 min, (c) AFM phase image of NaCl nanocrystals removed from nanopatterned substrates (scale bar: 200 nm).

A scanning electron microscope (SEM) investigation of the fractured specimen in Figure 2b revealed that the NaCl nanocrystals were not perfectly spherical (data not given). The nanoparticles had a shorter axis perpendicular to the substrate surface than parallel to the surface of the substrate. In Figure 2b, two neighboring NaCl nanocrystals appear to be connected, but due to the limitations of AFM, it is not clear whether each nanocrystal forms a separate particle. A grain boundary might exist between the two crystals. This conjecture was confirmed by the fact that the individual nanocrystals can be detached. AFM results (Figure 2c) showed the detached separate nanocrystals physically adsorbed onto a mica surface. The size of the detached particles (Figure 2c) was rather polydisperse, compared to the original particles. Indeed, the detached NaCl crystals collected onto a mica surface were allowed to dry. Under drying conditions, particle growth mechanisms follow Ostwald ripening (physical recondensation), that is, the dissolution of small particles and their growth on larger particles of the same composition.14 Also, the particles are not well dispersed in the detaching medium because of the strong attraction between the detached crystals. Successfully confined nanocrystallization was also obtained in fructose, 50 -guanosine monophosphate, and glycine solutions. The nanocrystals obtained using these materials are shown in Figure 3, and the effect of solution concentration and annealing time on the order of crystal alignment is also indicated. A specific solution concentration was critical when fructose was used. When the concentration was below 7 wt %, the concave PEO domains did not have sufficient fructose nanocrystal present (Figure 3a,b). The average number of nanocrystals per unit area of template increased with an increase in concentration of the solute. As can be seen in Figure 3c, the nanocrystals fully covered the PEO domains following the pattern of the PS-b-PEO copolymer. When the concentration of the solute was increased to 9 wt % (Figure 3d), the fructose nanocrystals became significantly larger, but the size distribution of nanoparticles remained relatively monodisperse, compared to the nanocrystals prepared from more typical emulsion or solvent evaporation methods.6 The time required for the dipping process should be

sufficient to allow wetting of the substrates with a solution and swelling of the water-soluble PEO chains. As a control, we confirmed that swelling of the PEO chains in pure water itself did not cause any significant differences in the AFM results for the PS-b-PEO surfaces. The wetting conditions are related to the numerical analy sis of the energies of droplets. In other words, to position one particle over one PEO domain, an optimum volume of material is needed. Our finding that an optimum solute concentration (Cop) exists for patterned nanocrystal formation is consistent with theoretical predictions.15 Finite element analysis of a droplet on a micropatterned substrate predicted that the lowest energy position of a droplet depends on its volume.15 Figure 3e,f shows the influence of annealing time. After several trials, 12 h and 90% RH were found to be sufficient to allow fructose nanocrystals to follow the patterns of PEO domains (Figure 3c). The rapid crystallization and patterning of NaCl nanocrystals without postannealing can be explained by the high mobility of ions and the attraction between PEO chains and ions. If molecules on the surface do not have fast diffusion rates, a postannealing process would be required to allow the molecules to arrange themselves in a pattern determined by the substrate. As reference values, the translational Brownian diffusion coefficients of NaCl, glycine, and sucrose in water at 310 K are 1.5  10-9, 1.0  10-9, 0.54  10-9 m2/s, respectively. The diffusion coefficients of fructose and glucose are similar.16 An example of the crystals formed when 50 -guanosine monophosphate was used is provided in Figure 3g. Among the materials investigated, only glycine produced less-thanperfect patterns. There may only be weak interactions between the PEO chains and glycine. Thus, to increase the interactions between PEO domains and glycine, a small quantity of NaCl was added to the glycine solution. Figure 3h indicates that addition of NaCl enhanced the selective positioning of glycine within the PEO domains, presumably due to enhanced ionic interactions. Herein, the salt components (cations) provide an effective interaction with the active site of polymer domain for coordination. In PEO, each backbone C-O site is negatively

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Figure 3. AFM topographic images (500  500 nm) of fructose particles grown on nanopatterned polymer substrates after immersion in (a) 5, (b) 7, (c) 8, and (d) 9 wt % fructose solutions for 12 h and annealing for 12 h under R.H. 90%, (e) immersion in 8 wt % fructose solution for 12 h without annealing, (f) immersion in 8 wt % fructose solution for 12 h and annealing for 6 h under R.H. 90%, (g) immersion in 9 wt % 50 guanosine monophosphate solution for 12 h and annealing for 12 h under RH 90%, (h) glycine particles formed on nanopatterned polymer substrates (dipped into a glycine/NaCl (10:1 w/w) solution for 12 h and annealed for 12 h under RH 90%).

charged. When NaCl is added to the glycine solution, the free cation (Naþ) gets coordinated to the electron-rich site of PEO backbone, mediating the interactions between glycine and PEO. Therefore, control of a solutions’s ionic nature may broaden the applicability of our approach. The crystallinity of nanoparticles was confirmed using electron diffraction, Raman spectroscopy, and grazing incidence X-ray diffraction (GI-XRD). The GI-XRD results of NaCl nanoparticles are given in Figure 4. The PS-b-PEO cylindrical domains show no significant crystalline peak but only a rather diffuse background ring, as can be seen in Figure 4a. NaCl nanocrystals on the PEO cylindrical domains and NaCl particles as received showed the same diffraction patterns, although the NaCl nanocrystals showed less distinct

peaks. From the center, the X-ray diffraction peaks of the {111}, {200}, {220} planes correspond to the three distinct rings in Figure 4b,c.17 The TEM image (Figure 5) indicates that the average size of the detached fructose crystals agrees with the size of confined crystals in Figure 3. Their electron diffraction patterns showed that the crystallinity of D-fructose is retained in its confined crystal state. These results showed the crystallinity of the nanoparticles formed on the confined surfaces. However, the detached fructose crystals collected onto a TEM grid show aggregation, but these aggregates were unlikely to correlate with the actual state, as the samples on TEM grids were allowed to dry (Figure 5). In an earlier report, it was observed during drying of the sample on a TEM grid where

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Figure 4. GI-XRD results: (a) PS-b-PEO thin film pattern, (b) after treatment of NaCl solution, (c) NaCl bulk powder.

Figure 5. TEM micrograph and its electron diffraction pattern: D-fructose nanocrystals removed from nanopatterned polymer substrates.

Figure 7. Raman spectra of glycine: (a) PS-b-PEO thin film having glycine nanocrystals, (b) glycine crystals formed on bare silicon wafer, (c) bare PS-b-PEO thin film.

Figure 6. Raman spectra of 50 -guanosine monophosphate (GMP): (a) bulk GMP (amorphous form), (b) bulk GMP (crystalline), (c) bare PS-b-PEO thin film, (d) PS-b-PEO thin film having confined GMP nanocrystals.

the nanoparticles stick to each other by the face with the largest surface area.18 When Raman spectroscopy was employed to identify the polymorphic forms of 50 -guanosine monophosphate (Figure 6) and glycine (Figure 7) from the confined nanocrystallization, an amorphous and crystalline mixture (50 -guanosine monophosphate) and an unstable polymorphic form (β-glycine) were identified. These are different from the structures of their corresponding bulk powder states. The polymorph of glycine nanocrystals was identified to be the β-form, which is known to be the least stable form.6 Consequently, the

unstable high energy polymorphic form could be energetically (or kinetically) favored under nanoscopic confinement, which has previously been observed in 3D confined systems.7,19 Because of confined nanocrystallization, the melting temperature of nanocrystals can significantly drop, and the formation of amorphous phases or changes in the relative stabilities of different polymorphs can occur.19 These unique changes due to confined nanocrystallization were also found in our nanoconfined system. The detailed thermal properties of nanocrystals are under investigation. A simple dipping and annealing process can successfully produce nanocrystals of various materials, which were patterned and could be detached later. Crystal formation on the concave pit surface of patterned cylindrical block copolymer requires selective wetting and an adequate postannealing process, allowing molecular diffusion. The key driving force of the process is the hydrophilic and hydrophobic interactions between patterned nanodomains and nanocrystal, producing a confinement effect. Therefore, this general underlying mechanism can be applied to various other materials to form and pattern monodisperse nanocrystals. If the phase structures of hydrophilic and hydrophobic blocks in templates are reversed, then this process could be applied to hydrophobic materials, as well. This simple method can be used for the preparation of novel drug delivery systems, sensors, and responsive surfaces. It might be also applicable to microcontact printing applications, since the particles that we observed on the patterned substrate are physically separable.

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Conclusion Confined nanocrystallization on the concave pits of an amphiphilic block copolymer produces patterned monodisperse nanocrystals. The nanocrystals could be detached from the surface by sonication. In the cases of crystalline NaCl and fructose, their crystal structures were similar to those of bulk materials. On the other hand, 50 -guanosine monophosphate was partially crystalline, and glycine showed a β-form. This simple formation and patterning method of monodisperse nanocrystals is applicable to various materials, since it uses the general underlying mechanism of hydrophilic and hydrophobic interactions. The efficiency of this process depends on the kinetics of molecular self-assembly following the regularity of nanotemplates. If the mobility of molecules or ions is sufficiently high, such as in the case of NaCl, a simple dipping instantaneously prepares patterned nanocrystals. Acknowledgment. This work was supported by a grant from the Fundamental R&D Program for Core Technology of Materials, funded by the Ministry of Commerce, Industry and Energy, Republic of Korea. M.K.L. would like to thank Human Resource Development BK21 (KRF) and the Human Resource Training Project for Strategic Technology (MKE & KOTEF).

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