Assembly of Hydrophobic Silica Nanoparticles
Microshells DOI: 10.1002/smll.200600613
Interfacial Assembly of Partially Hydrophobic Silica Nanoparticles Induced by Ultrasonic Treatment Dmitry Grigoriev, Reinhard Miller, Dmitry Shchukin,* and Helmuth Mçhwald
A
sonochemical approach has effectively been applied to prepare aqueous dispersions of air-filled nanostructured quartz silica shells from surface-engineered amorphous silica nanoparticles. The non-equilibrium nature of the cavitation process and high temperature and pressure in the cavitation microbubble can lead to partial crystallization of the amorphous silica nanoparticles producing the quartz phase and a high degree of interconnection between the silica nanoparticles in the microsphere shells. The very high stability of the silica shells against collapse and aggregation is determined by the hydrophobic nature of the silica nanoparticles. Because of the shell thickness and its high density caused by sintering of the silica nanoparticles, the gas (liquid) permeability through the shell is limited thus prolonging the life time of the air-filled nanostructured silica shells.
Keywords: · microspheres · cavitation · gas encapsulation · silica particles · sonochemistry
1. Introduction Studying the effects of ultrasound on the chemical and physicochemical processes in colloidal systems is a rapidly growing research area[1] with high potential applications in material chemistry, “green” decomposition technology, nanosynthesis, and chemical catalysis. The possibility of transforming the energy of sound waves into heat and chemical energy was demonstrated in the early 1900s after Lord Rayleigh had postulated the existence of cavitation bubbles.[2] However, the potential of ultrasound in the physical chemistry of interfaces has rarely been exploited and this area still remains challenging for the broad community of physical and interfacial chemists. The most important aspects of sonochemistry are its applications in the synthesis and modification of organic materials, decomposition of polymers, and waste processing.[3] Sonochemical enhancement of the reactivity of metals has [*] Dr. D. Grigoriev, Dr. R. Miller, Dr. D. Shchukin, Prof. H. Mçhwald Max-Planck Institute of Colloids and Interfaces 14424 Potsdam (Germany) Fax: (+ 49) 331-567-9202 E-mail:
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become a routine synthetic technique for many heterogeACHTUNGREneous organic and organometallic reactions, particularly those involving Mg, Li, or Zn. Rate enhancements of more than tenfold are common, yields are often substantially improved, and by-products are avoided.[4] Ultrasound has been used for the preparation of metal oxides, for example, ultrasonic irradiation produces mesoporous silica,[5] Fe3O4,[6] porous amorphous molybdenum oxycarbide,[7] and iron nitride.[8] Acoustic cavitation (the formation, growth, and collapse of bubbles) provides the primary mechanism for sonochemical effects.[9] Cavitation processes produce intense local heating, high pressures, and very short lifetimes, whereby the transient, localized hot spots drive high-energy chemical reactions. Measurements of the sonoluminescence spectra[10] and of the rates of thermosensitive chemical reactions of metal–carbonyl substitutions[11] confirmed the temperature inside a cavitation bubble to be approximately 5000 K, pressures of about 1000 atm (1 atm = 101.325 kPa), and heating and cooling rates at the interface of the cavitation microbubble above 1010 K s 1 were found. Hence, cavitation can create extreme physical and chemical conditions at the gas/ liquid interface of the microbubble although a low temperature in the bulk solution is maintained.
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The extreme conditions at the interface between the gaseous interior of the cavitation microbubble and the surrounding liquid medium can be utilized for carrying out various physical transformations and/or chemical reactions leading to the formation of spherical shells containing either gas or liquid in their inner cavity. In this case, a cavitation microbubble is employed as a quasi-stable template with unique physicochemical characteristics of the surface. Highintensity ultrasound applied to bovine serum albumin, avidin, and bovine hemoglobin solutions can produce organic liquid-filled microcapsules with a protein shell.[12] The shell of the spheres is formed due to sonochemical crosslinking by superoxides generated from water and oxygen during acoustic cavitation. Air-filled microshells were produced by ultrasonic treatment of the mixture of two nonionic surfactants (a sorbitan fatty acid ester (Span) and its polyoxyethylene adduct (Tween)[13] and were further significantly stabilized by layer-by-layer (LbL) deposition of polyelectrolytes (polyallylamine/poly(styrene sulfonate)).[14] The resulting polymeric microshells had a wide size distribution (1–20 mm), gaseous interior, and were stable for at least one week. An alternative way to achieve a stable gas-filled microstructure is the application of partially hydrophobic solid nanoparticles as shell constituents.[15] Being attached at the microbubble surface such nanoparticles remain there forming a so-called “armored” shell around the bubble. The solid shell can essentially reduce or even completely stop bubble disproportionation making them stable in solution for a long time. One of the strongest limitations of nanoparticulate stabilization of the gas bubble is the low diffusion coefficient of nanoparticles compared to those of molecules and, therefore, their approach to the microbubble surface is too slow. As a consequence, the time required for the saturation of the interfacial layer by nanoparticles to form the closed shell is longer than the life time of the bubbles. This limitation could be overcome by applying high-intensity ultrasound to form gaseous microbubbles by cavitation. However, the possibility to employ ultrasound and extreme conditions at the interface of the cavitation microbubble for the formation of micrometer-sized hollow structures from preformed surface-functionalized nanoparticles has not been shown so far despite their high potential to modify properties of the nanomaterials. One could also expect the formation of amorphous or metastable nanomaterials from initial preformed crystalline nanoparticles. The presented work is an attempt to use cavitation microbubbles as a reaction zone and, simultaneously, as a growing template for the formation of gas-filled microspheres from preformed silica nanoparticles functionalized by surfactants. We also investigate the influence of the ultrasonic cavitation on the structure and properties of the nanomaterials entrapped at the microbubble interface. The detailed consideration of the structure and properties of the individual nanoparticle-stabilized microbubbles may shed light on the mechanism of its stabilization by nanoparticles.
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2. Results and Discussion The surface of the preformed silica nanoparticles was modified by the adsorption of the cetyl trimethyl ammonium bromide (CTAB) (cationic surfactant). The main objective of the SiO2 surface modification is to achieve an appropriate hydrophobicity to facilitate the entrapment of the SiO2 nanoparticles at the interface of the cavitation microbubble. Hydrophobicity is one of the most crucial factors determining the ability of nanoparticles to remain stable at the interface. Usually, it is expressed in terms of the threephase contact angle q between a particle and two adjacent bulk phases. For instance, for a 15 nm particle the free energy of entrapment amounts to several thousands kT (k is the Boltzmann constant, and T is the temperature) already at q 308.[16] In turn, the particle contact angle is closely related to the ratio between free silanol groups on the surface of the silica nanoparticles and those coupled to oppositely charged CTA + ions. This substitution on the surface of the particles leading to the reduction of electrostatic repulsion between them can markedly influence the colloidal stability of their dispersion.[17] For these reasons, the ratio between SiO2 and CTAB concentrations as well as the pH value of the mixture were varied (see Table S1 in the Supporting Information). As seen from the table, the best concentration ratio was 0.05 mm CTAB per 1 g of SiO2. The accumulation of surfactants at the gas/solution interface is influenced by the oscillation intensity of the cavitation bubbles.[18] As was shown by Sostaric et al.[19] who explored sonochemistry and sonoluminescence in the presence of surfactants, the accumulation is determined not by the thermodynamic parameters of the particular surfactant but rather depends on the cavitation dynamics. The interface of cavitation bubbles is growing in the timescale of microseconds,[1] for instance at 20 kHz and 500 W the growth time is approximately 20–40 ms. The bubble growth is thus quite fast and most of the surfactant molecules adsorbed at the interface of the cavitation microbubbles were originally adsorbed at the gas/liquid interface of the initial heterogeneous cavitation centers or were present in solution within the hypothetic sphere of the cavitation microbubble before ultrasonic treatment. To increase the number of cavitation centers and, as a consequence, the number of gas-filled microbubbles, 0.8 m NaHCO3 was added to the SiO2/CTAB mixture. NaHCO3 partly decomposes to produce CO2 in aqueous solutions and thus providing an increased quantity of CO2-filled heterogeneous cavitation centers in the SiO2/ CTAB mixture. Moreover, the conversion of ultrasound energy into heat upon sonication can lead to the decrease of the CO2 water solubility and to liberation of CO2 from the solution to the cavitation microbubbles. Their growth rate is therefore strongly accelerated. The surface of the growing bubble, which is moving outwards very fast, collects all nanoparticles located in the expanding bulk solution near the interface. They are forced into an interfacial layer where fast formation of the nanoparticulate solid shell occurs. Confocal microscopy images of the resulting silica microspheres are presented in Figure 1. As seen from the images, the addition of NaHCO3 to the SiO2/CTAB mixture
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Figure 1. Confocal microscopy images of SiO2 microspheres (brightfield mode) ultrasonically fabricated in SiO2/CTAB (CTAB:SiO2 ratio is 0.05 mm of CTAB per 1 g of silica nanoparticles in 1 mm NaCl) mixture: A) without NaHCO3 present, and B) in the presence of 0.8 m NaHCO3.
is important to obtain silica microspheres using cavitation microbubbles as a template and CTAB-modified silica nanoparticles as building blocks. Microbubbles obtained in the SiO2/CTAB mixture without NaHCO3 are not stable enough (Table S1), forming a sort of shell debris made of silica nanoparticles (Figure 1 A). On the contrary, silica microshells obtained in the presence of NaHCO3 (Figure 1 B) are stable (1 month), individual, and well defined. The size of the microshells varies from 5 to 40 mm; they have a spherical shape and no aggregation is observed. A possible explanation could be the small size and number of the heterogeneous nucleation centers made by dissolved air in SiO2/ CTAB solution without NaHCO3. Addition of NaHCO3 to the SiO2/CTAB mixture increases the size and number of the gaseous heterogeneous cavitation centers before and also during ultrasonic treatment due to CO2 oversaturation caused by the elevated temperature of the solution ( 40 8C after 1 min treatment of the mixture at 500 W intensity) and NaHCO3 decomposition. Aging of the prepared silica shells leads to their fractionACHTUNGREation and size separation (Figure 2 A). SiO2 shells of large diameter move to the top of the solution forming a silica-based foam phase while the microspheres with a size small 2007, 3, No. 4, 665 – 671
below 25 mm (Figure 2 B) are deposited at the bottom. The smallest spheres of diameters less than 5 mm disappear due to the dissolution of the entrapped gas. Addition of a fluorescein isocyanate (FITC) solution to the suspension of the aged silica microspheres yields a fluorescence signal only from the media surrounding the SiO2 shells and not from the shell interior. This evidences the high thickness of the SiO2 shell and the gaseous interior of the microspheres (Figure 2 C). The deposition of the silica shells can be caused by the increasing thickness of the SiO2 layer during aging. Silica microspheres in solution could act as heterogeneous aggregation centers for silica nanoparticles present in solution and may not be involved in shell formation during sonication, which leads to the additional adsorption of free nanoparticles on the already formed silica shell increasing their thickness and weight. The thicker and consequently heavier multilayered microspheres form a precipitate at the bottom of the vessel (Figure 2 A). Gentle shaking of the sedimented microspheres in solution is enough to redisperse the milky suspension of individual microspheres. The size of the microspheres greatly depends on the intensity of the ultrasonic treatment applied to the solution (Figure 3 A). At low power (100 W) the size of the silica microshells is approximately 50 % larger than the average size of those produced at high power (500 W). However, after 9 days of aging the average size of the nanostructured silica microshells prepared both at 100 W of power and at 500 W of power becomes the same. Qualitatively, one can explain this phenomenon in a similar manner as the formation of inorganic crystals from the corresponding salt solution. At low ultrasonic intensity the number of the cavitation microbubbles is low, so there is no competition in the gas supply between neighboring cavitation bubbles, which can grow independently. At the other extreme, at high ultrasonic intensity a larger number of cavitation microbubbles are involved in a strong competition at the formation stage resulting in a larger number of microbubbles of smaller size. However, the degree of the reduction of the microshell average size during aging remains the same ( 50–60 %) for all ultrasonic intensities applied indicating the same mechanism of microshell ripening: Large microshells form a foam phase above the solution whereas for smaller ones the wall thickness increases and they precipitate. The size of the microbubbles is not influenced by the duration of the ultrasonic treatment if the sonication is carried out at constant (room) temperature; prolonged ultrasonic treatment without temperature control leads to the heating of the treated solution up to 70 8C and, consequently, to the rupture of the silica microspheres. The time-dependent stability of the silica microspheres in water is also correlated to the ultrasonic power applied during their preparation (Figure 3 B). The stability, which was estimated by confocal microscopy from the number of intact microshells after a certain time period divided by the total amount of microshells prepared, gradually increases with increasing ultrasonic power applied during their fabrication. However, approaching 500 W the increment of the stability growth becomes less pronounced. This could be associated with a different average size of the microshells fab-
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Figure 2. A) Photograph of the suspension of the silica microspheres 24 h after 1 min sonication of a SiO2/CTAB mixture in the presence of 0.8 m NaHCO3. Size fractionation of the silica shells is clearly seen. B, C) Confocal microscopy images of the deposited silica microspheres made in bright-field mode (B) and in FITC fluorescence mode (C) (signal from fluorescein isocyanate added to the suspension of silica microspheres).
suspension of silica microspheres with shell integrities that are stable for at least 1 month. Aged silica microspheres are stable upon drying and do not collapse (Figure 4 A) as previously reported for polyelectrolyte spheres.[14] Their stability can be explained by the dense silica shell of 700–800-nm thickness (Figure 4 B). The surface of the dried microspheres is rough and porous. The microspheres undergo a partial size reduction ( 30 %) while exposed to the open air. This evidences more loose packing of the SiO2 nanoparticles in silica microspheres present Figure 3. A) Average size of the freshly prepared and aged silica microspheres prepared by 1 min sonicain the water suspension as tion of the SiO2/CTAB mixture in the presence of 0.8 m NaHCO3 at different ultrasound intensity compared to the dried micro(250 microspheres counted). B) Fraction of the intact silica microspheres measured by counting confocal spheres. microscopy images (400 microspheres were counted and monitored). Silica microspheres were prepared by 1 min sonication of a SiO2/CTAB mixture in the presence of 0.8 m NaHCO3 at 1) 100 W, 2) 200 W, Raman spectra made of 3) 300 W, 4) 400 W, and 5) 500 W ultrasound intensity. suspensions of the as-prepared silica microspheres, suspensions of the aged silica microspheres, and the surrounding water solution are shown ricated at different ultrasonic intensity. A freshly prepared in Figure 5 A. A number of peaks observed for the silica mimicrosphere suspension contains fractions of large croshells within the range 400–2000 cm 1 can be assigned to ACHTUNGRE(>25 mm) as well as of small silica microspheres. During the first hours of aging, these microshells either form a foam the vibration and stretching modes of CTAB and LevasilH phase or are destroyed (approx. 30 % drop after 1 day of (an aqueous colloidal dispersion of amorphous silica) suraging). The rest of the microspheres remained in solution. face active components. The intensity of the broad band at However, the size of the microspheres fabricated at low ul3000–3600 cm 1, which corresponds to the vibration of the trasonic intensities is still large enough to form the foam OH groups of water, is considerably lower for silica microphase even on the sixth day of aging (see size distribution spheres as compared to the surrounding solution indicating diagrams in Figure S1). Only after 9 days of aging all samthe gaseous nature of the interior of the silica microspheres. ples achieve an appropriate size range (15–25 mm) forming a The gaseous core of the microspheres does not contribute
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Figure 4. A) Scanning electron microscopy (SEM) image of silica microspheres fabricated by 1 min sonication of a SiO2/CTAB mixture in the presence of 0.8 m NaHCO3 at 500 W ultrasound intensity after 9 days of aging. B) Transmission electron microscopy (TEM) image of an ultramicrotomed silica microsphere. Inset: structure of the silica shell at high magnification.
to the Raman signal at these wavenumbers and the small intensity of the peaks observed at 3000–3600 cm 1 is caused by the water molecules from the shell of the microspheres. Aging of the silica microspheres leads to the increase of the number of silica nanoparticles deposited on the wall, to an increase of the water content in the wall (Figure 5 A), as well as to their ability to entrap gas inside. Hence, both confocal permeability measurements (Figure 2 C) and Raman measurements (Figure 5 A) prove the gaseous core of the resulting nanostructured silica microshells. As seen in Figure 4 B, SiO2 nanoparticles composing the silica shell have a defined shape. To estimate the crystallinity of the ultrasonically treated SiO2 nanoparticles in the resulting microspheres, wide-angle X-ray analysis was performed (Figure 5 B). As seen from the X-ray diffraction (XRD) pattern, silica nanoparticles undergo partial crystallization (initial silica nanoparticles are amorphous) at the interface of the cavitation microbubble during ultrasonic treatment, thus forming a quartz[20] crystal phase of silica. Hence, the wall of the silica microspheres contains both initial amorphous silica nanoparticles, which are precipitated on the microsphere during aging, and partly crystalline quartz nanoparticles, which are formed at high temperature and pressure from the initial amorphous silica present inside small 2007, 3, No. 4, 665 – 671
Figure 5. A) Raman confocal microscopy spectra from 1) the suspension of fresh silica microspheres, 2) the suspension of aged silica microshells, and 3) the surrounding water solution. Signals between 400–2000 cm 1 correspond to the vibration and stretching modes of the CTAB and Levasil8 surface active components. B) XRD pattern of the hollow SiO2 microspheres. The arrows correspond to the patterns of the quartz crystal modification.
and at the gas/liquid interface of the cavitation microbubble during ultrasonic treatment of the aqueous CTAB/silica sol in the presence of NaHCO3. The very fast growth of the cavity and the essential hydrophobicity of the nanoparticles, which holds them strongly anchored at the interface, both lead to very high lateral pressures inside the SiO2 shells during the microsphere formation. This extreme condition together with high temperatures prevailing in the cavitation microbubble upon sonication can lead (at least, partially) to phase transformations of SiO2 in the shell. The possibility of phase transformations was predicted theoretically at even much smoother conditions[21] and later its real occurrence in surface-entrapped layers of silica nanoparticles was intensively discussed.[22] In general, the mechanism of formation of silica microspheres at the interface of the cavitation microbubble can be illustrated in three successive steps (Figure 6). During ultrasonic treatment of the CTAB/SiO2 aqueous sol in the presence of NaHCO3 initial gaseous nucleation centers rapidly expand forming micrometer-sized cavitation bubbles. CTAB molecules on the silica surface render the silica
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Figure 6. Schematic illustration of the mechanism of silica-microsphere fabrication at the interface of cavitation microbubbles as the reaction zone.
nanoparticles more hydrophobic, which promotes their anchoring at the gas/liquid interface thus forming preliminary silica shells around the gaseous nucleation core. This shell becomes denser during following microbubble expansion capturing additional SiO2 nanoparticles from the adjacent bulk solution. The adsorbed silica nanoparticles are exposed to the rapid heating/cooling cycles and high temperatures inherent in the cavitation microbubble.[10, 11] Such specific conditions at the cavitation interface result in partial crystallization of the amorphous silica nanoparticles producing the quartz phase and a high degree of interconnection between the silica nanoparticles in the microsphere shells. On the last stage, the separation of the polydisperse gas-filled silica microspheres is accomplished. The largest microspheres move to the top of the solution producing a foam phase while the smallest microspheres are destroyed dissolving their gaseous interior into the solution. The shells of the microspheres of medium size (20–25 mm) act as heterogeneous aggregation centers and the deposition of the silica nanoparticles from solution gradually occurs after sonication.
3. Conclusions Sonochemistry was effectively applied to prepare aqueous dispersions of air-filled nanostructured quartz silica shells from surface-engineered amorphous silica nanoparticles. The very high stability of the microbubbles against collapse and aggregation is determined by the properties of their surface multilayer (shell) made of hydrophobized silica nanoparticles. Because of the shell thickness and high density caused by sintering of the silica nanoparticles, the gas (liquid) permeability through the formed silica shell is practically limited prolonging the lifetime of the microbubbles. The demonstrated sonochemical approach can be used to prepare other types of hollow nanostructured materials from either surface-modified nanoparticles or metal precursors, which are difficult, if not impossible, to produce by other means. The non-equilibrium nature of the cavitation process and high temperature and pressure in the cavitation microbubble can lead to unique physicochemical parameters of the resulting microspheres. This may be expressed in highly sophisticated and hence high-value materials and also requires the investigation of the cavitation process in more detail in order to achieve control over the cavitation in the presence of surface-active materials. This will be a matter of future intense research.
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4. Experimental Section Materials: Levasil silica nanoparticles (average particle size 15 nm, specific surface area 200 m2 g 1, z-potential 56.9 mV) were purchased from HC Stark. NaHCO3, hexadecyltrimethylammonium bromide (CTAB), fluorescein isocyanate (FITC) were obtained from Aldrich and used without additional purification. The water used in all experiments was prepared in a three-stage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 1 MW cm. Ultrasonic treatment: Ultrasonic treatment was performed by a 500 W Sonics & Materials VCX500 ultrasonic processor operating at 20 kHz and equipped with a 1.3-cm diameter Ti horn. A cylindrical glass vessel (2.8-cm inner diameter) with a total volume of 20 mL was used for ultrasonic irradiation. The vessel was closed during sonication. Characterization: Confocal microscopy images of the silica microspheres in solution and statistical analyses were carried out on a Leica TCS SP scanning system equipped with a 60 B dry objective operating in the fluorescence and bright-field mode. For scanning electron microscopy (SEM) analysis a drop of each sample solution was applied to a glass wafer with sequential drying. Then the samples were sputtered with gold and measurements were conducted using a Gemini Leo 1550 instrument. Copper grids coated with carbon film were used to support the thin sections and a Zeiss EM 912 Omega transmission electron microscope operating at 300 kV was employed for transmission electron microscopy (TEM) analysis. The crystallinity of the silica shell was determined from wide-angle X-ray scattering (WAXS) with an Enraf–Nonius PDS-120. Raman spectra were recorded on a CRM200, Witec instrument.
Acknowledgements The work was supported by the EU project NANOCAPSULE (contract No. MIF1–2004–002642), by the European Space Agency (project FASES MAP AO-99–052), and by the MaxPlanck Society. The authors thank R. Pitschke and Dr. J. Hartmann for electron microscopy analysis, and Dr. Pantke for providing the Levasil silica colloid solution.
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