WO2007112356A2

WO2007112356A2 - Core-shell nanocapsules and applications thereof

Info

Publication number: WO2007112356A2
Application number: PCT/US2007/064941
Authority: WO
Inventors: Aleksa Vito Jovanovic; Randolph Stephen Duran
Original assignee: University Of Florida Research Foundation, Inc.
Priority date: 2006-03-24
Filing date: 2007-03-26
Publication date: 2007-10-04
Also published as: WO2007112356A3

Abstract

Core-shell nanocapsules containing a silica shell that is generally porous surrounds an oil core. A method to prepare the nanocapsules by preparing an oil-in-water-in-oil emulsion and carrying out a sol-gel synthesis of the silica from a tetraalkoxysilane in the water phase of the double micelle is described. The central oil core is prepared by the formation of a stabilized oil-in-water emulsion where an alkyltrialkoxysilane is included with the emulsion. The stabilized oil-in-water emulsion is then used as the aqueous phase to prepare the oil-in- water-in-oil emulsion. The nanocapsules can be used for drug detoxification applications.

Description

CORE-SHELL NANOCAPSULES AND APPLICATIONS THEREOF

FIELD OF THE INVENTION

[0001] The invention relates to core-shell nanocapsules, methods of preparation, and applications thereof.

BACKGROUND OF THE INVENTION

[0002] The compartmentalized sub-micron particles to provide an environment for a guest molecule have a variety of applications in the biomedical field. A nanoparticle that has a polar surface is of particular interest as the polar surface allows the use of the nanoparticle

in an aqueous systems. When the interior of these nanoparticles is hydrophobic, the particles

are well suited to application as carriers for drugs, fluorescent labels and reactive molecules for use in systems for drug delivery, diagnostics, and nanoreactors. Nanoparticles where the polar surface is that of silica is particularly attractive for a number of such biological applications as the shell is relatively non-toxic nor in other ways adverse to biological environments. The preparations of such core-shell particles, where a hydrophilic shell surrounds a hydrophobic core have been have been approached in a number of ways. One of the most common approaches is via block copolymers that form unimolecular micelles in solution due to the differing polarities of the blocks. The stability of the aggregates can then be realized by the cross-linking of the core or the shell. Another approach that has been pursued is the layer by layer deposition of oppositely charge macromolecules on the surface

of charged particles. A third approach to such core shell particles is where polymeric cores are synthesized in microemulsion droplets, followed by the synthesis of a shell on the surface. [0003] Microemulsions are thermodynamically stable dispersions of one liquid phase into another, stabilized by an interfacial film of surfactant. The interfacial tension between the two phases is generally extremely low due to the use of one or more surfactants. This dispersion may be either oil-in-water or water-in-oil. Small microemulsions, or nanoemulsions, are typically clear solutions, as the droplet diameter is approximately 100 nanometers or less. Micro and nanoemulsions have recently been used for the preparation of core shell particles with a ceramic shell via sol-gel methods on a template. For example, Hentze et al. Langmuir 2003, 19, 1068-74 discloses the formation of vesicle templates in water from a combination of surfactants and the formation of silica via a sol-gel process employing tetramethoxysilane (TMOS) where the condensation occurs at the bilayer of the unilamellar vesicle of an oil-in-water emulsion. This system is very sensitive to the ratio of

TMOS to surfactant and time and yields silica structures that range from small hollow spheres through giant vesicles through multilaminer structures to gels with variations in ratios and time.

[0004] Another approach to a core-shell silica particle is disclosed in Underhill et al.

Chem. Mater. 2002, 14, (12), 4919-25, this involves the mixing of a non-ionic surfactant and octyltrimethoxysilane (OTMS) with a small organic molecule, such as a fluorescent dye, or an ester with water to form an oil-in-water emulsion. The OTMS hydrolyzes and subsequently condenses to fix the geometry of the nanoparticle templates. Finally, a silica network is synthesized on the surface of the templates upon introduction of TMOS, where

TMOS is hydrolyzed and condensed with the free silanol groups of the template which

anchors the silica to the template. This gives a silica shell surrounding an organic core. Some condensation occurs in the aqueous phase and such systems are susceptible to droplet destabilization by changes of pH and temperature. Purification must be carried out before all TMOS is consumed and its removal must be carried out by dialysis. Outside of an acceptible window of time for removal of TMOS, a gel can form.

[0005] Hence there remains a need to prepare core-shell micro- and nanoparticles with silica shells. It is also desirable to use such particles to form microemulsions for use in biological applications, particularly where the particle sizes are sufficiently small such that the microemulsions are optically clear.

BRIEF DESCRIPTION QF THE DRAWINGS

[0006] A fuller understanding of the present invention and the features and benefits thereof will be accomplished upon review of the following detailed description together with the accompanying drawings, in which:

[0007] Fig. 1 shows a scheme for the preparation of oil filled or a hollow silica shell core-shell nanocapsules via an oil-in-water-in-oil (0/W/O) emulsion. [0008] Fig. 2 shows a stylized reaction scheme for the formation of core-shell

nanocapsules with a silica shell using a water-in oil (W/O) emulsion. [0009] Fig. 3 shows scanned images of a series of oil filled core-shell nanocapsules prepared by the 0/W/O formulation using the non-ionic surfactant Triton X-100 (top three rows) and a combination of Triton X-100 and the ionic surfactant Aerosol-OT by transmission electron microscopy.

[0010] Fig. 4 shows scanned images of an oil filled core-shell nanocapsules of Fig. 4

(top) and the resulting hollow silica nanocapsule after thermolysis as followed by thermogravimetric analysis (bottom) by transmission electron microscopy.

SUMMARY OF THE INVENTION

[00 U] A method to prepare core-shell nanocapsules with a silica shell involves: combining one or more surfactants, one or more oils, one or more alkyltrialkoxysilanes, and an aqueous phase; agitating the combination; adding an acid to promote hydrolysis and condensing the alkoxysilane groups of the alkyltrialkoxysilanes; neutralizing the acid wherein a stabilized oil-in-water emulsion forms. The oil-in-water emulsion is combined with one or

more surfactants and one or more oils; and the combination is agitated wherein an oil-in- water-in-oil emulsion forms. A tetraalkoxysilane is added to the emulsion, wherein the

alkoxysilane groups hydrolyze and condense in the vicinity of the oil and water interface to form a silica shell yielding stable core-shell nanocapsules. The surfactant can be a nonionic surfactant, an ionic surfactant, a zwitterionic surfactant or any combination of surfactants. The alkyltrialkoxysilane can be an alkyltrimethoxysilane or an alkyltriethoxysilane. The alkyl group can be a Cg to C₃₀ alkyl group. The oil can be a C₆ to C₃₀ hydrocarbon or C₆ to

C_3O carboxylic acid ester. The aqueous phase can be water or a salt solution. The tetraalkoxysilane can be tetramethoxysilane or tetraethoxysilane.

[0012] A core-shell nanocapsule has a silica shell, a core containing an oil solution, or is essentially hollow, and has a diameter of about 20 to about 800 tun. The shell is generally porous, wherein porous as used herein is defined as porosity sufficient to permit the transport of molecular species, such as draft in or out of the core, through the pores of the shell. [0013] A method of removing a drug from the bloodstream of a mammal includes: providing a suspension of core-shell nanocapsules of about 20 to about 800 nm in diameter with a porous silica shell and an oil solution core in an aqueous vehicle; and injecting the suspension into the bloodstream, where the drug is absorbed into the cores of the nanocapsules. The aqueous vehicle can be an injectable saline solution.

DETAILED DESCRIPTION OF THE INVENTION

[0014] Core-shell micro- and nano-particles can be prepared with silica shells by the use of an oil-in-water-in-oil (0/W/O) emulsion. The silica shell forms on a template constructed at an interface between a water and an oil phase by the hydrolysis and condensation of a trialkoxysilane with silanol groups present in a structured surfactant defined interface resulting from an alkyltrialkoxysilane. The inner most phase can contain an

agent for use in biological or other applications. When that agent is oil soluble an 0/W/O emulsion is preferred and where a water soluble or suspendable agent is used a W/O emulsion is preferred. In both cases the resulting core-shell nanocapsules exhibit hydrophobic tails derived from the surfactants and the alkyltrialkoxysilane being bound to the

external surface of a silica shell around a core that can be filled with an aqueous solution, an oil solution, or can be hollow after a heat treatment.

[0015] An OAV YO preparation according to the invention is outlined in Fig. 1. The synthesis begins with the formation of an oil in water (OfW) emulsion. As illustrated in Fig.

2, an exemplary oil and surfactant composition where an oil, ethyl butyrate, 2, is combined

with a nonionic surfactant, TWEEN-80™, 3, a zwitterionic surfactant, lecithin, 4, and an alkyltrialkoxysilane, octadecyltrimethoxysilane, 1, to give a emulsion when mixed with water. Upon subsequent acidification, hydrolysis of the methoxy groups and partial condensation of the resulting silanol groups results in the formation of a stabilized nanocapsule with an oil core. More oil, surfactants, and alkyltrialkoxysilanes can be added to the OW emulsions to form the 0/W/O emulsion as illustrated in the first step shown in Fig. 1. Any excess surfactant used for the formation of the OAV emulsion will partition into the

oil phase. A tetraalkoxysilane, such as tetraethoxysilane (TEOS), can then be added as in the second step shown in Fig. 1 along with a catalyst to promote the tetraalkoxysilane's

hydrolysis and condensation to form silica nanocapsules where the water of the central phase is partially or fully consumed by the tetraalkoxysilane. In general, water will remain, as levels of tetralkoxysilanes needed to consume all of the water will yield sufficient alcohol to destabilize the emulsion during formation of the nanocapsules. The resulting core-shell nanocapsules can be easily purified as the excess surfactant and the liberated ethanol can be readily separated from the nanoparticles by centrifugation and redispersing in water. This ease of purification is an additional advantage over OAV systems where isolation of the nanocapsules generally requires dialysis. [0016] Many different oils can be used separately or in combination during the preparation of the oil core. The oil may be a simple hydrocarbon such as hexadecane or an ester such as ethyl butyrate. The oil can be a silicone or any liquid non-polar compound that readily separates with an aqueous solution. The oils can contain 8 to more than 30 carbons. The oil can have a wide rage of viscosities including that of lower molecular weight oils such as ethyl butyrate and higher molecular weight oils such as ϊsopropyl myristate. Many different surfactants can be used. In addition to TWEEN-80™, ρoly(oxyethylene)[20]-sorbitan

monooleate, other non-ionic surfactants that can be used include TRITON X- 100™,

polyoxyethylene octyl phenyl ether, and other polyether based surfactants. An ionic

surfactant such as sodium octanoate and AEROSOL-OT™, sodium bis(2- ethyl- 1-

hexyl)sulfosuccinate, can be used in addition to or in place of the non-ionic surfactant. A zwitterionic surfactant such as lecithin can be included with the surfactants. Alkyltrialkoxysilanes can include those with alkyl group of 8 to about 20 carbons, and various alkoxy groups although methoxy and ethoxy groups are preferred. [0017] The size of the emulsion particles and the resulting core-shell nanocapsules can be controlled by the formulation parameters. The parameters include the surfactant to water ratio, the surfactant to oil ratio, the choice of the oil, and the choice of surfactant. For example the surfactant to water phase ratio (SAV) can be 1 to 5 but is preferably from 1.5 to

3, and most preferably about 2.4. The surfactant is used at about 5 to about 20 weight percent

of the oil of the final emulsion. The tetraalkoxysilane is generally used at about 5 to 25 weight percent of the water. Although larger amounts of the tetraalkoxysilane can be used relative to the amount of water, the alcohol generated upon hydrolysis can in some cases destabilize the emulsion at higher levels. Typically the components are combined with mixing. The mode of mixing employed can include stirring, shaking, forcing the suspension through a static inline mixer or a high sheer mixer and sonication. Mixing can be carried out over a variety of temperatures within the range of stabilities of the chemicals employed and

the physical limitation of the emulsions formed. Generally, the temperature range will vary from normal room temperatures to less than 100⁰C.

[0018] As is typical of silica prepared by sol-gel methods, the nanocapsules contain pores in the silica shell through which molecules can diffuse in or out of the pores as required for use and as driven by the physical and chemical gradients imposed by the environment to

which the nanoparticles are exposed. This permits the use of these nanocapsules for a host of applications- A simple oil filled core with a silica shell can be used for drug detoxification. For example, oil filled silica nanocapsules, having a diameter of less than 800 nm, can be injected into the bloodstream and can absorb an oil soluble drug. By doing so it is possible to reduce the level of an ingested drug in the bloodstream to a non-toxic level. [0019] Silica nanocapsules prepared via 0/W/O emulsions can include various organic compounds for use as imaging or therapeutic particles. Such nanoencapsulated compounds can be sensitive to the environment to which the nanocapsules are exposed and interact. Such an interaction can yield a targeted release, an optical change, etc. [0020] As can be seen in the scanned images shown in Fig. 3, in one series of experiments, core-shell nanocapsules prepared by a sol-gel reaction in an 0/W/O system using the surfactant TRITON X- 100™ and ethyl butyrate with a relatively high octadecyltrimethoxysilane concentration yielded core-shell particles that are approximately 140 nm in diameter with shells that are about 45 nm in thickness. The addition of an ionic surfactant, AOT, resulted in a silica surface that was less smooth than those excluding the ionic surfactant, but yielded nanocapsules of similar dimensions. As can be seen in the scanned images shown in Fig. 4, thermolysis during thermogravimetric analysis demonstrated that hollow nanocapsules can be produced, where the dimensions of the resulting silica shells remains approximately the same as that of the oil filled nanocapsules from which they are formed.

[0021] Features of preparing examples and practicing the method for some embodiments of the invention are presented in non-limiting examples and are provided for illustration purposes and do not encompass the entire scope of the invention.

Example

[0022] This example describes the preparation of a core-shell nanocapsule with an oil

core and a silica shell using a 0/W/O emulsion. The surfactant TWEEN-80™ (2.8g) and, the

oils Ethyl Butyrate (026g) and 1-dodecene (0.03 g) were added to filtered water or saline (0.9 wt.% NaCl in water). This mixture was stirred at 70 ⁰C for up to 15 minutes. Upon cooling to room temperature this emulsion clarified. Octadecyltrimethoxysilane (0.15g) was added and the mixture stirred at 70 ⁰C for 3 to 10 hours. Upon cooling to room temperature the

mixture clarified. In a first step, the hydrolysis of octadecyltrimethoxysilane was induced by

the addition of 0.5g of 0.5M HCl to lower the pH to about 3. After 30 minutes of stirring at room temperature, the solution pH was raised to 7.4 by the slow addition of 0.5M NaOH where in general 0.5-0.6 g of base solution is used. This oil-in-water emulsion was found to be stable for months at standard lab conditions.

[0023] In a second step, the surfactant TRITON X-100™ (4.82g) and co-surfactant hexanol (4.36mL) were added to 20.45 mL of Hexane as the oil phase to form a solution. The oil-in-water emulsion from the first step, (2 mL) was added to surfactant/oil solution and a clear bluish homogeneous emulsion resulted on agitation. To this emulsion was added

tetraethoxysilane (560 μL) with stirring. The mixture quickly became homogeneous, 40 μL

OfNH₃ was added to catalyze sol-gel reaction, and the mixture stirred at room temperature overnight. Ethanol was added dropwise to the slightly opaque mixture until phase separation occurred and a white precipitate formed on the bottom of the vessel. The upper liquid phase was carefully decanted and the lower phase was centrifuged for 15 minutes at 4000-10000 rpm. The supernatant was carefully decanted and the nanocapsules were resuspended in a water/ethanol mixture (50:50 v/v). Centrifugation was repeated, and the supernatant was subsequently decanted. The nanocapsules were resuspended in pure water, centrifuged, and the supernatant decanted from the nanocapsules, and the suspension, centrifugation and decantation repeated. Finally, the nanocapsules with a diameter of 99 run were resuspended

in 6 mL of water and stored at room temperature.

[0024] It is to be understood that while the invention has been described in conjunction with the preferred specific embodiments thereof, that the foregoing description as

well as the examples which follow are intended to illustrate and not limit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains.

Claims

1. A method to prepare core-shell nanocapsules with a silica shell, comprising: combining one or more first surfactants, one or more first oils, one or more alkyltrialkoxysilanes, and an aqueous phase to form a combination; agitating said combination; adding an acid to promote hydrolysis and condensation of the alkoxysilane groups of said alkyltrialkoxysilane; neutralizing said acid wherein a stabilized oil-in- water emulsion forms; combining said oil-in-water emulsion with one or more second surfactants and

one or more second oils; agitating said combination wherein an oil-in-water-in-oil emulsion forms; and

adding a tetraalkoxysilane to said emulsion, wherein said alkoxysilane hydrolyzes and condenses in the vicinity of an oil and water interface to form a silica shell to yield stable core-shell nanocapsules.

2. The method of claim 1, wherein said first and second surfactant are selected from the group consisting of nonionic surfactants, ionic surfactants, zwitterionic surfactants and

combinations thereof.

3. The method of claim 1, wherein said alkyltrialkoxysilane is an alkyltrimethoxysilane or an alkyltriethoxysilane.

4. The method of claim 3, wherein said alkyl group is a C₈ to C₃o alkyl.

5. The method of claim 1 , wherein said first and said second oil are selected from the group consisting of C₆ to C₃₀ hydrocarbons and C₆ to C₃0 carboxylic acid esters.

6. The method of claim 1, wherein said aqueous phase is water or a salt solution.

7. The method of claim 1 , wherein said tetraalkoxysilane is tetramethoxysilane or

tetraethoxysilane.

8. A core-shell nanocapsule, comprising: a silica shell; and a core comprising an oil solution inside said shell.

9. The nanocapsule of Claim 8, wherein said nanocapsules are about 20 to about 800 nm

in diameter.

10. The nanocapsule of Claim 8, wherein said shell is porous to at least one drug.