WO2013030240A1

WO2013030240A1 - Phase transfer reactions

Info

Publication number: WO2013030240A1
Application number: PCT/EP2012/066789
Authority: WO
Inventors: Iouri Gounko; Gemma-Louise DAVIES; Stephen Byrne
Original assignee: The Provost, Fellows, Foundation Scholars, And The Other Members Of Board, Of The College Of The Holy And Undivided Trinity Of Queen Elizabeth Near Dublin
Priority date: 2011-08-29
Filing date: 2012-08-29
Publication date: 2013-03-07

Abstract

Phase Transfer Catalysis (PTC) is one of the most efficient and versatile technologies, which is used in organic synthesis in industry and in research laboratories. PTC is widely used for the production of surfactants, polymers, flavours & fragrances, petrochemicals, agricultural chemicals, dyes, pharmaceuticals and explosives. The advantages of PTC include simple experimental operations, mild reaction conditions, use of inexpensive and environmentally friendly reagents and solvents, easy scalability. Disclosed herein is an inventive method for initiating phase transfer reactions by the application of a magnetic field. Also disclosed herein is an inventive method for the preparation of stable coated nanowire structures utilising magnetically induced phase transfer catalysis and/or condensation reactions.

Description

Title

Phase Transfer Reactions Field of the Invention

[0001] The invention relates to new approaches in phase-transfer reactions (PTR) and phase- transfer catalysis (PTC) using magnetic particles. Disclosed herein is an inventive method for initiating phase transfer reactions. In particular, the present invention provides for phase transfer reactions initiated by the application of a magnetic field. Also disclosed herein is an inventive method for the preparation of stable nanowire structures utilising magnetically induced phase transfer catalysis and/or condensation reactions.

Background to the Invention

[0002] PTC is one of the most efficient and versatile technologies, which is used in organic synthesis, both in industry and in research laboratories. PTC is widely used for the production of surfactants, polymers, flavours & fragrances, petrochemicals, agricultural chemicals, dyes, pharmaceuticals and explosives. The advantages of PTC include simple experimental operations, mild reaction conditions, use of inexpensive and environmentally friendly reagents and solvents, easy scalability.

[0003] Common PTC approaches involve the utilisation of quaternized ammonium or phosphonium based salts (e.g. tetramethylammonium chloride or tetrabutylphosphonium bromide) as catalysts. In this case, for example, a quaternary ammonium cation forms an ion pair with an anionic reactant, which equilibrates between an organic and an aqueous phase.

[0004] U.S. Patent No. 3992432 discloses a process for catalyzing heterogeneous ionic organic reactions in a system of multiple liquid phases in which at least two of the reactants (differing in polarity) are each located in a different phase with respect to the other. U.S. Patent No.

4460778 reports branched alkyldiorganoaminopyridinium salts used as phase transfer catalysts for facilitating nucleophilic aromatic substitution. U.S. Patent No. 4694104 describes polyoxyalkylene quaternized salts as phase transfer catalysts for liquid/liquid phase

cyclopropanation, substitution or addition reactions involving a normally liquid or solvent soluble olefinically unsaturated hydrocarbon and the process in which the quaternary compound is used. /V-secondary-alkyldiorganoaminopyridinium salts have also been disclosed as phase transfer catalysts for facilitating the preparation of fluoroaromatic compounds from the corresponding chloroaromatic compounds in U.S. Patent No. 4973771 . Similarly, U.S. Patent No. 4694104 claims a process for forming a siloxane oligomer from a mixture comprising at least one alkoxysilane, at least one phase transfer catalyst having a specified structure and water is described. [0005] The extraction of nanoparticles into different phases of bi-phasic systems has also been the subject matter of a number of patent applications. For example, U.S. Patent Application Publication No. 2005/0165120 relates to a process for the extraction of nanoparticles into an aqueous phase by complexation with water soluble surfactants such as cetyltrimethyl ammonium bromide (CTAB). A bi-phasic mixture is prepared in which hydrophobized nanoparticles are in an organic medium and surfactant molecules are in an aqueous phase. An emulsion is formed on vigorous shaking of the bi-phasic mixture and after phase separation, metal nanoparticles transfer into the aqueous medium.

[0006] European Patent No. 1337219 describes phase transfer catalysis of nanoparticles from a first phase to a second phase using DMAP based phase transfer catalysts. U.S. Patent Application Publication No. 2010/0215851 claims the production of core/shell composite nanoparticles by initially dispersing core particles (heat treated in advance to give them a crystal structure expressing the necessary characteristics) in an organic solvent by a first dispersant to prepare a first solution, adding a polar solvent to peel off the first dispersant from the core particles and making the nanoparticles agglomerate, allowing recovery. Next, the recovered core particles are dispersed in a second organic solvent by a second dispersant to form a second solution, and addition of a precursor of the shells to the second solution forms shells on the surfaces of the core particles.

[0007] Notwithstanding the state of the art there remains a need for alternative trans phase materials capable of initiating or catalysing a reaction at the interface of immiscible phases.

Summary of the Invention

[0008] The present invention utilises magnetic particles as trans-phase catalyst carriers and/or reagent carriers. The application of an external magnetic field is utilised to initiate or trigger the trans-phase reaction or process because the external magnetic field causes migration of the magnetic particles within a multi-phasic system. The external magnetic field can cause trans- phase migration of the magnetic particles (comprising or consisting of a catalyst or other reagent) toward the magnetic field source resulting in the initiation of a reaction in one of the phases.

[0009] Moreover, the present invention further provides for a method of producing coated magnetic micro- and nano-structures from nanoparticles by using an external magnetic field to transfer particles from a first phase in to a second phase and coat them in-situ using a one-step magnetic trans-phase delivery technique. The present invention also allows magnetically triggered reaction initiation or catalysis (e.g. polymerisation, condensation, etc.) at

aqueous/organic interfaces. [0010] Accordingly, in a first aspect the present invention provides for a method of promoting a reaction in a reaction system comprising at least a first phase and a second phase, wherein the first and second phases are immiscible, the method comprising:

providing at least one magnetically responsive particle in one of the first or second phases, wherein said particle consists of or comprises a reaction promoting component; and

applying a magnetic field such that the magnetically responsive particle migrates from one of the first or the second phase into the other of the first or second phase so that the reaction promoting component can promote a reaction therein.

[0011] As used herein, the term magnetically responsive particle refers to a particle that is attracted to a magnetic field. The magnetic field is provided by a magnetic field source, for example a magnet. In particular, the application of a magnetic field may cause the magnetically responsive particle to migrate towards the magnetic field source.

[0012] The construction of the term particle shall not be limited to material consisting of a single particle. It also covers particulate materials comprised of a plurality of particles and particle aggregates.

[0013] Within this specification it is recited that the magnetically responsive particle may consist of or comprise a reaction promoting component. This statement shall be construed as simply meaning that the magnetically responsive particle may be entirely composed of a material that is also a reaction promoting component, or that the magnetically responsive particle may be part composed of reaction promoting material or may have a reaction promoting component attached to, or adsorbed on the particle.

[0014] As used herein, the term "promote a reaction therein" should be construed as covering direct initiation of a reaction, indirect initiation of a reaction, or enhancement/catalysis of the rate of an already occurring reaction. For example, when the magnetically responsive particle migrates from one of the first or the second phase into the other of the first or second phase it may:

directly initiate a reaction in the other of the first or second phase;

react with a reagent in the other of the first or second phase to liberate a reaction initiating species (which in turn initiates the reaction), thereby indirectly initiating a reaction; or increase the rate of a pre-existing reaction.

[0015] In one embodiment, the reaction may comprise forming a coating on the surface of the magnetically responsive particle.

[0016] Advantageously, the coatings can be functionalised to modify the properties of the magnetic materials. For example, the coatings can be modified to include luminescent moieties (e.g. organic dyes or semiconductor quantum dots), radiolabeled moieties, a drug moiety, an electrochromic moiety and combinations thereof. [0017] Advantageously, the method of the present invention allows for clean and uncomplicated magnetic-based initiation of phase transfer reactions and processes without the use of any complex reagents and expensive systems. Moreover, the present invention provides for a quick, efficient and versatile method by which phase transfer reactions can be initiated.

[0018] With reference to the method of the present invention the magnetically responsive particle may be selected from the group consisting of a magnetic nanoparticle, a magnetic nanorod, a magnetic nanowire, a magnetic nanoworm, a magnetic nanotube, a magnetic nanoflake and combinations thereof.

[0019] The magnetically responsive particle may comprise a magnetic metal, a magnetic metal oxide, a magnetic metal alloy, a magnetic spinel, or combinations thereof. For example, the magnetically responsive particle may comprise iron, iron oxides, iron alloys, iron-based spinels, cobalt, cobalt oxides, cobalt alloys, nickel, nickel oxides, nickel alloys, chromium oxides, chromium-based spinels, and combinations thereof. The magnetically responsive particle may be selected from iron metal, iron oxides, cobalt ferrite, nickel ferrite, magnetic metal alloys such as CoPt, CoNi, FePt, FeAu, FeRu, and combinations thereof.

[0020] The present invention extends to a reaction system in which there are a plurality of immiscible phases. In this case, references to first and second phases simply refer to any two phases in said plurality of immiscible phases. In particular, the reaction system may be a biphasic system, i.e. consisting of two immiscible phases.

[0021] With reference to the method of the present invention, both the first and second phases may be liquid. The first and second liquid phases may form layers. The first and second liquid phases may form an emulsion or a microemulsion.

[0022] In one particular embodiment, one of the first and second phases is aqueous and the other of the first and second phases is organic.

[0023] The magnetically responsive particle may be dispersed in or held in suspension in one of the first or second phases. This may be achieved with surfactants or other dispersant molecules, or by surface modification/functionalisation of the magnetically responsive particles.

[0024] The magnetically responsive particle may be dissolved in solution in one of the first or second phases. The application of the magnetic field may cause migration of the magnetically responsive particle into a phase in which it is not soluble, whereupon it may precipitate out of solution.

[0025] The reaction promoting component may be a catalyst. The catalyst may cause direct initiation of a reaction, indirect initiation of a reaction, or increase of the rate of an already occurring reaction.

[0026] The reaction promoting component may be attached to or adsorbed on the surface of the magnetically responsive particle. The reaction promoting component may be physically or chemically adsorbed on the surface of the magnetically responsive particle. [0027] The present invention further provides for a container having a reaction system contained therein,

the reaction system comprising at least a first phase and a second phase, the first and second phases being immiscible,

at least one magnetically responsive particle is disposed in one of the first or second phases, the particle consisting of or comprising a reaction promoting component, and

a reagent disposed in the other of the first or second phases,

wherein the application of a magnetic field causes the magnetically responsive particle (and the reaction promoting component) to migrate into the other of the first or second phases such that the reaction promoting component can react with the reagent disposed in the other of the first or second phases.

[0028] The present invention also provides for a kit comprising:

i) a magnetic field source; and

ϋ) a container according to the present invention.

[0029] The magnetic field source may be a permanent magnet. The magnetic field source may be a variable strength magnet. The magnetic field source may be an alternating magnetic field. The magnetically responsive particle may be selected from the group consisting of a magnetic nanoparticle, a magnetic nanorod, a magnetic nanowire, a magnetic nanoworm, a magnetic nanotube, a magnetic nanoflake and combinations thereof.

[0030] The magnetically responsive particle may comprise a magnetic metal, a magnetic metal oxide, a magnetic metal alloy, a magnetic spinel, or combinations thereof. For example, the magnetically responsive particle may comprise iron, iron oxides, iron alloys, iron-based spinels, cobalt, cobalt oxides, cobalt alloys, nickel, nickel oxides, nickel alloys, chromium oxides, chromium-based spinels, and combinations thereof. The magnetically responsive particle may be selected from iron metal, iron oxides, cobalt ferrite, nickel ferrite, magnetic metal alloys such as CoPt, CoNi, FePt, FeAu, FeRu, and combinations thereof.

[0031] The kit extends to a reaction system in which there are a plurality of immiscible phases. In this case, references to first and second phases simply refer to any two phases in said plurality of immiscible phases. In particular, the reaction system may be a biphasic system, i.e. consisting of two immiscible phases. Both the first and second phases may be liquid. The first and second liquid phases may form layers. The first and second liquid phases may form an emulsion or a microemulsion. In one particular embodiment, one of the first and second phases is aqueous and the other of the first and second phases is organic.

[0032] The magnetically responsive particle may be dispersed in or held in suspension in one of the first or second phases. This may be achieved with surfactants or other dispersant molecules, or by surface modification/functionalisation of the magnetically responsive particles. The magnetically responsive particle may be dissolved in solution in one of the first or second phases. The application of the magnetic field may cause migration of the magnetically responsive particle into a phase in which it is not soluble, whereupon it may precipitate out of solution.

[0033] The reaction promoting component may be a catalyst. The catalyst may cause direct initiation of a reaction, indirect initiation of a reaction, or increase of the rate of an already occurring reaction. The reaction promoting component may be attached to or adsorbed on the surface of the magnetically responsive particle. The reaction promoting component may be physically or chemically adsorbed on the surface of the magnetically responsive particle.

[0034] In a further aspect the present invention provides for use of a magnetically responsive particle consisting of, or comprising a reaction promoting component in a reaction system comprising at least two immiscible phases, wherein the application of a magnetic field causes the magnetically responsive particle to migrate from one the phases into the other of phases so that the reaction promoting component can promote a reaction therein.

[0035] The invention also relates to a method of coating a magnetically responsive material, the method comprising the steps of:

providing a first phase having the magnetically responsive material and a coating promoting agent contained therein;

providing a second phase having a coating precursor contained therein, wherein the first and second phases are immiscible; and

applying a magnetic field such that the magnetically responsive material migrates from the first phase into the second phase, wherein coating promoting agent associated with the magnetically responsive material reacts with the coating precursor so as to form a coating on the magnetically responsive material.

[0036] As used herein, the term "coating promoting agent associated with the magnetically responsive material" shall be construed as a coating promoting agent that is attached to or adsorbed on the surface of the magnetically responsive material and also covers the case in which the coating promoting agent and the magnetically responsive material are the same material.

[0037] For example, the coating promoting agent may be:

i) the magnetically responsive material (i.e., the magnetically responsive material is also the coating promoting agent); or

ii) attached to or adsorbed on the surface of the magnetically responsive particle.

[0038] Advantageously, the coating method of the present invention offers a quick, efficient and versatile approach by which magnetic materials can be coated. The coating method of the present invention can be utilised with a variety of types of magnetic micro- and nano- objects and a host of different coating materials. The coating method of the present invention can provide for a wide range of coated materials, which can be further manipulated or modified after their initial preparation. Furthermore, the coating method is a one-step, one-pot, clean procedure, which allows straight-forward preparation of coated magnetic (nano)materials. Moreover, the coated (nano)materials can be recovered in high yields.

[0039] With reference to the coating method of the present invention, the magnetically responsive material may be selected from the group consisting of a magnetic nanoparticle, a magnetic nanorod, a magnetic nanowire, a magnetic nanoworm, a magnetic nanotube, a magnetic nanoflake and combinations thereof.

[0040] The magnetically responsive material may migrate into the second phase as a linear assembly so as to provide coated linear magnetic nanostructures. In particular, the

magnetically responsive material may migrate into the second phase as a linear assembly so as to provide coated magnetic nanowires.

[0041] Linear nanostructures may be more desirable because they are believed to exhibit superior performance than non-linear nanostructures in biomedical applications on account of their increased aspect ratio.

[0042] Advantageously, coated linear magnetic nanowires prepared by the present invention retain their linear shape even when the magnetic field has been removed. The coating keeps the linear shape. Further advantageously, coated linear magnetic nanowires prepared by the present invention are flexible and are responsive to an external magnetic field after formation, i.e. they can still be attracted by an external magnetic field.

[0043] The coating precursor may be selected from the group consisting of metal alkoxides, silicon alkoxides, siloxanes, C₁-C₁₀₀ polymerisable monomers, acetylacetonates, amides, and combinations thereof.

[0044] In one embodiment, the coating promoting agent may be selected from the group consisting of a basic solution, a polymerisation catalyst or a condensation catalyst. The basic solution may be aqueous. The basic solution may comprise a hydroxide salt. The basic solution may be selected from the group consisting of aqueous NH₄OH, aqueous NaOH, and aqueous KOH.

[0045] The magnetically responsive materials, such as magnetic nanoparticles, magnetic nanorods, magnetic nanowires, magnetic nanoworms and magnetic nanotubes can be coated with a variety of different coating materials. Advantageously, the coatings can be functionalised to modify the properties of the magnetic materials. For example, the coatings (or coating precursor) can be modified to include luminescent moieties (e.g. organic dyes or semiconductor quantum dots), radiolabeled moieties, a drug moiety, an electrochromic moiety and

combinations thereof. In particular, the coatings (or coating precursor) may be modified to include drug molecules and the coated magnetic nanoparticle may act as a drug delivery vehicle. [0046] The present invention further provides for a coated magnetically responsive material obtainable by the method of the present invention.

[0047] In a further aspect the present invention provides for a method of breaking an emulsion of immiscible liquids, the emulsion comprising a first liquid phase and a second liquid phase dispersed in the first liquid phase, the method comprising the steps of:

providing a magnetically responsive material in the (dispersed) second liquid phase of the emulsion; and

applying a magnetic field such that the magnetically responsive material migrates from the second phase into the first phase thereby destabilising or breaking the emulsion.

[0048] The magnetically responsive material may be held in solution or in suspension in the

(dispersed) second liquid phase.

[0049] In one particular embodiment, one of the first and second phases is aqueous and the other of the first and second phases is organic. The magnetically responsive material may be selected from the group consisting of a magnetic nanoparticle, a magnetic nanorod, a magnetic nanowire, a magnetic nanoworm, a magnetic nanotube, and combinations thereof.

[0050] Where suitable, it will be appreciated that all optional and/or preferred features of one embodiment of the invention may be combined with optional and/or preferred features of another/other embodiment(s) of the invention.

Brief Description of the Drawings

[0051] Additional features and advantages of the present invention are described in, and will be apparent from, the detailed description of the invention and from the drawings in which:

[0052] Figure 1 illustrates the preparation of coated nanowires utilising the phase transfer method of the present invention;

[0053] Figure 2 illustrates silica coated cobalt ferrite nanoparticles prepared using the phase transfer method of the present invention;

[0054] Figure 3 illustrates titania coated cobalt ferrite nanoparticles prepared using the phase transfer method of the present invention; and

[0055] Figure 4 provides Transmission Electron Microscopy (TEM) images of coated nanowires prepared using the phase transfer chemistry of the present invention.

Detailed Description of the Drawings

[0056] It should be readily apparent to one of ordinary skill in the art that the examples disclosed herein below represent generalised examples only, and that other arrangements and methods capable of reproducing the invention are possible and are embraced by the present invention. Application of Phase Transfer Chemistry Induced by a Magnetic Field in the Production of Coated Nanowires

[0057] The preparation of the nanowires was as follows, and a pictorial representation of the method is illustrated in Figure 1. Stabilised magnetic nanoparticles in aqueous solutions 101 were dispersed in a water/ethanol and ammonium hydroxide (NH₄OH) solution by manual shaking (referred to as the 'magnetic solution'). A solution of dichloromethane (DCM) and silicon or titanium alkoxide precursor (tetraethylorthosilicate [TEOS] or titanium isopropoxide) was prepared in a beaker 102. The magnetic solution 101 was then carefully transferred on top of the DCM/precursor solution 102 in order to form a bi-layer. The reaction vessel was then placed on top of a magnet for several hours; until brown magnetic material collected at the bottom of the beaker near the magnetic surface.

[0058] The mechanism of formation of these nanowire structures relies on the presence of an external magnetic field to pull the magnetic nanoparticles with an interstatically-attached surface layer of aqueous ammonium hydroxide catalyst. The magnet attracts the magnetic nanoparticle towards it, via the liquid interface 103 in the beaker. As the nanoparticles move to the interface, they begin to form into a 1 -dimensional (1 -D) array of nanoparticles 104. As this 1 -D array is pulled towards the magnet, a reaction is initiated by the catalyst at the surface of the aligned nanoparticles and causes them to be coated with a thin layer of oxide 105 (from the silica or titania precursor). These 1 -D arrays then collect at the bottom of the beaker next to the magnet. The presence of the oxide shell retains the nanowire arrangement, even when the sample is removed from a magnetic field.

[0059] A control experiment was carried out in the absence of a magnetic field, where there was no movement of nanoparticles towards the interface; therefore no coating reaction occurred and no nanowire structures were formed. Nanowires are only formed due to the presence of the magnetic field and the formation of an oxide coating at the interface.

Detailed Experimental

Preparation of Magnetic Nanoparticles

[0060] Initially, polyelectrolyte-stabilised magnetic nanoparticles were prepared according to Table 1 by mixing poly(sodium-4-styrene) sulfonate (PSSS) (0.05 g, 7.14 x 10^"7 mol [sample 1 ] or 0.1 g, 1 .43 x 10^"6 mol [sample 2] dissolved in 10 mL water) with cobalt nitrate hexahydrate (0.15 g, 0.5 mmol) and iron chloride tetrahydrate (0.2 g, 1 mmol) dissolved in 25 mL of deoxygenated Millipore water and carrying out a co-precipitation reaction using ammonium hydroxide (NH₄OH) (20 mL, 30v/v%) at 80-90 °C for 2 hours. The resulting magnetic colloid was washed with water until pH neutral and dried under vacuum to yield strongly magnetic black powder: referred to as PSSS-CoFe₂0₄ nanoparticles. The resulting nanoparticles prepared were composed of PSSS and metal ions in the molar ratios listed in Table 1 . Stabilised magnetic nanoparticles comprising various magnetic nanoparticles can be prepared using this method and can be utilised as the magnetic component in this invention.

Preparation of Nanowires Using Magnetically Induced Phase Transfer Reactions

[0061] The preparation of the nanowires was as follows. PSSS-magnetic nanoparticles (5 mL of a 4 x10^"5 g/mL suspension of nanoparticles in ethanol) were dispersed in a water (5 mL) and NH₄OH (0.1 mL, 30v/v%) solution by manual shaking to form aqueous solutions (referred to as the 'magnetic solution'). A solution of dichloromethane (DCM) (1 OmL) and silicon or titanium alkoxide precursor (tetraethylorthosilicate [TEOS] or titanium isopropoxide) (9 x 10^"3 mol) was prepared in a beaker. The magnetic solution was then carefully transferred on top of the DCM/precursor solution in order to form a bi-layer. The reaction vessel was then placed on top of a magnet for several hours; until brown magnetic material collected at the bottom of the beaker near the magnetic surface.

[0062] Figures 2 and 3 show TEMs of coated nanowires prepared according to the phase transfer method of the present invention. Figures 2A and 2B show different magnifications of silica coated cobalt ferrite nanowires. Figure 2C shows that the silica coated linear nanowires take up a non-parallel, scattered arrangement in the absence of a magnetic field. However, in Figure 2D we see that the silica coated nanowires remain responsive to an external field - they line up in a parallel arrangement upon application of the external magnetic field.

[0063] Figures 3A and 3B show different magnifications of titania coated cobalt ferrite nanowires. Figure 3C shows that the titania coated linear nanowires take up a non-parallel, scattered arrangement in the absence of a magnetic field. However, in Figure 3D we see that the titania coated nanowires remain responsive to an external field - they line up in a parallel arrangement upon application of the external magnetic field.

Preparation of a Magnetic Emulsion Based System

[0064] Emulsions consisting of magnetic nanoparticles stabilising water-in-oil and oil-in-water emulsions can be used to form stable suspensions of materials. Initially, magnetic

nanoparticles were prepared by co-precipitation of a solution containing iron chloride tetrahydrate (0.79 g, 4 mmol) and cobalt nitrate hexahydrate (0.58 g, 2 mmol) in 100 mL deoxygenated Millipore water using ammonium hydroxide (NH₄OH) (20 mL, 30v/v%) at 80-90 °C for 2 hours. The resulting magnetic colloid was washed with water until pH neutral and dried under vacuum to yield a strongly magnetic black powder: referred to as CoFe₂0₄ nanoparticles. An emulsion was prepared by mixing a dry sample of magnetic nanoparticles (0.1 g, 0.4 mmol) in water (0.7 mL), followed by the addition of oleic acid (24.6 mL, 0.078 mol). The reactants were mixed by sonication and vigorous stirring so as to create an emulsion. The emulsion can be destabilised by placing the sample vessel on top of a magnet, where the nanoparticles move the bottom of the flask, destroying the stable emulsion and yielding a layered bi-phasic system.

Parameter Variation

[0065] Investigations into the variation of reaction parameters shows that increasing the concentration of the aqueous ammonium hydroxide catalyst used to form a silica or titania shell around the nanowire structures does not cause larger wires to form (in terms of width, length or aspect ratio). Similarly, increasing the amount of silicon alkoxide (the silica precursor) or titanium alkoxide (the titania precursor) does not affect the thickness of the resulting silica shell. It should be noted that changing the concentration of the magnetic nanoparticles (i.e. in the top layer 101 ) does not alter the resulting nanowire structure.

[0066] Variation of the magnetic nanoparticle sample used to prepare the nanoparticles causes a change in the appearance of the resulting nanowire structures Figure 4. In one particular embodiment, the magnetic nanoparticles utilised were polyelectrolyte stabilised magnetic nanoparticles, namely poly(sodium-4-styrenesulfonate) stabilised cobalt ferrite (PSSS- CoFe₂0₄). PSSS is a negatively charged polyelectrolyte, which is used as a stabiliser during the preparation of CoFe₂0₄ nanoparticles and it hinders or prevents flocculation/agglomeration of the nanoparticles. The PSSS-stabilised CoFe₂0₄ nanoparticles were prepared with different concentrations of PSSS. Sample 1 was prepared using a high concentration of PSSS. Sample 2 was prepared using a lower concentration of PSSS, as shown in Table 1.

Table 1 : Details of PSSS-stabilised CoFe₂0₄ nanoparticles - their preparation and

characterisation.

Dcore - diameter of the core; D_hyd - hydrodynamic diameter; Pdl - Polydispersity index.

[0067] Sample 1 prepared with higher concentrations of poly(sodium-4-styrenesulfonate) exhibits lower magnetisation per kilogram owing to the larger amount of PSSS on the particles. Conversely, Sample 2 prepared with lower concentrations of poly(sodium-4-styrenesulfonate) exhibits higher magnetisation per kilogram owing to the smaller amount of PSSS on the particles. Particles from both samples are relatively monodisperse showing low Pdl values. [0068] In Figure 4, we can see the effect of using the different magnetic nanoparticle samples (i.e. Sample 1 or Sample 2 supra) to prepare silica coated nanowires, with all other parameters and concentrations remaining constant.

[0069] Using sample 2 PSSS-CoFe₂0₄ (d, e, f - images at different magnifications) produces nanowires which have a higher aspect ratio and a more ordered appearance.

[0070] TEM images a, b and c depict silica coated nanowires formed using Sample 1 PSSS- CoFe₂0₄. TEM images d, e and f depict silica coated nanowires formed using Sample 2 PSSS- CoFe₂0₄. TEM images a, b and c illustrate shorter, narrower nanowires when compared with TEM images d, e, and f, where the nanowires have a larger aspect ratio, are longer in length (they stretch across the TEM grid squares), have a thicker width, and have a more ordered appearance.

[0071] As used herein, the words "comprises/comprising" and the words "having/including" when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0072] It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination.

Claims

1 . A method of promoting a reaction in a reaction system comprising at least a first phase and a second phase, wherein the first and second phases are immiscible, the method comprising:

2. A method according to Claim 1 wherein the magnetically responsive particle is selected from the group consisting of a magnetic nanoparticle, a magnetic nanorod, a magnetic nanowire, a magnetic nanoworm, a magnetic nanotube, a magnetic nanoflake and combinations thereof.

3. A method according to any preceding Claim wherein the magnetically responsive particle is held in suspension in one of the first or second phases.

4. A method according to any preceding Claim wherein the reaction promoting component is a catalyst.

5. A method according to any preceding Claim wherein the reaction promoting component is attached to or adsorbed on the surface of the magnetically responsive particle.

6. A method according to any preceding Claim wherein both the first and second phases are liquid.

7. A method according to any preceding Claim wherein the reaction system is a biphasic system.

8. A method according to any preceding Claim wherein the reaction comprises forming a coating on a surface of the magnetically responsive particle.

9. A method according to Claim 8 wherein the coating further comprises a moiety selected from the group consisting of a luminescent moiety, a drug moiety, a radiolabeled moiety, an electrochromic moiety and combinations thereof.

10. Use of a magnetically responsive particle consisting of, or comprising a reaction promoting component in a reaction system comprising at least two immiscible phases, wherein the application of a magnetic field causes the magnetically responsive particle to migrate from one the phases into the other of phases so that the reaction promoting component can promote a reaction therein.

1 1 . A method of coating a magnetically responsive material, the method comprising the steps of:

12. A method according to Claim 1 1 wherein the coating promoting agent is:

i) the magnetically responsive material; or

13. A method according to any one of Claims 1 1 to 12 wherein the coating precursor further comprises a moiety selected from the group consisting of a luminescent moiety, a drug moiety, a radiolabeled moiety, an electrochromic moiety and combinations thereof.

14. A container having a reaction system contained therein,

a reagent disposed in the other of the first or second phases,

wherein the application of a magnetic field causes the magnetically responsive particle to migrate in to the other of the first or second phases such that the reaction promoting component can react with the reagent disposed in the other of the first or second phases.

15. A kit comprising:

i) a magnetic field source; and ii) a container according to Claim 14.