WO2005113705A1

WO2005113705A1 - Composite luminescent particles

Info

Publication number: WO2005113705A1
Application number: PCT/GB2005/002010
Authority: WO
Inventors: Gareth Wakefield; Mark Green
Original assignee: Oxonica Limited
Priority date: 2004-05-21
Filing date: 2005-05-23
Publication date: 2005-12-01

Abstract

A silica or other particle which comprises a plurality of particles of luminescent material incorporated therein or which comprises a composite particle of a luminescent polyoxometallate which possess a first coating of an organic material and a second coating over the first coating of an inorganic material.

Description

COMPOSITE LUMINESCENT PARTICLES

This invention relates to composite luminescent particles, including silica spheres which are particularly useful in biological tagging and polyoxometallates. The use of common organic dyes for tagging presents many problems, in particular due to photo bleaching and because narrow absorption bands make it difficult to excite different colours at once. Dye emission can also be broad, making multicolour imaging difficult. Previous attempts to utilise luminescent quantum dots for tagging applications have more recently been based principally on semi-conductors, with luminescence of various colours being generated by transitions across the quantum confined semi-conductor band gap. Semi-conductor nanoparticles can be processed to have narrow emission by size selective precipitation and the emission can be tuned by altering the particle size. However, making highly luminescent particles requires a further shell of semi-conductor and often a further shell of supporting material, typically silica. Transfer of the particle to water results in the loss of luminescence efficiency and eventual degradation of the nanoparticle surface which in turn quenches emission further. Thus the use of semi-conductor quantum dots can be problematic as they require excessive preparation and processing and are air/moisture sensitive, especially when not coated. The use of rare earth particles overcomes the majority of problems as they can be easily prepared, are not sensitive to atmospheric degradation and the emission colour is dependent upon the constituent rare earth ion, not the particle size. Rare earth materials have extremely narrow emission profiles irrespective of size with rare earth oxides being unaffected by oxidation processes. In our WO 03/038011 we disclose a process for preparing water soluble particles of a luminescent material by coating them with an organic acid or Lewis base such that the surface of the coating possesses one or more reactive groups. As a result, the resulting particles can be attached to one member of a ligand binding pair for biotagging purposes. However, to produce materials that excite in the appropriate range can require the preparation of complex solid state materials. This can result in various differing surfaces, with the surface chemistry of, say, a green emitting compound being vastly different from that of a red emitting compound. As a result, attachment of the relevant ligand to the emitting core may not be a particularly simple operation. The present invention provides a composite particle comprising luminescent material; said luminescent material being either in the form of second particles embedded in a first particle and/or in the form of a particle having a first coating of an organic material and a second coating of an inorganic material over the first coating. According to a first embodiment of the present invention, the problem of differing surfaces is overcome by incorporating the emitting material in a silica particle. As a consequence, the morphology and other characteristics of the emitting material, in particular its shape, size and surface chemistry, are no longer defining factors. The well documented surface chemistry of silica spheres therefore dictates all bioconjugation protocols. In effect, all the emitting particles are provided with a single universal capping ligand that can be used in bio-coupling reactions for all imaging materials. Accordingly, the first embodiment of present invention provides a first particle which comprises a plurality of second particles of luminescent material embedded or otherwise incorporated therein. The particle preferably comprises an amorphous metal oxide and said luminescent material. The amorphous metal oxide preferably comprises one or more of silica, alumina, titania and zirconia. The particle may have a polymer coating, and the polymer coating is preferably a substantially neutral material modified with a suitable surface termination. In one embodiment the polymer has an amine or carboxylic termination. The nature of the luminescent or emitting material is not critical but they are normally inorganic, although organic luminescentmaterials can be present as well. The term includes phosphors as well as semi-conductor compounds. Thus the emitting material is typically a rare earth containing phosphor, especially a rare earth oxide, hydroxide, tungstate, molybdate or uranyl compound or other rare earth based material. The rare earth moiety can also be a dopant in a lattice such as Y₂O₃:RE where RE is a rare earth atom. Other lattices include other oxides, mixed oxides, fluorides, iodides, mixed halides (such as BaFBr:Eu) alkaline earth metal fluorides (such as CaF₂, SrF₂ (LaAlCe)F₃ and LiYf₄, oxyhalides, borates, aluminates, silicates, phosphates, oxysulfides, tungstates, vanadates, sulphides, gallates and niobates. Typical phosphors which can be used in the present invention include those disclosed in WO 99/46204 having the formula Z₂O₃:Z^x+ where Z is a rare earth metal and x is from 3, and in WO 00/36051 A which are ternary oxides of the formula

where Z¹ is a metal of valency b, X is a metal or metalloid of valency a, such that 2y = b.z + a.x, and

RE is a dopant ion of a rare earth, in particular terbium, europium, cerium, thulium, samarium, holmium, erbium, dysprosium, praseodymium, manganese chromium or titanium. Further particles are disclosed in WO 00/36051. Also, WO 00/36050 discloses a process for preparing phosphor particles of a host oxide doped with a rare earth or manganese which comprises: preparing an aqueous solution of salts of the host ion and of the dopant ion and a water soluble compound which decomposes under the reaction conditions to convert said salts into hydroxy carbonate, heating the solution so as to cause said compound to decompose, recovering the resulting precipitate and calcining it at a temperature of at least 500°C. This gives rise to substantially monocrystalline particles. Such particles can also be used in the present invention. Other particles which can be used are disclosed in WO 03/037788. These are particles of a compound of the formula:

wherein X¹ is a rare earth metal or a metal of Group IIA, IIB, IVB or VB of the Periodic Table (see Chemistry of the Elements, Greenwood & Earnshaw, Pergamon, 1984), or a mixture of two or more thereof, Y is a metal which forms an anion with oxygen, or a mixture of two or more thereof, and a, b and c are such that the compound is stoichiometric. The rare earth atom is typically europium, terbium or yttrium. As will be appreciated, in some instances non-rare earth metal atoms can be used as dopants in certain lattices. These include transition metals such as manganese, copper and chromium, alkaline earth metals such as calcium and strontium and metals of Group IIB, IVB & VB such as lead, tin and antimony such as ZnS:Ca. By way of example, Mn can be doped into BaMg Al_MO₂₃, calcium or magnesium fluoride, calcium and titanium oxide, cadmium and zinc phosphate, magnesium, zinc and calcium silicate, strontium aluminate and cadmium borate. It will also be appreciated that there can be more than one dopant, for example erbium and ytterbium and more than one host lattice. Particularly preferred phosphors include those based on ZnS, ZnS:Mn, ZnO, YAg:Ce, YSiO₅:Si, Y₂O₂:Eu/Tb, Y₂O₂:Eu. Polyanion systems can also be used. A second embodiment of the invention will now be introduced. Polyoxometallates have a number of uses but principally as luminescent materials when they possess one or more metal ions capable of providing a luminescent centre. Although many polyoxometallates (POMs) are generally stable in aqueous solution in many instances their luminescent activity is quenched in water. While POMs can be coated with an inert material such as silica, this coating is difficult to achieve without incorporating water. It has now surprisingly been found, according to a second embodiment of the present invention, that luminescence of POMs can actually be enhanced in the presence of, for example, an organic amino group-containing compounds or other Lewis base (referred to as an enhancement agent) even though they can be in aqueous media. Accordingly, a second embodiment of the present invention provides a composite particle of a luminescent polyoxometallate, which possesses a first coating of an organic material (which preferably shields the polyoxometallate against water) and a second coating over the first coating of an inorganic material and optionally a polymeric coating over the first coating. Additional coatings may be provided. The coatings may be in the form of substantially continuous layers. Typical POMs which possess luminescent activity are those disclosed in WO 2004/018590. Such polyoxometallates possess the formula:

M²[M^!(POM)]

wherein: M¹ comprises an ion of a metal capable of providing a luminescent centre, or a mixture of such an ion and one or more other metals for example one or more other metals also capable of providing a luminescent centre, M² comprises a cation, generally an ion of hydrogen or of a metal of group I A or IIA of the Periodic Table or an optionally substituted ammonium ion, or a mixture of two or more said ions, and POM comprises a polyoxometallate, a polythiometalate or a polyoxothiometalate (optionally having a hetero atom X as explained below) of at least one metal of group VA or VIA of the Periodic Table, the amounts of X, M¹, M² and POM being such as to provide overall neutrality. Therefore by a "polyoxometallate" or (POM), as used herein, is meant not only a polyoxometallate proper but also a polythiometalate or a polyoxothiometalate, of at least one metal of group V(A) or VI(B) of the Periodic Table. Metals of group V and VI of the Periodic Table, (see Chemistry of the Elements, Greenwood & Earnshaw, Pergamon, 1984), notably vanadium, niobium, tantalum, molybdenum and tungsten, are able to form, in their highest oxidation state, metal oxygen/sulphur cluster ions known as polyoxo/thiometalates, and are collectively referred to herein for convenience as "POM". Reference will be made principally to oxo embodiments but it is to be understood that the comments apply also to the thio and mixed embodiments. The majority of structures are constructed from these transition metals as polyhedrons and may be viewed as arrangements of edge and vertex sharing MO_n where n is 4 or 6, especially MO₆ , octahedra and have one or two terminal oxygen ions although other polyhedral structures can be formed. At present we prefer POM structures that are symmetrical. M² can generally be any cation which does not adversely affect the luminescence of the particle. Therefore it should generally not be an M¹ metal and generally not a transition metal. It is typically an ion of hydrogen or group I A or II A of the Periodic Table or an optionally substituted amine or ammonium ion or an cation of a Lewis base. Typically, M² is potassium or sodium or NH₄. However, the ammonium ion may be substituted by, for example, one or more aliphatic or aromatic groups, typically alkyl groups of 1 to 6 carbon atoms such as methyl, ethyl, propyl, butyl or phenyl, or it can take the form of an amino acid anion of the type NH₂-CO₂H or a salt thereof such as one derived from histidine, arginine or lysine. All four hydrogen atoms can be replaced by substituents or 1 , 2 or 3 hydrogen atoms can remain. Typical organic counter ions include tetrabutyl ammonium, tetramethyl ammonium, trimethyl ammonium and isopropyl ammonium. When M² represents a said amino group containing cation or a cation of a Lewis base the luminescent activity of the POM is enhanced. In an alternative embodiment, though, which is preferred, the POM is coated, at least partially by an amino-group containing compound or a Lewis base The organic material should be capable of protecting the polyoxometallate ion from water which will otherwise tend to quench it. Since the latter will be negatively charged, the organic material will normally be positively charged. In fact it is believed that it is necessary for the organic material, which may be referred to as an enhancing agent, to possess a lone pair of electrons which are able to co-ordinate with the luminescent metal. It is believed that by such means hydroxyl ions are prevented from being co-ordinated around the luminescent ion. Apart from amine- group containing compounds Lewis bases which can be used include phosphines, phosphine oxides and thiols. The nature of the enhancement agent can be varied and can be monomeric, dimeric or polymeric. Suitable amino group-containing compounds apart from those mentioned above include monomeric compounds such as ethanolamine and other aliphatic hydroxy amines including substituted derivatives thereof, as well as amino acids including dimeric lysine and polymeric 1-lysine and polymeric arginine. The molecular weight of the polymeric material is typically from that of a dimer, say 50, to 300,000. The nature of the enhancing agent will depend on the precise nature of the POM. The most appropriate agent used can, of course, be determined by routine experiment. It has been found that best results for a europium- based POM involve the use of polylysine, polyarginine or ethanolamine while for dysprosium dimeric lysine has been found to be most appropriate. As indicated, M¹ is a metal capable of providing a luminescent centre and is typically a lanthanide although other metals include chromium, titanium and manganese can also be used. Typical ions include Eu³⁺, Tb³⁺, Dy³⁺, Tm³⁺, Er^3"1", Cr³⁺, Ce³⁺, Pr³*, Sm³⁺, Nd³⁺, Ho³⁺, Yb³⁺, Ti ⁺, Mn²⁺ and Mn⁴⁺. One or more such ions may be used. Preferred ions include Eu³⁺, Tb³⁺ and Dy³*. In this connection it should be pointed out that, europium, in particular, has in the past been incorporated in POMs but this has been for the purpose of examining crystal structure. In other words there has been no suggestion that the compounds could be used in biotagging or other such applications. It is possible for the POM to possess more than one active centre i.e. M¹ can be a mixture of two or more said metals. POM is a polyoxo/thiometalate of a group V or group VI metal, in particular vanadium, niobium, tantalum, molybdenum and tungsten, the last two being particularly preferred. As indicated in WO/2004/018590, the crystal structure of the POM is considered to be particularly important in providing a "shield" for the luminescent centre which is incorporated in it. In addition, in order to maintain a high quantum efficiency, energy transfer from charge transfer state to the optically active centre must be efficient. This is believed to be governed by the de-localisation of the electron wave function over the POM structure and this is in turn governed by the M- O-M or M-S-M bond angles. This structure can be modified by incorporation of a "heteroelement" X in the POM, where X is for example a transition metal or an element such as B, Al, Si, P, S, Ga, Ge, As, Se, In, Sb, Te, I, Pb or Bi. One or more such elements may be used. The use of Si or Al is particularly preferred. The precise nature of the heteroelement is generally unimportant provided that it will form O-M- O-X- linkages. It is believed that the nature of M governs the absorption wavelength of the light while, of course, M¹ governs the emission. As previously indicated, the heteroelement X can have an effect on the bond angles i.e. the structure which is important for the delocalisation of an electron. It is believed that X does not in itself govern either luminescence or emission. It is a particular feature of polyoxo/thiometalates that they are frequently formed from more than one metal. Typically M is a mixture, for example of molybdenum and tungsten. Also the number of oxygen atoms which are included in the structure frequently exceeds 10 and is typically 15 to 50. Examples include M₁₀O₃₆, M₆O_]8 and M_nO₃₉; of course the number of oxygen atoms is affected by M¹ and X. In connection in particular, though not exclusively, with the first embodiment, we note that apart from phosphors, luminescent semi-conductor compounds can also be used in which quantum confinement has resulted in visible light emission. Typically, CdSe and CdS can be used for this purpose. The number of semiconductors from which visible light may be observed is limited as a suitable material must have a direct, allowed band gap of about 1 eV. Other semi-conductors compounds include II-VI and III-V semi-conductor compounds. The particles which are incorporated into the silica spheres may themselves already be coated, for example with an organic acid or Lewis base as disclosed in our British application No. 0126283.1. The silica particles of the present invention are essentially of two types. First the particles are such that the luminescent particles are substantially around the surface of the silica particles, thus "decorating" them. The luminescent particles will nonetheless generally be at least partially embedded. They may therefore generally be regarded as being present in a surface layer or shell. In this instance a further layer of silica can be present so as to encase the emitting particles. In the second type, the particles are embedded in, and preferably substantially throughout (particularly substantially uniformly throughout), the silica particle. The POMs or other incorporated particles are preferably not exposed to the environment to the extent that enhanced luminescence is lost; nor should they constitute an excessive percentage of the exposed surface of the particles such that the surface is not able to interact as desired with the environment. Typically the silica particle will incorporate 10 to 500, for example 20 to 100, particles of light emitting material. It will be appreciated that by this means the silica particles can have a high loading of light emitting material. Thus the overall concentration of emitting material can represent up to 20% of the weight of the silica particles. The particles are generally substantially spherical (and are generally referred to herein as spheres) and will typically have a size not exceeding 500 nm, for example 5 to 400 nm, especially 20 to 200 nm, for example 20 to 40 nm, depending in the way in which they have been prepared. These dimensions are diameters, and in the case of non-spherical particles are diameters of spheres of the same volume. For particles where the light emitting particles are predominantly on the surface a preferred method of preparation involves an alcosol route. This process involves preparing a dispersion of the light emitting particles in water with a silicate, subjecting this dispersion to comminution and then adding an alcohol to cause the formation of small silica particles with light emitting particles, generally inside and on the surface. Typically, such a process will produce silica spheres, 20 to 40 nm in size. The process is typically carried out by preparing a dispersion of the light emitting material, for example phosphor material, in water. The concentration of material is, for example, from 0.1 to 10 g per litre, for example 0.15 to 1 g per litre. Silicate is then added to the dispersion, for example as an aqueous solution. In general, the amount of silicate added should be greater than the weight of the light emitting material, typically with a weight ratio of from 1.7:1 to 2.3:1, preferably about 2:1 silicate:luminescent centre. The silicate is typically an alkali metal silicate such as sodium silicate. Comminution is typically achieved by sonication. The resulting particles are then caused to precipitate by mixing with an alcohol. The alcohol is typically an aliphatic alcohol, for example 1 to 6 carbon atoms, such as ethanol. In another aspect of the first embodiment of the present invention, the particles are obtained by a reverse micelle procedure. This follows the procedure disclosed in J. Biomed. Optic, 2001, 6,160. The process involves preparing a water- in-oil emulsion with the light emitting particle present in the aqueous phase. This is then comminuted during which orthosilicate is added. Subsequently, an alkali such as ammonia is added to cause silica to be formed. The particles can then be obtained by the addition of a solvent which is immiscible with the oil of the original emulsion. Typically, the emulsion is formed with the aid of a surfactant, for example TX100 or other non-ionic surfactant of the polyethylene oxide type. The oil, typically a non-polar solvent such as a hydrocarbon, for example cyclohexane is used and to this is added a dilute dispersion of the light emitting material, typically phosphor. The volume of the oil generally exceeds that of the water, for example in a ratio by volume of 10 to 1 : 100 to 1 , for example 20: 1 to 50: 1. The resulting water- in-oil emulsion is then comminuted, for example shear mixed using a rotor/stator mixer such as a Silverson mixer, before an orthosilicate, for example an alkyl orthosilicate, typically tetraethyl orthosilicate, is added before further comminution takes place. In order to form the silica an alkali, typically ammonium hydroxide, is added. Typically after allowing the mixture to stand before the immiscible organic solvent, typically acetone, is added after which the silica spheres can be obtained by, for example, centrifugation. The size of the resulting particles can be tuned by adjusting the concentration of the original ingredients. The size of the microemulsion pools is in general governed by the ratio of water to surfactant. For example, a weight ratio of water to surfactant of 5 : 1 gives particles approximately ten times smaller than does a 10: 1 ratio. It will be appreciated that regardless of the process used, a mixture of organic and inorganic fluorescent compounds can be incorporated into the silica spheres. The present invention enables one to incorporate a large number of emitting centres within the sphere which results in a higher luminescent efficiency per sphere due to the higher loading of optically active material. Silica is well known to be biocompatible and hydrophilic, and there are numerous known methods for attaching biomolecules to a silica surface. For example, silica surfaces may be rendered amine terminated by immersion in a 1% v/v solution of trimethoxysilylpropyldiethylenetriamine and ImM acetic acid for 30 minutes. The surface can be further modified to be carboxylate terminated by treatment with 10% succinic anhydride in dimethylformamide under argon atmosphere for 6 hours. After such surface modification the silica spheres are bioactive. We return now principally to the second embodiment. The POM particles used in particular in the second embodiment can generally readily be made by a self-assembly procedure in an acidified medium. They can generally be prepared by mixing a water soluble source of M² with a water soluble salt of oxo/thiometalate in water or an aqueous solution. Acid, such as hydrochloric or acetic acid, is then added and then the mixture generally heated, typically to a temperature of 80° to 90°C. Generally the pH of the solution will be 8 to 9 and this is reduced to, say, 6.5 to 7.5 by the acid. In general, the degree of acidification affects the final structure of the compounds. If a strong acid is added too quickly to produce a pH of less than 3 locally, then this generates product that is non- luminescent. Finally, a water soluble salt of the metal capable of providing the luminescent centre is added, typically as a halide, for example chloride, which is preferred, or nitrate. This generally results in a precipitate enabling the particles to be recovered. Sometimes the precipitate dissolves on stirring. The resulting solution, optionally filtered, can then be cooled to cause the desired product to crystallise. It has been found that it is generally better to use the same cation for the ingredients as that of the desired M². It is also possible to form oxo/thiometalate by a solid state reaction, for example by high temperature firing; a final lanthanide-activated compound can be formed by reaction with a complex lanthanide at neutral or basic pH. Monomeric oxometalates such as WO₄ ^2" , and MoO₄ ^2" condense in an aqueous acidic media. As there are no electrons available from metallic bonding (due to the maximum valency state of the M ions) the materials are generally held together by closed loops of -O-M-O-M-O- linkages. The structure of the final material is governed by the reaction conditions, principally pH, temperature and the ratio of the reactants. In general, it is recognised that the reactants should be added in the molar proportions that are required in the final product. The POM particles obtained typically possess a particle size not exceeding 20 nm, desirably not exceeding 10 nm, preferably not exceeding 5 nm and more preferably not exceeding 2 nm. These sizes are diameters and, where the particles are not spherical, they correspond to diameters of spheres of the same volume. The particles may in general be substantially spherical. The particles of this invention can contain more than one such compound, in which case the different compounds may differ in any one or more of the M², M¹, POM or X that they may contain. The particles may consist primarily of (i.e. at least 50% by weight of), and preferably consist substantially entirely of (say at least 95% by weight of), the compounds defined above. However, where other materials are present, those other materials may comprises one or more other phosphors. Also, the particles may be mixed with or otherwise used in conjunction with other particles e.g. of other phosphors. This can result in a combination of, say, green and red emissions. Once the POM particles have been obtained they are then provided with a coating of the organic material. The particles can generally be coated simply by dissolving the POM in an aqueous solution of the organic material. The particles of the present invention have particular value for bio tagging and other diagnostic purposes. While a coating with, for example, an amino group containing polymer provides the necessary binding groups for further bio conjugation these binding groups may not be optional and it is therefore necessary to provide the particle with a surface of, for example, a common inorganic material such as silica or other chemically "neutral" material. Preferably the surface material will be unaffected by, for example, dissolution and pH change, but will be capable of reacting with a suitable organic molecule for biotagging purposes. The chemistry of such materials is in general well understood, and many such materials are routinely used in biological labelling work. Naturally the material should not detract unacceptably from the performance of the luminescent material. Accordingly it should be a material with a wide band gap, generally at least 3 electron volts. Other inorganic materials which can be used include certain other amorphous oxides such as alumina and yttria and phosphates such as calcium phosphate. It is desirable that the inorganic material be transparent so that it does not excessively, or at all, shield the luminescence. The presence of oxides such as titania which adsorb light should generally be avoided. Accordingly, the organic-coated particles are subsequently at least partially coated with an inorganic material such as silica, to which an intermediate "biological" molecule can be attached. The particles when coated with the inorganic material typically have a size from 20nm to 1 μm, generally from 35nm to 400nm. POMs can be incorporated into a silica sphere by, for example, alcosol methods (see, for example, J. Coll. Interface. Sci, 1996, 179, 318), a based catalysed reaction of the Stδber process type, for example as disclosed in Langmuir, 1992, 8, 2921, an acid catalysed reaction (see, for example, Langmuir, 2001, 17, 8376) and reverse micelle synthesis (see, for example, Science, 1999, 283, 963). Typical coating procedures include the deposition of silica by mixing alkali such as ammonium hydroxide with an orthosilicate, such as tetraethylorthosilicate, in the presence of the particle. Alternatively the particle can first be coated with a silane such as (3-mercaptopropyl) trimethoxy silane (MPS) and then silicate e.g. sodium silicate is added. The silane attaches to the particle surface and acts as a substrate for the silicate which then polymerises to form silica. Similar techniques can be used for other inorganic oxides. While the particles can be provided with the inorganic coating by a typical coating procedure it is also possible to incorporate the particle into a particle of the inorganic material i.e. the "coating" becomes the continuous phase. It is to be understood that the term "coating" as used herein covers this situation. Further details of this procedure are given above in connection with the first embodiment. When the inorganic material forms the continuous phase the particles are essentially of two types. First the particles are such that the luminescent particles are substantially around the surface of the silica particles, thus "decorating" them. The luminescent particles will nonetheless generally be at least partially embedded. They may therefore generally be regarded as being present in a surface layer or shell. In this instance a further layer of inorganic material can be present so as to encase the emitting particles. In the second type, the particles are fully embedded in, and preferably throughout, the silica particle. The POMs or other incorporated particles are preferably not exposed to the environment to the extent that enhanced luminescence is lost; nor should they constitute an excessive percentage of the exposed surface of the particles such that the surface is not able to interact as desired with the environment. Typically the particle will comprise 10 to 500, for example 20 to 100, POM particles. It will be appreciated that by this means the particles can have a high loading of light emitting material. Thus the overall concentration of emitting material can represent up to, say, 20% of the weight of the silica particles. For particles where the light emitting particles are predominantly on the surface, a preferred method of preparation involves an alcosol route. That process involves preparing a dispersion of the light emitting particles in water with a silicate (in the case of silica as the inorganic material), subjecting this dispersion to comminution, and then adding an alcohol to cause the formation of small silica particles with the organic coated POM particles, generally inside and on the surface. Typically, such a process will produce silica spheres, 20 to 40 nm in size. The process is typically carried out by preparing a dispersion of the light emitting material, for example phosphor material, in water. The concentration of material is, for example, from 0.1 to 10 g per litre, for example 0.15 to 1 g per litre. Silicate is then added to the dispersion, for example as an aqueous solution. In general, the amount of silicate added should be greater than the weight of the light emitting material, typically with a ratio of from 1.7:1 to 2.3:1, preferably about 2:1 silicate: luminescent centre. The silicate is typically an alkali metal silicate such as sodium silicate. Comminution is typically achieved by sonication. The resulting particles are then caused to precipitate by mixing with an alcohol. The alcohol is typically an aliphatic alcohol, for example 1 to 6 carbon atoms, such as ethanol. In another embodiment of the present invention, the particles are obtained by a reverse micelle procedure. This follows the procedure disclosed in J. Biomed. Optic, 2001, 6,160. The process involves preparing a water-in-oil emulsion with the light emitting particle present in the aqueous phase. This is then comminuted during which orthosilicate is added. Subsequently, an alkali such as ammonia is added to cause silica to be formed. The particles can then be obtained by the addition of a solvent which is immiscible with the oil of the original emulsion. Typically, the emulsion is formed with the aid of a surfactant, for example TX100 or other non-ionic surfactant of the polyethylene oxide type. The oil is typically a non-polar solvent such as a hydrocarbon, for example cyclohexane, and to this is added a dilute dispersion of the light emitting material, typically phosphor. The volume of the oil generally exceeds that of the water, for example in a ratio by volume of 10:1 to 100:1, for example 20:1 to 50:1. The resulting water-in-oil emulsion is then comminuted, for example shear mixed using a rotor/stator mixer such as a Silverson mixer, before an orthosilicate, for example an alkyl orthosilicate, typically tetraethyl orthosilicate, is added and then further comminution carried out.

In order to form the silica an alkali, typically ammonium hydroxide, is added.

Typically the mixture is allowed to stand before acetone or other immiscible organic solvent is added. The silica spheres can then be obtained by, for example, centrifugation. The size of the resulting particles can be tuned by adjusting the concentration of the original ingredients. The size of the microemulsion pools is in general governed by the ratio of water to surfactant. For example, a weight ratio of water to surfactant of 5 : 1 gives particles approximately ten times smaller than does a 10: 1 ratio. The resulting sphere with an inorganic coating, whether as a coating over the

POM particles or with POM particle in it, can be made bioactive by the addition of enzyme molecules, proteins and the like via the "biological" molecule in a routine manner as discussed in, for example, Analyst, 2001, 126, 1274-1278 and J. Biomed

Optics, 2001, 6, 160-166. It is believed that the use of an enhancing agent, preferably polymeric, has the effect of keeping the POM in the core or centre of the silica or other inorganic particle, i.e. the POM is completely surrounded by the inorganic coating. When such a polymer is not used some of the POM particle may not be coated with the inorganic material. In order to ensure that the particle has a "clean" surface for bio-tagging purposes it is generally desirable to carry out a second coating step. This can be done in a similar manner to the previous coating. If desired two or more different POMs can be coated together to produce composite particles containing two or more different POMs. The following examples further illustrate the present invention. Examples 1 and 2 relate to the first embodiment referred to above, and Examples 3, 4 and 5 relate to the second embodiment. EXAMPLE 1

A dispersion of phosphor material, 0.017g of Eu(SiMoW₁₀O₃₉)₂ ^13", was prepared in 10 mis water (0.25 g/1). To this was added 0.035 g sodium silicate solution (1.5 specific gravity), and sonicated. 0.2 ml of the silicate/phosphor solution was then added to 10 ml absolute ethanol, forming silica spheres (sub 100 nm diameter) with small phosphor particles inside and on the surface of the particles. The accompany Figures 1 and 2 show the nature of these particles, (size bar = 10 nm for Figure 1 and 20 nm for Figure 2).

EXAMPLE 2

42 ml cyclohexane, 10 aqueous ml TX 100 surfactant, and 10 ml hexanol were mixed together. To this was added 0.4 ml of dilute phosphor dispersion, 0.1 lg Eu(SiMoW₁₀O₃₉)₂ ^13" in 5ml water, (0.25 g/1), forming a water-in-oil emulsion. This was then shear mixed for 15 minutes and then 0.6 ml tetraethyl orthosilicate added. After a further 15 mins shear mixing, 0.4 ml ammonium hydroxide solution was added. This was left to stand over night, 100 ml acetone was then added and then the reagents were centrifuged to remove the silica spheres. Washing with water, and ethanol removed the surfactant molecules, leaving a white solid of particle size in the general region of about lOOnm.

EXAMPLE 3 200 nm poly-lvsine and polyoxometallate doped silica spheres.

200 ml of ethanol was mixed with 14 ml ammonium hydroxide. To this was added 0.15 g polyoxometallate in a solution of poly-1-lysine. This was stirred for 30 minutes, followed by isolation by centrifugation, washed with ethanol and redispersed. Figure 3 is an HRTEM of the particles obtained.

EXAMPLE 4 50 nm poly-lvsine and polyoxometallate doped silica spheres.

150 ml cyclohexane was rapidly mixed with 6 ml hexanol, 19 ml Triton TX- 100 and 7 ml of an aqueous solution of europium decatungstate (5.1xl0^-3 M) in polylysine (0.01% solution of molecular weight 50000-300000) . To this was added 1.7 ml ammonium hydroxide, followed by 10 ml tetraethylorthosilicate. This was then stirred for 3 days. 300 ml ethanol was then added, causing a precipitate, which was isolated by centrifugation. The material was washed 3 times with ethanol, followed by redispersion in water. Figure 4 is an HRTEM of the particles obtained.

EXAMPLE 5

0.2 g of dried 200 nm silica spheres doped with a red emitting polyoxometallate coated with polylysine was dispersed in 250 ml water and sonicated/stirred until fully dispersed. 0.15 ml ammonium hydroxide solution was then added, followed by 0.10 ml tetraethylorthosilicate. The reaction was stirred for 48 hours, and the spheres isolated by centrifugation. The white powder which resulted was then washed with 50 ml ethanol twice, giving an improved silica surface to the original silica spheres.

Claims

1. The present invention provides a composite particle comprising luminescent material being either in the form of second particles embedded in a first particle and/or in the form of a particle having a first coating of an organic material and a second coating of an inorganic material over the first coating.

2. A particle according to claim 1 comprising an amorphous metal oxide and said luminescent material.

3. A particle according to claim 2 wherein the amorphous metal oxide comprises one or more of silica, alumina, titania and zirconia.

4. A particle according to claim 1 , 2 or 3 having a polymer coating.

5. A particle according to claim 4 in which the polymer has an amine or carboxylic termination.

6. A particle according to any one of the preceding claims wherein the second particles are at the surface or in a surface shell thereof.

7. A particle according to any of claims 1 to 5 when the second particles are incorporated substantially throughout the body thereof.

8. A particle according to any one of the preceding claims wherein the second particles comprise phosphor particles and/or semi-conductor particles and/or polyoxometallate particles.

9. A particle according to claim 8 wherein the phosphor has the foπnula Y₂O₃:RE, Z-X-.O_y:RE or X-'CYO,,),. wherein RE is a rare earth metal, X¹ is a rare earth metal or a metal of Group IIA, IIB, IVB or VB or a mixture of two or more thereof, Z is a rare earth metal, Z¹ is a metal of valency (b) and X is a metal or metalloid of valency (a).

10. A particle according to claim 9 wherein the phosphor comprises one or more of ZnS, ZnS:Mn, ZnO, YAg:Ce, YSiO₅:Si, Y₂O₂:Eu/Tb and Y₂O₂:Eu.

11. A particle according to any one of the preceding claims wherein the second particles are individually coated.

12. A particle according to claiml 1 wherein the coating is of an organic acid or Lewis base.

13. A particle according to any one of the preceding claims which incorporates 10 to 500 of said second particles.

14. A particle according to any one of the preceding claims which has a size not exceeding 500 nm.

15. A particle according to claim 14 which has a size from 20 to 200 nm.

16. A process for preparing a particle as claimed in any one of the preceding claims which comprises preparing a dispersion of said second particles in water with a silicate, subjecting said dispersion to comminution and then adding an alcohol to cause the formation of the first particles.

17. A process according to claim 16 wherein the concentration of particles in the dispersion is from 0.1 to 10 g per litre.

18. A process according to claim 16 or 17 wherein the comminution is carried out by sonication.

19. A process for preparing a particle as claimed in any one of claims 1 to

15 which comprises preparing a water-in-oil emulsion with the second particles present in the aqueous phase, comminuting the emulsion during which an orthosilicate is added to it and subsequently adding an alkali to cause the silica particles to be formed.

20. A process according to claim 19 wherein the oil is a non-polar hydrocarbon solvent.

21. A process according to claim 19 or 20 wherein the ratio of the volume of oil to the volume of water is from 10:1 to 100:1.

22. A process according to any one of claims 19 to 21 wherein comminution is achieved by shear mixing.

23. A silica particle as defined in any one of claims 1 to 15 whenever prepared by a process claimed in any one of claims 16 to 22.

24. A silica particle according to any one of claims 1 to 15 and 23 to which one member of a ligand binding pair has been attached.

25. A particle according to claim 1 wherein the polyoxometallate comprises a compound of the formula:

M^M^POM)] wherein: M² comprises a cation, M¹ comprises an ion of a metal capable of providing a luminescent centre, and POM comprises a polyoxometallate, a polythiometalate or a polyoxothiometalate of at least one metal of group VA or VIA of the Periodic Table and, optionally, a hetero atom X, the amounts of X, M¹, M² and POM being such as to provide overall neutrality.

26. A particle according to claim 25 wherein M² is an ion of hydrogen or of a metal of group I A or IIA of the Periodic Table or of an optionally substituted amine or an ammonium ion, or a mixture of two or more said ions.

27. A particle according to claim 26 wherein M² is an ion of amino group- containing compound.

28. A particle according to any one of claims 1 and 25 to 27 wherein the organic material possesses a lone electron pair capable of reducing the possibility of hydroxyl ions co-ordinating around the polyoxometallate ion.

29. A particle according to any one of claims 1 and 25 to 28 wherein the organic material comprises an amino group containing compound or other Lewis base.

30. A particle according to any one of claims 1 and 25 to 29 which contains an amino group-containing compound.

31. A particle according to claim 29 or 30 wherein the amino group- containing compound is an amino acid or hydroxy alkyl amine.

32. A particle according to any one of claims 1 and 25 to 31 wherein the amino group-containing compound comprises a monomer, dimer or polymer of lysine, histidine or arginine or ethanolamine.

33. A particle according to any one of claims 1 and 25 to 32 wherein M¹ is one or more of Eu³⁺, Tb³⁺, O Tm³⁺, Er³⁺, Cr³*, Ce³⁺, Pr³*, Sm³⁺, Nd³⁺, Ho³⁺, Yb³⁺, Ti⁴⁺and Mn⁴⁺.

34. A particle according to claim 33 wherein M¹ is Dy^3"1" , Eu³⁺ and/or Tb³⁺.

35. A particle according to any one of claims 1 and 25 to 34 wherein M is vanadium, niobium, tantalum, molybdenum or tungsten.

36. A particle according to claim 35 wherein M is molybdenum or tungsten.

37. A particle according to any one of claims 1 and 25 to 36 wherein POM contains a heteroelement X.

38. A particle according to claim 37 wherein X is a transition metal or one or more of B, Al, Si, P, S, Ga, Ge, As, Se, In, Sb, Te, I, Pb and Bi.

39. A particle according to claim 38 wherein X is Si and/or Al.

40. A particle according to any one of claims 1 and 25 to 39 which has a diameter not exceeding 20 nm.

41. A particle according to claim 40 which has a diameter not exceeding 5 nm.

42. A particle according to any one of claims 1 and 25 to 41 which possesses a further coating of an inorganic material over the second coating.

43. A particle according to any one of claims 1 and 25 to 42 wherein the inorganic coating comprises an amorphous oxide.

44. A particle according to claim 43 wherein the amorphous oxide is silica.

45. A particle according to any one of claims 1 and 25 to 44 having a biological molecule on its surface.