GB2415427A - A process for the manufacture of magnetic particles - Google Patents

Abstract

A process for forming magnetic particles which comprises preparing a suspension of a colloidal magnetic material and forming a polymer coating on the colloidal material to form the magnetic particles. The polymer may be a urea -formaldehyde amino resin. The magnetic material may be Fe2O3. The particles may be 1-10 microns in size.

Description

24 1 5427 Microbead and a Process for Manufacture of Microbeads.

Chemical and biological separation processes are used widely in a variety of analytical and preparative procedures. The separations take many different forms, encompassing various physical techniques, for example sieving, filtration, or airborne particle classification, but also comprise chemical, molecular separations such as chromatographic procedures. Of the variety of separation approaches in any given instance, a method is chosen most appropriate to the application in question.

Separations based upon magnetic fields also find diverse applications, and are the subject of this invention. Examples of such separations include the removal of magnetic impurities in mining activity, separation of ferrous metals in scrap and recycling industry, and in the separation of ions in analytical mass spectrometers. Such examples rely on the intrinsic magnetic properties of the materials being separated.

The magnetic properties of materials can also be exploited in behaving as separation vehicles where, for example non-magnetic particulate solids or soluble species may be immobilised upon the surface of the magnetic carrier in one stage of the separation process. Subsequent exposure to an externally applied magnetic field causes efficient physical separation from other non-magnetic impurities to be effected. Examples of this can be found fluidised bed processing, and in affinity separations of biological or chemical solutes or particles. Examples of the latter are now commonplace in the fields of molecular biology in the manipulation and isolation of nucleic acids, proteins and other macromolecules of interest to the user. Medical applications are also to be found, in the sorting and/or separation of specific cell types from mixed cell populations.

There are many different types of magnetic separation vehicles presented in the scientific literature, each offering properties of size, shape or strength of magnetic response. In general, non-critical processes tend to be performed using magnetic materials of poor physical definition in that they may be finely divided, but having a wide range of particle sizes and/or topology. In many bulk applications this is perfectly satisfactory for the application giving good performance and may be obtained from economical sources. More critical processes such as those in, but not limited to, medical or scientific applications have a much more stringent requirement for definition in size and/or morphology of magnetic particles.

Higher definition in the magnetic particles is important for reasons of performance, and the behavioural aspects that the high definition confers.

The performance characteristics preferable for the more critical separations include rapid response to the applied magnetic field, for rapid separation, ease of re-suspension following the separation step, the absence of magnetic retained magnetization, and the absence of nonseparating fractions such as are commonly called "fines".

Currently there are a number of materials available from manufacturers for such separations. The following discusses some of the commonly used ones, but the list is not exhaustive. At the lower definition end of the range is the product of US Patent No. 4,152,210, comprising a matrix of cellulose particles entrapping the magnetic salt magnetite. Such material is effective in separations, but lacks definition, having amorphous particle form and can be slow to separate. Another commonly used material is the product of the US Patent No. 4,698,302, which is also amorphous in its particle shape giving slower separation and re- suspension. In particular such processes often use extensive grinding of bulk materials by means of ball-milling, which generates very fine fragments in addition to the particles sought. Various spherical forms of magnetizable particles are also available, being made in different matrix materials. Thus, there are examples of microbeads comprising soft gel matrix as exemplified in EP Patent No. 0 180 384 B1 (Cohen et al.), or alternatively the product of FR Patent No. 2,463,778, where the matrix is a polymer such as polystyrene. The disadvantage of these is that the manufacturing process relies upon emulsion- or dispersion-polymerisation techniques to obtain the spherical form for the particles. Whilst this works well as a general production method, the bead size resulting is frequently heterogeneous, being a consequence of the randomness of the droplet size generated during the emulsion formation. The size heterogeneity can be reduced to reasonable levels by wet sieving and other graduation methods but this is not ideal and results in an elongated and therefore costly manufacturing process. In addition the soft nature of the materials frequently causes compaction of the separated bead pellet during the separation process, the consequence of which is that re- suspension of the bead fraction can be slow at best or even practically impossible. Polystyrene latex matrices can also suffer from bulk adhesion to surfaces as a consequence of their hydrophobic character causing some repulsion from aqueous solvent.

Probably the best material in terms of performance is that represented by the process of Ugelstad (see, for example US Patent No. 4,459,378), where monodisperse bead of uniform size are manufactured. The process concept is elegant, whereby uniform seed latex is gradually and uniformly inflated by staged addition of further monomer. Magnetic material is subsequently deposited from a redox process from magnetic ions in solution. Whilst the performance of this material is generally accepted as being very good, the manufacturing process required results in product that is very costly to the user.

This invention seeks to provide a process for manufacturing rigid magnetic microspheres having excellent performance characteristics, which is very simple, convenient and economical to perform, and the particles formed by this process.

According to a first aspect of this invention there is provided a process for the manufacture of microbeads possessing magnetic properties comprising the steps: a) preparing a suspension of colloid magnetic material; and b) forming a polymer coating on said colloid magnetic material thus forming said microbeads. In this way it is possible to easily form microbeads of very consistent size particularly economically.

Preferably, the polymer coating is comprised of urea-formaldehyde aminoresin. This resin is simple to use yet forms a non-hydrophobic surface when coated on to the colloid particle.

A particularly preferred colloid of Fe2O3 is particularly advantageous as this may be readily formed and used.

The process may be performed in either a single step or in two or more sequential stages. In the process the microbead may be further derivatised chemically, depending upon the final use of the microbead.

According to a second aspect of the present invention there is provided a microbead comprising a magnetic core of mainly Fe2O3 encapsulated by a polymer sheath. Normally the core will be at least 90% Fe2O3.

The microbead advantageously may be provided with the polymer sheath hydrophilic. A particularly good sheath material is easily derivatisable and an example of such is where the polymer sheath comprises an aminoresin.

It is normally preferred for them microbead is about 1 micron to about 10 microns. Often the microbead is less than 7 microns, preferably about 3 microns, for many biological separations.

According to a third aspect of the present invention there is provided a separation material comprising a plurality of microbeads according to the second aspect of the invention.

According to a fourth aspect of the invention there is provided the use of the separation material of the third aspect of the invention in: a) an analytical procedure for qualitative or quantitative results; b) a prepartive process; or c) a separation process for controllable recoverable catalysis.

A preferred embodiment of the process of the invention will now be described, with reference to the drawing in Figure 1.

Figure 1 shows an illustration of the overall key process in the bead formation step where a, b, and c, represent various situations in time during that process: a) In particular a shows the generally homogeneous distribution of reactants at the start of the process, the black specks representing the colloidal material; b) In b, the situation as reaction proceeds is where partial coalescence of the polymer-coated colloid has occurred, forming nuclei for further bead formation; c) The complete, or near complete reaction is represented in c where coalescence is virtually complete and the finished beads are formed.

The process starts with the addition of a stabilised magnetic colloid suspension. Such material is not commercially available but may be simply prepared by the method of Massart (see Massart, R., IEEE Trans. Mag., 17(2), p1247, (1981)), where iron salts are firstly precipitated in alkaline solution and subsequently treated to obtain a stable magnetic fluid. This magnetic fluid is a suspension of colloidal particles considered mainly to be of y-Fe2O3, dispersed in aqueous solution. This iron salt has the advantage of being very responsive to magnetic fields but with relatively low mass, compared to the otherwise commonly used Fe3O4, or magnetite.

Next, urea and formaldehyde solution are added in the correct proportions to the colloid. Normally an excess of urea and formaldehyde are used to ensure that all the colloid is consumed. For example 1 mole of urea to 3 moles of formaldehyde to 1 mole of colloid. The whole is then acidified by the addition of acid to produce a low pH. A simple acid such as HCI, HNO3 or H2SO4 can be used to reduce the pH to about 2.0 to 3.0. The presence of urea and formaldehyde in low pH initiates an acid-catalysed polymerization and subsequent condensation to give an insoluble thermosetting aminoresin. Such aminoresin polymers are well established in the field of polymer chemistry and these are generally hydrophilic. As the insoluble polymer forms, it is first deposited at the interface between the solution holding the monomer, and the solid surface of the colloidal magnetic material. Once an initial layer of polymer has formed, giving polymer-coated colloid, at least two further processes occur. Firstly, continued deposition of polymer will occur on the solid polymer surface, and secondly Brownian motion of the colloids results in colloid- colloid particle contact. In the presence of the growing polymer layer this leads to adhesion between colloid particles. This fusion process continues sequentially throughout the reaction mixture until the majority of the colloidal particles have been sequestered in a kind of "snow- balling" process. The result is a suspension of magnetizable microbeads in aqueous solution. The product may then be isolated by a sequential process of washing, with the aid of a magnet as a separation tool, followed by filtration and drying for storage. The whole process is very simple and, depending upon reaction conditions may be completed within a short time. In the extreme this may be as little as twenty minutes. No stirring is required during reaction, and no other special requirements except attention to the maintenance of the chosen reaction conditions are needed. All of the raw materials are easily prepared or readily available and all are low cost.

Although the use of amino resin is preferred because of its simplicity and cost efficiency, other polymer chemistries could also be used for forming the polymer sheath. It is preferred to form a hydrophilic polymer sheath, but hydrophobic sheaths are also possible.

The microbeads that result are rigid, dark brown or black in colour, and are highly responsive to an externally applied magnetic field. It is important that the aminoresin fully encases the magnetic core of the microbeads. Upon removal from the magnetic field the microbead pellet falls away and is very easily re-suspended with gentle agitation.

Furthermore the polymer, being polar, is not hydrophobic, so avoiding the problems seen with many other bead types of clumping or smearing of the vessels used.

A further advantage of the process of this invention is that the colloidal magnetic materials become encapsulated in polymer at an early stage, so isolating that magnetic material from its further environment. Such attributes can be important for certain applications, such as those involving the use of enzymes or other proteins.

The characteristics of the product in terms of microbead size, degree of uniformity in diameter, magnetic content, and rate of reaction are controllable, by virtue of careful choice of the reaction parameters of relative quantities and conditions of e.g. time, temperature, pH, concentration and so on. Importantly the polymer surface bears a high concentration of chemical functionality, offering opportunity for further chemical modification. For example, to control microbead size, the encapsulation reaction simply requires quenching at the appropriate time.

Other variants of the process are also possible. In one variant size selection of the forming microbeads may be controlled by early termination and quenching of the reaction. Product microbeads may then be isolated from the quenched reaction mixture. In another variant of the process, a later dilution step of the reaction mixture can be used to afford magnetic core-shell beads, having magnetic material in the inner core bead, being wrapped in a polymer shell. Alternatively this result can also be obtained by using a secondary step by transferring the isolated microbeads and suspending them in a similar reaction mixture but omitting the magnetic component.

On the surface of the microbeads is a multiplicity of hydroxyl, and amide groups, which may be further exposed to a number of reactants to give a reactive layer to the surface, useful for the gentle, permanent attachment of biological substances. Thus, chemical functionalisation with home- or heterobifunctional compounds, followed by chemical bonding to binding proteins such as avidin, streptavidin, immunoglobulins, protein A, etc. results in microbeads having specific binding character. The resultant product is then rendered useful for a variety of biological applications.

The range of chemistry applicable to the microbead surface is not dealt with here in detail but is obvious to those skilled in the art of chemical immobilization methodology. Surface chemistries applied to these beads include organic reactants and linkers such as glutaraldehyde, and triazine, biological materials such as those listed above and also inorganic layers such as pure silica. In addition, surface charges may be applied allowing ion-exchange properties, or alternatively ion-chelation ligands.

Example 1

22.5 9 Purified urea was dissolved in 800 ml of reverse- osmosis/deionised water (resistively 18.5 MA cm), and HCI solution added dropwise to adjust the pH of the solution to pH 2.40 at room temperature. Colloidal iron oxide solution (prepared according to the Massart method), 200 ml of 7.5% w/w solution was added to the urea solution and mixed to give a homogeneous colloidal solution.

Immediately prior to reaction 1.96 litres of purified water was added to the solution followed by 36 ml of 37% formaldehyde solution and the whole solution mixed well. The reaction vessel was placed at 30 C for 4 h, and then was allowed to cool overnight. After 24 h, the solid beads which had collected at the bottom of the vessel were re-suspended, and then filtered to remove the supernatant. The bead filter cake was then washed with water, followed by ethanol, and finally was removed to a glass vessel and dried in an oven at 90 C for 1.5 h. The process gave 30.5 9 of dark brown microbeads as a solid free-flowing powder.

Microscopical examination of the product at the end of the reaction stage showed the product to consist mostly of 10 Am diameter beads.

Re-suspension of a sample of the dried, powdered product gave a brown suspension of microbeads, which separated rapidly on application of an external magnetic field, and which re-suspended instantly upon removal of the magnet and given gentle swirling.

Example 2

Urea, 7.5Og was dissolved in 200ml of filtered RO water, pre-warmed to 30 C, and the pH adjusted to 2.0 by the dropwise addition of HCI.

Formaldehyde solution (37%), 12 ml was added and mixed and the reaction allowed to proceed for 10 min at 30 C.

In the meanwhile, 65 ml of magnetic colloid (7.5%w/w) was diluted into 700ml of pre-warmed (30 C) and filtered purified water and mixed well, and degassed.

After 10 min of the polymerization reaction, the mixture was quickly poured into the colloid solution and mixed well. The bottle was then placed in the water bath and allowed to react at 30 C for 60 h. The bottle was shaken at around 30, and 90 min into the reaction period.

Microscopic examination of the reaction mixture showed that the particles were mostly of 5 1lm in diameter. The solid beads were re- suspended, and then filtered to remove the supernatant. The bead filter cake was then washed with water, followed by ethanol, and finally was removed to a glass vessel and dried in an oven at 90 C for 1.5 h. The process gave 4.97 g of dark brown microbeads as a solid free-flowing powder.

Re-suspension of a sample of the dried, powdered product gave a brown suspension of microbeads, which separated rapidly on application of an external magnetic field, and which re-suspended instantly upon removal of the magnet and given gentle swirling.

Claims (16)

1. Claims: 1. A process for the manufacture of microbeads possessing

   magnetic properties comprising the steps: c) preparing a suspension of colloid magnetic material; and d) forming a polymer coating on said colloid magnetic material thus forming said microbeads.
2. 2. A process according to Claim 1, wherein the polymer coating is comprised of urea-formaldehyde aminoresin.
3. 3. A process according to Claim 1 or Claim 2, wherein the magnetic properties are produced by a colloid of Fe2O3.
4. 4. A process according to any one of Claims 1 too, wherein the process may be performed in either a single step or in two or more sequential stages.
5. 5. A process according to any one of Claims 1 to 4, wherein the microbead is further derivatised chemically.
6. 6. A microbead comprising a magnetic core of mainly Fe2O3 encapsulated by a polymer sheath.
7. 7. The microbead of Claim 6, wherein the polymer sheath is hydrophilic.
8. 8. The microbead of Claim 6 or Claim 7, wherein the polymer sheath comprises an aminoresin.
9. 9. The microbead according to any one of Claims 6 to 8, wherein the microbead is derivatised. -,o
10. 10. The microbead of any one of claims 6 to 9, wherein the microbead is about 1 micron to about 10 microns.
11. 11. The microbead of Claim 10, wherein the microbead is less than 7 microns.
12. 12. The microbead of Claim 10, wherein the microbead is about 3 microns.
13. 13. A separation material comprising a plurality of microbeads according to any one of Claims 6 to 12.
14. 14. Use of the separation material of Claim 13 in: c) an analytical procedure for qualitative or quantitative results; d) a prepartive process; or c) a separation process for controllable recoverable catalysis.
15. 15. A process as hereinbefore described, with reference, to and/or as illustrated by, the accompanying drawings.
16. 16. A microbead or separation material as hereinbefore described with reference to, and/or as illustrated by, the accompanying drawings.