Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2001-01-08
2001-10-02
Le, Hoa T. (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C428S404000
Reexamination Certificate
active
06296937
ABSTRACT:
BACKGROUND OF THE INVENTION
Adsorbent materials having magnetic properties are known. Often magnetic properties such as ferromagnetism and superparamagnetism are desired to enable use of magnetic fields in various processes. Superparamagnetism is of special interest in many chemical applications where it is desired to collect and redisperse adsorbent particles. The particles of the prior art generally comprise a superparamagnetic material (e.g. magnetite) which is enveloped in or treated with some diverse material.
For many applications, the materials of the prior art have been considered adequate, however for certain applications, the materials of the prior art are inadequate due to their vulnerability to leaching in acidic environments or their lack of sufficient magnetic performance. In many applications such as processing of edible substances or sensitive biological materials (e.g. nucleic acid purification such as described in published patent application W095/06652), these deficiencies result in poor performance or simply prohibit use of these materials.
Thus, there is a need for improved coated superparamagnetic particles, especially particles useful as superparamagnetic adsorbents. Further, there is a need for improved methods of making such adsorbents.
SUMMARY OF THE INVENTION
The invention provides improved siliceous oxide-coated magnetic particles having a high resistance to leaching of the magnetic material on exposure to aqueous acidic environments while also possessing a hydrous siliceous oxide adsorptive surface and excellent magnetic response.
In one aspect, the invention encompasses particles having a superparamagnetic or low Curie Temperature core surrounded by a hydrous siliceous oxide coating wherein the particles exhibit little or no transition metal leaching on exposure to acidic media. The particles preferably exhibit low metal leaching even after sonication (i.e. ultrasonic treatment).
In another aspect, the invention encompasses methods for forming siliceous coatings on superparamagnetic or low Curie Temperature cores wherein the method comprises forming an aqueous dispersion of the cores and depositing a siliceous oxide coating on said dispersed cores wherein said deposition process has an end pH of about 9 or less. Preferably, the aqueous dispersion of the cores is achieved by precipitation of the cores (crystals) in an aqueous medium and maintaining the cores in contact with an aqueous medium from the moment of their precipitation through the deposition of the silica coating thereon.
In another aspect, the invention encompasses methods for adsorbing substances wherein particles having a superparamagnetic or low Curie Temperature core surrounded by a hydrous siliceous oxide coating are used to adsorb a substance from a system and the adsorbent is subsequently removed from the system by application of a magnetic field.
The core material is preferably a superparamagnetic material such as magnetite.
DETAILED DESCRIPTION OF THE INVENTION
The invention provides improved particles comprising siliceous oxide-coated superparamagnetic or low Curie Temperature cores having a high resistance to leaching of transition metal on exposure to aqueous acidic environments while also possessing a hydrous siliceous oxide adsorptive surface (i.e. a surface characterized by the presence of silanol groups) and excellent magnetic response. The particles of the invention may comprise agglomerates of these siliceous oxide-coated cores.
The particles of the invention are especially characterized by their (1) high resistance to leaching of metal from the magnetic core, especially on exposure to acidic environments, (2) excellent magnetic performance (e.g. their ability to be separated from a liquid by application of a magnetic field and their dispersibility in the absence of a magnetic field), and (3) excellent adsorption/desorption behavior, especially for adsorption/desorption of genetic biological material from a lysate.
For the purpose of describing this invention, the “core” refers to all the material which is surrounded by the siliceous oxide coating. The core of the particles of the invention may be any material which is amenable to the silica coating process which material exhibits superparamagnetic performance (including materials which exhibit a relatively low remanent magnetism) and/or has a low Curie Temperature. Preferably, the core comprises a superparamagnetic material having a remanent magnetism of less than about 10 emu/g, more preferably less than about 2 emu/g, most preferably 0 to 1 emu/g. Where a low Curie Temperature magnetic material is used, that material preferably has a Curie Temperature below about 100° C., more preferably between about -50° C. and 100° C., most preferably between about 0° C. and 90° C. The core materials are generally characterized by the presence of one or more transition metals. Of the superparamagnetic materials, metal oxides containing group VIII transition metal are preferred, magnetite iron oxide being most preferred. The core preferably consists essentially of iron magnetite.
In general, superparamagnetic materials are preferred over low Curie Temperature magnetic materials since they do not require a change in temperature to cause loss of magnetism on removal of an external field. Particles of the superparamagnetic materials may be easily redispersed on removal of any external magnetic field due to their low or non-existent remanent magnetism whereas the low Curie Temperature materials require elevation of the temperature to a level above the Curie Temperature to permit easy redispersion of the particles on removal of any external magnetic field.
The magnetic behavior of many materials often varies with the physical size and state of the material. Thus, materials may exhibit differing magnetic performance depending on their crystal size, their temperature or their physical surroundings (e.g. if they are embedded in a matrix). For magnetite and most other superparamagnetic materials, the material should have a crystal size (diameter) less than about 1000 Å, more preferably about 600 Å or less. In some cases, it may be possible to form the core from a combination of the above-mentioned magnetic materials. Alternatively, it may be possible to dilute the composition of the core with a portion of non-magnetic material depending on the magnetic material used and the desired degree of response to an applied magnetic field. In general, it is preferred that the core consist essentially of superparamagnetic material.
In some instances, it may be possible to form one or more intermediate layers of a diverse non-siliceous oxide material on the surface of the superparamagnetic or low Curie Temperature material. Preferably, intermediate layers of metals such as gold or noble metals are avoided. Most preferably, all intermediate layers are avoided such that the siliceous oxide coating directly contacts the surface of the superparamagnetic or low Curie Temperature material. In some instances, the superparamagnetic or low Curie Temperature material may possess a surface region of slightly higher oxygen content associated with incidental oxidation of the material surface or exposure to mildly oxidizing environments. Such regions of higher oxygen are not considered as intermediate layers for purposes of describing the invention.
The siliceous oxide coating may be of any composition which provides the desired silanol functionality as well as the desired porosity/surface area and barrier properties. In general, the siliceous oxide coating preferably contains at least about 80 wt. % (dry basis) SiO
2
, more preferably at least about 90 wt. % (dry basis), most preferably 95 to 100 wt. % (dry basis). The siliceous oxide coating preferably does not contain significant amounts of transition metals in a form prone to leaching. In general, the amount of transition metal in the siliceous oxide coating is preferably less than 1 wt. % (dry basis), more preferably less than 0.1 wt. %, most preferably 0-0.01 wt. %. The siliceous oxide coating ma
Crump Linda Lee
Pryor James Neil
Cross Charles A.
Le Hoa T.
W. R. Grace & Co.,-Conn.
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