# Patent

## Title
Harvesting micro algae

## Abstract

A reusable composite paramagnetic particle may comprise a paramagnetic core encased by a protective material to which is grafted a tendril layer comprising a plurality of polymeric chains. The polymeric chains may be designed to interact with a microorganism. The interaction between the microorganism and the polymeric chain may be electrostatic. The nanoparticle may be used in a method to isolate or recover microorganisms from solutions using an externally applied magnetic field.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
The present application is a divisional application of U.S. patent application Ser. No. 13/011,582, filed Jan. 21, 2011, now abandoned, which claims the benefit of priority under 35 U.S.C. §119(e) to U.S. provisional application 61/297,533 filed Jan. 22, 2010, the contents of which are hereby incorporated by reference in their entirety. The present application is related to U.S. provisional application Ser. No. 12/704,416, filed Feb. 11, 2010, titled NANOPARTICLES, COMPOSITIONS THEREOF, AND METHODS OF USE, AND METHODS OF MAKING THE SAME.


BACKGROUND
The subject technology relates generally to devices and methods for isolating microorganisms.
Microorganisms have many commercial applications. Bacteria, fungi, and algae may be used in various applications for the production of pharmaceuticals, food, supplements, and even fuel. For example, algae have applications in pharmaceutical, food, and biofuel production.
Microalgae is a term that may be used to distinguish single-celled, generally microscopic algae from multicellular algae. Algae may be found in fresh as well as salt water environments.
Use of microalgae in commercial applications may depend in part on understanding the biochemical and genetic makeup of microalgae. In addition, commercial application may also require cost-effective methods for handling microalgae. For example, at present microalgae harvesting techniques may limit their successful commercialization in the production of microalgae-based biofuels.
Some characteristics of microalgae may present challenges for their efficient harvest. For example, microalgae are generally small (a few to a few hundred micrometers), with low specific gravity, and a generally negative overall surface charge. In addition, microalgae may grow at low cell densities in water.
Current techniques used in harvesting microalgae may include centrifugation, filtration, flotation, flocculation, and ultrasound sedimentation etc. Each of the present harvesting techniques may have limitations. For example, centrifugation and ultrasound sedimentation may be slow processes with concomitantly high operation costs; filtration may be subject to clogging and shortened run times; flotation may require use of surfactants that may hamper downstream processes; and flocculation may require various chemical additives such as pro-oxidants (to induce liberation of extracellular organic matter), electrolytes (e.g. chitosan), or Al- and Fe-based compounds (to neutralize the surface charge and aid cell-to-cell adhesion). In addition, some of these techniques must be combined for efficient microalgae processing, for example flocculation may also require centrifugation in order to collect slowly-settled microalgae. Chemicals used in some of these (such as flocculation or flotation) may inhibit microalgae growth and may be detrimental for continuous growth-harvest cycled operation.
In some cases, microalgae flocculation has been modeled using theories of colloidal stability. For example, the use of polyelectrolyte-induced flocculation for microalgae harvesting can be understood by DLVO theory of colloidal stability (DLVO stands for Derjaguin, Landau, Verwey and Overbeek who made seminar contribution to the theory. See: R J Hunter, Foundations of Colloid Science, Clarendon Press, Oxford). DLVO theory models flocculation in terms of the interplay between electronic double layer repulsion, van der Waals attraction and entropic depletion interactions.
What is needed is an efficient method of harvesting microorganisms like microalgae, that has little or no adverse impact on downstream processes, and that is low-cost.
SUMMARY
The present disclosure is directed to composite particles comprising a paramagnetic core, the core being encased in a coating of a protective material, and the coating grafted to long, polymeric chains designed to interact with a microorganism.
In various embodiments of the present composite particle the paramagnetic core may comprise iron-oxide, the protective material coating may comprise silica, and the polymeric chains may be hydrophilic and/or carry a net charge in aqueous solution.
The size of the paramagnetic core may be between about 1 nanometer to about 10 micrometers, and may have a protective coating from about 10 nm to about 10 micrometers. The polymeric chains may be generally from about 0.1 to about 100 μm (micrometers) in length with a molecular mass up to about 107 Daltons (Da).
Methods for using the composite particle are also disclosed. In some embodiments, the particle may be used in harvesting microorganisms from a liquid medium comprising the use of an externally applied magnetic field. In various embodiments the particle may be used in a method of harvesting microalgae used in biofuel production. In various other embodiments the particle may be used in a method of harvesting microalgae in water treatment. In various other embodiments the particles may be used in a method of destabilizing colloidal mixtures.
The currently disclosed composite particle may be mass-produced at low cost and in some embodiments the particle is capable of re-use. In some embodiments the present method may require little or no post-harvest processing in order to remove chemicals, reagents, or materials. In various other embodiments, the method may involve a de-watering step which may require little maintenance, result in high concentration factors, consume little energy, and be operated continuously. Finally in many embodiments the currently claimed method may have little or no adverse effect on downstream processes.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a diagram of an embodiment of the presently disclosed composite paramagnetic particle.
FIG. 2 depicts the presently disclosed composite paramagnetic particles with grafted polymers adhering to microalgae.
FIG. 3A depicts rapid precipitation of the present composite paramagnetic Fe2O3 particle device by introduction of an external magnetic field. FIG. 3B depicts rapid precipitation of the present composite paramagnetic Fe3O4 particle device by introduction of an external magnetic field.
FIG. 4 depicts rapid coagulation of microalgae from a rich algal growth medium by applying a minuscule amount of the present composite paramagnetic particles (a few milligrams) and a moderate external magnetic field (provided by a magnetic stirrer bar).



DETAILED DESCRIPTION
The present disclosure is directed to composite particles comprising a paramagnetic core, the paramagnetic core being encased in a coating of a protective material, and the coating grafted to long, polymeric chains designed to interact with a microorganism.
In various embodiments of the present composite particle, the paramagnetic core may comprise iron-oxide, the protective coating may comprise silica, and the polymeric chains may be hydrophilic and/or carry a net charge in water.
The size of the paramagnetic core may vary from about 1 nanometer to about 10 micrometers. The thickness of the protective coating may be from about 10 nm to about 10 micrometers. The polymeric chains may be generally 0.1 to 100 micrometers in length with a molecular mass from about 101 to about 107 Daltons (Da).
Methods for using the composite particle as coagulation devices are also disclosed. In some embodiments, the particles may be used in harvesting microorganisms from a liquid medium involving the use of an externally applied magnetic field. In various embodiments the particle may be used in a method of harvesting microalgae used in biofuel production. In various other embodiments the particle may be used in a method of harvesting microalgae in water treatment. In various other embodiments the particle may be used in a method of destabilizing colloidal mixtures.
The currently disclosed composite particle may be mass-produced at low cost and in some embodiments the particle is capable of re-use. In some embodiments the present method may require little or no post-harvest processing to remove chemicals, reagents, or materials. In various other embodiments, the method may involve a de-watering step which may be of low maintenance, result in high concentration factors, consume little energy, and be operated continuously. Finally in many embodiments the currently claimed method may have little or no adverse effect on downstream processes.
As depicted in FIG. 1, the presently disclosed composite paramagnetic particle may comprise three general layers: (1) a core; (2) a protective coat or shell which encases the core; and (3) a tendril layer comprising polymer chains grafted on, to, or from the protective coating.
In various embodiments the paramagnetic core may comprise a γ-Fe2O3. The iron oxide allows the core to exhibit magnetism in response to an externally applied magnetic field, while exhibiting little or no magnetism in the absence of an external magnetic field. In various embodiments the core may comprise Fe3O4. In other embodiments the core may comprise other paramagnetic particles comprising, for example, Co, CoPt, CoO, CoFe2O4, Fe, FePt, Ni etc.
The protective shell may help prevent leaching of the paramagnetic material out of the core as well as create a surface for attachment or grafting of polymeric chains. The protective coating may also aid in imparting wettability to the particle.
In various embodiments the protective coating material may be silica. A silica shell may help provide a rich chemistry (silane chemistry) for modifying the surface of the particle.
The tendril layer may aid in interacting with microorganisms. The tendril layer may also help improve the dispersion of composite particles. The polymeric chains of the tendril layer may help promote aggregation of microorganisms. The polymeric chains may be designed to interact with microorganisms in various ways, for example through electrostatic interactions, van der Waals forces, and entropic depletion effects. In further embodiments the interactions may involve a combination of various forces.
In various embodiments, composite paramagnetic particles may be added to a solution containing a microorganism. The polymeric chains of the particles may interact with microorganisms within the solution. A composite paramagnetic particle may have multiple polymeric chains which may allow an individual particle to aggregate several microorganisms. After allowing the composite particles and microorganisms to interact, an external magnetic field may be applied. The application of an external magnetic field may result in attracting the paramagnetic core to the magnetic source. This attraction may result in concentration and/or precipitation of the microorganism.
The presently disclosed composite particles may be used to rapidly precipitate microorganisms from various solutions. In some embodiments the microorganism solution may be less than 10 liters. In other embodiments the microorganism solution may be greater than 10 liters. In some embodiments the composite particle may be used to continuously harvest microorganisms.
The presently disclosed composite particles may cause little or no adverse interference to common processing steps performed on microorganisms. For example, the disclosed composite particle may be used in lipid extraction methods, solvent extraction, mechanical press, and/or supercritical water extraction. Additionally, the particle may be used to recover microorganisms from solution without additional mechanical or chemical processing. After elution of the microorganism from the composite particle, the composite particle may be re-collected by magnetic force and re-used.
The choice of particle sizes, volume fractions, polymer chain length, compositions and charge states may be varied to optimize efficient harvesting of various target microorganisms.
Paramagnetic Core
Paramagnetic refers to substances that may exhibit magnetism in response to an external magnetic force. Paramagnetic substances generally do not exhibit magnetism in the absence of an externally applied magnetic force.
A wide variety of methods have been reported for making paramagnetic particles. For instance, paramagnetic γ-Fe2O3 particles can be prepared by injecting Fe(CO)5 into a hot surfactant mixture of octyl ether and oleic acid (see descriptions in: Hyeon T et al, J. Am. Chem. Soc., 2001, 123, 12798-12801), and paramagnetic Fe3O4 particles can be prepared by an autoclave reaction of FeCl3, ethylene glycol, NaAc, and polyethylene glycol mixture solution (see descriptions in: Deng H et al, Angew. Chem. Int. Ed. 2005, 44, 2782-2785). Preparation of other paramagnetic particles are also well documented (see for example: Jeong U et al, Adv. Mater. 2007, 19, 33-60 and references therein).
The composite particle may comprise a generally paramagnetic core. Various materials may be used to construct the paramagnetic core of the composite particle device. In some embodiments, the core material may comprise iron oxide. The magnetic core may also comprise other metal oxides such as Co. In further embodiments, the magnetic core may also include non-metal oxide materials like Co, CoPt, Fe, FePt, Ni etc. may be used. Further, those skilled in the art will recognize that different materials may be combined in the manufacture of these particles.
The paramagnetic core as described herein is generally between about 1 nanometer and about 10 micrometers in size. The paramagnetic core may have various shapes, including rods, spheres, and platelets.
In various embodiments, the paramagnetic core may measure greater than 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 200 nm, 210 nm, 220 nm, 230 nm, or 240 nm in at least one measureable dimension. In other embodiments, particles may measure less than 250 nm, 240 nm, 230 nm, 220 nm, 210 nm, 200 nm, 190 nm, 180 nm, 170 nm, 160 nm, 150 nm, 140 nm, 130 nm, 120 nm, 110 nm, 100 nm, 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, or 40 nm in at least one measurable dimension.
Protective Coating of Paramagnetic Core
In various embodiments, the paramagnetic core may be coated with a protective material. The protective material may help to encase the paramagnetic material of the core and may also provide a surface for attachment and/or grafting of polymeric chains.
The protective material may create a shell of variable thickness. In some embodiments, the shell may be about 100 nm. In other embodiments the protective shell may be generally less than 100 nm thick. In other embodiments the shell may be generally greater than 100 nm thick.
In various embodiments the material used to coat the paramagnetic cores comprises silica. Silanization of the particle may help create a functional coating for grafting or attaching polymer chains to the nanoparticle. Alternative embodiments may use non-silica materials, including but not limited to polymeric film, carbon, carbon nitride, boron nitride etc.
In various embodiments, silanization may include preparing a functional silane solution using anhydrous solvents and adding paramagnetic core material to the silane solution, followed by allowing the solution to react then washing and drying the particles. In other embodiments, silanization may include the use of reverse microemulsion where the paramagnetic cores are dispersed in a water-in-surfactant reverse microemulsion, and hydrolyzation of alkyl silane leads to the formation of silica coating around the paramagnetic core (see for example: Yi D K et al, Chem. Mater., 2006, 18, 614-619). In some embodiments the functional silanes may have a generic form of R1x—Si—(OR2)4-x, where x is 1, 2, or 3, R2 is usually an alkyl-group, R1 is an alkyl chain with a functional moiety as the end group. In some embodiments the functional moiety may be alkyl, alkene, alkyne, aryl, azide, hydroxyl, carboxyl, amine, amino, thio, epoxy, cyano, or halogen.
The protective coating may be modified prior to the graft-to or graft-from polymerization methods to aid in covalently bonding a variety of polymers with controllable chain length to the coating. For example, a silica coat may be modified, for instance, by (3-Aminopropyl)triethoxysilane or 5,6-Epoxyhexyltriethoxysilane, and anchored with RAFT (reversible addition-fragmentation chain transfer) functionalities. This modification may allow for “controlled/living polymerization” of grafted polymers (an example is as described in “Functional polymers from novel carboxyl-terminated trithiocarbonates as highly efficient RAFT agents” by Lai, J. T.; Filla, D.; Shea, R., Macromolecules 2002, 35, 6754-6756). Controlled/living polymerization describes polymerization in which termination of a growing polymer chain is inhibited, and the polymer chain may grow until monomers are consumed after which polymerization may again continue if new monomers are added to the reaction. RAFT is one example of controlled/living polymerization.
Polymeric Chain
In various embodiments, the outmost layer of the disclosed composite particles is comprised of grafted polymer chains. The polymeric chain length may be varied to aid in interacting with a target microorganism. In various embodiments the polymeric chains may vary from about 0.1 μm and about 100 μm. In various embodiments the polymeric chains may be less than 0.1 μm in length. In various embodiments the polymer chain length may be greater than 100 μm.
Polymeric chains may vary in molecular mass from about 1 kDa to about 10,000 kDa. In various embodiments the polymeric chains may be less than 1 kDa. In various embodiments the polymeric chains may be greater than 10,000 kDa.
In various embodiments the polymers are comprised of subunits that are hydrophilic. In some embodiments the polymer chain may be comprised of subunits that are generally negatively charged. In other embodiments the polymer chain may be comprised of subunits that are generally positively charged. In various other embodiments the polymeric subunits comprising the polymer chain may be neutral, negatively charged, and/or positively charged. In other embodiments the polymeric subunits may be chosen to select a generally uniform or non-uniformly charged polymer chain. In various embodiments the polymer chain may be neutral.
In various embodiments the number of polymeric chains per composite paramagnetic particle is between about 5 and about 20 chains. In various embodiments individual particles may have less than about 5 chains. In various other embodiments individual nanoparticles may have more than 20 polymeric chains.
The polymeric chains may be generally flexible to aid in interacting with target microorganisms.
In various embodiments the polymer chain may be generally cationic. In those embodiments the polymers include, but are not limited to, chitosan, poly (N-ethyl-4-vinypyridinium), poly(2,2-(dimethyl aminoethyl methacrylate), poly(ethylene imine), poly(allylamine), and poly(diallyl dimethyl ammonium chloride) etc.; In various embodiments the polymer chain may be generally anionic. In those embodiments the polymers include, but are not limited to, poly(acrylic acid), poly(styrene sulfonate), poly(vinyl sulfate), and poly(3-sulfopropyl methacrylate) etc.; In various other embodiments the polymer chain may be generally hydrophilic and neutral. In those embodiments the polymers include, but are not limited to, polyethylene glycol, poly(2-methyloxazoline, poly(2-ethyl-2-oxazoline), and polyacrylic amide etc.
The polymeric chains may be generally homogeneous on an individual composite particle, that is all chains on a composite paramagnetic particle may be substantially similar in length, weight, composition, charge, etc. In various other embodiments the polymeric chains on an individual composite particle may vary, that is, various chains on an individual particle may differ in length, mass, composition, and/or charge etc.
Various methods are available to elute microorganisms from the composite paramagnetic particles after recovery of the microorganism. For example, elution may be achieved by treating the composite particle-bound microorganism with a high or low ionic strength solution, organic solvent and/or ionic liquid extraction, and/or supercritical water treatment etc.
Polymerization
In various embodiments the grafted polymer chains are formed by step-growth or chain-growth polymerization. Step growth may be referred to polymerization that occurs in a stepwise fashion, for example monomer→dimer→trimer→etc. This type of polymerization may involve reaction of functional groups between monomers (such as —OH and —COOH) and as a result, the molar mass increases slowly. Chain grown may refer to a fast linkage of monomers by initiation (activate the unsaturated bonds). During chain growth, the molar mass may increase rapidly. In some embodiments the polymerization process occurs as controlled/living polymerization, such as for example RAFT polymerization. Controlled/living polymerization describes polymerization in which termination of a growing polymer chain is inhibited, and the polymer chain may grow until monomers are consumed after which polymerization again continue if new monomers are added to the reaction. RAFT is one example of living polymerization.
RAFT polymerization operates on the principle of degenerative chain transfer (see review: Moad G et al, Aust. J. Chem., 2005, 58, 379-410). Without being limited to a particular mechanism, Scheme 1 shows a proposed mechanism for RAFT polymerization. In Scheme 1, RAFT polymerization involves a single- or multi-functional chain transfer agent (CTA), such as the compound of formula (I), including dithioesters, trithiocarbonates, xanthates, and dithiocarbamates. The initiator produces a free radical, which subsequently reacts with a polymerizable monomer. The monomer radical reacts with other monomers and propagates to form a chain, Pn*, which can react with the CTA. The CTA can fragment, either forming R*, which will react with another monomer that will form a new chain Pm* or Pn*, which will continue to propagate. In theory, propagation to the Pm* and Pn* will continue until no monomer is left or a termination step occurs. After the first polymerization has finished, in particular circumstances, a second monomer can be added to the system to form a block copolymer.




RAFT polymerization involves a similar mechanism as traditional free radical polymerization systems, with the difference of a purposely added CTA. Addition of a growing chain to a macro-CTA yields an intermediate radical, which can fragment to either the initial reactants or a new active chain. With a high chain transfer constant and the addition of a high concentration of CTA relative to conventional initiator, synthesis of polymer with a high degree of chain-end functionality and with well defined molecular weight properties is obtained.
RAFT polymerization may be used in the synthesis of multifunctional polymers due to the versatility of monomer selection and polymerization conditions, along with the ability to produce well-defined, narrow polydispersity polymers with both simple and complex architectures.
In particular embodiments, RAFT polymerization is used to produce a variety of well-defined, novel biocopolymers as constructs for multifunctional systems for the surface modification of disclosed composite particles consisting of a paramegnetic core encased in a protective coating. The inherent flexibility of RAFT polymerizations makes it a candidate to produce well-defined polymer structures.
In various embodiments, the polymerization process is Atom Transfer Radical Polymerization (“ATRP”) ATRP is an example of controlled/living radical polymerization. In some embodiments, ATRP employs atom transfer from an organic halide to transition-metal complex to generate the reacting radicals, followed by back transfer from the transition metal to a product radical to form the final polymer product. One example of ATRP may be found in, Patten T E, Matyjaszewski K, “Atom Transfer Radical Polymerization and the Synthesis of Polymeric Materials”, which is incorporated herein in its entirety. (Adv. Mater., 1998, 10, 901-915).
In various embodiments, the polymerization process is ring-opening polymerization. Ring-opening polymerization is a form of chain-growth polymerization, in which the terminal end of a polymer acts as a reactive center, where further cyclic monomers join to form a larger polymer chain through ionic propagation. One example of ring-opening polymerization may be found in, Dechy-Cabaret O et al, “Controlled ring-opening polymerization of lactide and glycolide”, which is incorporated herein in its entirety. (Chem. Rev., 2004, 104, 6147-6176).
In various embodiments, the polymerization process is anionic polymerization. Anionic polymerization is a form of chain-growth polymerization that involves the polymerization of vinyl monomers with strong electronegative groups. One example of anionic polymerization may be found in Hsieh H and Quirk R “Anionic Polymerization: Principles and practical applications”, which is incorporated by reference in its entirety. (Marcel Dekker, Inc: New York, 1996);
In various embodiments, the polymerization process is cationic polymerization. Cationic polymerization is a type of chain growth polymerization in which a cationic initiator molecule binds and transfers charge to a monomeric unit, which becomes reactive as a result and reacts similarly with other monomeric units to form a polymer. One example of cationic polymerization may be found in Odian G, “Principles of Polymerization”, which is incorporated by reference in its entirety. (Wiley-Interscience, Hoboken, N.J., 2004)
In certain aspects, pre-formed polymers may be grafted to the protective coating instead of the graft-from polymerization on the protective coating.
Flocculation
Inter-particle forces, as well as hydrodynamics, and solution conditions (pH, ion strength, etc.) may aid in the harvesting of microalgae from solutions. For example, DLVO flocculation theory posits that identifiable distances may exist (primary and secondary minima) at which the forces of attraction may exceed those of electrostatic repulsion which may result in adhesion. DLVO theory may be used to model the present interaction between microalgae and the currently disclosed composite particle device.
Some studies in colloidal science have recently revealed that extremely low nanoparticle volume fractions (<10−5) could be used to stabilize or flocculate colloidal suspensions depending on the nature of colloid-nanoparticle interactions (see: Tohver, V. et al, Proc. Natl. Acad. Sci. U.S.A., 2001, 98, 8950-8954).
In various embodiments the composite particles may be added to a solution containing microorganisms and allowed to interact. In various embodiments the solution may be stirred to help facilitate interaction between the composite particles and microorganisms.
EXAMPLES
Example 1
The presently disclosed γ-Fe2O3@SiO2 composite particles may be well dispersed in solution. FIG. 3A, at left shows a solution of silica coated Fe-oxide nanoparticles in solution. The photo at right in FIG. 3A shows the solution shortly after introduction of an external magnetic field. This figure demonstrates that a homogeneous solution of the present composite particle devices may be rapidly, efficiently, and inexpensively concentrated. The presently disclosed composite particles can be precipitated out of solution rapidly under magnetic field (FIG. 3A). FIG. 3B depicts similar results using Fe3O4 particles.
Example 2
The presently disclosed γ-Fe2O3@SiO2@Polymer composite particles are used as coagulation agents to rapidly and efficiently concentrate microalgae from growth medium under external magnetic field (H), as schematically depicted in FIG. 2. The composite particles are able to be re-used after microalgae elution or biofuel extraction, thus greatly reduce the cost.
Example 3
Fe3O4@SiO2@Polymer composite particles were used as coagulation agents to rapidly and efficiently concentrate microalgae from a rich algal growth medium. A moderate external magnetic field (H) was provided by a magnetic stirrer bar. The microalgal strain used was Nannochloropsis, which has a small diameter ( ̃2 μm) and low specific density due to high oil content ( ̃50% by dry weight). Nannochloropsis is extremely hard to harvest via conventional methods. FIG. 4 shows nearly 100% coagulation of Nannochloropsis occurs in a few seconds after adding a few milligrams of the presently disclosed Fe3O4@SiO2@Polymer composite particles (right: before coagulation; left: after coagulation).
All directional references (e.g., upper, lower, upward, downward, left, right, leftward, rightward, top, bottom, above, below, inner, outer, vertical, horizontal, clockwise, and counterclockwise) are only used for identification purposes to aid the reader's understanding of the example of the invention, and do not create limitations, particularly as to the position, orientation, or use of the invention unless specifically set forth in the claims. Joinder references (e.g., attached, coupled, connected, joined, and the like) are to be construed broadly and may include intermediate members between a connection of elements and relative movement between elements. As such, joinder references do not necessarily infer that two elements are directly connected and in fixed relation to each other.
In some instances, components are described with reference to “ends” having a particular characteristic and/or being connected with another part. However, those skilled in the art will recognize that the present invention is not limited to components which terminate immediately beyond their points of connection with other parts. Thus, the term “end” should be interpreted broadly, in a manner that includes areas adjacent, rearward, forward of, or otherwise near the terminus of a particular element, link, component, part, member or the like. In methodologies directly or indirectly set forth herein, various steps and operations are described in one possible order of operation, but those skilled in the art will recognize that steps and operations may be rearranged, replaced, or eliminated without necessarily departing from the spirit and scope of the present invention. It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not limiting. Changes in detail or structure may be made without departing from the spirit of the invention as defined in the appended claims.
It will be apparent to those of ordinary skill in the art that variations and alternative embodiments may be made given the foregoing description. Such variations and alternative embodiments are accordingly considered within the scope of the present invention.

## Claims

1. A method of using a composite particle to recover a microorganism from a solution comprising:
adding at least one composite particle to a solution comprising microorganisms, wherein the composite particle has a paramagnetic core, a silica shell coating the core, and a plurality of polymeric chains having a molecular mass of 1 to 10,000 kDa grafted onto the silica shell;
allowing the composite particles and microorganisms to interact in the solution such that one or more microorganisms aggregate with a composite particle;
applying an external magnetic field to the solution; and
recovering the composite particle-microorganism aggregates from the solution.

2. The method of claim 1, wherein the polymeric chains interact with the microorganisms to promote aggregation of the microorganisms.

3. The method of claim 1, wherein the microorganism is microalgae.

4. The method of claim 3, wherein the microalgae is Nannochloropsis.

5. The method of claim 3, wherein the microalgae are producing biofuels.

6. The method of claim 1, wherein the solution is drinking water.

7. The method of claim 1, further comprising allowing the composite particles to be suspended in the solution after addition of the particles to the solution.

8. The method of claim 7, further comprising collecting and reusing the composite particles after elution of the microorganism from the composite particle.

9. The method of claim 1, further comprising eluting the microorganisms from the composite particle-microorganism aggregates.

10. The method of claim 1, wherein the paramagnetic core comprises iron oxide.

11. The method of claim 1, wherein the paramagnetic core comprises Fe2O3.

12. The method of claim 1, wherein the paramagnetic core comprises Fe3O4.

13. The method of claim 1, wherein the polymeric chains are hydrophilic.

14. The method of claim 1, wherein the polymeric chains have a net positive charge.

15. The method of claim 1, wherein the polymeric chains have a net negative charge.

16. The method of claim 1, wherein the polymeric chains have no net charge.

17. The method of claim 1, further comprising first producing the composite particle by grafting the polymeric chains onto the silica shell by one or more of step-growth polymerization, chain-growth polymerization, and controlled/living polymerization.

18. The method of claim 17, wherein grafting by controlled/living polymerization is one or more of reversible addition fragmentation transfer polymerization, atom transfer radical polymerization, ring-opening polymerization, anionic polymerization, and cationic polymerization.

19. The method of claim 18, wherein grafting is by reversible addition fragmentation transfer polymerization.

20. The method of claim 18, further comprising modifying the silica coating by 3-aminopropyl-triethoxysilane or 5,6-epoxyhexyltriethoxysilane prior to grafting the polymeric chains.