RECENT DEVELOPMENT FOR SYNTHESIS OF MAGNETIC …

RECENT DEVELOPMENT FOR SYNTHESISOF MAGNETIC NANOPARTICLESFOR BIOMEDICAL APPLICATIONS

IAN ROBINSON*,‡ and NGUYEN T. K. THANH†,‡,§

*Department of Chemistry, University of LiverpoolCrown Street, Liverpool, L69 7ZD, UK

†The Davy-Faraday Research LaboratoryThe Royal Institution of Great Britain

21 Albemarle Street, London, W1S 4BS, UK‡Department of Physics and AstronomyUniversity College London, Gower Street

London, WC1E 6BT, UK§[email protected]

An update is presented on some recent syntheses of magnetic nanoparticles developed in ourgroup for potential use in biomedical applications. Particular attention is paid to (i)the preparation of magnetic nanoparticles that are readily dispersed in aqueous solution (ii) thesynthesis of alloy magnetic nanoparticles and (iii) novel synthesis methods used to control thephysical properties of the nanoparticles.

Keywords: Nanoparticles; magnetism; nanoparticles; synthesis of nanoparticles; magneticnanoparticles; TEM; XRD; SQUID; hyperthermia; core�shell; hollow.

1. Introduction

Nanotechnology can be described as the develop-ment of materials at the atomic and molecular levelin order to instill them with special physical andchemical properties.1 Nanoparticles (NPs) representan area within nanotechnology, which deals withparticles typically between 1 nm and 100 nm in size.These NPs can be divided into di®erent typesaccording to their physical and chemical properties.For example, semiconductor NPs fabricated frommaterials such as CdSe, CdS and ZnS, which exhibitatom-like energy states due to con¯nement e®ects,can be described as quantum dots.2 The electronicproperties of these particles allows them to be usedas active materials in single electron transistors.3

Furthermore, the atom-like energy states also play a

role in the special optical properties observed inthese materials, such as particle-size dependentwavelength of °uorescence.1 Metallic NPs representanother class of nanomaterial that have been stu-died since the mid-1800s. For instance, gold NPshave been used in the past for medical applications4

and the optical properties of these particles havebeen found to di®er greatly from the bulk material.5

Metallic NPs of particular interest are the magneticNPs that can consist of metals such as iron,6,7

cobalt8 and nickel,9 alloys such as CoPt10,11 andFeCo12 as well as iron oxides.

Interest in magnetic NPs has grown rapidly inrecent years due to their diverse applications inbiomedicines,13 novel materials for engineering14

and devices,14 which arise from the unique

International Journal of NanoscienceVol. 10, Nos. 4 & 5 (2011) 883�890#.c World Scienti¯c Publishing CompanyDOI: 10.1142/S0219581X11009337

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combination of small size, exotic properties andprocessability of these materials. Magnetic NPs canbe extremely useful due to their potential biomedi-cal applications in areas such as magnetic resonanceimaging (MRI),15 targeted drug delivery,16 hyper-thermia treatment of solid tumors13 and cellseparation.13

The physical properties can be attributed to thesize and surface e®ect of the NPs as well as inter-particle interactions and unusual electron transportproperties.1 These include a large surface area tovolume ratio and ubiquitous tissue accessibilitymaking the particles very suitable for use in in vitrodiagnostic applications.13 The magnetic propertiesof the NPs is particularly important for in vivoapplications where superparamagnetic particles arepreferred as these do not retain any magnetismupon the removal of a magnetic ¯eld as opposedto ferromagnetic material that aggregate afterexposure to a similar ¯eld.17,18

As mentioned above, the physical properties ofthese nanostructures (optical, electrical, chemical,magnetic, etc) are dependent upon their size, com-position and crystal structural ordering. Therefore,the fabrication of tailored NPs reliably and pre-dictably is highly desirable. One of the ways toproduce well de¯ned NPs is to tailor the surfaceproperties of the particles, often by coating orencapsulating them in a shell of a particular ma-terial.19 This coating and encapsulation can alsoprotect the core from extraneous chemical andphysical changes.6,14,20 Various substances havebeen used to form the protective shell of NPs e.g.silica,7 polymer,21 peptide22,23 and noble metals,24,25

however achieving long term stability of the par-ticles still remains a problem.

Modi¯cation of the surface properties is par-ticularly important when considering in vivo appli-cations to ensure that particles are biocompatible,non-toxic and stable to the reticulo-endothelialsystem: the body's major defense mechanism.13

Particles possessing a hydrophobic surface arereadily coated with plasma components in the bloodstream and are therefore easily removed from thecirculation, while more hydrophilic particles canresist coating and are cleared much more slowly.26

Many common hydrophilic coatings used on NPsinclude derivatives of dextran, polyethylene glycol,polyethylene oxide, poloxamers and polyoxamines.27

The dense brushes of polymers can inhibit theopsonisation (the process that causes foreign bodiesto be recognized by the body's reticulo-endothelial

system) of the particles, thereby increasing theircirculation time.

2. Peptides

Peptides were used as capping ligands for the ¯rsttime by Thanh et al.23 in the in situ synthesis ofwater-soluble cobalt NP with the aim to use theseparticles for biological applications. The peptideTLVNN (threonine�leucine�valine�asparagine�asparagine) a®orded some degrees of stability uponthe NPs. The ability to tune the properties of pep-tides (by varying the length, and sequence of aminoacids) makes them a unique class of ligands forcombinatorial nanomaterial synthesis. In additionto the 20 amino acids that occur naturally in pro-teins, over 100 unnatural amino acids are availablefor peptide synthesis, which provide access to ahuge chemical combinatorial space.

3. Thermoresponsive Polymers

Using a similar single step method it was possible tosynthesize monodisperse water-soluble 7� 1 nm Coand 8� 1 nm �-Fe2O3 NPs that were coated with athermo-responsive polymer, based on poly-N-iso-propylacrylamide, with tert-butylacrylamide as a comonomer.28 The polymer can undergo a confor-mational change in response to changes in tem-perature that allow the synthesis of monodisperseparticles at high temperature, but facilitate the

Fig. 1. TEM images of Co NP synthesized in the presence ofpeptide and DMF after a few minutes dispersed in water. Scalebar, 100 nm.

884 I. Robinson & N. T. K. Thanh


transfer of the synthesized particles into the aqu-eous phase at room temperature (Fig. 2). The syn-thesized NPs displayed superparamagnetic behaviorat room temperature, with blocking temperature of6K and 92K of for the Co and �-Fe2O3 NPs,respectively. The pendent polymers had a goodcolloidal stabilizing e®ect across a wide pH rangeand furthermore, the fabricated NPs can exhibit\smart" behavior such as a nonlinear response to anexternal change, e.g., temperature or pH, which canbe used to control drug delivery together withhyperthermia cancer treatment.19

Thermo-responsive polymers can also be used ina two-step synthesis method involving, initially thethermal decomposition of the respective organo-metallic complexes in the presence of oleic acid toproduce hydrophobic Co and �-Fe2O3 NPs.

29 Then,using a ligand-exchange process with the thermo-responsive polymer, the NPs can be dispersed inaqueous solution. Among di®erent thermo-respon-sive polymers investigated, it was found that thepolymer based on poly(N-isopropylacrylamide)with a co-monomer component of acrylic acid andacrylamide can be used in the ligand-exchange tocoat Co and �-Fe2O3 NPs, respectively. The ligand

exchange process resulted in the formation of larger`spherical aggregates' of Co NPs in aqueoussolution, whereas, discrete �-Fe2O3 NPs wereobserved when they were dispersed in water, indi-cating that these NP did not aggregate in aqueoussolution. The results of the ZFC and FC magneti-zation curve were consistent with this observation,with Co NPs having a higher blocking temperature(190K) than the �-Fe2O3 NPs (12K). The NPs arefound to be water-soluble at temperatures belowcoil-to-globule phase transition of the coatingpolymer (Fig. 3). As with the NPs produced inRef. 28, the temperature dependent aggregationwas found to be reversible in that upon cooling theNPs re-dispersed into the aqueous solution.

4. Synthesis in Aqueous Solution

An alternative method used to synthesize mono-disperse water-soluble magnetic Co NPs used thefacile reduction of CoCl2 in aqueous media in thepresence of alkyl thioether end-functionalized poly(methacrylic acid) (PMAA-DDT) ligands.30 Thesize and shape of the NPs are both tunable byvarying synthesis conditions. The size of the

Fig. 2. Polymers used as stabilizing ligands can retain their physical properties.

(a) (b)

Fig. 3. The e®ect of temperature upon the stability in water of the polymer coated Co (a) and iron oxide (b) NPs.

Recent Development for Synthesis of Magnetic Nanoparticles for Biomedical Applications 885


spherical NPs can be tuned between 2�7.5 nm bychanging the concentration of the polymer (Fig. 4).This synthesis approach also provided a route forproducing much larger spherical NPs of 80 nm aswell as anisotropic nanorods of 15� 36 nm. Thespherical NPs were superparamagnetic at roomtemperature and were stable in water for up to eightweeks when 0.12mM PMAA-DTT with molecularweight of 13500 g mol�1 is used as ligand.

Other material such as CoPt hollow nano-structures have also been produced using this simplereduction method.31 The NPs were prepared inaqueous solution in the presence of poly(methacrylicacid) pentaerythritol tetrakis (3-mercaptopropio-nate) (PMAA-PTMP) polymer or a mixture ofO-[2-(3-mercaptopropionyl-amino)ethyl]-O'-methyl-polyethylene glycol (PEG-SH) polymer and

cysteine�cysteine�alanine�leucine�asparagine�asparagine (CCALNN) peptide ligands. The pre-sence of the multi-thiol functional group of theligands was found to be essential for the formation ofthe hollow nanostructures (Fig. 5) and their per-imeter size could be tuned within the range of7�54 nmbychangingpeptide concentrationor lengthof polymer. These hollow nanostructures were water-soluble and superparamagnetic at room temperature.They were stable in a wide range of pH from 1 to 10,high electrolyte concentration up to 2MNaCl, and incell culture medium giving them great potential to beutilized in biomedical applications.

The chemical reduction of a nickel salt in aqu-eous solution has been used to synthesize super-paramagnetic and °uorescent Ni NPs (consisting ofa single material), which were directly conjugatedto an enzyme, bovine pancreatic a-chymotrypsin(CHT).32 The temperature dependence of themagnetization M(T) taken in zero ¯eld cooling and¯eld cooling conditions, exhibits the main featuresof superparamagnetism (Fig. 6). The structuralperturbation to the structure of the enzyme afterconjugation with the Ni NPs was monitored usingcircular dichroism studies. The functional integrityof the enzyme conjugated to the Ni NPs was inves-tigated by monitoring the enzymatic activity ofthe Ni�CHT conjugates using UV-VIS absorptionspectroscopy and comparison with the unbound

Fig. 4. TEM image of the Co NPs synthesized in the presenceof the PMAA-DDT polymer (molecular weight MW = 8 610 gmol�1Þ (upper panel); size distribution histograms of the CoNPs synthesized in the presence of PMAA-DDT polymers withdi®erent molecular weights at a concentration of 0.12mM(bottom panel); the inset shows the average particle size as afunction of the polymer molecular weight.

Fig. 5. HRTEM image of the CoPt hollow nanostructuressynthesized in the presence of 0.12mM CCALNN and 0.24mMPEG-SH (Mw = 5,000 g mol�1Þ. The inset shows the corre-sponding Fast Fourier Transform (FFT) image which shows 3strongest rings with d-spacings of 2.213, 1.954 and 1.362Å.



enzyme under similar experimental conditions.The conjugation of Ni NPs to CHT, was con¯rmedby Forster resonance energy transfer (FRET)studies using a °uorescent probe, 4-nitrophenylanthranilate (NPA), known to bind at the enzy-matic active site of CHT, as the donor (D) andNi-NP-bound CHT as the acceptor (A). The studiesalso demonstrated that the FRET from the donorNPA to the acceptor Ni NPs in CHT can be mon-itored to follow the D�A distance and hence theprotein structure during thermal unfolding. Such amultifunctional superparamagnetic, °uorescentand biologically active nanobioconjugate may be ofrelevance in nanoparticle-based diagnostic andtherapeutic applications.

5. Magnetic Alloy Nanoparticles

Magnetic alloy NPs are a particularly interestingclass of nanomaterial because of their magnetic

properties and chemical stability. Ung et al.33

reported the controlled syntheses of Fe�Pt, Fe�Pdand Fe�Pt�Pd NPs having di®erent isolatedshapes including sphere, cube, octopod-cube, star,rod, bilobe, tetrahedron, or multipod with size of5�50 nm (Fig. 6). The formation of such a rich var-iety of shapes was made possible by controlling thesynthetic conditions (e.g., nature and concentrationof the precursors, reaction time, temperature andatmosphere) of a thermal decomposition and re-duction of precursors in the presence of surfactants.The results of the investigation gave insight to thepossible connection between formation of di®erentshapes and the symmetry of the nuclei as well as thedivergent kinetics of particle growth.

It can sometimes be di±cult to control thecomposition of magnetic alloy NPs when they areproduced from two or more precursors. This couldbe overcome by using a single precursor of bime-tallic carbonyl cluster in a thermal decomposition

(a) (b)

(c)

Fig. 6. (a) ZFC and FC magnetization as a function of temperature measured at an applied ¯eld of 100Oe for the Ni�CHTnanobioconjugates. Inset: an expanded view of the plot, clearly showing TB. (b) M�H plots of Ni NPs at 5K (symbol: square) and15K (symbol: diamond). (c) M�H plot at 300K of the Ni�CHT nanobioconjugates.



process (Fig. 7). This novel synthesis method hasbeen used to produce FeCo3, FeNi4, FePt, andFe4Pt alloy magnetic NPs, with average diametersof 7.0, 4.4 nm, 2.6 nm and 3.2 nm.34 The chemicalcomposition of the synthesized NPs re°ected that ofthe bimetallic carbonyl cluster used for their syn-thesis. Di®erent reaction conditions, such as ligandconcentration, ligand type, and reaction tempera-ture had very little e®ect upon the chemical andphysical properties of the synthesized NPs. Studiesof the magnetic properties and X-ray di®ractionpatterns suggested subtle di®erences in the physical

properties of the NPs can be achieved by use of theappropriate precursor. Due to the great diversity ofcarbonyl cluster chemistry, this versatile method forthe synthesis of magnetic alloy NPs and can beapplied to a wide variety of other nanomaterials.

6. Pulsed Laser Irradiation

As mentioned above, NPs have a wide variety ofpotential applications and each application requiresNPs that have tailored properties for that appli-cation e.g., size. Interest in producing magnetic NPs

(a) (b)

(c) (d)

(e) (f)

Fig. 7. Formation of spherical (a), cubic (b), octopod-cubic (c), star (d), rod (e) and bilobar (f) Fe�Pt NPs. The image of the FastFourier Transform (FFT) of self-assembled NPs in the inset of (a) indicate that they were in a fcc structure. The inset of (d) showszoom-in view on a few nanostars.



with controllable sizes has led to the development ofnew synthesis methods. For example, Robisonet al.35 synthesized sub 4 nm Co NPs by usingpulsed laser irradiation to decompose cobalt car-bonyl in a solution of stabilizing ligands. There issome concern about the use of Co in biomedicalapplications as its toxicological assessments havenot been thoroughly studied. However, if the NPshave a diameter of less than 8 nm, they can be ¯l-tered through the glomeruli of the kidney and cantherefore be rapidly cleared from the body.36 Usingthis synthesis method it was possible to control thesize of the NPs by varying the reaction conditionssuch as the ligand concentration and the wave-length of light used. The formation mechanism ofthe NPs was also investigated by changing theseconditions. It is possible that this technique couldbe applied to the synthesis of a variety of nanoma-terials with potential applications such as biomedi-cine, catalysis, and water puri¯cation.

7. Conclusion and Perspectives

Nanoparticles with cores composed of di®erentmagnetic materials have vast potential for appli-cation in many di®erent areas of biomedicine, fromdiagnostics to treatment of diseases. The e®ects ofnanoparticles must be predictable and controllable,and deliver the desired result with minimum cyto-toxicity. These criteria can be met by carefultailoring of the ligand shell, allowing stabilization,

speci¯c targeting and recognition of biochemicalspecies. For a given biomedical application, a widerange of physical, chemical, biological and physio-logical factors and conditions must be taken intoaccount. However, by tuning the nature of the core,shell and ligands, these factors can be takenadvantage of to provide the desired, biocompat-ibility and biofunctionality, making them suitablefor a very wide range of applications in biomedicine.

Acknowledgments

Nguyen TK Thanh thanks the Royal Society for herRoyal Society University Research Fellowship. IanRobinson thanks the EPSRC for his funding.

References

1. C. B. Murray, C. R. Kagan and M. G. Bawendi,Ann. Rev. Mater. Sci. 30, 545 (2000).

2. A. L. Efros and M. Rosen, Ann. Rev. Mater. Sci. 30,475 (2000).

3. D. L. Klein, R. Roth, A. K. L. Lim, A. P. Alivisatosand P. L. McEuen, Nature 389, 699 (1997).

4. H. M. Joshi, D. R. Bhumkar, K. Joshi, V. Pokharkarand M. Sastry, Langmuir 22, 300 (2006).

5. R. Elghanian, J. J. Storho®, R. C. Mucic, R. L.Letsinger and C. A. Mirkin, Science 277, 1078(1997).

6. M. Ohmori and E. Matijevic, J. Colloid InterfaceSci. 160, 288 (1993).

Nd:YAG Laser

Co2(CO)8�in DCB

3.7 nm NPs

5.3 nm NPsλ=266 nm

λ=355 nm

Photo and

thermolysis

Photolysisonly

Fig. 9. The decomposition of a bimetallic carbonyl cluster can be used to produce alloy magnetic NPs.

Fig. 8. The decomposition of a bimetallic carbonyl cluster can be used to produce alloy magnetic NPs.



7. P. Tartaj, T. G. Carreno, O. B. Miguel, C. J. Sernaand P. Bonville, Phys. Rev. B 69, 094401 1 (2004).

8. M. Giersig and M. Hilgendor®, J. Phys. D: Appl.Phys. 32, L111 (1999).

9. J. S. Jung, K. H. Choi, W. S. Chae, Y. R. Kim, J. H.Jun, L. Malkinski, T. Kodenkandath, W. L. Zhou,J. B. Wiley and C. J. O'Connor, J. Phys. Chem.Solids 64, 385 (2003).

10. Y. Sui, L. Yue, R. Skomski, X. Z. Li, J. Zhou andD. J. Sellmyer, J. Appl. Phys. 93, 7571 (2003).

11. C. N. Chinnasamy, B. Jeyadevan, K. Shinoda andK. Tohji, J. Appl. Phys. 93, 7583 (2003).

12. Z. H. Ban, V. O. Golub, V. Kolesnichenko and C. J.O'Connor, Abs. Papers Amer. Chem. Soc. 227,U1454 (2004).

13. Q. A. Pankhurst, J. Connolly, S. K. Jones andJ. Dobson, J. Phys. D: Appl. Phys. 36, R167 (2003).

14. D. V. Goia and E. Matijevic, New J.Chem. 22, 1203(1998).

15. L. M. Parkes, R. Hodgson, L. T. Lu, L. D. Tung,I. Robinson, D. G. Fernig and N. T. K. Thanh,Contrast Media Mol. Imaging 3, 150 (2008).

16. W. Schutt, C. Gruttner, U. Hafeli, M. Zborowski,J. Teller, H. Putzar and C. Schumichen, Hybridoma16, 109 (1997).

17. B. Bonnemain, J. Drug Targeting 6, 167 (1998).18. Y. X. J. Wang, S. M. Hussain and G. P. Krestin,

Eur. Radiol. 11, 2319 (2001).19. N. T. K. Thanh, I. Robinson and L. D. Tung,

Dekker Encycloped. Nanosci. Nanotechnol. 1, 1(2007).

20. R. Partch and S. Brown, J. Adhesion 67, 259(1998).

21. J. P. Stevenson, M. Rutnakornpituk, M. Vadala, A.R. Esker, S. W. Charles, S. Wells, J. P. Dailey and J.S. Ri®le, J. Magn. Magn. Mater. 225, 47 (2001).

22. R. Levy, N. T. K. Thanh, R. C. Doty, I. Hussain,R. J. Nichols, D. J. Schi®rin, M. Brust and D. G.Fernig, J. Amer. Chem. Soc. 126, 10076 (2004).

23. N. T. K. Thanh, V. F. Puntes, L. D. Tung and D. G.Fernig, J. Phys.: Confer. Ser. 17, 70 (2005).

24. L. Y. Wang, J. Luo, Q. Fan, M. Suzuki, I. S. Suzuki,M. H. Engelhard, Y. H. Lin, N. Kim, J. Q. Wangand C. J. Zhong, J. Phys. Chem. B 109, 21593(2005).

25. W. R. Lee, M. G. Kim, J. R. Choi, J. I. Park, S. J.Ko, S. J. Oh and J. Cheon, J. Amer. Chem. Soc.127, 16090 (2005).

26. U. Gaur, S. K. Sahoo, T. K. De, P. C. Ghosh, A.Maitra and P.K. Ghosh, Int. J. Pharm. 202, 1 (2000).

27. L. M. Lacava, Z. G. M. Lacava, M. F. Da Silva, O.Silva, S. B. Chaves, R. B. Azevedo, F. Pelegrini, C.Gansau, N. Buske, D. Sabolovic and P. C. Morais,Biophys. J. 80, 2483 (2001).

28. I. Robinson, C. Alexander, L. T. Lu, L. D. Tung,D. G. Fernig and N. T. K. Thanh, Chem. Commun.44, 4602 (2007).

29. I. Robinson, C. Alexander, L. D. Tung, D. G. Fernigand N. T. K. Thanh, J. Magn. Magn. Mater. 321,1421 (2009).

30. L. T. Lu, L. D. Tung, I. Robinson, D. Ung, B. Tan,J. Long, A. I. Cooper, D. G. Fernig and N. T. K.Thanh, J. Mater. Chem. 18, 2453 (2008).

31. L. T. Lu, L. D. Tung, J. Long, D. G. Fernig andN. T. K. Thanh, J. Mater. Chem. 19, 6023 (2009).

32. P. K. Verma, A. Giri, N. T. K. Thanh, L. D. Tung,O. Mondal, M. Pald and S. K. Pal, J. Mater. Chem.20, 3722 (2010).

33. D. Ung, L. D. Tung, G. Caruntu, D. Delaprtas,I. Alexandruo, I. A. Prior and N. T. K. Thanh,Cryst. Eng. Commun. 11, 1309 (2009).

34. I. Robinson, S. Zacchini, L. D. Tung, S. Maenosonoand N. T. K. Thanh, Chem. Mater. 21, 3021 (2009).

35. I. Robinson, M. Volk, L. D. Tung, G. Caruntu,N. Kay and N. T. K. Thanh, J. Phys. Chem. C 113,9497 (2008).

36. M. Longmire, P. L. Choyke and H. Kobayashi,Nanomedicine 3, 1703 (2008).



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