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Vol. 115 (2009) ACTA PHYSICA POLONICA A No. 1

Proceedings of the European Conference Physics of Magnetism (PM08), Poznan 2008

Magnetic Properties of Bacterial Nanoparticles

M. Timkoa, A. Dzarovaa, J. Kovaca, P. Kopcanskya, H. Gojzewskib,c

and A. Szlaferekd

aInstitute of Experimental Physics, Slovak Academy of Sciences

Watsonova 47, 04001 Kosice, SlovakiabInstitute of Physics, Poznan University of Technology

Nieszawska 13A, 60-965 Poznan, PolandcMax Planck Institute for Polymer Research

Ackermannweg 10, 55128 Mainz, GermanydInstitute of Molecular Physics, Polish Academy of Sciences

M. Smoluchowskiego 17, 60-179 Poznan, Poland

The objective of this study is to prepare and study magnetic properties of biological magnetic nanoparticles(magnetosomes) as a product of biomineralization process of magnetotactic bacteria Magnetospirillum sp. AMB-1.From temperature dependence of remanent magnetization and coercive field the Verwey transition is clearly seenat 105 K as a consequence of the large anisotropy along the chains of magnetosomes.

PACS numbers: 47.65.Cb, 51.60.+a, 75.30.Gw, 75.50.Tt, 75.60.Nt, 75.60.Ej, 75.75.+a

1. Introduction

It is known that upon cooling below Tv 120 Kmagnetite undergoes a sharp first-order metalinsulator

transition (so-called Verwey transition) at which heat ca-pacity has anomalous behavior, the resistivity increasessharply by two orders of magnitude [1] and transfor-mation from cubic to monoclinic structure appears [2].The results from measurements of remanence and co-ercive force measured in the monoclinic phase (belowTv) for fine grained magnetites with medium particlesizes of 0.037, 0.10 and 0.22 m have showed that coer-cive force Hc and remanence magnetization increase oncooling through Tv in submicron magnetite particles [3].These increases reflect the large increase in magnetocrys-talline anisotropy and magnetostriction in the cubic monoclinic transformation.

It is well known that bacterial magnetite nanoparti-cles (magnetosomes) in magnetotactic bacteria are ar-ranged in straight chains. After isolation from these bac-teria those chains tend to form closed loops. The reasonfor these phenomena is that the existence of lipid mem-brane surrounding magnetic core prevents them to sticktogether by electrostatic repulsion [4]. As was stated ear-lier, elasticity may play a major role in the magnetosomearrangement of the bent configuration [5]. The presenceof narrow shape and size distribution for magnetite crys-tals (magnetosomes) within magnetotactic bacteria sug-gests strongly that there are species-specific mechanisms

that control the process of biomineralization.The present paper reports on detailed hysteresis and

remanent magnetization measurements from 2 K up toroom temperature on magnetosomes with special atten-tion to the behavior at the Verwey transition.

2. Materials and methods

Bacterial magnetosomes investigated in this contribu-tion were synthesized by magnetotactic bacteria Magne-tospirillum sp. strain AMB-1 in laboratory conditions.This bacteria is a Gram-negative -proteobacteriumthat is more oxygen-tolerant and easier to grow on alarge scale. The detailed description of the cultivationprocess of magnetotactic bacteria and the isolation ofmagnetosomes from bacteria is given in our previouscontribution [6]. Owing to the presence of the enveloping

membrane, isolated magnetosome particles form stable,well-dispersed suspensions in water solution of HEPES(4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid).Typically, we were able to obtain 2.6 mg magnetosomesfrom a 1000 mL bacterial culture.

Atomic force microscopy (AFM) experiments in thispaper were performed with the Dimension 3100 atomicforce microscope retrofitted with the Nanoscope Vcontroller (Vecco, Digital Instruments, Santa Barbara,CA, USA). Images were obtained in the tappingmode with the super sharp silicone tip (MikroMaschDP15/HIRES/AIBS, Estonia), having less than 1 nm

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in radius, resonance frequency of about 325 MHz, andspring constant of about 46 N/m. In the tapping moderegime, the AFM cantilever oscillates close to its reso-nance frequency and the tip only slightly touches thesurface. All measurements were done in air (humidityof about 30%) at room temperature and a scan rate of1 Hz. Samples of magnetosomes in HEPES AFM inves-tigation were prepared by spin coating (SC) deposition

and droplet deposition on Gritek polished silicon wafers(GRINM Semiconductor Materials Co., Ltd., China) andhigh-grade mica wafers (Ted Pella, Inc., CA, USA), re-spectively. Preceding the magnetosomes deposition bySC technology, the silicone wafers were cleaned up byimmersion in the dichloromethane for about 10 min.For improvement of hydrophobicity, the substrates weremade hydrophilic in water (double purified MilliQ water,18 M cm1), ammonia and hydrogen peroxide solutionby immersion for about 20 min at 80C. Mica substrateswere freshly prepared immediately before droplet deposi-tion by cleavage. The studied magnetosomes were highlydiluted in HEPES to reduce the viscosity of the liquid.

To generate very thin films, in the SC process the rota-tion speed of 1500 rpm was applied (acceleration phaseof 500 rpm s1, deceleration phase can be negligible).The rotation time (60 s) facilitated the magnetosomesdispersion effect and support the evaporation of the so-lution. The nanoparticles were then fixed to the siliconesubstrates. One drop (about 1 L) was dropped on micasubstrates. The liquid was then spread over surface viaits excellent hydrophilic properties. Finally, all sampleswere placed in an oven for 12 h immediately after de-positions at temperature of 30C and pressure of about20 mbar to remove remaining solvent in the films.

A Quantum Design MPMS superconducting quantum

interference device magnetometer was used for magneticmeasurements reported here. The remanent magnetiza-tion line was determined from temperature dependence(2270 K) during warming process (RMW) after turningoff field-cooling (FC) magnetic field (500 Oe) when tem-perature reached 2 K. The annealing of this metastableremanent magnetization can provide useful informationregarding barrier and anisotropy. The coercive force wasobtained from hysteresis loops measurements at varioustemperatures in magnetic fields up to 5 T.

3. Results

According to our previous article [6], the X-ray diffrac-tion (XRD) using Co K radiations with a wavelengthof = 0.17903 nm has shown that powder diffractionpeaks of magnetosomes well fit with standard Fe3O4 re-flections which reveals that the magnetic nanocrystalswithin the magnetosome consisted of magnetite withoutanother phases. From the line-broadening of 311 peakof XRD powder diffraction pattern using the Scherrerequation the magnetosome diameter was calculated tobe 37 nm. On the other side, as we have stated previ-ously [6] and [7], the micrographs estimated from trans-mission electron microscopy (TEM) using JEOL 1200EX

Microscope working at 120 kV and 80 000 magnifica-tion by replication technique, showed that in our samplethe magnetosomes after isolation are arranged in chainswith tendency to create the bent and closed loops asa consequence of minimizing their magnetic stray fieldenergy. This arrangement of chains with 612 magneto-somes allows the existence of lipid membrane surroundingmagnetic core which prevents them to stick together by

electrostatic repulsion [5]. The mean size and standarddeviation estimated from TEM was 34 nm and 6 nm,respectively.

Figure 1 presents AFM topography image of studiedmagnetosomes deposited by spin coating. It was foundthat magnetosomes distributed on silicone and mica formclusters as well as single, isolated magnetite particles ofvarious sizes. Due to capillary forces (especially dur-ing solvent evaporation) and magneto-dipoledipole in-teractions clusters of few and many magnetosomes aremainly formed [8]. The geometrical sizes of magneto-somes (Fig. 1) were measured to be of about 20100 nm(in plane parallel to the silicon or mica surface), respec-

tively. Moreover, based on AFM images of separatedmagnetosomes (Fig. 1, right), it seems that their shapeis more ellipsoidal than spherical [9] which can contributeto the shape anisotropy of sample. The height and thelateral diameter measured by means of AFM do not cor-respond to each other, since such a cluster is formed.From analysis image (Fig. 1, right) the diameter of cho-sen magnetosome was estimated to be 54.8 nm which ishigher value as mean diameter estimated from TEM ex-periments which can be related to the various techniquesof measurement.

Fig. 1. AFM height image of magnetosomes depositedby spin coating on silicone wafers (left) and section anal-ysis (right) image (axes in nm).

In our previous work [6] the field cooling and zerofield cooling measurements of magnetization showed thesharp changes of magnetization in the vicinity of theVerwey transition ( 105 K). It seems that the bestway to reveal the magnetic signature of the Verweytransition is to measure the temperature dependenceof remanent magnetization (RMW) obtained afterswitching off the magnetic field applied during the fieldcooling experiment as this provides useful informationregarding to the barriers and anisotropy in sample. Inthis case the thermal energy kBT only competes with

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the dipolar collective barriers and magnetocrystallineanisotropy. The temperature dependences of the co-ercive force and remanent magnetization measured byMPMS instrument above described process are givenin Fig. 2. In temperature dependence curve of the

Fig. 2. Temperature dependence of the remanent mag-netization after switching off magnetic field of FC(500 Oe). The solid line shows the derivative of RWMcurve (top). Coercive fields vs. temperature estimatedfrom hysteresis loops measured for temperature belowand above the Verwey transition (bottom).

remanent magnetization a major drop due to the Verweytransition was observed between 90 and 110 K (Fig. 2,top) which is clearly shown in the derivative line of theRMW. Hysteresis loops were measured in a maximumfield of 5 T at selected temperatures from 2 K to 270 Kusing SQUID magnetometer. Values of coercive forcesat chosen temperatures are plotted in Fig. 2 (bottom).Coercive force is practically temperature independentabove the Verwey transition (105 K) and increases byfactor of 4 below this transition. The increase in Hcin the vicinity of Tv has been observed previously [3]for submicron magnetite nanoparticles with mediumparticle sizes 0.22 m. This increase reflects the large

increase in magnetocrystalline anisotropy in the cubicmonoclinic phase transition. The ratio of remanent andsaturation magnetization (Mr/Ms) was estimated to be0.47 is in excellent agreement with the theoretical valueof 0.5 for a random dispersion of single-magnetic-domainparticles. As it was stated in case of small nanoparticlesthe transition is smeared and remanent magnetizationand Hc are smaller. But in the case of magnetosomes

the chains act as long dipoles with enhanced effectiveanisotropy along the chain and thermal energy is insuf-ficient to overcome this barrier [10]. Therefore we canconclude that for magnetization behavior, large effectiveanisotropy coming from chains is responsible.

4. Conclusion

In this work we have studied the bacterial magnetitenanoparticles (magnetosomes) prepared by mineraliza-tion process. The AFM experiment showed that theshape of individual magnetosomes is more ellipsoidalthan spherical and shape anisotropy can contribute tothe character of transition, too. The sharp transition intemperature dependence of remanent magnetization andcoercive force at the Verwey transition (105 K) is con-nected with existence of chains of magnetosomes that actas long dipoles with enhanced effective anisotropy alongthese chains.

Acknowledgments

The Slovak Academy of Sciences, within the frame-work of Project VEGA No.6166 and Slovak Research andDevelopment Agency, within the framework of ProjectsAPVV No.APVV-0173-06, APVV 0509-07 and APVV-99-026505, supported this work.

References

[1] E.J.W. Verwey, Nature (London) 144, 327 (1939).

[2] A. Kosterov, Earth Planet. Sci. Lett. 186, 245 (2001).[3] O. Ozdemir, D.J. Dunlop, B.M. Moskowitz, Earth

Planet. Sci. Lett. 194, 343 (2002).

[4] D. S chuler, J. Mol. Microbiol. Biotechnol. 1, 79(1999).

[5] P. Valeri, J. Scherbakov, M. Winklhofer, Eur. Bio-phys. J. 26, 319 (1997).

[6] M. Timko, A. Dzarova, V. Zavisova, M. Koneracka,A. Sprincova, P. Kopcansky, J. Kovac, I. Vavra,A. Szlaferek, Magnetohydrodynamics 44, 3 (2008).

[7] M. Timko, A. Dzarova, P. Kopcansky, V. Zavisova,M. Koneracka, A. Sprincova, M. Vaclavkova,L. Ivanicova, I. Vavra, Acta Phys. Pol. A 113, 573

(2008).[8] K.R. Reddy, K.P. Lee, A.G. Iyengar, J. Appl. Polymer

Sci. 104, 4127 (2007).

[9] M. Rasa, A.P. Philipse, J. Magn. Magn. Mater. 252,101 (2002).

[10] R. Prozorov, T. Prozorov, S.K. Mallapra-gada, K. Surya, B. Narasimhan, T.J. Williams,D.A. Bazylinski, Phys. Rev. B 76, 054406 (2007).