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2008, 112, 3191-3193 Published on Web 02/13/2008
Magnetotransport of One-Dimensional Chains of CoFe2O4 Nanoparticles Ordered along DNA Joseph M. Kinsella† and Albena Ivanisevic*,†,‡ Weldon School of Biomedical Engineering, and Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47906 ReceiVed: December 21, 2007; In Final Form: February 1, 2008
We have investigated the magnetotransport properties of 5 nm CoFe2O4 nanoparticles aligned into one-dimensional chains using a DNA guide. The chemically synthesized particles were capped with a positively charged ligand molecule that drives the electrostatic assembly when incubated in solutions containing DNA. To test the transport, these nanoparticle-coated DNA strands were adsorbed onto microfabricated devices. Large room-temperature magnetoresistance is observed at high fields in these samples.
Introduction One-dimensional chains of nanoparticles exhibit unique magnetic properties due to geometric confinement, magnetostatic interactions, and nanoscale domain formation.1 These fundamental properties have many potential applications, for example, data storage devices, logic devices, and magnetic field sensing.2 Such systems also provide valuable insight into basic physical phenomena and the study of basic properties of nanoscale magnetism. For this to be realized, first, a method must be devised that will allow control over the arrangement of nanoparticles. Advances in focused ion beam and electron beam lithography can be used to fabricate platforms containing nanomagnets in the submicron range.3,4 Studies of these systems have provided important insight into magnetization reversal processes and magnetostatic interactions. Unfortunately, these lithographic techniques cannot create nanoscale structures less than tens of nanometers in overall size. Chemical synthesis on the other hand has been an effective way to create inorganic magnetic materials from single atoms to micron-sized particles.5-7 The organization of chemically synthesized nanomaterials into specific orientations or patterns on surfaces itself has been the focus of intense research efforts.8-11 A variety of less conventional techniques have been developed. Our chosen method relies on using sacrificial molecular guides. These guides typically consist of soft materials that can physically guide nanoscale structures into complex patterns or geometries using solution-based chemistries. DNA has been an extensively studied molecular guide for inorganic nanoparticles or ionic precursor salts to be developed using electroless deposition techniques.12-14 When used to organize nanoparticles, it has proven to be an effective way to create structures analogous to pearl necklaces in particle alignment.15 Unlike the more conventional top-down fabrication approaches, the bottom-up DNA guide method is limited to organizing only * To whom correspondence should be addressed. E-mail: albena@ purdue.edu. † Weldon School of Biomedical Engineering. ‡ Department of Chemistry.
10.1021/jp712002a CCC: $40.75
particles with diameters less than 10 nm. This yields very high aspect ratio features that are in the range of several microns in length and have diameters of less than 10 nm, which are defined by the diameter of the nanoparticles used. Here, we use a combination of chemical synthesis to produce nanoparticles and molecular templating using DNA to arrange the nanoparticles into one-dimensional chains. Once formed, the chains of nanoparticles are interfaced with conventionally fabricated microelectronics to provide a platform which can be measured with standard electrical characterization systems. We synthesized 5 nm cobalt ferrite nanoparticles that were capped with a positively charged ligand coating.16 When these particles are incubated in a solution containing DNA, which has a negative charge along its backbone, the particles align due to electrostatic interactions along the molecule. While any DNA can be used as a template, we chose λ phage DNA, which has a persistence length of 20 µm, has a mapped sequence, and is readily available. The molecular templating technique is advantageous since it is rapid, inexpensive, and performed on a bench top. Experimental Section Synthesis of the CoFe2O4 nanoparticles was performed via a thermal decomposition of iron(III) acetylacetonate and cobalt(II) acetylacetonate at ∼ 260 °C in 2-pyrrolidinone. The reactants were heated to 200 °C under a nitrogen atmosphere and held at this temperature for 30 min. The temperature was then raised to 260 °C and refluxed for 30 min. The high boiling point solvent, 2-pyrrolidinone, acts as a ligand for the CoFe2O4 particles formed in the reaction. The reaction is then brought to room temperature, and an equivalent volume of methanol is added to the solution to precipitate the nanoparticles from the solvent. The particles are further purified by iterative centrifugation in acetone and finally resuspended in a 10 mM Tris buffer containing 50 mM Mg2+. The particles are characterized using FTIR, XPS, and MFM to confirm ligand conjugation on the surface, chemical composition, and magnetic properties. The 5 © 2008 American Chemical Society
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Figure 1. Atomic force microscope (AFM) images of the samples used for magnetotransport studies. (A) A substrate that was not modified by adding any DNA or nanoparticles. (B) CoFe2O4-coated DNA connects the two gold electrodes spanning the 1.75 µm gap between.
nm poly-L-lysine gold nanoparticles are purchased (Ted Pella, CA) and reconstituted in 10 mM Tris buffer containing 50 mM Mg2+.
Letters To form the pseudo-1D chains of nanoparticles on the λ phage DNA molecules, equivalent masses of the positively charged nanoparticles and DNA are mixed into 10 mM Tris buffer solutions with 50 mM Mg2+. The resulting solutions are allowed to age at room temperature for a period of longer then 1 h but no longer then 24 h. The coated DNA is then adsorbed onto gold electrodes microfabricated on a silicon substrate. This is done by spotting 1 µL of particle-coated DNA solution onto the substrate and driving the spot parallel to the electrodes with a stream of nitrogen gas. The moving interface provides surface tension that is great enough, pN, to stretch the coiled DNA strands in solution but not great enough to break the covalent bonds holding the DNA together.17 The adsorbed coated DNA that results from this is verified by atomic force microscope (AFM) imaging (Figure 1). Samples containing DNA coated with CoFe2O4, gold, and with no coating are prepared using this method and verified by AFM. The gold and uncoated DNA samples are used as control samples for the experiments exploring the magnetotransport of the CoFe2O4-nanoparticlecoated DNA. The transport studies are performed using a Quantum Design physical properties measurement system (PPMS) interfaced with a Keithley 4200 semiconductor characterization system. Each sample is wire bonded to the PPMS sample puck using gold. The samples are treated as two wire resistors with voltage sweeps recorded from -250 to 250 mV while the current is measured. The applied external field is swept from -20 to 20 kOe, and data are collected at both room temperature and at 5 K. Results In control samples, uncoated DNA, and gold-coated DNA, there was no appreciable difference in IV curves for any applied field or at either temperature. At 300 K, the gold sample had a
Figure 2. (a) IV curves of the CoFe2O4-nanoparticle-coated DNA measured at varying fields. The current transitions from nA to µA levels as the applied magnetic field approaches positive values. In high fields (greater than 10 kOe), the conductivity of the samples shows significant increase as its resistance decreases to less than 100 kΩ. (b) Plot of Au-nanoparticle-coated DNA showing no change in conductivity at any applied field. (c) % Magnetoresistance (MR) response of the sample probed in (a). (d) Proposed mechanism for MR.
Letters resistance of 290 Ω with no applied field, 207 Ω when the strength of the applied field was -20 kOe, and 272 Ω at 20 kOe. The bare DNA control sample also had no significant change in resistance in any applied field. The resistance of the uncoated DNA sample at 300 K was in the GΩ range, which was also the case when a cleaned, unmodified device was measured. Similar results were obtained when the experiments were performed at 5 K. The samples probed containing a CoFe2O4 coating displayed significant changes in behavior when external fields were applied (Figure 2a,c). SQUID magnetometry data of bulk CoFe2O4 indicated that the particles are weakly ferromagnetic at room temperature, with a saturation magnetization of 89 emu/g occurring when the applied field is greater than 5 kOe.18 In the transport studies, the sample shows an increasing conductivity when in the presence of an increasing external field at 300 K. When no external or negative fields are applied, the resistance of the sample is an order of magnitude larger than what is observed at 20 kOe. The room-temperature magnetoresistance in the CoFe2O4 samples reaches 82.3% at high fields. The likely mechanism of the magnetoresistance effect observed arises from magnetostatic dipolar interactions (Figure 2c). As the nanoparticles are confined into one-dimensional arrays, the long-range interactions between the nanoparticles produce coinciding transitions of their magnetic moments. We have noticed this phenomenon in previous studies where the CoFe2O4 nanoparticles aligned along a DNA molecule displayed cooperative interactions.18 Charge transport in these systems appears to be dominated by a tunneling mechanism, of which electrostatic charging also imparts a portion. The density of the nanoparticles on the DNA backbone and magnetostatic interactions both cause the conductivity of these one-dimensional nanoparticle chains to be increased when an applied field of enough strength to saturate the particles is present. Conclusions The geometrical confinement of these nanoparticles has produced a significant change in magnetoresistance compared to that of bulk samples. Due to the sensitivity of the IV curves to the external field, this device may be useful in applications
J. Phys. Chem. C, Vol. 112, No. 9, 2008 3193 for MRAM read head technologies or in magnetic field sensing. Fabrication using a DNA guide is a universally adaptable method that can orient any type of positively charged nanoparticles, making it a valid candidate to produce further studies in fundamental mechanisms of magnetostatic interactions and transport in one-dimensional nanoparticle arrays. Acknowledgment. This work was supported by NSF under CMMI-0727927. The authors would like to thank Kristen Buchanan of the Center for Nanoscale Materials at Argonne National Laboratory for training and assistance with operation of the PPMS. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357. References and Notes (1) Adeyeye, A. O. Appl. Phys. Lett. 2002, 80, 2344. (2) Ross, C. Annu. ReV. Mater. Res. 2001, 31, 203. (3) Belle, B. D.; Schedin, F.; Pilet, N.; Ashworth, T. V.; Hill, E. W.; Nutter, P. W.; Hug, H. J.; Miles, J. J. J. Appl. Phys. 2007, 101, 09F517. (4) Shi, J.; Tehrani, S.; Scheinfein, M. R. Appl. Phys. Lett. 2000, 76, 2588. (5) Huber, D. L. Small 2005, 1, 482. (6) Lin, X. M.; Samia, A. C. S. J. Magn. Magn. Mater. 2006, 305, 100. (7) Sun, S. AdV. Mater 2006, 18, 393. (8) Cheon, J.; Park, J. I.; Choi, J.; Jun, Y.; Kim, S.; Kim, M. G.; Kim, Y. M.; Kim, Y. J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 3023. (9) Desvaux, C.; Amiens, C.; Fejes, P.; Renaud, P.; Respaud, M.; Lecante, P.; Snoeck, E.; Chaudret, B. Nat. Mater. 2005, 4, 750. (10) Gundiah, G.; John, N. S.; Thomas, P. J.; Kulkarni, G. U.; Rao, C. N. R.; Heun, S. Appl. Phys. Lett. 2004, 84, 5341. (11) Xiong, Y.; Chen, Q.; Tao, N.; Ye, J.; Tang, Y.; Feng, J.; Gu, X. Nanotechnology 2007, 18, 345301. (12) Dittmer, W. U.; Simmel, F. C. Appl. Phys. Lett. 2004, 85, 633. (13) Mertig, M.; Ciacchi, L. C.; Seidel, R.; Pompe, W.; De Vita, A. Nano Lett. 2002, 2, 841. (14) Richter, J.; Mertig, M.; Pompe, W.; Vinzenberg, H. Appl. Phys. A 2002, 74, 725728. (15) Nyamjav, D.; Kinsella, J. M.; Ivanisevic, A. Appl. Phys. Lett. 2005, 86, 093107. (16) Li, Z.; Sun, Q.; Gao, M. Angew. Chem., Int. Ed 2005, 44, 123. (17) Bensimon, A.; Simon, A. J.; Chiffaudel, A.; Croquette, V.; Heslot, F.; Bensimon, D. Science 1994, 265, 2096. (18) Kinsella, J. M.; Ivanisevic, A. Langmuir 2007, 23, 3886.
