Sudhanshu
Srivastava
,
Bappaditya
Samanta
,
Palaniappan
Arumugam
,
Gang
Han
and
Vincent M.
Rotello
*
Department of Chemistry, University of Massachusetts, USA. E-mail: rotello@chem.umass.edu; Fax: +1-413-545-4490; Tel: +1-413-545-2058
First published on 30th October 2006
Nanocomposite materials consisting of FePt nanoparticles and DNA were constructed via DNA-mediated “bricks and mortar” self-assembly. Electrostatic interaction between the cationic nanoparticles and the DNA through surface recognition led to the formation of extended composite aggregates. These DNA-assembled aggregates feature increased interparticle spacing arising from the DNA “mortar”. The enhanced structure and increased spacing in the bio-nanocomposite assembly was found to alter the magnetic properties of the assemblies, as demonstrated by a 54 K change in blocking temperature (TB).
The use of polymers6 and biomolecules7 as templates for the self-assembly of particles provides access to new materials featuring novel properties. The use of deoxyribonucleic acid (DNA) for generating assembled structures has been shown recently.8 DNA has been demonstrated to be a particularly versatile construction material due to its flexible length scale, rigidity and chemically programmable duplex structure.9 For example, Fitzmaurice and co-workers10 have shown that DNA was used as a template to assemble an array of gold nanoparticles between gold electrodes on a silicon oxide substrate. Alivisatos et al. also created discrete nanostructures of semiconductor nanoparticles/Au with DNA.11
In contrast to electronic materials, there have been relatively few examples of the use of DNA to assemble magnetic materials, e.g. Willner et al.12 controlled the magneto-mechanical properties of a cantilever by assembling magnetic particles on the cantilever surface with DNA. These studies, however, do not directly address the issue of interparticle spacing, a key factor in tuning the collective properties of nanometer-sized magnets.
In previous studies, dendrimer mediated self-assembly of Fe2O3 nanoparticles was used to demonstrate the controlled modulation of magnetic properties of nanoparticles, including blocking temperature (TB).13 Similarly, polymers functionalized with recognition elements assembled with magnetic nanoparticles also showed variation in the magnetic behavior.14 These magnetic nanoparticles provided a means to investigate the change in bulk magnetic properties as a function of both order and spacing. We report here the use of DNA to assemble magnetic nanoparticles, providing a route to magnetic bio-nanocomposites.
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Fig. 1 Structure of FePt (MPN 1) before and after place exchange. |
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Scheme 1 A) Magnetic nanoparticles assembled with DNA via electrostatic interaction. B) Schematic representation of magnetic dipole–dipole interaction between the nanoparticles before and after DNA assembly, showing partial denaturation of DNA during assembly. |
The interparticle spacing for MPN 1 and MPN 1 assembled with DNA was quantified by small angle X-ray scattering (SAXS) (Fig. 2). The MPN 1 and MPN 1 + DNA solutions were allowed to precipitate on thin Mylar sheets and the resulting films were analyzed. The q values, representative of average interparticle spacing, shift steadily downwards in the presence of DNA. The MPN 1 film alone shows a q value19 that corresponds to a center-to-center spacing of 7.8 nm while the DNA assembled nanocomposite exhibits a center-to-center spacing of 8.9 nm, an increase in interparticle spacing of 1.1 nm (Fig. 2).
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Fig. 2 SAXS profiles for MPN 1 alone and DNA assembled MPN 1 (normalized). |
To study the morphological differences between unassembled particles and those assembled with DNA, transmission electron microscopy (TEM) was performed. The TEM samples were prepared identically to the SAXS samples through slow precipitation of a film of MPN 1 + DNA on carbon-coated copper TEM grids. MNP 1 showed some self-association (Fig. 3A) due to van der Waals contact. In contrast, MNP 1 + DNA composites showed network-like aggregates7b throughout the grid indicative of rapid precipitation (Fig. 3B).
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Fig. 3 A) TEM image of MPN 1 (∼7 nm). B) Network-like aggregates of DNA assembled with MPN 1. |
A superconducting quantum interference device (SQUID) was used to acquire field-dependent magnetization analysis using zero-field-cooled (ZFC) and field-cooled (FC) measurements for the particles alone and after DNA assembly. MPN 1 (∼7 nm) nanoparticles are superparamagnetic at ambient temperature,20 becoming ferromagnetic at low temperatures. The temperature at which the superparamagnetic–ferromagnetic transition occurs is known as the blocking temperature (TB),21 which depends on the amount of magnetic material and interparticle spacings of the dipoles. The TB of a sample can be obtained through a ZFC/FC scan that sweeps at a chosen temperature range (300 K to 1.8 K) while recording the magnetic response of the sample to a constant applied field (100 Oe). In ZFC the magnetic dipoles of the particle will initially align in the direction of the magnetic field, with increasing temperature and due to its ferromagnetic behavior. Above the TB the dipoles will cancel each other due to thermal activation resulting in the magnetic moment approaching zero. The maximum point of this transition in the curve thus denotes the TB. In FC experiment the dipoles will again align in the direction of the field with decreasing temperature. In the final scan the dipoles will get locked in the magnetic field direction to maximize the magnetic moment.
The control sample (MPN 1) of precipitated particles showed a TB of 95 K while the DNA spaced sample showed a TB of 41 K (Fig. 4). This change in blocking temperature can be explained by a decrease in dipolar coupling arising from the increased interparticle spacing, as the TB of magnetic nanoparticles is directly related to their volume.22 Our samples were obtained from the exact same batch of nanoparticles, thus eliminating core size discrepancies. The observed decrease in TB is therefore a direct consequence of the DNA-mediated assembly: as the DNA assembles the particles, the dipolar coupling between particles decreases, resulting in an “effective” volume decrease compared to the control sample.
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Fig. 4 FC and ZFC magnetization plots for MPN 1 alone and assembled with DNA. |
For place exchange 20 mg of FePt nanocrystals were taken in 5 mL dichloromethane. 10-Carboxydecyltrimethylammonium bromide (60 mg) and 11-mercaptoundecyltrimethylammonium chloride (60 mg) in 0.5 mL of ethanol were added to the nanocrystal dispersion and stirred for three days. The black precipitate obtained was isolated using centrifugation. It was rinsed three times using a mixture of ethanol and dichloromethane (1 : 10) and was dissolved in milliQ water.
Footnote |
† Electronic supplementary information (ESI) available: CD data and complete synthesis of nanoparticles. See DOI: 10.1039/b613887j |
This journal is © The Royal Society of Chemistry 2007 |