Article pubs.acs.org/JPCC
Static and Dynamic Magnetic Characterization of DNA-Templated Chain-Like Magnetite Nanoparticles Debasish Sarkar and Madhuri Mandal* SN Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700098, India ABSTRACT: Here, we report the synthesis of chain like magnetite (Fe3O4) nanoparticles of different sizes. Chainlike formation of the nanoparticles is obtained using DNA as the template material. Being a chainlike molecule, DNA directs the growth of the particles in its own direction. Particle size is varied from 7 to 17 nm simply by varying the duration of the addition of coprecipitating agent. FTIR study confirms the bonding between metal ions and the phosphate backbone of the DNA chain. DC magnetic measurements reveal the decrease in anisotropy energy with increasing particle diameter. Smaller values of saturation magnetization of the nanoparticles than those of the bulk value are due to the presence of the magnetic dead layer in the nanoparticles (magnetic core and nonmagnetic shell), but in our case, the shell thickness decreases with an increase in particle size, which is opposite in trend as reported by other authors. Dynamic ac susceptibility measurements for the smallest particles (d ≈ 7 nm) show Neel−Arrhenius dependence of the blocking temperature with varying excitation frequency.
1. INTRODUCTION Magnetic nanoparticle systems of transition metal ferrites have attracted a great deal of interest from past decades for their possibility of applications in various technological fields1,2 as they are easy to prepare, have low fabrication cost, have higher stability, and have special electrical, optical, thermal, catalytic, and magnetic properties.3 Those fields include electronics, high density magnetic storage, magneto optics, magnetocaloric refrigeration,4 dynamic scaling, and also different bioinspired applications.3,5 Among all the ferrites, magnetite Fe3O4 has attracted special attention because of its extraordinary magnetic and optical properties.4 Bulk Fe3O4 has an inverse spinel structure with a Curie temperature of TC ≈ 850 K and nearly full spin polarization, which make magnetite very much useful for fabrication of giant magneto-electronic and spin-valve devices.5 Also, nanosized Fe3O4 particles have dimension comparable to the cells and biomolecules and have low toxicity, high biocompatibility, and high saturation magnetization.3 These additional unique properties of nanocrystalline Fe3O4 intensify their applications in different biomedical fields including biosensor, magnetic resonance imaging, magnetically targeted drug delivery, hyperthermia treatment, cell separation, and cancer therapy.3,4,6 Attachment of magnetic nanoparticles in different biomolecules has become a very interesting topic of research in recent years. Such type of attachments can prevent the magnetic nanoparticles from agglomeration and also increase their biocompatibility. Biology has provided a large class of molecules for templating. Among them, DNA has been extensively used for templating because it has a double helix structure and also has mechanical and self-assembling characteristics.7,8 DNA’s large aspect ratio, with the diameter being 2 nm and the length in micrometer range, provides a good platform © 2012 American Chemical Society
for the fabrication of nanowires. So, proper engineering with these types of nanobiocomposites may have amusing applications in nanotechnology, but before any further application of these types of nanoparticles in different fields, we must have to understand how the magnetic nanoparticles work together with the biomolecules and have to characterize their different properties for their controlled fabrication and also their operational behavior. Properties of the magnetic materials are found to vary significantly when their sizes are reduced from bulk to nanoscale due to a change in their band structure and also due to an increase in surface to volume ratio. When the sizes of the particles are reduced to nanometer scale (400
41.8
37.3 63.7 77.3
25.52 48.97 68.09
270.5
18.5 77 120
10.8 8.5 4.95
to 17 nm, respectively, though these values are much smaller than the MS value of bulk Fe3O4 (92 emu/g).17 This decrease of saturation magnetization with decreasing size is associated with the higher surface to volume ratio in smaller particles.17,18 Gangopadhyay et al. and Caruntu et al. proposed a theory that tells that a nonmagnetic shell is a disordered state of the surface spins of the nanoparticles, and this theory helps us to explain our results of such a decrease in M S . 3,17 The Fe 3 O 4 nanoparticles can be assumed to be spherical and to be composed of a single crystalline core (with saturation magnetization of MC = 92 emu/g and density ρC = 5.23 g/ cm3, corresponding to their bulk values) and a magnetically disordered shell.3,18 Disordered shell means spins in this shell are not oriented orderly. They have no type of ordering in them like ferromagnetic or antiferromagnetic type. Because of this type of random orientation, contribution of the surface spins to the total magnetization is very small or nil. That is why the surface layer or the shell is called a magnetically disordered shell. It is not a physical shell; it is a proposed magnetic shell. This poorly magnetic surface layer (shell) consists of iron ions with an unsaturated coordination environment, which may be due to the absence of some oxygen ions in spinel lattice and/or due to the bond formation of metal ions with the long chain organic molecules (here, DNA).3,9 For a particle of total radius r, consisting of a core surrounded by a shell of thickness dr ≪ r, the core diameter (dC) can be estimated using the following 3231
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equation:17,18
ligand.9 In the M−H measurements, we can also see that the saturation magnetization decreases with decreasing particle diameter. From here, we can conclude that the shell thickness (t) increases with decreasing total particle diameter. Larger particles have a thin surface layer and therefore high magnetization, whereas smaller particles have a thick nonmagnetic shell, which leads to small magnetization. The saturation magnetization at 80 and 300 K is found to decrease linearly when plotted against the inverse of the particle diameter, i.e., with the surface to volume ratio (1/d) (as shown in Figure 9). This type of behavior has been observed in several
dC = ⎤1/3 ⎡ (ρS/ρC) ⎥ d total ⎢ ⎢⎣ {(ρS/ρC) + (MC − MS)/(MS − Mshell)} ⎥⎦ (2) where ρS = magnetically disordered shell density, roughly assigned to 5 g/cm3; dC = core diameter; dtotal = total diameter of the particle; and Mshell = shell magnetization, which is zero considering the nonmagnetic shell. Using eq 2, we have calculated the core diameter from the total particle diameter and then plotted it as a function of total diameter, dtotal, at two different temperatures (as shown in Figure 8). From the
Figure 9. Saturation magnetization (MS) as a function of inverse of diameter (1/d) at 80 and 300 K. Straight lines are the regression fit to the data.
Figure 8. Core diameter (dC) calculated using eq 2 vs total diameter (dtotal) plot at 80 and 300 K.
ferrite systems.18 This type of linear dependence of magnetization with 1/d confirms the proposition that the magnetization is really effected by the surface of the particles.17,18 It is interesting to note that for the DNA-templated Fe3O4 particles, the coercivity decreases with increasing volume of particles at room temperature (Figure 7c). This type of behavior can be well explained by the Stoner−Wohlfarth theory, which predicts that for single-domain nanosized particles having uniaxial anisotropy axis, the coercivity (HC) depends on anisotropy constant (K) and also the saturation magnetization (MS) of the particles. The relationship can be written as follows:
regression fit to the data, the equations of the straight lines are found to be as follows: dC = (1.08dtotal − 2.44) nm and dC = (1.06dtotal − 3) nm at T = 80 K and T = 300 K, respectively. The corresponding values of dtotal, dC, and nonmagnetic shell Table 3. Values of dC and t calculated from dtotal using eq 2 at T = 80 and 300 K T = 80 K
T = 300 K
dtotal (nm)
dC (nm)
t (nm)
dtotal (nm)
dC (nm)
t (nm)
7.27 11 17.4
5.34 9.68 16.38
1.93 1.32 1.02
7.27 11 17.4
4.69 8.85 15.53
2.58 2.14 1.87
HC =
2K μ0MS
(3)
where μ0 is the permeability of free space.3,5,19 Thus, in our case, as the particles are single domain and have uniaxial anisotropy, the decrease of coercivity with increasing volume seems to be logical with both the decrease of anisotropy constant and the increase of the saturation magnetization. The magnetic relaxation dynamics was investigated using AC magnetic susceptometric analysis. Figure 10 shows the AC susceptibility measurements of the DNA-templated smallest Fe3O4 nanoparticles (7 nm diameter) at different frequencies ranging from 10 to 1000 Hz. Both the components χ′(T) and χ″(T) of AC susceptibility show the same trend of variation as expected for superparamagnetic particles. At each frequency, both the components exhibit a maximum at a temperature that corresponds to the blocking temperature (TB) of the system, which drifts toward a higher value with increasing frequency.5 Now, to provide a model independent classification between the freezing/blocking processes of canonical spin glasses, other magnetically disordered systems where freezing happens in
thickness (t) at different temperatures are summarized in Table 3. Here, the nonmagnetic shell thickness increases with temperature, which is obvious because thermal fluctuation helps in the canting of the superficial spins, which makes more spins to be in disordered state,17 but in our case, the thickness of the nonmagnetic shell decreases as the particle size varies from 7 to 17 nm at both temperatures. This trend of variation of shell thickness is against that reported by other groups, which states that the nonmagnetic shell thickness increases with increasing total particle diameter (dtotal).17,18 In the present case, an increase of shell thickness (t) with decreasing total particle diameter (dtotal) may be a consequence of a larger fraction of superficial iron ions in smaller particles.3 These iron ions possess uncompensated coordination spheres due to fewer fractions of oxygen ions at the surface and are also due to their bond formation with the phosphate backbone of the DNA 3232
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Figure 11. Arrhenius plot of the relaxation frequency f vs blocking temperature TB measured from the real component χ′(T) of AC susceptibility. Solid line is the best fit using eq 5 with Ea = 2417.2 K and f0 = 3.97 × 1011 Hz.
Figure 10. Temperature dependence of the in-phase (real) component χ′(T) of the magnetic susceptibility for the smallest DNA-templated Fe3O4 nanocrystals (7 nm diameter) at different excitation frequencies. Arrows indicate increasing frequencies. Inset: out of phase (imaginary) component χ″(T).
3.97 × 1011 Hz and 2417.2 K, respectively. As the calculated value of Larmor frequency falls in its accepted range (109−1013 Hz), we can conclude that the Neel−Arrhenus model perfectly describes this type of system. Now, from the expression Ea = KeffV sin2 θ, were Keff is the effective anisotropy constant, θ is the angle between the magnetic easy axis of the particle and the direction of the magnetic moment, and V is the volume of the particles, we can calculate the value of Keff by using the values of θ, V, and Ea. Ea is the energy that the spins need to cross the energy barrier that separates two energetically favorable ground states of spin. At the blocking temperature, the value of Ea should be maximum. For the maximum value of Ea, the θ is 90°. Putting the values of Ea, θ = 90°, and V = volume of 7 nm particles, we calculate Keff. For the smallest particle (7 nm), the value of Keff is found to be 1.84 × 105 J/m3. The value of Keff obtained here is in same order of magnitude but slightly higher than the value of the anisotropy energy constant as obtained earlier (K = 1.47 × 105 J/m3) from the Stoner−Wohlfarth theory. The obtained value of Keff is one order higher than the magnitude of its bulk value (Kbulk = 0.135 × 105 J/m3); the cause of which has been already discussed earlier.
steps and for the superparamagnetic particles, we can use an empirical parameter p, which is known as the frequency sensitivity of the blocking temperature (TB) and is represented as follows:20,21
p=
ΔTB TB Δlog10(υ)
(4)
where ΔTB is the difference between TB measured in the Δlog10(υ) frequency interval. In our case, the value of p for the smallest Fe3O4 nanoparticles is found to be 0.086, which is very close to the 0.10 value found for superparamagnetic particles.5 A slightly lesser value of p may be due to surface effect or interparticle interaction,5,22 but in this case, the former one is more effective, as discussed earlier. For a system of single domain noninteracting particles with uniaxial anisotropy direction, the thermal relaxation process follows the Neel−Arrhenus law given as5,11,23,24
⎛ E ⎞ f = f0 exp⎜ − a ⎟ ⎝ kBT ⎠
4. CONCLUSIONS We have successfully prepared chainlike magnetite nanoparticles of different sizes using DNA as the template material. The effect of surface anisotropy on the magnetic properties of the nanoparticles can be clearly observed from all the magnetization data. Bonding between metal cations and oxygen anions of the DNA chain also affect the anisotropy energy, thus playing an important role in changing the magnetic properties. Both static and dynamic magnetic characterization supports the noninteracting, single domain, superparamagnetic nature of the smaller magnetite particles. Such a type of biologically functionalized magnetic nanoparticles will be useful for hyperthermia treatment, bio-organ imaging, etc.
(5)
where f 0 is the Larmor frequency, which falls in the range of 109−1013 Hz for superparamagnetic particles, Ea is the anisotropy energy barrier, and kB is the Boltzmann constant. Now, to investigate if there is any type of interparticle interaction present among the particles, we have plotted ln( f) versus blocking temperature (TB), which are obtained from the in-phase component of AC susceptibility χ′(T) data measured at four different frequencies ( f = 10, 100, 500, and 100 Hz) from Figure 10, then, we tried to fit the data with the Neel− Arrhenus law (as shown in Figure 11). From Figure 10, the values of TB were estimated from the maximum of the real part of AC susceptibility graphs that were taken at four different frequencies ( f = 10, 100, 500, and 100 Hz). Then, these frequencies (in log scale) were plotted along the Y axis, the corresponding values of the TB at those frequencies were plotted along X axis, and then, the Neel−Arrhenus law was fitted with the experimental data to estimate Larmor frequency (f 0) and Ea. The values of f 0 and Ea were obtained from the best fitted data. From this fitting, the values obtained for the Larmor frequency and the anisotropy energy barrier for the DNA-templated Fe3O4 particles of the smallest size (7 nm) are
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
ACKNOWLEDGMENTS We are thankful to the Department of Science and Technology, Government of India, for the funding under the project SR/ FT/CS-090/2009. D. Sarkar is grateful to CSIR (India) for 3233
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providing his fellowship. We are thankful to Dr. D. Das, UGCDAE-CSR, Kolkata, for SQUID measurements.
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REFERENCES
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