ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 321 (2009) 1377–1380
Journal of Magnetism and Magnetic Materials
Probing temperature-sensitive behavior of pNIPAAm-coated iron oxide
nanoparticles using frequency-dependent magnetic measurements
Ã
Suchita Kalele a, Ravin Narain b, Kannan M. Krishnan a,
a Department of Materials Science and Engineering, University of Washington, 323 Roberts Hall, Box 352120, Seattle, WA 98195-2120, USA
b Department of Chemistry and Biochemistry, Laurentian University, Sudbury, ON, Canada P3E 2C6
a r t i c l e i n f o
a b s t r a c t
Available online 4 March 2009
Ferromagnetic iron oxide nanoparticles of about 33 nm in diameter were synthesized by high-
temperature decomposition of an iron–oleate complex, using octadecene as the solvent. These particles
were subsequently coated with polyN-isopropylacrylamide (pNIPAAm) by a surfactant exchange
method. Temperature-sensitive behavior of these particles was studied using ac susceptibility and
dynamic light scattering (DLS) measurements. Shifts in the imaginary part of the ac susceptibility are
correlated with swelling and collapse of pNIPAAm as a function of temperature.
Keywords:
Nanoparticle
Iron oxide
pNIPAAm
Biomedical application
Triggered drug delivery
Temperature-sensitive behavior
& 2009 Elsevier B.V. All rights reserved.
1. Introduction
imaginary component of the susceptibility. In this sense, the
nanoparticles can be used as a biosensor to detect binding events
of biomolecules of interest on the surface of the functionalized
particles [8]. Poly(N-isopropylacrylamide) or pNIPAAm is one of
the most studied temperature-sensitive polymers [9] and in
aqueous solution it is well-known to exhibit a sharp phase
transition, called the lower critical solution temperature (LCST), at
Magnetic nanocrystals have found a wide range of biomedical
applications such as in drug delivery, magnetic resonance imaging
(MRI), magnetic separation, and magnetic fluid hyperthermia [1].
Suspension of magnetic nanoparticles (ferrofluid) in an applied
´
alternating field relaxes by two different mechanisms, the Neel
a
temperature in the range 298–310 K depending on the
relaxation and the Brownian relaxation [2], that depend on their
composition. Below the LCST, the pNIPAAm random coil chains
are hydrated, hydrophilic in nature, and swollen. Above the LCST,
the chains become hydrophobic, dehydrated but weakly hydro-
gen-bonded with water molecules and collapsed. In this report,
using frequency-dependent magnetic measurements, we demon-
strate the possibility of detecting the changes in the hydrody-
namic volume of the pNIPAAm-coated iron oxide nanoparticles as
a function of temperature. The temperature-dependent physical
behavior is monitored by measuring the imaginary component of
the magnetic susceptibility, w00 above and below the LCST of
pNIPAAm. The clear trend in the peak in w00 is correlated with
swelling and collapse of pNIPAAm. We envision that the swelling
and collapse of such pNiPAAM-coated magnetic nanoparticles
could be used to trigger drug release.
´
size. For smaller unblocked particles, Neel relaxation dominates
and is caused by the rotation of the magnetization vector inside
the magnetic core against an energy barrier. Such relaxation is
typically accompanied by a phase lag between the applied ac field
and the magnetization, a process that can generate heat and is
preferred in bioapplications involving localized heating (such as
magnetic fluid hyperthermia) [3]. On the other hand, for larger
blocked particles, Brownian relaxation arises from the physical
rotation of the entire particle in the carrier fluid. It has been
proposed that by appropriate functionalization specific binding of
biomolecules to suspended colloidal magnetic particles can be
detected using frequency-dependent magnetic measurements
(i.e. Brownian relaxation of magnetic nanoparticles in an ac
magnetic field) with potential in biosensing [4–7]. Relaxation
frequency for Brownian rotation, fB is given by fB ¼ kBT/3
ZVH,
where is the viscosity of the ferrofluid and VH the hydrodynamic
Z
2. Synthesis and functionalization of nanoparticles
volume of the particle. Brownian relaxation is dependent on the
ability of the particles to rotate in their carrier fluid, thus changes
in the viscosity or hydrodynamic radius can be detected by
measuring the shift in frequency at which there is a peak in the
Ferromagnetic iron oxide nanoparticles (diameterꢀ33 nm)
were prepared and individually coated with pNIPAAm
(diameterꢀ80 nm). These particles have LCST between 298 and
310 K. Ferromagnetic iron oxide nanoparticles were synthesized
by high-temperature pyrolysis of the metal fatty acid salt
(ferric oleate), the corresponding fatty acid (oleic acid) and a
Ã
Corresponding author. Tel.: +1206 543 2814; fax: +1206 543 3100.
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved.