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Frequency-Dependent Magnetic Susceptibility of Magnetite and Cobalt Ferrite Nanoparticles Embedded in PAA Hydrogel
Van 't Hoff Laboratory for Physical and Colloid Chemistry, Debye Institute for Nanomaterials Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
* Author to whom correspondence should be addressed.
Received: 11 April 2013; in revised form: 29 April 2013 / Accepted: 7 May 2013 / Published: 14 May 2013
Abstract: Chemically responsive hydrogels with embedded magnetic nanoparticles are of interest for biosensors that magnetically detect chemical changes. A crucial point is the irreversible linkage of nanoparticles to the hydrogel network, preventing loss of nanoparticles upon repeated swelling and shrinking of the gel. Here, acrylic acid monomers are adsorbed onto ferrite nanoparticles, which subsequently participate in polymerization during synthesis of poly(acrylic acid)-based hydrogels (PAA). To demonstrate the fixation of the nanoparticles to the polymer, our original approach is to measure low-field AC magnetic susceptibility spectra in the 0.1 Hz to 1 MHz range. In the hydrogel, the magnetization dynamics of small iron oxide nanoparticles are comparable to those of the particles dispersed in a liquid, due to fast Néel relaxation inside the particles; this renders the ferrogel useful for chemical sensing at frequencies of several kHz. However, ferrogels holding thermally blocked iron oxide or cobalt ferrite nanoparticles show significant decrease of the magnetic susceptibility resulting from a frozen magnetic structure. This confirms that the nanoparticles are unable to rotate thermally inside the hydrogel, in agreement with their irreversible fixation to the polymer network.
Keywords: ferrohydrogel; hydrogel; ferrogel; poly(acrylic acid); iron oxide; cobalt ferrite; magnetic nanoparticles; magnetic susceptibility; Brownian relaxation; Néel relaxation
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van Berkum, S.; Dee, J.T.; Philipse, A.P.; Erné, B.H. Frequency-Dependent Magnetic Susceptibility of Magnetite and Cobalt Ferrite Nanoparticles Embedded in PAA Hydrogel. Int. J. Mol. Sci. 2013, 14, 10162-10177.
van Berkum S, Dee JT, Philipse AP, Erné BH. Frequency-Dependent Magnetic Susceptibility of Magnetite and Cobalt Ferrite Nanoparticles Embedded in PAA Hydrogel. International Journal of Molecular Sciences. 2013; 14(5):10162-10177.
van Berkum, Susanne; Dee, Joris T.; Philipse, Albert P.; Erné, Ben H. 2013. "Frequency-Dependent Magnetic Susceptibility of Magnetite and Cobalt Ferrite Nanoparticles Embedded in PAA Hydrogel." Int. J. Mol. Sci. 14, no. 5: 10162-10177.