A comparative study of magnetic transferability of superparamagnetic nanoparticles

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Abstract

The aspect of magnetic transferability was established using an automated magnetic particle transfer workstation. Maghemite (γ-Fe2O3) nanoparticles were synthesized via conventional co-precipitation procedure. Their transferability was determined in addition to several commercial nanoparticles that ranged in diameter, surface functionality, and composition. Transmission and scanning electron micrographs and infrared spectrum, respectively, provided size and surface information on the synthesized particles for comparison to commercially available magnetic nanoparticles.

Introduction

Magnetic nanoparticles have crossed the boarders of many disciplines currently thriving in biotechnology and biomedicine [1], [2], [3]. They have also spawned into the realms of magnetic fluids [4], catalysis [5], magnetic resonance imagining [6], and environmental remediation [7]. The assortment of commercially available magnetic nanoparticles attests to the wide range of applications that benefit from the ease of magnetic separation. The most common material used to provide magnetism is iron oxide in either magnetite (Fe3O4) or maghemite (γ-Fe2O3) form. The popularity of these oxide crystals is found in their unique response to an external magnetic force and subsequent lack of residual magnetism once the force is removed, deeming them with paramagnetic behavior. If the particle magnetic moment reverses at times shorter than the experimental time scales, the system is in a superparamagnetic state [8]. In the wide spread development of magnetic nanotechnology, many effects must be taken into account to improve the architecture of the final product before its applications.

The difficulty with such particles is their significantly small size range that makes them inherently unstable for long periods of time. The high chemical activity causes vulnerability to oxidation, agglomeration, and inevitable loss in magnetism [8]. Protective strategies to overcome these hindrances can be easily achieved by the incorporation of either an organic material, such as a polymer [9], surfactant [10], or inorganic coating of silica [11]. The presence of protective material can also be regulated to introduce further functionality that provides desirable physical and chemical properties.

The surface modification of magnetic nanoparticles is typically approached by the core–shell method in the post-functionalization of synthesized particles. Alternatively, the embedded method, where many magnetic nanoparticles reside in a guest matrix, has also been achieved. Matrix-dispersed magnetic nanoparticles can be created in a variety of different states: (1) they can be dispersed in a continuous matrix, (2) they can be presently dispersed in a coating on other larger particles (e.g. layer-by-layer method), or (3) they can form agglomerates of individual nanoparticles which are connected through their protective shells [8]. As a consequence, there are a great number of published literatures related to the properties of well-characterized iron oxide nanoparticles and their coated counterparts [12], [13], [14], [15].

In the broad range of magnetic nanoparticle compositions available, finite size effects and surface effects govern consequential magnetic properties. Finite size effects can be considered to be those that are originated by the discontinuity of some characteristic length due to the purely geometric constraint of finite volume. In contrast, surface effects arise from the lack of translational symmetry at the boundaries of the particle as a result of lower coordination number and the existence of broken magnetic exchange bonds which lead to surface spin disorder [16]. Amongst the extensive scope of approaches to evaluate different parameters of magnetic properties, the most commonly used techniques are vibrating sample magnetometry (VSM) and superconducting quantum interface device magnetometry (SQUID magnetometer) [17], [18], [19], [20]. Despite the sophisticated methods available, different and unique means of characterizing magnetic properties would provide greater insight to the continual development of the synthetic mechanisms involved.

In this paper, we present a new aspect of magnetic characterization of nanoparticles: magnetic transferability. Magnetic transferability can be defined as the susceptibility of superparamagnetic nanoparticles toward magnetic-field-assisted separation. The magnetic transferability of synthesized and commercial magnetic materials was characterized by an automated magnetic particle transfer device, BioSprint 15. It was important to compare their transferability due to the wide accessibility of commercial sources and, therefore, resultant magnetic particles. Magnetic γ-Fe2O3 nanoparticles were prepared by conventional co-precipitation of ferrous and ferric salts in basic solution. In addition, the physical properties of the resulting material were investigated for comparison with a number of commercially obtained magnetic particles.

Section snippets

Materials

Ferrous chloride (FeCl2·4 H2O) with 99% purity and anhydrous ferric chloride (FeCl3) with 98% purity were obtained from Sigma-Aldrich (St. Louis, MO, USA) and Honeywell Riedel de Haen (Seelze, Germany), respectively. Sodium hydroxide (NaOH) pellets of 97% purity were provided by Caledon Laboratory Chemicals (Georgetown, ON, Canada). Hydrochloric acid (38%) was purchased from Allied Chemical (Mississauga, ON, Canada). Methanol (MeOH) was purchased from Fisher Scientific (Fair Lawn, NJ, USA).

Comparison of magnetic particles’ properties

The commercially obtained magnetic particle suspensions composed an array of diameters, surface functionality, polymer incorporation, and consequent superparamagnetic properties. An anthology of these properties were compiled to establish the scope of magnetic materials investigated in this study. Turbobeads, which was a more recent product development, was the only provider of magnetic material that was composed of carbon coated cobalt nanoparticles <50 nm in diameter. Compared to other

Conclusion

This paper introduces the aspect of magnetic transferability using an automated workstation to provide more characteristic insight of commercially obtained and synthesized superparamagnetic nanoparticles. The uptake of magnetic samples was governed by either size or surface effects, whereas the dispersion of material into solution was dictated by surface–solvent interactions. Surface functionality of the particles played a much more pivotal role in magnetic transfer than expected. Consequently,

Acknowledgements

This project is financially supported by CRTI 06-0230 RD. The authors thank Dr. Dashan Wang for his help in transmission electron microscopy, and Molla Rafiquel Islam for his help with X-ray diffraction analysis in Prof. Sundar’s lab.

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