Research Article: Improving the reliability of the iron concentration quantification for iron oxide nanoparticle suspensions: a two-institutions study

Date Published: November 12, 2018

Publisher: Springer Berlin Heidelberg

Author(s): Rocio Costo, David Heinke, Cordula Grüttner, Fritz Westphal, M. Puerto Morales, S. Veintemillas-Verdaguer, Nicole Gehrke.


Most iron oxide nanoparticles applications, and in special biomedical applications, require the accurate determination of iron content as the determination of particle properties from measurements in dispersions is strongly dependent on it. Inductively coupled plasma (ICP) and spectrophotometry are two typical worldwide used analytical methods for iron concentration determination. In both techniques, precise determination of iron is not straightforward and nanoparticle digestion and dilution procedures are needed prior to analysis. The sample preparation protocol has been shown to be as important as the analytical method when accuracy is aimed as many puzzling reported results in magnetic, colloidal, and structural properties are simply attributable to inadequate dissolution procedures. Therefore, a standard sample preparation protocol is needed to ensure the adequate and complete iron oxide nanoparticle dissolution and to harmonize this procedure. In this work, an interlaboratory evaluation of an optimized iron oxide nanoparticle digestion/dilution protocol was carried out. The presented protocol is simple, inexpensive, and does not involve any special device (as microwave, ultrasound, or other high-priced digestion devices). Then, iron concentration was measured by ICP-OES (performed in ICMM/CSIC-Spain) and spectrophotometry (NanoPET-Germany) and the obtained concentration values were analyzed to determine the most probable error causes. Uncertainty values as low as 1.5% were achieved after the optimized method was applied. Moreover, this article provides a list of recommendations to significantly reduce uncertainty in both sample preparation and analysis procedures.

Partial Text

Iron oxide nanoparticles are widely used in ex-vivo bioassays for the detection and separation of small molecules, biomarkers, DNA, and bacteria [1], and also in-vivo medical diagnosis to enhance the contrast of magnetic resonance imaging (MRI) [2]. In cancer therapies, they have increased the efficiency of different drugs when used as delivery platforms assisted by magnetic fields to concentrate the drug or heat in the targeted tumor area [3]. They are also promising tools for gene therapy and tissue regeneration, among other applications in the biomedical field [4]. Other emerging applications of these nanoparticles include their use in catalysis, and as adsorbents or reactive agents in environmental applications [5]. Typically, magnetic nanoparticles are dispersed in aqueous media to form stable colloidal suspensions, which are commonly studied with a multitude of characterization techniques, focusing on structural, colloidal, and magnetic properties [6]. Many of these properties are strongly related to the concentration or overall amount of iron present in the suspension. Thus, the accurate determination of the iron concentration is crucial for the precise determination of physical and chemical parameters of the iron oxide nanoparticles, e.g., the saturation magnetization in Am2/Kg Fe, the specific absorption rate (SAR) in W/g Fe, or the concentration of functional groups on the particle surface in μmol/g Fe, respectively.

We have successfully harmonized the iron concentration quantification procedures between CSIC (ICP-OES) and nanoPET (spectrophotometry). The uncertainty values of the iron concentration from either spectrophotometry or ICP-OES were about 1.5% on average, when samples were prepared and analyzed in the same institution. Uncertainty values around 2.5–3%, were found only for the two samples with very high iron concentrations (> 8 mg/mL).




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