Magnetic Nanoparticles for in Vivo Use: A Critical Assessment of Their Composition

Abstract

Three different magnetic samples with particle sizes ranging from 10 to 30 nm were prepared by wet chemical methods. The powders were heated at 100, 150, 200, and 250 °C during 30 min under air. Ferrous and total iron contents were determined immediately after the synthesis and after the thermal treatments. All samples were characterized by X-ray diffraction, transmission and integral low-energy electron Mössbauer spectroscopy (ILEEMS) at 298 K. These samples are composed of a mixture of individual particles of maghemite and magnetite, which implies that once oxidation starts in this kind of material, it occurs throughout the entire particle volume. The existence of a maghemite/magnetite core-shell model was ruled out. A linear correlation between the average isomer shift and the magnetite content was found, allowing the estimation of the amounts of magnetite and maghemite in an unknown sample without the need of performing chemical analysis.

On the ‘centre of gravity’ method for measuring the composition of magnetite/maghemite mixtures, or the stoichiometry of magnetite-maghemite solid solutions, via57Fe Mössbauer spectroscopy

“…[ [25]. Here: the endpoints correspond to a = 1 and b = 0 for pure magnetite; and a = 0 and b = 5/3 for pure maghemite; and charge balance demands that for any intermediate solution 5a + 3b = 5.…”

“…Various analytical methods may be used: two of the most common are a colorimetric assay based on the reaction of Fe 2+ with 1,10-phenanthroline that forms a deep red solution [32,33]; and a redox titration assay with potassium dichromate using diphenylamine as an indicator [25,34]. However, as in all such analyses dealing with solid samples, the first step is one of acid dissolution, typically achieved using concentrated hydrochloric acid.…”

“…This dissolution step may take many hours or even days to complete [35], during which time one needs to be very careful that the dissolved Fe 2+ does not oxidise in situ-indeed, some researchers perform the dissolution in an anaerobic glovebox [33]. There are also other pitfalls that practitioners remark upon, such as the risk of ambient oxidation during the course of a slow-paced benchtop titration [25], or the oxidizing nitrates present in filter paper that can alter the iron-containing solution as it passes through [33].…”

“…One of the most promising of these has been 57 Fe Mössbauer spectroscopy, which has long been known to be an excellent method for discriminating between magnetite and maghemite [27,28]-as illustrated by the room temperature spectra shown in figure 2, where there are clear differences in both the number and the positions of the absorption lines for the two phases. However, this apparent simplicity belies an underlying complexity that arises whenever non-ideal samples are measured-where, for example, the Mössbauer spectra of fine particle samples can exhibit substantial line broadening [25,33,36], to the extent that profile-based discrimination between phases is impossible. Even in cases where the line profiles are well resolved the process of resolving the subspectra associated with Figure 1.…”

“…The method is based on that proposed by da Costa et al [25]-a team including one of the current authors-who were inspired in part by the work of Santoyo Salazar et al [39], who, in discussing a Mössbauer study of core-shell magnetite-maghemite nanoparticles, had noted that 'the mean stoichiometry can be estimated from the mean isomer shift'. The underlying physical principle here is quite straightforward, viz.…”