Artigos Científicos

Characterization of aluminum, aluminum oxide and titanium dioxide nanomaterials using a combination of methods for particle surface and size analysis


Link: https://pubs.rsc.org/en/content/articlelanding/2018/ra/c8ra00205c#!divAbstract

 

Issue 26, 2018

From the journal: RSC Advances



B. KrauseORCID logo *a   T. Meyer,b   H. Sieg,c   C. KästnerORCID logo d   P. Reichardt,a   J. Tentschert,a   H. Jungnickel,a   I. Estrela-Lopis,b   A. Burel,e   S. ChevanceORCID logo f   F. GauffreORCID logo f   P. Jalili,g   J. Meijer,h   L. Böhmert,c   A. Braeuning,c   A. F. ThünemannORCID logo d   F. EmmerlingORCID logo i   V. Fessard,g   P. LauxORCID logo a   A. Lampenc  and  A. Luch ORCID logo a  

 

Abstract

The application of appropriate analytical techniques is essential for nanomaterial (NM) characterization. In this study, we compared different analytical techniques for NM analysis. Regarding possible adverse health effects, ionic and particulate NM effects have to be taken into account. As NMs behave quite differently in physiological media, special attention was paid to techniques which are able to determine the biosolubility and complexation behavior of NMs. Representative NMs of similar size were selected: aluminum (Al0) and aluminum oxide (Al2O3), to compare the behavior of metal and metal oxides. In addition, titanium dioxide (TiO2) was investigated. Characterization techniques such as dynamic light scattering (DLS) and nanoparticle tracking analysis (NTA) were evaluated with respect to their suitability for fast characterization of nanoparticle dispersions regarding a particle's hydrodynamic diameter and size distribution. By application of inductively coupled plasma mass spectrometry in the single particle mode (SP-ICP-MS), individual nanoparticles were quantified and characterized regarding their size. SP-ICP-MS measurements were correlated with the information gained using other characterization techniques, i.e. transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). The particle surface as an important descriptor of NMs was analyzed by X-ray diffraction (XRD). NM impurities and their co-localization with biomolecules were determined by ion beam microscopy (IBM) and confocal Raman microscopy (CRM). We conclude advantages and disadvantages of the different techniques applied and suggest options for their complementation. Thus, this paper may serve as a practical guide to particle characterization techniques.

Solubility investigations on Al0 and Al2O3 NMs

To verify our hypothesis that Al0 NMs are more soluble than Al2O3, we performed ion release experiments in stock dispersions and DMEM. Adjusting the pH value was not necessary due to almost neutral pH values of 7.3 in stock dispersion compared to 7.2 in DMEM. Keeping in mind that aluminum is an amphoteric material due to the aluminum aqua complexes, one can assume that dissolution of Al0 NMs in complex media is highly pH-dependent. Compared to an acid, the aluminum aqua complex reacts as a base and vice versa. At a neutral pH value, reactivity is very low, thus only low ion release should occur. The possible aluminum aqua complexes at the different pH values are shown in Fig. S7.

The ion release for both Al0 and Al2O3 NMs after one hour was very low, about 0.2–0.4% in BSA and 0.3–0.5% in DMEM (with 10% FCS) (Table 2). As already described above, this was expected at a neutral pH value. For in vitro experiments, this will mean that the effects will originate mainly from particles and not from Al ions. The application of an artificial digestion procedure for mimicking in vivo situation showed, that the particle dissolution and complexation behavior was quite different in all three studied gastrointestinal compartments.24 After no significant changes in the saliva, the gastric environment leads to a significant increase of the dissolution rate as well as very strong agglomeration of NMs. The addition of intestinal fluid results in a nearly neutral pH value which leads to a decrease in the dissolution rate, a deagglomeration of particles and even de novo particle formation in ionic aluminum control.

 

Conclusions

An extensive characterization of NMs in their different states, from dispersion to in vitro conditions, should precede any toxicological testing. In this study, we have characterized two different types of NMs, one metal and one metal oxide. For this purpose, we used a wide range of complementary analytical techniques to characterize particle size, surface and composition in more detail. An in-depth insight into the dissolution, one of the most important determinants of NM toxicity, was achieved by investigation by the examples of Al0 and Al2O3 NMs. A very low dissolution rate and a small percentage of ion release was observed at a neutral pH value, while higher rates for acidic environment we already reported earlier.24 A significant difference between the surface and the extent of oxidation of Al0 and Al2O3 NM forms was detected. In contrast to the fully oxidized Al2O3 with a rather homogenous distribution of Al and O atoms, Al0 NM showed a core–shell structure with an oxide layer only a few nm thick. This was proven with XRD and the EELS technique, which emphasizes the benefit of using different techniques to get reliable results. It was demonstrated with the help of ToF-SIMS that both aluminum forms are subject to a surprisingly different complexation in biological media: while Al0 NMs were shown to form complexes with amino acids, Al2O3 NMs mostly formed polyoxo-complexes out of two or more Al2O3 molecules. Based on the collected results, all of the investigation methods applied have their own benefits. Thus, with the methods described in this study focusing on size, surface and complex formation, dissolution investigations can perhaps become a bit more predictable. Right now, the recommended limit for aluminum release from food is 5 mg kg−1 food. The various aluminum forms, e.g. ions, micro- and nanomaterials, pure Al0 or Al2O3, are not differentiated here. With this study, it becomes obvious that there should be differentiation for the different forms, as they may be taken up and react differently. For future analytical investigations on aluminum-containing NMs, we propose considering not only Al2O3 but also Al0 NMs and their interaction with the corresponding ions.

 



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