Abstract
Synthetic amorphous silica is widely used in food processing as a food additive (E551) due to its properties as a flavour carrier and anti-caking agent. The direct measurement of E551 suspended or embedded in complex matrices is difficult without prior removal of the matrix components. The isolation of nanoparticles from the matrix is hence the first step towards their comprehensive characterization. Due to its complexity, matrix removal is frequently not trivial and may cause modification of the number-size distribution of the silica particles. The isolation of engineered silica nanoparticles by removal of the matrix with microwave-assisted acidic digestion is demonstrated methodologically using both monodisperse (size standards) and polydisperse (E551) particles spiked into ultrapure water and tomato sauce. For the characterization of the isolated nanoparticles, asymmetric field flow fractionation (AF4) coupled to multi-angle laser light scattering (MALS) and inductively coupled plasma mass spectrometry (ICP-MS) were chosen. The combination of ICP-MS and ultracentrifugation allowed for the rapid and reliable measurement of the dissolved fraction of SiO2. The results show that microwave-assisted acidic digestion partially dissolves silica nanoparticles. Moreover, the digestion conditions, in particular the low pH value, lead to strong agglomeration of the particles. A complete deagglomeration is not achieved, even when exposing the suspension to elevated sonication doses. The consequence of these two findings is a size distribution of particles after acidic digestion that is different from the original distribution before digestion. This result may have an impact on the evaluation of whether the material is a nanomaterial according to the recommended definition of the European Commission.Graphical abstract
Highlights
The use of nanoparticles (NPs), including silica nanoparticles (SiO2-NPs), has increased intensely in the last years thanks to the technical benefits they offer [1, 2]
Due to the matrix complexity and the strong interactions between some of its components and the engineered nanoparticles (ENPs), matrix removal is not trivial [7], and many of the reported methods can modify the ENPs in terms of mass concentration and number-size distribution
This study aims at systematically assessing whether microwave-assisted acidic digestion is a suitable method for the separation of SiO2 nanoparticles from matrices
Summary
The use of nanoparticles (NPs), including silica nanoparticles (SiO2-NPs), has increased intensely in the last years thanks to the technical benefits they offer [1, 2]. Engineered nanoparticles (ENPs) in consumer products, such as cosmetics or foodstuffs, are usually suspended or embedded in complex matrices and can not be directly measured without prior removal of the matrix components. No universally applicable isolation and measurement method is available, and any procedure must be carefully tailored to the matrix/ material system. Chemical destruction of the organic components of the matrix can be achieved through acidic or alkaline digestion [2, 8,9,10] as well as through peroxy (Fenton) reaction [11, 12]. An alternative method for the separation of chemically sensitive materials from the matrix is enzymatic digestion, in which α-amylase for carbohydrate degradation and protease for protein degradation are added [9, 13, 14]
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