Analysis of nanoscale ingredients in commercial food and cosmetic products by field-flow fractionation and single particle ICP-MS
Authors:   , Naidoo Lyndon 
Published: January 2022

There is a growing need to disclose the possible presence of nanostructures on the labels of commercial food and cosmetic products in South Africa. Synthetic amorphous silica (SiO2) and titanium dioxide (TiO2) are the two widely used nanoparticles (NPs) selected for this study. This work was undertaken in two stages. The first part deals with an analytical method developed for the separation and characterization of TiO2 NPs capped with poly(ethylene glycol) (PEG) as a potential reference material for sunscreen analysis. The second aspect of this work focuses on SiO2, a highly attractive biomaterial widely used as a food additive (E551) to improve the flow properties of powdered food ingredients. Also, modern computational methods were implemented in both case studies, to better understand the nanoparticulate interactions with the organic substrates at an atomistic and molecular level. For the cosmetics study, asymmetric flow field-flow fractionation (AF4) coupled online to multi-angle light scattering (MALS) and dynamic light scattering (DLS) detectors were employed to assess the geometry and size of the PEGylated TiO2 NPs in terms of the evaluated radius of gyration (rg) and hydrodynamic radius (rh). Single particle inductively coupled plasma mass spectrometry (spICP-MS) and transmission electron microscopy (TEM) were implemented to provide information on the core diameter (d) of the TiO2 particle. To overcome agglomeration, Monte Carlo simulations were employed in this study to assess the effectiveness of PEG capped onto spherically constructed TiO2 anatase nanoclusters by systematically performing a series of adsorption studies. For PEG-TiO2 NPs, AF4-MALS-DLS reported rg and rh values of 28.7 nm and 40.3 nm respectively, with the shape factor (rg/rh) values generally reported in the range of 0.7 to 0.8, indicative of spherical particle geometry. SpICP-MS and TEM obtained complementary measurements of d = 32.0 nm and d = 38.4 nm, respectively. The computational modelling results demonstrated the strong binding affinity of PEG as a capping agent to TiO2, exhibiting stabilisation of TiO2 NPs in aqueous medium. Finally, the developed AF4-MALS-DLS method was applied to two commercial SPF 50 sunscreens, exhibiting promising separation and detection efficiency. These findings can contribute to regulatory measures in line with the South African National Nanotechnology Strategy for the cosmetics industry. With regards to the food application study, a multivariate method was developed for the detection and characterization of SiO2 particles also based on AF4-MALS-DLS. This analytical approach attempts to address the fate and the presence of nanoparticulate SiO2 additives (E551) in food products. The experimental design using SiO2 NP standards resulted in the following optimum conditions of the system: crossflow, 0.8 ml/min; injection time, 5 min and sonication time, 60 min. It was observed that the average geometric diameters (Dgeo) for SiO2 NPs in three selected commercial coffee creamers (A, B and C) detected by AF4-MALS were 286.7 nm, 129.8 nm and 190.7 nm respectively. Similar trends for the coffee creamers were observed for the hydrodynamic diameter (Dh) measurements using AF4-DLS i.e. 301.5 nm, 141.3 nm and 197.8 nm respectively. The rg/rh values were reported ranging from 0.7 to 0.8, indicative of spherical particle geometry. Also, the electrostatic interactions between SiO2 NPs and glucose/water mixtures, as evaluated by Monte Carlo simulations, revealed O-interactions dominating over the flat amorphous SiO2 surface. The strongest interaction observed (around - 239 kcal/mol) for the SiO2-water/glucose mixture demonstrates the hydrophilic nature of SiO2 NPs. The findings in both case studies provide fundamental information to improve the understanding of nanoparticulate interactions with additives and paves the way for the labelling of cosmetic and food products that potentially exhibit nanomaterials in complex matrices.