Analytical challenges in the assessment of intestinal fate, uptake and crossing of food-relevant nanomaterials

Characterising the fate of nanoparticles in the gastrointestinal tract as well as their uptake and crossing through the intestinal epithelium is essential for nanomaterials used in food-related applications. In chemico, in vitro and in vivo studies allow to simulate the human digestion process, assess transport across the intestinal barrier, and characterise the absorption and biodistribution of nanoparticles after oral exposure, respectively. However, these studies pose substantial analytical challenges. These include selecting the appropriate technique for the different physicochemical properties to be measured, dealing with complex sample matrices, distinguishing the particles of interest from any other interfering nanozised entity, discriminating the particulate fraction from any possible soluble (ionic or molecular) counterpart. The present review addresses these challenges by first providing an overview of the measurement techniques available for physicochemical characterisation and then examining the analytical strategies applied in the existing studies, from sample preparation protocols to the analytical measurement approaches, focusing on the information delivered, advantages and limitations. Emphasis is given to the combination of complementary analytical techniques addressing both the mass concentration and the size distribution of the particulate fraction, with special focus on mass spectrometry (MS)- and electron microscopy (EM)-based techniques. Crucial aspects such as dispersion protocols, analytical artefacts, development of approaches for complex and less studied organic nanomaterials are addressed. Based on the provided critical overview, a comprehensive multi-method analytical strategy to deal with these demanding physicochemical measurements is outlined.