ToF-SIMS analyses of brake wear particles in human epithelial Caco-2 cells

Air pollutants, especially fine particulate matter (diameter of <2.5 μm), are associated with severe health risks including increased cardiopulmonary and lung cancer mortality and development of neurodegenerative diseases. This study introduces a novel methodology that begins with in situ particulate pollution collection, proceeds to exposing non-animal human epithelial cell models which are then analyzed through high spatial resolution mass spectrometry imaging to differentiate the chemistry of particles among exposed cells. During regular train operations, brake wear particles (BWP) are primarily generated from brake pad abrasion. For this work, a custom train brake rig was used to generate BWP, which were then introduced to human epithelial Caco-2 cells. First, the BWP size distributions were characterized using an Electrical Low-Pressure Impactor and particles ranging from 1.1 to 2.7 μm were collected with a gravimetric impactor. A suspension of these BWP, characterized by Dynamic Light Scattering, was added to Caco-2 cells cultured on coverslips. After incubation, the cells were washed and fixed by freeze-drying to preserve the epithelial structure. Subsequent analyses with SEM and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) established substantial, partially agglomerated, BWP deposits on the surface of the epithelial cellular structures. Further analysis of the ToF-SIMS data using Multivariate Curve Resolution-Alternating Least Squares, achieved a better separation of underlying chemical distributions. This enhanced image contrast facilitated the observation of particle-cell interactions. The results demonstrate the efficacy and potential of ToF-SIMS mass spectrometry imaging in distinguishing and potentially characterizing particle-cell interactions. This methodology may be further refined by incorporating complementary analytical techniques such as ICP-MS to better quantify metal content in particles and attempts with smaller particles might help assess cellular particle penetration and accumulation.