In vitro localisation and degradation of few-layer MoS₂ submicrometric plates in human macrophage-like cells: a label free Raman micro-spectroscopic study

Monitoring the uptake, micro-environment and fate of micro or nano scaled particulate materials in cells is of paramount importance for the emerging fields of toxicology and medicine. Such particulate materials are known to interfere with colorimetric assays and many such assays record only a single end-point. Therefore, there is a need for a label-free, cost effective technique with little or no inference from the particulate materials.

Raman micro-spectroscopy was used to simultaneously interrogate the integrity of few-layer MoS₂ submicrometric plates in human macrophage-like cells, in vitro, as well as the biochemical characteristics of the local micro-environment in which they are encompassed. Firstly, the degradation profile of MoS₂ plates induced by hydrogen peroxidase was established using UV–vis absorption and Raman micro-spectroscopy. Raman micro-spectroscopic maps interrogated all aspects of the cell, including the nucleus, cytoplasm and perinuclear region, and the location/distribution of MoS₂ was monitored as a function of time (4, 24 and 72 h). Whereas only pristine MoS₂ was detectable after 4 and 72 periods, degradation in vitro was confirmed following a 24 h incubation. Analysis of the MoS₂ micro-environments revealed the presence of both phosphatidyl lipidic vesicles and enzymatic regions containing lysozyme, the former being most associated with the MoS₂ degradation. There was an increase and saturation of cytosolic neutral lipids detected following a 24 h incubation with MoS₂, which reduces following a prolonged incubation of 72 h. This study reveals that macrophage-like cells perform degradation of the material in vitro within lipidic vesicles subsequent to phagocytosis, which manifest as an increase in the production of lipid bodies as a mechanism of defense following exposure to industrial grade MoS₂. (paper)