Droplet microfluidic chip-ICP–MS-based single-cell analysis for study of cellular behavior of macrophages to thimerosal

Thimerosal (THI) is widely used as an antimicrobial preservative, but can hydrolyze to ethylmercury, causing potentially neurotoxicity. In this work, a THP-1 cell line was used to investigate the biological behavior of THI. An on-line droplet microfluidic chip system combined with time-resolved inductively coupled plasma mass spectrometry was used to quantify Hg in single THP-1 cells. The cellular uptake and elimination behaviors of THI were studied, and the toxicity of THI in terms of redox balance was discussed. The results showed that a small number of cells (<5%) exhibited a high uptake content (>200 fg/cell) for THI, and most of the cells (68.8-85.8% for different exposure groups at 25 h) exhibited a relatively low uptake content (<20 fg/cell). After stopping exposure to THI, the cells showed an elimination process for Hg, which was rapid in the first several hours and gradually slowed down. When the elimination time was 25 h, 7.4-26.3% of the cells in different exposure groups still contained a detectable amount of Hg (>2 fg/cell), indicating Hg could not be eliminated completely, which may cause cumulative toxicity to macrophages. Moreover, it was found that exposure to THI even at 50 ng/mL can cause cellular oxidative stress behavior, leading to an increase in reactive oxygen species level and a decrease in glutathione level. This trend would continue for a period of time after stopping THI exposure. With the elimination of Hg, the redox balance of cells showed a tendency to stabilize and restore, but cannot be restored to normal status, indicating a long-term chronic toxicity of THI to THP-1 cells.