Fluorescently visualizing the penetration of anionic surfactants across cytoplasmic membrane and the subsequent damage on human cells

The extensive application of chemically synthesized anionic surfactants would cause serious pollution of water and increase health risk to humans. However, the adverse impact of anionic surfactant on human cells has never been systematically demonstrated. In this paper, a series of fluorescent anionic surfactants containing a varying length of alkyl chain from C8 to C18 and a fixed hydrophilic head of 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) were synthesized and utilized for visualizing the interaction between surfactants and cells. The obtained molecules exhibited blue fluorescence presenting a decreasing fluorescent intensity with the increasing length of alkyl chain from C8 to C18 while showed the same sequence of HPTS-C16>HPTS-C18>HPTS-C12>HPTS-C8 on either surface activity, cellular adsorption, or cytotoxicity. In opposite, HPTS which contained no hydrophobic chain and thus exhibited no surface activity showed no cellular adsorption and cytotoxicity. It seems that the ligand of the appropriate chain length (C16) onto the hydrophilic HPTS molecules could cause the largest surface activity, the most distinguished cellular adsorption as well as the most adverse cytotoxicity. As reflected by the dynamic fluorescent visualization, the surfactant molecules of HPTS-C16 initially bound with cell membrane and entered into the intracellular lumen before finally localized at the endoplasmic reticulum (ER) and damaged it into a swollen structure. It is most likely that the structure of hydrophobic chain could determine the surface activities of surfactants and hence affect their cellular uptake and cytotoxicity. This study could help us to understand the adverse impact of anionic surfactant on human cells and its correlation with the surface activities or, in another word, the hydrophobic chain length.