3.3. Cell Cytotoxicity Assay and Fluorescence Imaging

The fluorescence imaging tests to biothiols were performed in MCF-7 cells. The cells were cultured in Dulbecco’s modified Eagle’s medium (DMEM) at 5% CO₂ and 37 °C for 24 h until the cells were overgrown. For the toxicity of PHPQ-SH, the cells were separately incubated with various concentrations (0.0–20.0 μM) of PHPQ-SH for 24 h in a 96-well plate, and then cell viability of the treated cells was measured by the MTT staining method. For confocal imaging, the MCF-7 cells were loaded on a confocal dish. The experiments were divided into three groups. The first group of MCF-7 cells was stained with PHPQ-SH (10.0 μM) for 30 min and photographed after washing the excess PHPQ-SH with PBS. Compared with the first group, the second group of MCF-7 cells was preincubated with NEM (1.0 mM) for 30 min and treated with PHPQ-SH (10 μM) for 30 min. In the third group, the MCF-7 cells were pretreated with NEM (1.0 mM) for 30 min, followed by incubation with PHPQ-SH (10.0 μM) for 30 min, and, after being placed in GSH at concentrations of 15.0, 50.0, 100 μM respectively, for another 30 min, the fluorescence imaging was performed.