3.9. In Vitro Cytotoxicity

The MTT assay was used to measure cell metabolic activity, cytotoxicity, proliferation and viability. Cell viability was determined by measuring the amount of tetrazolium salt in the viable cells of each sample using untreated cells as controls [36]. A cell culture model was used to determine the relative cytotoxicity of the nanoparticles by cell viability according to Liu et al.’s methods [37]. In a nutshell, 100 μL of HaCaT keratinocytes (1 × 10⁵/mL) was seeded onto a 96-well microtiter plate at 37 °C in a humidified atmosphere with 5% CO₂ for 24 h. The cells were treated with varying concentrations of PPNPS for another 24 h. The control group meant the cells without any treatment. Then cells were incubated with MTT (5 mg/mL) for 4 h to stain the cells. The MTT solution was then removed and 150 μL dimethyl sulfoxide (DMSO) was added and shaken well. The absorbance at 570 nm was determined by a microplate reader (Tecan Infinite, Switzerland). The results were calculated and expressed as the cell viability according to Equation (4):