1 São Paulo State University (UNESP), School of Sciences, Humanities and Languages, Assis, SP 19806-900, Brazil.
2 University of Sao Paulo (USP), São Carlos Institute of Physics, São Carlos, SP 13566-590, Brazil.
3 Concordia University, Department of Chemistry and Biochemistry and Centre for NanoScience Research, Montreal, QC H4B 1R6, Canada.

Rose Bengal (RB) is a promising photosensitizer (PS) for photodynamic therapy (PDT), but its application to colorectal carcinoma remains largely unexplored. Herein, we employ in vitro assays to demonstrate that incorporation of RB has substantial phototoxicity against Caco-2 cells, with more than 80% reduction in cell viability for 24 h incubation with 5 × 10^-6 mol/L RB followed by irradiation. In contrast, RB had minimal toxicity without irradiation. The mechanisms of RB action were further elucidated using confocal fluorescence microscopy, Langmuir monolayers as cell membrane models, and flow cytometry to determine the cell death pathways. Flow cytometry revealed that the primary mode of cell death was late apoptosis. RB incorporation affected Caco-2 plasma membrane morphology under light irradiation, and membrane interactions were confirmed using Langmuir monolayers of Caco-2 lipid extracts. Incorporation of RB into the monolayers shifted the pressure-area isotherms toward larger molecular areas, especially at low surface pressures and increasing RB concentrations (1, 10, and 25 × 10^-6 mol/L). RB adsorption also caused a decrease in the in-plane elasticity (Cs^1-) of the Caco-2 monolayers, with a large increase in monolayer flexibility as RB concentration increased. According to polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS), the anionic RB interacted electrostatically with positively charged phospholipid groups. Moreover, the changes in surface area observed in the monolayers upon RB incorporation and irradiation could be attributed to hydroperoxidation reactions triggered by the generation of singlet oxygen (¹O₂). These findings indicate that RB may be used as a PS in the PDT of colorectal cancer, providing detailed insights into its mechanism of action and phototoxicity.