Effect of concentration on singlet oxygen generation from xanthene-based photosensitizers

This study investigates the relationship between photosensitizer concentration and singlet oxygen (¹O₂) production, focusing on three xanthene-based dyes commonly used in photodynamic therapy (PDT): rose bengal (RB), erythrosin B (EB), and eosin Y (EY). ¹O₂ measurements were performed using an optical dosimeter capable of detecting ¹O₂ luminescence in the 1270–1280 nm infrared range in both ultra-pure water and saline (0.9 % NaCl) solutions for 10 concentrations ranging from 2.46 × 10^-5 to 1.97 × 10^-3 M. The results were fit with a model based on the Beer-Lambert law. Aggregation was quantified by analyzing the absorbance peak intensity ratios (measured using UV–vis spectroscopy). Our findings indicate that at lower concentrations (<2.46 × 10^-4 M), ¹O₂ production increases with rising photosensitizer concentration until it reaches a peak and then decreases at higher concentrations, as predicted with the Beer-Lambert model. Additionally, an aggregation effect is detected at higher concentrations in ultra-pure water and more pronounced in saline solutions, where the hydrophobic nature of the photosensitizers leads to enhanced aggregation which also affects the ¹O₂ generation. These results underscore the importance of optimizing photosensitizer concentration and solvent selection to maximize ¹O₂ generated while minimizing aggregation. Understanding this balance is crucial for improving the efficacy of PDT in clinical use.