Optimization of Reversible Photochromic Coatings Based on Methylene Blue: Formulation Screening, Performance Characterization, and Mechanistic Insights

Reversible photochromic coatings have emerged as promising materials for optical information storage, smart packaging, and UV detection, yet commercial products often suffer from high fabrication costs, poor color-switching sensitivity, and narrow absorption ranges. In this study, a methylene blue (MB) based reversible photochromic coating was developed via systematic single-factor optimization, focusing on seven critical variables: color-changing agent type, reducing agent type, oxidizing agent dosage, catalyst type, system temperature, UV wavelength, and auxiliary material type. The optimal formulation was determined as 0.02 g MB (color-changing agent), 0.2 g SnCl2 (reducing agent), and 20 g SiO2 (auxiliary material) in 0.5 mol/L HCl medium. Under 365 nm UV irradiation, this coating exhibited rapid color switching (blue → colorless → blue) with a color-changing time of 15-19 s and recovery time of 10-13 s over 8 cycles. Spectroscopic characterization via a V-1600 spectrophotometer revealed a broad absorption range of 330-685 nm, with a maximum absorption peak at 664 nm (consistent with MB’s characteristic absorption). Cost analysis showed the optimized coating cost only 12.66 CNY/kg, which is 85.8% lower than commercial photochromic coatings (87.71 CNY/kg). Additionally, incorporating 0.5 g tris(2,2′-bipyridine)ruthenium chloride hexahydrate ([Ru(bpy)3]Cl2·6H2O) as a catalyst expanded the color transition to orange-to-black, enhancing the material’s versatility. Mechanistic studies suggested that Sn2+ mediates MB’s reduction to leucomethylene blue (colorless), while UV irradiation (365 nm) drives reoxidation to MB (blue); SiO2 acts as a dispersant to improve coating uniformity and stability. This low-cost, high-performance MB-based coating provides a scalable solution for practical photochromic applications.