Optimizing the ratio of coomassie and methylene blue dyes for a cost-effective and rapid staining of PET, PVC, PP, PS, LLDPE, LDPE, and HDPE

Abstract The ubiquitous presence of plastic brought on by the extensive use of plastic products calls for efficient and rapid plastic detection methods to detect and evaluate pollution. The commonly used Nile red dye takes many hours and is expensive while also not equally efficient across all the common plastic waste. To address this, we investigated the staining efficiency and optimized the ratio of a combined Coomassie brilliant blue and Methylene blue dye. In the optimisation process, Methanol-based Coomassie and Methylene blue dyes effectively stained the Polyethylene Terephthalate (PET), Polypropylene (PP), Polystyrene (PS), Linear Low-Density Polyethylene (LLDPE), Low-Density Polyethylene (LDPE), and Linear Low-Density Polyethylene (HDPE) plastics without compromising the plastic's integrity. Image analysis showed a generally better staining efficacy compared to Nile red. Through systematic experimentation, we identified specific optimal ratios of Coomassie (C) brilliant blue: Methylene (M) blue for various plastics: 5:5 (mass) for PVC, 7:3 (mass) for PET, and 8:2 (mass) for PP, LDPE, and HDPE. Additionally, the ratio of 10:0 (mass) was found suitable for PS and LLDPE. Given the cost-effectiveness, efficiency, and accessibility of the blue dyes in labs, the optimized ratio of the blue dyes makes it suitable for large-scale plastic staining across the six tested types of plastic, replacing Nile red.