Lab-on-a-tip platform with cotton matrices for colorimetric detection of carmine in lipsticks

Carmine is a natural red dye derived from cochineal insects and is widely used in cosmetics and food products. Since carmine is derived from insects, individuals who follow vegan or vegetarian lifestyles and those who have religious beliefs may wish to ascertain its presence in the products they use. Additionally, carmine causes angioedema, urticaria, contact dermatitis, systemic anaphylaxis and even asthma in hypersensitive individuals. Conventional methods for detecting the presence of carmine require expensive equipment and skilled personnel; thus, robust, low-cost and rapid detection is needed. In this work, we fabricated a simple lab-on-a-tip device with reagents imbibed in two cotton matrices for the colorimetric detection of carmine. In this assay, carmine reacts with hydrochloric acid to produce carminic acid, which further reacts with ferric chloride to form a brown chromogenic complex whose intensity is proportional to the carmine concentration. Additionally, a comparative evaluation was performed between cotton and filter paper, in which cotton exhibited enhanced color intensity and sensitivity because of its three-dimensional intertwined fibrous network. The simple lab-on-a-tip device provides a reproducible and distinguishable color across the selected concentration range, with an experimentally determined limit of detection (LOD) of 0.5 mg mL^-1. FTIR spectral analysis confirmed the presence of characteristic functional moieties of carmine in commercial lipstick samples, as validated against standard carmine. The device was successfully applied to commercial samples where carmine was detected in some lipstick samples, while no detectable color change was observed in one lipstick sample. Overall, this work presents a simple, portable and cost-effective (USD $ 0.0055) device for onsite and rapid detection of carmine, with a green metric score of 0.74 determined using AGREE software, proving it to be a green method. Compared with conventional techniques, this device has minimal resource requirements and potential for point-of-care and field applications, especially in low-resource settings.