Solid-state sensor for direct and real-time assay of p-phenylenediamine in cosmetics and herbal products

p-Phenylenediamine (p-PDA) is a common ingredient in hair dyes, cosmetics and herbal products. The current study used graphene nanoparticles and phosphotungstic acid in a quality-by-design approach to develop and optimize a carbon-paste sensor for p-PDA. Twenty-three sensors with different compositions were tested, and the results were computationally analyzed to generate prediction models. The optimized sensor consisted of graphite, graphene, o-nitrophenyloctylether, and phosphotungstic acid in a 62.83: 4.01: 27.1: 5.34 (wt %) ratio. The optimized sensor displayed a Nernstian slope of 29.1 +/- 0.22 mV decade- 1, a detection limit of 1.0x10-7 mol L-1, thermal and pH stabilities and a fast equilibration time of 10 s over ten weeks. The sensor accurately measured p-PDA concentrations within a wide linear range (1.0x10-7-1.0x10- 3 mol L-1) with high sensitivity and selectivity (even over its isomers o- and m-phenylenediamine). The developed electrochemical method outperformed the commonly used high performance liquid chromatography methods in simplicity, speed, costeffectiveness, linear range width and enabled real-time detection. The sensor's microstructure, morphology, and particle size were examined using the scanning electron microscope and energy-dispersive X-ray analysis. Spiked samples (3.60x10- 3-10.8 % w/w) were analyzed for p-PDA content, including tea, henna, synthetic dye, black pepper, and cosmetics. The work provides quality control laboratories with a validated, fast, and reliable tool to support data-based decisions that can have important implications for health and safety.