Colorimetric detection of glutathione by an anionic pyridylazo dye-based Cu2+ complex in the presence of a cationic polyelectrolyte

The colorimetric sensing system for glutathione (GSH) was constructed by the mixture of three components, an anionic dye 2-(5-bromo-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol (BrPAPS), a cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDADMAC), and Cu²⁺ in an aqueous solution. BrPAPS was complexed with Cu²⁺ in a 2:1 molar ratio via the electrostatic interaction with PDADMAC (BrPAPS₂-Cu²⁺-PDADMAC). The addition of GSH to the complex caused a hypsochromic shift with a distinct colorimetric response from red to yellow since the regeneration of the original BrPAPS through the interaction between GSH and Cu²⁺ in a 1:1 stoichiometry. BrPAPS₂-Cu²⁺-PDADMAC allowed the selective recognition of GSH over other amino acids and the detection suffer little interference from them. Hence, BrPAPS₂-Cu²⁺-PDADMAC provides efficient colorimetric sensing of GSH with high selectivity and sensitivity in pure aqueous media.