Visible colour displacement sensing system for manganese(II)

A visible colour displacement system has been developed for the colorimetric and ratiometric detection of Mn(II) ions. The supramolecular displacement system is composed of 2-(5-bromo-2-pyridylazo)-5-[N-propyl-N-(3-sulfopropyl)amino]phenol (5-Br-PAPS), which binds Zn²⁺ with a change in colour and binds Mn²⁺ without changing colour, ethylene glycol-bis(2-aminoethylether)-N, N, N', N'-tetraacetic acid (EGTA) and Zn²⁺ at neutral pH. The sensing system exhibits the colour of 5-Br-PAPS itself since EGTA binds Zn²⁺ more strongly than 5-Br-PAPS does. However, upon presentation of Mn²⁺, Zn²⁺ is displaced from EGTA to bind 5-Br-PAPS to produce a pronounced colour change from yellow to purple. The absorbance decreases at 449 nm and increases at 552 nm, both linearly to Mn²⁺ concentration at low micromolar levels. This should be very advantageous since no suitable colorimetric reagent is available for the direct declination of Mn²⁺ at physiological pH.