Imine-linked chemosensors for the detection of Zn2+ in biological samples

A chemosensor 1 with a long hydrocarbon chain and polar end group is synthesized by the simple condensation reaction of a long chain amine with salicylaldehyde. A long chain hydrocarbon with a polar end group is used because of its solubility in an aqueous surfactant solution, which ensures that it can be used in a neutral water medium. The rationale for choosing an aryl aldehyde with -OH functionality is based upon the fact that a chelate ring consisting of an -OH group and an sp(2) nitrogen donor is always better for the selective recognition of Zn2+. The sensor shows selective binding to Zn2+ in 1% Triton-X-100 solution. Binding of Zn2+ by sensor 3 leads to an approximately 300% enhancement in the fluorescence intensity of the sensor, due to the combined effects of excited state intramolecular proton transfer (ESIPT) and the inhibition of the photo-induced electron transfer (PET) process by the -OH group. The fluorescence emission profiles of sensor 1 show some changes in the low and high pH ranges, however the sensor remains stable in the pH range 4-9, which makes it appropriate for use in biological fluids.