PEGylated meso-arylporpholactone metal complexes as optical cyanide sensors in water

Abstract

The colorimetric cyanide sensing ability of free base porpholactone, a pyrrole-modified porphyrin in which a porphyrin β,β′-double bond was replaced by a lactone functionality, and its zinc(II), platinum(II), and gallium(III) complexes in aqueous solution are reported. Water-solubility of the parent meso-pentafluorophenyl-derivatized porphyrinoids was assured by PEGylation of the p-aryl positions using a nucleophilic aromatic substitution reaction with thiol-terminated PEG chains. A central metal-dependent sensing mechanism was revealed: While the CN⁻ adds to the zinc(II) complex as an axial ligand, resulting in a minor response in its UV-vis spectrum, it undergoes a nucleophilic addition to the lactone moiety in the platinum(II) and gallium(III) complexes, leading to a much more prominent optical response. Nonetheless, these chemosensors are less sensitive than many other sensors reported previously, with detection limits at pH 7 for the zinc, gallium, and platinum complexes of 2 mM (50 ppm), 240 μM (6 ppm), and 4 mM (100 ppm), respectively. The gallium(III) complex is weakly fluorescent (ϕ = 0.8%) and cyanide addition leads to fluorescence intensity quenching; the cyanide adduct responds with a fluorescence switch-on response but the signal is weak (ϕ < 10⁻²%). Lastly, we report on the fabrication of a unique optical cyanide-sensing membrane. The PEGylated gallium-complex was incorporated into a Nafion® membrane (on a PTFE carrier film). It was shown to be stable over extended periods of time and exhibiting a reversible and selective response within minutes to cyanide, with a 5 mM (130 ppm) detection limit. This largely fundamental study on the ability to utilize the once rare but now readily available class of pyrrole-modified porphyrins as chemosensors highlights the multiple principle ways this chromophore platform can be modified and utilized.