Glyphosate sensing in acqueous solutions by fluorescent Zinc(II) complexes of [9]aneN3-based receptors

The binding abilities of Zn(II) complexes of [12]aneN4- (L1) and [9]aneN3-based receptors (L2 and L3) towards the herbicides N-(phosphonomethyl)glycine (glyphosate, H3PMG) and 2-amino-4-[hydroxy(methyl)phosphoryl]butanoic acid (glufosinate, H2GLU), and also aminomethylphosphonic acid (H2AMPA,) the main metabolite of H3PMG, and phosphate, are described. All ligands form stable Zn(II) complexes, whose coordination geometries allow a possible interaction of the metal center with exogenous anionic substrates. In the case of L2 and L3, the coordination sphere of the metal is not saturated by the ligand donors, making them potential metal-based receptors for anionic species. Differently from L2 and L3, L1 features a tetraamine macrocyclic unit and the quinoline nitrogen atom is close to the macrocyclic unit, so all five nitrogen atoms could participate to metal binding. However, Zn(II) could expand its coordination sphere, achieving hexa coordination in the presence of a coordinating exogenous ligand. Among the different substrates, H3PMG is selectively bound over the other analytes by the 1:1 Zn(II) complex of L2. Furthermore, only in the case of [ZnL2]2+ there is a wide pH window in which the ternary species [ZnL2(HPMG)] is the prevailing specie present in solution including neutral pH. The fluorescence sensing ability of the complexes parallels their binding efficiency. H3PMG is specifically signalled over the other substrate by the [ZnL2]2+ complex, whose emission is dramatically quenched in the presence of this analyte. The test strip also showed a good ability of the [ZnL2]2+ complex-based test paper for the selective detection of H3PMG over the other analytes considered.