From 8-hydroxy-5-sulfoquinoline to new related fluorogenic ligands for complexation of aluminium(III) and gallium(III)

Abstract

The hexadentate tripodal ligand O-TRENSOX (already known as a siderophore), incorporating three 8-hydroxy-5-sulfoquinoline (8-HQS) subunits, was investigated as a potential fluorogenic ligand of Al(III) and Ga(III). For the sake of comparison, every chelation study was also carried out with n-BUSOX, a ligand similar to one arm of O-TRENSOX. Chelations were studied at the optimal pH for fluorescence emission: pH = 4 for Al(III) and pH = 2 for Ga(III). An outstanding ‘tripod’ effect is exhibited by the values of the stability constants: with O-TRENSOX, log β₁₁₁  = 24.8 for Al(III) and 33.7 for Ga(III), whereas with n-BUSOX, log β₁₁₀  = 8.6 for Al(III) and 11.6 for Ga(III) at 25 °C. O-TRENSOX is nearly as efficient for Ga(III) chelation as for Fe(III). When increasing the [metal]/[ligand] ratio, fluorescence emission rose until either 1 : 1 chelation with n-BUSOX or 3 : 1 chelation with O-TRENSOX was achieved. Then, the resulting fluorescence intensity levelled off. The fluorescence emission intensity from n-BUSOX chelates was observed to be tenfold larger than that from O-TRENSOX chelates, suggesting that a self-quenching process occurs within the latter complexes. In terms of selectivity, ions such as Zn(II) or Cd(II), known to form strongly fluorescent complexes with 8-HQS, are not chelated at pH = 2 by n-BUSOX and O-TRENSOX. Thus, they are not potential interferences for Ga(III) determination, whereas Fe(III) strongly interferes, quenching the fluorescence. Conversely, although less stable at pH = 4, the chelates of Zn(II) and Cd(II) are possible interferences for Al(III) determination because of their strong fluorescence emission.