Selective Turn-On Fluorescence Sensing of Cyanide Using the Pyridoxal Platform of a Ni(II) Complex

Cyanide is a very toxic pollutant to aquatic life and
the environment.
Analytical methods for the quantitative assay of cyanide, which are
rapid, sensitive (low limit of detection), and cost-effective, are
in great demand. Colorimetric and fluorometric methods are ideally
suited for this purpose. In this report, we describe a Ni(II) complex
containing a pyridoxal platform for the rapid and sensitive fluorometric
estimation of cyanide. The square-planar Ni(II) complex, [Ni(L)(N3)]·3H2O, where the ligand LH = 4-[(2-dimethylamino-ethylimino)-methyl]-5-hydroxymtheyl-2-methyl-pyridin-3-ol,
a Schiff base formed between pyridoxal and (2-dimethylamino)ethyl
amine, was synthesized and characterized by various spectroscopic
techniques as well as by single-crystal X-ray structure determination.
The complex was found to selectively bind CN– in
the presence of other biologically important anions such as F–, Cl–, Br–, I–, OAc–, S2–, NO3–, PO43–, SO42–, and H2PO4– in tris-HCl/NaCl buffer [pH = 7.4], and it can be
monitored by fluorescence turn-on or by UV–visible spectroscopy.
The binding constant of the complex with CN– was
estimated to be 2.046 × 1014 M–2 and the limit of detection (LOD) was 9 nM, the LOD being considerably
lower than the maximum permissible level of cyanide ions (1.9 μM)
in drinking water, as recognized by the World Health Organization
(WHO). The effects of pH and temperature on the sensing are also investigated.
The Ni(II) complex is also found to bind to calf-thymus DNA very strongly,
and the apparent binding constant (Kapp) was determined to be 1.33 × 107 M–1 by the fluorescence quenching of the ethidium bromide–DNA
adduct by the complex.