Effective cyanide ions detection using neutral red receptor: Spectroscopic, colorimetric detection and theoretical deformation density analysis

Cyanides are toxic compounds and environmental pollutants that have destructive effects on human health and ecosystems. Nevertheless, the cyanide-containing compounds are widely used in industrial applications, making their detection crucial for avoiding human and environmental exposure. This research introduces the Neutral Red as an effective and sensitive colorimetric sensor for detecting ${\mathrm{CN}}^{-}$. The interaction of cationic Neutral Red (${\mathrm{NRH}}^{+}$) with ${\mathrm{CN}}^{-}$ was studied through a combination of experimental and quantum mechanical approaches. The UV–Vis and ¹H-NMR spectra and colorimetric response of ${\mathrm{NRH}}^{+}$ to ${\mathrm{CN}}^{-}$ indicated the formation of the $\mathrm{NRH}-\mathrm{CN}$ complex. Theoretical analyses based on density functional theory showed that the protonated pyrazine ring of the ${\mathrm{NRH}}^{+}$ is the favorable binding site for ${\mathrm{CN}}^{-}$. Theoretical deformation density analysis visualized the area of electron density accumulation and depletion in the $\mathrm{NRH}-\mathrm{CN}$ and revealed that the orbital relaxation and kinetic energy pressure have a competitive contribution to the total deformation density. The time-dependent density functional theory (DFT) analyses simulated the UV–Vis spectrum and the electron transition characteristics. Furthermore, the quantum theory of atoms in molecules (QTAIM) confirmed the existence of a noncovalent interaction between ${\mathrm{NRH}}^{+}$ and ${\mathrm{CN}}^{-}$. Overall, the experimental and theoretical analyses showed that the ${\mathrm{NRH}}^{+}$ chemosensor can effectively and selectively probe the cyanide.