A novel "turn-on" fluorescence probe for chromium(III) sensing: experimental validation and theoretical insights

Trivalent chromium exhibits a dual nature. It acts as a necessary micronutrient for lipid and carbohydrate metabolism at low levels in the body, but causes dangerous genotoxic harm at higher thresholds. Hence, precise tracking of trace trivalent chromium is absolutely essential. Compounding this, its intracellular detection remains highly obstructed by paramagnetic fluorescence quenching and a deficiency in selective probes. Addressing this material and analytical challenge, this work reports the development of an ultrasensitive "turn-on" fluorogenic chemosensor utilizing a novel azo-based dye, 2-[(E)-(4-methylphenyl)diazenyl]phenol (MPDP). Unlike traditional methods restricted to microgram boundaries, this newly developed strategy establishes a wide dynamic linearity descending to an extraordinary femtogram detection limit. The probe operates via a chelation-enhanced fluorescence (CHEF) pathway at pH 9.0, successfully validating chromium(III) quantification in plant matrices. Furthermore, integration of density functional theory (DFT) and non-covalent interaction (NCI) analysis successfully maps out the precise, thermodynamically favored octahedral coordination mechanics driving its extreme metal selectivity.

All the geometry optimizations of the 2-[(E)-(4-methylphenyl)diazenyl]phenol (MPDP) monomer, its corresponding isomers, and resultant chromium(III) complexes were performed utilizing density functional theory (DFT). Calculations were executed at the M06-2X level of theory, implemented within the Gaussian 16 software package. The standard 6-311++G(d,p) all-electron basis set was assigned for light atoms (C, H, N, O, Cl), paired with Los Alamos LANL2DZ effective core potentials (ECP) to accurately describe the core and valence electrons of the heavy chromium center. Vibrational frequency analyses were systematically conducted following geometry optimization to guarantee that all structures corresponded to true potential energy surface minima, verified by the total absence of imaginary frequencies. Thermodynamic properties and Basis Set Superposition Error (BSSE)-corrected binding energies were evaluated using the counterpoise method. To map out real-space weak interactions, non-covalent interaction (NCI) analysis and reduced density gradient (RDG) isosurface were visualized using Multiwfn and Visual Molecular Dynamics (VMD) software.