Development of a chromene-based fluorescent and colorimetric sensor for the sensitive detection of calcium ions in a complex medium: a detailed DFT and experimental approach

Abstract

Calcium plays a key role in various biochemical processes and cellular functions in the human body; its imbalance can cause severe hyper- and hypo-calcemic medical conditions. In this research work, we developed a novel AIE-active chromene-based fluorescent sensor, CFN (DMF, λ_exc = 330 nm, λ_em = 476 nm), through an easy Knoevenagel condensation reaction with solvatochromic characteristics. The synthesized CFN sensor exhibited excellent fluorescence enhancement with a blue shift in 90% aqueous media (9 : 1, H₂O/DMF, λ_exc = 330 nm, λ_em = 410 nm). Blue shift in the fluorescence spectrum indicates the formation of H-aggregates, which is further supported by dynamic light scattering (DLS) results. The CFN sensor shows a large Stokes shift of 120 nm in pure DMF and 54 nm in water/DMF. The CFN sensor was employed for extremely selective and sensitive colorimetric detection of Ca²⁺ ions via chelation-enhanced fluorescence quenching (CHEQ) mechanism. The calculated detection and quantification limits were 10.5 nM and 35 nM, respectively. Furthermore, UV-Vis spectroscopy, fluorescence spectroscopy, and ¹H NMR titration experiments were carried out to validate the interaction mechanism between CFN and Ca²⁺ ions. The application of CFN for the detection of Ca²⁺ ions in real samples were accessed through real-time fluorescence titration analyses in human plasma, commercial milk, and different water samples. The portable CFN sensor-coated strips were developed for on-site sensing of Ca²⁺ ions, and a logic gate was designed to mimic it as a point-of-need calcium ion sensor.