Bioengineered nanocellulose-Schiff base sensor for selective fluorometric detection of Cd²⁺ ions in aqueous media

Cadmium (Cd²⁺) is recognized as a highly detrimental, non-essential heavy metal and is classified as a Class I carcinogen, presenting significant hazards to both human health and aquatic ecosystems, even at minimal concentrations. Its prolonged biological half-life and propensity for bioaccumulation necessitate ongoing surveillance in both drinking water and wastewater. Traditional detection methods, including inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectroscopy (AAS), offer high sensitivity but are hindered by factors such as expensive equipment, extensive sample preparation processes, and limitations for field-based assessments. To address these limitations, we have engineered a sustainable, bio-derived fluorometric sensor specifically designed for the selective detection of Cd²⁺ ions in aqueous systems. This was achieved by immobilizing a Schiff base ligand, known as EDSB, onto nanocellulose (NC) derived from recycled office paper. The EDSB ligand, synthesized from 2,3-diaminomaleonitrile and 3-ethoxysalicylaldehyde, was characterized through Fourier-transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance (¹H NMR) analysis. The NC offered a renewable and high-surface-area platform for effective ligand immobilization. The resulting NC-EDSB sensor displayed immediate fluorescence quenching upon the binding of Cd²⁺ ions, functioning through ligand-to-metal charge transfer, and demonstrated a linear detection range spanning 0.033 to 0.70 ppm with a detection limit set at 0.033 ppm. The sensor exhibited high selectivity against a range of competing ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cu²⁺, Ni²⁺, Pb²⁺, Co²⁺, Zn²⁺, Hg²⁺, Fe²⁺, and Al³⁺, alongside various common anions), especially under near-neutral pH conditions. Response times were recorded at 60 s, and the sensor maintained over 85 % of its emission intensity throughout six regeneration cycles utilizing EDTA desorption. Notably, when evaluated with actual water samples, including tap and wastewater, the sensor achieved exceptional recovery rates ranging from 98.60 % to 99.77 %, thereby affirming its reliability and suitability for practical use. This study exemplifies how the combination of nanocellulose and cadmium-selective Schiff base ligands can yield an environmentally sustainable and field-ready platform for heavy metal detection.