MWCNT-coumarin hybrids as fluorescence sensors for selective Fe³⁺ detection: investigating roles of MWCNT characteristics via synthesis and functionalization

In this study, waste-derived benzene-toluene-xylene (BTX) mixtures were utilized as sustainable carbon feedstocks for surface-engineered nanomaterials. Pristine multi-walled carbon nanotubes (MWCNTs) with diameters of approximately 30 and 60 nm were synthesized using iron-based catalysts (Fe/Al₂O₃ and Fe/SiO₂), with commercial MWCNTs (∼10 nm in diameter) serving as benchmarks. The pristine MWCNTs were purified and functionalized through a one-pot acid treatment that simultaneously removed catalyst residues and introduced oxygen-containing surface groups, yielding high-purity functionalized MWCNTs (f-MWCNTs) with significant carboxyl content. Despite variations in nanotube diameter and purity, four f-MWCNT probes exhibited comparable selective fluorescence quenching toward Fe³⁺, with limits of detection (LODs) of 2.20–2.78 µM at probe concentrations of ∼2.0 µM. These results demonstrate that that sensing performance is governed primarily by surface chemistry and interfacial interactions. Covalent conjugation of coumarin derivatives to the f-MWCNT surfaces further tailored the organic–inorganic interface, improving the LODs to 1.82–1.92 µM at a fivefold lower probe concentration (∼0.4 µM) by enhancing fluorescence emission and suppressing aggregation-induced quenching. The coumarin-conjugated f-MWCNTs showed low cytotoxicity, maintaining >90% cell viability at 0.1–10 µg/mL. This work establishes a scalable CNT-based hybrid platform in which controlled surface functionalization and interface design enable robust fluorescence sensing for biomedical and environmental applications.