Sensitive fluorescence detection of Ni²⁺ ions using fluorescein functionalized Fe₃O₄ nanoparticles

This study describes a very sensitive fluorescence sensor for the selective nanomolar detection of Ni²⁺ ions. The Ni²⁺ ion sensing is based on fluorescence quenching of the fluorophore (fluorescein) in neutral aqueous medium. The fluorescence sensor is composed of a magnetic core and amino silica shell, functionalized with a fluorescein fluorophore. The morphology, physical and chemical properties of the sensing materials were studied by FT-IR spectroscopy, X-ray powder diffraction, vibrating sample magnetometer (VSM) and Transmission Electron Microscopy (TEM). UV-visible and fluorescence spectroscopy were used to characterize the fluorescein functionalized magnetic nanoparticles. The characterization measurements revealed that the fluorescent nanostructures were superparamagnetic in nature with an average particle diameter of 10 nm. The as-fabricated fluorescent nanosensor (Fe₃O₄@SiO₂-NH₂-fluorescein) showed an enhanced fluorescence quenching response towards Ni²⁺ ions in neutral pH medium. The response of the nanosensor was highly selective towards the target species whereas the possible interferences from other metal cations and biological molecules were negligible. The fluorescein probe has a very fast response, it is selective and has a sensitive detection limit (LOD = 0.83 nM) towards Ni²⁺ ions in neutral medium with a high binding constant (K) value of 3.2 × 10⁴ M^-1 for the complex formation between the sensor and Ni²⁺ ions. These features ensure the potential use of fluorescein functionalized magnetic nanomaterials as a new class of non-toxic biocompatible sensors for biological and environmental applications.