Visual determination of ferric ions in aqueous solution based on a high selectivity and sensitivity ratiometric fluorescent nanosensor

Abstract

A novel ratiometric fluorescent nanosensor for visual sensing of Fe³⁺ has been developed by integrating yellow-emissive rhodamine derivatives (RhB) onto the surface of silica nanospheres embedded with red CdTe QDs. Such a nanohybrid fluorescent sensor, RhB–CdTe@SiO₂ QDs, displays two distinct emission peaks 556 nm and 651 nm under a single excitation wavelength in the presence of Fe³⁺. The yellow fluorescence of RhB could be selectively increased by Fe³⁺ due to the strong chelating ability of RhB toward Fe³⁺, while the red fluorescence of CdTe QDs is almost constant, resulting in a distinct fluorescence color transformation from red to yellow, which can be applied for the visual detection of Fe³⁺. This approach shows high sensitivity and selectivity toward Fe³⁺, and the detection limit is determined to be 20.5 nM, which is lower than the maximum level (0.3 mg L⁻¹, equivalent to 5.4 μM) of Fe³⁺ ions permitted in drinking water by the U.S. Environmental Protection Agency (EPA). For practical application, we dope the mixture of RhB–CdTe@SiO₂ QDs and PVA onto the filter paper and fabricate test strips for direct visual inspection of Fe³⁺ in water. The paper based sensor has a visual detection limit of 1 μM by the naked eye, showing its promising application for onsite visual quantification of ferric ions without the need for elaborate equipment.