A fluorescent and colorimetric sensor based on ionic liquids for the on-site monitoring trace gaseous SO₂

SO₂ could cause severe environmental pollution and health threat, so real-time and on-site monitoring of SO₂ has attracted considerable attention. This work proposed a novel ionic liquid-based sensor, called trihexyl (tetradecyl) phosphonium fluorescein ionic liquid, which can accurately detect SO₂ with its fluorescent and colorimetric dual-readout assay without seventeen gases interference (eg: NO, N₂, CO₂, O₂, COS, HCl, CHCl₃). GC-MS was also used to verify the validation of the detection method. First, this fluorescein-based IL sensor exhibited fluorescence green and colorimetric yellow signals. When the sensor was exposed to gaseous SO₂, the green fluorescence quenched, and the colorimetric yellow color faded due to chemical bond interaction. Also, the proposed IL sensor exhibited good linearity in the SO₂ concentration range of 5.0-95.0 ppm with a detection limit of 0.9 ppm (fluorescence) and 1.9 ppm (colorimetry), and recoveries of 97%∼103% with RSD less than 1.21%. Besides, the IL sensor could be easily assembled into a paper device by simple immersion, and the paper strip was exploited to realize a semiquantitative visual detection of SO₂. These results indicated that the proposed fluorescence-colorimetric dual-signal chemosensor could be used as intelligent paper labels for real-time and on-site monitoring of SO₂ in ambient air.