Dye‐linked Upconverting Nanophosphor‐based Ratiometric Chemosensor for NIR‐excited and FRET‐mediated Ultrasensitive Detection of Sulfide Ions

Abstract In this article, a NIR‐excited ratiometric method for selective and ultrasensitive detection of sulfide (S 2− ) ions in an aqueous environment is presented, which is reliant on fluorescence resonance energy transfer (FRET). The chemosensor‐design is based on an upconverting nanophosphor (UCNP)‐linked coloured complex between a rhodamine‐B derivative and stannous ions (RBD‐Sn 2+ ). Under the excitation of NIR light, the green emission of UCNP (at 542 nm) is reabsorbed by the linked RBD‐Sn 2+ complex via FRET, which in turn emits in the orange (at 582 nm). S 2− ions specifically interact with the linked RBD‐Sn 2+ complex and quantitatively bleach its colour, leading to reduction in FRET and consequent recovery of the green emission of UCNP. We investigated the quantitative and ratiometric detection of S 2− ions in aqueous phase using this NIR‐excited FRET‐based approach. The concentration of S 2− ions is quantitatively connected to the ratiometric emission signal under NIR excitation. We have found that the detection limit for S 2− ions using our FRET‐based nanoprobe is around ten times lower than that of colorimetric or fluorescence‐based approaches. Owing to the use of NIR‐light as excitation source, our proposed sensor can demonstrate background‐free sensing of S 2− in complex environmental and biological samples.