Turn-On fluorescence resonance energy transfer (FRET)-based electrospun fibrous membranes: Rapid and ultrasensitive test strips for on-site detection of Mercury (II) ion

•Sub-ppb sensitive Hg2+ test strips display visually-observable results in a minute.•Novel organic dye shows Hg2+-specific fluorescence enhancement via FRET mechanism.•Dye’s large Stokes shift (248 nm) reduces self-absorption and background signal.•Surface chemistry of the electrospun fibers affects FRET behavior of immobilized dye. Creation of a highly sensitive test strip that can provide simple, rapid, and high-throughput determination of Hg2+ levels in consumer goods and industrial waste is extremely challenging since the portability and convenience of the paper-based platform often come at the expense of inferior sensitivity. In this work, a fluorescent electrospun fibrous membrane that can report mercury contamination down to sub-ppb level in less than a minute is developed. The key success of this user-friendly device relies on the design of novel mercury-favored organic dye (NF06) that displays specific fluorescence enhancement – “Turn-On” positive results – through fluorescence resonance energy transfer (FRET) mechanism and exhibits extremely large Stokes shift of 248 nm. The surface chemistry of the cellulose acetate(CA)/polycaprolactone(PCL) electrospun fibers on which the NF06 sensor is immobilized has been found to efficiently facilitate the FRET operation of the sensor. As a result, the detection limit of this NF06 strip reaches 0.309 ppb (0.697 nM), approximately an order of magnitude lower than the maximum mercury level permitted in drinking water (2 ppb) recommended by the US Environment Protection Agency. Instrument-free visual readout of Hg2+ level-dependent fluorescence enhancement after dipping the strips in Hg2+-spiked drinking water and skin whitening serum demonstrates that these low-cost test strips could certainly be used by general consumers for safety evaluation as well as by regulatory agencies for large-scale screening of a variety of commercial products without the need for expensive and highly sensitive detectors.