Ultrasensitive and Recyclable Upconversion‐Fluorescence Fibrous Indicator Paper with Plasmonic Nanostructures for Single Droplet Detection

Abstract Conventional indicator paper is disposable after use and requires a large amount of target solution. Herein, recyclable upconversion fluorescence indicator papers consisting of upconversion nanorods and plasmonic nanostructures are fabricated by electrospinning. This recyclable indicator paper exhibits good flexibility and superhydrophobicity with enhanced upconversion fluorescence. Rhodamine B (RhB) is selected as the target solution because it is prohibited as a food additive but often occurs in food, and conventional detection methods usually need a large volume of solution (>1 mL) to refine to higher concentrations because of the low detection sensitivity. Significantly different from the traditional detection system, this upconversion fluorescence fibrous indicator paper can detect RhB as low as 0.01 ppm, which is superior to most reported detection limits for fluorescence sensors. Moreover, this indicator paper can detect a single droplet (≈10 µL) of RhB without needing a large amount of solution. After detection, the target solution is easily removed for excellent recyclability, which is another advantage over the traditional detection system.