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• Ultrasensitive detection limit can reach as low as 0.01 ppm for riboflavin. • Just need a single droplet for detection using this indicator paper. • In-situ dual detection of riboflavin and pH with same droplet and indicator paper. • Excellent self-clean property makes droplet easy to remove and brings recyclability. Traditional indicator paper is disposable after use, requires a large number of target solutions and can only detect one item. In this study, recyclable upconversion-fluorescence indicator papers consist of upconversion nanorods and plasmonic nanostructures are fabricated by electrospinning. Human-body fluids containing different riboflavin concentration and pH are used as target solution, because these two items are usually detected in clinic to distinguish ambiguous diseases but often performed by two different testing processes that inevitably need more target solutions. Significantly different from conventional detecting system, this recyclable indicator paper can simultaneously achieve riboflavin and pH detections by using only a single droplet of target solution. As low as 0.01 ppm riboflavin can be sensitively detected, which is superior to most reported detection limit of fluorescence sensor. Moreover, this indicator paper can detect a single droplet (~10 μl) rather than a large amount of target solution (>1 ml) needed by traditional detecting system, and the pH can also be simultaneously in-situ detected using the same indicator paper. After the detection, the target solution is easily removed so that it brings about excellent recyclability, which is another advantage over traditional detecting system. [ABSTRACT FROM AUTHOR]