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. 2020 Sep 30;10:16169. doi: 10.1038/s41598-020-73149-2

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© The Author(s) 2020

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Transcutaneous spectroscopy of orally ingested fluorescein. (A) Example fluorescence spectrum recorded 16 min after ingestion of fluorescein (acquisition time = 250 ms). Blue shaded region indicates the wavelength range (500–580 nm) that was summed to calculate the normalised integrated fluorescence intensity (see details in “Methods”). (B) Normalised integrated fluorescence intensity as a function of time. The red arrow indicates the time at which the fluorescent dye was ingested by the participant (1920 s). (C) Close-up of the blue shaded region in (B) revealing an S-shaped uptake curve in the first hour after ingestion. (D) Spectra recorded in limit of detection experiment. Fluorescence from 5 mg of fluorescein was just detectable above the background (spectrum recorded 29 min after ingestion) while the signal from 25 mg of fluorescein was clearly observable (spectrum recorded 37 min after ingestion).