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. 2022 Mar 2;12:3427. doi: 10.1038/s41598-022-07408-9

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© The Author(s) 2022

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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Photos of the laminated paper-based analytical devices (LPADs) after the loading of each amino acid. (a) Original image of each LPAD after interaction with 0–100 μM tryptophan, glycine, histidine, and lysine. Color change was observed only in the detection area of substrate amino acid. (b) The images were color-inverted using the GNU Image Manipulation Program.