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. 2022 Mar 30;22(7):2649. doi: 10.3390/s22072649

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Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

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(a) The portable instrument setup, with the sample holder and the fixed LEDs. (b) The holder keeping the smartphone in a reading position. Reported from [37].