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(a) Schematic illustration of the colorimetric detection of formaldehyde based on the Methyl Yellow-impregnated PA-6 NFN membranes. Optical images of Methyl Yellow-impregnated PA-6 NFN membranes (b) before and (d) after exposure to 5 ppm formaldehyde. (c) FE-SEM image of the Methyl Yellow-impregnated PA-6 NFN membranes [281]. Reprinted with permission from [281]. © 2012 Elsevier B.V.
