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The present study investigates the influence of the addition of <span style="font-variant: small-caps;">l</span>-arginine to a matrix of carboxymethylated nanofibrillated cellulose (CMC-NFC), with the aim of fabricating a mobile carrier facilitated transport membrane for the separation of CO₂. Self-standing films were prepared by casting an aqueous suspension containing different amounts of amino acid (15&#8722;30&#8722;45 wt.%) and CMC-NFC. The permeation properties were assessed in humid conditions (70&#8722;98% relative humidity (RH)) at 35 &#176;C for CO₂ and N₂ separately and compared with that of the non-loaded nanocellulose films. Both permeability and ideal selectivity appeared to be improved by the addition of <span style="font-variant: small-caps;">l</span>-arginine, especially when high amino-acid loadings were considered. A seven-fold increment in carbon dioxide permeability was observed between pure CMC-NFC and the 45 wt.% blend (from 29 to 220 Barrer at 94% RH), also paired to a significant increase of ideal selectivity (from 56 to 185). Interestingly, while improving the separation performance, water sorption was not substantially affected by the addition of amino acid, thus confirming that the increased permeability was not related simply to membrane swelling. Overall, the addition of aminated mobile carriers appeared to provide enhanced performances, advancing the state of the art for nanocellulose-based gas separation membranes.