Harnessing Citrus limon Leaves for Natural Dyeing: Extraction, Antimicrobial Potential, and Application on Cotton Using Traditional Mordants

Lemon leaves (Citrus limon L., Rutaceae) are an abundant agricultural by-product, yet their potential as a textile dye has received limited attention. This study evaluated the dyeing performance of C. limon leaf extract on cotton and assessed its antibacterial and cytocompatibility properties. Among three solvent systems, 70:30 ethanol-water Soxhlet extraction at 60 °C for 2 h yielded the highest crude colourant (16% w/w). Sequential vacuum liquid chromatography, silica gel column chromatography, and preparative HPLC identified rutin (quercetin-3-O-rutinoside) as the principal colourant via FTIR, UV-Vis, and ¹H/¹³C NMR. UV-Vis maxima at 302 nm (Band II) and a shoulder at 370 nm (Band I) confirmed flavonol-type chromophores, while spectral shifts in mordanted samples suggested flavonol–metal complexation and co-pigmentation with minor phenolics. Cotton dyed at 100 °C with alum, SnCl₂, CuSO₄, and FeSO₄ exhibited generally acceptable fastness for natural dyes. Light fastness reached grades 4–4/5 (Blue Wool Scale), and wash and perspiration fastness ranged 3/4–5 (ISO grey scale). FeSO₄ post-mordanting provided the best overall fastness, whereas FeSO₄ pre-mordanting produced the highest colour strength, increasing K/S from 0.70 in unmordanted cotton to 1.30, accompanied by decreased lightness (L* from 73.0 to 58.5) and a hue shift from 74° to 80°. CuSO₄ gave the second-highest K/S (1.05), with alum and SnCl₂ yielding intermediate values (0.90–0.95). Dye uptake followed pseudo-second-order kinetics and fit the Freundlich isotherm. The extract also displayed concentration-dependent antibacterial activity and acceptable cytocompatibility, suggesting that rutin-rich extracts from C. limon leaves may be a promising natural cotton dye with added biofunctional properties.