Mild phenolation of isopropanol-extracted bamboo lignin for high-performance antioxidant nanoparticles

Isopropanol-extracted bamboo lignin showed the highest antioxidant activity. • Mild phenolation in 90% formic acid increased phenolic –OH by 93.1%. • β-O-4 linkages were selectively cleaved and p -hydroxyphenyl introduced. • DPPH radical scavenging activity increased by 110.68% after phenolation. • Acetone/water system yielded spherical lignin nanoparticles (134.64 nm, PDI 0.15). To develop nature-derived and potent antioxidants, bamboo was employed as a raw material to isolate lignin using alcohol/water systems. Among the tested solvents, isopropanol-extracted lignin exhibited the highest radical scavenging activity. A subsequent mild phenolation strategy followed by nanoformulation substantially enhanced its antioxidant performance. Four alcohol/water mixtures (9:1, v /v) comprising methanol, ethanol, n -propanol, and isopropanol were systematically compared, revealing that isopropanol-extracted lignin possessed superior free radical scavenging capability. Phenolation of the isopropanol lignin was conducted in 90% formic acid medium with phenol at 100 °C. Two-dimensional heteronuclear single quantum coherence nuclear magnetic resonance (2D-HSQC NMR) analysis demonstrated that the phenolation process efficiently cleaved β-O-4 linkages in lignin and introduced phenolic hydroxyl groups in the form of p -hydroxyphenyl units at the α-position. Quantitative analysis by phosphorus-31 nuclear magnetic resonance ( 31 P NMR) showed an 84.7% reduction in aliphatic hydroxyl content, a 93.1% increase in phenolic hydroxyl content, and a concomitant 110.68% enhancement in 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity after phenolation. Lignin nanoparticles were subsequently prepared via an antisolvent method. Three solvent systems (ethanol/water, isopropanol/water, and acetone/water) were evaluated for their influence on nanoparticle morphology and dispersibility. Notably, the acetone/water system yielded regular spherical nanoparticles with an average diameter of 134.64 nm, a zeta potential of −24.34 mV, and a polydispersity index of 0.15, indicating excellent dispersion stability. The proposed strategy proceeded at 100 °C and avoided the use of specialized equipment or strongly corrosive reagents. Relying on abundant bamboo feedstock and a recyclable formic acid medium, it may offer advantages for scale-up production, providing a feasible route for the preparation of renewable antioxidant nanomaterials. These findings suggest potential applications of phenolated lignin nanoparticles in polymeric materials, food packaging, and biomedical fields.