Biochemo-Catalytic Synthesis of Lignin Nanoparticles with Antioxidant Properties from Alkali Black Liquor Derived from Steam-Exploded Oil Palm Trunk

Alkali black liquor derived from oil palm trunk (OPT) serves as a recoverable source of technical lignin for conversion into functional materials. In this study, a biochemo-catalytic pathway was developed to convert steam-exploded alkali lignin recovered from this stream into antioxidant lignin nanoparticles (LNPs). The process was conducted at 60 °C and pH 4.5 in two synthesis stages. First, laccase was evaluated in sodium buffer with dosages ranging from 10 to 1000 POU/mg and reaction time from 30 to 90 min. The optimum condition (1000 POU/mg, 90 min) yielded the highest antioxidant activity. In the second stage, the optimum condition was combined with deep eutectic solvents (DESs), namely, lactic acid: betaine (LBDES) and lactic acid: choline chloride (LCDES). LBDES + laccase LNPs produced the highest activities (ABTS 3223.73 mg TE/g, DPPH 2300.94 mg TE/g, and TPC 921.61 mg GAE/g), exceeding those of LCDES + laccase LNPs by 27–108%. The resulting LNPs were uniform and colloidally stable with DLS indicating a mean size of 150.7 ± 48.7 nm and a ζ-potential of −35.4 mV. GPC confirmed the narrowest distribution. FTIR spectra revealed intensified O–H bands and distinct carbonyl peaks at 1704–1706 cm–1. Thermal analysis showed improved stability, with LCDES + laccase LNPs giving the highest onset temperature (301 °C), while laccase LNPs showed the highest mean residue at 800 °C (32.9 wt %). This approach provides a mild route for producing uniform lignin nanoparticles with improved thermal robustness and antioxidant performance. These results support the potential of this biochemo-catalytic system for lignin valorization, although process scalability, enzyme economy, and DES recovery remain to be established.