Highly hydroxylated monodisperse lignin colloidal nanospheres prepared through self-assembly for efficient photothermal conversion

High-purity lignin was extracted using a citric acid-LiBr molten salt system. • Nano-sized lignin colloidal spheres (LCSs) were prepared via solvent/anti-solvent self-assembly. • The Lignin-Pine shows higher photothermal property due to its higher –OH and β-O-4 retention. • The LCSs exhibit better photothermal property due to the ordered aromatic π–π stacking structure. • The Narrowest bandgap of HK (3.269 eV) explains the superior photothermal performance of LCSs-Pine. Lignin has emerged as a promising photothermal material due to its unique conjugated aromatic rings and phenolic hydroxyl groups. Herein, a citric acid–lithium bromide molten salt extraction method was employed to isolate high-purity lignin from pine, fir, poplar and bamboo. Uniform nano-sized lignin colloidal spheres (LCSs) with ordered supramolecular frameworks were subsequently fabricated through solvent/anti-solvent self-assembly driven by intermolecular hydrogen bonds. Such an ordered stacking structure of LCSs strengthens π–π interactions between aromatic moieties, expanding electron delocalization and boosting non-radiative transitions, thereby exceeding pristine lignin in photothermal conversion. LCSs-Pine displays superior photothermal performance due to the higher hydroxyl density (8.06 mmol·g −1 ), regular G-unit distribution and favorable retention of β-O-4 linkages (46.7%) of pine-derived lignin, contributing to its temperature rise from 30 °C to 84 °C within 10 min at 0.1 W·cm −2 . Density functional theory (DFT) simulations confirm that Lignin-Pine containing relatively higher Hibbert ketone (HK) units (49.5%) with the narrowest bandgap (3.269 eV) rationalizes its superior photothermal performance. The samples were characterized by Fourier-transform infrared spectroscopy (FTIR), quantitative 31 P nuclear magnetic resonance (NMR), two-dimensional heteronuclear single quantum coherence (2D-HSQC) NMR, gel permeation chromatography (GPC), scanning electron microscopy (SEM), dynamic light scattering (DLS), and thermogravimetric analysis (TGA). Moreover, the LCSs exhibit excellent photothermal cycling and thermal stability. This work provides a scalable, structure-guided route to convert lignin into efficient photothermal nanomaterials.