Enhancing the Broad-Spectrum Adsorption of Lignin through Methoxyl Activation, Grafting Modification, and Reverse Self-Assembly

Lignin nanospheres with broad-spectrum UV adsorption and excellent antioxidant properties were obtained by demethoxylation of alkali lignin (AL) and grafting the benzophenone moiety (UV-0) followed by a reverse self-assembly. The critical wavelength (λ c ) of the optimal AL-UV0 3 sample was ∼375 nm, while the UVA/UVB ratio reached 0.84. When it was applied as a unique active in sun cream, the sun protection factor value of the cream containing 10 wt % AL-UV0 3 was 22.81, which could further increase to 56.14 when it formed reverse colloidal spheres (LRCS) with a size of ∼130 nm. The self-assembly process and the UV-absorbing enhancement mechanism of LRCS were monitored and revealed by fluorescence, UV, and light scattering analysis. In addition, AL-UV0 3 exhibited superior photostability due to the three-dimensional network structure of lignin. The antioxidant activities of AL-UV0 3 and LRCS were better than that of AL and could increase continuously with the dosage. Both AL-UV0 3 and LRCS exhibited good biocompatibility. HaCaT cell activity remained 72–78% when their concentration was as high as 1.0 mg/mL. The novel lignin broad-spectrum UV blocker and antioxidant show good potential in skincare products and polymeric materials.