Green and Scalable Preparation of Colloidal Suspension of Lignin Nanoparticles and Its Application in Eco-friendly Sunscreen Formulations

Abstract

Lignin nanoparticles (LNPs) are applied in several industrial applications. The nanoprecipitation of LNPs is fast and inexpensive but currently still limited to the use of hazardous organic solvents, making it difficult to apply them on a large scale. Here, we report a scalable nanoprecipitation procedure for the preparation of colloidal lignin nanoparticles (cLNPs) by the use of the green solvents dimethylisosorbide and isopropylidene glycerol. Irrespective of the experimental conditions, cLNPs showed higher UV absorbing properties and radical scavenging activity than parent LNPs and raw lignin. cLNPs were successively used in the preparation of eco-friendly sunscreen formulations (SPF 15, 30, and 50+, as evaluated by the COLIPA assay), which showed high UV-shielding activity even in the absence of synthetic boosters (microplastics) and physical filters (TiO₂ and ZnO). Biological assays on human HaCaT keratinocytes and human skin equivalents demonstrated the absence of cytotoxicity and genotoxicity, associated with an optimal protection of the skin from UV-A damage.

Figure 1

SEM images (upper panel) and DLS diagrams (lower panel) of KL-cLNPs, OL-cLNPs, and AL-cLNPs. The SEM image shows that the grid allows the connection of different types of lignin with different types of primary solvents used in the nanoprecipitation process.

Figure 2

ATR FT-IR spectra of OL-LNPs, KL-LNPs, and AL-LNPs produced from different primary solvents. The analysis has been performed after the isolation of the particles. The spectra of native lignin are included as a reference.

Figure 3

UV absorbance capacity of LNPs and cLNPs. The analysis was performed with the appropriate sample (0.05 mg/mL) in the range from 190 to 700 nm at 25 °C under gentle stirring. Native amorphous lignin was used as a reference. Both UV-B and UV-A ranges are reported.

Figure 4

(A) Coloration of KL-cLNPs prepared in the three eco-sustainable organic solvents (DMI, IPG, and GVL). (B) Colorations of sunscreen formulations KL1–KL4 in relation to the amount of KL-cLNPs, B4, and COM50+ are reported as references.

Figure 5

Transmittance of KL1–KL4 and B1–B3 sunscreen formulations prepared with KL-cLNPs. The composition of samples KL1–KL4 is reported in the text. Samples B1–B3 did not contain natural UV filters different from KL-cLNPs.

Figure 6

(A) Cytotoxicity of KL-LNPs prepared from DMI. The cell viability was expressed as the variation of the optical density (OD) value at 550 nm. (B) UV-A damage protection of KL-LNPs prepared from DMI. The cell damage was quantitatively assessed by the MTT assay.

Figure 7

(A) Cytotoxicity of the KL4 sample. The cell viability was expressed as variation of the OD value at 550 nm. (B) UV-A damage protection of KL4 prepared from DMI. The cell damage was quantitatively assessed by the MTT assay.

Figure 8

Evaluation of the genotoxicity of KL4 by the skin equivalent assay (HSE). (A) Images of histological analysis. (B) Quantitative analysis of the TUNEL positive cells. Statistical analysis was performed by the one-way ANOVA method followed by the Bonferroni test.