Photochemical behavior of colloidal lignin particles under controlled UV exposure: Balancing self-stabilization and degradation.

The morphology control of lignin through particle size reduction to nanoscale seems to be a suitable conversion technology to overcome the intrinsic limitations of its native form to develop a wide range of biomaterials with high performance. Colloidal lignin particles (CLPs) in the range of 150–200 nm were synthesised from hardwood and softwood kraft lignins by the solvent shifting method. The initial molecular features of kraft lignins were evaluated in terms of purity, molecular weight distribution, and chemical functionalities. The impact of the lignin source and structure on the morphology, size distribution, and surface chemistry of CLPs was evaluated by particle size analyser, SEM, TEM and 1H NMR. The results evidenced the influence of the botanical origin on the morphology and surface chemistry of particles. Furthermore, the antioxidant properties and cytotoxicity of lignins and corresponding CLPs, towards lung fibroblast cells were compared. CLPs from hardwood kraft lignins exhibited higher antioxidant power against DPPH free radical and a higher cytotoxic effect (IC30 = 67–70 μg/mL) against lung fibroblast when compared to CLPs from softwood kraft lignin (IC30 = ~91 μg/mL). However, the cytotoxicity of these biomaterials was dose-dependent, suggesting their potential application as active ingredients in cosmetic and pharmaceutic products at low concentrations.

Enzymatically and chemically oxidized lignin nanoparticles for biomaterial applications.

Coating of colloidal lignin particles (CLPs), or lignin nanoparticles (LNPs), with proteins was evaluated in order to establish a safe, self-assembly mediated modification technique to tune their surface chemistry. Gelatin and poly- l-lysine formed the most pronounced protein corona on the CLP surface, as determined by dynamic light scattering (DLS) and zeta potential measurements. Spherical morphology of individual protein coated CLPs was confirmed by transmission electron (TEM) and atomic force (AFM) microscopy. A mechanistic adsorption study with several random coiled and globular model proteins was carried out using quartz crystal microbalance with dissipation monitoring (QCM-D). The three-dimensional (3D) protein fold structure and certain amino acid interactions were decisive for the protein adsorption on the lignin surface. The main driving forces for protein adsorption were electrostatic, hydrophobic, and van der Waals interactions, and hydrogen bonding. The relative contributions of these interactions were highly dependent on the ionic strength of the surrounding medium. Capillary electrophoresis (CE) and Fourier transform infrared spectroscopy (FTIR) provided further evidence of the adsorption-enhancing role of specific amino acid residues such as serine and proline. These results have high impact on the utilization of lignin as colloidal particles in biomedicine and biodegradable materials, as the protein corona enables tailoring of the CLP surface chemistry for intended applications.

Modulation of physicochemical and antioxidant properties of Pickering emulsions using colloidal lignin particles based on kraft softwood and hardwood acetone fractions

Due to lignin's complexity and heterogeneity, the influence of lignin structures on its nano or colloidal particle fabrication remains unclear. In this work, organosolv lignins representing a gradient of β-aryl ether linkage, phenolic OH groups, and molecular weights were extracted to fabricate size-tunable colloidal lignin particles (CLPs). DLS (dynamic light scattering) analysis revealed that the CLP size strongly correlates with the lignin structure and initial concentration. CLPs formed at low lignin concentrations (1 and 2.5 mg/mL) were smaller (200-400 nm), while higher lignin initial concentrations (10 and 20 mg/mL) yielded larger particles (500-750 nm). Lignin with fewer β-O-4 linkages, lower molecular weight, and higher phenolic OH content forms smaller CLPs at low lignin concentrations; in contrast, it tends to form larger CLPs at high lignin concentrations. The relationship between the CLP size and lignin typical structures provides new insights for fabricating size-tunable lignin particles with tailored applications.

Colloidal lignin particles (CLPs) are gaining attention as eco-friendly stabilizers for Pickering emulsions. Still, conventional lignin sources, like kraft lignin, are often limited by their dark color and strong odor. This study explores, for the first time, the use of a light-colored lignin derived from an aldehyde-assisted fractionation with glyoxylic acid (GA-lignin) for producing CLPs and derived Pickering emulsions. CLPs were produced by antisolvent precipitation with water (CLPs-W, pH6) and alkaline buffer (CLPs-B, pH8) as the antisolvents. The results revealed that the selected antisolvent significantly influenced the CLPs' properties. CLPs-W were larger, uniform in size, and hydrophobic, whereas CLPs-B were smaller, agglomerated into clusters, and exhibited greater hydrophilicity. Despite both CLPs' effectiveness in stabilizing oil-in-water emulsions, the stabilization mechanisms differed markedly; CLPs-W formed a robust membrane barrier at the oil-water interface, while CLPs-B facilitated oil droplet bridging. Overall, this work demonstrates that GA-lignin's light color nature offers advantages for Pickering emulsions design, surpassing a lignin typical limitation. This advancement highlights the versatility of GA-lignin-derived CLPs and supports the development of sustainable lignin-based products with significant commercial prospects.

Colloidal lignin particles (CLPs) from softwood kraft lignin were evaluated as a multifunctional ingredient to prepare bio-based oil-in-water (O/W) Pickering emulsions. After a preliminary screening, three Pickering emulsions systems were formulated using orange, coconut, and paraffin oils, at varying concentration of CLPs, oil/water ratio, and by applying two-step homogenisation processes (rotor-stator homogenisation followed by ultrasonication). Ultrasonication as a second homogenisation step considerably enhanced the emulsification efficiency, generating emulsions with smaller droplet size and less polydisperse distribution. Furthermore, the effect of ultrasonication on the characteristics of emulsions and the stability of the systems was evaluated over time. The oil content and type, the concentration of CLPs, and the homogenisation methods significantly influenced the characteristics of the emulsions and drop size. Higher concentrations of oil and CLPs favoured the formation of the emulsion and contributed to higher physical stability after 120 days of monitoring. The in vitro Sun Protection Factor (SPF) results demonstrated that the incorporation of natural oils containing phenolic compounds in Pickering formulations improves the SPF value of emulsions, showing an interesting synergic effect between lignin particles and vegetable oils, which was not observed in the case of paraffin oil. Furthermore, the broad-spectrum sun blocker of Pickering emulsions was confirmed by the values of UVA/UVB between 0.74 and 0.90 and Cλ > 380.

In the context of global climate change, the demand for new functional materials that are sustainable and environmentally friendly is rapidly increasing. Cellulose and lignin are the two most abundant raw materials in nature, and are ideal components for functional materials. The hydrophilic interface and easy film-forming properties of cellulose nanofibrils make them excellent candidates for natural biopolymer templates and network structures. Lignin is a natural UV-shielding material, as it contains a large number of phenolic groups. In this work, we have applied two routes for spray deposition of hybrid films with different laminar structures using surface-charged cellulose nanofibrils and water-based colloidal lignin particles. As the first route, we prepare stacked colloidal lignin particles and cellulose nanofibrils hybrid film through a layer-by-layer deposition. As the second route, we spray-deposite premixed colloidal lignin particles and cellulose nanofibrils dispersion to prepare a mixed hybrid film. We find that cellulose nanofibrils act as a directing agent to dominate the arrangement of the colloidal lignin particles in a mixed system. Additionally, cellulose nanofibrils eliminate the agglomerations and thus increase the visible light transparency while retaining the UV shielding ability. Our research on these colloidal lignin and cellulose nanofibril hybrid films provides a fundamental understanding of using colloidal lignin nanoparticles as functional material on porous cellulose-based materials, for example on fabrics.

Improving emulsion stabilizing capacity of sodium caseinate by colloidal lignin particles near the isoelectric point

Self-assembly of colloidal lignin particles in a continuous flow tubular reactor

Toward well-defined colloidal particles: Efficient fractionation of lignin by a multi-solvent strategy

The use of lignin, an abundant phenolic bio-polymer, allows us to transform our fossil-based economy into a sustainable and bio-based economy. The transformation of bulk lignin into colloidal lignin particles (CLPs) with well-defined surface chemistry and morphology is a possible way to cope with the heterogeneity of lignin and use it for material applications. These CLPs can be used as emulsifiers in so-called Pickering emulsions, where solid particles stabilise the emulsion instead of environmentally harmful synthetic surfactants. This work investigates the application of CLPs produced from various bulk lignins as a stabiliser in o/w Pickering emulsions with two different oil phases (solid and liquid state). The CLPs had a primary particle size of 28 to 55 nm. They were successful in stabilising oil-in-water Pickering emulsions with high resistance to coalescence and a strong gel-like network. This enables novel applications for CLPs in the chemical and cosmetic industries, and can replace fossil-based and synthetic ingredients.

The transformation of a molecularly complex and irregularly shaped lignin into a nanoscale spherical architecture is anticipated to play a pivotal role in the promotion of lignin valorization. From the standpoint of using colloidal lignin particles (CLPs) as building blocks for a diverse range of applications, it has become essential to study their interactions with soluble compounds of varied origin. However, the lack of model films with well-defined surface properties similar to those of CLPs has hindered fundamental studies using surface-sensitive techniques. Here, we report well-defined and stable thin films prepared from CLPs and demonstrate their suitability for investigation of surface phenomena. Direct adsorption on substrates coated with a cationic anchoring polymer resulted in uniform distribution of CLPs as shown with atomic force microscopy (AFM). Quartz crystal microbalance with dissipation monitoring (QCM-D) experiments revealed higher adsorbed mass of cationic lignin onto the CLP-coated substrate in comparison to the film prepared from dissolved lignin, suggesting preferential adsorption via the carboxylic acid enriched surfaces of CLPs. QCM-D further enabled detection of small changes such as particle swelling or partial dissolution not detectable via bulk methods such as light scattering. The CLP thin films remained stable until pH 8 and displayed only a low degree of swelling. Increasing the pH to 10 led to some instability, but their spherical geometry remained intact until complete dissolution was observed at pH 12. Particles prepared from aqueous acetone or aqueous tetrahydrofuran solution followed similar trends regarding adsorption, pH stability, and wetting, although the particle size affected the magnitude of adsorption. Overall, our results present a practical way to prepare well-defined CLP thin films that will be useful not only for fundamental studies but also as a platform for testing stability and interactions of lignin nanoparticles with materials of technical and biomedical relevance.

Lignin has interesting functionalities to be exploited in adhesives for medicine, foods and textiles. Nanoparticles (NPs) < 100 nm coated with poly (L-lysine), PL and poly(L-glutamic acid) PGA were prepared from the laccase treated lignin to coat nanocellulose fibrils (CNF) with heat. NPs ca. 300 nm were prepared, β-casein coated and cross-linked with transglutaminase (Tgase) to agglutinate chamois. Size exclusion chromatography (SEC) and Fourier-transform infrared (FTIR) spectroscopy were used to characterize polymerized lignin, while zeta potential and dynamic light scattering (DLS) to ensure coating of colloidal lignin particles (CLPs). Protein adsorption on lignin was studied by quartz crystal microbalance (QCM). Atomic force microscopy (AFM) was exploited to examine interactions between different polymers and to image NPs with transmission electron microscopy (TEM). Tensile testing showed, when using CLPs for the adhesion, the stress improved ca. 10 and strain ca. 6 times compared to unmodified Kraft. For the β-casein NPs, the values were 20 and 8, respectively, and for the β-casein coated CLPs between these two cases. When NPs were dispersed in adhesive formulation, the increased Young’s moduli confirmed significant improvement in the stiffness of the joints over the adhesive alone. Exploitation of lignin in nanoparticulate morphology is a potential method to prepare bionanomaterials for advanced applications.

Colloidal lignin particles out of lignocellulosic biorefineries have gained interest due to their improved properties and wide application range compared to standard lignin. In this work, an application as green adhesive was investigated. Solely a suspension of aqueous colloidal lignin particles was used to achieve anisotropic adhesion between two glass slides. The self-assembly of the colloidal lignin particles into macroscopic structures during drying lead to shear strength 8-times higher the tensile strength. Predefined lignin particles are crucial for the adhesion whereas dissolved lignin leads to no practicable adhesion. The high anisotropy of the adhesion and the fact that solely pure lignin is necessary opens new application and research fields and will propagate the application of colloidal lignin particles even more.