2.1. Anti-Solvent Precipitation

Whatever its provenance, lignin is generally insoluble in water or acidic solutions, but has good solubility in common organic solvents such as THF and DMF [8,10]. In the preparation of lignin nanoparticles, THF and DMF are commonly used as organic solvents to dissolve lignin due to their excellent performance and their lack of effect on the structure of lignin [36]. In addition, the method for preparing nano-lignin by dissolving and re-precipitating lignin has the advantages of being a relatively simple operation and requiring low level of equipment.

According to the difference in the solubility of lignin in organic solvents and water, the nanoparticles are precipitated from the solution due to the decreased solubility after the introduction of water. Table 1 shows the preparation details and properties of lignin nanoparticles by using water as anti-solvent.

Preparation of Lignin Nanoparticles by Using Water as Anti-solvent.

Lievonen et al. [47] prepared spherical lignin nanoparticles of about 200 nm in size by dialyzing a solution of softwood kraft lignin and THF in deionized water. The minimum average diameter appeared at 1 mg/mL lignin concentration and the particle size enlarged as the concentration increased until 10 mg/mL, which could be explained by nucleation-growth mechanism. The stability of the dispersion was excellent in pure water and NaCl solution in a wide pH range. This could be interpreted by the high negative zeta potential (−60 mV) allowing for the electrical double layer repulsion mechanism. The nanoparticles were more stable and it was easier to control their spherical shape, compared with others reports [48,49] using the same preparation method. However, this methodology presents some disadvantages: the difficulty to accurately control the size of nanoparticles and the utilization of THF organic solvent, this is a straightforward preparation.

Lintinen et al. [50] utilized a mixed solvent of tetrahydrofuran (THF), ethanol (EtOH) and water to dissolve soft wood kraft lignin. Colloidal spherical lignin particles around 200 nm with a zeta potential of −40 mV were generated after concentration and drying. The proof-of-concept process was designed to prepare colloidal lignin particles on an industrial scale, which included five steps: lignin dissolution, CLP generation, solvent evaporation, ultrafiltration and spray drying. The large-scale and closed cycle production of nano-lignin will benefit large potential applications of lignin. However, it is difficult to control energy consumption and nanoparticles yield due to the complexity and separability of the 5-step approach.

Chen et al. [51] obtained quasi-spherical lignin nanoparticles around 100 nm by introducing deionized water into lignin dispersed aqueous sodium p-toluenesulfonate (pTsONa) solution. Various types of lignin (kraft lignin, sulfonate lignin and alkaline lignin) could be completely dissolved in the pTsONa solution at room temperature. The nanoparticle diameter could be controlled by varying the pH of the solution. The size of nanoparticle decreased as the pH value increased, which could be explained by the synergistic dissociation of pTsONa and the phenolic OH and COOH functional groups of lignin nanoparticles. This method avoids some limitations of the solubility of lignin species and the use of organic solvents. Nevertheless, the revealed irregularity of nanoparticle morphology and the instability in different pH solutions are drawbacks.

Li et al. [52] produced spherical hollow nanocapsules of around 63 nm size via self-assembly by adding water to a simple mix of kraft lignin/ethanol solution. The diameter of the nanocapsules increased as the concentration of lignin increased and the speed of water addition decreased. The mechanism of π-π interaction among the aromatic groups was suggested during the nanocapsules formation, which was confirmed by ultraviolet and infrared spectroscopy. Despite the lack of studies on nanocapsules stability and the pH effects on solution, the use of green solvents and the simple operation are obvious advantages.

In addition, some other organic solvents were employed to dissolve various lignin for preparation of lignin nanoparticles. Camargos et al. [53] and Yearla et al. [54] utilized a solution of acetone/water to dissolve lignin extracted from corn biomass, hardwood lignin and softwood alkali lignin, respectively. The spherical lignin nanoparticles around 100 nm were obtained by controlling the solution pH and dropping double-distilled water rapidly. High purity lignin was dissolved in acetone by Richter et al. [55] to achieve flash precipitation of lignin nanoparticles. Li et al. [56] published a method to prepare uniform nanospherical particles about 300 nm with hollow cavity space by dissolving kraft lignin in dioxane.

The principle of acid precipitation for the preparation of lignin nanoparticles is similar to that of water as an anti-solvent, which is based on the difference in the solubility of lignin in acid solutions and organic solvents. Besides, according to the electrical double layer theory, lignin nanoparticles are easier to precipitate due to the large amount of H⁺ in the acid solution [57,58]. The preparation details and properties of lignin nanoparticles by using acid solution as anti-solvent are as shown in Table 2.

Preparation of Lignin Nanoparticles by Using Acid Solution as Anti-solvent.

Richter et al. [34] synthesized the low-sulfonated lignin nanoparticles about 84 nm with a zeta potential of −33 mV by introducing HCl into a solution of lignin in ethylene glycol. The lignin nanoparticles were pH-stable and biodegradable. The nanocomposites of lignin nanoparticles and Ag⁺ were successfully prepared, which displayed more excellent antimicrobial activity than silver nanoparticles. The lignin nanoparticles (50–250 nm) were prepared by Gupta et al. [59] following the same method, as shown in Figure 5. Compared with original lignin, improvements in crystallinity and thermal stability were revealed by X-ray diffraction analysis and DSC analysis. A similar approach was employed by Yang et al. [57] to prepare lignin nanoparticles around 50 nm with different HCl concentration. The nanoparticles were more uniformly distributed and stable than original lignin in a wide pH range. Furthermore, another approach of HNO₃ precipitation from NaOH aqueous solution was used by Frangville et al. [60] which prepared nanoparticles stable only at pH below 5. The excellent degradability and environmental compatibility were confirmed through dispersing lignin nanoparticles with microalgae and yeast.

Synthesis of lignin nanoparticles by self-assembly in ethylene glycol using HCl as anti-solvent.

Richter et al. [55] obtained kraft lignin nanoparticles (45–250 nm) through HNO₃ flash-precipitation from ethylene glycol solution. The surface of the nanoparticles was coated with a cationic polyelectrolyte, which made their surface properties adjustable and increased their stability in high pH system.

Beisl et al. [61] designed three different precipitation setups (batch, T-fitting, static mixer) with different mixing speeds to generate lignin nanoparticles, introducing H₂SO₄/H₂O solution into ethanol aqueous mixture. The smallest nanoparticles (almost 100 nm) could be produced by static mixer setup with the highest mixing speed. The molecular weight and chemical structure of lignin nanoparticles did not change during the precipitation process.

Supercritical flow technology has been widely used in the field of preparing nanoparticle materials on account of the many resulting unique physical and chemical properties [62,63]. The principle of supercritical antisolvent precipitation technology is that the solubility of lignin in supercritical fluid is less than the solubility in a solvent [64,65]. The solubility of lignin in the original solvent is reduced when the supercritical fluid is dissolved into the solution. A large degree of saturation is formed in a short time to precipitate high-purity lignin nanoparticles. The advantages of supercritical method are that the prepared nano-lignin has small particle size and narrow distribution due to the low viscosity and zero surface tension of supercritical fluid. Furthermore, the commonly used CO₂ is non-toxic and inexpensive [66,67]. Table 3 indicates the preparation details and properties of lignin nanoparticles by using supercritical CO₂ as anti-solvent.

Preparation of Lignin Nanoparticles by Using Supercritical CO₂ as Anti-solvent.

Myint et al. [68] successfully prepared the quasi-spherical lignin nanoparticles (38 nm) through introducing compressed CO₂ into kraft lignin/DMF solution, which exhibited high monodispersity and uniform size. The size of nanoparticles increased as the temperature increased and the pressure decreased. Furthermore, the influence of two different solution flow rates (0.03 and 0.06 kg/h) on the size was discussed: lower flow rates showing an increasing trend on nanoparticle diameter. These effects on the nanoparticles formation were attributed to the change in solubility between DMF and CO₂. Moreover, the nanoparticles exhibited excellent properties, for instance, favorable thermal degradation, outstanding dispersion stability, excellent UV absorption and non-cytotoxicity.

A similar process with supercritical CO₂ was used by Lu et al. [69] to prepare spherical nanoparticles around 144 nm in size. Lignin was dissolved in an acetone solution and CO₂ was added at 35 °C and 30 MPa. No change in the amorphous chemical structure was confirmed by FTIR analysis and XRD analysis in the supercritical antisolvent process. The nanoparticles possessed significantly improved solubility in water and antioxidant activity as a result of the enhanced specific surface area, compared with original lignin.

The above three anti-solvent precipitation methods are often used to prepare lignin nanoparticles, but there are some disadvantages. For example, lignin cannot be completely uniformly dispersed in the solvent, and agglomeration may occur in some cases. In the subsequent process, the solvent has to be removed by rotary evaporation or freeze-drying, which is unfavorable for precise control of the size and morphology of the nanoparticles. In order to satisfy the diversified requirements in different application fields, other preparation methods of lignin nanoparticles with various morphology and sizes attract the attention of research.