In recent years, different kinds of lignin-based nanoparticles have been prepared based on various types of lignin through different methods such as self-assembly, solvent exchange, acid precipitation, polymerization, ultrasonication, crosslinking, and CO2 antisolvent.
3.1. Self-assembly Method
Self-assembly is a process in which an ordered or organized structure is generated due to some specific intermolecular noncovalent interactions such as hydrophobic, electrostatic, hydrogen-bonding and Van der Waals interactions in absence of any external directions. This is a frequently-used method to prepare nanoparticles, which we would put emphasis on in this section.
Qian et al. [71
] used the self-assembly method to produce the uniform lignin-based colloidal spheres. After acetylation, the alkali lignin (AL) was transformed into acetylated lignin (ACL) and then dissolved into THF. With the gradual addition of water into the ACL/THF solutions, the ACL molecules started to associate to form colloidal spheres through the hydrophobic interaction. After rotary evaporation to remove THF, colloidal spheres with a hydrodynamic radius of 110 nm were successfully obtained. This study gives important enlightenment on how to convert the irregular lignin-based polymers into the ordered colloidal spheres. Qian et al. [72
] also reported a novel approach to fabricate lignin reverse micelles (LRMs) via self-assembly. In this method, LRMs were formed by adding cyclohexane into the alkali lignin/dioxane solutions. With the increasing amount of cyclohexane, LRMs were separated from the solutions in the form of precipitation. Deng et al. [73
] proposed a simple and feasible method in the formation of hollow lignin azo colloids. They first modified alkali lignin (AL) into the lignin-based azo polymer (AL-azo-H). Then, water was gradually added dropwise into the AL-azo-H/THF solutions. With the further addition of water above 53 vol%, AL-azo-H colloidal dispersions were obtained, and the average particle size of the formed spheres was approximately 170 nm. Li et al. [74
] prepared lignin hollow microspheres from the esterified organosolv lignin modified with maleic anhydride using the self-assembly method in the mixed solvent of THF and water. Richter et al. [75
] provided a simple self-assembly method for the synthesis of biodegradable lignin nanoparticles using organosolv lignin as a raw material. Specifically, organosolv lignin nanoparticles were obtained by gradually dropwise adding water into organosolv lignin/acetone solutions. Nanoparticles obtained from the above-mentioned preparation process showed a spherical shape and relatively uniform size (except organosolv lignin nanoparticles produced by Richter et al. (2016) due to the broad molecular weight distribution of organosolv lignin and inhomogeneous mixing, etc). However, there also are some limitations. They all utilized hazardous and expensive chemical reagents, such as acetyl bromide, cyclohexane, dioxane, NaNO2
, maleic anhydride, THF, and acetone, or involved complicated chemical modification reactions. Li et al. [55
] in their work, presented a simple, green, and low-cost preparation of nanocapsules through self-assembly from kraft lignin (KL) without any chemical reactions. During the process, water was added dropwise into the KL/ethanol solution by the peristaltic pump until the water content reached 90 vol%, at which the formation of KL nanocapsules was completed. The particle sizes of KL nanocapsules could be easily adjusted by changing the dropping speed of the water.
Qian et al. [76
] created an easy and practical self-assembly method to fabricate large, midsize, and small lignin-based colloidal spheres using the enzymatic hydrolysis lignin (EHL) and organosolv lignin (OL) as raw materials. Taking EHL as an example, large colloidal spheres were prepared by gradually adding NaCl aqueous solutions into the 10 g/L EHL acetone/water (8:1, vol/vol) solution. Midsize colloidal spheres were fabricated in much the same way, but NaCl aqueous solutions needed to be replaced with water. If small spheres were required, the concentration of EHL acetone/water (8:1, vol/vol) solution would be decreased to 0.1 g/L. Small nanoparticles were then formed by adding EHL solutions into water. Just by easily adjusting certain variables, lignin-based colloidal spheres with different sizes could be successfully obtained [74
]. Huang et al. [77
] separately used acetyl bromide, propionyl bromide, butyryl chloride, and valeryl chloride to perform hydrophobic modifications to AL in order to further understand the effect of the polarity and terminal alkyl chain length of acylation reagents on the lignin colloidal spheres. Then, the obtained four modified samples were prepared into colloidal spheres by the basically same process as created by Qian et al. [71
]. Results showed that the formed acetylated lignin colloidal spheres exhibit a hollow spherical structure with only one hole, but the spheres gradually became porous configurations with the increase of alkyl chain lengths. Yan et al. [78
] reported a facile self-assembly method to prepare size-controlled and super-term stable hollow or solid lignin-based nanospheres from kraft lignin. In this study, three different lignin samples labeled as KL1, KL2, and KL3 were first obtained by regulating the pH of black liquors to 6, 4, and 2 in the extraction, respectively. Then, water was uniformly added into the KL/THF solutions so as to prepare nanospheres (Figure 2
). The results showed that the nanospheres formed exhibited a hollow spherical structure from KL1 and KL2 but a solid spherical configuration from KL3 and the size showed a decreasing trend from KL1 to KL3. This was caused by the decreased phenol hydroxyl content and the increased S/G ratio. Trevisan et al. [79
] also separately produced lignin nanoparticles and acetylated lignin nanoparticles. The acid-alkali treatment was first performed on elephant grass to extract lignin for the acetylation and nanoparticle preparations. Then, a simple and easy self-assembly method was utilized to obtain nanoparticles in which water was poured into the acetone solution of lignin or acetylated lignin. In order to investigate the effect of hydrophobicity on particle size and stability, Li et al. [80
] synthesized four different kinds of corncob lignin (CL) sub-micro spheres from four alkylated CL samples (decane alkylated CL, dodecane alkylated CL, hexadecane alkylated CL, and octadecane alkylated CL) with different hydrophobic properties but similar chemical structures in the mixed DMF/H2
O system via self-assembly. The final results indicated that the particle size of spheres would decrease from 400 to 100 nm with the increasing length of n-alkane (Figure 3
), and all spheres formed from the four alkylated CL samples exhibited an excellent dispersity in both water and acid-base environment.
All the above-mentioned works are focused on the nanoparticle preparation of alkali, kraft, organosolv, or enzymatic hydrolysis lignin. However, the investigation of the formation of the lignosulfonate nanoparticle is rare. As is well known, lignosulfonate has strong hydrophilicity, which makes it possible to transform lignosulfonate into reverse micelles. Zhong et al. [81
] proposed an easy and green fabrication of sodium lignosulfonate reverse micelles (SLRMs). First, SL was dissolved into water to obtain aqueous SL solutions. Then, ethanol was gradually added, and the solutions started to become emulsified, thus forming SLRMs.
However, just due to this strong hydrophilicity, it is difficult for lignosulfonate to form normal micelles. To fill this gap, we presented a novel preparation method of lignosulfonate-based normal colloidal spheres via self-assembly in our previous work [7
]. In this process, the cationic surfactant cetyltrimethylammonium bromide (CTAB) was first introduced to perform hydrophobic modifications to lignosulfonate molecules by a simple mixing method. Since lignosulfonate is a negatively charged anionic surfactant. The introduced CTAB would attach to the surface of lignosulfonate through electrostatic attractions, causing the shielding of the hydrophilic functional groups and hence increasing the hydrophobicity. Then, colloidal spheres were successfully prepared from the lignosulfonate/CTAB complex system at the stoichiometric mass ratio in the mixed solvent of ethanol and water through self-assembly (Figure 4
). This whole preparation process was very simple, safe, and low-cost without any complicated chemical modification reactions, toxic or expensive chemical reagents. The formation of colloidal spheres from lignosulfonate not only provides a valuable and green approach to exploit the functionality of lignosulfonate and other technical lignin products but also gives some significant enlightenment on how to change the unordered and sophisticated lignosulfonate-based aggregates into ordered nanospheres. Soon afterward, other researchers in our group adopted a similar method to prepare lignin-based nanospheres. Instead of lignosulfonate, Li et al. [82
] employed alkali lignin (AL) as raw materials. In this work, AL was first modified into cationic quaternized alkali lignin (QAL) by the quaternization reaction. Then, anionic sodium dodecyl benzenesulfonate (SDBS) was added and colloidal spheres from the SDBS/QAL complex were successfully obtained in the mixed ethanol/water solvent through self-assembly.
3.2. Solvent Exchange Method
A straightforward solvent exchange method was developed by Lievonen et al. [83
] so as to prepare nanoparticles from waste lignin extracted from the kraft pulping process. In this study, the lignin was first dissolved into THF, and then the obtained lignin/THF solutions were placed into a dialysis bag, which was subsequently immersed in excess water. The lignin nanoparticles were formed after the dialysis process continued for at least 24 h. This whole preparation process was very simple which did not involve any chemical modification reactions. The size of nanoparticles could be adjusted by changing the pre-dialysis concentration of lignin/THF solutions. The obtained nanoparticles had excellent stability in the pure water at room temperature but tended to aggregate at a very high salt concentration or low pH. Lintinen et al. [84
] synthesized metal-organic nanoparticles from the iron isopropoxide treated softwood kraft lignin. In this method, Fe(OiPr)3
/THF or Fe(OiPr)2
/THF solutions and lignin/THF solutions were first prepared, and then the solution of Fe:lignin in THF was obtained by adding THF solutions of Fe(OiPr)3
or a mixture of both (2:1) into THF solutions of lignin. After hydrolysis reactions, the whole system was moved into a dialysis tube immersed in water for the solvent change. Finally, the metal-organic nanoparticles were successfully synthesized without any further purification. This work gave a simple method of preparing different metal-organic nanomaterials. Based on Lievonen et al. [83
] and Lintinen et al. [84
], Figueiredo et al. [85
] prepared pure lignin nanoparticles and iron(III)-complexed lignin nanoparticles. Fe3
-infused lignin nanoparticles were prepared by mixing an equal mass of THF solutions of lignin and THF solutions of oleic acid-coated Fe3
nanoparticles followed by water dialysis. Zikeli et al. [86
] also prepared nanoparticles from lignin separated from wood wastes according to Lievonen et al. [83
] by first dissolving lignin into DMSO and then performing dialysis with an excess of water.
3.6. Crosslinking Method
Yiamsawas et al. [95
] first synthesized the biodegradable hollow nanocontainers with a hydrophilic core from sodium lignosulfonate and alkali lignin. For the preparation of lignosulfonate nanocontainers, sodium lignosulfonate was first dissolved in aqueous NaCl solutions to generate the dispersed phase, which was subsequently mixed with cyclohexane containing the biocompatible surfactant PGPR (poly-glycerol polyricinoleate). The obtained pre-emulsion was then ultrasonicated so as to form a stable mini-emulsion. The polyaddition reaction occurred at the interface of the mini-emulsion droplets, which was initiated by dropwise adding toluene diisocyanate (TDI)/cyclohexane solutions into the mini-emulsion. After keeping at room temperature overnight, the lignosulfonate nanocapsule dispersions were successfully formed, which could still remain stable when being transferred into aqueous dispersions because of the presence of sulfonic groups. The preparation process of alkali lignin nanocapsules is basically the same, but sodium dodecyl sulfate should be added if the nanocapsules are required to redisperse in water. These obtained lignin nanocontainers had a particle size in the range of 150–200 nm and could keep stable in aqueous or organic dispersions over a long period (several weeks or even months). Tortora et al. [96
] created a novel synthesis of kraft lignin microcapsules by first preparing oil in water emulsions followed by ultrasound-assisted crosslinking of lignin at the oil/water interface. Taking the ultrasound preparations of lignin microcapsules in the presence of H2
as an example, the specific process was described as follows. Firstly, olive oil and H2
were added into lignin alkali solutions, and then the whole mixing system was sonicated. Next, lignin microcapsules were obtained by centrifuging and washing. The finally formed lignin microcapsules had an average particle size of 0.3–1.1 μm with a spherical configuration. The formation mechanism was also revealed by means of GPC and NMR measurements. Li et al. [97
] fabricated porous lignosulfonate spheres from the cross-linking reaction of lignosulfonate and sodium alginate using epichlorohydrin as a cross-linking agent followed by dropwise adding into CaCl2
solutions for gelation and solidification. Results showed that the formed lignosulfonate spheres exhibited an obvious porous structure with a large pore volume and a high porosity. Nypelö et al. [98
] synthesized lignin supracolloids by first adding lignin alkali solutions into octane containing surfactant mixtures of Span 80, Tween 80, and 1-pentanol to prepare microemulsions, and then adding epichlorohydrin into microemulsions. In another research carried out by Chen et al. [99
], pH-responsive lignin-based nanocapsules were successfully prepared by an interfacial mini-emulsion polymerization. In the first step, the water phase containing lignosulfonate and water without or with SDS was mixed with the oil phase containing butyl acetate, hexadecane (co-stabilizer), AIBN (an oil-soluble initiator), and trimethylolpropane tris(3-mercapto propionate) (a cross-linker), which was followed by sonication. In the second step, interfacial mini-emulsion polymerization was performed in the obtained mini-emulsion system to form nanocapsules. This work provided a facile method to make use of waste biomaterials from biorefinery industries.
3.7. CO2 Antisolvent Method
as an antisolvent to produce polymeric nanoparticles has attracted special interest due to advantages such as abundance, low costs, nontoxicity, nonflammability, and poor solubility for macromolecules [100
]. Lu et al. [101
] prepared nanoscale lignin from non-nanoscale lignin by means of the supercritical antisolvent (SAS) process. First, acetone was pressed into a precipitation chamber filled with SC-CO2
through the liquid pump in order to obtain stable precipitation reaction conditions. The acetone was stopped once the stable conditions were achieved. Then, the lignin/acetone solution was delivered into the precipitation chamber through a stainless-steel nozzle. Once the delivery was completed, the liquid pump was stopped, but the SC-CO2
continued to keep flowing to remove the residual organic solvents. Finally, the nano-scale lignin was obtained and could be taken out as the pressure of the precipitation chamber decreased to atmospheric pressure. Results indicated that the formed nanoscale lignin had an average particle size of about 0.144 μm, and there were no chemical changes when non-nanoscale lignin was converted into nanoscale lignin. Myint et al. [102
] utilized a similar method to Lu et al. [101
] to fabricate environmentally friendly nanoparticles from kraft lignin using DMF as an organic solvent. Then, they investigated the effect of different process parameters on the properties of the particles, revealed the formation mechanisms, and elucidated the quality of lignin nanoparticles by means of FESEM, HRTEM, BET, ATR-FTIR, XPS, XRD, DSC, TG/DTA, and UV-vis analyzers.