Lignin nanomaterials have emerged as a promising alternative to fossil-based chemicals and products for some potential added-value applications, which benefits from their structural diversity and biodegradability. This review elucidates a perspective in recent research on nanolignins and their nanocomposites. It summarizes the different nanolignin preparation methods, emphasizing anti-solvent precipitation, self-assembly and interfacial crosslinking. Also described are the preparation of various nanocomposites by the chemical modification of nanolignin and compounds with inorganic materials or polymers. Additionally, advances in numerous potential high-value applications, such as use in food packaging, biomedical, chemical engineering and biorefineries, are described.
1. Introduction
1. Introduction
With our continued in-depth understanding of the environmental pollution and resource crisis, the renewable and degradable properties of biomass materials are being increasingly valued [1][2]. As the second most abundant natural polymer material after cellulose, lignin has received extensive attention in recent years [3][4]. The development of bio-based products from lignin is an important part of any comprehensive biorefinery concept because of their biocompatibility and biodegradability [5]. They not only diversify the combination of products and markets, but also benefit waste recycling and economic sustainability [6][7]. Nevertheless, worldwide only 5% of the lignin is explored for high value development, therefore, there are still numerous challenges and opportunities for the in depth research and development of lignin applications [8].
The three-dimensional network structure of lignin is formed by three phenylpropane monomers (para-coumaryl alcohol, sinapyl alcohol and coniferyl alcohol, Figure 1) which connect to each other through ether bonds and carbon-carbon bonds [9][10]. It contains multiple active functional groups, such as aliphatic, aromatic, hydroxyl groups, etc. The isolation technology of cellulose, hemicellulose and lignin in biomass is directly related to the effective utilization of biomass. The complexity and diversity of lignin structures mainly depend on their different sources, types, extraction and purification methods [11]. Different extraction methods and pulping procedures will produce lignin with different structures and properties, which determine its subsequent development and applications [12].
Figure 1. Three monomer structures of lignin.
The conventional separation and extraction methods for lignin mainly include grinding, acid/alkaline/thioacid hydrolysis, cellulose enzymolysis, organic solvent extraction and ionic liquid extraction [13][14]. Organic solvent pulping reduces the dependence on alkali or inorganic acids in the traditional pulping process [15]. Organic solvents such as alcohols, esters and amines can be used to dissolve the lignin in the raw materials to achieve the purpose of separation from cellulose [16].
In addition, biological enzymes are used to selectively degrade cellulose to achieve lignin separation. This biological treatment separates the lignin with less impact on its structure and chemical properties [17]. Therefore, it is particularly important to select suitable and efficient extraction methods for different raw materials without destroying the lignin structure. More importantly, the presence of different types of ions and the ionic strength of solutions is the foundation for the association and isolation of lignin [18]. The macromolecular and colloidal properties of lignin can be investigated in depth by studying the self-aggregation kinetics at some specific solution conditions [19]. The study of lignin solutions/colloidal behavior and the associative/dissociative processes also facilitates the understanding of its physicochemical properties [20].
Wide applications of lignin as an additive in composite materials and bio-based products from lignin deconstruction are being intensively developed [21][22]. However, it is not possible to completely achieve the high-value applications of lignin due to some unfavorable byproducts. The diversity of lignin sources and the complexity of the structures make the research on the potential applications of lignin have certain opportunities and challenges [23][24]. Therefore, the formation of lignin-based nanomaterials will open up a different perspective for expanding the high-value applications of lignin.
As for other materials and biopolymers, such as cellulose and chitosan, the rationale for the preparation of lignin nanomaterials is to gain new attractive properties occurring only when matter is organized on a nanoscale [25]. This can be due to the so-called “quantum effect” bringing new tunable properties at the nanoscale or simply by the expanded surface to volume ratio of nanomaterials [26]. Compared with traditional composite materials, nanocomposites have obvious advantages, especially new materials from biorenewable and sustainable sources [27]. Due to the compatibility, degradability and environmental friendliness of bionanocomposites, their potential applications in the food packaging industry and pharmaceuticals are being exploited [28]. Polymer nanocomposites are being further used in additive manufacturing technology to produce more complex and diverse parts and components [29]. In addition, the application of multifunctional nanocomposite materials in the field of optics is also becoming increasingly prominent [30]. In the last few years, the research on multifunctional nanofibers is helpful for the development of potential applications in the field of medicine, biological tissue engineering, etc., especially chitosan electrospun nanofibers [31]. Electrospun nanofibers can be used to reinforce composite materials due to their specific molecular orientation and excellent mechanical properties [32].
As for lignin, with its aromatic and highly cross-linked network structure and chemical complexity, lignin contains plentiful of functional groups that are accessible for further surface modification [33]. This laat asset increases the potential activity of nanolignin to achieve high value-added applications of lignin [4]. In addition, the utilization of economical and environmentally friendly nanolignin as feedstock for the evolution of chemical industry conforms to green chemistry principles and sustainable development concepts [13].
Currently, the exploitation of nanolignin is the subject of a tremendous amount of research [34]. Lignin nanoparticles with different morphologies (smooth colloidal, hollow, spherical and quasi-spherical, Figure 2) have been successfully synthesized by controlling the reaction conditions of solvent/anti-solvent, the lignin concentration, the temperature and pH of solution, etc. [35][36]. Lignin nanoparticles have potential applications in antioxidants, thermal/light stabilizers, reinforced materials and nanomicrocarriers owing to their advantages of non-toxicity, environmental resistance, excellent thermal stability and biocompatibility [37].
Figure 2. Nanolignins with different morphologies and their applications in different fields.
With our continued in-depth understanding of the environmental pollution and
2. Synthesis Methods of Lignin Nanoparticles
The physical and chemical properties such as non-toxicity, corrosion and UV-resistance, antibacterial and anti-oxidation activity of lignin are attracting more and more attention [3][33]. Therefore, the use of low-cost and abundant lignin raw materials to prepare nanoparticles is an important aspect of expanding their high value-added utilization [38]. At the same time, finding simple, scientific and safe methods for preparing lignin nanoparticles is of great significance. The current preparation methods of nano-lignin mainly include anti-solvent precipitation, self-assembly, gradual addition and mechanical methods, etc [1][7][10]. Some emerging methods, such as ice segregation-induced self-assembly, aerosol-flow synthesis and electrospinning, are also being developed [35][39][40][41][42]. Figure 4 gives an overview of the synthesis and modification methods of nano-lignin. Lignin nanoparticles with different morphologies and sizes, which can be prepared through these different methods and conditions, will be of great value for their subsequent applications in different fields [43].
Figure 4. Overview of the synthesis and modification methods of nano-lignin.
resource crisis, the renewable and degradable properties of biomass materials are being3. The Value-Added Applications of Lignin Nanomaterials
increasingly valued [1,2]. As the second most abundant natural polymer material afterIn the process of exploring nanolignin materials, it was found that sometimes the ordinary nanolignin cannot satisfy the diverse requirements in different application fields. In addition to the basic thermal and mechanical properties of nanomaterials, some diverse nanostructures and specific performance such as metal adsorption are widely required [44][45]. Therefore, the ordinary nanolignin can be used to directly modify or compound with other nanomaterials, which can take advantage of nanolignin structure and further improve the performance of nanocomposite materials [46].
cellulose, lignin has received extensive attention in recent years [3,4]. The developmentLignin nanoparticles have attracted more and more attention because of their green, renewable and abundant source, antioxidant, antibacterial or ultraviolet absorption properties, biodegradability, biocompatibility, etc [47]. They are excellent substitutes for partially harmful nanomaterials, which are extensively used in the fields of drug release and control [48], food packaging, biomedicine, adsorbent materials, nanocarriers, environmental restoration and so on [49][50][51][52]. This not only solves the potential safety hazards of traditional nanomaterials from the source, but also broadens the value-added applications of lignin nanomaterials, which conforms to the principles of green chemistry development [53].
of bio-based products from lignin is an important part of any comprehensive biorefinery
concept because of their biocompatibility and biodegradability [5]. They not only diversify
the combination of products and markets, but also benefit waste recycling and economic
sustainability [6,7]. Nevertheless, worldwide only 5% of the lignin is explored for high
value development, therefore, there are still numerous challenges and opportunities for the
in depth research and development of lignin applications [8].
The three-dimensional network structure of lignin is formed by three phenylpropane
monomers (para-coumaryl alcohol, sinapyl alcohol and coniferyl alcohol, Figure 1) which
connect to each other through ether bonds and carbon-carbon bonds [9,10]. It contains
multiple active functional groups, such as aliphatic, aromatic, hydroxyl groups, etc. The
isolation technology of cellulose, hemicellulose and lignin in biomass is directly related to
the effective utilization of biomass. The complexity and diversity of lignin structures mainly
depend on their different sources, types, extraction and purification methods [11]. Different
extraction methods and pulping procedures will produce lignin with different structures
and properties, which determine its subsequent development and applications [12].
The conventional separation and extraction methods for lignin mainly include grinding,
acid/alkaline/thioacid hydrolysis, cellulose enzymolysis, organic solvent extraction
and ionic liquid extraction [13,14]. Organic solvent pulping reduces the dependence on
alkali or inorganic acids in the traditional pulping process [15]. Organic solvents such
as alcohols, esters and amines can be used to dissolve the lignin in the raw materials to
achieve the purpose of separation from cellulose [16].
In addition, biological enzymes are used to selectively degrade cellulose to achieve
lignin separation. This biological treatment separates the lignin with less impact on its
structure and chemical properties [17]. Therefore, it is particularly important to select
suitable and efficient extraction methods for different raw materials without destroying
the lignin structure. More importantly, the presence of different types of ions and the ionic
strength of solutions is the foundation for the association and isolation of lignin [18]. The
macromolecular and colloidal properties of lignin can be investigated in depth by studying
the self-aggregation kinetics at some specific solution conditions [19]. The study of lignin
solutions/colloidal behavior and the associative/dissociative processes also facilitates the
understanding of its physicochemical properties [20].
Wide applications of lignin as an additive in composite materials and bio-based
products from lignin deconstruction are being intensively developed [21,22]. However,
it is not possible to completely achieve the high-value applications of lignin due to some
unfavorable byproducts. The diversity of lignin sources and the complexity of the structures
make the research on the potential applications of lignin have certain opportunities and
challenges [23,24]. Therefore, the formation of lignin-based nanomaterials will open up a
different perspective for expanding the high-value applications of lignin.
As for other materials and biopolymers, such as cellulose and chitosan, the rationale for
the preparation of lignin nanomaterials is to gain new attractive properties occurring only
when matter is organized on a nanoscale [25]. This can be due to the so-called “quantum
effect” bringing new tunable properties at the nanoscale or simply by the expanded surface
to volume ratio of nanomaterials [26]. Compared with traditional composite materials,
nanocomposites have obvious advantages, especially new materials from biorenewable
and sustainable sources [27]. Due to the compatibility, degradability and environmental
friendliness of bionanocomposites, their potential applications in the food packaging
industry and pharmaceuticals are being exploited [28]. Polymer nanocomposites are being
further used in additive manufacturing technology to produce more complex and diverse
parts and components [29]. In addition, the application of multifunctional nanocomposite
materials in the field of optics is also becoming increasingly prominent [30]. In the last
few years, the research on multifunctional nanofibers is helpful for the development of
potential applications in the field of medicine, biological tissue engineering, etc., especially
chitosan electrospun nanofibers [31]. Electrospun nanofibers can be used to reinforce
composite materials due to their specific molecular orientation and excellent mechanical
properties [32].
As for lignin, with its aromatic and highly cross-linked network structure and chemical
complexity, lignin contains plentiful of functional groups that are accessible for further
surface modification [33]. This laat asset increases the potential activity of nanolignin
to achieve high value-added applications of lignin [4]. In addition, the utilization of
economical and environmentally friendly nanolignin as feedstock for the evolution of
chemical industry conforms to green chemistry principles and sustainable development
concepts [13].
Currently, the exploitation of nanolignin is the subject of a tremendous amount of
research [34]. Lignin nanoparticles with different morphologies (smooth colloidal, hollow,
spherical and quasi-spherical, Figure 2) have been successfully synthesized by controlling
the reaction conditions of solvent/anti-solvent, the lignin concentration, the
temperature and pH of solution, etc. [35,36]. Lignin nanoparticles have potential applications
in antioxidants, thermal/light stabilizers, reinforced materials and nanomicrocarriers
owing to their advantages of non-toxicity, environmental resistance, excellent thermal stability and
biocompatibility [37]
In recent years, the valorization of lignin, lignin nanoparticles and their nanocomposites
have been extensively reviewed. Figure 3 displays the number and subject areas of
published literature on lignin nanoparticles and nanocomposites over the last five years.
Tetyana et al. [38] focused on lignin-inorganic composite materials and applications in
energy storage. Mishra et al. [36] focused on non-covalent interactions and self-assembly
of lignin, briefly describing synthetic methods without mentioning specific applications.
Österberg et al. [39] provided only an overview of spherical lignin nanoparticles and their
applications in dispersants, coatings, adhesives and composites. Duval et al. [40] described
the structure and extraction process of lignin, as well as the applications of lignin-based
polymeric and micro/nano-structured materials.
Nanomaterials 2021, 11, x FOR PEER REVIEW 3 of 30
Currently, the exploitation of nanolignin is the subject of a tremendous amount of
research [34]. Lignin nanoparticles with different morphologies (smooth colloidal, hollow,
spherical and quasi-spherical, Figure 2) have been successfully synthesized by controlling
the reaction conditions of solvent/anti-solvent, the lignin concentration, the
temperature and pH of solution, etc. [35,36]. Lignin nanoparticles have potential applications
in antioxidants, thermal/light stabilizers, reinforced materials and nanomicrocarriers
owing to their advantages of non-toxicity, environmental resistance, excellent
thermal stability and biocompatibility [37].
In recent years, the valorization of lignin, lignin nanoparticles and their nanocomposites
have been extensively reviewed. Figure 3 displays the number and subject areas of
published literature on lignin nanoparticles and nanocomposites over the last five years.
Tetyana et al. [38] focused on lignin-inorganic composite materials and applications in
energy storage. Mishra et al. [36] focused on non-covalent interactions and self-assembly
of lignin, briefly describing synthetic methods without mentioning specific applications.
Österberg et al. [39] provided only an overview of spherical lignin nanoparticles and their
applications in dispersants, coatings, adhesives and composites. Duval et al. [40] described
the structure and extraction process of lignin, as well as the applications of lignin-based
polymeric and micro/nano-structured materials.
Therefore, in this review we would like to provide an outlook on this promising
research on nanolignin and its nanocomposites. We will emphasize lignin nanoparticle
synthesis methods and the characterizations of their structure and performance, along
with the effects of different reaction conditions on the morphology and dispersion of lignin
nanoparticles. Furthermore, the modification of lignin nanoparticles and multifunctional
lignin-based nanocomposites are described. The difference in the properties of the modified
lignin nanoparticles is emphasized. Finally, we will provide a novel perspective to the
value-added applications of lignin nanomaterials and many promising possibilities to
improve biotechnological developments.
2. Synthesis Methods of Lignin Nanoparticles
The physical and chemical properties such as non-toxicity, corrosion and UV-resistance,
antibacterial and anti-oxidation activity of lignin are attracting more and more attention
[3,33]. Therefore, the use of low-cost and abundant lignin raw materials to prepare
nanoparticles is an important aspect of expanding their high value-added utilization [41].
At the same time, finding simple, scientific and safe methods for preparing lignin nanoparticles
is of great significance. The current preparation methods of nano-lignin mainly
include anti-solvent precipitation, self-assembly, gradual addition and mechanical methods,
etc [1,7,10]. Some emerging methods, such as ice segregation-induced self-assembly,
aerosol-flow synthesis and electrospinning, are also being developed [35,42–45]. Figure 4
gives an overview of the synthesis and modification methods of nano-lignin. Lignin
nanoparticles with different morphologies and sizes, which can be prepared through these
different methods and conditions, will be of great value for their subsequent applications
in different fields [46].
3. The Value-Added Applications of Lignin Nanomaterials
In the process of exploring nanolignin materials, it was found that sometimes the
ordinary nanolignin cannot satisfy the diverse requirements in different application fields.
In addition to the basic thermal and mechanical properties of nanomaterials, some diverse
nanostructures and specific performance such as metal adsorption are widely required
[93,94]. Therefore, the ordinary nanolignin can be used to directly modify or
compound with other nanomaterials, which can take advantage of nanolignin structure
and further improve the performance of nanocomposite materials [95].
Lignin nanoparticles have attracted more and more attention because of their green,
renewable and abundant source, antioxidant, antibacterial or ultraviolet absorption properties,
biodegradability, biocompatibility, etc [96]. They are excellent substitutes for partially
harmful nanomaterials, which are extensively used in the fields of drug release and control
[97], food packaging, biomedicine, adsorbent materials, nanocarriers, environmental
restoration and so on [98–101]. This not only solves the potential safety hazards of traditional
nanomaterials from the source, but also broadens the value-added applications
of lignin nanomaterials, which conforms to the principles of green chemistry development
[102].
4. Conclusions and Future Perspectives
Overall, this review summarizes the current status of the preparation of nano-lignin by
sedimentation, mechanical, self-assembly and stepwise addition polymerization methods.
In order to provide a reference for the chemical deep processing of biomass resources
and the development of nano-lignin, the application characteristics of nano-lignin in UV
protection and anti-bacteria, nano-fillers and biomass-based carriers are also outlined.
Through the specific analysis of the preparation methods and application status of nanolignin,
it can promote the further research of nano-lignin and the development of novel
nano-lignin-based products. This is of great significance to the utilization and sustainable
development of lignin.
The multiple structures and the diverse properties of lignin make the prepared nanolignin
more complicated, which brings challenges to the research of the preparation and
performance of nano-lignin. Meanwhile, it provides broader prospects and opportunities
for the multi-functional and multi-field applications of nano-lignin. Nanometersized lignin
with high specific surface area and activity is a novel approach to achieve high value-added
utilization of lignin. Compared with the research of inorganic nanoparticles and renewable
nanocellulose, the preparation and application of nano-lignin are still in their infancy. The
scale and industrialization of nano-lignin-based products will become an important aspect
of future lignin research.
In view of the problems of complex process and toxic organic solvents in the preparation
of lignin nanoparticles, methods such as electrostatic spinning, self-assembly, ultrasonication
and homogenization can be utilized. In terms of multi-functional applications of
lignin-based nanomaterials, the range of use can be expanded by improving their strength,
electrical conductivity, thermal stability, crystallization performance, etc.
The following points need attention in the application of nanolignin materials. First
of all, achieving uniform dispersions of nanolignin in composite materials is a difficult
problem to solve. Lignin nanoparticles are extremely prone to agglomeration because of
the high surface energy and the large number of hydrogen bonds and Van der Waals forces
between the molecules. Therefore, completely solving the problem of particle agglomeration
and achieving the monodispersion of nano-lignin are the key to fully exerting the
nano-effect. Furthermore, the diversified morphology and size of lignin nanoparticles
are prerequisites for high value-added and multi-field applications of lignin. To solve the
interface and dispersion problems of nano-lignin materials, it is essential to find chemical
methods to modify nano-lignin and also supplement with effective physical dispersion
methods, such as mechanical stirring, ultrasonication and high shear homogenization. In
addition, the amount of nano-lignin added, the type of treatment agent and dispersing
equipment are all key factors that affect agglomeration, which need to be controlled and
improved during the preparation of lignin-based nanomaterials.
The structural and functional properties of lignin determine its extremely promising
applications in the field of biochemicals. Lignin and its derivatives have a wide range of
functionalities and can be used as dispersants, adsorbents/desorbents, oil recovery aids,
asphalt emulsifiers, etc. They can also be converted into aromatics, agrochemicals, polymers
and high-performance materials. However, all these processes depend on improvements
and innovations in the field of catalysis and product separation. Most importantly, to
realize the full industrial potential of lignin, further refinement of biopulp technology
is needed to achieve efficient separation of lignin and cellulose. It can be said that the
contribution of lignin to sustainable human development lies in its ability to provide a
stable and continuous source of organic matter. Thus it can truly guarantee sustainable
green development and energy supply.
In addition, the antioxidant and anti-UV effects of lignin are its most prominent properties,
so the application of lignin-based composites in food packaging and other fields has
been increasingly developed. The lignin-based nanocomposites are also excellent carriers
for a variety of metal ions and drug loading due to their unique nano-effects. They can be
used as bio-nanocomposite catalysts and reducing agents for heavy metal ions, which are of great importance for environmental protection and wastewater treatment. Furthermore,
the biocompatibility and non-toxicity of lignin are being intensively investigated.
Numerous scientific findings indicate that lignin-based nanocomposites have a very
promising future in the biomedical field. Specific lignins have significant anti-lipid peroxidation
and oxygen radical scavenging effects. Significant inhibitory effects of lignin
on the central nervous system or on cancer cell proliferation can also be observed. Most
importantly, lignin nanoparticles can be used as biological carriers for drug delivery and
targeted drug release. The molecular expression, biocompatibility and cytotoxicity of
lignin-based nanocomposites in cell lines have also been intensively studied. Conclusively,
the development of novel biomass materials and products will be applied to the medical
field and have a positive and effective impact on human life and health.
References
1. Figueiredo, P.; Lintinen, K.; Hirvonen, J.T.; Kostiainen, M.A.; Santos, H.A. Properties and chemical modifications of lignin:
Towards lignin-based nanomaterials for biomedical applications. Prog. Mater. Sci. 2018, 93, 233–269. [CrossRef]
2. Moon, R.J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J. Cellulose nanomaterials review: Structure, properties and
nanocomposites. Chem. Soc. Rev. 2011, 40, 3941–3994. [CrossRef]
3. Qian, Y.; Qiu, X.; Zhu, S. Lignin: A nature-inspired sun blocker for broad-spectrum sunscreens. Green Chem. 2015, 17, 320–324.
[CrossRef]
4. Sipponen, M.H.; Lange, H.; Crestini, C.; Henn, A.; Osterberg, M. Lignin for Nano- and Microscaled Carrier Systems: Applications,
Trends, and Challenges. ChemSusChem 2019, 12, 2039–2054. [CrossRef]
5. Dessbesell, L.; Paleologou, M.; Leitch, M.; Pulkki, R.; Xu, C. Global lignin supply overview and kraft lignin potential as an
alternative for petroleum-based polymers. Renew. Sustain. Energy Rev. 2020, 123. [CrossRef]
6. Zhu, J.; Yan, C.; Zhang, X.; Yang, C.; Jiang, M.; Zhang, X. A sustainable platform of lignin: From bioresources to materials and
their applications in rechargeable batteries and supercapacitors. Prog. Energy Combust. Sci. 2020, 76. [CrossRef]
7. Ago, M.; Tardy, B.L.; Wang, L.; Guo, J.; Khakalo, A.; Rojas, O.J. Supramolecular assemblies of lignin into nano- and microparticles.
MRS Bull. 2017, 42, 371–378. [CrossRef]
8. Anderson, E.M.; Stone, M.L.; Hülsey, M.J.; Beckham, G.T.; Román-Leshkov, Y. Kinetic Studies of Lignin Solvolysis and Reduction
by Reductive Catalytic Fractionation Decoupled in Flow-Through Reactors. ACS Sustain. Chem. Eng. 2018, 6, 7951–7959.
[CrossRef]
9. Wells, T., Jr.; Kosa, M.; Ragauskas, A.J. Polymerization of Kraft lignin via ultrasonication for high-molecular-weight applications.
Ultrason. Sonochem. 2013, 20, 1463–1469. [CrossRef]
10. Zhao, W.; Simmons, B.; Singh, S.; Ragauskas, A.; Cheng, G. From lignin association to nano-/micro-particle preparation:
Extracting higher value of lignin. Green Chem. 2016, 18, 5693–5700. [CrossRef]
11. Watkins, D.; Nuruddin, M.; Hosur, M.; Tcherbi-Narteh, A.; Jeelani, S. Extraction and characterization of lignin from different
biomass resources. J. Mater. Res. Technol. 2015, 4, 26–32. [CrossRef]
12. Tian, D.; Chandra, R.P.; Lee, J.S.; Lu, C.; Saddler, J.N. A comparison of various lignin-extraction methods to enhance the
accessibility and ease of enzymatic hydrolysis of the cellulosic component of steam-pretreated poplar. Biotechnol. Biofuels 2017, 10,
157. [CrossRef]
13. Gillet, S.; Aguedo, M.; Petitjean, L.; Morais, A.R.C.; da Costa Lopes, A.M.; Łukasik, R.M.; Anastas, P.T. Lignin transformations for
high value applications: Towards targeted modifications using green chemistry. Green Chem. 2017, 19, 4200–4233. [CrossRef]
Nanomaterials 2021, 11, 1336 25 of 30
14. Yoo, C.G.; Meng, X.; Pu, Y.; Ragauskas, A.J. The critical role of lignin in lignocellulosic biomass conversion and recent pretreatment
strategies: A comprehensive review. Bioresour. Technol. 2020, 301, 122784. [CrossRef]
15. Allegretti, C.; Fontanay, S.; Rischka, K.; Strini, A.; Troquet, J.; Turri, S.; Griffini, G.; D’Arrigo, P. Two-Step Fractionation of a
Model Technical Lignin by Combined Organic Solvent Extraction and Membrane Ultrafiltration. ACS Omega 2019, 4, 4615–4626.
[CrossRef]
16. Jääskeläinen, A.S.; Liitiä, T.; Mikkelson, A.; Tamminen, T. Aqueous organic solvent fractionation as means to improve lignin
homogeneity and purity. Ind. Crops Prod. 2017, 103, 51–58. [CrossRef]
17. Shen, X.-J.; Wen, J.-L.; Mei, Q.-Q.; Chen, X.; Sun, D.; Yuan, T.-Q.; Sun, R.-C. Facile fractionation of lignocelluloses by biomassderived
deep eutectic solvent (DES) pretreatment for cellulose enzymatic hydrolysis and lignin valorization. Green Chem. 2019,
21, 275–283. [CrossRef]
18. Dutta, S.; Theodore, M.; Garver, J.; Simo, S. Modes of association between kraft lignin components. ACS Symp. Ser. 1989, 397,
155–176. [CrossRef]
19. Magnus, N.; Håkan, E.; Lars, W. Aggregation of Lignin Derivatives under Alkaline Conditions. Kinetics and Aggregate Structure.
Langmuir 2002, 18, 2859–2865.
20. Lindström, T. The colloidal behaviour of kraft lignin. Part II. Coagulation of kraft lignin sols in the presence of simple and
complex metal ions. Colloid Polym. Sci. 1980, 258, 168–173. [CrossRef]
21. Liao, J.J.; Latif, N.H.A.; Trache, D.; Brosse, N.; Hussin, M.H. Current advancement on the isolation, characterization and
application of lignin. Int. J. Biol. Macromol. 2020, 162, 985–1024. [CrossRef] [PubMed]
22. Tribot, A.; Amer, G.; Abdou Alio, M.; de Baynast, H.; Delattre, C.; Pons, A.; Mathias, J.-D.; Callois, J.-M.; Vial, C.; Michaud, P.; et al.
Wood-lignin: Supply, extraction processes and use as bio-based material. Eur. Polym. J. 2019, 112, 228–240. [CrossRef]
23. Vishtal, A.G.; Kraslawski, A. Challenges in industrial applications of technical lignins. BioResources 2011, 6, 3547–3568. [CrossRef]
24. Dong, H.; Zheng, L.; Yu, P.; Jiang, Q.; Wu, Y.; Huang, C.; Yin, B. Characterization and application of lignin–carbohydrate
complexes from lignocellulosic materials as antioxidants for scavenging in vitro and in vivo reactive oxygen species. ACS Sustain.
Chem. Eng. 2019, 8, 256–266. [CrossRef]
25. Pinem, M.P.; Wardhono, E.Y.; Nadaud, F.; Clausse, D.; Saleh, K.; Guenin, E. Nanofluid to Nanocomposite Film: Chitosan and
Cellulose-Based Edible Packaging. Nanomaterials 2020, 10, 660. [CrossRef] [PubMed]
26. Willner, I.;Willner, B. Biomolecule-based nanomaterials and nanostructures. Nano Lett. 2010, 10, 3805–3815. [CrossRef]
27. Ates, B.; Koytepe, S.; Ulu, A.; Gurses, C.; Thakur, V.K. Chemistry, Structures, and Advanced Applications of Nanocomposites
from Biorenewable Resources. Chem. Rev. 2020, 120, 9304–9362. [CrossRef]
28. Sharma, R.; Jafari, S.M.; Sharma, S. Antimicrobial bio-nanocomposites and their potential applications in food packaging.
Food Control 2020, 112. [CrossRef]
29. Wu, H.; Fahy, W.P.; Kim, S.; Kim, H.; Zhao, N.; Pilato, L.; Kafi, A.; Bateman, S.; Koo, J.H. Recent developments in polymers/
polymer nanocomposites for additive manufacturing. Prog. Mater. Sci. 2020, 111. [CrossRef]
30. Won Jang, H.; Zareidoost, A.; Moradi, M.; Abuchenari, A.; Bakhtiari, A.; Pouriamanesh, R.; Malekpouri, B.; Jafari Rad, A.;
Rahban, D. Photosensitive nanocomposites: Environmental and biological applications. J. Compos. Compd. 2020, 2, 50–60.
[CrossRef]
31. Ambekar, R.S.; Kandasubramanian, B. Advancements in nanofibers for wound dressing: A review. Eur. Polym. J. 2019, 117,
304–336. [CrossRef]
32. Palazzetti, R.; Zucchelli, A. Electrospun nanofibers as reinforcement for composite laminates materials—A review. Compos. Struct.
2017, 182, 711–727. [CrossRef]
33. Laurichesse, S.; Avérous, L. Chemical modification of lignins: Towards biobased polymers. Prog. Polym. Sci. 2014, 39, 1266–1290.
[CrossRef]
34. Richter, A.P.; Brown, J.S.; Bharti, B.;Wang, A.; Gangwal, S.; Houck, K.; Hubal, E.A.C.; Paunov, V.N.; Stoyanov, S.D.; Velev, O.D.
An environmentally benign antimicrobial nanoparticle based on a silver-infused lignin core. Nat. Nanotechnol. 2015, 10, 817–823.
[CrossRef]
35. Beisl, S.; Miltner, A.; Friedl, A. Lignin from Micro- to Nanosize: Production Methods. Int. J. Mol. Sci. 2017, 18, 1244. [CrossRef]
[PubMed]
36. Mishra, P.K.; Ekielski, A. The Self-Assembly of Lignin and Its Application in Nanoparticle Synthesis: A Short Review. Nanomaterials
2019, 9, 243. [CrossRef] [PubMed]
37. Henn, A.; Mattinen, M.L. Chemo-enzymatically prepared lignin nanoparticles for value-added applications. World J. Microbiol.
Biotechnol. 2019, 35, 125. [CrossRef] [PubMed]
38. Budnyak, T.M.; Slabon, A.; Sipponen, M.H. Lignin-Inorganic Interfaces: Chemistry and Applications from Adsorbents to Catalysts
and Energy Storage Materials. ChemSusChem 2020, 13, 4344–4355. [CrossRef]
39. Österberg, M.; Sipponen, M.H.; Mattos, B.D.; Rojas, O.J. Spherical lignin particles: A review on their sustainability and applications.
Green Chem. 2020, 22, 2712–2733. [CrossRef]
40. Duval, A.; Lawoko, M. A review on lignin-based polymeric, micro- and nano-structured materials. React. Funct. Polym. 2014, 85,
78–96. [CrossRef]
41. Stewart, D. Lignin as a base material for materials applications: Chemistry, application and economics. Ind. Crops Prod. 2008, 27,
202–207. [CrossRef]
4. Conclusions and Future Perspectives
Overall, this review summarizes the current status of the preparation of nano-lignin by sedimentation, mechanical, self-assembly and stepwise addition polymerization methods. In order to provide a reference for the chemical deep processing of biomass resources and the development of nano-lignin, the application characteristics of nano-lignin in UV protection and anti-bacteria, nano-fillers and biomass-based carriers are also outlined. Through the specific analysis of the preparation methods and application status of nano-lignin, it can promote the further research of nano-lignin and the development of novel nano-lignin-based products. This is of great significance to the utilization and sustainable development of lignin.
The multiple structures and the diverse properties of lignin make the prepared nano-lignin more complicated, which brings challenges to the research of the preparation and performance of nano-lignin. Meanwhile, it provides broader prospects and opportunities for the multi-functional and multi-field applications of nano-lignin. Nanometersized lignin with high specific surface area and activity is a novel approach to achieve high value-added utilization of lignin. Compared with the research of inorganic nanoparticles and renewable nanocellulose, the preparation and application of nano-lignin are still in their infancy. The scale and industrialization of nano-lignin-based products will become an important aspect of future lignin research.
In view of the problems of complex process and toxic organic solvents in the preparation of lignin nanoparticles, methods such as electrostatic spinning, self-assembly, ultrasonication and homogenization can be utilized. In terms of multi-functional applications of lignin-based nanomaterials, the range of use can be expanded by improving their strength, electrical conductivity, thermal stability, crystallization performance, etc.
The following points need attention in the application of nanolignin materials. First of all, achieving uniform dispersions of nanolignin in composite materials is a difficult problem to solve. Lignin nanoparticles are extremely prone to agglomeration because of the high surface energy and the large number of hydrogen bonds and Van der Waals forces between the molecules. Therefore, completely solving the problem of particle agglomeration and achieving the monodispersion of nano-lignin are the key to fully exerting the nano-effect. Furthermore, the diversified morphology and size of lignin nanoparticles are prerequisites for high value-added and multi-field applications of lignin. To solve the interface and dispersion problems of nano-lignin materials, it is essential to find chemical methods to modify nano-lignin and also supplement with effective physical dispersion methods, such as mechanical stirring, ultrasonication and high shear homogenization. In addition, the amount of nano-lignin added, the type of treatment agent and dispersing equipment are all key factors that affect agglomeration, which need to be controlled and improved during the preparation of lignin-based nanomaterials.
The structural and functional properties of lignin determine its extremely promising applications in the field of biochemicals. Lignin and its derivatives have a wide range of functionalities and can be used as dispersants, adsorbents/desorbents, oil recovery aids, asphalt emulsifiers, etc. They can also be converted into aromatics, agrochemicals, polymers and high-performance materials. However, all these processes depend on improvements and innovations in the field of catalysis and product separation. Most importantly, to realize the full industrial potential of lignin, further refinement of biopulp technology is needed to achieve efficient separation of lignin and cellulose. It can be said that the contribution of lignin to sustainable human development lies in its ability to provide a stable and continuous source of organic matter. Thus it can truly guarantee sustainable green development and energy supply.
In addition, the antioxidant and anti-UV effects of lignin are its most prominent properties, so the application of lignin-based composites in food packaging and other fields has been increasingly developed. The lignin-based nanocomposites are also excellent carriers for a variety of metal ions and drug loading due to their unique nano-effects. They can be used as bio-nanocomposite catalysts and reducing agents for heavy metal ions, which are of great importance for environmental protection and wastewater treatment. Furthermore, the biocompatibility and non-toxicity of lignin are being intensively investigated.
Numerous scientific findings indicate that lignin-based nanocomposites have a very promising future in the biomedical field. Specific lignins have significant anti-lipid peroxidation and oxygen radical scavenging effects. Significant inhibitory effects of lignin on the central nervous system or on cancer cell proliferation can also be observed. Most importantly, lignin nanoparticles can be used as biological carriers for drug delivery and targeted drug release. The molecular expression, biocompatibility and cytotoxicity of lignin-based nanocomposites in cell lines have also been intensively studied. Conclusively, the development of novel biomass materials and products will be applied to the medical field and have a positive and effective impact on human life and health.