Lignin-Based Nanoparticles: a Review on Their Preparations and Applications
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polymers Review Lignin-Based Nanoparticles: A Review on Their Preparations and Applications Qianqian Tang 1, Yong Qian 2, Dongjie Yang 2, Xueqing Qiu 3, Yanlin Qin 3,* and Mingsong Zhou 2,* 1 College of Chemistry and Chemical Engineering, Henan Key Laboratory of Function-Oriented Porous Materials, Luoyang Normal University, Luoyang 471934, China; [email protected] 2 School of Chemistry and Chemical Engineering, State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China; [email protected] (Y.Q.); [email protected] (D.Y.) 3 School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China; [email protected] * Correspondence: [email protected] (Y.Q.); [email protected] (M.Z.); Tel.: +86-20-8711-4722 (M.Z.) Received: 29 September 2020; Accepted: 20 October 2020; Published: 25 October 2020 Abstract: Lignin is the most abundant by-product from the pulp and paper industry as well as the second most abundant natural renewable biopolymer after cellulose on earth. In recent years, transforming unordered and complicated lignin into ordered and uniform nanoparticles has attracted wide attention due to their excellent properties such as controlled structures and sizes, better miscibility with polymers, and improved antioxidant activity. In this review, we first introduce five important technical lignin from different sources and then provide a comprehensive overview of the recent progress of preparation techniques which are involved in the fabrication of various lignin-based nanoparticles and their industrial applications in different fields such as drug delivery carriers, UV absorbents, hybrid nanocomposites, antioxidant agents, antibacterial agents, adsorbents for heavy metal ions and dyes, and anticorrosion nanofillers. Keywords: lignin; lignin-based nanoparticles; preparations; applications 1. Introduction As a green, safe, low-cost, and sustainable natural renewable resource, lignocellulose is considered an excellent potential alternative to traditional petrochemical resources for a variety of applications [1]. In recent years, producing bioenergy and high-value green chemicals from lignocellulose has received worldwide attention due to the increasing resource, environmental, economic, and health issues [2–4]. Lignocellulose is derived from plant and wood, which mainly consists of cellulose, hemicellulose, and lignin [5]. Currently, the researches on the applicability of carbohydrates are mostly focused on cellulose and hemicellulose systems. However, lignin, as the second most abundant biopolymer next to cellulose on earth, is underutilized and its potential application value has not yet been well exploited [6–8]. Lignin is copious and also the largest reservoir of aromatic polymer on earth, which plays an important role in plants, including transporting water and minerals, providing mechanical supports, and protecting plants or wood from chemical or microbial attacks [9–11]. The molecular structure of lignin is highly dependent on the wood locations, species, and especially the extraction processes [10,12–16]. Different types of lignin contain different functional groups and show different molecular weight and elemental composition [17]. Therefore, the structure of lignin is extremely complicated and difficult to determine. However, it is generally accepted that lignin is a highly branched, amorphous, and Polymers 2020, 12, 2471; doi:10.3390/polym12112471 www.mdpi.com/journal/polymers Polymers 2020, 12, x FOR PEER REVIEW 2 of 22 Polymers 2020, 12, 2471 2 of 22 complicated and difficult to determine. However, it is generally accepted that lignin is a highly branched, amorphous, and three-dimensional network biomacromolecule, which consists of three three-dimensional network biomacromolecule, which consists of three basic phenylpropane monomers: basic phenylpropane monomers: guaiacyl, syringyl, and p-hydrophenyl [5] linked together by a guaiacyl, syringyl, and p-hydrophenyl [5] linked together by a number of bonds including several number of bonds including several types of carbon-carbon and ether linkage [18–23]. Figure 1 gives types of carbon-carbon and ether linkage [18–23]. Figure1 gives the possible structural representation the possible structural representation of lignin molecules [24]. From Figure 1, there are a large of lignin molecules [24]. From Figure1, there are a large number of active functional groups in lignin, number of active functional groups in lignin, such as aliphatic and phenolic hydroxyl groups, such as aliphatic and phenolic hydroxyl groups, carbonyl groups, methoxy groups, and phenyl groups, carbonyl groups, methoxy groups, and phenyl groups, which are important active sites for further which are important active sites for further chemical modifications of lignin by sulfonation, oxidation, chemical modifications of lignin by sulfonation, oxidation, graft copolymerization, or graft copolymerization, or hydroxymethylation reactions, etc. [25–30]. The obtained modified lignin hydroxymethylation reactions, etc. [25–30]. The obtained modified lignin products could be utilized products could be utilized in different industrial fields [5,31–35]. in different industrial fields [5,31–35]. Figure 1.1. The possible structural representation ofof ligninlignin [[24].24]. Besides the natural abundance, lignin is also present as a major byproduct of the pulp and paper Besides the natural abundance, lignin is also present as a major byproduct of the pulp and industry [36]. In the paper pulping process, most of the lignin is removed and discharged in the paper industry [36]. In the paper pulping process, most of the lignin is removed and discharged in form of spent liquors. Every year about 50 million tons of lignin is generated from the pulp and the form of spent liquors. Every year about 50 million tons of lignin is generated from the pulp and paper industry [37,38]. However, the majority is discarded as waste or burnt to recover heat and paper industry [37,38]. However, the majority is discarded as waste or burnt to recover heat and electricityelectricity [[39–41],39–41], causingcausing seriousserious environmentalenvironmental pollutionpollution andand resourceresource waste.waste. Only approximately 2%2% ofof the the produced produced lignin lignin is isolated is isolated and eandffectively effectively used for used various for productsvarious products [9], including [9], industrialincluding dispersantsindustrial dispersants [42–50], cleaning [42–50], agents cleaning [51], agents and dopants [51], and for conductivedopants for polymers conductive [36 ].polymers Theoretically, [36]. ligninTheoretically, should belignin a remarkable should be feedstock a remarkable for di fffeederentstock materials for different and could materials be widely and usedcould in be di widelyfferent fieldsused duein different to its aromaticity, fields due various to its reactive aromaticity, functional various groups, reactive excellent functional ultraviolet groups, and oxidationexcellent resistance,ultraviolet highand thermal oxidation stability, resistance, nontoxicity, high biodegradability, thermal stability, renewability, nontoxicity, and low costsbiodegradability, [22,24,52–55]. However,renewability, only and a very low small costs portion [22,24,52–55]. of lignin However, is utilized. only This a lowvery utilization small portion percentage of lignin mainly is utilized. results fromThis low the highlyutilization complex percentage and changeable mainly results molecular from structure,the highly poor complex miscibility and changeable with a host molecular polymer matrix,structure, and poor high miscibility polydispersity with a ofhost lignin polymer [56–58 matrix,]. Additionally, and high polydispersity the complexity of of lignin its isolation, [56–58]. purification,Additionally, chemical the complexity modifications, of its andisolation, structural purification, characterization chemical also modifications, inhibits its high and value-added structural applicationscharacterization [1]. Inalso any inhibits case, finding its high new value-added and high value-added applications applications [1]. In any of case, lignin finding recovered new from and pulpinghigh value-added waste liquor applications is imperative, of lignin which recovered has both from economic pulping and waste environmental liquor is imperative, benefits. In which light ofhas the both excellent economic properties and environmen and extensivetal benefits. applications In light of nanomaterials of the excellent in variousproperties fields, and it extensive has been consideredapplications to of prepare nanomaterials nanospheres in various from lignin,fields, whichit has been provides considered the possibility to prepare to utilize nanospheres lignin-based from Polymers 2020, 12, 2471 3 of 22 products in high value-added industrial fields, such as UV-blocking additives for thermoplastics, drug delivery, and Pickering emulsions [7]. In this review, we provide a comprehensive overview of the preparation of lignin-based nanospheres and their applications in different commercial fields. The aim of this article is to attract considerable attention from the target researches towards developing high value-added industrial applications of lignin-based products. 2. Main Types of Lignin Lignin could be extracted or separated from lignocellulosic biomass. Different extraction or separation processes would result in different physical and chemical properties of products [17,56], such as surface