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Research Article pubs.acs.org/journal/ascecg

Preparation and Formation Mechanism of Renewable Lignin Hollow Nanospheres with a Single Hole by Self-Assembly Fuquan Xiong,† Yanming Han,*,† Siqun Wang,†,‡ Gaiyun Li,† Tefu Qin,† Yuan Chen,† and Fuxiang Chu*,† †

Research Institute of Wood Industry, Chinese Academy of Forestry, Xiangshan Road, Beijing 100091, China Center for Renewable Carbon, University of Tennessee, 2506 Jacob Drive, Knoxville, Tennessee 37996, United States

‡

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S Supporting Information *

ABSTRACT: Lignin hollow nanospheres with a single hole were prepared through a straightforward self-assembly method, which included dissolving enzymatic hydrolysis lignin, a byproduct derived from biorefinery, in tetrahydrofuran and afterward dropping deionized water to the lignin/tetrahydrofuran solution. The formation mechanism and structural characteristics of the lignin hollow nanospheres were explored. The results indicated that the nanospheres exhibited hollow structure due to the effect of tetrahydrofuran on the selfassembly behavior. Hydrophobic outside surface and hydrophilic internal surface were formed via layer-by-layer self-assembly method from outside to inside based on π−π interactions. The chemical structure of lignin did not produce a significant change in the preparation process of lignin hollow nanospheres. With increasing of initial lignin concentration, the diameter of the nanospheres and the thickness of shell wall increased, while the diameter of the single hole, the surface area, and the pore volume of the nanospheres decreased. The surface area reached the maximum value (25.4 m2 g−1) at an initial lignin concentration of 0.5 mg/mL in setting concentration range. Increasing the stirring speed or dropping speed of water resulted in a decrease of the diameter of the hollow nanospheres. Moreover, an apparent change of the average diameter of the nanospheres was not observed after 15 days, and the nanosphere dispersions were stable at pH values between 3.5 and 12. The lignin hollow nanospheres with a single hole offer a novel route for a value-added utilization of lignin and would improve the biorefinery viability. KEYWORDS: Self-assembly, Lignin hollow nanospheres, Opening hole, Formation mechanism

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styrene/poly(3,4-ethylenedioxythiophene), 1 5 poly(methylsilsesquioxane),16 titanium dioxide,9 and silica.7,17 Enzymatic hydrolysis lignin (EHL) is a byproduct derived from biorefinery. With growing global environmental awareness and increasing of the shortage of fossil sources, increased attention has been paid to the development of new and alternative polymer sphere from renewable resources.18−20 Lignin is the second most abundant plant biopolymer on earth after cellulose,21 and it is a polyaromatic polymer composed of phenylpropanoid units. Lignin is generally regarded as a simple waste product or low-value byproduct.22 Only 2% of lignin is used for agricultural purposes and other industries such as adhesives.23 However, lignin-based nanoparticles offer an opportunity for value-added utilization of lignin products.24,25 Nanoparticles possess larger surface area for per unit volume comparing with micronsized particles.26 When nanoparticles are mixed with various polymers, they may produce close interaction with the polymer matrix.24 Furthermore, the lack of cytotoxicity is proved already for lignin, suggesting that lignin

INTRODUCTION

Hollow nano- or macrospheres usually show substantially different properties in terms of density, specific surface area, and surface permeability.1,2 They are expected to be applied in many fields such as delivery systems, catalysis, coating technology, and composite materials.3 When hollow spheres are used in carrying medicine, they usually have higher efficiency drug loading compared with those of spheres possessed of compact structure. Tremendous efforts have been made in fabrication of hollow spheres with controlled composition, tailored structure, and unique properties.4,5 Recently, as a new class of hollow structure, a hollow capsule with a single hole, which is also known as open-mouthed hollow capsule, has stimulated great interest due to the enhanced uptake capacity, diffusivity, and catalytic performance.6−9 In addition, hollow particles with controllable holes have potential value in the field of selective encapsulation.10 The hollow spheres with an opening hole have been reported using the preparation of various types of organic polymer and inorganic oxide, respectively, such as chiral phenylacrylonitrile tartaric acids,11 polystyrene,10,12 poly(acryamide-ethylene glycol dimeth acrylate),13 polystyrene/poly(divinylbenzene),14 poly© 2017 American Chemical Society

Received: October 27, 2016 Revised: February 6, 2017 Published: February 16, 2017 2273

DOI: 10.1021/acssuschemeng.6b02585 ACS Sustainable Chem. Eng. 2017, 5, 2273−2281

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. SEM (A−D) and TEM (E−H) images of lignin hollow nanospheres at different predropping lignin concentrations. Predropping lignin concentrations: (A, E) 0.5 mg/mL, (B, F) 1 mg/mL, (C, G) 1.5 mg/mL, and (D, H) 2 mg/mL.

can serve as a candidate in the field of biomaterials.27 Application of lignin nanoparticles in UV protection, in antibacterial and nanofiller applications, and as biomass-based carrier has been reported in the literature.28−32 For instance, Richter et al.31 prepared a biodegradable and green alternative to silver nanoparticles: lignin nanoparticles infused with silver ions and subsequently coated with a cationic polyelectrolyte layer. They found that the green silver nanoparticles possess the higher levels of antimicrobial activity and the smaller levels of environmental impact comparing with metallic silver nanoparticles. The preparation methods for lignin nanoparticles reported so far are the precipitation method, the mechanical method, polyaddition, and self-assembly. Frangville et al.33 developed two methods, i.e., precipitation of low-sulfonated lignin from an ethylene glycol solution and the acidic precipitation of lignin from a high-pH aqueous solution, for the fabrication of lignin nanoparticles. Myint et al.20 produced lignin nanoparticles by compressed fluid antisolvent technique. Nair et al.24 introduced a simple high-shear homogenizer to prepare nanolignin. Yiamsawas et al.34 prepared hollow lignin nanocapsules by interfacial polyaddition. However, the acquired lignin nanoparticles usually possessed an irregular shape. Qian et al.25 obtained uniform colloidal spheres using acetylated lignin via self-assembly, but this step required environmentally unfriendly chemicals such as acetyl bromide. Lievonen et al.35 produced spherical lignin nanoparticles using unmodified lignin, but the nanoparticles have compact structure. Recently, hollow spheres based on kraft lignin were reported,36 but the formation mechanism of the hollow spheres is still indeterminate. EHL exhibits greater chemical activity and applicability than lignosulfonate or kraft lignin and has a good solubility in ubiquitous organic solvent, such as tetrahydrofuran (THF).37,38 Currently, the increased demand for bioethanol produced from bioresources was driven by the depletion of fossil fuels that generally are used to prepare transportation fuel, leading to the fact that EHL was also generated in large quantities as a byproduct.39 However, EHL is usually burned to meet the internal energy use, that is, it has not been efficiently utilized.40 If EHL could be transformed to value-added materials, chemicals, or fuels through an energy- and cost-effective route, then it would greatly improve biorefinery viability.40 In light of the potential value of hollow nanospheres with a single hole and renewable enzymatic hydrolysis lignin,10 this study mainly focused on a simple one-step method for fabricating lignin hollow nanospheres with a single hole,

without the need for chemical modification of lignin, the aid of surfactants, and a template removal step during the preparation. The nanospheres were prepared by dissolving enzymatic hydrolysis lignin in THF and afterward dropping deionized water to the lignin/THF solution. The formation mechanism and structural characteristics of the lignin hollow nanospheres were explored by transmission electron microscopy (TEM), scanning electron microscopy (SEM), dynamic light scattering (DLS), BET analyzers, 31P nuclear magnetic resonance (31P NMR), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), elemental analysis, and UV−vis absorption spectra. Effects of various process parameters on the characteristics of lignin hollow nanospheres were respectively investigated as well.

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EXPERIMENTAL SECTION

Materials. Enzymatic hydrolysis lignin (EHL) was purchased from Hong Kong Laihe Biotechnology Co., Ltd. The hydroxyl content, the number-average molecular mass, and polydispersity of EHL are 2.67 mmol/g, 1430 g/mol, and 1.22, respectively. The EHL dissolved relatively well in THF. It was dried at 40 °C in a vacuum for 12 h and kept inside a reagent bottle with ground stopper before use. THF’s of analytical-grade purity and chromatographic grade were purchased from Beijing Chemical Reagent Company and Sinopharm Chemical Reagent Co., Ltd., respectively. They were used directly without further treatment. Preparation of Lignin Hollow Nanospheres. Lignin hollow nanospheres were easily formed by gradually adding deionized water into the EHL THF solution. EHL was dissolved in THF in various concentrations of 0.5, 1, 1.5, and 2 mg/mL. Specifically, 5 mg of EHL was dissolved into 10 mL of THF in the preparation of the EHL THF solution. Then the solution was stirred at room temperature (25 °C) under magnetic stirring about 600 rpm. Following these steps, 40 mL of deionized water was gradually dropped into the lignin/THF solution at the speed of 2 mL/min, and lignin hollow nanoparticles were formed. The suspension liquid containing lignin hollow nanoparticles was continuously stirred to evaporate THF. After 4 h, the suspension liquid was introduced into a dialysis bag (MWCO: 12 000−14 000, Spectrumlabs, USA) which was then immersed in excess of (periodically replaced) deionized water for removing residual THF before further measurements. Control of Process Parameters for EHL Nanospheres Synthesis. EHL was dissolved in THF at a fixed initial EHL concentration of 0.5 mg/mL. Then, deionized water was gradually dropped into the lignin/THF solution to 80 vol % in the case of magnetic stirring. The acquiescent stirring rate and dropping speed of water are 600 rpm and 2 mL/min, respectively. Characterization. Morphology of the samples were observed using SEM (Hitachi S-4800) and TEM (JEOL JEM-1230), respectively. The size distribution of lignin hollow nanoparticles in 2274


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Figure 2. Particle size analysis of lignin hollow nanospheres. (A) Effect of predropping lignin concentration on lignin hollow nanospheres size (insets are TEM images of lignin hollow nanospheres). (B) Effect of days after preparation on lignin hollow nanospheres size (insets are TEM images of lignin hollow nanospheres, and the nanosphere edges become obscure after 60 days). suspension was detected by dynamic light scattering (DLS) (Malvern Instruments, Malvern, UK). The specific surface area of the samples was obtained from the N2 adsorption isotherm at 77 K by the (Brunauer−Emmett−Teller) BET method,41 using Quantachrome Nova 1200e surface area and pore size analyzer, and the pore size distribution was calculated by the BJH method from the desorption branches. Total pore volume was estimated from the amount adsorbed at relative pressure (P/P0) = 0.98.42 FTIR spectra of EHL and lignin hollow nanoparticles was determined on a Bruker Fourier Transform Infrared spectrometer using a KBr disc. The chemical composition of EHL and lignin hollow nanoparticles was determined using 31P NMR on a Bruker AVIII 500 MHz (Bruker, Germany) spectrometer according to the previous literature.43−45 Elemental distribution of the surface of lignin hollow nanospheres after freeze dying and itself was analyzed by X-ray photoelectron spectroscopy (XPS, Axis Ultra, Kratos, England) and elemental analysis (Flash EA1112), respectively. UV−vis spectra of EHL and lignin hollow nanospheres were determined on a Shimadzu UV-2600 spectrometer in the region of 200−800 nm.

concentration has been observed in the synthesis of lignin nanospheres possessed of compact structure.35,46 Compared with conventional polymer hollow spheres, the lignin hollow spheres have smaller diameter.12,14,47 Figure 2B illustrates the stability of lignin hollow nanospheres at predropping lignin concentration of 0.5 and 2 mg/ mL, respectively. No apparent change of the average diameter of the two kinds of lignin hollow nanospheres was observed after 15 days. However, the nanospheres aggregation occurred after 45 days, with an increase in the average diameter. The nanosphere edges became obscure after 60 days. When lignin hollow nanospheres are dispersed in water, the phenolic hydroxyl groups and carboxyl groups provide the nanospheres a surface charge that promotes the formation of electrical double layers, which can stabilize the nanospheres dispersion via electrical double layer repulsion.35 However, lignin hollow nanospheres with asymmetric physical chemistry properties will affect electrical double layer structure, leading to the nanospheres aggregation. It is worth noting that the increased ratio of the diameter of the nanospheres at predropping lignin concentration of 0.5 mg/mL was higher than that of 2 mg/mL after 45 days, probably due to the fact that the diameter of the single hole of the former is larger than that of the latter. Surface Area, Pore Size, and Distribution of Lignin Hollow Nanospheres. Nitrogen adsorption−desorption isotherms of lignin and lignin hollow nanospheres is presented (Figure 3). The lower portion of the loop is traced out on

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RESULTS AND DISCUSSION Morphology and Size of Lignin Hollow Nanospheres. Typical SEM and TEM images of lignin hollow nanospheres at different initial lignin concentration are showed in Figure 1. The SEM images exhibited the hollow structure and a single hole (it was further demonstrated using AFM in Figure S1) on the surface of particle, and the particle was spherical (when the initial lignin concentration was fixed at 0.2 mg/mL, the particle was irregular, which was exhibited in Figure S2). It is worth noting that the diameter of the single hole of the nanospheres decreased with increasing initial lignin concentration. Their hollowness was further supported by using TEM. The TEM images displayed a clear contrast between the center and the shell that indicates the presence of a cavity. Furthermore, the thickness of shell wall increased with an increase of predropping lignin concentration. It may be attributed that higher predropping lignin concentration means that more lignin available for the formation of shell wall. This indicates that the size of the single hole and the thickness of shell wall can be adjusted by employing different predropping lignin concentration. The particle sizes of the hollow nanospheres in deionized water were tracked using DLS. With increasing predropping lignin concentration, an increase of the mean nanospheres size (from 419 to 566 nm) was observed in Figure 2A. A higher concentration of predropping lignin concentration means more lignin available for the formation of shell in the initial phase, leading to the fact that lignin hollow nanospheres size increases with an increase of predropping lignin concentration. An increase in particle size as a function of the initial polymer

Figure 3. Nitrogen sorption isotherms of lignin and lignin hollow nanospheres at different predropping lignin concentrations (0.5 and 2 mg/mL).

adsorption, and the upper portion is traced on desorption. A hysteresis loop was found in the adsorption−desorption isotherm of all samples, which resembles type IV of Brunauer’s classification.41 The very low amount of nitrogen adsorbed at low relative pressure indicates the nearly absence of microporosity. Moreover, the pore size distribution was calculated by the BJH method from the desorption branches. When 2275


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with that of raw material. Furthermore, the semiquantitative analysis of the FTIR spectra showed that the A1700/A1514 ratio of lignin was 0.5528, which was higher than that of lignin hollow nanospheres (0.5058 and 0.4828) at predropping lignin concentration of 0.5 and 2 mg/mL, respectively. It may be concluded that lignin molecules containing the high content of carboxy group may not produce aggregation in the process of preparation of lignin hollow nanospheres. The yields of lignin hollow nanospheres were 70.8 and 69.9% (w/w) at predropping lignin concentrations of 0.5 and 2 mg/mL, respectively, further suggesting that some lignin molecules are not involved in the formation of lignin hollow nanospheres. 31 P NMR spectra of the samples may be used to determine the amount of carboxyl, phenolic hydroxy groups, and aliphatic hydroxy groups in lignin.44 Lignin is marked with reagents containing phosphorus and then subjected to NMR analysis.51 Figure 4B summarizes the 31P NMR analysis results of the lignin and lignin hollow nanospheres. Compared with lignin raw material, concentration of aliphatic OH, carboxy groups, and phenolic OH decreased, except that aliphatic OH of lignin hollow nanospheres at the predropping lignin concentration for 0.5 mg/mL increased slightly. We suggest that some hydrophilic lignin molecules might not produce aggregation in the process of preparation of lignin hollow nanospheres, leading to the fact that concentration of hydrophilic groups in the lignin hollow nanospheres is lower than that in the raw lignin. Moreover, we highlight that the content of carboxy group of the lignin hollow nanospheres was decreased distinctly, hinting that it had significant influence on the hydrophilia of lignin, consistent with the result of the FTIR. XPS can be used to measure elemental composition for the top 1−5 nm depth of the surface region.52 The O/C ratio on the nanospheres surface region was measured by XPS (the typical measurement error is about 10%), while the O/C ratio of the nanospheres was measured using elemental analysis (the typical measurement error is about 0.1%). Since the hydrophilic groups of lignin hollow nanospheres are hydroxyl and carboxyl groups, the more hydrophilic chains must have a higher O/C ratio.25 As shown in Table 2, the O/C ratio on the nanospheres

predropping lignin concentration was 2 mg/mL, the hollow nanospheres exhibited pore radius distribution in the 1.5−14 nm range. It suggested that the sample was a mesopore material. As for the hollow nanospheres at predropping lignin concentration of 0.5 mg/mL, it showed the pore radius distribution was between 15 and 60 nm, demonstrating the presence of mesopores and macropore. The textural properties of lignin and lignin hollow nanospheres were summarized in Table 1. The surface areas of lignin Table 1. Porous Structure Properties of Lignin and Lignin Hollow Nanospheres at Different Predropping Lignin Concentration samples

BET Surface Area (m2 g−1)

total pore volume (cm3 g−1)

lignin 0.5 mg/mL 2 mg/mL

0.824 25.4 5.79

0.0024 0.1004 0.0219

hollow nanospheres were 25.4 and 5.79 m2 g−1 at predropping lignin concentrations of 0.5 and 2 mg/mL, respectively, which were higher than 0.824 m2 g−1 of lignin. Under the same conditions, the higher predropping lignin concentration leaded to the higher thickness of shell wall so that the surface area decreased. The surface area results are in good agreement with the results obtained from the corresponding TEM images. Furthermore, the changing trend of total pore volume was consistent with the surface area. Chemical Characteristics of Lignin Hollow Nanospheres. The lignin and lignin hollow nanospheres were characterized by FTIR analysis and the results are showed in Figure 4A. The absorbances are normalized to 1 at 1510−1514 cm−1 which is assigned to the maximum absorbance peak of aromatic skeleton stretch in studied samples. The lignin exhibited bands at 3400 cm−1 (the stretching vibrations of the hydroxyl group), 2900 cm−1 (C−H stretching of methyl or methylene groups), 1700 cm−1 (CO stretching), 1598 cm−1 (aromatic skeleton vibration), 1514 cm−1 (C−C stretching of aromatic skeleton), 1460 cm−1 (C−H stretching of aromatic skeletal), 1425 cm−1 (aromatic skeleton vibrations combined with C−H in-plane deformation), 1332 cm−1 (syringyl units vibration), 1118 cm−1 (aromatic C−H deformation in syringyl unit), and 1031 cm−1 (OH stretching of primary alcohol).22,48−50 In comparison to the raw lignin, these bands of lignin hollow nanospheres had no disappearance, hinting that the chemical structure of lignin did not produced a significant change in the preparation process of lignin hollow nanospheres. It is worth noting that the characteristic peak of the lignin hollow nanospheres at 1700 cm−1 was weakened compared

Table 2. Elemental Distribution of Lignin Hollow Nanospheres and Outside Surfacea elemental content nanospheres outside surface nanospheres a

analysis method

C (%)

O (%)

O/C ratio

XPS

81.1 ± 1.23

15.5 ± 0.69

0.19

elemental analysis

63.6 ± 0.10

29.4 ± 0.09

0.46

Pre-dropping lignin concentration was 2 mg/mL.

Figure 4. FTIR spectra (A) and structural characteristics calculated from the 31P NMR spectra (B) of lignin and lignin hollow nanospheres at different predropping lignin concentrations (0.5 and 2 mg/mL). Key: S, syringyl units; G, guaiacyl units; and H, p-hydroxyphenyl units. 2276


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Figure 5. Effects of stirring rate (A), dropping speed of water (B), and pH (C) on the characteristics of lignin hollow nanospheres. Insets are TEM images of lignin hollow nanospheres at pH values of 1.8 (a) and 12.3 (b), respectively.

fact that the negative charge of the nanospheres surface decreased at pH values less than 3.5, leading to the fact that the nanospheres produced aggregation.35 TEM images of the nanospheres at pH 1.8 exhibited blocking aggregation (Figure 5C-a). Moreover, the average particle size of the nanospheres was almost zero at pH values above 12, probably due to the fact that the nanospheres were dissolved in alkali solution above pH 12.35 As can be seen from the TEM morphology, the sample of the pH value of 12.3 was dissolved (Figure 5C-b). Formation Mechanism of Lignin Hollow Nanospheres. The nanospheres form a hollow structure due to the effect of THF levels. Usually, THF−water is miscible. However, we found that phase separation existed between THF of analytical-grade purity (AR-THF) and water. AR-THF− water has an obvious Tyndall phenomenon (Figure S5a) compared with THF of chromatographic grade (HPLC-THF) (Figure S5b), indicating that phase separation existed between AR-THF and water due to the formation of nanoemulsion.55 It is suggested that AR-THF may contain a small amount of impurity compared with HPLC-THF so that phase separation exists between the THF and water. To prove this, we contrasted the morphologies of the nanospheres prepared with two different THF’s, i.e., HPLC-THF and AR-THF, adding the same volume of HPLC-THF after AR-THF at natural volatilization. Figure 6A is the typical TEM image of the

surface region is 0.19, while the O/C ratio of the nanospheres is 0.46. It demonstrates that the hollow nanospheres have a relatively hydrophobic outside surface and a relatively hydrophilic internal surface, probably due to a layer of hydrophobic lignin membrane formed at two-phase interface between water and THF and lignin molecules then gradually aggregating in internal surface of the membrane. Effects of Stirring Rate, Dropping Speed of Water, and pH on the Characteristics of Lignin Hollow Nanospheres. Effects of stirring rate, dropping speed of water and pH on the characteristics of lignin hollow nanospheres were researched at a fixed initial EHL concentration of 0.5 mg/mL. The acquiescent stirring rate and dropping speed of water are 600 rpm and 2 mL/min, respectively. Without stirring, the hollow nanospheres exhibited a smaller diameter in average, but the uniformity is less satisfying (Figure S2). Figure 5A shows that an increase of the stirring rate from 500 to 800 rpm resulted in a decrease of the mean hollow nanospheres size from 480.4 to 353.5 nm. It is attributed that a higher stirring rates causes a greater shear force, leading to a smaller size of hollow nanospheres.53 The typical SEM and TEM images of lignin hollow nanospheres obtained at stirring rates of, respectively, 500 and 800 rpm show that the diameter of the hollow nanospheres was bigger at the stirring rate of 500 rpm than at the stirring rate of 800 rpm, consistent with the results of the DLS (Figure S3). In addition, the SEM images displayed that the diameter of the single hole was bigger at the stirring rate of 500 rpm than at the stirring rate of 800 rpm. Figure 5B exhibits the relationship between average diameter and dropping speed of water. As an increase of dropping speed of water, a decrease of the mean hollow nanospheres size (from 435.8 to 340.1 nm) was observed. When the dropping speed of water was increased to 20 mL/min, the average diameter decreased further (Figure S2). This is mainly because the preparation process of hollow nanospheres via self-assembly approach exhibit the competition of thermodynamics and kinetics.54 When the dropping speed of water increases, the ratio of kinetics in the preparation process increases, leading to the fact that lignin molecules were quickly transformed in a “frozen” state for the formation of lignin hollow nanospheres. Correspondingly, the typical SEM and TEM images of the lignin hollow nanospheres are illustrated at dropping speeds of water of 1 and 4 mL/min, respectively, consistent with the results of the DLS (Figure S4). The effect of the pH value of suspension on the characteristics of lignin hollow nanospheres is shown in Figure 5C. It can be seen that the nanosphere dispersions were stable at pH values between 3.5 and 12. When the pH value of the dispersions was less than 3.5, the average particle size of the nanospheres was gradually increased. This may be due to the

Figure 6. TEM images of lignin nanospheres prepared with two different THF: (A) HPLC-THF and (B) THF of adding the same volume of HPLC-THF after AR-THF at natural volatilize. The sample used here was prepared at predropping lignin concentration of 0.5 mg/ mL, at the stirring rate of 600 rpm and at a dropping speed of 4 mL/ min.

nanospheres prepared with HPLC-THF. It can be observed that the nanospheres exhibited the compact structure. However, the nanospheres prepared with THF of adding the same volume of HPLC-THF after AR-THF at natural volatilize exhibited the hollow structure (Figure 6B), suggesting that ARTHF containing a small amount of impurity is the reason for forming the hollow structure. In addition, Tyndall phenomenon was found in the mixture formed by adding the same volume of HPLC-THF after AR-THF at natural volatilization and water (Figure S5c). By further determination of the compositions of 2277


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Figure 7. TEM images of the samples obtained from the dispersions at different water contents. The predropping lignin concentration was 0.5 mg/ mL. Water content: (A) 0 vol %, (B) 20 vol %, (C) 40 vol %, (D) 50 vol %, (E) >80 vol %, after removing THF. (G) Schematic representation of formation process of the lignin hollow nanosphere in THF−H2O.

AR-THF by GC-MS, it was found that AR-THF contained some impurities, such as toluene, trimethylphosphine oxide, butylated hydroxytoluene, and triethyl citrate, compared with HPLC-THF (Figure S6). They may lead to less polarity of ARTHF so that phase separation exists between AR-THF and water, which forms a nanoemulsion system. Finally, the nanospheres with hollow structure is produced by means of nanoemulsion soft template. The formation process of the lignin hollow nanospheres is presented in Figure 7A−E. In state A, lignin was completely dissolved in THF, so no obvious particles could be seen on the corresponding TEM images (Figure 7A). When deionized water was dropped into the lignin/THF solution, phase separation took place, that is, continuous phase (THF) and dispersed phase (water). In state B, with water content added to 20 vol %, some lignin molecules with a stronger hydrophobic ability formed a layer of membrane at two-phase interface between water and THF, leading to the fact that the water was wrapped. Dynamic equilibrium between continuous phase and dispersed phase was formed.56 When the sample was dropped onto copper grids, the hollow spheres were formed with the evaporation of water (see the corresponding TEM image in Figure 7B). Further increase of water content would bring about an increase of pressure gradient between the inside and outside of the membrane so that the dynamic equilibrium was destroyed.57 When water content was 40 vol %, phase inversion took place in order to reduce in pressure gradient, and the membrane was broken (Figure 7C). The phase which was previously continuous became dispersed, leading to the fact that the relatively hydrophobic membrane was formed again. Therefore, the lignin−THF solution was wrapped by the membrane. As water content further increased, more and more water permeated through the membrane, leading to more lignin molecules aggregating on the internal surface. Moreover, pressure gradient inside and outside gradually increased so that the single hole could appear at the thinner side.58 The lignin hollow nanospheres with the single hole could be seen at a water content of 50 vol % in Figure 7D. The lignin hollow nanospheres were formed completely at a water content of 80 vol %; then, the THF in the solution was removed by the dialysis process. The stable lignin hollow nanospheres could be seen in the Figure 7E. The driving force for self-assembly consists of hydrogen bonding, van der Waals forces, electrostatic forces, and π−π interactions.59−62 The driving force of lignin molecules assembly is considered π−π interactions, which is demonstrated by UV−vis absorption spectra (Figure 8). The absorption peak at 275 nm, which belongs to the π−π transition of the

Figure 8. UV−vis absorption spectra of EHL and lignin hollow nanospheres (concentration 0.1 mg/5 mL). The nanospheres were obtained at the initial EHL concentration of 0.5 mg/mL, at the stirring rate of 600 rpm and a dropping speed of 2 mL/min.

characteristics of guaiacyl structural,63 was observed for the raw lignin in deionized water and THF, respectively. It is indicated that the peak position of the absorption spectra is not affected when the blank changes. The absorption peak of lignin hollow nanospheres in water is red-shifted to 284 nm, while the redshifted absorption peak returns to 280 nm after redissolving in THF. This demonstrated the existence of π−π interactions between lignin molecules in the formation process of lignin hollow nanospheres. Based on the above controlled experiments, a possible formation mechanism for the lignin hollow nanospheres is proposed, as shown schematically in Figure 7G. Lignin can be completely dissolved in THF. By adding water, lignin molecules with a stronger hydrophobic will form a layer of membrane at two-phase interface between water and THF, leading to the fact that the water was wrapped. Increasing water content will bring about an increase of pressure gradient between the inside and outside of the membrane so that phase inversion takes place. Lignin THF solution is wrapped by the membrane. As water content further increased, more and more water molecules permeate through the membrane, leading to the fact that more and more lignin molecules aggregate at the internal surface of the membrane by a layer-by-layer self-assembly method based on π−π interactions. Finally, the thinner side of the membrane will fracture to maintain stable pressure inside and outside. Therefore, it is reasonable to believe that a pressure difference between the inside and outside of the membrane leads to the formation of the hole in the shell of each hollow nanosphere.58 The primary holes are formed at the thinnest part of the micelle walls. With increasing predropping lignin concentration, more lignin molecules participate in the formation of the micelle walls so that the thickness of the micelle walls increase leading to a decrease of the diameter of the single hole. 2278


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FINT2014K01). Moreover, we thank Dr. Fengqin Dong from Institute of Botany of the Chinese Academy of Sciences for help with TEM observation.

Compared to the template method, i.e., direct polymerization and cross-linking reactions occurring at the surface of the template and then the complete removal of the template,13 the self-assembly method is very simple, convenient, and avoids the introduction of impurities and is therefore suitable for modern chemical preparation. We hope this novel method could contribute to the development of new strategies to fabricate nanolignin materials with novel structures and functions.

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CONCLUSIONS Lignin hollow nanospheres with a single hole can be obtained from renewable lignin by self-assembly method, which included dissolving EHL in THF of analytical-grade purity and afterward dropping deionized water to the lignin/THF solution. The nanospheres formed hollow structure due to the effect of THF levels. With increasing of the initial lignin concentration, the diameter of the nanospheres and the thickness of shell wall increased, while the diameter of the single hole, the surface area, and the pore volume of the nanospheres decreased. The surface area reached the maximum value (25.4 m2 g−1) at an initial lignin concentration of 0.5 mg/mL in setting concentration range. Increasing the stirring speed or dropping speed of water resulted in a decrease of the diameter of the hollow nanospheres. The nanospheres formed a relatively hydrophobic outside surface and a relatively hydrophilic internal surface via a layer-by-layer self-assembly method from outside to inside after phase inversion based on π−π interactions, and the chemical structure of lignin did not produce a significant change in the preparation process of lignin hollow nanospheres. Moreover, no apparent change of the average diameter of the nanospheres was observed after 15 days, and the nanosphere dispersions were stable at pH values between 3.5 and 12. The lignin hollow nanospheres with a single hole offer a novel route for a value-added utilization of lignin and would improve the biorefinery viability.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02585. AFM image of lignin hollow nanospheres, SEM and TEM images of lignin hollow nanospheres under different conditions, at different stirring rates, and different dropping speeds of water, photo of the THF− water solution illuminated with laser, and total ion chromagtogram of impurities in THF (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Fuquan Xiong: 0000-0001-7289-198X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from National Nonprofit Special Fund for Fundamental Research from Chinese Academy of Forestry (CAFYBB2016ZX002 and CA2279


Research Article

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