Functional Lignin Nanoparticles with Tunable Size and Surface
Properties: Fabrication, Characterization, and Use in Layer-by-Layer
Assembly Niloofar Alipoormazandarani, Tobias Benselfelt, Luyao Wang, Xiaoju Wang, Chunlin Xu, Lars Wågberg,
Stefan Willför, and Pedram Fatehi* Cite This: ACS Appl. Mater. Interfaces 2021, 13, 26308−26317
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* sı Supporting Information ABSTRACT: Lignin is the richest source of renewable aromatics and has immense potential for replacing synthetic chemicals. The limited functionality of lignin is, however, challenging for its potential use, which motivates research for creating advanced functional lignin-derived materials. Here, we present an aqueous- based acid precipitation method for preparing functional lignin nanoparticles (LNPs) from carboxymethylated or carboxy- pentylated lignin. We observe that the longer grafted side chains of carboxypentylated lignin allow for the formation of larger LNPs. The functional nanoparticles have high tolerance against salt and aging time and well-controlled size distribution with Rh ≤60 nm over a pH range of 5−11. We further investigate the layer-by-layer (LbL) assembly of the LNPs and poly(allylamine hydrochloride) (PAH) using a stagnation point adsorption reflectometry (SPAR) and quartz crystal microbalance with dissipation (QCM-D). Results demonstrate that LNPs made of carboxypentylated lignin (i.e.,
PLNPs with the adsorbed mass of 3.02 mg/m2) form a more packed and thicker adlayer onto the PAH surface compared to those made of carboxymethylated lignin (i.e., CLNPs with the adsorbed mass of 2.51 mg/m2). The theoretical flux, J, and initial rate of adsorption, (dΓ/dt)0, analyses confirm that 22% of PLNPs and 20% of CLNPs arriving at the PAH surface are adsorbed. The present study provides a feasible platform for engineering LNPs with a tunable size and adsorption behavior, which can be adapted in bionanomaterial production.
KEYWORDS: lignin nanoparticle, PAH, film, SPAR, QCM, adsorption, interface
■INTRODUCTION Global environmental and climate challenges call for renewable alternatives to replace fossil-based materials used in various industries. Lignin, extracted from lignocellulosic biomass, is the richest source of renewable aromatics and contains numerous functional groups, such as aliphatic and phenolic hydroxyl groups, which promote the functionalization of lignin for producing lignin-derived materials. Thus, it carries immense potential as a sustainable feedstock for replacing synthetic chemicals.1,2
Despite the progress in lignin production, the efficient conversion of lignin into value-added materials is still challenging due to its heterogeneous structure and poor miscibility with other polymeric materials.3,4 To address this problem, lignin nanoparticles (LNPs) have been recognized as starting materials to form functional materials. These emerging materials have been implemented in different applications, such as UV barriers,5 nanocomposite reinforcement,6 drug delivery and release,7,8 or adsorbent for methylene blue and rhodamine dyes,9 phosphates,10 nickel(II) and cadmium(II) ions,11 and proteins.12 Compared to the other biodegradable polymers, LNPs may have broader use in biomedical, emulsion, cosmetic, and composite applications due to their higher anti-UV and antioxidant properties as well as better miscibility with other polymers.13 Lignin nanoparticles are reported to have a size range of 5−500 nm and mostly uniform spherical, hollow, and square shapes.14 Among them, the spherical shape LNPs is preferred, as the spherical-shaped particles do not have sharp edges, which protect particles from breakage during composite, adhesive, or coating formulation processes.13 In this work, we focus on the formation of LNPs with a uniform spherical shape.
LNPs are fabricated through precipitation procedures initiated by acids, solvent exchange, water-in-oil micro- emulsion, or sonication methods.15 Among the mentioned techniques, acid-induced precipitation in ethylene glycol,16 sodium p-toluenesulfonate,17 or acetone18 is the most cost- Received:
February 22, 2021 Accepted:
May 17, 2021 Published: May 27, 2021 Research Article www.acsami.org
© 2021 American Chemical Society 26308 https://doi.org/10.1021/acsami.1c03496
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See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. effective method. Despite this progress, acid-induced precip- itation in aqueous systems is more preferred by the industry due to its simplicity and limited environmental footprint. In this study, we present an aqueous acidification system to produce LNPs.
In addition, the generation of functional LNPs was reported in the past. In one study, lignin nanoparticles were produced in a solvent/antisolvent system (acetone and water) and then coated with a cationic polyelectrolyte to produce cationic
LNPs.11 Alternatively, LNPs were generated in a solvent (ethylene glycol) and antisolvent (water) system and then chemically esterified or etherified.12 Similarly, LNPs were generated from lignin-2-(diethylamino)ethyl methacrylate (DEAEMA) polymer following a CO2/N2 switchable dis- persion/precipitation procedure.16 These materials have been reported to act as functionalized nanofillers and surfactants in
CO2/N2 switchable Pickering emulsions.19,20 However, design- ing LNPs with tunable particle size and functionality is a rather unexplored area. Previous investigations suggest that the alkyl chain length of the covalently attached group of polymers plays an important role in their structural arrangement, orientation, and packing behavior.21−24 To address the proposed research gap, we present a new method for fabricating LNPs from carboxymethylated or carboxypentylated lignin.
The development of polyelectrolyte layer-by-layer assembly has been established as a method to alter the interfacial properties of materials in different systems.25 As such, poly(allylamine hydrochloride), PAH, has extensively been used for improving paper strength,26 coating nanoparticles,27 and preparing xyloglucan28 and xylan derived coating and binder formulations.29 However, there is limited understanding of the key parameters controlling the formation of the nanostructured films constituting PAH and functional LNPs.
In this case, the surface properties and the adsorption/ interaction behavior of functional lignin nanoparticles with the different grafted alkyl side chains have not yet been studied.
Considering the sparse available literature, we have inves- tigated the influence of LNPs produced from carboxyalkylated lignin with different carbon spacers (i.e., chain lengths) and studied their adsorption behavior at solid−liquid interfaces in layer-by-layer assembly. This knowledge extends the applica- tion range and the commercial utilization of LNPs in different fields, such as surface coating formulations where wet adhesion and interfacial bonding of nanomaterials are of great importance.27
■EXPERIMENTAL DETAILS Materials and Methods. Analytical grades of sodium chloride (NaCl), sodium dodecyl sulfonate (SDS), poly(diallyl dimethyl- ammonium chloride) (PDADMAC), sodium nitrate, uranyl acetate, poly(ethylene oxide), sodium hydroxide (NaOH), endo-N-hydroxy-5- norbornene-2,3-dicarboxylic acid imide (eHNDI), chromium(II) acetylacetonate, dimethylformamide (DMF), isopropyl alcohol, 6- chlorohexanoic acid (CHA), 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxa- phospholane, pyridine, sodium chloroacetate (SCA), dimethyl sulfate
98.0% DMSO-d6, D2O isotopic purity (99.8%), and hydrochloric acid (HCl) were provided by Sigma-Aldrich Co. Cellulose acetate membrane dialysis tubes with a molecular weight cutoffof 1000 g/ mol were supplied by Spectrum Laboratories. Inc. HPLC grade water was produced by a Milli-Q water purifier and used in all experiments.
Silicon wafers used for reflectometry measurements were boron- doped with a thickness of 610−640 μm supplied by Addison
Engineering Inc. (single-side polished, San Jose, CA). The wafers were washed with Milli-Q water, ethanol, and water, dried with nitrogen gas, and oxidized at 1000 °C for 1 h to form SiO2, which was followed by hydrophilization in 10 wt % NaOH and finally cleaned with oxygen plasma (PCD 002, Harrick Scientific Corp., Ossining,
NY) for 2 min before use. Silica strips (10 × 50 mm2) were used as reflectometry substrates. QCM-D crystal sensors with a top layer of silicon oxide were supplied by the Q-Sense, Biolin, Gothenborg,
Sweden. Before measurements, the sensors were cleaned with the SDS solution in Milli-Q water (1 wt %) for 5 min at 60 °C and extensively rinsed with Milli-Q water. The sensors were further nitrogen dried and cleaned by a UV/ozone oxidation cleaner (NOVASCAN PSD
Series, digital UV ozone system) for another 10 min before use.
Poly(allylamine hydrochloride) (PAH) with the weight-average molecular weight, Mw, of 100000 g/mol was obtained from Alfa Aesar.
Different protocols have been developed to extract lignin from lignocellulosic biomass.30 Recently, the novel treatment of biomass with the aid of formic acid was used as a techno-economic biorefinery platform to fractionate biomass.31−33 In this work, lignin was extracted via the formic acid treatment of bamboo chips and used in carboxyalkylation reactions.34
Production of Carboxymethylated and Carboxypentylated
Lignin. The scheme of carboxymethylation and carboxypentylation of lignin is shown in Figure S1a. Lignin was carboxymethylated by the
SCA reagent following the method described by Konduri.35 Briefly, 60 mL of lignin solution was prepared at 16 g/L concentration in a three- neck glass flask. The solution was kept in an alkaline medium at pH
11 via adding 2 mL of NaOH (concentration 1 M) at room temperature under constant stirring until fully dissolved. Then, SCA was reacted with lignin at the SCA/lignin ratio of 4/1 mol/mol for
240 min at 60 °C. After completion, the reaction medium was neutralized, and the product was dialyzed for 48 h and freeze-dried.
Following the procedure described for the carboxypentylation of lignosulfonate,36,37 1.5 g of lignin was suspended in a mixture of isopropyl alcohol (45 mL) and 30 wt % NaOH (12 mL) at 25 °C for
30 min. The activated lignin was then reacted with CHA under the condition of 1 mol/mol CHA/lignin for 2 h and 80 °C in a three-neck glass flask while stirring at 300 rpm. The product was extensively washed with ethanol/water (40/10, v/v) and recovered by centrifugation (3500 rpm, 10 min). The precipitated product was dissolved in deionized water (50 mL) and purified by using membrane dialysis tubes for 2 days and dried in the oven at 80 °C.
Characterization of Carboxymethylated and Carboxypen- tylated Lignin. The molecular weight of the lignin was determined after acetylation with acetic anhydride38 and by gel permeation chromatography (GPC) with a low-temperature evaporative light scattering detector (LT-ELSD, Shimadzu, Japan) according to the procedure described by Zhang et al.39
The charge density, carboxylic acid, and molecular weight of samples were determined to be discussed in the Supporting
Information (Table S1).
Quantitative 31P NMR spectroscopic analysis of unmodified lignin, carboxymethylated lignin, and carboxypentylated lignin was also performed via phosphitylation (Figure S1b) as reported in the
Supporting Information.
The 1H NMR and 1H−1H COSY NMR spectroscopy analyses of lignin were also performed in D2O at room temperature via an
INOVA-500 MHz instrument (Varian, USA). Approximately 35 mg of unmodified, carboxymethylated, or carboxypentylated lignin was dissolved in 500 μL of D2O. The acquisition time was set to 3.983 s and 16 scans with 128 increments and 1 s relaxation time delay.
Furthermore, 0.05 g of oven-dried samples was used in Fourier transform infrared spectrophotometer analysis using a Bruker Tensor
37 (Germany, ATR accessory). The spectra were recorded in a transmittance mode in the range 600−4000 cm−1 with a 4 cm−1 resolution, and 32 scans per sample were conducted.
Functional Lignin Nanoparticle Production (LNP). Lignin nanoparticles were formed following the procedure described by
Zhang et al.6 Briefly, 0.1 g of carboxymethylated or carboxypentylated lignin was mixed with 100 mL of Milli-Q water, and the pH was increased by adding 0.1 M NaOH under constant stirring (300 rpm) until full dissolution was observed. Then, LNPs were formed by acid
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ACS Appl. Mater. Interfaces 2021, 13, 26308−26317 26309 precipitation through the addition of 0.1 M HCl to reach the desired pH value. Optimum properties were achieved at pH 6 in this study.
Therefore, further characterization and adsorption analysis of nanoparticles was conducted at this pH. The carboxymethylated lignin nanoparticles (with the charge density of −1.85 mequiv/g), denoted as CLNPs, and carboxypentylated lignin nanoparticles (with the charge density of −1.91 mequiv/g), denoted as PLNPs, were used in this analysis. Unmodified lignin nanoparticles were labeled as
LNPs.
The reversibility analysis of nanoparticles was studied via alkalizing initially acidified nanoparticles at pH < 4 to pH 11 by 0.1 M NaOH and then via acidifying the medium to pH 4 by 0.1 M HCl in four cycles. The particle size of the nanoparticles was measured after every cycle at pH 6.
Characterization of Functional LNPs. The particle size (Rh) and zeta potential (ζ-potential) of lignin nanoparticles were analyzed by the dynamic light scattering (DLS) using a Zetasizer Nano series (Malvern Instruments Ltd., UK), which was equipped with a folded capillary cell. Measurements were conducted in triplicates at a concentration of 0.1 wt %. The morphology of LNPs, CLNPs, and
PLNPs was imaged by using a JEM-1400 Plus transmission electron microscope (TEM, JEOL Ltd., Japan) at 80 kV acceleration voltage.
In this experiment, 5 μL of diluted LNP dispersions (0.1 mg/mL) was applied on a Formvar/carbon-coated copper grid for 3 min, which was followed by staining with 5 μL of uranyl acetate (2 wt %) for 1 min.
The stained grid with a uniform nanoparticle layer was then used for
TEM imaging.
Adsorption Analysis. SPAR Analysis. The amount of adsorbed polyelectrolyte and nanoparticle was determined by using stagnation point adsorption reflectometry provided by the Laboratory of Physical
Chemistry and Colloidal Science, Wageningen University, Nether- lands. The complete description of the method can be found in the report by Dijt et al.,40 and validation of lignin in this model is thoroughly explained elsewhere.41 Also, the applied procedure is explained briefly in the Supporting Information.
In this study, a baseline was formed in Milli-Q water with the same pH and ionic strength as the polyelectrolyte solutions on a clean SiO2 surface for 5 min. PAH (25 mg/L solutions at pH 6 in salt-free and 10 mM NaCl) was subsequently adsorbed on the surface. Then, lignin nanoparticles (25 mg/L with the same ionic strength and pH as PAH) were adsorbed onto the PAH layer after rinsing to form a multilayer model structure (i.e., silicon, SiO2, PAH, LNPs, PAH, LNPs, and water) as described in Figure S2. In this experiment, the adsorption sequence of PAH−rinse−LNPs−rinse was repeated at a flow rate of 1 mL/min under an ambient environment to form four bilayers. The modeling was performed with the aid of Professor Huygens software (Dullware, The Netherlands) supplied with the SPAR instrument. A layered optical model was used for calculating the adsorbed amount of polyelectrolyte from the SPAR data. The refractive index increment (dn/dc) of the PAH, CLNPs, and PLNPs was determined to be
0.225,42 0.193, and 0.195 mL/g, respectively (using a Brookhaven BI- DNDC differential refractometer, USA).
QCM-D Studies. In this work, the adsorption mechanisms of PAH,
CLNPs, and PLNPs were studied in a wet state by using a QCM-D
401, E1 (Q-Sense Inc., Gothenborg, Sweden). Adsorption was conducted by using a clean silicon oxide sensor. CLNPs, PLNPs, and
PAH were prepared at 0.1 g/L solutions, 10 mM NaCl, and pH 6.
Solutions were pumped by a peristaltic pump at the flow rate of 0.15 mL/min, and the temperature was set to 20 °C. Adsorption was initiated by establishing a baseline for 5 min, and then PAH was adsorbed on the sensor surface, which was followed by the adsorption of CLNPs or PLNPs. This procedure was repeated to assemble four layers. Buffer rinsing was conducted after every stage of adsorption, and results from the fifth overtone were presented. The details of
QCM-D principles are described in the Supporting Information.
Trapped Water Mass Fraction and Layer Thickness of Adlayer.
The swelling of adsorbed polymeric films can be calculated by combining the dry mass per unit area, Γdry, obtained by SPAR, and the wet mass per unit area, Γwet, obtained by QCM-D analysis. The mass fraction (x) of trapped water in the film can be estimated from eq 1:42 x
1 dry wet = − Γ Γ (1) To determine the thickness of the film theoretically (Ltheor), the water fraction (ϕ) of the film is determined from ideal mixing law following eqs 2 and 3: x x x (1
) p p s ϕ ρ ρ ρ = + − (2) Ltheor wet ρ = Γ (3) where ρs is the density of the NaCl solution, ρp is the density of the polyelectrolyte, and ρ is the density of the film. In the present case,
PAH and CLNP/PLNPs densities were taken as 1.15 and 1.35 g/cm3 as reported elsewhere.43,44 The density of the wet film is given by eq
4: (1 ) p s ρ ρ ϕ ρϕ = − + (4) Atomic Force Microscopy (AFM). The surface roughness of multilayer film after the deposition of four layers of PAH-LNPs- PAH-LNPs in SPAR was assessed by AFM, MultiMode 8 (Bruker,
Santa Barbara, CA) in a dry state in the air at ambient conditions.
RTESP 150 cantilevers with the spring constant and tip radius given by the supplier were used in the scanAsyst mode. The surface roughness was determined over an area of 1 × 1 μm2.
Error Calculations. All error intervals (x ± Δx) were at least 95% confidence calculated from the standard deviation (Sx) of repeat measurements (N), that is, Δx = Sx × t-value/(N)1/2, where the t- value is obtained from a t-distribution at a confidence level of 95% for
N −1 degrees of freedom.
■RESULTS AND DISCUSSION Characterization of Modified Lignin. Lignin was carboxymethylated and carboxypentylated by SCA and CHA reagents as explained in the Materials section. The hydroxyl groups of lignin samples were determined via 31P NMR, and the 31P NMR spectra of the samples are presented in Figure 1a, while the quantitative data are summarized in Table 1. The modification of lignin resulted in an insignificant change in the aliphatic −OH and some reductions in the total phenolic
−OH groups, indicating the phenolic hydroxyl group of lignin as a substitution site for the carboxyalkylation reaction.
Furthermore, the concentrations of carboxylate groups in carboxymethylated lignin (1.75 mmol/g) and carboxypenty- lated lignin (1.71 mmol/g) were higher than that in unmodified lignin (0.47 mmol/g), demonstrating the success- ful functionalization of lignin.
In the 1H NMR spectra shown in Figure 1b, the peak at 4.70 ppm corresponds to D2O solvent, and peaks at 6−8 ppm are attributed to the protons of the aromatic ring.36,37 The appearance of new signals in the range δ ≈1.40−3.70 ppm (assigned to the hydrogen of A at 3.70 ppm for carboxymethylated lignin or E at 3.70 ppm, C−D at 1.40−
1.55 ppm, B at 2.00 ppm, and A at 2.20 ppm for carboxypentylated lignin) (Figure 1b) confirmed the successful grafting of carboxylate groups to the lignin backbone.
1H−1H COSY NMR was also used for confirming the coupled spins of carboxypentylated lignin (Figure 1c). Several distinct peaks were observed, in which the diagonal peaks centered at F1 ≈F2 ≈4.70 ppm belonged to D2O. The appearance of four cross-peaks at {F1 = 1.55 ppm, F2 = 3.70 ppm}, {F1 = 1.55 ppm, F2 = 2.20 ppm}, {F1 = 1.55 ppm, F2 =
2.00 ppm}, and {F1 = 1.55 ppm, F2 = 1.40 ppm} in carboxypentylated lignin indicated the coupling of H−A, H−B,
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ACS Appl. Mater. Interfaces 2021, 13, 26308−26317 26310
H−C, H−D, and H−E.36 The cross-peak of {F1 = 3.70 ppm,
F2 = 3.70 ppm} overlapped with the diagonal line. The absence of other cross-peaks at 1.40−3.70 ppm confirmed the different chemical structures of carboxymethylated lignin (Figure S1c) and carboxypentylated lignin. Information about the 1H and
1H−1H COSY NMR techniques provides evidence for the carboxymethyl and carboxypentyl groups on the structure of modified lignin samples.
The FTIR spectra of samples are presented in Figure 1d. As seen, all three lignin samples showed a broad band at 3404 cm−1, which is assigned to stretching vibration of phenolic and aliphatic −OH groups.36,37 Other distinctive peaks related to the unmodified lignin characteristics are explained in the
Supporting Information. Also, carboxymethylated and carbox- ypentylated lignin had sharp absorption peaks at 1595 and
1042 cm−1, corresponding to CO and C−O stretching, respectively.36,37 This observation indicated that modified lignin samples contained considerably more carboxyl groups compared to the unmodified lignin. The molecular weight of unmodified lignin (11300 g/mol) was lower than that of carboxymethylated lignin (28700 g/mol) and carboxypenty- lated lignin (29900 g/mol), as listed in Table S1.
Controlling LNP Formation in Aqueous Solution. The preparation of LNPs commonly employs organic solvents such as tetrahydrofuran, ethyl glycol, dimethyl sulfoxide, and acetone.13 For the industrial viability of this technique, we replaced organic solvents with water. In this study, unmodified lignin, carboxymethylated lignin, and carboxypentylated lignin were fully dissolved in an aqueous system in the presence of
NaOH at pH 11. Then, the nanoparticles were formed by acid precipitation through the addition of HCl. The schematic illustration of nanoparticle fabrication is shown in Figure S3a.
The effects of concentration and pH on the stability and size of
CLNP and PLNP dispersion are discussed in the Supporting
Information (Figure S3b,c). As seen in Figure 2a, high dispersion stability was observed at a pH > 4, and the corresponding TEM images of CLNPs and PLNPs in Figure
2a showed their spherical structures (particle size range of 10−
60 nm with an unnoticeable difference for CLNPs and
PLNPs). The acidification, however, changed the appearance of the dispersions from transparent to opaque at pH < 4 (Figure 2a, left), which was attributed to the protonation of carboxylic acid at pH < 4.
The pH-responsive reversibility of the formed nanoparticles is shown in Figure 2b, and the corresponding particle size is reported in Figure S3d. The formation of functional CLNPs and PLNPs remained reversible in the first two cycles with minimal particle size variation. After the nanoparticle formation at pH 4, an increase in the pH from 4 to 11 assisted with the nanoparticle re-formation in the first two cycles, which was mediated by the carboxylic acids. This process became irreversible after two cycles, as observed by the cloudy solution and particle size growth. This is probably due to the loss of nanoparticle functionality via the cleavage of grafted alkyl side chains under strong alkalinity. In the case of
LNPs, the particle size increased in all pH cycles due to their lower charge density and water solubility. In other words,
LNPs remain agglomerated in the pH reversibility analysis, which shows the importance of the carboxyalkylation to control the formation and stability of the lignin nanoparticles. ζ-Potential and Rh of LNP Dispersions. The colloidal stability of LNPs is important for scale-up and utilization of the materials in different applications. Thus, we characterized
LNPs, CLNPs, and PLNPs in terms of ζ-potential and particle size (Rh). The ζ-potential analysis presented in Figure 3a shows that CLNP and PLNP dispersions were stable in the pH range of 5−11, attributing to strong double-layer repulsion
Figure 1. (a) 31P NMR spectra of unmodified, carboxymethylated, and carboxypentylated lignin, (b) 1H NMR spectra of unmodified, carboxymethylated, and carboxypentylated lignin, (c) 1H−1H COSY spectrum of carboxypentylated lignin in D2O, and (d) FTIR spectra of unmodified lignin, carboxymethylated, and carboxypentylated lignin.
ACS Applied Materials & Interfaces www.acsami.org Research Article https://doi.org/10.1021/acsami.1c03496
ACS Appl. Mater. Interfaces 2021, 13, 26308−26317 26311 between particles.18 As pH increased to 11, the carboxylate groups on the surfaces were fully disassociated, resulting in a ζ- potential of −40 mV for CLNPs and −45 mV for PLNPs.
Results highlighted that the negative ζ-potential values increased slightly as the grafted chain length increased from one carbon in CLNPs to five carbons in PLNPs, which is suggested to be due to a lower counterion condensation when charged groups are further apart, as described by the Bjerrum length.45 The ζ-potential of LNPs was lower than that of
CLNPs and PLNPs due to the lower concentration of charged groups (Table 1).
The particle size of the CLNPs and PLNPs remained stable with Rh ≤60 nm in the pH range 5−11 (Figure 3b). At the initial stage of nanoparticle formation, particles started to self- associate to form a nucleus, which was governed by a balance between electrodynamic and electrostatic forces.46 Thus, a high available charge in CLNP and PLNP dispersions resulted in a rather smaller nucleus (Figure 3b, schematic illustration), attributing to the enhanced repulsion between the negatively charged groups.47 Furthermore, longer grafted side chains resulted in less proximity of charged groups, and thus carboxypentylation allowed for the formation of larger nanoparticles. By contrast, the largest Rh of LNPs indicated that the low-charged lignin with a limited concentration of carboxyl groups coiled to form a larger cluster by van der
Waals or π−π interactions when there was less repulsion.48
The aggregation of LNPs was supported by its aggregated fractal structure observed in the TEM image (Figure S4). This is in agreement with the aggregation of lignin derivatives studied by Norgen et al.48
Below pH 4, the initially formed nucleus started to agglomerate in the CLNP and PLNP dispersions. Carboxylate groups are predominantly uncharged and protonated at pH <
4. Thus, the repulsive interactions induced by charged groups are diminished at this stage. This yielded a more compact and dense assembly of the lignin subunit chains,47 promoting lignin association due to van der Waals attractions.
To evaluate the sensitivity of the lignin nanoparticle dispersion to changes in chemical conditions, we studied the size alteration of the nanoparticles in the dispersion at different ionic strengths (Figure 3c). The Rh of CLNPs and PLNPs was
Table 1. Assignment of Different Functional Groups of Lignin Derivatives Determined by 31P NMR Spectra peak chemical shift (ppm) unmodified lignin (mmol/g) carboxymethylated lignin (mmol/g) carboxypentylated lignin (mmol/g)
1 141.7−143.1 syringyl −OH 1.71 1.58 1.60 138.3−140.3 guaiacyl −OH
1.35 1.02 0.88 137.3−138.3 p-hydroxyl −OH 1.64 1.20
1.04 total phenolic −OH 4.70 3.80 3.52 2 145.5−150.4 aliphatic −OH
2.68 2.62 2.64 3 134.0−135.9 carboxylate 0.47 1.75
1.71 Figure 2. (a) Illustration of LNP dispersion and corresponding TEM images of CLNPs and PLNPs obtained at pH = 6. (b) pH-responsive reversibility of nanoparticle formation.
Figure 3. (a) ζ-potential and (b) average particle size of lignin nanoparticles as a function of pH in the absence of salt and (c) particle size as a function of ionic strength at pH 6.
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ACS Appl. Mater. Interfaces 2021, 13, 26308−26317 26312 stable up to 50 mM NaCl (pH 6), and the particles agglomerated at 500−1000 mM NaCl when the surface charges of functional nanoparticles were screened and van der
Waals forces dominated. This observation was also supported by the existence of a clear reduction in the ζ-potential shown in Figure S5a and agrees with the DLVO theory, which is explained by a decrease in the osmotic double-layer repulsion when the bulk salt concentration approaches that in the counterion cloud.49 LNPs also indicated the same stability toward ionic strength variations. The stability of different nanoparticle dispersions was also evaluated as a function of time (Figure S5b), and the results indicated that CLNP and
PLNP dispersions were very stable in water with no significant aggregation after 20 days. However, nanoparticle size increased at pH below 4 after 20 days.
From the results discussed above, it can be concluded that
CLNPs and PLNPs were considerably more stable than LNPs over a wider pH range and were insensitive to variations in pH ranging from 5 to 11 and ionic strength up to 50 mM.
Considering these results and the nonspherical fractal structure of LNPs (Figure S4), we decided to not include LNPs further in this work. By contrast, the excellent colloidal stability of
PLNPs and CLNPs is interesting for many applications, for example, coatings formed by multilayer assemblies, which we discussed in the following section.
PAH and CLNP/PLNP Multilayer Formation. An interesting property of charged nanoparticles is that they can be assembled in the multilayered film by using the layer-by- layer assembly technique. Interestingly, lignin nanoparticles can be used to create coatings with UV-blocking or antioxidant properties. We formed multilayers with the lignin nanoparticles and the polycation PAH, and the assembly was studied by using SPAR and QCM-D techniques. The SPAR data in Figure
4 show that a higher adsorbed amount was observed in an electrolyte concentration of 10 mM NaCl (Figure 4b) compared to that in a salt-free system (Figure 4a). It is well- known that an increase in the salt concentration leads to a more coiled conformation of the polyelectrolyte chain and higher adsorption due to larger charge overcompensation.50
This was previously explained to cause a more pronounced three-dimensional structure of PAH at the interface of SiO2,51 which may partly explain the greater adsorption of CLNPs or
PLNPs on PAH at a higher salt concentration. In addition, the adsorption response of CLNPs/PNLPs on PAH to a higher salt concentration is affected by the lignin nanoparticle size at this ionic strength.
We also noted that the kinetics of PAH adsorption is rather fast, reaching saturation within 100 s upon addition. In this case, smaller and highly charged PAH molecules have a faster adsorption kinetic due to their faster diffusion and easier packing at the surface. CLNPs or PLNPs, on the other hand, exhibited rather slow adsorption at 10 mM salt with saturation times between 200 and 400 s. The large size of these particles results in slower diffusion toward the surface. Furthermore, when the surface started to be saturated, there was a significant repulsion and limited space for the following particles to adsorb, which led to a lower adsorption rate due to required rearrangements of the molecules on the surfaces.52
The multilayer formation presented in Figure 4 also shows different dynamics for CLNPs and PLNPs. PLNPs with five carbon alkyl side chains have a higher adsorption capacity compared to CLNPs with one carbon alkyl side chain even though they have a similar degree of modification. This is in
Figure 4. SPAR adsorption data showing the reflectometry signal (ΔS/S0) for the multilayer formation of PAH (the first and third layer) and CLNP/PLNP (the second and fourth layer) onto SiO2 surface at (a) 0 M NaCl and (b) 10 mM NaCl. The polyelectrolyte concentration is 25 mg/L. AFM images of four multilayers of (c)
PAH/CLNP and (d) PAH/PLNP onto SiO2 surfaces using SPAR at
10 mM NaCl and pH 6.
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ACS Appl. Mater. Interfaces 2021, 13, 26308−26317 26313 agreement with previous reports where polymers having a longer alkyl side chain showed enhanced adsorption proper- ties.24 PLNPs are larger, and hence they are likely to have a softer and more dynamic charged radiance that yields a better attachment to the PAH surface. Thus, the side chain of nanoparticles is an important factor in nanoparticle adsorption.
The morphology of the multilayer was further evaluated by
AFM imaging (Figure 4c,d), which shows a similar structure for CLNPs and PLNPs but with different surface roughness.
The images demonstrated that the PAH/PLNP multilayer with higher adsorption had a higher roughness of 3.3 nm (with the corresponding average height of 18 nm), while the CLNP surface had a roughness of 2.9 nm with an average height of
15.7 nm relating to its lower particle size. Moreover, there are some clusters formed on the surface upon the adsorption of both nanoparticles, indicating aggregation upon drying or aggregation in the vicinity of the surface during the adsorption.
The seemingly low surface coverage is commonly observed for this type of nanoparticle adsorption.53
Quantification of the Adsorbed Amounts in the Adsorbed Layers. We calculated the dry and wet adsorbed amounts using SPAR and QCM-D (Table 2). PAH adsorption onto silica is governed by ion-exchange interactions between charged silanol groups and partially ionized cationic PAH (∼50% protonation at pH 6).54 Likewise, the ion-exchange interactions are mainly responsible for the adsorption of
CLNPs and PLNPs on the PAH saturated surface. Based on the SPAR data, the adsorbed amounts of the first CLNP and
PLNP layers on the first PAH layer are 2.51 and 3.02 mg/m2, respectively. Based on the charge compensation mechanism involved in the adsorption, the theoretical mass required for neutralizing all available PAH charges on the surface is calculated to be 2.59 mg/m2 for CLNPs and 2.50 mg/m2 for
PLNPs based on the adsorbed mass of PAH and the charge density of the LNPs (calculation available in the Supporting
Information). The comparison of required theoretical adsorbed mass to actual adsorbed mass reported in Table 2 explains that the available charges of PAH were compensated for by CLNPs. By contrast, PLNPs induced a significant overcompensation where all PAH were neutralized by the anionic nanoparticles. Thus, alkyl side chain length plays a crucial role in neutralizing the opposite polyelectrolyte charges, and the size leads to a greater overcompensation.
The mass fraction of trapped water in the adsorbed layer is reported in Table 2. PLNPs adsorbed more and had a smaller trapped water mass fraction (xtrapped water = 0.27 ± 0.01), while
CLNPs adsorbed less but contained more trapped water xtrapped water = 0.30 ± 0.02. Therefore, adlayers formed by
PLNPs with a longer alkyl side chain are thicker, are more packed, and contain slightly less water. However, CLNPs with one carbon side chain induced a thinner and loosely bound adsorbed layer with a higher water fraction. Moreover, considering the low theoretical adsorbed layer thickness,
Ltheoretical, and the size of PLNPs and CLNPs, it can be concluded that nanoparticles undergo a substantial conforma- tional rearrangement when depositing on the PAH surface.
Results are also in line with the adsorption of poly(styrene- sulfonate) with the radius of gyration of 30−100 nm, forming a layer thickness of 0.5−2 nm on the amino-functionalized silica surface in SPAR studies.42
Flux of Particles and Rate of Adsorption. The kinetics governed by the transportation of material to the surface was assessed by using SPAR for the first adsorbed CLNP or PLNP layer. To achieve comprehensive information regarding the mechanism of adsorption, a comparison between the initial rate and rate of molecular arrival at the interface (given by theoretical flux, J) is discussed. The flux J (at the stagnation point where there is zero hydrodynamic flow)55 is given by
Dabros and van de Ven56 according to eq 5:
J k D C 1/3 2/3 b ν = × × × (5) The flux J is dependent on the transport coefficient k, kinematic viscosity (v), D (diffusion coefficient of the molecule), and Cb (concentration of molecules in bulk solution). The final flux J is calculated to be 0.0833 mg/(m2 s) for CLNPs and 0.115 mg/(m2 s) for PLNPs (explained in the Supporting Information and Figure S7). The higher flux achieved by PLNPs indicates the existence of a lower energy barrier or a more energetically favorable interaction in the case of longer grafted chains of the arriving PLNP at the interface.57
In the initial stage of the adsorption process, if all arriving particles at the interface adsorb, the initial adsorption rate ( ) t d d
0 Γ will be equal to the flux. Qn (i.e., quality factor) is used to determine this slope ( ) t t d d
0 Γ = according to eq 6:
Q S S t n 0 0 Γ = × Δ = (6) The slope of the initial adsorption is shown in Figure S8.
Slopes of 0.017 mg/(m2 s) for CLNPs and 0.025 mg/(m2 s) for PLNPs were observed and are significantly lower than the final flux J. This implies that approximately 20% of CLNPs and
22% of PLNPs arriving at the PAH interface would be adsorbed in the real scenario. There is no doubt that particle size and nanoparticle packing behavior (including localization/ distribution/reorientation) of adsorbing molecules play im- portant roles.58 This means that the larger PLNPs with longer grafted side chains had a greater affinity toward the PAH- coated surface.
Significance and Novelty. The importance of matching the lignin functions to the target application is frequently undervalued in the literature. In this study, we produced bamboo-based lignin nanoparticles with tunable negative charges and grafted side chain lengths for the first time. We showed that the grafted side chain length influences the assembly of the nanoparticles and their adsorption to the liquid−solid interface. These novel functional lignin nano- particles would offer some economical and environmental advantages applied in coating, glue, and composite material production based on the following features: (1) cost-effective one-step nanoparticle preparation method compared to previous reports,18−20,59 since aqueous acid precipitation rather than solvent systems was used for lignin nanoparticle production, (2) high stability over an extended pH range of
Table 2. Data for the CLNP or PLNP Adsorption onto
PAH-Treated SiO2 Surface at 0.1 M NaCla ΓQCM (mg/m2)
ΓSPAR (mg/m2) Δmtheor (mg/m2) xtrapped water (mg/mg)
Ltheor (nm) CLNP 3.58 ± 0.2 2.51 ± 0.1 2.59 0.30 ± 0.01
2.93 PLNP 4.88 ± 0.1 3.02 ± 0.1 2.50 0.27 ± 0.02 3.96 aAdsorbed mass of PAH was ΓSPAR = 0.55 mg/m2 and ΓQCM = 0.99 mg/m2 and is excluded from the reported values.
ACS Applied Materials & Interfaces www.acsami.org Research Article https://doi.org/10.1021/acsami.1c03496
ACS Appl. Mater. Interfaces 2021, 13, 26308−26317 26314
5−11 (Figure 3b), (3) high tolerance against ionic strength (Figure 3c), (4) control over the size (Figure 3b) and functionality (Table 1) of the nanoparticles, (5) excellent pH- responsive recyclability (Figure 2b), and (6) strong interaction toward polyelectrolytes (Figure 4).
■CONCLUSION We fabricated functional lignin nanoparticles with different grafted chain lengths and sizes via a simple acid precipitation process using carboxymethylated and carboxypentylated lignin.
Carboxypentylated lignin formed larger nanoparticles due to the dynamic properties of the longer alkyl side chain. The nanoparticles demonstrated great pH reversibility response, salt tolerance, and stability at a wide pH range. We showed that these particles can be used in a layer-by-layer assembly using cationic polyelectrolytes. PLNPs resulted in a greater adsorbed amount compared to CLNPs because of the flexibility of the pentyl chain on PLNPs. Adsorption studies illustrated an inverse relation between trapped water and adsorbed mass, with PLNP adsorption of Γ = 3.02 mg/m2 and xtrapped water = 0.27 mg/mg and the CLNP adsorption of Γ =
2.51 mg/m2 and xtrapped water = 0.3 mg/mg. The flux of adsorption demonstrated the dependence of adsorption on the particle size and nanoparticle packing behavior, where ∼20% of arriving nanoparticles were adsorbed on the PAH interface.
The practical benefit of this study is the precise control over nanoparticle size and how they can be tuned for the design of multilayer assemblies. Applications of the functional lignin nanoparticles range from composites to drug delivery where the adsorption behavior of particles is designed to obtain a specific multilayer matrix. Biological compatibility studies and antimicrobial properties of nanoparticles will be performed in future studies.
■ASSOCIATED CONTENT * sı Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.1c03496.
Proposed carboxyalkylation reaction scheme, phosphity- lation reaction, and
1H−1H COSY NMR spectra of carboxymethylated lignin (Figure S1), overview of parameters in the optical model at the interface (Figure
S2); lignin nanoparticle formation, particle size variation as a function of lignin concentration, dispersion stability of LNPs as a function of pH, and particle size of LNPs in pH-responsive reversibility analysis (Figure S3); TEM image of LNPs (Figure S4); effect of salt on zeta potential of nanoparticles, effect of time on hydro- dynamic diameter (Figure S5); frequency change of multilayer CLNPs and PLNPs with PAH (Figure S6); dimensionless α as a function of Reynolds number (Figure S7); SPAR initial adsorption (dΓ/dt) of CLNP and PLNP on PAH (Figure S8); charge density and molecular weight of samples (Table S1) (PDF)
■AUTHOR INFORMATION Corresponding Author Pedram Fatehi −Department of Chemical Engineering,
Lakehead University, Thunder Bay, ON, Canada; State Key
Laboratory of Biobased Material and Green Papermaking,
Qilu University of Technology, Jinan, Shangdong, China; orcid.org/0000-0002-3874-5089; Phone: 807-4343- 8697; Email: pfatehi@lakeheadu.ca; Fax: 807-346-7943
Authors Niloofar Alipoormazandarani −Department of Chemical
Engineering, Lakehead University, Thunder Bay, ON,
Canada; Laboratory of Natural Materials Technology, Åbo
Akademi University, Turku, Finland Tobias Benselfelt −Department of Fiber and Polymer
Technology, Division of Fibre Technology and Wallenberg
Wood Science Center, KTH Royal Institute of Technology,
Stockholm, Sweden; orcid.org/0000-0003-4388-8970 Luyao Wang −Laboratory of Natural Materials Technology,
Åbo Akademi University, Turku, Finland Xiaoju Wang −Laboratory of Natural Materials Technology,
Åbo Akademi University, Turku, Finland Chunlin Xu −Laboratory of Natural Materials Technology,
Åbo Akademi University, Turku, Finland Lars Wågberg −Department of Fiber and Polymer
Technology, Division of Fibre Technology and Wallenberg
Wood Science Center, KTH Royal Institute of Technology,
Stockholm, Sweden; orcid.org/0000-0001-8622-0386 Stefan Willför −Laboratory of Natural Materials Technology,
Åbo Akademi University, Turku, Finland Complete contact information is available at: https://pubs.acs.org/10.1021/acsami.1c03496
Author Contributions The research was conceived by N.A. and P.F. The manuscript was written by N.A. in close collaboration with T.B. and with contributions from the other coauthors. N.A. conducted the experimental analysis in cooperation with L.W. and T.B. X.W.,
C.X., L.W., and P.F. are the principal scientists of this work.
The authors have approved the final version of the manuscript.
Funding The authors acknowledge the Globalink MITACS national research organization, NSERC, Canada Foundation for
Innovation, Canada Research Chairs, Northern Ontario
Heritage Fund Corporation, and Ontario Research Fund programs for supporting this research.
Notes The authors declare no competing financial interest.
■ACKNOWLEDGMENTS The authors acknowledge the Electron Microscopy Laboratory of the University of Turku, Finland, for TEM imaging of this work. This work is also part of the activities within Johan
Gadolin Process Chemistry Centre at Åbo Akademi University and the Department of Fiber and Polymer Technology at KTH
Royal Institute of Technology. Lars Wågberg and Tobias
Benselfelt also acknowledge the Knut and Alice Wallenberg
Research Foundation through WWSC (Wallenberg Wood Science Centre).
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