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Mar 14, 2016 - Totally Green Process with Renewable Resources. Hao Li,. †,‡ ..... lower ρ value. The above results all indicate that the structures formed in.
Research Article pubs.acs.org/journal/ascecg

Preparation of Nanocapsules via the Self-Assembly of Kraft Lignin: A Totally Green Process with Renewable Resources Hao Li,†,‡ Yonghong Deng,*,‡ Bo Liu,‡ Yuan Ren,‡ Jiaqi Liang,‡ Yong Qian,‡ Xueqing Qiu,*,‡ Chunli Li,† and Dafeng Zheng‡ †

School of Chemical Engineering, Hebei University of Technology, Guangrong Street, Tianjin, PR China School of Chemistry and Chemical Engineering, South China University of Technology, Wushan Road, Guangzhou, PR China



S Supporting Information *

ABSTRACT: Kraft lignin (KL), the byproduct from the alkali pulping process in the paper industry, is an abundant renewable resource. It was found that KL could be used to form nanocapsules via self-assembly induced by adding water to an ethanol solution of KL. From the results of various imaging techniques and laser light scattering techniques, the hollow sphere feature of this product was confirmed. It was found that there is a higher percentage of ethanol enriched in the interior of the nanocapsules and that the different KL fractions can be spontaneously distributed in the shells according to their hydrophilic−lipophilic sequence. The π−π interactions between the aromatic rings are considered to be an important driving force in the assembly process of the KL nanocapsules. Moreover, the sizes of these KL nanocapsules can vary in the range of tens to hundreds of nanometers, depending on the preparation conditions, which enhance their flexibility to adapt to the potential applications in various fields. Preparation of KL nanocapsules requires only a simple mix of KL/ethanol solution and water, which is a totally green process utilizing a renewable resource. KEYWORDS: Kraft lignin, Nanocapsules, Self-assembly, Green chemistry, Renewable resources



INTRODUCTION Lignin, like cellulose and hemicelluloses, is a major component of plant materials. After cellulose, lignin is the most abundant renewable resource in nature and comprises 25−30% of the nonfossil organic molecules on earth.1 Most industrial lignin is obtained as a byproduct during the paper pulping process, in the emerging cellulosic ethanol industries and textile industries. In terms of the industrial chemical modification of lignin, the alkali pulping process in the paper industry is the dominant process. Kraft lignin (KL), which is recovered from the black liquor of alkali pulping, makes up the highest proportion in industrial lignin.2 At present, KL serves primarily as a heat source in the alkali recovery process, which caused its utilization efficiency to be very low. In addition, KL could also be used as biochemicals and biofuels after chemical degradation or pyrolysis,3,4 polymer materials by blending,5,6 heavy metal adsorbent, 7 hydrogels, 8 and many kinds of industrial dispersants after chemical modification.9−12 Overall, the applications of KL are mainly concentrated in ordinary industrial fields. Developing its higher value-added applications in the field of nanoscience and technologies is still worth exploring. As is well-known, the preparation and applications of polymer nanocapsules or vesicles has been an attractive field of study in recent years.13−16 Polymer nanocapsules are © 2016 American Chemical Society

nanometer-sized hollow container molecules or molecular assemblies which have a liquid core and a polymeric shell. Several methods to prepare synthetic polymer nanocapsules have been reported, which mainly include templated synthesis17,18 self-assembly,19,20 emulsion polymerization,21,22 core removal of dendrimers,23,24 and a direct polymerization reaction.25 Different preparation methods could give the inner cavity and the shells of the polymer nanocapsules different structures and surface properties in order to satisfy the different demands in various application fields. Because of their intrinsic ability to encapsulate a variety of guest molecules for wide-ranging applications, polymer nanocapsules have attracted a great deal of attention in recent years. Researchers have found that the polymer nanocapsules could offer many interesting applications, including drug delivery and release,26,27 material separation,28,29 microreactors and catalysis,30,31 supercapacitors,32 and medical examination and diagnosis.33,34 Yiamsawas35 used the water-soluble lignin fraction to prepare hollow nanocapsules by interfacial polyaddition in inverse miniemulsions. Tortora36 synthesized oil-filled microcapsules of kraft lignin by first creating an oil-in-water emulsion followed by a Received: September 13, 2015 Revised: March 12, 2016 Published: March 14, 2016 1946

DOI: 10.1021/acssuschemeng.5b01066 ACS Sustainable Chem. Eng. 2016, 4, 1946−1953

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directly without any further treatment. The ethanol used in the experiment was an analytical grade reagent, and the water is ultrapure water. The KL used for preparing the nanocapsules was the ethanol soluble component. The ethanol soluble KL was obtained as follows. First, 1 g of KL was added into in 50 mL of ethanol, and then ultrasound was used to accelerate the dissolution process for 2−3 min. After that, the solution was allowed to sit for 72 h in order to dissolve the KL as much as possible. It could be found that there were some insoluble components at the bottom of the solution. The insoluble components were separated by centrifugation. The supernatant was filtered through a 0.45 μm syringe filter. The filtered supernatant was used as the stock solution to prepare the nanocapsules. The solid ethanol soluble KL samples mentioned in Supporting Information were obtained from the filtered supernatant by rotary evaporation. Similarly, the solid ethanol insoluble KL samples were obtained from the insoluble components by vacuum drying. The characterizations for the ethanol soluble KL and the ethanol insoluble KL were carried out, and the results are shown in the Supporting Information. The functional groups characteristic in these two samples are nearly the same, and the differences for these two samples are mainly in their molecular weight. Only the fractions with the highest molecular weight did not appear in the ethanol soluble KL. Thus, it could be concluded that the ethanol soluble KL assuredly has the typical features of KL. Method for Nanocapsule Preparation. The KL/ethanol solution used for the preparation of nanocapsules was first obtained by diluting the stock solution with ethanol, and the concentration of this diluted KL/ethanol solution was 2.285 mg/mL and tested by the preprepared UV standard curve. Then, 3 mL of the diluted KL/ ethanol solution was added to a serum bottle with a magnet rotor in it. To prepare the nanocapsules, the ultrapure water was dropped to the diluted KL/ethanol solution in the serum bottle. (No pH adjustment was done throughout the experiment.) In the meantime, the solution was churned with magnetic stirring. The addition of water was stopped when the water content of the solution reached 90%. The dropping speed of the water was controlled by a peristaltic pump. The detailed experimental conditions for each kind of nanoshphere is summaried in Table S2. Characterization. The concentration of lignin in the ethanol solution was tested by the preprepared UV standard curve. The UV standard curve was obtained by testing the ethanol soluble KL samples with known concentrations. The UV−vis spectrum measurement was performed with a UV−vis spectrophotometer (UV-2450, Shimadzu Corp., Japan). TEM images were obtained using a JEM-2100HR transmission electron microscope. The TEM samples were prepared by dropping diluted solution onto copper grids coated with a thin carbon film and then dried at room temperature for several hours. AFM images were observed using a Park XE-100 instrument in noncontact mode. SEM observations were conducted on an FEI Nova Nano SEM 430 microscope. The samples for AFM and SEM were prepared by a vertical deposition technique: immersing the Si substrates into the KL nanocapsules solution with a certain angle and pumping out the solution at a very slow speed. After the solution was pumped out, the KL nanocapsules were fixed on the Si substrates. Then, the Si substrates with KL nanocapsules on them were used for testing. Static light scattering (SLS) and dynamic light scattering (DLS) experiments were performed on a commercial light scattering instrument (ALV/CGS-3, ALV GmbH, Germany) equipped with a multi-digital time correlator (ALV-7004) and a solid-state He−Ne laser (JDS-Uniphase, output power = 22 mW, 632.8 nm). A high performance laser−line band-pass filter (Edmund, NT47-494) was placed between the sample solution and the photomultiplier to avoid an overestimation of the molecular weight of lignin due to fluorescence.39 The refractive index increment value (dn/dc) was measured to be 0.3645 mL/g for KL nanocapsules in the mixed solvent of ethanol and water (10% ethanol content) by BI-DNDC (DNDC-2010, WEG, Kerpen, Germany). The temperature for the SLS and DLS measurement was set at 15 °C, and the viscosity of the 10% ethanol−water solution at this temperature was 1.792 mPa·s. The

high intensity, ultrasound-assisted cross-linking of lignin at the water/oil interface. During our recent studies, we were surprised to discover that KL could easily form nanocapsules via self-assembly in the mixed ethanol−water solvent. The most inspiring aspect is that these nanocapsules are obtained by a totally green process with the renewable resource of lignin. To our knowledge, it is the first time that the micellization of KL can be formed directly in green solvents without need for any pretreatment. Moreover, it was also found that the sizes and the shell thickness of these KL nanocapsules can be controlled within a certain range, which enhances its flexibility to adapt to applications in various potential fields. Compared to other nanocapsules, which are prepared by synthetic polymers or obtained by complex methods, KL nanocapsules obtained in this way have many obvious advantages. First, the whole preparation process is very simple and environment-friendly. Not only does it not require any chemical modifications but also it uses just two green solvents. Clearly, the use of ethanol−water solvent mixtures can conform to the principles of green chemistry. Second, the preparation is very inexpensive. For example, the Chinese market price of KL is only about $150−300 per ton, which is competitive with synthetic polymers or other biopolymers. Moreover, ethanol is also an inexpensive solvent, and it could be optionally recycled based on economic calculations or application requirements. Third, the physical and chemical properties of KL itself could also provide some inherent advantages when it is used as the shells of the nanocapsules. Because lignin comes from plants, it has the natural properties of biocompatibility, biodegradability, and low toxicity. Therefore, it has many congenital advantages when used in the life sciences, medicine, biology, agriculture, etc. In addition, lignin has been recently found to possess the property of ultraviolet resistance.37,38 So, we can speculate that if UV sensitive materials are wrapped in the KL nanocapsules, the KL shells could provide excellent protection. On the basis of these advantages, we can expect that the nanocapsules prepared by KL in this method would have wide application prospects. In our previous studies, we have found that KL, after chemically reacting with the acetylating agent, can form solid colloidal spheres in the mixed solvent of tetrahydrofuran and water.39,40 In this work, the newly developed self-assembled structure not only has obvious hollow features but also has the advantages of an easier preparation method, lower cost, more environmental friendliness, and does not need chemical modification, which has practical significance to extend the applications of KL. In this article, we directly prepared the KL nanocapsules via the self-assembly method in a mixed solvent of ethanol and water for the first time and then used transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), dynamic light scattering (DLS), and static light scattering (SLS) to characterize the structure of the KL nanocapsules and the formation process. In addition, the feasibility for controlling the particle size was also studied, which could lay the foundation for the exploitation and utilization of KL nanocapsules in different fields.



EXPERIMENTAL SECTION

Materials. KL was provided by Jinan Shengquan Group Co., Ltd. in Shandong province, China, which was separated from pulping black liquor using acid precipitation. The product from the factory was used 1947

DOI: 10.1021/acssuschemeng.5b01066 ACS Sustainable Chem. Eng. 2016, 4, 1946−1953

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ACS Sustainable Chemistry & Engineering samples with varying concentrations for SLS measurement were prepared by diluting the stock solution with the mixed solvent of ethanol and water (10% ethanol content).



RESULTS AND DISCUSSION Preparation and Characterization. The self-assembly process of KL in solution was first observed by static light scattering. Because the scattered light intensity is proportional to the polymer concentration and the aggregate molecular weight, any change of scattered light intensity should reflect the change of aggregate molecular weight if the concentration is nearly constant.41 Therefore, changes in the scattered light intensity could reflect changes in the molecular weights of the aggregates in the solution during the course of water addition. Figure 1 shows the plot of scattered light intensity against the

Figure 2. Typical TEM (a) and AFM (b) images of the morphologies of the KL nanocapsules. TEM samples were prepared by dropping dilute solution onto the copper grids, and AFM samples were prepared by a vertical deposition technique on Si substrates. The initial KL concentration in ethanol and the dropping speed of water for the nanocapsule preparation were 2.285 mg/mL and 0.117 mL/s, respectively.

side of the sample. Because the TEM image is obtained under dry conditions where the solvent in the interior of the sample would rush out during the drying process, the shells would be broken and leave an “open mouth” here. AFM could reflect the structure of the sample from a three-dimensional view. The AFM image (Figure 2b) of this sample also illustrates similar structural characteristics compared to those in the TEM image. Furthermore, it can be observed that the sample has obvious spatial structure. Although they appear as flattened spheres, it can be inferred that the flattened characteristic was caused in the drying process of a spherical structure. In addition, it can be determined that there is an upturned mouth (marked with a red arrow in the AFM image). However, this upturned “open mouth” only exists under the more crowded condition when there is not enough space in the flank of the capsules. For this condition, the interior solvent could only rush out of the shells from the top side and left an upturned hole there. In spite of the sample having a certain extent of deformation during the drying process, the central concave type of collapse has not appeared in the AFM image. This indicates that this sample could still have the rigidity and spatial stability within a certain degree under the drying process. DSL and SSL can provide a wealth of information about the nanocapsules in solution, hence they were used to characterize the shape and internal structure of the KL nanocapsules. To obtain a more accurate result from static laser light scattering, the sample with a relatively smaller particle size was selected for the measurement. The sample used here was prepared at the initial concentration of 2.285 mg/mL and the dropping speed of 0.581 mL/s; the particle size was smaller than the samples used for TEM imaging. DLS was used to determine the average hydrodynamic radius (Rh) and polydispersity of the nanocapsules in solution. For estimating those parameters, the cumulant method was used to describe the logarithm of the total autocorrelation function as a series expansion, where the first cumulant (Γ) yields the zaveraged diffusion coefficient, and the second cumulant (μ2) is a measure of polydispersity. Rh was obtained from the particle diffusion coefficient based on the Stokes−Einstein relationship. Figure 3a depicts the size distribution curve obtained from the DLS measurement. The z-averaged hydrodynamic radius (Rh) is 63.35 nm with a polydispersity (μ2/Γ2) of 0.015. The SLS was used to determine the weight-average mass of the KL nanocapsules (Mw,p) and the radius of gyration (Rg). The parameters obtained by the extrapolations regarding C and

Figure 1. Scattered light intensity against added water content in the solution. The initial concentration of KL in ethanol was 2.285 mg/mL. The insets were two photos, respectively, for KL in ethanol (left side) and KL nanocapsules in a mixed solvent of ethanol and water (right side). The mass concentration of these two solutions was kept at the same value.

water content for the KL solution, with a starting KL concentration of 2.285 mg/mL in ethanol and a water dropping speed of 0.117 mL/s. It can be determined that the scattering light intensity increases during the course of water addition. This indicates that the KL molecules begin to aggregate gradually. At the same time, the solution appears turbid. The insets in Figure 1 were two photos for KL/ethanol solution and KL nanocapsules solution with the same concentration. It is apparent that the KL/ethanol solution was the clear liquid with higher transparency, while the KL nanocapsule solution appears to have the characteristics of an emulsion. The structure and morphology of typical nanocapsules were first characterized by various imaging techniques, including TEM and AFM. Because the size of the KL nanocapsules would change with the preparation conditions (as described in the following section), only representative nanocapsules (with preparation conditions: initial concentration 2.285 mg/mL, water dropping speed 0.117 mL/s) were selected for imaging in this part. These two imaging techniques can reveal the morphology and structure characteristics of the nanocapsules from different points of view. The electron beam of TEM can penetrate the sample, and so it can reveal information on the internal structure. The TEM image (Figure 2a) shows that there is a hollow interior surrounded by a shell with a certain thickness and that there is often an “open mouth” at the lateral 1948

DOI: 10.1021/acssuschemeng.5b01066 ACS Sustainable Chem. Eng. 2016, 4, 1946−1953

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Figure 3. (a) Distribution of the hydrodynamic radius (Rh) values of the typical KL nanocapsules in the mixed solvent of ethanol and water. (b) Typical Zimm plot by light scattering photometry obtained for typical KL nanocapsules in the mixed solvent of ethanol and water. K is a constant for a given solution and temperature, and k is a constant to spread the plot. The initial KL concentration in ethanol and the dropping speed of water for the nanocapsules preparation were 2.285 mg/mL and 0.581 mL/s, respectively.

Table 1. DLS and SLS Experimental Results for the KL nanocapsules ⟨Rg⟩ (nm) 60.70

⟨Rh⟩ (nm) 63.35

⟨Rg⟩/⟨Rh⟩

Mw,m (g/mol)

Mw,p (g/mol)

⟨Nagg⟩

0.958

1.69 × 10

8.697 × 10

5.15 × 10

3

θ through the Zimm plot analysis (Figure 3b) are summarized in Table 1. The values of ⟨Rg⟩/⟨Rh⟩ can be used to characterize the morphology of the aggregates. Theoretically, for a uniform solid sphere, a thin shelled vesicle, a hyperbranched cluster, and a random coil, the ratios of ⟨Rg⟩/⟨Rh⟩ are 0.774, 1.0, 1.0−1.3, and 1.5−1.8, respectively.42,43 For a polymeric vesicle or a hollow sphere, ⟨Rg⟩/⟨Rh⟩ may be less or greater than 1.0, depending on the thickness and the density of the wall.42−44 The value of ⟨Rg⟩/⟨Rh⟩ estimated for this sample is approximately 0.958 (Table 1), which confirms that the samples are hollow spherical particles in the suspension. The average aggregate number in each nanocapsule is estimated to be 5.15 × 104, based on the equation ⟨Nagg⟩ = Mw,p/Mw,m. The average density of the nanocapsules (ρ) is calculated to be 0.135 g/cm3 by applying the following equation ⟨ρ⟩ = Mw,p/ (NA4π⟨Rh⟩3/3). Compared with the solid colloid spheres prepared by acetylated KL,39 these KL nanocapsules have a lower ρ value. The above results all indicate that the structures formed in the suspension are uniform nanocapsules. The nanocapsules can remain stable in the dispersion medium because of the repulsive electrostatic interactions of the surface charges. No obvious variation was observed from the DLS measurement and the TEM observation after storing the nanocapsule solution at room temperature for weeks. By the combination of imaging techniques and light scattering, the structure characteristics of the KL nanocapsules were clearly revealed. This detailed information could lay the foundation for exploring its applications. Composition Characteristics of KL Nanocapsules. Even though ethanol and water are mutually soluble, we still hold the view that the solvent wrapped in the nanocapsules contains a higher percentage of ethanol than the external environment of the nanocapsules. To prove this viewpoint, we contrast the morphologies of the nanocapsules before and after dialysis treatment. From Figure 7, it could be determined that the larger KL nanocapsules could leave a more obvious rupture feature; thus, the larger nanocapsules were chosen as a typical case to study. The sample used in this part was prepared at the initial concentration of 2.285 mg/mL and a lower dropping

7

⟨ρ⟩ (g/cm3) 4

0.135

speed of 0.0076 mL/s. As we know, the KL nanocapsules were prepared in the water−ethanol mixed solvent. After the nanocapsule was prepared, the sample was divided into two parts. One part was directly dropped on the copper grids to be observed by TEM and SEM. The other part was dialyzed in ultrapure water for 5 days to completely remove the ethanol in the solution and then the sample dropped on the copper grids to be observed by TEM and SEM. Figure 4a is the typical TEM image of the sample without dialysis treatment, and Figure 4b is the typical TEM image of

Figure 4. Typical TEM and SEM images of KL nanocapsules obtained before (a,c) and after (b,d) the dialysis treatment in water. The sample used here was prepared at the initial KL concentration of 2.285 mg/ mL and at a dropping speed of 0.0076 mL/s.

the sample after dialysis. It can be observed that the nanocapsules before dialysis have a longer “tail”, while the nanocapsules after dialysis treatment are nearly without the long “tail” feature. Figure 4, panels c and d are the SEM images for the nanocapsules before and after the dialysis treatment, respectively. Similar to Figure 4a and b, we can also observe the longer or the shorter “tails” corresponding to samples before 1949

DOI: 10.1021/acssuschemeng.5b01066 ACS Sustainable Chem. Eng. 2016, 4, 1946−1953

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ACS Sustainable Chemistry & Engineering and after dialysis. Furthermore, we can find that there is no concave type of collapse on the surface of the nanocapsules, indicating that the interior solvent rushing out of the nanocapsule occurs mainly from a large pore channel and that the shells of the nanocapsule could play a supporting role in its stereo structure. Because of the continuous dialysis, the final concentration of ethanol in the hollow interior of the nanocapsule would gradually decrease. Eventually, the interior ethanol would be completely replaced by water. Because the water inside the nanocapsule has a lower volatility than ethanol, its ability to break through the shells of the nanocapsule is also weaker than that of ethanol. Furthermore, the copper grid is a type of hydrophobic material which causes the spreading property of water on the copper grid to be much weaker than that of ethanol. Thus, when the water rushes out of the shells, it cannot spread to as large an area as ethanol. However, for the nanocapsule without dialysis, the interior ethanol has a stronger volatility and a better spreading ability on the copper grid. Consequently, the ethanol could carry more KL molecules when it rushed out of the shells and finally left a longer “tail” at the lateral side of the nanocapsule. Therefore, by contrasting the morphologies of the KL nanocapsule before and after dialysis, it can be proven that there is a higher percentage of ethanol enriched in the interior of the nanocapsule during the formation of the KL nanocapsules. According to the fact that ethanol is enriched in the interior of the nanocapsule, we can further speculate that the KL fractions distributed in the shells following a certain sequence according to their hydrophilic−hydrophobic properties: the more hydrophobic KL fractions mainly distribute on the inside of the shells, and the hydrophilic KL fractions mainly distribute on the outside of the shells. Because the KL is a type of natural polymer with a wide molecular weight distribution, the hydrophobicitie of different molecular weight fractions are also different from each other. Fortunately, the hydrophilic− hydrophobic differences of KL fractions could happen to provide the possibility for the formation and stability of KL nanocapsules. In the formation process of KL nanocapsules, the ethanol-favoring fractions of KL are preferentially located on the inside of the shells because there is a higher proportion of ethanol in the interior of the nanocapsule, while the aqueousfavoring fractions of KL are preferentially located on the outside of the shells because there is a higher proportion of water in solution. The final result is that different KL fractions can be spontaneously distributed in the shells according to their hydrophilic−lipophilic sequence. This feature is also the only way to make the KL nanocapsule reach the most stabilized state at its lowest energy. Formation Mechanism of KL Nanocapsules. Furthermore, although the method for preparing the KL nanocapsules is similar to that of some other nanocapsules made by block polymers,45 the assembly process for KL nanocapsules has its own unique characteristics. This is mainly caused by the obvious differences between the KL and the block polymers in their molecular structure and their way of aggregating. Previous studies have suggested that lignin is a flat and disk like macromolecule in solution46−48 and that these particles would tend to form aggregates in solution by the π−π interaction among the benzene groups.46,49,50 Thus, it is natural to remind us that the π−π interactions may also play important roles when the KL molecules gathered mutually to form the

nanocapsules. To prove this speculation, further studies were carried by ultraviolet (UV) spectroscopy (Figure 5).

Figure 5. UV−vis absorption spectra of KL and KL nanocapsules in an aqueous dispersion (50 mg/L). The KL nanocapsule in aqueous solution was prepared by a dialyzing procedure. The absorbance at λ = 280 nm was adjusted as similarly to each other as possible.

In the UV spectrum studies, two forms of KL samples were compared. One sample (abbreviated as “KL” in the following) is the ethanol soluble KL molecules which were directly dissolved in water; therefore, they cannot form nanocapsules. The other sample (abbreviated as “KL nanocapsules” in the following) is the KL nanocapsule prepared in the water− ethanol mixed solution. Moreover, before the UV test, the KL nanocapsule was first dialyzed against water for 5 days in order to ensure that the ethanol was completely removed. Thus, the chemical compositions of these two samples are all the same, and the only difference is their forms: one has formed the nanocapsule but the other has not. From Figure 5, it can be observed that the UV absorption characteristics of KL and KL nanocapsules in aqueous solution are quite different from each other, although the chemical compositions of KL and KL nanocapsules are all the same. It directly indicates that the aggregating environments of the aromatic rings are significantly different between KL and KL nanocapsules. In our previous study, it was proven that the aromatic groups of KL take Jaggregation as the typical π−π aggregation manner.50 JAggregation is a type of π−π aggregation in which the orientation of the adjacent aromatic rings is parallel to the longitudinal direction and looks like a head-to-tail arrangement. According to the molecular exciton coupling theory, the enhancement of J-aggregation would lead to a red shift of the UV spectrum; thus, the red shift in Figure 5 indicates that Jaggregation in the KL nanocapsules is enhanced. Enhancement of J-aggregation in the KL nanocapsules showed that the aromatic rings connected with each other in the manner of a head-to-tail arrangement in the nanocapsule. Obviously, this enhanced head-to-tail arrangement is beneficial to help the KL molecules to form the shells of the nanocapsules. In addition, the results of infrared spectroscopy (Figure S4) also demonstrate that the KL nanocapsule sample has a stronger π−π interaction among the aromatic groups than KL does. So, from the results of infrared and ultraviolet spectroscopy, it can be confirmed that the π−π interactions among the aromatic groups played a crucial role in the formation of KL nanocapsules. 1950

DOI: 10.1021/acssuschemeng.5b01066 ACS Sustainable Chem. Eng. 2016, 4, 1946−1953

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nanocapsules to grow freely. Once the “frozen” state has been formed, due to all of the KL single molecules having been used to form the nanocapsules and no longer remaining in solution, the size of the nanocapsules cannot increase over time. Correspondingly, the typical TEM images of KL nanocapsules obtained at four different dropping speeds of water are illustrated in Figure 7. It is obvious that all of the samples with

Size Control of KL Nanocapsules. For the various potential applications of nanocapsules, the particle size required for different fields may be different from each other.51 As an example, researchers have demonstrated that opsonization and subsequent recognition and phagocytosis by macrophages are strongly correlated with the size of the particle.52,53 The particles under 200 nm in diameter display a decreased rate of clearance and thus an extended circulation time compared to those with a larger diameter.54 Therefore, we explored the adjustable property for the particle size of KL nanocapsules. In general, many factors may impact the size of polymer nanocapsules, including the initial concentration of the polymer solutions, the dropping speed of the precipitation agent, additive (salt, acid, and alkali), temperature, etc. For this system, the dropping speed of water (the precipitation agent) is the simplest one to change among the above factors; thus, this factor was selected to explore the possibility of control over the particle size of KL nanocapsules. In this experiment, the influence of the dropping speed of water on the particle size of KL nanocapsules was studied at a fixed initial KL concentration of 2.285 mg/mL. The average sizes of KL nanocapsules were characterized by the average hydrodynamic radius (Rh) obtained from DLS measurements. Figure 6 describes the relationship between Rh and the

Figure 7. Four typical TEM images of the KL nanocapsules obtained at four different water dropping speeds: (a) 0.0076 mL/s, (b) 0.029 mL/s, (c) 0.236 mL/s, and (d) 0.581 mL/s. The initial KL concentration in ethanol was all fixed at 2.285 mg/mL.

different sizes have a hollow structure. It could also be observed that some of the nanocapsules would rupture after drying. Especially for the large-size nanocapsules, they are more likely to develop an obvious longer “tail” on the lateral side. The solvent wrapped in the nanocapsules has a strong tendency to escape from the shells. When the nanocapsules were spreading at the solid surface during the drying process, accompanied by the changing of surface tension, the nanocapsules were more likely to rupture on the junction of gas−liquid−solid three phase (the bottom side of the nanocapsules) where the nanocapsules are in contact with the copper grids. Finally, the solvent wrapped in the nanocapsules would break through the shells formed by KL, and the mark of the broken shells (“tail”) is thus left on the lateral side. The bigger nanocapsules contain more solvent and has thicker shells, so the solvent could take along more KL molecules which formed the shells when they flow from the nanocapsules, and thus the “tail” is much longer. For the smaller nanocapsules, not only do they contain less solvent and have a thinner shell but also their shells have a higher excess pressure because of the smaller radius of curvature. These reasons result in the smaller nanocapsules not easily rupturing or only leaving a shorter “tail”. In addition, the particle size of KL nanocapsules also could be controlled by adjusting the initial concentration of KL in ethanol (Figure S5). The Rh of nanocapsules would significantly increase as the initial concentration increases. By just controlling the dropping speed of water and the initial KL concentration, one can easily regulate the size of KL nanocapsules in a certain range, and these methods are simple to operate and cost-effective. Furthermore, the thickness of the shells would also be influenced by the particle size, which plays a crucial role in controlling properties such as mechanical strength, permeability, rigidity, degradation kinetics, etc.55 Easily adjustable properties for the size and shell thickness of KL nanocapsules laid the foundation for improving its potential

Figure 6. Average hydrodynamic radius (Rh) of KL nanocapsules as a function of the dropping speed of water. The initial KL concentration in ethanol was fixed at 2.285 mg/mL.

dropping speed of water. It can be determined that the dropping speed of water significantly affected the average ridus of the nanocapsules. As the dropping speed of water increases in a range from 0.0076 to 0.2 mL/s, the Rh decreases sharply. After the dropping speed reaches a value of approximately 0.4 mL/s, the Rh of the KL nanocapsules no longer significantly decreases as the dropping speed further increases. This changing regularity is the result of competition between the thermodynamic control and kinetic control during the selfassembly process of KL. The self-assembly process of the macromolecular micellization and its aggregation morphology are mostly driven by thermodynamics, and sometimes they could reach the thermodynamic equilibrium state. However, due to the faster dropping speed of water, there is not enough time to allow the nanocapsules to grow in time. The lack of time would cause the nanocapsules to be kept in a “frozen” state which hindered it from reaching the stable thermodynamic structure. That is to say, there is no time for the 1951

DOI: 10.1021/acssuschemeng.5b01066 ACS Sustainable Chem. Eng. 2016, 4, 1946−1953

Research Article

ACS Sustainable Chemistry & Engineering applications in different fields. For a certain application or purpose, the researchers could select the more suitable preparation conditions for their own studies.

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CONCLUSIONS We have presented a detailed study of the direct preparation of KL nanocapsules in the mixed solvent of ethanol and water, which does not require any chemical modification, template, and core removal. It is a totally green process which uses a renewable resource. The KL nanocapsule obtained by this method has many advantages, such as easy preparation, low cost, biocompatibility, biodegradability, and environmental friendliness. The sizes of KL nanocapsules can be easily controlled in the range of tens to hundreds of nanometers, which laid the foundation for improving its potential applications in different fields. The π−π interactions between the aromatic rings is considered to be a crucial contribution in the assembly process for the nanocapsule, which provide the driving force for KL to aggregate. On the basis of their advantages, KL nanocapsules would have wide application prospects in many fields.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01066. IR spectra and UV-vis absorption spectra of ethanol soluble KL and ethanol insoluble KL; weight-average Mw distributions and molecular weight values of ethanol soluble KL and ethanol insoluble KL; FTIR spectra of KL and KL nanocapsule; and preparation condition for the nanospheres (PDF)



AUTHOR INFORMATION

Corresponding Authors

*(Y.D.) Phone: +86-020-87114722. E-mail: yhdeng08@163. com. *(X.Q.) Phone: +86-020-87114722. E-mail: xueqingqiu66@ 163.com. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS We are grateful for the financial support from the National Basic Research Program of China (973 Program) (2012CB215302), the State Key Program of National Natural Science of China (21436004), the National Natural Science Foundation of China (21374032), and the Outstanding Young Scholars Program in Universities of Hebei Province, China (BJ2016015).



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DOI: 10.1021/acssuschemeng.5b01066 ACS Sustainable Chem. Eng. 2016, 4, 1946−1953

Research Article

ACS Sustainable Chemistry & Engineering

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DOI: 10.1021/acssuschemeng.5b01066 ACS Sustainable Chem. Eng. 2016, 4, 1946−1953