Research Article | Open Access
Generation of Spherical Cellulose Nanoparticles from Ionic Liquid Processing via Novel Nonsolvent Addition and Drying
A novel method to prepare spherical cellulose nanoparticles has been developed using imidazolium ionic liquid processing and regeneration from controlled acetonitrile nonsolvent addition and drying. Nanoparticles ranging from 100 to 400 nm have been prepared with high uniformity. Minimisation of moisture via solvent exchange drying led to discrete nanoparticles, whereas the presence of ambient moisture during regeneration contributed to aggregated morphologies. Chemical analyses of the spherical cellulose nanoparticles reveal a high-amorphous cellulose content. Furthermore, the range of particle sizes achieved with acetonitrile nonsolvent fractionation and solvent exchange drying suggest the size and uniformity of nanoparticle distributions reflect the fractionated cellulose weight fractions. This ionic liquid method is simple, energy efficient, and likely to have wide applicability across other biopolymers as well as potential to prepare surface functionalized spherical cellulose nanoparticles.
Cellulose is a highly abundant natural polymer. Of the 40 billion tons renewed annually, some 200 million tons are used as raw materials in industrial processing [1, 2]. The use of cellulose as a starting material for product development is a viable approach from both an environmental and economical perspective. Native cellulose is strongly resistant to breakdown due to a highly crystalline network structure and, due to its fibrous nature, is found in many applications ranging from textiles and films through to paper products. Over the past two decades, nanocellulose has gained high interest as a potential biomaterial within the industrial and scientific communities. Potential applications of this material range from new kinds of composite materials to uses in medical technology and the food and pharma industries. However, for some nanocellulose products, a high-energy input can be required for the initial defibrillation of the starting materials . Alternative methods which are less energy-intensive to produce nanoscale materials would open up greater utility for nanocellulose materials.
Ionic liquids are organic salts that are liquids at or near room temperature. There has been growing interest in utilising these materials in processing due to their unique properties. This includes processing and regeneration of cellulosic materials which has gained momentum since cellulose films were produced from imidazolium salts by Swatloski et al. in 2002 . The range of ionic liquids for processing cellulose materials has subsequently grown together with the cellulosic materials and applications of regenerated products [4, 5]. Recently, the application of ionic liquid processing of cellulose materials has extended to fractionation of celluloses by molecular weight on regenerating with water as nonsolvent . In related work, it was found that nonsolvent additions of an aprotic solvent can similarly fractionate cellulose [7, 8]. Moreover, choice of nonsolvent may induce differing morphologies of the regenerated materials which range from micron-sized particles to aggregated films. While there are many publications on cellulose nanoparticles, few describe spherical morphologies without hybrids, templating, or chemical modification . Generation of uniform spherical nanoparticles is difficult via mechanical, chemical, or ionic liquid approaches [10–12]. In the current paper, we report a new, facile method to create and isolate spherical cellulose nanoparticles from ionic liquid using acetonitrile nonsolvent addition. This novel regeneration of spherical nanoparticles is a simple process, producing spherical cellulose particles in the range of 100–400 nm. The potential applications of these materials include use in polymer composites, bioplastics, films and foams, implant materials, and biodegradable tissue scaffolds through to drug delivery and membrane preparation via surface modifications.
1-Ethyl-3-methylimidazolium acetate (EMIMAc) and microcrystalline cellulose were purchased from Sigma Aldrich. EMIMAc was vacuum-dried at 68°C for 24 hours before use. A stock solution of cellulose in EMIMAc solution was first prepared by heating EMIMAc (94 g) in a round bottom flask under a nitrogen atmosphere maintained at 80°C with magnetic stirring. Cellulose (e.g., 1.8 g, 2% w/v) was added and dissolved for over 16 hours maintaining a nitrogen atmosphere.
2.2. Regeneration of Cellulose from EMIMAc
For the regeneration method, a 2–8% cellulose solution (30 g) was transferred to a flask, and heat was maintained at 55°C. A nonsolvent (100 mL), either water or acetonitrile, was added to the cellulose solution with stirring. The samples were then centrifuged to recover the precipitate. The precipitates were separated from the filtrate and further washed with the respective nonsolvent up to four times. The recovered precipitates were then washed with water prior to freeze drying.
2.3. Preparation of Cellulose Nanoparticles by Solvent Drying
For the solvent drying method, the above fractionation procedure was followed by adding acetonitrile (100 mL) to a 2% cellulose EMIMAc solution (30 g) with stirring. The precipitated fraction was first washed with acetonitrile by centrifugation, decanted, and then suspended in ethanol and washed four times. After this, the precipitate was then repeatedly washed with acetone and finally washed with diethyl ether three times. In between the washings, sonication was used to suspend the sample in the solvent. After washing with diethyl ether, the gentle abrasion of the isolated material in a dry atmosphere was required to prevent the cellulose molecules from aggregating and bonding.
Scanning electron microscopy (SEM) images were acquired using a Philips XL30S field emission scanning electron microscope, operating at an acceleration voltage of 5.0 kV. The samples were mounted on aluminum studs using adhesive tape and sputter coated with platinum under vacuum using a standard technique.
Fourier transform infrared (FTIR) spectra were recorded using a Nicolet FTIR 8700 spectrometer in the transmission mode. Powdered cellulose samples were mixed with dried KBr and then pellets formed by compressing into discs. FTIR spectra were acquired between 4000 and 400 cm−1 using 64 to 128 scans for each sample.
Solid state 13C NMR spectra were obtained on a Bruker Avance DRX 200 instrument with a 7 mm doubly tuned 1H/X MAS probe (Bruker) at a frequency of 50.32 MHz. Hartmann–Hahn matching was conducted using glycine. Samples were packed into a zirconia rotor fitted with a Kel-F cap and spun at 5 kHz. A standard cross polarisation-magic angle spinning (CP-MAS) pulse program was used with a 1H preparation pulse of 5.56 μs, 1H decoupling field of 47 kHz, and an acquisition time of 20 ms.
X-ray diffraction was undertaken on a Bruker AXS D8 advance diffractometer at 25°C using Cu-Ka (λ = 1.5418 Å) radiation in the 2θ range 5 to 35°. Acquisitions were taken at 0.01° steps using intervals of 0.5 s per step.
3. Results and Discussion
Using either water or acetonitrile as a nonsolvent, each one was added to a cellulose solution in EMIMAc, an imidiazolium-based ionic liquid. Nonsolvent was added to regenerate cellulose into fractions which were dried, either by freeze-drying or solvent exchange methodology. Quantitative precipitation was achieved when water was used as a nonsolvent, whereas two fractions were obtained using acetonitrile. The SEM micrographs of the freeze-dried acetonitrile and water fractions are illustrated in (Figure 1). The cellulose regenerated with water was not of particle morphology being film-like in appearance. The fraction regenerated with acetonitrile had a particle morphology, with spherical micron-sized particles embedded in a gel matrix. Moreover, when a small region of a spherical particle was magnified, SEM revealed this to be aggregated nanoparticles with diameters less than 100 nm (as shown by arrow in Figure 1(b)). This aggregated nanoparticle morphology is comparable to that produced by enzymatic hydrolysis and ultrasonic  or homogenization  processing of cotton fibre.
Recovered fractionated samples were initially analysed to provide a qualitative assessment including chemical characterization with FTIR (Figure 2). A comparison of the original cellulose and spherical cellulose particles spectra reveals an absence of new bands confirming no chemical modification to cellulose. However, the symmetric CH2 bending absorption at 1430 cm−1 was observed to decrease, suggestive of a reduction in the degree of crystallinity of this sample. The C–O–C stretching band at 898 cm−1 evident for the amorphous component  was more intense in the particles than the original cellulose. The greater intensity of this β-(1→4)-glycosidic linkage absorption suggests the spherical cellulose particles had more amorphous cellulose content than the starting material which was further confirmed by 13C NMR and XRD measurements.
Results of 13C NMR and XRD analyses (Figure 3) suggest the fractionated material regenerated with both water and acetonitrile may also retain proportions of ordered cellulose. X-Ray diffractogram intensities indicate that the original cellulose crystallinity was lost during dissolution and regeneration. However, in applying the sensitivity of the C4 (81–93 ppm) and C6 (60–70 ppm) regions to order/disorder in cellulose samples , solid state 13C NMR results reveal the cellulose regenerated with water to have a C4 peak at 87 ppm, whereas the spherical particles have this peak at 83 ppm. Furthermore, in using the comparative assessments of CP/MAS 13C NMR and XRD by Isogai et al. for cellulose polymorphs , C4 signals for cellulose II and cellulose IVII are between 87.8–88.8 ppm and 83.5–84.6 ppm, respectively. Therefore, the NMR results and associated XRD patterns confirm that the cellulose regenerated with water had cellulose II crystalline character and the coalesced spherical particles obtained on acetonitrile nonsolvent addition have some cellulose IVII crystalline character likely induced by their agglomeration.
In comparing the regenerated cellulose morphologies achieved with water and acetonitrile, it was evident the relative polarity of these nonsolvents was a factor in particle generation. In the solid phase, imaging of individual natural cellulose molecules by AFM has revealed differing conformations due to surface polarity . Wan et al. report that individual cellulose chains adopt an extended conformation on a positively charged surface, whereas on a negatively charged surface, a compact globule conformation is adopted . By analogy, in EMIMAC solution, the ionic liquid provides a negatively charged surface for cellulose molecules to adopt a globular confirmation. When regenerated with the aprotic nonsolvent acetonitrile, this produces the observed particles as solvation is decreased. Conversely, when regenerated with water or with excess moisture, hydrogen bonding by the imidazolium ion is affected  which impacts cellulose solvation and may bring cellulose molecules together increasing chain entanglement, reforming of hydrogen bonds and inducing cellulose precipitation.
The above results using acetonitrile nonsolvent addition demonstrate that preparation of spherical cellulose nanoparticles is possible from ionic liquid. To further improve particle separation, cellulose was regenerated using acetonitrile addition to give two fractions , and the fractionated samples then dried by solvent exchange to reduce moisture influences on particle morphology (Figure 1). In this process, each regenerated fraction was first washed with acetonitrile to remove the residual ionic liquid, then with ethanol to remove any acetonitrile, and later with acetone to remove any residual ethanol and finally with ether to dry the sample. This method of drying eliminated the absorption of moisture to a reasonable extent, minimizing any moisture-induced agglomeration and coalescence of particles observed above (Figure 1). The SEM images of regenerated fractions dried by the solvent exchange method reveal visible improvement in the appearance and individuality of cellulose nanoparticles (Figure 4). This attempt to minimize the influence of moisture has led to fraction 1 exhibiting particles in an aggregated state with sizes ranging from 120 to 350 nm. The nanoparticles formed were of different sizes and state, being comparable to those isolated on acid hydrolysis and utlrasonication . In fraction 2, the particles were smaller and more uniform than observed in fraction 1. Moreover, with prior analyses revealing fractionation by molecular weight on nonsolvent addition [6, 7], the range and uniformity of nanoparticle sizes evident in (Figure 4) may suggest these particle distributions reflect fractionated cellulose molecular weights which can be achieved on acetonitrile nonsolvent addition .
This study has revealed cellulose dissolution in EMIMAC solvent and selection of acetonitrile as a nonsolvent for cellulose regeneration creates new potential for the production of spherical nano- and submicron-sized cellulose particles. The amount of moisture present when regenerating cellulose has a great influence on the resulting morphology of the material regenerated. Evident in (Figure 1) was a high rate of nanoparticle agglomeration likely promoted by a relatively high absorbed moisture content compared to nanoparticles produced using a solvent drying regime (Figure 4). Furthermore, we believe that nanoparticle aggregation can be avoided by working in isolation of any ambient moisture and results further improved by using more dilute cellulose solutions and performing both dissolution and regeneration under a strict anhydrous environment. In developing this new method to generate cellulose nanoparticles, it will be scalable and applicable to other plant polysaccharides or biopolymer systems also. Moreover, as the surface properties of nanocellulose affect its processability , these results offer the potential for differing types of modifications resulting in surface-functionalized cellulose nanoparticles which may be advantageous in different types of applications.
The data provided in this manuscript are freely available through a PhD thesis at the University of Auckland (URL: https://researchspace.auckland.ac.nz/handle/2292/5949).
Aspects of this article were highlighted at the BIOPOL 2013 conference in honour of the late Prof. Allan Easteal who was a colleague, supervisor, and contributor to this study.
Conflicts of Interest
The authors declare no conflicts of interest in publication of this study.
The authors wish to acknowledge the Biopolymer Network Ltd. for support for this work funded through the New Zealand Ministry of Business, Innovation and Employment. This supported a University of Auckland PhD stipend (KK) and thesis titled “Modifications of Cellulose using Ionic Liquids” (2010).
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