Patent Application
20200062865
This disclosure relates to spherical crystalline cellulose nanoparticles (SCCNPs) and a process for producing same from cellulosic materials.
Cellulose is a polysaccharide consisting of a linear chain of β(1→4) linked D-glucose units with a molecular formula as (C6H10O5)n. Hydroxyl groups of cellulose are involved in a number of intra- and intermolecular hydrogen bonds, resulting in various ordered crystalline arrangements (Scheme 1).
In scheme 1, n represents the degree of polymerization (DP). The n value is in the range of 300 to 700 units. DPs of cellulose materials vary depending on the source, production process, and treatment. DP values range from 100-3000 for commercial celluloses to 20,000 for cotton fiber secondary walls to 44,000 for Valonia (a species of algae).
The beta-1,4 glycosidic bonds per se are not too difficult to break. However, owing to intra- and intermolecular hydrogen bonds, cellulose can form very tightly packed crystallites to prevent the penetration of chemicals, enzymes, and even water. As the most abundant polymer, cellulose is present in plants, tunicates, algae, and some bacteria. Wood has 40-50% cellulose, compared to 90% for cotton fibers and 99% for bacterial cellulose produced by Acetobacter xylinum or Gluconacetobacter xylinus.
Cellulose fibers are formed by microfibrils; flexible hair strands composed of elementary fibrils. Such elementary fibrils formed during cellulose biosynthesis consist of 30 to 100 aggregated cellulose chains. Thus, elementary fibrils (also known as nanofibrils or nanofibers) are about 2-20 nm in diameter and a few micrometers in length with two distinct regions: crystalline and amorphous where the cellulose chains are arranged in highly ordered and disordered (amorphous) structures.
When subjected to acid hydrolysis (Revol et al., Int. J. Biol. Macromol., (1992) 14, 170-172,) or strong oxidation, particularly with ammonium persulfate (Leung et al., Small, (2011) 7(3), 302-305, 2011; Lam et al., Sustainable Chem. Eng., (2012) 1(2), 278-283,), cellulose microfibrils undergo transverse cleavage along the amorphous regions, resulting in the formation of crystalline nanocellulose crystals (CNCs) with a rodlike shape. CNCs have been attempted for diversified applications, and different production methods have been described in the literature.
The synthesis of spherical cellulose particles has been reported by Zhang et al. Carbohydrate Polymers, (2007) 69, 607-61. This procedure involves two concentrated acids (12.1 N HCl and 36.0 N H2SO4) and ultrasonication at 80° C. for eight hours, followed by centrifugation with continuous washings using deionized water, and neutralization with 2.00 N NaOH. The neutralized product is further washed with water, dialyzed and freeze-dried. The procedure results in cellulose nanospheres with sizes ranging from 60 nm to over 570 nm and the hydrolyzed nanocelluloses are predominantly cellulose II polymorphic crystalline structure.
One aspect of the disclosure relates to a process for producing spherical crystalline cellulose nanoparticles (SCCNPs) comprising contacting a cellulosic material with an effective amount of a reagent comprising potassium peroxomonosulfate (KHSO5).
One aspect of the disclosure relates to a nanosized cellulose particle, wherein the said particle is substantially spherical crystalline cellulose nanoparticles (SCCNPs) and said SCCNPs have an average diameter of about 3-10 nm or less as assessed by transmission electron microscope (TEM) micrographs.
One aspect of the disclosure relates to a substantially spherical crystalline cellulose nanoparticles (SCCNPs) prepared by the process as defined herein.
The present disclosure is illustrated with reference to the accompanying drawings, in which:
The present disclosure describes a process for producing substantially spherical crystalline cellulose nanoparticles (SCCNPs) by contacting a cellulosic material with a sufficient amount of the oxidizing agent.
The process is conducted in an aqueous medium. The reagent is dissolved in the aqueous medium (preferably in water alone) at a concentration ranging from about 0.20 M to about 1.25 M, or alternatively is 0.25 to 1.2 M, with the resulting pH of below about 2. The preferable concentration of the reagent is about 1 M with a corresponding pH of about 1.
The concentration of the cellulosic material in the aqueous medium can range from about 0.1 to about 2.5 wt/vol %, or about 0.2% to about 1 wt/vol %, or preferably is about 0.5 wt/vol % or higher. The amount is not over about 2.5 wt/vol % due to the gelation of the cellulosic material when the aqueous medium is subject to a temperature ranging from about 45-80° C.
The preferable ratio of the cellulosic material to the reagent (weight percentage) is about 1:1 to about 1:10; about 1:2 to about 1:10; 1:2 to 1:5 or 1:2 to 1:3.
The process is preferably conducted at a temperature, ranging from about 40° C. to about 80 ° C. with constant stirring. The preferable temperature is 60 ±5° C.
The process is preferably conducted at a temperature, ranging from about 40° C. to about 80° C. with constant stirring. The preferable temperature is 60±5° C.
The preferable contacting time between the cellulosic materials with the oxidizing agent is from about 8 hours to about 16 hours. The time may be lower, for example, 2-4 hours, if the temperature and/or Oxone® concentration is (are) above the minimum amounts described in the ranges previously described.
The resulting spherical crystalline cellulose nanoparticles (SCCNPs) may be recovered by any suitable method, including centrifugation, filtration and/or settling and decanting.
The resulting SCCNPs may further be dried by vacuum, freeze drying, or oven drying to form dried SCCNPs.
The present disclosure also produces carboxylic acid groups on the surface of spherical crystalline cellulose nanoparticles (SCCNPs). During the oxidation step, selective oxidation preferably occurs at the C6 primary hydroxyl group of the glucose ring to form carboxylic acid groups with a degree of oxidation (DO) of from 0.005 to 0.010 or 0.04 to 0.10.
Spherical crystalline cellulose nanoparticles SCCNPs have an average diameter of about 3-10 nm or less as assessed by transmission electron microscope (TEM) micrographs, depending the contacting time. As prepared SCCNPs with carboxylic groups can be neutralized with sodium hydroxide, potassium hydroxide or ammonium hydroxide to enhance water solubility and dispersion.
Spherical crystalline cellulose nanoparticles (SCCNPs) exhibit a similar crystallinity index (CRI), compared to microcrystalline cellulose, one of the substrates used for the preparation of SCCNPs. Advantageously, the CRI may be at least 10% greater than the CRI of the starting cellulosic material such as carton board and wood pulp.
In brief, the present process describes a procedure for producing SCCNPs with substantially similar or enhanced uniformity and crystallinity compared to the starting raw material. In contrast, both acid hydrolysis and ammonium persulfate oxidation produce rodshaped like materials. The present disclosure presents a new methodology of green chemistry, which uses an inexpensive reagent, and may even use, in certain embodiments, water as the sole aqueous medium, for the reaction and purification steps. The reaction is carried out at a temperature no higher than about 80° C. The waste product, potassium sulfate, resulting from the process, is a non-flammable white crystalline salt that is commonly used in fertilizers, providing both potassium and sulfur.
Spherical crystalline cellulose nanoparticles (SCCNPs) produced by the present process are carboxylated, which renders them more water soluble and amenable to bioconjugation for the synthesis of bionanocomposites. Biodegradable SCCNPs with biocompatibility, non-toxicity, and renewability will foster a plethora of diversified applications such as drug delivery, biosensing/bioimaging, pharmaceutical formulation, cosmetics, food, textiles, aerogels, etc. Cellulose nanoparticles degrade faster than metallic nanoparticles or carbon-based materials such as fullerenes and carbon nanotubes. SCCNPs are expected have low toxicity and low environmental risk, which are significantly lower than those for carbon nanotubes and other fibers. Thus, their applications for biomedical applications are promising towards the production of implants, face masks, drug delivery, cell carriers, artificial blood vessels, etc. They may be decorated with antimicrobial agents for applications in wound dressing, bandage, and hygienic products.
As used herein, the expression “cellulosic material” refers to suitable cellulose fibers with high initial cellulose contents such as MCC, cotton fibers, bacterial cellulose, etc. can be used. The use of MCC is a particular challenge due to its high crystallinity, which is less vulnerable to chemical or enzymatic attacks In this disclosure, any cellulose-based materials can be used as starting material, (e.g., Avicel) with different particles sizes or native cellulose fibers to produce nanosized cellulose particles.
As used herein, the reagent is comprising potassium peroxomonosulfate (KHSO5). Oxone®, for example, is a tri-salt comprising KHSO5.0.5 KHSO4.0.5 K2SO4 (MW=307.38), has the active component potassium peroxomonosulfate (KHSO5) as the oxidant. Oxone® is soluble and stable in water. At 20° C., the solubility of Oxone® in water is 277 g/L or 0.9 M. At 60° C. (140° F.), the solubility of Oxone® is about 387 g/L or 1.26 M.
In one embodiment, the reagent is comprising the tri-salt KHSO5.0.5 KHSO4.0.5 K2SO4.
In one embodiment, the reagent is consisting essentially (or consisting) of the tri-salt KHSO5 .0.5 KHSO4.0.5 K2SO4.
As used herein, the expression “aqueous medium” refers a process reaction medium comprising water. In one embodiment, the aqueous medium is water as the sole component.
Oxone® monopersulfate (KHSO5.0.5 KHSO4.0.5 K2SO4) and Avicel® PH-101 (20 to 50 μm in diameter) were obtained from Sigma-Aldrich. Avicel® has been known as microcrystalline cellulose with high crystallinity.
Preparation of crystalline cellulose nanoparticles. Avicel® PH-101 (0.2-1 g) was added to a 1 M Oxone® aqueous solution (100 mL, pH 1). The suspension was heated to 60° C. with vigorous stirring for up to 14-16 h to give a white suspension. The suspension was centrifuged (10,000 rpm) for 15 min. The supernatant was decanted, and about 50 mL of deionized water was added to the pellet, followed by 5 min of vigorous mixing. The centrifugation/washing cycles were repeated 5 times until the solution pH of 7 was reached. The product was placed in a vacuum chamber overnight to give a white powder. During the treatment of Avicel® with Oxone®, selective oxidation was effected at the C6 primary hydroxyl group of the glucose molecule subunit to form carboxylic acid groups. The Oxone® was effective in cleavage of the glycosidic bonds and etching out individualization of elementary fibrils to form crystalline cellulose nanoparticles.
Transmission electron microscopy (TEM) was performed by the Transmission Electron Microscope, JEM-1400, JEOL, equipped with a Bottom CCD 2×2k camera. For TEM imaging, the particle-containing sample was dispersed in deionized water and subject to bath sonication. Three small droplets from the resulting suspension were then applied on a carbon-coated copper TEM grid and vacuum dried in a covered plastic dish.
The X-ray diffraction (XRD) pattern was probed by a Bruker D8 Advance or with Philips PW1050 X-ray diffractometer (Cu Kα radiation, operating at 40 kV/30 mA with a 0.0019 step size and a 0.5 s step). The collected XRD data were analyzed using FullProf to provide peak position (2θ, 2theta), FWHM (full-width half maximum), peak deconvolution, and integration intensity for the estimation of the crystallinity index (CRI). Crystallite sizes were determined by the Scherrer equation as K λ/(FWHM. cos θ) with the Scherrer constant (K) 1 and λ=1.542 A. The dhkz-spacing is calculated as λ/2 sin θ
Detailed analysis of the XRD diffractograms of Avicel® and crystalline cellulose nanoparticles is summarized in Table 1.
An XRD was then used to follow the change in diffractogram of Avicel® during the course of treatment with the Oxone® oxidant. As shown in
The crystallinity index (CRI) of the pristine Avicel® and crystalline cellulose nanoparticles was estimated to be 84.87% and 83.51%, respectively. It should be noted that the CRI value of Avicel® is dependent upon the analytical procedure and calculation method (Park et al., Biotechnol. Biofuels. (2010) 3:10. 10.1186/1754-6834-3-10). This value could range from above 55% to over 95% (Kamaouri et al. J. Phys. Chem. B. (2016) 120, 309-319). However, the CRI value obtained for the Avicel® PH-101 using the same calculation procedure was in agreement with the result reported by Leung et al. in publication US 2012/0244357. The estimation is based on the crystalline peak area over the total peak area (crystalline area plus amorphous area). The peak height method for the estimation CRI, proposed by Segal et al. (Text. Res. J. (1959), 29, 786-794) is less applicable in this case (CRI=I002/(I002−IAmor) owing to the increase of the (101 and 10-1) peaks as mentioned previously. I002 is the at peak intensity at a 2θ angle close to 22° representing the crystalline part and IAmor is the peak intensity at 2θ close to 19° representing the amorphous part of the cellulose.
Atomic force microscopy (AFM) was performed by AFM Icon (Bruker) for imaging spherical cellulose nanoparticles using a silicon tip operated in tapping mode. In corroboration with TEM, atomic force microscopic (AFM) imaging was also conducted to show the synthesis of spherical cellulose nanoparticles by Oxone®.
Attenuated Total Reflection (ATR)-FTIR is performed using pristine Avicel® PH-101 and the vacuum-dried powder of crystalline cellulose nanoparticles (Bruker Optics, Billerica, Mass., USA, FT-IR spectrometer equipped with a diamond tip).
Solid State NMR measurements were performed on a Bruker 11.7T AvanceIII spectrometer equipped with a 4 mm VTN CPMAS probe at spinning rates of 8 and 10 kHz. 13C CP experiments employed a 1H 90° pulse of 2.4 μs, followed by a 2.5 ms CP contact time using ramped field on 1H (40 to 80 kHz) and 51 kHz field on 13C and composite-pulse 1H decoupling using the SPINAL64 sequence with RF field of 94 kHz during acquisition. 13C CPMAS spectra were collected with 20150 scans on Avicel® or crystalline cellulose nanoparticles with a recycle delay of 3 s.
The foregoing disclosure of the exemplary embodiments has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the claims to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CA2017/051230 | 10/17/2017 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62412325 | Oct 2016 | US |
