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Isolation of Cellulose Nanoparticles from Sesame Husk Bindu Sekhar Purkait,† Dipa Ray,*,† Suparna Sengupta,‡ Tanusree Kar,§ Amar Mohanty,|,⊥ and Manju Misra|,⊥ Department of Polymer Science & Technology, UniVersity of Calcutta, 92 A.P. C Road, Kolkata 700009, India, Calcutta Institute of Engineering & Management, Tollygunge, Kolkata 700040, India, Department of Materials Science, Indian Association for the CultiVation of Science, 2A and B Raja S.C Mallick Road, Kolkata 700032, India, School of Engineering, Thornbrough Building, UniVersity of Guelph, Guelph, N1G 2W1, ON, Canada, and Bioproducts DiscoVery and DeVelopment Centre, Department of Plant Agriculture, Crop Science Building, UniVersity of Guelph, Guelph, N1G 2W1, ON, Canada

Preparation of green cellulose nanoparticles (CNPs) from renewable resources is becoming an important area of research. An agro-waste such as sesame husk has been used for the first time to generate cellulose nanoparticles. Sesame husk, which is deep brown in color, was treated with 0.7% sodium chlorite solution, followed by alkali treatment. This chemically treated mass was subjected to acid hydrolysis with 35% sulfuric acid. Cellulose microwhiskers were released from the husk, forming a white colloidal suspension and leaving a solid residue at the bottom. These microwhiskers were examined by optical microscopy and transmission electron microscopy (TEM). The microwhiskers were observed to be multifaceted, existing in associated form and having width in the range of 1-2 µm. These microwhiskers were subjected to homogenization, after which spherical cellulose nanoparticles (CNPs) were produced having diameters in the range of 30-120 nm. X-ray diffraction study of these white cellulose nanoparticles and the residue showed a highly crystalline nature of the cellulose particles. Atomic force microscopy also confirmed the spherical shape of the cellulose nanoparticles. The block hardness and modulus of the CNPs were measured by nanoindentation tests. 1. Introduction Over the past few decades, there has been an enormous increase in interest toward natural fibers among material scientists and researchers. Instead of conventional petroleumbased plastic products, natural-fiber-reinforced biocomposites can be utilized in various fields such as automotives, packaging, and agriculture. Thus, the environment can be protected from pollution by using these ecofriendly polymers. Cellulose and many cellulose products are favorable polymers from the viewpoint of the environment, as they can be safely returned to the natural carbon cycle by simple rotting. Cellulose in its nanocrystalline form has a very high tensile strength and a high Young’s modulus and is a very good reinforcing filler for various composite materials.1-3 Jute micro-/nanofibers were effectively prepared by Wang et al.4 Spherical cellulose nanoparticles with sizes ranging from 60 to over 570 nm were developed by Zhang et al.5 Nanocrystalline cellulose (NCC) can be used for regenerative medicine as studied by Fleming et al6 and optical applications as reported by Revol et al.7 Cellulose nanofibrils or nanoparticles can be generated by mechanical or chemical treatments such as acid hydrolysis. During acid hydrolysis, hydrolytic cleavage of the glycosidic bonds takes place mainly in the amorphous regions of the cellulose, releasing individual crystallites. A technique for the production of nanocrystalline cellulose with a narrow size distribution was reported by Bai et al.8 Isolation and characterization of cellulose from sugar cane bagasse was reported by Sun et al.9 Wood fibers,10 cotton,11 sea animals,12 and sugar * To whom correspondence should be addressed. Tel.: +91-0332350 1397. Fax: +91-033-2351 9755. E-mail: [email protected]. † University of Calcutta. ‡ Calcutta Institute of Engineering & Management. § Indian Association for the Cultivation of Science. | School of Engineering, University of Guelph. ⊥ Department of Plant Agriculture, University of Guelph.

beet13 have been used as raw materials to isolate cellulose nanofibrils by chemical methods. Cellulose nanowhiskers from coconut husk fibers were prepared by Imam et al.14 A study of the mechanical, thermal, and morphological properties of microcrystalline cellulose particles prepared from cotton slivers using different acid concentrations was performed by Ray et al.15 Gas-phase surface esterification of cellulose microfibrils and whiskers was reported by Berlioz et al.16 Current international research into cellulose nanofibers and nanocomposites was described by Eichhorn et al.17 Microfibrillated cellulose from the peel of prickly pear fruits was prepared by Habibi et al.18 The shapes and size distributions of crystalline nanoparticles prepared by acid hydrolysis of native cellulose were reported by Samira et al.19 The effects of alkaline treatments on structural and morphological features of cellulose microfibrils from banana rachis were reported by Robin et al.20 Cellulose nanowhiskers from grass of Korea were prepared by Pandey et al.21 Properties of nanofibrillated cellulose from different raw materials and their reinforcement potential were discussed by Zimmerman et al.22 Characterization of microcrystalline cellulose and cellulose long fibers modified by iron salt was reported by Sain et al.23 Mechanical methods include a high-pressure refiner treatment,24 a grinder treatment,25 and a high-pressure homogenizer treatment.4,5 The fibers made by these mechanical methods could be bundles of microfibers or nanofibers. In the previous literature reports, various types of cellulosic resources were used as precursors for the generation of cellulose nanoparticles. In this work, we chose sesame husk as the starting material. The main objective was value addition to this agricultural waste, which is presently used only as cattle feedstock. The lignin content of deep-brown-colored sesame husk is high, around 40%. This separated lignin can also be recovered and utilized, which will make the process even more economical. This will open new avenues for using such

10.1021/ie101797d  2011 American Chemical Society Published on Web 12/14/2010

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abundant, renewable, and inexpensive agrowaste materials for developing value-added products. In the present work, cellulose nanoparticles (CNPs) were obtained from sesame husks by various chemical treatments. These CNPs were subjected to atomic force microscopy (AFM), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and nanoindentation studies to investigate their properties. Different cellulosic resources have been used by researchers to generate cellulose nanofibrils or nanowhiskers or nanoparticles, but use of agro-waste as a precursor is very rare. This is the novelty of this work, where a value has been added to a waste material apart from the generation of green, ecofriendly cellulosic nanofillers for diversified applications. 2. Experimental Section 2.1. Materials. Sesame husk was collected from a local sesame oil mill. Sodium chlorite and sodium bisulfite (LobaChemie Product) were used for delignification step. Sodium hydroxide and acetic acid, from Merck, were used for removal of the hemicellulose fraction. Sulfuric acid (laboratory-grade Merck product) was used for acid hydrolysis. A high-speed homogenizer (Remi, model RQ 127A) was used to reduce the particle size to the nanolevel. 2.2. Pretreatment of Sesame Husk. To isolate the cellulose nanoparticles, it is important to remove the lignin and hemicellulose fractions from sesame husk as far as possible. The rigidity of the highly crystalline cellulose particles cannot be fully exploited if they remain embedded within the amorphous cementing materials such as lignin and hemicellulose. Hence, in this study, we first separated these noncellulosic constituents from the sesame husk. The chemical treatment of the sesame husk is depicted in Figure 1. Delignification was done by standard sodium chlorite (0.7% NaClO2) treatment as reported by Ray et al.26 The raw sesame husk was treated with 0.7% sodium chlorite solution at pH 4 and kept for 2 h at near-boiling temperature with continuous stirring. Then, it was filtered and washed, and the material was put into 2% sodium bisulfite solution. The bisulfite-treated mass was filtered, washed, and dried in an oven until constant weight was observed. A deepyellow-colored solid residue was obtained. Nearly 35-40% weight loss was observed due to removal of the lignin. This dried mass was then subjected to alkali (17.5% NaOH) treatment for removal of the hemicellulose fraction. The total weight loss observed after alkali treatment was nearly 20%. Finally, this chemically treated mass (M) was subjected to acid hydrolysis. 2.3. Preparation of Cellulose Nanoparticles from Sesame Husk. The chemically treated husk (M) was hydrolyzed with 35% sulfuric acid with constant stirring for 3 h at 50 °C following the standard hydrolysis procedure of Dong et al.27 Hydrolyzed solution resulted in a white colloidal suspension and an amorphous solid residue at the bottom (R). This mixture was subjected to differential centrifugation to separate the colloidal suspension from the residue. This colloidal suspension was further transferred into centrifuge bottles and centrifuged for 15 min at 9000 rpm. The fractions were washed continuously by adding distilled water and by several centrifugation steps. A similar process has been reported by some researchers in their work.28,29 The cellulose suspension was then homogenized with high-speed homogenizer (Remi model RQ 127A) for 1 h at a speed of 2000 rpm and was sonicated for 2 h. When 1 g of chemically treated mass (M) was acid-hydrolyzed, the yield of

Figure 1. Flow-sheet diagram showing the preparation of cellulose nanoparticles from sesame husk.

cellulose whiskers was nearly 0.2 g. The preparation of cellulose nanoparticles from sesame husk is shown by a flow diagram in Figure 1. 3. Testing Method The prepared CNP suspension was subjected to TEM analysis at two stages: just after the acid hydrolysis and after homogenization. A transmission electron microscope [Technai G2 Spirit biotwin electron microscope (SEI)] with a voltage range of 210-240 V and a frequency range of 50-60 Hz, operated with a thermo ionic tungsten electron gun at 80 kV voltage, was used for the study. A drop of aqueous suspension of CNP was poured on a carbon-coated Cu grid (300 mesh) with a micropipet and dried prior to TEM examination. The CNPs were characterized by AFM using a Veeco MultiMode scanning probe microscope with a Nanoscope IIIa controller. A drop of aqueous suspension of CNP was poured on a glass coverslip with a micropipet and dried prior to AFM examination. Images were collected using tapping mode with a phosphorus-doped silicon tip (model RTESP) with a nominal frequency of 312 kHz. Field emissionscanning electron microscopy (FE-SEM) was performed using a model JEOL JEM-6700F instrument. An ADCON optical microscope was used for monitoring the change in sesame husk upon acid hydrolysis. CNP powder was obtained by freezedrying at -110 °C for 2 days. The XRD analysis of CNP powder was done with an X’Pert PRO model Rigaku MINISLEX instrument at a scanning rate of 4°/min. Nanoindentation of CNP pellet was performed using a Nanoindentor TM II apparatus. Pellets of CNP powder were made for the nanoindentation tests. To make the pellets, a measured amount of CNP powder was

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Figure 2. SEM micrograph of sesame husk.

Figure 4. TEM micrographs of cellulose microwhiskers.

Figure 3. Optical micrographs showing the gradual release of cellulose whiskers from the husk after (a) 45, (b) 120, and (c) 180 min of acid hydrolysis.

compacted in a metal mold in a compression molding machine, which was described in detail in our previous work.15 The measurements of hardness and elastic modulus were obtained using a Berkovich indentor. The reported values are the means from at least six indentations. 4. Results and Discussion Characterization of CNPs. The sesame husk (Figure 2) was initially chemically treated with sodium chlorite and alkali solution to remove the noncellulosic constituents. This chemi-

Figure 5. Particle size distribution of cellulose nanoparticles after (a) 45 min and (b) 1 h of high-speed homogenization.

cally treated mass, when subjected to acid hydrolysis, revealed a unique transformation. Very fine cellulosic microwhiskers of nearly 1-2 µm length were gradually released from the husk. The optical micrographs showing the gradual release of the cellulose microwhiskers from the husk are presented in Figure 3a-c. The images in parts a-c of Figure 3 were taken after

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Figure 6. AFM images of cellulose nanoparticles prepared by homogenization.

45, 120, and 180 min, respectively, of acid hydrolysis. The structures of the sesame husk were gradually disintegrated, and the fine microwhiskers came from them. These whiskers were observed by TEM to obtain an accurate idea of the shape and size of the cellulose whiskers. The TEM images (Figure 4) clearly reveal the many-sided structure of the whiskers and their association with one another. Crystalline arrays of cellulose nanofibrils are also evident from the diffraction and Kikuchi lines obtained from TEM (Figure 4c). Preparation of such cellulose microwhiskers, isolated from an agro-husk, is reported for the first time here. This indicates that such cellulose microwhiskers remain encapsulated by a noncellulosic husky cover and, if isolated, can be employed as a useful material. Acid hydrolysis finally resulted in a white colloidal suspension and a solid residue that settled at the bottom (R). The colloidal suspension of cellulose particles was separated, made acid-free, and then subjected to high speed homogenization. Particle size analysis was carried out after 45 min and 1 h of homogenization. The particle size distributions are shown in Figure 5a,b, and the maximum peak intensities were observed at 342 and 235 nm for 45-min and 1-h homogenized samples, respectively. The particles were investigated by AFM, shown in Figure 6. The particles were mostly spherical, but some were elliptical. The size and shape were also observed by FE-SEM after 2 h of high-speed homogenization, which revealed particles in the size range of 30-120 nm (Figure 7). A very uniform breakdown occurred that produced a uniform range of spherical cellulose nanoparticles. TEM also showed the spherical nature of the cellulose nanoparticles (Figure 8). Similar spherical cellulose nanoparticles were reported by Ibrahim et al in a recent study.28 The XRD graphs of these CNPs, the amorphous residue (R), and the precursor sesame husk (SH) are shown in Figure 9. The CNPs were highly crystalline, exhibiting a very sharp peak at 2θ ) 25°, whereas the residue (R) was fully amorphous. This sharp peak was evident in the XRD graph of the untreated, raw sesame husk (SH) (Figure 9 inset). A similar very sharp XRD peak was observed in the case of microcrystalline cellulose particles derived from newsprint, reported in our previous work.30 A pellet, prepared from CNP powder, was subjected to nanoindentation tests to investigate the block modulus and block hardness of the CNPs and to determine the viscoelastic behavior of the particles (Figure 10). There was a very high penetration of 19000 nm when a load of 100 mN was applied, which can be ascribed to the fragmentation of the pellet during nanoindentation, forming smaller plastically deformable cellulose nanoparticles. The block hardness and block modulus of the

Figure 7. FE-SEM images of cellulose nanoparticles prepared by homogenization.

Figure 8. TEM micrograph of cellulose nanoparticles prepared by homogenization.

CNPs were 1.76 VHN and 0.3 GPa, respectively. These observations fully conform to our previous work,15 where we reported the modulus and hardness of microcrystalline cellulose

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recovered, and suitable application of the lignin particles will be explored. Although extensive research to use cellulose nanofillers as reinforcements in polymer matrix composites is ongoing, some problems are yet to be solved. A major difficulty is faced during drying of the nanocellulose, because the fibers have a strong tendency to reaggregate and the nanosize effect is partially lost. Another problem is associated with the dispersion of hydrophilic nanocellulose particles into hydrophobic polymer matrixes. Therefore, the particles require surface modification to increase their suitability as reinforcing fillers in polymer cmatrixes. These points are now being investigated to increase the versatility and usefulness of such fillers. Acknowledgment

Figure 9. XRD graph of sesame husk (inset), cellulose nanoparticles, and noncellulosic amorphous residue.

D.R. is thankful to AICTE (All India Council for Technical Education), Government of India, for granting her CAYT project. The authors are thankful to Himel Chakrabarty (CSIR Fellow, BESU) for helping in nanoindentation tests. The authors are also thankful to Prof. Arup Mukherjee and Ms. Subhadra (CSIR Fellow) of the Dept. of Chemical Technology, University of Calcutta, for their help in particle size analysis. Literature Cited

Figure 10. Load-displacement curve of cellulose nanoparticles obtained by nanoindentation.

particles prepared by 35% acid hydrolysis of cotton. These observations indicate that 35% acid hydrolysis yields softer cellulose particles than 64% acid hydrolysis. 5. Conclusions The present work shows that cellulose whiskers can be isolated from sesame husk, which is an agro-waste material. Chemical treatment with sodium chlorite and alkali removed the noncellulosic constituents by nearly 50% by weight. The remaining portion, when hydrolyzed with 35% sulfuric acid, released very well-defined cellulose microwhiskers, which were 1-2 µm in diameter. These whiskers were further homogenized to reduce their size. High-speed homogenization resulted in a lowering of the size of the cellulose particles having diameters in the range of 30-120 nm. All of the microscopic investigations confirmed the uniform spherical nature of the particles. XRD analysis revealed a highly crystalline nature of the cellulose particles, and the residue separated during acid hydrolysis was fully amorphous. CNPs were made into a pellet that was subjected to nanoindentation. The block hardness and block modulus of the CNPs were found to be 1.7 VHN and 0.3 GPa, respectively. The separated lignin (almost 50%) will also be

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ReceiVed for reView April 20, 2010 ReVised manuscript receiVed November 16, 2010 Accepted November 23, 2010 IE101797D