Nano-crystalline

[Melissa Russian]

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The exploitation of various plant materials for the biosynthesis of nanoparticles is considered a green technology as it does not involve any harmful chemicals. The present study reports the synthesis of silver (Ag) nanoparticles from silver precursor using the bark extract and powder of novel Cinnamon zeylanicum. Water-soluble organics present in the plant materials were mainly responsible for the reduction of silver ions to nano-sized Ag particles. TEM and XRD results confirmed the presence of nano-crystalline Ag particles. The pH played a major role in size control of the particles. Bark extract produced more Ag nanoparticles than the powder did, which was attributed to the large availability of the reducing agents in the extract. Zeta potential studies showed that the surface charge of the formed nanoparticles was highly negative. The EC(50) value of the synthesized nanoparticles against Escherichia coli BL-21 strain was 11+/-1.72 mg/L. Thus C. zeylanicum bark extract and powder are a good bio-resource/biomaterial for the synthesis of Ag nanoparticles with antimicrobial activity.

Natural fibers such as kenaf have been studied extensively as a reinforcing phase and received major attention recently due to their renewability, biodegradability, and high strength comparable to other synthetic fibers. In this study, nano-crystalline cellulose (NCC) was produced from kenafcore wood using the acid hydrolysis method. Kenaf core was alkali treated with a 4 wt% of sodium hydroxide solution and subsequently bleached using sodium chlorite in acidic buffer. The resulting white, bleached kenaf core was hydrolyzed in 64 wt% sulfuric acid (H2SO4) to obtain NCC. The resulting NCC suspension was characterized using X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) analysis, and scanning transmission electron microscope (STEM). Hydrolysis with highly concentrated H2SO4 further increased the crystallinity of bleached kenaf core cellulose and reduced the dimension of cellulose to nano scale. FTIR results showed that with each subsequent treatment, hemicellulose and lignin were removed, while the chemical functionalities of cellulose remained after the acid hydrolysis treatment. XRD peaks shown by bleached kenaf core were characteristic of cellulose I, which was reaffirmed by the DSC results. The diameters of NCC obtained from kenaf core were found to be in the range of 8.5 to 25.5 nm with an average aspect ratio of 27.8.

Natural fibers such as kenaf have been studied extensively as a reinforcing phase and received major attention recently due to their renewability, biodegradability, and high strength comparable to other synthetic fibers. In this study, nano-crystalline cellulose (NCC) was produced from kenaf core wood using the acid hydrolysis method. Kenaf core was alkali treated with a 4 wt% of sodium hydroxide solution and subsequently bleached using sodium chlorite in acidic buffer. The resulting white, bleached kenaf core was hydrolyzed in 64 wt% sulfuric acid (H2SO4) to obtain NCC. The resulting NCC suspension was characterized using X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) analysis, and scanning transmission electron microscope (STEM). Hydrolysis with highly concentrated H2SO4 further increased the crystallinity of bleached kenaf core cellulose and reduced the dimension of cellulose to nano scale. FTIR results showed that with each subsequent treatment, hemicellulose and lignin were removed, while the chemical functionalities of cellulose remained after the acid hydrolysis treatment. XRD peaks shown by bleached kenaf core were characteristic of cellulose I, which was reaffirmed by the DSC results. The diameters of NCC obtained from kenaf core were found to be in the range of 8.5 to 25.5 nm with an average aspect ratio of 27.8.

Kenaf (Hibiscus cannabinus L., Malvaceae), an herbaceous dicotyledonous plant, consists of outer bast which resembles softwood fibers and a woody core which resembles hardwood (Pande et al.2000). Kenaf plant can grow very fast, reaching a height of more than 3 m in 3 to 4 months (Villar et al. 2009; Webber III and Bledsoe 2002). The annual yield of kenaf (whole stem) ranges between 12 and 30 tons ha-1, which is three times the maximum yield of Pinus radiata, depending on cultivars, soil type, climate, etc. (Villar et al. 2009). Kenaf core comprises 60 to 65 wt% of the whole stem of kenaf plant (Pande et al. 2000; Villar et al. 2009). Kenaf plant is largely used for its bast fibers, which are superior in mechanical strength as compared to kenaf core.

In 2009, designation of the International Year of Natural Fibers highlighted the importance of natural fibers and their impact towards people. Natural fibers have a good mechanical strength comparable to synthetic fibers and most importantly, are renewable and sustainable. In recent years, a particular natural fiber derivative received major attention for its superior mechanical properties i.e. cellulosic nano-fibers (Dufresne 2010; Eichhorn et al. 2010). They are most notably used as a reinforcing phase in composites and can also be used in a wide range of applications, such as drug delivery excipient, transparent paper, iridescent film, aerogels, etc. (Beck et al. 2010; Jackson et al. 2011; Nogi et al.2009; Sehaqui et al. 2010).

In this study, kenaf core wood was used to produce nano-crystalline cellulose (NCC) using the acid hydrolysis method. Kenaf bast is known to be higher in length and mechanical strength and therefore more suited for high mechanical strength application such as reinforcement in composites (Ishak et al.2010), cordage, making ropes, paper pulp (Villar et al. 2009), etc. Kenaf core wood was chosen because of its limited usage compared to bast fibers. On the other hand, NCC from kenaf core wood source has not been prepared and characterized yet. The produced NCC have been characterized using X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) analysis, and scanning transmission electron microscopy (STEM).

Five-month-old kenaf core wood was ground and sieved to obtain kenaf core powder (80 micron). It was then rinsed once with distilled water and strained using a cloth filter to remove impurities such as sand. Alkali treatment was conducted in NaOH solution of 4 wt% at 80 oC for 3 times. Alkaline treated fibers were subjected to NaClO2 bleaching of 1.7 wt% in acetic acid buffer at 80 oC for 4 times. The resulting fibers were strained and washed with distilled water and cloth filter until it reached neutrality. The kenaf core wood powder was then oven dried at 105 oC. Then it was deemed as bleached kenaf core.

Bleached kenaf core wood was hand ground before acid hydrolysis. The ground product was hydrolyzed in pre-heated H2SO4 (64 wt%) (Beck-Candanedo et al. 2005). The high concentration of acid was removed through centrifugation at 10,000 rpm for 10 min and repeated until the solution was turbid. The resulting NCC suspension was dialyzed using cellulose membrane in deionized water until the suspension reached a pH of around 5. The NCC suspension was freeze-dried to obtain NCC powder.

Kenaf core powder, alkaline-treated kenaf core, and bleached kenaf core were analyzed for Klason lignin and holocellulose content using standard methods described in TAPPI T222 om-99 and Wise et al. (1946), respectively.

DSC and TGA (Mettler Toledo) analysis was conducted for bleached kenaf core and NCC. Samples were heated at a rate of 10 oC/min from room temperature to 600 oC under N2 flow for DSC. For TGA, samples were heated at a rate of 10 oC/min from room temperature to 600 oC under N2 gas to analyze its thermal stability.

The dimensions of nano-crystalline cellulose were evaluated using STEM, Hitachi SU8000 at 30 kV. A drop of the diluted suspension of NCC was dropped onto a lacey carbon coated copper grid. From the micrographs, the diameters of NCC were calculated with the aid of integrated computer software. At least 60 measurements were taken.

Table 1 shows the proximate chemical composition of kenaf core powder, alkaline treated kenaf core, and bleached kenaf core. The holocellulose content of the alkaline treated kenaf core was found to be higher than kenaf core powder due to the removal of hemicellulose after alkali treatment, whereas the holocellulose in kenaf core powder consists of cellulose and hemicellulose. After alkali treatment, a majority of lignin fraction in sample still remains, therefore the Klason lignin is expected to be higher than that of the kenaf core powder. After repeated bleaching, the Klason lignin in bleached kenaf core was found to be 0.13%, whereas the holocellulose portion was 99.10%. High lignin content in sample is known to impede the acid hydrolysis process (Kumar et al. 2009); therefore low lignin composition is desirable for the production of NCC using acid hydrolysis.

Figure 1 shows the XRD intensities of kenaf core wood powder, alkaline treated kenaf core, bleached kenaf core, and NCC. The peaks obtained from the XRD intensities were the characteristic peaks for cellulose I. The (002) peak of samples shifted slightly towards higher angles with more intensive treatment, which is consistent with kenaf bast (Kargarzadeh et al. 2012) and other sources of cellulose. The crystalline peaks of cellulose are more profound with increasing treatment with the emergence of a doublet of (101) and . Both bleached kenaf core and NCC conform to peaks normally shown by cellulose such as Avicel PH-101, a type of microcrystalline cellulose (Park et al. 2010).

The crystallinity index (CrI) of kenaf core increased after alkali treatment due to the removal of amorphous hemicellulose. A combination of low NaOH concentration and relatively low temperature did not alter the cellulose structure, as shown by XRD results. Bleaching the alkaline-treated fibers further increased the CrI by removal of lignin. Further treatment using concentrated sulfuric acid increased the CrI from 48.1% for kenaf core powder, to 75.0% for NCC samples, which can be attributed to the removal of amorphous hemicellulose, lignin, and cellulose; the surface chains of remaining celluloses (NCC) may recrystallize causing an increment in CrI. On the other hand, aggregation of the products after treatment may also contribute to higher crystallinity (Leppnen et al.2009). The NCC produced in this study was freeze-dried; hence, the effects of recrystallization and aggregation were reduced.

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