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Synthesis and Characterization of Cellulose Nanoparticles and Its Derivatives using a Combination of Spectro-Analytical Techniques

Nanoparticles from different cellulosic sources were prepared via hydrolysis using three different acids. Cellulose, Methylcellulose (MC), Cellulose acetate (CA), Propylcellulose (PC) and Ethylcellulose (EC) was hydrolyzed using pulp-to-liquid ratio of 4.57:1 under 1/3/6 h hydrolysis. Different particles ranging from 70 1580 nm were obtained. The maximum yield (55.3 %) was attained at 3 h hydrolysis with H3PO4 acid, and particle size decreased with increasing reaction time as revealed by SEM and fiber length histograms and BET. Most fibers had pore area around 0.41 μm2. Nanoparticles obtained from all cellulose derivative used had low crystallinity index value attributed to substituent groups. The TGA showed that the prepared nanoparticles have thermal stability at T95 and T90 (5 and 10 wt % mass loss) in the temperature range of 297.51-411.09 oC and 315-508.55 oC with Tmax, 375-525 oC respectively. An extended temperature range was observed for some samples over the whole temperature range with only 50 wt % mass loss of the nanomaterials. FT-IR spectroscopy was employed to study the characteristic functionalities found in crystalline cellulose. BET analysis of samples gave evidences of the nanoparticles possessing large surface area of between 105.698 m2/g and 250.570 m2/g. The Langmuir surface area was 10336.157 m2/g, while the pore size and pore volume, ranged from 1.324 – 9.237 nm and 0.01427 – 0.07939 cm3/g respectively. The three different acids used gave various yields of nanocrystals with H3PO4 having the highest yield of 55.3 %, followed by HCl, (34.8 %) while H2SO4 has the lowest yield of 30 %. All of these yields were obtained under the same reaction conditions during 3 h hydrolysis. All samples hydrolyzed for 6 h gave low yield, 28.2 % of nanoparticles. The 3 h hydrolysis was the optimum reaction time for the generation of nanoparticles using 4.57 mL/g solid-liquid ratio.


Introduction
Cellulose is a linear biopolymer consisting of alternating amorphous and crystalline domains of D-anhydroglucopyranose units linked together by β1→4-glycosidic bonds, resulting in bundles known as microfibrils that can be easily cleaved by mineral acids ( Yanxia et al. 2013). It is a biodegradable and non-toxic biopolymer (Peng et al. (2011). It is a versatile starting material for the production of value-added products such as, cellulose-based threads and films as well as a variety of stable cellulose derivatives used in many areas of industry and domestic life (Atanu et al. 2006;Olatunji 2009). Derivatizing cellulose interferes with the orderly crystal-forming hydrogen bonding, so that even hydrophobic derivatives may increase the apparent solubility in water. Methylcellulose, Cellulose acetate, Ethylcellulose, Carboxymethylcellulose and native cellulose have been used in various industrial applications and these include; barriers films, thickener and emulsifier, lubricant, glue and binder, artificial tears and saliva, optical and biomedical devices, pharmaceutical raw materials, flame retardants, resins and filters, blends and composites. Because of their low cost, toughness, natural feel, transparency, softness, comfort and other favourable aesthetic properties (Rodionova et  Nanocrystalline cellulose is the individual nanocrystalline regions of elementary nanofibrils of crystalline cellulose obtained after the amorphous regions have been removed via hydrolysis with strong acids with their particle size depending on temperature, acid concentration and time (Rodionova et al. 2010;Koroleva et al. 2012;Yanxia et al. 2013). Nanocrystalline cellulose in the range of 100-300 nm long have been isolated and it is anywhere from a tenth to a quarter of the strength of carbon Nanotubes (Marthin, Orlando, Lucian and Mohini (2008). The incorporation of only a few percent of these cellulosic nanomaterials normally (1-5 %) into a polymer matrix can lead to vast improvements due to their large degree of surface area, stiffness, strength, toughness, thermal stability, barrier properties and flame retardancy compared to the pure polymer matrix Stephen (2010). Because of the nanometric size effect, composites from these materials show some unique and outstanding properties with respect to their conventional microcomposites counterparts. Since the pioneering work by the Toyota group according to Robert et al. (2011), polymer nanocomposites have attracted much more attention, particularly; the incorporation of nanoscale materials into conventional polymer matrices has created a broad range of materials with novel applications. These advantages are mainly derived from their high aspect ratio, high interfacial area and their extent of dispersion and percolation, which takes place when the nano filler particles start physically interacting, forming a continuous network due to large surface area for contact; greater fraction of atoms are exposed on the surface, and a high surface to volume ratio in contrast to their bulk counterparts (Stephen 2010;Tsagaye 2012). A decreased in the size of a nanoparticle leads to a fractional increase in the surface atoms and consequently, it increases reactivity and makes them highly reactive. Nanodimensional cellulose has already been used as a template to produce electrical wires of various metals through electroless deposition (Robert et al. (2011). Cellulose and cellulose nanocomposites for self-cleaning textiles as well as solar cell, biosensors and electronic storage devices have also been documented (Baptista, Ferreira and Borges, 2013; Ricardo, Marcia, Carlos and Tito 2012). Cadmium sulphide (CdS) nanowire has been made using a nanocellulose derivative (Lucian and Orlando 2009; Robert et al. 2011;Haorna et al. 2014). Other applications could be possible if one were able to combine other inorganics with cellulose as reported by Lucian and Orlando (2009). Cellulose has also been used for electrical devices (including artificial muscles), due to its piezoelectric nature Lucian and Orlando (2009). Smart cellulose/ electroactive paper (EAPap), a chemically treated paper with thin electrodes has been reported. Cellulose nanocrystals can be used as a starting material for the next generation of cellulose based materials of high mechanical strength, toughness, load bearing potentials, light weight, low density, low cost, biodegradability, enhanced energy recovery and chemical property stability (Robert et al. (2011). Under careful selection of reaction conditions, highly crystalline rod-like hydrophilic cellulose nanocrystals with properties comparable to other nano reinforcement materials can be obtained (Robert et al. 2011, Filpponen 2009). Cellulose nanocrystals have low cost compared to other nanoparticles and have high aspect ratio (length/diameter) varying between 30 -150 depending on the source and processing method from which it is obtained (Robert et al. 2011;Filpponen 2009). Nanocellulose has low density (1.6 g cm-3) and reactive surfaces of hydroxyl (-OH) sites which facilitates surface functionalization through grafting and click reactions to achieve various modifications with different surface properties. Some variety of CN composites produced to date can be, transparent, have tensile strengths greater than cast iron, and have very low coefficient of thermal expansion (LCTE). Besides them being relatively cheap and biodegradable materials, are their competitive mechanical properties. For example, cellulose nanocrystals have been reported to be stronger than steel and stiffer than aluminum. The elastic modulus and tensile strength have been reported to be 145 GPa and 7.5 GPa (Robert et al. 2011;Filpponen 2009;Kuga and Brown, 1987). Potential applications include but not limited to barrier films, antimicrobial films, transparent films and flexible displays, reinforcing fillers for polymers, biomedical implants, pharmaceuticals, drug delivery, fibers and textiles, templates for electronic components, separation membranes, batteries, super capacitors, electroactive polymers, development of magnetic materials for data storage, optoelectronics, medical diagnostics, photo-catalysts, sensors, alternative energy, communications, water purification, pollution reduction, medical and biomedical applications and many more (Robert et al. 2011;Jonas 2014; Tsegaye 2012; Haoran et al. 2014). Nanocrystalline cellulose materials have been used as strong template for impregnation of a range of different nanomaterials to form composites. For example, metals (Au, Ag, Pd, Ni, etc.), mineral (Cax(PO 4 )y, CaCO 3 and montmorillonite), and carbon (carbon nanotube and graphene) nanomaterials have been incorporated into nanocellulose, and materials with extraordinary electrical, optical and catalytic properties were obtained (Haoran et al. 2014). The objective of this paper was to discuss the results of synthesis of cellulose nanoparticles from cellulose, methylcellulose, cellulose acetate and ethylcellulose by acid hydrolysis and to also determine the optimum conditions for the reaction. The products synthesized were then characterized in relation to their morphologies by SEM, FT-IR, TGA, BET and XRD.

Materials
All the chemicals used in the synthesis of nanocellulose and its derivatives were of analytical grade and obtained from BDH Chemicals Ltd (Poole England). They were used without any further purification and these include; Cellulose, Cellulose acetate, Methylcellulose, Ethylcellulose, Propylcellulose and concentrated H 2 SO 4 , HCl and H 3 PO 4 .

Synthesis and Purification of Cellulosic Nanocrystals (CNCs)
The preparation of cellulose nanocrystals followed the procedures described by (Filpponen 2009, Rosa et al. 2010Jabbar and Timell 1960). In a typical experiment, 1 gram each of the cellulosic source material was hydrolyzed using 40 mL 64 % (4.57 mL/g) concentrated H 2 SO 4 , HCl and H3PO4 at 50 oC for 3/6 h under continuous mechanical stirring with a Teflon coated magnetic bar. The resulting suspension obtained had different colours depending on hydrolysis time. The obtained viscous suspension was neutralized using distilled water and then centrifuged at 3000 rpm for 30 min. and then washing was repeatedly carried out using centrifugation until the nanoparticle obtained was neutral. The thoroughly washed products were then kept in suspension by the addition of deionized water in plastic sample bottles and then stored in a refrigerator. These samples were used for SEM, XRD and surface area analysis. Soluble products obtained were allowed to stand to precipitate overnight. Methylated cellulose nanocrystals were obtained as hydrogels by salting out. All the products obtained were assigned codes as shown in the table below:

Scanning Electron Microscopy
The surface morphology of the samples was carried out using PhenomWorld ProX desktop scanning electron microscope with fully integrated and specifically designed EDS detector made in Eindhoven Netherlands.

FT-IR results of Cellulose, Methylcellulose and Cellulose acetate nanoparticles
The FT-IR spectra of samples were recorded FT-IR-8400S Fourier Transform Infrared Spectrophotometer in the spectra range of 4000-400 cm-1. Samples were run as (Kbr) pellets,

X-Ray Diffraction (XRD) Analysis of samples
X-ray diffractometry in reflection mode was carried out using a diffractometer (DLMAX-2550, Japan), with monochromatic Cu Kα radiation (λ = 0.154 nm), with a divergence and scatter at 1.00o, and a receiving slits at 0.30 mm, generated at 40 kV and 30 mA, at room temperature. The samples were scanned within 2.00 -70.00˚ 2θ in continuous scan mode with a step of 0.02˚ and a rate of 0.10 sec. The crystallite index of cellulose was calculated using the Kim's empirical method.

CI = (I002-Iam)/I002 x 100
Where CI is the crystallinity index, I002 is the maximum intensity and represents crystalline material, while Iam represents maximum intensity of the amorphous material.

Thermogravimetric Analysis (TGA)
Thermal behaviour of the prepared samples was examined by Thermogravimetric Analyzer model Schimadzu TGA-50H (Kyoto, Japan) from 28 °C to 900 °C. A heating rate of the analysis was 10 °C / min under nitrogen atmosphere and at a flow rate of 20 mL / min.

BET Analysis
The surface area of the prepared samples was examined by BET analysis using Quantachrome NovaWin-Data Acquisition and Reduction for NOVA instruments version 11.03.

Results and Discussion
Appearance    The implication of the synthesized nanoparticles is that the presence of the large surface area will aid the encapsulation and homogenous distribution of conventional adsorbents for improved performance. The pore area histogram, which is very important in revealing the frequency of average pore volume, is essential for pore size applications in filtration. The liquid-to-solid ratio in this work was 4.57:1 (4.57 mL/g) in contrast to most reports on cellulose nanocrystals employing liquid-to-solid (mL/g) ratio of 8.75:1 (8.75 mL/g) in the determination of sulphur contents in cellulose nanocrystals at various liquid-to-solid ratio and time Amit (2013). The implication is that, the generation of nanocellulose crystals is time, temperature and acid concentration dependant. The MC nanoparticles were larger than cellulose nanocrystals. The pore area of the MC nanoparticles was much greater than that for the cellulose nanoparticles and the acetate nanoparticles. The nanocellulose crystals had smaller particles with high porosity than those obtained from cellulose acetate fibers. The reason for the large size and low pore area of the nanoparticles from the acetate derivative could be due to the presence of acetyl group`s resistance to acid attack. The pore size and fiber diameter of these particles can be beneficial for cleaning applications such as for the removal of organic pollutants in water systems.   . The pore area of the particles showed that a high fraction of the fibers had size within 0.082 µm2 which is equivalent to 82000 nm2 with the highest frequency distribution count. This particles exhibited high kinks and homogenous surface area that is not found in cellulose acetate. This property can be apply for the removal of large organic molecules in waste and polluted water due to their small size. The presence of acetyl groups will necessitate hydrophobic interactions with organic molecules thus, enabling their removal in polluted and waste water. Different morphological properties were observed for these materials. Though, using the same acid concentration, SEM images of HCl hydrolyzed samples fibers did not showed the acid diffusion process and erosion of the fiber surface as did the SEM images of H 3 PO 4 hydrolyzed MC fibers which showed the gradual acid diffusion process, eroded fiber surface and spindlelike individual crystals. The acid strength is implicated for the gradual diffusion and erosion of the fiber surface. HCl being a stronger acid than H 3 PO 4 is able to quickly diffuse at a fast rate into the fiber. This process brings to our understanding of the fact that generating nanoparticles from biofibers does not depend only on acid concentration, temperature and time as earlier reported in many literatures, but also on acid strength according to the present work. This may be the reason H 3 PO 4 is not commonly used for the preparation of nanoparticles from biofibers. Fiber metric image measurement for the MC nanoparticle showed the presence of nanofiber in range of 68 nm -815 nm and these particles were the dominants as indicated by the fiber count histogram Figure  . Pore area measurement of fibers revealed the presence of most of the fibers with pore area which did not exceed 0.082 µm2 (82000 nm2). The above attributes of the synthesized nanoparticles has endowed this particles the potential to be useful in removing pollutants in water due to their small size and high tendency to form hydrogen bonding network with metal ions and organic molecules thereby, trapping pollutants out of the feed system.    .19 and A.25). The sample AZH8-CA-3h-H 3 PO 4 nanoparticle crystals synthesized had different clusters of particle with interconnected web-like net-work as revealed by SEM images (Figure A.22) below, unlike sample AZH 7-CELL-3h-H 3 PO 4 SEM images that showed a mixture of nano and short microcrystals ( Figure A.19). At low and high magnification, crystalline particles appeared to be homogenously dispersed with well define interconnected structural network, large and small pore, interfibrillar network structures. These attributes were not found in AZH 7-CELL-3h-H 3 PO 4 ( Figure A.19). The fiber length distribution intensity ( Figure  It can also be used as matrix for the microencapsulation (host-guest relationship) for sustainable/controllable drug release. The homogenous nature of these particles will allow for the even distribution of pollutants in the crystal lattice of the material which will lead to the effective removal of pollutants from the feed solution. Films with homogenous phase can be prepared from this material due to the uniformity of the pores. The porous nature of the material provides information on the possibility of using it for engulfing large organic and inorganic materials, atoms, ions and molecules that may be present in water/waste water not only on their surfaces, but throughout the bulk of the material by selective adsorption. CA is partially hydrophobic and hydrophilic, thus, it can absorb organic molecules from water as well as adsorption of water from organic solvent, it can acts as host molecules where they house organic guest molecules in their pores. For example, organic drugs for effective and control release for optimum performance and the adsorption of organic dyes.    . The large surface area is attributed to small pore size of the nanoparticles prepared and was in agreement with literature report where 32 Ǻ and 16 Ǻ (3.2 nm and 1.6 nm) have been specifically mentioned to be the most common pore size for large surface areas of nanoparticles by Alexander (2002). The pore volumes of the nanoparticles obtained in this study are larger than those obtained by Albert (2005) through BET analysis of cellulose using N2 adsorption isotherms, but lower than the pore volumes obtained through water adsorption isotherms. Based on the recent applications and uses of membrane and depth filters for removal of bacteria and viruses from water and blood systems, it is important to stress that the prepared nanoparticles posses the potentials to be applied in this areas. Because of the small pore size of the particles compared to the sizes of these viruses; Severe acute respiratory syndrome (SARS) corona virus 80 -120 nm in size, HIV virus, 100 -120 nm in diameter, Avian flu, 80 -120 nm in size, HBV 40 nm in size, mimivirus with 270 nm in diameter while some filoviruses having length up to 1400 nm with the capsid around 80 nm as described by (Zhao, Sugiyama, Miller and Miao (2008).

Time (min)
Since membrane filtration is based on pore size of the membrane particles, thus particles larger than the pores will not be able to penetrate the pores and are thus separated.

Thermal Stability
Thermal properties of cellulose and cellulosic materials have been intensively studied and well documented (   All H3PO4-MC nanoparticle and H3PO4-CA nanoparticle obtained under the 1, 3 and 6 h hydrolysis showed a unique sharp peak at (2theta = 6.1o, 10o, 22.3o) representing the crystalline area in MC and CA. The peak at (2theta = 18.1 in these samples was broad and thus, emanated from the amorphous regions and the randomly distributed acetyl and methyl groups which are difficult to re-orient themselves into a close packing and regular order again. For this reason, cellulose acetate is known to be a semicrystalline polymer Abdel-Naby and Ghamdi (2014) and a decrease in crystallinity is observed Loo, Hashim and Leh (2012). These samples had the lowest CI index value of (25 %) attributed to the presence of high amount of amorphous regions and short reaction duration employed for their synthesis. Another reason may be due to the formation of nano-hydrogel, very liable to absorbing water molecules during storage and may have resulted to decrystallisation of these samples. Another important reason for the low crystallinity index value for MC and CA may be attributed to substituent groups (methyl, in MC and acetyl, in CA) being organic and their contribution to hydrophobicity of the samples. In this work, samples were stored as suspension, and the long time storage of the samples under aqueous medium could have affected the CI values due to absorption of water molecules and subsequent re-organization of the hydrogen networks in CNCs.