Production of Green Nanomaterials Takuya Tsuzuki, Liyuan Zhang, Richa Rana, Qingtao Liu, Xungai Wang

Centre for Material and Fibre Innovation, Institute for Technology Research and Innovation, Deakin University, Geelong VIC 3217, Australia

Abstract— While our awareness towards sustainable society and environment grows, the importance of ‘green’ materials and manufacturing is gaining significant recognition. We have demonstrated that naturally-occurring fibers as renewable raw materials can be converted into nanoparticles and nano fibers using simple top-down methods without introducing hazardous chemicals. This new class of green nanomaterials will have a wide range of environmental and biomedical applications owing to the inherent biocompatible, biodegradable and carbon-neutral nature.

Keywords - green nanomaterials; natural fibers; protein nanoparticles; cellulose nanofibers;synthesis; top-down approach

I. INTRODUCTION Growing environmental awareness and government

regulations have led to increased pressure on manufacturers and users of nanomaterials to consider the environmental impact of the products at all stages of the life cycle including manufacturing, recycling and disposal processes [1]. Although the importance of organic nanomaterials in various applications has been widely recognized [2], they are normally synthesized mostly from petrol derivatives, causing environmental problems during production and disposal stages of the life cycle. Hence new types of materials and synthesis methods with minimum environmental impact are urgently sought [3].

Natural fibers such as silk, wool and plant fibers are carbon-neutral materials that normally consist of nano-scale fibrils of < 10 nm [4,5]. These structural units can be utilized for the production of nanomaterials. ‘Green’ properties such as biocompatibility and biodegradability are also expected from those naturally-occurring raw materials [1], which will be significantly advantageous for biomedical and environmental applications over synthetic organic and inorganic nanomaterials. Nanomaterials of natural origin also provide positive environmental benefits due to the possibility of carbon-neutral disposal.

To date, little study has been carried out on the methods to produce nanomaterials from natural fibers, and the reported few are based on bottom-up approaches via dissolution and reconstruction. For example, cellulose nanofibers with diameters from 200 nm to 1 μm were prepared by electrospinning cellulose acetate solutions in solvents such as acetone, dimethylformamide and trifluoroethylene [6]. Silk nanoparticles of 35–125 nm in diameter were produced by precipitating the particles out of silk aqueous solutions with addition of acetone [7].

However, original crystal structures and unique morphologies of nano-components in the raw fibers tend to be lost during the bottom-up processes, leading to undesired properties [7,8]. For example, regenerated cellulose fibers normally have the cellulose-II crystal structure that has poorer mechanical properties than the cellulose-I crystal structure in natural cellulose fibers [9]. Regenerated silk particles have notably poor crystal structures [10]. Moreover, the use of organic solvents for the dissolution of raw materials is not considered as ‘green’, as it could cause safety and environmental constraints [11].

It has recently been demonstrated that top down methods can produce high quality inorganic nanomaterials [12]. This approach, mainly represented by mechanical grinding and milling, has the potential to be used for the production of organic nanomaterials while retaining the original structures of raw natural fibers. Mechanical processes can take advantage of multilevel nanocomposite structures in natural fibers as a structural unit of resulting nanoparticles and nanofibers. Moreover, grinding processes can produce particles directly from natural fibers and avoid safety and environmental problems associated with the organic solvents in solution routes.

In this paper, we report the recent development in our institute to produce green nanomaterials from natural fibers using top down techniques. This report focuses on the preparation of cellulose nanofibers and protein nanoparticles.

II. PROTEIN NANOPARTICLES Organic nanoparticles have a wide range of applications in

biomedical fields such as drug delivery [13], enzyme immobilization [14] and wound care [15], as well as in the field of biomaterials and engineering composite materials [16]. For those applications, finer particles are more advantageous [15,17]. For example, their use as drug delivery agents normally requires the particles to be smaller than 100 nm.

Natural protein fibers such as wool consist of a hierarchical structure, made up of macrofibrils (~ 300 nm) that comprise a bundle of microfibrils (~10 nm) wherein α-keratin helix filaments of ~ 2 nm in diameter form the structural unit [4]. In our previous work, the production of silk and wool fine particles of < 5 µm in average diameter was demonstrated using grinding processes [18-20]. It was found that wool fine powders have significantly higher dye absorption volume and speed than conventional porous materials such as activated charcoals, despite the fact that the wool ultrafine particles had

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100 times smaller specific surface areas than activated charcoals [20]. Silk ultrafine particles of ~ 2 μm in diameter showed heavy-metal absorption capacities much higher than the commercial ion-exchange resins and the metal-ion binding was fast and reversible [21]. It is expected that these superior absorption characteristics of pulverized natural protein fibers will be enhanced when the particle size is reduced to the nano-scale, offering a great promise for the applications in environmental remediation and water purification.

In general, smaller grinding-ball sizes produce smaller particle sizes during mechanical milling [22]. However, if the grinding-ball size is too small compared to raw materials, effective milling cannot be achieved [22]. Therefore, in order to produce nano-meter scale particles from millimeter-scale raw fibers, two stage milling operations were performed, where grinding-ball diameters were reduced step wise.

First, raw wool fibers with the average diameter of 20 μm were washed with hot water to remove contaminants and then chopped into snippets of ~ 1.5 mm in length. The snippets were dipped in a 1 w/v% sodium dodecyl sulphate (SDS, Aldrich) aqueous solution for 15 min under constant stirring. The first stage of milling was conducted using an attrition mill (Union Process 1S). 2.5 L of a wool slurry with the wool to water mass ratio of 1:5 was introduced in the mill chamber with the capacity of 5.5 L, along with 20 kg of yttrium toughened zirconia milling balls of 5 mm in diameter. Milling was carried out up to 6 hrs with the rotor speed of 21,000 rpm.

Since the attrition mill was not capable of accommodating grinding balls smaller than 5 mm in diameter, the second stage of milling was carried out using a Spex 8000M shaker mill. In a typical run, 40 ml of an attrition-milled wool slurry with the wool to water mass ratio of 1:10 was placed in a polycarbonate container, along with 42 g of cerium-doped zirconia grinding balls with the average diameter of 0.5 mm.

Figure 1 shows volume average particle sizes of milled wool powders as a function of milling time, measured using the static light scattering technique with a Malvern Mastersizer 2000 instrument [23]. It is evident that attrition milling reduced the particle size down to ~ 10 μm in diameter.

Figure 1. Particle size of wool nanoparticles as a function of total grinding

time.

Attrition milling with SDS resulted in slightly smaller particle sizes than without SDS. The particle size reached a plateau after milling for 5 hrs.

During subsequent milling using the shaker mill, the sample that was attrition-milled without SDS was further ground down to 6 μm in diameter. This further reduction of particle size during the second stage of milling is attributable to the use of smaller grinding balls [22], though the difference in the mill type may be of consideration; attrition mills mainly induce shear force while impact force is dominant in shaker mills.

When SDS was present, shaker milling reduced the particle size down to 250 nm, significantly smaller than the case without SDS. It is thought that SDS’s ability to denature protein molecules and change their conformation contributed to the reduction of particle size. The milling action to mechanically deform the protein structure may have had a synergistic effect with SDS’s denaturing property. As can be seen in Fig. 2, the volume-weighted size distribution of the milled sample showed a broad log-normal size distribution around 250 nm. A small peak at ~ 5 μm is also evident, which may represent the powders trapped at the corners of the milling container and hence was not subjected to extensive milling. Figure 3 shows a typical scanning electron

Figure 2. Volume-weighted size distribution of wool nanoparticles after

milling for total 12 hrs in the presence of SDS.

Figure 3. SEM image of wool nanoparticles after milling for total 12 hrs in

the presence of SDS.

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microscopy (SEM) image of the sample milled with SDS for total 12 hrs. It is evident that the particles had neary spherical shapes.

III. CELLULOSE NANO FIBERS Cellulose is the most common organic polymer in the

world, representing 1.5 x 1012 tons of total annual biomass growth [5]. Since it is formed from CO2 gas in the atmosphere via photosynthesis in plants, it is a sustainable and renewable source of nanomaterials. Crystalline cellulose fibers have high strength and yet light weight. As such, wood has been used as a structural material since the dawn of civilisation.

Plant fibers consist of cellulose nanofibrils of 24 nm in diameter as a structural unit which made up of cellulose crystallites of ~4 nm in diameter [5]. Young’s modulus and tensile strength are as high as those of Kevlar fibers and about five times stronger than mild steel [9]. Their thermal expansion in the axial direction is as small as that of quartz [24]. As such, cellulose nanofibers find many applications in biomedical, environmental and industrial applications including biomedical membranes [6], artificial blood vessels [25], tissue scaffolds [26] and reinforced composites and bioplastics [27,28]. However, due to the strong hydrogen bonding between the nanofibrils, it is difficult to convert native fibers into nano-size [29,30]. In particular, the production of nanofibers using top-down methods is not a trivial task; the nanofibril structural-units need to be separated from each other without the destruction of their fibrous morphology.

Recently, we have demonstrated that a simple wet-grinding technique can be used to obtain cellulose nanofibers from dry wood pulp. The original fibers had a mean diameter of 38 μm. First, a pulp sheet was chopped into small fragments in water using a kitchen blender. The raw fibers were kept in water overnight to loosen the hydrogen bonding between them. The raw fibers were then ground using a Spex 8000M shaker mill. In a typical run, 25 g of a 1 wt% fiber suspension in water was milled using cerium-doped zirconia grinding balls with

Figure 4. SEM image of cellulose nanofibers produced by mechanical

grinding for 1 hr.

Figure 5. Diameter distribution of cellulose nanofibers produced by

mechanical grinding for 1 hr.

diameters between 0.1 and 1 mm. The weight ratio between the grinding balls and original fibers was 40 to 1.

Figure 4 shows a SEM image of the sample milled for 1 hr and subsequently dried. It is evident that nanofibers of less than 100 nm in diameter formed a porous network. Figure 5 shows a diameter distribution of cellulose nanofibers that was estimated by SEM image analysis. For each SEM image, the diameters of all the fibres that could be recognized in the image were measured. This was done manually by drawing across the fibre in the image and then measuring the distance using the Image-Pro Plus image analysis software. Each image was checked to ensure that an individual fibre in the image was only counted once. The measured fibre diameters were sorted into bins of width 5 nm, with the value of the bin centre being used for all calculations. The fiber diameter showed a log-normal distribution with an average diameter of 40 nm.

IV. SUMMARY There has been a growing expectation by consumers and

industry to use sustainable materials and a need to deploy green manufacturing processes that have minimal impact on the environment. This situation also extends to the field of nanotechnology. Our initial research has demonstrated that naturally-occurring fibers as renewable raw materials can be converted into nanoparticles and nanofibers using simple top-down methods without introducing hazardous chemicals. The methods thus developed will constitute the platform technology for the further investigation on the applications of nanomaterials of natural origin. The inherent biocompatible, biodegradable and carbon neutral nature of such green nanomaterials will be particularly advantageous in biomedical and environmental applications [21,31-33]. These nano-materials of natural origin will also assist traditional forestry and fiber industries to create new opportunities to engage with high technology industries by adding value to the natural fiber products.

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ACKNOWLEDGEMENTS The Authors thank Dr Warren Batchelor at the Australian

Pulp and Paper Institute, Monash University, for providing us with dry soft wood Kraft pulp as the raw material for the production of cellulose nanofibres.

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