**1. Introduction**

Chitin nanofibers (CNFs) are mainly extracted from crab and prawn shells [1, 2] and recetly found in small amount in edible species of mushrooms [3]. CNFs are composed of chitin compound. Chitin in powder form is obtained from fish industry wastes which is otherwise thrown as industrial waste. Since CNFs are biodegradable having typical width 10-20 nm and large surface-to-mass ratio thus they are being prepared, studied, and applied more recently world wide along with rapidly growing field nanotechnology dealing with the better proper‐ ties of materials when their sizes are smaller in the range 1-100 nm. Fibrilated chitin in the form of highly viscous gel suspension in water has found scope in pharmaceuticals [4], chiral separation [5], fillers in silsesquioxane [6]. When blended with inorganic metals to prepare advanced hybrid organic-inorganic composites they can have applications in electronics, electrical, optical devices and much needed solar energy production.

To introduce NFs, cellulose NFs are most important as cellulose is most abundant and readily available from plant cell walls and also produced by bacteria. Thus cellulose NFs must be most existing and easily available in nature. Attempt was successful to apply the cellulose NFs by using bacterial cellulose of the width 50 nm [7]. Though the diameter of NFs was 50 nm, larger compared to latter extracted by researchers [8], fibers worked as nanofillers in the cavities of polymerized acrylic resin. A visible light (400-800 nm) transparent flexible sheet of cellulose NFs reinforced acrylic resin polymer was obtained that showed a transmittance value of 85% at 600 nm wavelength when NFs content was 60 wt%. Prepared sheet was highly transparent due to the nanosized effect of NFs as the size of fibers was one-tenth of the wavelength of light that made material free from scattering therefore sheet was transparent. Thus authors claimed that the NFs of 50 nm width can have scope in optical devices such as displays. In year 2007, Abe et al. [8] extracted cellulose NFs of 15 nm width from Radiata pine tree wood powder

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using a series of chemical treatments followed by mechanical grinding. The width of the fibers was measured by field emission scanning electron microscopy (FE-SEM). The authors were first successfully reduced the size of extracted NFs from 50 to 15 nm from any natural resources.

**2. Preparation of CNFs from crab and prawn shells, and mushrooms**

removal of minerals, proteins, pigments, and lipids as shown in Fig. 1a and b.

Commercial grade dried crab shell flakes of species *Paralithodes camtschaticus* (red king crab) were used as a raw starting material to isolate NFs. Flakes from red king crab shell are so cheap and abundant that they are used in fertilizer industry. Crab shells were crushed to powder and purified according to the well established method. 1 wt.% slurry of crab chitin was prepared by a series of chemical treatment described in a previous chapter [14]. In brief minerals were removed by HCl treatment, suspension was filtered and washed thoroughly with distilled water, removal of proteins was done by refluxing the suspension with NaOH, pigments and lipids were removed by ethanol. After completion of above the treatments, suspension was filtered washed with distilled water and kept wet for mechanical grinding for fibrillation, this wet slurry was made to a concentration of 1 wt.% and called chitin slurry. Chemical treatment loosened the tightly bonded fibrils bundles to larger extent apart from

Chitin Nanofibers, Preparations and Applications

http://dx.doi.org/10.5772/57095

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**Figure 1.** FE-SEM images of crab shell surface after removal of matrix from shell surface by chemical treatment with‐ out mechanical grinding at different magnification scales; a) 1000 nm; b) 100 nm. Reprinted with permission from ref.

Bundles of NFs of width 30 nm are visible in micrographs without mechanical grinding. For fibrillation, 1 wt.% slurry was passed through a grinder of the model (MKCA6-3; Masuko Sangyo Co., Ltd.). After passing through the grinder, chitin slurry changed to highly viscous stable wet gel of CNFs. To record FE-SEM picture of sample, sheet of chitin material was prepared. Sheet was coated with 2 nm layer of platinum by an ion sputter coating before recording SEM micrographs. Chitin slurry was passed for one cycle through grinder at pH 7 and 3. As shown in Fig. 2a at neutral pH, fibers had width in wider range 10-100 nm. The bundles of embedded chitin-protein fibers were fibrillated successfully by grinding of wet chitin. It was easy to remove protein from water soaked chitin to isolate chitin fibrils. Authors [16] reported preparation method of CNFs from wet squid pen β-chitin at pH 3–4. In acidic

**2.1. CNFs from crab shells**

1. Copyright 2009, American Chemical Society.

Chitin is second most plentiful biomaterial [9] next to cellulose exists on earth with yearly production of 1011 tons. Chitin raw dried powder is manufactured from exoskeleton of sea food shellfish, crabs, shrimps, and insects and edible mushrooms of fungus species and sea weed algae. Chitin content in fish industrial waste is 8-33% which is thrown if not used. Thus our group is actively engaged in developing chitin research to make a number of products from atomized or fibrillated chitin in the form of chitin nanofibers (CNFs) and its derivatives [10-13]. Chitin obtained from its natural resources is highly crystalline and most of it is α-chitin conformation though the contents of α- and βchitin depends on the source.

We have published a number of review articles [11, 14, 15] covering back ground of CNFs in detail, method of preparation, sources, composition, physical and chemical properties, characterization, their composites and derivatives preparations, surface modification. Com‐ mercialization of dry chitin powder and CNFs has also been described. For atomization or fibrillation of 1 wt.% wet chitin to CNFs three types of methods were used and compared. A very recently developed [10] Star Burst atomization system which employed high pressure water jet system where slurry of chitin in high acetic acid medium is introduced in chamber of Star Burst system machine where it is fibrilized into NFs of width (18.0-19.0 nm). Atomiza‐ taion occurred in this newly developed machine chamber by collision to ceramic ball that throws out fine fibrillated NFs at extremely high pressure of 245 MPa through an out let nozzle. The two other commonly used apparatus used for fibrillation are a blender and grinder. The advantages of Star Burst system over blender or grinder for fibrillation have been described in article [10] published recently. CNFs obtained by Star Burst system were studied thoroughly recording FE-SEM images of fibers obtained after a number of passes up to ten. The width of NFs decreased from 19.0 nm to 16.5 nm when number of passes increased from one to ten, respectively. Effect of number of passes on the CNFs properties was investigated by FT-IR, XRD profiles of chitin. In review article [11] molecular structure of chitin, hierarchical organ‐ ization on the surface of crab shell exoskeleton and isolation from crab and prawn shell has been described. Method of isolation of CNFs from crab or prawn shell using a number of chemical treatments followed by grinder treatment has been explained. The width of NFs was determined by FE-SEM recordings, NFs of 10-20 nm diameter with high aspect ratio were obtained after one pass. FE-SEM images were recorded of stepwise isolation of NFs, just after removing the matrix components and after one pass treatment in acetic acid and without acetic acid condition. Without grinding treatment the fibers were like accumulated ribbons, after one pass treatment without acid the fibers were not separated but in acidic condition after one pass the fibers separated due to repulsion among the positive charges generated on the surface of fibers in acidic conditions. Chitin NFs were modified to produce novel green materials into nano-whiskers of width 6.2 nm and length 250 nm when fibers were deacetylated by treating with 33% NaOH. This contribution discusses most recent advances in preparation, derivati‐ zation, characterization and applications of CNFs. Most of the work has been conducted in our laboratory and we have also discussed the results from other groups as well.
