ABSTRACT
Lignin is one of the main constituents of wood and is available as a waste product of pulp-and-paper industry. Technical lignins are highly branched irregular polymers consisting of phenyl propane units linked by a set of ether and carbon-carbon linkages and contain a variety of functional groups such as hydroxyl, carbonyl, carboxyl, hydrosulfide, and sulphonate among others. Presence of functional groups imparts to lignin the capability to complex various substances from transition metals to polycyclic aromatic hydrocarbons and pesticides. Polyunsaturated nature of lignin favors its transformation into conducting polymer composite using appropriate doping. Ion-exchange and redox properties of lignin make it an attractive active substance for chemical sensing. Lignins are versatile materials, as the amount and the type of functional groups, molecular weight, chemical reactivity, and electrical conductivity depend on lignin origin and can be altered through appropriate modifications. The
present chapter discusses lignin properties and its applications in chemical sensing that have been put forward recently. 5.1 Lignin: Structure and Properties
5.1.1.1 Wood types and wood cell structure and compositionWood is produced by the seed-bearing plants or Spermatophytae. Wood has a complex hierarchic structure that determines mechanical and physical properties of all wood-derived products. Ultimately, these wood properties are governed by the wood structure and, in particular, by its anatomical organization and the cell wall ultrastructure. Wood is classified into softwoods and hardwoods. The formers are gymnosperms (conifers), which are normally evergreen, and the latter are angiosperms (deciduous or broad-leaf trees), which lose their leaves during the autumn. Table 5.1 Chemical composition of wood, wt.% [1,2]
into macrofibrils and lamellae, the latter organized into a concentric arrangement around the wood cell wall layers (Fig. 5.1). The HCs possess an amorphous structure and are associated and orientated along the cellulose, while lignin is amorphous and isotropic and encrusts both the HCs and cellulose. The major function of cellulose is the provision of support to the individual wood cell and tree and ultimately the final wood-derived products. Lignin fills the space between CMFs and HC fixating them toward each other, making the cell wall stiff and rigid.
