Biomedical Engineering Reference
In-Depth Information
12.2
Introduction
During the past decades, the synthesis of nanosized metal particles has been an impor-
tant research issue in nanoscience and nanotechnology because of their unique physical
and chemical properties in electronic and magnetic properties, and catalytic activity
(1, 2). An effective size control of the metal nanoparticles is critical in order to investi-
gate their novel electronic, optical, and catalytic properties. Some of these applications
require nonagglomerated metal particles in the micrometer or nanometer size range with
a narrow size distribution. A great number of methods including physical, chemical, and
electrochemical ones have been actively used to produce finely dispersed metals with
such well defined morphological characteristics.
The role of interfacial surface structure on material properties and attendant chemical
reactivity has attracted much attention in the field of nanotechnology. Intricate surface
structures that are present in natural biological materials along with ordered regions of
chemical functionality (e.g. hydroxyl groups) present an attractive template for develop-
ing tailored nanoscale materials with targeted properties. Hydroxy compounds, including
sugars and their derivatives, form a class of polymeric compounds that occur in nature
either freely or as constituents of other biomolecules. The saccharides are known to have
important roles in biological systems, not only in carbohydrate metabolism, but also to
some extent in the reduction and complexation of various toxic and nontoxic metal
ions (3).
The carbohydrate known as cellulose is present in cell walls and is the key biopolymer
used to fabricate hierarchically complex biological structures. It is chemically similar to
sugar but structurally different in that chain orientation fosters strong hydrogen bonding
with itself to form fibers that exist in both amorphous and crystalline forms. By reg-
ulating the molecular weight and the ratio of amorphous to crystalline content, better
structures and properties can be achieved. In a bio-refinery, the amorphous and crys-
talline components can be readily separated by means of acid hydrolysis; the amorphous
component is readily fermented into alcohol and the more chemically robust crystalline
co-component can be isolated as a bio-based product. Cellulose nanocrystals (CNXLs)
are readily obtained in the laboratory by means of acid controlled hydrolysis (diluted
H
2
SO
4
, HCl, HBr) and show much higher stability than the amorphous component (4).
The stability of the CNXL suspension is maintained by electrostatic hydration forces
as well as by hydrophobic interactions and hydrogen bonding. Since Ranby's (5) suc-
cessful hydrolysis of wood and cotton cellulose by sulfuric acid in 1951, many different
cellulose suspensions have been prepared from a variety of cellulose sources including
bacterial cellulose (6, 7), tunicate cellulose (8), soft-wood pulp (9, 10), and sugar beet
primary cell wall cellulose (11). CNXL has been applied as a reinforcing material for
the synthesis of nanocomposites with poly(diallyldimethylammonium chloride) (12) or
carboxymehtyl cellulose (13). The relative size of CNXLs is determined by the nature
of the cellulose source. Thus, algal and tunicate cellulose crystals can be up to several
micrometers in length while cellulose derived from wood or cotton tends to be signifi-
cantly shorter with diameters on the order of 10 nm. The properties of this molecularly
ordered material make it an attractive candidate template for directing growth of surface
inorganic structures.
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