Tese_Tânia Vieira.pdf - Ubi Thesis

Chapter I - Introduction
2012). Protective polymers can coordinate metal ions before reduction (Mohammed Fayaz et al.
2009), forming a polymer-metal ion complex, which can then be reduced to form zerovalent
metal colloids (Radziuk et al. 2007; Domingos et al. 2011). This process allows the production of
nanoparticles with a narrower size distribution, than those obtained without protective polymers
(Mohammed Fayaz et al. 2009). Once the reduction occurs, particles are attached to the much
larger protecting polymers that cover or encapsulate the metallic particles and thus stabilize
them to be used in biomedical field (Mohammed Fayaz et al. 2009). In order to be applied in the
referred field, is also required that both the stabilizing and the reducing agents must not
represent a biological hazard (Mohammed Fayaz et al. 2009).
Besides the problem of aggregation, as already described, typically AgNPs are likely to be
toxic for cells under physiological conditions, which limits their applications in biological systems
(Potara et al. 2011). The polymers and surfactants, used to stabilized AgNPs, are also used to
solve the cytotoxic problems (Ghosh et al. 2010) and can also lead to synergistic antibacterial
agents with new, improved optical, electrical and catalytic properties, unavailable in the
individual components themselves (Maneerung et al. 2008; Potara et al. 2011). Thus, the use of
the stabilizing agents in order to avoid aggregation, also provide a protective interfacial barrier
between the metal core and cells, which is especially important for preventing damage to the
surrounding healthy tissues (Schrand et al. 2008). Moreover, it was demonstrated that the
incorporation of AgNPs into polymers create a protective interfacial barrier that do not affect
the antibacterial properties of the nanoparticles and may increase them, as mentioned before
(Schrand et al. 2008). All these advantages have been widely employed in a vast number of
engineering and technical areas, especially in medical field, to produce biomedical devices with
specific properties (Prashantha et al. 2006; Liu et al. 2008; Maneerung et al. 2008).
Chitosan and dextran were used in this work to stabilize AgNPs in order to widening its
applications.
1.3.5.1. Dextran
Dextran (Figure 3) is a bacterial-derived polysaccharide generally produced by enzymes
from certain strains of Leuconostoc or Streptococcus (Xiao et al. 2009), with good
biodegradability and biocompatibility (Wang et al. 2011). It is built by glucose molecules
containing 17-20% sulfur coupled into long branched chains, mainly through 1,6-glucosidic and
some through 1,3-glucosidic linkages (Tiyaboonchai et al. 2007; Hwang et al. 2010; Anitha et al.
2011; Saboktakin et al. 2011; Wang et al. 2011).
Moreover, it is also colloidal, water-soluble, and inert to biological systems (Hwang et al.
2010; Jeong et al. 2011). Due to these properties, dextran has been studied to be used as a
carrier system for a variety of therapeutic agents including antidiabetic, antibiotic, anticancer,
peptides, and enzymes (Hwang et al. 2010). It has been also investigated to be used as an
antiviral agent, in the treatment of hypolipidemia, and for the prevention of free radical
damage, among other applications (Saboktakin et al. 2011).
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