Chemistry Reference
In-Depth Information
as a non-toxic reducing agent. A fractal gold nanofilm atop the gold electrode was produced by this method
in a way of completely green chemistry [64]. The growing trend of exploring bacteria, fungi, actinomycetes
and plant materials for the biosynthesis of nanoparticles is considered as eco-friendly and a green technological
approach. For example, the synthesis of gold nanoparticles from gold precursor using the extract derived
from the marine sponge,
Acanthella elongata
, has been reported. Water-soluble organics present in the marine
sponge extract were mainly responsible for the reduction of gold ions to nanosized Au particles. The sponge
extract added to 10
−3
M HAuCl
4
aqueous solution at 45°C changed to pinkish ruby red colour solution
confirming the bio-reduction within 4h with continuous stirring [65]. A gold nanoparticles-chitosan
nanocomposite gel, for the immobilization of K562 leukaemia cells, was also prepared using a whole process
that did not introduce any environmental toxicity or biological hazards according to the fundamental principles
of green chemistry. Gold nanoparticles were produced in situ in a solution of chitosan, which is a non toxic
natural polysaccharide. Moreover, the protocols for the gel preparation and the immobilization of cells were
simple and environmentally friendly. Further modification of a glassy carbon electrode with the prepared gel
produced an irreversible voltammetric response and an increased electron transfer resistance upon cell
immobilization with a good correlation to the logarithmic value of their concentration [66]. Also, a green
colloidal method was described for the fabrication of polymer stabilized gold nanoparticles and nanofilms
using ascorbic acid as reducing agent of Au
3+
ions and gum arabic as dispersing agent. Ascorbic acid is a
nontoxic reducing species that originates the rapid formation of gold atoms. Furthermore, gum arabic, which
is a mixture of saccharides and glycoproteins, acts as a nontoxic dispersing agent preventing particles
aggregation [67]. Using a related approach, a green method for the preparation of platinum nanoparticles by
reduction of H
2
PtCl
6
with nanocrystalline cellulose from cotton as the reducing agent, was also described
[68]. A simple and effective green chemistry route for the facile synthesis of nanowire-like Pt nanostructures
at one step was also developed. In the reaction, dextran acted as a reductive agent as well as a protective agent
for the Pt nanostructures. Simple mixing of precursor aqueous solutions of dextran and K
2
PtCl
4
at 80°C
resulted in spontaneous formation of the Pt nanostructures. The as-prepared nanowire-like Pt nanostructures
were immobilized on glassy carbon electrodes using an electrochemical coupling strategy, and the resulting
nanostructured electrode exhibited an excellent electrocatalytic activity for the reduction of oxygen and
the oxidation of NADH [69].
The preparation of silver nanoparticles by different green synthesis approaches involving the use of mixed-
valence polyoxometallates, polysaccharides, irradiation and biological methods, among others, has been
reviewed. Regarding biological methods, different bio-organisms may act both as reducing and capping
agents for Ag nanoparticles synthesis. The reduction of Ag
+
ions by combinations of biomolecules found in
their extracts such as enzymes/proteins, amino acids, polysaccharides, and vitamins is environmentally
benign. The extract of unicellular green algae
Chlorella vulgaris
was used to synthesize single-crystalline
Ag nanoplates at room temperature. Proteins in the extract provide dual function of Ag
+
reduction and shape-
control in the nanosilver synthesis. The carboxyl groups in aspartic and/or glutamine residues and the
hydroxyl groups in tyrosine residues of the proteins were suggested to be responsible for the Ag
+
ion reduction.
Plant extracts from live alfalfa, the broths of lemongrass, geranium leaves and others have also served as
green reactants in the synthesis of Ag nanoparticles [70].
14.3 Solvents
An important concern in the implementation of greener analytical methods is the substitution of the organic
solvents commonly used in many protocols of sample preparation and analytes determination for other more
environmentally friendly solvents. Nowadays the search for other alternative solvents also implies the use of
their characteristics to introduce additional improvements such as a reduction in the time of analysis or an
