The reaction mixtures were air-dried on silicon wafers when the
sample was prepared for the SEM studies. This resulted in a coffee
ring phenomenon. When liquids containing fine particles are
evaporated on a flat surface, the particles tend to accumulate along
the outer edge and form typical structures [27]. Gold nanoparticles
are also known to display this phenomenon. For example, there
is a report on the formation of colloidal gold films that exhibit the
characteristic coffee ring feature [28]. In the present study, the gold
nanoparticles accumulated towards the periphery of the dried drop.
They formed micronetworks as well as dendrite-like structures
(white arrows) that could be observed even at low magnifications
(Fig. 3c and e). High magnifications images (1000× or 2000×)
revealed that the networks were composed of gold microcubes
arranged in a specific array (Fig. 3d and f). Fig. 3f also shows the
presence of triangles and hexagons in the patterned microwires
(black arrows). BPE thus consistently mediated the synthesis of gold
nanoparticles which due to the coffee ring phenomenon aggregated
into microcubes and networks.
There are a few reports on the biological synthesis of nanocubes.
For example, there is a report on the production of such structures
(sized 10–100 nm) by the bacterium, Bacillus licheniformis [29]. In
addition, cysteine grafted chitosan has also been used to synthesize
gold cubes. These cubes had an edge length in the range of
1–2m [16]. In comparison to these two reports, the size of the
microcubes due to the coffee ring effect with BPE were much larger
and in the range of 10–20m (Fig. 2b, d and f). Moreover, unlike
the earlier two reports [16,29], the cubes formed in this study (due
to the coffee ring effect) aggregated into networks that were very
long (Fig. 2a, c and e). There is a report on the formation of gold
nanowire networks by the cell-free supernatant of the bacterium
Rhodopseudomoas capsulata [30]. Such structures were particularly
observed with higher concentrations of the gold salt and the diameters
of these polycrystalline gold nanowires were in the nanometer
range (50–60 nm) unlike the results obtained in the current study.
There is a recent report on the use of peach fruit extract for
the synthesis of hexagonal or triangular gold nanoplates [31]. An
advantage of the present study is the use of waste banana peels
rather than the edible fruit for the synthetic process. There has
been an increasing interest in the use of soluble polymers including
biopolymers as soft templates for directing crystal growth and
controlling self-assembly of inorganic nanoparticles [32]. Banana
peels are largely composed of hot water soluble pectin, cellulose
Fig. 3. (a) Representative spot EDS profile confirming the presence of gold in
microcubes and microwire networks. (b) Representative XRD profile of thin film
gold microcubes and microwire networks.
and hemicelluloses that constitute nearly 80% of the mass [33]. Cellulose
is known to mediate nanoparticle synthesis [17]. Pectin, on
the other hand, has been used to stabilize chemically synthesized
bimetallic nanoparticles of gold and platinum [34]. These natural
polymers present in BPE may be providing a template for the
assembly of nanoparticles into micronetworks. Literature survey
has shown that biosynthesis of gold nanoplates and nanocubes are
a common feature. However, the formation of large microcubes
(10–20m) and their assembly into long networks is an unusual
phenomenon that has not been hitherto reported. The micronetworks
so assembled in turn, may have applications in a variety of
fields.
The EDS attachment on the SEM provided chemical analysis of
the field of view as well as spot analyses of minute particles and confirmed
the presence of specific elements. Fig. 3a is a representative
plot of the spot EDS analysis. The SEM-EDS analysis displayed signature
spectra for gold and thus convincingly evidenced the presence
of this noble metal in the microcubes and microwires. These results
are consistent with other reports on the EDS analysis of gold structures
synthesized by using extracts derived from the fruits of pear
or the leaves of Magnolia kobus or Diopyros kaki [31,35].