were crystalline, stable, well dispersed and spherical in shape and had an average
diameter of 30.1–35.1 nm (Table 3).
Table 3 Characteristics of monometallic and bimetallic nanoparticles prepared
using Grape fruit extract treated by plasma discharge
Type NPs
Zeta potential, (mV)
Average diameter, nm
Ag
– 31.0
32.3±3.1
Ag/Au
– 40.8
30.1±2.2
On the basis of FT-IR analysis, it can be stated that the hydroxyl, carbonyl and
carboxyl functional groups present in bio-molecules of Grape fruit extract are
responsible for the reduction of Ag
+
, Au
3+
ions and stabilization of the obtained NPs.
For the formation of MNP
S
, the standard mechanism involves two stages, namely,
formation and polymerization of atoms. In the first stage, a part of metal ions in the
solution is reduced by available reduction groups. The atoms formed in this way act as
nucleation centers and catalyze the reduction of residual metal ions in the solution.
Compared with other water-soluble polymers, alginate is an anionic polymer with high
charge density. Negatively charged alginate facilitates the attraction of positively
charged silver ions to polymer chains, which are then reduced by means of existing
groups. Preformed metal atoms adsorb Ag
+
on the surface through dimerization or
association with excess ions due to the binding energy between metal atoms. Surface
ions are decreased again, then the atoms merge, leading to the formation of metal
clusters. Thus, the aggregation process does not stop and leads to the formation of
larger particles. The process is stabilized in the presence of functional groups,
preventing further coalescence. Probably, the metal clusters will be fixed through
strong bonds between the MNP
S
surface and the “O” atom of the (СOO–and OH)
functional group of extract [23]. So, the negative surface charge of functional groups
stabilizes metal nanoparticles against aggregation due to electrostatic and steric effects.
The DPPH free radical scavenging assay, total antioxidant capacity, reducing
power assay, total flavonoid contents and total phenolic contents determination
confirmed that synthesised Ag,
Ag/Au
NPs
are potent antioxidant agents; can be used
as an effective scavenger of free radicals.
The
Ag,
Ag-Au
NPs showed excellent catalytic activity for the reduction of p-
nitrophenol (4-NP) to p-aminophenol (4-AP) in the presence of NaBH
4
.
Fig. 3 shows
the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). 4-NP shows an
absorption peak at about 320 with a shoulder at about 400 nm.
It was seen that this peak undergoes a red shift from 320 to 400 nm immediately
upon the addition of aqueous solution of NaBH
4
, corresponding to a significant change
in solution color from light yellow to yellow-green due to formation of 4-
nitrophenolate ion. In the absence of Ag NPs catalyst, the absorption peak at 400 nm
remained unaltered for a long duration, indicating that the NaBH
4
itself cannot reduce
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