Present work reports exceptionally high reducing capacity of
Marriage of biology with nanotechnology was one of the most fruitful outcomes in material research, particularly in synthesis of metallic nanoparticles such as gold and silver. Living systems have learned over the tenure of millions of years of evolution to combat metal toxicity. In order to deplete the toxic effects of the metal ions, organisms release enzymes and other reducing agents to lower the oxidation states [
The most challenging endeavours in exploiting these living systems for commercial production of nanoparticles are as follows. Living organisms require very stringent parameters to produce nanoparticles of desired size and shape. Due to complex physiological circuits found in living systems, it becomes a daunting task to predict the Time required for the synthesis can vary from few minutes to few days. Purification of the nanoparticles sometimes becomes difficult, particularly in case of intracellular production of nanoparticles [
On the other hand advantages offered by living systems are manifold. Exceptional biocompatibility of the nanoparticles offers widespread avenues for biological application of metal nanoparticles. Water soluble metal nanoparticles can be obtained which can be used for biological proposes without stringent surface modifications. Materials and energy consumption by living systems for the production of nanoparticles are extremely less making it a green alternative. The process is thermodynamically efficient due to involvement of enzymes and short peptides [ Easy availability and low cost are involved in processing of the precursors.
Amongst most of the organisms which are used for the production of nanoparticles, plants are the most celebrated systems due to their inherent capacity to accumulate metals [
As formed SNPs were studied using UV-Vis spectrophotometer (Lambda-25 Perkin Elmer, USA), SNPs synthesized at different pH values were analysed using Field Emission Gun-Scanning Electron Microscope (Carl Zeiss, Germany) operating at 10 KV. Particle size distribution (PSD) was performed using Nano Sight LM20 (Amesbury, UK). Concentration of Ag+ ions before and after addition of plant extract was analysed using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) using ARCOS, Germany. Equation (
Surface charge of SNPs with respect to pH was determined using Zetasizer (Malvern, UK). Cyclic voltammetry (CV) studies to comprehend the reduction of SNPs in the solution triggered by the
SNPs synthesized at the above-mentioned pH values were centrifuged to remove excess reducing agents before performing stability test. Three mL of the SNPs synthesised at respective pH was taken in a quartz cuvette, and initial spectrum was recorded between 400 and 800 nm. 100
Flocculation parameters were calculated by measuring the integrated absorbance between 450 and 750 nm in case of SNPs [
One of the key requirements for efficient production of the metal nanoparticles is their rapidity of formation as well as monodispersity after addition of the reducing agents. In our work, reduction of the silver ions into SNPs was catalysed within 120 sec at 28 ± 2°C after addition of the peel extract. This is a very short time for synthesis of SNPs using plant extract unlike earlier observations using biological systems which report formation of the SNPs after 24 h at 30°C [
Figure
(a)
Any agglomeration of the nanoparticles in the solution can be ruled out due to sharpness of the peaks. This was also speculated due to the absence of turbidity in the SNP solution. A detailed study of agglomeration is presented in later part of the discussion. Abruptness in FWHMs with respect to time may be caused due to size and shape distributions of the nanoparticles during the rapid reduction process of silver ions to SNPs.
Exceptionally high reducing property of
Time dependent appearance of colour at respective pH values (2.8, 4.5, 5.5, 7, and 10) is shown in Figure
(a) Pictorial snapshots showing formation of SNPs at different pH values (in seconds).
At pH 2.8, slightly pink colour appeared after addition of the peel extract which became slightly blue after 120 sec (Figure
FE-SEM image (a) of SNPs biofabricated at pH (A) 2.8, (B) 4.5, (C) 5.5, (D) 7, and (E) 10 and dynamic light scattering (DLS) (b) of respective SNPs showing particle size distribution.
At pH 4.5 rapid formations can be seen, in stark contrast to acidic pH 2.8. As presented in Figure
SEM results show SNPs of roughly spherical morphology (Figure
At pH 5.5, comparatively faster reduction of Ag+ ions and hence appearance of colour can be seen in Figure
At pH 7, a peak became less intense and broader at 413 nm. In this case, a blue shift of 6 nm was observed (from 419 to 413 nm). Blue shift is an indicative of decrease in the diameter of the nanoparticles as well as its stabilization in the solution. The observation can also be verified from the SEM image (Figure
Extremely alkaline pH (pH 10) had profound influence on the formation of SNPs. Exceptionally fast reduction of the nanoparticles can be seen in Figure
Reduction of silver ions to form SNPs in solution was also verified by CV by assessing the change in the oxidation states of silver ions (Figure
(a) Cyclic voltammetry showing the rapid reduction of silver ions using silver electrode as reference and scan rate of 1000 mV S−1 (arrow is showing the event of addition of plant extract in the reaction mixture) and (b) open circuit potential during the synthesis of SNPs, points A-B represent the potential of KNO3 after addition of AgNO3 solution, C is the point where the plant extract was added, and D-E show wide constant potential of SNPs after reduction of Ag+ ions to Ag0.
Peaks observed at 0.46 and 0.28 V are assigned to reduction and oxidation potentials of AgNO3, respectively. In 25 mL 0.1 M KNO3 containing 25
Open circuit potential (OCP) under the above experimental conditions was performed to check time required for the reduction of the silver ions against potential. As shown in Figure
Powder X-ray diffraction shows the crystalline nature of biosynthesised SNPs (Figure S3). Arrangement of atomic arrangement within the crystal and size of SNP were calculated. Comparison with the standard (joint committee in powder diffraction standards file no. 04-0783) confirmed that the particles were SNPs as depicted by the peaks at
This equation exploits the reference peak width at angle
ICP-AES analysis of Ag+ content after reduction of AgNO3 by plant extract was found to be 1500 ppm at pH 7. Using (
TGA explains the surface protection of SNPs by some unique molecules, most probably short peptides (Figure S5). To remove organic peptide and protein capping of SNPs, peptidases and proteinases, respectively, could be used to degrade them into individual amino acids, and hence the nanoparticles can be used for further functionalization.
Stability of the SNPs was studied adding multiples of 100
The more the red shift, the less stable the particle at that salt concentration. Figure
UV-visible spectra showing the change in the optical properties of SNPs with respect to time after addition of the NaCl at pH (a) 4.5, (b) 5.5, (c) 7, and (d) 10.
Another important terminology to display the stability of the nanoparticles is flocculation parameter (FP) originally used by Wiesbecker et al. [
(a)
At pH 5.5, FP was found to be increasing with increase in time. This indicates agglomeration of the nanoparticles as evident from the colour of the solution. However, no signs of agglomeration are seen in the SEM image. Blue shift in this case indicated the agglomeration or increase in the multiple coatings on the surface of nanoparticles which may result in agglomeration. At alkaline pH (7 and 10), FP was found to increase initially but became constant and after 15 min finally dropped to some extent. Decrease in the FP value indicates more stability of the nanoparticles [
Sunil Pandey and Ashmi Mewada have equal contributions to the work.
The authors wish to acknowledge the financial support provided by the authorities of SICES, Ambernath, and specially Mr. K. M. S. Nair and Mr. K. M. K. Nair. They give special thanks to Mrs. Chalke, TIFR Mumbai, for carrying SEM analysis. They acknowledge IIT Bombay, SAIF department, for carrying out ICP-AES analysis. The authors are obliged to reviewers for their valuable suggestions.