Phytochemical Method Silver Nanoparticles: Synthesis and Characterization
The study of green synthesis of nanomaterials offers a valuable contribution to biomedicine at nanobiotechnology. This study focuses on the green synthesis of nanosilver from O. sanctum leaf extract and loading the nanosilver onto cotton fabrics and assessing their physical and biological properties.
In this study, O. sanctum leaf extract was used as reducing agent for the synthesis of silver nanoparticles. When the silver nitrate solution was mixed with leaf extract, the color changes occur immediately in silver nitrate solution. Initially, the leaf extract was green, which turned yellowish brown on adding the silver nitrate solution. The color changes indirectly indicate the formation of silver nanoparticles.
The color change was noted by virtual observation of O. sanctum leaf extract incubated with an aqueous solution of AgNO3. It started to change color from watery to yellowish brown at 4 h and dark pink at 24 h after incubation (Figure 1). It is due to the reduction of silver ions; this exhibits the formation of silver nanoparticles (Table 1). The color of the extract changed to intense brown along with threads after 24 h of incubation, and there was no significant change afterward.
Table 5. 1. Effect of leaf extract of O.sanctum on colour changes in silver nitrate solution at different time interval
Biosynthesis of nanoparticles by time-dependent absorption spectrum
The continuous formation of silver nanoparticles was investigated using UV-Vis spectroscopy, which has proven to be a useful spectroscopic method. The presence of silver nanoparticles was confirmed at a range of 200–600 nm. In UV-Vis spectra, silver nanoparticles can be shown by a SPR peak at around 400 nm, but a small shift (blueshift or redshift) in the wavelength of the peak could be related to obtaining —silver nanoparticles in different shapes, sizes, or solvent dependences. After 24 h of incubation, a typical peak of ?max at 421 nm was obtained due to the SPR of silver nanoparticles (Figure5. 2).
After the reaction time on adding of leaf extract reached 4 h, obtained silver nanoparticles showed a UV-Vis absorption peak, a characteristic SPR band for silver nanoparticles, centered at 400 nm (Figure 5.2).
Figure 2, the intensity of the SPR peak increased with the increase in the reaction time, which indicated the continued reduction of the silver nitrate ions, whereas the increase of the absorbance value with the reaction time indicated the increase in concentration of silver nanoparticles.
When the reaction time reached 12 h, the absorbance was increased and ?max value was slightly blueshifted to 435 nm. At reaction time of 24 h, the absorbance value was also increased and blueshifted to 435 and 421 nm, respectively. At the end of the reaction (24 h), the absorbance value was considerably increased and there was no significant change in ?max value (421 nm), compared with that at 12-h reaction time.
FTIR spectroscopy analysis of biosynthesized silver nanoparticles
FTIR measurements of the biosynthesized silver nanoparticle samples were carried out to identify the possible interactions between silver and bioactive molecules, which may be responsible for synthesis and stabilization (capping material) of silver nanoparticles. These were also to identify the possible biomolecules responsible for capping and efficient stabilization of the metal nanoparticles synthesized by leaf extract.
Figure 5.3 shows the FTIR spectra of aqueous silver nanoparticles prepared from O. sanctum leaf extract. The presence of the signature peaks of amino acids supports the presence of proteins in cell-free filtrate as observed in spectral analysis. The silver nanoparticle sample shows peaks at 3313.48, 3193, 2976.90, 2883, 1670, 1452, 1338, 1196.78, and 1112.75 cm?1 (Figure 5.3). The peaks corresponding to protein and silver nanoparticles were found commonly present in the nanoparticles synthesized by leaf extract.
X-ray diffraction analysis
The crystalline nature of silver nanoparticles was studied with the aid of XRD as shown in Figure 5.4. The dry powders of the biosynthesized silver nanoparticles were used for XRD analysis. The diffracted intensities were recorded from 20i‚° to 80i‚° at 2i?± angles.
Many strong Bragg diffracted peaks observed at 27.82, 32.25, 46.22, and 76.63 corresponding to 126, 199, 131, and 24 height of the face-centered cubic pattern of silver were obtained. The average grain size of the silver nanoparticles formed in the bio-reduction process was determined using Scherrer formula and it suggested that the synthesized silver nanoparticles were crystalline.
The size of the silver nanoparticles was found to be 26 nm, and it was determined using the width of the (126) Bragg’s reflection. In addition, yet some unassigned peaks were also observed suggesting the crystallization of biophase occurs on the surface of silver nanoparticles.
Fluorescence spectral analysis
Fluorescence spectroscopy is a type of electromagnetic spectroscopy which analyzes fluorescence from a sample. Figure 5.5 shows fluorescence emission spectrum from silver nanoparticles, dispersed in double distilled water. Fluorescence spectral analysis of silver nanoparticles used in the experiment was carried out to confirm the fluorescence emitted from the nanoparticles. A strong maximum at 431 nm wavelength and a quantum yield was 666.450 mV appeared in the fluorescence emission spectrum of O. sanctum leaf extract mediated silver nanoparticles.
Potentiometry analysis of biosynthesized silver nanoparticles
The biosynthesized silver nanostructure was shown and confirmed by the characteristic peaks observed in zeta potential, which will help to measure the diameter of nanoparticles with corresponding average zeta potential values, and also used for suggesting higher stability of silver nanoparticles. The reduction of silver ions to form nanoparticles was also monitored using a potentiometer.
The large negative potential value could be due to the capping of polyphenolic constituents present in the extract. Figure 5.6 shows the results of time-dependent zeta potential analysis from 0 to 24 h of incubation period. A pointed reduction in the potential could be observed on 4 h of interaction, further indicating the formation of nanoparticles at this stage. The potential decrease from an initial value of 0.436 V for silver ions to 0.153 V at the end of 11 h (Figure5.6) was observed, after which the decrease in potential was gradual, decreasing up to 0.048 V at the end of 24 h.
Zeta potential analysis of synthesized silver nanoparticles
The zeta potential analysis was used to measure the electrophoretic mobility of the silver nanoparticles. The complex zeta potential is a parameter that is used to learn the surface charges and stability of nanoparticles. The zeta potential charges significantly affect the particle distribution and agglomeration of nanoparticles. The high zeta potential value indicates a high electric charge on the surface of the nanoparticles. It describes strong repellent forces among the particles, which prevent aggregation and lead to stabilization of the nanoparticles in the medium. The zeta potential of the nanoparticles formulated was only measured in systems that did not sediment after overnight equilibration.
The alteration in zeta potential with a moment in time is shown in Figure 5.7. It can be observed that there was charge stabilization from 11 to 16 h, with the charge stabilized at around ?57 mV. The zeta potential was ?62 mV for the 14 h interacted samples, which further decreased to ?35 mV for the 24 h interacted samples.
SEM analysis of silver nanoparticles
The morphology of silver nanoparticle was observed using a SEM instrument. The shape and size of silver nanoparticles were analyzed after 24 h of incubation using SEM as shown in Figure 5.8. In general, the nanoparticles were spherical with varying size ranged from 7 to 28 nm. Most of the nanoparticles were combined, with only a few of them scattered, as observed under SEM.
The biosynthesized silver nanoparticles were mostly spherical. These were used to characterize the morphology, size, and distribution in aqueous suspension and were prepared by dropping the suspension onto a clean glass plate and allowing water to completely evaporate. It was evident that the ends of silver nanoparticles are brighter than the middle, suggesting the particles are encapsulated by biomolecules such as proteins in the Basil leaf extract (Figure 5.8).
EDS analysis of silver nanoparticles
The EDS spectrum (Figure 5.9) clearly identified the elemental composition of the synthesized nanoparticles, which suggests the presence of silver as the ingredient element. The vertical axis shows the counts of the X-ray and the horizontal axis shows energy in keV. The strong signals of silver correspond to the peaks in the graph confirming presence of silver. Biosynthesized silver nanoparticles typically show an optical absorption peak at 3.2 keV due to SPR. However, other elemental signals along with silver nanoparticles were also recorded, which were not observed for the biosynthesis of many other nanoparticles.
TEM was used to visualize the size and shape of silver nanoparticles. Figure 5.10 shows the typical TEM micrograph of the synthesized silver nanoparticles. It is observed that most of the silver nanoparticles were spherical. A few agglomerated silver nanoparticles were also observed in some places, thereby indicating possible sedimentation at a later time. It is evident that there is variation in particle sizes, and the average size was estimated to be 26 nm and the particle size ranged from 8 to 45 nm. The natural products, namely glycosides, flavanones, and reducing sugars, are the main constituents of the O. sanctum leaf extract