The Alstonia scholaris plant leaves were collected from surrounding of hilly region of Khandesh (Maharashtra, India). Other chemicals required for the synthesis of silver nanoparticles and core-shell nanoparticles such as silver nitrate, iron nitrate were procured from Merck, India.

The collected samples were brought to the laboratory and washed using fresh water to remove the contaminants from the leaf surface (Fig. 1), which were shade dried for 15 days under dust free condition. These dried leafs were cut into small pieces and grounded into fine powder on grinder. This powder was sieved using 75 µm mesh size sieve shaker. The extract was prepared using this powder (25 gm) in millipore water (100 ml). This mixture was sonicated for 40 min (B03-Ultrasonic Processor make E-Chrom Tech., Taiwan) at pulse rate of 5 sec on and off with intensity 20 KHz. The color of the solution turns to a light yellow (Fig. 2). This leaf extract was allowed to cool at room temperature and filtered using Whatmann filter paper No.1. This extract was stored in refrigerator to protect it from fungal growth. Further this extract was used as reducing and capping or stabilizing agent for the synthesis of AgNPs, α-Fe₂O₃NPs and Ag-Fe₂O₃ CNPs.

Fig. (1) Alstonia scholaris plant leaves.

Fig. (2) Solutions of i) Alstonia scholaris plant leaf extract, ii) silver nitrate solution, iii) iron nitrate solution, iv) Colloidal suspension of AgNPs, v) Colloidal suspension of Ag-Fe2O3CNPs.

0.01 M silver nitrate (AgNO₃) was added drop wise into Alstonia scholaris leaf extract (25 ml) under controlled ultrasound assisted cavitation technique. The pulse rate was maintained at 5 sec on/off till the complete addition. The mixture was sonicated for additional one hour at same pulse rate and intensity (20 KHz). During this process, the color of colloidal suspension was change from yellowish to light brown. Further the particles were isolated from this colloidal suspension using ethanol as a non-solvent. The Ag particles were dried in hot air oven at 80 ^oC for one hour. Similar procedure was adopted for the synthesis of α-Fe₂O₃ NPs using 0.01 M iron nitrate.

The Ag-Fe₂O₃CNPs were prepared by taking colloidal suspension of AgNPs (25 ml) to which the colloidal solution of α-Fe₂O₃NPs were added drop wise under controlled ultrasound caviation technique. The pulse rate was 5 sec on/off throughout the reaction at intensity 20 KHz. The color of colloidal suspension was appears to be dark during the process. After complete sonication, the particles were isolated from colloidal suspension using ethanol as a non-solvent. Ag-Fe₂O₃CNPs particles were dried in hot air oven at 80 ^oC for one hour. The core was of AgNPs, while the shell of α-Fe₂O₃NPs.

UV-Vis absorption spectra at resolution of 2 nm were recorded on a double beam spectrophotometer 1800 (Shimadzu, Tokyo, Japan) in a wavelength range between 300-600 nm.

X-ray diffraction (XRD) analysis was conducted to know about the size of particles on Brukers D8 advance X-ray diffractometer (Germany) with CuKα₁ radiation (λ=1.5404 Å) at 30 kV & 40 mA at scanning range of 10-80^o with intensity 2500 cps.

A transmission electron microscope (TEM) (Philips CM200, Netherlands) was used to assess the morphology and size of nanomaterials at a resolution of 2.4 Aº. The samples were dispersed in ethanol and deposited on a copper grid before viewing under the microscope.

The FE-SEM analysis was done on field emission-scanning electron microscopy (FE-SEM) (Hitachi S-4800, Tokyo, Japan) used to assess the surface morphology and energy dispersive X-ray spectroscopy (EDAX) for the elemental analysis. The samples were dispersed in ethanol and mounted on specimen tub with a gold coating before viewing under the microscope.

A Fourier transform infrared (FTIR) spectrum was recorded on FTIR-8400 spectrophotometer (Shimadzu, Tokyo, Japan) within the spectral range of 500 - 4000 cm^-1 with a resolution of 4 cm^-1. The samples were prepared in the pellet form by mixing potassium bromide (KBr) as a binder.

Fig. (3) shows a schematic presentation of AgNPs, α-Fe₂O₃NPs and Ag-Fe₂O₃CNPs. Ultrasound assisted synthesis involves the cavitation mechanism, which refers to the formation, growth and collapse of bubbles due to transmission of acoustic waves through a medium which ruptures the molecular structure of the medium (Fig. 3). During cavitation, bubbles grow due to the diffusion of Ag⁺ into it by implosive collapse and get concentrated at the interface where they react with the high energy species and along with beginning of nucleation. The formed nanoparticles are bounded by the layer(s) of various stabilizing agents, which are present in plant extract, which protects the particles from agglomeration arise due to strong charges over the surface of ions such Ag and Fe ions. On reaching its maximum size, the bubble undergoes implosive collapse, which resulting high temperature of the solution attaining along with faster cooling rate. At such fast kinetics the growth of the nuclei is hindered and consequently, nanosized silver particles are formed and reduction takes place. These nanoparticles have a large number of dangling bonds on their surface. For surface stability bond formation is necessary, which leads to growth of nuclei. But when the growth is stopped prematurely, the stabilization of the silver nanoparticles occur through bond formation with stabilizing agents present in the plant extract of Alstonia scholaris which prevents aggregation of nanoparticles (Fig. 6a). Further the formation of CNPs using Ag and iron oxide was continued by in-situ method under ultrasound cavitation technique. The shell formation was took place over the core of AgNPs. The core and shell bonding is took place due to presence of bonding agent in Alstonia scholaris leaf extract.

Fig. (3) Mechanism of AgNPs, α-Fe2O3NPs and Ag- Fe2O3CNPs formation.

AgNPs and Ag-Fe₂O₃CNPs formation by Alstonia scholaris leaf extract of green synthesis approach was studied using UV-Vis spectroscopy. The sharp peak at visible region is evidence for the formation of AgNPs and Ag-Fe₂O₃CNPs. Metal and core-shell particles (AgNPs and Ag-Fe₂O₃CNPs) posses free electrons abundantly, which moves through conduction and valance band. These free electrons are responsible for surface plasmon resonance (SPR) absorption band and broad peak with red shift from the core for core-shell particles. It was observed that AgNPs exhibits surface plasmon resonance (SPR) absorption at 375 nm, while Ag-Fe₂O₃CNPs shows broad peak with a red shift from the core of AgNPs (Figs. 4a, b). The large shift of the SPR peak can be accounted by the local dielectric effect, as the oxide shell has higher refraction index of 2.4, which turns to red-shift from SPR absorption band according to classical Mie theory. A similar phenomenon has been observed elsewhere [27Shi W, Zeng H, Sahoo Y, et al. A general approach to binary and ternary hybrid nanocrystals. Nano Lett 2006; 6(4): 875-81.
[http://dx.doi.org/10.1021/nl0600833] [PMID: 16608302] ].

Fig. (4) UV-Vis of AgNPs and Ag- Fe2O3CNPs.

The crystallite domain size was calculated from the width of the XRD peaks, assuming that they are free from non-uniform strains, using the following Scherer’s formula.

(1)

Where, D is the average crystallite domain size perpendicular to the reflecting planes, λ is the X-ray wavelength, β is the full width at half maximum (FWHM), and θ is the diffraction angle. (Fig. 5) shows the XRD pattern of Ag-Fe₂O₃CNPs, AgNPs and α-Fe₂O₃NPs. The peak at 24^o, 31^o, 39^o, 43^o, 65^o, 67^o and 75^o corresponds to crystal planes (012), (104), (111), (200), (220), (300) and (311) The sets of crystal plane such (012), (104), (116) and (300) other the crystal planes of AgNPs and α-Fe₂O₃NPs indicates the formation of core-shell. The peaks at 39^o, 43^o, 65^o and 75^o corresponds to (111), (200), (220) and (311) planes. These planes can be indexed as face-centered –cubic structure (FCC) of AgNPS, whereas crystal planes at (111), (200), (220), (311), (400), (422) and (511) for α- Fe₂O₃NPs. There is no any additional peak, which is the indication of formation of high purity product. This effective formation of Ag-Fe₂O₃CNPs AgNPs and α- Fe₂O₃NPs was due to synthesis using Alstonia scholaris leaf extract, which doesn’t form any side products.

Fig. (5) (a) Ag- Fe2O3CNPs (b) AgNPs (c) α-Fe2O3NPs.

The morphology of Ag-αFe₂O₃CNPs and AgNPs was studied using transmission electron microscopy. It was observed that AgNPs having spherical as well as triangular pyramidal shape with minimal aggregated structures. The size of AgNPs was found to be ~15-38 nm Figs. (6a, b) shows the TEM of Ag-αFe₂O₃CNPs. It was found that the core (AgNPs) having dark black shade, while the shell (αFe₂O₃NPs) is having faint grey shade. The overall diameter of Ag-αFe₂O₃CNPs was ≈50 nm, while diameter of AgNPs core was ≈10 nm with αFe₂O₃NPs thickness over core is ≈4 nm. Fig. (6c) in inset clearly shows the Ag-αFe₂O₃CNPs. Fig. (6c) shows the clear view of core-shell nanoparticles.

Fig. (6a) TEM of AgNPs.

Fig. (6b) TEM of Ag- Fe2O3CNPs (c) Inset of core-shell structure.

Fig. (6c) TEM of Ag- Fe2O3CNPs (Clear image).

Fig. (7a-b) shows FE-SEM of AgNPs & Ag-αFe₂O₃CNPs synthesized using Alstonia scholaris leaf extract. It can be observed that most of AgNPs were of spherical shape having size ~17-51 nm, while Ag-αFe₂O₃CNPs having diameter ≈ 50 nm with core of AgNPs and shell of αFe₂O₃NPs. The particle size and shape as seen in FESEM is in line with TEM. Moreover, Figs. (8a, b) shows the energy-dispersive X-ray spectroscopy (EDAX) for AgNPs & Ag-αFe₂O₃CNPs synthesized using Alstonia scholaris leaf extract to examine the composition and purity. Elemental silver could be seen in the graph presented by the EDAX analysis, which indicates the reduction of silver ions to elemental silver by Alstonia scholaris leaf extract. Moreover the presence of elemental silver ion along with the iron oxide indicates the formation of Ag-αFe₂O₃CNPs. The vertical axis displays the number of X-ray counts while the horizontal axis displays energy in keV.

Fig. (7) FESEM AgNPs and Ag- Fe2O3CNPs.

Fig. (8) EDAX of AgNPs and Ag- Fe2O3CNPs.

FT-IR spectroscopy was applied to identify the potential functional groups presented in the bio molecules of Alstonia scholaris leaf extract, which are responsible for reducing the silver ions as well as formation of core-shell nanoparticles. A representative FTIR spectrum of the synthesized AgNPs, Ag-αFe₂O₃CNPs & αFe₂O₃NPs was shown in Figs. (9a-c). The peaks at 3400, 1568, 1492, 1383, 903, 510 cm^-1 in the region of 500-3400 cm^-1 was the indication of formation of AgNPs using Alstonia scholaris leaf extract as reducing agent Fig. (9a). The indicative absorption peak at 3364 cm^-1 attributed to N-H stretching for primary and secondary amine or amide functional groups. Besides this the peak at 1568-1492 cm^-1 might be due to asymmetric stretching vibrations of –N-O and peak at 1383 cm^-1 was for symmetric stretching vibration of –N-O functional group. The absorption peaks 903 cm^-1 can be assigned as the absorption peaks of – C-N waging of primary or secondary amines and peak at 510 cm^-1 can be attributed due to C-X stretch of alkyl halide (X is any halide group) [28Luo L-B, Yu S-H, Qian H-S, Zhou T. Large-scale fabrication of flexible silver/cross-linked poly(vinyl alcohol) coaxial nanocables by a facile solution approach. J Am Chem Soc 2005; 127(9): 2822-3.
[http://dx.doi.org/10.1021/ja0428154] [PMID: 15740096] ]. Fig. (9b) shows the FT-IR of Ag-αFe₂O₃CNPs. Number peaks were observed and the major indicative peaks are 3400, 1532, 1442, 1072, 550 cm^-1 in the region of 500-3400 cm^-1. Moreover the same phenomenal observation was in case of αFe₂O₃NPs Fig. (9c). The indicative absorption peak at 3380 cm^-1 attributed to N-H stretching for primary and secondary amine or amide functional groups.

Fig. (9) FTIR of (a) AgNPs (b) Ag- Fe2O3CNPs (c) α-Fe2O3NPs.