Green synthesis of cobalt nanoparticles by using methanol extract of plant leaf as reducing agent

In the present work total reducing strength or phenolic compounds in leaf extract of Conocarpus erectus and Nerium indicum were determined and then Cobalt nanoparticles (Co NPs) were synthesized by using only methanol extract of Conocarpus erectus leaves as reducing agent because of its higher values of total phenolic compounds (296 ± 9 μg/g) in comparison to Nerium indicum (185 ± 6 μg/g). Characterization of the green synthesized Co NPs was performed by SEM (Scanning Electron Microscope) and XRD (X-Ray Diffractometer) techniques. The size of Co NPs was estimated in the range of 20-60 nm. The usage of plant extract for the preparation of Co NPs makes the process cost effective, non-toxic and green method.


Research Article
Green synthesis of cobalt nanoparticles by using methanol extract of plant leaf as reducing agent Introduction Cobalt nanoparticles (Co NPs) show a variety of properties such as electrical, magnetic, and catalytic because of their large surface area.They also show below a critical size of 20 nm as singe-domain particles displaying quantum size effects, super para magnetism and large magnetic anisotropies [1].Future applications of Co NPs in the area of catalysis, separation technology biomedicine and information storage systems, require the Co NPs to be discrete, identical in size and shape, uniform in composition and crystal structure.Several in-solution methods (liquid-phase reduction methods) are relatively simple, less expensive quicker to

Preparation of extract of samples
Samples (Conocarpus erectus and Nerium indicum) were obtained from the Main Campus of NED University Karachi and they were shad-dried for one week and then they were grinded.100 g of shade-dried leaves were grinded to form powder then it was added to 500 mL methanol, ethanol and distillated/deionized water in 1L flask and mixed vigorously.The preparation of plant extract was performed by using hot plate/magnetic stirrer at 50° C for 1 h.The acquired plant extract was filtered by vacuum filtration assembly.Determination of total reducing strength or total phenolic compounds Total reducing strength or total phenolic compounds in all plant extracts were determined as described by singleton et al.
[9] with the help of spectrophotometer.Briefly 0.5 mL of extract added with 10 % Folin-Ciocalteu's reagent (2.5 mL in equal voume of 7.5 % NaHCO3).Blank was prepared by adding methanol (0.5 mL), 10 % Folin-Ciocalteu's reagent (It was dissolved in water and 7.5 % NaHCO3 in equal volumes of 2.5 mL).The reaction mixtures were incubated at a temperature of 45° C for 45 minutes in an incubator/thermostat.The absorbance of solution was noted at 765 nm wavelength using spectrophotometer.Standard solution of gallic acid was used as standard.

Green synthesis of cobalt nanoparticles
The cobalt nanoparticles were prepared in a 250 mL conical flask in which 50 mL cent molar solution of cobalt nitrate was mixed with 10 mL of the plant extract (100 g of dried leaves powder of was added to 500 mL methanol, ethanol and deionized/distillated water in 1L flask) along with vigorous shaking on a hot plate till the appearance of dark brown colour.

Characterization of Co NPs
The external appearance and size of produced Co NPs were characterized by using SEM (Scanning Electron Microscope) and X-Ray Diffractometer (XRD).

Results and discussions Total phenolic compounds
Leaves extracts (water, ethanol and methanol) of Conocarpus erectus and Nerium indicum were investigated for total phenolic compounds and results are presented in table 1.It can be seen that the total phenolic compounds were found higher in methanol extracts (Conocarpus erectus, 296 ± 9 µg/g; Nerium indicum, 185 ± 6 µg/g) lower in water extract (Conocarpus erectus, 59 ± 8 µg/g; Nerium indicum, 27 ± 2 µg/g).As methanol leaf extract of Conocarpus erectus contained higher values of phenolics so in the present study it was used for the green synthesis of Co NPs.  have reported that the total phenolic compounds were higher in ethyl acetate fraction of fruits and flowers (303.45 and 301.15 mg/g GAE respectively) whereas they were lower (186.21and 181.61 mg/g GAE) in leaves and stem.On the contrary our results shows the methanol extract of leaves contain higher values which might be due to environmental effect and choice of solvent [10].Vinayagam and Sudha,[7] has reported in scientific literature that the total reducing strength or total phenolic compounds was found to be higher in Nerium indicum flower extract (449 mg/100g), as compared to leaves 227mg/100g.Our selection of leaves extract as raw material for the green preparation of cobalt nanoparticles is only due to the abundance and availability of leaves throughout the year.

Green synthesis of cobalt nanoparticles
In the present work our main focus is on the synthesis of Co NPs using reducing properties of total phenolic compounds inside the plant leaves according to the scheme/mechanism (Fig. 1).Of course, the effect of other phytochemicals inside the plant is also possible [4].

SEM analysis
The SEM analysis is helpful in determining the structure of the Nanoparticles (or reaction products) that were fashioned.The SEM image (Fig. 2) disclosed a number of discrete and spherical Co NPs as well as some larger groups.The SEM image of Co NPs also revealed that spherical shaped and to the some extent irregular nanoparticles were fashioned with the diameter range 20-60 nm whereas grouped particles were fashioned above the range 100 nm.Similar types of images for Co NPs were also reported by various researchers [1-3].

Figure 1 .
Figure 1.Scheme/Mechanism for the formation of Co NPs by phenolic compound

Figure 2 .
Figure 2. Scanning electron microscope image of green synthesized Co NPs by Conocarpus erectus XRD analysis The powdered sample was used for XRD Analysis in order to confirm the presence of Co NPs.Relative intensities and Peak positions [Seven peaks at 2θ values of 30, 35, 39, 43, 54, 57 and 62 degrees corresponding to (330), (311), (222), (400), (422), (511) and (440) planes of cobalt] for green synthesized Co NPs (Fig.3) were matched to values from JCPDS (Joint Committee on Powder Though extract of both compounds have been reported rich in phenolic compounds but Conocarpus leaf extract was reported a large variety of these compounds [7, 8].The aim of present work is to synthesize cobalt nanoparticles by using leaf extracts of plant (Conocarpus erectus or Nerium indicum) having higher values of total phenolic compounds because to the best of our knowledge through literature it was the first time to use plant leaf extract (Conocarpus erectus or Nerium indicum) for the green synthesis of Cobalt nanoparticles.