Green Synthesis of Silver Nanoparticles Using Aqueous Flower Extract of Cassia Angustifolia

The present study reports an eco-friendly, green synthesis of silver nanoparticles using aqueous flower extract of Cassia angustifolia (Ca-AgNPs) for the first time. Preliminarily the synthesis of Ca-AgNPs from the flower extract was visually confirmed by color change. Further, synthesized nanoparticles were characterized by UV-Visible spectroscopy, SEM, TEM, EDX, XRD, and Zeta potential analyser. SEM and TEM microscopic observation showed that the synthesized Ca-AgNPs were spherical shape with average size of 4-60 nm. XRD studies revealed FCC crystallite phase of synthesized Ca-AgNPs. The photocatalytic activity of green synthesized Ca-AgNPs was evaluated by the degradation of crystal violet (CV) cationic dye under UV light. Synthesized nanoparticles exhibited significant enhanced degradation of CV. After, 25 min of UV light irradiation more than 95% of CV dye was degraded.

Introduction

A reliable, eco-friendly and non-toxic synthesis of nanoparticles is a key step in the field of nanoparticles synthesis. Among the metal nanoparticles, silver nanoparticles (AgNPs) have much focused due to their varied applications such as photocatalyst and biocidal agent. There are number of synthesis methods are available to synthesis AgNPs such as phase transfer process, microwave assists process, electrochemical, sono-chemical, and more. Most of these methods are extremely hazardous and also uses toxic chemicals, which may leads to potential health and environmental problems.

In this study, we have used phyto-compounds as reducing and capping agents to synthesis AgNPs due to their non-toxic, eco-friendly and simple facile synthesis process. Literature reports suggest that the removal of cationic dyes using AgNPs is a better choice than the other common dye removal techniques. Photocatalytic degradation of cationic dyes by AgNPs synthesized using plants such as Cocos nucifera , Cordia dichotoma , Coccinia grandis and more has been reported.

In this paper, we have demonstrated the synthesis of AgNPs using flower extract of Cassia angustifolia for the first time. The catalytic performance was evaluated by photodegradation of crystal violet dye in aqueous solution under UV light irradiation.

Materials and Methods

Synthesis of Ag-nanoparticles

Plant material were collected from Botanical garden, Tamilnadu Agricultural University, Coimbatore and identified at Botanical survey of India division (BSI). About 20 g of dried flowers were extracted with 100 mL of purified distilled water using soxhlet extractor, and the synthesis of AgNPs using C.angustifolia flowers (Ca-AgNPs) was carried out according to the method of Bharathi et al.. Briefly, 90 mL of AgNO3 was taken into a conical flask and 10 mL of flower extract was added and kept at room temperature for synthesis, and color change. After the synthesis, purified Ca-AgNPs were obtained by centrifugation at 10000 rpm for 10 min. Extra impurities were removed by washing the synthesized Ca-AgNPs with distilled water followed by centrifugation at 10,000 rpm for 10 min and then purified nanoparticles were dried at 60˚C.

Characterization techniques

Formation of Ca-AgNPs was monitored by UV-Visible spectroscopy (JASCO-V-670). UV spectral analysis was studied in the range from 300-800 nm at a resolution of 2 nm. Surface morphology and average size of synthesized Ca-AgNPs was determined using scanning electron microscopy (SEM, Model-JEOL JSM-6400) and Transmission electron microscopy (TEM, JEM 2001, Japan). Presence of elements in synthesized Ca-AgNPs were identified using energy dispersive X-ray spectroscopy (EDX) attached with SEM. X-ray diffraction (XRD) analysis was carried out for the determination of crystalline structure of prepared Ca-AgNPs. X-ray diffraction recorded in the 2θ range from 20-80 at 40 kV/40 mA current with CuKa radiation (SHIMADZU, XRD-7000). Zeta potential value of the synthesized Ca-AgNPs was measured using dynamic light scattering instrument (DLS, Malvern Instruments Ltd, Malvern, UK).

Photocatalytic activity

The photocatalytic activity of Ca-AgNPs was assayed by evaluating the degradation of CV under UV (300W, UV lamp: λ≥420 nm,) irradiations as described by Arunachalam et al. [10]. About 25 mg of synthesized Ca-AgNPs were added to 100 mL of CV dye aqueous solution (0.1mg/100mL). Prior to irradiation, the suspension was stirred for 60 min in dark conditions to achieve adsorption/desorption equilibrium between the dye and catalyst. After the addition of Ca-AgNPs, the suspension was subjected to irradiations. Control setup was maintained without adding Cd-AgNPs to the test dye. At different time intervals, 5 mL of suspension was taken and centrifuged at 6000 rpm for 5 min and the absorbance spectrum was measured using UV-Visible spectrophotometer. The degradation percentage was calculated from the formula, E (%) = (C0-Ct / C0) × 100, Where E is the degradation efficiency, C0 is the absorbance before irradiation, and Ct is the absorbance at different time (t).

Results and discussion

After the addition of C.angustifolia flower extract into 1 mM AgNO3, the change in color was observed from yellow to brown color within 2-3 min (Fig. 1a). The brown color change in the reaction samples confirmed the formation of AgNPs and the color change may due to the excitation of surface plasmon resonance (SPR) in Ag reaction mixture. Phytochemicals present in the flower extract may responsible for the synthesis of AgNPs. The formation of Ca-AgNPs was monitored by UV-Visible spectroscopy and the strong UV- absorbance peak was observed at 452 nm (Fig. 1b), and consistent with other’s reports.

SEM and TEM microscope observation was performed to determine the morphology, size and shape of the Ca-AgNPs. SEM and TEM microscopic observation showed the shape of the synthesized Ca-AgNPs were mostly spherical shape with varied size range from 4-60 nm (Fig. 1c-e). Similar to our study, Aloe arborescens coated AgNPs exhibited spherical shape with varied size range from 40 to 50 nm [11].

EDX spectral analysis from Fig. 1f represents the signal from Ag along with Cl and O. Signal from 3 keV shows that, Ag has been correctly identified [5]. Another absorbance speaks of Cl and O in spectrum was most possibly due to the excitation of X-rays from bio-organic phase of existing minimal trace phyto-compounds in Ca-AgNPs.

Figure 2 shows the XRD pattern of Cd-AgNPs. From the XRD pattern, 2θ peaks observed at 32.81º, 46.95º, 64.84º and 77.18º which corresponding to 111, 200, 220 and 311 planes, respectively. The obtained peaks indicate the face centred cubic (FCC) crystallite phase of formed Cd-AgNPs. These planes were matched with JCPDS, file no. 4-0783 values for Ag. The extra peaks (*) in the given XRD data may due to the crystallization of trace phyto-organic compounds in the AgNPs. The mean average crystalline size of Ca-AgNPs was calculated using Debye-scherre’s formula:

D = 0.9λ / β cos θ,

Where “λ” is the wavelength of X-ray, β is FWHM in radians and θ diffraction angles. The average mean crystallite size was determined to be approximately 12 nm from breath of the refraction (111). Zeta potential value for the synthesized Ca-AgNPs was found to be negative values (-9.1mV) (Fig. S1). Negative potential values from zeta results supported the high stability and dispersity of Ca-AgNPs due to negative repulsion.

The photo catalytic activity of Ca-AgNPs was assayed for CV dye under UV light irradiation. The UV-Vis absorbance spectrum of CV dye was tested at different time interva. In order to determine CV dye degradation, a maximum absorption peak of CV at 590 nm is noticed and the light exposure time is monitored. The plot of C/Co versus time interval presents a gradual decrease with the increase of time interval in the presence of Ca-AgNPs, indicating a rapid decomposition of CV under UV light irradiation. Synthesized Cd-AgNPs exhibited more than 95% of dye degradation activity within 25 min for CV cationic dye. The synthesized Ca-AgNPs was found to be active for five cycles without any major deactivation, and more than 95% degradation activity was achieved in all the five experiments. This reusability test also supports the stability of synthesized nanoparticles. Similar to our study, AgNPs synthesized from plant compounds showed time dependent photo catalytic performance. It was reported that the photo catalytic activity can be strongly depend on the shape, size of the nanoparticles and process of generation, transfer, and consumption of the photo generated carriers.

Conclusion

In this study, we have demonstrated the simple eco-friendly green synthesis of AgNPs using C.angustifolia flowers. The spherical morphology and average size of the synthesized nanoparticles were characterized by SEM and TEM. Furthermore, green synthesized Ca-AgNPs exhibited significant enhanced photocatalytic activity for CV cationic dye. Overall, our finding suggested that eco-friendly green synthesized AgNPs can be used as a significant dye degradation agent for cationic CV dye with a good reusability feature.