Green synthesis, structural characterization, and catalytic activity of silver nanoparticles stabilized with Bridelia retusa leaf extract

Chemical reagents and glassware

Silver nitrate and RhB dye were acquired from Merck, India. Sodium tetrahydroborate (NaBH₄) was procured from SRL Pvt. Ltd., Mumbai, India. All the glassware were properly acid-cleaned and then rinsed with Millipore-MilliQ water.

Preparation of leaf extract

Fresh B. retusa leaves were collected in the month of March, in the Manipal University premises, washed initially with tap water to eliminate dirt and other stuck-on particles, and then rinsed with Millipore-MilliQ water. After allowing them to dry under the shade, the washed leaves were chopped into fine pieces and 10,000 mg of the sample was boiled in 100 ml of Millipore-MilliQ water. The resultant contents were cooled and filtered, which yielded a clear yellow filtrate. The BRLE thus collected was preserved at 4°C for future purposes.

Synthesis of SNPs

The synthesis of SNPs was initiated by mixing 10 ml of BRLE to 90 ml of 0.001 mol/l silver nitrate solution in a flask. The flask was incubated in a temperature-controlled water-bath at 80°C for 10 min. One control experiment (without BRLE) was also conducted with the same conditions. The synthesis process was monitored with respect to time to examine the plasmon resonance band pattern using UV-visible spectroscopy. The control sample containing only silver nitrate did not develop any color change. In contrast, the color of the sample containing silver nitrate and BRLE changed from yellow to deep brown, which connoted the synthesis of SNPs.

Characterization of SNPs

The optical properties of SNPs were analyzed using UV-Visible spectrophotometer (Shimadzu, Model 1700) in a range of 200–800 nm. The morphologies and sizes of the SNPs were determined by scanning electron microscope (EVO MA 18). The energy dispersive X-ray (EDX) analysis (Oxford) confirmed the elemental composition of SNPs. The X-ray diffractometer (XRD, Rigaku Miniflex 600) having Cu Kα radiation source and Ni filter was used to ascertain the crystallinity of SNPs. The scanning was done within 1–2 min at a range of 20°–80°. Fourier transform-infrared spectrometer (FT-IR, SHIMADZU-8400 S) was employed to analyze (4000–400 cm⁻¹ range) the functional groups involved in the synthesis of MNPs. Dynamic light scattering (DLS) instrumentation (Malvern-Zetasizer nanosizer) was employed to determine the hydrodynamic size of the SNPs.

Catalytic degradation of RhB

The catalytic potential of SNPs was tested against a model dye, RhB (λ_max=555 nm) in the presence of sodium tetrahydroborate as a reductant. The method suggested in our previous study [35] was carried out with few modifications. Briefly, two clean quartz cuvettes were taken, after which 2 ml of requisite concentration of RhB and newly prepared sodium tetrahydroborate (500 μl) was mixed to each of them thoroughly. To one of the cuvettes, 500 μl of SNPs was added. The extent of dye degradation process of both cuvettes was monitored with respect to time by checking the spectral details using UV-Visible spectrophotometer.

Visual and UV-visible studies

The addition of the silver nitrate solution (colorless) to the BRLE (yellow color) resulted in deep brown color (Figure 2), a characteristic feature of SNPs. The change in color is due to the reduction of Ag⁺ to Ag^o by the biological components found in the leaf extract.

Figure 2:

Formation of SNPs using BRLE.

In order to ascertain this reduction, UV-vis spectroscopic analysis was performed (Figure 3). In general, SNPs exhibit a UV-vis absorption maximum between 400 and 500 nm which depends on the size and shape of the SNPs formed and the biomolecules present in the medium [36]. In the present investigation, a characteristic surface plasmon resonance (SPR) peak at 420 nm was observed in the spectrum. This result is consistent with the observations made by other researchers [37], [38]. In addition, the narrow peak corroborates the fact that the synthesized SNPs are monodispersed [39].

Figure 3:

UV-vis spectra of SNPs synthesized using BRLE.

SEM with EDX analysis

SEM was used to analyze the surface morphology of the SNPs (Figure 4). The SNPs formed in this investigation were spherical in shape. Similar results were obtained by many experts for SNP synthesis using other plant extracts [40], [41]. The aggregate formation of SNPs may be due to the procedures involved during sample preparation [42] and the electrostatic interactions between the biomolecules present in the BRLE and SNPs [43].

Figure 4:

SEM image of SNPs synthesized using BRLE.

The elemental composition of the SNPs was determined by EDX analysis. Figure 5 reveals the strong signal for silver (3 keV). This finding ascertained the existence of metallic silver in the sample. The appearance of other peaks, such as Ca and Si, may be due to the glass slide that bears the sample [44].

Figure 5:

EDX spectrum of SNPs synthesized using BRLE.

XRD analysis

The X-ray diffractogram of synthesized SNPs is shown in Figure 6. The diffractogram affirms the crystallinity of the SNPs as (1 1 1) plane of the face-centered-cubic corresponding to a 2θ value of 38.65°. This result is consistent with the standard data file JCPDS No. 42-0783. This crystalline structure is comparable with the other published literature [45]. Moreover, the sharp and single peak obtained in the present study signified the purity of the synthesized SNPs. Scherrer equation (Equation 1) was used to calculate the crystallite size and is given by

Figure 6:

XRD pattern of SNPs synthesized using BRLE.

where d, λ, β_0.5, K, and θ are the size of SNPs, wavelength (0.154 nm) of the X-ray source, full width at half maximum of the diffraction peak, Scherrer constant (0.9–1), and Bragg angle in the radian, respectively. The mean size of SNPs was calculated as 16.21 nm. The lattice parameter was 0.4032 nm, which was coherent with the standard lattice parameter of 0.40729 nm for metallic silver [46].

DLS studies

The average particle size distribution of SNPs is shown in Figure 7. The average hydrodynamic diameter of 201.68 nm obtained in the present study is acceptable because the DLS measures only the hydrodynamic diameter and not the actual size of the nanoparticle [47]. The single peak indicates the uniform distribution of nanoparticles. The extent of distribution of particle size is given by poly dispersity index, and the low value attained in this method (0.283) affirms the monodispersity of the nanoparticles [48].

Figure 7:

Particle size distribution of SNPs synthesized using BRLE.

The zeta potential analysis of the synthesized SNPs is shown in Figure 8. The zeta potential value obtained from the analysis is −18.1±4.46 mV, conveying good stability of SNPs [49]. It is proposed that the negative value is due to the capping effect of biomolecules in the BRLE. The repulsion among the negatively-charged particles prevents the aggregation, thereby resulting in stable nanoparticles [50].

Figure 8:

Zeta potential distribution of SNPs synthesized using BRLE.

FT-IR studies

FT-IR measurements were performed to determine the probable functional groups accountable for the reducing and capping properties of leaf extract. The BRLE is known to contain polyphenolic compounds that could act as electron donors for the conversion of Ag⁺ to Ag⁰. The major absorption bands are shown in the Figure 9.

Figure 9:

FT-IR spectra of SNPs synthesized using BRLE.

The FT-IR spectrum of the SNPs showed the presence of –OH stretching (3564.21 and 3417.63, sharp bands) owing to the existence of natural flavanols in the BRLE. This fact is further confirmed by the presence of a sharp band at 1026.06 due to the C–O stretching in alcohols [51]. The intense band at 2916.17 depicts the aliphatic –C–H stretching. The small band at 2352.99 denotes –C–NH⁺ stretching [52]. The presence of carbonyl group is confirmed by the band at 1774.39 (C=O stretching). Another band at 1604.66 denotes the bending vibrations of amide I group present in the BRLE which substantiate the electrostatic attraction of charged groups [53].

The presence of methyl groups (–CH₃) are revealed from the bands at 1450.37 and 1388.65. The small bands at 1126.35 and 1026.06 signifies the presence of the C–O stretching of esters or C–N stretching vibrations present in the BRLE. The observed bands in this study corroborates the fact that the phytocompounds in the BRLE (as mentioned in the “Introduction” section) adsorb on the surface of SNPs [54]. Many reports have indicated that the phytocompounds in the leaf extract participate in the formation of weak interactions with SNPs and are responsible for the capping and stabilization of SNPs [55], [56], [57].

Catalytic degradation of RhB

The catalytic activity of SNPs was investigated by using them on the reduction of RhB in the presence of NaBH₄. The reduction of RhB by NaBH₄ in the absence of SNPs is shown in Figure 10. The UV-Vis absorption spectrum of the degradation of dye at 5-min intervals for a period of 1 h is depicted in Figure 10. At the start of the reaction, the characteristic band of RhB dye at 555 nm was observed, which originated from n→π* transitions in its conjugated structure [58]. The decrement in the intensity of absorption band at 555 nm was slow and it took 45 min to completely degrade the dye. The inset of Figure 10 indicates that the degradation process is a pseudo first order reaction (linear correlation between ln (A/A₀) and the reduction time). The degradation constant was estimated as 0.0709 min⁻¹.

Figure 10:

Sequential UV-vis spectra of degradation of RhB in the presence of NaBH₄.

(Inset plot shows the relationship between ln (A/A₀) vs. time).

Figure 11 shows the UV-Vis spectra of dye degradation in the presence of NaBH₄ and SNPs catalyst for the same time period. The introduction of SNPs to the degradation of dye process accelerated the reaction rate, which can be construed from the spectra. The complete degradation of dye was accomplished within 9 min in the presence of SNPs, thus indicating a faster reaction rate as compared with the earlier case (without SNPs). Similar results were obtained by [59] for the degradation of methylene blue by Ag and Au nanoparticles. In the current study, the degradation constant in the presence of SNPs catalyst was estimated as 0.1323 min⁻¹ (inset of Figure 11).

Figure 11:

Sequential UV-vis spectra of degradation of RhB in the presence of NaBH₄ and SNPs.

(Inset plot shows the relationship between ln (A/A₀) vs time).

The added SNPs act as a redox catalyst (electron relay effect) by playing the role of electron transfer mediator between the RhB dye and NaBH₄ [60]. To be exact, the BH₄⁻ ions donate electrons to the SNPs, and the dye molecules capture the electrons from SNPs; hence, the RhB dye is reduced to its colorless form [61]. Another noteworthy observation in Figure 11 is the appearance of SNPs peak (at 420 nm), which remains constant during the degradation process. There is neither a shift nor a change in the intensity of this peak thus demonstrate the role of catalyst in the degradation of RhB.