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SN Applied Sciences

, 1:1368 | Cite as

Novel green synthesis of silver nanoparticles using clammy cherry (Cordia obliqua Willd) fruit extract and investigation on its catalytic and antimicrobial properties

  • Femina Kanjirathamthadathil Saidu
  • Anjaly Mathew
  • Asna Parveen
  • Vijeesh Valiyathra
  • George Vazhathara ThomasEmail author
Research Article
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Part of the following topical collections:
  1. 1. Chemistry (general)

Abstract

In this study, highly monodispersed, exceptionally stable, spherical silver nanoparticles (AgNPs) were successfully synthesized by the microwave assisted rapid and cost-effective green method. Aqueous extract of clammy cherry (Cordia obliqua Willd) fruit was used as the green reductant, and capping agent for the synthesis of AgNPs and the effect of different synthesis parameters on the optical properties of the synthesized AgNPs was also studied. The characterization of synthesized AgNPs by Fourier transform infrared spectroscopy, X-ray diffraction studies, UV–visible spectroscopy, scanning electron microscopy and transmission electron microscopy (TEM) revealed the formation of small AgNPs with narrow size distribution. TEM studies corroborated that the AgNPs are highly crystalline and spherical with an average diameter of 7.13 nm. The cyclic voltammetry profile of AgNPs modified electrode in NaOH depicted prominent redox peaks evidencing an impressive electrochemical response. The AgNPs manifested high catalytic activity towards reduction of methyl orange and rhodamine blue with apparent rate constant 0.3038 min−1 and 0.1542 min−1 respectively. Additionally, the prepared AgNPs exhibited strong antibacterial efficacy against the tested microbes.

Graphic abstract

Green synthesis of silver nanoparticles using clammy cherry fruit extract

Keywords

Silver nanoparticles Microwave Green Reduction Catalysis Surface plasmon resonance Methyl orange Rhodamine blue 

1 Introduction

Nanotechnology is a fast developing research field making its impact on all spheres of human life. Currently, the synthesis of noble metal nanoparticles has drawn considerable attention by nanochemists because of their exceptional physio-chemical characteristics and their wide biomedical, sensing and catalytic applications [1, 2, 3, 4, 5]. Silver nanoparticles (AgNPs) are extensively studied compared to nanoparticles of Au, Pt, and Pd because of its ease of synthesis and lower cost. AgNPs shows excellent optical, electronic, antimicrobial and catalytic properties [6, 7, 8]. Versatile organic transformations for the production of industrially and therapeutically important organic molecules have been successfully carried out with high efficiency and selectivity using nanosilver based catalytic systems [9, 10, 11]. The catalytic activity of metal nanoparticles has extensively investigated in the field of water pollution remediation. Many research groups have demonstrated the effective utilization of noble metal NPs or metal NP incorporated composites systems as a suitable redox catalyst for the degradation of various organic pollutants like dyes, drugs and aromatic nitro compounds [12, 13, 14, 15, 16, 17, 18, 19]. Additionally unique electrochemical features and high conductivity of AgNPs identifies AgNPs as a good candidate for selective and sensitive monitoring of biomolecules and pharmaceuticals [20, 21, 22]. Earlier reports have shown that size, shape, and surface features are critical in deciding performance of their so far mentioned catalytic and sensing applications [21, 22, 23, 24, 25]. It is reported that the cytotoxicity of AgNPs against diseases like HIV and cancer are also size dependent [2, 26, 27]. In this context, the method of synthesis, especially the nature of stabilizing agents, reducing agent and other synthesis parameters have a critical role in deciding the properties of the AgNPs.

Due to the potential applications in diverse fields, there is a constant urge for the development of the fast synthesis of AgNPs with controllable size and uniqueness in good yield. To date, a number of chemical reduction procedures have been reported for the synthesis of AgNPs [7, 28]. Even if this synthetic route allows the fast synthesis of AgNPs in bulk scale, most of the reagents used as reducing and/stabilizing agents are toxic and not biocompatible. Hence the AgNPs obtained by such chemical routes cannot be used for the biomedical application, such as drug carrier and or in vivo imaging studies.

Recently, the biosynthesis of metal nanoparticles (MNPs) have been proposed as a cost-effective and eco-friendly substitute to chemical methods. Many studies have been demonstrated the biosynthesis of AgNPs mediated by the microorganism or by phytochemicals or enzymatic synthesis [11, 18, 19, 29, 30, 31, 32, 33]. Among the biological methods, photosynthesis which involves reduction by phytochemicals derived from plants seems to be the superior choice. Phytochemical mediated synthesis is considered as a green alternative as it utilizes nontoxic, green reducing and stabilizing agent, thus making the method simple, economical environment-friendly and biocompatible. However, synthesis of AgNPs by the phytochemical reduction methods are rather slow compared to conventional chemical reduction and it was considered as the major limitation that faced during the earlier periods of green synthesis. Currently, by coupling plant-mediated synthesis with microwave (MW) assisted synthetic techniques, the biosynthesis can be conveniently carried out rapidly with good yield [4, 34, 35]. MW assisted chemical transformations a simple yet versatile eco-friendly process to achieve the fast synthesis of metal NPs.

Here, we investigated the role of fruit extract of Cordia obliqua Willd as the green reductant and stabilizing agent during the formation of AgNPs Cordia obliqua Willd commonly known as clammy cherry is a plant that belongs to Boraginaceae family which has widely distributed warmer regions of India and Ceylon [36, 37]. Traditionally the plant is of great medicinal value. The phytochemical screening studies on clammy cherry extract reported earlier revealed the presence of numerous poly-functional molecules like carbohydrates, proteins, amino acids, flavonoids, phenolic compounds, alkaloids, glycosides and sugar in different part of the plant [36, 37].

We have demonstrated the ability of this clammy cherry derived phytochemicals as reducing and capping agent during the formation of AgNPs. In the present work, we could synthesize spherical AgNPs with narrow size distribution under microwave irradiation of few minutes. The use of water for extraction and reaction medium is a further add-on to the green chemistry policy. The effect of the MW power, irradiation time, and concentration of silver nitrate solution on AgNPs characteristics was also investigated. Impressive electrochemical response and high catalytic activity towards the reduction of organic dyes are highly promising for its futuristic applications in diverse fields including catalysis and sensing. This study has established a rapid, cost-effective and eco-friendly procedure for the synthesis of highly stable AgNPs having hopeful application potentials.

2 Experimental

2.1 Materials and methods

The chemicals, silver nitrate (AgNO3), sodium borohydride (NaBH4), graphite powder, paraffin liquid and sodium hydroxide used in the study were purchased from Merck Chemicals Ltd, Mumbai, India. The organic dyes methyl orange (MO) and rhodamine blue (RhB) have purchased from Nice Chemicals, India. Microwave oven [wave (LG) Model 1S2021CW at 2450 MHz] was used for the AgNPs synthesis.

2.2 Preparation of clammy cherry extract

Ripened clammy cherry fruits were collected from the Maharajas college campus, Kerala, India. 10 g of fruit was washed thoroughly with deionized water several times, then peel was removed, and the pulp along with seed was refluxed with 100 mL water under microwave heating for 2 min at 350 W. The aqueous extract is cooled, filtered with Whatmann 40 filter paper and filtrate is used for synthesis.

2.3 Synthesis of silver nanoparticles (AgNPs)

Silver NPs was prepared by heating a solution of AgNO3, and clammy cherry extract in a domestic MW oven. In a typical procedure, 10 mL of the clammy cherry extract (10 g/100 mL) was mixed well with 20 mL of 1 mM AgNO3 solution and irradiated in MW oven at 2.45 GHz under 350 W for about 8 min. Wherein, the color of the solution changes to light yellowish to reddish brown indicating the formation of the AgNPs. The effect of different parameters such as MW power, the period of exposure, the concentration of AgNO3 and composition of clammy cherry extract were also investigated. The entire process was monitored by recording the UV–Vis spectrum using the reaction mixture collected at regular intervals.

2.4 Characterization

Thermoscientific evolution 160 UV–VIS Spectrometer is used for recording the UV–Vis spectrum of the periodically collected reaction mixtures. The surface morphology of AgNPs was analyzed by scanning electron microscope (SEM) using VEGA3 TESCAN. Transmission electron microscopic images were done using a JEOL JEM-2100 microscope. X-ray diffraction (XRD) studies were carried out using powdered AgNPs which are collected by ultracentrifugation followed by drying in a vacuum oven for 24 h. Fourier transform infrared spectroscopy (FTIR) spectra of the vacuum dried clammy cherry extract and AgNPs were recorded in the range 4000–450 cm−1 using Perkin Elmer FTIR spectrometer.

2.5 Antibacterial assay

Biological studies were conducted for determining the bactericidal activity of synthesized AgNPs against four human pathogenic bacteria namely Escherichia coli, Bacillus circulans, Pseudomonas aeruginosa, and Staphylococcus aureus. Mueller–Hinton agar (MHA) well diffusion method was employed to investigate the antibacterial activity of clammy cherry stabilized silver NPs. 25 µL spore suspension (10–100 spore/mL) of S. aureus, E. coli, B. circulans, P. aeruginosa were added to a sterile Muller Hinton medium fast before solidification, then poured into sterile Petri dishes (9 cm in diameter) and spread using a cotton swab. Sterilized disc (6 mm) is taken, and 10 µL of nanoparticles solution was dropped into it and was kept in the Petri dish. Sealed the Petri dish with cling film and incubated for 16 h. Inhibition zones were detected around the disc and measured.

Minimum inhibitory concentration (MIC) is the lowest concentration of a chemical which prevents visible growth of a bacterium. MIC was determined by resazurin based microtiter dilution assay (RMDA) method. In a standard procedure, RMDA was done using, 96 well microtiter plates (HiMedia) under sterile conditions. The first row of the plate is filled with 100 μL of AgNPs (1 mg/mL) dissolved in sterilized water. All the wells of microtiter plates are packed with 50 μL of Luria broth. By transferring 50 μL AgNPs solution from the first row to the next wells in the next row of the same column, a two-fold serial dilution is achieved and so that each well has 50 μL of test material in serially descending concentrations. 2 μL of resazurin indicator solution was added into each well to impart purple color. Finally, 10 μL bacterial suspension was added to each well to achieve a concentration of 5 × 106 CFU/mL. Then each plate was covered with cling film to avoid the dehydration of bacterial culture. There was a set of 3 controls for each microtiter plate, a first column with the positive control (Ampicillin), a second column with all reagents but without AgNPs solution and the third column with all solutions along with 10 μL of Luria broth instead of the bacterial solution. The entire plates are incubated at 37 °C for 1 day. Any color change observed from purple was taken as positive. The lowest concentration of the sample at which no color change occurred was recorded as the MIC value. All the experiments are performed in triplicates and average was taken as the MIC of synthesized AgNPs.

2.6 Catalytic activity studies

The catalytic activity of the clammy cherry stabilized AgNPs was evaluated by monitoring the reduction of organic pollutants like methyl orange (MO) and rhodamine blue (RhB) under pseudo-first order condition in presence of relatively high concentration of sodium borohydride. The reaction is monitored by recording the absorption spectra in each min in the range of 200–800 nm at room temperature using Thermo scientific evolution 160 UV–Vis Spectrophotometer.

2.7 Electrochemical studies

The electrochemical response of the AgNPs was studied by cyclic voltammetric (CV). The CV was recorded with AgNPs modified carbon Paste Electrode (CPE) was carried out in Metrohm Auto lab Potentiostat/Galvanostat (Model No. AUT87141) furnished with NOVA 2.1 software. A three-electrode electrochemical setup containing modified carbon paste electrodes as working electrode, Pt wire as a counter electrode and Ag/AgCl reference electrode is used for recording CV. Carbon paste prepared by thorough mixing graphite powder and paraffin oil (weight ratio of 70:30) was packed in a clean glass tube. Silver wire was inserted into carbon paste for electrical contact. The CPE surface was modified by drop casting 10 µL of the AgNPs to get AgNPs/CPE.

3 Result and discussion

3.1 Synthesis of AgNPs and UV–Vis spectral studies

A fast synthetic protocol for the synthesis highly stable AgNPs by exploiting the reducing and stabilizing capabilities of the phytochemicals present in the aqueous extract of the Clammy Cherry was established here. The presence of numerous poly-functional molecules like carbohydrates, proteins, amino acids, flavonoids, phenolic compounds, alkaloids, glycosides and sugar in different part of the clammy cherry have been established in their phytochemical screening studies on clammy cherry reported earlier [36, 37]. The formation of AgNPs was evidenced by the apparent color change of the solution from colorless to dark red within a short period. These biomolecules are considered to be responsible for the fast reduction of silver ions and stabilization of AgNPs. Here the use of water as the solvent for extraction and as the reaction medium was a further add-on to green chemistry policy. Additionally, MW-assisted route has radically reduced the reaction time and increased the yield of AgNPs during the current process. A schematic representation of MW assisted synthesis of AgNPs using clammy cherry extract is depicted in Fig. 1.
Fig. 1

Pictorial representation of MW assisted synthesis of AgNPs using clammy cherry extract

As the MW heating of the reaction mixture containing clammy cherry extract and AgNO3 solution proceeds, light yellow color has appeared within few seconds, and the color got intensified on increasing the time of MW exposure as illustrated in Fig. 2.
Fig. 2

The visual color change observed during MW exposure of AgNO3 (1 mM) with clammy cherry extract (10 g/L)

UV–Vis spectroscopy was used to monitor the progress of AgNPs formation. No absorption peaks are seen for both AgNO3 and clammy cherry extract solution in the visible region (red and black curve of Fig. 3 respectively), and absorption peaks were observed at 304 nm and 283 nm for AgNO3 and Clammy cherry extract solution respectively. However, after exposing the mixture of two solutions to MW radiation, a typical absorption band with λmax around 409–425 nm corresponding to surface plasmon resonance (SPR) of AgNPs were observed which confirmed the formation of AgNPs having a size between 5 and 20 nm [38].
Fig. 3

UV visible spectra of clammy cherry extract (black), silver nitrate solution (red), and AgNPs solution after 6 months (blue)

The stability of prepared AgNPs is a valuable parameter to be analyzed and achieved. Excellent stability and durability of AgNPs are highly appreciated for their sensing and catalytic applications. It is observed that the Clammy cherry reduced AgNPs are stable for more than 6 months without coagulation under refrigeration. The UV–Vis spectra of the AgNPs recorded after 6 months presented in Fig. 3 (blue line) still shows SPR with absorption maxima at 420 nm confirming the existence of highly dispersed AgNPs.

The progress of the bioreduction was followed by UV–Vis spectral studies and the spectra are presented in Fig. 4a–d. Results reveal that rate of formation, size, and optical properties are found to be influenced by the reaction parameters such as the concentration of AgNO3, the composition of extract, MW power and time of exposure. Among the four synthesis parameter investigated, it is clear from spectral studies that like time of exposure, MW power has a profound effect on the nature of AgNPs which is reflected in Fig. 4a. At low MW power as low as 75 W, no color change was observed for 5 min of exposure, while the reduction was much fast at higher MW power. Hence, it is be attributed that minimum activation energy required for the reduction and formation of AgNPs. An increase in the absorption intensity of SPR absorption with the time of MW exposure was observed suggesting more population of AgNPs in the reaction medium due to more reduction (Fig. 4b). After 7 min of MW treatment under 350 W power, the intensity of absorption remains almost the same confirming the complete reduction of silver ion. It is observed that on increasing the time of MW exposure, a slight shift in SPR maxima towards lower wavelength was observed. Figure 4c revealed that even low concentration of extract (10 g/L) is sufficient for the effective reduction and stabilization of AgNPs. From Fig. 4d it is clear that the intensity of absorption and hence population of AgNPs shows a direct dependence with the concentration of AgNO3 solution. At higher AgNO3 concentration coagulation was observed resulting in a black precipitate which is attributed to the fast reduction without proper passivation. This might be because of the ineffective passivation of AgNPs lesser number of plant metabolites compared to silver ions. Most of the UV–Vis spectra observed were sharp and contains single SPR absorption maxima was indicating the spherical AgNPs are of narrow size distribution. There is no agglomeration since the particles are better stabilized, thus UV–Vis spectral study corroborates the unique capping effect of the phytochemicals present in the clammy cherry extract. The different synthesis conditions adopted and its effect on SPR absorption maxima of corresponding AgNPs formed are summarized in Table S1 (Supporting information).
Fig. 4

UV–Vis spectra recorded at different synthesis condition a effect of MW power, b effect of time of exposure, c effect of clammy cherry extract concentration, and d effect of concentration AgNO3

3.2 FTIR studies

To identify the possible functional groups involved in the reduction and stabilization of the AgNPs, FT-IR spectral studies of the vacuum dried clammy cherry extract and AgNPs were carried out, and it is presented in the Fig. 5 (curve black and red respectively). FTIR spectrum of Clammy cherry extract shows some distinct peaks corresponding to functional groups such as –OH, –NH, –CONH, –COOH, C=O, and C–O–C linkages and aromatic C=C linkages. The broad peak at 3247 cm−1 and sharp peak 1575 cm−1 are assigned to H-bonded amino/hydroxyl stretching and bending respectively. The peak around 1390 cm−1 is assigned to the –COO stretching from amino acid groups. The peak at 1719 cm−1 suggests the presence of carboxyl and/kenotic functionalities. Hence it is inferred that Clammy cherry extract contains biomolecule having an active functional group which can reduce silver ion to metallic silver and can take part in stabilizing interaction with AgNPs. Earlier studies have confirmed that carbonyl, ester, amino, hydroxyl, and phenolic groups effectively bind with the noble metals through their coordinating and polar interactions [4]. In the present study, it is suggested that these identified functional groups may interact with silver and forms a capping layer over the AgNPs which prevents their clustering ensuring proper stabilization. Further, the FTIR spectrum of dried AgNPs showed peaks similar to those of the clammy cherry extract hence it is suggested that AgNPs might be covered with a layer of phytochemicals derived from clammy cherry.
Fig. 5

FTIR spectra of clammy cherry fruit extract and synthesized AgNPs

3.3 XRD studies

The XRD pattern observed for the clammy cherry extract reduced AgNPs is presented in the Fig. 6. The XRD spectra show sharp peaks corresponding to the characteristic diffraction from (111), (200), (220), and (311) planes of the fcc lattice of metallic silver proving that the prepared AgNPs are highly crystalline (JCPDS: 01-089-3722). No prominent peaks corresponding to silver oxide or any other silver compounds were observed confirming that the AgNPs are composed of pure metallic phase [2].
Fig. 6

XRD pattern of the clammy cherry extract reduced AgNPs

3.4 SEM analysis

Figure 7 depicts the SEM images clammy cherry stabilized AgNPs. It is observed that the clustered AgNPs forms well-defined spherical microstructures and the aggregated microspheres are coated by a thin film composed of various phytochemicals contained in the mucilage or gum of aqueous extract of clammy cherry. Similar observations were also made during plant-mediated synthesis of the metal nanoparticle by some other research groups [39, 40].
Fig. 7

SEM images of AgNPs under a low and b high magnification

3.5 TEM analysis

The size, morphology and crystal structure of synthesized silver nanoparticles were further studied in detail by transmission electron microscopy (TEM) and the TEM images were shown in Fig. 8a, b. The TEM images, confirms the AgNPs are almost spherical and monodisperse. The small size and narrow size distribution are attributed to the fast reduction under MW irradiation. The particle size distribution was shown in the histogram (Fig. 8d). The average particle size measured from the TEM images is observed to be 7.13 nm. Selected area electron diffraction (SAED) pattern shows concentric rings which reveal the crystalline nature of the synthesized Ag NPs. The particles are well isolated from each other by the capping phytochemicals. Hence no particle aggregation was observed as evident from the TEM image and results confirms the effective capping ability of the clammy cherry derived phytochemicals.
Fig. 8

TEM of Ag NPs under different magnification (a, b), SAED pattern (c) and particle–size histogram (d)

3.6 Cyclic voltammetry studies

The electrochemical response of the AgNPs modified CPE was analyzed by recording CV in 0.1 M NaOH solution, and it was compared with bare CPE. The AgNPs modified electrodes show substantial enhancement in current response compared to unmodified CPE. In the CV curve of the AgNPs/CPE (Fig. 9 red curve), all prominent peaks corresponding to particular redox transitions of metallic silver to silver oxide were observed [8]. The significant current response is due to the large electrochemical surface area which is attributed to the smaller size, large surface to volume ratio and better dispersion of highly conducting AgNPs. The good electrocatalytic and sensing performance was anticipated based on the CV results, and studies on electrochemical sensing of biomolecules using synthesized AgNPs are underway.
Fig. 9

The CV of AgNPs modified CPE (red curve) and bare CPE (black) recorded in 0.1 M NaOH against Ag/AgCl reference electrode and, Pt as a counter electrode at a scan rate of 50 mV s−1

3.7 Antibacterial studies

In this study, we have investigated the in vitro antibacterial activity of the synthesized Ag NPs against the common pathogenic bacteria both Gram-positive bacteria [Bacillus circulans (B. circulans), Staphylococcus aureus (S. aureus)] and Gram-negative bacteria [Pseudomonas aeruginosa (P. aeruginosa) and Escherichia coli (E. coli)] using Mueller–Hinton Agar (MHA) well diffusion method. The bacterial strains impregnated with AgNPs were incubated for 24 h in the dark at 37 °C, and the corresponding photographs are shown in Fig. 10. The observation of distinct zones of inhibition around the discs suggests the strong antibacterial activity of the prepared AgNPs. Additionally, the photographs indicate that the extent of antibacterial activity has a direct dependence with the dosage of AgNPs colloid, and zone observed for 20 µL of AgNPs colloid is higher than that corresponds to 10 µL in all cases.
Fig. 10

Photographs showing the zone of inhibition observed for on addition of AgNPs colloid (T1-10 µL and T2-20 µL) against bacterial strains. a Bacillus circulans, b Escherichia coli, c Pseudomonas aeruginosa, d Staphylococcus aureus and e histogram showing the antibacterial activity of AgNPs

The MIC values AgNPs for each bateria determined by the RMDA methods are shown in Table 1 and the values are found to be significantly lower than that of standard positive control ampicillin, The observed low MIC values again confirm high antibacterial activity of the clammy cherry stabilized AgNPs (photographed image showing results of RMDA is shown in Fig. S1 (supporting information).
Table 1

MIC values of prepared AgNPs obtained from RMDA method

Bacteria

Minimum inhibitory concentration (MIC) (µg/mL)

Standard drug ampicillin (µg/mL)

Pseudomonous aeruginosa

0.0275

3.125

Staphylococcus aureus

0.0551

0.3906

Bacillus circulans

0.0551

25

Escherichia coli

0.1103

0.7812

Antibacterial activity of silver nanoparticle and silver ions are well known, but still, an exact mechanism of action is not precise. Different research groups have suggested various modes of bacteriostatic action. It is proposed that the bacterial cell death is due to the structural and morphological changes induced by AgNPs. When AgNPs comes in contact with the bacteria, they adhere to the cell wall and cell membrane. Once bound, some of the silver may penetrate through the cell wall and interacts with DNA and RNA and blocks the bacterial cell’s replication [2, 4, 29, 41]. Although the proposed mechanism varies for every type of cell, as there is great variation in their cell wall composition, AgNPs with an average size of 10 nm or less shows better interactions that significantly increase their bactericidal activity. It has been reported that the size of the particle significant role in antimicrobial activity [26, 29, 42]. The smaller particles can penetrate the cell wall and interact with the cell metabolites quickly. Here the excellent bactericidal activity of as-synthesized AgNPs is attributed to the smaller size and high surface to volume ratio.

3.8 Catalytic studies

The catalytic efficiency of the biosynthesized AgNPs was investigated towards the reductive degradation of rhodamine blue (RhB) and methyl orange (MO) with an excess of sodium borohydride as a reductant in the aqueous medium. The progress of both catalytic processes was followed by recording in situ UV–Vis spectrum at each minute, and the time-dependent UV–Vis spectra are presented in Fig. 11. As time proceeds there is a visually observable color change in the reaction mixture and the solution becomes colorless within 9 min for MO (Fig. 11a) and 14 min for RhB (Fig. 11b). As the reductant concentration was high, the reaction is supposed to proceed under pseudo-first order condition, and the kinetics of the AgNPs catalyzed reduction as studied by noting the absorbance at 464 nm and 554 nm for MO and RhB respectively. The pseudo-first-order rate constant for the reaction determined by plotting log(C/C0) against time ‘t’, are presented in Fig. 11 (picture ‘c’ and ‘d’ for MO and RhB respectively). The apparent rate constant Kapp was calculated by assuming pseudo first order condition was found to be 0.3038 min−1 and 0.1542 min−1 for MO and RhB respectively.
Fig. 11

Successive UV–Vis. Absorption spectra for the NaBH4 reduction of MO and RhB catalyzed by AgNPs at 28 °C (picture a and b respectively) and plot of log(C/C0) against time ‘t’ (picture ‘c’ and ‘d’ for MO and RhB respectively)

As proposed by previous literature, the conversion of RhB and MO to their colorless reduced form involves the fast transfer of the electron from donor borohydride to the acceptor dye molecules assisted by added AgNPs [4, 24, 43]. No significant decrease in the intensity of absorption maxima of MO/RhB was observed in the solution containing MO/RhB and NaBH4 even after 20 min in the absence of AgNPs, as the reductive degradation of MO/RhB was slow. On adding synthesized AgNPs to the above solution, the color of the solution decreases much faster indicating enhanced degradation process. The BH4 and MO/RhB molecules possibly get adsorbed on the surface of AgNPs without affecting their activity. The AgNPs surface will be refreshed and active throughout the reduction process as the surface of AgNPs are well passivated by Clammy cherry derived biomolecules keeping the AgNPs well dispersed and thus providing a large surface for the adsorption of the reactants. The adsorption of nucleophilic BH4 ions and electrophilic dye molecule over the AgNPs alters its standard electrode potentials to facilitate faster electron transport, where the AgNPs acts as a redox center and transport electrons from BH4 to MO/RhB. Figure 12 represents the proposed scheme of the AgNPs catalyzed the reduction of MO/RhB.
Fig. 12

A proposed scheme for reduction of MO/RhB by NaBH4 in the presence of AgNPs

Table 2 evaluates the catalytic performance of the prepared AgNPs related to that of metal-based catalytic systems reported earlier.
Table 2

Comparison of catalytic activity of the synthesized AgNPs towards the reduction of MO and RhB with reported catalytic systems

Dye

System

Rate constant (min−1)

References

Methyl orange (MO)

Ag–Zeolite X

0.305

[44]

AgNPs

0.0186

[45]

PdAu/Dens-OH

0.0948

[46]

Pt NPs

0.0029

[47]

AuNPs

0.0049

[47]

AgNPs

0.5853

[47]

ZnO/carbon black-cellulose acetate

0.09

[48]

Au/CeO2–TiO2

0.433

[49]

Fe–Chitosan

0.1698

[50]

Fe3O4–PPy–MMA/Ag

0.312

[51]

Co-DNB

0.091

[52]

RGO-Co-DNB

0.2912

[52]

Ag–HNT

0.222

[53]

AgNPs

0.3038

This work

Rhodamine blue (RhB)

Au NPs

0.453

[54]

rGO–SiW

0.127

[55]

AgNPs

0.600

[56]

CuOS

0.036

[57]

Ag–Fe2O3–SiO2

0.152

[58]

Au/CeO2–TiO2

0.222

[49]

Au–PVP

0.033

[59]

Ag–PVP

0.007

[59]

Pt–PVP

0.305

[59]

Fe–Chitosan

0.3804

[50]

Co-DNB

0.108

[52]

RGO-Co-DNB

0.3367

[52]

Ag–Fe3O4

0.42

[60]

AgNPs

0.1542

This work

It is evident from the comparison that the clammy cherry stabilized AgNPs has competing catalytic efficacy towards the reduction reaction. The good catalytic activity is being attributed to the better stabilization, and small size of the AgNPs synthesized by Clammy cherry mediated reduction. Therefore, clammy cherry protected stable AgNPs based catalytic systems can be conveniently used for the effective removal of dye pollutants in the water bodies.

4 Conclusion

Here, the green synthesis of AgNPs by reduction of silver ions using clammy cherry extract under microwave irradiation is reported for the first time. The synthesis of AgNPs was comparatively faster and prepared AgNPs colloid was exceptionally stable for a considerably longer period of time. The formation AgNPs of the average size of 7.13 nm and narrow size distribution established that the polyfunctional molecules present in the clammy cherry extract are effective for reduction and proper stabilization of AgNPs. The clammy cherry reduced AgNPs exhibited excellent antibacterial activity, high catalytic efficacy, and better electrochemical response. It is concluded that the present study is highly promising as it could demonstrate a simple and fast synthesis method for the size and shape controlled AgNPs, having unique characteristics for biological, catalytic and sensing applications.

Notes

Acknowledgements

The authors would like to thank the financial assistance to Femina K.S. Granted by University Grants Commission (under Faculty Development Program: Grant No. FIP/12th Plan/KLMG 009 TF 12 dated 20/04/2017), Government of India. The authors would like to thank SAIF STIC and Biotechnology Department of CUSAT, and Kerala, India for characterization facilities. The authors thank Dr. Bindu Sarmila and Dr. Neena George, Maharajas College, Ernakulam, Kerala, India for their valuable suggestions during the manuscript writing.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Supplementary material

42452_2019_1302_MOESM1_ESM.docx (538 kb)
Supplementary material 1 (DOCX 537 kb)

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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  • Femina Kanjirathamthadathil Saidu
    • 1
    • 2
  • Anjaly Mathew
    • 3
  • Asna Parveen
    • 2
  • Vijeesh Valiyathra
    • 3
  • George Vazhathara Thomas
    • 1
    Email author
  1. 1.Department of ChemistrySt. Joseph’s CollegeMoolamattam, IdukkiIndia
  2. 2.Department of ChemistryMaharaja’s CollegeErnakulamIndia
  3. 3.Department of ChemistrySree Neelakanta Govt. Sanskrit CollegePattambi, PalakkadIndia

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