Momordica charantia fruit mediated green synthesis of silver nanoparticles

1 Introduction

Nanotechnology is a promising and rapidly growing field with applications in science and technology [1]. Noble metal nanoparticles such as silver, gold and platinum are broadly used in medicinal applications. Silver nanoparticles (AgNPs) are important materials that have been studied widely. There is a growing need to develop an eco-friendly method for the synthesis of nanoparticles (NPs) that does not utilize toxic chemicals. In general, NPs are prepared by a variety of physical and chemical methods [2, 3] which are not eco-friendly. Nowadays, green chemistry procedures are using various biological systems such as bacteria, fungi, yeast, and plant extract [4, 5] for the synthesis of NPs. Among them, the plant-extract-based green biosynthesis of metal NPs, especially gold and silver with controlled physicochemical properties have been reported by many researchers [6, 7]. The recent reports include the green biosynthesis using neem leaf [8], tansy fruit [9], mango peel [10], Pinus eldarica bark extract [11], jackfruit seed [12], blue dawn flower [13], Azolla pinnata whole plant extract [14], microorganism [15], and so on. AgNPs prepared by using biological materials have the properties of a high surface area, smaller sizes and high dispersion. These prepared nanomaterials have many applications, including spectrally selective coatings for solar energy absorption, optical receptors, generation of intercalation materials for storage batteries [16], catalysis in chemical reactions, biolabeling, and antibacterial agents. It is well known, that silver is an effective antibacterial agent and possesses a strong antibacterial activity against bacteria, fungi and viruses, even though the mechanisms are still not well known [17]. The high antibacterial activity of AgNPs is a result of well-developed surface, providing maximum contact with the environment [18].

Momordica charantia Linn.

Momordica charantia L. usually known as bitter melon, bitter gourd or bitter squash belongs to the Cucurbitaceae family and it is a commonly available medicinal plant, which is used for the synthesis of AgNPs. Bitter gourd is both a nutritious and healthy food with a distinctive bitter flavor, and it is also widely exploited in traditional medicine. The fruits contain many phytochemicals such as flavonoids, triterpenoids, saponins, lectins, and phenolic compounds, etc. [19, 20]. The fruits of M. charantia are reported to possess a wide range of pharmacological activities such as anti-diabetic [21], anti-oxidant [20], anti-HIV [22] and the inhibition of p-glycoproteins [23]. The fruits are used traditionally as anthelmintic, carminative, purgative and for the treatment of jaundice, anemia, malaria, cholera, etc. [24]. In this present study, the fruits of M.charantia were used for the rapid, simple and viable biosynthesis of AgNPs and the biosynthesized NPs were characterized by UV-vis spectroscopy and the capping agent for the AgNPs synthesis was confirmed by Fourier transform infrared spectroscopy (FTIR) and TEM analysis.

2 Materials and methods

2.1 Materials

The fresh fruit of M. charantia (Figure 1) was collected from a local vegetable garden. The fruit was kept at 0°C until further analyses. Silver nitrate (AgNO₃) was purchased from Sigma Aldrich Chemicals (Malaysia Branch, USA). Chemicals were of analytical reagent grade and were used without further purification. All solutions were freshly prepared using deionised distilled water and were kept in the dark to keep away from any photochemical reaction. Glass wares were properly washed with distilled water and dried in an oven before use.

Figure 1:

Photograph of M. charantia fruit used in this study.

3 Results and discussion

Reduction of Ag+ into AgNPs during the exposure to the fruit of M. charantia extract could be seen by the color change. The color of fresh suspension of M. charantia extract was gray-brown. However, after the addition of the AgNO3 solution and the incubation for 30 min at room temperature, the mixture turned brown-yellow. Color changes in aqueous solutions are due to the surface plasmon resonance phenomenon. The results showed the fruit of M. charantia have a good potential for synthesizing the AgNPs as a reducing agent. According to the study of Shameli et al. [26], the chemical equations for the biosynthesis of the AgNPs are possible as follows:

After dispersion of silver ions in the M. charantia aqueous extract [Eq. (1)], the formed [Ag (M. charantia)]⁺ complex reacted with the aldehyde groups in the molecular structure of the extract to obtain [Ag (M. charantia)] due to the reduction of silver ions by the oxidation of the aldehyde to the carboxylic acid groups [Eq. (2)].

The formation of AgNPs from 1 mm solution of AgNO₃ was confirmed by using UV-Vis spectral analysis. Metal AgNPs have free electrons, which give rise to a surface plasmon resonance (SPR) absorption band [27], due to the combined vibration of electrons of metal NPs in resonance with the light wave [28]. Surface plasmon spectra were obtained for brown-yellow colored silver solutions in the range of 300–700 nm.

3.1 Optimization study

3.1.1 Effects of contact time

The formation of AgNPs started within 15 min and the spectra were recorded after that at 15 min, 30 min, 1, 2, 3, 4 and 5 h. The effect of the contact time on AgNPs synthesis was evaluated with UV-Vis spectra and it was noted that with the increase in interaction time the SPR peak became sharper (Figure 2). The reaction time found in the development of NPs in this study was found to significantly increase up to 5 h and had a comparatively shorter reaction time than in earlier reports [29].

Figure 2:

Effect of contact time on AgNPs synthesis.

3.1.2 Effects of silver ion concentration

The effect of AgNO₃ concentration on the formation of AgNPs was analyzed using UV-Vis spectrophotometer (Figure 3). It is clear from Figure 3 that the formation of AgNPs depends on the AgNO₃ concentration. Plasmon resonance spectra for AgNPs was obtained in 440 nm with brown-yellow colors, at different metal ion concentration. Moreover, it was found that the peak absorbance value increases with the increase in AgNO₃ concentration (0.1 mm to 5 mm) which means that the rate of formation of AgNPs is higher for higher concentration of AgNO₃ than for lower concentration.

Figure 3:

Effect of silver ion concentration on AgNPs synthesis.

3.1.3 Effects of fruit quantity

Different quantities of M. charantia fruit were used for the synthesis of AgNPs. The fruit extract quantity was varied from 0.5, 1.0, 1.8, 2.8, 3.8, 4.8 ml in 50 ml of 1 mm AgNO₃ solution which was used in the synthesis of AgNPs. Based on the UV-Vis spectra, the sharpness of the absorption peak depends on the concentration of the fruit extract, which gets further sharpened at a higher concentration (Figure 4). With an increase in fruit extract quantities from 0.5 to 4.8 ml, a consistent increase in the peak absorbance was found (Figure 4). Here the results show the increase in the formation of AgNPs was maximum for the higher fruit extract quantity as well. Similarly, visual examination of the solutions revealed color changes from light yellow to deep yellow on silver solutions with an increase of fruit extract quantity in the reaction solution.

Figure 4:

Effect of fruit extract quantity on AgNPs synthesis.

3.1.4 FTIR studies

FTIR has emerged as an important tool for understanding the involvement of surface functional biological groups in metal interaction. FTIR spectroscopy analysis was carried out to identify the possible biomolecules that were responsible for the stabilization of the AgNPs synthesized by M. charantia fruit extract (Figure 5). The FTIR spectra was recorded for the fruit extract and also for the AgNPs. The FTIR illustrates the absorbance bands observed at 3336 cm^-1, 1664 cm^-1, 1208 cm^-1 and 1284 cm^-1 in the 4000–500 cm^-1 region. A major peak was observed at 3336 cm^-1 that could be responsible for O-H stretching [30]. The peak located at 1660 cm^-1 indicates the presence of C=O stretching in carboxyl or C=N bending in the amide group [31]. The band with a peak at 1208 cm^-1 and 1284 cm^-1 corresponds to C-O stretching of esters, ethers and phenols [32].

Figure 5:

FTIR spectra of samples before and after the treatment producing AgNPs.

M. charantia fruits contain a variety of flavonoids and phenolic compounds which may be involved in the biosynthesis of AgNPs and act as a reducing agent for the reduction of Ag⁺ to Ag⁰. Phenolic compounds possess carboxyl and hydroxyl groups, which are capable to bind to metal [33]. Flavonoids can also directly strip molecular species of active oxygen. Their antioxidant activity resides mainly in their capability to provide electron or hydrogen atoms [34]. We conclude from the overall observations that the synthesized NPs were encircled by the different functional groups, such as carboxyl, carbonyl, amide, ester, ether and phenol. From the investigation of FTIR studies, we observed that these functional groups have stronger ability to bind metal NPs to prevent aggregation and provide higher stability. It is clear from the above discussion that the biological molecules can probably perform the dual functions of formation and stabilization of AgNPs in the aqueous medium [35].

3.1.5 TEM images

The structure and morphology of the green synthesized AgNPs using the extract of M. charantia were further confirmed by the TEM (Tecnai G² 200 kV TEM, Hillsboro, Oregon, USA) micrograph images. Figure 6A and B show various TEM images with different magnifications, depicting that the AgNPs were spherical in shape, with a particle size distribution between 8 and 47 nm.

Figure 6:

TEM images of AgNPs using M. charantia fruit extract at different nanometers.

4 Conclusion

In this work, the AgNPs were synthesized using an aqueous extract of M. charantia fruit. The AgNPs were characterized by UV-visible, FTIR spectroscopy, and TEM analysis. The biosynthesis of AgNPs using green resources like M. charantia is a good method over chemical synthesis because this method is environmently-friendly. M. charantia extract was prepared and successfully employed for the development of AgNPs. The results showed that the formation of AgNPs was strongly dependent on the process parameters such as M. charantia extract concentration, silver ion concentration and the reaction time of the solution. This simple, low cost and greener method for the development of AgNPs may be valuable in biotechnological, biomedical and environmental applications.