Synthesis of Silver and Gold Nanoparticles Using Cashew Nut Shell Liquid and Its Antibacterial Activity Against Fish Pathogens

Abstract

This study reveals a green process for the production of multi-morphological silver (Ag NPs) and gold (Au NPs) nanoparticles, synthesized using an agro-industrial residue cashew nut shell liquid. Aqueous solutions of Ag⁺ ions for silver and chloroaurate ions for gold were treated with cashew nut shell extract for the formation of Ag and Au NPs. The nano metallic dispersions were characterized by measuring the surface plasmon absorbance at 440 and 546 nm for Ag and Au NPs. Transmission electron microscopy showed the formation of nanoparticles in the range of 5–20 nm for silver and gold with assorted morphologies such as round, triangular, spherical and irregular. Scanning electron microscopy with energy dispersive spectroscopy and X-ray diffraction analyses of the freeze-dried powder confirmed the formation of metallic Ag and Au NPs in crystalline form. Further analysis by Fourier transform infrared spectroscopy provided evidence for the presence of various biomolecules, which might be responsible for the reduction of silver and gold ions. The obtained Ag and Au NPs had significant antibacterial activity, minimum inhibitory concentration and minimum bactericidal concentration on bacteria associated with fish diseases.

Introduction

The creation of nanoscale materials for advanced structures has led to a growing interest in the area of biomineralization. The cashew tree, Anacardium occidentale L. is a botanical species native to eastern Brazil that was introduced into other tropical countries such as India, Africa, Indonesia and South East Asia in the 16th century [1, 2]. The true fruit of a cashew is the nut, a kidney-shaped structure approximately 2–3 cm in length, which is attached to the end of a fleshy bulb, generally called the cashew apple. The shell comprises 50 % of the weight of the raw nut, the kernel represents 25 % and the remaining 25 % consists of the natural cashew nut shell liquid (CNSL), a viscous reddish brown liquid [2]. Crude CNSL represents one of the major and cheapest sources of naturally occurring non-isoprenoid phenolic lipids, including anacardic acids, cardols, cardanols, methylcardols, and polymeric materials, this might be also responsible for the production of nanoparticles [3]. CNSL has found important commercial usage as a phenolic raw material for the manufacture of certain resins and plastics due to its unusual electrical and frictional properties [3]. Chemical and physical modes of nanoparticle synthesis aimed at controlling the physical properties of the particles are currently employed in the production of metal nanoparticles [4]. Earlier report has shown some agro-industrial residues have the capacity to produce nanoparticles [5]. Nanoparticles have attracted considerable attention in recent years because they have diverse applications [6, 7]. In this study, we describe the synthesis and characterization of silver (Ag NPs) and gold (Au NPs) nanoparticles using CNSL obtained from the cashew nut separation process. In summation, the antibacterial activity of the Ag and Au NPs against fish pathogens was evaluated.

Materials and Methods

CSNL Preparation

The shells (50 g) of cashew nuts (Anacardium occidentale) procured from a cashew nut separating plant (Panruti, Tamilnadu, India) were washed thoroughly dried and ground using a stone grinder. Twenty grams of cashew nut shell were boiled in a 500 mL beaker, using 250 mL of Milli-Q Ultrapure water, for 15 min at 100 °C. The beaker was covered with aluminum foil in order to prevent excess evaporation. The contents of the beaker were cooled, mixed thoroughly and filtered through Whatman No. 1 filter paper to obtain the extract. This extract was prepared freshly for each experiment.

Syntheses of Silver and Gold Nanoparticles

Silver nitrate (AgNO₃) acquired from DaeJung Chemicals, South Korea, and chloroauric acid (HAuCl₄) from Kojima Chemicals, South Korea, was used for the synthesis of Ag and Au NPs, respectively. In 100-mL Erlenmeyer flasks, 45 mL of Milli-Q Ultrapure water and 5 mL of CNSL was added to separate Ag and Au NPs. Combination of nanoparticle was performed by the addition of 1 mM AgNO₃ for silver and 1 mM HAuCl₄ for gold into the reaction mixture (water and CSNL extract).

Isolation of Silver and Gold Nanoparticles

After conformation of the nanoparticles formations. The reaction mixture was filtered through 0.22-μm Steritop Millipore filters and centrifuged at 12,000 rpm for 15 min for Ag and Au NPs isolation. The obtained NPs were freeze-dried to obtain a powder.

Characterization of Silver and Gold Nanoparticles

The Ag and Au NPs were confirmed by scanning the absorption maximum of the reaction mixtures between 200 and 800 nm on a UV-1800 UV–VIS spectrophotometer (Shimadzu, Japan).Scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS, JEOL-64000, Japan) analysis confirmed the formation of Ag and Au NPs. The morphologies, and size distributions of Ag and Au NPs were analyzed using transmission electron microscopy (TEM, Hitachi, H-n650, Japan). X-ray diffraction (XRD) measurements of Ag NPs and Au NPs were analyzed on a drop-coated glass substrate in a Rigaku instrument, Japan. FT-IR spectra of Ag and Au NPs were obtained with a Perkin-Elmer Fourier transform infrared spectroscopy (FTIR) spectrophotometer (Norwalk, USA) in the diffuse reflectance mode at a resolution of 4 particles cm⁻¹ in KBr pellets.

Antibacterial Assay

Bacterial Strains

Aeromonas hydrophila-MTCC 646, Aeromonasbestiarum-MTCC 10106, Pseudomonas fluorescens-MTCC 67, and Edwardsiella tarda-MTCC 2400 acquired from microbial type culture collection (MTCC) Chandigarh, India was used as test strains. Strains were maintained and grown on nutrient agar.

Agar Well Diffusion Assay

The well diffusion method was used to study the antibacterial activity of the synthesized Ag and Au NPs [8]. The bacterial suspensions were prepared by growing a single colony overnight in Luria–Bertani broth and by adjusting the turbidity to 0.5 McFarland standards [8, 9]. Mueller–Hinton agar plates were inoculated with each bacterial suspension and 0.1 mg of the Ag and Au NPs were dissolved in 1 mL deionized water separately. Approximately 50 μL of the resulting solution was added to the center of well with a diameter of 8 mm. Control plates were made using wells containing CSNL only. Tetra-cycline was used as a positive control. The plates were incubated at 37 °C for 24 h in a bacteriological incubator, and the zone of inhibition (ZoI) was measured.

Minimal Inhibitory Concentration/Minimum Bactericidal Concentrations

The minimal inhibitory concentrations (MIC) and minimum bactericidal concentration (MBC) of the Ag and Au NPs were determined by applying the MTT assay by using a 96-well microtiter plate [9]. The 5 mL of Luria–Bertani broth medium containing 10–100 μg/mL of Ag and Au NPs were prepared with single dilution. For the determination of MIC, suspending a single isolate from the Luria–Bertani agar plates and inoculates into 50 μL of Luria–Bertani broth. After 24 h of incubation, suspensions were diluted in Milli-Q Ultrapure water to obtain final inoculums of 5 × 10⁵–5 × 10⁶ colony-forming units/mL. Purity examination of the isolates were checked by Gram staining and colony morphology during the study. Twofold serial dilutions of Ag and Au NPs solutions were prepared in Luria–Bertani broth in 98-well plates starting from a stock solution of 10⁻² M. Microtiter plate containing 0.05 mL of the serial compound dilutions was filled with an equal volume of each bacterial inoculum. After incubation for 18–24 h at 35 °C, MIC was determined with polar star optima micro plate reader (BMG LABTECH GmbH, Germany). The absorbance was compared with the negative control wells are determined, broth with Ag and Au NPs or broth alone and without inoculum with the lowest concentration of the compound [10, 11]. The mean values of three independent replicates were expressed in results.

Statistical Analysis

Each experiment was finished in triplicates, and the resulting bacterial growths on replicates were reported in corresponding to a particular sample in the mean ± standard deviation (n = 3). The experimental analysis was based on three independent sample analyses for MIC and MBC.

Results

Syntheses, Characterizations and Morphology of Ag and Au Nanoparticles

The additions of 1 mM each of AgNO₃ and HAuCl₄ to the respective reaction mixtures resulted in solutions with a yellowish brown color and a dark purple color, indicating the formations of Ag and Au NPs, respectively (Fig. 1). The color developments may be explained by the excitation of surface plasmon vibrations in the metal nanoparticles [10, 11]. The silver surface plasmon resonance (SPR) band occurred at 440 nm, and the intensity steadily increased as a function of time, without any shift in the peak wavelength (Fig. 1) [11, 12]. In the gold ion reduction, the SPR band occurred at 546 nm (Fig. 1) [12, 13]. The EDS of the NPs synthesized after treatment with CNSL and 1 mM of either silver AgNO₃ or HAuCl₄ solution are shown in Fig. 2a, b, which illustrate the presence of Ag and Au NPs. Review shows different characterizations of Ag and Au NPs using EDS [12–14]. The energy-dispersive spectroscopy analysis of the X-rays (EDS; Fig. 2a, b) shows the presence of elemental silver and gold in the freeze-dried powder. These results agree with other studies on Ag and Au NPs preparation using biomaterial such as plant extracts and microorganisms reported by Shahverdi et al. [14]; Song and Kim [15]; Twu et al. [16]. The XRD patterns recorded for Au and Ag NPs show six intense peaks for silver and eight intense peaks for gold over the whole spectrum of 2θ, with values ranging from 10 to 90 (Fig. 3a, b). Intense peaks of 111, 200, 220, and 311 nanoparticles were observed, indexed as the crystalline silver and gold face-centered cubic phase. The intensity of the Bragg reflections suggests that there are strong X-ray scattering centers in the crystalline phase that could possibly arise from inorganic compounds present in the nanoparticles during synthesis [16, 17]. Further, the morphologies and sizes of the nanoparticles were determined with the help of TEM. The average size range of the Ag and Au NPs was 5–50 nm, as illustrated in Figs. 4a, b, 5a, b, respectively. The TEM images indicate that the Ag NPs are spherical in shape and that the Au NPs showed multiple morphologies, including spherical, triangle, truncated triangles, and decahedral morphologies with relatively uniform diameters. Figure 6a shows FT-IR spectra of the Ag NPs. Spectra obtained from the NPs manifested absorption peaks positioned at about 790.1, 1000.10, 1610.2, 2350.5, and 3400.6 in the region of 5000–4000 cm⁻¹. The FT-IR spectra revealed the presence of different acids and functional groups, such as anacardic acid, carboxylic acids, cardanol, cardol, 2-methyl-cardolketones, aldehydes, aromatic and hydroxyl functional groups in alcohols and phenolic compounds (3400.6 cm⁻¹), and amino acid residues present in the CSNL. These groups may lie between those of inorganic molecules and synthesized Ag NPs, which gives rise to the well-known signatures in the infrared region of the electromagnetic spectrum [17, 18].

Fig. 1.

Inset show the formation of Ag and Au NPs

Fig. 2.

SEM-EDS spectra of a silver and b gold nanoparticles synthesized using CSNL

Fig. 3.

XRD patterns of a silver and b gold nanoparticles synthesized using CSNL

Fig. 4.

TEM micrographs of Ag NPs [scale bar at 100 nm (a) and 20 nm (b)] synthesized using CSNL

Fig. 5.

TEM micrographs of the Au NPs [scale bar at 100 nm (a) and 20 nm (b)] synthesized using CSNL

Fig. 6.

FT-IR spectra of silver (a) and gold (b) NPs synthesized using CSNL

Our results corroborate the reports of Kaliswaralal et al. [18] the component present in CSNL can bind to nanoparticles through either free amine groups or aldehydes, aromatic, hydroxyl functional groups in alcohols, and phenolic compounds in the CSNL. With the case of Au NPs, strong bands at 1090.2, 1400.8, 1610.2, 1720.2, 2900.6, and 3400.6 cm⁻¹ were observed, and some weak bands at 710.2 and 790.1 cm⁻¹ were also recorded (Fig. 6b). The bands at 790.1, 1610.2, 1090.2, and 3400.6 cm⁻¹ were similar to those of the Ag NP peaks; consequently, they were attributed to the same compounds and amino acid residues. Residual peaks at 1400.8, 1720.2, and 2900.6 cm⁻¹ might represent the carbonyls of ketones, ester –NH stretching of the amide (II) band, carbonyl groups, or secondary amines [11, 16].

Surface plasmon resonance spectra for Ag and Au NPs were obtained at 440 and 546 nm with brown-yellow and purple color, respectively. Ag and Au NPs have free electrons, which cause an (SPR) absorption band arising from interband transitions due to the combined vibration of electrons of metal nanoparticles in resonance with the light wave [12, 13]. In this study, Ag and Au NPs are found to have (SPR) absorption bands centered at 440 and 546 nm, respectively, as shown in Fig. 1.

Antibacterial Activity

The antibacterial activity of the synthesized Ag and Au NPs was tested against the Gram-negative fish pathogenic bacteria A. hydrophila, A.bestiarum, P. fluorescens, and E. tarda. The results of the antibacterial activity of Ag and Au NPs against fish pathogens are observed after 24 h of incubation at 37 °C was presented in Fig. 7 and Table 1. The negative control CSNL (10 mg/mL) has no remarkable interests on zone formation. Ag NPs are effective in inhibiting all the above said bacterial strains, however for Au NPs only arresting the growth of A.bestiarum and P. fluorescens. The ZOI of around 15.3 ± 0.3 (Ag NPs) was observed for E. tarda, 14.8 ± 0.4 P. fluorescens, 14.1 ± 0.6 A.bestiarum and 12.5 ± 0.2 for A. hydrophila. However in the case of Au NPs the ZOI of around 12.6 ± 0.4 and 11.2 ± 0.9 for A.bestiarum and P. fluorescens, respectively.

Fig. 7.

Plates showing the inhibition zones around wells for Aeromonas hydrophila, Aeromonas bestiarum, Pseudomonas fluorescens, and Edwardsiella tarda

Table 1.

Antibacterial activity of CSNL mediated synthesized Ag and Au nanoparticles (ZOI, MIC and MBC) against Aeromonas hydrophila, Aeromonas bestiarum, Pseudomonas fluorescens, and Edwardsiella tarda fish pathogens

Bacterial strains	Gram class	ZOI (mm)	MIC (μg/mL)	MBC (μg/mL)	Control positive TC (mm)
Silver nanoparticle
Aeromonas hydrophila	Gram-negative, rod-shaped	12.5 ± 0.2	250 ± 10.2	322 ± 9.2	21.1 ± 0.3
Aeromonas bestiarum	Gram-negative, rod-shaped	14.1 ± 0.6	312 ± 9.4	205 ± 10.5	20.4 ± 0.3
Pseudomonas fluorescens	Gram-negative, rod-shaped	14.8 ± 0.4	180 ± 14.6	183 ± 14.6	23.1 ± 0.1
Edwardsiella tarda	Gram-negative, straight rod-shaped	15.3 ± 0.3	124 ± 9.1	128 ± 11.0	16.4 ± 0.2
Gold nanoparticle
Aeromonas hydrophila	Gram-negative, rod-shaped	NZ	–	–	21.1 ± 0.3
Aeromonas bestiarum	Gram-negative, rod-shaped	12.6 ± 0.4	294 ± 12.8	294 ± 9.4	20.4 ± 0.3
Pseudomonas fluorescens	Gram-negative, rod-shaped	11.2 ± 0.9	386 ± 12.7	363 ± 16.2	23.1 ± 0.1
Edwardsiella tarda	Gram-negative, straight rod-shaped	NZ	–	–	16.4 ± 0.2

Mean ± standard deviation was used to expressed the values as (n = 3). 10–100 μg/mL concentration of Ag and Au nanoparticles was used

Ag silver, Au gold, ZOI zone of inhibition, MIC minimum inhibitory concentration, MBC minimum bactericidal concentration, CSNL cashew nut shell liquid, TC tetracycline, NZ no zone

Minimal Inhibitory Concentration/Minimal Bactericidal Concentration

The MIC was determined as the lowest concentration at which no visible growth of the fish pathogen was observed. The MIC of Ag NPs were found to be 124 ± 9.1 μg/mL for E. tarda, 180 ± 14.6 μg/mL, P. fluorescens, 312 ± 9.4 μg/mL, A.bestiarum, and 250 ± 10.2 μg/mL, A. hydrophila, respectively and for Au NPs found to be 294 ± 12.8 μg/mL A.bestiarum and 386 ± 12.7 μg/mL for P. fluorescens respectively (Table 1). The MBC values for Au NPs is about 128 ± 11.0 μg/mL for E. tarda, 183 ± 14.6 μg/mL P. fluorescens, 205 ± 10.5 μg/mL A.bestiarum, and 322 ± 9.2 μg/mL A. hydrophila. For Au NP the MBC was 363 ± 16.2 μg/mL P. fluorescens and 294 ± 9.42 μg/mL A.bestiarum, respectively (Table 1).

Discussion

This study evaluated the efficacy of the inhibitory effect of Ag and Au NPs synthesized using CSNL against bacterial fish pathogens. Especially with regards to those that cause important disease outbreaks in the aquaculture industry that have severe economic impacts [19]. Therefore, emerging nanoparticle antimicrobial therapies are of great interest ecological conservations. Through various instrumental analysis, it shows that CSNL has the capacity to produce various morphology of Ag and Au NPs. This present study concluded that synthesized Au NPs has maximum inhibitory effect against all the bacterial strains used. However, Au NPs had less effect on P. fluorescens and A.bestiarum. In application point of view Au NPs are not suitable, due to its economical value when compare to Ag NPs. The bacterial growth inhibition around the well is due to incorporation of metallic silver particles, which supports is to obtain high specific surface area and a high fraction of surface atoms of NPs will lead to high bactericide activity [8]. The differential sensitivity of bacteria towards Ag and Au NPs depends upon size of particles, route of synthesis, structure of bacterial cell wall, and the mode of contact with organisms with nanoparticles [11, 20]. The difference in activity against these two nanoparticles could be attributed to the compound present in the metals and the size of nanoparticles obtained might be a reason to high/low antibacterial activity. A recent study describes, when irradiation applied at 4 kGy without Ag NPs and at 2 kGy with Ag NPs against total aerobic mesophilic bacteria, Enterobacteriaceae, Escherichia coli and Clostridium perfringens shows greater inhibition zones in antimicrobial treatments of the packaging films using Ag NPs impregnation and gamma irradiation [20–22]. The CSNL mediated synthesized Ag and Au NPs with bactericidal activity might became an alternative to antibiotics used in fishery and aquaculture industry [22–24]. In the fact of controlling chemical toxicity in the environment, occurred through the antibiotic used in aquaculture industry can be replaced by eco-friendly biosynthesized Ag NPs [25].