Introduction

Metal nanoparticles have emerged as a new avenue of opportunities and novel applications in the field of nanotechnology due to their unique properties which vastly differ from their bulk counterparts. Among the different noble metal nanoparticles, gold nanoparticles are gaining a lot of significance in the recent years because of their enticing applications in the catalysis, biomedical, drug delivery, sensing, optics, and imaging [1,2,3]. The utilization of gold nanoparticles as both homogenous and heterogenous catalyst in various reactions is clearly evident in their key applications [4,5,6]. This high catalytic potential is attributed towards their physico-chemical properties due to the small particle size, morphology, quantum confinement, and large surface area [7].

The synthesis of metal nanoparticles can be achieved by means of chemical, sonochemical, photochemical, microemulsion, radiolytic, and microwave techniques [8,9,10]. Upon environmental concerns and focus on green chemistry, the strategy of microwave synthesis is the frequently adopted method. Microwave irradiation provides uniform heating, homogenous nucleation, and requisite growth conditions for the nanoparticle production within very short reaction time [11]. Overall, the procedure is fast, cost-effective, clean, and ecofriendly. Energy transfer from microwave radiation to the reactants is achieved by the interaction of the radiation with water or other high dielectric constant solvents or solvent molecules with large dipole moments [12]. Based on the ease of natural availability, simplicity, and safety in handling, plant extracts are the mostly preferred non-chemical reducing and stabilizing agent than other biological entities like natural products, biopolymers, or microorganisms. The incorporation of microwaves to this approach offers a simple route for the preparation of gold nanoparticles, which results in the successful completion of reaction with better yield of products involving lower energy utilization [13]. Moreover, this methodology precludes the use of toxic chemicals and coarse reaction requirements which were fundamental in the traditional means. The disadvantage of requiring longer reaction times for biological means to produce nanoparticles is mitigated by the action of unconventional microwave strategies and does not interfere with the required green reaction conditions [14, 15]. Green synthesis of gold nanoparticles using different plants including Euphorbia tirucalli [16], Aerva lanata [17], and Garcinia kola [18] with the aid of microwave energy is reported.

Recently, environmental issues regarding fresh water contamination and shortage of adequate drinking water are increasing consideration everywhere throughout the world. The disposal of untreated industrial effluents and domestic wastage into the natural aquatic environment is of global concern. Vital toxic pollutants mainly include synthetic dyes and more than 100,000 different dyes are produced annually [19]. Dyes are class of organic compounds which find extensive use in paper, leather, glass, textiles, cosmetics, ceramics, and pharmaceutical industries [20]. Their ability to impart color into the medium even at mild concentrations makes them a major contaminant in the waste water effluents. They are potent carcinogen, mutagen, eye, and skin irritant and may cause major health hazards upon inhalation or ingestion [21].

Aromatic nitro compounds are another set of hazardous organic pollutants which are a major threat to the water environment. They are produced from different industrial and agriculture processes including the manufacture of herbicides, pesticides, synthetic dyes, explosives, drugs, and other products [22]. They dangerously affect the human reproductive and central nervous systems [23]. The stability, water solubility, and inhibitory nature of nitroaromatics adulate their toxicity.

Besides human health problems, the discharge of organic contaminants into the water sources causes deprivation of oxygen in fresh water streams, decreases sunlight penetration, and thereby affects the aquatic life [24]. Thus, alleviation of these anthropogenic pollutants in the waste water becomes a prerequisite to their disposal. The high chemical stability and microbial resistance of these pollutants obturate the conventional water treatment methods like biodegradation, adsorption, and ultrafiltration to become effectual [25]. Latest reports suggest that catalytic activity of green nanomaterials has been expanded to the sector of waste water treatment also [14, 15]. From the ecological point of view, the development of such an effective and facile method for the abatement of organic pollutants from waste water is significantly adorable.

In this paper, we report a novel microwave-assisted green synthesis of gold nanoparticles using aqueous extract Amaranthus tricolor as the reducing and stabilization agent. Amaranthus tricolor is a medicinal plant which belongs to the family of Amaranthaceae. They are used in the treatment of piles, bronchitis, leucorrhoea, blood disorders, and even as laxative, stomachic, diuretic, hemorrhage, antipyretic, and hepatoprotective agent [26]. Amaranthin is the main betacyanin violet pigment in the plant [27]. The phytochemical analysis of its aqueous extract shows the presence of alkaloids, tannins, flavonoids, saponins, amino acids, proteins, and phenolic compounds [28]. Microwave heating aids in the complete reduction of Au3+ to Au0 within 1 min. The Amaranthus tricolor–mediated gold nanoparticles (AuNP-AT) are characterized using UV–vis., FT-IR, HR-TEM, EDX, DLS, and zeta potential analyses. The prospective catalytic efficiency of AuNP-AT is examined for reductions of 2-nitrophenol and eosin Y. Also, the cytotoxic and antibacterial properties of the nanoparticles were also evaluated.

Experimental

Materials

All the chemicals purchased were of analytical grade and were used as such with no further purging. Hydrogen tetrachloroaurate (III) trihydrate (HAuCl4.3H2O, 99.99%) was purchased from Sigma-Aldrich. Eosin Y (99%), 2-nitrophenol (98%), and sodium borohydride (98%) were obtained from Merck. All solutions were prepared using double distilled water in the entire experiment. Fresh leaves of Amaranthus tricolor were identified and collected.

Methods

Preparation of Amaranthus tricolor (AT) Leaf Extract

About 25 g of the leaves of Amaranthus tricolor was chopped into fine pieces and boiled for 1 h with 100 mL distilled water. The aqueous extract is filtered and stored at 4 °C for further studies.

Green Synthesis of Gold Nanoparticles Using Amaranthus tricolor Leaf Extract (AuNP-AT)

Amaranthus tricolor (AT) leaf extract and 1 mM solution of the gold salt were mixed in a 250-mL beaker in a specific ratio. It was then subjected to microwave irradiation for 60 s in a domestic microwave oven (Sharp R-219 T(W)), 2450 MHz). The reduction of Au3+ to Au0 was visually identified by the color change of the reaction mixture from pale pink to violet. Confirmation on the formation of gold nanoparticles was done by monitoring the UV–vis absorption spectrum in the range of 200–800 nm. The optimization by means of composition ratio of AT extract to the gold salt, microwave irradiation time, and power was also investigated.

Catalytic Studies

The catalytic reduction of eosin Y (1 × 10−4 M) and 2-nitro phenol (4 × 10−4 M) was studied. It involves the mixing of 2.5 mL of the organic pollutant, cold solution of 0.5 mL freshly prepared NaBH4 (0.6 M), and 0.25 mL AuNP-AT in a quartz cuvette of 1 cm path length. The degradation of both pollutants was identified by its solution decolorization and were monitored using UV–vis spectrophotometer in the range of 200–800 nm at regular intervals of time.

In Vitro Cytotoxic Studies Using MTT Assay

The cytotoxic effect of Amaranthus tricolor (AT) leaf extract and AuNP-AT against A549 (Human Lung cancer) cells was determined using MTT (3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay. The cell lines were procured from National Centre for Cell Sciences (NCCS), Pune, India, and was maintained in Dulbecco’s modified Eagles medium (DMEM) supplemented with 10% FBS, L-glutamine, sodium bicarbonate (Merck, Germany), and antibiotic solution containing the following: penicillin (100U/mL), streptomycin (100 µg/mL), and amphoteracin B (2.5 µg/mL). Two-day-old confluent monolayer of cells was trypsinized and the cells were suspended in 10% growth medium, and 100 µL cell suspension (5 × 103 cells/well) was seeded in 96-well tissue culture plate. The cultured cell lines were kept at 37 °C in a humidified 5% CO2 incubator (NBS Eppendorf, Germany). The viability of cells was evaluated by direct observation of cells by inverted phase contrast microscope and followed by MTT assay method.

Various amounts of the plant extract and gold nanoparticles (6.25, 12.5, 25, 50, and 100 μg/mL) were added to the grown cells and were incubated for 24 h. The untreated cells serve as the control. After incubation period, entire plates were observed using inverted phase contrast tissue culture microscope (Olympus CKX41 with Optika Pro5 CCD camera) and microscopic observation was recorded as images.

After 24 h of incubation period, the sample content in wells was removed and 30 µL of reconstituted MTT solution was added to all test and cell control wells, and the plate was gently shaken well, then incubated for 4 h (at 37 °C in 5% CO2 incubator). After the incubation period, the supernatant was removed and 100 µL of MTT solubilization solution (dimethyl sulphoxide, DMSO, Sigma-Aldrich, USA) was added and the wells were mixed gently by pipetting up and down in order to solubilize the formazan crystals. The absorbance values were measured by using microplate reader at a wavelength of 540 nm. The whole experiment is done in triplicate. The percentage of cell viability was calculated using the following equation.

$$\%\;cell\;viability=\frac{Mean\;OD\;of\;Samples}{Mean\;OD\;of\;control}\times100$$

where “OD” stands for optical density. The LC50 values were calculated using ED50 PLUS V1.0 software and mean ± standard deviation was also identified.

Antibacterial Studies

The potential of synthesized gold nanoparticles as antibacterial agents was accessed by using the agar well diffusion method. Petriplates containing 20 mL Muller Hinton agar medium were seeded with bacterial culture of Escherichia coli, Pseudomonas aeruginosa, Streptococcus mutans, and Staphylococcus aureus. Wells of approximately 10 mm were bored using a well cutter and 100 µL of the samples was added. The plates were then incubated at 37 °C for 24 h. The antibacterial activity was assayed by measuring the diameter (in mm) of the inhibition zone formed around the well and distilled water was used as the control.

Characterization Techniques

The green-synthesized AuNP-AT was characterized by Shimadzu UV-2450 spectrophotometer (UV–vis spectra), Perkin Elmer-400 FT-IR spectrometer (FT-IR spectra), Bruker AXS D8 Advance X-ray diffractometer (XRD diffractions), JEOL-2100 model microscope (HR-TEM and EDX) and Horiba SZ-100 scientific nanoparticle analyzer (DLS and zeta potential).

Results and Discussion

UV–vis Spectroscopy

The aqueous extract of AT acts as both a reducing and capping agent for the synthesis of AuNP-AT. The microwave-assisted gold nanoparticles synthesis was monitored using UV–vis spectroscopic technique in the range of 200–800 nm. The green-synthesized AuNP-AT was primarily identified by the color change of the solution mixture from pale pink to violet upon 1 min of microwave heating. It was confirmed by the absorbance band at 534 nm which corresponds to the surface plasmon resonance (SPR) in the UV–vis spectrum (Fig. 1). Also, the AT extract did not show any peak in this range.

Fig. 1
figure 1

a Photograph of Amaranthus tricolor and b UV–vis spectra of AT extract and AuNP-AT

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In the phytomediated synthesis, formation and nucleation of gold nanoparticles were resulted by the transfer of charge from the various biomolecules in the plant extract such as proteins, amino acids, enzymes, flavonoids, and terpenoids to Au3+ ions. While few molecules of phytochemicals get adsorbed on the nanoparticles surface by means of coulombic forces of attraction and form electrostatic double layers [29]. Due to the van der Walls forces of attraction, a diffuse double layer was formed around this electrostatic double layer through adsorption of various layers of phytochemicals. Electrostatic or steric barriers take place around the surface of gold nanoparticles due to the variations in the molecular structures of phytochemicals present in the AT extract. This led to the effective capping of the synthesized nanoparticles by restricting its agglomeration and enhancing their stability.

The green synthesis of AuNP-AT was optimized by varying the compositional ratio between AT extract and gold salt, microwave irradiation power, and irradiation time. Different compositions of AT extract and tetra chloroaurate solution were tried for the green synthesis of AuNP-ATas shown in Fig. 2a. For 1:3 ratio, nanoparticle formation did not occur. In case of 1:5 ratio, the formation of nanoparticles was identified but the absorption band was not sharp enough. A broadened peak appeared in 1:7 composition. While 1:9 composition exhibited a smooth sharp peak at 534 nm in the absorbance spectrum which manifests the monodispersitivity of the formed AuNP-AT particles. Thus, this 1:9 is well chosen over other ratios for further studies.

Fig. 2
figure 2

UV–vis spectra showing the effect of a compositional ratio between AT extract and HAuCl4 where 1:9 is optimum, b microwave power level where 800 W is optimum, and c microwave irradiation time where 60 s is optimum for the formation of AuNP-AT

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The effect of microwave power on the AuNP-AT synthesis using 1:9 composition was studied between 200 and 800 W, by maintaining the time of irradiation a constant (Fig. 2b). The formation of nanoparticles was not initiated at 200 W. Out of the various smooth absorbance bands obtained for 400, 600, and 800 W of AuNP-AT synthesis, the most intense and sharp one was that of 800 W (Fig. 2b). The optimum irradiation power was thus fixed at a power of 800 W.

The dependance of microwave irradiation time on AuNP-AT synthesis was monitored at every 15 s of heating. An increase in the absorbance of the SPR band around 534 nm with irradiation time was observed and reached a maximum within 60 s (Fig. 2c). No considerable change in the position and absorbance of the peak was detected even after 1 min of microwave heating due to the absence of nanoparticle agglomeration.

FT-IR Spectra

FT-IR analysis helps in the identification of various functional groups present in the plant extract which are responsible for the reduction of gold salt. Figure 3 shows the FT-IR spectra of AT extract and AuNP-AT. A strong absorption band at 3371 cm−1 corresponds to the stretching vibration of -O–H bond. The bands at 2949, 1625, 1225, and 1090 cm−1 are due to the aliphatic C-H, -C = O, C-O, and C–O–C stretching vibrations of different phytochemicals present in the AT extract [30]. Bands at 1388 and 1016 cm−1 confirmed the presence of aromatic and aliphatic amines, respectively [31]. Thus, the hydroxyl, carboxyl, and amine groups played an important role in both reduction and stabilization process of the synthesized AuNP-AT. A slight displacement in peak positions of AT extract is observed in AuNP-AT.

Fig. 3
figure 3

FT-IR spectra of a Amaranthus tricolor extract, and b AuNP-AT

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XRD Analysis

The XRD pattern aids in determining the highly crystalline nature of the formed AuNP-AT. The highly distinct and intense diffraction patterns at 2θ values of 38.31°, 44.41°, 64.61°, and 77.82° matches the reflections from (111), (200), (220), and (311) planes of face-centered cubic lattice of metallic gold nanoparticles (Fig. 4). These values are much similar to that of JCPDS file no. 04–0784 [32]. Out of the four diffraction peaks, the one which corresponds to the (111) plane is the more preferred plane of orientation by the AuNP-AT.

Fig. 4
figure 4

XRD pattern of AuNP-AT

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HR-TEM and SAED Analysis

The TEM micrographs at different magnifications of the synthesized AuNP-AT are depicted in Fig. 5a and b. The polydispersive nature of the gold nanoparticles is simply evident from the various morphologies adopted such as triangular plate, spherical, and rod shapes. Majority among them are of almost spherical morphology and the average diameter of particles is found to be 18.33 nm (Fig. 5c). The HR-TEM image of the formed AuNP-AT shows the clear lattice fringes (Fig. 5e). Bright circular rings in the selected area diffraction (SAED) pattern validate the pure crystallinity of the nanoparticles. The SAED fringes in Fig. 5d correspond to the (111), (200), (220), and (311) planes of the face-centered gold which are in good agreement with the XRD pattern.

Fig. 5.
figure 5

a and b TEM images of AuNP-MF at different magnifications, c particle-size histogram, d HR-TEM image, and e SAED pattern of the formed AuNP-AT

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EDX Analysis

The EDX analysis of AuNP-AT showed signals at 2.3, 8.2, 9.8, 11.5, and 13.4 keV, which confirms the presence of elemental gold (Fig. 6). The sharp signals indicated the complete reduction of gold ions to elemental gold. Signals of carbon may arise from the phytochemical constituents in the leaf extract of AT.

Fig. 6
figure 6

EDX spectrum of AuNP-AT

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DLS and Zeta Potential Measurements

The particle size obtained from the DLS analysis is found to be 148.5 nm which is much larger than their TEM results (Fig. 7a). This is so because in TEM, individual particle diameter is determined. While DLS measures the intensity of the scattered light from the core nanoparticles and the phytochemical cloud around the gold nanocore is analyzed as a function of time and thus the hydrodynamic size of nanoparticles is determined. The effect of Brownian motion and size measurement using Stokes–Einstein relationship is of much importance in DLS analysis. Thus, the nanoparticle measured size is somewhat larger than the actual size measured from TEM analysis. The zeta potential value of − 21.3 mV for AuNP-AT particles confirms its stability (Fig. 7b). The DLS and zeta potential measurements were carried out at an instrumental controlled setup of 25 °C.

Fig. 7
figure 7

a DLS and b zeta potential measurement of AuNP-AT

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Catalytic Reduction of Eosin Y

Eosin Y is a water soluble heterocyclic dye having four bromine atoms, which belongs to the xanthene family. They are used in fluorescent pigments, histological staining, textile, and leather industries [33, 34]. The direct discharge of eosin Y into water streams is a serious threat to the environment due to its dark orange color, toxicity, and high stability because of their complex aromatic structure [35, 36].

The absorption spectrum of eosin Y shows a prominent peak at 515 nm (Fig. 8a). The UV–vis spectra of the dye with NaBH4, in the absence of AuNP-AT catalyst, did not show a much appreciable change in their absorbance as well as in the orange color of its aqueous solution even after several hours of the reaction (Fig. 8b). Upon the addition of 0.02 mg/mL AuNP-AT catalyst, a progressive reduction of the dye initiated and the reaction was completed within 14 min (Fig. 9a). The completion of catalyst-supported dye reduction was identified by the visual color change from orange to colorless. This shows that the degradation is not effectual in the absence of gold catalyst or in the presence of reducing agent, NaBH4 alone. The UV–vis spectra at 515 nm were monitored at every 1-min interval during the course of the degradation.

Fig. 8
figure 8

UV–vis spectra of a eosin Y, and b degradation of eosin Y in the presence of NaBH4 alone

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Fig. 9
figure 9

UV–vis absorption spectra for the degradation of eosin Y by NaBH4 in presence of a 0.02, b 0.03, and c 0.04 mg/mL concentration of AuNP-AT catalyst

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Metal nanoparticles accelerate the degradation process by means of their electron relay between the donor–acceptor molecules. The large surface area availability and intermediate reduction potential of the gold nano catalyst between the borohydride (donor) and dye (acceptor) moieties contribute towards their catalytic activity. The reduction mechanism involves the effective adsorption of both reactants, dye and borohydride ions, onto the surface of AuNP-AT catalyst and efficient electron transfer between them [37]. The phytoconstituents surrounding the metal nanoparticles aid in bringing the reactant groups in a closer proximity to the catalyst surface by means of electrostatic interactions. Thus, a kinetically forbidden reaction becomes an allowed one.

The kinetics of the reaction is studied by monitoring the changes in absorbance of the peak at 515 nm with time. As the concentration of NaBH4 used in the degradation is much higher than that of the dye and AuNP-AT catalyst, it remains a constant throughout the reaction, and hence, it follows a pseudo-first-order kinetics. The reaction kinetics is represented as ln[A]/[A0] =  − kt, where k is pseudo first order rate constant, [A0] is the concentration of eosin Y at time t = 0, and [A] is the concentration at time “t.” The value of [A] is acquired from the absorbance value of the peak at 515 nm. The rate constant of the catalytic reduction can be directly obtained from the slope of ln [A] versus time plot.

In order to study the effect of catalyst concentration on the rate of reduction, reactions were carried out using different concentrations of AuNP-AT catalyst, keeping all other parameters a constant. Straight line plots with good correlation coefficients were obtained in the ln [A] versus time graph for all catalytic reactions (Fig. 10). A linear increase in the first-order rate constants with increase in concentration of the catalyst was also observed (Table 1). The schematic representation for the reduction of Eosin Y is shown in Fig. 11.

Fig. 10
figure 10

Plot of ln [A] against time for the reduction of eosin Y

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Table 1 Catalytic activity of AuNP-AT in the reduction of eosin Y
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Fig. 11
figure 11

Schematic representation for the reduction of eosin Y

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Catalytic Reduction of 2-Nitrophenol

Nitrophenols are a major class of toxic intermediates in the production of rubber chemicals, dyestuffs, pigments, and fungicides [38]. They are extensively used in laboratories as a good acid–base indicator. Nitrophenols are enlisted as priority pollutants by US-EPA due to their carcinogenic and mutagenic potential [39]. Decomposition of aqueous nitrophenol is very difficult even at low concentration. Therefore, their reduction into aminophenol is often appreciated due to its pharmaceutical importance of being an antecedent for the manufacture of various drugs [40].

The aqueous solution of 2-nitrophenol shows an absorption peak at 340 nm in UV–vis spectrum as shown in Fig. 12a. On the addition of NaBH4, the characteristic peak at 340 nm shifted to 400 nm, due to the evolution of phenolate ions [41, 42]. Moreover, the pale yellow color of 2-nitrophenol changed to intense greenish yellow. The color of solution and peak intensity at 400 nm remains unaltered even after a long time of response. Although NaBH4 exhibits strong reducing power, only a very gradual reduction of 2-nitrophenol to 2-aminophenol can be attained by borohydride ions alone (Fig. 12b).

Fig. 12
figure 12

UV–vis spectra of a 2-nitrophenol and 2-nitrophenolate ion, and b reduction of 2-nitrophenol by NaBH4 alone

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After the addition of a very small amount of AuNP-AT catalyst with increasing concentration, an enhanced rate of reduction was observed (Fig. 13). The reactions were portrayed by the regular diminishment of peak at 400 nm with time in the absorption spectrum and the peak at 282 nm was red-shifted to 290 nm. The emergence of a new peak around 290 nm and the drop of peak intensity at 400 nm correspond to the effectual reduction of 2-nitrophenol to 2-aminophenol by AuNP-AT catalyst [43]. As the concentration of NaBH4 used was in excess, kinetics of the reduction follows pseudo-first-order with respect to 2-nitrophenol. Reaction kinetics was evaluated using the equation ln[A]/[A0] =  − kt, where k is the pseudo-first-order rate constant, and [A] and [A0] are the concentrations of 2-nitrophenol corresponding to their absorbances at 400 nm on initial and “t” timings, respectively. Reaction constants were calculated from slopes of the linear plots of ln [A] against time (Fig. 14).

Fig. 13
figure 13

UV–vis absorption spectra for the degradation of 2-nitrophenol by NaBH4 in the presence of a 0.02, b 0.03, and c 0.04 mg/mL concentration of AuNP-AT catalyst

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Fig. 14
figure 14

Plot of ln [A] against time for the reduction of 2-nitrophenol

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The nanocatalyst facilitates the hydrogenation reaction by the coherent transfer of electrons from borohydride donor to nitrophenolate ion acceptor which was initiated upon the adsorption of both substrate ions onto the AuNP-AT catalyst surface. The electrons ejected to the metal surface from the borohydride ions are gained by phenolate ions and get easily reduced. To assess the dependence of catalyst concentration on the reaction, reductions were accomplished using different concentrations of AuNP-AT catalyst, keeping other parameters a constant. The first-order rate constants for the reactions using various concentrations of AuNP-AT catalyst with their respective correlation coefficients and time for degradation are shown in Table 2. Thus, the environmentally benign green gold nanoparticles act as a valid candidate for the productive use in waste water treatment. Figure 15 shows the schematic representation for the reduction of 2-nitrophenol. A comparison on the catalytic activity between AuNP-AT and various other green synthesized nanoparticles is shown in Table 3.

Table 2 Catalytic activity of AuNP-AT in the reduction of 2-nitrophenol
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Fig. 15
figure 15

Schematic representation for the reduction of 2-nitrophenol

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Table 3 Comparison of catalytic activity of AuNP-AT with various reported green synthesized nanoparticles
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Cytotoxic Studies

The antiproliferative effect of aqueous AT leaf extract and AuNP-AT was investigated on human lung cancer cell line, A549 using MTT assay. Morphological changes including shrinking and granulation of the lung cancer cells caused by the interaction of AT extract and AuNP-AT are shown in Fig. 16a and b, respectively. Nanoparticles of gold penetrate into the cell membrane and associate with the proteins or nucleic acids and hence destroy the infected cells selectively [50]. The antiproliferative effect of nanoparticles is greatly attributed to its biocompatibility and high surface to volume ratio [51]. The presence of various bioactive components on the surface of nanoparticles is responsible for the generation of reactive oxygen species inside the cells which may also effectively contribute towards damaging of carcinoma cells. The LC50 values, concentration required for 50% cell death, were determined to be 118.851 ± 0.12 and 102.39 ± 0.36 μg/mL for AT extract and AuNP-AT, respectively, and were calculated using ED50 PLUS V1.0 software. The obtained results show that LC50 value is low for AuNP-AT than the AT extract itself. Figure 17 depicts the dose-dependent cell viability (%) induced by AT extract and AuNP-AT towards A549 cancer cells.

Fig. 16
figure 16

Morphological changes induced on treated A549 cancer cell by a aqueous AT extract, and b AuNP-AT

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Fig. 17
figure 17

The cell viability (%) of AT extract and AuNP-AT towards A549 cancer cell. Values are represented as the mean ± SD (n = 3)

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Antibacterial Studies

Antibacterial properties of the synthesized AuNP-AT nanoparticles were examined using agar diffusion well method against two gram negative bacteria: Escherichia coli and Pseudomonas aeruginosa; and two gram positive bacteria: Streptococcus mutans and Staphylococcus aureus as shown in Fig. 18. The antibacterial efficiency of AT extract and AuNP-AT is measured in terms of inhibition zone diameter (mm). The antibacterial activity of aqueous AT extract is attributed to the presence of alkaloids, tannins, flavanoids, saponins, proteins, and phenolic compounds in it. The synergetic effect from both nanosized gold and the bioactive phytoconstituents attached on its nanosurface contributed towards the higher antibacterial efficiency of the AuNP-AT particles. In case of gram negative bacteria: Escherichia coli and Pseudomonasaeruginosa, the zone of inhibition exhibited by the AuNP-AT is greater than that of AT extract. And the difference between their inhibition zones is much relevant. The close binding of nanoparticles with the phosphorus- and sulfur-containing components like DNA of the bacterial cell membrane leads to protein dysfunction and ultimate damage of the cell [52]. But in case of gram positive bacteria: Streptococcus mutans and Staphylococcus aureus, the zone of inhibition shown by the AuNP-AT is only slightly greater than that of AT extract. The very thin peptidoglycan layer present on the cell wall of gram negative bacteria permits the easy penetration of AuNP-AT into the bacterial cell and disturbs proper functioning of the cells by hindering the normal respiration and permeability [53]. While the presence of very thick peptidoglycan layers of the cell wall hinders the easy penetration of nanoparticles into the gram positive pathogens. Thus, the antibacterial efficiency of AuNP-AT is more significant in case of gram negative pathogens than the positive ones (Table 4).

Fig. 18
figure 18

Illustration of the antibacterial activity of AuNP-AT against E. coli, P. aeruginosa, S. aureus, and S. mutans using agar well diffusion method. [A: AT extract, B: AuNP-AT and C: control]

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Table 4 Zone of inhibitions (mm) of AT extract, AuNP-AT, and control against various bacteria
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Conclusions

Microwave-assisted green synthesis of highly crystalline stable gold nanoparticles using the aqueous extract of Amaranthus tricolor leaves as both reducing and stabilizing agent is established. This is a simple, rapid, and eco-friendly method for the synthesis of noble metal gold nanoparticles. The synthesis of gold nanoparticles was optimized and the formed AuNP-AT were characterized by UV–vis., FT-IR, XRD, HR-TEM, EDX, DLS, and zeta potential measurements. The prepared metal nanoparticles exhibited catalytic properties in the degradation of eosin Y and 2-nitrophenol. Both the catalytic degradations followed pseudo-first-order kinetics model with good correlation coefficients and rate of the reaction increased linearly with increasing catalytic dosage. The antiproliferative studies proved that the prepared nanoparticles can be used as a mighty weapon against human lung cancer cell lines, A549 and may be used in preparation of anticancer drug. The nanoparticles were found to be highly toxic towards various bacteria, which proved its antibacterial efficacy. This study emphasizes on the green synthetic protocols adopted for gold nanoparticles and their stupendous applications in the fields of catalysis and biomedicine.