Introduction

Nanoregime size attributes awesome properties to noble metals which can be tailored according to our provision (Oemrawsingh et al. 2012). Designing and synthesizing novel products through less hazardous domain is a green chemistry perspective (Anastas and Kirchhoff 2002; Sheldon 2012). The inculcation of plant extracts in the metal nanoparticles synthesis is an environmentally benign approach and is a fast alternative to conventional chemical and physical methods (Ahmed et al. 2016a; Huo et al. 2017). Microwave-assisted synthesis is a green method, which eliminates the use of toxic chemicals and reduces the preparation period (Joseph and Mathew 2015a). It is a nonclassic source of energy which improves quality and quantity of the product (Baghbanzadeh et al. 2011).

Plant extract supported gold and silver nanoparticles synthesis has been plentifully reported in the recently (Lallawmawma et al. 2015). The medicinal plant Aerva lanata, a rich reservoir of alkaloids and flavonoids, reduced HAuCI4 and AgNO3 to spherical gold and silver nanoparticles (Joseph and Mathew 2015b). Colloidal silver and gold nanoparticles were prepared at room temperature using aqueous and methanolic seed extract of Dolichos biflorus Linn (Basu et al. 2016). Gold and silver nanoparticles were prepared and stabilized using leaf extract of Mentha piperita which exhibited spherical geometry (MubarakAli et al. 2011). Phenolic hydroxyls present in the fruit extract of Punica granatum were used in the reduction of Ag+ and Au3+ ions to polydispersed silver and gold nanoparticles (MeenaKumari and Philip 2015). Using aqueous root extract of Glycyrrhiza uralensis, spherical gold and silver nanoparticles were prepared by simple heating technique (Huo et al. 2017).

Green nanoparticles synthesis is simple and possesses innumerable applications (Mohanpuria et al. 2007; Abbasi et al. 2014). Biological applications of noble metal nanoparticles include their antimicrobial power (Ahmed et al. 2016a; Chung et al. 2016), in vitro antioxidant potential (Seralathan et al. 2014; Hong et al. 2016), anticoagulant capacity (Hamedi et al. 2017), and their cytotoxicity towards different cancerous cells (Das et al. 2013; Rathi Sre et al. 2015; Balashanmugam et al. 2016; Kim et al. 2016). Effectiveness of silver nanoparticles synthesized using plant extracts towards mosquito control was recently established (Benelli 2015).

The increased use of organic toxic chemicals contributed to water pollution as per the US Environmental Protection Agency (Ibrahim et al. 2016). Huge amounts of thousands of dyes and pigments were produced every year, 10–15% ejected out as effluents from industries (Subbaiah and Kim 2016). Environment polluting and human cancer-causing dyes were effectively removed from water bodies using the electron relay effects shown by the silver and gold nanocatalysts (Joseph and Mathew 2014a; Lim et al. 2016). Marine algae-reduced gold nanoparticles were catalytically employed in the reduction of various nitro compounds using NaBH4 (Ramakrishna et al. 2016). Zero-valent metals in nanoscale help in environment remediation process (Li et al. 2016). Responsible usage of drinking water, strict control of water pollution, and novel technologies for waste water purification were inevitable for the survival of life in our planet.

Mussaenda glabrata (M. glabrata), a showy plant, belongs to Rubiaceae (coffee) family. It is found in wild forests in the Western Ghats of India. They have broad elliptical leaves, star-shaped deep red flowers, and beautiful white sepals which catch vision from a long distance. They flower almost all seasons throughout the year. M. glabrata is used to cure ulcers and asthma (Akter et al. 2013). The plant leaves were used as herbal shampoo, which improves hair growth and prevents dandruff. Recent reports of Mussaenda species unveiled that silver nanoparticles produced from AgNO3 precursor using Mussaenda erythrophylla leaf extract advent a novel nanocatalyst for the decay of methyl orange (Varadavenkatesan et al. 2016).

In the present study, gold and silver nanoparticles were prepared using M. glabrata leaf extract from their respective metal salt precursors by microwave assistance. The synthesis under room temperature was also performed. We are able to demonstrate that the synthesized nanoparticles manifest excellent antioxidant and antimicrobial potentials which facilitate a broad envelop of biomedical functionalities embedded on them. The unquantifiable hazardous effects of environment polluting dyes rhodamine B and methyl orange were eliminated using the developed gold and silver nanoparticles. The catalytic ability of the nanoparticles to accelerate the reduction of 4-nitrophenol by the reductant NaBH4 was also investigated.

Materials and methods

All chemicals used were of analytical grade. Chloroauric acid brought from Sigma-Aldrich. Silver nitrate, sodium borohydride, 4-nitrophenol, methyl orange, and rhodamine B were obtained from Merck India Ltd. All the reagents were applied without further purification.

Preparation of M. glabrata leaf extract

The fresh M. glabrata leaves were collected from wild realm in the month of August and were taxonomically identified (Fig. 1). Washed out all the dirt using double distilled water and 20 g of the sliced pieces was refluxed with 100 mL of double distilled water for 30 min at 70 °C. Then, filtration using Whatmann No.1 filter paper was done and the filtered extract was kept at 4 °C.

Fig. 1
figure 1

Photograph of the Mussaenda glabrata (M. glabrata) plant

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Room temperature synthesis of gold and silver nanoparticles

Using aqueous leaf extract of M. glabrata, gold and silver nanoparticles were prepared. A 1 mM solution of HAuCI4.3H2O/AgNO3 was mixed well with the leaf extract (9:1) and kept it for 5 and 10 min, respectively, at room temperature (28 °C). Color change of the reaction mixtures was noted, and the spectroscopic confirmation for the formation of nanoparticles was done using electronic spectrophotometer.

Microwave-assisted synthesis of gold and silver nanoparticles

A house hold microwave oven (Sharp R-219T (W)) supplied the source of microwave energy. Gold/silver metal salt solution and plant extract were mixed in the ratio 9:1. Stirred the contents well and placed inside the oven for uniform microwave nurturing. After a period of 0.5 and 1 min, the nanoparticles formation was affirmed by UV-vis spectroscopy.

Purification of nanoparticles

The phytocomponents adhered on the microwave-synthesized gold and silver nanoparticles were removed by repeated centrifugation under 12,000 rpm in a refrigerated centrifuge. Decanted the supernatant liquid and redispersed the nanoparticles in double distilled water. The metal nanoparticles were dried in vacuum. The purified nanoparticles were used for analyses.

Characterization methods

UV-vis spectra were recorded using Shimadzu UV-2450 spectrophotometer. FT-IR spectra of the purified samples were done by Perkin Elmer Spectrum Two FT-IR spectrometer. The crystallographic information of the pure samples was documented by Bruker AXS D8 Advance X-ray diffractometer. Surface morphologic parameters were drawn from a JEOL JEM-2100 transmission electron microscope equipped with EDX attachment. AFM studies were conducted using WITec Alpha300 RA machine working in tapping mode.

Antioxidant potential assessment

The antioxidant potential of the leaf extract of M. glabrata and the nanoparticles derived from it were evaluated using DPPH assay. 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical is highly stabilized by the complete delocalization of an odd electron and it possesses a pink color with an absorption maximum at 517 nm in ethanol medium (Alam et al. 2013). When scavenged by hydrogen-donating substrates, color of the medium turned yellow. The decrease in absorbance at 517 nm is direct measure of free radical scavenging of a sample in terms of its hydrogen-donating ability (Chang et al. 2001). Different concentrations (12.5, 25, 50, 100, and 200 μg/mL) of antioxidant solutions were mixed with 0.1 mM DPPH solution in DMSO under dark condition and kept it at room temperature for 20 min under incubation. The absorbance at 517 nm for the samples was measured using UV-vis spectrophotometer. Plant extract and synthesized gold and silver nanoparticles were employed in the assessment process. Ascorbic acid functioned as the standard. Three milliliter of DPPH was taken as control. Inhibition (%) of the DPPH radical scavenging is calculated by the formula, Inhibition (%) = (AbsControl − AbsSample)/AbsControl × 100. The assessment was repeated in triplicate. IC50 values were estimated using Graph pad Prism software (Kharat and Mendhulkar 2016; Phull et al. 2016).

Antimicrobial study

Antimicrobial property of the set aside gold and silver nanoparticles was figured out by the well diffusion pathway using four bacterial and two fungal stains (Perugu et al. 2015; Rajan et al. 2015; Rathi Sre et al. 2015). Freshly prepared Petri plates with Muller Hinton Agar Medium were seeded with the bacteria namely, Bacillus pumilus (MTCC 1640), Staphylococcus aureus (MTCC 96), Pseudomonas aeruginosa (MTCC 424), Escherichia coli (MTCC 443). Aspergillus niger (MTCC 1344) and Penicillium chrysogenum (MTCC 5108) were spread on potato dextrose agar medium. All microorganisms were procured originally from Microbial Type Culture Collection, Institute of Microbial Technology, Chandigarh, India. Wells of 6 mm were bored using a well cutter and the nanoparticles were added to it. Standard drugs streptomycin and griseofulvin were used as positive control for antibacterial and antifungal studies. Sterilized water used as negative control. The diameter of the zone (mm) matured after 24 h incubation time around the well revealed the restriction capacity of the nanoparticles against the tested bacterial stains (Balouiri et al. 2016). After 1 week of incubation, the antifungal activity was measured as zone of inhibition in millimeter. All experiments were replicated and the mean and standard deviation of zone of inhibition were found out.

Statistical analysis

All the biological data were expressed as mean ± standard deviation, and the results were analyzed by one-way ANOVA followed by Tukey’s post hoc analysis using Graph pad Prism software. A value of p ˂ 0.05 was considered as statistically significant.

Catalytic capacity evaluation

Reduction of 4-nitrophenol

The potential of synthesized gold and silver nanoparticles to function as heterogeneous catalysts in aqueous media was exploited in the hydrogenation of 4-nitrophenol by NaBH4 (Joseph and Mathew 2015b). A quartz cuvette containing 2 mL 4-nitrophenol (8 × 10−5 M), 0.5 mL of NaBH4 (0.06 M), and 0.5 mL of the metal nanoparticles (0.02 mg/mL) were continuously monitored under UV-vis spectrophotometer at regular intervals. The rate mechanism was studied by noting the depletion in absorbance at 400 nm of the reaction aliquot.

Dye degradation reactions

The environment pollution caused by organic coloring pigments viz. rhodamine B and methyl orange can be removed by NaBH4 using the synthesized gold and silver nanoparticles as catalysts. 2 mL rhodamine B (5 × 10−5 M) or methyl orange (8 × 10−5 M) and 0.5 mL (0.06 M) NaBH4 were taken in a quartz cuvette of 3 mL capacity. To this, a fixed amount of gold/silver nanoparticles was added. The degradation of rhodamine B and methyl orange was monitored by time based UV-vis spectra. In order to investigate kinetics of the reaction, optical density at 553 and 464 nm for rhodamine B and methyl orange was measured, respectively, at different intervals (Joseph and Mathew 2015c).

Results and discussion

UV-vis spectral analysis

The genus Mussaenda is rich in alkaloids, steroids, flavonoids, and tannins. Most of the species had attractive antioxidant, antimicrobial, antipyretic, diuretic, and wound healing capacities. The species Mussaenda was used in traditional folk medicine in Chinese and Fijian culture (Vidyalakshmi et al. 2008). The UV-vis spectrum of M. glabrata leaf extract is shown in Fig. 2(a). The water-soluble components of M. glabrata leaves caused the reduction of trivalent gold and monovalent silver ions to the corresponding metal nanoparticles.

Fig. 2
figure 2

UV-vis spectra of a M. glabrata leaf extract, b and c AuNP- M. glabrata and AgNP- M. glabrata prepared at room temperature, d and e AuNP- M. glabrata and AgNP- M. glabrata prepared under microwave irradiation

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The preliminary observation for the formation of noble nanoparticles was the rapid color change of the reaction medium. The formation of gold nanoparticles is identified by the development of vivid ruby red color to the system. The onset of reddish brown shade indicated the generation of nascent silver nanoparticles. The gold and silver nanoparticles formation has been clearly rooted in their well-established surface plasmon resonance (SPR) absorptions in UV-vis spectra (Mittal et al. 2013). The SPR is the peculiar property of certain metal nanoparticles which arises due to the resonant oscillations of surface electrons with the frequencies of the electromagnetic radiations (Mulvaney 1996). The UV-vis spectrum of the gold and silver nanometals recorded after 5 and 10 min followed by the addition of M. glabrata extract to their metal salt solutions at room temperature (28°) is shown in Fig. 2(b and c). The SPR band corresponds to gold nanoparticles was 555 nm and that corresponds to silver was 427 nm.

Microwave irradiation of the similar reaction mixtures, for 0.5 and 1 min, has brought the gold and silver nanoparticles and the corresponding UV-vis spectra are shown in Fig. 2(d and e). The SPR peaks were found to be 541 nm and 415 nm, respectively, for gold and silver nanoparticles. It was well known that microwave synthetic route generates narrow-sized nanoparticles in a rapid manner compared to the other pathways (Joseph and Mathew 2014b). Formation of gold nanoparticles takes place more rapidly than silver nanoparticles due to the difference in reduction potential (Joseph and Mathew 2015b). The synthesized gold and silver nanoparticles were abbreviated as AuNP- M. glabrata and AgNP- M. glabrata. The photographs of M. glabrata leaf extract, AgNP- M. glabrata, and AuNP- M. glabrata are shown in Fig. 3.

Fig. 3
figure 3

Camera images of a aqueous M. glabrata leaf extract, b AgNP- M. glabrata, and c AuNP- M. glabrata

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FT-IR spectra

The high potency of M. glabrata to perform the reduction process and to prevent the aggregation of the nanoparticles was due to the chemical components present in the aqueous leaf extract. The functional groups of the biomolecules were identified by Fourier transform infrared spectroscopy. FT-IR spectra of (a) M. glabrata, (b) AuNP- M. glabrata, and (c) AgNP- M. glabrata are exposed in Fig. 4. A broad peak at 3210 cm−1 of the leaf extract is due to N–H stretching of amines that is hydrogen bonded, and the peaks around 1732 and 1605 cm−1 are due to C = O stretching and C = C stretching of aromatic group, respectively. The sharp peaks centered at 1372 and 1015 cm−1 arise from –O–C– and –C–O–C– stretching modes. The aromatic –C–H bending vibrations showed peak at 824 cm−1. The FT-IR spectra of gold and silver nanoparticles clearly coincide with that of M. glabrata leaf extract, proved its assistance in the formation and stabilization of nanoparticles.

Fig. 4
figure 4

FT-IR spectra of a M. glabrata, b AuNP- M. glabrata, and c AgNP- M. glabrata

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X-ray powder diffractogram

The X-ray diffraction pattern of AuNP- M. glabrata exhibited fair peaks at 2θ positions 37.96°, 44.07°, 64.56°, and 77.59°. The X-ray powder diffractogram of the gold nanoparticles justified the crystalline nature with face centered cubic lattice. The diffraction of X-rays on AgNP- M. glabrata showed peaks at 37.83°, 43.99°, 64.26°, and 77.19o originated from (111), (200), (220), and (311) planes of face centered cubic lattice of nanosilver (Fig. 5). The most intense peak from AuNP- M. glabrata and AgNP- M. glabrata crystals was from (111) plane, revealed the preferred orientation of the crystals towards (111) plane (Kora et al. 2012; Sujitha and Kannan 2013).

Fig. 5
figure 5

X-ray diffraction spectra of a AuNP- M. glabrata and b AgNP- M. glabrata

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TEM and EDX analyses

The electron micrographs investigated the surface features of the phytosynthesized nanoparticles. The morphological peculiarities of the microwave-generated gold nanoparticles like shape of the particles and allocations of the particle size were clearly visualized from the TEM images (Fig. 6). Spherical and triangular geometry of the nanogold is obviously depicted in the HR-TEM images (d). The gold nanoparticles have mean diameter of 10.59 nm. The elemental dispersive spectrum (g) confirmed the presence of gold.

Fig. 6
figure 6

a–d TEM images of AuNP- M. glabrata in diverse magnifications. e SAED patter. f The particle histogram. g EDX spectrum of AuNP- M. glabrata

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The average size of the synthesized AgNP- M. glabrata is found to be 51.32 nm and has a notable spherical geometry (Fig. 7). The polydispersed and polycrystalline nature of the silver nanoparticles was also clear from the images (a–d). The selected area diffraction pattern (e) showed sharp diffraction spots in a circular arrangement arising from lattice reflections from different planes of nanosilver. The EDX spectrum demonstrated the elemental presence silver (g).

Fig. 7
figure 7

a–d TEM images of AgNP- M. glabrata in different magnifications. e The SAED pattern. f The particle histogram. g EDX spectrum of AgNP- M. glabrata

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Atomic force microscopy

The topographic information of materials was best studied by its response towards the atomic force microscope. This most versatile technology gave accurate two- and three-dimensional surface maps of the nanoparticles (Logeswari et al. 2015). Fig. S1 corresponds to AFM images of microwave-synthesized AgNP- M. glabrata and AuNP- M. glabrata.

The micrographic images revealed a nonuniform morphology arising from both the agglomerated and well-separated gold and silver nanoparticles (Jayaseelan et al. 2013). The topographic information of nanoparticles drawn by TEM is supplemented and supported by their atomic micrographs (Engelbrekt et al. 2009). The root mean square roughness obtained for AuNP- M. glabrata and AgNP- M. glabrata is 13.55 and 3.81 nm, respectively (Swamy et al. 2015).

Antioxidant capacity-DPPH method

The essential element oxygen creates highly reactive free radicals and resulted in various health disorders (Tong et al. 2015). Antioxidants were of natural or synthetic origin which counteracts the oxidation progress (Nimse and Pal 2015). Fresh fruits and vegetables were the best antioxidants, since they were rich in vitamin C, vitamin E, and β-carotene (Shashirekha et al. 2013; Moo-Huchin et al. 2015). Supplementation of antioxidants through diet or medication was a major strategy in medical science (Carlsen et al. 2010).

The potent antioxidant power of gold and silver nanoparticles (Fig. S2) showed a direct concentration dependency in the tested range (Kumar et al. 2016). Among silver and gold nanoparticles and plant extract, silver nanoparticles showed greater free radical scavenging potential than gold nanoparticles and plant extract. Silver and gold nanoparticles have greater capacity to undergo single electron transfer and to perform reduction of DPPH than the plant extract (Shahidi and Ambigaipalan 2015). IC50 values shown by AgNP- M. glabrata, AuNP- M. glabrata, and M. glabrata were 44.1 ± 0.82, 57.92 ± 1.33, and 169.13 ± 4.25 μg/mL, respectively, in comparison with 14.9 ± 0.33 μg/mL of standard, ascorbic acid. Biological activity expressed in terms of IC50 which reflected the concentration of the antioxidant needed to halve the initial concentration of DPPH. The antioxidant efficacy generally correlated to the oxidisable phenolic and flavonoid content of the plant extracts (Kalpanadevi and Mohan 2012). In vitro antioxidant potential of the prepared noble nanoparticles may offer remedy for oxidative stress.

Antimicrobial investigations

Substances which act against various functions of microorganisms were called antimicrobial agents. Plant-originated metal nanoparticles were proved to be potential antimicrobials (Balouiri et al. 2016) and are used to cure various infectious diseases (Rai et al. 2015). In vitro antimicrobial susceptibility of the microwave-synthesized silver and gold nanoparticles were tested by the agar well diffusion method. Fig. S3 exhibited the snapshots of different bacterial stains and fungal stains after an incubation period of 24 h and 7 days, respectively. The ability of nanoparticles to inhibit growth and action of microorganisms was expressed as the zone of inhibition (mm) formed around the well (Fig. S4).

The zone of inhibition certified that AgNP- M. glabrata and AuNP- M. glabrata were toxic to both gram positive and gram negative bacteria. The nanoparticles get diffused out of the well and interact with the microorganisms. The effected inhibition of microbial growth by the nanoparticles resulted in more or less circular zone. Many mechanisms were known for the antimicrobial action of metal nanoparticles. Nanoparticles may destroy cell membranes or prevent the functions of DNA mainly metabolism and replication (Prabhu and Poulose 2012; Hong et al. 2016). The metal nanoparticles showed different degree of inhibition towards the tested gram-positive and the gram-negative bacterial stains due to their difference in cell wall composition (Gurunathan 2014). The silver nanoparticles proved a better effectiveness than gold nanoparticles and are explained as the immobilization caused by agglomeration (Ramamurthy et al. 2013). The microwave-generated and M. glabrata-reduced gold and silver nanoparticles showed a moderate antifungal activity against the fungi A. niger and P. chrysogenum (Ahmed et al. 2016b). The bactericidal activity shown by nanoparticles mainly silver nanoparticles has an important use in water treatment (Hong et al. 2016).

Catalytic studies

Reduction of 4-nitrophenol to 4-aminophenol

An organic pollutant that generated by human involvement in the environment is 4-nitrophenol. It had an absorption maximum of 317 nm which was shifted to 400 nm by the introduction of NaBH4 in aqueous medium resulting in the formation of 4-nitrophenolate ion (Rajan et al. 2015). Based on the reduction potential perspective, the conversion of 4-nitrophenol to 4-aminophenol using the reducing agent NaBH4 was a thermodynamically allowed reaction (Otari et al. 2014). But it found that the required reduction was kinetically and practically forbidden even for a few couple of days by NaBH4 alone (Gangula and Podila 2011).

The reduction of 4-nitrophenol was achieved by reducing agent NaBH4 and utilizing the gold/silver nanocatalysts (Vilas et al. 2016). Adding 0.5 mL of AuNP- M. glabrata or AgNP- M. glabrata to the reaction cuvette containing 4-nitrophenol and NaBH4, the reaction proceeds in a faster rate and reduction phenomena wonderfully ended up at 7 and 9 min (Fig. 8). The damping of absorbance at 400 nm indicated the progress of the reaction. The electron transfer from the BH4 − to the electron accepter phenolic moiety was assisted and substantiated by the nanocatalysts. The gold and silver nanoparticles had ample surface area for the adsorption of the 4-nitrophenol and NaBH4 and performed the electron relay process. The previously reported works forced to propose Langmuir-Hinshelwood model for this heterogeneous catalysis (Baruah et al. 2013). The reactions followed pseudo-first order kinetics with respect to 4-nitrophenol concentration, since the amount of NaBH4 was surplus and thus practically remained constant all over the reaction. The pseudo-first-order rate constants with respect to 4-nitrophenol were also calculated from the kinetic plots following the rate equation \( k=\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$ t$}\right. \ln \left[{A}_0\right]/\left[ A\right] \), where k is pseudo-first-order rate constant, [A 0] is the initial concentration of 4-nitrophenol, and [A] is concentration at time t (Yu et al. 2016; Manjari et al. 2017).

Fig. 8
figure 8

Time dependent UV-vis spectra for the reduction of 4-nitrophenol by NaBH4 catalyzed by a AuNP- M. glabrata (0.02 mg/mL) and b AgNP- M. glabrata (0.02 mg/mL)

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Dye degradation studies

Degradation of rhodamine B

Metal-chelating reagent rhodamine B is a dye used in paper, drug, and cosmetic industries. This dye pollutant causes an adverse effect on human eyes and skin. The taking away of this contaminating pigment from waste water is inadequate for healthy aquatic life (Wilhelm and Stephan 2007). In the absence of catalyst, the removal of rhodamine B takes place too slowly by NaBH4 (Joseph and Mathew 2015c). AuNP- M. glabrata (5 μg/mL) and AgNP -M. glabrata (0.02 mg/mL) activated the complete removal of rhodamine B by 5 and 9 min. The deterioration reaction was monitored periodically by UV-vis absorption spectra (Fig. 9). The kinetic examination was conducted by listing the absorbance at 553 nm at different time gaps. The graph generated by plotting ln[A] verses time followed a linear behavior, and the reaction was found to pursue a pseudo-first-order with respect to rhodamine B concentration (Jeyapragasam and Kannan 2016). Catalytic rate constants for gold and silver nanoparticles were 0.7250 and 0.4464 min−1, respectively.

Fig. 9
figure 9

Degradation of rhodamine B a in presence of AuNP- M. glabrata (5 μg/mL) and b in presence of AgNP- M. glabrata (0.02 mg/mL)

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The dye removal was happened through the shuttling of electrons between BH4 − and the dye moiety at the surface of gold and silver nanocatalysts and rhodamine B get reduced to leuco rhodamine B (Paul et al. 2016). The surface of nanoparticles acted as heterogeneous catalyst following a Langmuir-Hinshelwood mechanism (Bastús et al. 2014; Bhargava et al. 2016).

Degradation of methyl orange

Methyl orange is a toxic textile dye and possesses an appalling impact on aquatic kingdom. This azo dye has an absorption maximum of 464 nm in the UV-vis absorption spectra, due to N = N functional group. The reducing agent NaBH4 alone was impotent in bringing the degradation of methyl orange (Vidhu and Philip 2014). The glabrata-reduced gold and silver nanocolloids executed the abolition of methyl orange within a period of 4 and 7 min, respectively, in a heterogeneous catalytic pathway (Fig. 10). Either the disappearance of peak at 464 nm or the appearance of peak at 256 nm due to the formation of product, hydrazine, indicates the removal of the methyl orange in the aqueous phase. The degradation of methyl orange passes through the transfer of an electron from donor to acceptor via the surface of the nanocatalysts (Varadavenkatesan et al. 2016). The catalytic degradation of methyl orange followed pseudo-first-order kinetics and can be beneficially exploited in water pollution government mission.

Fig. 10
figure 10

Methyl orange degradation by NaBH4 catalyzed by a AuNP- M. glabrata (0.01 mg/mL) and b AgNP- M. glabrata (0.01 mg/mL)

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Conclusions

Gold and silver nanoparticles were synthesized by environmentally safe and facile route, namely, the microwave-assisted synthesis. The leaf extract of the wild medicinal plant M. glabrata acts as both reducing agent and antiagglomeration agent. The components of plant responsible for reduction and stabilization of gold and silver nanoparticles were identified by FT-IR spectroscopic analysis of the crude leaf extract. Colloidal metal nanoparticles were clearly characterized by UV-vis, powder X-ray diffraction, TEM, and AFM techniques. Gold and silver nanoparticles have excellent antioxidant characteristics, making them useful as therapeutic agents in future. Both nanoparticles were verified as good antimicrobial agents against different human disease-causing pathogens. The prepared gold and silver nanometals functioned as heterogeneous catalysts in different dye degradation perspectives. They offer new avenues for the perennial issues pertinent to waste water purification by degrading toxic contaminants.