1 Introduction

Metallic nanoparticles have occupied an important place in the field of nanoscience due to their intriguing chemical, optical, electronic properties and etc. Metallic nanoparticles are synthesized by using reducing, protective and stabilizing agents. Such kind of agents are highly toxic, flammable, may adsorb on the surface of nanoparticles and are also unfavorable in their applications [1, 2]. Due to these confines, the researchers more concentrate on green approach for synthesis of nanoparticles. Several biological systems are existing for synthesis of metallic nanoparticles like as microbes, fungi, and plant materials. Recently, nanoparticles (silver, gold, copper, platinum, palladium etc.,) have been synthesized by using plants. This is due to large scale process, rapid synthesis and also produce more stable nanoparticles compare with other microorganisms. The development of green chemistry has three important factors, they are suitable solvent, ecofriendly reducing agent and non-toxic chemicals are used in synthesis of nanoparticles [3, 4]. Among various metallic nanoparticles, researchers have more interest on gold nanoparticles (AuNPs) which is due to its unique properties, tunable SPR and applications [5].

Colloidal gold is a sol that exerted curative properties for several diseases [6]. Approximately 5.02 billion tons of gold produced by Philippines and ranked third in the world [7]. Gold nanoparticles have more commercial applications in various biomedical fields. Synthesized AuNPs have lot of applications like catalysis, cancer therapeutics, optical molecular sensing, electronics, optoelectronics, photonics diagnostic, drug delivery, biomedical sensing and imaging [8, 9]. Upon interacting with light, excited free electrons of AuNPs directly state to collective oscillation called as a strange Surface Plasmon Resonance (SPR) phenomenon [10, 11]. It can be tuned to different size or shape of the AuNPs, which cause the extensive investigation [12]. Various plant materials are used for synthesis of AuNPs such as Magnolia kobus & Diopyros kaki [13], Acacia nilotica [14], Dillenia indica [15], Edible mushroom [16] and Cinnamomum zeylanicum [17].

In this study, we have examined the synthesis of AuNPs using medicinal plant Justicia adhatoda leaf extract, belongs to Acantheceae family. Phytochemical components of the leaf contain phenols, tannins, alkaloids, anthraquinones, saponins, flavonoids and reducing sugars, vasicine, vasicinone alkaloid and quinazoline [18,19,20]. The leaves of J. adhatoda contains considerable amount of alkaloids, flavonoids, phenols, proteins and carbohydrates (reducing sugar). These bioactive secondary metabolites are associated with their medicinal applications.

Pollution caused by heavy metals and dyes from industrial effluents are the chief origin of environmental pollution. In recent, more than 10,000 of different commercial dyes and pigments exist and about 7x105 tones are produced annually in worldwide [21,22,23]. It is expected that 10–15% of dye is lost during the dyeing process from the effluent. Dyes are broadly used in various industries like textile, color photography, pharmaceutical, food, cosmetics, paper, plastics and other industries [24]. They are using dye for coloring their products and discharge the effluents into water bodies. Such contaminated water is harmful to living organisms, in particularly Methylene blue and Congo red dye, it causes serious problem to the eco-system [25, 26]. Methylene blue is a cationic dye and it is a heterocyclic aromatic compound (C16H18N3SCl). Methylene blue can cause permanent injury to eyes, nausea, vomiting, diarrhea, skin irritation and severe serotonin toxicity [27, 28]. Congo red is a heterocyclic aromatic compound (C32H22N6Na2O6S2). It is a brownish-red crystal dye. It is chemically called as sodium salt of benzidine diazo-bis-1-naphthylamine-4-sulfonic acid. Congo red is a water soluble, highly stable and resistant to degrade. Congo red is a carcinogenic and cause irritation in skin, eye, gastrointestine and also cause blood clotting, allergic reactions and respiratory problems [29, 30]. Numerous technologies and methods have been progressed for the removal of dyes from the industrial effluent. Various conventional methods existing such as coagulation, flocculation [31], ozonation [32], electrochemical method [33], fungal decolonization [34] adsorption on activated carbon [35,36,37,38], ultrafiltration and reverse osmosis are already in practice. These methods are costly, unsuccessful one and it cannot be effectively treated in broad range. Therefore, removal of these dyes are an important one, without using any other toxic chemicals. In recent decades, nano catalysts are attractive one to remove dyes from the industrial effluent. The nano catalyst is biocompatible, ecofriendly and facile route which is used to remove the dyes. Based on the above, the present study was focused on green synthesized AuNPs to evaluate their responsibility to degrade the dyes. We report on synthesize of stable AuNPs using leaf extract of J. adhatoda and project their size dependent catalytic activity in dye degradation (Congo red and Methylene blue) process.

2 Methodology

2.1 Chemicals

HAuCl4∙3H2O (99.9%), Congo red, Methylene blue, and Sodium borohydride were obtained from Sigma-Aldrich.

2.2 Preparation of aqueous leaf extract

The leaf of Justicia adhatoda was collected from Cumbum valley, Tamil Nadu, India and it is identified by experts. The leaves were washed with DD water three times to remove the dust particles and dried (shade, dry) for 10–14 days and ground into coarse powder. The coarse powder was refluxed at 80 °C for 20 min in a 250 mL Erlenmeyer flask containing 100 mL of DD water. After cooling, the extract was filtered through Whatman no. 1 filter paper and was kept in refrigerator for further use.

2.3 Synthesis of AuNPs

For synthesis of AuNPs, 1 mL aqueous extract was added to 7 mL of HAuCl4∙3H2O (1 mM) with constant stirring for 10 min. The color of the reaction mixture changes from pale yellow to ruby red indicates the formation of gold nanoparticles due to the reduction of Au3+ to Au0, no precipitate was found. The synthesized gold nanoparticles were confirmed for the further characterizations.

2.3.1 Effect of metal ion concentration

One mL leaf extract was added to 7 mL of three different concentrations (0.5, 1.0 and 1.5 mM) of HAuCl4, and synthesized AuNPs are labeled as a1, a2 and a3 respectively. The reduction and formation of AuNPs were monitored by using UV–Vis spectrophotometer.

2.3.2 Effect of different quantities of leaf extract on formation of AuNPs

Gold nanoparticles (AuNPs) were prepared by adding 1, 2 and 3 mL leaf extract of J. adhatoda with 7 mL of HAuCl4∙3H2O (1 mM) independently, immediate pink-ruby red color was observed. The formation of AuNPs was monitored by UV–Vis spectrophotometer.

2.4 Characterization of AuNPs

The formation of the gold nanoparticles was recorded in the range of 700–400 nm is using UV–Visible spectrophotometer (Perkin Elmer Lambda-45). The size and shape of the nanoparticles were analyzed from SEM and TEM (Hitachi and TEM-JEOL model 1200EX) analyzer along with EDX operating at 200 kV. XRD analysis of the AuNPs was performed on Powder X-Ray Diffractometer (EnrafNonius CAD-4), operating at a voltage of 40 kV with Cu Kα (λ = 1.54 Å) radiation.

2.5 Catalytic studies

2.5.1 Catalytic activity of Congo red degradation using AuNPs

The catalytic activity of AuNPs was evaluated against Congo red dye as a substrate in the presence of NaBH4. For catalytic degradation, 1 mL of Congo red (1 mM) diluted with 10 mL of DD water and then 1 mL of freshly prepared NaBH4 (0.1 and 0.2 M) was added respectively [39]. Size-dependent work was done by the addition of 500 μL colloidal AuNPs (a1, a2 and a3) with the mixture in UV light. The color of the samples gradually changed from deep red to colorless.

The catalytic activity of synthesized AuNPs was assessed against Methylene blue dye as a substrate in the presence of NaBH4. In a typical assay, 1 mL of Methylene blue (1 mM) diluted with 10 mL DD water and then added to 1 mL of freshly prepared NaBH4 [39, 40]. Two blank samples were prepared by various concentrations of NaBH4 (0.1 and 0.2 M). The above procedures were repeated for the addition of a1, a2 and a3 of samples (500 μL) with Methylene blue in the presence of NaBH4 separately. Finally, color changed into leuko Methylene blue color.

3 Results and discussion

Justicia adhatoda act as a biological reductor, to reduce gold chloride ions. The leaf of J. adhatoda constitutes water (15.3%), proteins (6.5%) and reducing sugar (16.4%) (d-glucoside, d-galactose, β-glucoside-galactose, β-sitosterol-d-glucoside) [19]. During aqueous extract preparation, the reducing sugar broke down into smaller monosaccharide (glucose) unit exposing extra –CHO group [41]. The reducing activity of aldehyde group (–CHO) has been exploited by Tollens test, in which silver ions are reduced [42]. Similarly, the aldehyde group has reduced the gold ions into AuNPs. Maximum number of aldehyde group behave as a reducing agent for gold chloride ions [43].

3.1 Justicia adhatoda extract as a reducing agent for Au NPs

The possible redox reaction between gold ions and d-glucose has shown in schematic diagram (Fig. 1). It is continuously undergo nucleophillic substitution reaction, where O–H radicals present in the sixth carbon of glucose unit. First of all, Au3+ ions gets attached with the oxygen of the aldehyde group present in glucose of aqueous extract, this is because of its free electrons. Due to instability of the carbon atom in the presence of O–H, those electrons gets transferred to Au3+ and gets reduced into Au0. At last, the oxygen of C=O group recovers the double bond and it is converted into gluconic acid [43].

Fig. 1
figure 1

Schematic diagram represents the reduction of Au3+ ion into Au0

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3.2 Justicia adhatoda extract as a stabilizing agent for AuNPs

Silver (Ag) and gold (Au) nanoparticles are stabilized by hydroxyl (-OH) groups present in the starch molecules [44]. The terminal d-glucose unit gets oxidized into gluconic acid in the presence of Au ions. The negative charge of the gluconic acid interacts with the surface positive charge of AuNPs by electrostatic interaction; this is the reason for bond formation between them. Thus, the capping agent is responsible for AuNPs stabilization.

3.3 UV–Visible spectroscopy studies

3.3.1 Effect of contact time

Formation of gold nanoparticle (AuNPs) was monitored with the help of UV–Vis spectroscopy. A gradual color change could be observed from pale yellow to ruby red color after the addition of gold chloride solution which indicates the formation of AuNPs. The Surface Plasmon resonance (SPR) band is due to the oscillation of free conduction electrons induced by an interacting electromagnetic field on the surface of gold nanoparticles. The SPR band of AuNPs were related to their size, shape and aggregation conditions [45, 46]. The formation of AuNPs was confirmed by UV–Vis spectra (Fig. 2). The SPR peak is observed 542 nm which is gradually increased with the function of time (10–60 min) and this is clearly exhibiting that the redox reaction has occurred between the aqueous extract and metal ions. Both of them were responsible for the formation of the stable AuNPs [47]. Early reports show that the temperature and pH parameters are vital factors which affecting the size and shape of the AuNPs [48,49,50,51]. The rate of formation of AuNPs increases with increase in the SPR band.

Fig. 2
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UV–Vis spectra of AuNPs (Contact time for J. adhatoda leaf extract with HAuCl4)

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3.3.2 Effect of quantity of leaf extract

With an increasing ratio of leaf extract, subsequent color changes were noted from pink into reddish pink. In general, AuNPs is synthesized by direct deal with the quantity of aqueous extract, higher dosage of reducing agent will produce maximum yield of nanoparticle. The SPR peak of blue shift (542, 541 and 540 nm) for 1, 2 and 3 leaf extract respectively (Fig. 3). The formation of a blue shift indicates decreasing of the particle size [52,53,54,55].

Fig. 3
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UV–Vis spectra of AuNPs at different volume of J. adhatoda plant extract (PE)

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3.3.3 Effect of metal concentrations

To examine the effect of metal salt concentration, the volume of leaf extract and HAuCl4 were kept constant (1:7), but the concentration of metal salt varied (0.5, 1.0 and 1.5 mM). The rapid formation of dark pink color was noticed at high concentration of metal salt [56]. Figure 4 shows that the SPR peak intensity increases with increase in metal salt concentrations. The intensity of SPR peak and sharpness is increased at higher metal concentration which indicates increase of the size of nanoparticle [56, 58]. By increasing the metal concentration, increases the size of AuNPs (a1–a3) was also found in SEM and TEM images (Figs. 4 and 5). Dubey et al. [57] have also reported that the rate of formation of AuNPs was found to be faster at highest metal salt concentration. Dwivedi et al. [59] reported that the less number and smaller size of nanoparticles was observed in lowest metal salt concentration.

Fig. 4
figure 4

UV–Vis spectra of AuNPs at various concentration of HAuCl4 (0.5, 1.0 and 1.5 mM)

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Fig. 5
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SEM images of AuNPs from J. adhatoda leaf extract a a1, b  a2 and c a3

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3.4 Morphological studies

3.4.1 SEM analysis

The SEM images explain the morphological study of AuNPs (a1, a2 and a3) synthesized from J. adhatoda. The obtained AuNPs (a1, a2 and a3) were found to be spherical shape with and without aggregation. Shape of sample a1 is uniform, but a2 and a3 samples are poly-dispersed in nature with slight aggregation are shown in Fig. 5a–c.

3.4.2 TEM analysis

The size and shape of the nanoparticles were further confirmed by TEM analysis. The TEM images in Fig. 6a–f prove the synthesized AuNPs are spherical in shape and their average sizes are 27.1, 33.3 and 46.8 nm for sample a1, a2 and a3 respectively. Some AuNPs are agglomerated in some places, thereby indicating possible deposition of nanoparticles after few days. Philip et al. [16, 60, 61] also discussed that the synthesized spherical shape of gold nanoparticles is due to the sharp SPR peak. TEM images also confirmed that the spherical shape and its sizes was increased with increase in the concentration of HAuCl4. Beside, analysis of the AuNPs by Energy Dispersive Spectroscopy (EDAX) confirmed the occurrence of the signal feature of elemental gold (Fig. 6g). Maximum peaks are noticed for crystalline Au peaks and few peaks are observed for Cu comes from the copper grid. Thus, there are no foreign substances present in the sample. Figure 6h shows Selected Area Electron Diffraction pattern (SAED) of the AuNPs and it is measured from one of the nanoparticles. The SAED images clearly exhibited the diffraction rings with interplanar spacing which denotes AuNPs are crystalline nature. SAED spots are related to the different crystallographic planes of FCC structure of elemental gold (Fig. 6h) The SAED spots are related to the different crystallographic plane families [111], [200] and [220] of pure face-centred cubic gold structure [62].

Fig. 6
figure 6

TEM images of AuNPs synthesized from J. adhatoda leaf extract a, b a1, c, d  a2, e, f  a3, g EDX and h SAED

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3.5 Xrd

The typical XRD pattern of fabricated AuNPs displayed, Bragg’s reflection planes at (111), (200), (220) and (311) corresponds to 2θ values of the peaks at 38.3°, 44.3°, 64.5° and 77.6° respectively (Fig. 7) for a1–a3. All peaks of AuNPs (a1-a3), which are well matched with the standard data file (JCPDS file No. 99-101-3542). From this result, a number of prominent Bragg reflection peaks are based on Face Centered Cubic (FCC) crystalline nature of AuNPs. The reflection peaks are broader which indicates the smaller dimension of particles [63]. No other peaks are found in XRD pattern, which is confirmed the purity of AuNPs. Hence, the obtained pure crystalline nature of AuNPs was enclosed with biomolecules.

Fig. 7
figure 7

XRD pattern of AuNPs for a  a1, b a2 and c a3 

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The peak (38.3°) corresponding plane is more intense than other planes. Thus, the plane is signifying one; this is due to the predominant progress of AuNPs along the direction of plane (111). The average nanocrystallite size is calculated by using Debye–Scherer equation and it is found to be 20.2, 40.1 and 40.3 nm for the corresponding samples a1, a2 and a3 respectively, which slightly varied from the obtained particle sizes of TEM analysis of AuNPs (Fig. 6a–f). It shows slight changes of the spherical shape of particles that is needed the Debye–Scherer formula.

The average nanocrystalline material size has been calculated by Debye–Scherrer formula,

$${\text{D}} = {\text{k}}\uplambda/\upbeta\cos\uptheta,$$

where D is particle diameter size, k = 1(constant), λ wavelength of X-ray source (0.1541 nm), β full width at half maximum (FWHM) and θ diffraction angle corresponding to the lattice plane (111).

3.6 FTIR

In order to inspect the AuNPs, they are capped with the biomolecules. FTIR analysis was carried out in order to detect the potential biomolecules of J. adhatoda plant extract which are responsible for the reduction, capping and efficient stabilization of AuNPs. FTIR spectrum of aqueous leaf extract shows the stretching vibration peaks at 3404, 2938, 2846, 1636, 1408, 1050, 1257, 1082, 827 and 886 cm−1 (Fig. 8a). The synthesized AuNPs exhibit the peaks at 3402, 2850, 1636, 1404, 1081, 661, 600 and 522 cm−1 (Fig. 8b). The broad IR band at 3402 cm−1 is due to stretching vibrations of O–H group [64]. A peak at 1634 cm−1 is attributed to C-O stretching vibrations in the amide I shifted to 1636 cm−1 the protein molecules are possible to bind with AuNPs via carboxylate groups (–COO−) [25, 64,65,66]. The peak at 1408 cm−1 is due to COO− symmetric stretching of chloroplasts shifted to 1404 cm−1 [67]. A minor peak at about 661 cm−1 is due to C–Cl or C–Br stretch of alkyl halides [68]. In FTIR, such kind of shifting vibrational spectra has been ascribed that the O–H (3402 cm−1) and (NH) C = O groups (1636 cm−1) can bind with AuNPs [25, 64]. Based on the above mentioned facts, it is confirmed that the reducing sugar (d-glucoside) and protein molecules are responsible for reduction of Au3+ and capping agent of AuNPs.

Fig. 8
figure 8

FTIR spectra of a J. adhatoda plant and b AuNPs

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3.7 Catalytic activity studies

3.7.1 Catalytic activity of synthesized gold nanoparticles in Congo red

The present study demonstrates the size-dependent, catalytic activity of AuNPs against Congo red dye in the presence of the NaBH4. The absorption band of Congo red is about 486–498 nm [39]. The absorption peak of Congo red was observed at 496 nm and the peak intensity decreased during the reduction. This is due to the degradation of Congo red by AuNPs in the presence of NaBH4. Schematic diagram exhibits the possible structure of Congo red dye degradation (Fig. 9). Figure 10a, b illustrates the catalytic degradation of Congo red by using two different concentrations of NaBH4 (0.1 and 0.2 M) in the absence of AuNPs independently. The degradation was slow in both cases when increasing the concentration of NaBH4 (0.1 and 0.2 M) meanwhile after addition of AuNPs, the degradation was faster. Narayanan et al. [69,70,71] discussed that NaBH4 is a weak reducing agent which was used in a lot of industrial processes as well as in wastewater treatment. The reduction process found to speed up by adding samples a1–a3. Figure 11a–c shows the samples a1, a2 and a3 degraded the dye within 12, 20 and 24 min respectively. The catalytic reduction of a1 was faster than other samples a2 and a3. From this observation, the degradation time decreased while using smaller size of the nanoparticles (a1). The sample a1 increases the catalytic property, their larger surface area will enhance the catalytic activity [72, 73]. A good linear correlation was observed between ln(A/A0) versus time (Fig. 11d). The overall reduction reaction was hypothetical pseudo first order reaction. The rate of reaction also increased with decrease in nanoparticles sizes [5]. The obtained rate constant values from the plot ln A/A0 versus time for all five samples are shown in Table 1. The electron transfer from donor (BH4−) to acceptor (Congo red dye) through the surface of AuNPs [39]. The plant extract having different functional groups are enhancing the catalytic activity of AuNPs [74]. This study exhibits an overwhelming performance as a catalyst, depending on its size.

Fig. 9
figure 9

Schematic diagram for Congo red degraded into protonated Congo red in the presence of synthesized AuNPs

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Fig. 10
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UV–Vis spectra for degradation of Congo red dye without using AuNPs in the presence of a 0.1 M and b 0.2 M NaBH4

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Fig. 11
figure 11

UV–Vis spectral analysis of Congo red using NaBH4 (0.1 M) with addition of 500 μL of AuNPs a a1, b a2, c a3 and d linear plot of kinetic data for a1–a3 samples

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Table 1 Rate constants of Congo red dye reduction
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3.7.2 Catalytic activity of synthesized AuNPs in Methylene blue

The catalytic activity of AuNPs was demonstrated by the degradation of Methylene blue along with NaBH4. The absorbance peak of Methylene blue was observed at 664 nm and with a shoulder peak at 614 nm is due to n − π* transition. The decreasing tendency of maximum absorption denotes the reduction of Methylene blue [72]. A possible mechanism for Methylene blue dye degradation has shown in schematic diagram in Fig. 12. Figure 13a and b demonstrate the UV–Vis absorption spectra of the reduction of Methylene blue by using two various concentrations of reducing agent (0.1 and 0.2 M) without adding AuNPs. The degradation was slow in 30 min period of time even increasing the concentrations of NaBH4 [39, 40]. When adding AuNPs (a1–a3) to the reaction mixture the reduction was fast respectively. Hence, the reduction reaction was found to be faster in sample a1 rather than a2 and a3 samples. The degradation of dye when using a1, a2 and a3 samples within 5, 8 and 15 min respectively (Fig. 14a–c). Based on the above observation, that the degradation time was decreasing with decrease in the size of AuNPs (a1 − 27.1 nm) [39, 40]. The smaller size of AuNPs will increase the catalytic activity, due to their larger surface area [69]. The rate of reaction was also increased with decrease in nanoparticles sizes [5]. The rate of catalytic degradation increased with increase in the dosage of catalyst [75]. A good linear correlation plot was observed between ln A/A0 versus time graph (Fig. 14d). From the plot of ln A/A0 versus time, the rate constant values for all the five samples are shown in Table 2. AuNPs are assisting the electron transmission from donor (BH4−) to acceptor (Methylene blue) in the redox reaction which is termed as an electron relay effect [71]. The percentage of dye degradation can be calculated using the following equation.

$$\% {\text{ of dye degradation}} = \frac{{{\text{A}}_{0} - {\text{A}}}}{{{\text{A}}_{0} }} \times 100$$

A0 initial absorbance, A absorbance at time.

Fig. 12
figure 12

Schematic diagram for Methylene blue degraded into Leuko Methylene blue in the presence of synthesized AuNPs

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Fig. 13
figure 13

UV–Vis analysis for the degradation of Methylene blue without addition of AuNPs using different concentration of NaBH4 a 0.1 M and b 0.2 M

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Fig. 14
figure 14

UV–Vis spectra for Methylene blue using NaBH4 with addition of 500 μL of AuNPs a a1, b a2, c a3 sample and d linear plot of kinetic data for a1–a3 samples

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Table 2 Rate constants of Methylene blue dye reduction
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Therefore, the catalytic reduction reactions (Congo red and Methylene blue), was carried out as-synthesized AuNPs acted as a potential catalyst. Table 3 represents a comparative literature survey of dye degradation, against Congo red and Methylene blue by different nanoparticles.

Table 3 Review of various nanoparticles utilized for degradation of Methylene blue (MB) and Congo red dye (CR)
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4 Conclusion

We disclosed an ecofriendly approach for synthesis of stable AuNPs using J. adhatoda leaf extract. The concentration of metal was essential in synthesis of various sizes of AuNPs. The spherically crystalline nature of AuNPs was encircled with the d-gulcoside and protein molecules. When there is increase in concentration of reducing agent, the degradation did not show better results, meanwhile after the addition of AuNPs, the degradation was good. Then, the obtained AuNPs was efficient in catalytic dye degradation. This study reveals that the smaller size of AuNPs enhances the catalytic activity due to their large surface area. From the above result, we concluded the most excellent degradation was found in Methylene blue than Congo red. Finally, green synthesized AuNPs are effective nano-catalyst and could be made potentially exciting for large scale synthesis of AuNPs.