1 Introduction

Investigation and synthesis of nanoparticles (NPs) via eco-friendly materials in the form of whole cells, metabolites, or extracts from plants are gaining more popularity in nanoscience [1, 2]. Compared to other methodologies, green synthesis has some distinct advantages and therefore being used to synthesize various metal NPs. This is mainly due to the high usage of chemicals or different energy forms in conventional methods, which leads to a negative impact on the environment [3,4,5,6]. Published scientific literature suggests that fabricated NPs can be used in numerous biological and physicochemical applications, which is due to their specific size and morphology [6,7,8,9,10,11,12].

Among all the metallic NPs, AgNPs have a prime position due to their numerous unique properties, which alluded to many applications. The optical, catalytic, electrical, biological, and electrochemical properties can significantly affect in diverse areas such as food, medicine, environmental, textile, and catalytic applications [8, 13, 14].

Numerous types of plant wastes (Cocos nucifera coir, corn cob, fruit seeds and peels, wheat bran, rice bran, palm oil mill effluent, kola seed shell, kola pod, cocoa pod husk, and avocado fruit peel) have been used to synthesize various metal NPs [1, 15,16,17]. The usage of agro-wastes is an eco-friendly and sustainable route for the effective usage of plant waste [1, 18]. As per the published literature, agro wastes of G. mangostana L.(Mangosteen) and N. lappaceum L (Rambutan) are rich in secondary metabolites; therefore, these plant extracts can be used as reducing, stabilizing, and capping agent to synthesize AgNPs [19].

Mangosteen (G. mangostana L.) is a fruit-bearing tree, found in South and Southeast Asian countries, including Sri Lanka. The tree is abundant in Kaluthara District, which is situated in the Western Province of Sri Lanka. The tree grows up to about 6–25 m in height, contains leathery round leaves, and has slightly acidic sweet-flavored fruits that are round in shape. These fruits turned to a deep reddish-purple when entirely ripped [20]. Various plant parts (fruit, leaves, bark, stem, and roots) of G. mangostana are used in ethnomedical treatments and proven to have anti-effective properties including antioxidant, anti-inflammatory, antibacterial, and antiviral activities [20, 21].

Rambutan (Nephelium lappaceum L.) is a tropical fruit originated in South and Southeast Asia and now spreading as an agricultural crop in many countries, including Singapore, Australia, and South Africa [22]. This evergreen tree grows up to 12–20 m in height, contains dark green, oval-shaped leaves. The oval or ellipsoid shaped fruits, once fully ripened, would have a color of yellow to bright red depending on the variety [22]. The fruit can be separated into hairy peel (2–4 mm in thickness), palpable fleshy fruit, and single seed. N. lappaceum is used to produce consumer products, and the plant parts, including the fruit, have shown to possess numerous biological activities including antioxidant, anti-inflammatory, antimicrobial, and antiviral activities [16, 22,23,24,25].

Industrial wastage generated from production processes such as textile or pharmaceutical carries dyes that can cause environmental threats. As many compounds being used are aromatic, they are not readily decomposable, can last in water bodies, and cause enduring damage to aquatic nature and life [26, 27]. Therefore, water purification technologies have also been developed through a range of chemical, physical and biological approaches, including coagulation, flocculation, filtration, precipitation, ion exchange, membrane filtration, adsorption strategies, biodegradation approaches, and catalytic mechanisms [13]. Photocatalysis plays a crucial role in these methods, as it splits organic contaminants into the simplest gaseous types without producing secondary pollutants [17, 28].

The majority of the photocatalysts are UV active; therefore, the UV lights are necessary for efficient oxidation of organic contaminants. Therefore, the photocatalyst process under both UV and visible light are attractive, cost-effective, and efficient, which can use for industrial effluent treatment plants [13, 29]. Methylene Blue (MB) is one such major organic contaminant of industrial effluents, and it is stable at acidic pH [26, 29, 30]. Moreover, MB is used in medical fields as a disinfectant and biological stain [31]. MB is a toxic substance at high concentrations and low pH [26]; hence, the present study is focused on evaluating the photocatalytic effect of AgNPs on MB dye.

Rhodamine B is a conjugated polymer which has been used with a wide range of applications as chemical and biochemical sensor devices since it has a high fluorescence quantum yield. Fluorescence quenching of Rhodamine B can be achieved by inducing its fluorophore with quencher molecules through various molecular interactions to decrease the quantum yield [32]. Hence, Rhodamine B was utilized as an indicator to monitor the quenching behavior of biogenic AgNPs.

AgNPs have been synthesized using different approaches such as traditional heating, ultrasonic, laser irradiation, γ irradiation, and microwave irradiation, which originated from top-down and bottom-up approaches. However, the techniques mentioned above trigger high temperatures, high pressure, and other harmful substances that cause contamination in the environment [33]. Further, the previous studies have used ethanolic plant extracts to synthesize AgNPs that maximize the usage of chemicals [34, 35]. Hence this study is focused on synthesizing rapid, one step, eco-friendly, and cost-effective greener NPs using water extracts.

In contrast to the published literature related to the plant extract mediated synthesis of AgNPs, this remains the first attempt in synthesizing AgNPs using agricultural wastes generated from the G. mangostana and N. lappaceum fruits catalyzed by direct Sunlight and by UV irradiation [19, 36, 37]. The principal objective of this research was to study the comparison of the shape and the size of the biogenic AgNPs synthesized by irradiating direct Sunlight and UV light. Considering the physical parameters, direct Sunlight irradiated biogenic AgNPs were used to evaluate for their fluorescence quenching efficacy on Rhodamine B and photocatalytic activity by MB degradation. During the literature survey, it was found that the fluorescence quenching ability of biogenic AgNPs on Rhodamine B was under-explored. Therefore this study was carried out to evaluate the fluorescence efficacy of AgNPs on Rhodamine B. The lack of published literature regarding the details on the acidity or basicity of the reaction medium in which photocatalytic assays of biogenic AgNPs was also brought into attention.

2 Materials and methodology

2.1 Materials

Silver nitrate (purchased from Sigma-Aldrich, Anala R grade), Ethanol (purchased from BDH, Anala R grade), Sodium hydroxide pellet (purchased from Sigma-Aldrich, Anala R grade), Hydrochloric acid (purchased from Sigma-Aldrich, Anala R grade), Rhodamine B (purchased from Sigma-Aldrich), Methylene Blue (purchased from Sigma-Aldrich).

2.2 Sample collection and preparation of the extracts

Previously published methods were used with appropriate modification to prepare G. mangostana and N. lappaceum extracts [6, 38, 39]. The ripened fruits of the common variety of G. mangostana available in Kaluthara district were gathered from households. The N. lappaceum fruits were also collected in the same manner from the Colombo district. Ripened fruits were washed with normal water, and then the peel and seed of N. lappaceum and fruit rind and the seed of G. mangostana were separated with a sharp knife. The separated parts were washed with water, cut into small portions, and air-dried for 2 days in the shade. Plant extracts were prepared by placing 3.00 g (Analytical balance-RADWAG Wagi Electroniczne, AS-220. R2) of powdered plant material in a flask containing 300.0 mL of deionized water and heating for 10 min at 100 °C. The crude extracts were filtered using Whatman No. 1 filter paper to separate the plant material from the aqueous water extracts.

2.3 Synthesis of AgNPs

Prepared plant waste extracts were mixed with different concentration of silver nitrate (AgNO3) solutions in different volume ratios, irradiated with direct Sunlight or Ultraviolet light [UV light, the distance between the reaction mixture and the UV lamp (Philips, E/70/2 HPW-125-W) − 5 cm] and kept in the dark. Based on the optimization results, a suitable time was identified to collect the NPs. Resulted reaction mixtures were centrifuged at 4500 rpm (GEMMYCO, PLC-036 h) for 15 min to precipitate the NPs. Then the AgNPs were washed with deionized water followed by Ethanol and oven-dried (Universal oven-UN 55) at 50 °C for 12 h.

AgNPs synthesis from G. mangostana peel extract: Preparation of AgNPs under direct Sunlight: Extract obtained from G. mangostana peel (MPE) was mixed with AgNO3 solution (1 mM) in 2:5 volume ratio of plant extract to AgNO3. This reaction mixture was irradiated with direct Sunlight for 10 min and was kept in the dark for up to 3 h. Preparation of AgNPs under UV light: MPE was mixed with AgNO3 solution (1 mM) in 2:5 volume ratio of plant extract to AgNO3 solution and was irradiated under UV light for 60 min and then kept in the dark up to 24 h.

AgNPs synthesis from G. mangostana seed extract: Preparation of AgNPs under direct Sunlight: Extract obtained from G. mangostana seed (MSE) was subjected to centrifugation (4500 rpm for 15 min) followed by the addition of Ethanol to remove the pectin sedimentation. Isolated supernatant of MSE was collected and mixed with AgNO3 solution (10 mM) in a 1:1 volume ratio of plant extract to AgNO3. This reaction mixture was irradiated with direct Sunlight for 10 min and was kept in the dark until 1 h. Preparation of AgNPs under UV light: MSE was mixed with AgNO3 solution (10 mM) in a 1:1 volume ratio of plant extract to AgNO3 solution and was irradiated with UV light for 30 min and then kept in the dark up to 1 h.

AgNPs synthesis from N. lappaceum peel extract: Preparation of AgNPs under direct Sunlight: Peel extract obtained from N. lappaceum (RPE) was mixed with AgNO3 solution (5 mM) in 1:5 volume ratio of plant extract to AgNO3. This reaction mixture was irradiated under direct Sunlight for 10 min and was kept in the dark for up to 1 h. Preparation of AgNPs under UV light: RPE was mixed with AgNO3 solution (5 mM) in a 1:5 volume ratio of plant extract to AgNO3. The mixture was irradiated under UV light for 60 min.

AgNPs synthesis from N. lappaceum seed extract: Preparation of AgNPs under direct Sunlight: Seed extract obtained from N. lappaceum (RSE) was subjected to centrifugation (4500 rpm for 15 min) followed by the addition of Ethanol to remove the pectin sedimentation. Isolated supernatant (RSE) was collected and mixed with the AgNO3 solution (10 mM) in a 1:1 volume ratio of plant extract to the AgNO3 solution. The obtained reaction mixture was irradiated under direct Sunlight for 30 min and was kept in the dark for up to 1 h. Preparation of AgNPs under UV light: RSE was mixed with AgNO3 solution (10 mM) in a 1:1 volume ratio of plant extracts to AgNO3, irradiated under UV light for 120 min.

2.4 Characterization of AgNPs

Ultraviolet–Visible spectroscopy (UV–VIS spectroscopy) (U-2910–Hitachi, Japan, and Vernier, Go DirectTM SpectroVis Plus) was used to monitor the fabrication of AgNPs. The synthesized NPs were further characterized by Fourier Transform Infrared spectroscopy (Horizon ABB-MB 3000 ATR FT-IR) (FTIR). The morphological features such as shape and particle diameter of NPs were characterized by Scanning Electron Microscope (SEM) (Carl Zeiss, evo) and Transmission Electron Microscope (TEM) (JEOL JEM-2100 Japan) with Energy Dispersive Energy (EDX) system.

2.5 Fluorescence quenching ability of the synthesized AgNPs

The fluorescence quenching of Rhodamine B compound by the AgNPs was evaluated by fluorescence spectrophotometry using a previously published method with appropriate modifications [40]. AgNPs synthesized under direct Sunlight irradiation were used to carry out the fluorescence quenching ability due to the yield and the particle size. A Rhodamine B solution (0.05 mM) was prepared in deionized water, and from the prepared solution, 1000 µL was treated with 50 µL, 100 µL, 250 µL, 500 µL, 750 µL, and 1000 µL volumes of AgNPs (0.2 mg/mL) in deionized water, synthesized using MPE, MSE, RPE, and RSE. The solutions were topped up to 3000 µL with deionized water. The test samples were mixed by vortexing, and then the fluorescence intensity was recorded using Spectroflurometer (F-2700 FL Hitachi, Japan).

2.6 Photocatalytic activity of the synthesized AgNPs

Degradation of MB induced by direct Sunlight at different pH (pH 2, pH 6, and pH 10) in the presence of NPs was used to evaluate the photocatalytic activity of the synthesized AgNPs [41]. AgNPs synthesized by irradiating direct Sunlight was used to study the photocatalytic activity due to the yield and the particle size. Degradation of the MB at different pH: A pH study was carried out to evaluate the effect of the acidic/basic nature of the medium on the degradation of the dye. The initial pH of the MB was recorded (pH = 6.53, GENWAY, 3510, benchtop meter). The pH of the MB was varied by adding 0.1 M HCl to make it acidic and by adding 0.1 M NaOH to alter it to basic. The prepared MB solutions at pH 2, 6, and 10 were kept under direct Sunlight, and the photocatalytic activity was recorded periodically. AgNPs (1.00 mg) were dispersed (Advance vortex mixture-VELP Scientifica ZX3) in 5.00 mL of distilled water. A 25.00 mL of MB solution (5 ppm, pH 2) was added into the prepared AgNPs solution. The reaction mixtures were placed under direct Sunlight (temperature of the environment: 35 ± 3 °C) with continuous stirring. Control was kept in the dark to monitor the activity in the absence of Sunlight. Degradation of the MB was monitored by withdrawing 3.00 mL aliquots from the reaction mixture at every 30-min interval, and UV–VIS absorbance was measured at 661 nm.

2.6.1 Adsorption isotherm study

The adsorption studies of MB were analyzed by varying the MB concentration (2.4 ppm, 2.8 ppm, 3.2 ppm, 3.6 ppm, 4.0 ppm) in the presence of a constant concentration of AgNPs (0.2 mg/mL) and by adjusting the pH to 2. UV–VIS spectrophotometer was used to measure the absorbance of the samples at 661 nm. Langmuir adsorption isotherm and Freundlich adsorption isotherm were plotted using the following equation. The below equation denote the amount of dye adsorbed per unit mass of adsorbent at equilibrium (qe):

$$q_{e} \left( \frac{mg}{g} \right) = \left[ {{\text{C}}_{0} \left( \frac{mg}{L} \right) - {\text{C}}_{e } \left( \frac{mg}{L} \right)} \right] \times \frac{{{\text{V}} \left( L \right)}}{{{\text{W}}\left( g \right)}}$$

where Co is the initial concentration of MB (mg/L), Ce is the concentration of MB at equilibrium (mg/L), V is the volume of the solution (L), and W is the weight of the sorbent (g) [42, 43].

2.7 Statistical analysis

Means of three replicates and standard error (SEr ±) were determined for all analysis obtained.

3 Results and discussion

3.1 Synthesis and characterization of AgNPs

The synthesis of the AgNPs was optimized using different concentrations of AgNO3 solution (0.8 mM, 1 mM, 2 mM, 2.5 mM, 3.0 mM, 5 mM and 10 mM) with using various plant extract to volume ratio and irradiation time of sunlight/UV light [38, 44]. During the optimization step, when the concentration of the AgNO3 solution increases, the larger shift to the longer wavelength was observed in UV spectrometry due to the formation of larger AgNPs. For this study, the AgNO3 concentration, 1 mM for MPE, 5 mM for RPE and 10 Mm for MSE and RSE was chosen by considering the stability, yield and the optimum wavelength of AgNPs.

UV–VIS Spectroscopy: One of the remarkable features of the metal NPs is their optical properties, which changes with the shape and the size [45, 46]. Dispersed metal NPs can be identified from the localized surface plasmon resonance (LSPR) phenomenon through UV–VIS Spectroscopy. [47]. The resonance frequency of localized surface plasmon of metallic NPs also depends on the shape, size, composition, and the degree of aggregation of metallic NPs [48]. Usually, the LSPR band of metallic NPs occurs in the UV range; however, in the case of Ag, the plasmon moves closer to the visible region due to the presence of electrons in the s atomic orbital. The AgNPs formation was confirmed by UV–VIS spectroscopy once the reaction medium changed its color from the initial color of light yellow to grey or dark red/orange color, which is due to the optical property of AgNPs [6].

During different time intervals (10 min–24 h), an aliquot of the reaction mixture was withdrawn to confirm the presence of AgNPs by UV–VIS spectroscopy. The synthesized AgNPs were within the range of 400–440 nm, as reported in the previous literature [38, 49, 50]. The wavelength of the NPs formed was considered in selecting optimum reaction time. UV light irradiation was carried out to evaluate the effect of UV light on the particle size and shape compared with the Sunlight irradiated samples. Figure 1a and b denote the UV–VIS absorbance spectra obtained for the synthesis of AgNPs using MPE under Sunlight and UV light, respectively. The (under Sunlight) wavelengths of the reaction mixture were recorded by periodical monitoring for up to 24 h as shown in Fig. 1a. The optimum reaction condition was identified as λmax 434 nm at 3 h, and therefore the AgNPs were collected at that time, which is in accordance with the published literature [51]. Figure 1b presents the UV–VIS absorbance spectra obtained for the synthesis of AgNPs with MPE under UV irradiation and wavelengths were monitored periodically for 24 h. were monitored periodically, as shown in the spectra. Even though the lowest λmax value was recorded at 408 nm after keeping under UV light for 60 min, the NPs were collected after 24 h due to the high yield.

Fig. 1
figure 1

UV–Visible spectra obtained for the synthesis of AgNPs using MPE a under sunlight b under UV light and MSE c under sunlight, d under UV light

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Figure 1c and d represent the UV–VIS absorbance spectra for the synthesis of AgNPs using MSE under direct Sunlight and UV light, respectively. The NPs were collected at 420 nm due to the high yield for both Sunlight and UV irradiated MSE samples.

In Fig. 2a and b the UV–Visible absorbance spectra obtained for the synthesis of AgNPs from RPE under the direct Sunlight and UV light irradiations are given, respectively. Similar to NPs synthesized from MPE, considering the yield and the wavelength, the optimum time frame was identified. The λmax value obtained was in agreement with the published results [52]. Within a short period, a more significant shift to the longer wavelengths was observed for the direct Sunlight, as well as UV light irradiated samples, prepared by RPE compared to MPE.

Fig. 2
figure 2

UV–Visible spectra obtained for the synthesis of AgNPs using RPE a under sunlight b under UV light and RSE c under sunlight, d under UV light

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Subsequent Fig. 2c and d provide the UV–VIS absorbance spectra obtained for AgNPs synthesized using RSE under direct Sunlight and UV light, respectively. Due to the considerable bathochromic shift, which changed the position of the spectral band to longer wavelengths, monitoring of the reaction time after 2 h period was ceased. The bathochromic shift of the spectrum is mainly due to the morphology and the size of the NPs [53]. Considering the Sunlight mediated synthesis of NPs, UV light radiated synthesis took a longer time to initiate the reaction by changing the color.

3.2 Mechanism of formation of biogenic AgNPs

The present study experimented with the photoinduced synthesis of AgNPs utilizing the agro-waste of G. mangostana L. and N. lappaceum L. which was initiated by the absorption of photons (Sunlight and UV) by photosensitive biomolecules present in the plant extracts.

The high content of phytochemicals such as Alkaloids, Flavanoids, Triterpenoids, Terpenoids, Tannin, Saponin, Quinone, Protein, and Sugars are present in many plant extracts which are photosensitive [1, 15, 19]. Phytochemicals such as Flavonoids, Tannins, and many sugars act as reducing agents and while other phytochemicals act as capping or stabilizing agents in the process of synthesizing AgNPs.

Numerous polyphenolic compounds are present in G. mangostana L. and N. lappaceum L plant extracts, which can be involved in many reduction pathways. The possibility of polyphenols or tannins to act as reductants is comparatively higher due to the prominent quantity of these chemicals in G. mangostana L. and N. lappaceum L. [54, 55]

The tannins molecules contain many − OH groups, which can get oxidized to quinoid form by donating electrons to Ag+ to form Ag0. Many terpenols, flavin binding proteins, and most of the phytochemicals are photoresponsive; they either itself can donate the electrons or can induce other compounds to donate electrons to the Ag+ in the presence of photons, which provides clear proof of photoinduced synthesis of AgNPs. Some alternative reducing reactions can also take place due to the reducing sugars present in the plant extracts. Whereas sugars, proteins, or some other phenolic compounds present in the plant extracts can behave as stabilizers for the synthesis of AgNPs [56, 57] (Fig. 3).

Fig. 3
figure 3

Proposed reaction mechanism for the AgNPs formation [56]

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3.3 FTIR analysis of the plant extracts and synthesized AgNPs

Plant extracts contain many secondary metabolites that act as the reducing and capping agents in the green synthesis of metal NPs [6]. FTIR analysis was carried out in order to identify the functional groups present in the biomolecules which are responsible for the synthesis and stabilization of the synthesized AgNPs [58]. The FTIR spectra of the N. lappaceum and G. mangostana plant extracts were separately gathered as a means of identifying functional groups of the secondary metabolites in plant extracts that could contribute to the reduction of Ag+ [51]. Infrared (IR) absorption of the AgNPs was arisen due to the molecules that adhered to the surface of the NPs. A comparison between the plant extracts and the synthesized NPs could provide information on the surface adherence of molecules. Washing step with methanol could be a reason for the low intensity of IR peaks as this wash away the secondary metabolites adhering to the surfaces.

Figures 4 and 5, denote the FTIR spectra obtained for plant extracts, and AgNPs synthesized using the plant extract of MPE, MSE, RPE, and RSE, respectively. The common prominent peaks in all the spectra from plant extracts appeared in 3268–3290 cm−1, 2347–2363 cm−1, 2099–2103 cm−1 and 1635–1639 cm−1. These high-intensity peaks could be attributed to − OH vibration, –CN stretching, C≡C stretching, and C=O stretching, respectively [8, 59, 60]. Peaks at 2914/2919 cm−1 and 2851/2852 cm−1 appeared in both the seed extracts, which correspond to the C–H vibration in the − CH2 group. Seeds usually contain a high level of fatty acids/fatty acids like molecules in contrast to high flavonoid or phenolic compounds present in peel and edible portions. That could be mainly due to the CH2 groups present in the seeds.

Fig. 4
figure 4

FTIR spectra a MPE (in black) and AgNPs synthesized using MPE (in red) b MSE (in black) and AgNPs synthesized using MSE (in red)

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Fig. 5
figure 5

FTIR spectra a RPE (in black) and AgNPs synthesized using RPE (in red). b RSE (in black) and AgNPs synthesized using RSE (in red)

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3.4 SEM analysis of the synthesized AgNPs

The physical characteristics of the synthesized AgNPs were evaluated through SEM. The analysis was carried out in different magnitudes (15,000, 30,000, and 75,000) at 10 000 kV to investigate the particle shape and the size. AgNPs synthesized using MPE and RPE showed spherical particles which gathered in the form of clusters in both Sunlight and UV light treated samples, as shown in Fig. 6a, b, e and f. However, AgNPs obtained through seed extracts showed high agglomeration for both Sunlight and UV light treated samples that may be due to the high-fat content, as shown in Fig. 6c, d, g and h.

Fig. 6
figure 6

SEM images of AgNPs synthesized via a MPE with sunlight treatment b MPE with UV light treatment c MSE with sunlight treatment d MSE with UV light treatment e RPE with sunlight treatment f RPE with UV light treatment g RSE with sunlight treatment h RSE with UV light treatment

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The information on NPs size was observed as given by the SEM images. The details obtained through UV–VIS spectroscopy and SEM are summarized in Table 1. By observing the UV–VIS spectra of the NPs and the SEM images, it can be concluded that Sunlight is far effective in the NPs fabrication process than the UV light irradiation. A possible reason may be the structural deformation and loss of antioxidant activity that might occur in the secondary metabolites in the presence of UV light.

Table 1 Physical characteristics obtained from UV–VIS analysis and the SEM images
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3.5 TEM and EDX analysis of the synthesized NPs

TEM analysis results depict that all the NPs were fallen within the nano range, and the NPs were approximately spherical, which also was confirmed by the SEM images. Figure 7 depicts the TEM images of the synthesized AgNPs via MPE (Fig. 7a), MSE (Fig. 7b), RPE (Fig. 7c), and RSE (Fig. 7d). The average diameter of the AgNPs synthesized via MPE, MSE, RPE and RSE were detailed as \(14.9\, \pm \,5.7\;{\text{nm}}\), \(19.2\, \pm \,7.2\;{\text{nm}}\), \(31.3\, \pm \,9.2\;{\text{nm}}\), and \(12.1\, \pm \,5.9\;{\text{nm}}\), respectively. Moreover, the TEM images revealed that the AgNPs synthesized from seed extracts had high agglomeration compared with the AgNPs synthesized via peel extracts, which was exhibited similarly in SEM images.

Fig. 7
figure 7

TEM images of AgNPs synthesized via a MPE, b MSE, c RPE and d RSE

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The results reported by Kumar et al. indicates that the NPs synthesized via N. lappaceum peel was fallen within the range of 132.6 ± 42 nm [19]. However, in this study, the AgNPs were synthesized successfully with an average particle size of \(31.3\, \pm \,9.2\;{\text{nm}}\). This variance may be due to the differences in the concentration of the plants or AgNO3 and the radiation media used.

Atomic resolution cross-section of AgNPs synthesized via MPE, MSE, and RSE TEM image is given in Fig. 8d–f, respectively, which demonstrated the precise and uniform fringers. The atomic interlayer distance of all three AgNPs was estimated, and that was recorded as 0.24 nm, which was closer to the lattice spacing of 0.232 of the (111) lattice planes of bulk Ag [61, 62].

Fig. 8
figure 8

TEM images of AgNPs Synthesized via a MPE, b MSE, c RSE and line profile of the HRTEM image of AgNPs synthesized via d MPE, e MSE, f RSE

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The elemental composition of the AgNPs synthesized via MPE, MSE, RPE, and RSE was investigated by EDX analysis. The EDX spectrum illustrated (Figs. 9, 10, 11 and 12) that they have well-defined peaks related to Silver, Oxygen, and Carbon. The Carbon peak can be responsible for either to the biomolecules that are capped to the surface of AgNPs and/or to the coating of the copper grids. Besides, EDX spectra have shown the presence of extremely pure NPs without any other impurity-related peaks [63].

Fig. 9
figure 9

EDX a mapping and b spectrum of AgNPs synthesized via MPE

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Fig. 10
figure 10

EDX a mapping and b spectrum of AgNPs synthesized via MSE

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Fig. 11
figure 11

EDX a mapping and b spectrum of AgNPs synthesized via RPE

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Fig. 12
figure 12

EDX a mapping and b spectrum of AgNPs synthesized via RSE

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3.6 Fluorescence quenching activity of the synthesized AgNPs

The fluorescence quenching ability depends on the nature of fluorophore, quencher concentration, quencher molecule, and polarity of the solvent medium [64]. The fluorescence quenching efficacy of the synthesized NPs using MPE, MSE, RPE, and RSE dispersed in water was studied using fluorescence spectroscopy. Figure 13 summarizes the fluorescence emission spectra of Rhodamine B treated with AgNPs synthesized using MPE, MSE, RPE, and RSE, respectively. The spectral lines obtained for the fluorescence emission of Rhodamine B with AgNPs of each sample for 50 µL, 100 µL, 250 µL, 500 µL, 750 µL, and 1000 µL are given below. It was observed that in all the samples treated with AgNPs synthesized from MPE, MSE, RPE, and RSE, the fluorescence intensity of the Rhodamine B decreased gradually with an increasing volume of the AgNPs solution. The use of green synthesized AgNPs as quencher molecules exhibit various advantages such as a cost-effective, eco-friendly, and easy process.

Fig. 13
figure 13

Fluorescence emission spectra of Rhodamine B treated with AgNPs synthesized using a MPE with sunlight b MSE under sunlight and c RPE under sunlight d RSE under sunlight

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Fluorescence quenching can be due to either dynamic quenching or static quenching [40]. The Stern–Volmer equation is used to describe the dynamic quenching mechanism [40]. The data obtained through the fluorescence emission spectra were used to generate the Stern–Volmer plots for AgNPs treated with Rhodamine B is represented in Fig. 14. High linearity was observed for all the samples with R2 > 0.9596. At the lower concentration, all four plots appeared to be linear, corresponding with the presence of a dynamic quenching mechanism. However, a positive deviation at higher concentration (0.035–0.07 M) was observed in the AgNPs synthesized via RPE, MSE, and RSE. In order to confirm this experimental observation, further studies have to be carried out with a narrow interval of concentrations. The reasons for deviation from the linearity may be due to the presence of dynamic and static quenching at the same time and also due to the formation of excited-state collisions or chemical complex [65, 66].

Fig. 14
figure 14

Stern volmer plots obtained from the data gathered from the fluorescence emission of Rhodamine B dye treated with AgNPs of MPE (-orange), MSE (-blue), RPE (-yellow) and RSE (-blue)

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3.7 Photocatalytic activity of synthesized AgNPs

Various studies reported the catalytic effect of biofabricated nanomaterials in the process of reducing hazardous organic substances. Interestingly, AgNPs act as an efficient catalyst in the decolorization of dyes, which gives evidence that nanocatalyst has an industrial role due to high efficiency and reaction rate [67]. Hence, the present study has been carried out utilizing phytofabricated AgNPs via agricultural wastes of G. mangostana and N. lappaceum for the degradation of MB.

When the photons irradiated to the AgNPs dispersed medium, the collective oscillations of electrons from the outermost band to a higher energy state take place. The plasmonic excitation of a surface electron is absorbed by molecular oxygen, which is present in the reaction medium, and it is converted into radical oxygen species. The positive charge generated in AgNPs (5 sp band) is filled with the electrons captured from the MB; thus, the MB dye adsorbed to the surface of the AgNPs. As a result, the MB dye gets oxidized. Meantime the hydrogen ions get generated from the reaction medium. The generated oxygen radical species willing to react with hydrogen ions and form hydroperoxyl radicals. The generated reactive radical species are responsible for the reduction of MB dye, and the proposed mechanism is given in Fig. 15 [68, 69].

Fig. 15
figure 15

Reaction mechanism of degradation of MB dye utilizing biosynthesized AgNPs. (Extracted from Singh, Jagdeep ect al.)

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The pH of the medium is a vital factor in the photodegradation process. The surface charge properties of the photocatalyst and the size of aggregates it forms depends on the pH. Hence, the photodegradation of the MB was carried out at pH 2, 6, and 10 to observe the effect of pH that is having on dye (without the presence of any catalyst). The experimental results revealed the higher photodegradation of MB dye in pH 10 followed by pH 6 and pH 2 as shown in Fig. 16a–c respectively. Based on the results obtained, pH 2 was identified as a suitable pH to carry out to evaluate the photocatalytic activity of the AgNPs since the MB solution does not decompose naturally under Sunlight. To the best of the knowledge, this remains the first report wherein a study demonstrates the effect of the pH on the dye degradation using NPs. It was revealed that the MB dye was manifested insignificant degradation at pH 2 while compared with higher pH (which, therefore, could be facilitated to monitor the degradation effect of the synthesized NPs without the interference of self-degradation that the dye may undergo). In most of the previous studies, it has been reported that the MB degradation was carried out at neutral or basic pH. The research carried out by Jagdeep Singh ect al. has obtained 83% MB dye degradation at 180 min (pH 7) in the presence of AgNPs.

Fig. 16
figure 16

UV–VIS absorbance spectra gathered for the degradation of MB dye at a pH 2, b pH 6 and c pH 10

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The control (contains the AgNPs and the MB solution at pH 2), which kept under dark, did not show any degradation with time. However, the UV–VIS absorbance spectra of the MB treated with AgNPs synthesized using MPE, MSE, RPE and RSE under Sunlight demonstrated significant degradation of the dye, as shown in Fig. 17. It can be observed in Fig. 17 that the absorption intensity decreased with time considerably, within the first 30 min, in all samples. Among all the four synthesized AgNPs, rapid degradation of MB was observed in AgNPs synthesized using RSE. As per the TEM images, the AgNPs/RSE shows the highest surface area compared with the other AgNPs.This may be the reason for the rapid degradation of MB via AgNPs/RSE.

Fig. 17
figure 17

Degradation of MB dye with the treatment of AgNPs synthesized via a MPE b MSE and c RPE d RSE

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Calculation of the efficiency of the photocatalytic activities and the rate constants were performed following published literature [40]. Absorbance at the 661 nm was taken as proportional to the concentration of the MB at the determined time intervals, according to the Beer–Lambert law. The following equation was used to calculate the degradation efficiency of the MB, where initial absorbance and absorbance at a time were denoted by Co and C, respectively.

$${\text{Degradation}}\;{\text{Efficiency}} = \left( {1 - \frac{C}{{C_{0} }}} \right) \times 100\%$$

The intersection of curves [C/Co and (1 − C/Co)*100] was taken as the Half-life of MB, in the presence of AgNPs [39, 70]. Degradation efficiency for each AgNPs sample were measured at 360-min time period, and plots were generated for calculating Half-life and degradation efficiencies. Information gathered on photocatalytic activity are summarized in Table 2. This study reveals that the biogenic AgNPs behave as efficient catalysts at pH 2 (360 min).

Table 2 Photocatalytic efficiency of the synthesized AgNPs on the degradation of MB at pH 2
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3.7.1 Adsorption isotherm study

Langmuir and Freundlich isothermal models (Figs.18, 19) were utilized to study the adsorption on MB on the sorbent surface. The figure equations are as follows:

Fig. 18
figure 18

Langmuir Isotherm and Freundlish Isotherm a MPE/AgNPs and b MSE/AgNPs

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Fig. 19
figure 19

Langmuir Isotherm and Freundlish Isotherm a RPE/AgNPs and b RSE/AgNPs

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Langmuir equation

Freundlich equation

AgNPs/MPE

\({\text{y}}\, = \,0.0272{\text{x}}\, + \,0.0434 \, \left( {{\text{R}}^{2} \, = \,0.9945} \right)\)

\({\text{y}}\, = \,0.5358{\text{x}}\, + \,2.5536 \, \left( {{\text{R}}^{2} = 0.3305} \right)\)

AgNPs/MSE

\({\text{y}}\, = \,0.0272{\text{x}}\, + \,0.0434 \, \left( {{\text{R}}^{2} \, = \,0.9945} \right)\)

\({\text{y}}\, = \,0.4175{\text{x}}\, + \,2.7241 \, \left( {{\text{R}}^{2} \, = \,0.9885} \right)\)

AgNPs/RPE

\({\text{y}}\, = \,0.0582{\text{x}}\, + \,0.0188 \, \left( {{\text{R}}^{2} \, = \,0.969} \right)\)

\({\text{y}}\, = \,0.2718{\text{x}}\, + \,2.7816 \, \left( {{\text{R}}^{2} \, = \,0.7616} \right)\)

gNPS/RSE

\({\text{y}}\, = \,0.0847{\text{x}}\, + \,0.0693 \, \left( {{\text{R}}^{2} \, = \,0.9757} \right)\)

\({\text{y}}\, = \,0.5512{\text{x}}\, + \,2.4945 \, \left( {{\text{R}}^{2} \, = \,0.8036} \right)\)

Accordingly, with respect to the linearity of these plots, the Langmuir model is appropriate for the adsorption of MB on AgNPs. As per this model, monolayer sorption happens, and the distribution of surface-active sites of AgNPs is homogeneous.

The Langmuir isotherm follows the below equation:

$$\frac{{C_{e} }}{{q_{e} }} = \frac{1}{{q_{max} b}} + \frac{{C_{e} }}{{q_{max} }}$$

Ce—equilibrium concentration of the MB (mg/L). qe—amount of MB adsorbed per unit mass of adsorbent (mg/g). b—maximum adsorption capacity and surface energy (the affinity between sorbent and sorbate) (mg/L).

For this study, the highest qmax and b were achieved as 36.76 mg/g and 0.627 mg/L for both AgNPs/MPE and AgNPs/MSE. The qmax observed for AgNPs/RPE and AgNPs/RSE were 17.18 mg/g and 11.81 mg/g respectively. The b constant observed for AgNPs/RPE and AgNPs/RSE were 3.10 mg/L and 1.22 mg/L respectively.

4 Conclusions

The results of this study suggest that the plant extracts of G. mangostana and N. lappaceum peel and seed extracts can be used as an eco-friendly reducing and capping agents in the preparation of AgNPs. The study aims at environmental sustainability through green processes to minimize agricultural wastes. The UV–Visible spectroscopic results revealed that the NPs synthesized from MPE, MSE, RPE, and RSE lie within the range of 400–440 nm and the LSPR band of green synthesized AgNPs was confirmed with the published literature. According to the SEM analysis, the NPs synthesized from MPE and RPE under direct Sunlight, and UV light was spherical with low aggregation. However, the NPs synthesized from MSE and RSE under direct Sunlight, and UV light showed higher agglomeration. The TEM analysis for all four samples reveals a nano range size distribution, which was fallen within the range of 12.1–31.3 nm. Furthermore, the atomic interlayer distance of AgNPs synthesized via MPE, MSE, and RSE was recorded as 0.24 nm, which demonstrated the precise and uniform fringers. Considering the yield and the particle size of the AgNPs, sunlight irradiated synthesis was chosen to study the fluorescence quenching and photocatalytic activity of the NPs. To the best of our knowledge, this remains the first attempt that used biogenic AgNPs to quench the fluorescence ability of aqueous Rhodamine B solution. All the AgNPs were able to quench the fluorescence emission of Rhodamine B with increasing AgNPs concentration. The high photocatalytic activity was observed for the degradation of MB at pH 2 by all the NPs. As per the adsorption study, it was confirmed that the adsorption process is best described by the Langmuir isotherm model. The results of this study provided an ecofriendly approach to eliminate hazardous dyes from industrial effluents.