1 Introduction

Nowadays, people are more susceptible to harmful microbes and high pollution levels because of the population boom and continual modifications in daily life [1, 2]. Additionally, the overuse and abuse of antibiotics in food and medicine have increased bacteria’s resistance to them [3]. The spread of contagious diseases, the cost of medical treatments, and even the death rate among the population have all increased significantly due to antibiotic resistance [4, 5]. Metal oxide nanostructures are crucial to the development of antimicrobial and environmental remediation technologies. They are flexible materials with tremendous potential for addressing pollution and fighting microbial infections due to their capacity to harness light energy for the breakdown of organic contaminants [6,7,8]. Furthermore, the presence of non-biodegradable dyes in drinking water and wastewater poses a severe risk to the ecosystem, and many researchers have reported the use of nanomaterials for the effective photocatalytic degradation of these dyes [9,10,11,12,13]. For instance, Congo red (CR), used as a coloring ingredient in many chemical industries, harms the environment when it is discharged with effluents [14]. Diverse techniques have been used to remove this persistent dye, including flocculation, adsorption, and chemical oxidation. The problem is that because the final product of the oxidation process is unstable and toxic, some strategies increase secondary pollution and reduce the efficiency of the process [15,16,17]. Therefore, novel antimicrobial and photocatalytic nanomaterials that may be implemented to antimicrobial surfaces, air, and water are urgently needed [18, 19].

Over the past few decades, nanotechnology has become a noteworthy, interdisciplinary research field on a global level. The unusual physicochemical features of nanoparticles (NPs) have drawn more attention to their design, manufacture, characterization, and uses [20]. Nanoparticles typically range in size from 1 to 100 nm are incredibly small particles and have entirely different features from bulk counterparts. Because of the variety in the other specific parameters, such as size, distribution, and shape, they have a larger surface-area-to-volume ratio [21]. The improved catalytic and biological properties of the NPs are the result of their larger surface area [22, 23]. Due to their extraordinary physicochemical, optical, and biological characteristics, metal oxide NPs are widely used in a variety of pharmaceutical and industrial applications [24,25,26].

Amidst metal oxides, Bi2O3 is a significant p-type semiconductor that comes in numerous polymorphs (α, β, γ, δ, and ε) of Bi2O3 NPs, with diverse morphologies such as nano-flowers, thin films, nanowires, nano-plates, nano-belts, and nano-rods, for environmental cleanup [27]. Bi2O3 has drawn a plethora of attention due to its diverse uses as a photocatalyst [28], electrodes [29], lithium-ion batteries [30], sensors [31], biological agents [32], and catalyst [33]. The creation of Bi2O3 NPs has been carried out using various methods, including precipitation [34], sol-gel [29], laser ablation [35], microwave-assisted [36], hydrothermal [37], and chemical reduction approach [38]. Unfortunately, these methods utilize high pressure, high temperature, and poisonous chemicals. In order to make metal oxide NPs, researchers have discovered a viable, facile, speedy, non-toxic, affordable, and environmentally benign green technique employing natural materials like bacteria, fungi, algae, and plant extracts [39,40,41,42]. There is no demand for isolation, culturing, or maintenance because the rate of NP creation is higher [43].

Rubus ellipticus is an annual plant found in the subtropical zone of the globe. Naturally, the plant maintains a variety of habitats, including mountain valleys, roadsides, hilly terrain, forests, and slopes, throughout south and south-east Asian nations [44, 45]. In the Himalayan region, its delectable edible fruits are taken raw, and the plant’s fruits, leaves, and roots are vital parts of Ayurveda, traditional Chinese medicine, and many folk remedies [46,47,48]. Since ancient times, diverse plant portions of this species have been used to treat dysentery, diarrhea, fever, cough, constipation, wounds, colic, vomiting, uterine relaxant, and gastric trouble. Numerous pharmacologically helpful bioactive compounds (Fig. 1) with health-promoting properties have been discovered from the species [49, 50].

Fig. 1
figure 1

Biologically active phytochemicals in Rubus ellipticus

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There have not been many reports of studies that use plant extract to produce α-Bi2O3 NPs. Table 1 provides a summary of certain plant extracts used for the green synthesis of α-Bi2O3 NPs. In the present study, we synthesized α-Bi2O3 NPs from Rubus ellipticus fruits and leaves extract using the modified solgel method. The employed method is useful for plant extracts, which have a highly acidic nature and prohibit the reduction of nanoparticles. However, the proposed method provides the reduction of a whole metal precursor and its transformation into metal oxide, along with enhancements in their photocatalytic and biological activities. The innovative aspect of this study is developing a green approach for modifying the surface morphology of α-Bi2O3 NPs to improve photocatalytic activity and antimicrobial efficacy. XRD, FTIR, FE-SEM, HR-TEM, UV-vis DRS, and XPS were explored to characterize the textural properties of α-Bi2O3 NPs. The intrinsic mechanism of photocatalytic action has been determined. Furthermore, the antibacterial activity of the α-Bi2O3 NPs against gram-positive and gram-negative bacteria was tested to validate their multi-functionality. Additionally, α-Bi2O3 NPs exhibit a broad range of antioxidant potential as well.

Table 1 Green synthesis of α-Bi2O3 NPs utilizing different plant extracts
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2 Experimental

2.1 Materials

The synthesis of Bi2O3 NPs was carried out using the chemicals bismuth nitrate pentahydrate (Bi(NO3)3.5H2O, Sigma-Aldrich, ACS reagent, ≥98.0%), citric acid (HOC(COOH)(CH2COOH)2·H2O, Sigma-Aldrich, ACS reagent, ≥99.0%), polyethylene glycol (H(OCH2CH2)nOH, Sigma-Aldrich, PEG 400), deionized water (DW), methanol fruit extract, and methanol leaf extract. All chemicals were used without any purification.

2.2 Plant material collection and extract preparation

The Rubus ellipticus plants of the Rosaceae family were found in the district of ShimLa at an altitude of 2000, Himachal Pradesh, India. The authentication of plant material (herbarium sheets of selected plants) was done by the Botanical Survey of India (BSI), Dehradun, India, with Accession no: 117. Selected R. ellipticus plants’ fruits and leaves were gathered in polybags and transported to the lab for analysis. The gathered produce was air dried at ambient temperature, then ground into a coarse powder using a grinder before being kept in airtight containers. To get the coarse powers of the leaves and fruits extracts, 10 g of dried fruits/leaves samples was extracted with 100 mL of methanol in an incubator shaker for 48 h. The extracts were filtered through Whatman filter paper no. 41 and dried in a hot air oven at 37 °C, collected separately. The dried crude methanolic extracts of R. ellipticus fruits and leaves were further stored at 5 °C in a refrigerator for further analysis.

2.3 Fabrication of bismuth oxide nanoparticles (Bi2O3-NPs)

The synthesis of Bi2O3-NPs using methanol extacts of fruits and leaves of R. ellipticus firstly involves dissolving citric acid monohydrate 10 g in 100 mL of distilled water and 10 g of bismuth nitrate pentahydrate in 50 mL of DW in separate beakers using a magnetic stirrer. The aqueous solution of bismuth nitrate was added to the 100 mL aqueous solution of citric acid and stirred for 1 h at 60 °C. The 100 mg of dried fruit extract was dissolved in 30 mL of methanol separately and was further added to the aforementioned solutions and stirred for 30 min at 60 °C with 10 mL of polyethylene glycol (PEG). Aforementioned solution was transferred to a 500 mL round bottom flask and placed on a heating mantle for 24 h at 50 °C. The black-colored dried product was obtained, thoroughly crushed into a fine powder and calcinated at 500 °C for 5 h. The given method was employed for the synthesis of NPs using leaves extract. Finally, the yellow-colored powder was obtained, and further sent for characterization.

2.4 Characterization of fabricated nano-particles

The crystalloid structure and phase purity of the manufactured samples were verified using X-ray diffraction patterns (XRD) generated by a SmartLab 9kW rotating anode X-ray diffractometer. The surface morphology and texture of the produced NPs were investigated using high-resolution transmission electron microscopy (HRTEM, FP 5022/22-Tecnai G2 20 S-TWIN) and scanning electron microscopy (SEM, FEI-FP 5022/22-Tecnai G2 20 S-TWIN). X-ray photoelectron spectroscopy was used to probe the elemental composition (XPS, Nexsa base). Fourier transform infrared spectroscopy was used in order to detect vibrations in chemical bonds (FTIR, RZX, Perkin Elmer). Dynamic light scattering (DLS) was utilized to obtain particle size distribution and zeta potential of the nanoparticles using the Malvern Nano Zs-90 size analyzer. The UV-vis diffuse reflection spectrum was used to determine the energy bandgap and UV-vis absorption spectra of the produced NPs (UV 2450 Shimadzu).

2.5 Photocatalytic activity

To assess the photocatalytic activity of the synthesized samples, the photocatalytic degradation of Congo red in the presence of visible light was performed. In brief, a dye solution of 100 mL was prepared, and 0.05 g of photocatalyst was dispersed for the photocatalytic reaction. To achieve adsorption equilibrium, the solution was stirred in the dark for 30 min prior to the photocatalytic degradation. A halogen lamp was used as a visible light source to induce dye photodegradation, with a distance of 25–28 cm between the photocatalytic degradation solution and the light source. The catalyst was recovered by periodically collecting 5 mL of the mixture and centrifuging it at 12,000 rpm for 5 min. The maximum absorbance of the purified Congo red dye solution was determined to be at 498 nm using a UV-visible spectrophotometer. Photodegradation efficiency of the photo-catalyst was calculated using the following Eq. (1) [56]:

$$\%\kern0.5em \textrm{degradation}\ \textrm{efficiency}=\frac{{\textrm{C}}_0-{\textrm{C}}_{\textrm{t}}}{{\textrm{C}}_0}\times100\kern13.25em$$
(1)

where C0 states the concentration of the initial dye solution at maximum wavelength, and Ct represents the final concentration of solution at maximum wavelength after the finishing point of the photocatalytic reaction.

2.6 Antioxidant activity (DPPH; Blois, 1958)

UV-Vis spectrophotometer readings at 517 nm were used to compare the free radical scavenging activities of REL-NPs and REF-NPs against the 2,2 diphenyl-1-picryl hydrazyl radical. The sample’s activity was evaluated after being prepared in methanol at several doses (25–100 μg/mL). After adding 1 mL of DPPH reagent (0.4% DPPH in 100 mL of methanol), the absorbance was measured against a standard of ascorbic acid. The percentage of DPPH decolorization of the sample was calculated according to Eq. (2) [57]:

$$DPPH\ \left(\%\right)=\left[\left( Ab\hbox{--} Aa\right)/ Ab\right]\times 100$$
(2)

Graphs showing radical scavenging activity vs. nanoparticle concentration were constructed. Here, Ab is the absorption of the blank sample and Aa is that of the nanoparticles. The inhibitory concentration (in micrograms per milliliter, or IC50) at which 50% of DPPH radicals were scavenged is shown. The antioxidant ascorbic acid was as a reference standard.

2.7 Antimicrobial assay

The antibacterial efficacy of the synthesized samples was tested against various bacterial strains, including Bacillus subtilis (MTCC 441), Staphylococcus aureus (MTCC 737), Escherichia coli (MTCC 739), Klebsiella pneumoniae (MTCC 109), and Pseudomonas aeruginosa (MTCC 424). Additionally, two fungal pathogenic strains, Fusarium oxysporum (SR266-9) and Rosellinia necatrix (HG964402.1), were selected for antifungal assays. The bacterial and fungal strains were obtained from IMTech in Chandigarh and the Faculty of Biotechnology at Shoolini University in Solan, India.

2.8 Antibacterial activity

The antibacterial efficacy of REL-NPs and REF-NPs was assessed using the disc diffusion assay. Nutrient agar plates were inoculated with 100 μL of bacterial culture using sterile cotton swabs to ensure even distribution. To create a stock solution (DMSO), 50 mg of each nanoparticle type was dissolved in 1 mL of dimethyl sulfoxide. Next, 40 μL of nanoparticles was spread onto 6 mm sterilized paper discs, and the plates were incubated at 37 °C for 24 h. The size of the zone of inhibition was measured using the antibiotic zone scale once the incubation period had ended. Each antimicrobial test was repeated thrice to ensure accuracy. Ampicillin (5 mg/mL) and DMSO (solvent) were used as positive and negative controls, respectively.

2.9 Antifungal assay

The poison food method was utilized for the antifungal assay, of Dhatwalia et al. (2020) to determine antifungal activity [58]. Before usage, both the F. oxysporum and R. necatrix fungi were grown on potato dextrose agar for 7 days at 25 °C. Before pouring each plate, 24 mL of PDA was combined with 1 mL of nanoparticles (2 mg/mL). Fungal discs with a diameter of 6 mm were cut from a 7-day-old culture of both fungal strains using a flame-sterilized cork borer, then put in the middle of Petri plates and cultured at 25 °C for 7 days. On day 7 of incubation, the fungal strains were compared based on the diameter of their colonies. There were three separate tests done. The percentage inhibition was calculated by using the provided formula:

$$\mathrm{Inhibition}\;(\%)=\frac{\left(\mathrm C-\mathrm T\right)}{\mathrm C}\times100$$

where C is the diametric growth of the colony in control, and T is the diametric growth of the nanoparticles and extract.

2.10 Minimum inhibitory concentration (MIC)

In a 96-well microtitre plate, the MIC test was used to assess the fruit extract and nanoparticles for their antibacterial and antifungal properties. Sterilized nutritional broth and potato dextrose broth were used to fill the 0.1 mL wells in each of the microtiter plate’s 12 rows. To serially dilute the nanoparticles, we transferred 100 L of the test stuff from the first row to the succeeding wells in the next row of the same column, so that each well contained 100 μL of test material in serially falling concentrations. This process was repeated for wells 2 through 11. Finally, a total of 10 μL of a bacterial and fungal solution (5×106 CFU/mL) was applied to each well. Each plate was lightly covered with cling film to prevent the microorganisms from drying out while in culture. The deep wells were heated to 37° for 24 h. Each well was given 10 μL of a resazurin solution for use as a color indicator. A visual inspection of the well allowed us to see the color shift. The minimal effective concentration (MIC) was calculated as the concentration of extract needed to induce a detectable color change.

3 Result and discussion

3.1 XRD, FTIR, DLS, and TGA analysis

The XRD pattern of REF-NPs and REL-NPs is given in Fig. 2a, which shows the sharp peaks with high intensity, which can be related to the high crystallinity of both the prepared samples. The XRD patterns of the synthesized NPs are in good agreement with PDF card no. 71-2274 without any presence of secondary phases [59]. The diffraction peaks position in XRD plots for [REF NPs, REL NPs] and their associated (hkl) values are as follows [19.80, 19.83]: (110); [21.76, 21.79]: (020); [24.59, 24.62]: (−102)(021); [25.79, 25.75]: (002); [26.97, 26.94]: (−112)(111); [27.44, 27.45]: (120)(−121); [28.05, 28.07]: (012); [32.54, 32.49]: (−211); [33.07, 33.11]: (−122)(121); [33.28, 33.26]: (−202)(200); [34.02, 34.04]: (022); [35.03, 35.07]: (210); [35.43, 35.48]: (031); [35.94, 35.99]: (102); [36.99, 37.03]: (130); [37.64, 37.69]: (122); [40.09, 40.11]: (−222)(220); [41.5, 41.53]: (131); [41.92, 41.94]: (211); [42.40, 42.37]: (122). The rietveld refinement patterns shown in Fig. 2b and c were obtained using FullProf software, confirming the monoclinic phase of the synthesized α-Bi2O3 NPs with space group P 121/c1. The obtained lattice parameters for REF-NPs are a = 5.8463 Å, b = 8.1585 Å, c = 7.5042 Å; and REL NPs are a = 5.8434 Å, b = 8.1573 Å, c = 7.5011 Å. It can be observed that lattice parameters decreased for NPs synthesized using leaf extract with crystallite size of 66.51 nm and 68.56 nm for REF-NPs and REL-NPs. The crystallite size was calculated using Scherrer’s formula given in Eq. (3) [60]:

$$\textrm{d}=0.9\uplambda /\upbeta \textrm{cos}\uptheta$$
(3)
Fig. 2
figure 2

a XRD pattern of Bi2O3 NPs synthesized using fruit (REF) and leaf (REL) extract; Rietveld refined patterns of b REF-NPs and c REL-NPs, d FTIR spectra of REF-NPs and REL-NPs

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where d is the crystallite size, λ is the wavelength, β is the full-width half maxima, and θ is the diffraction angle. The high crystallite size value can be attributed to the higher crystallinity of the samples induced due to calcination temperature. Figure 2d represents the FTIR spectra of α-Bi2O3 structure for REF-NPs and REL-NPs in the range of 400–4000 cm−1 wavenumber. In general, metal oxide semiconductor exhibits FTIR absorption bands below 1000 cm−1 due to the vibration of the metal-oxygen interatomic interaction. For pure α-Bi2O3, the broad peaks 425–510 cm−1 belong to the Bi-O stretching vibration of non-bonding oxygen of the distorted Bi-O polyhedral and Bi-O-Bi stretching vibration [61]. The absorption peaks at 840 cm−1 correspond to the monoclinic phase of the Bi2O3 structure [62]. The weak absorption band located at 1400 cm−1 can be assigned to small interlayer nitrate groups. The synthesized REF and REL Bi2O3 NPs in deionized water were analyzed for their size and zeta potential using dynamic light scattering (DLS) technique. The zeta potential (mV) of the synthesized REF NPs and REL NPs was measured to be −19.25 mV and −30.5 mV; the particle size distributions of the NPs are 189.46 nm and 609.43 nm. The synthesized Bi2O3 NPs showed excellent colloidal stability as evidenced by their narrow size distribution and high zeta potential value at room temperature.

Thermal analysis measurement of the dried precursor gel was carried using TGA and DTA. Figure 3a and b show the TGA and DTA curves of REF and REL dried precursor gel where the analysis was done in the temperature range from room temperature to 600 °C in the nitrogen atmosphere. It can be observed that the TGA curves show the weight loss in three regions at temperature around 200 °C, 310 °C, and 450 °C. The first weight loss is due to removal of water molecules content trapped inside the gel, and second major weight loss is due to the decomposition of citric acid, nitrate salts, and organic compounds present in the gel [63]. The exothermic peak can be observed at around 310 °C in DTA curve and is present due to the initiation of crystallization process which suggests that formation of Bi2O3 NPs initiates at this temperature. The final weight loss occurred due to phase change from unstable β-Bi2O3 to α-Bi2O3 and removal of residual impurities [64, 65].

Fig. 3
figure 3

TGA-DTA of a REF NPs and b REL NPs

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3.2 SEM analysis

Figure 4a and b reveal the SEM micrographs of REF-NPs and REL-NPs with their EDX pattern. Figure 4a shows the self-assembly ultrathin nano-sheets to form a microsphere structure. In Fig. 4b, the REL-NPs assemble to form irregularly shaped diffused nanostructures with quasi-spherical nanoparticles.

Fig. 4
figure 4

a SEM with EDX pattern for REF-NPs. b SEM image with EDX pattern for REL-NPs

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3.3 TEM analysis

Figures 5a–d and 6a–d reveal the TEM micrographs, particle size distribution, and SAED patterns of REF-NPs and REL-NPs. Figures 5a and 6a show the self-assembly of smaller nanoparticles for bulk structures in REF-NPs and REL-NPs. The average particle size was determined using ImageJ software and plotting particle size distribution graph, which reveals the average particle size of 8.39±0.15 nm and 7.65±0.07 nm for REF-NPs and REL-NPs. In Fig. 5c and 6c, the inter-planar distance and associated (hkl) values were determined using the FFT function of the ImageJ software. The SAED patterns associated with REF-NPs and REL-NPs are given in Figs. 5d and 6d, which reveals that REF-NPs have high crystallinity due to sharp and bright points. At the same time, REL-NPs show scattered bright spots forming circular rings revealing polycrystalline nature.

Fig. 5
figure 5

a TEM micrograph, b particle size distribution, c inter-planar distance, and d SAED pattern of REF-NPs

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Fig. 6
figure 6

a TEM micrograph, b particle size distribution, c inter-planar distance, and d SAED pattern of REL-NPs

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3.4 XPS analysis

XPS measurements were performed to determine the elemental composition and binding states for the α-Bi2O3 NPs. Figure 7a and b show the Bi 4f core-shell spectra comprising two peaks, Bi 4f7/2 and Bi 4f5/2 [66]. The decovulation of these two peaks in Fig. 7a reveals the presence of Bi3+ and Bi2+ species in the REF-NPs. The peaks at 158.44 eV and 163.74 eV are associated with Bi3+ oxidation states, while peaks at 157.05 eV and 162.25 eV correspond to Bi2+ species [67]. It can be observed that Bi3+ species are dominant in REF NPs with a slight presence of Bi2+ species. Figure 7b reveals that the REL NPs only comprise Bi3+ species with associated peaks at 158.2 eV and 163.55 eV. Figure 7c and d depict the O1s spectra of REF-NPs and REL-NPs, which comprise three contributing peaks named as OL, OV, and OC, corresponding to oxygen species associated with lattice of α-Bi2O3, oxygen vacancies, and chemisorbed oxygen species [68]. It can be observed that there is a higher percentage of the chemisorbed oxygen species associated with the REF-NPs, which in turn contributes towards enhanced photocatalytic activity and improved antimicrobial properties by providing active sites for these processes [69].

Fig. 7
figure 7

Bi 4f spectra of a REF-NPs and b REL-NPs and O1s spectra c REF-NPs and d REL-NPs

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3.5 UV-visible spectroscopy

The optical properties of REF and REL α-Bi2O3 NPs were determined using UV-visible spectroscopy. Figure 8a shows the absorbance peak at 396 nm for REF-NPs and REL-NPs in the 200–800 nm range. The direct energy optical band gap (Eg) for REF-NPs and REL-NPs was determined using the Tau’s relation (Eq. 4) [70]:

$$\kern7.75em {\left(\upalpha \textrm{h}\upnu \right)}^2=\textrm{A}\left(\textrm{h}\upnu \hbox{--} \textrm{Eg}\right)$$
(4)

where α is the absorption coefficient and hν corresponds to the photon energy. Hence, the plot between (αhν)2 and (hν) gives the energy band gap (Eg). Figure 8b reveals the direct energy band gap of 2.8 eV for both the REF-NPs and REL-NPs, which match well with the previously reported values of Eg for α-Bi2O3 NPs [62, 71].

Fig. 8
figure 8

a UV-visible absorbance spectra. b Direct energy bandgap (Eg) of REF-NPs and REL-NPs

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3.6 Photocatalytic studies

The photodegradation activity of biogenic samples was scrutinized by opting for a toxic dye, viz., Congo red. The dye concentration was found to drop steadily with time for both synthesized REF-NPs and REL-NPs. After 110 min of visible light irradiation, only about 5% of the dye was found degraded without photocatalysts, exemplifying that Congo red is a highly stable dye. The absorbance of the dye solution was studied after the solution was taken from the reactor at a time interval after visible light illumination. Figure 9a and b depict the absorption spectra for both samples, such as (a) REF-NPs and (b) REL-NPs, clearly indicating the enhanced photocatalytic activity of these nano-photocatalytic semiconductors. The photocatalytic activity of all synthesized samples was estimated in the order of REF-NPs < REL-NPs with photo-degradation efficiency of 84.22% and 89.24%, respectively, for Congo red (Fig. 9c, d). Photo-degradation experiments revealed that the fabricated α-Bi2O3 owns the enhanced degradation performance for toxic Congo red dye. First pseudo order kinetics of degradation results were estimated using Eq. (5), Langmuir Hinshelwood model:

$$-\mathrm{Ln}(\mathrm C/{\mathrm C}_0)\;=\;\mathrm{kt}$$
(5)

where C represents the initial and C0 displays the final concentration of dye, respectively, at t (min), and k is the rate constant value for the degradation of Congo red dye.

Fig. 9
figure 9

Wavelength vs. absorbance graph for a REF-NPs and b REL-NPs, photocatalytic degradation of Congo red dye using sample c REF-NPs and d REL-NPs under visible light using fabricated biogenic semiconductor photocatalysts. [Reaction conditions (RC): (dye) = 1 × 10–5 mol.dm–3; (fabricated sample) = 50 mg/100 mL; pH = 7 and intensity of light = 750 lx]

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The rate order obtained was estimated as 0.0010 min−1 for the REF-NPs sample (Fig. 10a) and 0.0014 min−1 for the REL-NPs sample (Fig. 10b) for the photocatalytic degradation of Congo red dye. Comparing both samples, REL-NPs exhibited the highest rate, which was found to be 1.4 times higher than the REF-NPs sample (Fig. 10c). COD and CO2 removal experiments were carried throughout photocatalytic examination of toxic dye for both samples. As represented in Fig. 10d, REF-NPs removed 93.11% of COD, and the REL-NPs sample eliminated 99.55% of COD for photo-catalytic removal of Congo red dye. Under certain reaction parameters, the complete degradation of Congo red dye has significantly aided COD removal tests. The COD elimination results were supplemented by CO2 approximation experiments (Fig. 10a). CO2 production during dye degradation recommends that the degradation of Congo red dye emits CO2 and H2O as byproducts. The mineralization ability of the REL-NPs sample was found to be higher than that of the REF-NPs sample for photodegradation of Congo red. Aside from photocatalytic efficacy, the reusability and stability of photocatalysts are important considerations in practical research. To test the re-usability of fabricated plant extract-based samples, we ran a photocatalytic process continuously recycled for photo-degradation of Congo red. After 5 cycles, there was no visible decrease in dye degradation. Thus, both fabricated samples, viz., REF-NPs and REL-NPs were discovered to be highly stable (Fig. 10b, c). According to the results of re-cyclic investigations, the fabricated REL-NPs photocatalyst sample exhibits extraordinarily high stability. We performed a control experiment with no radical scavengers to exclude the possibility of dye degradation by radical scavengers. For superoxide radicals (∙O2−), hydroxyl radicals (∙OH), and holes (h+), respectively, P-benzoquinone (BQ), Tert-butyl alcohol (TBA), and sodium ethylene diamine tetraacetic (Na2-EDTA) were employed. Figure 11 illustrates scavenging tests for Congo red photo-degradation using REF-NP (Fig. 11d) and REL-NP samples (Fig. 11e). The photo-degradation percentages of both samples were reduced in the presence of BQ, TBA, and Na2-EDTA, with 52.1%, 78.4%, and 28.4% using REF-NP samples and 56.3%, 81.9%, and 32.4% using REL-NPs sample, respectively. These data revealed that superoxide and hydroxyl radicals were the predominant active species in the photo-degradation of Congo red hazardous dyes in 110 min utilizing α-Bi2O3 under visible light.

Fig. 10
figure 10

First-order kinetics fitting data for photocatalytic degradation a REF-NPs, b REL-NPs, c rate constant value (k) for photocatalytic degradation for both samples, and d COD elimination using both samples. [Reaction conditions (RC): (dye) = 1 × 10–5 mol.dm–3; (fabricated sample) = 50 mg/100 mL; pH = 7 and intensity of light = 750 lx]

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Fig. 11
figure 11

a CO2 approximation, reusability tests b REF-NPs, c REL-NPs and scavenging experiments d REF-NPs, e REL-NPs. [Reaction conditions (RC): (dye) = 1 × 10–5 mol.dm–3; (fabricated sample) = 50 mg/100 mL; pH = 7 and intensity of light = 750 lx]

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3.7 Photocatalytic mechanism

Mulliken electronegativity theory [72] was used to determine potential band positions in order to explore the photocatalytic degradation mechanism (Eqs. (6) and (7)):

$$E(VB)=X- Ee+0.5 Eg$$
(6)
$$E(CB)=E(VB)- Eg$$
(7)

where Ee represents electron-free energy on the hydrogen scale (+4.50 eV) and Eg defines the band gap energy of the semiconductor photocatalyst. The estimated band gap of plant extracted α-Bi2O3 was found to be 2.8 eV for both REF-NP and REL-NP samples. The band potentials were estimated as −0.34 eV for ECB and +3.14 eV for EVB. The photoinduced charge-transfer mechanism is presented in Fig. 12 based on the aforementioned optical characterization, estimated energy band positions, and ROS trapping investigations, with matched band positions of synthesized α-Bi2O3. Under visible light, photocatalysts get stimulated, and electron-hole pairs are photogenerated; the superoxide generation will take place on the conduction band of the Bi2O3 as the ECB position of Bi2O3 is enough for photo reduction O2/O2−∙. The more negative reduction potential ECB of Bi2O3 (−0.34 eV) than the reduction potential O2/O2−∙ (−0.33 eV vs. NHE) [72], the electrons at ECB of Bi2O3 competently reduce O2 into O2−∙ radicals. Then, photo-induced superoxide (O2−∙) radicals oxidized the dye molecules to H2O and CO2. Furthermore, the EVB potential of Bi2O3 is directly greater than the conventional redox potential of H2O/∙OH (+2.5 eV vs. NHE) [73]. As a result, the holes are capable of photoreduction of H2O radicals to ∙OH radicals. Scavenging studies, on the other hand, demonstrated that O2−∙ and ∙OH were active radical species throughout the photo-degradation process. Conversely, the EVB potential of Bi2O3 (+3.14 eV) is more favorable than the normal redox potential of H2O/O2 (+0.82 eV vs. NHE); thus, holes (h+) in Bi2O3 EVB immediately react with H2O molecules to generate O2. Thus, photo-generated molecular oxygen-induced O2−∙ and then ∙OH contribute to enhanced photodegradation performance which was found consistent with the findings of scavenging experiments.

Fig. 12
figure 12

Proposed possible photocatalytic mechanism using semiconductor under visible light

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4 Phytochemical screening of plant extracts

The results showed the presence of phenols, tannins, terpenoids, flavonoids, saponins, and glycosides in the methanol extract of leaves and fruits of R. ellipticus (Table 2). Similar to the present study, Saklani et al. (2012) and Shibu Prasanth and Chandran (2017) also reported the presence of tannins, phenols, saponins, flavonoids, glycosides, terpenes, and proteins in the methanol extract of R. ellipticus leaves and fruits [74, 75]. The phytochemical constituents in R. ellipticus plants act as stabilizing agents for synthesizing bismuth oxide nanoparticles. The plant extracts act as a reducing and capping agent that reduces particle size and increases the antimicrobial activity of nanoparticles [76, 77].

Table 2 Preliminary phytochemical screening of methanol extract of R. ellipticus
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4.1 Antioxidant activity

The antioxidant activity of REL-NPs and REF-NPs has presented in Fig. 13. The results indicated that as the concentration of REL-NPs, REF-NPs, and ascorbic acid (AA) increased (25–100 μg/mL), the percentage inhibition was also increased Fig. 13. At a concentration of 100 μg/mL, the maximum percentage inhibition was found to be 48.28±0.34% with REF-NPs, while the minimum inhibition was found to be 43.13±0.40% with REL-NPs. The IC50 value of REL-NPs, REF-NPs, and AA were observed as 130.26 μg/mL, 113.20 μg/mL, and 24.32 μg/mL, respectively. The lower IC50 value observed during the present study indicated the higher antioxidant activity of nanoparticles. The REF-NPs showed higher antioxidant potential as compared to REL-NPs. A previous study also discussed the antioxidant activity of bismuth oxide nanoparticles from the Delftia species with an IC50 value of 307 μg/mL [78]. Similarly, Das et al. also reported that the metallic bismuth nanoparticles were prepared from leaves extracts of Moringa oleifera and showed good antioxidant activity [79].

Fig. 13
figure 13

Antioxidant activity of nanoparticles

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4.2 Antimicrobial assay

4.2.1 Antibacterial activity

REL-NPs and REF-NPs produced from the aqueous extract of R. ellipticus were evaluated against Gram-positive (S. aureus and B. subtilis) and Gram-negative (E. coli, K. pneumoniae, and P. aeruginosa) bacteria for their antibacterial activity. The results of the disc diffusion assay and MIC assay are presented in Figs. 14 and 15. The nanoparticles showed higher antibacterial activity against Gram-positive bacteria than Gram-negative bacteria. The REL-NPs showed maximum zone of inhibition and MIC value against S. aureus (16.66±1.0 mm; 250 μg/mL) and B. subtilis (16±1.0 mm; 250 μg/mL) followed by E. coli (13±1.0 mm; 500 μg/mL), P. aeruginosa (12.6±0.5 mm; 1000 μg/mL), and K. pneumonia (10.6±0.5 mm; 1000 μg/mL). The REF-NPs revealed the maximum antibacterial activity against S. aureus and B. subtilis (125 μg/mL) than the remaining strains of bacteria (500 μg/mL). In comparison to both nanoparticles, REF-NPs showed higher antibacterial potential than REL-NPs. The antibacterial activity varies with the concentration, surface area, morphology, and crystalline nature of the nanoparticles [80]. The present study results were similar to the results of Jassim et al. and Motakef-Kazemi and Yaqoubi et al. who investigated the antibacterial activity of bismuth oxide nanoparticles against E. coli, P. aeruginosa, S. aureus, and Salmonella typhi [53, 81]. Similarly, Das et al. also reported the good antibacterial potential of Moringa oleifera leaves extract-mediated metallic bismuth nanoparticles against Gram-positive and Gram-negative bacteria. The difference between Gram-positive and Gram-negative bacteria may be due to the cell wall structure, physiology, and cell surface receptors [79].

Fig. 14
figure 14

Petri plats showing zone of inhibition of REL-NPs and REF-NPs against E. coli (A), P. aeruginosa (B), K. pneumoniae (C), B. subtilis (D), S. aureus (E). Here, a REL-NPs (2 mg/mL), b REF-NPs (2 mg/mL), c positive control (Amp: 50 μg/mL), d negative control (DMSO 10 μL)

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Fig. 15
figure 15

Antibacterial activity of REL-NPs and REF-NPs against Gram-positive and Gram-negative bacteria

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4.2.2 Antifungal activity

Using the food poison approach and the minimum inhibitory concentration (MIC) method, the antifungal activity of REL-NPs and REF-NPs was assessed for two fungal strains (F. oxysporum and R. necatrix), and the findings are shown in Figs. 16 and 17. The REL-NPs showed the maximum inhibition of R. nectatrix (58.0±0.6%; 250 μg/mL) and then F. oxysporum (52.2±1.0%; 500 μg/mL). Similarly, REF-NPs also showed higher antifungal activity against R. nectatrix (63.0±0.6%; 125 μg/mL) than F. oxysporum (56.1±0.7%; 250 μg/mL). Among both nanoparticles, REF-NPs had higher antifungal potential than RFL-NPs, as revealed with lower MIC values. The lower MIC value indicated higher antifungal potential, as in Table 3. In the previous study, the bismuth oxide shows good antifungal activity against Candida species [82, 83]. Oxygen molecules present on the surface of nanoparticles could be responsible for killing microorganisms [84].

Fig. 16
figure 16

Petriplates showing the percentage inhibition of REL-NPs and REF-NPs against F. oxysporum and R. necatrix [A R. necatrix and F. oxysporum control (without extract and nanoparticles), B REL-NPs (2 mg/mL), C REF-NPs (2 mg/mL), and D positive control (Hyg: 0.1 mg/mL)

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Fig. 17
figure 17

Antifungal activity of REL-NPs and REF-NPs against fungi

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Table 3 MIC of REL-NPs and REF-NPs against bacterial and fungal strains
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The antimicrobial mechanism of inorganic compounds has yet to be understood, and their specific action method against bacteria and fungi still needs to be fully elucidated (Fig. 18). Nanoparticles release metal ions, allowing electrostatic contact between negatively charged cell membranes and positively charged nanoparticles. The production of ROS (reactive oxygen species) plays a crucial role in antimicrobial activity. The Singlet oxygen, OH radicals, and H2O2 all play significant roles in the mechanism, including alteration of enzymes and proteins, DNA damage, and causes membrane disruption [85].

Fig. 18
figure 18

Probable mechanism of antimicrobial action using nanoparticles

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5 Conclusion

The current study demonstrates the green synthesis of α-Bi2O3 NPs utilizing leaves and fruit extracts of R. ellipticus. The crystal structure, shape, and optical properties of the α-Bi2O3 NPs were also comprehensively investigated. REF-NPs and REL-NPs were tested for the visible light photocatalytic degradation of the hazardous Congo red dye. The photocatalytic performance of REF-NPs and REL-NPs was found with photodegradation efficiency of 84.2% for REF-NPs and 89.2% for REL-NPs. Compared to the REF-NPs sample, REL-NPs had the highest rate, which was found to be 1.4 times higher. Photodegradation studies demonstrated that the plant extract-based green Bi2O3 had improved Congo red dye degradation ability. Compared with REL-NPs, REF-NPs displayed higher antioxidant activity, which could be attributed to the higher ascorbic content in the fruits of R. ellipticus than in the leaves. Additionally, REL-NPs and REF-NPs displayed higher antimicrobial potential against Gram-positive bacteria than Gram-negative bacteria, possibly due to more phytocompounds. The current study presents a straightforward green technique for fabricating innovative α-Bi2O3 as a spectacular nanomaterial for aquatic bodies to break down dangerous contaminants via visible light photo-degradation and an antibacterial agent. This study would also motivate others to use bismuth-based nanomaterials and a practical strategy to building-wide band gap bismuth-based photocatalysts for efficient visible light photo-degradation in environmental remediation as well as biomedical applications.