Abstract
Bacteria develop resistance against antimicrobial drugs, and new remediations are constantly being introduced in the market. Silver and its compounds have strong resistance against different bacteria. The vegetable waste extract-synthesized silver nanoparticles (VWE-AgNPs) have distinct properties and potential applications because of their unique size and morphology. The fundamental purpose of this study was to develop an environment-friendly method for the synthesis of VWE-AgNPs to avoid the use of hazardous chemicals that cause danger to the environment as well as recycling vegetable waste material. The VWE-AgNPs were synthesized by mixing 1 mM AgNO3 solution and VWE at room temperature. The VWE-AgNPs were characterized by UV-visible spectroscopy, FTIR, SEM, and EDX. The synthesized particles showed good antibacterial properties against poultry bacteria Salmonella gallinarum and Salmonella enteritidis (growth reduction of 31 mm and 18 mm at 80 mg/mL AgNPs, respectively). The results demonstrated that VWE-AgNPs inhibited the growth of tested bacterial strains. The fabricated VWE-AgNPs also had the potential to act as a green photocatalyst for degradation of 87.7% of methylene blue (MB) and 90.76% of methyl orange (MO) nearly at 3.25 h and 1 h sunlight exposure time, respectively. The highest antifungal activity, which was determined to be 36.5 mm and 31.8 mm against Alternata sp. and C. albican, was discovered to be in VWE-AgNPs.
1. Introduction
Nanotechnology has gained significant attention due to the fabrication and application of nanoparticles (NPs) for various uses [1, 2]. NPs possess novel properties and have multidimensional applications in different fields such as cosmetics, optoelectronics, medicine, biomedical devices, and environmental remediation [3]. Among different metal nanoparticles (MNPs), silver nanoparticles (AgNPs) are considered more promising due to their unique physical, chemical, and biological properties [4, 5]. Moreover, AgNPs have the essential quality of bactericidal effects against many strains [6]. Different methods have been used for the fabrication of AgNPs including UV irradiation [7], gamma and microwave irradiation [8, 9], chemical and photochemical reduction [10, 11], and sonoelectrochemical [12]. All these methods produce hazardous by-products, required high power consumption, difficulty in purification, multistep processes, low yield, and high cost. Therefore, the need to develop a green way of synthesis with good reproducibility and high yield is the demand of time [13, 14].
Green chemistry is helping in reducing the use of chemicals that are hazardous to the environment [15]. So, different natural resources such as plants, fruits, vegetables, enzymes [16], microorganisms, and marine algae [17] have been used for the synthesis of AgNPs. The green routes have great importance due to their low cost, eco-friendly nature, and easy handling because they do not need high temperatures, pressure, energy, and toxic chemicals [18, 19]. Recent studies explore that AgNPs fabricated using different sources have potential antibacterial, antifungal, antiinflammatory, and antioxidant activities. Industrial effluents contain toxic organic dyes and cause water contamination. The contaminated water directly affects the life cycle of aquatic species. Therefore, to save the aquatic species, toxic organic dyes should be removed from industrial effluents. The available physical and chemical methods for the fabrication of MNPs have a serious environmental issue due to the use of expensive and hazardous chemicals. The aqueous extracts of bottle gourd peel [20] and fresh vegetable waste [21] for the biosynthesis of AgNPs had already been reported.
Synthetic organic dyes are extensively used in the paper, food, cosmetic, textile, and leather industries. They make water resources harmful to aquatic species. The elimination of these toxic organic dyes from industrial effluents is a crucial issue for the environment. Different techniques such as flocculation, activated carbon sorption, coagulation, redox treatment, and UV-light degradation are routinely used for the purification of water [22]. Since these techniques do not fully support the breakdown of organic dyes, there is room to design an efficient substitute method. The most recent research MNPs reported as nanocatalysts have potential photocatalytic applications. Of late, the special focus on AgNPs and AuNPs as a photocatalyst in industrial dye degradation is due to their distinctive optical, optoelectronic, and photothermal properties brought about by their localized surface plasmon resonance (LSPR) [23–25]. AuNPs, AgNPs, and PdNPs as a nanocatalyst can also reduce the aromatic pollutants (p-nitroaniline and methyl red) [26].
The current study is focused on the green synthesis of AgNPs by recycling the huge vegetable waste generated in the kitchen. To the best of our knowledge, the study on extracts of kitchen waste of Solanum melongena, Cucurbita moschata, Phaseolus vulgaris, and Brassica oleracea for the fabrication of AgNPs has not been reported. The antibacterial activities of fabricated VWE-AgNPs were analyzed on Salmonella gallinarum and Salmonella enteritidis. The novelty of the current study is the utilization of an aqueous extract of kitchen waste for the preparation of VWE-AgNPs and their antibacterial efficacy against poultry pathogens. The photocatalytic activity of the resulting VWE-AgNPs was also evaluated against MB and MO dyes.
2. Experimental
The chemicals required for this research were purchased from Sigma–Aldrich and distilled water was obtained from the university laboratory. The VWE-AgNPs synthesis experiment was performed in the laboratory of the University of Management and Technology (UMT) in Lahore, Pakistan.
2.1. Vegetable Waste Collection and Extract Preparation
The vegetable waste material was collected from the kitchen which was a mixture of brinjal (Solanum melongena), pumpkins (Cucurbita moschata), beans (Phaseolus vulgaris), and cauliflower (Brassica oleracea). The collected waste was first washed with tap water. After that, it was strategically washed with distilled water and sanitized with a sodium hypochlorite solution. The mixed vegetable waste (50 g) was dried and crushed into powder. The powdered waste (10 g) was added to 100 mL of deionized water and boiled for 20 min at 95°C. The resulting extract was filtered, centrifuged for 5 min at 4000 rpm, and saved for further experimental work.
2.2. Biosynthesis of VWE-AgNPs and Characterization of AgNPs
AgNPs were fabricated by using VWE. 10 mL of VWE was added dropwise into 20 mL of 1 mM AgNO3 solution. It was allowed to react for 45 min in sunlight. The color change of the solution from yellow to black-brown was observed due to Ag+ ion reduction with phytochemicals present in the aqueous VWE. During the synthesis, phytochemicals in VWE act as both reducing as well as stabilizing agents of VWE-AgNPs [27, 28]. The dispersed VWE-AgNPs were centrifuged at 4000 rpm for 10 min and washed 5 times with distilled water and ethanol to remove soluble impurities. The VWE-AgNPs pellets were ground into fine powdered, calcined at 600°C, and stored in dry bottles for further analysis.
2.3. Antibacterial Studies with VWE-AgNPs
The biosynthesized VWE-AgNPs were tested against two poultry pathogens: Salmonella gallinarum and Salmonella enteritidis which were obtained from the Department of Microbiology, University of Veterinary and Animal Sciences Lahore, Pakistan. The inoculum was prepared by the colony suspension method in 10 mL of normal saline solution. To assess the antibacterial activity of VWE-AgNPs against these pathogens, a sterile cotton swab was immersed in a freshly prepared culture of cell density (105-106 CFU/mL) on a nutrient agar plate in 3 to 4 layers and allowed to grow overnight in sterile conditions. Wells of 5 mm diameter and 4 mm depth were made using a sterile borer [29, 30]. The VWE-AgNPs suspension solutions of different concentrations 20 mg/mL, 40 mg/mL, 60 mg/mL, and 80 mg/mL were dispensed into respective wells. The plates were incubated at optimum temperature (37°C) and the zone of inhibition was measured after 24 h.
2.4. Photocatalytic Activity of VWE-AgNPs and Organic Dyes Degradation
The photocatalytic ability of VWE-AgNPs was evaluated by methylene blue (MB) and methyl orange (MO) dye degradation. The stock solutions of MB and MO dyes were prepared using 5 mg of each in 1000 mL of double-distilled water. The experiment was performed by adding 5 mg of VWE-AgNPs to 100 mL of MB and MO dye solution. Initially, before exposure to sunlight, both solutions were well homogenized by a magnetic stirrer for 30 min to make equilibrium. Afterward, we put them under sunlight and monitor them. After 15 min, 5 mL of each solution was filtered and used for the evaluation of photocatalytic degradation. The UV-Vis study was done to measure the absorbance after 15 min until the solution became completely colorless. The degraded concentration of MB and MO was calculated by the absorbance value at 662 nm and 491 nm, respectively. To check the reusability of VWE-AgNPs as the green catalyst, MB solution (after 3 h) was centrifuged and particles obtained were washed 5 times with double-distilled water and ethanol which were then reused for degradation. The percentage of dye degradation was calculated by the following formula:where A0 and At are absorbance at initial and at time t, respectively.
3. Results and Discussion
3.1. UV-Visible Spectroscopy
The presence and reduction of silver ions in the solution were monitored and characterized by absorption measurement of the solution using a UV-VIS spectrometer (Shimadzu, UV-1240, Kyoto, Japan). The distilled water was used as a blank. The four samples of VWE-AgNPs were scanned in the 250–650 nm range (Figure 1). The maximum absorbance peak was observed at 400 nm which confirmed the existence of AgNPs as reported in the literature [31, 32].

3.2. FTIR Analysis of VWE-AgNPs
The FTIR analysis provided information of functional groups which were involved in the capping and stabilization of AgNPs [33]. The spectrum of VWE-AgNPs (Figure 2) showed a peak at 3580 cm−1 which corresponds to OH stretching vibration. The peak found at 1584 cm−1 is due to N-H deformation whereas bending vibrations for the CH bond were observed at 1287 cm−1. The green synthesized AgNPs are reported to be capped by different phytochemicals including proteins that have the strong ability to bind with metal to prevent agglomeration and thus provide stability in aqueous solution in addition to their role in the formation of VWE-AgNPs [34].

3.3. SEM and EDX Analysis
The structural, as well as morphological studies of prepared VWE-AgNPs were carried out by scanning electron microscope (SEM). SEM images of VWE-AgNPs show spherical and irregular structures with a diameter in the range of 10–100 nm (Figure 3). Past studies reported that AgNPs prepared from different plant extracts have different structures. They may be spherical, triangular, or hexagonal depending on the chemical composition of plant extracts and synthesis conditions [35–37]. The result of previous research has shown a characteristic absorption peak of silver at 3 keV with different weight percentages indicating the AgNPs [38, 39].

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EDX spectrum of tested material (Figure 4) depicts a significant peak of silver (69.6%). The EDX of the tested material also depicted the presence of different elements such as 5.57% C, 15.96% O, 2.73% Na, 1.07% Mg, and 5.06% Cl. The presence of Ag and O elements confirmed the fabrication of AgNPs. Sodium and magnesium, which are present in the extract, play important role in the neutralization of organic anions. The use of VWE is one of the most important benefits to prepare AgNPs instead of using chemicals.

3.4. Antibacterial and Antifungal Properties Analyses
Salmonella gallinarum and Salmonella enteritidis (poultry pathogens) strains were used to investigate the antibacterial study of different concentrations of fabricated VWE-AgNPs. The antibacterial results of VWE-AgNPs are shown in Figure 5. Both poultry pathogens species gave positive responses towards all concentrations of VWE-AgNPs studied. VWE-AgNPs showed the highest zone of inhibition (18 mm) against Salmonella enteritidis at 80 mg/mL concentration and Salmonella gallinarum (31 mm) at 80 mg/mL. It was observed that the zone of inhibition slightly increased with an increase in concentration and hence the antibacterial activity of VWE-AgNPs against both pathogens species is dependent on the precursor concentration used. The fabricated VWE-AgNPs were active against both poultry pathogen species with a slightly better activity toward Salmonella gallinarum. The antibacterial activity depends on the quantity and rate of silver released by the sample. The possible hypothetical mechanism to denature the pathogens includes VWE-AgNPs providing the large surface area, release of Ag ions, and attachment with biomolecules containing phosphorus (DNA and RNA), sulfur, or nitrogen (protein) on bacterial cell wall causing the high oxidative stress and prevent the replication process and cause cell death [40]. The antibacterial activities of AgNPs are reported against a wide range of bacterial pathogens [41–43] but have not been reported against poultry pathogens to the best of our knowledge. The comparison of antibacterial species is added in Table 1. On the other hand, according to the antifungal activity results, the zone inhibition values of VWE-AgNPs for Alternata sp. and C. albicans are 36.5 mm and 31.8 mm, respectively (Table 2).

3.5. Photocatalytic Degradation Study of Methylene Dye
The photocatalytic degradation of the aqueous solution of MB was carried out using a green catalyst (VWE-AgNPs) in sunlight exposure. The dye degradation process was monitored by visual observation and UV-visible spectroscopy. The UV-Vis spectrum of pure MB stock solution (5 mg/L) showed maximum absorbance (λmax) at 662 nm which is in the visible region. Initially, the degradation process was slow; however, the absorption peak of MB decreased gradually with an increase in radiation exposure time which indicates the degradation efficiency of a green-added catalytic agent. The absorbance intensity of MB at 662 nm decreased while that of AgNPsat 420 nm increased. The completion of MB photocatalytic degradation was recognized after a constant decrease in absorbance when absorbance at 662 nm reached zero after 3 h 15 min of radiation exposure time. With a constant decrease in dye concentration, the UV spectra showed a characteristic surface plasmon resonance (SPR) band for VWE-AgNPs at 420 nm (Figure 6). The reported MB dye degradation is 84% in 16 h and 92% and 95% in 72 h of radiation exposure [48–51]. The percentage photocatalytic degradation efficiency of fabricated VWE-AgNPs was increased with radiation exposure time (Figure 7) and was calculated to be 87.7% at 3 hr 15 min (Table 3).


3.6. Photocatalytic Degradation Study of Methyl Orange Dye
The UV-visible spectrum of blank MO (blank) showed strong absorption intensity at 491 nm due to the presence of the azo group. However, the addition of a green catalytic agent (1 mM VWE-AgNPs) in 100 mL of MO significantly accelerated the degradation of the reaction mixture as observed in UV-vis spectra. The visual observation and decrease in the absorption intensity peak at 491 nm confirmed the degradation of dye (Figure 8). The suspension in the presence of a green catalytic agent was examined spectrophotometrically after different time intervals (Figure 9). The dye degradation rate increased with exposure time mainly due to VWE-AgNPsSPR phenomena in azo dyes. After 1 h of radiation exposure, 90.76% dye degradation occurred as shown in Table 4. The AgNPs are highly efficient; green catalytic agents and a decrease in MO intensity had been reported within 4.3 h and 10 h under sunlight[52, 53].


3.7. Comparison of Dye Degradation with Reported Data
The MB and MO dye degradation comparison with reported research of AgNPs and the current study is shown in Table 5. The current study on dye degradation percentage indicates that the synthesized VWE-AgNPs have efficient photocatalytic activity for industrial dye degradation.
4. Conclusion
Several reports suggested methods for the utilization of fruit and plant extracts for the fabrication of nanoparticles as nonconventional, nontoxic, and economic procedures as compared to traditional physical and chemical techniques. A convenient, eco-friendly, nontoxic, and cost-effective green technique was developed in this study by using vegetable waste extract (VWE) as a reducing agent for the fabrication of VWE-AgNPs. The characterization of fabricated AgNPs was carried out by UV, FTIR, SEM, and EDX. The phytochemicals responsible for reducing, capping, and stabilization of AgNPs were present in VWE as confirmed by FTIR. The fabricated VWE-AgNPs were spherical with a size of 10–100 nm. The elemental analysis performed by EDX showed Ag (69.6%) and O (15.96%) as the main components of prepared AgNPs with small amounts of C (5.57%), Na (2.73%), Mg (1.07%), and Cl (5.06%). The biosynthesized VWE-AgNPs proved to be an excellent antibacterial agent against poultry pathogens and were found to have higher inhibition against Salmonella gallinarum as compared to Salmonella enteritidis. The degradation results showed that VWE-AgNPs can be used as potent catalysts for the photocatalytic degradation of MB and MO dyes for the treatment of industrial wastewater.
Data Availability
The datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This research work has been financially supported by the HEC Pakistan under the NRPU project 20-15745/NRPU/R&D/HEC/2021. The researchers would like to acknowledge the Deanship of Scientific Research, Taif University, for funding this work. This research was funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2023R316), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia. The authors extend their appreciation to the Research Center at AlMaarefa University for funding this work.