Exploring the Dipteryx alata fruit endocarp as a novel source for the synthesis of silver nanoparticles: antibacterial and photocatalytic insights for water purification Open Access

Nanotechnology is a rapidly advancing field focused on manipulating and utilizing materials at the nanoscale. Within this domain, metallic nanoparticles (MNPs) have garnered significant attention due to their unique properties, such as a high surface area-to-volume ratio, enhanced reactivity, and distinct optical and magnetic characteristics (Manikandan et al. 2021; Azeez et al. 2023). These attributes make MNPs highly desirable for various applications across electronics, medicine, materials science, energy, and environmental remediation (Singh et al. 2019; Nguyen et al. 2020; Abbas & Amin 2022; Hossain et al. 2022).

Silver, gold, platinum, iron, and copper are among the commonly used metals for nanoparticle synthesis, owing to their inherent properties and chemical stability. Notably, silver nanoparticles (AgNPs) have been a focal point of extensive research in recent years. AgNPs exhibit distinct optical characteristics attributed to surface plasmon resonance, along with noteworthy antimicrobial, catalytic, and electronic properties, making them highly sought after in various fields (Rafique et al. 2017; Abdelghany et al. 2018). In the medical field, AgNPs find applications in drug delivery systems, wound healing, and antibacterial coatings (Mousavi et al. 2018; Tarannum et al. 2019; Huq et al. 2022). They also play a crucial role in biosensors designed to detect biological molecules and environmental pollutants (Alex et al. 2020). AgNPs exhibit promising potential in combating multidrug-resistant bacteria, showcasing their value in antimicrobial therapy (Lakkim et al. 2020). Additionally, AgNPs find applications in water purification, serving as catalysts and acting as adsorbents for both organic and inorganic compounds (Dixit et al. 2018; Singh et al. 2019; Rohaizad et al. 2020).

Traditionally, MNPs are synthesized using chemical and physical methods, such as the reduction of metallic salts, thermal decomposition, co-precipitation, radiation-assisted methods, and pyrolysis (Ishak et al. 2019). However, these methods have drawbacks, including high energy requirements and the use of hazardous chemicals that can be harmful to human health and the environment (Rolim et al. 2019; Hawar et al. 2022). In contrast, biogenic methods for synthesizing MNPs have gained popularity due to their simplicity, cost-effectiveness, and the absence of toxic compounds. Furthermore, these methods can be conducted safely at room temperature and pressure (Sasikala et al. 2015; Rafique et al. 2017).

Various biological systems have been utilized for MNP synthesis, including plants, bacteria, fungi, and algae (Ishak et al. 2019; Saeed et al. 2020; Dadayya et al. 2023; Kingslin et al. 2023). Plant extracts are widely used due to the presence of phytocompounds such as terpenoids, flavonoids, alkaloids, saponins, and tannins, which act as reducing and stabilizing agents (Ishak et al. 2019). Recent studies have successfully employed several plant species, including Carica papaya (Jain et al. 2020), Ctenolepis garcinia (Narayanan et al. 2021), Alhagi graecorum (Hawar et al. 2022), Ligustrum vulgare (Singh & Mijakovic 2022), Citrus limon (Khane et al. 2022), and Allium cepa (Baran et al. 2023), for the synthesis of AgNPs. However, the synthesis of AgNPs utilizing the Dipteryx alata fruit endocarp (DAE) extract has not been previously explored. D. alata is a tree species native to the Brazilian Cerrado, belonging to the Fabaceae family. Its nuts, known as baru, are rich in fibers, healthy fats, vitamins, minerals, and antioxidants and are consumed in Brazil as roasted nuts or as an ingredient in typical gastronomy (Viana et al. 2023). The DAE is a woody residue discarded during the agro-industrial processing of the nuts and presents itself as a new source of phytochemicals for reducing and capping nanoparticles.

Given the imperative for developing sustainable nanoparticle synthesis methods, this study explored the utilization of DAE extract for the first-time synthesis of AgNPs. The synthesized AgNPs were characterized using various techniques such as UV–Vis spectroscopy, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), dynamic light scattering (DLS), and X-ray diffraction (XRD). Furthermore, the antibacterial activities of AgNPs were evaluated against Escherichia coli and Enterococcus faecalis, while their photocatalytic properties were tested for the degradation of rhodamine B (RhB) in an aqueous solution.

The DAE was thoroughly washed with distilled water to remove impurities and then oven-dried at 60 °C for 48 h. It was subsequently finely shredded into particles smaller than 500 μm using a high-power grinder. To prepare the extract, a mixture of DAE and distilled water in a ratio of 1:100 (m/v) was stirred at 80 °C for 60 min (Singh et al. 2019). The resulting extract was separated through filtration and stored at 4 °C until further use.

The procedure for synthesizing biogenic AgNPs was developed based on methods described in the literature (Aisida et al. 2019; Hossain et al. 2022; Baran et al. 2023). Initially, 20 mL of the DAE extract was added to 100 mL of a silver nitrate solution (2 mM), and the pH of the mixture was adjusted to 10.0 using a dropwise addition of a NaOH solution (0.1 M). The reaction mixture was stirred in the dark for 1 h at room temperature. The volume of 20 mL of the DAE extract used for AgNP synthesis was selected as optimal after testing various volumes of DAE (1, 10, 20, and 50 mL). The development of a reddish-brown color from a light yellow mixture indicated the formation of AgNPs. The synthesized AgNPs were collected by centrifugation (12,000 rpm, 15 min), washed twice with distilled water to remove ions and residues from the DAE extract, and dried in an oven at 105 °C for 2 h.

The AgNPs were characterized using spectroscopic and imaging techniques. UV–Vis spectra were recorded from 250 to 750 nm with a resolution of 1.0 nm using a UV–Vis Hach DR 6000 spectrophotometer. FTIR analysis was conducted in the range of 500–4,000 cm⁻¹ to identify the functional groups of phytochemicals responsible for stabilizing the nanoparticles. AgNP samples were prepared as KBr discs, and the spectra were acquired using a Shimadzu IRAffinity-1 spectrometer. The morphology of the AgNPs was investigated by SEM using a Zeiss EVO-MA10 instrument at an accelerating voltage of 20 kV, and the particle size and frequency distribution were confirmed by DLS (Malvern Zetasizer Nano ZS, Malvern, UK). The crystalline nature of the nanoparticles was determined by XRD analysis on a Bruker D8 Advance diffractometer. The analysis was performed with Cu Kα radiation, operating at a voltage of 40 kV and a current of 40 mA.

The antibacterial activity of biogenic AgNPs was assessed against E. coli (ATCC 25922) and E. faecalis (ATCC 19433) using the disc diffusion method (Jain et al. 2020). Initially, bacterial strains were cultured in nutrient broth at 37 °C until reaching an optical density of 0.5 (at λ = 600 nm). Subsequently, bacterial suspensions were evenly spread on the surface of Mueller–Hinton agar plates using a sterile cotton swab. Five sterile discs were placed on each plate, and varying masses of AgNPs (10, 25, 50, and 75 μg) were applied to the discs. Discs containing ciprofloxacin (5 μg) and gentamicin (10 μg) served as positive controls. The plates were incubated at 37 °C, and after 24 h, the diameter of the inhibition zones was measured. The experiments were conducted in triplicate.

The photocatalytic activity of the AgNPs was assessed by studying the degradation of RhB under ultraviolet radiation. In the experiment, 20 mg of AgNPs was introduced into a 100 mL aqueous solution containing 25 mg L⁻¹ of RhB. The dye solution was continuously agitated on a magnetic stirrer and exposed to UV light (15 W, 254 nm). The degradation of RhB was monitored at regular intervals using UV–Vis spectrophotometry.

Cucumber seeds (Cucumis sativus, seed germination = 99%) were exposed to the degraded product of RhB to assess its toxicity, following the recommendation by the U.S. Environmental Protection Agency for toxicity testing and environmental assessment (U.S. EPA 1998). In Petri dishes (90 mm diameter), 10 seeds were positioned on filter paper moistened with 5 mL of the test solution. The plates were incubated at 22 °C for 120 h. Distilled water served as the negative control and all assays were performed in triplicate. Following the incubation period, the number of germinated seeds as well as the lengths of the radicle and hypocotyl were measured.

The FTIR analysis was conducted to evaluate the interaction between synthesized AgNPs and the phytochemicals in the extract. The absorption bands at 3,847, 3,730, and 3,643 cm⁻¹ in the FTIR spectrum (Figure 2(b)) are likely attributed to the –OH stretching vibrations of phenols and alcohols (Singh et al. 2020). The bands at 2,914 and 2,850 cm⁻¹ corresponded to C–H stretching vibrations and at 2,349 cm⁻¹ to C ≡ N stretching vibrations (Singh et al. 2019). Bands at 1,737 and 1,631 cm⁻¹ are associated with C = O stretching vibrations of aldehydes, ketones, and carboxylic acids (Sangaonkar & Pawar 2018). Furthermore, stretching vibrations of C = C and C–O at 1,502 and 1,269 cm⁻¹, respectively, were also observed (Ayinde et al. 2019). Particularly, the absorption bands at around 850–750 cm⁻¹ indicated the bonding of AgNPs with oxygen (Singh et al. 2019). Therefore, the FTIR analysis confirmed that the extract played a crucial role in reducing and capping AgNPs.

SEM was utilized to study the morphology of the AgNPs. As depicted in Figure 2(c), the nanoparticles exhibited a spherical shape with some degree of agglomeration, which was attributed to the interaction between biomolecules on the nanoparticle surface (Sasikala et al. 2015; Aisida et al. 2019).

The nanoparticle size distribution was determined from the DLS analysis (Figure 2(d)). The average particle size was 137.5 ± 59.3 nm with a polydispersity index (PDI) of 0.695. The PDI, ranging from 0 to 1, indicates the size uniformity of nanoparticles in a colloid suspension. PDI values ≤0.5 suggest that the samples are monodisperse, with nanoparticles having similar shapes and sizes (Majoumouo et al. 2019). Since the AgNPs had PDI values greater than 0.5, the nanoparticles can be classified as polydisperse.

XRD analysis confirmed the crystalline nature of AgNPs, with four main peaks observed in the XRD diffractogram (Figure 2(e)). These peaks at 2θ values of 37.96°, 44.10°, 64.38°, and 77.27° correspond to the (111), (200), (220), and (311) crystallographic planes, respectively, indicative of the face-centered cubic structure of AgNPs (Aisida et al. 2019). Two additional peaks were observed at 29.345° and 33.52°, potentially indicating the presence of organic compounds in the extract. Similar results were obtained in other studies (Aisida et al. 2019; Korkmaz et al. 2020; Nguyen et al. 2020).

The precise mechanisms underlying the antibacterial action of AgNPs are still under investigation. The main hypotheses suggest that AgNPs physically damage bacterial cell walls and membranes, leading to rupture, cellular content loss, and bacterial death (Tang & Zheng 2018). AgNPs may also penetrate bacteria and induce the formation of reactive oxygen species (ROS) that damage proteins, lipids, and nucleic acids (Aziz et al. 2015). These nanoparticles can also disrupt the electron transport chain, affecting energy production and reducing metabolic capacity (Singh et al. 2020).

During the degradation of organic contaminants, it is expected that degraded products are non-toxic. Thus, toxicity tests were conducted with cucumber seeds (C. sativus) to assess the potential toxic effects of degraded RhB solution. Control (distilled water), RhB solution (25 mg L⁻¹), and degraded RhB solution allowed 100% germination (Table 2). However, the development of the seedlings was negatively affected by the undegraded RhB solution (Figure 8). The mean radicle length and hypocotyl lengths of C. sativus were 1.73 and 1.44 cm, respectively, in the RhB solution, while in distilled water, the radicle and hypocotyl lengths were 7.74 and 2.48 cm, respectively (Table 2). In the case of the degraded RhB solution, radicle and hypocotyl lengths of 7.54 and 2.13 cm were observed. Although the radicle and hypocotyl lengths of the degraded RhB solution were slightly lower compared to those in distilled water, statistical analysis (analysis of variance (ANOVA)) did not reveal any significant difference between them and the control. This suggests the absence of toxicity and validates the RhB remediation through the photocatalytic process using AgNPs. These results are consistent with the findings reported by Shaikh et al. (2020), who also observed that the products of the photocatalytic degradation of RhB by AgNPs showed no phytotoxicity to Cicer arietinum.

Textile wastewater treatment presents significant environmental challenges due to the presence of various hazardous compounds, including dyes, heavy metals, and aromatic compounds. Annually, approximately 140,000 tons of dyes are arbitrarily discharged into global water resources (Islam et al. 2023). These compounds can alter water color and transparency, reducing sunlight penetration and impeding photosynthesis, crucial for the growth of aquatic plants and algae (Golin et al. 2022). Additionally, many dyes contain toxic chemicals that can harm aquatic organisms, disrupting their physiological processes and leading to population declines (Castro et al. 2017). In our study, AgNPs synthesized from DAE effectively degraded RhB, offering a promising solution for removing dye pollutants from textile effluents. Given AgNPs' excellent photocatalytic activity against dyes, there is an opportunity to apply them for degrading other emerging contaminants such as pharmaceuticals, pesticides, and personal care products. This further expands their potential application in purifying industrial and urban wastewater. The ability of AgNPs to deactivate bacteria is another characteristic observed in this study that can be exploited for water disinfection. AgNPs have proven to be effective in eradicating over 700 microorganisms that are commonly encountered in sewage treatment plants (Epelle et al. 2022). Therefore, leveraging the potential of AgNPs in water purification emerges as a pivotal step toward tackling the urgent challenges of water contamination, providing a sustainable and efficient solution for ensuring water quality, safeguarding public health, and protecting the environment.

In this study, AgNPs were successfully synthesized using DAE, a residue from agroindustry. The characterization of these AgNPs involved various analytical techniques, including UV–Vis spectroscopy, FTIR, SEM, DLS, and XRD. The AgNPs exhibited a spherical shape with a diameter of 137.5 ± 59.3 nm and a PDI of 0.695, classifying them as polydisperse. FTIR analysis revealed that the phytochemicals present in the extract played a crucial role in reducing and capping the AgNPs. The crystalline nature of the AgNPs was confirmed through XRD analysis, showing characteristic peaks corresponding to the face-centered cubic structure of silver. Notably, these AgNPs demonstrated significant antibacterial activity against E. coli and E. faecalis. The photocatalytic properties of the AgNPs were evaluated by their ability to degrade RhB, achieving a 99.3% degradation of this dye. The kinetics of this photocatalytic degradation were best described by second-order kinetics. The good reusability of AgNPs was confirmed through four cycles of degradation. Additionally, phytotoxicity studies with C. sativus confirmed the non-toxic nature of the degraded products of RhB, a crucial aspect when considering the environmental impact of organic contaminant degradation. In conclusion, this research highlights a promising strategy for sustainable nanomaterial synthesis with significant implications for environmental and water quality management. Moreover, the success in repurposing agro-industrial waste for synthesizing AgNPs contributes to effective waste management and opens avenues for exploring other agro-wastes.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.