Green synthesis of Ag and Pd nanoparticles for water pollutants treatment

Synthetic dyes from industries cause severe water pollution, which causes serious environmental problems (Zhao et al. 2004). Most of the dyes are very harmful contaminants for water sources due to their toxicity and carcinogenic effects (Rasheed et al. 2019) and commonly result in severe problems such as inhibiting the growth of microorganisms, reducing light penetration, and increasing chemical oxygen demand (COD). An enormous amount of dyes has been discharged in recent years, which affects the water resources, soil fertility and aquatics to a great extent (Fan et al. 2018; Perez-Rodriguez et al. 2018). So, it is urgent to develop a strategy to manage the dyes pollution (Kramer et al. 2018).

Hence, there are numerous methods such as ozonation, membrane filtration, and electrochemical/photochemical degradation, biodegradation that have been developed for dye removal from water (Kariyajjanavar et al. 2010; Krejčíková et al. 2012; Zhang et al. 2014). However, the aforementioned techniques exhibit certain limitations such as formation of hazardous by-products, high cost, low efficiency or regeneration difficulties (Shen et al. 2001; Nawahwi et al. 2013; Singh et al. 2015). These limitations were effectively tackled by the nanostructured materials through their unique characteristics such as high surface area and electron communication features, narrow band gap, chemical stability, and favorable optoelectronic and light absorbing ability (Dong et al. 2013; Yusoff et al. 2013).

Nanostructured metals are of great interest due to their widespread applications in modern society (Moores & Goettmann 2006; Cepriá et al. 2016; Luo et al. 2016; Celania & Mudring 2019). The ultimate aim of the utilization of nanoparticle-mediated dye degradation is to decrease toxicity in the environment (Safari-Amiri et al. 2017).The nanoparticles such as NiO, TiO₂, ZnO, ZnS and metal ion-doped titania were utilized for organic dye degradations such as orange-II, reactive black, methyl orange and methylene blue (Ge et al. 2009; Saggioro et al. 2011; Bonyadinejad et al. 2012; Liu et al. 2013). However, the synthesized nanoparticles from physical and chemical methods are not environmental friendly due to the usage of hazardous reduction and stabilization agents, which may lead to some adverse effects.

To decrease the utilization of hazardous reduction and stabilization agents, bio-mediated synthesis using microorganisms, plants and algae has been proposed. Recently, the gaseous pollutant acetaldehyde was effectively degraded by Jatropha curcas-mediated cerium oxide (CeO₂) (Magudieswaran et al. 2018). Carica papaya extract was utilized for the preparation of copper oxide (CuO) nanoparticles, and has been used for degradation of coomassie brilliant blue dye (Sankar et al. 2014). Especially, Ag and Pd nanoparticles could be synthesized by bio-mediated process and were effectively utilized to degrade the environmental pollutant. For example, bio-mediated Ag nanoparticles were effectively applied for removal of azo dyes (Reddy et al. 2018; Rasheed et al. 2019). Non-toxic and renewable Boswellia serrata-mediated Pd nanoparticles were utilized for the degradation of anthropogenic dye pollutants (Kora & Rastogi 2016).

Among the existing dyes in the xanthene family, rhodamine 6G (Rh-6G) has extensive applications as a dyeing agent, non-linear optical chemical and photosensitizer material (Saini et al. 2005; Khalfaoui et al. 2012). It has been experimentally proven that Rh-6G exhibits carcinogenic effects and neurotoxicity, which are detrimental to humans and animals (Aarthi & Madras 2007; Lutic et al. 2012) Nanoparticles catalyzed photo-degradation was considered to be a promising approach for Rh-6G removal (Carreño et al. 2008; Nagaraja et al. 2012). Thus, development of green photocatalysts for Rh-6G removal is important. Here, we developed a green method to use Daucus carota leaves extracts as the ecofriendly reducing agent for synthesis of Ag nanoparticles (D-Ag) and Pd nanoparticles (D-Pd). Moreover, the catalytic activity for Rh-6G removal and antibacterial activity by these green synthesized nanoparticles were also evaluated.

Daucus carota leaves were obtained from a local supermarket (Zhenjiang, Jiangsu province, China). Silver nitrate (AgNO₃), palladium chloride (PdCl₂), sodium borohydride (NaBH₄) and rhodamine 6G (Rh-6G) were obtained from Aldrich Chemicals. The leaves of Daucus carota were obtained from the local premises.

The freshly derived Daucus carota leaves were washed and chopped into small pieces (∼1 cm × 1 cm). For the preparation of 20 wt. % extract (20 g of Daucus carota leaves in 100 ml de-ionized water), an appropriate amount of Daucus carota leaves was boiled in de-ionized water at 80 °C for 10 min. Then the solution was filtered by using a Whatman filter paper (no. 1) and the filtrate was considered as the Daucus carota leave extract and stored at 4 °C for further use.

Daucus carota extract (20 wt %) was gradually added into the aqueous solution of AgNO₃ (1 mM) and the mixture was magnetically stirred for 30 min. The color change was observed from yellow to brown, which suggests the formation of D-Ag nanoparticles (Raouf et al. 2019). Similarly, D-Pd nanoparticles were prepared by using an aqueous solution of PdCl₂ (1 mM) containing Daucus carota extract (20 wt %). Finally, the solution turns a pale yellow color, which suggests the formation of D-Pd nanoparticles (Kettemann et al. 2015).

The catalytic activity of D-AgNPs and D-PdNPs was examined by analyzing the UV–Vis spectra of Rh-6G dye on treatment with sodium borohydride. The concentration of Rh-6G dye was monitored by using the UV-Vis spectrophotometric method (Kalaiselvi et al. 2015). Typically, 0.5 ml of dye solution (10 μM) was added to an equal volume of sodium borohydride (0.1M) solution. Then 0.1 ml of the D-Ag NPs solution was added to a mixture of dye and NaBH₄, and stirring was continued for an additional 5 min. Absorbance value was continuously monitored during the dye degradation process in UV–Vis spectra at respective wavelengths at room temperature. Additionally, the control reactions were also carried out in parallel without NPs. Similarly, D-Pd NPs were analyzed for the degradation study.

The surface plasmon resonance of the prepared nanoparticles and catalytic Rh-6G dye degradation were monitored by using a UV-Vis spectrophotometer. The crystalline character of the synthesized D-Ag and D-Pd nanoparticles was ascertained by X-ray powder diffractometer (XRD) (Rigaku D/max-2500). Morphological images and dimensions of the prepared nanoparticles were evaluated by using a JEOL JEM-2010 transmission electron microscope. Structural characterization of the prepared nanoparticles was examined using a Perkin Elmer Fourier transform infrared (FT-IR) spectroscopy in KBr pellets.

To evaluate the possibility to synthesize D-Ag and D-Pd nanoparticles with Daucus carota leaf extracts, the color change of the solution during synthesis was monitored and the synthesized product was characterized. After addition of the extract into AgNO₃ or PdCl₂ solution, the solution gradually changed to brown and pale yellow color, suggesting the reduction of AgNO₃ or PdCl₂ had occurred, respectively (Figure 1).

The formation of D-Ag nanoparticles was confirmed by UV-Vis spectrum, which shows a sharp absorption band at 393 nm (Figure 2(a)). By adjusting the pH of the solution (pH = 6) to basic medium (pH = 9), the color of the nanoparticles was further intensified and attributed in the variation of dissociation constant (pKa) of the functional groups, which is attached on the phyto-chemicals of the biomass (Mukherjee et al. 2001; Vigneshwaran et al. 2006; Silva et al. 2007; Wang et al. 2018). Biomass of the extract is helpful for reduction and stabilization of nanoparticles and the inclusion of sodium hydroxide (pH = 9) sharpened the SPR band, which indicates the reduction in dimension of the prepared nanoparticles. The incorporation of aqueous Daucus carota extract is responsible for the formation of D-Pd nanoparticles, identified by the vanished SPR band at 400 nm, and the colour of the solution becomes pale yellow (Figure 2(b)).

Morphological structures of synthesized D-Ag and D-Pd nanoparticles were discussed with TEM images (Figure 3(a)). Most of the nanoparticles were formed as spherical in shape and of different sizes with a smooth surface. In addition to the spherical-shaped nanoparticles, the triangular shape of D-Ag nanoparticles was also observed from the morphological images. The average size of the D-Ag nanoparticles was obtained as in the range of 18–25 nm and the diameter of the triangle shaped nanoparticles was around 40–50 nm (Celebioglu et al. 2019). Rod like structures of D-Pd were clearly observed from the morphological images and the diameter of D-Pd nanoparticles was in the range of 20 nm and the length of the nanoparticles varied from 38–48 nm (Figure 3(b)) (Nadagouda & Varma 2008).

The crystalline nature of D-Ag and D-Pd nanoparticles was further confirmed by X-ray diffraction (XRD) pattern, respectively (Figure 4(a)). The index peaks were observed at angles of 38.1°, 46.2°, 64.5° and 76.8° and can be assigned to (111), (200), (220), and (311) reflections, respectively (Wen et al. 2006). It confirms the pure crystalline nature and fcc (face-centered cubic) structure of the synthesized nanometric D-Ag. The highly crystalline nature of D-Pd nanoparticles was confirmed from the highly intensified peaks observed at the angles of 39°, 45.5°, 64.3°, 81.2° and 83.6° and can be assigned to (111), (200), (220), (311) and (222) respectively (Yang et al. 2010). A characteristic crystalline peak observed in the XRD pattern is attributed to the crystalline nature of the biomass.

FT-IR spectra of the synthesized D-Ag and D-Pd nanoparticles were analyzed with plausible functional groups (Figure 4(b)). FT-IR spectrum of prepared nanoparticles showed intensive bands at 3,434, 2,358 cm^-1 that were attributed to N-H and C = O stretching vibrations, respectively (Prathna et al. 2011). In generally, 1,379 cm^-1 is assigned to the germinal methyl groups and the mentioned band has shifted to 1,384 cm⁻¹. Bio-mediated nanoparticle peaks were located around 1,598 cm^-1 and attributed to C = C stretching (Tripathy et al. 2010). From the analysis, it is clear that the flavanones and terpenoids could have appeared on the metal nanoparticle surface by a possible interaction via carbonyl groups. Reducing sugar and terpenoids were helpful for the π bond formation on the carbonyl group in biosynthesized nanoparticles. Physiosorbed functional groups may cause electrostatic or steric walls around the surface of bio-mediated nanoparticles to provide stabilization of nanoparticles, which may improve their catalytic performance for antibacterial or dye decolorization (Rasheed et al. 2018).

It is well known that Ag or Pd nanoparticles have good antibacterial activity. The antimicrobial action of Ag⁺ and Pd⁺ is closely associated with the interface with sulfhydryl groups in enzymes and proteins. For instance, Ag⁺ and Pd⁺ can bind to proteins that are present in the cell membrane to form stable bonds, resulting in protein deactivation, which would be detrimental to bacterial cells (Klueh et al. 2000; Giessen & Silver 2016). Antibacterial activities for the D-Ag and D-Pd nanoparticles were tested against Klebsiella pneumonia, Vibrio cholera and Escherichia coli and the activity compared by the zone of inhibition method (ZOI) (Figure S1 in Supplementary Material). From the results, the antibacterial activity of D-Ag and D-Pd nanoparticles against Klebsiella pneumonia (ZOI = 36 mm for D-AgNPs, 24 mm for D-PdNPs) and Vibrio cholera (ZOI = 44 mm for D-AgNPs, 18 mm for D-PdNPs) was higher than against Escherichia coli (ZOI = 18 mm for D-AgNPs, 16 mm for D-PdNPs). In this study, D-Ag nanoparticles have good antibacterial behavior towards Gram-negative bacteria stains with the same concentration. Also, the synthesized D-Pd nanoparticles have shown an obvious inhibition zone against the tested microorganisms and the observed activity was similar to the D-Ag nanoparticles. Furthermore, antibacterial activity of prepared nanoparticles was evaluated by the colony-forming unit (CFU) method with different concentrations against Gram–negative Escherichia coli for 16 h of incubated time at 37 °C. The maximum antibacterial activities for D-Ag and D-Pd nanoparticles were around 88.9% and 92.7% at 1,200 μg/ml respectively (Figure 5). The results indicated that the synthesized nanoparticles with Daucus carota extracts have good antibacterial activity, which might be used for water disinfection.

The catalytic efficiency of prepared D-Ag and D-Pd nanoparticles towards the degradation of Rh-6G dye using NaBH₄ as the reducing agent was carried out at room temperature. Rh-6G exhibited a characteristic SPR band at 526 nm corresponding to characteristic π–π* transition (Bakkialakshmi et al. 2012). The degradation of Rh-6G dye by D-Ag and D-Pd nanoparticles with NaBH₄ was monitored by UV-Vis absorption spectroscopy.

The intensity of absorptions of Rh-6G was only slightly decreased as a function of time with an addition of NaBH₄, which can be explained by the electrons from NaBH₄ being transferred to the Rh-6G molecule and reducing the dye molecules (Song et al. 2014). However, the dye degradation process was saturated at 30 min with minimal degradation by NaBH₄ (Figure 6(a)). The addition of Daucus carota extract decreased the UV-Vis adsorption of the dye molecule, and may be ascribed to the dilution effect (Figure 6(b)). Then, the D-Ag and D-Pd nanoparticles synthesized with Daucus carota extract were used as the catalyst for Rh-6G reduction with NaBH₄. Although the color intensity of Rh-6G dye was decreased as a function of time, complete decolorization was not observed for D-Ag nanoparticles (Figure 6(c)). However, the addition of D-Pd nanoparticles completely removed Rh-6G within 2 min (Figure 6(d)). The change in the color of the dye was monitored up to 30 min to confirm the removal efficiency of D-Pd nanoparticles, revealing the independence of dye reduction from adsorption. According to the reduction profile, NaBH₄ reduced 30% of dye with at maximum ability. D-Ag nanoparticles exhibited a gradual degradation and reduced 89% of dye, which explains the change in initial concentration of dye with function of time. Similarly, the concentration of Rh-6G dye rapidly decreased in the presence of NaBH₄ mediated D-Pd nanoparticles reduced 98% of dye within 2 min and the degradation rate was much higher than the Ag nanoparticles (Figure 6(e)). These results substantiated that these nanoparticles synthesized with the developed green method could be applied for catalytic water pollution treatment.

D-Ag and D-Pd nanoparticles were synthesized using Daucus carota leaves mediated bio-synthesis process. Daucus carota broth extract acts as an effective reducing and stabilizing agent in the preparation of nanoparticles and was examined by FTIR analysis. The synthesized nanoparticles were effectively applied in the degradation process for the cost-effective removal of pollutants. The catalytic decolorization of Rh-6G dye and pathogen deactivation was achieved with D-Ag and D-Pd nanoparticles synthesized with the green method. These findings prove that Daucus carota extracts could be an eco-friendly reducing agent for functional nanoparticle synthesis. The plant-extract based nanoparticles showed the advantages of cheap, environmentally friendly, self-capping and stabilization, which would be useful for practical application in water pollutant management.

This work was supported by the Priority Program Development of Jiangsu Higher Education Institutions (China).

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