Ecofriendly synthesis of Salmalia Malabarica gum stabilized palladium nanoparticles: antibacterial and catalytic properties

Effluents containing dyes, discharged by various industries, have become a significant contributor to water pollution. This study explores the use of green-synthesized palladium nanoparticle (PdNP) catalysts, which offer enhanced catalytic per - formance compared to traditional methods. The research focuses on the synthesis of palladium nanoparticles using Salma-lia Malabarica (SM) gum via a microwave-assisted process and investigates their catalytic and antibacterial properties. SEM analysis confirms the even distribution of PdNPs on the surface of SM gum. Furthermore, TEM analysis reveals a PdNPs size distribution of 10 ± 2 nm. XPS study was used to identify the chemical state of Pd in the synthesized nanoparticles. The results demonstrate that PdNPs are highly effective catalysts for the degradation of dyes such as Methylene Orange (MO), Rhodamine-B (Rh-B), and 4-Nitrophenol (4-NP), even after being reused five times. The catalytic activity of PdNPs was remarkable, achieved 99% dye degradation in four minutes. The degradation data of PdNPs on 4 -NP, MO and Rh-B dyes are followed by pseudo-first-order kinetics with 0.0087, 0.0152 and 0.0164 s − 1 , respectively. Additionally, PdNPs exhibit exceptional antimicrobial activity against both bacterial and fungal strains. This synthesis process proves to be cost-effective, devoid of toxic chemicals, and remarkably rapid. The findings suggest promising applications for PdNPs in fields like nanomedicine and environmental remediation, reflecting their potential for addressing water pollution issues.


Introduction
Effluents from industrial processes often carry a significant load of organic dyes, which can contain both organic and inorganic toxic compounds [1]; among these dyes commonly used in various industries, including Rhodamine -B, Methyl Orange, and Methyl Blue [2].Furthermore, 4-Nitrophenol (4-NP) serves as a critical and versatile precursor in the production of agricultural chemicals, pharmaceutical drugs, and dyes.Notably, colorants and dyestuffs find extensive applications in sectors such as textiles and food production, further contributing to the release of nitrogencontaining organic compounds into the environment [3,4].The escalating use of these coloring agents and nitrogenbased organic substances stands out as a leading cause of water contamination, especially in surface water bodies.The pollutants discharged into the liquid phase pose a significant threat to aquatic ecosystems and their inhabitants.Exposure to these contaminants can result in various health issues, including but not limited to jaundice, cyanosis, quadriplegia, Heinz body formation, tissue necrosis, and more [5].
A significant challenge in dealing with dye pollutants is their large size, intricate molecular structure, high stability, and resistance to biodegradation.Due to their recalcitrance, these dyes must undergo treatment before being discharged into the environment.Conventionally, the treatment of wastewater containing such pollutants has relied on various techniques, including chemical treatments, electrochemical processes, ultrafiltration, coagulation, physisorption, chemical precipitation, and ion exchange methods [6].In the quest for alternative methods to address this issue, researchers have explored the use of natural and synthetic adsorbents, such as animal charcoal, char/biochar, fibers derived from materials like coconut and jute, and graphene-oxide nanocomposites [7,8].Notably, graphene-oxide nanocomposites have shown promise in efficiently removing dyes from wastewater [9].However, it's important to acknowledge that the aforementioned conventional methods have limitations.They often exhibit slow processing, high operational costs, and sometimes fall short in achieving complete removal of pollutants.Consequently, there is a pressing need for more efficient processes to effectively eliminate industrial effluents from the environment.
Currently, an efficient method for mitigating pollution in wastewater involves the utilization of nanocomposites or metal nanoparticles [10] in conjunction with NaBH 4 .Numerous studies have explored the degradation of colored pollutants using various metal nanoparticles, including Cu, Au, and Ag [11][12][13][14], as well as metal oxide nanoparticles like TiO 2 , ZnO, etc. [14][15][16].Notably, PdNPs are renowned for their exceptional catalytic properties, which stem from their specific properties and chemical activity.PdNPs stand out as one of the most effective catalysts among metal nanoparticles.They have demonstrated their catalytic prowess in various processes, such as the reduction of nitroarenes and Suzuki coupling reactions [17].These catalytic properties make PdNPs a valuable tool in the quest to combat water pollution, offering a rapid and effective means of eradicating pollutants from wastewater [18,19].
Metal nanoparticles exhibit well-established antimicrobial applications.Many chronic diseases in humans started from bacterial and fungal infections, which are commonly treated with antibiotics due to their potent antimicrobial properties [20].However, the excessive and inappropriate use of antibacterial agents has led to the development of antibiotic resistance, a growing global concern.Recent reports have raised the alarm about the emergence of super bacteria that may be impervious to all known antibiotics [21].Bacterial resistance to antibiotics results in disease recurrence, posing a significant health challenge.
Inorganic nanoparticles, particularly metallic nanoparticles, have emerged as promising antibacterial agents to address these concerns [22].Nanoparticles crafted from metals demonstrate remarkable efficacy in combating bacterial and microbial resistance and are well-suited for targeted drug delivery.Notably, PdNPs have come to the forefront in recent years, offering innovative solutions as antibacterial and antifungal agents [18,23].Their unique properties make them valuable contributors to the ongoing battle against antimicrobial resistance and infectious diseases.
PdNPs can be synthesized through various methods, including biological, chemical, sonochemical, electrochemical, or biochemical approaches [24,25].However, traditional chemical and physical methods have several drawbacks, such as the use of toxic chemicals that harm the environment and the need for costly, energy-intensive equipment operating at high temperatures and pressures.As a result of the use of harsh chemicals, chemical methods cause harm to the environment.These factors have prompted researchers worldwide to seek alternative, more cost-effective, environmentally friendly, and sustainable synthetic routes for PdNP production.Therefore, biological methods have been reported to synthesize NPs using biological materials such as plant extracts and biological species by researchers.
Biogenic synthesis offers a method to prepare nanoparticles using various plant and animal sources [26,27].Recent research has indicated that plant metabolites play a crucial role in capping and reducing agents during the green synthesis process [28].A variety of leaf extracts and plant extracts were also used for the production of palladium nanoparticles, including Solanum trilobatum [28], Anacardium occidentale [29] and pectin versus acaciagum as polysaccharides [30].In this regard, we have developed a simple and green method for synthesising palladium nanoparticles from Na 2 PdCl 4 by using a non-toxic, renewable natural plant extract of Salmalia Malabarica (SM) gum.SM gum is abundantly available in nature.Notably, SM gum boasts a wide range of medical applications, including its use in treating dysentery, diarrhea, menorrhagia, styptic, hemoptysis, influenza, pulmonary tuberculosis, enteritis, and more [31].The gum comprises various compounds like terpenoids, glycosides, sugars, coloring agents, fatty compounds, and more.This study was undertaken to explore the promising applications of palladium nanoparticles in general and the potential medicinal properties of SM gum.
Herein, we present a rapid and straightforward approach for the synthesis of PdNPs using SM gum, employing the microwave (MW) technique.The MW synthesis method, known for its advantages over traditional approaches, is simple, scalable, environmentally friendly, unambiguous, and highly efficient.Notably, the MW method significantly reduces reaction time due to its intense energy supply and unique heating mode.Importantly, this study marks the first instance in the literature of biogenic PdNP synthesis using SM gum with the microwave irradiation method and explores their catalytic and antimicrobial applications.SM gum's stabilizing and reducing properties further enhance its effectiveness.Our research involves the synthesis of PdNPs with SM gum using microwave irradiation, followed by a comprehensive characterization by utilizing UV-Visible, FTIR, XRD, XPS, SEM, and TEM techniques.We also conducted reactions involving MO, Rh-B, along with 4-NP in the existence of NaBH 4 , employing the synthesized PdNPs as a catalyst.Additionally, we studied the recyclability of the PdNPs and investigated their antibacterial activity against different bacterial strains.

Materials
Salmalia Malabarica (SM) gum was procured from Hyderabad, India.All chemicals, reagents, and materials of analytical grade used in this study were sourced from Sigma Aldrich and SD Fine-Chem Limited (Mumbai, India).

Preparation of SM gum extract
We mixed 0.5 g of finely powdered SM gum in 100 mL of double distilled (DD) water in a beaker as the reaction mixture was stirred continuously for 3 h at 60 ºC.Unreacted materials were removed by cooling the reaction mixture to room temperature and filtering it with a Whatman filter.

Biosynthesis of PdNPs with Salmalia Malabarica gum extract
The 18 mL of SM gum extract was mixed with 12 mL of 0.5 mM Na 2 PdCl 4 solution in a beaker.750 watts of microwave irradiation was applied to the reaction mixture for 7 min.The mixture was turned into a grey colour after microwave irradiation [32].It suggests the palladium nanoparticles formation.After centrifuging for 10 min at 5000 rpm, the dark grey residue was obtained.We discarded the supernatant solution.PdNPs residue was washed three times with deionized water/methanol (1:1 v/v).These steps were used to remove unwanted substances from PdNPs.PdNPs were dried at 65 °C for 2 h.Furthermore, the obtained pure palladium nanoparticles were characterized and assessed for their biological and catalytic properties.The catalytic [32] and antibacterial activity procedure [33] and characterization techniques were present in the supplementary information file.

UV-visible spectroscopic analysis
UV-Vis spectra were used to monitor the green synthesis of PdNPs using SM gum by a microwave irradiation method through a color evolution of the solution.A paleyellow solution of Pd 2+ ions changed to black when the SR gum was added.It indicates that PdNPs were formed by reducing Pd 2+ to Pd 0 .Figure 1 shows UV-Vis spectra of SR gum, Na 2 PdCl 2 solution, and PdNPs.Absorption peaks were observed for Pd 2+ ions at 360 and 450 nm [34], but no peak was detected for gum.A wavelength maximum was not observed for the PdNPs (gum + Pd 2+ ).The results Fig. 1 UV-Vis absorption spectra of SM gum, Pd (II) solution, and PdNPs contained irregular and monodispersed palladium nanoparticles [39].The energy-dispersive X-ray (EDX) spectrum displayed a distinct Palladium metal peak (Fig. 3b), providing unequivocal confirmation of the presence of palladium nanoparticles [40].The EDX spectrum also suggested the successful immobilization of palladium nanoparticles on SM gum materials.This observation strongly indicates that palladium nanoparticles are uniformly distributed across the surface of the SM gum matrix.

XRD and TEM analyses
PdNPs were investigated by XRD to determine their crystallinity.XRD (Fig. 4) showed four characteristic peaks at 2θ = 39.65°,45.24°, 65.14°, and 78.65°, respectively, and demonstrate that there are no Pd 2+ ions in the solution, indicating that all Pd 2+ ions have been reduced to Pd 0 .UV-visible PdNPs do not exhibit plasmonic properties due to their non-absorbing properties [35].

FTIR spectroscopic analysis
The FTIR spectrum of green synthesized PdNPs using SM gum is shown in Fig. 2. The bands at 3395 cm − 1 and 2912 cm − 1 were associated with -OH and aliphatic -C-H bonds in the FTIR spectrum of SM gum (black line in Fig. 2).These two strong absorption bands observed at 1790 and 1690 cm − 1 were attributed to C = O stretching in the spectrum of aliphatic ester and aromatic amide, respectively [36,37].The absorption bands found at 1508, 1302, and 1058 cm − 1 were due to bending vibrations of N-H, C-O, and CO-O-CO for aromatic amides, aromatic esters, and anhydrides, respectively.The gum-capped PdNPs showed high-intensity vibrational bands (pink line in Fig. 2) at 3445, 2917, 1734, 1615, 1427, 1227, and 1041 cm − 1 .Shifts in the bands were observed from 3395 to 3445, 2912 to 2917, 1790 to 1734, 1690 to 1615, 1508 to 1427, 1302 to 1227, and 1058 to 1041 cm − 1 [38].These results indicate that the synthesized PdNPs might be capped by functional groups present in the gum.

SEM and EDX analysis
The SEM analysis was conducted to assess the surface morphology and shape of the palladium nanoparticles.The results revealed that the PdNPs were predominantly quasispherical in shape.However, a few exhibited irregular shapes, as depicted in SEM images (Fig. 3a).Further examination indicated that the surface of the SM gum matrix  Bragg angle; β represents the full width at half maximum of the strongest peak, and k is a shape-dependent Scherrer's constant [42].The crystallite size of PdNPs calculated from XRD was in good agreement with the TEM analysis.
The TEM images (Fig. 5a) provide a clear depiction of the PdNPs that were synthesized, displaying their precise sizes and shapes.These particles were predominantly spherical in shape and uniform in size, with an average size of approximately 10 ± 2 nm.Furthermore, the PdNPs exhibited excellent dispersion, and high-resolution TEM images revealed a smooth surface [42].As shown in Fig. 5b, there is a distinct spherical shape of 5 nm PdNPs.The TEM analysis clearly illustrates that the SM gum created a protective surface layer over the PdNPs, effectively preventing their aggregation.Moreover, the SEAD analysis (Fig. 5c) revealed well-defined lattice parameters, indicating that the synthesized PdNPs possess a high degree of crystallinity.A histogram developed from the analysis of 100 PdNPs is presented in Fig. 5d.

X-ray photoelectron spectroscopy (XPS) analysis
In order to determine the chemical state of Pd in nanoparticles, XPS was employed, which is a surface-sensitive analytical technique.The XPS spectra (Fig. 6) of Pd exhibited two distinct peaks: one at a high energy band of 340.98 eV and another at a lower energy of 335.97 eV.These peaks correspond to the Pd 3d 3/2 and 3d 5/2 , respectively and fully indexed to (111), ( 200), ( 222) and (311) planes of PdNPs (Joint Committee on Powder Diffraction Standards files No. 89-4897).XRD spectrum of the synthesized PdNPs showed that PdNPs were crystallized in a face-centered cubic structure [41].The high intense peak (111) observed in the XRD pattern represents that PdNPs are more favoured to grow along the (111) orientation.Furthermore, the XRD pattern demonstrated the highly crystallized nature of the PdNPs.From Scherrer's equation D = kλ/βcosθ, an average crystallite size of 9.40 nm was found for PdNPs.In Scherrer's equation, λ represents the wavelength θ represents the  Pd (II) ions to elemental palladium while undergoing oxidation themselves to form carbonyl and carboxyl groups [45].Besides, the FTIR analysis corroborated the essential role of hydroxyl and carbonyl groups in both the synthesis and stabilization of PdNPs.The FTIR spectra obtained before and after PdNPs synthesis revealed a noticeable decrease in the hydroxyl and carbonyl bands [39].At the same time, the COO-band at 1790 cm − 1 was due to the groups on the surface of the nanoparticles.These results confirmed that the reduction and stabilization of PdNPs were mediated by the hydroxyl and carbonyl groups in SM gum [46].

Catalytic properties of PdNPs
Using 4-NP, MO, and Rh-B as substrates, PdNPs were evaluated for their catalytic activity.UV-Vis spectroscopy was used to monitor the reactions.

The catalytic reduction of 4-NP to 4-AP
A reduction of 4-NP to 4-AP in the presence of NaBH 4 was used to determine the catalytic effectiveness of PdNPs.Chemical reactions cause the colour to change from light yellow to deep yellow.The UV-Vis spectrum of 4-nitrophenol showed an absorption band at 315 nm [47].Nitrophenolate ions are formed in solution when NaBH 4 is added to 4-NP, as indicated by the strong absorption peak at 405 nm (Fig. 8a).The peak intensity at 405 nm was retained for up to 120 min without the addition of PdNPs.This indicates that p-nitrophenolate was not reduced fully by NaBH 4 alone.By adding 5 mg of PdNPs to the reaction mixture (p-nitrophenolate ion), the catalytic effectiveness of PdNPs was tested and recorded over a variety of time intervals using a UV-Visible spectrophotometer [48].Figure 8b reveals that the absorption peak intensity at 405 nm was decreased rapidly.There is an electron transfer process between PdNPs match to reduced Pd(0)NPs.These findings are consistent with previous studies [43].Therefore, the XPS spectrum analysis confirmed the successful preparation of zero-valent PdNPs using SM gum as a reducing and stabilizing agent [44].

Mechanism for PdNPs formation
SM gum played a dual role as both a reducing and stabilizing agent in the synthesis of PdNPs (as shown in Fig. 7), facilitating the reduction of Pd(II) to Pd(0).The composition of SM gum includes a variety of compounds such as terpenoids, carbohydrates, alcohols, esters, carboxylic acids, and carbonyl derivatives.Notably, the carboxylic, carbonyl, and hydroxyl groups within the gum were pivotal in reducing Pd (II) ions to their elemental Pd(0) state.Phytochemicals present in the gum possess potent antioxidant properties and high reducing capacity.This allowed them to effectively reduce linear relationship was presented between ln C t /C 0 and reaction time [52,53].The rate constant at the room temperature reaction was determined as 0.0152 s − 1 .

Rhodamine-B dye reduction studies
Rh-B is used in textiles, paper, paints, leather, textile dyeing and other applications.Rh-B dye is highly soluble in water and has a basic nature.However, it can cause serious environmental and biological problems, perhaps irritate the eyes and skin and cause cancer.Utilizing PdNPs and NaBH 4 in conjunction with Rh-B dye, this study evaluated its catalytic degradation.In the presence of NaBH 4 , Rh-B reduction shows absorption peaks at 615 and 665 nm in the UV-Vis spectrum [54,55].Even after 60 min, there was still a sharp peak at 550 nm in the case of Rh-B + NaBH 4 (Fig. 10a), suggesting that NaBH 4 alone could not be reduced by Rh-B effectively.PdNPs induced Rh-B reduction in 240 s (Fig. 10b).As the reaction time increased, the intensity of absorption bands at 550 nm decreased, accompanied by a change in colour from red to colourless [56,57].In the presence of PdNPs, Rh-Rh-B was reduced completely [58,59].Figure 10c showed a linear relationship between ln C t /C 0 with reaction time, suggestive of pseudo-first-order kinetics and 4-nitrophenol [49].After 220 s, we observed that the absorption peak at 405 nm gradually declined, and a new peak at 298 nm was developed, which implies the formation of 4-nitrophenol.4-Nitrophenol was catalytically reduced in 220 s.As shown in Fig. 8c, ln (A t /A 0 ) is plotted against reaction time.The plot indicates that the reaction followed a pseudo-first-order kinetic model, with a rate constant of 0.0087 s − 1 at room temperature.

MO dye reduction studies
PdNPs catalytic efficacy was evaluated by reducing MO dye. Figure 9a shows the absorption spectrum of MO dye, which exhibits two distinct bands at 260 nm (n -π* electronic transition) and 450 nm (π -π* electronic transition).Figure 9a shows the absorption spectrum of MO dye when reduced with NaBH 4 without PdNPs.The two absorption bands remained unchanged even after 180 min without the PdNPs catalyst.It assumes that MO dye reduction was not completed [50,51].After adding 5 mg of PdNPs to the reaction mixture, the peak intensities (in Fig. 9b) were rapidly decreased.The dye solution was turned colourless from orange within 240 s, which indicates a complete reduction.A pseudo-first-order kinetic model is evident from Fig. 9c.A

Catalytic recyclability of PdNPs
PdNPs catalyst reusability was assessed by undergoing a variety of reduction cycles with 4-NP, MO, and Rh-B.Afterwards, the catalyst was washed, dried with methanol and water, and reused after being cleaned and dried [9,63].Figure 12 shows the recycling capability of PdNPs in the reduction reactions of 4-NP, MO, and Rh-B.After five cycles, PdNPs showed no significant loss in catalytic activity [64].

Antibacterial activity of PdNPs
Antibiotics employ several mechanisms to target bacteria, affecting critical processes like DNA replication, cell wall synthesis, and translation.However, despite these multifaceted attacks, bacteria have developed resistance mechanisms, rendering antibiotics ineffective.In contrast, the mode of action of metal nanoparticles, such as palladium nanoparticles, directly targeting the bacterial cell wall at room temperature, with a constant rate of 0.0164 s − 1 .The increased activity of PdNPs may be due to their higher surface-to-volume ratio, high surface coverage, enhanced electron transfer properties, and effective reduction of kinetic barriers [60,61].

The catalytic reduction mechanism of the dyes
Figure 11 shows a possible catalytic reduction mechanism for 4-NP, Rh-B and MO, utilizing PdNPs as catalysts.Analytes' molecules (i.e., 4-NP, MO, and Rh-B) were absorbed on the surface of PdNPs during the first stage of the reduction process.PdNPs transferred electrons from BH 4− (donor) to analytes (acceptor) through electrostatic interactions [32].These reactions have occurred at the surface of PdNPs [62].By transferring electrons, dyes on the surface of PdNPs were reduced.As the PdNPs were desorbed from their surface, their byproducts diffused into the bulk of the solution.

Conclusions
This study focused on the eco-friendly and rapid synthesis of PdNPs using SM gum via microwave irradiation, presenting an efficient and environmentally benign method.The average particle size of PdNPs measured by XRD and TEM was 10 ± 2 nm with crystalline and spherical structures.The XPS spectra of PdNPs displayed two distinctive peaks at the high energy band 340.98 eV and low energy band 335.97 eV, which are the characteristics of Pd 3d 3/2 and 3d 5/2 , respectively, and matches to fully reduced Pd (0) NPs.The catalytic activity of PdNPs was remarkable, achieving a 99% reduction of 4-NP, MO, and 98% reduction of Rh-B dyes.Significantly, the PdNPs catalyst could be reused multiple times without substantial loss of catalytic activity.Furthermore, these PdNPs exhibited strong antibacterial properties, with a zone of inhibition diameters measuring 19 ± 0.30, 15 ± 0.25 and 15 ± 0.27 mm for Bacillus subtilis, Bacillus cereus and Escherichia coli, respectively.Bacillus subtilis showed good antibacterial activity, while others showed moderate to good antibacterial activity.This study underscores the potential of PdNPs to swiftly reduce environmental pollutants and suggests their suitability for future biomedical applications.circumvents the issue of resistance compared to antibiotics [65].Consequently, metal nanoparticles offer a promising alternative for combating microbes.Palladium nanoparticles, in particular, have garnered significant attention due to their unique biological properties, including their antimicrobial and anticancer capabilities [12].These nanoparticles interact with bacteria in various ways.Smaller PdNPs can penetrate directly into bacterial cells, while larger nanoparticles remain outside the cells.This results in a continuous release of Pd 2+ ions.These released ions disrupt the bacterial cell membrane by binding to it [66].Once the cell wall is destabilized, palladium nanoparticles gain entry into the bacterial cell, where they interact with lipids, proteins, and DNA.This interaction leads to the dysfunction of the bacterial cell, ultimately inhibiting its growth and survival.
The PdNPs antibacterial tests were conducted against Gram-positive (Bacillus subtilis and Bacillus cereus) and Gram-negative (Escherichia coli).Disc diffusion was used to measure zone inhibitions of tested bacterial cultures [67,68].The zones of inhibition were observed with PdNPs (Table 1 & S. Figure 1).There were inhibition zones of 19 ± 0.30 mm and 15 ± 0.25 mm, respectively, for Bacillus subtilis and Bacillus cereus (Gram-positive strains).The inhibition zone for Gram-negative Escherichia coli was 15 ± 0.57 mm.Good antibacterial activity was demonstrated against Bacillus subtilis by PdNPs, but moderate activity was observed for other bacteria [69].The antibacterial effect of PdNPs was significant for both Gram-positive and Gramnegative bacteria [64].The antibacterial activity of PdNPs was increased by increasing its concentration.A comparison of the antibacterial activity of PdNPs is presented in Table 2.

Fig. 3
Fig. 3 SEM image of PdNPs stabilized with SM gum and its EDX spectrum

Fig. 9
Fig. 9 UV -Visible Absorption spectra of (a) MO in the presence of NaBH 4 and in the absence of PdNPs, (b) Time-dependent MO reduction with NaBH 4 and PdNPs catalysts, and (c) Linear plot of (C t /C 0 ) vs. time

Fig. 11
Fig. 11 Catalytic reduction mechanism of MO, Rh-B dyes, and 4-NP by PdNPs in the presence of NaBH 4 .

Fig. 10
Fig. 10 UV -Visible Absorption spectra of (a) Rh-B in the presence of NaBH 4 and the absence of PdNPs, (b) Time-dependent Rh-B reduction with NaBH 4 and PdNPs catalysts, and (c) Linear plot of (C t /C 0 ) vs. time

Table 1
Antibacterial activity of PdNPs (Zone of inhibition in mm)