1 Introduction

Since organic dyes are resistant to biodegradation and persist for a long time in the environment, their presence in industrial effluents poses major safety problems [1]. Therefore, it is crucial to control the industrial wastewater produced because of the use of these hazardous dyes. Photocatalysis is recognized as an effective, energy-saving technique that does not generate secondary pollutants and is a potential alternative for managing the degradation of existing dyes in wastewater [2].

Out of all the dyes, the azo compounds, which are used in a range of products such inks, textiles, paints, foods, household goods, and cosmetics, are the ones that are used the most frequently. Advanced oxidation processes (AOPs) and chemical oxidation are two preventative strategies that are frequently advised. It however, revealed certain financial restrictions [3].

For the same purpose, photocatalysts may be utilized [4]. These catalysts are very effective in destroying the targeted dye molecule by creating reactive oxygen species (ROS) when light is present. Ag NPs, TiO2 NPs, ZnO NPs, and Pd NPs are some of the most often utilized modern photocatalysts, which is why scientists have turned to nanotechnology and the creation of new effective nanomaterials [5,6,7].

Numerous studies of nanomaterials have been carried out in many applications especially thanks to the optical, electronic, magnetic, and chemical properties of metal and metal oxide nanoparticles and the synthesis of these nanoparticles with a particularly environmentally friendly green chemistry method has become the focus of many researchers [8,9,10]. Metal nanoparticles can usually be synthesized by various biological, chemical, and physical procedures. The most widely used method is the chemical synthesis method using many organic and inorganic reductants. However, this method has many disadvantages such as being difficult to separate, expensive, toxic, and dangerous [11]. So, the clean, non-toxic, and environmentally friendly green method has been developed for metal nanoparticle synthesis [12]. Green synthesis methods allow the removal of hazardous chemicals that cause toxicity, surface-active agents/dispersants from large-scale industrial products have created a unique area by eliminating the effects of various harmful chemicals on the human being and at the same time being harmless to the environment [13]. Various metal nanoparticles (gold, silver, zirconium, titanium, etc.) have various uses in biomedical and catalytic fields [14, 15]. Especially, in developing nanotechnology, transition metals with higher yields and capacities are becoming more preferred. Palladium nanoparticles are frequently used in medical applications because palladium is capable of binding to single-stranded DNA without damaging the DNA structure [16], and studies of using chitosan-graphene-palladium nanoparticles as biosensors for glucose measurements are also known [17]. The use of metals in the Phyto-reduction of nanoparticles has been extensively explored, particularly for Au and Ag, and the search for metals other than this is promising in terms of nanotechnology. Therefore, in this study, Rosemary (Rosmarinus officinalis L.) was used as a reductant for palladium nanoparticles. This extract served both as a reducing agent and a stabilizing agent [18, 19]. The researchers showed that rosemary extract can be used chemically to target-specific pathways via the activating of apoptosis and reduced cell survival. Also, rosemary extract may be used as neutraceutical to can be used as neutraceutical to raise the anticancer effects of existing chemotherapeutics. This can result in lower doses and less toxicity in healthy tissues [20].

In this study, biogenic synthesis of Pd NPs was performed using rosemary extract, and its morphological and chemical structure was characterized by different techniques. Then, both the biological and catalytic activities of Pd NPs were investigated. The biological activities of Pd NPs were tested as antimicrobial, antioxidant, and DNA cleave activity, and the results are promising for biotechnology applications. The catalytic activity of Pd NPs was investigated for photocatalytic activity for dye removal from wastewater and energy production by alcohol oxidation.

2 Materials and Methods

Palladium chloride (99.9%, PdCl2), DPPH, and all chemicals were purchased from Sigma Aldrich. The rosemary herb used in the experiment was purchased from the local market.

The rosemary extract was prepared using the microwave method, for this purpose, 5 g of rosemary powder and 100 mL of 70% ethanol were mixed and were left in the microwave at a power of 90 W for 5 min. The resulting extract was strained off the pellet with filter paper (Whatman 40) and preserved at + 4 °C. The synthesis method (Scheme 1) of nanomaterials is included in the Supporting file.

Scheme 1
scheme 1

Overview of the synthesis steps of Pd NPs

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Photocatalytic degradation of MB was evaluated under a sunlight simulator. Here, the MB aqueous solution (10 mg/L) was mixed with 10 mg of the produced nanomaterial. For 30 min in the dark, the solution was magnetically agitated to achieve an adsorption–desorption equilibrium. The solution was subjected to sunshine irradiation using the simulator for several time intervals (0–120 min) while at ambient conditions and being stirred. Absorption values were measured in UV–Vis absorption spectrometry to monitor the degradation process of MB.

The obtained nanostructure was finally tested in anodic reactions in fuel cells. In this study, electro-catalytic reactions were investigated at room temperature (25 °C) using a typical three-electrode cell and a computer-assisted Potentiostat/Galvanostat device (Gamry Reference 3000). In each set of experiments, the cell was inserted with nitrogen (N2) gas (99.99%). A glassy carbon electrode (GCE) was used as the working electrode, silver/silver chloride (Ag/AgCl) as the reference electrode, and a platinum plate as the counter electrode. The geometrical surface area of the GCE is approximately 0.0717 cm2. The GCE surface was cleaned with alumina powder before each experiment and then sonicated. In each experiment, the electrode surface was coated with 15 µl of electrode solution containing Pd nanostructure by drop casting method and dried for 30 min. Cyclic Voltammogram (CV) cycling was performed in 0.5 M Potassium Hydroxide (KOH) aqueous solution with and without 0.5 M methanol over a potential range of −0.8 V to + 0.2 V. Chronoamperometry (CA) experiment was carried out at − 0.2 V for 5000 s to investigate the relative stability of the electrodes.

3 Results and Discussion

Recently, the production of nanoparticles has made use of extracts from a variety of plant components, including seeds, fruits, stems, roots, leaves, gums, barks, and flowers [21]. Many salts have strong reduction potentials, but Pd salts, like chlorides and nitrates, have very strong potentials because the metals may bind to the acetate and chloride portions and have a propensity to donate electrons. Consequently, metals could raise the conjugation salts’ electron density. In fact, the reduction reaction causes the ionic forms of metals to rapidly separate from the anionic portions, making them extremely stable when plant extracts are used. A sustainable method of producing NPs is by the biosynthesis of Pd NPs by plants, as will be covered below:

4 Pd salt + Plant source → Pd NPs biocompatible + biocompatible By-products

The proposed mechanism for reduced Pd NPs using Rosemary plant’s extract was that first because the metal ions are reduced from their salt predecessors by metabolites of plant macromolecules with reduction capacity, the metal ions first pass through the activation phase, during which the development rate of particles is typically slow. After that, during the nucleation phase, new nanoparticles are created when metal ions combine via ionic bonding with biometabolite reducing agents like flavonoids or terpenoids to cause reduction [22].

The Rosemary extract used to synthesize Pd NPs as both a stabilizer and a reducing agent was determined by FT-IR spectroscopy analysis. The bands corresponding to phyto molecules in rosemary extract were also observed in the FTIR spectrum of green synthesized Pd NPs. In Fig. 1a, Rosemary extract spectra show the O–H peak at ~ 3400 cm−1 [23]. Peaks at ~ 1025 cm−1, ~ 1500 cm−1, and ~ 2214 cm−1 express the existence of stretching of C–O, C=C, and C=O, respectively. Although the absorption bands of the rosemary extract were in the same positions, sometimes they were observed in the FT-IR spectrum of Pd NPs in minor shifts, for instance, the bands at ~ 3400, ~ 2900, ~ 1686, and ~ 1160 cm−1. The presence of these IR bands in the spectra of rosemary-mediated green produced Pd NPs suggests that the organic compounds in rosemary extract, which function as biological reducing agents, serve as capping ligands on the surface of the Pd NPs [24].

Fig. 1
figure 1

a FTIR spectra, b UV–vis absorption spectrum, c XRD Pattern, d 50 nm and e 100 nm scale TEM image, and f size histogram of Pd NPs

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Figure 1b illustrates the UV–vis spectra of Pd NPs. The transformation of the initially colourless liquid to black color indicates that Pd NPs were reduced by the Rosemary extract [25]. It was observed that no peak for Pd NPs [26]. This indicates that the Rosemary extract is a stabilizing and biologically reducing agent, as well as the Phyto molecules of this extract functionalizing the surface of Pd NPs [27].

Pd NPs’ crystal structure and typical crystal size were determined using XRD. The XRD diffraction analysis of synthesized Rosemary-mediated green of Pd NPs is shown in Fig. 1c.

Lattice plane clusters (111), (200), (220), and (311) are represented by the peaks at values of 40.26, 46.57, 68.03, and 81.86. It is obvious from the XRD pattern that the Pd NPs produced through green synthesis have a facial-centered cubic (FCC) structure. The crystal average particle size of Pd NPs was calculated as 4.80 nm. These values obtained for FCC structures are consistent with the literature [28,29,30].

The images in Fig. 1d and e illustrate the TEM image of Pd NPs at the scale of 50 nm and 100 nm, respectively. Figure 1f shows the size distribution of the rosemary-mediated green synthesized Pd NPs with average diameters of 4.91 nm. TEM images showed the spherical shape of Pd NPs and the presence of an organic component (bright contrast color) around the Pd NPs. The prepared NPs tend to agglomerated because of Van der Waals gravitational forces and the concentration of the coating must be controlled to prevent it. The results of the TEM images' measurements of particle size are consistent with those of previously published studies on the particle size of green-produced Pd NPs (< 20 nm) [24].

After finishing of characterization of the prepared Pd NPs, they have been tried for DPPH radical scavenging abilities. Normally, DPPH is a dark-coloured compound containing nitrogen radicals. This procedure was performed to test the activity of the antioxidative compounds functioning as hydrogen donors or proton radical scavengers [31]. Figure 2 shows the antioxidant capacities of both rosemary extract and Pd NPs. As shown in Fig. 2, the DPPH radical scavenging abilities of Rosemary-mediated green synthesized Pd NPs were concentration dependent. The DPPH scavenging percentage were determined as 5.6%, 12.8%, 21.4%, 40.5%, 71.2% and 82.7% at 10, 25, 50, 100, 200 and 500 mg/L, respectively. The highest DPPH scavenging percentage was 82.7% at 500 mg/L and the lowest DPPH scavenging percentage was 5.6% at 10 mg/L. Standard antioxidants exhibited higher DPPH scavenging activities than Rosemary-mediated green synthesized of Pd NPs at all tested concentrations. DPPH radical scavenging percentage for the green synthesized using gum ghatti (Anogeissus latifolia) Pd NP reported by Kora and Rastogi (2018) was 81.9% [32]. The antioxidant activities of the different solid materials (such as Schiff base, phthalocyanines, etc.) were also investigated [32, 33]. As a result, this investigation showed the high antioxidant activity by using Rosemary-mediated green synthesized Pd NPs.

Fig. 2
figure 2

DPPH scavenging activity of Pd NPs and standards

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The capacity to cleave DNA is said to work by plasmid supercoiled DNA relaxing into nicked circular and/or linear shapes. The DNA of closed circular forms (Form I) will migrate the fastest when the technique is applied to circular plasmid DNA. The supercoiled will transform into a slower-moving nicked version (version II) if one strand is severed. A linear form (Form III) will be produced if two strands are severed, and it lies between Form II and Form I [34]. The DNA cleavage activity of Pd NPs is shown in Fig. 3. The agarose gel electrophoresis test findings displayed that Pd NPs were able to create DNA damage at all concentrations. Form I changed into Form II at 200 mg/L for 30 min (Lane 2) while Form I changed into Form II and Form III at 500 mg/L for 30 min (Lane 3). Besides, Form I changed into Form II and Form III at 200 mg/L for 90 min and at 500 mg/L for 90 min (Lane 4 and Lane 5). Several metal complexes such as iron, silver, copper, cobalt, palladium, etc. can be induced primarily classical electrostatic interactions with DNA molecules. These interactions also can be caused by DNA binding or DNA cleavage. Yeginer et al. [36] reported that transition metal (II) complexes with a novel azo-azomethine Schiff base ligand showed DNA cleavage activity. Our results exhibited good agreement with their results.

Fig. 3
figure 3

Pd NP DNA Cleavage. After 30 min of incubation, the following lanes were added: Lane 1, pBR 322 DNA; Lane 2, pBR 322 DNA + 200 mg/L of Pd NPs; Lane 3, pBR 322 DNA + 500 mg/L of Pd NPs; After 90 min of incubation, the following lanes were added: Lane 4, pBR 322 DNA + 200 mg/L of Pd NPs; and Lane 5, pBR 322 DNA + 500 mg/L of Pd NPs

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The minimum inhibitation concentration (MIC) was used to assess the antimicrobial properties of Pd NPs. The results are presented in Table 1. The MIC values of Pd NPs were 32 µg/mL for E. coli, 128 µg/mL for E. hirae, 128 µg/mL for S. aureus, 64 µg/mL for P. aeruginosa, 128 µg/mL for B. cereus, 16 µg/mL for L. pneumophila and 256 µg/mL for C. albicans. The newly synthesized Pd NPs exhibited the strongest and the weakest antimicrobial activity against L. pneumophila and C. albicans, respectively. The antibacterial activity findings displayed that the Pd NP was more powerful to Gram (-) bacteria than Gram ( +) bacteria. Different mechanisms on the antimicrobial ability of metal-based nanoparticles have been recommended such as cell membrane disruption, binding and cleaving the nucleic acids, production of reactive oxygen species (ROS) which damage the cell [37]. Gram-negative bacteria are more easily penetrated by plant derived green Pd NPs compared to Gram-positive bacteria, which have a thick peptidoglycan coating, which may explain their antibacterial effects. Similar results were reported by Umadevi et al. (2011) [38] and Anjana et al. (2019) [39]. As a result, these findings are in line with the literature.

Table 1 MIC values of Pd NPs
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The optical absorption spectra of the MB aqueous solution with Pd NPs after exposure to sunlight irradiation for various time intervals are shown in Fig. 4a. Figure 4b shows the photodegradation rates of Pd NPs against MB dye at different times. As the irradiation period is increased, it can be observed that the strength of the MB absorption peak at 665 nm drops, which suggests that the Pd nanomaterial is degrading the MB molecules. The relative concentration (Ct/C0) of MB is depicted as a function of time in Fig. 4c, where Ct represents the concentration of MB at the moment of irradiation t and C0 represents the concentration of the dye prior to irradiation. In addition, ln (C/C0) versus time was plotted and the photodegradation rate constant of Pd NPs was calculated to be approximately 0.012 min−1 (Fig. 4d). Pd NPs showed 79.9% photodegradation against MB at 120 min. This indicates that Pd NPs have a good potential for photocatalytic activity.

Fig. 4
figure 4

a The change in the absorbance spectrum of the MB dye solution, b Photocatalytic degradation of MB dye, c C/C0 plot versus irradiation time under solar light, and d Kapp of MB degradation over Pd NPs

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Here is the proposed process of MB dye's photodegradation by PdNPs, when the surface of Pd NPs was exposed to solar light irradiation, electrons were attracted to the conduction band (CB) with the induction of holes in the valance band (VB). Light absorption, charge transfer, and separation on the prepared material's surface are often expected to have a significant role in influencing its catalytic efficiency [40, 41]. It was determined that the small size and charge transfer from PdNPs to the attached phytoconstituents, which effectively minimize the recombination of electrons leading to an increase in the degradation efficiency of PdNPs, are responsible for the fast photodegradation of MB dye [40, 42].

For the electrochemical study, firstly, the CVs of GCE electrodes coated with Pd NPs obtained by green synthesis in the potential range of − 0.8 V to + 0.2 V at a scan rate of 50 mV/s were investigated in methanol oxidation reaction (MOR). 1 M methanol (CH3OH) was used in the study. Since the activity of the prepared materials against methanol was measured in the study, the methanol ratio was not changed. During anodic scanning, a large peak around − 0.2 V appeared for methanol oxidation. This showed that the nanostructure is a good material for MOR in an alkaline medium [43]. In the study, the oxidation peak/forward current (If) was obtained as approximately 48.22 mA/cm2. It can be said that the reduction of palladium oxides is caused by this peak value. In the reduction/backflow (Ib) scans, the oxidation peak is 26.42 mA/cm2 due to the oxidation of methanol by fresh adsorption in the forward scan. The Ib/If ratio was obtained as approximately 0.548. If/Ib range indicates the resistance of the material to CO poisoning [44]. This value is ideal for the material obtained by green synthesis. Since CO poisoning will be at low levels when used as an anodic material in fuel cells, energy efficiency will remain stable for a certain period [45]. Figure 5 shows the CV voltammetry for the Pd NPs obtained by green synthesis. The figure also shows the CV obtained without CH3OH.

Fig. 5
figure 5

Cyclic voltammetry of biogenic Pd NPs; measurement at 50 mV/s scan rate of the cell in 0.5 M KOH and 1 M CH3OH (Bare; CV graph of Pd NPs without methanol addition, PdNPs; CV graph of Pd NPs after adding methanol)

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The electrochemical mechanism will provide a better understanding of absorption and CO poisoning. The possible reaction for the CH3OH oxidation reaction in an alkaline medium can be described by the following mechanisms (Eq. 1) [46].

$${\text{Pd }} + {\text{ C}}{{\text{H}}_3}{\text{OH}} \to {\text{Pd }}-- \, {\left( {{\text{C}}{{\text{H}}_3}{\text{OH}}} \right)_{{\text{ads}}}}$$
(1)
$${\text{Pd }} + \, {\left( {{\text{C}}{{\text{H}}_3}{\text{OH}}} \right)_{{\text{ads}}\;\;}} + {\text{ O}}{{\text{H}}^- }\;\; \to {\text{Pd }}-- \, {\left( {{\text{C}}{{\text{H}}_3}{\text{OH}}} \right)_{{\text{ads}}}} + \, {{\text{H}}_2}{\text{O }} + \, {{\text{e}}^- }$$
(2)
$${\text{Pd }} + \, {\left( {{\text{C}}{{\text{H}}_3}{\text{OH}}} \right)_{{\text{ads}}\;\;}} + {\text{ O}}{{\text{H}}^- }\; \to {\text{Pd }}-- \, {\left( {{\text{C}}{{\text{H}}_2}{\text{OH}}} \right)_{{\text{ads}}}} + \, {{\text{H}}_2}{\text{O }} + \, {{\text{e}}^- }$$
(3)
$${\text{Pd }} + \, {\left( {{\text{C}}{{\text{H}}_2}{\text{OH}}} \right)_{{\text{ads}}\;\;}} + {\text{ O}}{{\text{H}}^- }\;\; \to {\text{Pd }}-- \, {\left( {{\text{CHO}}} \right)_{{\text{ads}}}} + \, {{\text{H}}_2}{\text{O }} + \, {{\text{e}}^- }$$
(4)
$${\text{Pd }}-- \, {\left( {{\text{CHO}}} \right)_{{\text{ads}}}} + {\text{ O}}{{\text{H}}^- }\;\; \to {\text{Pd }}-- \, {\left( {{\text{CO}}} \right)_{{\text{ads}}}} + \, 4{{\text{H}}_2}{\text{O }} + \, {{\text{e}}^- }$$
(5)
$${\text{Pd }}-- \, {\left( {{\text{CO}}} \right)_{{\text{ads}}}} + \, 2{\text{ O}}{{\text{H}}^{ - \;\;}} \to {\text{Pd }}--{\text{ C}}{{\text{O}}_{2}}_{{\text{ads}}} + \, {{\text{H}}_2}{\text{O }} + \, 2{{\text{e}}^- }^-$$
(6)

According to the mechanism, a six-electron CH3OH electrooxidation process takes place. By chemisorption, methoxy species can weaken the CO bond of the methanol adsorbed on the biogenic Pd NPs electrocatalyst, and the alkaline environment can also lead to the formation of carbonate ions. By connecting the If and Ib amplitudes, it is also able to assess the tolerance to CO formation and other carbons (intermediate adsorbed species) [46,47,48].

Another electrochemical analysis is on the behaviour of the prepared catalyst at an increasing scan rate. The goal of this investigation used in fuel cells is to investigate how the designed catalyst for the MOR process's diffusion control mechanism works. The scan rates are 50–300 mV/s. For the experiment of the study, a 0.5 M KOH buffer system containing 1 M CH3OH was used in the application. The current values grew linearly as the scan rate increased, as seen in Fig. 6a and b. The result proves the system is diffusion controlled. There was no apparent shift to the right or left as the scan rate increased. Such changes are thought to be brought on by the MOR's production of by products, the generation of CO, and inadequate methanol adsorption on the catalyst surface [49]. However, it can be said that the adsorption mechanism works well on the Pd NP catalyst formed because of green synthesis. Furthermore, limited poisoning of the Pd catalyst due to the scarcity of CO and intermediates, no significant change in the Ib range was observed. Observation of the effect of different scan rates on the electrocatalyst gave important results.

Fig. 6
figure 6

a The impact of scan rate on MOR activity, and Current density values for Pd NPs, b the square root of scan rate from a

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Long-term testing of prepared materials to be used for fuel cells is also important. In this context, long-term stability tests were carried out by CA method for 5000 s in 0.5 M KOH medium containing 1 M CH3OH (Fig. 7). The lifetime test of the prepared material was also investigated in the study. The initial current value decreased rapidly during CA measurement at a potential of 0.55 V for 5000 s and then stabilized. It is thought that CO and other intermediate products resulting from the coating of MOR on the catalytic surface cause the initial current value to decrease over time. However, the current density never reached zero value. According to this situation, it can be said that the the prepared material exhibited stable behaviour for 5000 s [49,50,51].

Fig. 7
figure 7

CA for the long-term durability of biogenic Pd NPs in 0.5 M KOH to 50 mV/s

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The results obtained were compared with other publications in the literature (Table 2). According to the comparison results, the If ratio was found to be close to or better than the literature. Accordingly, it can be said that the obtained material can be used in different fields.

Table 2 The comparison of obtained results with other studies
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5 Conclusion

Rosemary extract has been used to generate a biogenic process for the making of Pd NPs. In brief, Pd (II) was reduced to Pd (0) using a rosemary-ethanolic extract. The resulting Pd NPs were characterized by UV spectroscopy, XRD, FTIR, and TEM techniques. In the TEM results, it was shown that the average particle size for Pd NPs was 4.91 nm. Pd NPs showed no absorption peak in their UV–vis spectra. Following other biological activities of Pd NPs, Pd NPs’ antioxidant, DNA interaction, and antibacterial efficacy may be helpful in pharmaceutical and medical applications. The results showed that the maximum DPPH scavenging percentage of Pd NPs was 82.7% at 500 mg/L and the lowest DPPH scavenging percentage was 5.6% at 10 mg/L. The Pd NPs showed a good antibacterial effect which shows the MIC values as follows: 32 µg/mL for E. coli, 128 µg/mL for E. hirae, 128 µg/mL for S. aureus, 64 µg/mL for P. aeruginosa, 128 µg/mL for B. cereus, 16 µg/mL for L. pneumophila and 256 µg/mL for C. albicans. In addition, Pd NPs showed 79.9% high photocatalytic activity against MB after 120 min. The current density obtained at 48.22 mA/cm2 for alcohol oxidation of Pd NPs was quite ideal compared to Pd-derived NPs obtained by green synthesis. The study proved the usability of the prepared materials for different applications. The simple, cost-effective, environmentally friendly method of this new method for nanoparticle production can have very good results in energy, environmental, biomedical, and biotechnological applications.