Nanoengineering of eco-friendly silver nanoparticles using five different plant extracts and development of cost-effective phenol nanosensor

The production of environmentally friendly silver nanoparticles (AgNPs) has aroused the interest of the scientific community due to their wide applications mainly in the field of environmental pollution detection and water quality monitoring. Here, for the first time, five plant leaf extracts were used for the synthesis of AgNPs such as Basil, Geranium, Eucalyptus, Melia, and Ruta by a simple and eco-friendly method. Stable AgNPs were obtained by adding a silver nitrate (AgNO3) solution with the leaves extract as reducers, stabilizers and cappers. Only, within ten minutes of reaction, the yellow mixture changed to brown due to the reduction of Ag+ ions to Ag atoms. The optical, structural, and morphology characteristics of synthesized AgNPs were determined using a full technique like UV–visible spectroscopy, FTIR spectrum, XRD, EDX spectroscopy, and the SEM. Thus, Melia azedarach was found to exhibit smaller nanoparticles (AgNPs-M), which would be interesting for electrochemical application. So, a highly sensitive electrochemical sensor based on AgNPs-M modified GCE for phenol determination in water samples was developed, indicating that the AgNPs-M displayed good electrocatalytic activity. The developed sensor showed good sensing performances: a high sensitivity, a low LOD of 0.42 µM and good stability with a lifetime of about one month, as well as a good selectivity towards BPA and CC (with a deviation less than 10%) especially for nanoplastics analysis in the water contained in plastics bottles. The obtained results are repeatable and reproducible with RSDs of 5.49% and 3.18% respectively. Besides, our developed sensor was successfully applied for the determination of phenol in tap and mineral water samples. The proposed new approach is highly recommended to develop a simple, cost effective, ecofriendly, and highly sensitive sensor for the electrochemical detection of phenol which can further broaden the applications of green silver NPs.

www.nature.com/scientificreports/ leaves of Eucalyptus, Melia, Ruta, Geranium, and Basil respectively, due to the excitation of the SPR of AgNPs. In this study, the synthesis of AgNPs is approved by UV-visible spectrum in the range of 400-422 nm which is the characteristic wavelength range of silver nanoparticles 40 .
FTIR analysis. FTIR help us investigating the NPs functional groups and determining the biomolecules that lead to the formation of AgNPs. Figure 2 shows the FTIR spectrum of AgNPs synthesized from different leaf extracts. The large band at 2800-3500 cm −1 is attributed to the stretching vibration of N-H 41 . The bands observed between 1702 and 1730 cm −1 , and 1570-1611 cm −1 are related respectively, to the -C=O group and amide I band of protein 42 . Then, the two other bands between 1383 and 1447 cm −1 and 1023-1087 cm −1 are characteristic of the stretching vibrations of C-N, respectively, for aromatic and aliphatic amines. It has been shown previously that proteins can be linked to NPs via free amine groups or cysteine residues in proteins. This leads to the suggestion that the protein could be the reducing and stabilizing agent of the AgNPs 43 . Figure 3 shows the mechanism suggested for the biosynthesis of AgNPs using a leaf extract.
XRD analysis. XRD analysis was used to investigate the crystalline AgNPs structure. So, the vacuum-dried nanoparticles were reported to be elemental silver. As displayed in Fig. 4, the Bragg's reflections showing (111), (200), (220), and (311) plans of FCC crystalline structure due to metallic silver, are presented at the range of 38°-41°, 48°-51°, 65°-67°, and 72°-75°, respectively. The AgNPs average size was calculated using the Scherrer formula, as mentioned elsewhere 40 . The size of the different types of NPs was around 40, 30, 50, 21, 26 nm for AgNPs-B, AgNPs-G, AgNPs-E, AgNPs-M, and AgNPs-R, respectively. It has been reported that the electrocatalytic activity of NPs is highly dependent on their size and their morphology 44,45 . Besides, the presence of smaller nanoparticles could be beneficial for sensor development than the larger ones. Here, AgNPs-M is smaller in size than other types of nanoparticles like AgNPs-B, AgNPs-G, AgNPs-R, and AgNPs-E.  www.nature.com/scientificreports/ In this perspective, AgNPs-M seems to be the most suitable for the electrochemical study, due to its small size, and hence its use in our following work.

Application of AgNPs-M for the development of phenol sensors. Nanostructural properties: SEM
and zeta sizer studies. EM analysis was utilized to identify the morphology and the size of stabilized AgNPs-M at several magnifications (100×, 1000×, 50,000× and 100,000×) ( Fig. 5A-D). SEM results showed that AgNPs-M has spherical morphologies and 23 ± 3 nm as an average size, with a high percentage of nanoparticles (Fig. 5E). However, we can find some bigger NPs with a low percentage of about 15%, probably due to the agglomeration of small ones. In addition, the zeta potential results of green AgNPs indicate a negative value of − 13.1 mV, which indicates the good stability and the correct dispersion of AgNPs.
Furthermore, EDX analysis was employed to define the chemical composition of AgNPs-M. As shown in Fig. 5F, at 3 keV, we notice a high signal corresponding to elemental silver, confirming the AgNPs presence 46,47 . Besides, O, K and Ca atoms signals, obtained due to the plant residues capping the AgNPs (Table 1).  We notice the high sensitivity, of about 0.410 A/M, of our developed sensor. The LOD was calculated according to the equation: (3 × SD blank )/slope was 0.42 µM (n = 3). Table 2 shows several of the LODs mentioned in the literature. We can see that our obtained LOD is one of the lowest values.
Reproducibility, repeatability, and stability at AgNPs-M/GCE. Reproducibility, repeatability, and stability are the main elements of sensor performance. Using three different similarly prepared electrodes and in 5 μM of phenol, reproducibility was verified by DPV, showing an RSD of 3.18% (n = 3). Repeatability was tested through the response of 6 µM of phenol using the same electrode, three times in succession, showing an RSD of 5.49% (n = 3). The stability of our proposed sensor was estimated by conserving the electrode for four weeks and measuring the phenol twice a week. After this period, the sensor maintained 93.11% of its initial response for the determination of phenol, indicating good stability of AgNPs-M/GCE. The results obtained indicate the good reproducibility of our proposed sensor besides a good repeatability and long-term stability.

Selectivity of the developed sensor towards other phenolic compounds.
To estimate the selectivity of the developed phenol sensor towards other phenolic compounds (nanoplastics) such as bisphenol A (BPA) and catechol (CC), particularly for their detection in mineral water contained in plastic bottles, we have carried out their simultaneous analysis. The obtained results showed that the peak potential of phenol was quite stable with a slight decrease of the oxidation current peak by less than 10% in the presence of different concentrations of BPA and CC. Therefore, our proposed simple, cost effective and eco-friendly nanosensor exhibits good selectivity for the determination of phenol with a potential opportunity of application for its fast analysis in the water contained in plastics bottles.

Practical applications in water analysis.
To assess the response, in real applications, of the AgNPs-M/ GCE structure based phenol sensor, the presence of phenol was investigated in samples of tap and mineral waters. In this context, each of these real samples was prepared by adding 5 ml of water and 5 ml of PBS, then the determination was studied by DPV under optimized experimental conditions. The first test was without phenol, then we have added 5 and 8 µM of phenol to the water sample. Table 3 shows the recovery results which are about 100% and 107.4%. These good results of our AgNPs-M/GCE based phenol nanosensor, showed that it is highly adequate for application in real water samples.

Conclusions
We have demonstrated, that the synthesis of AgNPs is possible by a simple and rapid biological route using various extracts from the leaves of plants such as Basil, Geranium, Ruta, Eucalyptus, and Melia at room temperature. Biosynthesized nanomaterials have been well characterized by several techniques such as UV/visible, XRD and www.nature.com/scientificreports/ FTIR spectroscopy, and electron microscopy (SEM and EDX), resulting that the silver nanoparticles synthesized from Melia azedarach leaves extract (AgNPs-M) have less size at about 23 ± 3 nm compared to the other types of nanoparticles, which would be interesting for electrochemical sensors activity. The AgNPs-M were used as catalysts in electroanalysis, and were drop-casted onto the surface of Glassy Carbon transducers to build a novel AgNPs/GCE modified nanosensor. This sensor was employed for determination of phenol, obtaining excellent figures of merits concerning: high sensitivity, low LOD of 0.42 µM, good stability with a lifetime of about 1 month and also a good selectivity towards other phenolic compounds with a deviation less than 10%, as well as  www.nature.com/scientificreports/ www.nature.com/scientificreports/ repeatable and reproducible results with an RSD of 5.49% and 3.18% respectively, thanks to the green AgNPs as cost-effective nanomaterials. Besides, our proposed nanosensor has been employed successfully for phenol determination in tap and mineral water samples showing a good recovery percentage values between 100 and 107.4%. We think that our results provide an exciting perspective for sustainable large-scale nanoplastics sensors production by glassy carbon electrode surface engineering using green nanoparticles for water quality monitoring.

Methods
Materials. The plants such as Basil, Geranium, Eucalyptus, Melia, and Ruta were gathered from the ISA-CM (Tunisia). AgNO3 was purchased from Sigma-Aldrich, Germany. Phenol, bisphenol A (BPA), and catechol were obtained from Sigma-Aldrich. Dipotassium phosphate (K 2 HPO 4 ) and monopotassium phosphate (KH 2 PO 4 ) from Merck (Germany), were utilized in the preparation of the phosphate buffer solution (PBS).
Declaration statement. We declare that the collection of plant material is in accordance with relevant institutional, national and international guidelines and legislation.
Preparation of plant leaf extracts. The plant leaves were collected and washed multiple times to eliminate all dirt particles. The cleaned leaves were parched for 12 h in an oven at 60 °C temperature and then powdered (Fig. 8). An amount of 2 g of this powder was mixed, for 5 min, with 50 ml of boiling water. Finally, the obtained solution was spun 15 min, at 5000 rpm, in a centrifuge, filtered using filter paper, and then stocked in refrigerator at 4 °C for future experiences.
Green synthesis of AgNPs. The synthesis procedure was explained in our previous work 40 . A volume of 10 ml of 0.025 M AgNO 3 solution was mixed with 40 ml of each aqueous plant extract. Before being filtered and dried, the mixed solution was put to heat for 10 min at 40 °C, and as evidence of the creation of AgNPs, we can see a deviation from yellow to brown. Finally, the obtained NPs were stored for further experiments.
Instrumentation. The synthesized silver nanoparticles formation, was assured using UV-visible spectroscopy. Their optical properties were done using UV-Vis spectrophotometer Unico S-2150, from 200 to 800 nm, with 1 nm resolution.
The size and crystalline structure of biosynthesized AgNPs were studied using XRD analysis by a Panalatical X'pert PRO diffractometer (PXRD) using CuKα radiation (λ = 1.54 nm) with the scanning 2θ angle in the range of 30°-80° at room temperature.  www.nature.com/scientificreports/ The morphologies and the average size of the AgNPs-M were obtained using a high-resolution FEI Q250 Thermo-Fisher ESEM with 2.9 nm resolution at 30 kV.
EDX was utilized to study the composition of the AgNPs-M. The detector EDX was bound with the SEM instrument.
Fabrication of AgNPs-M/GCE based phenol sensor. The fabrication of our electrochemical sensor based on GCE modified with synthesized silver nanoparticles from a Melia leaf extract (AgNPs-M/GCE), has been carried out by casting 8 µl of AgNPs-M onto the clean surface of GCE (Fig. 9). Then, the prepared AgNPs-M/GCE was dried in darkness at ambient temperature for 24 h.
Electrochemical measurements. The EC measurements were performed, by gradually raising the phenol concentration in the PBS solution, in an electrochemical cell containing three electrodes, the working electrode is GCE, the counter electrode is made of Pt, and the reference electrode is Ag/AgCl.
The electroanalytical technique used is the differential pulse voltammetry (DPV) in the potential range of 0.1-0.9 V and as parameters we have the PT of 100 ms, SS of 60 mV s −1 , PA of 100 mV.   www.nature.com/scientificreports/