## 3 Results and Discussion
### Nanoparticle Characterization  
Scanning and transmission electron microscopy (SEM and TEM, respectively) were used to characterize NP size and morphology. The SEM micrographs showed a spherical nanoparticle with a size of approximately 5 nm without the surfactant and quercetin (Figure 2a). When the PVP and the quercetin were added to the reaction, an envelope was observed (Figure 2b,c). Agglomeration in PdNP synthesis with PVP has been shown [44; 45]. However, [PERSON] et al. [46] showed a palladium cobalt nanoparticle synthesized with PVP without agglomeration. Agglomeration could be attributed to PdNP synthesis, because, in this experiment, a reduction method synthesis was used. The influence of method synthesis in size and morphology has been shown in previous works [47; 48]. Figure 1: Nanoparticles UV/visible analysis. The absorbance was shown for all the components involved in the functionalization quercetin (Q), polyvinylpyrrolidone (PVP), Palladium nanoparticles (PdNPs)-PVP-Q. The TEM analysis showed that the nanoparticles have a hemispherical shape regardless of the PdNP type evaluated (Figure 2a-f). Similar results were found with the reduction of palladium chloride by tetraethylene glycol with KOH [45] and with reduction mediated by heat and the use of carboxymethylcellulose [41]. Nevertheless, other authors have reported PdNP synthesis as spherical, using methanol as a reducing agent [47], square-shaped, with citric acid as a reduction agent, heat, and PVP as a stabilizing agent [49], and flower-shaped and porous [16]. TEM analysis showed the differences in size with the PVP and quercetin addition (Figure 2g-i). The PdNPs alone showed a size between 3.7 and 14.47 nm, with almost 30% of PdNPs in a range from 6.42 to 7.42 nm (Figure 2g). The surfactant addition produces nanoparticles with a size range between 1 and 8.74 nm, with 35% of PdNPs in a size from 2.12 to 3.14 nm (Figure 2h). When the quercetin was added to PdNPs-PVP, the nanoparticles showed a tighter distribution from 2.3 to 8.29 nm (Figure 2i). Additionally, the PdNPs-PVP-Q showed 50% of PdNPs in a size range from 3.6 to 5.09 nm (Figure 2i). Quercetin helps nanoparticles to have a more uniform size, even more so the surfactant (PVP). Change in the distribution size could be attributed to the addition of quercetin on  
Figure 2: Characterization of size and morphology by electron microscopy. (**a**–**c**) Scanning electron microscopy: (**a**). PdNPs alone, (**b**). PdNP-PVP, and (**c**). PdNP-PVP-Q. (**d–f**) Transmission electron microscopy: (**d**). PdNPs alone, (**e**). PdNP-PVP, and (**f**). PdNP-PVP-Q. (**g–i**). Distribution analysis of primary size: (**g**). PdNPs alone, (**h**). PdNP-PVP, and (**i**). PdNP-PVP-Q. The white and black lines represent the scale in every micrograph in a-f.  
the PdNPs-PVP surface, which avoids nucleation and increases in size. It has been reported that NP size decreased when surfactant was added to the nanoparticle synthesis due to the NPs' stabilization [48; 50][49; 51; 52]. This stabilization depends on the molecule size employed, the charge, and the molecule's nature [9; 12]. Regarding NP synthesis mediated by an extract from plants, it was suggested that the reduction of metallic salts could be carried out by flavonoid molecules such as apigenin and luteolin [51]. Additionally, these molecules and phenols provide stability in the nanoparticle formed [52; 53]. Nanoparticles agglomerate in a solution depending on the solution's ionic strength and the nanoparticles' superficial charge [54]. Because of this, determining the size of the nanoparticles in suspension is important. To determine these nanoparticle characteristics, hydrodynamic size and zeta potential were analyzed (Figure 3). In each part of the NP synthesis, the addition of PVP or quercetin changes the hydrodynamic size and zeta potential (Figure 3a and 3b, respectively). Hydrodynamic size showed a diminution depending on the NP type (Figure 3a).