## 5 Discussion  
Conventionally, Au-NPs are synthesized by reducing HAuCl\({}_{4}\) with citrate and other types of organic reducing agents (e.g., cyclohexanone and 3-glyceroloxypophylltrimethoxysilane); their nano geometry can be between 10 and 33 nm [48; 49; 50]. Polyethyleneimine (PEI) has been introduced as a stabilizer and reducing agent for metal-nanoparticle synthesis [51; 52]. PEI is a cationic polymer used in various industrial applications and drug delivery, especially nucleic acid delivery in mammalian cells [53; 54]. However, the previously reported synthetic routes are tedious and time-consuming. We have previously reported a nano-geometry-controlled and rapid synthesis protocol for producing gold and silver nanoparticles using cyclohexanone and formaldehyde as reducing agents, and we evaluated the antimicrobial activity of these materials [55; 56; 57]. In this study, we synthesized PEI-stabilized AuNPs and functionalized them with vancomycin. In this procedure, vancomycin interacts with the gold cation in an unusual way that has not been previously reported; however, some previous studies have reported a unique interaction of vancomycin with copper (II) and iridium (III) ions [58; 59]. However, the coordination mode associated with the chemical groups interacting with vancomycin and the gold ion remains to be determined without crystals suitable for X-ray structural assignment. The unclear coordination mode may be dependent on the various Lewis bases in vancomycin: the two adjacent, but not consecutive, nitrogen atoms in the amide group and the -NHCH\({}_{3}\) group, the 2, 2-biphenolic units, the carboxylate on the C-terminus, and the oxygen atom of the asparagine side chain. All of these can potentially serve as coordination sites between vancomycin and the metal center [58; 59]. Vancomycin is a glycopeptide with cationic properties and micellar behavior, and it is most likely that PEI participates in the nucleation and stabilization of gold nano geometry. Furthermore, the functionalization of the surface of the gold nanoparticles with vancomycin was confirmed by DLS and fluorescence spectroscopy. An increase in the hydrodynamic radii compared to bare gold nanoparticles and a shift in the emission maxima  
Figure 9: TEM micrograph of PEI-AuNP@Van-treated conidia of _A. fumigatus_ and _A. flavus_: (**a**) represents the untreated conidia of _A. fumigatus_, and (**b**) treated with PEI-AuNP@Van; (**c**) represents the untreated conidia of _A. flavus_, and (**d**) treated with PEI-AuNP@Van. (335-418 nm) at the same excitation wavelength (280 nm) confirmed the functionalization (Figures 1 and 2) [33]. Vancomycin exhibits a unique mode of action and is used as an antibacterial agent, mostly against Gram-positive bacteria. It establishes hydrogen bonds with terminal d-alanyl-d-alanine moieties in the NAM/NAG peptides, which prevents the incorporation of the NAM/NAG-peptide subunits within the peptidoglycan matrix [29]. Additionally, there is evidence that vancomycin modifies cell membrane permeability and prevents ribonucle-cleic acid synthesis. Unlike Gram-positive bacteria, the conidial wall of Aspergillus has a complex structure composed of unique polysaccharides (chitin, chitosan, galactosamine-galactan, and galactomannan) and glycoproteins (mannoproteins). Galactosaminogalactan comprises \(\alpha\)-1-4 linked galactose and \(\alpha\)-1-4 linked N-acetyl galactosamine residues [60]. _A. flavus_ conidia are heterogeneous in size; they exhibit surface-exposed polysaccharides as well as an amorphous layer that interfaces with the surface rod-let layer. In contrast, _A. fumigatus_ conidia exhibited homogeneity in terms of size, with a rod-shaped layer covering their surface. _A. fumigatus_ conidia are uniformly covered by the hydrophobic rod-let layer; _A. flavus_ conidia exhibit additional amorphous surface patches that are associated with the lowering of hydrophobicity [61]. _A.