Impact of Protecting Ligands on Surface Structure and Antibacterial Activity of Silver Nanoparticles

Abstract

Silver nanoparticles (Ag NPs) have attracted much attention in the past decade because of their unique physicochemical properties and notable antibacterial activities. In particular, thiol-protected Ag NPs have come to the forefront of metal nanoparticle studies, as they have been shown to possess high stability and interesting structure-property relationships. However, a clear correlation between thiol-protecting ligands, the resulting Ag NP surface structure, and their antibacterial properties has yet to be determined. Here, a multielement (Ag and S), multi-edge (Ag K-edge, Ag L3-edge, S K-edge) X-ray absorption spectroscopy (XAS) methodology was used to identify the structure and composition of Ag NPs protected with cysteine. XAS characterization was carried out on similar-sized Ag NPs protected with poly(vinylpyrrolidone) (PVP), in order to provide a valid comparison of the ligand effect on surface structure. The PVP-Ag NPs showed a metallic Ag surface and composition, consistent with metal NPs protected by weak protecting ligands. On the other hand, the Cys-Ag NPs exhibited a distinct surface shell of silver sulfide, which is remarkably different than previously studied Cys-Ag NPs. The minimum inhibitory concentration (MIC) of both types of Ag NPs against Gram-positive (+) and Gram-negative (-) bacteria were tested, including Staphylococcus aureus (+), Escherichia coli (-), and Pseudomonas aeruginosa (-). It was found that the MICs of the Cys-Ag NPs were significantly lower than the PVP-Ag NPs for each bacteria, implicating the influence of the sulfidized surface structure. Overall, this work shows the effect of ligand on the surface structure of Ag NPs, as well as the importance of surface structure in controlling antibacterial activity.

Characterization of silver nanoparticles and evaluation of their antimicrobial effect on Salmonella

“…Although the Salmonella serotypes showed different resistance profiles, the MIC for the bacteria tested was considered lower than other studies with gram positive bacteria Research, Society and Development, v. 9, n. 9, e103996435, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i9.6435 effective on gram negative bacteria than gram positive bacteria (Buszewski et al, 2016;Panácek et al, 2017). In contrast, recent studies have shown that the interaction of Ag-NPs with the bacterial surface depends on how their synthesis was performed and the type of coating used, considering that according to the bacterial structures these Ag-NPs can be coated, which will allow them to more easily permeate the plasma membrane of cells, regardless of whether they are gram positive or negative (Padmos et al, 2015;Zheng et al, 2018).…”

Structure of Tiopronin-Protected Silver Nanoclusters in a One-Dimensional Assembly

“…22,23,46,50 However, based on the two types of Ag-Ag bonding from the WT-EXAFS analysis, we performed a three-path refinement of the FT-EXAFS data similar to a technique recently used to study thiolate-protected gold nanoclusters of a similar size (~1-2 nm). 51 In this way, the FT-EXAFS was refined with Ag-S and Ag-Ag paths first, and then the Ag-S path was held constant in the subsequent three-path refinements with Ag-S, Ag-Ag 1 (metal-type) and AgAg 2 (sulfide-type).…”

“…The NCs were also more effective against gram negative bacteria than gram positive bacteria, which is consistent with similarly run MIC assays of other Ag NPs that were protected by cysteine (Cys-Ag NPs). 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 …”

“…13,21 Slightly larger Ag 62 S 13 (SBu t ) 32 (BF 4 ) 4 NCs (Ag 62 NCs for short) exhibit even more highly sulfidized surfaces with very little metallic Ag in their cores, and their enhanced luminescence was attributed to this. 22 Furthermore, larger Ag nanoparticles (NPs) protected by thiols have also shown high sulfidation at their surfaces, 23,24 and this was suggested to hinder their antibacterial effects in some cases. 23 Hence, the sulfidation at the surface of Ag NCs or NPs as a result of the thiol ligands is an emerging trend in both NC and NP synthesis, and has far reaching implications for both understanding the surface structures of Ag NCs/NPs and their application to various fields.…”

“…22 Furthermore, larger Ag nanoparticles (NPs) protected by thiols have also shown high sulfidation at their surfaces, 23,24 and this was suggested to hinder their antibacterial effects in some cases. 23 Hence, the sulfidation at the surface of Ag NCs or NPs as a result of the thiol ligands is an emerging trend in both NC and NP synthesis, and has far reaching implications for both understanding the surface structures of Ag NCs/NPs and their application to various fields. Therefore, understanding the effect of other thiol ligands on the surface structure of Ag nanomaterials is an important and ongoing topic of research.…”