# Matrix-Dependent Stability and Antibacterial Efficacy of Silver Nanoparticles: A Comparative Study of Anionic Carbopol vs. Non-Ionic Pluronic Hydrogels

## TL;DR

This study compares how two hydrogel types affect the stability and antibacterial power of silver nanoparticles, finding that anionic Carbopol gels perform better despite some instability.

## Contribution

The study reveals matrix-dependent behavior of AgNPs in hydrogels, particularly their stability and antibacterial efficacy on solid media.

## Key findings

- Carbopol–AgNP hydrogels showed superior antibacterial activity against Gram-negative bacteria in a size-dependent manner.
- Pluronic–AgNP hydrogels remained stable on agar but had lower antibacterial efficacy after accounting for the polymer's baseline activity.
- Carbopol–AgNP hydrogels aggregated on agar but outperformed Pluronic systems in net antibacterial efficacy against Gram-positive bacteria.

## Abstract

Background/Objectives: Hydrogels infused with silver nanoparticles (AgNPs) are widely used for their antibacterial properties, yet their stability, specifically upon contact with solid growth media (agar), remains poorly explored. This study compared two hydrogel matrices, anionic Carbopol 934 and non-ionic Pluronic F127, incorporating AgNPs of three different sizes. The evaluation focused on colloidal stability and antibacterial efficacy against Gram-positive and Gram-negative bacteria. Methods: In this study AgNPs (~20, ~55, and ~65 nm) were synthesised via a wet-chemical method and characterised by UV–visible spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM). AgNPs were incorporated into Carbopol 934 and Pluronic F127 hydrogel matrices. Colloidal stability was monitored over four months of storage and upon contact with tryptic soy agar (TSA). Antibacterial activity was assessed using agar diffusion assays. Results: Showed that both hydrogel systems maintained AgNP stability during storage, comparable to aqueous suspensions. However, upon contact with TSA, aggregation of Carbopol–AgNP hydrogels occurred, whereas Pluronic–AgNP hydrogels remained stable. In antibacterial assays, both hydrogels produced larger zones of inhibition (ZOI) than AgNP suspensions against Gram-negative bacteria (E. coli, P. aeruginosa), with Carbopol–AgNP hydrogels demonstrating superior efficacy in an inverse size-dependent manner. Against Gram-positive bacteria (S. aureus, S. epidermidis), Pluronic–AgNP hydrogels initially showed larger ZOIs due to the polymer’s intrinsic antibacterial activity. However, after correcting for this baseline, Carbopol–AgNP hydrogels exhibited superior net efficacy, with S. epidermidis showing greater susceptibility than S. aureus. Conclusions: While both Carbopol 934 and Pluronic F127 stabilise AgNPs during storage, the matrix type significantly influences behaviour at the biological interface. Carbopol–AgNP hydrogels aggregate upon contact with solid agar yet deliver superior, size-dependent antibacterial activity compared to the stable but less potent Pluronic systems.