PLASMONIC NANOPARTICLES ASSOCIATED WITH 1,9-DIMETHYLMETHYLENE BLUE FOR THE INACTIVATION OF Candida albicans

Candida albicans infections remain a significant medical challenge due to the increasing resistance of this pathogen to conventional antifungal drugs. For this reason, the World Health Organization (WHO) has classified C. albicans as a high-priority fungal pathogen, particularly due to its growing resistance to azole antifungals. In this context, photodynamic inactivation (PDI) has emerged as a promising alternative for combating pathogenic microorganisms, particularly resistant fungi. PDI is based on the photoactivation of a photosensitizer (PS) in the presence of oxygen resulting in the production of reactive oxygen species (ROS), which damage cellular components and lead to microbial cell death. Among the PSs used in PDI, phenothiazines stand out, including 1,9-dimethylmethylene blue (DMMB), due to its cationic nature and higher lipophilicity, which may favor stronger cellular interaction. In addition, metallic nanoparticles (NPs) can enhance PDI through their localized surface plasmon resonance (LSPR), reduce PS aggregation, and improve PS-microbial cell interactions. Therefore, this study aimed to associate prismatic silver NPs (AgNPrs) with DMMB and evaluate their performance in the PDI of C. albicans. The AgNPrs were synthesized via chemical reduction, and the AgNPrs-DMMB systems were prepared in different proportions (1:4 and 4:1 v/v). Both the AgNPs and the systems were characterized using optical techniques and Zeta potential measurements. It was observed that AgNPrs-DMMB systems exhibited a less negative Zeta potential, indicating the association of the cationic DMMB with the anionic AgNPs. For PDI, planktonic cells were incubated with the PS or the systems for 10 min, followed by irradiation at ~24.2 mW/cm² for 3 min. The antimicrobial activity was evaluated by counting colony-forming units per milliliter (CFU/mL). The systems (1:4 and 4:1, containing 2 µM of PS) promoted complete eradication of the yeasts, whereas DMMB alone (4 µM) resulted in only a 3 log₁₀ reduction. Therefore, these results demonstrate that the association of AgNPrs with DMMB enhances antifungal efficacy, representing a promising strategy for combating C. albicans.