Methylene blue@silver nanoprisms conjugates as a strategy against Candida albicans isolated from balanoposthitis using photodynamic inactivation

Abstract

Balanoposthitis can affect men in immunocompromised situations, such as HIV infection and diabetes. The main associated microorganism is Candida albicans, which can cause local lesions, such as the development of skin cracks associated with itching. As an alternative to conventional treatment, there is a growing interest in the photodynamic inactivation (PDI). It has been shown that the association of photosensitizers with metallic nanoparticles may improve the effectiveness of PDI via plasmonic effect. We have recently shown that the association of methylene blue (MB), a very known photosensitizer, with silver prismatic nanoplatelets (AgNPrs) improved PDI of a resistant strain of Staphylococcus aureus. To further investigate the experimental conditions involved in PDI improvement, in the present study, we studied the effect of MB concentration associated with AgNPrs exploring spectral analysis, zeta potential measurements, and biological assays, testing the conjugated system against C. albicans isolated from a resistant strain of balanoposthitis. The AgNPrs were synthesized through silver anisotropic seed growth induced by the anionic stabilizing agent poly(sodium 4-styrenesulfonate) and showed a plasmon band fully overlapping the MB absorption band. MB and AgNPrs were conjugated through electrostatic association and three different MB concentrations were tested in the nanosystems. Inactivation using red LED light (660 nm) showed a dose dependency in respect to the MB concentration in the conjugates. Using the highest MB concentration (100 µmol⋅L−1) with AgNPr, it was possible to completely inactivate the microorganisms upon a 2 min irradiation exposure. Analyzing optical changes in the conjugates we suggest that these results indicate that AgNPrs are enhancers of MB photodynamic action probably by a combined mechanism of plasmonic effect and reduction of MB dimerization. Therefore, MBAgNPrs can be considered a suitable choice to be applied in PDI of resistant microorganisms.