Methylene blue aggregation in the presence of human saliva

Abstract

Photodymanic antimicrobial therapy (PAT) has been proposed to treat oral infections and the phenothiazinic compound Methylene Blue (MB) has been considered as a suitable photosensitizer for this application. MB is a known methachromatic compound and the dimerization process may lead to different photochemical reactions, into the oral cavity, where the complete isolation of the saliva may not be possible. The aim of this study is to monitor the dimerization process of MB in the presence of human saliva through absorption spectroscopy. Absorption spectra of 30μM MB solutions in water and in human saliva were recorded in the wavelengths ranging from λ=400nm to λ=700nm. The spectra were recorded immediately after mixture and 1 min, and 5 minutes after blending. The results were evaluated by spectral analyses and through the calculus of the dimer/monomer ratio. The results demonstrated that immediately after mixture a hypochromic effect characterized by the diminishing on the total absorption in the visible range of the spectrum (λ= 400nm - 700nm) is observed, but the aggregation process is not detectable. After 1 minute the ratio between dimer and monomer absorption increase and this increase became higher upon increasing the contact time. The results indicate that the addition of saliva into the mixture leads to and hypochromic effect follow by the dye aggregation. Aggregation is probably an important variable to be analyzed when choosing the pre-irradiation time in oral cavity application, because it may lead to different photochemical routes.