Diffusion of Methylene Blue in Phantoms of Agar Using a Photoacoustic Technique

Abstract

In this work, the kinetics of diffusion of methylene blue in agar aqueous solution is studied using a photoacoustic technique. Two agar phantoms solutions in water with a relation of mass/volume of 0.01% and 0.05% were analyzed. The study was performed using a modified Rosencwaig photoacoustic cell that is enclosed by transparent windows, on both sides. The sample is deposited directly on top of the upper window. A red light beam, at a fixed modulation frequency, is sent through the lower window illuminating the sample and inducing the photoacoustic effect inside the closed chamber of the cell. At the beginning of the experiment, a droplet of 100μL of agar solution is deposited; afterwards, the signal stabilizes, and 10μL of methylene blue aqueous solution (0.0125 g · mL−1) is added to the surface of the agar. During the first seconds of the experiment, the photoacoustic signal amplitude increases followed by a gradual and long decay. Results for modulation frequencies in the range from 10Hz to 80Hz for both agar concentrations are presented. A simple theoretical approach is presented to analyze the experimental data. It is demonstrated that the kinetics of the process can be parameterized as a function of the changes of an effective optical absorption coefficient. From these results, the characteristic time, in which the dye diffusion process stabilizes, is obtained. It is found that this time is larger for samples with a higher agar concentration. These differences provide important results for biomedical sciences in which agar gels are used as phantoms resembling some of the properties of living organs and tissues.