New optoacoustic spectroscopy system to quality and characterize absorbers in turbid mediaexperimental mesaurements and analysis of gold nanoparticles for biomedical applications

Resumo

The limited penetration depth of optical energy in biological media is primarily due to the high level of optical scattering. This optical attenuation places an upper limit on optical, diagnostic, and therapeutic techniques for in in-vivo applications. The optoacoustic technique combines the unique advantages of light and sound, providing high levels of contrast, while maintaining the spatial resolution of acoustic techniques. The importance of colloidal gold solutions, as optical contrast agents in optoacoustic biomedical applications, is to increase the absorption of optical energy in areas where scattering is dominant and has prompted the study presented in this thesis on their characterization and quantification. It has been demonstrated that the acoustic transients generated from the optical irradiation are directly proportional to the wavelength dependent absorbed optical energy. Advantage has been taken of this fact to design two separate optoacoustic schemes. The first has been implemented to quantify the optical absorption, at a single laser wavelength, of spherical gold nanoparticles embedded within a medium that simulates the high scattering nature of soft tissue. The second has been designed to obtain the spectroscopic profile of such gold nanostructures. The results obtained from the optoacoustic analysis are compared to those from a reference measurement scheme, based on collimated optical transmission, which is in parallel and under the same conditions. This comparative analysis has confirmed the interest for the proposed technique of optoacoustics for laser spectroscopy of gold nanoparticles. In this thesis results obtained, for the first time, on the quantification and spectroscopic analysis of gold nanoparticles embedded in a liquid phantom using the optoacoustic technique are presented. The results have been confirmed by the reference measurement scheme and an offline commercial spectrophotometer. To further demonstrate the potential of the optoacoustic technique, information regarding the position and dimensions of the absorbing solutions have been extracted from measurements. The spectroscopic analysis has been carried out in steps of 5 nm over the complete wavelength range from 450 nm to 650 nm for a spherical gold nanoparticle solution and 410 nm to 1000 nm for a gold nanorod solution. This research work has opened up new lines of investigation in the field of monitorization of nanoparticle contrast agents embedded in biological media for optoacoustic diagnostics and biomedical applications.