Discriminating between absorption and scattering coefficients in optical characterisation measurements on gold nanoparticle based photoacoustic contrast agents

Plasmon resonant nanoparticles such as gold nanoshells and gold nanorods can be tuned to possess sharp interaction peaks in the near-infrared wavelength regions. These have great importance as contrast agents in photoacoustic imaging and as photothermal agents for therapeutic applications due to their high absorptions. While the optical properties of the particles are can be described using Mie theory and/or numerical methods such as the Discrete Dipole Approximation, discriminating between their optical absorption and scattering in experiments is not easy. In this paper we discuss for the first time a novel method based on a two-fiber spectrometer that allows measurement of the scattering and absorption coefficients of gold nanoparticles in solution. This technique, called Differential Path length Spectroscopy, has been developed earlier for measurement in highly diffusive media such as tissue. We demonstrate this concept on gold nanospheres and nanoshells of various sizes. We believe that this will develop into a fast and reliable method able to work on small samples (<1 ml) of nanoparticles to obtain scattering and absorbing spectra