Measurement of the Optical Properties of Gold Colloids by Photoacoustic Spectroscopy

The particular features of gold have generated widespread interest for applications in different areas of science and technology. Notably, gold nanoparticles can be prepared with different sizes and forms and can be easily functionalized with a wide range of ligands. Developing effective experimental techniques to characterize such properties is thus important. In this work, photoacoustic spectroscopy was used to explore the relationship between the nanoparticle size and the optical absorption coefficient (at 405 nm and 532 nm) of gold colloid solutions, according to the Beer-Lambert's law. A correlation between this optical parameter and the nanoparticle size was found. In addition, for comparison purposes, conventional UV-visible spectroscopy was used for measuring the absorbance at these two wavelengths. Very good agreement was obtained between the optical properties measured by the two methodologies at 405 nm. However, large discrepancies were obtained when measurements were performed at 532 nm. At the latter wavelength, the extent of radiation dispersion is too large for the Beer-Lambert's law to be valid when the conventional spectroscopy technique is used. Unlike the UV-visible spectroscopy, the photoacoustic method is minimally affected by radiation dispersion effects. Thus, the photoacoustic method presents fewer limitations in that regard in characterizing the optical properties of metallic colloidal suspensions.