Extinction coefficients of gold nanoparticles with different sizes were determined by Zetasizer particle analyser and UV ndash;vis absorption spectroscopic measurement. Four different types of gold nanoparticles were prepared and studied: citrate-stabilized nanoparticles in two different sizes; ionic liquid-protected gold nanoparticles and CTAB-protected gold nanoparticles. A linear relationship between the logarithms of extinction coefficients and core diameters of gold particles was found ...