The size of GNPs with diameters smaller than 35 nm cannot be determined using eq 10, and hence another method has to be employed in this size range. Since, for decreasing particle size, Aspr is increasingly damped relative to the absorbance at other wavelengths, the ratio of the absorbance at different wavelengths may be used to determine the particle size without knowledge of the concentration. Figure 6a shows theoretical results for the ratio of absorbance (A1/A2) of GNPs in dependence of the particle diameter, where the wavelengths are stated in the figure; for example, 450/600 refers to A1 at λ ) 450 nm divided by A2 at λ ) 650 nm.