We found that the particle size can be determined with higher precision if the initial concentration of gold (cAu in mol per liter) used to synthesize the particles is known. In Figure 7, the calculated molar decadic extinction coefficient at the surface plasmon resonance ( spr in M-1 nm-1) per unit volume of gold (nm-3) is plotted against ln(d) for particle sizes between 5 and 50 nm. The agreement between theory and experiment is excellent, and the linearity of both experiment (R2 ) 0.996) and theory (R2 ) 0.998) is striking. The particle diameter (in nanometers) can thus be calculated from cAu (in mol per liter) and the absorbance at the surface plasmon resonance (Aspr) according to