Here, C1 and C2 are parameters calculated from the slope and the intercept in Figure 7 using eq 4, the molar volume (197 cm3) and the density of gold (19.28 g cm-3). With the use of the theoretical values (C1 ) -4.70 and C2 ) 0.300) the average error for the calculation of particle sizes according to eq 12 is ∼9%. With the use of the experimental values (C1 ) -4.75 and C2 ) 0.314) this error is reduced to ∼6%, and hence eq 12 provides a precise way to calculate the size of particles in the range between 5 and 50 nm. If the size of the GNPs has been determined by one of the methods described above, then the number density of the particles (N) can be determined from the absorbance of the hydrosol. It was found that the calculated extinction efficiency at 450 nm, can be perfectly fitted with a Gauss function over the whole range of particle sizes (R2 ) 0.9999; CHI2 ) 10-4):