Here, B1 is the inverse of the slope (m) of the linear fit in Figure 6b and B2 ) B0/m where B0 is the intercept. Agreement with the experimental data is not perfect, and some spread is observed, resulting in an error of ∼18% for the calculation of the particle sizes if the best fit parameters are determined from the theoretical data in Figure 6b (B1 ) 3.55; B2 ) 3.11). If the experimentally determined fit parameters are used (B1 ) 3.00; B2 ) 2.20) the calculation of particle diameters using eq 11 results in an improved average deviation of ∼11%. Numerical data of d(Aspr/A450) which allow the reader to determine the size of GNPs without the need to perform calculations are presented in the Supporting Information