The evaluation of the ECSA from the Au NPs reduction peak in acid, see Figure , are reported in Figure . The smaller the NPs, the higher the ECSA, as expected . The ECSA decreases in the order A>B>C>D>E, with 6.9 ± 0.4, 5.4 ± 0.9, 5.5 ± 1.5, 4.1 ± 0.5 and 3.1 ± 0.9 m 2 gAu -1 , respectively, which, taking into account the uncertainty of the measurements, follows rather well the order of increasing size A<B<C<D<E. These results illustrate the benefits and the validity of the presented SF colloidal synthesis method to study size effects in (electro)catalysis. The sample F, although it shows the largest size of 22 nm, shows a relatively high ECSA of 4.1 ± 0.8 m 2 gAu -1 . This is discussed further in the The normalization for the ECSA assumes a 100% conversion of the Au precursor during NP synthesis.