The shell thickness of the final Au@ZnO NPs can be controlled by varying the number of gold nanoparticles in the initial reaction solution while keeping the concentration of the remaining reactants constant. The Au@ZnO NPs shown in Figure , are obtained with an initial concentration of 3.1×10 AuNPs/ml leading to a final diameter of the Au@ZnO core@shell nanoparticles of 137 nm, corresponding to a ZnO shell thickness of 47 nm. In Figure Precise knowledge of the AuNPs concentration allows us to estimate the yield of the ZnO synthesis reaction. Figure shows the extinction spectra of the particles shown in Figure , after the hydrothermal treatment but before the final annealing step, i.e. corresponding to Au@Zn(OH)2 core@shell NPs. As expected, suspensions with a larger initial concentration of AuNPs have higher overall extinction across the visible and a slightly blue-shifted LSPR, indicating a thinner Zn(OH)2 shell (Figure ). Figure shows the total volume of ZnO in the final Au@ZnO core@shell nanoparticle solutions of Figure , calculated from the initial AuNP concentration, 𝐶𝐶 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 , the radius of the gold cores, 𝑟𝑟 𝐴𝐴𝐴𝐴 , and the radius of the final Au@ZnO core@shell NPs, 𝑟𝑟 𝐴𝐴𝐴𝐴@𝑍𝑍𝑍𝑍𝑍𝑍 , according to: