The result of a typical Au@ZnO synthesis is shown in Figure , for Au cores of 43 ± 3 nm in diameter and a ZnO shell thickness of 47 ± 4 nm. Figure shows the extinction spectrum of the reaction solution, containing AuNPs, L-ascorbic acid (AA), cetyltrimethylammonium bromide (CTAB), hexamethylenetetramine (HMTA), and zinc nitrate (Zn(NO3)2), prior to hydrothermal synthesis. At this stage, light extinction is solely attributed to the LSPR of the gold nanoparticles with a peak at 534 nm. Figure shows the extinction spectrum of the as-synthesized Au@Zn(OH)2 dispersion after the hydrothermal reaction. The formation of a hydrated Zn(OH)2 shell with an average thickness of 59 ± 5 nm induces a ~15 nm redshift in the LSPR wavelength to 549 nm, due to the slight increase in the refractive index of the gold surrounding from water to Zn(OH)2. The nanoparticle dimensions and core@shell structure are confirmed by scanning electron microscopy. To convert the hydrated Zn(OH)2 shell into crystalline ZnO, we centrifuge the Au@Zn(OH)2 NPs, redisperse them in water, dry them in air at 150 °C for 30 min, and calcinate them at 500 °C in air for 2 h. The obtained powder is then let to cool down and redispersed in water. Figure shows the extinction spectrum of a suspension of Au@ZnO core@shell nanoparticles, highlighting a further ~86 nm red-shift of the LSPR to 635 nm, due to the higher refractive index of ZnO compared to Zn(OH)2. The spectrum also shows the emergence of a peak at 375 nm, corresponding to the band gap energy of ZnO (E ≈ 3.37 eV) and confirming the presence of crystalline wurtzite phase ZnO, as confirmed via X-ray diffraction (XRD) measurement (see SI 1.1). The SEM images demonstrate a uniform particle shape and narrow size distribution, as also indicated by the narrow LSPR peaks observed in the UV-Vis extinction spectra. The inset of backscattered electron SEM image in Figure (middle row) shows the presence of a Au core in all the Au@ZnO NPs.