Doping zinc oxide with aluminum causes a decrease in refractive index of the surrounding medium of the Au cores, resulting in a blue-shift of the LSPR. The extinction spectra measured on Au@AZO NPs suspensions confirm this trend (Figure ) as the LSPR shifts from 617 nm for Au@ZnO to as low as 575 nm for a Au@AZO core@shell NP with a nominal shell doping of 4 at.%. In literature, the successful doping of ZnO with Al 3+ in thin AZO films is typically monitored by the lowering of the AZO bandgap with increasing dopant concentration. However, the high scattering of the Au@AZO NPs causes a strong increase in extinction at shorter wavelengths. This high scattering background affects the shape and spectral position of the ZnO bandgap, preventing us from using it as an indicator of dopant concentration.