Characterization of AuNPs and AuNPs-ZnPc conjugated systems. For plasmonic enhancement with AuNPs, the NPs need to be large enough (>10 nm) so that the plasmonic response from the free electron cloud is sufficiently strong and the quantum size effects are not significant. The ~ 14.1 ± 1.3 nm NPs as obtained by the synthesis procedure are thus suitable for the present work. As shown in Figure (a) and (b), the TEM imaging of the NPs confirms the spherical shape and relatively monodisperse size distribution. It is known that the extent of plasmonic near-field enhancement increases with nanoparticle size up to ~50 nm and decreases thereafter. It is important to note that with increasing size, the surface area per unit of mass of the NPs also decreases. Thus, for the present work, the ∼ 15 nm NPs are small enough to ensure large surface area while still exhibiting a strong plasmonic response. To combine these NPs with the photosensitizer via click chemistry, a linker molecule is required that can strongly bond with the AuNPs. The azidothiol molecules form the strong -S-Au-bond and an azide group remains at the free end. The click chemistry catalyzed by the Cu(I) can thus facilitate the bonding between the Pc and the azide group to form a strong AuNPs-Pc linkage as shown in Scheme 1. The successful modification of AuNPs with azidothiol was further confirmed by measuring the zeta potential. All the observed peaks correspond to standard Bragg's reflections and are consistent with the reported data. Due to the amorphous nature of the Pcs, broad peaks in the range of 17° -26° are observed in Figure (b) with typical lattice indices (400) and (600). Previous studies have indicated that the presence of metalated Pc composites gives rise to distinctive XRD peaks within the 2θ range of 21° to 30°. These peaks can be attributed to the electron density originating from Pc, leading to modifications in the tilt angle and alterations in the intensity distribution observed in the XRD diffraction pattern. The diffraction peak of AuNPs can be seen in Figure (c) after click reaction with ZnPc as a very broad noisy peak at 17°-26°.