3. Silver nanoclusters supported by silver nanoparticles or silver oxideAs explained in the Introduction, silver nanoparticles larger than 2 nm exhibit plasmon resonance effects and typically do not present fluorescence. However, there are recent reports about luminescent silver nanoparticles. The emission of light from large silver particles is likely produced from surface-bound nanoclusters or subnanodomains within silver nanoparticles.Polycrystalline silver nanoparticles with diameter of 18 nm synthesized by thermal reduction of silver ions in glycinematrix are stable in aqueous solution and exhibit bright luminescence (Fig. 4a).71 The authors showed that the nanoparticles with single-crystalline or twined structures with domain sizes averaging 8 nm are not luminescent, whereas the polycrystalline nanoparticles with numerous small domains in the range of 1–2 nm exhibit luminescence. The luminesence is claimed to originate from these small domains, with sizes comparable to the electron Fermi wavelength (approx. 0.5 nm). The small domains present in the nanoparticles result in discrete energy states that allow optical transitions.