Appendix 2. Analytical model to describe plasmon resonance
Gold nanoparticles

Metallic nanoparticles are increasingly used in both fundamental research and industrial applications. However, given the nanometric size of these objects, it is often difficult to obtain an immediate diagnosis of NP size and quantity using microscopy methods. In many cases, optical absorption spectroscopy can provide useful information. The analytical model proposed here is very simple to implement and enables an initial quantitative analysis of absorption spectra of metallic NP suspensions.