Abstract: The present invention refers to Biodegradable, biocompatible, water-suspensable nanoparticle (1), for generating a second- or third-harmonic light signal upon illumination, as well as a method for preparing an aqueous suspension comprising said nanoparticle, a method for second-harmonic generation imaging of the nanoparticle (1) as and a use of the nanoparticle (1) for second-harmonic generation imaging. The nanoparticle (1) according to the invention comprises —a shell layer (2) comprising a biodegradable polymer (3), wherein the shell layer (2) encloses —a plurality (40) of oligopeptides (4), wherein the plurality (40) of oligopeptides (4) is structured such that a second-harmonic light signal is generated upon illumination of the nanoparticle (1) with light.