Polymeric nanoparticles comprising a medium for photon up-conversion

A polymeric nanoparticle comprising a medium for photon up-conversion and a stabilizing agent, said medium comprising at least two components and a polymeric organic matrix component, said polymeric organic matrix component forming a polymeric matrix in which polymeric matrix said at least two components are distributed, wherein a first component of said at least two components is capable of absorbing light at a first wavelength region w≦λ1≦x, which first component acts as a sensitizer in said medium, and wherein a second component of said at least two components is capable of emitting light at a second wavelength region y≦λ2≦z, which second component acts as an emissive component in said medium, wherein λ2≦λ1, and wherein, upon absorption of light by said first component at said first wavelength region w≦λ1≦x, said emissive component emits light at said second wavelength region y≦λ2≦z, wherein said first component and said second component are organic compounds.