Organo-soluble decane thiol monolayer-protected gold nanoparticles of different shapes and sizes are homogenously mixed with strong fluorescent perylene diimide (PDI) dye molecules in both solution and solid states. The effect of the doped gold nanoparticles on PDI is investigated by UV–vis, fluorescence, transmission electron microscopy, scanning electron microscopy, and Raman experiments. The gold nanoparticles are found to modify the strong fluorescence properties of PDI depending on particle size and shape. The smallest spherical gold nanoparticles (2.6 nm) exhibit the strongest quenching effect. When the smallest spherical gold nanoparticles are cast on the PDI soaked paper followed by drying and/or sealing in a polydimethylsiloxane film, the area with gold nanoparticles shows significant quenching effects under a UV light but appears visually identical to the surrounding area under daylight. This demonstrates the potential to introduce gold nanoparticles into practical applications such as antifraud technologies.