Gold cluster coatings enhancing Raman scattering from surfaces: Ink analysis and document identification

Based on the method of laser-ablation in liquids (LAL) with a strategy of bubbling nitrogen through a custom-made chamber, we prepared chemically-pure gold clusters which were found to be metastable for an extensive period of time beyond months. A practical use of discrimination among different surfaces is demonstrated here by applying the gold clusters as surface coatings which result in surface-enhanced Raman scattering (SERS) due to the surface plasmon resonance (SPR). This technique identifies various documents from different printers/copiers and written with different pen-inks. The stable and additive-free gold clusters enable repetitive examinations without impurity interference.