Spontaneous Formation of Fractal Aggregates of Au Nanoparticles in Epoxy-Siloxane Films and Their Application as Substrates for NIR Surface Enhanced Raman Spectroscopy

We present a facile, inexpensive route to free-standing, thermo-mechanically robust and flexible epoxy-siloxane substrates embedded with fractal aggregates of Au nanoparticles, and demonstrate their efficiency as substrates for surface enhanced Raman spectroscopy (SERS) at NIR wavelengths. The metallodielectric films are prepared by generating Au nanoparticles through the in-situ reduction of gold (III) chloride trihydrate in epoxypropoxypropyl terminated polydimethyl siloxane (EDMS). The metal nanoparticles spontaneously aggregate into fractal structures in the colloid, which could then be drop-cast onto a substrate. Subsequent UV-initiated cationic polymerization of epoxide moieties in EDMS transforms the fluid colloid into a thin, free-standing film, which contains a dense distribution of fractal aggregates of Au nanoparticles. We used electron and optical microscopy as well as UV⁻Vis⁻NIR spectrometry to monitor the evolution of nanoparticles and to optically and structurally characterize the resulting films. Raman spectroscopy of the chromophore Eosin Y adsorbed onto the metallodielectric films showed that they are excellent SERS substrates at NIR excitation with an enhancement factor of ~9.3 × 10³.