Preparation and characterization of SERS substrates: From colloids to solid substrates

Surface enhanced Raman spectroscopy is an important technique for detection of wide number of analytes. Today, there is a growing need for the development of stable, sensitive, reproducible and portable SERS-active substrates for use in different fields. Some of the most extensively used SERS substrates are metal colloids. However, the use of the colloid solutions for SERS deals with important issues, like stability and reproducibility. Other popular types of substrates are rough or nanoporous surfaces, such as porous silicon, coated with noble metals. Porous silicon (pSi) is a semiconducting material typically obtained by electrochemical etching in hydrofluoric acid (HF). Due to its high surface-to-volume ratio pSi is an interesting material for photonic and sensing devices, drug delivery systems as well as for use in SERS. In this paper, attempts are made to prepare a stable and uniform noble metal colloidal solution as well as solid substrate using porous silicon (pSi) with deposited silver and gold nanostructures. SERS substrates obtained by different methods are evaluated for SERS efficiency using methylene blue and rhodamine 6G (R6G) at 514.5 and 633 nm. The prepared substrates showed good stability and reproducibility. For some substrates the concentrations detected were below nanomolar range.