Superhydrophobic bull's-eye for surface-enhanced Raman scattering.

We present a micro-patterned silicon structure that enables the preparation of a substrate for surface-enhanced Raman scattering (SERS) and pre-concentration of the analyte molecules. The structure is designed to produce a hydrophobicity gradient. As a result, a water droplet placed on it will remain centred on the structure as it dries, enabling delivery of materials to its centre. The structure is therefore referred to as a superhydrophobic bull's-eye. A water droplet containing gold colloids placed on it dries to produce a cluster at the bull's-eye centre. A second water droplet placed on it, this time containing analyte molecules, dries such that the molecules are delivered to the gold colloid cluster. We demonstrate the detection of molecules at low concentrations (Rhodamine 6G at 10(-15) M) from small droplets.