3-D Nanoporous optofluidic device for high sensitivity SERS detection

We report the demonstration of an optofluidic surface enhanced Raman spectroscopy (SERS) device that leverages nanoporous microfluidics to dramatically increase the SERS performance. A number of optofluidic approaches have been used to improve the detection limit of SERS in microfluidic channels, including active concentration of nanoparticles and/or analyte and passive concentration of nanoparticles. Previous reports have used a single nanofabricated fluidic channel to trap metal nanoparticles and adsorbed analytes. In this work, we utilize a significantly simpler fabrication approach by packing silica beads in a microfluidic channel to create a 3D nanofluidic concentration matrix. The device is fabricated using polydimethylsiloxane (PDMS) on glass using typical soft lithography methods. Due to the larger area of the nanoporous fluidic channel, this approach should be less prone to clogging than single nanofluidic inlets, and the loading time is decreased compared to previous reports. Using this microfluidic device, we achieved a detection limit of 4 femtomoles of Rhodamine 6G in 2 minutes. Compared to an open microfluidic channel, the 3D nanoporous concentration matrix increased the SERS signal by a factor of 250 due to the trapping of silver nanoclusters. Fiber optic cables are integrated into the PDMS to deliver excitation light directly to the detection volume and to collect Raman-scattered photons. As a result, the use of a laser diode and alignment-free integrated fiber optics implies the potential for the device to be used in portable and automated applications, such as the on-site detection of pesticides, water contaminants, and explosives.