Point-and-Shoot Synthesis of Metallic Ring Arrays and Surface-Enhanced Optical Spectroscopy

The realization of optically active structures with direct-write printing has been challenging, particularly in spatially constrained microfluidic devices which are essential for point-of-care (POC) applications. The existing techniques are limited by resolution, accessibility, and multistep fabrication constraints. Point-and-shoot strategies to achieve site-specific fabrication of optically active Ag rings and on-demand targeted surface-enhanced optical spectroscopy are reported. Stable microbubbles over an Au nanoisland (AuNI) substrate are generated using a continuous-wave laser at low power (approximate to 0.5mW mu m(-2)). Analytical modeling of bubble generation process substantiates the evolution of ring morphology and its power dependence. The tunable Ag rings exhibit surface plasmon resonances in the mid-IR regime from 3.8 to 4.6 mu m, while the AuNI shows visible region response. The Ag ring over the AuNI imparts intensified surface-enhanced Raman spectroscopy (SERS) activity owing to amplified hot spots at Ag ring/AuNI interface. As an example, SERS and surface-enhanced infrared spectroscopy of rhodamine 6G, crystal violet, and 2,4,6-trinitrotoluene molecules, respectively, are demonstrated. The applicability of this technique to perform in situ fabrication and SERS sensing in microfluidic channels is shown. Using a simple in situ approach toward optically active structures, our technique can synergize multiple surface-enhanced optical spectroscopies to facilitate POC applications.