Multifunctional Nanowire Evanescent Wave Optical Sensors

Compact, reusable chemical sensors are highly desirable for on-site detection in the field, including the identification of water contaminants, hazardous biochemical compounds, or blood serum content. Ideally, such a sensing platform should be portable, and employ several complementary sensing modalities that allow quantitative chemical identification of extremely small sample volumes. Optical spectroscopy is a powerful analytical tool for characterizing biological and chemical systems, but making a standard optical laboratory transportable is a major challenge. However, with recent advances in the synthesis and assembly of nanomaterials, [1] it is timely to begin integrating these materials into functional device architectures for sensing and monitoring. Of the well-studied inorganic nanostructures, chemically synthesized 1D semiconductor systems have gained significant interest from the photonics community as passive and active components for miniaturized spectroscopic devices. This is due, in part, to their ability to guide a significant portion of the confined electromagnetic energy outside the measurement cavity (i.e., in the evanescent field) while operating below the diffraction limit of light. [2] Because the evanescent field efficiently travels through fluidic and air dielectrics, [3] it is possible to integrate the waveguides into microfluidic devices and sense molecules located near the surface of the cavity. We demonstrate this by performing absorbance, fluorescence, and surface-enhanced Raman spectroscopy (SERS) measurements on sub-picoliter volumes of solution. The chemical specificity of SERS is obtained by decorating the waveguide with silver nanocubes, thus enhancing the field around the nanoribbon. Our nanowire optical sensing platform complements nanowire field-effect sensors [4] with the ability to monitor optical attenuation across the wire element. However, the use of photons instead of electrons allows optical spectroscopy to be carried out on the analyte. Fiber-based detection is a unique alternative to free-space sensing, because it localizes chemical recognition at the surface of a waveguide. Among the most popular sensing schemes that rely on the evanescent field of a fiber are absorption [5–7] and fluorescence. [8–11] Typically, these set-ups involve multimode silica fibers with diameters much larger than the free-space wavelength of light. The evanescent field in these experiments has been used to measure refractive indices of liquids, [12] monitor volatile compounds in water, [13] and detect shifts in localized surface-plasmon resonances of coupled metal colloids. [14] Recently, the use of subwavelength silica fibers in a Mach–Zehnder-type interferometer to detect index changes caused by molecules interacting with the surface of the fibers has been proposed. [15] Although these various sensing configurations are promising for high sensitivity, fast cycling times, and reversibility, they do not provide versatility in their spectroscopic detection, nor enable a chemical read-out of the analyte. To move beyond fiber sensors that operate solely as on/off detectors it is vital to develop materials that can sustain multiple analytical modes for chemical identification.