© (2006) Wiley

The marriage of nanomaterials and optics leads chemical sensing into a new age. Compactness, reusability, portability and chemical identification from small sample volumes are some of the main goals of chemical sensing to date, especially for on-site detection. Optical spectroscopy has proved useful but in general, appliances are too bulky to be portable. Nanomaterials are anticipated to overcome the limitation caused by size. Appreciating their merits and the possibility of scattering of the evanescent field in fluidic and air dielectrics, a research team from Lawrence Berkeley National Laboratory report a photonic sensor based on subwavelength nanowires that is capable of detecting the concentration and the refractive index of different solutions1.

By performing absorbance, fluorescence and surface-enhanced Raman spectroscopy (which maximizes the otherwise weak signal retrieved from probing vibrational modes), the researchers are able to carry out measurements on subpicolitre volumes. They use a nanoribbon structure made of single-crystalline tin oxide to implement efficient waveguiding across channels that have chemical analyte flowing through them. On optical pumping, the emission intensity from the nanoribbon waveguide is attenuated by the absorbance of the analyte species, allowing analysis to be carried out. To enhance the evanescent field around the nanoribbon and thereby enhance the Raman signal, the team add 50-nm silver nanocubes to the waveguide surface This additional signal strength provides the ability to monitor optical attenuation across the wire element on excitation of surface plasmons, hence giving extraordinary chemical specificity. The authors expect that use of their technique could advance the design of compact optical sensors.