Recently, there is widespread interest in the development of chemical and biochemical sensors based on plasmonic resonance of metallic nanostructures. In the localized surface plasmon resonance (LSPR) for nanometer-sized metallic structures, a resonant oscillation of the conduction electrons within the metallic nanostructures gives rise to an enhanced scattering and absorption of light. The LSPR sensing requires simpler instrumentation than surface plasmon resonance (SPR).
Nanodots are popular nanostructures used in LSPR research. Their spectra position is dependent on the size and shape of the nanodots, the composition of the nanodots, the interaction of the nanodots, and the dielectric environment surrounding the nanodots. Controlling the size and the periodicity of an array of gold nanodots, one can engineer the structure to operate at the desired wavelength window. The property of spectra position of metallic nano-structure depends on the dielectric environment, and is widely used as the working principle for label-free chemical and biological sensing applications. These investigations have been on planar substrates such as glass or sapphire.
In addition to the benefits of LSPR of metallic nanostructures on planar substrates, the use of optical fiber as a platform has been investigated for more advantages, including compact and high portability, and immunity to electromagnetic interference. Sensors based on ordered arrays of nanoholes that are fabricated on the gold-coated optical fiber tip using focused ion beams (FIBs) have been reported. In previous publications, high-sensitivity LPR biochemical sensing based on transmission spectra of ordered arrays of metallic nanodots fabricated on the optical fiber tip using electron beam lithography has been reported.
This innovation is comprised of an optical fiber having a metallic dot array on its tip, a light source coupled to the optical fiber via a light coupler, and a spectrometer coupled to the optical fiber via the coupler. The light source is configured to transmit light within a range of wavelengths along the optical fiber. When the light reaches the dot array, the light excites surface plasmons of the dot array and causes the plasmons to resonate. The dots have a specific affinity for a particular substance, and the resonance frequency of the dots changes when the substance is present, thereby changing an absorption peak of the light. The light is reflected back through the optical fiber to the spectrometer, and the spectrometer detects a parameter indicative of a change in the absorption peak. Presence of the particular substance is determined based upon the change in the absorption peak.