Takamitsu Yoshida

Colloidal gold submonolayer-coated thin-film glass plates for waveguide-coupled surface plasmon resonance sensors

A novel optical sensor based on spectroscopic measurement of the plasmon absorption of a colloidal gold submonolayer immobilized upon a planar waveguide has been developed for label-free detection of biomolecular interactions at the sensors surface. We fabricated the sensor by locally modifying a 50-microm-thick glass plate with hemoglobin (Hb) and then self-assembling gold colloids from the aqueous solution onto the Hb-modified area of the glass plate. Polychromatic light from a xenon-arc lamp was launched into the thin-film glass plate by use of a broadband fiber-coupling method. With the use of a CCD detector to monitor the light beam emitted from an end face of the glass plate, the plasmon absorption spectrum of the colloidal gold submonolayer was determined to depend on the polarization states of the guided light and to change with the refractive index of the medium in contact with the colloids. In addition to simplicity of fabrication and the ease of use, the sensor yields a larger absorbance sensitivity than the normal transmission measurement.

Multimode Optical Waveguide Spectrscopy of Aqueous Gold Sol

Optical waveguide (OWG) spectroscopy of aqueous Au sol was performed using 50-µm-thick glass plates as multimode waveguides. Measurements of the TE- and TM-polarized absorbance spectra in the wavelength range 400 – 800 nm demonstrated that Au nanoparticles in water were surface-inactive for hydrophilic glass waveguides due to electrostatic repulsion but they could be easily immobilized on the silanized and protein-modified glass surfaces to form the aggregated monolayer.

Prism-Free Broadband Coupling Approach for Spectroelectrochemical Characterization of Surface-Immobilized Molecules

Indium-tin oxide (ITO)-coated quartz plates with tapered ends have been utilized as stand alone waveguides for spectroelectrochemical characterization of immobilized dyes and protein films. Light coupling into the core is achieved through a 60-degree tapered end on both sides of the waveguide that conveniently eliminates the need for a pair of coupling prisms. This is a significant improvement in our slab optical waveguide (SOWG) platform that utilizes a pair of LaSF8 prisms for light coupling into the waveguide. We observed that the prism coupling approach is limited only to within a certain period of time owing to the volatility of the coupling liquid used to span the prism/waveguide gap. The new prism-free geometry gives a more stable response with time and transmits a broad range of wavelengths from about 350 up to 750 nanometers. Using a representative dye, methylene blue, and a model protein, cytochrome c, the efficacy of the tapered end coupling approach in studying surface-bound molecules is established and results are reported in this paper.