Abstract
A highly sensitive technique based on optical absorption using a single-mode, channel integrated optical waveguide is described for broad spectral band detection and analysis of heme-containing protein films at a glass/water interface. Fabrication steps and device characteristics of optical waveguides suitable for operation in the wavelength range of 400 - 650 nm are described. Experimental results reported here show a limit of detection smaller than 100 pg/cm(2) for a submonolayer of ferricytochrome c by probing the Soret transition band with a 406-nm semiconductor diode laser propagating in a single-mode, ion-exchanged channel waveguide. By taking advantage of the exceptionally low limit of detection, we examined the adsorption isotherm of cytochrome c on a glass surface with unprecedented detail. Unlike other surface-specific techniques (e.g. SPR, integrated optic Mach-Zehnder interferometer) that probe local refractive-index changes and therefore are very susceptible to temperature fluctuations, the integrated optical waveguide absorption technique probes molecular-specific transition bands and is expected to be less vulnerable to environmental perturbations.
Highlights
Binding characteristics of proteins at various interfaces are critical for a wide range of biotechnological fields and play a key role in the development/screening of novel pharmaceutical drugs [1]
Due to the superior signal-to-noise ratio (SNR) in our setup, the adsorption isotherm of cyt-c molecules on a glass/aqueous interface was examined with unprecedented detail, and limit of detection (LOD) results were determined from the measured SNR and absorbance values
The step-index profile is clearly seen for ion concentrations shown in Fig. 2 and the effective thickness of the waveguide is less than 1μm
Summary
Binding characteristics of proteins at various interfaces are critical for a wide range of biotechnological fields and play a key role in the development/screening of novel pharmaceutical drugs [1]. By using a silver-film ion-exchange process, we developed here highly confined, singlemode, integrated optical waveguides that exhibit sufficiently low propagation loss in the short wavelength range and provide high detection sensitivity; in addition this work employs channel waveguides which is a superior configuration to interface the probing device with fiber coupled sources and detectors when compared to slab waveguides. These glass waveguides, which provide low optical losses and are fairly inexpensive, can potentially enable the fabrication of miniature disposable integrated optical biosensors. Due to the superior SNR in our setup, the adsorption isotherm of cyt-c molecules on a glass/aqueous interface was examined with unprecedented detail, and LOD results were determined from the measured SNR and absorbance values
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