Development of a rapid response biosensor for detection of Salmonella typhimurium.

An integrated optic interferometer for detecting foodborne pathogens was developed. The interferometer is a planar waveguide with two thin antibody-coated channels of immunochemically selective agents that interact with antigen molecules. One channel is coated with antibody to Salmonella as a sample, and the other is coated with human immunoglobulin G as a reference channel by using reductive amination. Salmonella was introduced onto the sensing channels through the flow cell on the channels. Phase shift (pi) generated by refractive index variation, as determined by interfering the perturbed sample channel with an unperturbed reference channel and observing the fringe shift, was used for detection. Salmonella Typhimurium (heat-treated or boiled) was detected by binding to antibody against Salmonella common structural antigen immobilized on a silane-derived sensor surface at concentrations in the range of 1x10(5) to 1x10(7) CFU/ml. Salmonella (1x10(7) CFU/ml) mixed with Escherichia coli (1x10(7) CFU/ml) were readily detected without any decrease in sensitivity by the direct assay. Application of a sandwich assay with a second antibody or a gold-conjugated antibody increased the detection limit to 1x10(5) CFU/ml within a 10-min reaction time. Various methods for the immobilization of the capture antibody to the biosensor channels were compared. The greatest binding response was observed in a direct reductive amination method with a long reaction period and increased the detection limit of direct binding of Salmonella antigen to 1x10(4) CFU/ml. The biosensor was able to detect Salmonella Typhimurium in chicken carcass wash fluid originally inoculated at a level of 20 CFU/ml after 12 h of nonselective enrichment. The planar optic biosensor shows promise as a fast, sensitive, reliable, and economical means of detecting food pathogens in the future.

Hartman interferometer: versatile integrated optic sensor for label-free, real-time quantification of nucleic acids, proteins, and pathogens.

The Hartman interferometer, a proprietary integrated optic sensor, provides a basis for a broad range of biomedical diagnostics, including antibody-based and gene probe-based assays. As with other evanescent-wave optical sensors, the interferometer measures the refractive index change resulting from biomolecular binding on a waveguide surface. The exciting promise of evanescent-wave sensors lies, in general, in their potential to be used as label-free, real-time transducers that can operate in a true mix-and-read fashion and provide fast, quantitative results. One of the major issues facing their development, however, is creating a simple, low-cost configuration for multianalyte testing. The Hartman interferometer addresses this challenge by relying on linearly polarized light and a planar waveguide format, thereby avoiding the problems associated with circular polarization and channel waveguides. We report preliminary experiments that demonstrate the applicability of this sensor configuration to detection of a wide range of protein, nucleic acid, and pathogen analytes.

Phase stability of ferroelectric liquid crystals upon repeated switching and static temperature characteristics.

Surface-stabilized ferroelectric liquid crystals (FLC's) are promising materials for semiconductor integrated-circuit-based spatial light modulators. For coherent optical processing applications, phase stability upon repeated switching is critically important. The phase characteristics of an FLC device were measured at switching rates of up to 1 kHz and found to be very stable. The change in the total optical path length through the cell was found to be < 0.0025λ at a wavelength of 632.8 nm. The static optical characteristics were measured for a range of temperatures at and above room temperature in order to be able to identify any temperature-induced phase changes upon switching. The temperature of the FLC cell was externally varied, and changes in the birefringent optical path difference, the optical path length, and the optic axis tilt angle were measured. However, because of the observed phase stability of the FLC, the change of temperature caused by switching was determined to be < 0.046°C. It is clearly shown that FLC's can exhibit the stability needed for critical coherent and incoherent optical data-processing applications.

Coherent optical characterization of magnetooptical spatial light modulators.

A precision optical system, developed for characterizing the amplitude and phase properties of spatial light modulators, was used to characterize a 48 x 48 pixel magnetooptic spatial light modulator (MOSLM). Considerable variations in the amplitude (+/-25%) and phase transmittance (+/-50%) over the area of a given pixel were observed with coherent light illumination. The pixel-to-pixel variations in the average amplitude (+/-5%) and average phase (+/-6%) were considerably less. The contrast ratio and the polarization rotation for full frame monochromatic illumination were ~10:1 and 11.25 degrees , respectively. For illumination within a single pixel, the contrast ratio and polarization rotation were ~100:1 and 14.0 degrees , respectively. A theoretical model is presented showing that the reduced values for full frame illumination may be described by the coherent addition of light of unrotated polarization (transmitted between pixels and around the edges) with the polarization rotated light. The amplitude and phase characteristics of the MOSLM were found to be a very stable with repeated switching of the pixel and with switching of neighboring pixels. This stability is a central requirement in coherent optical information processing.

Antireflection gold surface-relief gratings: experimental characteristics.

A systematic procedure using the effective index method and impedance matching has recently been developed Appl. Opt. 26, 3123 (1987)] for the design of antireflection high-spatial-frequency rectangulargroove gratings on lossy materials including high conductivity metals. The design procedure in turn can be used as a starting point to design antireflection metallic gratings with lower spatial frequencies using rigorous coupled-wave analysis. These lower spatial-frequency gratings have the advantage of being easier to fabricate. In the present work, a particular antireflection gold grating design (having a period of 1.0 microm, a filling factor of 50%, and a groove depth of 147.5 nm for use at a freespace wavelength of 500 nm, normal incidence, and polarization parallel to the grooves) was fabricated and its diffraction characteristics experimentally measured. The grating indeed showed very nearly zero specular reflection in the blue region of the spectrum. Unlike previously reported antireflection anomalies, the effect is broadband occurring over a broad range of wavelengths and angles of incidence, and for both orthogonal polarizations. This work clearly shows that the systematic design of zero specular reflection grating surfaces is possible.