Mark J. Feldstein

Array biosensor for detection of biohazards

A fluorescence-based biosensor has been developed for simultaneous analysis of multiple samples for multiple biohazardous agents. A patterned array of antibodies immobilized on the surface of a planar waveguide is used to capture antigen present in samples; bound analyte is then quantified by means of fluorescent tracer antibodies. Upon excitation of the fluorophore by a small diode laser, a CCD camera detects the pattern of fluorescent antibody:antigen complexes on the waveguide surface. Image analysis software correlates the position of fluorescent signals with the identity of the analyte. This array biosensor has been used to detect toxins, toxoids, and killed or non-pathogenic (vaccine) strains of pathogenic bacteria. Limits of detection in the mid-ng/ml range (toxins and toxoids) and in the 10(3)-10(6) cfu/ml range (bacterial analytes) were achieved with a facile 14-min off-line assay. In addition, a fluidics and imaging system has been developed which allows automated detection of staphylococcal enterotoxin B (SEB) in the low ng/ml range.

Array biosensor: optical and fluidics systems.

Optical and fluidics systems have been developed as central components for an automated array biosensor. Disposable planar waveguides are patterned with immobilized capture antibodies using a physically isolated patterning (PIP) method. The PIP method enables simultaneous deposition of several antibodies and completely circumvents cross-immobilization problems encountered with other array deposition processes. A multi-channel fluidics cell allows numerous assays to be performed on the patterned waveguide. The sensing arrays are optically interrogated using a diode laser with a tailored output to optimize coupling to and maximize excitation uniformity within the waveguide. A patterned cladding is employed to optically isolate the waveguide from perturbations induced by the permanently attached flow cells. Compact optics image the evanescently excited fluorescence onto a large area, cooled CCD array. The image data is processed and automated signal analysis corrects for local background and noise variations.

Optical dephasing on femtosecond time scales: Direct measurement and calculation from solvent spectral densities

The connection between dephasing of optical coherence and the measured spectral density of the pure solvent is made through measurements and calculations of photon echo signals. 2‐pulse photon echo measurements of a cyanine dye in polar solvents are presented. Signals are recorded for both phase matched directions enabling accurate determination of the echo signal time shift. Echo signals are calculated by two approaches that employ the response function description of nonlinear spectroscopy; (i) a single Brownian oscillator line shape model, and (ii) the line shape obtained using the solvent spectral density. The strongly overdamped Brownian oscillator model incorporates only a single adjustable parameter while the experimental data present two fitting constraints. The second model incorporates the measured solvent spectral density. Both give very good agreement with the experimental results. The significance of the second method lies in this being a new approach to calculate nonlinear spectroscopic sign...

Rapid-scan pump–probe spectroscopy with high time and wave-number resolution: optical-Kerr-effect measurements of neat liquids

A system for the rapid-scan (RS) acquisition of time-resolved nonlinear spectroscopic signals, capable of femtosecond resolution over a range of tens of picoseconds, is presented. Operationally, the system is based on a magnetically driven, commercially available velocity transducer that continuously scans a probe delay line relative to a fixed delay line while data are recorded on the fly and in real time. A simple calibration and data time-scale linearization are carried out and tested on optical-heterodyne-detected optical-Kerr-effect measurements. These results are compared with data acquired with a detection system that is based on a stepped delay-line lock-in amplifier. It is found that the RS system is favorable in several areas of signal acquisition, including signal-to-noise ratio, acquisition time, spectral resolution in the Fourier-transformed data, and immunity to artifacts such as baseline distortions.

Femtosecond Optical Spectroscopy and Scanning Probe Microscopy

Results from a new experimental technique for simultaneous spatial and temporal resolution studies of optically initiated dynamics at interfaces are presented.

Array biosensor for multi-analyte sensing

As biosensors become more sophisticated and commercially available, the general appreciation for their capabilities also increases. We now focus on multi-analyte sensors and address the problems inherent in discriminating multiple simultaneous signals without loss in assay speed, specificity or sensitivity. Furthermore, the goals of portability, simplicity and low cost have not diminished in importance. NRL is developing a multi-analyte sensor designed to be portable, inexpensive, and easy to use. To achieve these goals, we use a room temperature CCD, a diode laser, and a disposable waveguide. While our goals of using automated fluidics and automated image processing are not yet completely realized, we have fabricated a prototype biosensor which fits into a tackle box with a associated portable computer. Simple microscope slides are used as waveguides and precoated with arrays of immobilized antibodies. Fluorescence immunoassays are performed on these waveguides using as many as 6 samples at a time and assaying for up to 5 different analytes in each samples Sensitivities in the 1-10 ng/ml range have been achieved in 10-minute assays. Initial studies in clinical fluids indicate that assays can be run on samples such as whole blood, plasma, urine, saliva and nasal secretions.

Observation of coherent multiple scattering of surface plasmon polaritons on Ag and Au surfaces

Abstract We report the observation of coherent multiple scattering of surface plasmon polaritons in Ag and Au films through attenuated total reflectance in the Kretschman geometry. The pump-probe measurements reveal exponential tails in the amplitude of the reflected light that extend well beyond the pulse correlation time. The exponential tails reflect the temporal decay of surface plasmon polaritons (SPP). This observation and interpretation of SPP decay through scattering from surface roughness suggest the possibility of SPP localization on metal surfaces. An alternative interpretation of the signals, pulse reshaping due to the constant angle SPP modes, is ruled out.

Femtosecond correlated optical reactivity and scanning tunneling microscopy studies of metal surfaces

A new experimental technique, correlated optical reactivity and scanning tunneling microscopy (CORSTM), is shown to be a uniquely powerful tool for the study of spatially localized reactivity of surfaces. In particular, CORSTM measurements directly correlate electromagnetic field enhancements that affect chemical dynamics and reactivity with surface topography on the length scale of a few nanometers. These measurements are based on the detection of surface plasmon polariton mediated multi-photon ionization from metal surfaces using ultrafast optical excitation and scanning probe microscopy photoelectron detection. The CORSTM approach is extended to a pump-probe scheme facilitating spatially localized measurement of hot electron dynamics. The experimental results provide direct confirmation of the optimal structural topographies for surface enhanced spectroscopy predicted by electromagnetic theories. CORSTM will provide a better understanding of phenomena that involve plasmons through the direct measurement of structure-function correlations.

Time-resolved Carrier Dynamics Near The Insulator-metal Transition

Carrier dynamics in polyaniline and colloidal Au films have been examined using a combined approach of time resolved laser spectroscopy and atomic force microscopy (AFM). These systems exhibit insulator-metal transitions (IMT) in conjunction with synthetic modification of their structure. The relationship between structure and reactivity, in terms of hot carrier lifetimes and transport, has been identified by correlati ng changes in the dynamics with the directly measured morphology. Physical insight into the processes affecting carrier lifetimes and localization and the nature of the IMT has been derived from an analysis of the experimental data. The results and conclusions presented herein have implications for directing research and development in device applications based on thin film technologies.

Ultrafast STM-tip Localized Responses from Nanostructured Surfaces

Spatially correlated reactivity and femtosecond time-scale responses from mesoscopic rough films are reported. The STM-tip spatially-localized transient SHG dynamics of nanometer scale metallic structures are used to establish localized surface plasmon dynamics.