A microsphere-fiber probe used for spectroscopic analysis of material samples is presented. The monolithic structure is formed by melting the end of a section of optical fiber forming a bead at the distal end of the fiber. Light guided through the fiber and emerges from the sphere focusing to a point beyond the surface. Raman scattering is used to demonstrate the efficacy of the probe, which operates in a bidirectional manner efficiently collecting the scattered light, re-imaging it back into the waveguide, and transmitting it to a spectrometer for dispersion. The probe demonstrates an order of magnitude improvement over the spatial resolution conventional fiber probes. This improvement in spatial resolution and corresponding collection efficiency will aid in critical analyses such as cancer margin detection and material characterization.
Non-spherical aerosols, particularly aggregates and those comprised of rough surfaces, produce complex light scattering patterns that deviate considerably from those of their spherical counterparts. Consequently, discerning particle morphology from the complex scattering pattern, i.e., the inverse problem, is difficult at best. Additional information is required to associate uniquely the interference pattern resulting from scattered light and the particle's morphology (size, shape, etc.). This uniqueness challenge of the inverse problem may be overcome by incorporating digital holographic imaging into the light scattering apparatus. Using a color CCD camera, we demonstrate that two-dimensional light scattering patterns and digital holograms from individual owing aerosol particles may be recorded simultaneously at different wavelengths revealing the complex scattering pattern along with the size, shape, and orientation of the particle at the instant the scattering occurs. Knowing the exact scattering pattern associated with an exact particle morphology will improve the understanding the radiative characteristics of non-spherical atmospheric aerosols, and reduce uncertainties of important physical parameters such as radiative forcing of aerosols.
Whispering gallery mode resonators provide a robust and sensitive platform for the trace detection of chemical and/or biological analytes. The conventional approaches exhibit some challenges in simultaneous multi-channel (i.e., multi-species) detection. We will present an alternative monitoring scheme that allows for the spatial multiplexing of whispering gallery mode resonators with the simultaneous observation of the resonance spectra from each of them. The resonance spectra and corresponding shifts due to analyte adsorption are readily observed from different spherical resonators.
Light scattering patterns from non-spherical particles and aggregates exhibit complex structure that is only revealed when observing in two angular dimensions. However, due to the varied shape and packing of such aerosols, the rich structure in the two-dimensional angular optical scattering (TAOS) pattern varies from particle to particle. We examine two-dimensional light scattering patterns obtained at multiple wavelengths using a single CCD camera with minimal cross talk between channels. The integration of the approach with a single CCD camera assures that data is acquired within the same solid angle and orientation. Since the optical size of the scattering particle is inversely proportional to the illuminating wavelength, the spectrally resolved scattering information provides characteristic information about the airborne particles simultaneously in two different scaling regimes. The simultaneous acquisition of data from airborne particulate matter at two different wavelengths allows for additional degrees of freedom in the analysis and characterization of the aerosols. Whereas our previous multivariate analyses of aerosol particles has relied solely on spatial frequency components, our present approach attempts to incorporate the relative symmetry of the particledetector system while extracting information content from both spectral channels. In addition to single channel data, this current approach also examines relative metrics. Consequently, we have begun to employ multivariate techniques based on novel morphological descriptors in order to classify “unknown” particles within a database of TAOS patterns from known aerosols utilizing both spectral and spatial information acquired. A comparison is made among several different classification metrics, all of which show improved classification capabilities relative to our previous approaches.
Optical probes to identify tumor margins in vivo would greatly reduce the time, effort and complexity in the surgical removal of malignant tissue in head and neck cancers. Current approaches involve visual microscopy of stained tissue samples to determine cancer margins, which results in the excision of excess of tissue to assure complete removal of the cancer. Such surgical procedures and follow-on chemotherapy can adversely affect the patient’s recovery and subsequent quality of life. In order to reduce the complexity of the process and minimize adverse effects on the patient, we investigate ex vivo tissue samples (stained and unstained) using digital holographic microscopy in conjunction with spectroscopic analyses (reflectance and transmission spectroscopy) in order to determine label-free, optically identifiable characteristic features that may ultimately be used for in vivo processing of cancerous tissues. The tissue samples studied were squamous cell carcinomas and associated controls from patients of varying age, gender and race. Holographic microscopic imaging scans across both cancerous and non-cancerous tissue samples yielded amplitude and phase reconstructions that were correlated with spectral signatures. Though the holographic reconstructions and measured spectra indicate variations even among the same class of tissue, preliminary results indicate the existence of some discriminating features. Further analyses are presently underway to further this work and extract additional information from the imaging and spectral data that may prove useful for in vivo surgical identification.
The whispering gallery mode (WGM) biosensor is a micro-optical platform capable of sensitive label-free detection of
biological particles. Described by the reactive sensing principle (RSP), this analytic formulation quantifies the response
of the system to the adsorption of bioparticles. Guided by the RSP, the WGM biosensor enabling from detection of virus
(e.g., Human Papillomavirus, HPV) to the ultimate goal of single protein detection. The latter was derived from insights
into the RSP, which resulted in the development of a hybrid plasmonic WGM biosensor, which has recently
demonstrated detection of individual protein cancer markers. Enhancements from bound gold nanoparticles provide the
sensitivity to detect single protein molecules (66 kDa) with good signal-to-noise (S/N > 10), and project that detection of
proteins as small as 5 kDa.
The reactive sensing principle applied to hybrid plasmonic whispering gallery mode biosensor has recently demonstrated detection of individual protein cancer markers. The rough surface of a gold nanoparticle affixed to the resonator surface acts like a nanoscopic antenna, significantly boosts the local electric field within the cavity mode. Adsorption of a target protein onto this nanoscopic antenna results in an enhanced response of the resonator system to the binding event. We have demonstrated detection of individual protein molecules (66 kDa) with good signal-to-noise (S/N > 10), and project that detection of proteins as small as 5 kDa are possible.
Our hybrid plasmonic whispering gallery mode biosensor has recently demonstrated detection and characterization of the smallest known RNA virus. A gold nanoparticle affixed to the resonator surface acts like a nanoscopic antenna, enhancing locally the electric field within the cavity mode. When a target analyte binds with this nanoscopic antenna the result is an enhanced response (spectral shift) of the resonator system to the binding event. We have observed shift enhancements ~70× over the response of the bare resonator, thereby permitting the detection and characterization of all known viral particles and even some large protein molecules.
We report the label-free detection and sizing of the smallest individual RNA virus, MS2 by a spherical microcavity. Mass of this virus is ~6 ag and produces a theoretical resonance shift ~0.25 fm upon adsorbing an individual virus at the
equator of the bare microcavity, which is well below the r.m.s background noise of 2 fm. However, detection was
accomplished with ease (S/N = 8, Q = 4x105) using a single dipole stimulated plasmonic-nanoshell as a microcavity wavelength shift enhancer. Analytical expressions based on the “reactive sensing principle” are developed to extract the radius of the virus from the measured signals. Estimated limit of detection for these experiments was ~0.4 ag or 240 kDa below the size of all known viruses, largest globular and elongated proteins [Phosphofructokinase (345 kDa) and Fibrinogen (390 kDa), respectively].
We describe and demonstrate a physical mechanism that substantially enhances the label-free sensitivity of a
Whispering-Gallery-Mode biosensor for the detection of individual nanoparticles in aqueous solution. It involves the
interaction of dielectric nanoparticle in an equatorial carousel orbit with a plasmonic nanoparticle bound on the orbital
path. As a 60 nm dielectric particle parks on plasmonic hot spots we observe frequency shifts that are considerably
enhanced consistent with a simple reactive model. Using the same model the label free detection of a single bovine
serum albumin (BSA) molecule is projected.
We extend our previous work on two-dimensional angular optical scattering (TAOS) patterns to water droplets with inclusions and absorption studies at two different wavelengths. Using an acoustic levitator, the TAOS from water droplets containing polystyrene latex sphere inclusions is observed as the droplet evaporates leaving only a dry aggregate of spheres. Dramatic changes in the TAOS patterns occur even when the droplet still has a large volume fraction of water. Two wavelengths (266 nm and 532 nm) are used to examine the TAOS patterns from droplets containing tryptophan. The effect absorption has on the angular scattering peaks is presented. Such dual wavelength measurements can provide useful information to those working on the inverse problem.
Real-time methods which is reagentless and could detect and partially characterize bioaerosols are of current interest. We present a technique for real-time measurement of UV-excited fluorescence spectra and two-dimensional angular optical scattering (TAOS) from individual flowing biological aerosol particles. The fluorescence spectra have been observed from more than 20 samples including Bacillus subtilis, Escherichia coli, Erwinia herbicola, allergens, dust, and smoke. The S/N and resolution of the spectra are sufficient for observing small lineshape differences among the same type of bioaerosol prepared under different conditions. The additional information from TAOS regarding particle size, shape, and granularity has the potential of aiding in distinguishing bacterial aerosols from other aerosols, such as diesel and cigarette smoke.
Our group has been developing a system for single-particle fluorescence detection of aerosolized agents. This paper describes the most recent steps in the evolution of this system. The effects of fluorophore concentrations, droplet size, and excitation power have also been investigated with microdroplets containing tryptophan in water to determine the effects of these parameters on our previous results. The vibrating orifice droplet generator was chosen for this study base don its ability to generate particles of well- known and reproducible size. The power levels required to reach saturation and photodegradation were determined. In addition, the collection of fluorescence emission was optimized through the use of a UV achromatic photographic lens. This arrangement permitted collection of images of the droplet stream. Finally, the use of a dual-beam, conditional firing scheme facilitated the collection of improved signal- to-noise single-shot spectra from individual biological particles.
The detection and characterization of micro-particles, particularly airborne biological particles, is currently of great interest. We present a novel technique for recording the 2D angular scattering pattern from a single airborne microparticle. Angular scattering measurements were performed in both the near-forward and near-backward regions for a variety of particles including for ethanol droplets, single polystyrene latex spheres, psl clusters, and clusters of Bacillus subtilis spores, all of various sizes. Because the angular scattering pattern is sensitive to size, shape and refractive index, the angular feature associate with clusters may be used to better characterize such airborne micro-particles. A watershed image processing routine has also been implemented. Through this routine, the number of intensity patches per solid angles is found to increase with cluster diameter.
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