Matthew B. Hart

Characterizing and monitoring respiratory aerosols by light scattering

The elastic-scattering intensity pattern from a single particle as a function of spherical coordinate angles theta and phi provides detailed information on the patterns morphology. By use of an ellipsoidal reflector and a CCD camera, a single-laser-shot intensity pattern from a large angular range (theta from 90 degrees to 168 degrees and phi from 0 degrees to 360 degrees) was detected from a single aerosol (e.g., a Bacillus subtilisspore, a 1-microm-diameter polystyrene latex sphere, or a cluster of either of these) flowing through the reflectors focal volume at 5 m/s. Noticeable difference in the large-angle-range two-dimensional angular optical scattering (LATAOS) suggest that the LATAOS pattern could be useful in differentiating and classifying life-threatening aerosols from normal background aerosols.

Light backscatter by surfaces composed of small spherical particles.

We present measurements of phase angle curves of intensity and degree of linear polarization of powdery surfaces at two spectral bands centered near 0.44 and 0.63 microm. Three powder samples consisting of nonabsorbing spherical particles of sizes comparable with the wavelengths 0.5, 1.0, and 1.5 microm were examined. The particulate surfaces were measured in the phase angle range of 0.2 degrees-50 degrees by two different photometers and/or polarimeters. At small phase angles, powdery samples consisting of spherical particles (having very high albedo that resulted in significant multiple scattering) showed prominent features that corresponded to single-particle scattering. These features became more prominent after compressing the surfaces when we changed the packing density of the powders from 0.29 to 0.48. Noticeable differences were observed between polarimetric curves corresponding to different wavelengths. All the samples demonstrated prominent opposition intensity spikes at phase angles <2 degrees likely caused by the coherent backscatter enhancement due to multiple scattering within the particulate surface. The intensity phase curves at these two wavelengths were similar. The photopolarimetric measurements may have broad applications to the interpretation of photometry, spectroscopy, and polarimetry of the ice regoliths of high albedo satellites.

Optical measurements from single levitated particles using a linear electrodynamic quadrupole trap.

We have recently made advancements in a linear electrodynamic quadrupole (LEQ) device for capturing and levitating either single or multiple micro-particles that provides significant improvements in capture efficiency, reliability, and optical measurement access. We have used our LEQ to trap particles ranging from 30 to less than 0.5 μm in size and provide a controlled environment to study particle physical/chemical dependencies on temperature, relative humidity, and gas constituents. To demonstrate this approach, we present data and analysis of liquid-droplet evaporation rates for two materials: glycerol and dibutyl sebacate. Droplet size was monitored as a function of time by two independent optical methods: direct imaging and fixed-angle light scattering. This new approach provides a means to rapidly characterize a wide range of aerosol particle properties and a platform for development of new aerosol optical-diagnostic measurements.

Metallic-nanoparticles-enhanced fluorescence from individual micron-sized aerosol particles on-the-fly

Fluorescence spectra from individual aerosol particles that were either coated or embedded with metallic nanoparticles (MNPs) was acquired on-the-fly using 266 nm and 355 nm excitation. Using aqueous suspensions of MNPs with either polystyrene latex (PSL) spheres or dissolved proteins (tryptophan or ovalbumin), we generated PSL spheres coated with MNPs, or protein clusters embedded with MNPs as aerosols. Both enhanced and quenched fluorescence intensities were observed as a function of MNP concentration. Optimizing MNP material, size and spacing should yield enhanced sensitivity for specific aerosol materials that could be exploited to improve detection limits of single-particle, on-the-fly fluorescence or Raman based spectroscopic sensors.

Backscattering of agglomerate particles

We examine how aggregation affects the light-scattering signatures, especially the polarization in the near-backward-scattering direction. We use the discrete dipole approximation (DDA) to study the backscatter of agglomerate particles consisting of oblong monomers. We examine the effects of monomer number and packing structure on the resulting negative polarization branch at small phase angle. We find large a dependence on the orientation of the monomers within the agglomerate and a smaller dependence on the number of monomers, suggesting that the mechanism producing the negative polarization minimum depends strongly on the interactions between the individual monomers. We also examine experimental measurements of substrates composed of biological cells. We find that the light-scattering signatures in the backward direction are not only different for different spore species, but for spores that have been prepared using different methodologies. These signatures are reproducible in different substrates composed of the spores from the same batches.

Rapid identification of biological particles using on-the-fly fluorescent marking

We are developing a novel method to fluorescently label specific biological aerosols on-the-fly using an in-line electrospray technique. Fluorescently labeled biomarkers such as molecular beacons, aptamer beacons, or those constructed from antibodies, will be used to coat aerosol particles in an air stream. Single biological particles with appropriate receptors will be tagged with biomarkers that fluoresce at a particular wavelength allowing the particle to be identified in near real time using a simple laser induced fluorescence technique. The fluorescent markers are normally quenched in the absence of their target analyte, permitting the use of mixtures of different biomarkers for simultaneously identifying multiple types of biological particles. The technique can also be applied to inorganic particulate with a molecular surface composition that lends itself to epitopic binding. Some of the issues that are currently being investigated include the kinetics of biomarker binding in an aerosol stream, optimal electrospray geometries and the nondestructive charging of biological particles on the fly.

Developments in on-the-fly biomarking: a new method to rapidly identify chemical and biological aerosols

We report on the advances made in the basic research to label specific chemical or biological aerosols on-the-fly using an electrospray technique. Fluorescent biomarkers that have been created for specific targets, and that produce a detectable change in emission characteristics only upon binding, will be used to coat all aerosols in an air stream. Aerosols with appropriate receptors will be labeled in this manner, allowing them to be identified in near real-time using a simple laser-induced fluorescence technique. In effect, an immunoassay is quickly performed on the surface of single chemical or biological particles as they flow in an air stream, labeling specific ones for rapid, single-particle interrogation and identification among a diverse and dynamic background. This method permits the use of solutions containing mixtures of different biomarkers to simultaneously identify multiple types of chemical or biological aerosols. Some issues that are currently being investigated include the kinetics of biomarker surface binding to an aerosol in flight and the control of charged aerosols for efficient single particle interrogation.