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.
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.
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.