Nicholas F. Fell

Mitigating phosphate interference in bacterial endospore detection by Tb dipicolinate photoluminescence

A new technique for the detection of bacterial endospores using terbium dipicolinate photoluminescence has been previously reported in the literature. The performance of this technique is adversely affected by the presence of substances that contain phosphate ions. This deficiency must be overcome before this method can be used as a viable means of detecting bacterial endospores. We have experimentally investigated methods for mitigating the phosphate problem and demonstrate that the addition of AlCl3 is a solution to the problem. Proper experimental procedures for this method are also presented.

Optimization of substrates for surface-enhanced Raman spectroscopy of bacteria

The use of surface-enhanced Raman spectroscopy (SERS) has recently seen a revitalization of interest with advances in surface coating technologies and other related areas. Recent reports have indicated that enhancement factors of up to 14 orders of magnitude can be achieved, providing the sensitivity requisite to trace level detection of target analytes. Due to the short range of the SERS effect, the interference of background materials may be reduced if the target analyte can be selectively brought near the SERS surface. SERS also holds the promise of providing the ability to determine the identity of bacterial species through recognition of the unique spectrum of a given species. The first major hurdle to its application to this problem is the design and optimization of appropriate surfaces for SERS of bacteria. This is complicated by the negative surface charge of the metal surface and the bacterium that results in a repulsive force that must be overcome. Our efforts have focused on selection of the best SERS substrate for this purpose. We are examining four potential SERS substrates: Au colloids in suspension with the bacteria, Au colloids immobilized on a surface, electrochemically roughened Au surfaces, and Ag periodic particle arrays provided by Prof. Richard van Duyne.

Development of a broadband lidar system for remote determination of aerosol size distributions

We describe a broad bandwidth lidar technology for the estimation of size distributions and refractive indices of aerosol clouds. The concept is to illuminate an aerosol cloud with a pulse of intense broadband light from a specially designed laser system, collect the backscattered light as a function of wavelength and analyse it with novel inversion algorithms. The laser system has a simultaneously broadband output between 1.4-1.8 µm in each pulse. This is in an eye-safe region and is optimal for detection of particles in the one to ten micrometre size range. A lidar simulation was performed to demonstrate the potential for the method and the broad bandwidth source is described and characterized for power and spectral output.

Detection and characterization of explosives using Raman spectroscopy: identification, laser heating, and impact sensitivity

Raman spectroscopy has been shown to be a useful tool for characterizing neat crystalline explosive samples and for identifying principle components in many propellant and explosive formulations. Recently, we have been investigating changes in Raman spectra of explosives and propellant formulations which occur as the temperature approaches the melting point of the sample. We report recent measurements of Raman spectra of explosives and propellant formulations during bulk heating, and recent measurements of laser heating of the samples during measurement of Raman spectra. The results of these measurements are important to investigators using Raman spectroscopy to measure vibrational spectra at the surface of burning propellant samples.

Photonic nanostructures as SERS substrates for reproducible characterization of bacterial spores

Surface enhanced Raman spectroscopy (SERS) has been used as a tool to investigate spectral differences of bacterial endospores. Ultimately, this method could be used as a smart and rapid on-site detector for biological warfare agents. However, due to the spectral complexity and the relative size of spores to the substrate features, a rigidly defined substrate is necessary for reproducible characterization. We are investigating many of the reported substrate classes such as: Nano-sphere lithography (NSL), Film over nano-sphere (FONS), nano-shells, electrochemically roughened metals, and dispersed and immobilized colloids. The key aspects of this work include discerning what architectural pattern provides the largest enhancement and reproducibility when sampling the spore coat and whether some method of immobilization, or attraction, of the spores to the surface is necessary. We will present preliminary results of bacterial spore identification as well as a comparison of the substrates studied.

Trace chemical vapor detection by photothermal interferometry

Photothermal interferometry has been demonstrated as a technique that can detect vapors with extremely high sensitivity (parts-per-trillion levels). Our present research uses a photothermal detection scheme that incorporates tunable sources and a modified Jamin interferometric design to provide high selectivity and sensitivity for organo-phosphate vapor detection. Two possible tunable excitation sources are being studied for this sensor technology, a tunable CO2 laser and difference frequency mixing of a tunable NIR laser with a fixed wavelength NIR laser in a nonlinear crystal. The modified Jamin design imparts superior vibrational immunity by ensuring both interferometer beams encounter common optical elements. Examining the two complementary optical outputs of the interferometer, phase shifts on microradian levels have been detected. Trace chemical vapor detection is accomplished by introducing the tunable excitation laser source across the path of one interferometer beam providing a phase shift due to absorptive heating. Preliminary results indicated parts-per-billion level detection of both DMMP and DIMP using ~ 400mW of CO2 laser power at appropriate wavelengths.

Characterization of photonic nanostructures used as surface-enhanced Raman substrates for bacterial spores

Efforts to develop a single solution for detecting hazardous chemicals and biological organisms for both military and civilian communities often produce conflicting requirements. The detection of biological threats, specifically spores, presents us with the most challenging problem. Raman spectroscopy is an excellent method for unique chemical and biological identification. The applicability of Raman spectroscopy to bacterial identification and analysis has been previously demonstrated. Surface-enhanced Raman scattering (SERS) is a well-known method for improving the signal level in Raman scattering. In order to form a uniform noble metal surface architecture, and therefore reproducible surface enhanced spectra, novel fabrication techniques have been developed. Here we report on our recent efforts using silver-shells around latex spheres as a SERS substrate for bacterial endospores.

PLASTICIZER MIGRATION IN NITROCELLULOSE-BASED PROPELLANTS

Abstract An “environmentally-friendly” technique for determining plasticizer concentration profiles in propellant sheets has been developed. The method was developed in response to a need for a quick, accurate means to study plasticizer migration in experimental gun propellents. Evidence of rapid migration between laminated sheets of nitrocellulose-based propellant containing different amounts of two nitrate ester plasticizers was obtained. An estimate of the diffusion coefficients for one of these plasticizers was also obtained.

Investigating photonic nanostructures for reproducible characterization of bacterial spores

Raman spectroscopy has proven to be a plausible solution to the difficult challenge of on-site detection of biological threats. Adding to the challenge is the fact that many biological species, spores specifically, have relatively low scattering cross sections. The intrinsic need to detect these threats at low concentrations and in the presence of strong background signals necessitates the need for surface enhancement schemes. With an available technique to quickly identify bacterial spores, we hope to find spectral differences between target species in order to incorporate library technologies with the on-site sensor. We are investigating many of the reported substrate classes such as: Nano-sphere lithography (NSL), Film over nano-sphere (FONS), nano-shells, electrochemically roughened metals, and dispersed and immobilized colloids. The key aspects of this work include discerning what architectural features provide the largest enhancement and reproducibility. We will present preliminary results of bacterial spore identification as well as a comparison of the substrates studied.

Surface-enhanced Raman substrate optimization for bacterial identification

The threat of biological agents to soldiers and the civilian community was amply demonstrated in the fall of 2001. We are examining the feasibility of using surface-enhanced Raman spectroscopy (SERS) to detect and identify bacteria. In order to use SERS for bacterial detection and identification, it is necessary to determine the most appropriate type of SERS substrate to use. We are examining gold colloids in suspension, immobilized gold colloids, electrochemically roughened gold, periodic particle arrays (PPA), and film over nanosphere substrates (FONS). Briefly, PPA’s are prepared by depositing gold or silver in the interstitial spaces in a close-packed array of polystyrene nanospheres, while FONS are prepared by depositing approximately half a nanosphere diameter of gold or silver on top of a close-packed array of polymer nanospheres. We are evaluating each of these substrate types to determine which will have a high affinity for bacteria, whether we need to modify the surface of the substrate to attract bacteria, and the degree to which each type of substrate enhances the Raman scattering from the bacterial targets. We will present the results of our initial evaluations of substrates and the spectra obtained for several species of bacteria.