Final Report: Singlet Delta Oxygen Airflow Sterilization for Building Protection

EPA Contract Number: EPD06082
Title: Singlet Delta Oxygen Airflow Sterilization for Building Protection
Investigators: Henshaw, Thomas L.
Small Business: Neuman Information, Inc. d.b.a. Directed Energy Solutions
EPA Contact: Manager, SBIR Program
Phase: II
Project Period: April 1, 2006 through June 30, 2007
Project Amount: $224,667
RFA: Small Business Innovation Research (SBIR) - Phase II (2006) Recipients Lists
Research Category: Hazardous Waste/Remediation , SBIR - Homeland Security , Small Business Innovation Research (SBIR)

Description:

The attached final report describes work conducted by Directed Energy Solutions (DES) over the technical period spanning the years 2006–2008 under the EPA contract EPD06082 toward demonstration of a system for building protection via sterilization using flowing singlet-delta oxygen (SDO). DES proposed direct optical excitation at 764 nm of ground-state (|X,0>) oxygen molecules to their second excited electronic state (|b,0>), whereby SDO (first excited electronic state, |a,0>) molecules would be produced after being quenched by |X,0> molecules. This is a very attractive approach for decontamination applications as SDO is highly reactive toward chemical and biological agents.

Room-temperature semiconductor diode laser arrays (DLAs), based on the AlGaAs ternary compound material system and grown by an epitaxial process such as metal-organic chemical  vapor deposition (MOCVD) to emit at 808 nm, are now commonplace at power levels in excess of several tens of Watts.  In this perspective, DES proposed to leverage this mature technology to develop a high-power, narrow-lined optical source by cascading an 808-nm external-cavity diode laser array (ECDLA) with a volume Bragg grating (VBG) for optical feedback within a narrow spectral band.  The ECDLA was fixed to an aluminum plate that interfaced a Dewar of liquid nitrogen (LN2) in a cryostat for conductive cooling to 77 K so that the laser wavelength could be shifted to 764 nm at cryogenic (cryo) operating temperature for more efficient absorption by oxygen molecules.  The VBG was mounted on a separate aluminum plate that was at room temperature and integrated with the cold plate inside the cryostat; thermoelectric coolers (TECs) installed in close proximity of the VBG enabled fine-tuning of the output wavelength with a high degree of precision to match a given absorption line in molecular oxygen.  The high-power 764-nm radiation was coupled into an off-axis optical resonator that had, in addition, input and output mechanical ports for a constant stream of air flow to enhance the pump intensity and generate SDO molecules.  This approach represented a sub-scale and mobile system that could be used for local decontamination in a single room while providing proof-of-concept for a larger scale sterilization system that would be based upon coupling light from an assembly of several DLAs into a building’s air duct.  The latter was the technical vehicle proposed and theoretically studied by DES for whole-building sterilization; the experimental results from the small-scale system serve as a practical guide for scaling the photoreactor (SDO generator).  

Summary/Accomplishments (Outputs/Outcomes):

A numerical model was developed to study the requirements for high-efficiency SDO generation in an air duct as well as in the sub-scale SDO generator system.  Furthermore, it aided in the optimization of the laser source and generator designs.  Our analysis indicated that an SDO mass flow rate (MFR) of 0.01–0.1 mmole/s can be achieved by flowing air or oxygen through the small-scale generator at a speed of ~15 m/s when 4.8–48 W of 764-nm optical power was coupled into the off-axis resonator.  For an air duct of cross-sectional area 6x12and with inner reflective walls, the model predicts that 30–300 W of laser power is needed to produce SDO at 0.01–0.1 mmole/s MFR when flowing ambient air at a rate of 500 CFM.  This model can be adapted for device scaling studies in the future for advanced decontamination systems.

The salient experimental results for the cryo-VBG-ECDLA include a maximum measured power of 40 W at 764 nm, spectral linewidth of ~8 GHz, 91 percent of the total power in the band for oxygen absorption, and side-mode suppression of 21.5 dB, thereby constituting a world-record performance for such a device.  Flexibility in tuning the laser wavelength to match a specific absorption line was demonstrated by controlling the VBG temperature with a TEC. Long-term operation of the cryo-VBG-ECDLA yielded a constant output power at 25 W for more than 45 minutes; this constant-power operation was interrupted when the drive current on the DLA was changed deliberately to increase the output power to 28 W.  The test indicated potential field applications at a constant high output power.

In collaboration with the Air Force Research Laboratory, Biosciences and Protection Division, Counterproliferation Branch (AFRL/HEPC), DES conducted a series of sub-scale tests to study the potential for bio-decontamination by exposing bacillus thuringiensis (B.T.) spores to a flow of optically generated SDO.  The results indicated that an SDO concentration-time (CT) value of 0.13 Torr-Hr was required to obtain a 6-log reduction in colony forming units (CFU) of the B.T. spores that acted as simulants for a biological weapon agent (BWA).

 A prototype SDO generator, including a cryo-VBG-ECDLA as the optical pump source, was assembled on an optical breadboard that can be incorporated into a mobile system for room decontamination on a small scale.  

Conclusions:

Significant progress was made during Phase II of this program toward demonstration of a practical decontamination system based on SDO generation in flowing air with a compact, narrow-line, 764-nm laser source.

Because of a lack of time coupled with funding constraints toward the end of the program, DES was unable to test the fabricated small-scale SDO generator with the demonstrated cryo-VBGECDLA.  The performance of the SDO generator for future bio-decontamination applications can be evaluated by coupling the 764-nm radiation into the off-axis resonator and using the fluorescence technique described in the final report to gauge SDO concentration as a function of laser power.

A mechanism for SDO deactivation on B.T. spores could not be elucidated because of a limited data set during this program.  The available literature data on SDO biocidal efficacy is mainly limited to aqueous media and does not provide ready comparisons to airborne (gas-phase) deactivation.  Additional decontamination tests would give a more accurate picture of B.T. deactivation by SDO and would enable an engineering assessment for scaling the SDO generator power.


Journal Articles on this Report : 1 Displayed | Download in RIS Format

Other project views: All 2 publications 1 publications in selected types All 1 journal articles
Type Citation Project Document Sources
Journal Article Meng LS, Nazimov B, Madasamy P, Brasseur JK, Henshaw TL, Neumann DK. High power 7-GHz bandwidth external-cavity diode laser array and its use in optically pumping singlet delta oxygen. Optics Express 2006;14(22):10469-10747. EPD06082 (Final)
  • Full-text: Optics Express PDF
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  • Abstract: Optics Express
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  • Supplemental Keywords:

    small business, SBIR, EPA, airflow decontamination, chemical attack, homeland security, biological attack, singlet delta oxygen, SDO, chemical agent, biological agent, air, recirculated air, air purification, air sterilization, chemical weapons agent, safe buildings, biological and chemical pathogens, airflow sterilization, volatile organic compounds, VOCs, toxic chemicals, HVAC, heating, ventilation, air conditioning, ecosystem protection/environmental exposure & risk, air, scientific discipline, RFA, engineering, chemistry, physics, environmental engineering, environmental chemistry, monitoring/modeling, environmental monitoring, biological warfare agents, chemical warfare agents, chemical characteristics, bioterrorism, air decontamination, airflow sterilization, air quality, chemical composition, monitoring, real-time monitoring, environmental measurement, analytical chemistry, chemical attack, homeland security, SDO Generator, air duct, cryogenically-cooled laser source, external-cavity diode laser, Volume Bragg Grating, narrow-lined laser , RFA, Scientific Discipline, Air, Ecosystem Protection/Environmental Exposure & Risk, Environmental Chemistry, Monitoring/Modeling, Environmental Monitoring, Engineering, Chemistry, & Physics, Environmental Engineering, monitoring, homeland security, chemical characteristics, environmental measurement, bioterrorism, biological warfare agents, photochemical generator, chemical composition, singlet delta oxygen, analytical chemistry, air quality, real-time monitoring, chemical warfare agents, chemical attack, air decontamination


    SBIR Phase I:

    Singlet Delta Oxygen Airflow Sterilization for Building Protection  | Final Report