Final Report: Singlet Delta Oxygen Airflow Sterilization for Building ProtectionEPA Contract Number: EPD05023
Title: Singlet Delta Oxygen Airflow Sterilization for Building Protection
Investigators: Henshaw, Thomas L.
Small Business: Directed Energy Solutions
EPA Contact: Manager, SBIR Program
Project Period: March 1, 2005 through August 31, 2005
Project Amount: $69,065
RFA: Small Business Innovation Research (SBIR) - Phase I (2005) RFA Text | Recipients Lists
Research Category: SBIR - Homeland Security , Small Business Innovation Research (SBIR) , Health Effects
In this Phase I project, Directed Energy Solutions (DES) demonstrated a diode laser-pumped singlet delta oxygen (SDO) generator that can produce high densities of SDO for the decontamination of chemical and biological warfare (CBW) agents. To DES’s knowledge, this is the first ever generation of effective SDO concentrations for CBW decontamination using diode laser excitation. DES is able to generate SDO at densities of 8 × 1014 cm-3 from a 500-mW external cavity diode laser. The approach for the airflow sterilization device involves the use of an optical generator of metastable SDO, a powerful yet selective oxidant that shows excellent potential to be an effective CBW decontamination agent (see References 1-6). The device produces high SDO concentration and has a service-free operational lifetime greater than 10,000 hours, which overcomes the shortcomings of device instability, poor lifetime and low SDO concentrations found in other approaches.
One important commercial application of SDO is the sterilization of biological agents—such as mold, viruses and bacteria—for improving indoor air quality. With increased incidences of indoor respiratory illnesses that stem from poor indoor air quality, facilities mangers are faced with growing pressures to sterilize the indoor air environment. There is also increasing concern among commercial owners to decontaminate their building air supply in the event of a terrorist attack of some type.
SDO is a known sterilization agent for a variety of chemical and biological pathogens (see References 7-9). The diode-pumped SDO air sterilization device offers excellent capability for commercial indoor air quality and CBW protection. For building air sterilization of robust CBW agents, which represents the most difficult scenario for decontamination, a 500-CFM duct in a 5,000-square-foot building would require device scaling of 1,000 times. Current diode laser technology can generate more than 10 W of laser power in a single diode array with line narrowing. By using diode laser arrays and making simple improvements to the device, one to two orders of magnitude improvement in the performance of SDO generator can be accomplished. DES’ current device-scaling path, based on a single diode operation demonstrated here, suggests that 10 laser devices multiplexed into a single cavity will generate sufficient power to sterilize a commercial building of this size.
The SDO lifetime in air is about 100 to 200 ms, which is long enough to allow in situ air sterilization. But the SDO lifetime is also short enough to leave no residual effects in the atmosphere. Several system advantages are evident in this approach. The device is HVAC-compatible, operates at a minimum pressure drop and will:
- Use nonhazardous SOD as a proven decontaminant.
- Quickly and permanently destroy the chemical or biological agent.
- Potentially have universal utility so that the specific chemical or biological agent need not be known.
- Produce no toxic residuals such as ozone or chlorine dioxide.
- Operate in both standby (routine purification) and high-power (attack-protection) modes.
- Use durable and efficient off-the-shelf laser diodes with “set and forget” operation and no personnel training.
The approach for the optical SDO generator uses a waveguide to enhance the output power of an external cavity diode laser and to efficiently couple laser diode light into oxygen for SDO generation. Figure 1a shows the energy-level diagram of oxygen and the pumping scheme using a 764-nm laser source. Oxygen is optically excited from the ground state, O2(X3Σ), to the second electronically excited state, O2(b1Σ), via 764-nm excitation, as shown in equation (1). Singlet sigma oxygen is then quenched by ground-state oxygen, yielding exclusively SDO. The SDO has lifetimes on the order of about 200 ms at 100 Torr oxygen. The schematic configuration of the diode-pumped SDO generator is shown in Figure 1b. In this configuration, the diode output, tuned to the oxygen absorption at 763.8 nm, is confined using an optical waveguide for efficient coupling of optical energy to chemical energy in the form of SDO.
DES has developed several approaches that use electrically powered light sources—such as high-intensity discharge lamps, light-emitting diodes and fiber lasers—for the production of SDO in a compact, high-efficiency device. Photosensitized generation of SDO using organic or organometallic materials has the potential to become a viable method for neutralizing chem-bio agents. To be considered an effective method for chem-bio agent decontamination, the sensitizer must not only demonstrate efficient singlet oxygen generation, but also be benign with respect to quenching of SDO and oxidation by SDO. However, most organic photosensitizers are susceptible to photodegradation/oxidation upon prolonged illumination. Indeed, a recent analysis performed under DES’ High Energy Lasers Multidisciplinary Research Initiative program (AFOSR Grant No. F49620-02-1-0331) indicates xanthene-based sensitizers, such as Rose Bengal, are precluded due to their susceptibility to self-oxidation by SDO and potentially low SDO yields. Accordingly, DES has determined the Rose Bengal dye approach is not viable and was not pursued in this project.
In this Phase I project, DES pursued a diode-pumped SDO generator approach described in the Project Description section. Diode-laser pumping offers an efficient, cost-effective and durable decontamination approach to high-density SDO generation. The approach for the optical SDO generator uses a waveguide to enhance the output power of an external cavity diode laser and to efficiently couple laser diode light into oxygen for SDO generation. The Pound-Drever-Hall laser-locking technique is used to stabilize the pump laser to oxygen absorption line; the frequency stability of the laser was tested overnight (see References 10 and 11). A gas cell with Teflon channels inside matching the beam size of the cavity is used for the production of SDO. The SDO production efficiency of the device is characterized by measuring the fluorescence signal of SDO using an optical multichannel analyzer spectrometer. A sample fluorescence spectrum of SDO is shown in Figure 2. SDO concentration was calibrated by using a microwave cavity that generates a known quantity of SDO (see Reference 12). The measured SDO concentration at a flow velocity of 0.5 m/s is 8.1x1014/cm3.
The effectiveness of SDO against a CW simulant was studied during this Phase I project. A bench-scale test bed was designed and fabricated for measuring the rate constants of SDO quenching by CBW simulants, and to assess efficiency of CBW agent decontamination using SDO. The measured total quenching rate constant (chemical and physical) of SDO by 2-chloro phenyl ethyl sulfide (CEPS) suggests that oxidation of CEPS will occur rapidly under the conditions of atmospheric pressure and concentration of SDO greater than 1014 molecules/cm3, which are actual conditions under which the decontamination will be done. The measured rate constants as the dimensionless slopes of the kquench× [CEPS] × t versus the CEPS absorbance plots is shown in Figure 3. Further work is needed, which should include measurement of the chemical reaction component of the total quenching rate constant and extending the measurements to the other HD stimulants, such as CEES, to build a database of the rate constants.
A diode-pumped SDO generator for CB agent decontamination was assembled and tested for the first time, which produced effective SDO densities for decontamination of CB agents. The rate constant for the interaction of SDO with CEPS, an HD stimulant, was measured: k = 5.4 ± 0.7 × 10-14 cm3s-1. The rate constant measurement sets the SDO requirement for oxidizing HD to be about 2 x 1014 /cm3. With a 500-mW diode source, SDO densities of 8x1014cm-3 were obtained, which is about four times the required SDO concentration. Based on previous studies, SDO oxidizes CEPS to form benevolent products. Further studies are required to accurately measure the branching ratios of oxidation products of CWA by SDO.
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