FEATURE
Update on
Environmental
Biosensors Scientific understanding and technological development are advancing, but commercialization, with a few exceptions, has been slow. K I M R.
ROGERS
AND C L A R E L.
GERLACH
T
he forecast has improved for commercialization of environmental biosensors since last reported three years ago (i). Several previously introduced technologies have been demonstrated in the field, and several new technologies are expected to become commercially available in the next year for environmental screening or analysis applications. Already, several systems are in the process of formal field testing and are being considered for inclusion in the U.S. EPA Office of Solid Waste method compendium, SW-846 (2). Significant progress has been made in the technological development of biosensors, particularly advances made in signal transducer technology and the increased sensitivity of biosensors. Moreover, technological advances in microchips have resulted in microminiaturized systems capable of dispensing, mixing, separating (by chromatographic or electrophoretic methods), and detecting analytes and tracers. These techniques will have a significant impact on biosensor development and should improve their operational characteristics, so that they will more easily overcome many of the technical obstacles to commercialization. Several market obstacles remain. Many of these same technological advances, such as lab-on-a-chip, also contribute to improvements in competing methods—miniaturized GC, LC, MS, and electrophoresis. Consequently, a critical issue for commercial success of environmental biosensors involves their ability to find unique cost, convenience, and performance advantages in the competitive environmental monitoring market. Although initially used for performing clinical and diagnostic analyses—biosensors currently account for a small but growing share of this market (3)—their use for making environmental measurements is relatively recent. Biosensors used for this latter
5 0 0 A • DECEMBER 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
Not subject to U.S. copyright. Published 1999 American Chemical Society
purpose potentially fill an important need for fieldportable methods that are both sensitive to analytes of interest and rugged with respect to harsh field conditions and complex matrices. The sensors are composed of a biological recognition element and a signal transducer. Recognition elements usually fall into one of three groups: biocatalytic (enzymes); bioaffinity (antibodies, receptors, DNA); or microorganisms (bacteria, yeasts). Signal transducers typically used in environmental biosensors include electrochemical, optical, or acoustic devices (see Figure 1). Both theoretically, and, to a significant extent, as demonstrated in practice, any biological recognition element can be coupled to any signal transducer. Yet despite this flexibility, which enables biosensors and biosensor-related techniques to be used in a broad range of potential environmental applications, they must still overcome a number of market-related and technical obstacles to assure commercial viability in the highly competitive area of field analytical methods. Performance factors—detection limit sensitivity, competitiveness compared with existing field analytical methods, and whether potential market size justifies development costs—must be considered when evaluating the suitability of new biosensor technologies. Although decreases in environmental project budgets have increased the need for measurement methods that are more cost-effective, small market potential can discourage innovation. Moreover, in part because of the prescriptive and sometimes com-
plex environmental regulations, there has been a reluctance of users to try innovative technologies. Biosensor development and use are also influenced by the cost per analysis and system start-up costs, including that of the biosensor instrument itself (and other nondisposable parts), which can be substantial compared with immunoassay or chemical test-kit methods. Biosensor assays, however, are typically low in cost per analysis, generate fewer contaminated disposable wastes, and in some cases, show potential for multianalyte analysis. There are other challenges to environmental biosensor development (see Figure 2) besides obstacles in the marketplace. These include relatively high development costs for single-analyte systems, limited shelf life and operational lifetimes for premanufactured biorecognition components, and relatively high assay complexity {4-6).
Currently available systems The use of biosensors for potential environmental monitoring applications is reported for a wide range of specific compounds and compound classes. Most such reports concern laboratory prototype devices or commercially available instrument designs adapted for specific biological recognition elements and assay formats. Since 1996, a number of new biosensor systems have been introduced, and previously available biosensor t e c h n i q u e s have b e e n improved to better meet some of the environmental monitoring challenges (see Table 1).
Current field-deployable biosensor instrumentation, clockwise from left: RAPTOR portable, fourchannel fluorometric assay and Flow Assay Sensing and Testing System (FAST 2000), both courtesy of Research International, Inc., and Biolyzer hand-held detector instrumentation, courtesy of Environmental Technologies Group, Inc.
DECEMBER 1, 1999/ ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS » 5 0 1 A
In one example application, biological oxygen demand (BOD) sensors take advantage of the high reaction rates of A biosensor schematic microorganisms interfaced to elecThe biological recognition element, which interacts with one (or more) target analytes, trodes to measure the oxygen deplemay, for example, be an enzyme, antibody, receptor, DNA, or a microorganism. tion rate—standard BOD methods reElements are interfaced with sensor material, which is itself coupled to a signal quire five days to complete compared transducer. with about 15 minutes for a biosensorbased analysis. A better understanding of the actions of microorganisms in sewage treatment plants and water purification plants, coupled with technological advances in electroanalysis, has allowed development and improvement of BOD sensors in several countries, including Germany, Japan, and the United States. A biosensor for BOD was developed in Japan that uses Pseudomonas putida (a common bacterium found in soil) (7). This sensor is capable of determining low BOD levels in river water and secondary effluents and shows negligible response to interferents such as chloride and heavy metal ions. The instrument (the BOD-2000 for desktop use or the BOD-2200 for mobile laboratories) is commercially available through Central Kagaku Corp., Tokyo. The BOD senFIGURE 2 sor is routinely used in Japan for environmental monitoring and is accepted as a method by the JapaBiosensor development challenges nese Industrial Standard. FIGURE 1
Commercialization of biosensors for enviromentally relevant applications faces significant hurdles, many of which still remain to be adequately addressed. As a result, development cycles tend to be long and commercial success remains limited.
At the U.S. Naval Research Laboratory in Washington, D C , two promising techniques based on biosensors are being tested for acceptance by the U.S. EPA Office of Solid Waste and being considered for inclusion in their method compendium, SW-846. A continuous-flow immunosensor (the FAST 2000) for chemical and biological warfare agents has been assigned SW-846 method number 4655 (Explosives Analysis in Soil and Water Using Environmental Immunosensors) and is expected to be included in the fourth update to that document. Analyte 2000 is a four-channel, single-wavelength fluorometer optimized for performing evanescent-wave fluoroimmunoassays. It is used for explosives and biowarfare agents and has been tentatively assigned method number 4656 (Explosives Analysis in Soil and Water Using Fiber-Optic Immunosensors) (8). Both the FAST 2000 and the Analyte 2000 were developed by researchers at the Naval Research Laboratory and Research International, Inc. The availability of these sensors and other comparable field analytical methods is important for the analysis of groundwater in or near military sites and munitions storage areas. Explosives such as TNT and RDX are mobile in soil and may contaminate groundwater. By using field analytical methods for on-site monitoring of these components, the U.S. Department of Defense can cut the cost of a single analysis from $250-$350 to $3-$8. Even with higherdensity sampling to improve site characterization and frequent confirmation by more expensive methods, such as the reverse-phase-high-performance liquid
5 0 2 A • DECEMBER 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
chromatography method (RP-HPLC), the national savings would be considerable (9).
In the pipeline Although relatively few biosensors for environmental applications have become commercially available since 1996, technological advances both in hardware and bioanalytical chemistry have continued to improve the sensitivity and selectivity of biosensor and biosensor-related systems reported as laboratory prototypes. This trend (see Table 2) is evidenced by the development of increasingly sensitive (lower detection limits) biosensor-related immunoassay techniques for the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). Because many of these techniques use monoclonal antibodies derived from the same clone, recent increases in sensitivity are largely due to improvements in assay formats and transduction technology. The general trend toward assay formats that are better adapted to miniaturization, improved assay formats, and the increased sensitivity already noted tend to make these methods more attractive for future commercial development.
TABLE 1
Analytes for which biosensors have been reported Analyte
Biochemical mechanism
Pesticides Triazines 2,4-D Oganophosphates/carbamates Alachlor Imidazolinones Cyclodienes Dithiocarbamates
immunosensors immunosensors, GEM a sensors enzyme sensors, immunosensors immunosensors immunosensors immunosensors enzyme-electrodes immunosensor enzyme-electrode
Diphenylureas Sulfonylureas Organic BTEX" Chlorobenzoates Formaldehyde (aqueous/vapor) Peroxides Organonitriles Explosives (TNT, RDX, HMX) Phenol (aqueous/vapor) Chlorophenols Cyanide Polychlorinated biphenyls (PCBs) Benzo(a)pyrene Nitrate Sulfite Naphthalene
compounds GEM sensor GEM sensor enzyme-electrodes enzyme-electrodes enzyme-electrodes immunosensors enzyme-electrodes, bacterial sensor: enzyme-electrodes enzyme-electrodes, GEM sensors immunosensors, GEM sensors immunosensors bacterial sensors enzyme sensors GEM sensor
Heavy metals Zn, Hg, Cu, V, Ni, A g , Pb, Cr, Co
enzyme sensors, GEM sensors
Biological parameters Recent reports about biosensors used for environmental applications reMulticompound toxicity" (microorganism) GEM, yeast, bacterial sensors Multicompound toxicity"* (enzyme) enzyme sensors flect the increasingly international naPhotosystem inhibition enzyme-electrodes ture of biosensor research. More than Biological oxygen demand (BOD) GEM, bacterial, enzyme sensors 80% of the reports shown in Figure 1 DNA damage DNA-intercalation sensors and Tables 1 and 2, originate in EuBacterial immunosensors, DNA rope, Japan, or Russia. Japanese com(identification/enumeration) sequence sensors panies and universities, along with the a Genetically engineered microorganisms. Japanese government, are leading the 6 Benzene, toluene, ethylbenzene, and xylene. development and commercialization of c These biosensors have been tested for multicompound class toxicity. Pollutants tested in various chemical and biosensor technology in systems include the following: heavy metals, phenols, herbicides, and surfactants. " These biosensors have been tested for multicompound class inhibition of enzyme activity. Pollutants general. The European Union has intested in various systems include the following: heavy metals, chlorophenols, triazine herbicides, vested heavily in biosensor technoldithiocarbamate fungicides, carbamate insecticides, benzoic acid, thiourea, mercaptoethanol, and cyanide. ogy with significant emphasis on environmental monitoring. A variety of biosensor development activities have been reported from Russia and former Eastern block countries. Examples of biosensor deattenuation of total reflectance. Once the response velopments in Russia include devices and techhas achieved steady state, it is possible to employ niques for compounds such as the pesticides 2,4-D signal averaging to improve detection limits. Sigand 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), simnal averaging is reproducing the signal many times azine, atrazine, and a number of organophosphoand combining these measurements to minimize or rus pesticides {10). In addition to environmental cancel out the random noise and improve the signalbiosensor reports from England, Ireland, Spain, Italy, to-noise ratio. This is an important feature in meaGermany, Poland, Czech Republic, Ukraine, Swesuring analytes at very low concentrations. The den, France, Netherlands, Greece, Austria, China, and Bio-MEMS team at the University of Cincinnati is Canada, there are a significant number of reports also developing a generic microfluidic system cafrom the United States. pable of detecting microorganisms in liquids, including water, blood, and urine {12). Early reA recent study at the University of Cincinnati in search indicates that this is a feasible technology and Ohio {11) examined key issues in signal processthat electrochemical, immunoassay-based, biologing for spectroelectrochemical sensors. These senical-chemical detection systems may be useful in sors are made up of a selective film coated over an environmental and clinical applications. Microchipoptically transparent electrode. The sensor detects DECEMBER 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 5 0 3 A
Biosensor-related immunoassays for detection of 2,4-dichlorophenoxyacetic acid (2,4-D) Methodological improvements and new analysis techinques have greatly improved the sensitivity of detection techniques for some molecules in environmentally relevant settings.
Method type ELISAa Fluorescence polarization ELISA ELISA ELISA Immunosensor (piezoelectric) RaPID Agglutination immunoassay Immunosensor (piezoelectric) ELISA (chemiluminescent) Immunosensor (fiber optic) EnviroGard immunoassay test kit Flow injection immunoassay Flow injection immunoassay Immunosensor (electrochemical) Inhibition assay Immunosensor (fluorescence) Flow injection immunoassay (amperometric) Immunosensor (electrochemical) ELISA (chemiluminescent) ELISA (chemiluminescent) Immunosensor (amperometric) 3
Detection limit (pg/L)
Date reported
100 100 22 3 1 1 0.7 0.5 0.3 0.2 0.2 0.1 0.1 0.1 0.1 0.05 0.03 0.03 0.01 0.006 0.005 0.002
1989 1993 1996 1996 1994 1994 1992 1994 1997 1998 1997 1992 1996 1996 1998 1993 1998 1996 1997 1996 1997 1997
ELISA stands for renzyme-l-nked immuunosobent tassy."
based immunoassay techniques have also been demonstrated at the University of Alberta, Canada {13). Researchers at Cornell University (Ithaca, N.Y.) have investigated the use of liposome-based sensors for detecting and quantifying polychlorinated biphenyls (PCBs) at low parts per billion (ppb) levels (14). These techniques have been used to detect PCBs in equipment wipe-test samples taken at a Superfund site. Liposome-based amperometric immunoassay techniques have also been demonstrated for detection of triazine pesticides by researchers at the University of Stuttgart, in Germany (15). Liposomes offer certain advantages over enzymes as biological recognition elements because they provide immediate amplification, can be stored for a long time and are resistant to temperature fluctuations seen in the field. There is great interest in the ability to detect more than one analyte per "run." A disposable array-based immunosensor, developed at the Naval Research Laboratory, has proven effective in measuring multiple toxins at the subnanomolar range in a single 150-pL sample (16). In this application, the toxins detected were ricin, Yersinia pestis (the organism responsible for Plague), and staphylococcal enterotoxin B. The planararray design used here may be adaptable to multianalyte biosensors for varieties of compounds of environmental interest. The device is portable and can be used at room temperature. S 0 4 A • DECEMBER 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
At the U.S. EPA National Exposure Research Laboratory in Las Vegas, Nev., biosensor research is being conducted to provide rapid, sensitive, and costeffective field screening and biosensor methods for such compounds as phenols, 2,4-D, organophospha.es, and PCBs. In addition, collaborative efforts have been established with New Mexico State University in Las Cruces, the University of California-Riverside, and the Environmental Technologies Group (ETG) in Baltimore, Md., to develop and evaluate optical (17) and electrochemical (18) biosensor techniques. The Biolyzer, which is being developed at ETG, is an expansion of its commercially available Metalyzer. It consists of a handheld electrochemical instrument capable of accepting disposable sensors based on screen-printed electrodes. The patented sensor design (19) contains all of the reagents necessary to perform an analysis and eliminates the need to measure samples or handle chemical reagents in the field. The instrument is currently being tested for the analysis of phenols (20) and several heavy metals. Future development plans include modifying the reagents and electrode coating (polymer, enzyme, or antibody) to create sensors for pesticides and a wide variety of other organic and inorganic compounds. The Biolyzer automatically recognizes which type of analysis is being performed by reading a calibration chip This chip is inserted into the instrument prior to analysis and contains the analytical parameters and calibration curve for a specific lot of sors The ease of use enables nontechnical field personnel to make reliable measurements in the field TVie irprsatiliHj a n d e c o n o m y nffprpH h\7 t t l i s tamp o f
biosensor format are important in field analytical chemistrv Texas Instruments has developed an integrated analytical biosensor based on a surface plasmon resonance platform whose gold-sensing surface can be coated directly with a variety of biological substances, such as creatine kinase hybrid, bovine serum albumin, or 2,4-dinitrophenolated glycine. More sophisticated methods of biofilm attachment, such as the use of the avidin-biotin-binding interaction, can also be used to attach the biological element to the surface (21). Another demonstrated method uses a gold-binding peptide (22). This work demonstrates the feasibility of using simple biofilm preparations to ready a biosensor for robust field applications Known as Spreeta this device has been demonstrated for TNT in soil (23) The developers expect that by switching out the biofilm coating Spreeta can be adapted to the analysis of a wide variety of low molecular weight analytes
Future developments Support for biosensor development continues to grow (see box on next page). Biosensors of the future will most likely contain structurally and chemically modified biorecognition proteins or biomimetic polymers that possess a wide and controllable range of affinities and selectivities. These recognition elements will be arranged on microchips in large arrays to allow for multianalyte analysis in a single run. They may be used as stand-alone sensors with neu-
ral network algorithms or as detectors for microscale or nanoscale chromatographic or electrophoretic separation systems. Aspects of these technologies have already been reported [24, 25). With respect to potential environmental monitoring applications for biosensors, one of the areas that seems to be lagging behind is the integration of sample preparation and handling techniques. This presents a problem due to the wide variety of environmental matrices (ranging from drinking water to sewage sludge) and complex mixtures of environmental pollutants. Although reported biosensor methods for environmental applications do not typically list a variety of matrices, some efforts have been made in this area. For example, a. screen-printed, immunosensor for 2,4-D was recently developed and tested using aqueous and organic solvent soil extracts in the field in a tandem evaluation that included an immunoassay test kit. Although comparable results were achieved, the immunosensor afforded greater simplicity and had a shorter extraction time requirement [26) In addition an LC technique using an enzyme biosensor detector was applied to the evaluation of phenols in contaminated soils and soilleachate samples from Superfund sites [27) In addition to environmental regulatory applications, another area in which biosensors show particular promise for environmental monitoring is for toxicity and genotoxicity screening. A number of microbial biosensors have been reported for detection of mixed pollutant toxicity. Although environmental pollution is not regulated in terms of toxicity, there are a number of potential applications. For example, the pulp and paper industry could benefit from rapid, cost-effective technologies to monitor the potential toxicity of waste effluents. In addition, these techniques could be of value for monitoring contaminated sediments from hightraffic ports and waterways. Another area in which biosensor technology shows future promise for environmental applications involves the analysis of nucleic acids. Considerable efforts have been made in the biomedical field toward development of microsensor techniques to measure sequencing, PCR, and hybridization processes [28). Recently, biochips have been developed as a new generation of biosensors using DNA probes—the DNA biochip [29). The DNA biochip is based on an integrated circuit microchip using nucleic acid bioreceptors designed to detect specific gene biomarkers of interest in environmental applications. Associated areas that could be of value to environmental monitoring involve the identification and enumeration of pathogenic microorganisms usint? seof DNA captured on microchip electrodes [30) adducts such as benzopyrene tetrol (an important biomarker for DNA damage due to exposure to benzo(a)pyrene [31) or the detection of DNA damage using biosensor techniques [32) The commercialization of biosensors for environmental applications has shown only modest progress over the past three years. Technological advances, however, continue to result in higher sensitivity and more versatile operational characteristics. Increased miniaturization of biosensors will also al-
Biosensor information and support Although commercialization has moved few environmental biosensors off the bench and onto the shelf, research continues in government laboratories and universities in the United States and other countries. In the United States, resources are available from various government agencies for research and development that will lead to commercialization of new technologies. These resources can be in the form of monetary grants or cooperative research and development agreements. Sources of funding include the Small Business Innovative Research grants and Small Business Technology Transfer grants offered by a number of federal agencies. Agencies that have funded biosensor-related projects in the past include EPA, the National Institute for Standards and Technology (NIST), and the U.S. Departments of Energy and Defense. In addition, NIST funds sensorrelated projects through the Advanced Technology Program. This program was designed to foster high-risk research and exploratory development in areas such as sensor technology (3). Information concerning biosensors for environmental applications can be accessed from a variety of sources, including scientific journals in specialized areas, such as analytical chemistry, field analytical chemistry, toxicology, and microbiology. A variety of conferences and meetings every year are primarily devoted to sensors. Some of these include the World Congress on Biosensors and the International Meeting on Chemical Sensors, Sensors, and Eurosensors. In addition, several organizations such as the American Chemical Society, the International Society for Optical Engineering, the International Association of Environmental Analytical Chemistry, and the Pittsburgh Conference regularly sponsor symposia focused on biosensors. The Internet has also become a valuable source of information on this subject. For example one particularly useful Web site is the Biosensor Information Exchange at www.biosensor-network.de. This site which currently has 176 members from 33 countries provides access to a number of meeting announcements companies and other portals
low these devices to be incorporated as detectors in microchip systems. Although a variety of environmental market issues will likely remain as obstacles for commercial development of biosensors, these devices show the potential for application as standalone techniques, detectors in chromatographic systems, and as microsensors integrated into multitarget compound analysis systems.
Acknowledgment The U.S. Environmental Protection Agency, through its Office of Research and Development, funded the work involved in preparing this article. It has been subject to the agency's peer review and has been approved for publication. The U.S. government has the right to retain a nonexclusive, royalty-free license in and to any copyright covering this article.
References (1) Rogers, K. R.; Gerlach, C. L. Environ. Sci. Technol. 1996, 30(11), 486A-491A. (2) Test Methods for Evaluating Solid Waste, PhysicallChemical Methods (SW-846); EPA Office of Solid Waste, U.S. Environmental Protection Agency, U.S. Government Printing Office: Washington, DC, 1986 (with updates). DECEMBER 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 5 0 5 A
(3) Weetall, H. H. Biosens. Bioelectron. 1999, 14 (2), 237Sensor for Metal Analysis and Method of Using Same. U.S. 242. Patent 5,830,344, 1998. (4) Rogers, K. R.; Mascini, M. Field Anal. Chem. Technol. 1998, (20) Schmidt, J. C. Field. Anal. Chem. Technol. 1998,2 (6), 3512 (6), 317-331. 361. (5) Kissinger, E T. Curr. Cont. 1997, 16 (3), 101-103. (21) Elkind, J. L.; Stimpson, D. I.; Strong, A. A.; Bartholomew, D. U.; Melendez, J. L. Sens. Actuators B 1999, (6) Williams, L. R. Field Anal. Chem. Technol. 1998,2 (6), 31554 (1-2), 182-190. 316. (7) Chee, G. J.; Nomura, Y.; Karube, I. Anal. Chim. Acta 1999, (22) Woodbury, R. G.; Wendin, C; Clendenning, J.; Melen379 (1-2), 185-191. dez, J.; Elkind, J.; Bartholomew, D.; Brown, S.; Furlong, (8) Lesnik, B.; Fordham, O. M., Jr. Environ. Testing Anal. 1998, C. E. Biosens. Bioelectron. 1998, 13 (10), 1117-1126. 7 (6), 9-19. (23) Strong, A. A.; Stimpson, D. I.; Bartholomew, D. U.; Jenkins, T. E; Elkind, J. L. SPIE Newsl. 1998, 3710, 362-372. (9) Field Demonstration and Method Validation ofNRL Environmental Immunosensors; Environmental Security (24) Marshall, A.; Hodgson, J. Nature Biotechnol. 1998,16 (1), Technology Certification Program, U.S. Department of 27-31. Defense, U.S. Government Printing Office: Washington, (25) Jacobson, S. C; Culbertson, C. T.; Daler, J. E.; Ramsey, J. DC, in press. M. Anal. Chem. 1998, 70 (16), 3476-3480. (26) Kroger, S.; Setford, S. J.; Turner, A. E F. Anal. Chem. 1998, (10) Lukin, Y. V; Generalova, A. N.; Tyrtysh, T. V; Eremin, S. 70 (23), 5047-5043. A. In Immunochemical Technology for Environmental Applications; Aga, D. S., Thurman, E. M., Eds.; ACS Sym(27) Rogers, K. R.; Mascini, M. Field. Anal. Chem. Technol. 1999, posium Series 657; American Chemical Society: Wash3 (1), 55-66. ington, DC, 1996; pp. 97-105. (28) Freemantle, M. Chem. Eng. News 1999, 77 (8), 27-36. (11) Slaterbeck, A. E; Ridgway, T. H.; Seliskar, C. J.; Heine(29) Vo-Dinh, T; Alarie, J. E; Isola, N.; Landis, D; Wintenberg, man, W. V.. Anal. Chem. 1999, 71 (6), 1196-1203. A. L.; Ericson, M. N. Anal. Chem. 1999, 71 (2), 358-363. (12) Ann, C. H.; Henderson, T.; Heineman, W; Halsall, B. Pa(30) Cheng, J.; Sheldon, E. L.; Wu, L.; Uribe, A; Gerrue, L. O; per presented at Micro Total Analysis Systems (uTAS) 1998 Carrino, J.; Heller, M. J.; O'Connell, J. E Nature BiotechWorkshop, Banff, Canada, Oct. 13-16, 1998. nol. 1998,16 (6), 541-546. (13) Harrison, J.; Chiem, N. H. Clin. Chem. 1998, 44 (3), 591(31) Vo-Dinh, T.; Alarie, J. P.; Johnson, R. W; Sepaniak, M. J.; 598. Santella, R. M. Clin. Chem. 1991, 37 (4), 532-535. (14) Roberts, M. A.; Durst, R. A. Anal. Chem. 1995, 67 (3), 482(32) Wang, J.; Rivas, G.; Oszos, M.; Grant, D. H.; Cai, X. H; Par491. rado, C. Anal. Chem. 1997, 69 (7), 457-460. (15) Baumner, A. J.; Schmid, R. D. Biosens. Bioelectron. 1998, 13 (5), 519-529. Kim R. Rogers is an analytical chemist and biosensor (16) Wadkins, R. M.; Golden, J. P.; Pritsiolas, L. M; Ligler, E S. Biosens. Bioelectron. 1998, 13 (3-4), 407-415. researcher at the U.S. EPA National Exposure Re(17) Rogers, K. R.;Wang, Y.; Mulchandani, A.; Mulchandani, search Laboratory, Human Exposure and Atmospheric P.; Chen, W. Biotechnol. Prog. 1999, 15 (3), 517-521. Sciences Division, in Las Vegas, Nev. Clare L. Gerlach is (18) Wang, J.; Tian, B.; Rogers, K. R. Anal. Chem. 1998, 70 (9), the task leader for technical transfer at Lockheed Mar1682-1685. tin Environmental Services in Las Vegas, Nev. (19) Priddy, R. V; Schmidt, J. C; Studer, J. E., Jr. Disposable
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