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Sensors 2005 , 5 , 4-
sensors
ISSN 1424-
© 2005 MDPI
http://www.mdpi.org/sensors
Overview of Sensors and Needs for Environmental Monitoring
**Clifford K. Ho ***^ , Alex Robinson, David R. Miller and Mary J. Davis
Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185, USA. Tel: (505) 844-2384, Fax: (505) 844-7354.
Received: 31 August 2004 / Accepted: 01 December 2004 / Published: 28 February 2005
Abstract: This paper surveys the needs associated with environmental monitoring and long- term environmental stewardship. Emerging sensor technologies are reviewed to identify compatible technologies for various environmental monitoring applications. The contaminants that are considered in this report are grouped into the following categories: (1) metals, (2) radioisotopes, (3) volatile organic compounds, and (4) biological contaminants. United States regulatory drivers are evaluated for different applications (e.g., drinking water, storm water, pretreatment, and air emissions), and sensor requirements are derived from these regulatory metrics. Sensor capabilities are then summarized according to contaminant type, and the applicability of the different sensors to various environmental monitoring applications is discussed.
Keywords: environmental monitoring, sensors, regulatory standards.
Introduction
Environmental monitoring is required to protect the public and the environment from toxic contaminants and pathogens that can be released into a variety of media including air, soil, and water. Air pollutants include sulfur dioxide, carbon monoxide, nitrogen dioxide, and volatile organic compounds, which originate from sources such as vehicle emissions, power plants, refineries, and industrial and laboratory processes. Soil and water contaminants can be classified as microbiological (e.g., coliform), radioactive (e.g., tritium), inorganic (e.g., arsenic), synthetic organic (e.g., pesticides), and volatile organic compounds (e.g., benzene). Pesticide and herbicides are applied directly to plants and soils, and incidental releases of other contaminants can originate from spills, leaking pipes, underground storage tanks, waste dumps, and waste repositories. Some of these contaminants can persist for many years and migrate through large regions of soil until they reach water resources, where they may present an ecological or human-health threat. The United States Environmental Protection Agency (U.S. EPA) has imposed strict regulations on the concentrations of many environmental contaminants in air and water. However, current monitoring methods are costly and time-intensive, and limitations in sampling and analytical techniques exist. For example, Looney and Falta [1] report that the Department of Energy (DOE) Savannah River Site requires manual collection of nearly 40,000 groundwater samples per year, which can cost between $100 to $1,000 per sample for off-site analysis. Wilson et al. [2] report that as much as 80% of the costs associated with site characterization and cleanup of a Superfund site can be attributed to laboratory analyses. In addition, the integrity of the off-site laboratory analyses can be compromised during sample collection, transport, storage, and analysis, which can span several days or more. Clearly, a need exists for accurate, inexpensive, long-term monitoring of environmental contaminants using sensors that can be operated on site or in situ. However, the ability to deploy and use emerging sensors for these applications is uncertain due to both cultural and technological barriers. The purpose of this report is to assess the needs of long-term environmental monitoring applications in the U.S. and to summarize the capabilities of emerging sensor technologies (with an emphasis on Sandia-developed sensor technologies). A market survey is presented that elucidates the costs, drivers, and potential benefits of using in-situ sensors for long-term environmental monitoring. Regulatory metrics for different environmental monitoring applications are then presented to provide requirements for the sensor technologies. Emerging sensor technologies that are being developed at Sandia National Laboratories are then evaluated that can be used to monitor environmental contaminants, particularly for long-term environmental stewardship. We limit our focus to four categories of contaminants: (1) metals, (2) radioisotopes, (3) volatile organic compounds, and (4) biological contaminants. For each contaminant, we seek portable sensors that can provide rapid responses (relative to current methods and technologies), ease of operation (for field use), and sufficient detection limits.
Market survey
In 2001, U.S. companies generated $213 billion in environmental industry revenue, with a growth of 2.1% and exports representing 11% of this figure [3]. Overall, the environmental industry is in a state of evolution. The U.S. environmental remediation/industrial services markets have topped out
Based on information gathered in equipment user surveys, an analysis of the market for environmental field instrumentation determined that field instrumentation has been expanding due to cost savings from on-site analysis and improved regulatory and customer acceptance of on-site methods [6]. The environmental field instrument market is expected to enjoy an average growth of 7% annually for the foreseeable future. The market will expand with technology developments and increasing regulatory acceptance. However, given the current regulatory environment, field instruments may never completely replace laboratory analysis, and therefore never realize its maximum market potential. Remediation opportunities will wane and be replaced with smaller, longer-term opportunities related to post-closure monitoring and long-term stewardship. This should open doors to new instruments and measurement technologies and remote information management systems. The market consists of many niche applications, which are met by a number of different technologies. The long- term nature of post-closure monitoring and surveillance will be required at a wide variety of nuclear sites, uranium mill tailing sites, low-level and mixed-waste burial grounds, and hazardous waste sites that may create new areas for application. This market overlaps with other markets, such as for chemical industry process monitoring. Technology developments that can crosscut multiple areas within the environmental industry have a greater potential for success within the industry. Long-term stewardship is not unique to the DOE. The EPA is currently determining its stewardship responsibilities through its Federal Facilities Restoration and Reuse Office. Both EPA Region IV and X have released policy documents on the use of institutional controls at Federal facilities. However, the specific ways in which long-term institutional control issues are implemented vary considerably at state and local offices. The Department of Defense (DoD) conducts cleanup activities at more than 10,000 sites, nearly 2,000 military installations and more than 9,000 formerly used defense properties. The Department of Interior (DoI) is responsible for overseeing approximately 13,000 former mining sites, some of which have been abandoned by the original owners. The nation’s commitment is also not limited to federal properties. For example, sanitary and hazardous landfills, industrial facilities, and former waste management operations likely require long-term monitoring that will be funded by state and local governments. The DOE conducts its stewardship activities in compliance with applicable laws, regulations, and inter-agency agreements. In general the DOE is required to implement some land-use controls at waste disposal facilities in perpetuity. Groundwater-monitoring timeframes are expected to be 30 years or greater. Costs of post-cleanup stewardship activities are currently unknown. However, a DOE Office of Inspector General audit found that the “DOE groundwater monitoring activities were not being conducted economically as they could have been since some sites had not adopted innovative technologies and approaches to well installations, sampling operations, and laboratory analysis.” The report concluded that in part this occurred because innovative groundwater monitoring techniques were either unavailable or had not been effectively disseminated, evaluated for applicability at other sites and implemented” [7]. In summary, the development of sensors for long-term groundwater monitoring may fill a niche that could have a wide-ranging application for long-term environmental monitoring.
Regulatory requirements, standards and policies
Drinking water
National Primary Drinking Water Regulations apply to public water systems and are legally enforceable standards. These primary standards are intended to protect public health by limiting the levels of contaminants that can be found in drinking water. Although these standards are applicable to public water systems (i.e., at the tap), they are often applied by remediation regulators in the aquifer (i.e., at the monitoring wellhead). The following tables (Tab. 1 to Tab. 6) summarize the drinking water standards imposed by the U.S. Environmental Protection Agency (EPA). Additional information regarding potential health impacts and sources of contamination can also be found at their web site (http://www.epa.gov/safewater/mcl.html).
Table 1. EPA national primary drinking water standards for microorganisms.
Contaminant
Maximum Contaminant Level Goal (mg/L)
Maximum Contaminant Level (mg/L) Cryptosporidium zero See footnote* Giardia lamblia zero See footnote* Heterotrophic plate count n/a See footnote* Legionella zero See footnote* Total Coliforms (including fecal coliform and E. Coli ) zero 5.0%** Turbidity n/a See footnote* Viruses (enteric) zero See footnote* *EPA's surface water treatment rules require systems using surface water or ground water under the direct influence of surface water to (1) disinfect their water, and (2) filter their water or meet criteria for avoiding filtration so that the following contaminants are controlled at the following levels:
- Cryptosporidium (as of1/1/02 for systems serving >10,000 and 1/14/05 for systems serving <10,000) 99% removal.
- Giardia lamblia: 99.9% removal/inactivation.
- Viruses: 99.99% removal/inactivation.
- Legionella: No limit, but EPA believes that if Giardia and viruses are removed/inactivated, Legionella will also be controlled.
- Turbidity: At no time can turbidity (cloudiness of water) go above 5 nephelolometric turbidity units (NTU); systems that filter must ensure that the turbidity go no higher than 1 NTU (0.5 NTU for conventional or direct filtration) in at least 95% of the daily samples in any month. As of January 1, 2002, turbidity may never exceed 1 NTU, and must not exceed 0.3 NTU in 95% of daily samples in any month.
- HPC: No more than 500 bacterial colonies per milliliter.
- Long Term Enhanced Surface Water Treatment (Effective Date: January 14, 2005): Surface water systems or (GWUDI) systems serving fewer than 10,000 people must comply with the applicable Long Term 1 Enhanced Surface Water Treatment Rule provisions (e.g., turbidity standards, individual filter monitoring, Cryptosporidium removal requirements, updated watershed control requirements for unfiltered systems).
- Filter Backwash Recycling: The Filter Backwash Recycling Rule requires systems that recycle to return specific recycle flows through all processes of the system's existing conventional or direct filtration system or at an alternate location approved by the state.
- **More than 5.0% samples total coliform-positive in a month. (For water systems that collect fewer than 40 routine samples per month, no more than one sample can be total coliform-positive per month.) Every sample that has total coliform must be analyzed for either fecal coliforms or E. coli if two consecutive TC-positive samples, and one is also positive for E.coli fecal coliforms, system has an acute MCL violation.
Table 5. EPA national primary drinking water standards for organic chemicals.
Contaminant Max. Contaminant Level Goal (mg/L) Max. Contaminant Level (mg/L) Carbon Polychlorinated
- Alachlor zero 0. Acrylamide zero Treatment Technology*
- Atrazine 0.003 0.
- Benzene zero 0.
- Benzo(a)pyrene (PAHs) zero 0.
- Carbofuran 0.04 0.
- Chlordane zero 0.
- Chlorobenzene 0.1 0.
- 2,4-D 0.07 0.
- Dalapon 0.2 0.
- 1,2-Dibromo-3-chloropropane (DBCP) zero 0.
- o-Dichlorobenzene 0.6 0.
- p-Dichlorobenzene 0.075 0.
- 1,2-Dichloroethane zero 0.
- 1,1-Dichloroethylene 0.007 0.
- cis-1,2-Dichloroethylene 0.07 0.
- trans-1,2-Dichloroethylene 0.1 0.
- Dichloromethane zero 0.
- 1,2-Dichloropropane zero 0.
- Di(2-ethylhexyl) adipate 0.4 0.
- Di(2-ethylhexyl) phthalate zero 0.
- Dinoseb 0.007 0.
- Dioxin (2,3,7,8-TCDD) zero 0.
- Diquat 0.02 0.
- Endothall 0.1 0.
- Endrin 0.002 0.
- Ethylbenzene 0.7 0. Epichlorohydrin zero Treatment Technology*
- Ethylene dibromide zero 0.
- Glyphosate 0.7 0.
- Heptachlor zero 0.
- Heptachlor epoxide zero 0.
- Hexachlorobenzene zero 0.
- Hexachlorocyclopentadiene 0.05 0.
- Lindane 0.0002 0.
- Methoxychlor 0.04 0.
- Oxamyl (Vydate) 0.2 0.
- Pentachlorophenol zero 0.
- Picloram 0.5 0.
- Simazine 0.004 0.
- Styrene 0.1 0.
Contaminant Max. Contaminant Level Goal (mg/L) Max. Contaminant Level (mg/L) Tetrachloroethylene zero 0. Toluene 1 1 Toxaphene zero 0. 2,4,5-TP (Silvex) 0.05 0. 1,2,4-Trichlorobenzene 0.07 0. 1,1,1-Trichloroethane 0.20 0. 1,1,2-Trichloroethane 0.003 0. Trichloroethylene zero 0. Vinyl chloride zero 0. Xylenes (total) 10 10 *Each water system must certify, in writing, to the state (using third-party or manufacturer's certification) that when acrylamide and epichlorohydrin are used in drinking water systems, the combination (or product) of dose and monomer level does not exceed the levels specified, as follows:
- Acrylamide = 0.05% dosed at 1 mg/L (or equivalent).
- Epichlorohydrin = 0.01% dosed at 20 mg/L (or equivalent).
Table 6. EPA national primary drinking water standards for radionuclides.
Contaminant Max. Contaminant Level Goal Max. Contaminant Level Alpha particles zero 15 picocuries per Liter (pCi/L) Beta particles and photon emitters zero 4 millirems per year Radium 226 and Radium 228 (combined) zero 5 pCi/L Tritium zero 20,000 pCi/L Uranium zero 30 ug/L (as of 12/08/03)
Storm water monitoring
Under the National Pollutant Discharge Elimination System (NPDES) regulations, all facilities which discharge pollutants from any point source into waters of the United States (US) are required to obtain a permit. The NPDES storm water regulations cover the following classes of storm water dischargers: operators of municipal separate storm sewer systems (MS4s); industrial facilities in any of eleven identified categories that discharge to an MS4 or to a water of the US; and operators of certain construction activities. Storm water regulations are implemented by the EPA or authorized states. NPDES permits may be issued as individual or general permits. In either case, NPDES permits generally require the development of a storm water pollution prevention plan, implementation of best management practices, and monitoring and reporting of storm water discharge data. Most industrial facilities elect coverage under a general permit because the permitting process is designed to be more efficient. EPA has developed a multi-sector general permit (MSGP) for storm water dischargers, providing both general requirements and sector-specific requirements. The specific requirements apply to each of 30 industrial sectors and their associated subsectors. The current MSGP was published in the Federal Register on October 30, 2000 [8]. Authorized states may use alternative permits and/or may impose additional requirements. Three types of monitoring may be required under the MSGP: visual examination, analytical monitoring, and compliance monitoring. Visual examinations are intended to provide a simple, inexpensive evaluation of storm water quality. Analytical monitoring is required for only specified
Table 7. Pretreatment standards for manufacturers of organic chemicals, plastics, and synthetic fibers [10].
The type of industry regulated under the pretreatment program is wide-ranging, including grain mills, feedlots, electroplating facilities, iron and steel manufacturers, and fertilizer manufacturers. For many industries, the monitoring required is limited to several effluent characteristics, such as biological effluent demand, total suspended solids, and pH. For other industries monitoring of a select set of priority pollutants, such as a specified subset of metals, is required. In a few instances, monitoring of all priority pollutants is required. Pretreatment standards for indirect discharges from manufacturers of organic chemicals, plastics, and synthetic fibers are provided in Table 7.
Ambient air quality
A number of substances are identified as hazardous air pollutants (now termed “toxic air pollutants” by EPA) under the Clean Air Act and are regulated under the National Emission Standards for Hazardous Air Pollutants program. The American Conference of Governmental Industrial Hygienists (ACGIH) established airborne concentration limits called Threshold Limit Values (TLV) of various hazardous air pollutants. The TLVs are believed to represent conditions under which nearly all workers could be exposed day after day without adverse health effects. The TLVs are based on information from industrial experience and experimental studies on humans and animals. Table 8 lists a few hazardous air pollutants (HAP) and the associated ACGIH TLVs. Additional information on these compounds can be found from the following web sites:
- www.epa.gov/ttn/atw/hlthef/benzene.html
- www.epa.gov/ttn/atw/hlthef/xylenes.html
- www.epa.gov/ttn/atw/hlthef/tri-ethy.html
Table 8. Threshold Limit Values for several hazardous air pollutants [11].
Threshold Limit Value (ppm) Hazardous Air Pollutant 8-Hour Time Weighted Average
15-Minute Short-Term Exposure Limit Benzene 0.5 2. Xylenes 100 150 Trichloroethylene 50 100
In 1998, the City of Albuquerque adopted a policy for regulating emissions from industries. An analysis for each relevant HAP at a site is performed to determine if the emissions from the stack result in an exceedance of the ACGIH TLV for any of the relevant substances. If the ACGIH TLV at the stack is exceeded, the concentration of that substance must be analyzed at the “fence line” (i.e., property boundary). The concentration at the fence line should not exceed 1/100th^ the ACGIH TLV. For any HAP that has uncontrolled emissions which result in an exceedance of the ACGIH TLV at the stack and 1/100 th^ of the ACGIH TLV at the fence line, air-pollution controls will be required to reduce the concentrations to 1/100th^ the TLV at the fence line. An air quality permit will also be required to ensure proper operation of the control equipment. Additional air quality standards have been compiled from 20.11.1 NMAC - Title 20, Environmental Protection - Chapter 11, Albuquerque/Bernalillo county Air Quality Control Board - Part 1 General Provisions (see Table 9).
Trace metal sensors
-Nanoelectrode array
Nanoelectrode arrays have been fabricated to identify and quantify dissolved metals [12- 13]. Signals from the electrodes are obtained by monitoring current and voltage during application of an electrical potential. Approximately 1 million individual electrodes can be placed on a 1 square inch substrate using electron-beam lithography or chemical vapor deposition. The sensing electrodes are integrated with the reference electrode, eliminating the need for buffers and permitting non- contaminating sensing in ultra-pure water. The small electrode size coupled with a very high density produces a signal with up to 10^3 times better signal-to-noise ratio than standard electrodes. Using multiple electrodes, coatings, and electrochemical techniques, target analytes can include toxic industrial chemicals and metals, such as trichloroethylene, methyl-t-butyl ether, arsenic, lead, and chromium.
-Laser-induced breakdown spectroscopy (LIBS)
As its name implies, LIBS uses a laser to rapidly heat a very small area (usually solid or liquid), generating a plasma from the atomic constituents present at the focal point. Radiative relaxation of the plasma is then observed using sensitive spectroscopic instrumentation (see Figure 1). LIBS is also known as Laser Spark Spectroscopy (LASS).
Figure 1. Stand-off LIBS probe head. Laser ablation energy and spectroscopic collection occurs through fiber optics.
LIBS can be used for rapid analysis of hazardous metals and other inorganic contaminants in water, soil, and mixed waste sites [14- 15]. It can be used to detect almost all elements, though certain metals exhibit orders of magnitude greater emission. Detection limits are a function of each specific metal, and the spectroscopic and detector hardware. Low ppb levels are typical. Contaminants targeted in Sandia projects include As, Be, Hg, Se, Pb, Cd, Cu, Zn, Ag, Cr, Fe, and Mn. Recently, a LIBS system was set up for measuring metal emissions in the waste streams of a thermal treatment facility [16]. Currently, a field deployable LIBS system is configured at Sandia-Livermore employing an image intensified CCD array, which provides sufficient signal intensity for single laser pulse LIBS. Delivery of the laser light to remote location via a fiber-optic cable has been performed. Spectral emission
likewise can be readily be transported over hundreds of feet for analysis [15]. LIBS can be extended to biodetection by looking for rapid, temporal increases in the presence and/or ratios of Ca, Na, K.
-Miniature chemical flow probe sensor
The miniature chemical flow-probe sensor can detect metals, especially copper. See “Miniature chemical flow probe sensor” below for details (page 19).
Radioisotope sensors
RadFET (Radiation field-effect transistor)
The RadFET concept for measuring gamma radiation dose has been around for many years. It is based on ionizing radiation permanently promoting high mobility electrons into low mobility holes. This creates an irreversible shift in the FET’s threshold voltage. Sandia has microfabricated miniature RadFETs [17]. Sensitivities depend in part upon fabrication structure, and range from 0.01 to 5 mV per rad. An energy spectrometer can be made by fabricating filters of varying threshold energies on RadFET arrays (Figure 2). With consideration of threshold barriers, RadFETs are universal ionizing radiation detectors. The sensitivity of RadFETs increases with application of increasing bias voltage. However Sandia has fabricated designs that are moderately sensitive with no voltage source.
Figure 2. The 1 mm^2 RadFET element fits on a standard TO-18 package header. Over 5000 RadFETs can be microfabricated on a single 4 inch wafer.
Cadmium zinc telluride (CZT) detectors
CZTs are semiconductor gamma and neutron radiation detectors, producing current flow under the influence of a gate voltage, upon exposure to high energy radiation. They can be fabricated in arrays to perform imaging or spectroscopy [18] (see Figure 3). While these are promising and sensitive sensors, their performance, and thus calibration, degrades with cumulative exposure. Long term performance is hard to track, as damage may be progressive with radiation energy levels [19]. Sandia performed experiments to improve the fabrication process for industry (see also Figure 3). Commercial sensors and spectrometers are available from EV Products or AmpTek.
Geiger counters were one of the first radiation detectors available, and the first to provide quantitative measurements of radiation. They use very simple electronics and cover a wide radiation range, but they are bulky compared to some of the sensors described above. Commercial options: Radiation Experiments and Monitors (REM) makes a commercial radiation FET sensor with a sensitivity of –10 mV/rad when biased to +20V. TLDs can be purchased from Teledyne Isotopes. CZT detectors can be purchased from Mitsubishi Electric and Communication Electronics, Inc. (Ann Arbor, MI). Geiger counters can be purchased from Mineralab (Prescott, AZ).
Volatile organic compound sensors
-Evanescent fiber-optic chemical sensor
An evanescent wave is the energy that penetrates a dielectric interface when electromagnetic radiation undergoes total internal reflection. This wave can interact with matter within the penetration depth. By using specialized coatings as the fiber-optic cladding, chemical species can be preferentially concentrated from a matrix into the evanescent interaction zone. Polymer optical wave guides have been used for sensing organic compounds in aqueous solutions at low ppm levels [ 24]. pH measurements can be made using sol-gel coatings. For sensing applications, near infrared (NIR) spectroscopy is used for quantitative measurements. With excellent light transmission in this region, sensing can be performed over great distances. However, the spectroscopic signal from mixtures must be deconvolved using multivariate analysis.
-Grating light reflection spectroelectrochemistry
Grating light reflection spectroscopy (GLRS) is a technique for spectroscopic analysis and sensing. A transmission diffraction grating is placed in contact with a liquid sample to be analyzed, and an incident light beam is directed onto the grating. At certain angles of incidence, some of the diffracted orders are transformed from traveling waves to evanescent waves. This occurs at a specific wavelength that is a function of the grating period and the complex index of refraction of the sample. The intensity of a diffracted order is also dependent upon the sample’s complex index of refraction. The real part of the theoretical equations correspond to the speed of light in the material, and the imaginary part corresponds to light absorption. This technique was used at Sandia in combination with electrochemical modulation of a gold-coated metallic spectroscopic grating for the detection of trace amounts of aromatic hydrocarbons [25]. The grating was configured as the working electrode in an electrochemical cell containing water plus trace amounts of TNT and a dye. Cyclic electrochemical modulation produced lower limits of detection, 50 parts per million and 50 parts per billion, respectively.
-Miniature chemical flow probe sensor
This down-hole probe is designed to measure organic analytes diffusing through a semi-permeable membrane [15]. The analytes react with a reagent, forming spectrally distinct products. Absorption bands from a flash lamp are then observed with a spectrometer system, using fiber-optics to carry the light in both directions. Target analytes can be volatile organic compounds in air or water (particularly chlorinated halocarbons), or dissolved metals (copper gives particularly strong response).
-SAW chemical sensor arrays
An acoustic sensor is typically used by measuring a decrease in its active resonant frequency that is related to trace mass loading on the active surface (Figure 4). Polymers, sol-gels, and high surface area coatings are often applied to enhance mass absorption/adsorption, and to provide a degree of chemical class selectivity. Acoustic sensors used at Sandia include flexural plate wave (FPW) sensors, quartz crystal microbalances (QCM), and surface acoustic wave (SAW) sensors. By placing coatings of various chemical properties on a 6-SAW array, chemical speciation and quantification of vapors have been performed [26]. In one test the responses of these materials to each of 14 different analytes, representing the classes of saturated alkane, aromatic hydrocarbon, chlorinated hydrocarbon, alcohol, ketone, organophosphonate, and water, was evaluated. The results revealed a qualitative "chemical orthogonality" of the films useful for pattern recognition analysis. SAWs are the most sensitive of the above-mentioned acoustic sensors, and a number of technological advances have been made to facilitate their use in other chemical systems. Perhaps the most important of these advances is an ASIC (application specific integrated circuit) that converts DC power to the required high frequency impulse, and a reverse conversion for monitoring the frequency shift as a proportional DC shift [27].
1 mm1 mm
Figure 4. Four SAW sensor elements aligned vertically on an application specific integrated circuit. One delay line is left uncoated to compare frequency shifts of the other polymer or sol-gel coated lines.
-MicroChemLab (gas phase)
The gas phase MicroChemLab is a miniature gas chromatography (GC) system originally designed for chemical warfare agent detection for national security needs. Due to the high versatility of GC it has widespread utility. The MicroChemLab can likewise be configured for a variety of applications,
-Hyperspectral imaging
Multiple infrared images of the same location (microscopic or macroscopic) are obtained using different filters. Thus, a color spectrum of each pixel is obtained. These multidimensional images can be processed for quantitative species mapping [30]. This is a stand-off method and could be used from a UAV or satellite for surface soil monitoring. These methods have also been used for biological and biomedical applications [31].
-Chemiresistor array
The chemiresistor sensor is a chemically sensitive resistor comprised of a conductive polymer film deposited on a micro-fabricated circuit [32]. The chemically-sensitive insulating polymer is dissolved in a solvent and mixed with conductive carbon particles. The resulting ink is then deposited and dried onto thin-film, parallel, non-intersecting platinum traces on a solid substrate (chip). When chemical vapors come into contact with the polymers, the chemicals absorb into the polymers, causing them to swell. The swelling changes the physical conformation of the conductive particles in the polymer film, thereby changing the electrical resistance across the platinum-trace electrodes, which can be measured and recorded using a data logger or an ohmmeter. The swelling is reversible if the chemical vapors are removed, but some hysteresis can occur at high concentration exposures. The amount of swelling corresponds to the concentration of the chemical vapor in contact with the chemiresistor, so these devices can be calibrated by exposing the chemiresistors to known concentrations of target analytes. The architecture of the microsensor integrates an array of chemiresistors with a temperature sensor and heating elements [33]. The chemiresistor array has been shown to detect a variety of VOCs including aromatic hydrocarbons (e.g., benzene), chlorinated solvents (e.g., trichloroethylene (TCE), carbon tetrachloride), aliphatic hydrocarbons (e.g., hexane, iso-octane), alcohols, and ketones (e.g., acetone). The on-board temperature sensor comprised of a thin-film platinum trace can be used to not only monitor the in-situ temperature, but it can also be used in a temperature control system. A feedback control system between the temperature sensor and on-board heating elements can allow the chemiresistors to be maintained at a fairly constant temperature, which can aid in the processing of data when comparing the responses to calibrated training sets. In addition, the chemiresistors can be maintained at a temperature above the ambient to prevent condensation of water, which may be detrimental to the wires and surfaces of the chemiresistor. A robust package has been designed and fabricated to house the chemiresistor array [34]. This cylindrical package is small (~ 3 cm diameter) and is constructed of rugged, chemically-resistant material. Early designs have used PEEK (PolyEtherEtherKetone), a semi-crystalline, thermoplastic with excellent resistance to chemicals and fatigue. Newer package designs have been fabricated from stainless steel (Figure 5). The package design is modular and can be easily taken apart (unscrewed like a flashlight) to replace the chemiresistor sensor if desired. Fitted with Viton O-rings, the package is completely waterproof, but gas is allowed to diffuse through a GORE-TEX®^ membrane that covers a small window to the sensor. Like clothing made of GORE-TEX®, the membrane prevents liquid water from passing through it, but the membrane “breathes,” allowing vapors to diffuse through. Even in water, dissolved VOCs can partition across the membrane into the gas-phase headspace next to the chemiresistors to allow detection of aqueous-phase contaminants. The aqueous concentrations can be
determined from the measured gas-phase concentrations using Henry’s Law. Mechanical protection is also provided via a perforated metal plate that covers the chemiresistors. The chemiresistors are situated on a 16-pin dual-in-line package that is connected to a weatherproof cable, which can be of any length because of the DC-resistance measurement. The cable can be connected to a hand-held multimeter for manual single-channel readings, or it can be connected to a multi-channel data logger for long-term, remote operation.
Figure 5. Stainless-steel waterproof package that houses the chemiresistor array. Left: GORE-TEX® membrane covers a small window over the chemiresistors. Right: Disassembled package exposing the 16-pin dual-in-line package and chemiresistor chip.
Biological sensors
-Fatty acid methyl esters (FAME) analyzer
This method uses microhotplates and micro-chromatography columns (μGC) from the MicroChemLab to analyze whole biological cells [35]. A liquid sample is placed on the hotplate along with a methylating agent. When the hotplate is thermally ramped (to 500°C in tens of milliseconds) the
