WO2001025780A1

WO2001025780A1 - Chemical sensor and coating for same

Info

Publication number: WO2001025780A1
Application number: PCT/US2000/026092
Authority: WO
Inventors: Guojun Liu; Ruth Shinar; Marc D. Porter
Original assignee: Iowa State University Research Foundation, Inc.
Priority date: 1999-10-06
Filing date: 2000-09-21
Publication date: 2001-04-12
Also published as: AU7607500A

Abstract

An acoustic wave-based chemical sensor utilizing a crystal substrate and a coating of polymer beads is disclosed. The polymer beads preferably include a high glass transition temperature polymer. Transducers are connected to the crystal substrate to generate an alternating potential across the crystal substrate, which in turn causes the crystal to resonate due to the converse piezoelectric effect. The coating absorbs the analyte e.g. a volatile organic compound, thus changing the mass of the chemical sensor, and accordingly changing its resonant frequency. The transducers detect the change in resonant frequency to indicate that the analyte is present. The polymer bead coating has a large surface area which facilitates mass uptake of large amounts of VOCs, improved acoustic properties even with relatively thick coatings, and a wide operational temperature range.

Description

CHEMTCAI, SENSOR AND COATING FOR SAME

FIELD OF THE INVENTION The present invention generally relates to systems for

monitoring environmental contaminants and, more particularly, to systems

for monitoring fugitive emissions from process equipment.

BACKGROUND OF THE INVENTION

Industrial plants which handle volatile organic compounds

(VOCs) typically experience unwanted emissions of such compounds into the

atmosphere from point sources, such as smoke stacks, and non-point

sources, such as valves, pumps, and fittings installed in pipes and vessels

containing the VOCs. Such VOCs include, but are not limited to, aromatics

(e.g. , benzene, toluene, ethylbenzene, and xylenes), halogenated

hydrocarbons (e.g. , carbon tetrachloride, 1 ,1,1-trichloroethane, and

trichloroethylene), ketones (e.g. , acetone, and methyl ethyl ketone), alcohols

(e.g. , methanol, ethanol, and propanol), ethers (e.g. , dimethyl ether and

methyl t-butyl ether), and aliphatic hydrocarbons (e.g., natural gas and

gasoline).

Emissions from non-point sources, referred to as fugitive

emissions, typically occur due to the leakage of the VOCs from joints and seals. Fugitive emissions from control valves can occur as the result of leakage through the packing between the valve stem and the body or bonnet of the valve. Valves employed in demanding service conditions involving frequent movement of the valve stem and large temperature fluctuations typically suffer accelerated deterioration of the valve stem packing, which

results in greater fugitive emissions than valves employed in less demanding

service.

While improvements in valve stem packing materials and designs have reduced fugitive emissions and lengthened the life of valve packing, the monitoring of fugitive emissions has become important as a

means to identify and reduce fugitive emissions, and to comply with the more stringent regulation of emissions. For example, the Environmental

Protection Agency (EPA) has promulgated regulations for specifying the

maximum permitted emission of certain hazardous air pollutants from

control valves, and requires periodic surveys of emissions from control

valves.

Current methods of monitoring fugitive emissions involve manual procedures using a portable organic vapor analyzer. This manual

method is time consuming and expensive to perform, and also can yield

inaccurate results due to ineffective collection of the fugitive emissions from

the equipment being monitored. If measurements are made on a valve

exposed to wind, emissions from the valve may be dissipated before the analyzer can properly measure the concentration of the emissions. Also, if the

analyzer is not carefully moved around the valve to capture all the emissions from the valve, an inaccurate measurement will result. Manual measurement methods also require plant personnel to dedicate a significant amount of time to making the measurements, thereby distracting plant personnel from other

duties.

Automated monitoring and detection of fugitive emissions can yield significant advantages over existing manual methods. The EPA regulations require surveys of fugitive emissions at periodic intervals. The

length of the survey interval may be monthly, quarterly, semi-annually, or

annually, with the required surveys becoming less frequent if the facility operator can document a sufficiently low percentage of control valves

exhibiting excessive leakage. Thus, achieving a low percentage of leaking

valves reduces the number of surveys required per year. In a large industrial facility, where the total number of survey points can range from 50,000 to

200,000, a reduced number of surveys can result in large cost savings. By

installing automated fugitive emission-sensing systems on valves subject to the

most demanding service conditions, and thus, most likely to develop leaks,

compliance with the EPA regulations can be more readily achieved for the

entire facility.

However, employing chemical sensors in an industrial

environment requires designing sensors that perform satisfactorily in the presence of high relative humidity over a broad temperature range. The sensors must be able to discriminate between the emissions of interest and other environmental contaminants, while retaining sufficient sensitivity to detect low concentrations of the fugitive emissions. A provision also must be made to enable periodic calibration of the sensors. The output signals from the fugitive emission sensing system must be suitable for input into plant

monitoring and control systems typically found in process plants. This permits simple and inexpensive integration of the sensing system into existing plant

process control systems.

The fugitive emission sensing system must be inexpensive to

manufacture, and use a power source that is readily available in a typical process plant in order to keep installation costs to a minimum. The system must be suitable for use in hazardous areas subject to risk of explosion- requiring electrical equipment to be intrinsically safe or of an explosion-proof design. It also must be able to operate in harsh environments, including areas

subject to spray washing, high humidity, high and low temperatures, and

vibration. The system also must be simple and reliable, in order to minimize

maintenance costs.

In certain applications, the sensors used to detect fugitive

emissions are provided in the form of piezoelectric-based sensors having high

sensitivities to surface mass changes, such that when an alternating potential is applied across the sensors, changes in resulting wave characteristics in the sensors, specifically the resonant frequency, indicate the presence of the analyte. More specifically, the sensors typically include a quartz crystal substrate with an outer layer made of material selected to most effectively absorb the analyte. Such outer coatings are selected to increase sensitivity, while reducing acoustic wave damping effects. In addition, such materials

should be environmentally robust to accommodate the aforementioned wide

temperature ranges, humidity ranges, and high levels of dust particles and other contaminants.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a

chemical sensor is provided which includes a substrate, at least two electrodes

connected to the substrate, and a coating positioned over the substrate and at

least one of the electrodes, with the coating comprising polymer beads.

In accordance with another aspect of the present invention, the

polymer beads can be (a) unmodified polymer beads such as polystyrene,

poly(2, 6-dimethyl-p-phenylene oxide), polystyrene cross-linked with

polyvinylbenzene, polystyrene butadiene, polystyrene/vinyltoluene, poly(methyl

methacrylate), poly(vinyltoluene), poly(bromostyrene),

poly(vinylbenzylchloride), poly(vinylnaphthalene), poly(butyl methacrylate), and poly(acrylic acid); (b) modified polymer beads, such as carboxyl modified polystyrene, hydroxyl modified polystyrene, amino modified polystyrene, protein modified polymer beads, enzyme modified polymer beads, dye-immobilized polymer beads, and magnetic polymer beads; and (c) mixtures of unmodified and modified beads.

In accordance with other aspects of the present invention, the

polymer beads have a diameter of about 0.01 to about 5 microns. In preferred embodiments, the polymer beads have a glass transition temperature of about 25 °C to about 200°C. A sensor of the present invention can detect VOCs in an amount as low as 10 ppb, and operate in a temperature range of about -10°C to about 85 °C. The resulting coating has a thickness ranging from

about 0.5 microns to about 12 microns.

In accordance with yet another aspect of the present invention, a

coating is provided for an acoustic wave-based chemical sensor, wherein the coating comprises polymer beads.

These and other aspects and features of the present invention

will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRTEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a block diagram of a fugitive emissions sensing system

employing the present invention; Fig. 2 is a schematic diagram of a chemical sensor circuit including a chemical sensor embodying the present invention;

Fig. 3 is a graph showing a typical adsorption isotherm of toluene vapor on a polystyrene bead coating at 25 °C; Fig. 4 is a graph plotting the frequency shifts of a sensor built in accordance with the present invention and a sensor built using a

poly(diphenoxy phosphazene) coating vs. time upon exposure to toluene

vapor; and

Fig. 5 is a schematic diagram of an alternative embodiment of a

chemical sensor according to the present invention.

While the invention is susceptible of various modifications and

alternative constructions, certain illustrative embodiments thereof have been shown in the drawings and will be described below in detail. It should be

understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the intention is to cover all

modifications, alternative constructions, and equivalents, falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, and with specific reference to

Fig. 1, a fugitive emissions sensing system utilizing the present invention is generally depicted by reference numeral 20. However, it is to be understood that the present invention is primarily directed to a chemical sensor 22 which

can be employed in a variety of applications, including applications separate from the fugitive emissions sensing system 20.

By way of overview, Fig. 1 is a block diagram of an illustrative fugitive emissions sensing system 20 employing the chemical sensor 22. An emission source 24 is shown, from which a sample stream 26 is drawn into sample retrieval system 28. The sample retrieval system 28 includes an

accumulator 30, a sensor chamber 32, and an ejector 34. A chemical sensor

array 36 is located within the sensor chamber 32. The sample stream 26 is

drawn from the accumulator 30 into the sensor chamber 32, exposing the

chemical sensor array 36 to the sample stream 26. The chemical sensor array 36 contains one or more chemical sensors 22 (Fig. 2). The sample stream 26

then passes into the ejector 34. A compressed air source 40 provides compressed air 42 to the ejector 34, creating a pressure drop within the ejector

34 which draws a sample stream 26 through the sensor chamber 32 and into the

ejector 34. The compressed air 42 and sample stream 26 are mixed within the

ejector 34 and exhausted to atmosphere as a mixture 44.

The chemical sensor array 36 is connected to a sensor interface

circuit 50, which processes the signals from the chemical sensor array 36 and provides process signals to a microcontroller 52. The microcontroller 52 stores

the data from the sensors 22 in a memory 54, and uses the sensor data received

from the fugitive emissions sensing system 20 to initiate control actions to reduce or eliminate the emissions For example, the microcontroller 52 could close a valve upstream from the emissions source 24 to stop the flow of fluid through the emissions source 24 in order to stop emissions caused by the leakage of the fluid. Alternatively, the microcontroller 52 could alter operating conditions of the emissions source 24 itself to reduce or eliminate the fugitive

emissions The microcontroller 52 may use a communication interface circuit

56 to provide control signals to the upstream valve, the emission source 24, or

any other equivalent that may be used to reduce or eliminate the emissions It can therefore be seen that the fugitive emissions sensing

system 20 may be used to detect the presence of, or measure the concentration of, various types of fluids emitted from the emissions source 24. The system

may be used to detect hazardous, toxic or polluting substances emitted from

the source, or to detect leakage of non-hazardous substances, the loss of which

may be a cause of concern The fugitive emission sensing system 20 may be

used to detect emissions from any kind of source, particularly industrial process

equipment from which hazardous substances may leak. Examples include

control valves, block valves, pumps installed on lines carrying hazardous gases,

agitators, screw conveyors, or other equipment installed on process vessels containing hazardous fluids, heat exchanges, reactors, etc When emissions are detected by the fugitive emissions sensing system 20, this data may be used by

the fugitive emissions sensing system 20 to control the process in such a way as

to reduce or eliminate the emissions As indicated above, the chemical sensor array 36 may include one or more chemical sensors 22 responsive to a particular analyte or fugitive emission being monitored. In the embodiment depicted in Fig. 2, the chemical sensor 22 is a quartz crystal microbalance (QCM) chemical sensor, but can be

another type of piezoelectric acoustic wave devices, including surface acoustic wave (SAW) devices, acoustic plate mode (APM) devices, and flexural plate

wave (FPW) devices. The sensor could also be in the form of a fiber optic sensor or electrochemical sensor.

As shown in Fig. 2, the chemical sensor 22 may be connected to

an oscillator circuit 62 for monitoring gas emissions. In an alternative embodiment, the chemical sensor 22 could be connected to a network analyzer.

More specifically, the oscillator circuit 62 may include NAND gates 64 and 66,

and an AND gate 68, connected in series. A resistor 70 may be connected

between the output of the NAND gate 66 and the circuit power supply voltage

72, and a resistor 74 may be connected between the output of NAND gate 66

and circuit power supply voltage 72 A resistor 75 may be connected across the NAND gate 64, connecting a first input to the output. A select signal 76

may be connected to the second input of the NAND gate 64, and the same

select signals may also be connected to an input of the AND gate 68. An

enable signal 78 may be connected to an input of the NAND gate 66. When

the select signal 76 and the enable signal 78 are both high, the NAND gates 64 and 66 act as high-gain inverting amplifiers and cause an oscillator 80 to oscillate between high and low voltage, producing an oscillating square wave output. The oscillating voltage from the oscillator output 80 may be transferred through the AND gate 68 and applied across the chemical sensor 22

causing the chemical sensor 22 to physically resonate. In order to appreciate the significance of this resonance, it is first important to understand that the chemical sensor 22 utilizes the converse piezoelectric effect. By way of background, the piezoelectric effect holds that a mechanical stress applied to the surfaces of various crystals, including quartz, affords a corresponding electrical potential across the crystal having a

magnitude proportional to the applied stress. The electrical charge generated

in the quartz crystal under stress is due to the shift of dipoles resulting from the

displacement of atoms in the crystalline material. The converse piezoelectric

effect holds that application of a voltage across certain crystals, including

quartz crystals, results in a corresponding mechanical strain in the crystal. In quartz, this strain or deformation is elastic. It follows that an alternating potential across the crystal causes a vibrational motion in the quartz crystal,

i.e., the aforementioned resonance. The chemical sensor 22 therefore includes

a crystal substrate 82 which interacts with the oscillating circuit 62, and in turn

causes the oscillator circuit 62 to oscillate at the resonant frequency of the

chemical sensor 22. Thus, the frequency of the oscillator output 80 will vary as

the resonant frequency of the chemical sensor 22 varies. The resonant frequency of the chemical sensor 22 can vary based on a number of parameters, including the mass, size, shape, and cut of the quartz crystal substrate 82 Quartz crystal exhibits a natural resonant frequency that is a function of the mass and structure of the crystal The precise size, type of cut, and thickness of the quartz crystal substrate 82 are selected to result in a particular resonant frequency For example, an AT-cut crystal with a nominal resonant frequency of 8-30 megahertz is suitable for chemical sensor applications Suitable quartz crystal substrates may be obtained from Standard Crystal Corporation of California Other types of

suitable materials to serve as the substrate include lithium niobate (LiNbO₃), which is particularly suited for a surface acoustic wave (SAW) based-sensor, and aluminum nitride (A1N), which is particularly suited for a thin film resonator based-sensor

In order to apply the alternating potential across the substrate

82, first and second electrodes 84 and 86 are connected to the crystal substrate

82 and may be constructed of chromium/gold, although other suitable corrosion-resistant and acoustically compatible conductors may be used,

possibly including aluminum, palladium, chromium, gold-on-chromium, and

graphite The electrodes 84 and 86 may serve as both the conductors for

generating the alternating current across the crystal substrate 82, and as

transducers for sensing parameters related to changes in resonant frequencies resulting in the crystal substrate 82 As indicated above, the resonant frequency of the chemical sensor 22 is a function of the total mass of the device. Therefore, the mass of any coating provided around the crystal substrate 82 also affects the total mass of the device, and thereby affects the resonant frequency of the chemical sensor 22. The coatings provided about the crystal substrate 82 are selected to absorb

molecules of the analyte. When analyte molecules are absorbed by the coating, the mass of the coating is slightly increased, which in turn increases the mass of

the entire sensor 22, and thus changes the resonant frequency of the chemical

sensor 22. The resonant frequency of the chemical sensor 22 is also a function

of the viscoelastic properties of the coatings and mechanical stresses caused by temperature effects in the sensor mounting structure. However, these effects are either negligible or can be compensated for. Thus, a very sensitive gas detector may be constructed by selecting a coating that has a chemical affinity

with the particular analyte of interest. The quantity of molecules absorbed and

deposited, and the resulting change in the operating frequency of the oscillator

circuit 62, is a function of the concentration of the chemical being measured in

the environment surrounding the chemical sensor 22. The frequency changes linearly with changes in chemical concentration, within certain limits.

Thus, a change in the concentration of the analyte can be

measured by measuring the change in the frequency of the oscillator output 80.

The chemical sensor 22 can be calibrated by exposing the sensor 22 to known

concentrations of the analyte and recording the resulting frequency of the oscillator output 80. The chemical sensor 22 then can be used to measure the absolute concentration of the analyte by comparing the measured frequency to the aforementioned recorded values.

The particular coating chosen for the crystal substrate 82 should preferably readily absorb the molecules of the analyte, to provide a high degree

of sensitivity to the analyte, but do so without damping the generated waves. The coating 88 also should be usable over a wide temperature range, and provide fast response and recovery times. The present invention provides such a coating in the form of a polymer bead coating 88.

Low glass transition temperature polymers and some materials

with low melting points are attractive as coating materials for such sensors due

to their selective, rapid, and reversible responses to volatile organic

compounds. However, low glass transition temperature polymers have low shear modulus, and therefore exhibit a relatively large damping effect on

acoustic waves. The damping or attenuation of the acoustic waves increases as

coating thickness increases, or as ambient temperature increases. This combination of features dictates that coatings of low glass transition

temperature polymers be of a limited thickness and be exposed to a limited

temperature range. However, because the chemical detection sensitivity of

acoustic wave-based sensors is generally proportional to coating thickness,

coatings of low glass transition temperature polymers, which are necessarily

thin, are accordingly limited in their detection sensitivity. The performance of low melting point materials as coatings generally resembles that of low glass transition temperature polymers. Additionally, the operational temperature range of low melting point materials used as coatings is necessarily limited by

the low melting point. High glass transition temperature polymers exhibit characteristics generally opposite to those of low glass transition temperature polymers when used as chemical sensing coatings. More specifically, since high glass transition temperature polymers have relatively large shear moduli, they exhibit less damping or attenuating effects on acoustic waves over a wide

temperature range. Consequently, thicker coatings of high glass transition temperature polymers can be used which in turn increases sensitivity by

allowing mass uptake of the analyte. However, high glass transition

temperature polymers exhibit extremely slow and hysteresis responses unless used as very thin coatings.

The polymer bead coating 88 of the present invention preferably

utilizes a high glass transition temperature polymer to capitalize on the aforementioned acoustic wave benefits of high glass transition temperature polymers. Polymer bead coating 88 comprises small polymer beads, which

increases coating surface area, thus enabling faster responses primarily through

surface absoφtion. Moreover, the small polymer beads, and resulting high

surface area of the coating 88, enable mass uptake of large amounts of VOCs. The polymer beads also provide detection sensitivity over a wide range of temperatures, e.g., about -10°C to about 85°C.

Many materials can be effectively employed as the polymer bead material including unmodified polymer beads and microspheres, such as, but not limited to, polystyrene (PS), poly(2, 6-dimethyl-p-phenylene oxide),

polystyrene cross-linked with polyvinylbenzene (PS/DVB), polystyrene/butadiene (PS/B), polystyrene/vinyltoluene (PS/NT), poly(methyl

methacrylate) (PMMA), poly(vinyltoluene) (PNT), poly(bromostyrene),

poly(vinylbenzylchloride), poly(vinylnaphthalene), poly butyl methacrylate),

and poly(acrylic acid). Polystyrene (PS) and poly(phenylene oxide) are preferred materials of construction of the polymer beads. Surface modified polymer beads and microspheres wherein the surfaces are modified to tailor the affinity of the analyte to the sensor also can be employed. Such modified polymer beads and microspheres include, but are not limited to, carboxy

modified polystyrene (COOH), hydroxy modified polystyrene (OH), and amino

modified polystryrene (ΝH₂), protein and enzyme modified beads and

microspheres for biosensors, dye-immobilized polymer beads and microspheres

for fiber optical sensors, and magnetic beads and microspheres for magnetic

sensors. The polymer beads have a diameter of about 0.01 to about 5

microns, with a diameter of about 0.02 to about 1 micron being preferred, and

a diameter of about 0.02 to about 0.1 microns being most preferred. This small bead size increases detection sensitivity with regard to VOCs because as the size of the polymer bead decreases, the number of beads in a given volume increases, and thus the surface area of the coating 88 increases. A large surface area allows the uptake of larger amounts of VOC vapor. For example, the coating 88 has a detection limit as low as about 10 ppb for toluene vapor at

25 °C, and an upper detection limit at the saturated vapor pressure for toluene

The polymer beads preferably have a glass transition temperature of about 25 °C to about 200 °C. The coating 88 preferably has a thickness of about 0.5

to about 12 microns. In manufacturing a sensor 22 in accordance with the present

invention, sufficient adhesion of the beads to the substrate 82 and electrodes

84, 86 is important. Conventional methods for applying coatings to sensors, such as dipping, spraying, and spin coating, are not particularly well suited to application of beads. Rather, the sensor 22 is preferably manufactured by

soaking an absorbent fabric, such as a cotton swab, in an aqueous suspension

of beads, typically at a nonlimiting concentration range of about 2 to about

10%, by weight. The beads preferably are applied to the substrate 82 and

electrodes 84 and 86 by rubbing the wet cotton swab against the substrate 82

and the electrodes 84 and 86 In this way, the beads are strongly bonded to the

substrate 82 and electrodes 84, 86, forming uniform adherent coating 88 The

uniformity of the application process was verified using various microscopic techniques including scanning electron microscopy and atomic force microscopy. The substrate 82 and electrodes 84, 86 can be manufactured from any of the aforementioned types and materials, with a 9-10 MHZ quartz crystal substrate and gold electrodes being preferred.

The resulting coating exhibits a detection capability for toluene

in the sub ppm level, i.e., 10 ppb, at ambient temperature. Sensor response time, i.e., the time needed for the response to attain 90% of its saturation value, is about one to about nine minutes, and is preferably less than eight minutes. The operational temperature range for the sensor is about -10°C to about 85 °C. Figure 4 shows the response curves of a sensor using a polystyrene bead (PSB) coating according to the present invention, and a sensor using a

poly(diphenoxy phosphazene) (PDPP) coating. More specifically, the PSB

coating has a thickness of 8.3 microns, while the PDPP coating has a thickness

of 2.5 microns. The graph shows exposure to toluene vapor at concentration

levels of 100-1000 ppm in the presence of 40% relative humidity at 40 °C. The

data indicate that sensitivity of the PSB coating to toluene is six times larger than that of the commonly used PDPP coating. The response time of the PSB coating is longer than that of PDPP, but is orders of magnitude shorter than that of bulk polystyrene coatings with a thickness of less than 1 micron.

Fig. 3 shows a typical adsorption isotherm of toluene vapor on

the PSB coatings at 25 °C. The linear correlation between the frequency

response and the partial pressure of toluene vapor in log-log scales suggests

that the adsorption follows the Fruedlich adsoφtion isotherm. The low detection limit for a signal to noise ratio of 5 is estimated to be about 10 ppb. The data also indicate that PSB coatings have a tendency to selectively absorb

aromatic VOCs, and are less sensitive to polar and hydrogen-bonding vapors, including ketones and alcohols. It has been found that humidity has a minimal effect on the detection of aromatic VOCs.

The sensor 22 was tested using a network analyzer. The network analyzer outputs a sine wave signal in a preprogrammed frequency

range, typically from 8.5 to 12.5 MHZ, and enables the sensor 22 to oscillate at its fundamental resonant frequency. From the signal reflected from the sensor under test, its resonant frequency, series resistance, inductance and capacitance

are extracted using the equivalent circuit analysis.

In an alternative embodiment shown in Fig. 5, a second coating

100 can be positioned over the coating 88 of polymer beads. The second

coating 100 is chosen to enhance responsiveness characteristics and detection sensitivity. For example, a second coating that is hydrophobic can reduce

interference of water vapor. A second coating that is hydrophilic or contains

tailored functional groups, such as amines, carboxylates, or OH groups, can

enhance selectivity and specificity in analyte detection. The second coating 100 may be provided at a range of thicknesses, with a thickness of about 0.2 microns to about 1.5 microns being preferred.

From the foregoing, it can therefore be seen that the present

invention provides an improved acoustic-wave based sensor having a coating of polymer beads, preferably of a high glass transition temperature polymer. Such a coating exhibits excellent acoustic properties which tend not to damp acoustic waves even at relatively large thicknesses and elevated temperatures, and exhibits excellent responsiveness in terms of volumetric uptake capacity, response time, and reversibility.

Claims

What Ts Claimed Is:

1. A chemical sensor, comprising:

a substrate;

at least two electrodes connected to the substrate; and a coating positioned over the substrate and at least one of the

electrodes, the coating comprising polymer beads.

2. The chemical sensor of claim 1 wherein the polymer

beads are unmodified polymer beads.

3. The chemical sensor of claim 2 wherein the unmodified

polymer beads are selected from the group consisting of polystyrene, poly(2, 6-

dimethyl-p-phenylene oxide), polystyrene cross-linked with poly(vinylbenzene),

polystyrene/butadiene, polystyrene/vinyltoluene, poly(methyl methacrylate), poly(vinyltoluene), poly(bromostyrene), poly(vinylbenzylchloride),

poly(vinylnaphthalene), poly(butyl methacrylate), poly (acrylic acid), and

mixtures thereof.

4. The chemical sensor of claim 1 wherein the polymer beads are modified polymer beads.

5. The chemical sensor of claim 4 wherein the modified polymer beads are selected from the group consisting of carboxy modified polystyrene, hydroxy modified polystyrene, amino modified polystyrene , protein modified polymer beads, enzyme modified polymer beads, dye- immobilized polymer beads, magnetic polymer beads, and mixtures thereof.

6. The chemical sensor of claim 1 wherein the polymer

beads comprise unmodified polymer beads, modified polymer beads, or a mixture thereof.

7. The chemical sensor of claim 1 wherein the polymer

beads have an average diameter of about 0.01 to about 5 microns.

8. The chemical sensor of claim 1 wherein the coating has a

thickness of about 0.5 to about 12 microns.

9. The chemical sensor of claim 1 wherein the substrate is selected from the group consisting of quartz crystal, lithium niobate, and

aluminum nitrite.

10. The chemical sensor of claim 1 wherein the polymer

beads have a glass transition temperature of about 25 °C to about 200 °C.

11. The chemical sensor of claim 1 wherein the chemical

sensor is functional at a temperature of about -10°C to about 85 °C.

12. The chemical sensor of claim 1 having a response time in the range of about one minute to about nine minutes.

13. The chemical sensor of claim 1 further including a second coating positioned over the coating of polymer beads.

14. A coating for an acoustic wave based-chemical sensor,

the coating comprising of polymer beads.

15. The coating of claim 14 wherein the polymer beads are

unmodified polymer beads.

16. The coating of claim 15 wherein the unmodified polymer

beads are selected from the group consisting of polystyrene, poly(2, 6-

dimethyl-p-phenylene oxide), polystyrene cross-linked with poly(vinylbenzene),

poly(vinylnaphthalene), poly(butyl methacrylate), poly(acrylic acid), and

mixtures thereof.

17. The coating of claim 14 wherein the polymer beads are

modified polymer beads.

18. The coating of claim 17 wherein the modified polymer

beads are selected from the group consisting of carboxy modified polystyrene,

hydroxy modified polystyrene, amino modified polystyrene , protein modified

polymer beads, enzyme modified polymer beads, dye-immobilized polymer beads, magnetic polymer beads, and mixtures thereof.

19 The coating of claim 14 wherein the polymer beads comprise unmodified polymer beads, modified polymer beads, or a mixture

thereof

20 The coating of claim 14 wherein the polymer beads have an average diameter of about 0 01 to about 5 microns

21 The coating of claim 14 wherein the coating has a thickness of about 0 5 to about 12 microns

22 The coating of claim 14 wherein the polymer beads have a glass transition temperature about 25 °C to about 200 ^CC

23 The coating of claim 14 wherein the coating is functional

at a temperature of -10° C to about 85 °C

24 The coating of claim 14 having a response time of about one to about nine minutes

25 The coating of claim 14 further including a second

coating positioned over the coating of polymer beads

26. A method of detecting volatile organic compounds using a sensor comprising: a substrate; at least two electrodes connected to the substrate; and a coating positioned over the substrate and at least one of the

electrodes, the coating comprising polymer beads.

27. The method of claim 26 wherein the polymer beads are

unmodified polymer beads.

28. The method of claim 27 wherein the unmodified polymer

beads are selected from the group consisting of polystyrene, poly(2, 6-

dimethyl-p-phenylene oxide), polystyrene cross-linked with poly(vinylbenzene), polystyrene butadiene, polystyrene/vinyltoluene, poly(methyl methacrylate),

poly(vinyltoluene), poly(bromostyrene), poly(vinylbenzylchloride), poly(vinylnaphthalene), poly(butyl methacrylate), poly (acrylic acid), and

mixtures thereof.

29. The method of claim 26 wherein the polymer beads are

modified polymer beads.

30. The method of claim 29 wherein the modified polymer

beads are selected from the group consisting of carboxy modified polystyrene, hydroxy modified polystyrene, amino modified polystyrene , protein modified polymer beads, enzyme modified polymer beads, dye-immobilized polymer beads, magnetic polymer beads, and mixtures thereof.

31. The method of claim 31 wherein the polymer beads comprise unmodified polymer beads, modified polymer beads, or a mixture

thereof.