- Email: [email protected]
The design of functionalized silicone polymers for chemical sensor detection of nitroaromatic compounds
Sensors and Actuators B 65 Ž2000. 5–9 www.elsevier.nlrlocatersensorb
The design of functionalized silicone polymers for chemical sensor detection of nitroaromatic compounds R. Andrew McGill a
a,)
, Todd E. Mlsna b, Russell Chung b, Viet K. Nguyen b, Jennifer Stepnowski b
NaÕal Research Laboratory, Code 6375, 4555 OÕerlook AÕe. NE, Washington, DC 20375, USA b Geo-Centers, 1801 RockÕille Pike, Suite 210, RockÕille, MD 20852, USA Accepted 12 October 1998
Abstract The solubility properties of a series of nitroaromatic compounds have been determined and utilized with known linear solvation energy relationships to calculate their sorption properties in a series of chemoselective polymers. These measurements and results were used to design a series of novel chemoselective polymers to target polynitroaromatic compounds. The polymers have been evaluated as thin sorbent coatings on surface acoustic wave ŽSAW. devices for their vapor sorption and selectivity properties. The most promising materials tested, include siloxane polymers functionalized with acidic pendant groups that are complimentary in their solubility properties for nitroaromatic compounds. The most sensitive of the new polymers exhibit SAW sensor detection limits for nitrobenzene ŽNB. and 2,4-dinitrotoluene in the low parts per billion Žppb. and low parts per trillion Žppt. concentration range, respectively. Polymers with favorable physicochemical properties exhibit low water vapor sorption, and rapid signal kinetics for NB, reaching 90% of signal response in 4 s. Studies with an in situ infrared spectroscopy technique are used to determine the mechanism of interaction between nitroaromatic compounds and the chemoselective polymer. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Nitroaromatic compound; Chemoselective polymer; Solubility; Saw sensor
1. Introduction The detection of chemical signatures associated with explosives is of great interest in the location of unexploded ordnance such as land-mines w1x. It is well known that dogs are able to locate explosives, including buried land-mines. It is clear that the canine olfactory system is capable of detecting and identifying land-mine chemical signatures. Of particular interest are the polynitroaromatic compounds identified in the chemical signature w2x. This work focuses around the characterization of the solubility properties of nitroaromatic compounds and the design of complementary sorbent polymer coatings. The polymer coatings are to be used to enhance the performance of a variety of chemical sensors being developed to locate buried land-mines from associated chemical signatures. To characterize the sorption properties of new poly)
Corresponding author. Tel.: q1-202-767-0063; fax: q1-202-7675301; e-mail: [email protected]
meric materials, surface acoustic wave ŽSAW. devices coated with the polymers have been exposed to vapors including: nitroaromatic compounds, common organic solvents, and water vapor. When exposed to a vapor, a polymer-coated SAW device sorbs vapor which results in a perturbation of the SAW propagation velocity. Vapor sorption is monitored as a shift in signal frequency. The nature of the interaction between the coating and vapor molecules determines the selectivity, sensitivity, signal kinetics, and the reversibility of the sensor w3x. The sorption of a vapor in a polymer coating can be described by the gas–polymer partition coefficient Ž K p ., which is defined as the ratio between the concentration of the vapor in the gas phase, C v , and the concentration of the sorbed vapor in the polymer phase, Cp , at equilibrium. The K p distribution of a vapor between the gas phase and a polymer-coated sensor is illustrated in Fig. 1. A solvation equation has been developed over the last 2 decades to model physicochemical and biochemical processes w4x. For the gas–polymer partition coefficient, the solvation equa-
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 3 5 1 - 2
R.A. McGill et al.r Sensors and Actuators B 65 (2000) 5–9
6
Table 1 Solubility parameters for UXO-related solutes and comparison compounds Solute vapor
R2
p 2H
b 2H
a 2H
log L16
246TNT 24DNT 26DNT 135TNB 2NT NB Cyclohexanone Toluene n-Octane Ethanol Acetone Nitromethane Water
1.430 1.150 1.150 1.430 0.866 0.871 0.403 0.601 0.000 0.246 0.179 0.313 0.000
2.23 1.60 1.60 2.23 1.11 1.11 0.86 0.52 0.00 0.42 0.70 0.95 0.45
0.61 0.47 0.45 0.61 0.28 0.28 0.56 0.14 0.00 0.48 0.56 0.27 0.35
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.37 0.04 0.12 0.82
7.85 6.26 6.30 – 4.88 4.56 3.79 3.33 3.68 1.49 1.70 1.89 0.26
Fig. 1. The gas–polymer vapor partition coefficient.
tion is given by the linear solvation energy relationship ŽLSER.: log K p s c q r R 2 q s p 2H q a a 2H q b b 2H q 1 log L16
Ž 1.
where log K p is the log of the gas–polymer partition coefficient, and relates to a series of vapors for a polymer. K p is a measure of the strength of the interaction between vapor molecules and polymer molecules, where larger K p values indicate that the vapor molecules are more soluble in the polymer solvent phase. The variables are solute vapor parameters: R 2 , an excess molar refraction that models polarizability contributions from ‘‘n’’ and ‘‘p’’ electrons: p 2H , the dipolarityrpolarizability; a 2H , the hydrogen-bond acidity; b 2H , the hydrogen-bond basicity; and log L16 , where L16 is the gas–liquid partition coefficient for n-hexadecane. The middle three variables are developed from chromatographic measurements and are more correctly denoted as summation parameters referring to solute molecules surrounded by solvent molecules; for simplicity, this notation is used. The coefficients resulting from a regression of log K p values characterize the solubility properties of the polymer. The most important of these are: s, the dipolarity; a, the hydrogen-bond basicity; b, the hydrogen-bond acidity; and
l, which reflects a combination of solvation cavity effects and dispersion interactions. The magnitude of ‘‘l’’ reflects the ability of the polymer to discriminate between a homologous series. The coefficient ‘‘r ’’ reflects the polarizability properties of the polymer, and ‘‘c’’ is a constant resulting from the regression. The size and relative magnitude of all the coefficients are used to identify polymers with high selectivity and sorption characteristics for vapors or gases of interest. A wide range of chemoselective polymers have been characterized with solvation equation w3x. From the known LSER equations, log K p values for more than 2000 vapors and gases can be calculated. Unfortunately, when this work was initiated, the available database of solute parameters did not contain the nitroaromatic compounds of interest. The first goal of this work was therefore to determine these parameters and utilize them with known LSERs to calculate their sorption properties in a series of chemoselective polymers. These results were used to design and synthesize a series of novel chemoselective polymers with complementary solubility properties for polynitroaromatic compounds. These polymers have been evaluated as thin sorbent coatings on SAW devices for their vapor sorption and selectivity properties.
2. Experimental ST-cut quartz, 250 MHz, SAW two-port resonator devices ŽMicrosensor Systems wMSIx, Bowling Green, KY. were used in these experiments. The SAW devices were
Table 2 Chemoselective polymer LSER parameters Polymer
Constant Žc.
Polarizability Žr.
Dipolarityr polarizability Žs.
Hydrogen-bond basicity Ža.
Hydrogen-bond acidity Žb.
Dispersion and cavity Žl.
PIB SXPHB PEM SXCN PVTD PECH PVPR OV202 P4V SXFA FPOL ZDOL
y0.77 y0.85 y1.65 y1.63 y0.59 y0.75 y0.57 y0.39 y1.33 y0.08 y1.21 y0.49
y0.08 0.18 y1.03 0.00 y0.02 0.10 0.67 y0.48 y1.54 y0.42 y0.67 y0.75
0.37 1.29 2.75 2.28 0.74 1.63 0.83 1.30 2.49 0.60 1.45 0.61
0.18 0.56 4.23 3.03 2.44 1.45 2.25 0.44 1.51 0.70 1.49 1.44
0.00 0.44 0.00 0.52 0.22 0.71 1.03 0.71 5.88 4.25 4.09 3.67
1.02 0.89 0.87 0.77 0.92 0.83 0.72 0.81 0.90 0.72 0.81 0.71
R.A. McGill et al.r Sensors and Actuators B 65 (2000) 5–9
7
Fig. 2. Chemoselective materials with known LSER equations.
cleaned and coated with the polymers as described before w6x, to provide a signal frequency shift of 250 kHz, which corresponds to about 50 nm of film thickness. The SAW frequency was monitored with a SAWPRO-250 system ŽMSI.. NB and other vapor tests were carried out with an automated VG400 vapor generator ŽMSI., and 2,4-dinitrotoluene Ž24DNT. vapor exposure tests were performed with vapor generated by passing dry N2 through a column packed with 24DNT-coated sand. FTIR experiments to examine nitroaromatic vapor– polymer interactions were carried out by attenuated total reflectance ŽATR. spectroscopy. ATR elements were coated with the polymer, and exposed to vapor generated as described above. 3. Results and discussion The solute solubility parameters for a wide range of nitroaromatic compounds have been determined and those of direct interest are detailed in Table 1. Nitroaromatic
compounds such as 2,4,6-trinitrotoluene Ž246TNT., 24DNT, and nitrobenzene ŽNB. consist of an aromatic ring with one or more nitro groups attached directly to the ring. The nitro groups are electron-rich sites, and exhibit hydrogen-bond basic Žhbb. properties. As the number of nitro groups is increased, the hbb properties of the nitroaromatic compound increase. The electrons in the aromatic ring of nitroaromatic compounds are delocalised and this allows electron density to be shifted from one nitro group and the aromatic ring to another nitro group, that is interacting with a dipolar or hydrogen-bond acidic Žhba. site of a neighboring polymer molecule. For polynitroaromatic compounds, the oxygen atoms of the nitro groups preferentially lie above and below the plane of the aromatic ring, so that, delocalization and polarizability are not maximized. As a result of the out of plane nitro configuration, the interaction of a nitro group attached to an aromatic ring with a neighboring molecule should be possible in a head-to-head position, or from above or below the plane of the aromatic ring.
Fig. 3. Ža,b. 246TNT gas–polymer sorption properties.
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R.A. McGill et al.r Sensors and Actuators B 65 (2000) 5–9
Fig. 4. 246TNT showing the electron-rich and hbb nitro groups. The aromatic triol provides complementary hba sites.
Fig. 5. Aromatic siloxane polymer with the HFIP functional group.
For 12 chemoselective polymers Žsee Fig. 2. with known LSER equations Žsee Table 2., the solubility parameters for 246TNT and the other nitroaromatic compounds have been used to calculate the gas–polymer partition coefficients. The K p values for 246TNT are summarized in Fig. 3a and b. Fig. 3a and b differ only in that the P4V polymer is deleted in Fig. 3b in order to use a more suitable scale for the remaining polymers. From the 12 polymers used in the LSER studies, the P4V dominates the different materials for the sorption of polynitroaromatics. P4V is a strong hba, and is strongly dipolar, which are complementary solubility properties to the polarrpolarizable and hbb solubility properties of polynitroaromatics. Unfortunately, this polymer is crystalline and does not have ideal physical characteristics for a chemical sensor coating. The dipolarrpolarizable and spatial hbb solubility properties of polynitroaromatics provide characteristic features that should allow the design of chemoselective materials with complementary solubility properties that can strongly sorb these compounds. To illustrate this concept, an aro-
matic triol is shown in Fig. 4, with complementary hba sites for 246TNT that spatially match the hbb nitro groups. In order to build similar features into a polymer with rubbery properties, a series of siloxane polymers with functionalized aromatic and other pendant groups have been synthesized w5x, and their sorption properties characterized against different vapors. In order to maximize the hba properties of the polymers, the aromatic pendant groups were functionalized with one or more hexafluoroisopropanol ŽHFIP. groups. The arrangement of the hba HFIP groupŽs. on the aromatic ring spatially match the complementary hbb nitro groups for nitroaromatic compounds. An example siloxane polymer, SXPHFA, with an hba aromatic pendant group is shown in Fig. 5. In Fig. 6a, the SAW response is graphed for an SXPHFA-coated SAW device exposed to 30 ppm of NB vapor. Fig. 6b shows an expanded portion of Fig. 6a during the first 5 s of vapor exposure. The rapid response provides 90% of signal within 4 s of vapor exposure. For a SAW device coated with SXPHFA and exposed to 24DNT at a concentration of f 400 ppb, a reversible response was obtained with a signal of 8500 Hz. Based on an extrapolation, the SAW detection limit Žwith a signalto-noise level of 3. for 24DNT is 235 ppt. When SXPHFA is exposed to NB, changes in the IR spectrum Žsee Fig. 7a. of the OH stretching region of the polymer are observed. The NB is adsorbed at free-OH sites of the polymer reducing the intensity of this IR absorption. This is reflected in the negative peak that occurs in the difference spectrum ŽFig. 7c.. The OH groups that are associated with the NB show a red shift in their OH stretching frequency giving rise to a shoulder near 3460 cmy1 in the absorbance spectrum ŽFig. 7a. and a peak in the difference spectrum ŽFig. 7c..
4. Summary and conclusions A series of HFIP functionalized aromatic siloxane polymers have been synthesized, characterized, and evaluated
Fig. 6. Ža. Response of SXPHFA-coated SAW device to 30 ppm of nitrobenzene. Žb. Expanded view for response of SXPHFA-coated SAW to 30 ppm of nitrobenzene.
R.A. McGill et al.r Sensors and Actuators B 65 (2000) 5–9
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signal in 4 s. ATR-FTIR studies, provide strong evidence that nitroaromatic vapor molecules interact with hba polymers by formation of a hydrogen-bond between the nitro group and the hydroxyl groups of the polymer.
Acknowledgements
Fig. 7. ATR-FTIR spectrum of SXPHFA in the OH stretching region before Žb. and after exposure Ža. to NB at 30 ppm, and the difference spectra Žc..
as vapor sorptive coatings for use with chemical sensors. These strong hba polymers readily and reversibly sorb hbb vapors such as nitroaromatics. Based on a signal-to-noise ratio of 3 and an extrapolated value, the detection limit for 24DNT vapor with a 250-MHz SAW device, coated with the polymer SXPHFA, is approximately 235 ppt. The SAW devices were utilized as a tool to evaluate the sorption properties of new and old polymers synthesized at the Naval Research Laboratory. More sensitive transducers such as the thin film resonator being developed by Rockwell Science Center w7x are expected to lower the detection limits described here. In addition, from modeling studies and some preliminary experimental studies, the sorption of 246TNT in the SXPHFA coating is stronger for equivalent vapor concentrations than for 24DNT. A significant advantage of the SXPHFA coating is that rapid vapor sorption is possible, and for NB vapor exposures, rapid SAW sensor responses were observed that provided 90% of equilibrated
Financial support for this work was provided by DARPA with funds administered by Dr. Regina Dugan. Support from the Office of Naval Research is also acknowledged. Technical assistance is greatly appreciated from Dr. Michael Abraham at University College London and Dr. Robert Mowery.
References w1x A.M. Rouhi, Chemical and Engineering News, March 10 Ž1997. 14–22. w2x D.C. Leggett, T.F. Jenkins, R.P. Murrmann, CRREL Report 77–16 Ž1973. 1–50. w3x R.A. McGill, M.H. Abraham, J.W. Grate, CHEMTECH 24 Ž9. Ž1994. 27–37. w4x M.H. Abraham, Chem. Soc. Rev. Ž1993. 73–83. w5x T.E. Mlsna, R.A. McGill, in preparation. w6x R.A. McGill, J.W. Grak, M.R. Anderson, in: T. Mallouk, J. Harrison ŽEds.. Interfacial Design and Chemical Sensing, ACS Symposium Series 561, American Chemical Society, Washington DC, 1994, pp. 280–294. w7x P. Kobrin, C. Seabury, C. Linnen, A. Harker, R. Chung, R.A. McGill, P. Matthews, Proc. SPIE Aerospace Conference, Orlando, FL, USA, 1998.
The design of functionalized silicone polymers for chemical sensor detection of nitroaromatic compounds R. Andrew McGill a
a,)
, Todd E. Mlsna b, Russell Chung b, Viet K. Nguyen b, Jennifer Stepnowski b
NaÕal Research Laboratory, Code 6375, 4555 OÕerlook AÕe. NE, Washington, DC 20375, USA b Geo-Centers, 1801 RockÕille Pike, Suite 210, RockÕille, MD 20852, USA Accepted 12 October 1998
Abstract The solubility properties of a series of nitroaromatic compounds have been determined and utilized with known linear solvation energy relationships to calculate their sorption properties in a series of chemoselective polymers. These measurements and results were used to design a series of novel chemoselective polymers to target polynitroaromatic compounds. The polymers have been evaluated as thin sorbent coatings on surface acoustic wave ŽSAW. devices for their vapor sorption and selectivity properties. The most promising materials tested, include siloxane polymers functionalized with acidic pendant groups that are complimentary in their solubility properties for nitroaromatic compounds. The most sensitive of the new polymers exhibit SAW sensor detection limits for nitrobenzene ŽNB. and 2,4-dinitrotoluene in the low parts per billion Žppb. and low parts per trillion Žppt. concentration range, respectively. Polymers with favorable physicochemical properties exhibit low water vapor sorption, and rapid signal kinetics for NB, reaching 90% of signal response in 4 s. Studies with an in situ infrared spectroscopy technique are used to determine the mechanism of interaction between nitroaromatic compounds and the chemoselective polymer. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Nitroaromatic compound; Chemoselective polymer; Solubility; Saw sensor
1. Introduction The detection of chemical signatures associated with explosives is of great interest in the location of unexploded ordnance such as land-mines w1x. It is well known that dogs are able to locate explosives, including buried land-mines. It is clear that the canine olfactory system is capable of detecting and identifying land-mine chemical signatures. Of particular interest are the polynitroaromatic compounds identified in the chemical signature w2x. This work focuses around the characterization of the solubility properties of nitroaromatic compounds and the design of complementary sorbent polymer coatings. The polymer coatings are to be used to enhance the performance of a variety of chemical sensors being developed to locate buried land-mines from associated chemical signatures. To characterize the sorption properties of new poly)
Corresponding author. Tel.: q1-202-767-0063; fax: q1-202-7675301; e-mail: [email protected]
meric materials, surface acoustic wave ŽSAW. devices coated with the polymers have been exposed to vapors including: nitroaromatic compounds, common organic solvents, and water vapor. When exposed to a vapor, a polymer-coated SAW device sorbs vapor which results in a perturbation of the SAW propagation velocity. Vapor sorption is monitored as a shift in signal frequency. The nature of the interaction between the coating and vapor molecules determines the selectivity, sensitivity, signal kinetics, and the reversibility of the sensor w3x. The sorption of a vapor in a polymer coating can be described by the gas–polymer partition coefficient Ž K p ., which is defined as the ratio between the concentration of the vapor in the gas phase, C v , and the concentration of the sorbed vapor in the polymer phase, Cp , at equilibrium. The K p distribution of a vapor between the gas phase and a polymer-coated sensor is illustrated in Fig. 1. A solvation equation has been developed over the last 2 decades to model physicochemical and biochemical processes w4x. For the gas–polymer partition coefficient, the solvation equa-
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 3 5 1 - 2
R.A. McGill et al.r Sensors and Actuators B 65 (2000) 5–9
6
Table 1 Solubility parameters for UXO-related solutes and comparison compounds Solute vapor
R2
p 2H
b 2H
a 2H
log L16
246TNT 24DNT 26DNT 135TNB 2NT NB Cyclohexanone Toluene n-Octane Ethanol Acetone Nitromethane Water
1.430 1.150 1.150 1.430 0.866 0.871 0.403 0.601 0.000 0.246 0.179 0.313 0.000
2.23 1.60 1.60 2.23 1.11 1.11 0.86 0.52 0.00 0.42 0.70 0.95 0.45
0.61 0.47 0.45 0.61 0.28 0.28 0.56 0.14 0.00 0.48 0.56 0.27 0.35
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.37 0.04 0.12 0.82
7.85 6.26 6.30 – 4.88 4.56 3.79 3.33 3.68 1.49 1.70 1.89 0.26
Fig. 1. The gas–polymer vapor partition coefficient.
tion is given by the linear solvation energy relationship ŽLSER.: log K p s c q r R 2 q s p 2H q a a 2H q b b 2H q 1 log L16
Ž 1.
where log K p is the log of the gas–polymer partition coefficient, and relates to a series of vapors for a polymer. K p is a measure of the strength of the interaction between vapor molecules and polymer molecules, where larger K p values indicate that the vapor molecules are more soluble in the polymer solvent phase. The variables are solute vapor parameters: R 2 , an excess molar refraction that models polarizability contributions from ‘‘n’’ and ‘‘p’’ electrons: p 2H , the dipolarityrpolarizability; a 2H , the hydrogen-bond acidity; b 2H , the hydrogen-bond basicity; and log L16 , where L16 is the gas–liquid partition coefficient for n-hexadecane. The middle three variables are developed from chromatographic measurements and are more correctly denoted as summation parameters referring to solute molecules surrounded by solvent molecules; for simplicity, this notation is used. The coefficients resulting from a regression of log K p values characterize the solubility properties of the polymer. The most important of these are: s, the dipolarity; a, the hydrogen-bond basicity; b, the hydrogen-bond acidity; and
l, which reflects a combination of solvation cavity effects and dispersion interactions. The magnitude of ‘‘l’’ reflects the ability of the polymer to discriminate between a homologous series. The coefficient ‘‘r ’’ reflects the polarizability properties of the polymer, and ‘‘c’’ is a constant resulting from the regression. The size and relative magnitude of all the coefficients are used to identify polymers with high selectivity and sorption characteristics for vapors or gases of interest. A wide range of chemoselective polymers have been characterized with solvation equation w3x. From the known LSER equations, log K p values for more than 2000 vapors and gases can be calculated. Unfortunately, when this work was initiated, the available database of solute parameters did not contain the nitroaromatic compounds of interest. The first goal of this work was therefore to determine these parameters and utilize them with known LSERs to calculate their sorption properties in a series of chemoselective polymers. These results were used to design and synthesize a series of novel chemoselective polymers with complementary solubility properties for polynitroaromatic compounds. These polymers have been evaluated as thin sorbent coatings on SAW devices for their vapor sorption and selectivity properties.
2. Experimental ST-cut quartz, 250 MHz, SAW two-port resonator devices ŽMicrosensor Systems wMSIx, Bowling Green, KY. were used in these experiments. The SAW devices were
Table 2 Chemoselective polymer LSER parameters Polymer
Constant Žc.
Polarizability Žr.
Dipolarityr polarizability Žs.
Hydrogen-bond basicity Ža.
Hydrogen-bond acidity Žb.
Dispersion and cavity Žl.
PIB SXPHB PEM SXCN PVTD PECH PVPR OV202 P4V SXFA FPOL ZDOL
y0.77 y0.85 y1.65 y1.63 y0.59 y0.75 y0.57 y0.39 y1.33 y0.08 y1.21 y0.49
y0.08 0.18 y1.03 0.00 y0.02 0.10 0.67 y0.48 y1.54 y0.42 y0.67 y0.75
0.37 1.29 2.75 2.28 0.74 1.63 0.83 1.30 2.49 0.60 1.45 0.61
0.18 0.56 4.23 3.03 2.44 1.45 2.25 0.44 1.51 0.70 1.49 1.44
0.00 0.44 0.00 0.52 0.22 0.71 1.03 0.71 5.88 4.25 4.09 3.67
1.02 0.89 0.87 0.77 0.92 0.83 0.72 0.81 0.90 0.72 0.81 0.71
R.A. McGill et al.r Sensors and Actuators B 65 (2000) 5–9
7
Fig. 2. Chemoselective materials with known LSER equations.
cleaned and coated with the polymers as described before w6x, to provide a signal frequency shift of 250 kHz, which corresponds to about 50 nm of film thickness. The SAW frequency was monitored with a SAWPRO-250 system ŽMSI.. NB and other vapor tests were carried out with an automated VG400 vapor generator ŽMSI., and 2,4-dinitrotoluene Ž24DNT. vapor exposure tests were performed with vapor generated by passing dry N2 through a column packed with 24DNT-coated sand. FTIR experiments to examine nitroaromatic vapor– polymer interactions were carried out by attenuated total reflectance ŽATR. spectroscopy. ATR elements were coated with the polymer, and exposed to vapor generated as described above. 3. Results and discussion The solute solubility parameters for a wide range of nitroaromatic compounds have been determined and those of direct interest are detailed in Table 1. Nitroaromatic
compounds such as 2,4,6-trinitrotoluene Ž246TNT., 24DNT, and nitrobenzene ŽNB. consist of an aromatic ring with one or more nitro groups attached directly to the ring. The nitro groups are electron-rich sites, and exhibit hydrogen-bond basic Žhbb. properties. As the number of nitro groups is increased, the hbb properties of the nitroaromatic compound increase. The electrons in the aromatic ring of nitroaromatic compounds are delocalised and this allows electron density to be shifted from one nitro group and the aromatic ring to another nitro group, that is interacting with a dipolar or hydrogen-bond acidic Žhba. site of a neighboring polymer molecule. For polynitroaromatic compounds, the oxygen atoms of the nitro groups preferentially lie above and below the plane of the aromatic ring, so that, delocalization and polarizability are not maximized. As a result of the out of plane nitro configuration, the interaction of a nitro group attached to an aromatic ring with a neighboring molecule should be possible in a head-to-head position, or from above or below the plane of the aromatic ring.
Fig. 3. Ža,b. 246TNT gas–polymer sorption properties.
8
R.A. McGill et al.r Sensors and Actuators B 65 (2000) 5–9
Fig. 4. 246TNT showing the electron-rich and hbb nitro groups. The aromatic triol provides complementary hba sites.
Fig. 5. Aromatic siloxane polymer with the HFIP functional group.
For 12 chemoselective polymers Žsee Fig. 2. with known LSER equations Žsee Table 2., the solubility parameters for 246TNT and the other nitroaromatic compounds have been used to calculate the gas–polymer partition coefficients. The K p values for 246TNT are summarized in Fig. 3a and b. Fig. 3a and b differ only in that the P4V polymer is deleted in Fig. 3b in order to use a more suitable scale for the remaining polymers. From the 12 polymers used in the LSER studies, the P4V dominates the different materials for the sorption of polynitroaromatics. P4V is a strong hba, and is strongly dipolar, which are complementary solubility properties to the polarrpolarizable and hbb solubility properties of polynitroaromatics. Unfortunately, this polymer is crystalline and does not have ideal physical characteristics for a chemical sensor coating. The dipolarrpolarizable and spatial hbb solubility properties of polynitroaromatics provide characteristic features that should allow the design of chemoselective materials with complementary solubility properties that can strongly sorb these compounds. To illustrate this concept, an aro-
matic triol is shown in Fig. 4, with complementary hba sites for 246TNT that spatially match the hbb nitro groups. In order to build similar features into a polymer with rubbery properties, a series of siloxane polymers with functionalized aromatic and other pendant groups have been synthesized w5x, and their sorption properties characterized against different vapors. In order to maximize the hba properties of the polymers, the aromatic pendant groups were functionalized with one or more hexafluoroisopropanol ŽHFIP. groups. The arrangement of the hba HFIP groupŽs. on the aromatic ring spatially match the complementary hbb nitro groups for nitroaromatic compounds. An example siloxane polymer, SXPHFA, with an hba aromatic pendant group is shown in Fig. 5. In Fig. 6a, the SAW response is graphed for an SXPHFA-coated SAW device exposed to 30 ppm of NB vapor. Fig. 6b shows an expanded portion of Fig. 6a during the first 5 s of vapor exposure. The rapid response provides 90% of signal within 4 s of vapor exposure. For a SAW device coated with SXPHFA and exposed to 24DNT at a concentration of f 400 ppb, a reversible response was obtained with a signal of 8500 Hz. Based on an extrapolation, the SAW detection limit Žwith a signalto-noise level of 3. for 24DNT is 235 ppt. When SXPHFA is exposed to NB, changes in the IR spectrum Žsee Fig. 7a. of the OH stretching region of the polymer are observed. The NB is adsorbed at free-OH sites of the polymer reducing the intensity of this IR absorption. This is reflected in the negative peak that occurs in the difference spectrum ŽFig. 7c.. The OH groups that are associated with the NB show a red shift in their OH stretching frequency giving rise to a shoulder near 3460 cmy1 in the absorbance spectrum ŽFig. 7a. and a peak in the difference spectrum ŽFig. 7c..
4. Summary and conclusions A series of HFIP functionalized aromatic siloxane polymers have been synthesized, characterized, and evaluated
Fig. 6. Ža. Response of SXPHFA-coated SAW device to 30 ppm of nitrobenzene. Žb. Expanded view for response of SXPHFA-coated SAW to 30 ppm of nitrobenzene.
R.A. McGill et al.r Sensors and Actuators B 65 (2000) 5–9
9
signal in 4 s. ATR-FTIR studies, provide strong evidence that nitroaromatic vapor molecules interact with hba polymers by formation of a hydrogen-bond between the nitro group and the hydroxyl groups of the polymer.
Acknowledgements
Fig. 7. ATR-FTIR spectrum of SXPHFA in the OH stretching region before Žb. and after exposure Ža. to NB at 30 ppm, and the difference spectra Žc..
as vapor sorptive coatings for use with chemical sensors. These strong hba polymers readily and reversibly sorb hbb vapors such as nitroaromatics. Based on a signal-to-noise ratio of 3 and an extrapolated value, the detection limit for 24DNT vapor with a 250-MHz SAW device, coated with the polymer SXPHFA, is approximately 235 ppt. The SAW devices were utilized as a tool to evaluate the sorption properties of new and old polymers synthesized at the Naval Research Laboratory. More sensitive transducers such as the thin film resonator being developed by Rockwell Science Center w7x are expected to lower the detection limits described here. In addition, from modeling studies and some preliminary experimental studies, the sorption of 246TNT in the SXPHFA coating is stronger for equivalent vapor concentrations than for 24DNT. A significant advantage of the SXPHFA coating is that rapid vapor sorption is possible, and for NB vapor exposures, rapid SAW sensor responses were observed that provided 90% of equilibrated
Financial support for this work was provided by DARPA with funds administered by Dr. Regina Dugan. Support from the Office of Naval Research is also acknowledged. Technical assistance is greatly appreciated from Dr. Michael Abraham at University College London and Dr. Robert Mowery.
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