Carbosiloxane polymers for sensors
Polymers for sensors can be synthesized, cross-linked, grafted, and photopatterned using hydrosilylation chemistry.
Sorbent and functionalized polymers play a key role in a variety of fields, including chemical sensors, separation membranes, solid-phase extraction techniques, and chromatography. Sorbent polymers are critical to several sensor array or electronic nose systems. The responses of the sensors in the array give rise to patterns that can be used to distinguish one compound from another, provided that a sufficiently diverse set of sensing materials is present in the array. Each sensor in the array has a different coating, each one giving a characteristic response to a given analyte. Using hydrosilylation as the bond-forming reaction, we have developed a versatile and efficient approach to developing sorbent polymers with diverse interactive properties for sensor applications. The chemical and physical properties of these polymers are predictable and tunable by design.
Sensor arrays and electronic noses
Many kinds of sensor devices can and have been used in sensor arrays for gas-phase vapor detection. These arrays consist of a variety of interactive coatings on multiple sensors. The resulting multivariate data are analyzed by pattern recognition techniques. Array detectors are sometimes called electronic noses because like human noses, they use multiple receptors to produce signals that are processed by neuronal pattern recognition processes. Some of the best-known examples of gas-phase sensor arrays use polymers as the interactive coatings.
Arrays based on acoustic wave sensors use quartz crystal microbalances (QCM), surface acoustic wave (SAW) devices, or flexural plate wave (FPW) devices as the sensing transducers (1). The generated signals are proportional to the mass of the vapor sorbed by each of the polymer coatings on the device surfaces. The observed signals may also include a polymer modulus change contribution.
Vapor sorption by polymers can also be transduced using chemiresistor configurations in which the insulating polymer is loaded with (typically) 20% carbon black particles. Vapor sorption swells the insulating polymer and increases the resistance through the polymercarbon black composite. These kinds of chemiresistor sensors have served as the basis for sensor arrays (2).
Alternatively, sorbent polymers can be used as the matrix for fluorescent dyes such as Nile red. Vapor sorption alters the fluorescence signal from the incorporated dye molecules. Arrays have been prepared with various dyes in various polymers on the ends of fiber-optic bundles (2).
Thus, sorbent polymers are important as sensing materials on several array-based electronic noses, either as neat polymers, as composites containing conducting particles within the polymer film, or as composites incorporating fluorescent dyes. All of these approaches can benefit from rational polymer design and synthesis.
Polymers for chemical vapor sensing
Polymers are useful materials for chemical vapor sensing for several reasons:
The chemically interactive properties of polymers for chemical vapor sensing have been systematically examined using linear solvation energy relationships (3, 4). These linear free-energy relationships model the sorption of vapors by polymers in terms of fundamental interactions, such as dispersion interactions, dipole and induced-dipole effects, and hydrogen bonding. Models derived by this approach can be used for prediction and understanding, the latter usually being the most important.
The role of fundamental vaporpolymer interactions and linear solvation energy relationships for designing sensor arrays was first detailed in 1991 (4). Subsequent treatments have demonstrated applicability in many related aspects of chemical sensor development (1, 5, 6). Designing a sensor array for detecting organic vapors entails selecting a set of polymers in which each emphasizes a different interaction. This approach leads to requirements for polymers that are nonpolar, polarizable, diplolar, hydrogen-bond basic, and hydrogen-bond acidic.
For a polymer to be useful for chemical sensing, just having the required chemical interaction properties is not enough. A sensing polymer also must have the desired physical properties. Rapid chemical sensor responses, which are usually desirable, are promoted by polymers with glass-to-rubber transition temperatures (Tg} below the operating temperature of the sensor. Vapor diffusion in and out of polymers is rapid under these conditions.
The method by which the material will be applied to a chemical sensor as a thin film may also impose requirements on the polymer or prepolymer formulation. For example, solubility in organic solvents may be necessary, or a certain viscosity for a prepolymer may be necessary as part of a coating process. Particular sensor platforms may impose their own requirements, such as refractive index requirements for an optical sensor. Therefore, the synthesis of a polymer for a sensor or sensor array application must consider not only the chemical interactions, which have been studied in detail, but also the desired physical properties for film application and sensor performance.
It would be useful to have a single synthetic strategy that is versatile enough to yield a variety of individual polymers, so that each new sensing material would not present a new and unique synthesis problem. These optimized sensing polymers would have the right combinations of chemical and physical properties to meet a broad spectrum of performance criteria. The synthesis strategy must also be able to create diverse sets of such materials for various applications and arrays. It also would be desirable to be able to tune and formulate these polymers for various sensor devices, each of which may impose different requirements on physical properties and thin-film deposition methods.
With these considerations in mind, we have explored the use of hydrosilylation polymerization for design and synthesis of polymers for sensors and sensor arrays. In its most typical form, the hydrosilylation reaction forms a siliconcarbon bond by adding a silicon hydride bond across a carboncarbon double bond in the presence of a noble metal catalyst such as platinum (7).
Linear polymers with alternating monomers can be formed when one monomer has two
Polymers combining siloxy linkages and CC bonds in the polymer backbone are called carbosiloxane polymers. When the monomer with two
In most of the original examples of hydrosilylation polymerization, the substituents on silicon were typically methyl, ethyl, or phenyl; hexachloroplatinic acid was used as the catalyst (8). The reaction of isoprene with dihydridododecamethylhexasiloxane (Figure 1, bottom) was an early example of an organicinorganic material prepared by hydrosilylation polymerization.
Even though the use of the hydrosilylation reaction for polymerization was demonstrated more than 35 years ago, Itsuno and co-workers noted as recently as 1993 that the direct use of the hydrosilylation reaction in preparation of organosilicic polymers has been limited so far (9). Furthermore, Dvornic and Gerov noted in 1994 that early studies on the use of hydrosilylation polymerization resulted only in low molecular weight oligomeric products and indicated that the versatile reaction has not yet been successfully developed for the synthesis of truly high molecular weight linear polymers (10). Dvornic went on to demonstrate that truly high molecular weight methyl-substituted polymers could be obtained by this reaction using a platinum divinyltetramethyldisiloxane catalyst (Karstedts catalyst) rather than hexachloroplatinic acid.
Several examples of functionalized polymers and novel architectures were described in a brief review of hydrosilylation polymerization (8). The hydrosilylation reaction has been used to make liquid-crystal polymers, chiral polymers, hyperbranched and dendritic structures, and macromolecules with redox-active centers. Hydrosilylation chemistry is also being used to functionalize silicon surfaces (11). In the remainder of this article, we focus on developing functionalized polymers and polymer thin films for chemical sensors and arrays using hydrosilylation chemistry.
Rational polymer design and synthesis
Hydrosilylation polymerization methods can be used to incorporate a variety of organic structures and functional groups into a polymeric structure. The hydrosilylation reaction is selective and tolerates many functional groups including esters, nitriles, amines, amides, nitro groups, ketones, ethers, phosphates, sulfides, and sulfones (7, 12). It also is well known that inserting oligosiloxane segments into a polymer often lowers the Tg. Therefore, this method meets two primary criteria for the synthesis of chemical-sensing polymers: Diverse polymers with various functional groups can be prepared, and the resulting polymers should exhibit low Tg. Functional groups can be selected to obtain the vaporpolymer interactions discussed earlier. The length of the oligodimethylsiloxane segment can be varied to influence Tg and other physical properties.
The approach has several other desirable features. The bond-forming reaction produces SiC bonds and does not introduce any polar functionalities into the final material. Thus, the bond-forming reaction does not bias the selectivity of the resulting polymers, which would be the case if all polymers were formed by amide or ester linkages. The method allows control of end group functionality. Polymers or oligomers may be terminated with primarily
Figure 2 illustrates four of the carbosiloxane polymers that we have prepared by this method. Each polymer in Figure 2 is designed to emphasize different properties and interactions. The methyl-substituted carbosiloxane polymer CSME is a nonpolar material that is good for sorbing aliphatic hydrocarbons. It is the same or similar to other carbosiloxane polymers prepared previously (see Figure 1) (10) and has sorbent properties similar to poly(dimethylsiloxane) and polyisobutylene.
The phenyl-substituted polymer CSPH was prepared because phenyl groups provide greater polarizability than simple aliphatic groups. On sensors, these polarizable polymers offer sensitivity to aromatic hydrocarbons such as benzene, toluene, ethylbenzene, and xylenes (BTEX), as well as to chlorinated hydrocarbons. Both of these classes of compounds are environmental contaminants of concern.
Although other phenyl-substituted polymers with low Tg are available (e.g., the gas chromatographic stationary phase OV-25), we wanted to prepare our own carbosiloxane polymer in order to have control over composition, properties, and formulation for sensing films. OV-25 is a polysiloxane with 75% phenyl substituents and 25% methyl substituents. Commercial OV-25 has a slight hydrogen bond acidity that is probably attributable to residual SiOH groups; and it compromises the desired chemical selectivity. In our experience, commercial polymers often contain functionalities or contaminants that are not indicated by the nominal polymer structure, and sometimes these can be seen in the infrared spectrum. In addition, we observed erratic behavior of OV-25coated SAW devices under humid conditions. Our carboxysilane polymer, CSPH, was designed to have the desired high ratio of phenyl to methyl groups (67:33). We found that CSPH-coated SAW sensors were not ill-behaved in humid atmospheres.
We prepared the polymer containing the urea group, UR3, as a basic polymer for chemical sensor arrays. Urea groups are very basic and dipolar. The hydrosilylation polymerization proceeded smoothly and yielded well-behaved chemical sensors when applied to SAW devices. Other basic polymers such as poly(vinylpyrrolidone) and poly(ethyleneimine) have basic properties and have been applied to SAW devices as sensor coatings; however, poly(vinylpyrrolidone) does not have the desirable low Tg, and poly(ethyleneimine) has yielded sensors with poor reproducibility. Again, shortcomings of commercial materials for our application prompted us to synthesize our own low-Tg basic polymer.
The bisphenol-containing polymer BSP3 is one of our most interesting and useful polymers (13, 14). This polymer was rationally designed to have strong hydrogen bond acidic properties that are desirable for sorbing basic vapors. Many basic organic solvents that one might want to detect are used industrially. In addition, some vapors of national security concern, such as nerve agents, are strong hydrogen bond bases. Polymers in this class are also useful for detecting nitroaromatic explosives (15).
Considering the chemical structures that have hydrogen bond acidic properties leads to a choice of fluorinated alcohols and phenols as the functionality that should be incorporated into a polymer (4). Commercial polymers, however, with these functionalities and low Tgs have not been available. Accordingly, several synthetic materials have been prepared and investigated as sensor phases, most of which incorporate hexafluoroisopropanol moieties as the hydrogen bond acidic group (1317).
In a study published in 1991, Abraham and co-authors used inverse gas chromatography and linear solvation energy relationships to compare the hydrogen bond acidities of several propyl- or allyl-substituted bisphenol structures (18). They showed that fluorinated bisphenol-A structures were substantially more hydrogen-bond acidic than nonfluorinated analogues; fluorination improved sorption of basic compounds by factors of 100 or more. These findings provided the rational design criteria for synthesis of polymers containing the fluorinated bisphenol structure in the polymer chain. The BSP3 polymer shown in Figure 2 has three silicon atoms in the repeat unit. Variants have also been synthesized with longer oligosiloxane units. Interaction of a BSP polymer with a basic vapor is shown in Figure 3.
Experiments on SAW sensors have shown that these phenolic polymers are useful in sensing basic vapors and nerve agent simulants, and have properties that are equal to or superior to previous materials in this category (13, 14, 19). Figure 4 shows the calibration curves for SAW sensors coated with BSP3, fluoropolyol (FPOL), and poly(dimethylsiloxane) (PDMS) when tested against dimethyl methylphosphonate, a nerve agent simulant. FPOL has been used in the past as a sensor coating for nerve agents (20), and PDMS is as an example of a sorbent polymer lacking functionalities designed for sorption of basic vapors. Signals of >20,000 Hz are observed at a concentration of only 8 mg/m3 using BSP3. This corresponds to a concentration of 12 ppm, indicating that detection limits for a minimum detectable signal of 10 Hz would be ~1 ppb (13).
Together, the polymers in Figure 2 can provide the chemical diversity that is desirable in small sensor arrays (1, 4). A wide variety of monomers can be combined in this synthetic approach. Many organic dienes exist that could be combined with α,Ω-dihydridooligosiloxanes to yield carbosiloxane polymers. Although this is not combinatorial chemistry or parallel synthesis per se (so far we have prepared polymers in individual batches rather than in parallel), it does illustrate the principle that a variety of materials can be prepared efficiently by using various combinations of a limited number of starting monomers.
We have prepared a ferrocene-derivatized monomer in which ferrocene is appended to an oligosiloxane chain that is terminated by SiH groups. We successfully combined this monomer with a bisphenol monomer to produce a polymer with a repeat unit that is shown in Figure 5. Cyclic voltammetry on solutions and thin films of this resulting material confirmed the presence of redox-active ferrocene groups. The usefulness of electrofunctional polymers in the analytical sciences and especially in sensor development has been noted (21). In principle, it should be possible to prepare a whole series of functionalized redox-active polymers by combining the ferrocene-containing monomer with various monomers or with other organic dienes.
It is possible to vary the properties of these materials to meet other sensor requirements. We were interested in coating a hydrogen bond acidic polymer as the cladding on silica optical fibers. This platform imposed the requirement that the refractive index of the polymer be lower than that of silica (~1.46) to guide light efficiently. In addition, the material had to be formulated as a prepolymer that could be coated on a fiber pulled freshly from the melt using an automated optical fiber drawing tower. The prepolymer is then cured in a tube furnace before the clad fiber is wound onto the drum of the fiber drawing system. We wanted a viscosity in the range of 10003000 centistokes (cS) to permit delivery of prepolymer to a cladding cup through 1/4-in. or 3/8-in. tubing. BSP3 is unsuitable for direct use in this application because its refractive index is 1.48, and it is a gum phase.
We prepared a liquid prepolymer with a viscosity in the range of 1500 cS and a refractive index of 1.42; we used a much longer α,Ω-dihydridooligosiloxane as a macromonomer (~80 dimethylsiloxane repeat units) under conditions leading to a lower molecular weight product. Two equivalents of fluorinated bisphenol were reacted with 1 equiv of this macromonomer, yielding an allyl-terminated prepolymer incorporating the desired bisphenol. This material was combined with a platinum catalyst and miscible cross-linker, phenyltris(dimethylsiloxy)silane (Figure 6, center), containing three SiH groups and applied to fibers as described earlier. Sections of fiber clad using this method were as effective at guiding light as commercially available plastic fibers or silica fibers clad in our laboratory with commercial poly(dimethylsilicone) cladding formulations. This example illustrates the versatility of the approach for synthesizing polymers and oligomers containing a desired functionality while tuning the physical properties for a particular sensing platform.
The carbosiloxane polymers prepared by hydrosilylation polymerization are intrinsically terminated with groups that can be used for cross-linking. By design, these can be essentially all terminal siliconhydride or all terminal vinyl or allyl groups. Adding a platinum catalyst and a multifunctional cross-linker, such as those shown in Figure 6, yields a formulation that will cross-link on curing. Hydrosilylation is one of several reactions that are conventionally used for cross-linking vinyl-modified polysiloxanes. Cross-linking is required for some sensor formats, such as the optical fibers, and may be desirable for others. A cross-linked film will have mechanical stability, and it is less likely to fail by dewetting the surface. In addition, cross-linked films may offer enhanced sensor stability and life expectancy.
Rapp and co-workers investigated the UV-initiated free-radical cross-linking of polysiloxane films on SAW devices (22). They found that cross-linking produced sensors with superior stability compared with uncross-linked polysiloxanes. Hydrosilylation chemistry provides a clean reaction for cross-linking that can often be conveniently carried out in air.
Our overall scheme for polymer synthesis, cross-linking, and grafting is shown in Figure 7. Hydrosilylation polymerization generates polymers or oligomers incorporating interactive groups (indicated in the figure by squares with the letter I inside) for chemical selectivity. These may also have a redox-active center or reporter group incorporated in the structure, as indicated by R in the figure. By design, this approach yields chains with terminal vinyl or silicon hydride groups for cross-linking. Formulation with multifunctional cross-linkers (vinyl- or SiH-substituted) and catalyst produces curable films. The same procedure on modified surfaces (vinyl- or SiH-modified) can yield cross-linked and grafted films. Thus, hydrosilylation is used in polymer synthesis, polymer cross-linking, and polymer grafting.
Patterned polymer films
The fact that solubility varies depending on cross-linking and grafting suggests that hydrosilylation could be used for photopatterning polymer films. This would simply require a catalyst that was inactive until exposed to light. Platinum(II) bis(β-diketonates) such as platinum bis(acetylacetonate) [Pt(acac)2] offer these properties. These compounds have been used as photoactivated hydrosilylation catalysts for solution reactions, polymerization of vinyldimethylsilane in solution, and for curing and patterning preceramic polymer films (2426).
We have found that this chemistry is useful for patterning sorbent and functionalized siloxane and carbosiloxane polymers.
A variety of other functionalized sorbent materials can be photopatterned by these methods. It is important to use cross-linkers that are miscible with the vinyl-functionalized polymer and to allow an adequate dark reaction time before developing the pattern by dissolving away uncross-linked materials. Simultaneous grafting to the surface is often necessary to retain the pattern. We have patterned phenyl- and cyano-substituted siloxane formulations this way.
Whereas the reactions above are based primarily on photoactivated hydrosilylation cross-linking, it is also possible to photoactivate polymerization as part of the patterning process. For example, we have formulated a prepolymer film containing Pt(acac)2, the bisphenol monomer, and a SiH-terminated oligomer that was prepared and isolated from the reaction of the same monomer with excess of an α,Ω-dihydrodimethylsiloxane. A small amount of cross-linker was also included in the prepolymer. Photoactivated polymerization and pattern development on a vinyl-modified silicon surface gave the lines shown in Figure 9.
Alternatively, patterns of highly cross-linked network polymers can be obtained by formulating functionalized monomers or oligomers with multifunctional cross-linkers; however, excessive cross-linking can raise the Tg significantly. Obtaining sufficient cross-linking to retain polymer patterns on the substrate after rinsing with the pattern development solvent, while maintaining a low Tg, requires a delicate balance of polymer formulation, cross-linker, surface modification for grafting, dark reaction time, and solvent selection.
We have found that a variety of functionalities, including all those in Figure 2, as well as cyano groups, can be incorporated in photopatterned siloxane or carbosiloxane polymer films. Thus, the patterning approach has great potential for use in sensors and sensor arrays.
Acoustic wave sensor array on a chip
Acoustic wave sensors such as quartz crystal microbalances and SAW devices are often the basis for polymer-coated sensor arrays (1). FPW devices can also be coated with polymers for chemical sensing (27). In this case, the active surface of the device is at the bottom of an etch pit in a silicon chip. It is possible to fabricate several of these devices on a single chip, leading to an array on a chip. Different polymers then can be applied to the active devices in their individual etch pits.
A picture of a packaged FPW array on a chip is shown in Figure 10. Sensor responses to the vapors from two industrial solvents, toluene and methyl isobutyl ketone, are shown in Figure 11. The array was coated with three of the carbosiloxane polymers shown in Figure 2 (BSP3, UR3, and CSPH) and three other commercial polymers [poly(isobutylene), OV-275, and Eypel-F]. These two bar graphs illustrate the generation of distinguishable patterns for different vapors. Pattern recognition analysis of data from FPW array sensors was described in detail by Zellers and co-workers. (28). The patterns in Figure 11 support some of the chemical selectivity principles that are the basis for our rational design approach. Of the six polymers shown in Figure 11, the polarizable CSPH polymer is the most sensitive to toluene. The most sensitive polymer for basic methyl isobutyl ketone is BSP3. (The BSP3 polymer has also been used on a SAW array on a chip .)
Hydrosilylation: A versatile tool
Hydrosilylation chemistry offers tremendous versatility in the development of sorbent and functionalized polymers and thin films. Diverse sets of polymers can be prepared with control over chemical and physical properties. The chemistry can also be used in the cross-linking, grafting, and patterning of thin films. The materials can be adapted to a variety of sensor types. These kinds of materials and films are useful, or potentially useful, for chemical sensors, sensor arrays, membranes, solid-phase extraction, chromatography, and lab-on-a-chip applications.
We thank Mary Bliss, Richard Craig, Glen Dunham, and Jing Li for their contributions. We are also grateful for support from the Department of Energy Office of Nonproliferation and National Security, NN-20. The Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute.
Jay W. Grate leads an interdisciplinary research group in microsensors and microfluidics in the Environmental Molecular Sciences Laboratory at the Pacific Northwest National Laboratory (PO Box 999, Richland, WA 99352; 509-376-4242; email@example.com). He worked at the Naval Research Laboratory in Washington, DC, from 1984 to 1992, including a year of sabbatical research at The Scripps Research Institute in La Jolla, CA. He received his Ph.D. in chemistry from the University of California, San Diego, in 1983. His research integrates aspects of the chemical sciences, material sciences, and measurement sciences into new microanalytical principles, methods, and systems. His work on developing chemical sensor arrays has focused on the interactions of organic vapors with polymeric sensing materials, rational polymer design, acoustic wave sensors, and multivariate data analysis techniques.
Steven N. Kaganove is an associate scientist at the Michigan Molecular Institute (1910 W. St. Andrews Rd., Midland, MI 48640). He received a B.A. degree in chemistry from Grinnell College in Iowa and a Ph.D. in organic chemistry from the University of California, Los Angeles. He did postdoctoral research on synthesizing organic superconductors at Indiana University, Bloomington, and on developing polymeric materials for organic vapor sensing at Pacific Northwest National Laboratory. His current research interests include organic thin films, chemical sensors, silicon-containing polymers, dendritic polymers, and novel polymer architectures.
David A. Nelson is a staff scientist at the Pacific Northwest National Laboratory. He received his Ph.D. in organic chemistry from Montana State University, Bozeman. He was presented with an R&D 100 award for the organometallic extraction of hydrogen from hydrogen sulfide. He conducts research with polysiloxanes, polycarbosiloxanes, polyphosphazenes, membranes, photopatterned thin films, and detection systems for noxious chemicals. He is also examining the direct use of carbon dioxide for polycarbonate manufacture.