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Sensor based on piezoresistive microcantilever technology
Sensors and Actuators A 88 (2001) 47±51
Sensor based on piezoresistive microcantilever technology T.L. Porter*, M.P. Eastman1, D.L. Pace, M. Bradley Department of Physics, Northern Arizona University, Flagstaff, AZ 86011, USA Received 12 June 2000; received in revised form 21 August 2000; accepted 24 August 2000
Abstract A new type of micromechanical sensor has been developed that incorporates piezoresistive microcantilevers, such as those used in scanning force microscope (SFM) instruments. In these devices, the microcantilever is in direct contact with an active sensing material, which may be a common organic polymer or a biologically active sensing layer. Upon analyte exposure, the cantilever directly measures the swelling or volume expansion experienced by an active sensing material. Individual microcantilever sensors may be incorporated into an array in which pattern recognition techniques are used to identify a wide variety of chemical or biological analytes. In addition, these devices are compact, lightweight, and require only simple, inexpensive support electronics. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Microcantilever; Pezoresistive; Chemiresistor; Polymer
1. Introduction A number of advancements have been made in recent years in the development of microsensors capable of recognizing chemical or biological agents [1±3]. One new class of chemical sensors makes use of the ``swelling'' that many common organic polymers such as poly(vinyl acetate) (PVA), poly(iso-butylene) (PIB), or poly(ethylene vinyl acetate) (PEVA) undergo as analyte vapor molecules partition into the material [3±7]. In order for these polymer-based sensors to operate, the response to the analyte vapor, i.e. the polymer swelling, must be reliably measured. Incorporation of carbon black particles into the polymer may be used to provide a means to measure this swelling [4±6,8]. Upon incorporation of the carbon black particles, the normally non-conducting polymer becomes partially conductive. When the composite material swells due to analyte exposure, the conductivity of the polymer/carbon composite material drops. If this chemiresistor material is deposited onto suitable electrodes, such as an interdigitated array, the conductivity changes due to analyte exposure can be readily measured [3,7]. Other chemiresistor-based sensors use conductive polymers as their active element, these devices do not require the addition of materials such as carbon black [9]. The largest measurable responses occur when the *
Corresponding author. Tel.: 1-520-523-2661; fax: 1-520-523-1371. E-mail address: [email protected] (T.L. Porter). 1 Co-corresponding author.
percentage of carbon black loading into the polymer is such that the swelling that occurs during analyte exposure drops the carbon conductivity through its percolation threshold [3]. Arrays of these sensors may be produced, using 10 or more carbon loaded common organic polymers as sensing elements [3,7]. In such an array, many of the individual sensing elements may respond to a given analyte vapor, but each analyte will produce a particular, unique signature when the degree of response for each element in the array is considered. Pattern recognition algorithms enable sensor arrays to identify different vapor analytes. There are some disadvantages to these chemiresistorbased sensors. Recent scanning force microscope (SFM) studies indicate that the carbon particles may be expelled over time from the host polymer matrix as the material undergoes many swelling/recovery cycles [10]. Over time, this effect could render the sensor less sensitive, or perhaps completely inoperative. The carbon particles themselves may absorb analyte vapors, with subsequent slow release of these vapors, extending the recovery times for the sensors [7]. Fabrication and calibration of these devices may be dif®cult due to the precise ratios of the materials needed, and the spatial distribution/homogeneity required for the carbon particles within the polymer matrix. Microcantilever-based sensors are also a current interest. In these devices, an SFM type cantilever is coated with a chemically or biologically active material [1,2,11,12]. In many of these devices, the cantilever is then driven (by external circuitry) into oscillation at a resonance frequency.
0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 0 ) 0 0 4 9 8 - 2
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T.L. Porter et al. / Sensors and Actuators A 88 (2001) 47±51
Analyte molecules bind or adsorb to the active layer on the cantilever, increasing the mass of the vibrating cantilever slightly. The increase in mass results in a slight shift in the cantilever vibration frequency or amplitude, measurable by external means. Depending on the active sensing layer used, both chemical species and certain biological species may be detected. While arrays of these ``vibrating cantilever'' sensors may be fabricated, the external circuitry required to both drive the cantilevers and measure their frequency/ amplitude shifts may be both cumbersome and expensive. Crosstalk and interference between close-packed arrays of vibrating cantilevers may also result in unwanted or spurious signals. Finally, the sensing layer itself may not adhere over time to a cantilever bending or oscillating at tens of hundreds of kiloHertz. In this paper, we describe a new type of microsensor that utilizes piezoresistive microcantilevers to directly measure analyte produced swelling or volumetric changes in a tiny bead of active material. The active sensing material may be an organic polymer, or a biologically active layer. These microsensors may be fabricated in small arrays, operated in gaseous as well as liquid environments, and operated with only simple, inexpensive support electronics.
elements in our tests. No carbon black or other compounds were mixed with the polymers. Piezoresistive micro-cantilevers were obtained from Park Scienti®c, Inc. (now Thermomicroscopes, Inc., Sunnyvale, CA). These cantilevers were designed for use in contactmode SFM, and carried a nominal piezo-channel resistance of 2 kO. The tip shape was pyramidal (no sharpening). The cantilevers were used as is, without any further modi®cation. Resistance changes during sensing experiments were obtained with standard precision multimeters. The sensing experiments were carried out in a modi®ed scanning probe microscope instrument (SPM) from Thermomicroscopes (Fig. 1). Dry nitrogen was passed through two ¯ow meters, one for controlling the background nitrogen gas level, and the other for controlling the percentage of saturated analyte vapor that was added to the background gas. Saturated analyte vapor was obtained by passing clean, dry nitrogen through a gas bubbler assembly containing the analyte liquid. The analytes used in these experiments included water vapor, ethanol, hexane, toluene and acetone. Exposures of the sensing polymers to the analyte vapors ranged from 1 to 3 min., and recovery times (dry nitrogen exposure only) ranged up to 5 min.
2. Experimental
3. Results and discussion
Organic polymers PVA, PIB, or PEVA were prepared by dissolving the polymers using the solvents deionized distilled water, chlorobenzene, or chloroform. These were then deposited onto stainless steel substrates and allowed to dry. The polymers used were purchased from Aldrich Chemical Company (Milwaukee, WI) or Scienti®c Polymer Products (Ont., NY), and used as received. The thickness of these ®lms as measured by stylus pro®lometry was approximately 2 mm. These ®lms were then used as the active sensing
A diagram of a single piezoresistive sensor element is shown in Fig. 2. An active sensing layer, or ``generalized sensing element'' is deposited onto the substrate (i.e. stainless steel or polished Si). The lateral dimensions of this layer may be as small as a fraction of a micrometer. The only requirement for the composition of this active sensing layer is that it swell, or expand in the vertical direction upon analyte exposure. Using a simple mechanical approach mechanism, the piezoresistive microcantilever is brought
Fig. 1. Schematic diagram of modified SPM used for piezoresistive sensor element testing.
T.L. Porter et al. / Sensors and Actuators A 88 (2001) 47±51
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Fig. 2. Diagram of single element sensor. The generalized sensor element consists of any layer (organic polymer or other) that undergoes ``swelling'' upon analyte exposure.
into contact with the surface of this sensing layer (a highpitch screw mechanism is suf®cient for this task). These cantilevers are only 100±200 mm long, and about 50 mm wide. They contain an internal channel of piezoresistive material, connected to two tiny external electrodes. The zero-strain resistance of these cantilevers is on the order of 2 kO [13], but changes rapidly and measurably in response to any bending of the cantilever. In fact, these cantilevers are sensitive enough to measure bending strains of only a few Ê (a typical response for one of these cantilevers is 1± tens of A 4 mO for each angstrom of de¯ection). Any swelling of the active layer in contact with the cantilever tip will result in an immediate, easily measurable change in the cantilever channel resistance. This change will be in exact proportion to the amount of the vertical swelling; a simple multi-meter is thus suf®cient to record the sensing activity. As a test of this sensor design, we prepared piezoresistive microcantilever sensor elements utilizing the three polymers (or elastomers) PVA, PIB, or PEVA. Each of these was exposed to the vapors of water, ethanol, hexane, toluene and acetone (these analytes were chosen due to their wide range of solubility parameters). The response of these sensor elements when exposed to the analyte vapors under the conditions P=Psat 0:5 is summarized in Table 1 (measured in Ohms). The exposure consisted of a 50±50 mix of saturated analyte vapor with dry nitrogen gas. In most cases, the sensor response was achieved almost instantly. Note that while all three polymers responded to most of the analytes, a unique ``pattern'' for each individual analyte is obtained. Table 1 Cantilever response measured in Ohms for the three polymers PVA, PEVA, and PIB during exposure to the analyte vapors indicateda Polymer
Water
Hexane
Ethanol
Toluene
Acetone
PVA PEVA PIB
0.52 0.11 0.0
0.07 0.73 0.98
0.23 0.12 0.05
0.26 1.02 0.58
0.14 0.29 0.23
a Each analyte produces a unique response from the ``array'' of three individual sensors.
Fig. 3. (A) Plot of sensor response vs. solubility parameter for the three sensors used in our study (squares: PIB, circles: PEVA, double triangles: PVA). (B) Resistance measurements during exposure to hexane at P=Psat 0:1, 0.2, 0.3, 0.4, and 0.5 for the PIB-based sensor.
In general, the swelling of an organic polymer as a result of the partitioning in of gaseous molecules may be modeled using Hildebrand solubility parameter concepts [14]. Each of the analytes used in our experiments is associated with a particular solubility parameter, ranging from a high of 48 MPa1/2 for water vapor to a low of 16.7 MPa1/2 for hexane [15]. Partitioning of the analyte molecules into the polymer is postulated to occur to the greatest extent when there is a close match between the polymer and the analyte solubility parameters [7]. Correspondingly, less partitioning occurs the greater the difference in these two numbers. A plot of sensor response versus solubility parameter for the three sensors used in our study is presented in Fig. 3A. For example, in our study the polymer PIB (solubility parameter of 15.5 MPa1/2) experienced the largest swelling (and concomitant cantilever resistance change of 0.98 O, or Ê swelling) upon exposure to hexane vapor (solubility 500 A parameter 16.7 MPa1/2). The other large responses (0.58 and 0.23 O, respectively) occur during exposure to toluene vapor (18.2 MPa1/2) and acetone vapor (20.5 MPa1/2). We obtain little or no response to either ethanol (26 MPa1/2) or water vapor (48 MPa1/2). Chemiresistor work by Eastman et al. [7] on PIB-based sensors shows the largest drops in conductivity (swelling) occur upon exposure to the vapors of hexane, acetone, and toluene. A much smaller conductivity drop is obtained during ethanol exposure, and a zero response for water vapor was noted [7].
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T.L. Porter et al. / Sensors and Actuators A 88 (2001) 47±51
For our PVA-based cantilever sensor (solubility parameter 19.62 Mpa1/2), the largest responses occurred for water vapor (0.52 O), toluene (0.26 O), and ethanol (0.23 O). A smaller response occurred for acetone vapor, and we measured little cantilever response in the case of hexane exposure. These results also compare favorably with previous data utilizing PVA in chemiresistor-based sensors. In the chemiresistor study by Longeran, et al. [3], a PVA/carbon black composite chemiresistor sensor showed the largest response (resistance drop) for acetone exposure, a moderate response for ethanol and toluene, and only a tiny response to hexane vapor. Water vapor was not used in this study. In the study by Eastman et al. [7], a similar PVA/carbon black composite chemiresistor sensor also showed a large response to water vapor, and smaller responses to all other analytes. We note that in the Eastman et al. [7] study, only a negligible response to acetone was reported, however our data more closely tracks the large response noted in the Longeran et al. [3] study. This difference in the two chemiresistor sensor studies may be attributable to differences in carbon loading, i.e. small percentage differences in the carbon black component may result in large differences in sensor response for a given analyte. In the case of PEVA-based chemiresistors (solubility parameter 18.6), the Longeran et al. [3] data showed large responses for toluene, hexane, and acetone (in order or response magnitude), with a smaller response to ethanol (water vapor was not tested). For a similar PEVA-based chemiresistor in the study by Eastman et al. [7], the largest responses also occurred for acetone, hexane and toluene. Somewhat lesser responses were measured for ethanol. The smallest response was measured for water vapor. Our measured responses once again agree well with this previous chemiresistor data, with large responses for toluene (1.02 O), hexane (0.73 O), and acetone (0.29 O). We measure a smaller response for ethanol exposure as well as exposure to water vapor. For the three polymers used in our study, generally good agreement is obtained with the solubility parameter concept, i.e. the largest swelling responses occur when the solubility parameter of the analyte is close to the solubility parameter for the polymer. Also, our data tracks previous chemiresistor response data well, with the exceptions noted above. It is clear (in theory at least) that the solubility parameter concept presents a method that will allow for the detection of virtually all known solvent vapors by an array containing only a relatively small number of distinct polymer-based cantilever sensors [3,7]. For such an array, pattern recognition techniques are used to recognize the unique response from the individual polymer sensors for each different analyte. We have also obtained preliminary data regarding the linearity of these piezoresistive microcantilever sensors (Fig. 3B). Using the polymer PIB, we obtained resistance measurements during exposure to hexane at P=Psat 0:1, 0.2, 0.3, 0.4, and 0.5. The cumulative resistance increases
were 0.04, 0.14, 0.27, 0.48, and 0.68 O. We did not achieve the full 0.98 O response at P=Psat 0:5, however, due to large variations in the PIB ®lm thickness. While this measured response to hexane is not linear (especially in the lowest exposure ranges), it is reproducible and therefore may be easily used in a pattern recognition scheme. While the data obtained clearly demonstrates the operation of piezoresistive cantilever-based microsensors, there may be other advantages to this general form of sensor operation. It may also be possible to use piezoresistive-based microsensors for biological sensing applications. As previously mentioned, vibrating cantilever biosensors have already been described in the literature, but have certain disadvantages associated with them, such as complicated support electronics, low portability, adhesion problems, and possible interference effects. For a piezoresistive cantileverbased sensor to operate as a biosensor, a biologically active sensing layer, which ``swells'' upon exposure to an analyte is needed. For example, alkanethiol molecules will form selfassembled layers on gold ®lm surfaces [16,17]. These molecules chemisorb on the gold surface through the thiol headgroup to form densely packed, robust, well ordered layers. The bio-sensing properties of these ®lms may then be controlled by choosing the appropriate chemical or biological functionality of the alkanethiol molecules terminal functional group. A piezoresistive cantilever in contact with this layer may experience a measurable strain as analyte moieties attempt to bind with the bio-sensing layer. We are currently working on testing of such a device. Work has also been done on DNA self-assembled monolayers. In these devices, single-stranded DNA molecules immobilized on a substrate surface act as the biological probes [18]. Thiol-derivatized DNA may readily bind to gold surfaces. Each speci®c DNA sequence then acts as a surface receptor site for its complimentary strand, which will bind strongly to the appropriate receptor site. A piezoresistive cantilever in contact with this surface-bound DNA layer may sense the additional analyte DNA binding to the sensor layer. Arrays of individual sensors could again be easily assembled. Alternately, various biological bilayers containing vesicles or lipids may be assembled which undergo large physical changes when receptor sites pick up speci®c species in solution, lending themselves to sensing applications using microcantilevers. Such bilayers, with the correct receptors, could ``swell'' when the chosen analyte molecule or virus enters the layer. Piezoresistive microcantilever-based sensors could be operated in both gaseous and liquid environments. Once the initial contact between the cantilever and the active sensing layer/material is established, the environment surrounding each sensor element is no longer critical. Also, piezoresistive microcantilevers can be manufactured using standard semiconductor fabrication techniques. If multiple cantilevers are produced on a single chip, arrays of individual sensors are easily assembled (Fig. 4). In the case of polymer-based chemical sensors, deposition techniques
T.L. Porter et al. / Sensors and Actuators A 88 (2001) 47±51
Fig. 4. Arrays of piezoresistive cantilevers may be fabricated onto a single chip using standard semiconductor processing techniques. This makes possible the simple construction of multi-element cantilever sensor arrays.
such as common ink-jet technology may be used to produce close-packed beads of various polymers on a ¯at substrate [7]. The cantilever array is then mechanically positioned in contact with the polymer sensing layer, and ®xed in place. A single multiplexing ohmmeter may then be used to extract the individual sensor responses. In the case of biosensor arrays, tiny regions on a ¯at substrate are functionalized with an appropriate antibody/vesicle sensing layer. Contact with the cantilever array is then made in the same fashion as with the chemical sensor array. 4. Conclusions We have designed and tested a new type of microsensor device that incorporates advantages of microcantileverbased sensors and chemiresistor-based sensors. These sensors make use of piezoresistive cantilevers in contact with sensing layers that ``swell'' upon analyte exposure. Using the organic polymers PVA, PEVA, and PIB, we have fabricated sensors that respond uniquely to the analytes water vapor, ethanol, hexane, toluene and acetone. These sensors are small, portable, may operate in gaseous as well as liquid environments, and require only simple support electronics. Also, this type of sensor may also be used in certain biosensing applications. Acknowledgements This work is supported by the National Science Foundation (DMR-0071672 and DMR-9703840), Sandia National Laboratories, and the NASA Space Grant Consortium. We would also like to thank R.C. Hughes (Sandia National Laboratories) for polymer preparation and helpful discussions. References [1] D.R. Baselt, G.U. Lee, R.J. Colton, Biosensor based on force microscope technology, J. Vac. Sci. Technol. 14 (2) (1996) 789±793.
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[2] E.A. Wachter, T. Thundat, Micromechanical sensors for chemical and physical measurements, Rev. Sci. Instrum. 66 (6) (1995) 3662±3667. [3] M.C. Longeran, E.J. Severin, B.J. Doleman, S.A. Beaber, R.H. Grubbs, N.S. Lewis, Array-based vapor sensing using chemically sensitive carbon black-polymer resistors, Chem. Mater. 8 (1996) 2298±2312. [4] P. Talik, M. Zabkowskawaclawek, W. Maclawek, J. Mater. Sci. 27 (1992) 6807. [5] G.R. Ruschau, R.E. Newnham, J. Runt, B.E. Smith, Sens. Actuators 20 (1989) 269. [6] B. Lundberg, B. Sundqvist, Resistivity of a composite conducting polymer as a function of temperature, pressure, and environment: applications as a pressure and gas concentration transducer, J. Appl. Phys. 60 (1986) 1074. [7] M.P. Eastman, R.C. Hughes, G. Yelton, A.J. Ricco, S.V. Patel, M.W. Jenkins, Application of the solubility parameter concept to the design of chemiresistor arrays, J. Electrochem. Soc. 146 (10) (1999) 3907± 3913. [8] R.H. Norman, Conductive Rubbers and Plastics, Elsevier, Amsterdam, 1970. [9] G. Harsanyi, Polymer Films in Sensor Applications: Technology, Materials, Devices and their Characteristics, Technomic Publishing Co., Lancaster, 1995. [10] T.L. Porter, M.P. Eastman, D.L. Pace, M. Bradley, Polymer-based materials to be used as the active element in microsensors: a scanning force microscopy study, Scanning, in press. [11] T. Thundat, R.J. Warmack, G.Y. Chen, D.P. Allison, Thermal and ambient-induced deflections of scanning force microscope cantilevers, Appl. Phys. Lett. 64 (21) (1994) 2894±2896. [12] J. Fritz, M.K. Baller, H.P. Lang, H. Rothuizen, P. Vettiger, R. Meyer, H.-J. Guntherodt, C. Gerber, J.K. Gimzewski, Translating biomolecular recognition into nanomechanics, Science 288 (2000) 316±318. [13] Private communication, Park Scientific Instruments. Inc., Sunnyvale, CA, USA. [14] D.W.V. Krevelen, Properties of Polymers, Elsevier, New York, 1976. [15] E.A. Grulke, Polymer Handbook, Wiley, New York, 1989. [16] G.E. Poirier, E.D. Pylant, The self-assembly mechanism of alkanethiols on Au(1 1 1), Science 272 (1996) 1145. [17] G.E. Poirier, Characterization of organosulfur molecular monolayers on Au(1 1 1) using scanning tunneling microscopy, Chem. Rev. 97 (4) (1997) 1117. [18] T.M. Herne, M.J. Tarlov, Characterization of DNA probes immobilized on gold surfaces, J. Am. Chem. Soc. 119 (1997) 8916±8920.
Biographies Timothy L. Porter, Professor, Department of Physics, Northern Arizona University. PhD obtained from Arizona State University, Tempe, AZ in 1988. Current research interests include inorganic/organic host±guest composite matcrials, prebiotic synthesis of oligopeptides and oligonucleotides, microsensor design and fabrication, scanning force rnieroscopy, and matrix assisted laser desorption/ionization time of flight mass spectrometry. Michael P. Eastman, Regents Professor, Department of Chemistry, Northern Arizona University. PhD obtained from Cornell University, Ithaca, NY in 1968. Current research interests include chemical sensors, electron spin resonance spectroscopy, synthesis of novel inorganic/organic composite materials, clay minerals, and prebiotic chemistry. Danielle L. Pace is student at Northern Arizona University, majoring in Chemistry. Michelle Bradley is also student at Northern Arizona University, majoring in Chemistry.
Sensor based on piezoresistive microcantilever technology T.L. Porter*, M.P. Eastman1, D.L. Pace, M. Bradley Department of Physics, Northern Arizona University, Flagstaff, AZ 86011, USA Received 12 June 2000; received in revised form 21 August 2000; accepted 24 August 2000
Abstract A new type of micromechanical sensor has been developed that incorporates piezoresistive microcantilevers, such as those used in scanning force microscope (SFM) instruments. In these devices, the microcantilever is in direct contact with an active sensing material, which may be a common organic polymer or a biologically active sensing layer. Upon analyte exposure, the cantilever directly measures the swelling or volume expansion experienced by an active sensing material. Individual microcantilever sensors may be incorporated into an array in which pattern recognition techniques are used to identify a wide variety of chemical or biological analytes. In addition, these devices are compact, lightweight, and require only simple, inexpensive support electronics. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Microcantilever; Pezoresistive; Chemiresistor; Polymer
1. Introduction A number of advancements have been made in recent years in the development of microsensors capable of recognizing chemical or biological agents [1±3]. One new class of chemical sensors makes use of the ``swelling'' that many common organic polymers such as poly(vinyl acetate) (PVA), poly(iso-butylene) (PIB), or poly(ethylene vinyl acetate) (PEVA) undergo as analyte vapor molecules partition into the material [3±7]. In order for these polymer-based sensors to operate, the response to the analyte vapor, i.e. the polymer swelling, must be reliably measured. Incorporation of carbon black particles into the polymer may be used to provide a means to measure this swelling [4±6,8]. Upon incorporation of the carbon black particles, the normally non-conducting polymer becomes partially conductive. When the composite material swells due to analyte exposure, the conductivity of the polymer/carbon composite material drops. If this chemiresistor material is deposited onto suitable electrodes, such as an interdigitated array, the conductivity changes due to analyte exposure can be readily measured [3,7]. Other chemiresistor-based sensors use conductive polymers as their active element, these devices do not require the addition of materials such as carbon black [9]. The largest measurable responses occur when the *
Corresponding author. Tel.: 1-520-523-2661; fax: 1-520-523-1371. E-mail address: [email protected] (T.L. Porter). 1 Co-corresponding author.
percentage of carbon black loading into the polymer is such that the swelling that occurs during analyte exposure drops the carbon conductivity through its percolation threshold [3]. Arrays of these sensors may be produced, using 10 or more carbon loaded common organic polymers as sensing elements [3,7]. In such an array, many of the individual sensing elements may respond to a given analyte vapor, but each analyte will produce a particular, unique signature when the degree of response for each element in the array is considered. Pattern recognition algorithms enable sensor arrays to identify different vapor analytes. There are some disadvantages to these chemiresistorbased sensors. Recent scanning force microscope (SFM) studies indicate that the carbon particles may be expelled over time from the host polymer matrix as the material undergoes many swelling/recovery cycles [10]. Over time, this effect could render the sensor less sensitive, or perhaps completely inoperative. The carbon particles themselves may absorb analyte vapors, with subsequent slow release of these vapors, extending the recovery times for the sensors [7]. Fabrication and calibration of these devices may be dif®cult due to the precise ratios of the materials needed, and the spatial distribution/homogeneity required for the carbon particles within the polymer matrix. Microcantilever-based sensors are also a current interest. In these devices, an SFM type cantilever is coated with a chemically or biologically active material [1,2,11,12]. In many of these devices, the cantilever is then driven (by external circuitry) into oscillation at a resonance frequency.
0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 0 ) 0 0 4 9 8 - 2
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T.L. Porter et al. / Sensors and Actuators A 88 (2001) 47±51
Analyte molecules bind or adsorb to the active layer on the cantilever, increasing the mass of the vibrating cantilever slightly. The increase in mass results in a slight shift in the cantilever vibration frequency or amplitude, measurable by external means. Depending on the active sensing layer used, both chemical species and certain biological species may be detected. While arrays of these ``vibrating cantilever'' sensors may be fabricated, the external circuitry required to both drive the cantilevers and measure their frequency/ amplitude shifts may be both cumbersome and expensive. Crosstalk and interference between close-packed arrays of vibrating cantilevers may also result in unwanted or spurious signals. Finally, the sensing layer itself may not adhere over time to a cantilever bending or oscillating at tens of hundreds of kiloHertz. In this paper, we describe a new type of microsensor that utilizes piezoresistive microcantilevers to directly measure analyte produced swelling or volumetric changes in a tiny bead of active material. The active sensing material may be an organic polymer, or a biologically active layer. These microsensors may be fabricated in small arrays, operated in gaseous as well as liquid environments, and operated with only simple, inexpensive support electronics.
elements in our tests. No carbon black or other compounds were mixed with the polymers. Piezoresistive micro-cantilevers were obtained from Park Scienti®c, Inc. (now Thermomicroscopes, Inc., Sunnyvale, CA). These cantilevers were designed for use in contactmode SFM, and carried a nominal piezo-channel resistance of 2 kO. The tip shape was pyramidal (no sharpening). The cantilevers were used as is, without any further modi®cation. Resistance changes during sensing experiments were obtained with standard precision multimeters. The sensing experiments were carried out in a modi®ed scanning probe microscope instrument (SPM) from Thermomicroscopes (Fig. 1). Dry nitrogen was passed through two ¯ow meters, one for controlling the background nitrogen gas level, and the other for controlling the percentage of saturated analyte vapor that was added to the background gas. Saturated analyte vapor was obtained by passing clean, dry nitrogen through a gas bubbler assembly containing the analyte liquid. The analytes used in these experiments included water vapor, ethanol, hexane, toluene and acetone. Exposures of the sensing polymers to the analyte vapors ranged from 1 to 3 min., and recovery times (dry nitrogen exposure only) ranged up to 5 min.
2. Experimental
3. Results and discussion
Organic polymers PVA, PIB, or PEVA were prepared by dissolving the polymers using the solvents deionized distilled water, chlorobenzene, or chloroform. These were then deposited onto stainless steel substrates and allowed to dry. The polymers used were purchased from Aldrich Chemical Company (Milwaukee, WI) or Scienti®c Polymer Products (Ont., NY), and used as received. The thickness of these ®lms as measured by stylus pro®lometry was approximately 2 mm. These ®lms were then used as the active sensing
A diagram of a single piezoresistive sensor element is shown in Fig. 2. An active sensing layer, or ``generalized sensing element'' is deposited onto the substrate (i.e. stainless steel or polished Si). The lateral dimensions of this layer may be as small as a fraction of a micrometer. The only requirement for the composition of this active sensing layer is that it swell, or expand in the vertical direction upon analyte exposure. Using a simple mechanical approach mechanism, the piezoresistive microcantilever is brought
Fig. 1. Schematic diagram of modified SPM used for piezoresistive sensor element testing.
T.L. Porter et al. / Sensors and Actuators A 88 (2001) 47±51
49
Fig. 2. Diagram of single element sensor. The generalized sensor element consists of any layer (organic polymer or other) that undergoes ``swelling'' upon analyte exposure.
into contact with the surface of this sensing layer (a highpitch screw mechanism is suf®cient for this task). These cantilevers are only 100±200 mm long, and about 50 mm wide. They contain an internal channel of piezoresistive material, connected to two tiny external electrodes. The zero-strain resistance of these cantilevers is on the order of 2 kO [13], but changes rapidly and measurably in response to any bending of the cantilever. In fact, these cantilevers are sensitive enough to measure bending strains of only a few Ê (a typical response for one of these cantilevers is 1± tens of A 4 mO for each angstrom of de¯ection). Any swelling of the active layer in contact with the cantilever tip will result in an immediate, easily measurable change in the cantilever channel resistance. This change will be in exact proportion to the amount of the vertical swelling; a simple multi-meter is thus suf®cient to record the sensing activity. As a test of this sensor design, we prepared piezoresistive microcantilever sensor elements utilizing the three polymers (or elastomers) PVA, PIB, or PEVA. Each of these was exposed to the vapors of water, ethanol, hexane, toluene and acetone (these analytes were chosen due to their wide range of solubility parameters). The response of these sensor elements when exposed to the analyte vapors under the conditions P=Psat 0:5 is summarized in Table 1 (measured in Ohms). The exposure consisted of a 50±50 mix of saturated analyte vapor with dry nitrogen gas. In most cases, the sensor response was achieved almost instantly. Note that while all three polymers responded to most of the analytes, a unique ``pattern'' for each individual analyte is obtained. Table 1 Cantilever response measured in Ohms for the three polymers PVA, PEVA, and PIB during exposure to the analyte vapors indicateda Polymer
Water
Hexane
Ethanol
Toluene
Acetone
PVA PEVA PIB
0.52 0.11 0.0
0.07 0.73 0.98
0.23 0.12 0.05
0.26 1.02 0.58
0.14 0.29 0.23
a Each analyte produces a unique response from the ``array'' of three individual sensors.
Fig. 3. (A) Plot of sensor response vs. solubility parameter for the three sensors used in our study (squares: PIB, circles: PEVA, double triangles: PVA). (B) Resistance measurements during exposure to hexane at P=Psat 0:1, 0.2, 0.3, 0.4, and 0.5 for the PIB-based sensor.
In general, the swelling of an organic polymer as a result of the partitioning in of gaseous molecules may be modeled using Hildebrand solubility parameter concepts [14]. Each of the analytes used in our experiments is associated with a particular solubility parameter, ranging from a high of 48 MPa1/2 for water vapor to a low of 16.7 MPa1/2 for hexane [15]. Partitioning of the analyte molecules into the polymer is postulated to occur to the greatest extent when there is a close match between the polymer and the analyte solubility parameters [7]. Correspondingly, less partitioning occurs the greater the difference in these two numbers. A plot of sensor response versus solubility parameter for the three sensors used in our study is presented in Fig. 3A. For example, in our study the polymer PIB (solubility parameter of 15.5 MPa1/2) experienced the largest swelling (and concomitant cantilever resistance change of 0.98 O, or Ê swelling) upon exposure to hexane vapor (solubility 500 A parameter 16.7 MPa1/2). The other large responses (0.58 and 0.23 O, respectively) occur during exposure to toluene vapor (18.2 MPa1/2) and acetone vapor (20.5 MPa1/2). We obtain little or no response to either ethanol (26 MPa1/2) or water vapor (48 MPa1/2). Chemiresistor work by Eastman et al. [7] on PIB-based sensors shows the largest drops in conductivity (swelling) occur upon exposure to the vapors of hexane, acetone, and toluene. A much smaller conductivity drop is obtained during ethanol exposure, and a zero response for water vapor was noted [7].
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For our PVA-based cantilever sensor (solubility parameter 19.62 Mpa1/2), the largest responses occurred for water vapor (0.52 O), toluene (0.26 O), and ethanol (0.23 O). A smaller response occurred for acetone vapor, and we measured little cantilever response in the case of hexane exposure. These results also compare favorably with previous data utilizing PVA in chemiresistor-based sensors. In the chemiresistor study by Longeran, et al. [3], a PVA/carbon black composite chemiresistor sensor showed the largest response (resistance drop) for acetone exposure, a moderate response for ethanol and toluene, and only a tiny response to hexane vapor. Water vapor was not used in this study. In the study by Eastman et al. [7], a similar PVA/carbon black composite chemiresistor sensor also showed a large response to water vapor, and smaller responses to all other analytes. We note that in the Eastman et al. [7] study, only a negligible response to acetone was reported, however our data more closely tracks the large response noted in the Longeran et al. [3] study. This difference in the two chemiresistor sensor studies may be attributable to differences in carbon loading, i.e. small percentage differences in the carbon black component may result in large differences in sensor response for a given analyte. In the case of PEVA-based chemiresistors (solubility parameter 18.6), the Longeran et al. [3] data showed large responses for toluene, hexane, and acetone (in order or response magnitude), with a smaller response to ethanol (water vapor was not tested). For a similar PEVA-based chemiresistor in the study by Eastman et al. [7], the largest responses also occurred for acetone, hexane and toluene. Somewhat lesser responses were measured for ethanol. The smallest response was measured for water vapor. Our measured responses once again agree well with this previous chemiresistor data, with large responses for toluene (1.02 O), hexane (0.73 O), and acetone (0.29 O). We measure a smaller response for ethanol exposure as well as exposure to water vapor. For the three polymers used in our study, generally good agreement is obtained with the solubility parameter concept, i.e. the largest swelling responses occur when the solubility parameter of the analyte is close to the solubility parameter for the polymer. Also, our data tracks previous chemiresistor response data well, with the exceptions noted above. It is clear (in theory at least) that the solubility parameter concept presents a method that will allow for the detection of virtually all known solvent vapors by an array containing only a relatively small number of distinct polymer-based cantilever sensors [3,7]. For such an array, pattern recognition techniques are used to recognize the unique response from the individual polymer sensors for each different analyte. We have also obtained preliminary data regarding the linearity of these piezoresistive microcantilever sensors (Fig. 3B). Using the polymer PIB, we obtained resistance measurements during exposure to hexane at P=Psat 0:1, 0.2, 0.3, 0.4, and 0.5. The cumulative resistance increases
were 0.04, 0.14, 0.27, 0.48, and 0.68 O. We did not achieve the full 0.98 O response at P=Psat 0:5, however, due to large variations in the PIB ®lm thickness. While this measured response to hexane is not linear (especially in the lowest exposure ranges), it is reproducible and therefore may be easily used in a pattern recognition scheme. While the data obtained clearly demonstrates the operation of piezoresistive cantilever-based microsensors, there may be other advantages to this general form of sensor operation. It may also be possible to use piezoresistive-based microsensors for biological sensing applications. As previously mentioned, vibrating cantilever biosensors have already been described in the literature, but have certain disadvantages associated with them, such as complicated support electronics, low portability, adhesion problems, and possible interference effects. For a piezoresistive cantileverbased sensor to operate as a biosensor, a biologically active sensing layer, which ``swells'' upon exposure to an analyte is needed. For example, alkanethiol molecules will form selfassembled layers on gold ®lm surfaces [16,17]. These molecules chemisorb on the gold surface through the thiol headgroup to form densely packed, robust, well ordered layers. The bio-sensing properties of these ®lms may then be controlled by choosing the appropriate chemical or biological functionality of the alkanethiol molecules terminal functional group. A piezoresistive cantilever in contact with this layer may experience a measurable strain as analyte moieties attempt to bind with the bio-sensing layer. We are currently working on testing of such a device. Work has also been done on DNA self-assembled monolayers. In these devices, single-stranded DNA molecules immobilized on a substrate surface act as the biological probes [18]. Thiol-derivatized DNA may readily bind to gold surfaces. Each speci®c DNA sequence then acts as a surface receptor site for its complimentary strand, which will bind strongly to the appropriate receptor site. A piezoresistive cantilever in contact with this surface-bound DNA layer may sense the additional analyte DNA binding to the sensor layer. Arrays of individual sensors could again be easily assembled. Alternately, various biological bilayers containing vesicles or lipids may be assembled which undergo large physical changes when receptor sites pick up speci®c species in solution, lending themselves to sensing applications using microcantilevers. Such bilayers, with the correct receptors, could ``swell'' when the chosen analyte molecule or virus enters the layer. Piezoresistive microcantilever-based sensors could be operated in both gaseous and liquid environments. Once the initial contact between the cantilever and the active sensing layer/material is established, the environment surrounding each sensor element is no longer critical. Also, piezoresistive microcantilevers can be manufactured using standard semiconductor fabrication techniques. If multiple cantilevers are produced on a single chip, arrays of individual sensors are easily assembled (Fig. 4). In the case of polymer-based chemical sensors, deposition techniques
T.L. Porter et al. / Sensors and Actuators A 88 (2001) 47±51
Fig. 4. Arrays of piezoresistive cantilevers may be fabricated onto a single chip using standard semiconductor processing techniques. This makes possible the simple construction of multi-element cantilever sensor arrays.
such as common ink-jet technology may be used to produce close-packed beads of various polymers on a ¯at substrate [7]. The cantilever array is then mechanically positioned in contact with the polymer sensing layer, and ®xed in place. A single multiplexing ohmmeter may then be used to extract the individual sensor responses. In the case of biosensor arrays, tiny regions on a ¯at substrate are functionalized with an appropriate antibody/vesicle sensing layer. Contact with the cantilever array is then made in the same fashion as with the chemical sensor array. 4. Conclusions We have designed and tested a new type of microsensor device that incorporates advantages of microcantileverbased sensors and chemiresistor-based sensors. These sensors make use of piezoresistive cantilevers in contact with sensing layers that ``swell'' upon analyte exposure. Using the organic polymers PVA, PEVA, and PIB, we have fabricated sensors that respond uniquely to the analytes water vapor, ethanol, hexane, toluene and acetone. These sensors are small, portable, may operate in gaseous as well as liquid environments, and require only simple support electronics. Also, this type of sensor may also be used in certain biosensing applications. Acknowledgements This work is supported by the National Science Foundation (DMR-0071672 and DMR-9703840), Sandia National Laboratories, and the NASA Space Grant Consortium. We would also like to thank R.C. Hughes (Sandia National Laboratories) for polymer preparation and helpful discussions. References [1] D.R. Baselt, G.U. Lee, R.J. Colton, Biosensor based on force microscope technology, J. Vac. Sci. Technol. 14 (2) (1996) 789±793.
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[2] E.A. Wachter, T. Thundat, Micromechanical sensors for chemical and physical measurements, Rev. Sci. Instrum. 66 (6) (1995) 3662±3667. [3] M.C. Longeran, E.J. Severin, B.J. Doleman, S.A. Beaber, R.H. Grubbs, N.S. Lewis, Array-based vapor sensing using chemically sensitive carbon black-polymer resistors, Chem. Mater. 8 (1996) 2298±2312. [4] P. Talik, M. Zabkowskawaclawek, W. Maclawek, J. Mater. Sci. 27 (1992) 6807. [5] G.R. Ruschau, R.E. Newnham, J. Runt, B.E. Smith, Sens. Actuators 20 (1989) 269. [6] B. Lundberg, B. Sundqvist, Resistivity of a composite conducting polymer as a function of temperature, pressure, and environment: applications as a pressure and gas concentration transducer, J. Appl. Phys. 60 (1986) 1074. [7] M.P. Eastman, R.C. Hughes, G. Yelton, A.J. Ricco, S.V. Patel, M.W. Jenkins, Application of the solubility parameter concept to the design of chemiresistor arrays, J. Electrochem. Soc. 146 (10) (1999) 3907± 3913. [8] R.H. Norman, Conductive Rubbers and Plastics, Elsevier, Amsterdam, 1970. [9] G. Harsanyi, Polymer Films in Sensor Applications: Technology, Materials, Devices and their Characteristics, Technomic Publishing Co., Lancaster, 1995. [10] T.L. Porter, M.P. Eastman, D.L. Pace, M. Bradley, Polymer-based materials to be used as the active element in microsensors: a scanning force microscopy study, Scanning, in press. [11] T. Thundat, R.J. Warmack, G.Y. Chen, D.P. Allison, Thermal and ambient-induced deflections of scanning force microscope cantilevers, Appl. Phys. Lett. 64 (21) (1994) 2894±2896. [12] J. Fritz, M.K. Baller, H.P. Lang, H. Rothuizen, P. Vettiger, R. Meyer, H.-J. Guntherodt, C. Gerber, J.K. Gimzewski, Translating biomolecular recognition into nanomechanics, Science 288 (2000) 316±318. [13] Private communication, Park Scientific Instruments. Inc., Sunnyvale, CA, USA. [14] D.W.V. Krevelen, Properties of Polymers, Elsevier, New York, 1976. [15] E.A. Grulke, Polymer Handbook, Wiley, New York, 1989. [16] G.E. Poirier, E.D. Pylant, The self-assembly mechanism of alkanethiols on Au(1 1 1), Science 272 (1996) 1145. [17] G.E. Poirier, Characterization of organosulfur molecular monolayers on Au(1 1 1) using scanning tunneling microscopy, Chem. Rev. 97 (4) (1997) 1117. [18] T.M. Herne, M.J. Tarlov, Characterization of DNA probes immobilized on gold surfaces, J. Am. Chem. Soc. 119 (1997) 8916±8920.
Biographies Timothy L. Porter, Professor, Department of Physics, Northern Arizona University. PhD obtained from Arizona State University, Tempe, AZ in 1988. Current research interests include inorganic/organic host±guest composite matcrials, prebiotic synthesis of oligopeptides and oligonucleotides, microsensor design and fabrication, scanning force rnieroscopy, and matrix assisted laser desorption/ionization time of flight mass spectrometry. Michael P. Eastman, Regents Professor, Department of Chemistry, Northern Arizona University. PhD obtained from Cornell University, Ithaca, NY in 1968. Current research interests include chemical sensors, electron spin resonance spectroscopy, synthesis of novel inorganic/organic composite materials, clay minerals, and prebiotic chemistry. Danielle L. Pace is student at Northern Arizona University, majoring in Chemistry. Michelle Bradley is also student at Northern Arizona University, majoring in Chemistry.