Electrically stretched capacitive membranes for stiffness sensing and analyte concentration measurement

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Abstract

A novel method for stiffness sensing is developed using an electret capacitive sensor. The electret membrane is coated with a recognition layer that responds with a change in its stiffness/elasticity in the presence of a target analyte. Since the electret membrane is stretched by electrostatic pulling towards a metallic base plate, a change in stiffness of the composite membrane results in deflection of the membrane. This deflection is measured as a change in capacitance of the sensor. The sensitivity of the sensor to stiffness changes depends on the strength of the preset electric field. The developed sensor operates in a quasi-static mode and eliminates the need for resonant monitoring. The sealed capacitive sensor is ideal for monitoring analytes in both gas and liquid environments. The final sensor package with the capacitance measurement circuitry has a low power consumption (<30 mW). A proof-of-concept carbon dioxide gas concentration sensor is developed by coating the electret sensor with a single-walled nanotube film whose stiffness changes in the presence of carbon dioxide. Experimental results prove the viability of the sensing technique.

Introduction

There is enormous interest in development of solid-state analyte sensing techniques that are inexpensive, sensitive and simultaneously robust to interfering species. Several types of solid-state sensors exist for sensing concentration of analytes such as gases, biological molecules, etc. These sensors may be classified based on the physical quantity that is measured in response to analyte concentration changes. The physical quantities used in the past include a material’s resistance [1], mass [2], dielectric constant [3], elasticity [4], electrolytic properties [5], etc. Several methods further have been developed for the measurement of a single physical quantity itself. For example, resistance measurement has been performed using simple current measurement as well as using surface acoustic wave sensors [6] and field-effect transistors [7]. Similarly, dielectric constant measurement has been performed using capacitance measurement [8] as well as resonant sensors [9].

In this paper we report a novel sensing platform for detecting stiffness/elasticity changes in recognition layers for analyte sensing. Among other techniques, stiffness sensing has gained importance since the discovery of their potential for selective analyte sensing. It has been proven that a recognition layer’s stiffness changes due to intermolecular reorientation in the presence of an analyte. Stiffness measurements have already been proven to be useful for gas sensing [10], DNA detection [11] and cancer testing [12].

However, unlike physical properties such as conductivity, dielectric constant and mass; techniques for portable and sensitive measurement of stiffness are not well-developed. For example, the conventional method of stiffness measurement requires optical methods for detecting cantilever deflection in the presence of an analyte [13], [14]. While the solid-state sensing element itself is small, the use of large optical systems complicates its use in portable apparatus. Indeed, a chief motivation behind solid-state sensing research is the reduction in cost. However, optical analyzers typically increase the system cost unacceptably.

Apart from the disadvantages of optical sensing, the cantilever design of conventional stiffness sensors also suffers from a few drawbacks. Cantilever-based methods typically have the disadvantage of non-specific analyte adsorption on the uncoated surface in liquid environments [15]. In gaseous environments, capacitive measurements of cantilever deflection are affected by gas seepage due to change in dielectric constant between the electrodes. Further, cantilever actuation and position measurement require development of independent mechanisms. Most techniques also require frequency sweeping for resonant frequency identification. Such analyzers increase the power requirement of the final sensor. Researchers have used piezoelectric [16], thermal and magnetic actuation [17], [18] of resonance in cantilevers. Piezoelectric, capacitive [19] and piezoresistive [20] monitoring of cantilever motion have also been published. However, all these methods suffer from the one or more of the drawbacks discussed above.

Attempts have been made to find alternate methods of stiffness sensing for the above reasons. Metal oxide semiconductor (MOS) transistors have been used to detect change in stiffness of piezoresistive films [21]. But, such methods apply only to large changes in surface stress of mechanical members. Also, the measurement of resistance change used in these techniques is not suitable for analyte sensing, when the recognition layer changes its resistance along with its stiffness in the presence of analytes (as seen in carbon nanotubes). Other techniques using SAW sensors [22] too are not suitable for monitoring elastic property changes of recognition layers because of their extreme sensitivity to conductivity changes in such films.

Recently, Satyanarayana et al. [15], Rodriguez et al. [23] and Tsouti et al. [24] have suggested capacitive measurement of curvature changes in surface micromachined membranes for measurement of stiffness variations in the presence of analytes. While such capacitive techniques eliminate the need for optical measurement and membrane vibration, the sensitivities of proposed sensors are limited by the membrane stiffness post-micro-fabrication. This work develops a capacitive stiffness sensor using electrically stretched capacitive membranes. Electrostatic stretching of membranes is used to augment their sensitivity to stiffness changes as discussed in the following section. Subsequently, an electret sensor with a precharged membrane is shown to be an excellent solution for self-actuated stiffness measurement. Finally, as proof-of-concept, one embodiment of the sensor coated with single-walled carbon nanotubes (SWNTs) is shown to track changes in carbon dioxide (CO2) concentration.

Section snippets

Sensitivity enhancement using electrostatic actuation

This section describes the sensing principle for stiffness measurement using electrically stretched capacitive membranes. As mentioned earlier, capacitive stiffness measurement techniques permit realization of miniature analyte sensors. Micromachined capacitive membranes are typically pre-stressed during microfabrication due to the high temperature deposition of the membranes and its subsequent cooling down to room temperature. A change in the capacitance of the sensor is effected by the change

Design of stiffness sensor

Membranes with high stiffness require large bias voltages to increase the sensitivity. Indeed, Eq. (5) suggests that sensitivity reduces with increasing membrane stiffness for a given bias potential. Hence, it is beneficial to use a membrane with low nominal stiffness. A membrane with low nominal stiffness is also required for studying stiffness changes in mechanically soft recognition layers. This is because the stiffnesses of the membrane and the recognition layer act in a parallel

Experimental setup

MEO96PD-00-604-NF FET-less omnidirectional electret condenser microphones obtained from ICC Intervox are used as stiffness sensors in this work. The nominal capacitance of these microphones is measured to be around 10 pF. Other no-FET back-electret microphones such as the TSB-1460, -160, -165, etc. from Transound International could also be used equally. Capacitance measurement is achieved using MS3110 universal capacitive readout IC from Irvine Sensors Corporation. The capacitance readout IC

Results and discussion

The measured change in the microphone’s capacitance is almost entirely due to the stiffness change of the SWNT-coated electret membrane. The effect of mass change due to CO2 adsorption is found to be negligible upon estimating the force exerted on the membrane due to mass change. The approximate force is computed by assuming an average diameter of 453 pm for a CO2 molecule [29]. Given that the diameter of the microphone’s electret membrane is less than 1 cm, it is possible to calculate the

Conclusion

This paper presents the application of a stiffness monitoring technique using an electret microphone for portable CO2 gas sensing. Compared to earlier resonant frequency techniques, the presented method greatly simplifies measurement, reduces size and cost and improves the ease of use. Though the sensor is designed to specifically respond to CO2 gas by coating the electret microphone with SWNTs, the developed technique could be equally used for other sensing applications by appropriately

Acknowledgements

TMP acknowledges the National Science Foundation (DUE-0535763) for financial support and Owen R. Kinsky for assistance in the preparation of P3HT. SS and RR thank Prof. Kent R. Mann and Prof. Xun Yu for advice and help on the experimental setup. Film fabrication and characterization were performed at the Nano-fabrication Center and the Characterization Facility at the University of Minnesota, Twin Cities which are supported by the NSF’s National Nanotechnology Infrastructure Network (NNIN).

Shyam Sivaramakrishnan is pursuing a Ph.D. in mechanical engineering at the University of Minnesota, Twin Cities. He received his B.Tech. degree in mechanical engineering from the Indian Institute of Technology, Madras, in 2004. His research interests include carbon nanotube sensors, passive wireless sensors and surface acoustic wave devices.

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    Shyam Sivaramakrishnan is pursuing a Ph.D. in mechanical engineering at the University of Minnesota, Twin Cities. He received his B.Tech. degree in mechanical engineering from the Indian Institute of Technology, Madras, in 2004. His research interests include carbon nanotube sensors, passive wireless sensors and surface acoustic wave devices.

    Rajesh Rajamani received his B.Tech. degree from the Indian Institute of Technology, Madras, in 1989, and his M.S. and Ph.D. degrees from the University of California at Berkeley, in 1991 and 1993, respectively, all in Mechanical Engineering. He is currently a Professor in the Department of Mechanical Engineering at the University of Minnesota. His research interests include sensors for biomedical and automotive applications.

    Ted M. Pappenfus received his B.A. degree in chemistry from St. John’s University (Collegeville, MN) in 1995 and then his Ph.D. in inorganic chemistry from the University of Minnesota in 2001 while working with Kent R. Mann. Following his post-doctoral research with William H. Smyrl at the University of Minnesota, he became an assistant professor of chemistry at the University of Minnesota, Morris. His research interests include the design and synthesis of organic and inorganic materials.

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