CNTs are novel 1-D nanostructures and offer unique capabilities for sensing stud- ies. The combination of useful mechanical, electrical, thermal, and optical prop- erties of CNTs can be made use of for building nanoscale sensors for a range of
analytes such as gases, chemicals, and biomolecules. Integration of such structures into micro- and nanoelectromechanical systems will lead to miniaturization of sen- sor devices. Although advances so far are impressive, more needs to be done to control the CNT growth and integration. CNT-FET devices show great poten- tial for biosensing and biomolecular detection. In microfluidics, combining the FET-based sensing capabilities of CNTs can be taken advantage of for building composite multifunctional devices using techniques of MEMS. Together such devices can offer unique opportunities for biosensing and bioassay studies and enable real-life pathogen detection and disease diagnosis. Combining these capa- bilities with NIR optical emission, CNTs offer great potential for targeted drug delivery and studies on cancer, where important advances have already been made.
Advances so far imply that the CNTs offer unique advantages as sensors, such as high sensitivity and selectivity for range of analytes such as gases, chemicals, bio- molecules, and pathogens. With these advances in place, the CNTs could be useful in gas detection, environment monitoring, biomedicine field, and counterterrorism applications. Meanwhile, more studies are needed to address sampling and cycling behavior for sensor devices and systems based on CNTs. With these important leads, CNT-based miniature sensor devices could find applications in industries such as aerospace, automotive, medicine, and defense.
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31
Carbon-Nanotube-Based Fluidic Shear-Stress
Sensors
Winnie W.Y. Chow, Yanli Qu, and Wen J. Li
Contents
2.1. Overview of Carbon Nanotube Sensors ...32 2.2 Types of Shear-Stress Sensors ...33 2.2.1 Direct Measurement ...33 2.2.2 Indirect Measurement ... 34 2.3 Operating Principle of the CNT Sensor Shear-Stress Sensor ...37 2.4 Dielectrophoretic Batch Manipulation of CNTs ...39 2.4.1 Theoretical Background ...39 2.4.2 Manipulation of CNTs ... 40 2.5 Integrated SWCNT Sensors in Micro-Wind Tunnel for Airflow
Shear-Stress Measurement ... 42 2.5.1 Experimental Details ... 42
2.5.1.1 Fabrication Process of the Integrated CNT
Sensor Chip ... 42 2.5.1.2 Experimental Setup ...45 2.5.2 Results and Discussions ... 46 2.5.2.1 Characteristics of SWCNTs ... 46 2.5.2.2 Sensor Response Toward Airflow Inside
a Micro-Wind Tunnel ...47 2.5.3 Summary ...50
2.1 Overview of Carbon Nanotube Sensors
Carbon nanotubes (CNTs), as one class of nanostructured materials, are getting much attention due to their remarkable mechanical and electronic properties as well as their high thermal, chemical stability, and excellent heat conduction. CNTs are as hard as diamond and have as good an electrical conductivity as graphite.
Since the electronic property of CNTs is a strong function of their atomic structure and mechanical deformation, such relationships make them useful when develop- ing extremely small sensors that are sensitive to the chemical or physical environ- ment. Hence, in recent years, various types of chemical sensors, physical sensors, and biosensors have been developed.
CNTs are usually manipulated across electrodes to form sensing elements that are used to perform sensing tasks. Manipulation techniques include direct growth, external forces, and polar molecular patterning. For example, chemiresistors are built by growing CNTs directly on the platform by chemical vapor deposition (CVD) to detect NO2 and NH3 (Qi et al. 2003). A single CNT was successfully manipulated across electrodes by the deposition of organic polar molecules (Miller and Williams 2003) and by atomic force microscopy (AFM) (Chan et al. 2005).
However, these techniques are often complex and time consuming or operate at high temperature. Therefore, a fast and efficient manipulation technique is needed for CNT sensor fabrication. In recent years, our group has developed a promising process to batch manipulate CNT bundles onto gold microelectrodes (Wong and Li 2003). Using the technique of electric-field-assisted assembly, that is, dielectropho- resis (DEP), makes the batch fabrication of nanodevices with nanotubes as compo- nents feasible. By using this technique, multiwalled carbon nanotube (MWCNT) sensors were built for different sensing purposes. Thermal (Fung et al. 2004), 2.6 Ultralow-Powered EG-CNT Sensors for Aqueous Shear-Stress
Measurement in Microfluidic Systems ...51 2.6.1 Experimental Details ...51 2.6.1.1 Sensor Design and Fabrication ...51 2.6.1.2 Experimental Setup ...53 2.6.2 Results and Discussions ...54 2.6.2.1 Characteristics of EG-CNTs ...54 2.6.2.2 Sensor Sensitivity ...57 2.6.2.3 Thermal Dissipation Principle ...60 2.6.2.4 Transient Heat Transfer under Nature Convection ...60 2.6.2.5 Dynamic Response under Forced Convection ...62 2.6.3 Summary ... 64 2.7 Comparison of Different Shear-Stress Sensors ... 64 2.8 Conclusions ...65 Acknowledgments ... 66 References ... 66
pressure (Fung et al. 2005), and alcohol (Sin et al. 2007) sensors were success- fully built. Their sensing abilities were studied and analyzed by the change of the MWCNT bundles’ resistance when a constant current (CC) was applied. Based on the same manipulation technique, we further investigate more sensing applications of the CNT sensor and develop it into different types of sensors.
2.2 Types of Shear-Stress Sensors
A fluid flowing past a surface boundary exerts normal and tangential stresses on the surface. The tangential stress is called surface or wall shear stress (Hanratty and Cambell 1996). The study of wall shear stress has been investigated for decades by fluid dynamicists, particularly in turbulent flows, because of its importance in determining the onset turbulence and in understanding fluidic drag phenomenon.
Different measurement methods for wall shear stress such as the usage of Pitot tubes and hot films were developed in the twentieth century and are commonly used by fluid dynamicists for experimental studies. Among all types of shear stress measurement devices, hot-wire sensors are widely used because they induce rela- tively small disturbance to flow field. However, for conventional hot-wire sensors, their complex fabrication techniques not only limit the size of the sensors, but also make mass production and integration difficult.
With the emergence of microelectromechanical system (MEMS) technology, the study of different kinds of flow profile in microchannels becomes feasible with the use of a micro-shear-stress sensor. It is especially important in boundary turbu- lent flow study (Kimura et al. 1999). Moreover, micro-shear-stress sensors are also used to study the shear stress of blood vessels (Soundararajan et al. 2004) and cells (Barakat et al. 2002), which can eventually help in the diagnosis of many diseases.
There are many techniques for measuring wall shear stress. They can be mainly divided into direct and indirect measurements.
2.2.1 Direct Measurement
The floating-element shear-stress sensor is one of the typical techniques for direct measurement. A sensing element is suspended by springs or anchors and flush mounted into the wall restricting the flow (see Figure 2.1). The floating element is free to displace laterally. The shear force produced by the flow acting on the float- ing element is then transduced to its displacement. An electrostatic mechanism is commonly applied to measure the floating element displacement (Pan et al. 1995, Zhe et al. 2005). The floating plate overlaps with an electrode, forming a parallel- plate capacitor. The displacement is then determined by the differential capaci- tance. Piezoresistive transduction is another technique used in floating-element sensors (Shajii et al. 1992, Barlian et al. 2006). The plate element is suspended by four piezoresistive tethers. The tethers experience shear stresses during the flow and
these stresses in turn generate axial strain fields. The measurements can be deter- mined by the change of resistance of the piezoresistive tethers. The advantage of using a floating-element sensor is that it can measure the shear stress directly from the displacement of the floating-element and no conversion or further calculation is needed to determine the shear stress. However, the fabrication process of these sensors is extremely complicated, inefficient, and time consuming. Meanwhile, the sensors suffer from complex configuration, unacceptably large chip size (several millimeters on a side), and unstable mechanical properties due to environmental condition (e.g., moisture) variations (Schmidt et al. 1988, Goldberg et al. 1994, Padmanabhan et al. 1997).
2.2.2 Indirect Measurement
In direct measurement techniques, shear stress is directly measured by the displace- ment of the floating-element sensor. On the other hand, indirect techniques require an empirical or theoretical correlation, typically valid for very specific conditions, to relate the measured property to the wall shear stress. The MEMS community has produced a variety of indirect transduction schemes such as hot-film sensors and micro-optical systems to measure near wall velocity gradients. For instance, the thermal shear-stress sensor which measures flow-imposed surface shear stress based on the amount of convective heat transfer from an electrically heated sensing element to the surrounding fluid flow (Hanratty and Cambell 1996).
Among all the measurement techniques, hot-film/hot-wire sensors are widely used in indirect measurement of shear stress. Hot-film/hot-wire sensors measure shear stress based on the thermal-transfer principle. When an element is heated, a thermal-boundary layer is developed. A thermal-boundary layer is characterized as a region where temperature gradients are present in the flow. Within the thermal- boundary layer, flow temperature decreases with increasing distance away from the heated element until the temperature reaches that of the mean stream flow. Then, the thermal element is placed at the wall in the velocity boundary layer in which
Flow profile
Floating-element
Gap
Springs
Figure 2.1 Schematic cross-sectional view of a floating-element sensor.