can effectively detect everything of interest in every possible environment. Rather, selecting the optimum sensing approach from a group of technologies may be the best method to address a sensing need. There are a range of types to choose from, i.e., within the electronic and electrochemical sensor classes, the possible sensor platforms include Schottky diodes, metal oxide semiconductors, electrochemical cells, and calorimetric devices.2 A given sensor can therefore be described from the target analyte viewpoint – sensor for CO or ; or from the platform viewpoint – surface acoustic : Engineering wave (SAW) sensors, electrochemical sensors, or optical sensors, mass based sensors, or nanowire sensors. Structures and Materials Many newer sensors are built by techniques and this provides a host of advantages including from Micro to Nano lower power consumption, small size, and weight.3 These microfabrication by Joseph R. Stetter, Peter J. Hesketh, techniques can be used to produce a and Gary W. Hunter range of different basic microsensor platforms. These microplatform structures can be tailored to optimize its Sensors science and engineering is relevant to virtually use for a given detection problem. For all aspects of life including safety, security, surveillance, example, the microfabricated pattern for monitoring, and awareness in general. Sensors are central an electrochemical cell to detect gases can be formed and repeatedly fabricated. to industrial applications being used for process control, However, varying the selection of monitoring, and safety. Sensors are also central to medicine electrolyte and electrodes to be deposited being used for diagnostics, monitoring, critical care, and public in the microstructure can result in very different gas sensor types. Figure 2 health. This article is a brief introduction to world of sensors. shows, in effect, a family tree of sensor platform approaches and the wide range Sensors are devices that produce a stress, strain, position, particles, or force; of sensor types and measurement options measurable change in output to a known (ii) chemical sensors for concentration which can result from using these input stimulus. This stimulus can be or identity of a chemical substance like platforms.2 Each chemical microsensor a physical stimulus like temperature ethanol, , gasoline, or platform has it own strengths, ideal and pressure or a concentration of other molecules; and (iii) biosensors for range of application, and provides a specific chemical or biochemical biologically active substances be they different types of information about the material. The output signal is typically cellular like toxic plague bacteria or environment. For example, different proportional to the input variable, anthrax spores, supramolecular like flu sensor platforms have different responses which is also called the measurand. For viruses, or molecular like protein toxins to the reactant gas (e.g., exponential, example, temperature sensors respond like Staphylococcus enterotoxin B (SEB). logarithmic, power law, etc.). Which with a voltage, resistance, color, or Sometimes biosensors are considered sensor class, platform, or combination other change when the temperature a subset of chemical sensors. New of platforms one uses, and how those is changed. Sensors can be used in all nanotechnology and novel materials sensors are tailored, depends on the three phases of matter although gas and provide exciting contributions to sensor needs of the application. Considerations liquid sensors are the most common. technology.1 include: Sensors are transducers, in that the There are six sensor classes typically 1. Does the application require high incoming property to be measured, , e.g. divided by the energy transduced in the sensitivity or a broader range of temperature, is changed or transduced sensor. These are optical, mechanical, detection? by the sensor into an electrical or thermal, magnetic, electronic, and other convenient output for the user. 2. Can the application needs be met electrochemical sensors. This division The thermostat in your home takes by careful choice of the operating is illustrated in Fig. 1 wherein the temperature and turns it into mechanical parameters of the sensor or will electronic and electrochemical sensor motion by unfolding a bimetallic strip a combination of technologies be classes are combined for convenience, that turns a dial calibrated to read needed to sort out the contribution and because they are often related and in units of temperature. In this way, of various similar analyte? integrated. For example, in an optical the thermal energy associated with a class chemical sensor, the presence 3. Does the application’s operating specific temperature is transduced into of a chemical stimulus modulates an environment require special mechanical motion. optical property of the sensor providing materials or fabrication procedures? Generally, sensors can be divided a change in optical signal (intensity The ECS Sensor Division typically into three major categories: (i) physical or wavelength of light). There is not a is concerned with the topic areas of sensors for properties like T, P, flow, single sensing class or technology that chemical and biochemical sensors and

66 The Electrochemical Society Interface • Spring 2006 4 Chemical and biochemical Sensors by Transducer Platform barrier layer on a semiconductor. The – Ions semiconducting properties of the silicon – Amperometry are used for sensor operation. The diode – Optical: Absorption, Emission – Potentiometry, pH Electrochemical – Conductimetry characteristics are such that current – Fiber optic evanescent wave Electronic • Voltages – Piezoelectric can easily flow in one direction and • Currents – QMB be restricted in another direction. The Radiant • Impedance Mechanical – SAW,… •Frequency • Weight metal layer serves as a gate for the diode. •Intensity • Size • Shape When a gaseous component selectively Paramagnetism – �T absorbs onto the surface of the gate layer, Hall effect – Catalytic the Schottky energy barrier will change Magnetic Thermal – Pyroelectric •Field strength • Heat flow – Calorimetric and this change can be measured and •Field Direction • Heat content correlated with the gas concentration. Electrical Signal Output The diode can be operated in a mode in which the response to changes in gas concentration is exponential. Biosensors are built on the same platform but have bio-directed [enzyme, cell, haptan, …] parts! A Schottky diode based hydrogen gas sensor has been reported.5,6 The sensor FIG. 1 Chemical and biochemical sensors by transducer platform. employs a palladium-alloy gate and the MEMS Technologies for Gas Sensors sensor is fabricated using silicon-based processing techniques. Figure 3 shows the sensor structure. Hydrogen can be SENSING selectively absorbed in a palladium- PRINCIPLE alloy gate lowering the Schottky energy barrier.4-7 The change in the diode characteristics can be used to quantify Schottky Electro-chemical Resistor Nanotechnology Ion Mobility Diode Cell the hydrogen gas presented. A platinum thin film resistance temperature detector

MEMs Particle SiC Si Single Wall ZrO NASICON (RTD) and heater are integrated into the Nanocrystalline SnO 2 Detector 2 sensor structure. This permits the sensor to be operated at a controlled elevated SiC Diode with Pd Alloy Pd Alloy Catayliticdoping undoped Amperometric Amperometric Pd Alloy Gate MOS Diode Diode temperature enhancing the response time of the sensor. With varying cell voltage The Schottky diode platform can be

Aerosols Smoke Dust HC H2 CO NO NOx CO HC H NH HC H CN CO LM O2 H2O CO2 modified to detect a range of gases by altering the gate material or changing the Fig. 2. Electronic and electrochemical gas sensor platform technology. Given a limited number of semiconductor to, for example, silicon platforms, a wide range of gases can be detected. Each platform is shown and the target detection species carbide. Changing the gate alloy can is noted for that configuration of the platform. (Courtesy of Makel Engineering, Inc.2) change the viable concentration range of hydrogen detected.6 Hydrocarbons, which dissociate at higher temperatures, can be detected by replacing the silicon substrate by a high temperature semiconductor like silicon carbide.7 Cantilever based sensors show promise for highly sensitive, low power, and compact transducers. The transduction can be based upon either surface stress, which results in bending of the cantilever, or changes in the resonant frequency due to mass changes.8 Sensitivities as low as 14 ppt have been demonstrated for explosives with selective self-assembled monolayer (SAM) coatings. Arrays of cantilevers could then provide sensitive and broad spectrum sensing platforms for FIG. 3. Schottky diode based microsensor (hydrogen sensor). complex mixtures. Arrays of sensors that are responsive to the different analytes can be integrated into a single microfabrication techniques, in addition ECS Sensor Division explores, ranging measurement system. This technology to miniaturization of chemical analysis from sensor fundamentals to the is generally known as machine olfaction systems. These topics are broadly development and application of complete or the artificial nose, and an entire ECS multidisciplinary and, in addition analytical systems. proceedings was dedicated to this topic.9 to transductions principles, include One example microsensor is a In the E-nose, sensors with a somewhat areas of chemistry and biochemistry Schottky diode hydrogen sensor. less than perfect discrimination between as well as engineering topics such as This sensor is composed of a metal analytes are combined to provide a microfabrication, microfluidics, and in contact with a semiconductor or fingerprint or selective response of the signal processing. The following are a metal in contact with a very thin examples of the types of topics that the (continued on next page)

The Electrochemical Society Interface • Spring 2006 67 Sensors... (continued from previous page)

(a) (b)

A

B

C

FIG. 4. (a) (A) silicon mold for fabrication of a miniature GC column, (B) microfabricated 1 m long parylene column, and (C) parylene column with an integrated heater. (b) Scanning electron microscopy (SEM) image of a bistable microvalve, where 1 = Orthonol base; 2 = Au microcoil; 3 = CoMnNiP magnet; 4 = membrane; 5 = circular support; and 6 = soft magnetic dome (Permalloy); 7 = membrane supported legs. analyte that can be discriminated above gas chromatography and liquid transducer. Examples include those background using pattern recognition chromatography with micromachined based upon enzymes, antibodies, methods. This approach is analogous separation columns illustrates how deoxyribonucleic acid (DNA), or cell to the biological olfactory system in miniaturization of chemical analytical membrane receptors. Proteomics is which each cell receptor responds to a methods reduces the separation time an exciting area and immunoassays number of analytes and the response to a time short enough that one may can provide medical diagnostics pattern is indicative of the odor. Sensor consider the measurement equivalent to which are suitable for analysis in a arrays could ultimately lead to artificially real-time sensing. Low thermal mass gas microfluidic format. For example, in intelligent robots that are aware of their chromatography (GC) columns, shown a sandwich assay where the primary surroundings. in Fig. 4a, require 50 mW to achieve an antibody is bound to paramagnetic A micrototal analytical system, operation temperature of 100˚C and can beads, an effective means for mixing 10 sometimes called the lab-on-a-chip, be utilized in miniature GC systems. and separation can be achieved. contains many or all the elements A microvalve is also an important Detection of bound target antigen is typically found in larger analytical building block for control in fluidic then determined with an enzyme- instrumentation including a sampling systems. A microfabricated latching labeled secondary antibody. When system with pre-concentration, sample electromechanical valve built on a an enzymatic substrate is introduced, conditioner, separation system, and a single substrate is shown in Fig. 4b. the quantity of bound complex may detection system. Sensor arrays can be The actuator is 1 mm in diameter and be determined by amperometric used for characterizing the chemical comprises a permanent magnet defined measurements with microelectrodes, or biochemical space and the sampler on a movable Permalloy membrane, thereby simplifying instrumentation and computer or data processor for data approximately 400 µm in diameter, compared to other techniques. A further acquisition, data manipulation, and supported by two cantilever beams. advantage is that the small sample display. Sensor arrays are able to mimic When current is applied to the coil a volume increases the sensitivity of the 12 our human sensor systems, i.e., the eye magnetic field is generated which attracts enzyme labeled immunoassay. Other is an optical sensor array, the nose is a the membrane, hence closing off the microfluidic approaches include the use chemical gas sensor array, the tongue is flow channel in the center. Once closed, of electrophoretic separation based upon a liquid chemical sensor array, the ear the permanent magnet is strong enough molecular properties. Microfluidics has is an array of acoustic sensors. Sensor to maintain the valve in the closed also been applied to DNA sequencing as arrays can be homogeneous (made of condition without current applied to well as DNA separation and hybridization many of the same kind of sensors) or the coil. This actuator has an efficient and polymerase chain reaction (PCR) heterogeneous (made of different or magnetic design so that low energy on-a-chip. The commercial markets for orthogonally responding sensors) and consumption of only 3 mJ is achieved biomedical applications are large and performance is generally better for when actuated. The microfabricated because they often require disposable a heterogeneous array of orthogonal integrated design results in arrays of sensors, the technology selected must sensors. miniature latching microvalves with a be suitable for low cost making plastic microelectromechanical systems (MEMS) The issue of manipulation of the low dead-volume, rapid actuation, and low power consumption.11 Realization and plastic manufacturing process are sample and its introduction to a attractive. chemical sensor array is often achieved of valves on a single substrate has the with microfluidics technology. advantage of integration for bio and Microinstrumentation activities have Combining photolithographic processes chemical analysis systems. The valves do produced sensor-sized miniaturized to define three-dimensional (3D) not have to be built on silicon; because mass-spectrometers, gas chromatographs, structures can accomplish the necessary the temperature processing is low, a NMR, and, more recently, micro-Raman fluid handling, mixing, and separation plastic substrate may also be considered. instruments. The key advantages of through chromatography. For example, Biosensors include a biological miniaturization are more rapid analysis, recent demonstrations of miniature recognition element, along with a and hence a higher sample throughput

68 The Electrochemical Society Interface • Spring 2006 and smaller required sample volumes. seed technologies are now being Schiffman, H. T. Nagle, and J. W. Gardner, However, to date, the miniature developed for a long-term vision Handbook of Machine Olfaction, Wiley-VCH, New York (2003). systems have done so at the expense that includes intelligent systems that 10. H. S. Noh, P. J. Hesketh, and G. Frye-Mason, of analytical performance and there is are self-monitoring, self-correcting J. Microelectromech. Sys., 11, 718 (2002). much more work to be done to optimize and repairing, and self-modifying 11. J. S. Bintoro, A. D. Papania, Y. H. Berthelot, microinstruments. or morphing not unlike sentient and P. J. Hesketh, J. Micromech. Microeng., 15, 1378 (2005). Given the range of the technologies beings. The ability for a system to see 12. J. H. Thomas, S. K. Kim, P. J. Hesketh, available, in the end, the application or (photonic technology), feel (physical measurements), smell (electronic noses), H. B. Halsall, and W. R. Heineman, Anal. customer does not really care if there is a Biochem., 328, 113 (2004). hear (ultrasonics), think/communicate sensor in the little sensor box, an array, 13. S. Kumar, S. Rajaraman, R. A. Gerhardt, or a complete analytical system. They (smart electronics and wireless), and Z. L. Wang, and P. J. Hesketh, Electrochim. care about getting the measurement and move (sensors integrated with actuators), Acta, 51, 943 (2005). information they need. However, the is progressing rapidly and suggests an 14. G. Hunter, AIAA 2003-3045 presented exciting future for sensors.14 at AIAA/ICAS International Air & Space sensing mechanism used is important to Symposium and Exposition, The Next 100 developers and in effective application Recent symposia held at ECS meetings Years, AIAA (2003). and must be chosen carefully to include: MEMS and sensors, nanosensors, About the Authors understand how the overall performance electrochemical sensors, acoustic sensors, relates to the sensor system. The surface cantilever sensors, and electronic noses.9 JOSEPH R. STETTER is the Director of the coating on the sensor often defines the In addition, the miniaturization of MicroSystems Innovation Center, Physical Sciences Division. He is a full professor in the Chemistry specificity, sensitivity, response time, and instruments for chemical analysis (lab- Division, and Director of the Sensor Research stability and selective functionalization on-a-chip) has become an increasingly Group, both at the Illinois Institute of Technology; of the sensor is critical. In miniature active area at ECS meetings. The and is President of Transducer Technology, Inc. He analytical instruments, a batch process chemical and biosensors symposium and is a past chair of the ECS Sensor Division; and was is used so that there are several ways to MEMS symposium are held regularly. named the 2002 Entrepreneur of the Year by the Technology Management Association of Chicago. obtain improved sensor performance: The quality of the symposia is directly He serves on refereed journal editorial staffs, improved sensitivity of detectors, dependent on the inputs of the scientific international scientific committees, and several improved sampling systems, and and engineering communities; new corporate boards of directors for startup companies. improved separation or preconcentration ideas, visions, and members are actively His research and business interests focus on new of analytes. sought in these fields which can have so sensors, artificial senses; chem/biosensors; novel drug and vaccine delivery microsystems; vacuum much impact on the way people live.  What is exciting in sensor research microelectronic devices; space ion sources; and and development today? This is a tough References micro/ and structures. He may be question. There are many significant reached at [email protected]. 1. J. R. Stetter and G. J. Maclay, in Advanced PETER J. HESKETH is a professor of mechanical innovations and inventions being made Micro and Nano Systems, Vol. I, Chap. 10, engineering at the George Woodruff School of daily. Micro- and nanotechnology, novel p. 357, H. Baltes, O. Brand, and G. K. Mechanical Engineering, Georgia Institute of materials, and smaller, smarter, and more Fedder, Editors, Wiley-VCH Verlag-GmbH Technology. He is a member of the Technical and Co., Weinheim, Germany, (2004). effective electronic systems will play an Affairs Committee and a past chair of the ECS important role in the future of sensors. 2. C. C. Liu, P. Hesketh, and G. W. Hunter, Sensor Division. His research interests include Electrochem. Soc. Interface, 13(2), 22 (2004). To fulfill the promise of ubiquitous design and micro/nanofabrication for chemical 3. M. Madou, Fundamentals of Microfabrication, and biosensors and their integration with sensor systems providing situational CRC Press, Boca Raton, FL (1997). awareness at low cost, there must be a microfluidic systems. In addition, the use of 4. I. Lundstrom, M. S. Shivaraman, and C. M. magnetic materials, polymers and methods for demonstrated benefit that is only gained Svensson, J. Appl. Phys., 46, 3876 (1975). nanomaterial integration with MEMS/NEMS. He through further miniaturization. For 5. G. W. Hunter, C. C. Liu, D. B. Makel, in is a Fellow of the American Association for the example, new nanowire-based materials MEMS Handbook, Chap. 22, M. Gad-el-Hak, Advancement of Science and a member of the that have unique sensing properties Editor, CRC Press LLC, Boca Raton, FL ASME, ASEE, AVS, ECS, and IEEE. He may be (2001). can provide higher sensitivity, greater reached at [email protected]. 6. G. W. Hunter, L. Chen, P. G. Neudeck, G. W. HUNTER is Technical Lead for the Chemical selectivity, and possibly improved D. Makel, C. C. Liu, Q. H. Wu, and 13 Species Gas Sensors Team and Intelligent Systems stability at a lower cost and such D. Knight, Tech. Rep. 34th AIAA/ASME/ Hardware Lead in the Sensors and Electronics improvements are necessary to the sensor SAE/ASEE Joint Propulsion Conference, Branch at NASA Glenn Research Center in Cleveland, OH (1998). future. Cleveland, Ohio. He has been actively involved 7. G. W. Hunter, P. G. Neudeck, L. Y. Chen, in the development of chemical sensors, as well Sensors can improve the world D. Knight, C. C. Liu, and Q. H. Wu, Silicon as hardware for intelligent systems, for use in a through diagnostics in medical Carbide, III-Nitrides, and Related Materials, range of aerospace and industrial applications. In Proceedings of International Conference on SiC applications; improved performance 1995, he was corecipient of an R&D 100 Award and Related Materials, p. 1093 (1997). of energy sources like fuel cells and for development of an Automated Hydrogen Leak 8. L. A. Pinnaduwage, H.-F. Ji, and T. Thundat, batteries and solar power; improved Detection System. In 2005, he was corecipient of IEEE Sensors J., 5, 774 (2005). an R&D 100 Award for development of a Multi- health and safety and security for 9. Artificial Chemical Sensing 8/ Olfaction and people; sensors for exploring space and Parameter, MicroSensor-Based Low False Alarm the - ISOEN 2001, J. R. Stetter Fire Detection System. He is chair of the ECS the known universe; and improved and W. R. Penrose, Editors, PV 2001-15, The Sensor Division and also a member of the ASME. environmental monitoring. The Electrochemical Society Proceedings Series, He may be reached at [email protected]. Pennington, NJ (2001); T. C. Pearce, S. S.

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