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©KEN EWARD/BIOGRAFX 1994
Centennial Retrospective on Chemical
In the past 20 years, chemical sensors have become an accepted part of analytical chemistry.
Jir˘í Janata
Georgia Institute of Technology
et us begin by looking at where we are now, at the turn of the century. There are several journals specializing in chemical sensors; large professional societies have sensor divisions; and several series of international conferences on sensor take place regularly. In the last fundamental review article I coauthored for Analytical Chemistry, more than 2000 sensor-related were papers published per year, and the rate of publication had been steadily increasing over the past decade (1–5). During the past 20 years, chemical sensors have become an accepted part of analytical chemistry because they satisfy the expanding need for data. Like all of science, chemical sensors have benefited from the growing power of computers, new materials, and design and processing tools. Sensors offer a means to optimize process control, pharmaceutical therapy, and environmental and security monitoring. A general awareness of the value of chemical sensors is reflected in the frequently used phrase “…. and it can be used also as a chemical sensor,” which has become a typical default statement for publications describing a solution looking for a problem. The meaning of the term “chemical sensor” has become somewhat fuzzy with the introduction of miniaturized automatic analytical procedures, particularly micro total analysis systems. To draw the boundaries for this article, I want to define a chemical sensor as a device that provides continuous information about its environment. In contrast, a chemical sensing system is an apparatus on which chemical assays are performed in a series of discrete steps, thereby obtaining information in a batch manner. There is a good reason for emphasizing this frequently and sometimes deliberately ignored difference. In some cases, the interaction of the analyte with the sensing layer is irreversible or so strong that, to return the system to its baseline, a step or series of steps must be performed (usually automatically). This can be accomplished in a sensing system. Similarly, to achieve a desired low detection limit or selectivity, a lengthy incubation period, a separation, or a preconcentration step must be included in the analytical sequence. As an example, consider DNA analysis or an assay for biological pathogens. As specific and sensitive as DNA interactions are, they have not been implemented as true chemical sensors precisely because of the irreversibility of the binding at normal temperature. However, DNA assays are the most rapidly growing type of chemical analysis. Some automatic micro total analysis assays provide information even faster than slow chemical sensors. Moreover, researchers could conceivably design DNA-based sensors that operate at or near the melting temperature of the hybrid complex. Therefore, sensors and assays have their advantages and disadvantages and, in some cases, they are complementary. This seemingly insignificant difference between the two modes of information acquisition becomes very important in the quest for direct sensors for pathogenic organisms. To my knowledge, there are none, but automatic
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FIGURE 1. Thirty-year trends in technological parameters of conventional (dashed lines) and chemical (solid lines) electronics. Lines 2 and 5 are the logarithm of transistor density (transistors/chip). Lines 1 and 4 are the logarithm of the typical diameter of a silicon wafer. Lines 3 and 6 are the logarithm of the characteristic feature size (e.g., transistor channel length).
biological assays for pathogens are a viable alternative. This distinction will be one of the most important parameters shaping the future of the chemical sensor field.
Unde vadere? (Where did we come from?) Very often, sensors have been “add-ons” to some seemingly unrelated technology or invention. Thus, bulk and surface quartz oscillators, otherwise useful components in electronic timing and transmission circuits, became one of the hottest platforms for mass chemical sensors. Likewise, optical fibers that were developed primarily for high-speed data transmission became an immensely successful platform for optical sensors. Chemically sensitive field-effect transistors (CHEMFETs) were no more than an afterthought on the insulated gate field-effect transistor. This pattern will be repeated again and again in the future. The quest for better selectivity has dominated chemical sensor research for the past 50 years. Between 1950 and 1980, ion-selective electrodes revolutionized the approach to the sometimes difficult analysis of inorganic ions. The single crystal fluoride electrode is the prime example of that success (6). Going back even further to the 1930s, the coupling of a glass electrode to a vacuum tube voltmeter and its successful commercialization became the foundation of one of the most successful analytical instrument companies—Beckman Instruments. More patents for different formulations of glass for pH electrodes are available than for any other chemical sensor. Biologically derived selectivity has given rise to an entirely new subclass of chemical sensors—biosensors. However, taking advantage of this selectivity and translating it into a rugged sig152 A
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nal has been a much bigger problem than originally anticipated. On the other hand, this is precisely where chemical sensing systems have their biggest advantage over chemical sensors. In the past 20 years, the development of chemical-sensing arrays has become an alternative strategy in the search for better selectivity, which has been enabled by miniaturized computational power. Thus, a long-expected marriage between chemometrics and chemical sensors is taking place. Introducing sensing arrays has brought another phenomenon into the chemical-sensing arena—miniaturization. Originally, the almost exclusive domain of silicon electronics, miniaturization has expanded to other domains. Not surprisingly, miniaturization and microfabrication have offered some hitherto unrealized fundamental and practical advantages. The first chemical sensors fabricated by more-or-less conventional silicon fabrication technology were CHEMFETs, which appeared on the scene in 1970s. At that time, the dimensions of conventional electronics and socalled chemical electronics were comparable (7 ). However, as development continued, it became obvious that Moore’s law of conventional electronics (the number of transistors on a chip doubles every 18 months, Figure 1) will not apply to microfabricated chemical sensors. FETs are directly comparable devices. Their chip density has remained at ~10/chip. Although the characteristic feature size of conventional electronics decreased to 20,000 references in its database and mid-range IR sensors will open new avenues of opticaland grows at the rate of ~10%/year clearly has a bright future— sensing capabilities. The so-called broadband revolution, the one should never argue with success. The demand for more in- vigorous growth of information transmission technology, is unformation continues, and as always, analytical chemists will strive doubtedly going to affect the chemical sciences and particularto deliver this information in a better and more efficient form. ly the chemical-sensing field, which is the prime supplier of raw The specific application will always define the need for and the data. Transmitting data about the local chemical environment optimal attributes of any information acquisition scheme, be it a within a precisely defined geographical grid over long distances sensor or a microassay. For example, the µChemLab, developed at is already a feasible task. The availability of reliable sensors for Sandia National Laboratories, is a sensing system that is capable of chemical telemetry and remote sensing provide further impetus both LC and GC (9). It has a footprint of 0.5 ⫻ 0.5 cm and can for developing integrated microsensors. resolve parts-per-billion levels of explosives and warfare agents in The main transduction principles—thermal, mass, electro30 s and 1 min, respectively. The size of the overall system, in- chemical, and optical—were defined in the last century. Most cluding the pumps, mobile-phase reservoirs, and controls, is not progress is likely to come from coupling new chemistry and quite clear. However, without question, this is a challenging biochemistry to the coming advances in optics, electronics, and benchmark of performance for any chemical sensor. To justify de- data processing and transmission. veloping a new sensor for this type of application, the sensor or sensing array would have to better the µChemLab in speed, physical size, weight, cost (both Table 1. Conventional and chemical electronics in 1996. capital and operational), power requirements, and continuous operation. Therefore, the challenge is clearly defined by existing sensor system technology. Conventional transistors Chemical electronics Worldwide volume per year 1017 99% ~60% ent sensors under given conditions (10). It is deMetallization Aluminum, copper Platinum, gold Top passivation SiO2 Si3N4 fined as the ratio of the method (sensor) sensitivity Feature size 0.1 µm x,y ~ 4 µm (z ~ 100 µm) m and the standard deviation s of the signal at the given concentration of the analyte (␥ = m/s). Therefore, in terms of sensitivity, the choice of an optimum sensor can be guided by this simple relationship. Dif- Jir˘í Janata is professor at the Georgia Institute of Technology. His referent applications may require different selection criteria, such search interests include chemical sensors, electrochemistry, and environmental chemistry. Address correspondence about this article to as speed of response, cost, or detection limit. The availability of different transduction modes opens the Janata at School of Chemistry and Biochemistry, Georgia Institute of possibility of using mixed sensing platforms to enhance the Technology, Atlanta, GA 30327 (
[email protected]). overall information acquisition process. Sensing the same analyte with several different types of sensors is similar to using a References hyphenated technique, such as GC/MS or differential scanning (1) Janata, J.; Bezegh, A. Anal. Chem. 1988, 60, 62 R. calorimetry–IR spectroscopy. The term “higher-order” chemical (2) Janata, J. Anal.Chem. 1990, 62, 33 R. sensors was coined for this type of sensing arrangement (11). (3) Janata, J. Anal.Chem. 1992, 64, 196 R. Perennial sensor problems such as baseline stability, drift, and (4) Janata, J.; Josowicz, M.; DeVaney, D. M. Anal.Chem. 1994, 66, 207 R. interferences can be avoided by increasing the order of the ana- (5) Janata, J.; Josowicz, M.; Vanysek, P.; DeVaney, D. M. Anal. Chem. 1998, lytical technique. Chemical sensors are no exception. For exam70, 179 R. ple, simultaneous optical and electrochemical responses from the (6) Frant, M. S.; Ross, J. W. Science 1966, 154, 473. same sensing layer would have the inherent advantages of a hy- (7) Zemel, J. M. Anal.Chem. 1975, 47, 255 A. phenated analytical technique. Increasing sensing order and mix- (8) Ricco, A. J.; Crooks, R. M.; Janata, J. Interface 1998, 7, 18. ing sensing platforms are developments that are not too far away. (9) Zubritsky, E. Anal. Chem. 2000, 72, 392 A. Speed of information acquisition by sensors is another im- (10) Barsan, N.; Stetter, J. R.; Findlay, Jr., M.; Göpel, W. Anal. Chem. 1999, 71, 2512. portant parameter for applications, such as process control or se- (11) Wise, B. M.; Janata, J. In Encyclopedia of Energy Technology and the Envicurity. Speed can be improved by estimating the final steadyronment; Wiley & Sons: New York, 1995; pp 1247–1258. state or equilibrium response from the first 20–30% of the (12) Ishida, H.; Kobayashi, A.; Nakamoto, T.; Moriizumi, T. IEEE Trans. Robotics response curve (12). This approach requires explicit knowledge and Automation 1999, 15, 25. M A R C H 1 , 2 0 0 1 / A N A LY T I C A L C H E M I S T R Y
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