56.4 Biosensors Biological measurands are biologically produced substances, such as antibodies, glucose, hormones, and enzymes Biosensors are not the same as biomedical sensors, which are any sensors used in biomedical appli cations, such as blood pressure sensors, or electrocardiogram electrodes. Many biosensors are biomedical ensors; however, they are also used in industrial applications, e.g., the monitoring and control of fermentation reactions. Table 56 1 does not include biological signals as a primary signal because they can be classified as either chemical or physical in nature. Biosensors are of special interest because of the very high selectivity of biological reactions and binding. However, the detection of that reaction or binding is often elusive. A very familiar commercial biosensor is the in-home pregnancy test sensor, which detects the presence of human rowth factor in urine. That device is a nonelectrical sensor since the output is a color change which the eye senses. In fact, most biosensors require multiple transduction mechanisms to arrive at an electrical output signal. Two examples are given below: an immunosensor and an enzyme sensor. Rather than examine a specific species, the examples describe a general type of sensor and transduction mechanism, since the same principles can be applied to a very large number of biological species of the same type Immunosensor Commercial techniques for detecting antibody-antigen binding utilize optical or x-radiation detection. An optically fluorescent molecule or radioisotope is nonspecifically attached to the species of interest in solution. The complementary binding species is chemically attached to a glass substrate or glass beads which are packed into a column. The tagged solution containing the species of interest, say the antibody, is passed over the antigen-coated surface, where the two selectively bind. After the binding occurs, the nonbound fluo- rescent molecules or radioisotopes are washed away, and the antibody concentration is determined by fluores- ence spectroscopy or with a scintillation counter, respectively. These sensing techniques are quite costly and bulky, and therefore other biosensing mechanisms are rapidly being developed. One experimental technique uses the change in the mechanical properties of the bound antibody-antigen complex in comparison to an unbound surface layer of antigen. It uses a shear mode, surface acoustic wave(Saw) device(see Chapter 51 and Ballentine et al, 1997)) to sense this change as a change in the propagation time of the wave between the generating electrodes and the pick-up electrodes some distance away on the same piezoelectric substrate. The ubstrate surface is coated with the antigen and it is theorized that upon selectively binding with the antibody, this layer stiffens, changing the mechanical properties of the interface and therefore the velocity of the wave The advantages of this device are that the saw device produces an electrical signal (a change in oscillation frequency when the device is used in the feedback loop of an oscillator circuit) which is dependent on the amount of bound antibody; it requires only a very small amount of the antigen which can be very costly; the entire device is small, robust and portable; and the detection and readout method is inexpensive. However, there are numerous problems which currently preclude its commercial use, specifically a large temperature sensitivity and responses to nonspecific adsorption, i.e, by species other than the desired antibo Enzyme sensor Enzymes selectively react with a chemical substance to modify it, usually as the first step in a chain of reactions to release energy(metabolism). A well-known example is the selective reaction of glucose oxidase(enzyme) with glucose to produce gluconic acid and peroxide, according to C6H12O6+0,glucose oxidase gluconic acid +H, O, +80 kilojoules heat An enzymatic reaction can be sensed by measuring the rise in temperature associated with the heat of reaction or by the detection and measurement of byproducts. In the glucose example, the reaction can be sensed by measuring the local dissolved peroxide concentration. This is done via an electrochemical analysis called amperometry [Bard and Faulkner, 1980]. In this method, a potential is placed across two inert metal c 2000 by CRC Press LLC© 2000 by CRC Press LLC 56.4 Biosensors Biological measurands are biologically produced substances, such as antibodies, glucose, hormones, and enzymes. Biosensors are not the same as biomedical sensors, which are any sensors used in biomedical appli￾cations, such as blood pressure sensors, or electrocardiogram electrodes. Many biosensors are biomedical sensors; however, they are also used in industrial applications, e.g., the monitoring and control of fermentation reactions. Table 56.1 does not include biological signals as a primary signal because they can be classified as either chemical or physical in nature. Biosensors are of special interest because of the very high selectivity of biological reactions and binding. However, the detection of that reaction or binding is often elusive. A very familiar commercial biosensor is the in-home pregnancy test sensor, which detects the presence of human growth factor in urine. That device is a nonelectrical sensor since the output is a color change which the eye senses. In fact, most biosensors require multiple transduction mechanisms to arrive at an electrical output signal. Two examples are given below: an immunosensor and an enzyme sensor. Rather than examine a specific species, the examples describe a general type of sensor and transduction mechanism, since the same principles can be applied to a very large number of biological species of the same type. Immunosensor Commercial techniques for detecting antibody-antigen binding utilize optical or x-radiation detection. An optically fluorescent molecule or radioisotope is nonspecifically attached to the species of interest in solution. The complementary binding species is chemically attached to a glass substrate or glass beads which are packed into a column. The tagged solution containing the species of interest, say the antibody, is passed over the antigen-coated surface, where the two selectively bind. After the specific binding occurs, the nonbound fluo￾rescent molecules or radioisotopes are washed away, and the antibody concentration is determined by fluores￾cence spectroscopy or with a scintillation counter, respectively. These sensing techniques are quite costly and bulky, and therefore other biosensing mechanisms are rapidly being developed. One experimental technique uses the change in the mechanical properties of the bound antibody-antigen complex in comparison to an unbound surface layer of antigen. It uses a shear mode, surface acoustic wave (SAW) device (see Chapter 51 and [Ballentine et al., 1997]) to sense this change as a change in the propagation time of the wave between the generating electrodes and the pick-up electrodes some distance away on the same piezoelectric substrate. The substrate surface is coated with the antigen and it is theorized that upon selectively binding with the antibody, this layer stiffens, changing the mechanical properties of the interface and therefore the velocity of the wave. The advantages of this device are that the SAW device produces an electrical signal (a change in oscillation frequency when the device is used in the feedback loop of an oscillator circuit) which is dependent on the amount of bound antibody; it requires only a very small amount of the antigen which can be very costly; the entire device is small, robust and portable; and the detection and readout method is inexpensive. However, there are numerous problems which currently preclude its commercial use, specifically a large temperature sensitivity and responses to nonspecific adsorption, i.e., by species other than the desired antibody. Enzyme Sensor Enzymes selectively react with a chemical substance to modify it, usually as the first step in a chain of reactions to release energy (metabolism). A well-known example is the selective reaction of glucose oxidase (enzyme) with glucose to produce gluconic acid and peroxide, according to An enzymatic reaction can be sensed by measuring the rise in temperature associated with the heat of reaction or by the detection and measurement of byproducts. In the glucose example, the reaction can be sensed by measuring the local dissolved peroxide concentration. This is done via an electrochemical analysis technique called amperometry [Bard and Faulkner, 1980]. In this method, a potential is placed across two inert metal C H O O gluconic acid H O kilojoules heat glucose oxidase 6 12 6 2 2 2 + æææææææÆ + + 80