INSTRUMENTATION
Hamamatsu Hollow Cathode Lamps are now available from major lab suppliers. Hamamatsu single and multielement Hollow Cathode Lamps offer superior stability, spectral purity and output intensity, even for such elements as arsenic and selenium. They are compatible with most commercial spectrophotometers, including Beckman, Zeiss and Perkin-Elmer. And best of all, they're available from your local lab supplier.
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Another method of detection by semiconductor devices is based on the use of thermistors to monitor changes associated with enzyme reactions. The heat of reaction, which can be quantitatively determined in a closed reaction cell, represents a direct observation of the free energy changes associated with product evolution (24). This is not identical to observation of the free energy of binding (thermal energy is a product of a reaction) and is, in comparison, a much more complicated physicochemical process. The systems are suitable only for selective coatings based on enzyme reactions and require relatively large quantities of substrate for reproducible signal generation as determined by heats of reaction. These devices are completely reversible, but they must be carefully designed to avoid interference from the reaction products. Piezoelectric sensors. Piezoelectric devices generally are based on specially cut quartz crystals that mechanically oscillate when subjected to an alternating electrical potential. Although other piezoelectric materials can be used, the availability of appropriate quartz crystals has led to extensive investigation of two modes of operation. Surface acoustic wave (SAW) devices have been described as gas sensors, but apparently they cannot operate simply in liquid media (25). Bulk acoustic wave (BAW) devices have been used extensively as gas sensors and recently also as liquid-phase devices with potential for biosensing, as determined by studies of antibodies located on one surface of a device (26). The mechanical oscillation of the crystals (usually at MHz frequencies) is very regular and can be perturbed by minor mass or microviscosity (physicochemical interactions of surface chemistry with bulk environment) changes caused by selective binding interactions at the surface of the crystal. Frequency changes smaller than 1 Hz can sometimes be measured reproducibly, providing nanogram sensitivity with respect to adsorption to the surface of the device. Evolution of products from selective reactions is not necessary for transduction to take place. The major limitations of these devices are governed by nonselective adsorption, surface occlusion, and receptor denaturation. Because many of the problems encountered during electrochemical experiments (such as mixed potential effects) do not greatly affect mass response, these devices offer practical advantages over other electrochemical devices. Optical sensors. Significant innovation has been achieved with optical
CIRCLE 58 ON READER SERVICE CARD 402 A · ANALYTICAL CHEMISTRY, VOL. 63, NO. 7, APRIL 1, 1991
sensors by using optical fibers that act as light guides to transmit optical information to and from a remote, compartmentalized target reaction (2730). The most common configuration of such devices is the extrinsic sensor, in which a selective reagent is immobilized at the terminus of a fiber. When an intrinsic sensor is used, optical fibers are directly coated along the surface with selective reagents, thereby permitting optical transduction by evanescent wave stimulation. Sensors that use optical fibers are sometimes referred to as optodes, in analogy to the electrode. The sensitivity of an optical system based on absorption of electromagnetic radiation is greatly limited by short pathlengths, and most fiber-optic biosensors use fluorescence processes for analytical signal generation. Although many of these sensors are designed to detect the evolution of products from enzymatic processes (e.g., a fluorescent dye sensitive to pH changes), it is possible to directly monitor the selective binding event. For example, a fluorescent label such as dansyl chloride located near the binding site of an Fai, antibody fragment may experience significant electrostatic and motional alterations induced by an analyte. The altered fluorescence intensity (average and time-resolved) provides an analytical signal that results from direct interaction of the probe with the free energy changes induced by the binding event and represents a generic process suitable for many different antibodies (31). An external label perturbs the binding event by virtue of the location of the label(s) near or at the receptor site. An alternative sensing strategy based on observation of intrinsic fluorescence from selective chemistry may be possible. For example, Trettnak and Wolfbeis (32) reported a biosensor that monitored fluorescence from the flavin-based cofactor of a glucose oxidase reaction. Fluorescence processes offer the possibility of multidimensional analysis by concurrent observation of wavelength, intensity, polarization, and event lifetime (33,34). In combination, these analytical parameters can be used to define unique solutions to both qualitative and quantitative analyses. Although there is no structural information available in a single type of binding process, the distinct contributions from selective and nonselective binding can be identified in some cases. For example, a selective binding process may alter both the emission wavelength and the intensity of a label close to the binding site. Labels far from the
