Blood chemical sensors

One important application of amperometry is in the construction of chemical sensors. One of the first amperometric sensors to be developed was for dissolved O2 in blood, which was developed in 1956 by L. C. Clark. The design of the amperometric sensor is shown in Figure 11.38 and is similar to potentiometric membrane electrodes. A gas-permeable membrane is stretched across the end of the sensor and is separated from the working and counter electrodes by a thin solution of KCl. The working electrode is a Pt disk cathode, and an Ag ring anode is the... 

Amperometry is a voltammetric method in which a constant potential is applied to the electrode and the resulting current is measured. Amperometry is most often used in the construction of chemical sensors that, as with potentiometric sensors, are used for the quantitative analysis of single analytes. One important example, for instance, is the Clark O2 electrode, which responds to the concentration of dissolved O2 in solutions such as blood and water. 

Comparable is the CHEMFET (Chemical Field Effect Transistor), a chemical sensor on a FET, e.g., for H , Na, K and Ca2+ in blood, four CHEMFETs had been mounted on one plate [Clin. Chem., 30 (1984) 1361. 

Fig. 5 shows the instrumental arrangement of the commercially most successful optical chemical sensor between 1984 and 2000. It is used in about 70% of all critical care operations in the US to monitor pH, pC02 and p02 in the cardiopulmonary bypass operations35. It contains 3 fluorescent spots, each sensitive for one parameter, in contact with blood. Fluorescence intensity is measured at two wavelengths and the signals are then submitted to internal referencing and data processing. 

Fluorescent pH indicators offer much better sensitivity than the classical dyes such as phenolphthalein, thymol blue, etc., based on color change. They are thus widely used in analytical chemistry, bioanalytical chemistry, cellular biology (for measuring intracellular pH), medicine (for monitoring pH and pCC>2 in blood pCC>2 is determined via the bicarbonate couple). Fluorescence microscopy can provide spatial information on pH. Moreover, remote sensing of pH is possible by means of fiber optic chemical sensors. 

Chemical sensors (i) Gases (e.g. blood oxygen electrode, carbon monoxide detector) (ii) pH and ions (e.g. pH meter, potassium-selective elecbode) and (iii) optical oximetry (e.g. pulse oximeby for non-invasive monitoring of blood oxygenation). 

Potential applications of chemical sensors are diverse and numerous, and the environment where the sensor is used varies. Therefore, chemical sensor often requires to be tailor-made or semi-tailor-made to meet the needs in the special circumstance. For example, sensing of oxygen in an automobile exhaust or in water or in blood can be accomplished by using an electrochemical-based sensor. However, the selection of electrolyte and a diffusion-limited layer or protective membrane will be different in each case. Therefore, the platform chemical sensor technology is discussed in general. Special applications of a chemical sensor under a particular circumstance need to be addressed separately. 

Sensor temperature coefficient and time response are also variables to be understood. Temperature may shift the pKa of the dye and change the cell thickness, and will certainly affect the actual value of the blood gas variables of the blood that is adjacent to the sensor. For these reasons, a complete blood gas sensor includes a local temperature sensor, particularly if the sensor is to be placed in a peripheral artery where local temperature may not be equal to central body temperature. The chemical sensor temperature coefficient must be well characterized so that it will accurately measure the local blood gas value. Bench analysers usually measure blood samples at 37 °C so the in vivo system must then adjust the measured value to that temperature. The temperature coefficient of the blood gas variables, in blood, may be several percent per degree, and, in the case of Foj, depend very strongly on the actual value. Thus, to make the in vivo sensor agree with bench analysers, local temperature sensing must have an accuracy of better than 1 °C. Size and accuracy requirements can be met by a miniature thermocouple. The system designer has to make sure that the temperature circuit can handle the microvolt signals with adequate accuracy and stability as well as meet patient electrical isolation requirements. 

Chemical sensors have been developed by companies such as DuPont and Cygus Therapeutic Systems for the measurement of blood electrolytes and gases, and ion selective membranes are common in many clinical analyzer systems. While the use of chemical sensors for such determinations will continue to increase, sensor applications in clinical diagnostics will favor development and application of biosensors due to the high specificity residing in the biological component of these sensors. 

Current and projected markets for chemical sensors are shown in table 23.8. The biomedical market consists primarily of chemical sensors for blood gases and electrolytes. This area is growing as portable, real time systems such as the i-STAT clinical analyzer are increasingly used at point of care and remote testing locations. The demand for faster, more reliable, and cheaper detectors for environmental and workplace monitoring for toxic gases and volatile organics... 

FIGURE 6.7 Structural diagram of an integrated fiber optic blood gas catheter. (Taken from Otto S. Wolfbeis, Viher Optic Chemical Sensors and Biosensors, Vol. 2, CRC Press, Boca Raton, FL, 1990.)... 

---