NEW MICROWAVE DIGESTION BOMBS
continuous monitoring of glucose con centration under physiological condi tions. The crucial feature of this novel sensor configuration is that, unlike pre vious designs, the restriction of parallel diffusion of glucose and oxygen into the enzyme-containing membrane is removed. Oxygen can diffuse into the membrane from two directions, where as glucose diffuses from only one. The result is sensitivity to a wide range of glucose levels, even at very low oxygen tensions. Among the many enzyme electrode configurations that have been devised, a high proportion are potentiometric and combine an ion-selective electrode with an immobilized enzyme (9). An important example is the penicillin electrode, which is widely used to mon itor the penicillin content of fermenta tion broths. This electrode is based on a p H probe coated with immobilized penicillinase: Penicillinase, Penicillin Penicilloic acid +
The probe responds to changes in [H ] arising from dissociation of penicilloic acid. Electron transfer mediators
Now in two sizes, 23 ml and 45 ml. The speed and convenience of microwave heating can now be applied to the digestion of inorganic, organic, or biological materials in a Teflon Lined Bomb. The new Parr Microwave Digestion Bombs have been designed to combine the advan tages of closed high-pressure and high temperature digestion with the requirements of microwave heating. Many samples can be dissolved or digested with less than one minute heating times. As with all Parr Digestion Vessels, careful design and test ing effort have gone into the safety and sealing aspects of this unique vessel and operating environment. Call or write for Bulletin 4781 with complete technical details.
The difficulties in achieving direct electron transfer between enzymes and electrodes has encouraged the use of small-molecule, electroactive media tors to enhance the rate at which the transfer of electrons occurs. The role of the mediator is to shuttle electrons effi ciently between electrode and enzyme, as depicted in Figure 1. Ferrocenes. In recent years a new generation of mediators has been intro duced based on ferrocene (bis(i) s -cyclopentadienyl)iron, Fecp2) and its deriv atives. Many substituted ferrocenes are available with different overall charges and a wide range of solubilities in various solvents. The formal poten tial of the ferrocene is responsive to the substituents on either or both of the cyclopentadienyl rings, but the elec tron transfer reactions retain their de sirable characteristics of rapidity and
Creatine phosphate
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reversibility. However, most signifi cant in the context of biosensor design is the fact that they can be used to modify other molecules, including pro teins, and retain the properties of a simple, one-electron redox couple. Ferrocene and its derivatives, in their oxidized, ferricinium ion forms, will act as electron acceptors to a num ber of flavoproteins (enzymes whose function is associated with bound fla vin coenzymes), including pyruvate ox idase, xanthine oxidase, diaphorase, and alcohol dehydrogenase. Without a doubt the most successful application of ferrocenes as mediators has been in an amperometric enzyme electrode for the determination of glucose (10). A substituted ferrocene (Fecp 2 R) medi ates electron transfer between immobi lized glucose oxidase and a graphite electrode. The system is not dependent on oxygen; the ferricinium ion replaces oxygen as the cofactor for glucose oxi dase. Once reduced, the ferricinium ion can be regenerated at the electrode. Glucose + GOD (ox) — Gluconolactone + GOD ( r e d ) GOD ( r e d ) + 2Fecp 2 R + GOD (ox) + 2Fecp 2 R + 2 H + 2Fecp 2 R *=* 2Fecp 2 R + + 2e~ A linear current response, proportional to glucose concentration, is observed over the range commonly found in dia betic blood samples (1-30 mM), and measurements can be made in plasma and whole blood. The electrode also can be coupled to analytes other than glucose, using en zymes that compete with glucose oxi dase for its substrate. Assays based on this strategy have been developed for adenosine triphosphate and creatine kinase (11) (Figure 2). Creatine kinase activity is measured by the rate of con version of glucose to glucose-6-phosphate in a bulk medium to which hexokinase, creatine phosphate, and adeno sine diphosphate have been added. The removal of glucose as a substrate for the glucose electrode causes a decrease in the steady-state current arising from
GI ucose-6-phosphate
ADP
Creatine kinase
Hexokinase
Α Λ
Creatine
ATP
Glucose enzyme electrode Glucose
Figure 2. Coupled reaction for determination of creatine kinase or ATP. CIRCLE 125 ON READER SERVICE CARD 934 A · ANALYTICAL CHEMISTRY, VOL. 59, NO. 15, AUGUST 1, 1987
