Direct electron transfer of GOD

Mediators capable of direct electron transfer with GOD... 

The difficulties in achieving direct electron transfer between GOD and electrodes have led to the use of small electroactive molecules, called mediators, which shuttle electrons efficiently between the enzyme and the electrode. A new design of glucose electrodes depends (79) on ferricinium (Fc) ions as the mediator ... 

The direct electron transfer between GOD and an electrochemically pretreated platinum electrode has been studied by Durliat and Comtat (1984). Spectroelectrochemical investigations showed that GOD was quantitatively reduced at the electrode. A GOD solution (100 pmol/1) was entrapped in a reaction chamber of 0.04 mm thickness in front of the electrode by a dialysis membrane. The electrode was pretreated by cyclic sweeping between -700 mV and +900 mV for 8 h. In anaerobic solution this sensor responded to glucose at a potential of +450 mV with a response time of 6 min and a linear concentration range between 0.01 and 7 mmol/1. 

In the third approach, the polymers are functionalised not only as immobilised matrices but also as electron mediators. In this approach, the nature of the enzyme reaction is exploited. For example, in the case of GOD, the enzyme is reduced (or oxidised) by D-glucose (substrate) and then this reduced (oxidised) enzyme is oxidised (reduced) by an acceptor. The mediator could also be a redox polymer. Direct electron transfer between GOD and the electrode can also occur. 

By analyzing the dynamic behaviour of GOD ring-disk electrodes, Albery and Bartlett (1985) have shown that the sensor response to low glucose concentration is limited by substrate diffusion through the covering membrane. In contrast to Kulys (1986) they postulated a direct electron transfer between enzyme and electrode surface on the grounds that (i) TCNQ is not soluble enough, and (ii) the reoxidation of GOD is too slow to explain the measured current as a result of dissolved TCNQ-. 

A novel principle for accelerating the electron transfer has been the direct chemical modification of GOD by electron-mediating groups such as ferrocene derivatives (Heller and Degani, 1987). The distance between the mediator molecules was at most 1 nm and the relays had to be attached in the vicinity of the prosthetic group. The binding of ferrocene to GOD was therefore conducted in 2 mol/1 urea. After refold-... 

Os redox polymer detn. of glucose with GOD, direct electron transfer [165]... 

In contrast to GOD, some enzymes such as IIRP can undergo direct electron transfer with unusual facility. For this reason, no mediator was required when those enzymes were used [100, 105, 107, 108]. In general, the sensitivity of the detection of a target substrate could be significantly improved by the use of a mediator. For the construction of reagentless biosensors, it is necessary to immobilize the mediator together with an en me inside the polymer film. So, ferrocene [74,104, 109, 111], quinone and related compounds [106], redox dye [97], and mono- [102] and bipyridinium [92-94, 96] have been incorporated into ECP films. 

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