The Developments Of Glucose Biosensing Technology

The second model is also known as the ‘Incipient Hydrous Oxide Adatom Mediator’ (IHOAM) model, and it was proposed by Burke [20]. Burke, this model originally formulated on the observation of active metal atoms on the electrode surface, which will take place low lattice stabilization and enhanced reactivity. These atoms undergo a monolayer oxidation step, during which an incipient hydrous oxide, OHads, the layer is formed to mediate the oxidation of glucose at the electrode surface. Figure 3 shows a schematic illustration of the IHOAM model. The reductive metal adsorption site is M*, and the oxidative absorbed hydroxide radical is M [OH]ads [20]. This figure shows that both oxidative and reductive processes are catalyzed at the metal electrode surface. Although these models are under scrutiny, they can be still used to explain the oxidation of glucose for different electrode materials.

Enzymatic Glucose Biosensors

The glucose sensor is governed by the transduction mechanism. Different types of glucose sensing mechanisms have been presented and are discussed as under. The detection of glucose using electrochemical methods can be broadly categorized under enzymatic and non-enzymatic methods. The generations of glucose biosensors are defined by the process by which the enzyme is carried out the oxidation and reduction processes. Based on the technology developments of electrochemical glucose biosensors, three generations of glucose biosensors have been presented [20, 21].

1. 5. 1 First Generation Glucose Biosensors

The first generation glucose biosensors estimated glucose concentration in the sample based on hydrogen peroxide production by glucose oxidase (GOx) utilizing dissolved oxygen as given by eq 1. 1:

Glucose+O_2→Gluconic Acid+ H_2 O_2 (1. 1)

For a Pt working electrode a negative potential is applied for a reductive detection of oxygen consumption as given by eq 1. 2:

〖 O〗_2+〖4H〗^++〖4e〗^-→〖2H〗_2 O (1. 2)

The above reaction lies in the redox center of the GOx (FAD) and it performs the initial electron acceptor. The glucose molecules interact with the flavin adenine dinucleotide (FAD) of GOx results in its reduction and given by eq 1. 3:

Glucose+GOx(FAD)→Gluconate+GOx(FADH_2 ) (1. 3)

The transformation of the cofactor of enzyme GOx occurs in the presence of molecular oxygen, resulting in the formation of hydrogen peroxide (H2O2) as given by equation by eq 1. 4,

GOx(FADH_2 )+O_2→GOx(FAD)+H_2 O_2 (1. 4)

Thus, the reduction of oxygen is directly proportional to the glucose concentration that is enumerated by either measuring the reduced oxygen concentration or increased of hydrogen peroxide concentration.

Hydrogen peroxide thus produced as a byproduct is oxidized at Pt anode electrode. The electrons transferred are recognized by the electrode and thus the number of electrons transferred is directly proportional to the number of glucose molecules present [20]. Eq 1. 5 shows the electrons produced during a chemical reaction.

〖 H〗_2 O_2→O_2+〖2H〗^++〖2e〗^- (1. 5)

This glucose biosensing technology of Clark was transferred to Yellow Spring Instrument Company and on the same principle, in 1975 they launched the product (Model 23A YSI analyzer) first commercial glucose biosensor in the market for the direct measurement of glucose.

Major drawbacks of first-generation glucose biosensor [20]:

Electroactive species interference present in blood, such as uric acid, ascorbic acid and other constituents of blood, for amperometric measurement of hydrogen peroxide the high operational potential (+0. 6V) is required.

Sensor involves natural oxygen as the electron acceptor due to the presence of enzyme oxidase in general face errors resulting from fluxes in oxygen tension due to the limited solubility of oxygen in biological fluids. This decreases the linear range of the biosensor.

A number of methodologies have been proposed for addressing this problem, Joseph Wang and group introduced a biosensor with high solubility oxygen based on a fluorocarbon (Kel-F oil) pasting liquid. Thus the inner fluctuation of oxygen supports the reaction catalyzed by the enzyme, even in the absence of oxygen in glucose solution [21]. Another methodology was proposed by Gough’s group, in which they designed a two-dimensional cylindrical electrode in which diffusion of glucose is allowed only from one direction, while the oxygen is allowed to diffuse from both the directions into the region immobilizes at enzyme. The above approach was achieved by developing a two-dimensional sensor design containing a cylindrical gel with GOx and covering the outer part with a silicone rubber tube and it does not allow the glucose but it is highly permeable to oxygen.