Biosensor linear range

Attempts to reduce interference and minimize the effect of variations in oxygen tension have resulted in the development of biosensors with improved linear ranges which operate at lower electrode potentials. They incorporate artificial electron acceptors, called mediators, to transfer electrons from the flavoenzyme (e.g. glucose oxidase) to the electrode and thus are not dependent on oxygen. Ferrocene (bis(i75-cyclopentadienyl)iron) and its derivatives are examples of redox mediators for flavoenzymes. The reaction now becomes... 

An SPCE modified with CoPC was employed as an H202 transducer in a cholesterol biosensor fabricated by the drop-coating of ChOx, followed by a cellulose acetate membrane [50]. The resulting cholesterol biosensors were operated in stirred solutions using an applied potential of +400 mV vs. screen-printed Ag/AgCl, and displayed a linear range of 0.06-5 mM. 

A characteristic that should presently be improved is the limited linear range of the biosensors which could cause some problem in the cases of hyperglycemic levels. Studies are in progress to solve this problem by finding a suitable microdialysis probe which would be able to recover the subcutaneous glucose in the desired range of concentrations. 

A system underpinned by commercially made screen-printed electrochemical cells was described by Palmisano et al. [19]. The cells were converted into biosensors for lactate in milk and yoghurt by addition of an electrochemically polymerised barrier to interference and a layer composed of lactate oxidase, glutaraldehyde and BSA. These steps appeared to have been carried out by hand. As there was no outer diffusion-limiting membrane, the linear range of the sensors was quite small (0-0.7 mM). They were incorporated into a FIA with a microdialysis unit based on a planar membrane and a buffer reservoir (earlier work used a microdialysis fibre with a platinum electrode [29]. The concentration of lactate was determined in various milks (0.27-1.64 mM), and in raw milk (c. 0.5-0.9 mM) left to degrade on the laboratory bench. The recovery of the microdialysis unit, 2.6%, implied that the sensor had an ability to return measurable currents for very low concentrations of lactate. A further implication is that the electro-polymerised layer was very effective at preventing interference. 

GOx was also immobilized on Pt modified with chemically oxidized SWCNTs through covalent attachment using EDC [93]. The amperometric experiments at 0.40 V gave a sensitivity of 18.7 mAM cm", a linear range up to 12 mM glucose and an apparent Km of 13.1 mM. The biosensor showed a good stability, keeping a 90 % of the activity after 4 months. 

The actual glucose sensor (a platinum electrode covered by three membranes ceUulose acetate, nylon net with covalently Unked GOD, and a polycarbonate protective membrane) is located in a miniaturized waU-jet cell. The sensor exhibits excellent performance, with a linear range extending up to 27 mM, thanks to the microdialysis dilution effect which was estimated to be 1 10 for the probe length used and for the flow rate set by the instrumentation. Long-term stability tests revealed that the biosensor stiU maintains its initial activity after incubations of 4 weeks at 45°C, 11 weeks at 37°C, and 32 weeks at room temperature (see Table 12.2). From these results, a shelf life of more than 2 years at 2-8°C can be extrapolated [119]. 

In the previously mentioned approach, a relatively high flow rate (20 /il/min) was used for the continuous monitoring and this resulted in high time resolution as well as the previously mentioned extension in the linear range of the glucose biosensor. The absolute concentration of the interstitial fluid was not determined, but variations in glucose concentration in the subcutaneous tissue were well correlated with those of the blood values and in general there was no phase lag between the two series of measures. 

Table I provides a comparison of the performance of the dendrimer-based electrochemical glucose biosensors. It is found that many of the dendrimer electrode assemblies show reasonable linear range and detection limit. Their stability assessment and high sensitivity values indicate good scope for commercialization. The bi-enzyme model is better than the conventional mono-enzyme model in terms of lower detection potential which can help to circumvent the interferences due to other biomolecules during the measurements.

Analyte Biosensor Detection Limit Upper Linear Range Sensitivity Operating Potential (and Mode) Ref. 

Ti02 nanotubes composite electrode has been constructed for effective immobilization of cytochrome c and successful realization of its direct electrochemistry and electrocatalysis. The immobilized Cyt c/Ti02 bioactive electrode exhibits favorable electrocatalytic activity toward the reduction of H202 with good stability and sensitivity. The linear range was obtained is 2 x 10"6 to 3.49 x 10 mol /L"1 with detection limit of 1.21 x 10 6 mol/L"1. The fabricated biosensor retains 98.7% of the initial response after 30 days [27],... 

Sol-gel derived hybrid Ti02 film deposited on glassy carbon electrode has been used to construct the phenol biosensor. The resulting biosensor is selective towards phenol with a linear range from 7.5 x 10 8 - 6 x 10"6 M with detection limit 1 x 10"8 and has response time as 10 s. The biosensor exhibits maximum response at 45 °C. The initial response current of the bioelectrode decreases to 95 % after 2 months [85],... 

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