Membranes biosensor performance

Since the first attempt of Chapman and co-workers in 1966 [860], IR spectroscopy has become one of the most frequently used tools for elucidating lipid properties and the mutual effects of different lipids and proteins, which are of interest for different aspects of bioscience and biosensor design (see Refs. [333, 748, 861-864] for review). The IR methods used are transmission, ATR (MIR), and IRRAS for model monolayer, bilayer, and multibilayer membranes and biological membranes. To perform in situ measurements on the membranes of intact individual cells (e.g., as a function of cell membrane potential), planar miniature waveguides can be used instead of the ATR optics [865]. PM-IRRAS has been applied to obtain high-performance spectra of model membranes at the AW interface [866-875]. The experimental data focus mainly on the correlation between the structure of the matrix amphiphile or phospholipid film and the structure of the constituent species, the subphase composition, the surface pressure, and other external conditions, as well as the interaction of such monolayers with peptides and proteins (for reviews, see Refs. [332-334, 876, 877]). 

When constmcting biosensors from cells and tissues there can be several factors that can affect the performance of the sensor. The amount of cells immobilized, the type of membrane used, the degree of polymerization, and the amount of cross-linking agent can also influence the performance of the biosensor. Additionally, the pH, composition, and temperature of the solution tested can influence the biosensor performance dramatically. Even small changes in pH can reduce the enzymatic activity, which is often a function of pH, or even irreversibly destroy the cells or tissues. Furthermore, the composition of the solution is important as certain compounds could inhibit the enzymatic function. 

Since ideally, a biosensor should be reagentless, that is, should be able to specifically measure the concentration of an analyte without a supply of reactants, attempts to develop such bioluminescence-based optical fibre biosensors were made for the measurements of NADH28 30. For this purpose, the coreactants, FMN and decanal, were entrapped either separately or together in a polymeric matrix placed between the optical fibre surface and the bacterial oxidoreductase-luciferase membrane. In the best configuration, the period of autonomy was 1.5 h during which about twenty reliable assays could be performed. 

If silicon technology is involved all thermal sensors suffer from the high thermal conductivity of silicon, which dramatically decrease their sensitivity [12]. However, by use of micromachining and integrated silicon technology a powerful thermal biosensor can be realized. Using a thermopile integrated on a thin micromachined silicon membrane reduces thermal loss due to the substrate and so excellent performance can be accomplished [13]. 

Acetylcholineesterase and choline oxidase Enzyme immobilized over tetra-thiafulvalene tetracyanoquinodi-methane crystals packed into a cavity at the tip of a carbon-fiber electrode. The immobilization matrix consisted of dialdehyde starch/glutaraldehyde, and the sensor was covered with an outer Nafion membrane. The ampero-metric performance of the sensor was studied with the use of FIA system. An applied potential of +100 mV versus SCE (Pt-wire auxiliary electrode) and a carrier flow rate of 1 mL/min. The Ch and ACh biosensors exhibited linear response upto 100 pM and 50 pM, respectively. Response times were 8.2 s. [97]... 

The enzymatically coupled FET is reviewed in this chapter (32). First, a brief review is given. Determining how to deposit an enzyme-immobilized membrane on the surface of a FET was one of the most difficult problems to solve before tin enzymatically coupled FET could be developed. Therefore, some enzyme-immobilized membrane deposition methods and the photolithographic enzyme-immobilized membrcme patterning method developed by the authors are described in detail. Concomitantly, the performances of some FET biosensors with an enzyme-immobilized membrane made by this method are described. Finally, recent applications of an enzymatically coupled FET are surveyed. 

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]. 

A different lactate biosensor was proposed by Pfeiffer et al. [152], who used an enzyme sandwich membrane that was commercially available for whole blood lactate analysers. The membrane was inserted into a flow cell connected to a microdialysis probe. This membrane showed a significant day-to-day variation in sensitivity ( 50%), but no trend in sensitivity decrease. The problem of rejecting interference has not been completely solved by this system. However, the continuous monitoring of subcutaneous lactate was feasible at least in small rodents, and results were consistent with liquid chromatographic measurements performed on dialysate samples collected during the in vivo experiment. 

Usually in the operation of biosensors the flow conditions are adjusted to provide a mass transfer rate from the solution to the membrane system which is fast as compared with the internal mass transfer (exception implanted sensors). On the other hand, variations of the diffusion resistance of the semipermeable membrane are being used to optimize the sensor performance. A semipermeable membrane with a molecular cutoff of 10 000 and a thickness of 10 pm only slightly influences the response time and sensitivity. In contrast, thicker membranes, e.g. of polyurethane or charged material, significantly enhance the measuring time, but may also lead to an extension of the linear measuring range. 

Assolant-Vinet and Coulet (1986) used a preactivated Biodyne Im-munoaffinity Membrane (Pall, USA) to cover two electrodes working in differential mode. Immobilization was performed by simply dropping the enzyme solution onto the membrane, i.e., the biosensor-users may prepare the enzyme membranes themselves. Good results were obtained with GOD, lactate oxidase, and oxalate oxidase (Coulet, 1987). 

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