Monolayer-coated sensors

Figure 7. Amperometric response of glucose sensors prepared using LB film-coated electrodes. ( ) multilayer GOx (o) monolayer GOx sensors.

Hydrazine activation can also be used as the basis for site-directed immobilization or directional immobilization [16]. Directional immobilization is the immobilization of the sensor detector molecules in a highly ordered and reproducible manner. For example, in antibody-based biosensors requiring a monolayer coating of antibody on the transducer, it is critical to sensor functionality and sensitivity that as many of the antibodies as possible are immobilized such that their antigen binding sites are directed outward and easily accessible by antigen (analyte). 

Table 2. Responses of a grating coupler sensor coated with multilayers of monoclonal antibodies to the respective analytes. The response values are related to that of an adsorbed antibody monolayer (B2M p2-microglobulin, HCG human choriogonadotropin, HRP horseradish peroxidase.

Attaching the catalyst molecules to the electrode surface presents an obvious advantage for synthetic and sensor applications. Catalysis can then be viewed as a supported molecular catalysis. It is the object of the next section. A distinction is made between monolayer and multilayer coatings. In the former, only chemical catalysis may take place, whereas both types of catalysis are possible with multilayer coatings, thanks to their three-dimensional structure. Besides substrate transport in the bathing solution, the catalytic responses are then under the control of three main phenomena electron hopping conduction, substrate diffusion, and catalytic reaction. While several systems have been described in which electron transport and catalysis are carried out by the same redox centers, particularly interesting systems are those in which these two functions are completed by two different molecular systems. 

A C felt sensor coated with a glucose oxidase monolayer was used in an FI manifold in the presence of dissolved potassium hexacyanoferrate as mediator for the on-line determination of interferents [89]. 

Fig. 12.6. Influence of typical interferents on mercury sensors based on changes of lateral resistance of thin gold layers. Sensors bare gold layers (open symbols), gold layers coated by self-assembled monolayers of 1-hex-adecanethiol (filled symbols). Interferents 100% humidity (a), saturated vapour of sulphuric acid (b), 10 pg/1 of 1-butanethiol (c), 10 pg/1 of iodine vapour (d), Thickness of gold layers 42 nm [25].

Self-assembled monolayers on solid substrates have significant technological applications as coatings and sensors and in nanofabrication. 

Another important area where gold-thiol monolayers might find promising applications is gas- and biosensing. Simple sensors sensitive to certain types of compounds, based on such detection methods as surface plasmon resonance or surface acoustic wave, have been described454,455,531-533. This type of device is usually made of a gold plate coated with a functionalized monolayer. The terminal functional group of such a monolayer is responsible for selective interactions with the analyte, and adsorption of the latter is then detected by the appropriate method. 

Direct detection of a cytokine protein, recombinant human interferon-y, using an IBIS SPR sensor is presented in [60]. Several types of sensor chip coatings, including self-assembled monolayers and hydrogel-derivatized SAMs, were characterized in terms of their ability to resist non-specific adsorption from plasma. The best results with respect to plasma adsorption and surface regenerability were obtained with antibodies immobilized on the dextran-modified 11-mercaptoimdecanoic acid SAM. The detection limit for detection of human interferon-y in 1 100 diluted plasma was established at 250ngmL . ... 

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