Surface immobilization techniques

Since none of the liposomal immunoassay approaches described in the scientific literature thus far took advantage of surface immobilization techniques, one could envision a double-amplification biosensor in which surface modification plays an important role [35]. For example, consider a dehydrogenase enzyme marker system which requires an electroactive cofactor such as NAD+. In the enzymatic reaction scheme ... 

A new trend in Mizoroki-Heck reactions is the apphcation of supported palladium catalysts with the aim of easy catalyst recycling and higher selectivity. The application of such catalysts results in a higher regioselectivity, which might be rationalized by the increased steric hindrance of the catalyst at the surface. Immobilization techniques use catalyst on a carrier, catalyst and ionic liquid on a carrier, ionic liquid and ligand on a carrier with and without catalyst, fixation of the base and the starting material. 

Advances have been achieved in recent years, such as the use of CL reagents as labels to derivatize and sensitively determine analytes containing amine, carboxyl, hydroxy, thiol, and other functional groups and their application in HPLC and CE [35, 36], the synthesis and application of new acridinium esters [37], the development of enhanced CL detection of horseradish peroxidase (HRP) labels [38], the use of immobilization techniques for developing CL-based sensors [39-42], some developments of luminol-based CL in relation to its application to time-resolved or solid-surface analysis [43], and the analytical application of electrogenerated CL (ECL) [44-47], among others. 

As a technique for selective surface illumination at liquid/solid interfaces, TIRF was first introduced by Hirschfeld(1) in 1965. Other important early applications were pioneered by Harrick and Loeb(2) in 1973 for detecting fluorescence from a surface coated with dansyl-labeled bovine serum allbumin, by Kronick and Little(3) in 1975 for measuring the equilibrium constant between soluble fluorescent-labeled antibodies and surface-immobilized antigens, and by Watkins and Robertson(4) in 1977 for measuring kinetics of protein adsorption following a concentration jump. Previous rcvicws(5 7) contain additional references to some important early work. Section 7.5 presents a literature review of recent work. 

Presently, specific immobilization of various enzymes is studied under the aspect of the orientation and the local surface environments. The deeper understanding of biocatalytic systems together with suitable surface coating techniques may lead to biologically inspired and more complex catalytic systems grafted on solid supports. 

Physical or electrochemical adsorption uses non-covalent forces to affix the nucleic acid to the solid support and represents a relatively simple mechanism for attachment that is easy to automate. Adsorption was favoured and described in some chapters as suitable immobilization technique when multisite attachment of DNA is needed to exploit the intrinsic DNA oxidation signal in hybridization reactions. Dendrimers such as polyamidoamine with a high density of terminal amino groups have been reported to increase the surface coverage of physically adsorbed DNA to the surface. Furthermore, electrochemical adsorption is described as a useful immobihzation strategy for electrochemical genosensor fabrication. 

A large number of metJiods for immobilizing biomolecules on the surface of solid substrate have been proposed in the past few decades, in which the molecules are immobilized on a carrier using covalent bonds ( i, ionic bonds (2), physical adsorption (3), cross-linkage of the biomolecules (4), or by microencapsulation (5). Immobilizing techniques are indispensable to treat biomolecules in an experiment. The provision of an immobilization process is one of the most essential processing steps that are required in order to obtain practical biomolecule carriers such as... 

Several examples of catenanes and rotaxanes have been constructed and investigated on solid surfaces.1 la,d f 12 13 26 If the interlocked molecular components contain electroactive units and the surface is that of an electrode, electrochemical techniques represent a powerful tool to study the behavior of the surface-immobilized ensemble. Catenanes and rotaxanes are usually deposited on solid surfaces by employing the Langmuir-Blodgett technique27 or the self-assembled monolayer (SAM) approach.28 The molecular components can either be already interlocked prior to attachment to the surface or become so in consequence of surface immobilization in the latter setting, the solid surface plays the dual role of a stopper and an interface (electrode). In most instances, the investigated compounds are deposited on macroscopic surfaces, such as those of metal or semiconductor electrodes 26 less common is the case of systems anchored on nanocrystals.29... 

To characterize the properties of molecules and polymer films attached to an electrode surface, a wide variety of methods have been used to measure the electroactivity, chemical reactivity, and surface structure of the electrode-immobilized materials [9]. These methods have been primarily electrochemical and spectral as indicated in Table I. Suffice it to say that a multidisciplinary approach is needed to adequately characterize chemically modified electrodes combining electrochemical methods with surface analysis techniques and a variety of other chemical and physical approaches. 

Continuous-flow microreactors were successfully fabricated by etching channels in silicon and immobilizing urease onto channel surfaces by a layer-by-layer self-assembly technique. Preliminary results show urea conversion. The potential advantages of this surface-coating technique in microreactors warrant continued investigation. 

---