Chip Surface Considerations

An important consideration for the direct physical measurement of adhesion via pull-off measurements is the influence of the precise direction of the applied force. In AFM the cantilever does not usually lie parallel to the surface, due to the risk that another part of the cantilever chip or chip holder will make contact with the surface before the tip. Another problem relates to the fact that the spot size in the optical beam deflection method is usually larger than the width of the lever. This can result in an interference effect between the reflection from the sample and the reflection from the cantilever. This is reduced if the cantilever and sample are not parallel. Most commercial AFM systems use an angle in the range of 10°-15° between the sample and the cantilever. Depending on this angle and the extent to which the cantilever is bent away from its equilibrium position, there can be a significant fraction of unintentional lateral forces applied to the contact. 

While large-scale commercialization of the 3D chip has taken considerably more hme than expected, several companies now offer such surfaces. Motorola Life Sciences abandoned the Mirzabekov gel (Chapter 3) in favor of SurModic s PhofoLink surface chemistry (www.surmodics.com), chemistry which it then brought to market as the CodeLink . Amersham Biosciences acquired CodeLink from Motorola in 2002 and obtained a license to the Southern patents in 2003 to expand the product into the clinical diagnoshc arena. 

Scanning electron microscopy (SEM) is a powerful tool used to study the surface topography of potato chips. Figure 11.5 shows that potato chips lose a considerable quantity of surface oil after they are washed in petroleum ether, which allows clear observation of the cellular microstructure of the surface (Figures 11.5bl, 11.5b2 and 11.5b3) (Pedresehi etal., 2008). 

With conventional chips, it is well known that thiolated-DNA can be easily immobilized on gold but does not interact by ordered self-assembly, due to nonspecific interactions between the DNA bases and the gold. Different alkanethiols have been used to improve the hybridization efficiency and the selectivity, followed by immobilization of thiolated-DNA on the substrate [59,61,69]. Hence, this method requires two steps and needs consideration concerning the experimental conditions. However, DNA-conjugated polymer does not only prevent desorption of probe DNA from the surface, because the DNA is covalently bound to the self-assembly polymer, but it also... 

The performance of protein or antibody microarrays is dependent on various factors. One of these is the use of an appropriate microarray surface for the immobilization of the protein or antibody samples. Most conventional microarray surfaces have been adapted from DNA chip technology. DNA can easily be immobilized by electrostatic interactions of the phosphate backbone onto a positively charged surface. In contrast to DNA, as already mentioned, proteins are chemically and structurally much more complex and show variable charges, which may influence the efficiency of protein attachment. Additionally, proteins lose their structure and biochemical activity easily. For example, globular proteins consist of a hydrophilic exterior and a hydrophobic interior. When immobilized on a hydrophobic surface, the inside of the protein turns out, which may destabilize the structure and, simultaneously, the activity of the protein. These considerations demonstrate the complex requirements for protein immobilization. 

For a given chip power there is an optimal array configuration in terms of jet spacing and diameter. The concept of using arrays of liquid microjets was successfully implemented. The module has proved capable of dissipating 129 W, with a heat flux of 300 W/cm at a surface temperature of 80 °C, which is a considerable achievement at the present time. 

Due to downscaling of the microchannel, the surface free energy46,47 ( surface tension ) becomes considerable. As a consequence, the contribution of capillary pressure to the MALDI-chip pumping mechanism will be significant. For a flat rectangular channel the capillary pressure Pc [Pa] is given by35,48... 

The electrochemical properties of diamond films are very promising. Films doped for better conductivity feature a wide potential window, and owing to their stability and fast response times, etc., they suit very well to a use as electrode material in electroanalysis. Diamond films with their surface being suitably modified further suggest themselves for the analysis of biological material. Due to a low unspecific adsorption, interactions will occur only at those positions carrying the respective structure. This is of considerable interest for the development of the so-called lab on a chip. 

The miniaturization of sensors is permitting the creation of very small nanomechanical sensors at IBM that could create very high sensitivities, e.g. the detection of a few molecules on the surface. Also, at the moment the use of an optical transducer looks promising. Optical sensors based on the physical vibration bands of molecules (e.g. infra-red spectrometers) are not particularly sensitive but combined with new materials the sensitivity can be impressive. For example, a recent report on colour-sensitive dyes based on Lewis acid ase reactions offers considerable promise [8]. Figure 8 shows an electronic nose based on a set of dyes that changes colour when the target analyte is introduced and this is detected using a CCD array chip. The unit has been shown to have ppb sensitivities to certain compounds. 

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