Microarrays provide a powerful analytical tool for the simultaneous analyses of thousands of parameters, be they DNA or proteins, in a single experiment. However, a number of challenges continue to hinder the widespread application of microarray assays for analytical and diagnostic purposes. Advances in detection methods can help overcome some of these challenges by improving sensitivity and reliability in signal detection and by enabling real-time detection of binding events. Thus, the aim of this review is to highlight the most promising optical microarray biosensing techniques. The principles of techniques making use of labels, including scanner type, total internal reflection type, fiber-optics-based, and SPRenhanced fluorescence, are described, along with a brief summary of labeling strategies. State of the art label-free techniques, including imaging SPR and imaging ellipsometry, are also reviewed. Examples of microarray-based assays using each technique are given to illustrate both their usefulness and their limits of detection. Furthermore, the most competitive commercial microarray systems are presented and compared with one another in the context of their detection system. Finally, a discussion of the remaining challenges as well as trends and future applications of microarrays are presented in the context of optical sensing.
Simple methods for the determination of refractive indices of transparent polymers and inorganic and organic solids of irregular geometry or with scratched or corrugated surfaces are rare. A classical procedure is based on the invisibility of a body immersed in a liquid with the same refractive index as that of the body. In order to avoid the laborious procedure connected with the search for a liquid with matching refractive index and to find an approach which is independent of the observation by eye, we describe here a modified immersion method which allows the ready determination of the refractive index of solids. The present method is based on the interpolation of the maximum transmission (n Tmax ) of a solid immersed in liquids with different, typically non-matching, refractive indices. Illustrations with quartz glass, crown glass and poly(vinylidene fluoride) (PVDF) films showed that n Tmax can be determined with a reproducibility of ±0.003. By comparison with refractive indices determined by ellipsometry, it was concluded that the refractive index of a solid can be determined with the modified immersion method within an accuracy better than ±0.01 when systematic errors resulting from the fit method are also taken into consideration. C 2005 Springer Science + Business Media, Inc.
The production of hierarchical nanopatterns (using a top-down microfabrication approach combined with a subsequent bottom-up self-assembly process) will be an important tool in many research areas. We report the fabrication of silica nanoparticle arrays on lithographically pre-patterned substrates suitable for applications in the field of nanobiotechnology. Two different approaches to reach this goal are presented and discussed: in the first approach, we use capillary forces to self-assemble silica nanoparticles on a wettability contrast pattern by controlled drying and evaporation. This allows the efficient patterning of a variety of nanoparticle systems and—under certain conditions—leads to the formation of novel branched structures of colloidal lines, that might help to elucidate the formation process of these nanoparticle arrays. The second approach uses a recently developed chemical patterning method that allows for the selective immobilization of functionalized sub-100 nm particles at distinct locations on the surface. In addition, it is shown how these nanocolloidal micro-arrays offer the potential to increase the sensitivity of existing biosensing devices. The well-defined surface chemistry (of particle and substrate) and the increased surface area at the microspots, where the nanoparticles self-assemble, make this patterning method an interesting candidate for micro-array biosensing.
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