Metal clusters deposited on a substrate and positioned at a nanometric distance from a wave-reflecting layer act as nanoresonators able to receive, store and transmit energy within the visible and infrared range of the spectrum. Among the unique effects of these metal nanocluster assemblies are high local field enhancement and nanoscale resonant behaviour driving optical absorption in the visible and infrared range of the spectrum. In these types of devices and sensors the precise nanometric assembly coupling the local field surrounding a cluster is critical for allowing resonance with other elements interacting with this field. In particular, the cluster–mirror distance or the cluster–fluorophore distance gives rise to a variety of enhancement phenomena (e.g. resonant-enhanced fluorescence, REF). Depending on the desired application this ‘resonance’ distance is tuned from 5 up to 500 nm. High-throughput transducers using metal cluster resonance technology are based on surface enhancement of light absorption by metal clusters (surface-enhanced absorption, SEA). These devices can be used for detection of biorecognition binding as well as structural changes in nucleic acids, proteins or any polymer. The optical property made use of in the analytical application of metal cluster films is so-called anomalous absorption. An absorbing film of clusters is positioned 10–400 nm from an electromagnetic wave-reflecting layer. At a well-defined mirror–cluster distance the reflected electromagnetic field has the same phase at the position of the absorbing cluster as the incident field. This feedback mechanism strongly enhances the effective cluster absorption coefficient. These systems are characterized by a narrow reflection minimum whose spectral position shifts sensitively with interlayer thickness, because a given cluster–mirror distance and wavelength defines the optimum phase.
Based on the understanding of the absorption behavior of metal nanoparticles we aimed at the direct detection of sub-monomolecular layers of DNA with the naked eye. This extremely sensitive detection needs optical amplification techniques to be used in replacement of nanoparticle-aggregates applied e.g., in agglutination assays. We focus on the nanolayer-coated metallized-PET-chip setup and on the synthesis of DNA-nanoparticle conjugates suitable for 'resonance enhanced absorption'-point of care-tests and the application of those particles in the direct visualization of DNA-DNA binding events. Stabilization of nanoparticles and their sequence specific binding was proven with direct optical visibility of sub-monolayers of colored nanoclusters. Synthetic routes leading to suitable conjugates as well as stability tests and a biorecognition test are described in detail adding to the repertoire of tools that contribute to the application of nanoparticles in novel nano-enhanced devices.
The resonance-enhanced absorption (REA) by metal clusters on a surface is an effective technique on which to base bio-optical devices. A four-layer device consisting of a metal mirror, a polymer or glass-type distance layer, a biomolecule interaction layer and a sub-monolayer of biorecognitively bound metal nano-clusters is reported. Experiments indicate a strong influence of the resonator homogeneity on the absorption maximum. Layer stability plays an important role in the overall performance of the device. Techniques and optimised lab protocols to set up biochips that use the REA process in the detection are presented. The sensors show one to three narrow reflection minima in the visible and or infra-red (IR) part of the spectrum and therefore they do not suffer from the spectral limitations associated with spherical gold colloids. Metal clusters (synthesised by thermal step reduction) as well as metal- dielectric shell clusters (synthesised by various shell deposition processes) are used to precisely shift the readout of the device to any frequency in the visible and near IR range. Disposable single-step protein chips, DNA assays as well as complex biochip arrays are established that use various DNARNA, antigen-antibody and protein-protein interaction systems.
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