Absorbing mirrors are optical thin film systems consisting of an absorbing metal layer held by a transparent interlayer in defined distance to a (metal) mirror layer. In the reflected light, such three-layer systems show vivid color effects due to the absorption in the metal. This absorption is modified and enhanced by interference effects. The interference condition can be tuned by the thickness of the transparent interlayer. We show by experimentation and by stratified medium model calculations the importance of a nanoscale granular structure in the absorbing metal layer for obtaining a wide spectral range of the color effect.
Spatially tuned resonant nano-clusters allow high local field enhancement when exited by electromagnetic radiation. A number of phenomena had been described and subsequently applied to novel nano- and bionano-devices. Decisive for these types of devices and sensors is the precise nanometric assembly, coupling the local field surrounding a cluster to allow resonance with other elements interacting with this field. In particular, the distance cluster-mirror or cluster-fluorophore gives rise to a variety of enhancement phenomena. High throughput transducers using metal cluster resonance technology are based on surface-enhancement of metal cluster light absorption (SEA). The optical property for the analytical application of metal cluster films is the so-called anomalous absorption. At a well defined nanometric distance of a cluster to a mirror the reflected electromagnetic field has the same phase at the position of the absorbing cluster as the incident fields. This feedback mechanism strongly enhances the effective cluster absorption coefficient. The system is characterised by a narrow reflection minimum.
Based on this SEA-phenomenon (licensed to and further developed and optimized by NovemberAG, Germany Erlangen) a number of commercial products have been constructed. Brandsealing(R) uses the patented SEA cluster technology to produce optical codings. Cluster SEA thin film systems show a characteristic color-flip effect and are extremely mechanically and thermally robust. This is the basis for its application as an unique security feature. The specific spectroscopic properties as e.g. narrow band multi-resonance of the cluster layers allow the authentication of the optical code which can be easily achieved with a mobile hand-held reader developed by november AG and Siemens AG. Thus, these features are machine-readable which makes them superior to comparable technologies.
Cluster labels are available in two formats: as a label for tamper-proof product packaging, and as a direct label, where label and logo are permanently applied directly and unremovable to the product surface. Together with Infineon Technologies and HUECK FOLIEN, the SEA technology is currently developed as a direct label for e.g. SmartCards.
Molecular interaction especially biorecognitive binding can be visualized by metal cluster enhanced fluorescence. Fluorescent molecules that are bound within the electromagnetic field of a layer of metal clusters exhibit a strong boost in excitation as well as emission. We present a study, using novel surface enhanced chips in glass-slide-format, their set up, the micro arraying onto the surface, and the hybridization of oligonucleotides on these chips. Compared to standard (glass slide) DNA chips, performance, fluorescent signals as well as signal to noise ratio were considerably higher.
Metal clusters excited by light exhibit high local field enhancement and nanoscale resonant behavior. Absorptive properties of these metal clusters bound to a surface are the basis of various new and highly promising setups to transduce biorecognitive interactions into an optical signal. Multilayered highly resonant systems had been proposed and recently demonstrated employing a metal mirror, a nanometric polymer distance layer, a biomolecule interaction layer and biorecognitively bound metal nano clusters. The optochips clearly exhibit strong reflection minima induced by the resonant behavior of the metal cluster layer. At least one narrow reflection minimum can be shifted to the red or infra red spectral range and therefore far away from spherical gold colloids (less than 520 nm) and human plasma absorption. The setup enabled us to replace conventional binding assays (like ELISA) overcoming the various technological limits as there are multiple incubation steps, harmful reagents and spatial resolution. A modified setup (the metal island coated swelling polymer over mirror system) employing an optical thin-layer system consisting of a metal mirror, an active analyte-induced swelling polymer, and a metal cluster (island) film as the topmost layer was used to transduce human plasma ion concentrations.
The choice of metal clusters as signal transducers of molecular binding events is based on their about 1000 times higher extinction coefficients compared to conjugated chromophores. Using cluster based assays it is possible to visualize the binding of biomolecules at a given surface by a bound layer of ligand-modified metal clusters. The success of cluster visualization was mainly based on the significant signal stability contrary to chromophores and especially fluorophores. Cluster probes are not only efficient direct markers but within the past years became the basis of new devices employing cluster resonance, cluster field enhancement, and cluster-cluster interactions. Multilayered highly resonant systems clearly exhibit strong reflection minima induced by the resonant behavior of the metal cluster layer. At least one narrow reflection minimum can be shifted to the red or infra red spectral range and therefore far away from spherical gold colloids to be ready by a 10 micrometer resolution optical scanner type high density device. Even without employing near-field optics spatial resolution is within 100 - 500 nm. The setup enabled us to replace conventional ELISA assays overcoming the various technological limits as there are e.g. multiple incubation steps and spatial resolution. The focus of the development was to provide an optical biochip which allows detection of analytes based on arrays of proteins, DNA and high throughput-screening targets for drug discovery.
Surface enhanced absorption of metal nano-clusters enabled us to transduce bioaffinity interactions highly amplifying the optical effect of changes in sensor surface coverage. The sensors were built depositing multiple nanoscale layers: at first silver or gold were sputtered onto oxygen plasma activated polycarbonate substrates to obtain a semitransparent metal cluster layer. Alternatively the primary metal layer was built of gold colloids covalently coupled to the activated polycarbonate. Next a chemically inert distance layer was applied e.g. by polymer-spinning. Finally a second cluster layer of e.g. gold colloids was coupled via bioaffinity interactions to the surface of the inert distance layer. The optical properties of the senor were found to be dependent on the size, shape and number of the metal-clusters as well as the distance between both metal cluster layers. For biomedical sensing the number and the spatial arrangement of biorecognitive bound metal clusters was transduced into an optical signal with high sensitivity. Since the defined spatial approach of colloids to the sensor surface alone creates the signal we could visually follow molecular binding events in real time. The first setups constructed were based on lectin-hexose or antibody-antigen interaction. The analytes were quantified via a distinct change of the spectral reflectivity of the sensor chip visible to the eye or measured by a miniaturized photometric device.
The device presented is based on a supported membrane incorporating modified ion channels. Various analytes induced down regulation of the ion flux by interaction with specific binding sites or artificial ligands attached near the channel entrance. To set up such a device new types of stabilizing membrane supports had to be developed. The new support presented was deposited by electyropolymerization of 1,2-diaminobenzene onto metal electrodes thus exhibiting a highly charged surface. Negatively charged lipids formed SA- membranes tightly bound to that gel surface. Mixtures of biological lipids and archaeabacteria type bolaamphiphilic lipids minimized lipids minimized floating of the membrane layer. Various ligands were bound to the modified Bisgramicidin-A to interact with their specific antibodies. It turned out that it is vital to accurately coupled the ligand at a single functional hydroxy-group. Sensors were optimized using a metal/ligand and 2,4- dinitrophenol/polyclonal antibody setup. Summing up, highly stable supported membranes were formed, ion channel were functionally integrated into these membranes and molecule interactions of analytes with embedded ion channels were monitored.
A new technique for direct monitoring of biochemical binding events of individual molecules was developed. Artificial gated ion channels in supported membranes of circular bolaamphiphilic lipids mixed with lipid phase modifiers were used to monitor the binding events of analyte molecules to the ion channel inducing a change of the ion concentration in a small sub membrane compartment. Ion flux or the ion concentration within the small compartment was determined electrochemically or by using fluorescent indicator dyes. Binding of the analyte to ligand modified peptide channels resulted in an on/off-response of the channel current due to channel closure or distortion. Trans-membrane permeability changes were quantified by applying a trans-membrane potential or a transient pulse of pH or ion concentration. Doubtless, the need for a direct more specific measurement in complex matrices leads to the increased interest in this form of miniaturized analytical device. Nevertheless most of the biosensors developed up to now use the biochemical processes of correlated bioanalytic assays. In bioanalytical assays where the analyte binds to an artificial membrane ligand a new strategy had to be developed. This paper presents new sensor devices using optical and electrochemical signal transduction. The biorecognitive interaction of an analyte at a ligand modified Bisgramicidin A ion channel results in a nearly digital on/off response of the channel current due to peptide channel closure or distortion. The current response of the sensor induced by analyte binding depends on an accurately positioned small ligand and on the binding of a large analyte molecule as e.g. an antibody. The trans-membrane leakage of membrane through membrane channels. Sodium and potassium ions as well as protons are widely spread in biological fluids. For this reason they are utilized as mobile species which are forced to move through the ion channels by an electric field or a concentration gradient. This gradient was induced either electrochemically or by ion concentration shift. The data presented show the optimization of the setup and first studies with this new device monitoring analyte gated ion channels in supported lipid membranes on sensor chips.