Metal nanoparticles posses the property of changing their optical properties as a function of both internal characteristics (size, shape, dielectric function) and refractive index of the local environment. A special class of applications in the field of biosensing uses the dependency of the nanoparticle's plasmonic peak localization on the local refractive index change. The response of this type of sensors is usually monitored by the change of the extinction spectrum of an ensemble of nanoparticles where analytes interact with functionalized nanoparticles in solution or immobilized at an interface; detection is done with a spectrophotometer. This type of sensors has a limited sensitivity. This can be overcome by using single nanoparticle based biosensors. This type of sensors measures the changes of the scatter spectrum of a collection of individually addressable functionalized nanoparticles in the presence of analytes.Here we report on a new detection method of binding events of analytes to functionalized gold nanoparticle using a standard colour camera attached to a darkfield microscopy setup. This setup is capable of parallel detection of the spectral shifts of thousands of 60 nm antibody-functionalized gold spheres as a result of binding events of protein analyte molecules. This setup can be the basis for multiplexing and quantification.
We present a fabrication procedure for batch production of MEMS devices directly on top of an optical fiber. The procedure relies on the approach introduced earlier by our group (Gavan et al 2011 Opt. Lett. 36 2898–900), which has been optimized here to obtain higher yield and increased reliability. We describe in details the eight steps of the procedure and we show its application to the fabrication of several cantilever-based structures. Overall, we report a process yield of 80% functioning MEMS devices in our final batch.
The light scattering and absorption properties of gold nanoparticles (GNPs) can be utilised for the detection of DNA. Binding of molecules to the GNP influences the local refractive index. The increase in refractive index can be measured as proportional red-shift of the GNPs extinction maximum; therefore GNPs are suitable for use as nanoparticle chemical sensors. Utilizing this method it is possible to detect DNA in naturally occurring quantities.In bulk measurements we have shown a red-shift of 7 nm of the absorption maximum (λ max ) upon binding of thiolated ssDNA. Subsequently, we were able to follow the interaction between two sets of GNPs functionalised with complementary strands.Randomly immobilised GNPs were visualised with an inverted darkfield microscope. The use of a colour camera enables us to analyse the colour change of each individual particle in the field of view. A change of λ max of 1 nm can be detected by the colour camera, which corresponds to ~100 20mer ssDNA molecules. For the detection of a single DNA binding events we are developing an assay for DNA detection, utilizing a second set of GNPs. The interaction of two GNPs within a range of 2.5 times the radius of each other results in a shift of ~7 nm in λ max for the presence of one DNA strand. This increased shift makes the method not only more accurate but also easier to detect.
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