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| Research Focus & Application: |
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Applications for nano particles touch hundreds of areas from biosensors to microelectronics to textiles to construction materials with more innovative uses every day. Most work in Nano particles is typically being done below 1 micron in size and often only a few tens of nanometers you may wonder how optical microscopes can help when operating below the optical Rayleigh "resolution" limit. One such application in Nanotech is related to characterizing nano particles (spheres, wires, other shapes, etc.). These particles typically range from 10nm to 2micron. And since some applications do not call for resolution but for detection… an optical microscope can be used to "detect" particles well below the commonly considered resolution limit of ~200nm. The common optical method of choice to view / detect nano particles is DARKFIELD microscopy. This technique makes the particles visible through the scattering of light (i.e. Mie scattering effect). Much like dust particles are made visible by a ray of sun light in a dark room. Darkfield illuminates at an oblique angle and the specular reflected light passes outside the cone of the objective lens while only the scattered light is captured by the objective lens to form the image. This technique can be either transmitted or reflected light which is sample and application dependant. Whether it be the ultra condenser for transmitted light darkfield or the new “Advanced” darkfield, where Zeiss has a strong optical configuration, designed to reduce stray light for a very high quality and efficient darkfield. The smaller the particle the more light may be needed to have sufficient scattered light to detect it (i.e. X-Cite source). The general size of the particle in the intermediate image plane (if it is sub-resolution) is based on the general size of the Airy disc formed by the objective used (PSF:point spread function)and highly dependent on numerical aperture. The particles intensity may also give hints as to if more than 1 particle is present in close proximity to each other.
Once being able to visualize the particle has been accomplished now real interesting things happen based on size, shape and surrounding medium…quantitative information can be gleaned from the scattered light spectrum. It seems, due to Mie scattering and some other atomic effects(i.e. Plasmon resonance), the scattered light spectrum may have an absorbance peak(s) that may be in different locations of the spectrum based on particle characteristics which gives rise to varying colored particle. This scattered light may now be passed to a spectrometer for spectral analysis and correlation.
| Microscopy and Imaging Methods: |
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Darkfield reflected light with 100x 0.95NA EpiplanApo objective on AxioImager microscope
| Why the instrument was selected for this application: |
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Highly efficient darkfiled with low stray light. | |
40 nanometer Au (gold) particles in aqueous solution using 100x EC Epiplan Apo Brightfield/Darkfield objective. |