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(a) A schematic view of the core of a generic SPR system. A transparent prism is metal coated on one face where a light beam is shone in total internal reflection conditions. Plasmon Resonance is detected monitoring with a CCD camera the reflectivity properties of the metal surface. This surface is treated with a ligand. When a liquid containing the target molecule is circulated, analyte, and ligand bind to form a larger complex. As a result the overall RI and the reflectivity curve are changed. (b) Reflectivity SPR curve as integrated by the CCD camera in the scheme (a). The curve is calculated for a gold coating of about , in water solution and illuminated by laser beam. A positive variation in index of refraction (about 1%) results in an increased resonance angle.
The comparative analysis of the response of a glass prism based SPR sensor and a higher refraction index material based one. The grayed group of curves is the simulated SPR response to different RI samples, ranging from to . The calculation are made considering a laser beam, a thin gold metal layer over a prism. The other curves are calculated in the same conditions for a standard glass prism. In the case of glass substrate only the first two lower RI curves show resonance peaks. With high RI material deeper and sharper peaks appear for all the RI range. In an actual sensor this will result in wider dynamic, better signal-to-noise ratio and sensitivity.
The transmission spectra of the ceramic used is compared with quartz. This is a special transparent material based on the so called BMT mixture: Ba(Sn,Zr,Mg,Ta)O3. This ceramic can be molded to any shape, has a very high RI and a wide transmission range.
The core of the setup. (a) A schematic representation of the ceramic sensor in the flow cell. A diode laser illuminates a thick gold layer deposited on the face of the sensor exposed to the solution circulating in the flow cell. The laser beam is incident at a variable angle , and it is internally reflected on the gold surface at . The reflected light is finally collected by a detector (CCD). The angle is 45°. (b) A picture of the sensor. The letter “c” indicates the flow-cell block, one of the in/out pipes is visible to the right indicated by an arrow. The ceramic “prism” sensor is encased in the center, recognizable by its reddish color. To the side of the flow cell another isolated one is shown marked by the symbol “p.”
The differential response of the BMT ceramic based sensor. The signal plotted results from the reflectivity difference between pure water and a 3% sucrose solution. In the inset the raw reflectivity curves are shown within the angular range grayed on the main plot.
The SPR sensor linear response to different glucose concentration of a standard glassy SPR sensor. In the inset, differential response for ceramic substrate reaches about at 3% glucose concentration, one order of magnitude better than with the glass based sensor.
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