PhotonicSys Surface Plasmon Resonance Sensors:
Surface waves are waves propagating along the interface of two different media. They can be gravity waves such as at the surface between two fluids with different densities, elastic waves such as seismic waves (Rayleigh or Love waves) or electromagnetic (light waves) between two media having two different optical properties. One of the intriguing phenomena occurring at the interface of metals and dielectric media is surface plasmon resonance (SPR) wave. This wave maybe described as a collective oscillation of the free electrons Fermi gas at the metal surface. Its excitation requires light with wavelength incident at an angle such that its momentum along the interface is high enough to match that of the surface plasmon wave. This can be achieved using several techniques; the mostly used are shown in the figure below.
The wavelength of light or the incidence angle at which the SPR is excited, are strong functions of the refractive index of the dielectric medium adjacent to the metal surface. The dielectric medium can be a fluid such as water or blood with varying refractive index (RI) due to variations in the concentration of different biomolecules or other biological entities in the fluid (see figure below). Hence using this phenomenon one can build a biosensor to probe interactions between an analyte in solution and its corresponding recognition element immobilized on the SPR sensor surface. Based on unique SPR system design and substrates, our technology provides a compact non-invasive, label free system for studying biomolecular interactions. Using the same concept one can develop a range of applications for determination of food and drinks quality and safety, environmental monitoring, medical diagnostics, biochemical industry process inspection and other applications in research and development.
The miniature design allows integration into other systems such as microscopy to image the bioentities being monitored with SPR on the metal surface, for example with fluorescence microscopy, phase contrast, confocal and other techniques. It can also be integrated with a spectrometer for a more quantitative analysis.
One of the important properties of SP waves is the fact that the electromagnetic field (EM) is evanescent, that is decaying inside the analyte solution. The penetration depth is an important parameter for determining the sensor performance and it is on the order of 100-250nm when wavelengths in the visible range are used and without any special designs. This fact helps building sensor with high specificity by treating the surface with special immobilization molecules that attach specifically to the analyte to be detected. Because the EM field exists only near the surface, it can detect variations in the analyte concentration only within the evanescence region. Hence small molecules and viruses are easy to detect using the standard SPR structure and the SPR curve shifts monotonically with the analyte concentration in this case. However large bioentities such as cells extend beyond the penetration depth and therefore the signal shifts non-monotonically with their concentration. In order to be able to detect bacteria and cells the penetration depth has to increase to the microns scale.
With our proprietary methodology we are able to tune the penetration depth from few hundreds of nm till few microns, thus making the sensor suitable both for small and for large entities. As it can be shown in the figure below using our special design the penetration depth in our case covers more than two layers of bacteria. Hence it can be used for monitoring biofilms growth even.
The present version of the PhotonicSys SPR system can record sensograms with detection limit down to 10-6 refractive index units (RIU) in a user friendly graphical interface (see figure below). Translation from RI units to concentration units can be done easily using a simple formulae or a numerical table. Based on continuous referenced measurement, the temperature fluctuations and instabilities due to mechanical vibrations are compensated.