epDifferential surface plasmon resonance imaging (dSPR), is an ultra-sensitive surface interrogation technique which can provide the basis for chemical and biological sensors. Two of the cameras designaed within iBIOS, have been applied to such dSPR experiments as part of an ongoing research project with the University of Exeter. Like SPR imaging, dSPR imaging takes advantage of a phenomenon that occurs at an interface of two differing media under total internal reflection in certain conditions.
At an interface between two homogeneous media of differing refractive indices n1 and n2 where n1>n2, light incident from above a certain critical angle of exhibits total internal reflection. All incident light is reflected the electromagnetic field component penetrates a short distance (tens or hundreds of nanometres) into a medium of a lower refractive index creating an exponentially diminishing evanescent wave.
It has been shown that when a thin metal/dielectric interface is inserted between the two media, a beam of monochromatic p-polarized light (often called TM polarisation) focussed onto it through a prism (n1 media) results in a sharp shadow in the reflected light at a specific incident angle due to the resonance energy transfer, as shown in the figure to the left.
If linearly polarised light containing both s and p polarisations is incident upon the surface plasmon set-up, the p-polarised light undergoes a phase change, whereas the s-polarised light is unaffected. the fiogure to the right shows the behaviour of both s and p-polarised light at the plasmon angle. The phase change of one of the two orthogonal components results in the reflected light being elliptically polarized. The phase changes rapidly at the minima of the plasmon dip, thus the ellipticity and orientation of polarization ellipse also changes rapidly. Crucially, the phase response is sharper than the intensity dip in p-polarized reflectivity, leading to a system more sensitive than standard intensity based SPR techniques.A polarizer placed in the path of the elliptically polarized light produces a signal with a cos2(Φ)dependence, along with an amplitude and constant offset due to the ellipticity [Hooper, 2008]. The angle at which the maximum occurs in this dependence is the azimuth of the polarization ellipse
In the dSPR system, a photo elastic modulator (PEM) is introduced after the first polarizer (before the Kretschmann-Raether Cell) as shown below.
The light from the LED is collimated with a lens, the light is then polarized by the input polarizer. A sinusoidal dither is introduced on the polarization state of the incident light to the SP setup with a photo-elastic modulator (PEM), which has a resonant frequency of 47 KHz, prior to being focussed onto the Kretschmann-Raether Cell. A further polarizer analyses the reflected signal prior to detection by the phase-sensitive camera.
A device such as a modulated light camera, clocked at the same frequency as the dither, can be used as a phase sensitive detector to monitor the reflected signal from the system. The differential of the cos2(Φ) curve can be extracted as a function of the angle of the polarizer in front of the detector, the zeros being the azimuth or azimuth +/- π/2 rad Of the polarization ellipse. Changes to the refractive index at the boundary of the two media alter the angular position of the zero.
The sensitivity delivered by this system allows for sensitive measurements such as the surface sensing of binding. The detection of molecular binding is possible using a set-up as shown below, where antibodies are attached to the surface of the gold layer with a sample solution flowing across it. Any binding at the surface causes a change in the diffractive index that is detectable via the shift in the angular position of the zero.