![]() In addition, identical interactions at differing distances from the sensor surface will give rise to different signal shifts. The high signal-to-noise ratios achievable with evanescent field sensors are partially due to this relative insensitivity towards changes in the sample bulk. One important consequence of the exponential decay of the evanescent field is that a typical SPR biosensor is unable to detect events beyond 600 nm from the sensor chip surface. SPR biosensors work measure alterations in the SPR angle as a sample passes over and analyte molecules bind to immobilized ligands at or close to the sensor surface. As there is a linear relationship between the amount of surface-bound material and the change in SPR angle, this can be used to calculate the amount of analyte bound to the surface. This refractive index will alter as macromolecules adsorb to the surface, in turn altering the angle at which SPR occurs. The value of the SPR angle is dependent on the refractive index of the surface and its immediate surrounds, up to a distance of approximately several hundred nanometres. The majority of commercial instruments utilise light with a wavelength between 600 and 800 nm, and thus d is in the range of 300 – 400 nm. This decay length is usually defined as the distance over which the intensity of the evanescent field drops to 1/e, approximately 37% of initial intensity. In the sample buffer above the metal, the decay length of the field (d), is approximately half the wavelength of the light involved. The field in this perpendicular direction is said to be evanescent, reflecting the bound, non-radiative nature of surface plasmons. The combination of these characters affects the surface-perpendicular component of the electric field, causing enhancement near the surface, but exponential decay as one moves further and further away. Surface plasmons which occur at the interface between a metal and a dielectric material contain both electromagnetic wave and surface charge characteristics. Thus the ‘SPR angle’, the angle at which SPR waves are induced, is associated with a sharp ‘dip’ or decrease in reflected light intensity. Energy from the photons must be diverted to create these waves, and so the intensity of the reflected light will be dramatically decreased. If the incoming light is p-polarised, occurring at a certain wavelength and angle, the evanescent field will induce the formation of surface plasmon polaritons – electromagnetic waves travelling within the free electrons of a thin metal film (usually gold). Although all of the photons are reflected from the interface, part of the electromagnetic field component will penetrate through to create an electromagnetic evanescent field. Total internal reflection occurs when all of the inbound light is reflected from this interface, and usually occurs at shallow reflective angles. Evanescent fields, characterised by exponential intensity decay, are generated when light undergoes total internal reflection at a boundary between two substances of differing refractive index, for example glass and air. The initial phase of SPR requires the formation of an evanescent field at the sensor surface. ![]() Alterations in the physical conditions at the surface have a significant effect on this resonance, and the detection of these changes form the basis of the SPR biosensor. Surface Plasmon Resonance is caused by the interaction of electromagnetic radiation and the free-flowing cloud of electrons within a metal, a process which induces electromagnetic waves and, under certain conditions, resonance effects. ![]()
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