Effect Of MgF₂ Dielectric Film On Sensitivity Of Long-range Surface Plasmon Resonance Based-biosensor

The technique of SPR-based biological sensing offers several distinctive advantages, such as label-free detection, real time monitoring and high sensitivity. A new type SPR-Long-Range Surface Plasmon Resonance(LRSPR) has been introduced in this paper. The dependence of the LRSPR curve shape on the thicknesses of the metal film and the intermediate dielectric film is studied from the theoretical and experimental point of view.The condition and fundamentals about the resonance of surface plasmon and evanescence wave which is originated from p-polarized light after total reflection are analyzed. Long range surface plasmon resonance is discussed further when thin metal films are sandwiched between two dielectrics with similar refractive indices.A set of SPR monitor instrument based on the angular spectroscopy is developed, which consists of a SPR sensor, a sampling system, a data acquisition system, and analysis software. The full scale of angular scan is between 30 degree and 70 degree, and the angular resolution is 0.005 degree.A four-phase numerical model which has a symmetrical layer structure is derived from Fresnel's formulas and multiple-reflection theory of Hansen. Characteristic of this model is simulated with the professional software-MATLAB. The relationship between the width of the resonance curve at half-maximum height and the collocation of the silver film and MgF2 dielectric buffer layer is discovered. MgF2 film and silver film are prepared by magnetron sputtering so that the thicknesses can be controlled precisely. Deionized water is detected by our SPR instrument. The result shows that the width of the resonance curve at half-maximum height will be reduced at LRSPR mode when the thickness of MgF2 film increased and the thickness of silver film decreased correspondingly. The width of the resonance curve at half-maximum height is 0.59°when the optimum thickness of MgF2 film reaches 651nm and the corresponding thickness of silver film is 46nm,while the width of the resonance curve at half-maximum height is 1.25°at the conventional SPR mode when the optimum thickness of silver film is 57nm。The width of the resonance curve at half-maximum height can be reduced by 52% for LRSPR mode compared to conventional SPR mode. The refractive index of biological solution is near water; therefore the LRSPR mode with MgF2 as dielectric buffer layer material can increase the sensitivity of biosensor effectively.