I think it might due to the change in the refractive indices of two different mediums. Thus, the condition to observe SPR at the water/chip interface also changes. Hope it helps!

Tu Anh Ngo thank you for your reply. Of course the refractive index is changing and this is what I expect. This is going to change the SPR dip position. I want to observe the dip position shift. But the problem is it disappears completely.

When refractive index of sensing medium varies, the wave wave vector of surface plasmon wave (SPW) also shifted so it is possible that the wave vector of incident photon (LED) is not matches with SPW. You should use different source of photon {633 to 1550nm}.

Shivam Singh thank you for reply. I am using 650 nm point source emitter as a led. It works in the air. But in the water there is no SPR dip. After a few research I found that the problem is the glass type of the prism which is BK7. For refractive index of BK7 the SRP goes to the extreme limits which is about 87 degree. At these angles there is interference and it is making difficult to observe the actual SPR dip. Now I am looking for a solution without changing the prism.

You should vary the thickness of metal layer or sensor chip. May be SPR Dip shift towards lower degree.

Hi

did you find the solution?

Sara Sadeghi yes. The problem was the material of the prism. When you use BK7 and the medium is water the dip shift to the edge of the range which is very difficult to observe. You have 2 option to solve the situation. First, you can use equilateral triangle shaped prism instead of right angled one. Second you can change the material for example sf11 is a good option.

Thank you so much. I have another question.

did you fixed the SPR angle in air environment or in water?