In this study a diffraction based sensor has been developed for bio molecular sensing applications and performing assays in real time. A diffraction grating fabricated on a glass substrate produced diffraction patterns both in transmission and reflection when illuminated by a laser diode. We used zeroth order I(0,0) as reference and first order I(0,1) as signal channel and conducted ratiometric measurements that reduced noise by more than 50 times. The ratiometric approach resulted in a very simple instrumentation with very high sensitivity. In the past, we have shown refractive index measurements both for bulk and surface adsorption using the diffractive self-referencing approach. In the current work we extend the same concept to higher diffraction orders. We have considered order I(0,1) and I(1,1) and performed ratiometric measurements I(0,1)/I(1,1) to eliminate the common mode fluctuations. Since orders I(0,1) and I(1,1) behaved opposite to each other, the resulting ratio signal amplitude increased more than twice compared to our previous results. As a proof of concept we used different salt concentrations in DI water. Increased signal amplitude and improved fluid injection system resulted in more than 4 times improvement in detection limit, giving limit of detection 1.3×10-7 refractive index unit (RIU) compared to our previous results. The improved refractive index sensitivity will help significantly for high sensitivity label free bio sensing application in a very cost-effective and simple experimental set-up.
In this work, we explore the sensing applications of Surface Plasmon Resonance (SPR) enhanced
transmission of light through 1-D metal gratings on commonly available compact discs (CDs). We show that SPR
on CDs (CD-SPR) can be used to build a simple and compact angular displacement measurement system with submicro-
radian resolution. In addition we show that by controlling the azimuthal angle of the grating vector with
respect to incident k-vector, it is also possible to measure angular displacement in two planes which is not possible
with thin film SPR. The major advantage of this method is the compact form factor which will enable CD-SPR
based angular measurement systems to be integrated into other experimental setups with the least burden.
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