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Planar Total Internal Reflection Biofouling Sensors


Planar, integrated microscale sensors utilizing prism-coupler type angular interrogation sensing technique have been demonstrated. The main structure of the sensor consists of an optical prism coupled to a built-in waveguide to introduce Fraunhofer diffraction when light ray comes into the prism from the waveguide. The Fraunhofer diffraction creates spectrum of consecutive rays over the sensing edge of the prism such that there is no need for the bulky scanning mechanisms typically used in other macro scale sensing systems. Two types of sensors are presented: (1) total internal reflection based critical point detection (CPD) sensor, and (2) surface plasmon resonance (SPR) based resonance point detection (RPD) sensor.

The CPD sensor is fabricated by a simple, two-mask process which creates a right angle prism with three sides with lengths of 1, 0.86, and 1.33 mm, respectively in the prototype design and a waveguide with a cross sectional area of 4~0.25 um2. The 0.25 um-thick core and the 2.5 um-thick cladding layers of the waveguide are made of silicon nitride and silicon dioxide, respectively. The CPD sensing technique measures the shift of the critical point of the total reflection as the results of change of refractive index due to biofouling. Optical simulations are used to validate the working principle and the calculated biofouling sensitivity is comparable to the other optical sensing methods. A baseline measurement has been conducted to verify the operation of the sensor with an error of less than ± 0.002 R.I.U. During a 9-hour biofouling measurement using milk as the media, a change in the refractive index as much as 0.0089 is recorded as the result of biofouling.

The RPD sensing technique employs surface plasmon resonance as its sensing mechanism by measuring the shift of the resonance point with respect to the change of the incident angle. The design and fabrication process is similar to the fundamental structure of CPD sensors with an additional deposition of a thin metal layer on the sensing edge of the prism. The theoretical sensitivity is calculated as 90 deg RIU-1, which is comparable with the state-of-the-art optical sensors at 127 deg RIU-1. The refractive index measurement for selected liquids agrees with the values in the literature with an error range of less than ± 0.002 R.I.U. Furthermore, the refractive index change of biofouling formation is measured to be 0.0078 for a 9-hour experiment using milk as the testing media.

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