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Integrated optic sensor for pH and ammonia

Planar waveguides have evanescent fields sensitive to index of refraction changes in the volume immediately above the waveguide surface. Optically combining one guided sensing beam with a reference beam in an interferometric configuration generates measurable signals. Applying a chemically selective film over the sensing arm of the interferometer provides the basis for a chemical sensor. Acid-base chemistry either on the waveguide surface or incorporated into a polymer film, deposited on the waveguide, allows for reversibly sensing pH in solution or ammonia in air. The pKas of the sensing groups can be spaced across the pH range to permit continuous change over the entire range. Alternatively, sensing and reference arms can have different sensing groups with pKas, which bracket the acidity or basicity of the target analyte. A polymer film thicker than the evanescent field shields the optical beam from environmental changes, but also permits facile transfer of either protons or ammonia. Multiple interferometers can be fabricated on a single integrated optic chip. Channels not used for acid-base sensing can be used to cancel out interferants, or to measure other analytes. Sensitivities achieved to date are in the ppbv range for ammonia and <0.01 pH in the pH sensor.