Planar waveguides have evanescent fields sensitive to index of refraction changes in the volume immediately above the waveguide surface. These fields extend up to 5000 Å above the surface. Placing a chemically sensitive polymer film within this region provides the basis for a chemical sensor. Polymer-analyte interactions change the index of refraction causing the propagating light velocity to change in a direction opposite to that of the index change. To measure this change, a reference propagating beam, adjacent to the sensing beam, is optically combined with the sensing beam creating an interference pattern of alternating dark and light fringes. When chemical or physical changes occur in the sensing arm, the interference pattern shifts. Real-time Fourier transform signal processing converts the time-dependent pattern to total phase shift which, is a measure of total analyte absorbed. Employing different polymer detection layers produces phase shifts whose pattern of response is used to identify and quantify the analytes present. Data taken from contaminated well sites measured using this interferometric sensor, and verified by independent laboratory measurements, is presented.
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.
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. Tailored chemistries can be passive (e.g.; inducing swelling or dissolution in a film) or active (e.g.; containing reactive or binding sites). Fast and reversible chemistries are the goal, in most cases for both gaseous and liquid applications. Passive mechanisms are used when the target analyte is relatively inert, i.e. aromatic and chlorinated hydrocarbons. Active chemistries developed include tailoring the acid-base strength of the sensing film for pH or ammonia response, and antibody-antigen binding. Currently the integrated optic waveguide platform consists of thirteen interferometers on a 1 X 2-cm glass substrate. A different sensing film deposited on each channel allows for multiple analyte sensing, interferant cancellation, patterned outputs for analyte identification, or extended dynamic range. Sensitivities range from the low ppm to low ppb for both vapor and aqueous applications, 0.01 pH units and ng/mL for biologicals.
An integrated optic chemical sensor has been developed to monitor benzene, toluene, ethylbenzene and xylene (BTEX) in water. The sensor uses planar waveguide interferometry, where the evanescent field associated with a guided wave probes the refractive index changes immediately above the waveguide surface. Currently, up to thirteen interferometers are fabricated on a 1 X 2 cm glass chip. One arm of each interferometer is coated with a chemically interactive film, and the other arm is buried under an inert layer of silicon dioxide (SiO2). The interference pattern formed by combining the guided waves from the two arms is read by a linear photodiode array, and onboard electronics convert the raw optical intensities into analyte concentrations. The sensor is packaged in a 1.5 inch diameter, 18 inch long stainless steel housing suitable for use in monitoring wells of with cone penetrometers. It is plug-and-play compatible with E-SMARTTM monitoring networks.
Planar waveguide interferometers provide an attractive sensing platform for biosensor applications. Advantages include small size, real-time sensing, multiple analyte detection on a chip, performance independent of wavelength and optical power, and nulling of thermal and mechanical noise. Limitations include slow diffusion time of the analyte to the functionalized surface, interference from non-specific binding and bulk index of refraction changes and a lack of reversibility. Combining certain techniques used in affinity chromatography and enzyme-linked immunosorbent assays and with an amplifying chemoselective film on the waveguide produces a sensor that is versatile, reusable and overcomes most of the above limitations. Work will be presented using an optical pH and ammonia sensor for detection.
An integrated optic sensor for monitoring NH3 volatization as related to agricultural fertilizer applications is described. The sensor is capable of monitoring NH3 levels over a range from less than 100 parts per billion by volume to levels approaching 1000 parts per million by volume. The sensor is based on a planar waveguide operating in an interferometric mode. The device functions by monitoring a refractive index change resulting from a reversible chemical reaction occurring on the waveguide surface.
An integrated optic biosensor for detecting foodborne pathogens is described. The sensor is based on a planar waveguide operating in an interferometric mode. The device functions by detecting the direct binding of an antigen molecule to a functionalized waveguide surface. It is capable of detecting biomolecules at subnanogram/milliliter concentrations and has been used to detect proteins specific to Salmonella.