A Field-Deployable Dual-Wavelength Fiber-Optic pH Sensor Instrument Based on Solid-State Optical and Electrical Components

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Abstract
The construction, operation, and performance characteristics of a compact, dual-wavelength, solid-state photometer and fiber-optic probe for use with absorbance-based thin-film chemical sensors are described. The instrument has been specifically designed for field deployment and uses low-cost components while maintaining a precision and accuracy comparable to fluorescence-based laboratory sensors. The computer-controlled photometer employs red and green light-emitting diodes (LEDs) as fixed-wavelength light sources and silicon photodiodes as detectors, and has no moving parts. Graded refractive index lenses control light propagation through a beamsplitter in the photometer and through the fiber-optic probe. The beamsplitter directs light simultaneously onto a sample and a reference detector in a double-beam in-space mode which compensates for fluctuations in the outputs of the LEDs. A chemically modified polymeric film is mounted on the fiber probe, and serves as an optical pH sensor. The previously described sensing film is constructed by the immobilization of the diprotic acid-base indicator Congo Red at a base-hydrolyzed cellulose acetate film and responds to changes in pH between 0 and 4.2. An absorbance-based internal calibration scheme, which takes advantage of the optical properties of each of the reactive forms of the immobilized indicator, is presented. The potential capability of this instrumentation, based on its small size and minimal need for calibration, to function as a durable low-maintenance pH sensor for on-site deployment in environmental monitoring applications, is discussed.

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