Research on optical chemical sensors blossomed over the past 15-20 years as a result of the availability of novel and inexpensive waveguides and the ever-increasing need for sensors for a wide range of chemical applications. Most chemical sensors however require calibration since sensor response is not consistent between sensors nor is it stable over time.
Reagent-based Optical Chemical Sensors (hereafter referred to as ROCS) generated substantial excitement early on in their development because they promised improved performance and versatility over their electrochemical sensor analogs. These potential advantages included greater stability and selectivity and a simple, characterizable response. But, after nearly 20 years of research and development, these advantages have yet to be fullly realized 1,2.
The majority of ROCS use fiber optics to direct light into a small membrane-enclosed volume, resulting in single-ended electrode-like probes3. In most ROCS designs, a calorimetric or fluorimetric reagent is entrapped within the membrane at the fiber tips. The membrane serves a variety of important purposes- it contains and protects the analyte-selective reagent, it provides an additional level of selectivity for groups of different compounds (e.g. gases, hydrophilic or hydrophobic species), and it acts as a diffuse reflector of the light transmitted through, or emitted from, the selective reagent. Chemical sensing is generally accomplished by simply monitoring the change in light intensity in the presence of the analyte of interest. Light intensity changes unrelated to the analyte such as light source fluctuations or changes in the fiber optic transmission are compensated for by using a reference light intensity at a wavelength insensitive to the analyte concentration. Even so, these ROCS designs have significant stability problems that primarily originate from the changes in the composition of the entrapped reagent chemistry1. To improve ROCS performance, researchers began investigating methods for renewing the reagent4,5.
Although steps have been taken in achieve consistency and stability among optical chemical sensors, a calibration-free sensor displaying long-term stability and an identical response from sensor to sensor has not yet been described. A need therefore exists for such a calibration-free sensor that can be applied to a wide range of biomedical, industrial and environmental applications.
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