Calibration-free optical chemical sensors

Calibrations are necessary for most chemical sensors because the response is not consistent between sensors nor stable over time. If chemical sensors could be designed to have identical behavior from sensor to sensor and no drift, the need for sensor calibrations would be reduced. In the present paper, the feasibility of calibration-free optical chemical sensors is explored. An indicator-based pCO₂ (partial pressure of CO₂) sensor is designed that has excellent sensor-to-sensor reproducibility and measurement stability. This superior level of performance is achieved by using the following strategy: (1) renewing the sensing solution, (2) allowing the sensing solution to reach equilibrium with the analyte, (3) calculating the response from a ratio of the indicator solution absorbances, and (4) through careful solution preparation, wavelength calibration, and stray light rejection. Three pCO₂ sensors are calibrated, and the response curves are essentially identical within the uncertainty of the calibration. Long-term laboratory and field studies are presented that show the response has no drift over extended periods (months). The theoretical response, determined from thermodynamic characterization of the indicator solution, also predicts the observed calibration-free performance. Other absorbance-based sensors, such as optrodes, can be designed and operated in a similar fashion, making calibration-free optical chemical sensors available for a wide range of biomedical, industrial, and environmental applications.