Sensors based on intensity measurements, e.g., ab-A lifetime-based optical NH 3 sensor based on the sorbance or fluorescence, are often subject to signal principle of fluorescence resonance energy transfer drift resulting from leaching and photobleaching of the was developed. The sensor consisted of sulforhodaindicator dye, variations of light source intensity, and mine 101 as the donor, bromocresol green as the acstability of a photodetector. Although wavelength-raceptor, ethyl cellulose as the polymer support, and tritiometric techniques provide a means of overcoming butyl phosphate as the plasticizer. When the these difficulties, few ratiometric fluorophores are concentration of NH 3 changed, it caused a change in available. Lifetime-based sensing, on the other hand, the decay time of the SR101, which was measured by is independent of fluorophore concentration and variaphase-modulation fluorometry. At 100 MHz, increasing tions in the intensity of the light source and is an interthe concentration of NH 3 from 0 to 175 ppm resulted nally referenced method. Therefore it displays excelin a decrease in phase angle of about 31Π and an inlent long-term stability and ease of calibration.
crease in modulation of about 18%. Oxygen and carbon
The lifetime of a fluorophore can be measured by dioxide did not interfere with the sensor. However, a either time-domain or frequency-domain methods (6,7).