A lifetime-based fluorescence resonance energy transfer sensor for ammonia

Abstract

A lifetime-based optical NH3 sensor based on the principle of fluorescence resonance energy transfer was developed. The sensor consisted of sulforhodamine 101 as the donor, bromocresol green as the acceptor, ethyl cellulose as the polymer support, and tributyl phosphate as the plasticizer. When the concentration of NH3 changed, it caused a change in the decay time of the SR101, which was measured by phase-modulation fluorometry. At 100 MHz, increasing the concentration of NH3 from 0 to 175 ppm resulted in a decrease in phase angle of about 31 degrees and an increase in modulation of about 18%. Oxygen and carbon dioxide did not interfere with the sensor. However, a 30% relative humidity could cause a downward shift of the response by 5 degrees, while additional increase in the relative humidity to 100% showed little further effect. For a film thickness of 40 microns, the typical response and recovery times for 90% of total signal change were 1 and 2.5 min, respectively. The phase angle measurements for the same sample were reproducible for 5 days, with no special care of the film sample.

FIG. 1.

Normalized absorption and emission spectra of SR101 in EC/TBP film (solid lines); the dotted lines represent the absorption spectra of BCG in EC/TBP film in NH₃ and in air.

FIG. 2.

SR101/BCG/EC/TBP film sensor in 0, 6.83, 15.1, 30.8, 43.9, 61.0, 110, and 175 ppm NH₃ (from top to bottom). Excitation wave length of 488 nm is indicated by the arrow. Emission was measured with an Andover 600FH90 long-wave-pass filter.

FIG. 3.

SR101/BCG/EC/TBP film sensor (solid lines) in 175, 135, 87.5, 52.3, 30.8, 10.5, and 0 ppm NH₃ (from bottom). SR101 alone in EC/TBP film is shown with dashed lines. Excitation is at 488 nm, with emission collected through an Andover 600FH90 long-wave-pass filter.

FIG. 4.

Calibration curves of phase angle, φ, versus P_NH3 for SR101/BCG/EC/TBP film sensor. Excitation is at 488 nm, with emission collected through an Andover 600FH90 long-wave-pass filter.

FIG. 5.

Calibration lines of tan (φ₀)/tan (φ) versus P_NH3 for SR101/BCG/EC/TBP film sensor. Data derived from Fig. 4.

FIG. 6.

Time-dependent response and recovery of SR101/BCG/EC/TBP film sensor in an alternating gas supply of pure N₂ and 171 ppm NH₃ balanced with N₂. Solid line, dry N₂; dotted line, humid N₂. Ex, 488 nm; em, 600FH90 filter; frequency, 100 MHz.

FIG. 7.

Long-term stability of SR101/BCG/EC/TBP film sensor in N₂ and in 161 ppm NH₃. Ex, 488 nm; em. 600FH90 filter; frequency,: 100 MHz.