Resolving Cross-Sensitivity Effect in Fluorescence Quenching for Simultaneously Sensing Oxygen and Ammonia Concentrations by an Optical Dual Gas Sensor

Abstract

Simultaneous sensing of multiple gases by a single fluorescent-based gas sensor is of utmost importance for practical applications. Such sensing is strongly hindered by cross-sensitivity effects. In this study, we propose a novel analysis method to ameliorate such hindrance. The trial sensor used here was fabricated by coating platinum(II) meso-tetrakis(pentafluorophenyl)porphyrin (PtTFPP) and eosin-Y dye molecules on both sides of a filter paper for sensing O₂ and NH₃ gases simultaneously. The fluorescent peak intensities of the dyes can be quenched by the analytes and this phenomenon is used to identify the gas concentrations. Ideally, each dye is only sensitive to one gas species. However, the fluorescent peak related to O₂ sensing is also quenched by NH₃ and vice versa. Such cross-sensitivity strongly hinders gas concentration detection. Therefore, we have studied this cross-sensitivity effect systematically and thus proposed a new analysis method for accurate estimation of gas concentration. Comparing with a traditional method (neglecting cross-sensitivity), this analysis improves O₂-detection error from -11.4% ± 34.3% to 2.0% ± 10.2% in a mixed background of NH₃ and N₂.

Keywords: PtTFPP; cross-sensitivity; dual gas sensor; eosin Y; fluorescence quenching; fluorescence-based sensor; optical gas sensor.

Figure A1

(a) A flow chart showing the synthesis processes of a PtTFPP-containing solution. (b) Schematic diagram representing a trial sensor adsorbing O₂ and NH₃ gases.

Figure A2

(a) Emission spectra of the trial sensor under systematically varying O₂ concentration conditions in a NH₃-free environment. The (b) I₁, (c) I₂, and (d) I₁/I₂ as a function of oxygen concentration are plotted based on the spectra in (a).

Figure A3

(a) Emission spectra of the trial sensor under systematically varying NH₃ concentration in a O₂-free environment. The (b) I₁, (c) I₂, and (d) I₁/I₂ as a function of ammonia concentration are plotted based on the spectra in (a).

Figure 1

(a) A flow chart showing the synthesis processes of O₂- and NH₃-sensing solutions. (b) Schematic diagram representing the fabrication concept of a trial dual sensor.

Figure 2

Topside SEM images of a piece of filter (a) before and (b) after treated with sensing solutions. The treatment process is schematically represented in Figure 1.

Figure 3

Schematic diagrams of the system setup for optical gas sensing.

Figure 4

Systematic study of the emission spectra of a trial dual sensor under (a) 0 ppm of NH₃, (b) 0% of O₂ and (c) 200 ppm of NH₃. The insets show the enlarged areas for NH₃-sensitive peaks in the corresponding spectra. The intensity units for the insets are arbitrary units.

Figure 5

A typical example showing the Gaussian fitting of the emission spectra under conditions of 40% O₂ and 400 ppm NH₃.

Figure 6

Sensitivity (I₀/I) of (a) fitted NH₃-sensitive peak as a function of ammonia concentration under an oxygen-free environment and (b) fitted O₂-sensitive peak as a function of oxygen concentration under an ammonia-free environment. Equation (2) is used to fit the data points as shown by the red curves.

Figure 7

(a) Sensitivity (I₀/I) of fitted NH₃ sensitive peak as a function of ammonia concentration under systematically varying environmental oxygen concentration. Equation (2) is used to fit the data points as shown by the colored curves. (b) f (red squares) and K_SV (blue dots) as a function of oxygen concentration based on the fitted colored curves in (a). The f and K_SV are parameters in Equation (2).

Figure 8

(a) Sensitivity (I₀/I) of a fitted O₂-sensitive peak as a function of oxygen concentration under systematically varying environmental ammonia concentrations. Equation (2) is used to fit the data points, as shown by the colored curves. (b) f (red squares) and K_SV (blue dots) as a function of ammonia concentration based on the fitted colored curves in (a). The f and K_SV are parameters in Equation (2).

Figure 9

Estimated O₂-concentration error as a function of case number for the direct (blue squares) and modified (red dots) methods. The experimental conditions of the various cases are presented in Table 1.