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Fiber optic oxygen sensor based on phosphorescence quenching of erythrosin B trapped in silica-gel glasses
Analytica Chimica Acta 408 (2000) 33–37
Fiber optic oxygen sensor based on phosphorescence quenching of erythrosin B trapped in silica-gel glasses M.A. Chan a , J.L. Lawless b , S.K. Lam c , D. Lo c,∗ a
c
Potential Star Ltd., 4428 Cosco Tower, 183 Queen’s Road Central, Hong Kong, China b Redwood Scientific Inc., 1005 Terra Nova Blvd., Pacifica, CA 94044, USA Physics Department, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China Received 7 July 1999; received in revised form 3 November 1999; accepted 6 November 1999
Abstract Experimental results from a phosphorescence-based fiber-optical oxygen sensor are presented. The optical oxygen sensor module has overall dimensions of 6 × 6 × 12 mm3 . Erythrosin B immobilized in sol–gel silica, which showed strong phosphorescence quenching by oxygen, was used in the sensor probe. Oxygen sensing is effective from 0.014 to 600 mbar. On account of the long phosphorescence lifetime (∼0.28 ms) and high phosphorescence yield (∼2%) of erythrosin B in sol–gel silica at room temperature, the sensitivity of the sensor improves by a factor of 10 as compared to transition-metal complex-based optical fiber oxygen sensors. The phosphorescence quenching effect is highly selective to oxygen. The sensor is inert to commonly found gases such as nitrogen and argon. Time-decay of phosphorescence is also studied. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Sensor; Fiber-optics; Oxygen; Sol–gel; Erythrosin B
1. Introduction The area of optical oxygen sensors has witnessed much research activity because of the great interest of the sensor applications in environmental, biomedical and industrial monitoring. The main advantages of optical sensors over the traditional electrochemical oxygen electrodes are low oxygen consumption, compatibility with advanced integrated optical technology, and the absence of electrode poisoning by the organic compounds or by external electromagnetic field. Most optical oxygen sensors in the literature are ∗ Corresponding author. Tel.: +852-2609-6103; fax: +852-2603-5204. E-mail address: [email protected] (D. Lo).
based on fluorescence quenching of the fluorophore, such as transition-metal complexes (e.g. ruthenium(II) (Ru(II)) complexes), embedded in polymer or in sol–gel matrices [1–3]. Much progress has been made on transition-metal complex-based oxygen sensors. These sensors however are susceptible to several limitation such as short lifetime (ns–s range) [4] and low sensitivity (fluorescence intensity variation less than 10 times between an oxygen concentration of 0–100%) [2]. High speed and high sensitivity opto-electronics are often needed for the detection, making it expensive and difficult for full integration into simple and low-cost oxygen sensing systems. An alternative approach to fluorescence sensing is to sense phosphorescence, which has a lifetime ranging from milliseconds to several seconds [5].
0003-2670/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 8 4 9 - 1
34
M.A. Chan et al. / Analytica Chimica Acta 408 (2000) 33–37
Higher sensitivity of the sensor is therefore possible. Although phosphorescence of organic dyes has been known to be sensitive to ambient oxygen concentration, oxygen sensing based on phosphorescence is less well studied. This is partially due to the weak emission intensity of phosphorescence of most materials at room temperature. The only phosphorescence based oxygen sensors that have appeared in the literature were based on platinum/palladium complexes-doped polymeric materials [5]. Recently, sol–gel derived inorganic glasses have been described as promising materials in many photonic applications [6,7]. In comparison with organic polymers, sol–gel glasses have superior transmission behavior in the UV and in the near IR, and higher values of refractive indices. Both properties are important in the design consideration of opto-electronic devices. Sol–gel glass can also provide a chemically inert cage to large numbers of organic and inorganic dopants at very high concentrations without aggregation [7]. Examples of dye-doped sol–gel glasses in use in photonics are the recently developed sol–gel fiber pH sensor [8] and the dye-doped glass planar waveguide laser [9]. A recent study of the photophysical properties of dye in sol–gel materials also shows that a number of dyes trapped in sol–gel materials have high phosphorescence quantum yield at room temperature [10–12]. Dye-doped sol–gel materials thus appear to hold promise for application in oxygen sensors. In this letter, we report the phosphorescence quenching of an optical fiber coupled oxygen sensor based on erythrosin B-doped sol–gel silica. We have made time-transient spectroscopic measurements of the phosphorescence of erythrosin B in sol–gel silica in a previous work [11] and the phosphorescence
yield was found to be 2% [12]. This sensor shows a 10 times increase in sensitivity over the conventional optical sensors based on fluorescence detection on account of the long phosphorescence decay lifetime and high quantum yield.
2. Experiments Erythrosin B-doped sol–gel silica was prepared according to a base catalyzed sol–gel process. Certain amounts of tetramethoxysilane (TMOS), methanol, deionized water and NaOH were mixed under magnetic stirring at room temperature. The erythrosin B dye was added to the mixture and the resulting solution (sol) was casted in acrylic cuvettes which were sealed with sticky tapes. The sol gelled within 10 min at room temperature. The gels were allowed to further dry, shrink and age at ambient temperature. After 2 weeks, the gels were baked at 115◦ C for 10 h. The typical silica gels measured are 6 mm3 × 6 mm3 × 12 mm3 . The ends of a multi-mode optical fiber were soaked in methylene chloride for 1 min. The jacket and the cladding of the fibers were then removed mechanically. Subsequently, the cleaved fiber end was bonded to the silica gel surface using poly(methyl methacrylate) (PMMA) toluene solution as adhesive. Fig. 1 shows the schematic diagram of the sensor probe. All measurements were conducted in a stainless steel vacuum chamber at room temperature. Details of the experimental apparatus can be found in [11]. The samples were degassed in vacuum for 1 day before measurements were taken. 5 ns full width at half maximum (FWHM) pulses of the second harmonic (λ = 532 nm) of an Nd:YAG laser were used as
Fig. 1. Schematic diagram of fiber optical sensor for oxygen sensing.
M.A. Chan et al. / Analytica Chimica Acta 408 (2000) 33–37
excitation source and were launched into the optical fiber bonded onto the sol–gel silica bulk surface. Luminescence of the oxygen probe was collected by the same fiber coupled to a fiber bundle which was connected to a 0.3 m grating monochromator. A gatable intensified charge coupled device (ICCD) was used to collect the emission spectra. In order to measure only the phosphorescence spectra, the delay time (the time between the onset of the laser pulse and the onset of the gate pulse which was used to switch on the ICCD for light collection) was set to 70 ns and the gate width (exposure time of the ICCD) to 80 ms. The 70 ns delay time is longer than the decay time of fluorescence but much shorter than the decay times of phosphorescence. The influence of fluorescence is therefore screened off. Also, the gate width was set to 80 ms to allow time-integration of the entire phosphorescence and delayed fluorescence signals.
3. Results and discussions Fig. 2 shows the phosphorescence spectra of erythrosin B trapped in sol–gel silica under various oxygen pressures. The spectra show a two peaks structure. The small peak at short wavelength (∼570 nm) can be attributed to delayed fluorescence and the strong peak at long wavelength (∼680 nm) to phosphorescence. Phosphorescence emission decreases as oxygen pressure increases. The peak of the phosphorescence drops by approximately a factor of 4 as the oxygen
Fig. 2. Phosphorescence and delayed fluorescence spectra of oxygen sensor in vacuum, 0.078, 0.214, and 2 mbar of oxygen pressure, respectively.
35
Fig. 3. Stern–Volmer plots of phosphorescence quenching at pressures ranging from vacuum to 600 mbar of O2 , N2 and Ar.
pressure increases from vacuum (the pressure reading was actually 0.014 mbar according to the pressure gauge, but the oxygen level was 0 as we did not let in any oxygen) to 2 mbar. Quenching of luminescence is generally described by the Stern–Volmer equation written as I0 = 1 + KSV [O2 ] I where I0 and I are the luminescence intensities in the absence and in the presence of quencher, respectively, [O2 ] is the oxygen concentration, and KSV is the Stern–Volmer quenching constant. I0 and I can be deduced directly from the phosphorescence emission spectra. The value of the phosphorescence peak measured at 0.014 mbar, the lowest pressure reading that can be registered by our pressure gauge, is the I0 used to calculate the Stern–Volmer ratio I0 /I. Fig. 3 shows the Stern–Volmer plot of the phosphorescence of erythrosin B-doped sol–gel silica. The phosphorescence emission in vacuum (0.014 mbar) is 100 times that at 600 mbar. In other words, the ratio I0 /I increases from 1 to over 100 when the oxygen pressure increases from vacuum (0.014 mbar) to 600 mbar. Such sensitivity is much higher than that of most optical oxygen sensors. For example, the Stern–Volmer ratios of Ru(II) complex or other fluorescence based oxygen sensor are usually smaller than 20 [13,14]. For Platinum(II) (Pt(II) complex based oxygen sensors, which also employ phosphorescence for oxygen sensing, this ratio is often smaller than 40 [5,15].
36
M.A. Chan et al. / Analytica Chimica Acta 408 (2000) 33–37
Our I0 /I ratio does not show a linear dependence on oxygen pressure as prescribed by the Stern–Volmer equation. Such nonlinear behavior is often encountered in solid phase or heterogeneous systems. This is usually explained by a multiple emitting sites model or a log-Gaussian multisite-quenching model [16]. Although mathematical treatments of these two models are different, both models imply that excited dye molecules at different locations are dynamically quenched with different efficiencies. Multiple emitting species such as emissive isomer or various ionic forms of organic dye may also contribute to the nonlinear Stern–Volmer behavior. Further study is needed to clarify the mechanism responsible. Selectivity of the erythrosin B doped sol–gel silica was studied by monitoring the phosphorescence emission in the presence of nitrogen or argon. The phosphorescence spectra collected in either nitrogen or argon atmosphere show little variation as pressure changes. The Stern–Volmer plots for nitrogen or argon as the quencher as shown in Fig. 3. At very high nitrogen concentration, weak quenching of phosphorescence is observed. This is in fact due to impurity species present in the N2 gas (e.g. oxygen at 1.5 ppm for the nitrogen gas used in the experiments.) In comparison, the sensor response to O2 is much more drastic. The sensitivity is higher by at least two orders of magnitude. By contrast, quenching is totally absent when high purity Ar is used (Fig. 3). Quenching of phosphorescence by oxygen implies that the reaction with molecular oxygen is an effective de-excitation channel for triplet erythrosin B. As O2 pressure increases, we expect both the phosphorescence emission and the lifetime to decrease. To measure the decay of phosphorescence at various ambient oxygen pressures, the monochromator was set to the position of phosphorescence peak (∼680 nm). A photo-multiplier tube (Hamamatsu 4220P) in combination with a multi-channel scaler (Stanford Research Systems SR450) were configured for photon-counting
Fig. 4. Lifetime decay curves of phosphorescence by the sensor in vacuum, 1.043 and 11 mbar of oxygen pressure, respectively.
Fig. 5. Time response of the sol–gel sensor to a step change in oxygen concentration.
measurements of the time decay profile. The decay curves of phosphorescence were recorded and are plotted in Fig. 4. It is found that the phosphorescence does not decay exponentially but can be fitted using a bi-exponential function. Data are listed in Table 1. The decay lifetimes are then determined by least square fitting of the decay curves with bi-exponential functions. Both the fast and the slow components of the lifetime constants decrease with increased oxygen
Table 1 Values of the fast and slow components of lifetime and their ratios from a bi-exponential fit for the oxygen sensor measured at three different oxygen pressures Oxygen pressure (mbar)
Lifetime (s)
τ 10 /τ 1
Lifetime (s)
τ 20 /τ 2
0.014 1.04 106
60.11 15.03 1.82
1 4.5 33.0
287.59 156.05 15.49
1 1.84 18.57
M.A. Chan et al. / Analytica Chimica Acta 408 (2000) 33–37
pressure. The ratios τ 10 /τ 1 , and τ 20 /τ 2 , where τ 10 and τ 20 denote the fast and slow components of lifetime constant in vacuum and τ 1 and τ 2 are for those at ambient oxygen pressure of 106 mbar, were 33.0 and 18.57 for the fast and the slow components, respectively. Again, the presence of two lifetime components suggests the use of the multiple emitting site/multiple emissive species model. The sensor shows a fast response toward a step change in oxygen concentration. When oxygen is suddenly introduced into the vacuum chamber, the peak phosphorescence intensity at 690 nm decreases and reaches a steady state value of 100% oxygen within 1 s. The intensity change of phosphorescence corresponding to the abrupt change in oxygen concentration from 0 to 100% is shown in Fig. 5. The oxygen pressure was varied from vacuum to 300 mbar. Oxygen was introduced into the chamber by turning on a mechanical valve manually. The response time as observed in Fig. 5 is in fact limited by both the turn-on time of the valve and the equilibration time of the introduced gas. The true response time of the sensor is expected to be far less than 1 s.
4. Conclusion In summary, we reported phosphorescence quenching by oxygen based in erythrosin B-doped sol–gel silica. Phosphorescence measurements of our new sensor probe indicates high sensitivity and high selectivity in response to O2 . The phosphorescence emission drops by a factor of 100 when oxygen pressure increases from 0.014 to 600 mbar. The decay time profile can be fitted using bi-exponential functions. The two lifetime constants, the fast and the slow components, decrease at increased pressure of O2 . Our results demonstrate the possibility of its further development into a lifetime-based sensor. The sensor shows a fast response toward the change of oxygen concentration. Further study will be done on coating erythrosin B
37
sol–gel silica on the end of multi-mode fiber for miniaturization purpose. With the use of inexpensive light sources (e.g. LED), the simple fiber coating fabrication technique may facilitate the production of a low cost, compact, and high performance optical oxygen sensor.
Acknowledgements The work of S.K.L. and D.L. in this paper is supported in part by RGC Earmarked Grant of Hong Kong SAR ref. no. CU 97504.
References [1] I. Kilment, M. Kuhl, R.N. Glud, G. Holst, Sensors and Actuators B 29 (1997) 38. [2] B.D. MacCraith, C.M. McDonagh, G. O’Keeffe, A.K. McEvoy, T. Butler, F.R. Sheridan, Sensors and Actuators B 29 (1995) 51. [3] W. Trettnak, Optical sensors based on fluorescence quenching, in: O.S. Wolfbeis (Ed.), Fluorescence Spectroscopy, Springer, Berlin, 1992, p. 79. [4] M. Lippitsch, J. Pusterhofer, M. Leiner, O. Wolfbeis, Anal. Chem. Acta 205 (1988) 1. [5] D.B. Papkovsky, Sensors and Actuators B 29 (1995) 213. [6] D. Avnir, D. Levy, R. Reisfeld, J. Phys. Chem. 88 (1984) 5956. [7] D. Avnir, Acc. Chem. Res. 28 (1995) 328. [8] J.Y. Ding, M.R. Shahriari, G.H. Sigel Jr., Electron Lett. 27 (1991) 1562. [9] Y. Sorek, R. Reisfeld, I. Finkelstein, S. Ruschin, Appl. Phys. Lett. 66 (1995) 1169. [10] D. Levy, D. Avnir, J. Photochem. Photobiol. A: Chem. 57 (1991) 41. [11] S.K. Lam, D. Lo, Chem. Phys. Lett. 281 (1997) 35. [12] S.K. Lam, E. Namdas, D. Lo, J. Photochem. Photobiol. A: Chem. 118 (1998) 25. [13] M.L. Bossi, M.E. Daraio, P.F. Aramendia, J. Photochem. Photobiol. A: Chem. 120 (1999) 15. [14] W.W. Lee, K.Y. Wong, X. Li, Anal. Chem. 65 (1993) 255. [15] S.Y. Lee, I. Okura, Anal. Chim. Acta 342 (1997) 181. [16] A. Mills, Sensors and Actuators B 51 (1998) 69.
Fiber optic oxygen sensor based on phosphorescence quenching of erythrosin B trapped in silica-gel glasses M.A. Chan a , J.L. Lawless b , S.K. Lam c , D. Lo c,∗ a
c
Potential Star Ltd., 4428 Cosco Tower, 183 Queen’s Road Central, Hong Kong, China b Redwood Scientific Inc., 1005 Terra Nova Blvd., Pacifica, CA 94044, USA Physics Department, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China Received 7 July 1999; received in revised form 3 November 1999; accepted 6 November 1999
Abstract Experimental results from a phosphorescence-based fiber-optical oxygen sensor are presented. The optical oxygen sensor module has overall dimensions of 6 × 6 × 12 mm3 . Erythrosin B immobilized in sol–gel silica, which showed strong phosphorescence quenching by oxygen, was used in the sensor probe. Oxygen sensing is effective from 0.014 to 600 mbar. On account of the long phosphorescence lifetime (∼0.28 ms) and high phosphorescence yield (∼2%) of erythrosin B in sol–gel silica at room temperature, the sensitivity of the sensor improves by a factor of 10 as compared to transition-metal complex-based optical fiber oxygen sensors. The phosphorescence quenching effect is highly selective to oxygen. The sensor is inert to commonly found gases such as nitrogen and argon. Time-decay of phosphorescence is also studied. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Sensor; Fiber-optics; Oxygen; Sol–gel; Erythrosin B
1. Introduction The area of optical oxygen sensors has witnessed much research activity because of the great interest of the sensor applications in environmental, biomedical and industrial monitoring. The main advantages of optical sensors over the traditional electrochemical oxygen electrodes are low oxygen consumption, compatibility with advanced integrated optical technology, and the absence of electrode poisoning by the organic compounds or by external electromagnetic field. Most optical oxygen sensors in the literature are ∗ Corresponding author. Tel.: +852-2609-6103; fax: +852-2603-5204. E-mail address: [email protected] (D. Lo).
based on fluorescence quenching of the fluorophore, such as transition-metal complexes (e.g. ruthenium(II) (Ru(II)) complexes), embedded in polymer or in sol–gel matrices [1–3]. Much progress has been made on transition-metal complex-based oxygen sensors. These sensors however are susceptible to several limitation such as short lifetime (ns–s range) [4] and low sensitivity (fluorescence intensity variation less than 10 times between an oxygen concentration of 0–100%) [2]. High speed and high sensitivity opto-electronics are often needed for the detection, making it expensive and difficult for full integration into simple and low-cost oxygen sensing systems. An alternative approach to fluorescence sensing is to sense phosphorescence, which has a lifetime ranging from milliseconds to several seconds [5].
0003-2670/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 8 4 9 - 1
34
M.A. Chan et al. / Analytica Chimica Acta 408 (2000) 33–37
Higher sensitivity of the sensor is therefore possible. Although phosphorescence of organic dyes has been known to be sensitive to ambient oxygen concentration, oxygen sensing based on phosphorescence is less well studied. This is partially due to the weak emission intensity of phosphorescence of most materials at room temperature. The only phosphorescence based oxygen sensors that have appeared in the literature were based on platinum/palladium complexes-doped polymeric materials [5]. Recently, sol–gel derived inorganic glasses have been described as promising materials in many photonic applications [6,7]. In comparison with organic polymers, sol–gel glasses have superior transmission behavior in the UV and in the near IR, and higher values of refractive indices. Both properties are important in the design consideration of opto-electronic devices. Sol–gel glass can also provide a chemically inert cage to large numbers of organic and inorganic dopants at very high concentrations without aggregation [7]. Examples of dye-doped sol–gel glasses in use in photonics are the recently developed sol–gel fiber pH sensor [8] and the dye-doped glass planar waveguide laser [9]. A recent study of the photophysical properties of dye in sol–gel materials also shows that a number of dyes trapped in sol–gel materials have high phosphorescence quantum yield at room temperature [10–12]. Dye-doped sol–gel materials thus appear to hold promise for application in oxygen sensors. In this letter, we report the phosphorescence quenching of an optical fiber coupled oxygen sensor based on erythrosin B-doped sol–gel silica. We have made time-transient spectroscopic measurements of the phosphorescence of erythrosin B in sol–gel silica in a previous work [11] and the phosphorescence
yield was found to be 2% [12]. This sensor shows a 10 times increase in sensitivity over the conventional optical sensors based on fluorescence detection on account of the long phosphorescence decay lifetime and high quantum yield.
2. Experiments Erythrosin B-doped sol–gel silica was prepared according to a base catalyzed sol–gel process. Certain amounts of tetramethoxysilane (TMOS), methanol, deionized water and NaOH were mixed under magnetic stirring at room temperature. The erythrosin B dye was added to the mixture and the resulting solution (sol) was casted in acrylic cuvettes which were sealed with sticky tapes. The sol gelled within 10 min at room temperature. The gels were allowed to further dry, shrink and age at ambient temperature. After 2 weeks, the gels were baked at 115◦ C for 10 h. The typical silica gels measured are 6 mm3 × 6 mm3 × 12 mm3 . The ends of a multi-mode optical fiber were soaked in methylene chloride for 1 min. The jacket and the cladding of the fibers were then removed mechanically. Subsequently, the cleaved fiber end was bonded to the silica gel surface using poly(methyl methacrylate) (PMMA) toluene solution as adhesive. Fig. 1 shows the schematic diagram of the sensor probe. All measurements were conducted in a stainless steel vacuum chamber at room temperature. Details of the experimental apparatus can be found in [11]. The samples were degassed in vacuum for 1 day before measurements were taken. 5 ns full width at half maximum (FWHM) pulses of the second harmonic (λ = 532 nm) of an Nd:YAG laser were used as
Fig. 1. Schematic diagram of fiber optical sensor for oxygen sensing.
M.A. Chan et al. / Analytica Chimica Acta 408 (2000) 33–37
excitation source and were launched into the optical fiber bonded onto the sol–gel silica bulk surface. Luminescence of the oxygen probe was collected by the same fiber coupled to a fiber bundle which was connected to a 0.3 m grating monochromator. A gatable intensified charge coupled device (ICCD) was used to collect the emission spectra. In order to measure only the phosphorescence spectra, the delay time (the time between the onset of the laser pulse and the onset of the gate pulse which was used to switch on the ICCD for light collection) was set to 70 ns and the gate width (exposure time of the ICCD) to 80 ms. The 70 ns delay time is longer than the decay time of fluorescence but much shorter than the decay times of phosphorescence. The influence of fluorescence is therefore screened off. Also, the gate width was set to 80 ms to allow time-integration of the entire phosphorescence and delayed fluorescence signals.
3. Results and discussions Fig. 2 shows the phosphorescence spectra of erythrosin B trapped in sol–gel silica under various oxygen pressures. The spectra show a two peaks structure. The small peak at short wavelength (∼570 nm) can be attributed to delayed fluorescence and the strong peak at long wavelength (∼680 nm) to phosphorescence. Phosphorescence emission decreases as oxygen pressure increases. The peak of the phosphorescence drops by approximately a factor of 4 as the oxygen
Fig. 2. Phosphorescence and delayed fluorescence spectra of oxygen sensor in vacuum, 0.078, 0.214, and 2 mbar of oxygen pressure, respectively.
35
Fig. 3. Stern–Volmer plots of phosphorescence quenching at pressures ranging from vacuum to 600 mbar of O2 , N2 and Ar.
pressure increases from vacuum (the pressure reading was actually 0.014 mbar according to the pressure gauge, but the oxygen level was 0 as we did not let in any oxygen) to 2 mbar. Quenching of luminescence is generally described by the Stern–Volmer equation written as I0 = 1 + KSV [O2 ] I where I0 and I are the luminescence intensities in the absence and in the presence of quencher, respectively, [O2 ] is the oxygen concentration, and KSV is the Stern–Volmer quenching constant. I0 and I can be deduced directly from the phosphorescence emission spectra. The value of the phosphorescence peak measured at 0.014 mbar, the lowest pressure reading that can be registered by our pressure gauge, is the I0 used to calculate the Stern–Volmer ratio I0 /I. Fig. 3 shows the Stern–Volmer plot of the phosphorescence of erythrosin B-doped sol–gel silica. The phosphorescence emission in vacuum (0.014 mbar) is 100 times that at 600 mbar. In other words, the ratio I0 /I increases from 1 to over 100 when the oxygen pressure increases from vacuum (0.014 mbar) to 600 mbar. Such sensitivity is much higher than that of most optical oxygen sensors. For example, the Stern–Volmer ratios of Ru(II) complex or other fluorescence based oxygen sensor are usually smaller than 20 [13,14]. For Platinum(II) (Pt(II) complex based oxygen sensors, which also employ phosphorescence for oxygen sensing, this ratio is often smaller than 40 [5,15].
36
M.A. Chan et al. / Analytica Chimica Acta 408 (2000) 33–37
Our I0 /I ratio does not show a linear dependence on oxygen pressure as prescribed by the Stern–Volmer equation. Such nonlinear behavior is often encountered in solid phase or heterogeneous systems. This is usually explained by a multiple emitting sites model or a log-Gaussian multisite-quenching model [16]. Although mathematical treatments of these two models are different, both models imply that excited dye molecules at different locations are dynamically quenched with different efficiencies. Multiple emitting species such as emissive isomer or various ionic forms of organic dye may also contribute to the nonlinear Stern–Volmer behavior. Further study is needed to clarify the mechanism responsible. Selectivity of the erythrosin B doped sol–gel silica was studied by monitoring the phosphorescence emission in the presence of nitrogen or argon. The phosphorescence spectra collected in either nitrogen or argon atmosphere show little variation as pressure changes. The Stern–Volmer plots for nitrogen or argon as the quencher as shown in Fig. 3. At very high nitrogen concentration, weak quenching of phosphorescence is observed. This is in fact due to impurity species present in the N2 gas (e.g. oxygen at 1.5 ppm for the nitrogen gas used in the experiments.) In comparison, the sensor response to O2 is much more drastic. The sensitivity is higher by at least two orders of magnitude. By contrast, quenching is totally absent when high purity Ar is used (Fig. 3). Quenching of phosphorescence by oxygen implies that the reaction with molecular oxygen is an effective de-excitation channel for triplet erythrosin B. As O2 pressure increases, we expect both the phosphorescence emission and the lifetime to decrease. To measure the decay of phosphorescence at various ambient oxygen pressures, the monochromator was set to the position of phosphorescence peak (∼680 nm). A photo-multiplier tube (Hamamatsu 4220P) in combination with a multi-channel scaler (Stanford Research Systems SR450) were configured for photon-counting
Fig. 4. Lifetime decay curves of phosphorescence by the sensor in vacuum, 1.043 and 11 mbar of oxygen pressure, respectively.
Fig. 5. Time response of the sol–gel sensor to a step change in oxygen concentration.
measurements of the time decay profile. The decay curves of phosphorescence were recorded and are plotted in Fig. 4. It is found that the phosphorescence does not decay exponentially but can be fitted using a bi-exponential function. Data are listed in Table 1. The decay lifetimes are then determined by least square fitting of the decay curves with bi-exponential functions. Both the fast and the slow components of the lifetime constants decrease with increased oxygen
Table 1 Values of the fast and slow components of lifetime and their ratios from a bi-exponential fit for the oxygen sensor measured at three different oxygen pressures Oxygen pressure (mbar)
Lifetime (s)
τ 10 /τ 1
Lifetime (s)
τ 20 /τ 2
0.014 1.04 106
60.11 15.03 1.82
1 4.5 33.0
287.59 156.05 15.49
1 1.84 18.57
M.A. Chan et al. / Analytica Chimica Acta 408 (2000) 33–37
pressure. The ratios τ 10 /τ 1 , and τ 20 /τ 2 , where τ 10 and τ 20 denote the fast and slow components of lifetime constant in vacuum and τ 1 and τ 2 are for those at ambient oxygen pressure of 106 mbar, were 33.0 and 18.57 for the fast and the slow components, respectively. Again, the presence of two lifetime components suggests the use of the multiple emitting site/multiple emissive species model. The sensor shows a fast response toward a step change in oxygen concentration. When oxygen is suddenly introduced into the vacuum chamber, the peak phosphorescence intensity at 690 nm decreases and reaches a steady state value of 100% oxygen within 1 s. The intensity change of phosphorescence corresponding to the abrupt change in oxygen concentration from 0 to 100% is shown in Fig. 5. The oxygen pressure was varied from vacuum to 300 mbar. Oxygen was introduced into the chamber by turning on a mechanical valve manually. The response time as observed in Fig. 5 is in fact limited by both the turn-on time of the valve and the equilibration time of the introduced gas. The true response time of the sensor is expected to be far less than 1 s.
4. Conclusion In summary, we reported phosphorescence quenching by oxygen based in erythrosin B-doped sol–gel silica. Phosphorescence measurements of our new sensor probe indicates high sensitivity and high selectivity in response to O2 . The phosphorescence emission drops by a factor of 100 when oxygen pressure increases from 0.014 to 600 mbar. The decay time profile can be fitted using bi-exponential functions. The two lifetime constants, the fast and the slow components, decrease at increased pressure of O2 . Our results demonstrate the possibility of its further development into a lifetime-based sensor. The sensor shows a fast response toward the change of oxygen concentration. Further study will be done on coating erythrosin B
37
sol–gel silica on the end of multi-mode fiber for miniaturization purpose. With the use of inexpensive light sources (e.g. LED), the simple fiber coating fabrication technique may facilitate the production of a low cost, compact, and high performance optical oxygen sensor.
Acknowledgements The work of S.K.L. and D.L. in this paper is supported in part by RGC Earmarked Grant of Hong Kong SAR ref. no. CU 97504.
References [1] I. Kilment, M. Kuhl, R.N. Glud, G. Holst, Sensors and Actuators B 29 (1997) 38. [2] B.D. MacCraith, C.M. McDonagh, G. O’Keeffe, A.K. McEvoy, T. Butler, F.R. Sheridan, Sensors and Actuators B 29 (1995) 51. [3] W. Trettnak, Optical sensors based on fluorescence quenching, in: O.S. Wolfbeis (Ed.), Fluorescence Spectroscopy, Springer, Berlin, 1992, p. 79. [4] M. Lippitsch, J. Pusterhofer, M. Leiner, O. Wolfbeis, Anal. Chem. Acta 205 (1988) 1. [5] D.B. Papkovsky, Sensors and Actuators B 29 (1995) 213. [6] D. Avnir, D. Levy, R. Reisfeld, J. Phys. Chem. 88 (1984) 5956. [7] D. Avnir, Acc. Chem. Res. 28 (1995) 328. [8] J.Y. Ding, M.R. Shahriari, G.H. Sigel Jr., Electron Lett. 27 (1991) 1562. [9] Y. Sorek, R. Reisfeld, I. Finkelstein, S. Ruschin, Appl. Phys. Lett. 66 (1995) 1169. [10] D. Levy, D. Avnir, J. Photochem. Photobiol. A: Chem. 57 (1991) 41. [11] S.K. Lam, D. Lo, Chem. Phys. Lett. 281 (1997) 35. [12] S.K. Lam, E. Namdas, D. Lo, J. Photochem. Photobiol. A: Chem. 118 (1998) 25. [13] M.L. Bossi, M.E. Daraio, P.F. Aramendia, J. Photochem. Photobiol. A: Chem. 120 (1999) 15. [14] W.W. Lee, K.Y. Wong, X. Li, Anal. Chem. 65 (1993) 255. [15] S.Y. Lee, I. Okura, Anal. Chim. Acta 342 (1997) 181. [16] A. Mills, Sensors and Actuators B 51 (1998) 69.