- Email: [email protected]
Examination of various shape of sol-gel optodes for indirect fiberoptic sensors
Examination of various shape of sol-gel optodes for indirect fiberoptic sensors Damian Andrzejewski, Halina Podbielska Bio-Optics Group, Institute of Physics, WrocΩaw University of Technology, 50 370 WrocΩaw, Wybrzez˙e Wyspian´skiego 27, Poland
Abstract: The sol-gel derived matrices for indirect fiberoptic sensors are described. Variously shaped fiber tips with attached optodes are prepared: flat one with macroptode, as well as tapered and shallow ones with microoptodes. The angular light intensity distribution near the fiber tip was measured for these three types of optodes. Advantages and disadvantages of the above solutions are discussed. The Ruthenium complex based optodes were constructed and the sensors responses after 36 hours of constant illumination were examined, as well. The smallest signal decrease was observed for shallow fiber tip with an internal microptode. Key words: Sol-gel optodes – fiber tips – angular light intensity distribution
1. Introduction Generally, there are two solutions for fiberoptic sensors: direct and indirect sensing techniques (fig. 1). In direct sensing method the changes in the environment (e.g. pH, temperature, pressure of concentration of different compounds) are measured directly, for example by monitoring of the absorption or changes in light polarization. Depended on the analytical method used, the optical sensors can be broadly divided into physical and chemical ones. As an example of direct physical fiberoptic sensor serves the sensor exploiting birefringent fibers [1]. The indirect sensing involves the construction of special element, called optode, which acts as a transducer between environment and fiberoptic guide connected to the photodetector and analyzing unit. So thus, the optode transduces the non-optical signal from the environment to the optical one, readable by the photodetector. Various indirect optical sensors and their applications are described in literature [2]. The optode can work as a chemical sensor that detects certain analytes in aqueous solutions or in gases, on chemical way. It means that changes in the environment cause the changes in the photosensitive material, which is immobilized in the optode matrix. These chemical changes influence the observed light intensity (for example due to absorption) or one can Received 25 October 2000; accepted 20 January 2001. Correspondence to: H. Podbielska Fax: ++48-71-3283696 E-mail: [email protected] Optik 112, No. 4 (2001) 158–162 © 2001 Urban & Fischer Verlag http://www.urbanfischer.de/journals/optik
analyze the intensity or time decay of luminescence signals. There are numbers of publications devoted to the family of optical chemical sensors [3]. Construction of the optode for optical biosensor requires immobilization of sensitive compounds in the host matrix. There are several methods enabling molecules entrapment. One can use gels, polymers, saccharose, various meshes and membranes [4]. In case of fiberoptic indirect sensors optode must be attached to the fiber tip. Nowadays, there are two commonly used optode host materials: sol-gel materials and polymers. Silica gels seem to be ideal materials for construction of optodes for indirect fiberoptic sensors. Their visible transparency, porosity enabling the transport of gases or liquids through the material, thermal and chemical stability, and ability to be filled with additional active phases are the key properties of sol-gels that can be exploited in various sensor applications [5, 6]. The sol-gel process, which leads to the silica glass formation, is carried out at room temperature. In addition, it is easy to prepare the sol-gel elements of various shapes. All these features are making the sol-gels very attractive materials for indirect fiberoptic sensors.
Fig. 1. Schematic set-up of a) direct and b) indirect fiberoptic sensing systems. Light from the light source LS is guided through the optical fiber OF to the environment. In case of indirect sensor an optode O is placed at the fiber end. The output signal is transmitted back and after reflection from dichroic mirror DM is recorded by photodetector PD and analyzed by electronic signal processing unit ESP. 0030-4026/01/112/04-158 $ 15.00/0
D. Andrzejewski, H. Podbielska, Examination of various shape of sol-gel optodes for indirect fiberoptic sensors
Fig. 2 Fig. 2. Fig. 3. Fig. 4.
Fig. 3
159
Fig. 4
Microscopic picture of flat cut optical fiber tip – basis for the sensor type T1 with macrooptode. Tapered optical waveguide tip – base for construction of sensor T2 with microoptode. Microcavity inside the optical fiber – basis for fiberoptic sensor T3 with microoptode.
In order to act as a transducer, optode must be attached to the optical fiber. The big bulks (>1 mm) of sol-gel matrix can be easily glued to the fiber tip, especially, if the polymer fiber is used [7]. The smaller optodes can be attached to fiber end by dip-coating method or simply by direct painting of the fiber-tip with a liquid gel. Depending on the optode dimension, two constructions are possible: micro- or macrooptodes. If the optode diameter is comparable with the diameter of optical fiber or bigger, it is defined as a macrooptode. If it is smaller, then we have a microoptode construction. Typical sizes of macrooptodes vary from 125 mm up to 2 mm and of microoptodes from 50 mm down to 25 mm. Features of both types will be discussed in this paper.
2. Sol-gel host matrix The chemistry of the sol-gel process comprises several steps that are well described in the literature [6, 8]. First, silicate precursor (e.g. Tetraethylorthosilicate TEOS or Tetramethylorthosilicate TMOS) is mixed with water and catalyst and stirred for a few hours. This process leads to hydrolysis of the Si–O–R bonds. Acids or bases catalyze the hydrolysis reaction. The longer the hydrolysis, the larger amount of the Si–OR groups undergo hydrolysis to the Si–OH form. For the purpose of this study the matrices were produced by exploiting the acid catalyzed route. After hydrolysis the
pH of the obtained homogenous hydrolyzate was gradually brought up to ca. 6 by means of a diluted ammonia solution. This resulted in quick (several minutes) gelation and formation of so called “wet” gel. Subsequently, the obtained gel can be aged for a few days in water or ammonia solution. This process reduces the mechanical stress during the drying of the gel and prevents, to some extent, the risk of the sample cracking. The precursor for the sol-gel matrix used in our experiments was TMOS (Tetramethoxysilan) mixed with MeOH (methanol) and with addition of 0.1N HCl as a catalyst. The proportions were: 2.46 ml TMOS: 12.30 ml MeOH: 1.22 ml HCl. TMOS and MeOH were mixed together with magnetic stirrer for 4 h at room temperature. At the end of the hydrolysis process the sensitive material (here the Ruthenium complex) was added to the solution (2.1 g for 100 ml hydrolizate). After hydrolysis, the early polycondensation began and the liquid gel was placed on the fiber tip by dip-coating method.
3. Fiber tips and optodes The most common shape of the sensor fiber tip is the flat one. It is easy to prepare, since only a fiber cutter is needed. Typical flat fiber tip prepared with the fiber cleaver is demonstrated in fig. 2. This fiber tip will be used further for construction of the optical macrosensor T1.
160
D. Andrzejewski, H. Podbielska, Examination of various shape of sol-gel optodes for indirect fiberoptic sensors
Fig. 5. Microcavity shape in the optical waveguide tip depending on the etching time: a), b) and c) are the phases according to the irritation time depicted by table 1. Table 1. The microcavity depth depending on the etching time. The microchambers are schematically depicted by fig. 5. Bath time [min]
Chamber depth [mm]
Figure
2 3 10
10–20 30–50 >100
5a 5b 5c
The sensor of type T1 was prepared by dipping the cleaved fiber tip into the liquid sol-gel. No additional glue was used. The silica nature of both substrates bounds it tightly together. However, this bounding is not very robust and it can be destroyed mechanically. The flat fiber end can be modified, so thus to obtain different shapes, like tapered, beveled, conical ones or even in form of microlenses [9, 10]. In this study, tapered fiber tip was prepared by means of hot torch method. The corresponding picture is shown in fig. 3. The sensor type T2 was constructed analogically as the sensor type T1 – by means of dipping method. The thin layer of sol-gel coating creates a microoptode on the distal end of the tapered fiber. Microoptodes offer generally higher resolution of the measurements. However, as in the previous case, such an external optode can be easily scratched and damaged. To avoid the problem of optode damage, the good solution is an internal optode, for example placed in the microchamber inside the fiber tip. Figure 4 demonstrates mentioned here microchamber inside the optical fiber. This kind of the tip (T3) was made chemically by etching the fiber with the 40% HF acid (Merck) bath at room temperature. Depending on the etching duration, different depths of microchamber can be achieved what fig. 5 and table 1 demonstrate. To prepare the sensor of type T3 the liquid sol-gel should be poured into the fiber cavity. It is quite challenging, since the input window diameter is approximately 120 mm and the chamber volume is ca. 4 ´ 10– 13 m3. For this study we used 10 minutes exposure for the fiber etching. These three different sensors described above were based on the same sensitive material: Ruthenium(II)-tris-4,7diphenyl-1,10-phenanthroline. It is the oxygen sensitive luminescent compound and its application is described already in the literature [11]. The luminescence time of this complex depends on the oxygen concentration – in absence of oxygen an average luminescence decay time approximately equals 6 ms. Thus, it can be exploited for monitor-
Fig. 6. Experimental set-up for measuring of angular intensity distribution of output beam. PD – photo detector (photodiode), OF – optical fiber, OWT – optical waveguide tip (here: magnification of the tapered fiber).
ing of oxygen concentration in the environment. For all experiments described here, the standard waveguides were used (140/100 mm – Radial).
4. Measurement of the angular light intensity distribution The angular light intensity distribution was examined in order to find out the influence of the tip shape on the light intensity near the fiber end. The profile of light distribution is important from two reasons. First, it informs how much light can be coupled into the sensor. Second, the knowledge about the energy density allows designing the optode with specially tailored distribution of photosensitive dye in order to avoid problems with photobleaching. Photobleaching phenomena that occur in photosensitive materials may result in worsen performance of optical sensors [12]. Investigations were performed in the set-up demonstrated in fig. 6. As a light source the light emitting diode LED with maximum at l = 635 nm was used. The light beam was coupled into the optical fiber and guided to the fiber end with attached optode. The photodetector (photomultiplier tube – PMT H5702-50, Hamamatsu Photonics K.K.) was placed on a circulating arm at the distance 1 cm from the fiber output and connected to the voltammeter. For all types of optodes the optical fiber had the same length equals to 1 m. The experimental results are depicted in fig. 7. The solid line represents the angular light intensity distribution for the flat fiber tip. It has the maximum at the angle equals to 0°, which means on optical axis. Due to the fiber numerical aperture (0.275 + 0.015), light beam propagates in rather narrow output angle not exceeding 15°. The “+” symbol line represents the light distribution in the case of the tapered fiber. The output angle is in this case much wider (up to 25°) but there is less light at the direction of optical axis comparing to the flat fiber tip. An interesting light distribution is seen near the fiber with the microcavity inside (see the line with “x” symbol). There
161
D. Andrzejewski, H. Podbielska, Examination of various shape of sol-gel optodes for indirect fiberoptic sensors
Fig. 7. The angular distribution of light intensity. Black line represents the light intensity around the flat fiber tip, “+” line ´” line is for the fiber tip with stands for the tapered fiber tip, “´ microcavity.
is less light observed at the angle 0° than in the both previous cases. However, very close to the fiber tip, one can see wider beam distribution with two additional maximums corresponding to the angles 30° and 40°. One can expect that different beam outputs will result in different sensor answers.
5. Luminescence response of the optodes of different shapes In many practical applications sensors should preserve their ability to measure the environmental parameters for longer period. It is sometimes difficult to achieve in case of chemical optical sensors due to the photobleaching of photosensitive dyes. The effect of dye deterioration causes that the sensor response signal decreases with time. This phenomenon should be taken into consideration in calibration procedures. In order to examine the optodes responses, the luminescence signals were measured in the optical set-up shown in fig. 8. The set-up consists of the light source (LED), optical filters (F1, F2), optical-fiber with sensing optode (O), beamsplitter (B) and the detector (PMT). The excitation light from LED (maximum emission at 470 nm) is guided via the optical fiber to the optode placed on the fiber tip. The part of the light beam is reflected by the beamsplitter and it is transmitted to the intensity monitoring unit, which is responsible to maintain the LED signal constant for the duration of experiment. During the experiments, the sensors were constantly illuminated for 36 hours. The illumination intensity was set to maximum and kept constant. The stable illumination signal was controlled by specially constructed light intensitymeter based on a photodiode with an internal
Fig. 8. Schematic set-up for long-time monitoring of sensor responses. LED – light source (blue led), F1, F2 – optical filters for separating excitation and luminescence signals, OF – optical-fiber with sensing optode OP, B – beamsplitter, PMT – photomultiplier (detector), Th – thermostatic bath, IM – intensity monitoring unit.
Table 2. The sensor luminescence responses depending on the illumination time and optode type. Sensor type
Signal [mV] Day 0
Signal [mV] Day 1
Signal [mV] Day 2
Signal [mV] Day 3
T1 T2 T3
2570 3000 1210
2430 2750 1200
2310 2420 1170
2190 2070 1120
amplifier (IPL 10530 DAL). The achieved accuracy of excitation light stability was better than 0.1%. To ensure the stable experimental conditions, the constant environmental temperature was kept. The sensor head (fiberoptic tip with optode) was inserted into the thermostatically controlled water bath (25°). The filters F1 (cut-off at 485 nm) and F2 (560 nm) are used to separate the excitation and phosphorescence signals, thus improving signalto-noise ratio. The experimental results as measured on the optical axis are shown in table 2. The initial signal was highest in the case of the tapered fiber tip T1 (with macrooptode) and lowest in the case of internal microoptode. In all three cases, the decrease of the signal intensity with the illumination time due to the photobleaching effect was observed. However, after 12 hours of constant exposure the sensor T2 demonstrated the highest decrease in response signal – 92% of initial value, whereas for sensor T1 it was 95% and 99% for T3. After 36 hours of constant illumination the signal from T2 sensor reached 69% of initial value. The luminescence signal from
162
D. Andrzejewski, H. Podbielska, Examination of various shape of sol-gel optodes for indirect fiberoptic sensors
cence intensities of the investigated sensors were different. Type T3 sensor seems to be a good solution. The internal microoptode demonstrated certain resistance against the photobleaching. It features higher robustness than the other designs, as well. Therefore, it can be an interesting alternative for the future sensor probes. Acknowledgements. The authors expressed their acknowledgement to the State Committee of Scientific Research KBN for partial support of this work (Grant No 8 T11E 029 15). Prof. O. S. Wolfbeis and his team from the Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg are gratefully acknowledged for supplying some of the equipment to perform these experiments.
References Fig. 9. Sensor response signals decrease depending on the duration of illumination.
T1 decreased up to 85%. Again, the sensor T3 with microoptode demonstrated the smallest decrease (up to 92.5%). These results are visualized in diagrams in fig. 9. Analyzing the results, one can conclude that the sensor T3 shows the best overall performance. It means that photobleaching is not so effective in this case. Such a behavior is due to the quantity of the sensing material in T3. It is much higher comparing to T2 and T1, where on the tip very thin sol-gel layer is deposited, even hardly to see it under microscope. In case of the T3, the optode total volume is much bigger. It results in prolonged activity of such type of sensor. The internal optode offers the better mechanical stability, as well.
6. Conclusion Three different types of sol-gel based optodes were constructed. The angular light intensity distribution was measured for flat fiber tip (T1), tapered one (T2) and for fiber with an internal microchamber (T3). Basing on these three fiber tips, three different optodes were prepared and their luminescence responses were examined. In all three cases the probe-heads were built with success. However, lumines-
[1] Bock WJ, Urbanczyk W, Voet MRH: Selected applications of fiber-optic sensors based on highly birefringent fibers in engineering mechanics. In Lamprapoulos GA (Ed.): Proc. on Application of Photonic Technology: Sensing, Signal processing and Communication, pp. 311–316, Plenum Press, 1995 [2] Wolfbeis OS: Fiber Optic Chemical Sensors and Biosensors. CRC Press, Boca Raton 1991 [3] Wolfbeis OS (Ed.): Biochemical and Medical Sensors. Proc. SPIE 2085 (1993) [4] Koncki R, Mohr G, Wolfbeis OS: Enzyme biosensor for urea based on a novel pH bulk optode membrane. Biosensors & Bioelectronics 10 (1995) 653–659 [5] Reisfeld R, Jorgensen CK (Eds.): Chemistry, Spectroscopy and Applications of Sol-Gel Glasses. Springer-Verlag, Berlin 1992 [6] Klein LC (Ed.): Sol-Gel Optics: Processing and Applications. Kluwer Academic Publishers, Boston 1994 [7] Baldini F, Bracci S, Cosi F, Falciai R: Italian patent: No. FI 93 A125, 6 July (1993) [8] Brinker CJ, Scherer GW: Sol-Gel Science. Academic Press, San Diego 1999 [9] Verdaasdonk RM, Borts C: Ray tracing of optically modified fibertips 1. Spherical probes. Appl. Opt. 30 (1991) 2159–2171 [10] Verdaasdonk RM, Borts C: Ray tracing of optically modified fibertips 2. Laser scalpels. Appl. Opt. 30 (1991) 2172–2177 [11] Preininger C, Klimant I, Wolfbeis OS: Optical fiber sensor for biological oxygen demand (BOD). Anal. Chem. 66 (1994) 1841–1846 [12] Hartmann P, Leiner MJ, Kohlbacher P: Photobleaching of a ruthenium complex in polymers used for oxygen optodes and its inhibition by singlet oxygen quenchers. Sens. Actuators B 51 (1998) 196–202
Abstract: The sol-gel derived matrices for indirect fiberoptic sensors are described. Variously shaped fiber tips with attached optodes are prepared: flat one with macroptode, as well as tapered and shallow ones with microoptodes. The angular light intensity distribution near the fiber tip was measured for these three types of optodes. Advantages and disadvantages of the above solutions are discussed. The Ruthenium complex based optodes were constructed and the sensors responses after 36 hours of constant illumination were examined, as well. The smallest signal decrease was observed for shallow fiber tip with an internal microptode. Key words: Sol-gel optodes – fiber tips – angular light intensity distribution
1. Introduction Generally, there are two solutions for fiberoptic sensors: direct and indirect sensing techniques (fig. 1). In direct sensing method the changes in the environment (e.g. pH, temperature, pressure of concentration of different compounds) are measured directly, for example by monitoring of the absorption or changes in light polarization. Depended on the analytical method used, the optical sensors can be broadly divided into physical and chemical ones. As an example of direct physical fiberoptic sensor serves the sensor exploiting birefringent fibers [1]. The indirect sensing involves the construction of special element, called optode, which acts as a transducer between environment and fiberoptic guide connected to the photodetector and analyzing unit. So thus, the optode transduces the non-optical signal from the environment to the optical one, readable by the photodetector. Various indirect optical sensors and their applications are described in literature [2]. The optode can work as a chemical sensor that detects certain analytes in aqueous solutions or in gases, on chemical way. It means that changes in the environment cause the changes in the photosensitive material, which is immobilized in the optode matrix. These chemical changes influence the observed light intensity (for example due to absorption) or one can Received 25 October 2000; accepted 20 January 2001. Correspondence to: H. Podbielska Fax: ++48-71-3283696 E-mail: [email protected] Optik 112, No. 4 (2001) 158–162 © 2001 Urban & Fischer Verlag http://www.urbanfischer.de/journals/optik
analyze the intensity or time decay of luminescence signals. There are numbers of publications devoted to the family of optical chemical sensors [3]. Construction of the optode for optical biosensor requires immobilization of sensitive compounds in the host matrix. There are several methods enabling molecules entrapment. One can use gels, polymers, saccharose, various meshes and membranes [4]. In case of fiberoptic indirect sensors optode must be attached to the fiber tip. Nowadays, there are two commonly used optode host materials: sol-gel materials and polymers. Silica gels seem to be ideal materials for construction of optodes for indirect fiberoptic sensors. Their visible transparency, porosity enabling the transport of gases or liquids through the material, thermal and chemical stability, and ability to be filled with additional active phases are the key properties of sol-gels that can be exploited in various sensor applications [5, 6]. The sol-gel process, which leads to the silica glass formation, is carried out at room temperature. In addition, it is easy to prepare the sol-gel elements of various shapes. All these features are making the sol-gels very attractive materials for indirect fiberoptic sensors.
Fig. 1. Schematic set-up of a) direct and b) indirect fiberoptic sensing systems. Light from the light source LS is guided through the optical fiber OF to the environment. In case of indirect sensor an optode O is placed at the fiber end. The output signal is transmitted back and after reflection from dichroic mirror DM is recorded by photodetector PD and analyzed by electronic signal processing unit ESP. 0030-4026/01/112/04-158 $ 15.00/0
D. Andrzejewski, H. Podbielska, Examination of various shape of sol-gel optodes for indirect fiberoptic sensors
Fig. 2 Fig. 2. Fig. 3. Fig. 4.
Fig. 3
159
Fig. 4
Microscopic picture of flat cut optical fiber tip – basis for the sensor type T1 with macrooptode. Tapered optical waveguide tip – base for construction of sensor T2 with microoptode. Microcavity inside the optical fiber – basis for fiberoptic sensor T3 with microoptode.
In order to act as a transducer, optode must be attached to the optical fiber. The big bulks (>1 mm) of sol-gel matrix can be easily glued to the fiber tip, especially, if the polymer fiber is used [7]. The smaller optodes can be attached to fiber end by dip-coating method or simply by direct painting of the fiber-tip with a liquid gel. Depending on the optode dimension, two constructions are possible: micro- or macrooptodes. If the optode diameter is comparable with the diameter of optical fiber or bigger, it is defined as a macrooptode. If it is smaller, then we have a microoptode construction. Typical sizes of macrooptodes vary from 125 mm up to 2 mm and of microoptodes from 50 mm down to 25 mm. Features of both types will be discussed in this paper.
2. Sol-gel host matrix The chemistry of the sol-gel process comprises several steps that are well described in the literature [6, 8]. First, silicate precursor (e.g. Tetraethylorthosilicate TEOS or Tetramethylorthosilicate TMOS) is mixed with water and catalyst and stirred for a few hours. This process leads to hydrolysis of the Si–O–R bonds. Acids or bases catalyze the hydrolysis reaction. The longer the hydrolysis, the larger amount of the Si–OR groups undergo hydrolysis to the Si–OH form. For the purpose of this study the matrices were produced by exploiting the acid catalyzed route. After hydrolysis the
pH of the obtained homogenous hydrolyzate was gradually brought up to ca. 6 by means of a diluted ammonia solution. This resulted in quick (several minutes) gelation and formation of so called “wet” gel. Subsequently, the obtained gel can be aged for a few days in water or ammonia solution. This process reduces the mechanical stress during the drying of the gel and prevents, to some extent, the risk of the sample cracking. The precursor for the sol-gel matrix used in our experiments was TMOS (Tetramethoxysilan) mixed with MeOH (methanol) and with addition of 0.1N HCl as a catalyst. The proportions were: 2.46 ml TMOS: 12.30 ml MeOH: 1.22 ml HCl. TMOS and MeOH were mixed together with magnetic stirrer for 4 h at room temperature. At the end of the hydrolysis process the sensitive material (here the Ruthenium complex) was added to the solution (2.1 g for 100 ml hydrolizate). After hydrolysis, the early polycondensation began and the liquid gel was placed on the fiber tip by dip-coating method.
3. Fiber tips and optodes The most common shape of the sensor fiber tip is the flat one. It is easy to prepare, since only a fiber cutter is needed. Typical flat fiber tip prepared with the fiber cleaver is demonstrated in fig. 2. This fiber tip will be used further for construction of the optical macrosensor T1.
160
D. Andrzejewski, H. Podbielska, Examination of various shape of sol-gel optodes for indirect fiberoptic sensors
Fig. 5. Microcavity shape in the optical waveguide tip depending on the etching time: a), b) and c) are the phases according to the irritation time depicted by table 1. Table 1. The microcavity depth depending on the etching time. The microchambers are schematically depicted by fig. 5. Bath time [min]
Chamber depth [mm]
Figure
2 3 10
10–20 30–50 >100
5a 5b 5c
The sensor of type T1 was prepared by dipping the cleaved fiber tip into the liquid sol-gel. No additional glue was used. The silica nature of both substrates bounds it tightly together. However, this bounding is not very robust and it can be destroyed mechanically. The flat fiber end can be modified, so thus to obtain different shapes, like tapered, beveled, conical ones or even in form of microlenses [9, 10]. In this study, tapered fiber tip was prepared by means of hot torch method. The corresponding picture is shown in fig. 3. The sensor type T2 was constructed analogically as the sensor type T1 – by means of dipping method. The thin layer of sol-gel coating creates a microoptode on the distal end of the tapered fiber. Microoptodes offer generally higher resolution of the measurements. However, as in the previous case, such an external optode can be easily scratched and damaged. To avoid the problem of optode damage, the good solution is an internal optode, for example placed in the microchamber inside the fiber tip. Figure 4 demonstrates mentioned here microchamber inside the optical fiber. This kind of the tip (T3) was made chemically by etching the fiber with the 40% HF acid (Merck) bath at room temperature. Depending on the etching duration, different depths of microchamber can be achieved what fig. 5 and table 1 demonstrate. To prepare the sensor of type T3 the liquid sol-gel should be poured into the fiber cavity. It is quite challenging, since the input window diameter is approximately 120 mm and the chamber volume is ca. 4 ´ 10– 13 m3. For this study we used 10 minutes exposure for the fiber etching. These three different sensors described above were based on the same sensitive material: Ruthenium(II)-tris-4,7diphenyl-1,10-phenanthroline. It is the oxygen sensitive luminescent compound and its application is described already in the literature [11]. The luminescence time of this complex depends on the oxygen concentration – in absence of oxygen an average luminescence decay time approximately equals 6 ms. Thus, it can be exploited for monitor-
Fig. 6. Experimental set-up for measuring of angular intensity distribution of output beam. PD – photo detector (photodiode), OF – optical fiber, OWT – optical waveguide tip (here: magnification of the tapered fiber).
ing of oxygen concentration in the environment. For all experiments described here, the standard waveguides were used (140/100 mm – Radial).
4. Measurement of the angular light intensity distribution The angular light intensity distribution was examined in order to find out the influence of the tip shape on the light intensity near the fiber end. The profile of light distribution is important from two reasons. First, it informs how much light can be coupled into the sensor. Second, the knowledge about the energy density allows designing the optode with specially tailored distribution of photosensitive dye in order to avoid problems with photobleaching. Photobleaching phenomena that occur in photosensitive materials may result in worsen performance of optical sensors [12]. Investigations were performed in the set-up demonstrated in fig. 6. As a light source the light emitting diode LED with maximum at l = 635 nm was used. The light beam was coupled into the optical fiber and guided to the fiber end with attached optode. The photodetector (photomultiplier tube – PMT H5702-50, Hamamatsu Photonics K.K.) was placed on a circulating arm at the distance 1 cm from the fiber output and connected to the voltammeter. For all types of optodes the optical fiber had the same length equals to 1 m. The experimental results are depicted in fig. 7. The solid line represents the angular light intensity distribution for the flat fiber tip. It has the maximum at the angle equals to 0°, which means on optical axis. Due to the fiber numerical aperture (0.275 + 0.015), light beam propagates in rather narrow output angle not exceeding 15°. The “+” symbol line represents the light distribution in the case of the tapered fiber. The output angle is in this case much wider (up to 25°) but there is less light at the direction of optical axis comparing to the flat fiber tip. An interesting light distribution is seen near the fiber with the microcavity inside (see the line with “x” symbol). There
161
D. Andrzejewski, H. Podbielska, Examination of various shape of sol-gel optodes for indirect fiberoptic sensors
Fig. 7. The angular distribution of light intensity. Black line represents the light intensity around the flat fiber tip, “+” line ´” line is for the fiber tip with stands for the tapered fiber tip, “´ microcavity.
is less light observed at the angle 0° than in the both previous cases. However, very close to the fiber tip, one can see wider beam distribution with two additional maximums corresponding to the angles 30° and 40°. One can expect that different beam outputs will result in different sensor answers.
5. Luminescence response of the optodes of different shapes In many practical applications sensors should preserve their ability to measure the environmental parameters for longer period. It is sometimes difficult to achieve in case of chemical optical sensors due to the photobleaching of photosensitive dyes. The effect of dye deterioration causes that the sensor response signal decreases with time. This phenomenon should be taken into consideration in calibration procedures. In order to examine the optodes responses, the luminescence signals were measured in the optical set-up shown in fig. 8. The set-up consists of the light source (LED), optical filters (F1, F2), optical-fiber with sensing optode (O), beamsplitter (B) and the detector (PMT). The excitation light from LED (maximum emission at 470 nm) is guided via the optical fiber to the optode placed on the fiber tip. The part of the light beam is reflected by the beamsplitter and it is transmitted to the intensity monitoring unit, which is responsible to maintain the LED signal constant for the duration of experiment. During the experiments, the sensors were constantly illuminated for 36 hours. The illumination intensity was set to maximum and kept constant. The stable illumination signal was controlled by specially constructed light intensitymeter based on a photodiode with an internal
Fig. 8. Schematic set-up for long-time monitoring of sensor responses. LED – light source (blue led), F1, F2 – optical filters for separating excitation and luminescence signals, OF – optical-fiber with sensing optode OP, B – beamsplitter, PMT – photomultiplier (detector), Th – thermostatic bath, IM – intensity monitoring unit.
Table 2. The sensor luminescence responses depending on the illumination time and optode type. Sensor type
Signal [mV] Day 0
Signal [mV] Day 1
Signal [mV] Day 2
Signal [mV] Day 3
T1 T2 T3
2570 3000 1210
2430 2750 1200
2310 2420 1170
2190 2070 1120
amplifier (IPL 10530 DAL). The achieved accuracy of excitation light stability was better than 0.1%. To ensure the stable experimental conditions, the constant environmental temperature was kept. The sensor head (fiberoptic tip with optode) was inserted into the thermostatically controlled water bath (25°). The filters F1 (cut-off at 485 nm) and F2 (560 nm) are used to separate the excitation and phosphorescence signals, thus improving signalto-noise ratio. The experimental results as measured on the optical axis are shown in table 2. The initial signal was highest in the case of the tapered fiber tip T1 (with macrooptode) and lowest in the case of internal microoptode. In all three cases, the decrease of the signal intensity with the illumination time due to the photobleaching effect was observed. However, after 12 hours of constant exposure the sensor T2 demonstrated the highest decrease in response signal – 92% of initial value, whereas for sensor T1 it was 95% and 99% for T3. After 36 hours of constant illumination the signal from T2 sensor reached 69% of initial value. The luminescence signal from
162
D. Andrzejewski, H. Podbielska, Examination of various shape of sol-gel optodes for indirect fiberoptic sensors
cence intensities of the investigated sensors were different. Type T3 sensor seems to be a good solution. The internal microoptode demonstrated certain resistance against the photobleaching. It features higher robustness than the other designs, as well. Therefore, it can be an interesting alternative for the future sensor probes. Acknowledgements. The authors expressed their acknowledgement to the State Committee of Scientific Research KBN for partial support of this work (Grant No 8 T11E 029 15). Prof. O. S. Wolfbeis and his team from the Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg are gratefully acknowledged for supplying some of the equipment to perform these experiments.
References Fig. 9. Sensor response signals decrease depending on the duration of illumination.
T1 decreased up to 85%. Again, the sensor T3 with microoptode demonstrated the smallest decrease (up to 92.5%). These results are visualized in diagrams in fig. 9. Analyzing the results, one can conclude that the sensor T3 shows the best overall performance. It means that photobleaching is not so effective in this case. Such a behavior is due to the quantity of the sensing material in T3. It is much higher comparing to T2 and T1, where on the tip very thin sol-gel layer is deposited, even hardly to see it under microscope. In case of the T3, the optode total volume is much bigger. It results in prolonged activity of such type of sensor. The internal optode offers the better mechanical stability, as well.
6. Conclusion Three different types of sol-gel based optodes were constructed. The angular light intensity distribution was measured for flat fiber tip (T1), tapered one (T2) and for fiber with an internal microchamber (T3). Basing on these three fiber tips, three different optodes were prepared and their luminescence responses were examined. In all three cases the probe-heads were built with success. However, lumines-
[1] Bock WJ, Urbanczyk W, Voet MRH: Selected applications of fiber-optic sensors based on highly birefringent fibers in engineering mechanics. In Lamprapoulos GA (Ed.): Proc. on Application of Photonic Technology: Sensing, Signal processing and Communication, pp. 311–316, Plenum Press, 1995 [2] Wolfbeis OS: Fiber Optic Chemical Sensors and Biosensors. CRC Press, Boca Raton 1991 [3] Wolfbeis OS (Ed.): Biochemical and Medical Sensors. Proc. SPIE 2085 (1993) [4] Koncki R, Mohr G, Wolfbeis OS: Enzyme biosensor for urea based on a novel pH bulk optode membrane. Biosensors & Bioelectronics 10 (1995) 653–659 [5] Reisfeld R, Jorgensen CK (Eds.): Chemistry, Spectroscopy and Applications of Sol-Gel Glasses. Springer-Verlag, Berlin 1992 [6] Klein LC (Ed.): Sol-Gel Optics: Processing and Applications. Kluwer Academic Publishers, Boston 1994 [7] Baldini F, Bracci S, Cosi F, Falciai R: Italian patent: No. FI 93 A125, 6 July (1993) [8] Brinker CJ, Scherer GW: Sol-Gel Science. Academic Press, San Diego 1999 [9] Verdaasdonk RM, Borts C: Ray tracing of optically modified fibertips 1. Spherical probes. Appl. Opt. 30 (1991) 2159–2171 [10] Verdaasdonk RM, Borts C: Ray tracing of optically modified fibertips 2. Laser scalpels. Appl. Opt. 30 (1991) 2172–2177 [11] Preininger C, Klimant I, Wolfbeis OS: Optical fiber sensor for biological oxygen demand (BOD). Anal. Chem. 66 (1994) 1841–1846 [12] Hartmann P, Leiner MJ, Kohlbacher P: Photobleaching of a ruthenium complex in polymers used for oxygen optodes and its inhibition by singlet oxygen quenchers. Sens. Actuators B 51 (1998) 196–202