FIO 24(3-4) #52245
Fiber and Integrated Optics, 24:201–225, 2005 Copyright © Taylor & Francis Inc. ISSN: 0146-8030 print/1096-4681 online DOI: 10.1080/01468030590922731
Luminescence-Based Optical Fiber Chemical Sensors P. A. S. JORGE
C. C. ROSA
Unidade de Optoelectrónica e Sistemas Electrónicos Porto, Portugal, and Faculdade de Ciências da Universidade do Porto Porto, Portugal
Unidade de Optoelectrónica e Sistemas Electrónicos Porto, Portugal, and Faculdade de Ciências da Universidade do Porto Porto, Portugal
A. G. OLIVA
Unidade de Optoelectrónica e Sistemas Electrónicos Porto, Portugal, and Instituto Politécnico de Viana do Castelo Viana do Castelo, Portugal
Universidade Nova de Lisboa Oeiras, Portugal
J. L. SANTOS Unidade de Optoelectrónica e Sistemas Electrónicos Porto, Portugal, and Faculdade de Ciências da Universidade do Porto Porto, Portugal
J. C. G. ESTEVES DA SILVA Faculdade de Ciências da Universidade do Porto Porto, Portugal
F. FARAHI University of North Carolina at Charlotte Charlotte, North Carolina, USA A scheme for the simultaneous determination of temperature and analyte concentration for application in luminescence-based chemical sensors is proposed. This scheme is applied to an optical oxygen sensor, which is based on the quenching of the fluorescence of a ruthenium complex. Temperature measurement is performed using the excitation radiation and an absorption long-pass filter. Preliminary results are presented that show the viability of an oxygen measurement that is independent of temperature and optical power level. The possibility of self-referenced temperature measurements with semiconductor nanoparticles is also investigated. In order to optimize the sensor design, several different optical fiber probe geometries for oxygen sensing are tested and compared, including different methods of coupling radiation Received 30 October 2004. Address correspondence to P. A. S. Jorge, Unidade de Optoelectrónica e Sistemas Electrónicos, INESC Porto, Rua do Campo Alegre, 687, Porto 4169-007, Portugal. E-mail: [email protected]
P. A. S. Jorge et al. into the optical fiber system. Polyvinyl alcohol (PVA) and polyacrylamide membranes are tested as supports for sensor immobilization in fiber-optical pH sensing devices in aqueous solution. Some results are presented that show the feasibility of using fiber-optical pH indicators for remote monitoring. Keywords metals
luminescence sensors, optical fiber, oxygen, temperature, pH, heavy
Introduction The measurement of chemical and biological parameters using optical sensors is an expanding area of research with growing importance, especially in biomedical and environmental applications. Intrinsic immunity to electromagnetic interferences, increased sensitivity, fast response times, and suitability to remote monitoring, when used with optical fibers, are some of the characteristics that make optical sensors an appealing alternative to conventional technologies. The combination of optical fiber technologies with fluorescence spectroscopy techniques, together with the development of new materials for sensor immobilization, has greatly contributed to the progress of optical chemical sensors [1–4]. At present, with the environment and, consequently, the human health constantly at risk, this is a very important prior area of technological development and, therefore, a strategic area of research. In this context, luminescence-based optical sensors are especially attractive due to their high sensitivity and selectivity . An important class of luminescent sensors is based on the dynamic quenching of fluorescence. In this type of sensor the analyte to be measured quenches the luminescent emission of a sensing dye. This sensing mechanism is widely used for the determination of several analytes like oxygen, halides, and various metals [6–8]. The intensity, I , and the excited state lifetime, τ , of the luminescent emission of the sensing dye are both quenched in the presence of the analyte. These luminescence parameters are related to the analyte concentration by the Stern-Volmer (SV) equation : I0 τ0 = = 1 + KSV [A] I τ
KSV = kq τ0
where I0 and τ0 are the luminescent intensity and the excited state lifetime in the absence of the quencher, respectively; KSV is the SV constant; and [A] is the analyte concentration. The SV constant is related to the dynamic quenching rate constant, kq , a characteristic of the molecular species involved in the process, which describes the dynamic collisional deactivation of electronic energy. The constant kq is directly proportional to the quenching efficiency and to the diffusion coefficient. This way, dynamic quenching mechanisms are diffusion-dependent processes that are clearly affected by temperature. Therefore, knowledge of the sample temperature is necessary for the correct determination of the analyte concentration by luminescence quenching methods. Equation (1) shows that measurements of either the luminescence intensity or the excited state lifetime can be used to determine the analyte concentration. However, the direct measurement of either of these parameters has its associated problems. When
intensity measurements are performed, a reference scheme is needed in order to compensate for fluctuations of the excitation optical power. Otherwise, changes in the sensor output caused by optical source instability, coupling efficiency fluctuations, leaching, and photo bleaching of the indicator, can be mistaken with changes in analyte concentration. Conversely, direct lifetime measurements demand for high-speed electronics and fast pulsed optical sources. These problems can be minimized using frequency domain techniques . If sinusoidal modulation is applied to the optical source, the resulting luminescent emission will also be modulated with the same frequency. However, a phase delay, φ, will be present between the excitation and the luminescent emission signal that is related to the lifetime by: tan(φ) = 2πf τ
where f is the modulation frequency. Using Equations (1) and (3) the phase delay information can easily be related to the analyte concentration. With this technique, lowcost, high-brightness optical sources, like blue LEDs, which are suitable for excitation of many sensing dyes, can be used in association with standard photodetection. This very simple but powerful method, together with the development of new materials for immobilization of the sensing chemistry in the optical fiber (sol-gels, polymers), has greatly contributed to the development of a new class of analytical instrumentation. Several fiber optic–based chemical sensing devices are already commercially available [11, 12]. However, several problems still persist and there is much room for improvement. The fact that the phase is directly proportional to the excited state lifetime implies that it is also dependent on temperature. Although some optical temperature reference schemes have been proposed, commercially available luminescent sensors still rely on conventional technology for the measurement of temperature [13, 14]. This is incompatible with certain applications where an all-optical fiber probe must be used. Phase detection would be very sensitive and independent of the signal intensity if the signal-to-noise ratio (SNR) was high. However, low efficiency of the luminescence process, added to the use of broadband optical sources with standard photodetection and optical fibers, makes it very difficult to fulfill this condition. In this perspective, the design of the sensing system (sensing head geometry, level of optical power, etc.) is critical in order to optimize sensor performance. Another important class of luminescent sensors is based on fluorescent pH indicators. The optical measurement of pH offers many advantages over traditional glass electrodes. The use of sensitive luminescent methods for pH determination, when used in combination with optical fibers, introduces the possibility of miniaturization and remote monitoring. Additionally, pH indicators provide a base for other detection systems, like biosensors and heavy metal ions. These are key features for environmental and biomedical applications [15, 16]. An important step for implementation of optical fiber–based pH sensing is the immobilization of the fluorescent indicators in a solid matrix on the fiber probe, as it determines the sensor performance. An ideal immobilization membrane would effectively entrap the indicator and preserve its optical properties, avoiding leaching and photobleaching, and would establish a fast and reversible equilibrium with the aqueous environment. Some progress has been made in this regard with the development of new sol-gel and polymer materials; however, there is no universal solution available, since each application has its own specific chemistry, and this is still an active area of research.
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The determination of pH with measurements based on the luminescence intensity need reference due to leaching and photobleaching and other sources of optical power drift. Ratiometric detection is the most popular method for this purpose and it involves the ratio of two fluorescent intensities at different wavelengths . In this context, the application of frequency domain techniques, an established technology in optical oxygen sensing, is highly desirable. A scheme based on non-radiative energy transfer from a luminescent long-lived ruthenium(II) complex (the donor) to a short-lived colorimetric pH indicator (the acceptor) was proposed that allows conversion of the pH information into a luminescence decay time signal . In practical applications, the operation of pH sensors in the near infrared region of the spectrum (NIR) is a major step toward low-cost instrumentation. Due to the development of telecommunications, NIR optical fiber and semiconductor technology are widely available at low cost. Additionally, excitation and detection in this wavelength range avoid background noise from environmental and biological samples. Several NIR fluorescence chemosensors for pH determination have already been developed . However, fluorescence chemosensors for toxic heavy metal detection and the corresponding optical fiber instrumentation are still in the first steps of development. In this article several problems of luminescence-based optical fiber sensors are addressed. We propose an all-optical scheme for simultaneous determination of oxygen concentration and of temperature that can be applied in any luminescence based sensor. The possibility of using nanoparticle quantum dots in a luminescent probe for temperature determination is also assessed. Different methods of coupling the excitation radiation into the fiber system are evaluated. Additionally, four different oxygen fiber probe geometries are tested and compared. Finally, the properties of some membranes for the immobilization of pH sensors in optical fibers are addressed. Some results are presented that show the viability of NIR optical fiber pH probes. Preliminary tests are made to assess the possibility of adapting the pH fiber probes to the detection of metal ions.
Experimental Temperature Measurement with a Long Pass Filter Oxygen is a well-known quencher of many luminescent dyes. Ruthenium complexes are specially suited for oxygen sensing applications. They are efficiently excited by blue LEDs (470 nm), their emission (620 nm) is strongly quenched by O2 , and their lifetimes are relatively long (0.5 to 6 µs), which make them suitable for frequency domain applications. Additionally, the immobilization of these sensing dyes in sol-gel and polymer matrices is a well studied and developed subject [20–22]. Some optical solutions for the measurement of oxygen are already commercially available. For these reasons, the temperature measurement scheme and the sensing probes geometry were evaluated using a phase detection scheme applied to an oxygen sensor. The experimental setup can be observed in Figure 1. In order to obtain a temperature measurement that is independent of the oxygen concentration, part of the excitation radiation (which is back-reflected from the sensing probe) can be used. The spectral characteristics of this radiation are independent of the oxygen concentration but can be made temperature dependent when propagating through a colored glass long-pass filter placed in the distal end of the sensing probe. The position of the cut-off wavelength of these filters has well-known linear temperature dependence . If the edge of the filter is made to coincide with the source spectral peak,
Figure 1. Experimental setup for simultaneous measurement of oxygen and temperature. Insert: the transmittance of low-pass filter for two different temperatures and the LED emission spectrum.
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the back-reflected blue radiation becomes temperature dependent and can be used to determine temperature. For the implementation of the proposed configuration, a high-brightness blue LED (470 nm—Nichia) was used as excitation source and was sinusoidally modulated at 90 kHz. Light was injected into a multimode silica fiber coupler with core/cladding diameters of 550/600 µm and a nominal 50/50 coupling ratio. The oxygen-sensing fiber probes and the temperature-sensing fiber probe were connected to different outputs of the fiber coupler and placed close together in an oxygen/temperature controlled environment. For the temperature probe a mirror and a long-pass absorption filter with cut-off wavelength of 455 nm (GG455—Schott glass) were placed in the tip of the fiber. The cutoff wavelength of this filter is shifted to higher wavelengths with increasing temperature by 0.08 nm/◦ C. The luminescent signal (620 nm) from the ruthenium complex carrying the oxygen information and the blue signal (470 nm) with the temperature information were guided back to one of the input arms of the fiber coupler. These signals were separated at the fiber output by a dichroic beam splitter. The luminescent signal was fed into a lockin amplifier and was compared with the reference signal that was used to modulate the optical source. In these conditions the lock-in output was an amplitude/phase signal proportional to the oxygen concentration. Conversely, the spectrum of the blue signal was spatially distributed by a diffraction grating. Two photodiodes were then used to obtain two signals, centered at 470 nm (P1 ) and at 500 nm (P2 ), with a bandwidth of ≈3 nm. While signal P1 overlapped with the edge of the filter and became strongly temperature dependent, signal P2 overlapped with a transmission maximum of the filter and remained independent of temperature. This way, a signal that was independent of optical power drifts and contained the information of the sensing head temperature could be obtained with the ratio P1 /P2 . Temperature Measurement with Quantum Dots Semiconductor nanoparticles promise to be a powerful tool with potential to solve many problems in luminescence-based applications [24–26]. Quantum dots (QDs) are nanometer-sized particles of semiconductor material (e.g., CdSe). Due to the quantum confinement effect, they have unique properties. QDs have relatively high quantum yields, narrow fluorescence spectrum (FWHM of 30 nm), very broad absorption, outstanding photostability, and the ability to tune optical properties by simply changing the size of the particles. At present, they are readily available in a variety of emission wavelengths (from 350 to 2000 nm) and can be provided in the form of colloidal suspensions suitable to be immobilized in polymers or sol-gel–based materials. QDs have been reported to have favorable characteristics to be used as temperature probes in optical sensing applications . It was observed that the wavelength maximum, width, and intensity of the QDs photoluminescence emission were all strongly temperature dependent. The possibility to perform temperature measurements independent of the optical power level in the system was tested. Due to the presence of a wavelength shift, a simple detection scheme can be implemented in order to obtain self-referenced temperature measurements with QDs. If two signals, S1 and S2 , corresponding to two narrow spectral windows on opposite sides of the spectrum are normalized according to (S1 − S2 )/ (S1 + S2 ), the resulting normalized output will be proportional to temperature and independent of the optical power level in the system. In order to test the applicability of
QDs as temperature probes in the context of luminescent chemical sensors, core-shell CdSe-ZnS quantum dots with emission wavelengths ranging from 517 nm to 610 nm were immobilized in non-hydrolytic sol-gel. Several samples were excited by a blue LED (λmax at 470 nm, from Nichia). Optical power was carried from the LED to the sample through a 4-mm-diameter fiber bundle. An Ocean Optics S2000 miniature spectrometer (with a 600-µm fiber cable), connected to a PC, was used for detection of the photoluminescence emission. The temperature of each sample was controlled using a peltier cooling device. The emission spectrum of each sample was recorded after thermal equilibrium was reached. To assess independence of optical power drift, temperature measurements were performed using the proposed processing scheme for different levels of the LED optical power. Oxygen Fiber Probes For this particular application, Tris(2,2 -bipyridine) ruthenium(II) chloride hexahydrate [Ru(bpy)3 ] was used as the oxygen sensor with a peak emission at 620 nm and a lifetime of approximately 1 µs. The sensing dye was used to dope a tetraethoxysilane (TEOS)based sol-gel solution. Several fiber probes were prepared by dip-coating silica fibers with core/cladding ratio of 550/600 µm into the resulting solution. After adequate thermal treatment, probes coated with a thin film of porous glass were obtained. The detailed preparation procedures are described elsewhere . Four different oxygen sensing probe geometries were tested. To choose the best performing sensing head, a simplified setup was used: the temperature probe was removed and the corresponding output of the fiber coupler was immersed in index-matching gel. Also, the detection of the luminescent signal was made using a photodetector with two cascaded colored glass long-pass filters (OGG 550: cutoff at 550 nm, extinction coefficient 10−3 ). The four geometries tested can be observed in Figure 2. In all the configurations the excitation radiation and the luminescent signal were carried by the same fiber. The first configuration, Figure 2a, is an extrinsic sensor where the sol-gel sensing film was deposited on a glass slide that was placed inside the O2 chamber. The fiber tip, in direct contact with the ruthenium-doped film, was used to excite and collect the luminescence signal. All the other configurations were intrinsic: the sensing films were deposited directly onto the fiber tips. In configuration (b) the sensing film was deposited on a polished
Figure 2. Fiber probe geometries: (a) glass slide; (b) fiber tip; (c) uncladded fiber tip; (d) fiber taper.
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fiber tip. In configuration (c), the cladding of a 2-cm section of the fiber tip was removed by chemical etching with hydrofluoric acid (HF) and then coated. Finally, in configuration (d), 2 cm of the fiber tip were tapered (550 µm at the base, 300 µm at the tip) by slow dipping in HF, and then coated. In order to compare the sensitivity of the different geometries, some parameters were evaluated. One of them was the quenching response defined by: Q=
IN2 − IO2 IN2
where IN2 and IO2 are the luminescence intensities in N2 - and O2 -saturated atmospheres, respectively. To assess phase sensitivity independently of the modulation frequency, the lifetime difference was measured: τ = τN2 − τO2
where τN2 and τO2 are the excited state lifetimes in saturated atmospheres of N2 and O2 , respectively. Lifetime was estimated by plotting tan[φ] (Equation (3)) as a function of modulation frequency. Additionally, the amount of detected luminescence signal, Pdet , and the back-reflected excitation radiation, Pblue , were measured and compared. Coupling Systems Using the same simplified setup, the efficiency of three different techniques of coupling LED radiation into the fiber system were tested and compared: a standard microscope objective (10×) system; a 10-mm-diameter ball lens; and direct butt-coupling of the fiber to an LED whose encapsulation was partially removed and polished. The configuration with best performance was used to test the simultaneous measurement of oxygen and temperature. pH Fiber Probes The properties of two different membranes for immobilization of pH sensors were tested and compared: polyvinyl alcohol (PVA) (cross-linked with glutaraldehyde) and polyacrylamide gel. The detailed preparation of these membranes and its doping with pH sensors is described elsewhere [17, 29]. Both membranes were doped with 5(6)-carboxynaphtofluorescein (CNF). This pH indicator was chosen for its versatility. It has a fluorescent response on the physiological pH range (6.0 to 8.0), which is of particular interest to biomedical applications, it can be indirectly used as sensor for metal ion recognition when it is coupled to a chelating agent, it has absorption bands at 460 nm and 630 nm that are suitable to LED and diode laser excitation, and it is a dual emission dye suitable for ratiometric detection (it emits at 550 nm in acidic form, and at 700 nm in basic form) . Immobilization was tested both in extrinsic (configuration (a)) and intrinsic (configuration (c)) fiber probes. For the extrinsic membranes the polymerization process was executed in small molds. Conversely, the PVA membrane was applied to the fibers by dip coating. The polyacrylamide membrane was covalently bounded to the silica core of the fibers. The fibers were immersed into a glass capillary, with internal diameter of 650 µm, filled with polymerization solution, and illuminated with a blue light (430 nm) (Hilux Curing Light 200). After photo-induced polymerization the membrane was bounded to the silanized core but not to the glass capillary, which was easily removed.
Figure 3. Experimental setup used to test the pH sensing membranes.
The membranes were tested in the experimental setup shown in Figure 3. The fiber probes were connected to a multimode fiber coupler (50/50 coupling ratio). An He-Ne laser (633 nm) was used as excitation source. For the detection of the luminescent signal the output fiber was connected to a miniature CCD spectrometer (USB2000 Ocean Optics). A magnesium ion [Mg(II)] sensor was tested by co-immobilization on PVA of the CNF sensor with Eriochrome Black T (EBT). The absorption spectrum of EBT overlaps with the absorption spectra of CNF. When Mg(II) is present the absorption spectrum of Mg(II)-EBT shifts to shorter wavelengths, provoking the increase of the CNF fluorescence. Preliminary tests were performed with the setup in Figure 3 by dipping the sensing fiber into the modified solution in order to access the response of the luminescent signal to the presence of Mg(II).
Results and Discussion Coupling Systems The first step in assembling the experimental setup was to optimize the coupling system. The results obtained with the different configurations tested can be observed in Table 1. The lowest efficiency (2.6%) was obtained with a traditional coupling method (collimating and focusing with 10× microscope objectives). The main reason for this poor Table 1 Power coupling efficiencies of the different techniques Coupling system
Coupling efficiency (%)
Microscope Ball lens Butt coupling 550 µm core Butt coupling 1 mm core
2.6 8.0 8.0 20.0
Very sensitive Sensitive Straightforward Straightforward
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result was the high divergence of the optical source (≈15◦ ) and the high insertion losses of the optical system. The efficiency of this configuration was very sensitive to the position of the optical fiber relative to the focus of the optical system. A higher efficiency was obtained with a 10-mm ball lens (8%). This improvement was mainly due to the reduced insertion loss of a single glass sphere when compared with the two microscope objectives. This system was less sensitive to misalignment when compared with the previous one. After polishing the LED encapsulation, the semiconductor emitting surface was only 100 µm away from the fiber tip and an efficiency of ≈8% was achieved by direct butt-coupling. In this setup, fiber alignment was straightforward. After polishing, the LED divergence angle increased to 45◦ , however, the proximity to the emitting area was also increased. As a result, the power density reaching the fiber top was much higher. Considering that the LED emitting area (≈900 µm diameter) was bigger than the fiber core (≈550 µm diameter), better results are expectable for the case where larger fiber core diameters are used. Indeed, for a fiber with a 1-mm core diameter, light coupling efficiencies of ≈20% were achieved. In the experimental setup of Figure 1 the butt-coupling configuration was used with a 550-µm core fiber. Oxygen Fiber Probes The glass slide (configuration (a)) was used to tune the sol-gel properties. These properties have a significant influence on the final characteristics of the sensing films. The resulting glass porosity has a known dependence on the solution aging time and determines the oxygen accessibility to the sensing dye and thus the sensitivity of the resulting sensor . After determining the more suitable sol-gel parameters, several probes of geometries (a), (b), (c), and (d) were coated under the same conditions: sol-gel solution aging time of more than 48 h; dip-coating speed of 3 mm/s; and curing at ambient temperature for 24 h. The merit parameters of the probes with best characteristics are shown in Table 2. Observation of the films obtained with configuration (a) under an optical microscope revealed uniform films with no cracks. A perfilometer was used to measure the films’ thicknesses; thicknesses of 600 nm to 800 nm were found. The response times of the sensing films when they underwent O2 /N2 saturation cycles were the same and were approximately 9 s (framed to the 10% and 90% reference levels). In configuration (a) the detected luminescence emission was very weak (Pdet ≈ 0.36 nW), and due to a low signal-to-noise ratio the phase signal was unstable.
Table 2 Comparative results for the tested fiber probe configurations Geometry
Response time (s)
Slide Tip Uncladded Taper
40 34–44 55 65
176 246 250 350
9 20–60 20 11–13
1.00 1.81 2.00 3.58
— 1.00 0.16 0.15
With configuration (b) no uniform thin films could be obtained because they were thicker in the tip than in the lateral surface, a consequence of the fabrication process. However, the film deposited smoothly in the fiber lateral surface and in the tip the film structure was also relatively flat. The response time of several fiber tips with this geometry, with the same sol-gel parameters, varied from 20 s to 60 s. This shows that, with dip-coating, it is not possible to repeatedly obtain films of uniform thickness at the fiber tip. In spite of that, due to the increased thickness and a more efficient luminescence coupling, a significant increase in SNR was observed. In comparison to configuration (a) the value of detected luminescence, Pdet , increased by a factor of 1.8. Although the coating parameters were similar in all geometries, smaller values for Q and τ were obtained with this configuration. This can be related to film thickness and lower oxygen accessibility. In configuration (c), uniform thin films were obtained in the side of the fiber where the cladding was removed. However, in the tip of the fiber the resulting films were non-uniform and too thick. Response time of approximately 20 s was obtained. This probably results from the averaging of a faster response from the side of the fiber with a much slower response from the top. Also, some improvement in Q and τ parameters was observed. However, no significant enhancement in luminescence coupling efficiency was observed in comparison with configuration (b). This may indicate that, in spite of the much larger emission area, the contribution from the film in the fiber side to the luminescence signal is small due to poor guiding from this region. With configuration (d), uniform thin film was obtained in the side as well as at the fiber tip due to its geometry. Shorter response times clearly indicate that the overall film thickness is similar to the one obtained with a glass slide. This is also shown by the increase in Q and τ which also denote better quenching efficiency and sensitivity. With this configuration, several sensing probes with very similar parameters were obtained, showing good reproducibility. A great improvement in the SNR was registered in the tests performed with this configuration, and Pdet was increased by a factor of 3.5 relative to the glass slides. In Figure 4 a photograph of a fluorescent tapered fiber in a 100% N2 environment is shown. In the last column of Table 2, the degree of detected back-scattered blue radiation in each configuration (Pblue ) is presented. Configuration (b) shows a 6-fold increase in the level of back-scattered radiation when compared with configurations (c) and (d). Considering the respective levels of detected radiation (Pdet ) this implies that even after attenuation by a factor of 1000 (obtained with a single filter), a significant amount of blue light is present with the luminescent signal. The attenuated power at the excitation wavelength (Pblue × 10−3 ) is still 18% of Pdet in configuration (b), 2.6% of Pdet in configuration (c), and 1.4% of Pdet in configuration (d). Straightforward numerical calculations verify that this will introduce phase errors of 5.5, 0.7, and 0.4%, respectively . In the experimental setup used to test the fiber probes, this problem was avoided by using two cascaded long-pass filters corresponding to a 10−6 attenuation factor. In this case the error dropped below a negligible 0.005% in all configurations. In general, double filtering is not a desirable solution from a practical point of view, but it can be bypassed by using filters with higher performance. Anyway, the desirable solution to this problem is to use sensing probe geometries, which minimize the level of backscattered radiation. We should note that when the back-scattered blue radiation is reduced, a more efficient and uniform excitation in the sensing device is automatically accomplished. The results obtained clearly show that tapering the fiber is a right step to producing an optimal sensing probe and that both the intensity and phase measurement schemes will
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Figure 4. Tapered fiber in a 100% N2 environment.
benefit from its properties. Therefore, a tapered fiber was used to test the simultaneous measurement of oxygen and temperature (setup of Figure 1). The intensity and the phase of the sensor output signal were recorded while the sensing head was submitted to successive N2 /O2 saturation cycles. The respective results can be observed in Figures 5a and 5b. From these results, a response time of approximately 10 s can be estimated. Also, it can be observed that the phase signal is more unstable, indicating the need to improve the SNR in order to take full advantage of the frequency domain detection scheme. The sensor calibration curve, i.e., the Stern Volmer plot, was obtained by performing measurements of the system response to several values of oxygen concentration. The corresponding plot can be observed in Figure 6. The nonlinearity of the response curve indicates that the sensing dye is not uniformly distributed within the sensing film and that sites with different oxygen accessibilities coexist in the sol-gel matrix. This behavior can be more accurately described with a more complex bi-exponential model . Additionally, an adequate thermal treatment can render the sensor response more linear at the expense of loosing some quenching efficiency.
(b) Figure 5. Response of tapered fiber to O2 /N2 saturation cycles: (a) luminescence intensity response; (b) phase response.
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Figure 6. Stern-Volmer plot obtained from the phase response of the tapered fiber.
In order to assess the sensitivity of the oxygen sensing probe to temperature fluctuation, the output phase and the luminescent intensity were measured for several values of temperature with a constant oxygen concentration of 21%. The results can be seen in Figures 7a and 7b. It is clearly shown that both the lifetime and the luminescence intensity of the sensing dye depend not only on the oxygen concentration, but also on the temperature. In order to extract the information about the oxygen concentration from the luminescent signal, the temperature must be simultaneously determined. Temperature Measurement with a Long Pass Filter The configuration of Figure 1 was used to measure the temperature using the spectral information of the blue radiation reflected by the temperature probe. The measurements were performed under a constant oxygen concentration of 21%. As it can be seen from Figure 8, the spectral characteristic of the reflected excitation radiation is rendered temperature dependent by the presence of the long-pass filter. The temperature dependence is maximum around 465 nm, nearby the LED spectral peak and coinciding with the band edge of the filter. This dependence vanishes for longer wavelengths (>480 nm), where the filter transmittance is maximum (insert in Figure 1). As the temperature increases, the band edge of the filter shifts toward the red (0.08 nm/◦ C), and less radiation is transmitted at 465 nm (insert in Figure 8). In order to evaluate the influence of optical power level in the temperature measurement, the outputs P1 and P2 were measured for two different radiation coupling conditions, identified by HIGH and LOW. In the LOW condition the optical power reaching the detector was 20% less than in HIGH condition. In Figure 9 the temperature dependency of signal P1 at two different conditions, HIGH and LOW, are shown.
(b) Figure 7. Response of oxygen probe to temperature: (a) luminescence intensity response; (b) phase response.
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Figure 8. Spectral response of temperature probe (temperature range 19◦ C, 54◦ C). Insert: temperature dependence of the return optical power at 465 nm.
Figure 9. Temperature response of P1 detected at 470 nm for 100% optical power (HIGH) and 80% optical power (LOW) (temperature range 15◦ C, 45◦ C).
It can be seen that the optical power (P1 ) of the LED at the central wavelength is a linear function of temperature. However, it is also clear that this measurement is not independent of the optical power level (the 20% decrease in optical power can clearly be observed in the resulting plots). To achieve immunity to optical power fluctuations, the curves in Figure 9 were divided by the optical power, P2 , which has negligible temperature dependence. To optimize the processing scheme, the ratio P1 /P2 was performed at two different reference wavelengths. P2 was detected at 496 nm (Ratio A) and at 500 nm (Ratio B). The corresponding results are shown in Figures 10a and 10b. Comparing the processed signal with the original data, it can be observed that an optical power drift of approximately 20% in P1 is reduced to a 4% variation in Ratio B (Figure 10b), and to a 1% variation in Ratio A (Figure 10a). Both the 496 nm and the 500 nm optical power signals have negligible temperature dependence. However, for higher wavelengths the optical power available decreases very fast and noise is introduced in the ratio operation. If the operation (P1 − P2 )/(P1 + P2 ) is performed instead, noise is slightly decreased and the 20% change in P1 induces a 0.7% change in Ratio A and 1.1% in Ratio B. This shows that data dispersion comes from a low SNR and an increase in the LED output power will benefit the sensor performance. Nevertheless, these results indicate that the reference wavelength can be tuned in order to reduce further the residual dependence of the system performance to the optical power fluctuations. Temperature Measurement with Quantum Dots Several samples of CdSe-ZnS QDs immobilized in sol-gel were submitted to temperature changes while their spectral characteristics were monitored. In Figure 11, the spectral behavior of the fluorescent emission of a thin sol-gel film (∼5 µm) doped with CdSe-Zn QD (emission peak at 610 nm) can be seen. The sample temperature was increased from 11 to 48◦ C. Data analysis showed that both the wavelength and the fluorescence intensity changed in a linear and reversible way with temperature. The behaviors of different samples (different emission wavelength, different sol-gel parameters) were very similar. The rate of change in the intensity with temperature varied from −0.7%/◦ C to −1.6%/◦ C. On the other hand, the rate of increase in the wavelength with increase in temperature in all samples is approximately 0.2 nm/◦ C. This indicates that the differences in the intensity response could be due to different immobilization environments, distinct film thickness, etc. Because the wavelength dependencies of the fluorescence signals to temperature are identical, it is reasonable to assume that the intrinsic responses of the various QD samples to temperature are similar. Also, deviations in linearity were observed in the intensity response (Figure 12a) for the lower temperature range, which were due to some water condensation on the samples surface, which changed the coupling conditions. This was not observed in the wavelength response, which was linear through all temperature intervals. Therefore, this mechanism is particularly well suited to perform self-referenced temperature measurement. To test the self-reference scheme, two signals, S1 and S2 , corresponding to the spectral windows 595–600 nm and 620–625 nm were detected using a CCD spectrometer and adequate software. The two signals were then normalized by calculating (S1 −S2 )/(S1 +S2 ). Temperature measurements were performed using three different levels of the LED optical power (PLED ): 100% PLED , 90% PLED , and 80% PLED . This was achieved by changing the LED drive current. Figure 12a shows the QD luminescence
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(b) Figure 10. Temperature dependence of P1 /P2 , for 100% optical power (HIGH) and 80% optical power (LOW). (a) P1 /P2 with P2 detected at 496 nm; (b) P1 /P2 with P2 detected at 500 nm.
Figure 11. Temperature behavior of the emission spectrum of CdSe-ZnS QDs immobilized in sol-gel (temperature range 11◦ C, 48◦ C).
intensity response to temperature changes for three different power levels. The corresponding normalized outputs can be seen in Figure 12b. Comparison of the plots in Figures 12a clearly shows that the luminescent emission response strongly depend on the change in the excitation optical power. This possible source of error can be eliminated by applying the proposed normalization scheme, Figure 12b. The dispersion of the normalized signal was within the noise limit of the system. Although the emission spectrum of the QDs used to obtain the results in Figures 11 and 12 overlaps with the emission of most common oxygen sensors (620 nm), very similar behavior was observed with other QDs with lower emission wavelengths. Therefore, these results confirm that QDs could be used to configure self-referenced temperature probes for many chemical sensors. pH Fiber Probes Both the PVA and the polyacrylamide membranes showed good compatibility with the CNF sensor. When immersed into different buffer solutions the polyacrylamide sensing membrane showed a fast and reversible response to pH changes in the surrounding medium. Both the color and the fluorescent emission of CNF changed with change in pH. Very little leaching was observed and it occurred mainly in the basic form. Due to covalent bonds the adhesion to the fiber core was relatively strong. However, in its hydrated form the membrane was soft and could be easily damaged. Additionally, due to the fabrication procedure the end of the fiber tip was not efficiently coated. Hence, a very low signal-tonoise ratio was obtained with the intrinsic configuration. Tapered fiber design is expected to improve both the SNR and adhesion to the fiber tip.
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(b) Figure 12. Temperature response of QDs for three different levels of excitation optical power (PLED = 100, 90, and 80%), for a temperature range from 11◦ C to 48◦ C. (a) Luminescence intensity response; (b) normalized signal response.
With the PVA membrane, a very fast and reversible response to pH was observed. There was no apparent leaching of the sensor into the buffer solutions. The membranes obtained were very robust and elastic and not easily damaged. However, adhesion to the fiber core was very weak. Some modifications are in progress to improve the adhesion of membrane to the fiber. In this work, only extrinsic membranes were tested. Figure 13 shows the spectral response of CNF immobilized in PVA when the membrane was immersed into solutions of pH 6.5, 7.5, and 9.9, respectively. The resulting plot indicates an increase in the luminescence signal as the pH of the buffer solution increases. The emission peak of the CNF fluorescence is around 700 nm and can easily be discriminated from the laser emission. However, it can be seen that the back-reflection of the laser is stronger than the fluorescence signal from CNF. The use of optical high-pass filters may be needed in order to increase noise rejection and improve the sensor signal-to-noise ratio. Also the use of tapered fiber probes will improve SNR. Figure 14 shows a plot of intensity of the fluorescence peak as a function of the solution pH over a pH range of 3.9–9.9. These results indicate a good compatibility between the CNF sensor and the PVA as immobilization membrane. Currently, work is in progress to improve adhesion to the fiber to fabricate an intrinsic fiber probe. The CNF sensor was combined with a chelatometric indicator (Eriochrome Black T, EBT) in order to become responsive to metal ions. The sensitivity of the modified sensor to the presence of magnesium was tested in pH = 9 buffer solutions. The spectral response of the modified sensor to increased concentration of Mg(II) can be observed in Figure 15. The results show an enhancement of the luminescent response that is proportional to the concentration of Mg(II) in the solution (insert in Figure 15). The enhancement is due to reduction of the interference of the absorption band of free EBT. This shows that modified pH indicators could be used to detect heavy metal ions in aqueous environments. Work is in progress to improve the sensitivity to Mg(II) and to other metal ions.
Figure 13. Emission spectra of CNF immobilized in PVA as function of pH (excitation at 633 nm).
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Figure 14. Response of the PVA fiber probe as a function of solution pH (from 3.9 to 9.9).
Figure 15. Response of the modified CNF to the presence of increasing concentrations of magnesium in sample solutions. Insert: Integrated luminescence intensity as function of Mg2+ concentration.
Conclusion Several technological challenges to optimize the luminescence-based optical fiber sensors were addressed. Four different fiber probes were tested using a luminescence phase detection scheme. The tapered fiber tip structure showed the best performance and, in particular, the results obtained from this design were very reproducible. This sensor design provides larger excitation efficiency and reduced level of back-scattered excitation radiation. Light was coupled into the fiber with higher efficiency and alignment constraints were more relaxed with a butt-coupling geometry. The performance of the sensing system was assessed using both phase and intensity measurements. An all-optical configuration for simultaneous measurement of oxygen concentration and temperature was demonstrated. Encouraging preliminary results were obtained. Further integration of the sensing head can be achieved if the mirror and the absorption filter are placed on the top of the oxygen fiber probe. A single fiber will then be enough and part of the fluorescence radiation that was lost in the previous configuration can now be detected. Controlling the LED operating temperature, its central wavelength can be tuned within a few nanometers. This can be used to optimize the overlap of the filter transfer function and of the LED spectrum increasing the efficiency of the optical power reference scheme. The principle of operation of this configuration can easily be extended to other optical fiber sensors that are based on the quenching of fluorescence. Preliminary results showed that QDs can be used to design a selfreferenced temperature probe in fluorescence-based sensors. Their availability in a wide range of wavelengths opens the possibility of using them with different sensors and in multiplexing applications. The PVA and acrylamide as immobilization membranes together with CNF as sensing dye present promising characteristics for environmental and biomedical applications. With excitation at 633 nm, only one emission peak (700 nm) of CNF is excited, and there is no possibility of implementing a ratiometric reference scheme. However, work is in progress to implement frequency domain interrogation techniques and eliminate the dependency to optical power level. The results obtained during this course of research clearly show that luminescence technique offers a practical path with a great potential to develop many novel fiber optic sensing systems for environmental and biomedical applications.
Acknowledgments Pedro Jorge acknowledges the financial support of FCT (Fundação para a Ciência e a Tecnologia) and FLAD (Fundação Luso Americana para o Desenvolvimento). Financial support from Fundação para a Ciência e Tecnologia (Lisbon) (FSE-FEDER) (project POCTI/QUI/44614/2002) is also acknowledged.
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Biographies Faramarz Farahi has been working in the area of optical devices for 19 years. He has more than 130 papers and several patents in this area. He is currently a professor of physics and optical science in the University of North Carolina at Charlotte. José L. Santos graduated in applied physics (optics and electronics) from the University of Porto, Portugal, in 1983. In 1993 he was awarded a Ph.D from the same university for research in fiber optic sensing. He is an associate professor of the physics department of the University of Porto and is in charge of the Optoelectronics and Electronic Systems Unit of INESC, Porto. His main research interests are in the optical fiber sensing field. He is a member of OSA and SPIE. Joaquim C. G. Esteves da Silva graduated in chemistry (inorganic and environmental) from the University of Porto (1985), with a Ph.D. from the same University in 1994. Presently he holds the position of associate professor of the chemistry department of the University of Porto. Currently, his research interests focus in luminescence spectroscopy (fluorescence, bioluminescence, and chemiluminescence) and in the development of new luminescent methodologies for the analysis of chemical, environmental, and biochemical systems. Abel Oliva received his agricultural engineering degree from the University of Buenos Aires in 1984 and his Ph.D. at the Hohenheim Universitaet, Stuttgart, Germany, in 1989. He started a biosensors laboratory in the Instituto de Tecnologia Química e Biológica, of the Universidade Nova de Lisboa/Portugal in 1992. His research interests and scientific activity include optical immunosensors and immunoassays for clinical diagnostics and bioprocess monitoring. Carla Carmelo Rosa received her technological physics engineering degree (1996) and electrical engineering and computers sciences M.Sc. (1999) from Instituto Superior Técnico, Lisbon, Portugal. Presently in the physics department of the sciences faculty of Universidade do Porto, she is working in her Ph.D. in the Optoelectronics Group of INESC-Porto. Her research interest are on fiber-based sensors with applications in bio-medical fields, in particular low-coherence interferometry techniques and biosensor development. Pedro Jorge graduated in applied physics (optics and lasers) from the University of Minho in 1996. He obtained his M.Sc. degree in lasers and optoelectronics from the University of Porto in 2001. Currently he is a researcher in the Optoelectronics and Electronic Systems Unit at INESC Porto working toward his Ph.D. on optical fiber sensors based on fluorescence spectroscopy in the physics department of the University of Porto. Paulo Caldas graduated in applied physics (optics and lasers) from the University of Minho in 1999. He received the M.Sc. degree in lasers and optoelectronics in the physics department of the University of Porto in 2003. Currently he is a researcher in the Optoelectronics and Electronic Systems Unit at INESC Porto and is teaching physics at the Polythenic Institute of Viana do Castelo. He has been working in the area of optical fiber sensors and biosensors.