Effect of polymeric dye network bonding on fluorescence thermometry for optical fiber temperature sensor

Fluorescein and two of its derivatives (allyl Fluorescein and diallyl Fluorescein) are solidified in glycidyl methacrylate (GMA) polymer matrix for the development of fluorescence-based fiber optically temperature sensor. These two Fluorescein derivatives were investigated in order to study the influence of degree of covalent bonds to GMA polymer backbone on Fluorescein dye sensitivity for temperature changes. The dye-polymer discs have been investigated separately in order to determine their sensitivities in the temperature ranges from 10 to 300 K and from 297 to 370 K. The dye-polymer discs are excited by Argon ion laser (488 nm, 50 mW) and the fluorescence intensity is measured using lock-in technique. Below room-temperature Fluorescein dye in (GMA) polymer shows very weak sensitivity toward temperature changes in the range from 10 up to 297 K. The fluorescence intensity peaks of allyl Fluorescein dye in (GMA) polymer exhibit tendency for decreasing slightly, while the dye is warmed up from 40 to 245 K. Also, the fluorescence intensity peaks of diallyl Fluorescein dye in (GMA) are reported in the temperature range from 49 to 300 K. The intensity peak values decrease gradually, whereas at the temperature 171 and 229 K the peaks show hazard change. Above room temperature, the fluorescence intensity peaks decrease with increasing temperature in the temperature range from 302 to 370 K.


Introduction
The fiber-optic communication industry improved the telecommunication industry by providing higher performance, more reliable telecommunication links with decreasing bandwidth cost. Fiber-optic sensor technology has been a major user of these developing technologies [1][2][3][4][5][6][7][8][9]. The general diagram of fiber-optic sensor consists of optical source, optical fiber as transmission channel, sensing element, optical detector, and end processing device [10][11][12]. Fiberoptic sensors are classified depending upon different parameters [2,[13][14][15][16][17]. There are a number of configurations for these sensors. Two of the most common are the end tip configuration and the side-etched configuration of the optical fiber [18]. Optical temperature sensors and high accuracy of sensor sensitivity are a key parameter to determine the industry application. Thus, it requires high luminescence efficiency and high-temperature stability.
Compared to many traditionally used electric sensors such as thermocouples and thermal resistances [19,20], optical fiber temperature sensors become one of the most commonly used tools because of their distinctive advantages as small sizes, long-range sensing, immunity to electromagnetic interference, resistance to environments, and distributed sensing networking potentials [21,22]. Therefore, many optical fiber temperature sensors have been investigated and used in various applications, such as fiber Bragg grating [23], photonic crystal fibers [24], and Fabry-Perot fiber sensors [25,26]. Here, in our work, we prepared three solid discs of fluorescein derivatives dyes of different covalent bonds in polymeric host to explore the impact of different connections of covalent bonds between fluorescence probe and polymer chain on temperature sensing. The monitoring temperature range was from 10 to 370 K, using Argon ion laser (488 nm, 50 mW) as pumping source. As well, photostability and thermal stability were carried out to confirm the reproducibility of studies.
1.1 Experimental procedure and data processing methods

Polymeric disc samples preparation
For the preparation of polymer samples covalently bonded with the Fluorescein (F), allyl Fluorescein (AF), and diallyl Fluorescein (Di-AF) dye, (5 9 10 -3 M) of the required dye was dissolved in freshly purified (GMA). The resulting mixture was placed in an ultrasonic water bath for 10 min until monomer-dye was completely dissolved. The required amount of Azobisisobutyronitrile ''AIBN'' was added and allowed to dissolve in the ultrasonic bath for about 5 min. The resulting solution was filtered into soda glass tube (2.2 cm diameter and 20 cm length). After careful de-aeration by purging with inert gas, the test tube was sealed. Copolymerization reaction was performed in a temperature-controlled oven; the temperature is maintained at 60°C for 7 days and then it was reduced in steps of 10°C per day until room temperature was reached [27]. All the prepared polymer samples were cut and polished for 2 mm thickness for required measurements. The molecular structures of GMA monomer, (F) dye, (AF) dye, and (DA-F) dye are shown in Fig. 1a, b, c, and d, respectively.

Characterization
The absorption spectra A(k) were measured using single-beam spectrophotometer (Camspec model M501 UV-Vis) in the spectral range 300-700 nm. The resulting spectrum pattern is represented as a graph of absorbance versus wavelength. The thermo-gravimetric analysis is performed in the temperature range from room temperature to 400°C in an atmo- sphere of nitrogen. The temperature rate is 10.00°C/ min. The detector is DTG-60H.

Laser-induced fluorescence measurements
Fluorescence measurement of each dye is carried out as explained in the schematic experimental setup in Fig. 2. Light from Lexel laser Inc. model 95 Argon ion laser with 488-nm wavelength was used as the excitation source. As shown in figure, the sample is attached to the cold finger inside the closed cycle Helium gas cryostat, where the sample temperature can be controlled using the temperature controller, Scientific Instruments Inc. 9620-1 silicon diode, from 10 to 370 K. The Ar laser light 488 nm falls onto the sample through the quartz window of the cryostat by 45°angle and the emitted light is collected using a converging lens and filtered by a 515 OG filter to eliminate the reflected 488-nm Ar laser light. The emitted light is dispersed and detected using the Spex-monochromator 750 mm followed by a photomultiplier tube. The signal is amplified using the lock-in technique. The laser power selected is as low as possible to avoid the sample photodegradation. The fluorescence is recorded in the wavelength range 450-750 nm.

Photostability
In order to study the photostability due to the long irradiation time during fluorescence measurement, further experimentation on the dye-polymer discs was applied. The intensity of fluorescence peak is measured as a function of time during irradiation by 488-nm Ar laser light (50 mW) for 1 h. Figure 3 contains two photos taken for the dye-polymer disc before and after the measurement (i.e., before and after exposure to the laser light), respectively.
2 Results and discussion

Absorption measurements
Absorption spectra of (F), (AF), and (Di-AF) in (GMA) were measured in the wavelength range from 300 to 700 nm. The resultant spectra are presented in Fig. 4. The maximum peak in the absorption spectra was noticed at 459, 458, and 458 nm, respectively, with different absorbance intensities.
The principal absorption bands in the three dyes are due to the p ? p* transitions in the molecules. In general, these transitions are influenced by different factors, such as the conjugation length, degree of conjugation, and the different substituents. These factors effect on the lowest occupied molecular orbital (LUMO) and the highest unoccupied molecular orbitals (HOMO) through them absorption transition occur [28][29][30][31][32][33]

TGA analysis for the dyes
In order to assure that results acquired for intensity changes with temperature changes in the temperature range from room temperature of about 297 to about 370 K, for the used dyes in polymer (GMA), are with no decomposition of the dye-polymer samples from heat, TGA analysis in the temperature range from about 300 to about 675 K is performed. The following Fig. 5 is obtained. In Fig. 5, the analysis of (F) disc sample starts at 524 K (251°C) and when it reaches 550 K (277°C) a weight loss of -1.795% occurs. At 675 K (402°C), the overall weight loss reaches to -48.278%. While in case of (AF) disc sample, the analysis starts at 475 K (202°C) and at 494 K (221°C) a weight loss of -4.252% occurs and the overall weight loss was -73.54% at 674 K (401°C). In (Di-AF) disc sample, the first temperature recognized for which weight loss appears to begin was at 475 K (202°C). This range of weight loss ends at 618 K (344°C) was -70.351% of the sample weight.

Laser-induced fluorescence measurements
Argon Ion laser of 488-nm wavelength and moderated power (50 mW) was used for exciting the fluorescent three samples at different temperature ranges, below and above room temperature, including heating and cooling condition, for assigning the thermo-sensitivity of samples for opto-electronic device applications.

Fluorescence of fluorescein in (GMA)
for the temperature range (10 ? 297 K) Figure 6 shows the fluorescence spectra obtained for Fluorescein dye in (GMA) polymer when measured at different temperatures in the cryogenic temperature range (10-297 K). The peak intensities are found at 556 nm in the range of temperature measurement. Figure 7 shows the line of the best mathematical relation between peak intensity changes and the corresponding temperature assigned to each increment in order to formulate a suitable mathematical formula that describes the relation.
As shown in Fig. 6, Fluorescein dye shows very weak sensitivity toward temperature changes. The values of the peak intensities are almost constant along the range of temperature measurement.
The fluorescence peak intensities are determined at each temperature in the range from 10 to 297 K. These data are represented and fitted mathematically as shown in Fig. 7.
The data were fitted with a polynomial equation of the second degree with an R 2 value of 0.6758 suggesting constancy of fluorescence intensity throughout the temperature range (10-297 K). The best fit equation is as follows: 2.5 Fluorescence of fluorescein in (GMA) for the temperature range (302 ? 370 K) and (370 ? 305 K) Figure 8 shows the fluorescence of Fluorescein dye in (GMA) obtained in the temperature range increased (by heating) from room temperature 302 up to 370 K.
The fluorescence peak intensities are positioned at 556-nm wavelength. Fluorescence peak position shows temperature independency, while the fluorescence peak intensity shows temperature dependency. The peak height shows fast decrease from the temperature 302 to 312 K and then regular decrement steps from 312 up to 370 K are observed.
The reproducibility of the measurement was tested by measuring the fluorescence as a function of temperature in the decreasing range (370-305 K). The fluorescence spectra of the dye Fluorescein in (GMA) are shown in Fig. 9 when the dye Fluorescein in (GMA) is illuminated by 488-nm wavelength Ar laser of 50-mW power.
As shown in Fig. 9, the fluorescence peak intensities occur at 560 nm and the fluorescence exhibits a gradual increase in the intensity at every decrease in temperature in the range (370-305 K). The intensities increase in small amounts along the range of measurement. The hysteresis of data with increasing the temperature in ascending steps from 302 to 370 K and with decreasing the temperature in descending steps from 370 to 305 K is represented in Fig. 10. It shows that the temperature range increasing from 320 to 370 K and in reversible steps down again to 320 K shows negligible hysteresis. At low temperatures from 305 to 320 K there is a noticeable hysteresis.
In Fig. 7, heating behavior shows the relation between increasing the temperature from 302 to 370 K and the fluorescence peak intensity values that could be presented mathematically by with an R 2 value equal to 0.8332. The intensity decreases exponentially with the temperature gradual increments. The intensity change/temperature change (sensitivity) varies along the exponential relation. For application purpose the range of temperature measurement can be divided into two ranges. The first range is (312-340 K) which shows the greatest slope of the tangent at the midpoint of the range and equals -2.68 9 10 -7 . The second range of temperature is (340-370 K) and the slope of its tangent at the midpoint is smaller than the slope of the first range and equals -7.3 9 10 -8 . The slope of the tangent is negative, i.e., the intensity decreases with increasing the temperature. This is one of the disadvantages for practical use, where the sensitivity depends on the range of temperature measurement. As well in Fig. 10, it noticed continuous increase in the fluorescence peak intensities values with decreasing the temperature from 370 down to 320 K (cooling behavior). A decrease in the fluorescence peak intensities at the temperature values of 312 and 375 K is recorded. The best fit equation with an R 2 value of (0.8998) at 556 nm is given by 2.6 Fluorescence of allyl fluorescein dye in (GMA) for the temperature range (40-245 K) Allyl Fluorescein (AF) dye in (GMA) is tested for its fluorescence peak intensity changes when illuminated by 488-nm Ar laser of 10 mW in the temperature range from 40 to 245 K. Figure 11 shows the obtained fluorescence spectra. The peak intensities are found at 559-nm wavelength. Figure 12 presents the closest mathematical relation between peak intensity changes and the corresponding temperature assigned to each increment in order to formulate a suitable mathematical formula that describes the relation. In Fig. 11, the spectra are graphed when the dye is heated in gradual steps in the temperature range from 40 to 245 K. Each step is separated from the next increment step by about 20-K degrees.
The fluorescence intensities exhibit tendency for decreasing gradually, while the dye is warmed up from 40 to 245 K. The spectra are close and the peaks are observed at 559 nm. While, Fig. 12 shows the values reached by the fluorescence peaks in Fig. 11 with respect to the corresponding temperature values at which the spectra were recorded.
The best fit equation at 559 nm with R 2 value of 0.9795 is given by the equation: The points fit linearly intercepting the y-axis at 0.001 and with a slope of 1 9 10 -6 showing a decreasing tendency in the range of temperature measurement from 40 to 245 K. The fluorescence spectra of the dye allyl Fluorescein in (GMA) is measured and plotted when the dye is illuminated by 488-nm wavelength Ar laser with a power of 10 mW, in the range of temperature measurement from 301 to 370 K. Figure 13 shows the obtained spectra at that range of temperatures. Fluorescence peak position at 560 nm shows temperature independency in studied range of temperature, as well as the full-width at half-maximum (FWHM).
As shown in Fig. 13, the fluorescence peak intensity shows temperature dependence that decreases with increasing temperature.
The reproducibility of the measurement can be tested by measuring the fluorescence as a function of decreasing temperature in the range (370-309 K). The fluorescence spectra of the dye allyl Fluorescein in (GMA) are recorded in Fig. 14, while the dye is illuminated by 488-nm wavelength Ar laser of 10-mW power.
As shown in figure the fluorescence peaks intensities are observed at 560-nm wavelength in the descending temperature range from 370 to 309 K. The intensities increase in small amounts at the temperatures from 370 to 340 K. There is a noticeable random jump in peak intensity starting at 340 to 330 K. At 322 K the peak intensity increases, but at 313 K it decreases and generally the peak intensity increases as the temperature decreases from 370 to 309 K. Figure 15 shows the data summarized in Figs. 13 and 14 in order to test the coincidence of the points obtained in the two behaviors (i.e., the ascending range of temperature (301-370 K) and the descending range (370-309 K)).
As shown in Fig. 15 there is a coincidence in the ascending and the descending curves at high temperatures, i.e., in the range (350-370 K) but the curves are not coincident at low temperatures, i.e., (301-350 K). The coefficient of the exponential curve on heating (301-370 K) is -0.043, while the coefficient of the exponential curve on cooling (370-309 K) is -0.013. This means that the fluorescence peak intensity decreases faster in ascending than in descending temperature range. This noticeable hysteresis may be attributed to photodegradation effect of the excitation laser power.
In heating behavior, the relationship between the temperature and the fluorescence peak changes in the temperature range (301-370 K) that is expressed by an exponential function. The best fit equation of the plot with R 2 value of 0.993 is as follows: For the exponential behavior the Intensity change/ Temperature change (sensitivity) varies along the exponential line. For application purpose, the range of temperature measurement can be divided into three ranges. The first range of temperature is (301-321 K) which shows the greatest slope of the tangent at the midpoint of the range and equals -1.38 9 10 -5 .
The second range of temperature is (321-340 K), and the slope of its tangent at the midpoint is smaller than the slope of the first range and equals -5.66 9 10 -6 . In the third range of temperature (340-370 K), the slope is much lower and equals -1.85 9 10 -6 that means a decrease in the sensitivity (Intensity change/Temperature change). This is disadvantageous in the practical use, where the sensitivity depends on the range of temperature measurement.
On the other hand, the cooling behavior shows the relation between fluorescence peak intensities and temperature obtained for the dye allyl Fluorescein sample in multiple temperatures in the range from 370 down to 309 K. It represents the best fit for the experimental points. The best fit equation of the plot of R 2 value of 0.9138 is as follows: 2.8 Fluorescence of diallyl fluorescein (DA-F) dye in (GMA) for the temperature range (49 ? 300 K) The dye diallyl Fluorescein in (GMA) is tested for fluorescence thermo-sensitivity in the cryogenic temperature range from 49 up to 300 K when illuminated with 488-nm wavelength Ar laser of 10-mW power. Figures 16 and 17 show the spectra obtained and the calibration curve that represents the fluorescence spectra peaks and the temperature, respectively. The mathematical equation derived from the curve that describes the relation is recorded. As shown in Fig. 16, the fluorescence intensity peaks are located at 559-nm wavelength. The intensity peak values decrease gradually, whereas at the temperature 171 and 229 K the peaks increase. Figure 17 contains the relationship between the fluorescence intensity peaks of the spectra obtained in the range of temperatures from 49 up to 300 K of the dye diallyl Fluorescein sample.
As shown in Figure, despite some irregularities in intensity of the peak values at the temperatures 70, 171, and 229 K, the relationship can be fitted exponentially. The best fit equation of the plot with R 2 value of 0.9385 is as follows: For the exponential behavior, the Intensity change/ Temperature change (sensitivity) varies along the exponential line. For the application purpose the range of temperature measurement can be divided into three ranges. The first range of temperature (49-130 K) shows the greatest slope of the tangent at the midpoint of the curve which equals -3.89 9 10 -5 . The second range (130-210 K) shows a slope that equals -2.56 9 10 -5 . In the third range (210-300 K) the slope is (-2.61 9 10 -5 ). This is disadvantageous for practical use as mentioned before where the sensitivity depends on the range of temperature measurement.

2.9
Fluorescence of diallyl fluorescein dye in (GMA) for the temperature range (297-365 K) and (365-305 K) (DA-F) dye in (GMA) polymer is tested for thermosensitivity in the temperature range from 297 up to 365 K. The dye is illuminated by 488-nm wavelength Ar laser of 10 mW and heated from room temperature 297 gradually to 365 K in steps of about 10-K degrees length as shown in Fig. 18. Fluorescence peak intensity position at 560 nm shows temperature independency in the studied temperature range as well as the FWHM. Also, the reproducibility of the measurement can be tested by measuring the fluorescence as a function of the temperature in the decreasing range (365-305 K). The fluorescence spectra of the dye diallyl Fluorescein in (GMA) are recorded in Fig. 19 when decreasing the temperature from 365 down to 305 K, while the dye is illuminated by 488-nm wavelength Ar laser of 10-mW power.
As shown in Fig. 19 the maximum intensity of the fluorescence is noticed on each curve to be at 560-nm wavelength. The fluorescence peak intensity rises with lowering the temperature in the temperature range (365-305 K). Figure 20 compares the data summarized in Figs. 18 and 19 in order to test the coincidence of the points obtained in the two plots in the ascending range of temperature (297-365 K) and the descending range (365-305 K).
The resultant graph shows coincidence at higher temperatures, i.e., in the range (335-365 K), but at lower temperatures, i.e., in the range (297-365 K), the two curves do not coincide. The coefficient of the exponential curve on heating (297-365 K) is -0.035 (Eq. 8), while the coefficient of the exponential curve on cooling (365-305 K) is -0.014 (Eq. 9). This means that the fluorescence peak intensity decreases fast on increasing the temperature, but it increases slowly on decreasing the temperature. This noticeable hysteresis may be attributed to photodegradation effect of the laser power.
As shown in heating behavior in figure, the fluorescence peak intensity decreases when temperature is elevated. Exponential dependence of the fluorescence peak intensity with temperature is recorded. The best fit equation with R 2 value of 0.9825 is given by For this exponential curve the Intensity change/ Temperature change (sensitivity) varies along the exponential line. For application purpose the range of temperature measurement can be divided into three ranges. The first range (297-315 K) shows the greatest slope of the tangent at the midpoint of the range which equals -1.189 9 10 -4 . The second range is (315-335 K) which has a smaller slope that equals -6.75 9 10 -5 . In the third range (335-365 K) the slope has the smallest value of -1.45 9 10 -5 .
While in cooling behavior of (DA-F) dye in figure, the fluorescence peak values increase with lowering the temperature. The best fit equation for the plotted relation with R 2 value of 0.9522 is given by: 2.10 Photodegradation: effect of continuous exposure to laser The three dyes solid discs are subjected to 50-mW Ar laser of 488 nm for about one hour. The monochromator is stopped at the peak intensity of the fluorescence reached for each dye. The readings of the intensity are recorded manually at close time  Figure 21 illustrates the data obtained for intensity peak changes with time for each dye.
As seen in Fig. 21 the peak intensity of the fluorescence at 556, 560, and 560 nm of (F), (AF), and (Di-AF) in (GMA), respectively, decrease linearly on continuous exposure to laser light fitting with R 2 values of 0.9038, 0.6965, and 0.9145, respectively. The photostability of the dye-polymer discs depend on the inter-molecular and intra-molecular interactions of the dye molecule with the surrounding chemically active molecules of polymer macromolecule end groups, unreacted monomers, absorbed atmospheric oxygen molecules, and another dye molecule. In general there are three accepted reasons for degradation of the dye molecule: photo-deactivation from the excited state caused by chemical oxidation reaction, formation of dimers that absorbs irradiation without any fluorescence, and thermal destruction because of the low thermal conductivity of the (GMA) polymer host.

Conclusion
The hydrolysis of Oxarine ring in GMA structure during polymerization process results in its interaction and formation of an additional covalent bond between GMA backbone and three pigments. In the cryogenic range down to 10 K the three dyes showed poor sensitivity, good sensitivity, and enhanced sensitivity, respectively. In the range of temperatures above room temperature to 370 K, (F) polymeric disc showed poor sensitivity unlike that was stated in literature for (F) dye in silica gel as a host. Allyl Fluorescein and diallyl Fluorescein showed higher sensitivity. In general diallyl Fluorescein showed enhancement in thermo-sensitivity compared to (F) and (AF). This conclusion is applicable in both ranges of temperatures from 49 to 300 K and from 297 to 365 K. The best significant value of the fluorescence thermo-sensitivity of (Di-AF) was achieved because there are three covalent bonds between (Di-AF) and backbone of polymer matrix during polymerization reaction, while (AF) and (F) have only two and one covalent bonds, respectively. This led to high inter-molecular interaction by three covalent bonds that facilitates dissipation of absorbed energy through covalent bonds (that not excited to higher energy level of dye molecules) to GMA chain molecules. Also, inherently prevents the dye from having transitional motion within pores of the polymer matrix as well as decreasing the number of dynamic interactions producing higher fluorescence thermosensitive device.

Author contributions
All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by TMM, MTHAK, and GAF. The first draft of the manuscript was written by NAN and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Data availability
Raw data were generated at the [Cairo University] large-scale facility. Derived data supporting the findings of this study are available from the corresponding author upon request.

Declarations
Competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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