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Optical oxygen sensor coating based on the fluorescence quenching of a new pyrene derivative
Sensors and Actuators B 104 (2005) 15–22
Optical oxygen sensor coating based on the fluorescence quenching of a new pyrene derivative Bharathibai J. Basu∗ , A. Thirumurugan1 , A.R. Dinesh2 , C. Anandan, K.S. Rajam Surface Engineering Division, National Aerospace Laboratories, Bangalore 560017, India Received 8 December 2003; received in revised form 12 April 2004; accepted 26 April 2004 Available online 7 June 2004
Abstract A new pyrene derivative 1-decyl-4-(1-pyrenyl) butanoate (DPB) was synthesized by the esterification reaction of 1-pyrene butyric acid with n-decanol. The photophysical properties of this pyrene ester were studied in toluene and in silicone polymers. The emission spectra of DPB exhibited excimer emission in solution and in silicone polymer coatings and the emission was effectively quenched by oxygen. A comparison of the oxygen sensor performance of DPB was made with that of pyrene and 1-pyrene butyric acid in silicone polymer coatings. The effect of different silicone resins on the properties of the DPB-based oxygen sensor coating was studied and the most suitable binder was selected. The new pyrene derivative was found to have potential as a substitute for pyrene in pressure-sensitive paints used for the measurement of surface pressure distribution on wind tunnel models. © 2004 Elsevier B.V. All rights reserved. Keywords: New pyrene derivative; Fluorescence quenching; Oxygen quenching sensitivity; Pressure-sensitive paint
1. Introduction Pyrene and some of its derivatives are widely used for oxygen sensor applications because of their long excited state lifetime, high luminescence quantum yield and oxygen quenching efficiency [1–8]. An optical oxygen sensor is based on the dynamic quenching of luminescence of some materials by oxygen molecules. The oxygen sensor films and coatings are prepared by dispersing the fluorescent probe molecules in a suitable polymer support. The polymer support also has an active role in deciding the sensor performance. Often silicone resins are used as the polymer matrix or binder mainly because of their high oxygen permeability compared to other polymers [9]. Pyrene forms excimers in solution and in silicone polymer matrix. An excimer is formed due to the interaction of a pyrene molecule in ground state with a pyrene molecule in excited state [10]. The excimer formation is a diffusion-controlled process and because of the high diffusion coefficient of pyrene
∗ Corresponding author. Tel.: +91-80-5086251; fax: +91-80-5210113. E-mail address: [email protected] (B.J. Basu). 1 Present address: Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India. 2 Present address: Department of Chemical Engineering, University of Louisiana at Lafayette, Lafayette, LA 70503, USA.
0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2004.04.092
in silicone polymers, the excimer peak intensity of pyrene is high even at low concentrations. The excimer emission band of pyrene occurring at 480 nm is more sensitive to oxygen quenching than the monomer emission peaks [11]. Optical oxygen sensors are widely used in the area of biology and medicine. In recent times, oxygen-sensitive coatings have gained popularity for measuring surface pressure distribution on wind tunnel models. The sensor coating applied on the surface of a wind tunnel model for this purpose is known as pressure-sensitive paint (PSP) [12–18]. The basic principle of PSP is oxygen quenching of luminescence which is the same as that of optical oxygen sensors. The details of the PSP technique and the paint are described elsewhere [12–18]. A good PSP should have high pressure sensitivity and low temperature coefficient in addition to high fluorescence quantum efficiency and good photo stability. Pyrene is often used as an active luminophore in PSP because of its low temperature coefficient compared to other sensors like metal-organic complexes [18]. But pyrene-based PSPs often lack stability due to loss of pyrene from the matrix by evaporation/sublimation. We have studied the mechanism of degradation of pyrene-based PSPs and identified the diffusion-assisted evaporation as the cause for the degradation [19]. The diffusion coefficients of pyrene in silicone polymer coatings were determined and it was found that the diffusion coefficient of pyrene was quite high and was independent of the viscosity of the silicone resin [20].
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In order to improve the stability of the PSP coatings, it is necessary to arrest the diffusion and evaporation of luminophore molecules from PSP coatings. The high diffusion rate of pyrene in silicone coatings is due to the side group rotation and segmental motion of polymer chains of silicone polymers. It has been reported that pyrene derivatives like 1-pyrene butyric acid (PBA) have suppressed molecular mobility in silicone films [3]. Our study of the photophysical characteristics of several commercially available pyrene derivatives had shown that the silicone coatings of these compounds had poor oxygen quenching for excimer emission [11]. Hence efforts were made to prepare a new pyrene derivative with high oxygen quenching sensitivity and lower diffusion coefficient in silicone polymer matrix. These investigations have resulted in the synthesis of a promising compound, namely 1-decyl-4-(1-pyrenyl) butanoate (DPB). In the present study, photophysical characteristics of DPB were examined with a view to use it as a substitute for pyrene in the pressure-sensitive paint and the results are discussed.
2. Experimental 2.1. Materials Pyrene and 1-pyrene butyric acid were procured from Sigma Aldrich Corporation. 1-Decanol was obtained from BDH and p-toluene sulfonic acid from Burgoyne and Co. The silicone resins, RTV IS 9188, RTV IS 8002, RTV 615 (clear), TSE 3455T and RTV 11 were obtained from GE Silicones and Silastic 733 from Dow Corning. RTV IS 9188, RTV IS 8002 and Silastic 733 are one-component silicone resins and RTV 11, RTV 615 and TSE 3455T are two-component silicone resins. These resins were used as binders for the PSP coatings. The reference luminophore, Y2 O2 S:Eu (red phosphor, type QKL63/N-C1), was procured from M/s Phosphor Technology, UK. AR grade toluene, n-hexane and ethyl acetate from Ranbaxy Fine Chemicals were used as solvents. 2.1.1. Synthesis of 1-decyl-4-(1-pyrenyl) butanoate A direct esterification reaction between PBA and 1-decanol was used for the preparation of DPB. p-Toluene sulphonic acid was used as catalyst. A higher molar ratio of decanol to PBA was used to ensure maximum conversion of PBA to ester. 0.10 g (0.346 mM) of PBA was mixed with 0.316 g (2.0 mM) of 1-decanol in 30 ml toluene in a 250 ml round bottom flask. 0.05 g of p-toluene sulphonic acid was added as catalyst. The above solution was refluxed for 6 h under nitrogen atmosphere. During the reaction, water was removed via a Dean-Stark trap. The brownish yellow reaction mixture was cooled to room temperature, then stirred with 10 g of activated silica gel (60–200-mesh) for 2 h in order to remove unreacted starting material. The filtered solution was light yellow. The filtrate containing the product
in toluene was evaporated to about 5 ml. The product was purified using silica gel column chromatography. A mixture of hexane and ethyl acetate (3:1) was used for eluting the product. DPB was eluted after toluene followed by excess decanol present in the mixture. The fraction containing DPB was evaporated to remove the elutants. The product was a brownish yellow viscous liquid soluble in toluene. It was found that yield of the product was not improved if the reaction time was longer than 6 h. TLC analysis indicated only one species. The infrared spectrum of the product showed signals at 1734.66, 1244.83 and 1178.29 cm−1 (duo to C=O group) which clearly indicates that the product is ester. The absence of signal around 3500 cm−1 shows the absence of –OH group of decanol or PBA and hence the product is pure and free from decanol and PBA. 2.1.2. Preparation of sensor coatings Oxygen sensor coatings were prepared by mixing 1 g of silicone resin with a known amount of DPB dissolved in 5 ml toluene. Other paint ingredients like red phosphor and blue pigment also were added to the mixture as described earlier [19]. The mixture was sprayed onto 15 cm ×5 cm aluminum sheets of 0.3 mm thickness coated with a white reflecting undercoat. The coating was allowed to cure for 24 h at room temperature. Five sensor specimens of 3 cm × 5 cm were cut from each aluminum sheet sprayed with each mixture. 2.2. Methods 2.2.1. Fluorescence measurements Fluorescence emission spectra were recorded using a fiber optic spectrometer, model SD1000 from Ocean Optics Inc., USA, and a 300 W Xenon Arc Lamp (model no. 6258 from Oriel Instruments) as source [19,20]. A bandpass filter (Oriel cat. no. 51650) was used to transmit UV radiation from the source in the wavelength range of 300–380 nm and to illuminate the specimens with UV radiation. The specimens were mounted in a sample chamber fabricated in our laboratory so that emission was measured by the front face technique. Purified nitrogen was passed through the sample chamber to record the emission spectra in the absence of oxygen. The fluorescence emission spectra of the coatings were recorded in air and in the presence of nitrogen.
3. Results and discussion 3.1. Photophysical characteristics of DPB in solution and in silicone coatings The absorbance spectrum of a solution of DPB in toluene is shown in Fig. 1. The spectrum was very similar to that of pyrene and pyrene butyric acid. The absorbance peaks of DPB are shifted to the longer wavelength side by about 5 nm compared to that of pyrene. The absorbance maxi-
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250
1 Intensity (counts)
2
Absorbance
0.8 0.6 0.4 0.2
200 150 100 1
50 0 350
0 300
400
500
600
400
(A)
450
500
550
600
650
600
650
Wavelength (nm)
Wavelength (nm) 400
mum was around 345 nm. This is in agreement with the results reported earlier for other probes based on PBA [21]. It has been reported that when pyrene is connected to other functional groups through methylene bridge on pyrene, then the absorbance spectra of the resulting probe exhibited well-resolved spectra similar to that of unsubstituted pyrene. The fluorescence emission spectrum of a solution of DPB in toluene is shown in Fig. 2. The excimer emission band at about 480 nm was more prominent than the monomer emission peaks. The solution of DPB had high UV stability and there was no decrease in the fluorescence intensity on continuous UV exposure of the solution for several minutes. The excimer fluorescence of DPB in solution was oxygen sensitive as the emission intensity increased about four times after de-oxygenation of the solution with pure nitrogen (Fig. 2). Silicone coatings of DPB were prepared as described in Section 2.1.2 using RTV IS 9188 resin as binder. Fig. 3A shows the fluorescence emission spectra of a DPB coating in air and in the presence of nitrogen. The spectra of DPB coating exhibited monomer peaks in addition to the broad excimer peak. The peaks between 600 and 650 nm (red emission) were due to reference luminophore and the red intensity was independent of the partial pressure of oxygen. The blue and red emission intensities were measured at 475 and 626 nm respectively in air and in the presence of nitrogen. The intensity ratio, Iblue /Ired , was calculated in each 120
2
Intensity (counts)
100 80
Intensity (counts)
Fig. 1. Absorbance spectrum of a solution of DPB in toluene. 300 200 2
100 1
0 350
(B)
400
450
500
550
Wavelength (nm)
Fig. 3. Fluorescence emission spectra of silicone polymer coatings of (A) DPB and (B) PBA (1) in air and (2) in the presence of nitrogen.
case. Normalized intensity was determined using the equation (Iblue /Ired ) × 100. The oxygen quenching sensitivity (OQS) or the pressure sensitivity (PS) of the sensor coating was calculated using the equation (IN2 − Iair ) × 100/IN2 , where IN2 and Iair were the normalized intensity values of the coating in nitrogen and air respectively. The OQS of DPB in the silicone coating was about 75%. This value is higher than the PS of silicone coatings containing any other commercially available pyrene derivative at the same concentration. For comparison, the fluorescence emission spectra of a silicone coating of PBA (with the same silicone resin) in air and in the presence of nitrogen are shown in Fig. 3B. It can be seen that the excimer emission of PBA coating has poor oxygen sensitivity (<10%) whereas excimer emission of DPB coatings have higher quantum efficiency and higher oxygen sensitivity. For PSP applications, the excimer emission is utilized because of its larger Stokes’ shift, higher oxygen quenching and higher quantum efficiency than that of the monomer emission [18]. Hence DPB may be used as active luminophore in PSP formulations.
60 40
3.2. Effect of DPB concentration on fluorescent intensity and pressure sensitivity
1
20 0 300
400
500
600
700
Wavelength (nm)
Fig. 2. Fluorescence emission spectra of a solution of DPB (1) before deaeration and (2) after deaeration.
Effect of DPB concentration on the photophysical characteristics of the paint coatings was studied. Paint coatings were prepared using GE RTV IS 9188 resin as described in Section 2.1.2. The concentrations of red phosphor and blue pigment in the coatings were kept constant. The DPB con-
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Table 1 Effect of DPB concentration on the fluorescent intensity and pressure sensitivity of a DPB-based binary paint using the silicone resin GE RTV IS 9188 DPB concentration (mM)
Normalized intensity in air (counts)
Pressure sensitivity (% bar−1 )
2.0 4.0 8.0 12.0 16.0
4.6 6.2 11.0 17.3 24.7
51.0 66.1 72.9 74.1 72.5
centrations in the coatings were varied from 2 to 16 mM. The blue and red emission intensities of these coatings were measured in air and nitrogen. Normalized intensity and pressure sensitivity of the coatings were calculated. The results are shown in Table 1. A comparison of the effect of luminophore concentration on fluorescent intensity and pressure sensitivity (PS) of DPB-based and pyrene-based PSP coatings is shown in Fig. 4. It was found that the normalized intensity values increased with increase in luminophore concentration for both coatings (Fig. 4A). The PS of DPB coating was maximum at a DPB concentration of 12 mM and decreased at lower and higher concentrations whereas the PS decreased with increase in pyrene concentration for pyrene-based PSP coatings as shown in Fig. 4B. The Stern–Volmer calibration plot (intensity ratio versus air pressure) of a DPB-based PSP coating was recorded using the PSP calibration chamber of the PSP system at NAL and it was found that the plot was fairly linear with a slope of 0.75 bar−1 .
Intensity (counts)
40 30
2
20 1
10 0 0
Pressure sensitivity (%/bar)
(A)
5 10 15 Luminophore concentration (mM)
20
2
I0 τ0 4πNσατ0 P(pO2 ) = = 1 + kq τ0 [O2 ] = 1 + I τ 1000
1
Table 2 Fluorescence characteristics of DPB coatings with different silicone resins
60 40
Silicone resin
Blue intensity in air (counts)
Pressure sensitivity (% bar−1 )
IS 9188 SIL 733 IS 8002 RTV 11 RTV 615 TSE 3455T
44.7 40.7 45.0 45.3 45.8 48.0
73.1 73.2 72.1 76.3 75.6 74.5
20 0 0
(B)
It has been reported that binder has an important role in deciding the oxygen sensor performance [22]. Commercially available silicone prepolymers contain solvents, fillers, low molecular weight cross-linkers, catalysts and other additives. The effect of these components on the oxygen quenching of the sensor molecules is not known. Therefore the effect of different silicone resins on the oxygen quenching of DPB-based coatings was studied. Six commercially available RTV silicone resins of different types of curing were selected for this study. These were single-component resins like GE RTV IS 9188 (clear), GE RTV IS 8002 (white), Silastic 733 (clear) (Dow Corning) and two-component resins like GE RTV 11 (white), RTV 615 (clear) and TSE 3455T. Binary PSP coatings of DPB were prepared using these silicone resins as binder and the spectral properties of the coatings were studied. A comparison was made of the photophysical characteristics of DPB-based coatings with these different resins. Six sets of PSP coupons were prepared with about 10 mM DPB using these six resins as binders. Coatings with RTV615 and TSE 3455T were prepared without blue pigment as the paint did not cure in the presence of pigment. The blue and red emission intensities of these coatings were measured in air and nitrogen. Normalized intensity and PS were calculated. The results are shown in Table 2. It was found that the blue excimer intensity of the coating was nearly independent of the type of silicone resin used. Maximum PS was obtained for DPB coatings with RTV 11 (76% bar−1 ). The DPB coatings with the three single-component silicone resins showed a slightly lower PS of about 72–73% bar−1 . The DPB coatings of RTV 615 and TSE 3455T had intermediate values of PS. It is possible to explain these results on the basis of the oxygen quenching kinetics [23]. Dynamic quenching or diffusion-controlled quenching is described by the Stern–Volmer equation:
where I and I0 are the intensity, and τ and τ 0 are the lifetimes of the luminophore, at a molar concentration of oxygen [O2 ] and in the absence of oxygen, respectively, kq the quench-
100 80
3.3. Effect of binder on the oxygen quenching of DPB-based silicone coatings
5 10 15 Luminophore concentration (mM)
20
Fig. 4. Effect of luminophore concentration on (A) intensity and (B) pressure sensitivity for (1) DPB coating and (2) pyrene coating.
B.J. Basu et al. / Sensors and Actuators B 104 (2005) 15–22
3.4. Effect of normal aging on the spectral properties of DPB-based PSP coatings The effect of normal aging on the emission intensity of DPB-based PSP coatings was studied. The emission intensities of the coatings were measured at regular intervals starting from the first day after the paint was cured. Thus the duration for which a coating can be stored without deterioration of its intensity (the shelf life of the coating) was determined. Fig. 5 shows the effect of storage time (or normal aging) on the normalized intensity of PSP coatings of four DPB concentrations for 30 days. It was seen that the intensity increased slightly for coatings with low DPB concentrations. At 4 mM DPB, the increase in normalized emission intensity was about 12% in 30 days and at 8 mM it was about 6%. When the DPB concentration was 12 mM, the normalized intensity of the paint coupon remained nearly constant for 30 days. At 16 mM DPB, the normalized intensity was constant for about 23 days and decreased after that. Further normal aging of the coupons showed that a coating with 12 mM DPB could be stored for about 60 days without much change in their emission properties.
Normalized Intensity (counts)
40
4mM 8mM 12mM 16mM
35 30 25 20 15 10 5 0 0
5
10
15 20 Time (days)
25
30
35
Fig. 5. Effect of normal aging on the intensity of coatings with different DPB concentrations.
A comparison was made of the normal aging behavior of coatings of DPB and pyrene with about 10 mM concentration prepared with the same binder, RTV IS 9188. The results are shown in Fig. 6. It was seen that the intensity of pyrene-based PSP coating degraded rapidly by about 40% in 12 days whereas the intensity of DPB-based coating remained constant. The evaporation loss of luminophore at room temperature was found to be negligible for DPB-based coatings whereas it was high for pyrene coatings. Thus it was found that DPB-based PSP coatings are more stable than pyrene-based PSP coatings under ambient storage conditions. 3.5. Effect of thermal aging on the spectral properties of DPB-based coatings In order to check the stability of DPB-based PSP coatings, a thermal aging test was carried out by exposing the coatings to 60 ◦ C and 0.1 bar for 1 h in a vacuum oven and measuring the emission intensity before and after thermal aging. The effect of thermal aging on the normalized intensity and pressure sensitivity of PSP coatings of four DPB concentra20 Normalized Intensity
ing rate constant, α the probability that a collision leads to quenching which is assumed to be equal to unity in many cases, N the Avogadro’s number, σ the collision radius of the oxygen–dye complex, pO2 the oxygen partial pressure in contact with the surface of the coating under ambient pressure and temperature conditions and P the oxygen permeability of the polymer matrix. P is the product of DO2 and sO2 where sO2 is the solubility of oxygen in the coating and DO2 is the diffusion coefficient of oxygen in the polymer. It is evident from this equation that the pressure sensitivity of a PSP coating depends on the oxygen permeability of the binder and unquenched luminescence lifetime, τ 0 , of the luminophore. It has been reported that silicone coatings with higher cross-linking exhibit decreased oxygen permeability [22]. Even though the composition of the commercial silicone resins is unknown, our results indicate that the coatings of IS 9188, IS 8002 and SILASTIC 733 may have higher cross-linking due to branched chains in their molecular structure. RTV 11 may have slightly higher oxygen permeability than the other resins as seen from these results. The effect of binder on the UV degradation of the coating was studied. A set of paint coupons prepared with the six binders were exposed to UV radiation continuously for 300 s and the intensity values were recorded before and after UV exposure. It was found that coatings with IS 9188 had better UV stability than coatings with other resins. DPB coatings prepared with resins containing white fillers (IS 8002 and RTV 11) degraded faster than coatings with clear and transparent resins (IS 9188, RTV 615 and SILASTIC 733). Based on these results, IS 9188 resin was selected as binder for preparation of the PSP coatings.
19
2
15
1
10
5
0 0
5
10 15 Time (days)
20
25
Fig. 6. Comparison of the normal aging stability of the coatings of (1) DPB and (2) pyrene.
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Table 3 Effect of DPB concentration on the thermal stability of DPB coatingsa Pressure sensitivity (% bar−1 )
DPB concentration (mM)
Normalized intensity Before thermal aging
After thermal aging
Before thermal aging
After thermal aging
4 8 12 16
6.34 11.37 18.5 27.3
6.19 10.2 16.3 22.7
66.1 72.9 74.1 72.5
63.2 71.4 74.2 73.3
tions was studied. The DPB concentrations in the coatings were 4, 8, 12 and 16 mM. Thermal aging at 60 ◦ C and 0.1 bar for 1 h was done separately for paint coupons with different DPB concentrations. The results are summarized in Table 3. It was found that thermal degradation increased with increase in DPB concentration. Thermal stability was good at low DPB concentration but the emission intensity was poor. Also there was a decrease in PS after thermal aging at low DPB concentrations whereas the change in PS was negligible at 12 mM DPB concentration (Table 3). Therefore, an optimum DPB concentration of 12 mM DPB was selected for preparation of PSP coating. Thermal aging was done for several such DPB-based PSP coatings. The average decrease in normalized intensity after thermal aging was found to be about 15%. Fig. 7 shows the fluorescent emission spectra of a typical DPB-based PSP coating in air and in the presence of nitrogen before and after thermal aging. It can be seen that there was about 15% decrease in the intensity in air after thermal aging but PS was not affected. A comparison was made with the thermal aging behavior of a DPB-based PSP coating with that of a pyrene-based PSP with 10 mM pyrene concentration. Fig. 8 shows the fluorescent emission spectra of a pyrene-based PSP coating in air and in the presence of nitrogen before and after thermal aging. The decrease in normalized intensity for pyrene coating after thermal aging was about 50–60% for a coating of 30 m thickness. 400 A1,N1- Before thermal aging A2,N2- After thermal aging
Intensity (counts)
−2.4 −10.5 −11.8 −16.6
Thickness of the coating = 35 ± 10 m.
300
A1,N1- Before thermal aging A2,N2- After thermal aging
500 N1
400 300 200
N2
100
A1 A2
0 350
400
450
500
550
600
650
Wavelength (nm)
Fig. 8. Fluorescence emission spectra of a pyrene-based coating (1) before and (2) after thermal aging in air (A1, A2) and in the presence of nitrogen (N1, N2).
3.6. Effect of thickness of the coating on thermal stability The effect of thickness of the DPB coating on thermal stability was studied. Coatings of different thickness ranging from 20 to 60 m were prepared with a paint mixture of 12 mM DPB concentration. It was seen that blue intensity of the coating increased with increase in thickness of the coating and was maximum at 60 m thickness. Pressure sensitivity was almost independent of the thickness of the coating. Coatings of different thickness were thermally aged at 60 ◦ C and 0.1 bar for 1 h and the intensities were measured before and after thermal aging. The results are shown in Table 4. It was found that the degradation in intensity after thermal aging test was independent of the thickness of the coating in
N1
200
Table 4 Effect of thickness of DPB coatings on the thermal stability
N2
100 A1
0 400
600
Intensity (counts)
a
Change in intensity after thermal aging (%)
A2
450
500
550
600
650
Thickness (m)
Normalized intensity Before thermal aging
After thermal aging
20 30 40 60
12.20 15.34 16.12 24.17
9.60 13.63 14.11 20.53
700
Wavelength (nm)
Fig. 7. Fluorescence emission spectra of DPB coating (1) before and (2) after thermal aging in air (A1, A2) and in the presence of nitrogen (N1, N2).
Change in normalized intensity (%) −21.3 −11.1 −12.5 −15.1
B.J. Basu et al. / Sensors and Actuators B 104 (2005) 15–22 Table 5 Comparison of paint properties of DPB-based and pyrene-based PSP coatings Paint properties
DPB coating
Pyrene coating
Blue intensity (counts) Pressure sensitivity (% bar−1 ) Normal aging stability/shelf life Average thermal degradation (%) UV degradation (% min−1 )
40–50 74.0 Good −15 −0.30
60–70 79.0 Poor −50 −0.20
the range of 30–60 m. When the thickness of the coating was ≤20 m, the degradation was found to be >20%. Thus the thermal stability of DPB-based PSP coating was found to be independent of thickness of the active layer at coating thickness ≥ 30 m. In the case of pyrene-based PSP coatings, the thermal stability decreased with decrease in thickness of the coating [19]. Coating thickness was a critical parameter for the thermal stability and shelf life of pyrene-based paints since pyrene molecules have high mobility due to diffusion in silicone polymer matrix. 3.7. Response time and photostability The response and recovery profile of the DPB coating when exposed to 100% oxygen followed by 100% nitrogen was studied. The 90% response and 90% recovery times of the coating to an alternating atmosphere of oxygen and nitrogen were calculated. The coating exhibited fully reversible response and recovery times of 10 and 40 s respectively. It was found that the response is faster compared to the recovery time. The photostability of the coating was evaluated. The decrease in fluorescence intensity of the coating after continuous irradiation for 1 h was <5%. There was no interference from carbon dioxide or water vapor in amounts as present in air under ambient conditions. Table 5 shows a comparison of properties between DPB coatings and pyrene coatings containing 10 mM of active luminophore. It can be seen that the stability was higher for DPB-based coatings even though the PS and quantum efficiency were higher for pyrene-based PSP.
4. Conclusions The photophysical characteristics of the new pyrene derivative were found to be similar to that of pyrene. A binary pressure-sensitive paint was developed using DPB as the active luminophore, red phosphor as reference luminophore and an RTV silicone resin as binder. The excimer emission intensity and pressure sensitivity of DPB-based PSP coatings depended on the concentration of the luminophore in the coating. The emission intensity of the coating increased linearly with increase in concentration of
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DPB. The pressure sensitivity was maximum (75% bar−1 ) at 10–12 mM DPB concentration in the coating and decreased at lower and higher DPB concentrations. The selection of binder for the paint was based on the UV stability and thermal stability of the coating. The results of the static aging tests have shown that DPB coatings have better stability than pyrene-based PSP. DPB-based PSP coatings had good shelf life for about 60 days when stored under ambient conditions. It was found that there was about 15% degradation in intensity for DPB coating after thermal aging at 60 ◦ C and 0.1 bar for 1 h. The change in pressure sensitivity after thermal aging test was <1%. Thickness of the coating in the range of 30–60 m had little effect on the thermal stability of the DPB coating. Thus DPB was found to have potential as a substitute for pyrene in pressure-sensitive coatings used for the measurement of surface pressure distribution on wind tunnel models.
Acknowledgements The authors are grateful to Dr. B.R. Pai, Director, NAL, for his support and permission to publish the work and Dr. T.S. Prahlad, Ex-Director, NAL, for his constant encouragement during the course of the study. We thank Dr. Channa Raju for calibration of a DPB coating in the PSP calibration chamber.
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[11] B.J. Basu, K.S. Rajam, Comparison of the oxygen sensor performance of some pyrene derivatives in silicone polymer matrix, Sens. Actuators B 99 (2004) 459–467. [12] M. Gouterman, Oxygen quenching of luminescence of pressure sensitive paint for wind tunnel research, J. Chem. Educ. 74 (1997) 697–702. [13] J. Kavandi, J. Callis, M. Gouterman, G. Khalil, D. Wright, E. Green, Luminescent barometry in wind tunnels, Rev. Sci. Instrum. 61 (1990) 3340–3347. [14] B.G. McLachlan, J.L. Kavandi, J.B. Callis, M. Gouterman, E. Green, G. Khalil, D. Barns, Surface pressure field mapping using luminescent coatings, Exp. Fluids 14 (1993) 33–41. [15] J.H. Bell, E.T. Schairer, L.A Hand, R.D. Mehta, Surface pressure measurements using luminescent coatings, Annu. Rev. Fluid Mech. 33 (2001) 155–206. [16] R.C. Crites, Measurement Techniques, Lecture Series, Von Karman Institute for Fluid Dynamics, 1995–2001. [17] T. Liu, B.T. Campbell, S.P. Burns, J.P. Sullivan, Temperature and pressure sensitive luminescent paints in aerodynamics, Appl. Mech. Rev. 50 (1997) 227–246. [18] Y. Mebarki, Pressure Sensitive Paints: Application in Wind Tunnels, ONERA NT 1998-6, 1998, pp. 1–143. [19] B.J. Basu, C. Anandan, K.S. Rajam, Study of the mechanism of degradation of pyrene-based pressure sensitive paints, Sens. Actuators B 94 (2003) 257–266. [20] C. Anandan, B.J. Basu, K.S. Rajam, Investigations of the effect of viscosity of resin on the diffusion of pyrene in silicone polymer matrix using steady state fluorescence technique, Eur. Polym. J. 40 (2004) 335–342. [21] L. Bucsiova, P. Hrdlovic, S. Chmela, Spectral characteristics of fluorescence probes based on pyrene in solution and in polymer matrix, J. Photochem. Photobiol. A: Chem. 143 (2001) 59–68. [22] H. He, R.J. Fraatz, M.J.P. Leiner, M.M. Rehn, J.K. Tusa, Selection of silicone polymer matrix for optical gas sensing, Sens. Actuators B 29 (1995) 246–250. [23] X. Lu, M.A. Winnik, Luminescence quenching in polymer/filler nanocomposite films used in oxygen sensors, Chem. Mater. 13 (2001) 3449–3463.
Biographies Bharathibai J. Basu received her Masters degree in chemistry from Calicut University, Kerala, India, in 1973 and Ph.D. from Indian Institute of Science, Bangalore, India, in 1995. She is working as a senior scientist in National Aerospace Laboratories, Bangalore, India. Research areas of her interest are trace element analysis, electroanalytical chemistry and spectroscopy. Her current interest is in the development of pressure-sensitive paints for wind tunnel studies. A. Thirumurugan obtained his M.Sc. degree in chemistry from American College, Madurai, India, in 2001. He worked as a trainee in the project on the development of pressure-sensitive paints in National Aerospace Laboratories, Bangalore, from 2001 to 2002. Presently he is pursuing his Ph.D. in chemistry in Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India. A.R. Dinesh received his B.E. degree from Bangalore University, Karnataka, India, in 2000. He worked as a trainee in the project on the development of pressure-sensitive paints in National Aerospace Laboratories, Bangalore, from 2001 to 2002. He obtained his M.S. in chemical engineering from Department of Chemical Engineering, University of Louisiana at Lafayette, USA, in 2003. C. Anandan obtained his M.Tech. in materials science from Indian Institute of Technology, Kanpur, in 1981 and Ph.D. from University of Wales, College of Cardiff, in 1990. He has worked at National Physical Laboratory, New Delhi, India, from 1983 to 1999 in the area of thin film materials and devices and surface analysis techniques. Currently he is working in the Surface Engineering Division of National Aerospace Laboratories, Bangalore. K.S. Rajam holds a doctorate from Bangalore University. Currently she is the head of the Surface Engineering Division, National Aerospace Laboratories, Bangalore, India. Her research interests include developing surface modification technologies for aerospace and other engineering applications.
Optical oxygen sensor coating based on the fluorescence quenching of a new pyrene derivative Bharathibai J. Basu∗ , A. Thirumurugan1 , A.R. Dinesh2 , C. Anandan, K.S. Rajam Surface Engineering Division, National Aerospace Laboratories, Bangalore 560017, India Received 8 December 2003; received in revised form 12 April 2004; accepted 26 April 2004 Available online 7 June 2004
Abstract A new pyrene derivative 1-decyl-4-(1-pyrenyl) butanoate (DPB) was synthesized by the esterification reaction of 1-pyrene butyric acid with n-decanol. The photophysical properties of this pyrene ester were studied in toluene and in silicone polymers. The emission spectra of DPB exhibited excimer emission in solution and in silicone polymer coatings and the emission was effectively quenched by oxygen. A comparison of the oxygen sensor performance of DPB was made with that of pyrene and 1-pyrene butyric acid in silicone polymer coatings. The effect of different silicone resins on the properties of the DPB-based oxygen sensor coating was studied and the most suitable binder was selected. The new pyrene derivative was found to have potential as a substitute for pyrene in pressure-sensitive paints used for the measurement of surface pressure distribution on wind tunnel models. © 2004 Elsevier B.V. All rights reserved. Keywords: New pyrene derivative; Fluorescence quenching; Oxygen quenching sensitivity; Pressure-sensitive paint
1. Introduction Pyrene and some of its derivatives are widely used for oxygen sensor applications because of their long excited state lifetime, high luminescence quantum yield and oxygen quenching efficiency [1–8]. An optical oxygen sensor is based on the dynamic quenching of luminescence of some materials by oxygen molecules. The oxygen sensor films and coatings are prepared by dispersing the fluorescent probe molecules in a suitable polymer support. The polymer support also has an active role in deciding the sensor performance. Often silicone resins are used as the polymer matrix or binder mainly because of their high oxygen permeability compared to other polymers [9]. Pyrene forms excimers in solution and in silicone polymer matrix. An excimer is formed due to the interaction of a pyrene molecule in ground state with a pyrene molecule in excited state [10]. The excimer formation is a diffusion-controlled process and because of the high diffusion coefficient of pyrene
∗ Corresponding author. Tel.: +91-80-5086251; fax: +91-80-5210113. E-mail address: [email protected] (B.J. Basu). 1 Present address: Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India. 2 Present address: Department of Chemical Engineering, University of Louisiana at Lafayette, Lafayette, LA 70503, USA.
0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2004.04.092
in silicone polymers, the excimer peak intensity of pyrene is high even at low concentrations. The excimer emission band of pyrene occurring at 480 nm is more sensitive to oxygen quenching than the monomer emission peaks [11]. Optical oxygen sensors are widely used in the area of biology and medicine. In recent times, oxygen-sensitive coatings have gained popularity for measuring surface pressure distribution on wind tunnel models. The sensor coating applied on the surface of a wind tunnel model for this purpose is known as pressure-sensitive paint (PSP) [12–18]. The basic principle of PSP is oxygen quenching of luminescence which is the same as that of optical oxygen sensors. The details of the PSP technique and the paint are described elsewhere [12–18]. A good PSP should have high pressure sensitivity and low temperature coefficient in addition to high fluorescence quantum efficiency and good photo stability. Pyrene is often used as an active luminophore in PSP because of its low temperature coefficient compared to other sensors like metal-organic complexes [18]. But pyrene-based PSPs often lack stability due to loss of pyrene from the matrix by evaporation/sublimation. We have studied the mechanism of degradation of pyrene-based PSPs and identified the diffusion-assisted evaporation as the cause for the degradation [19]. The diffusion coefficients of pyrene in silicone polymer coatings were determined and it was found that the diffusion coefficient of pyrene was quite high and was independent of the viscosity of the silicone resin [20].
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In order to improve the stability of the PSP coatings, it is necessary to arrest the diffusion and evaporation of luminophore molecules from PSP coatings. The high diffusion rate of pyrene in silicone coatings is due to the side group rotation and segmental motion of polymer chains of silicone polymers. It has been reported that pyrene derivatives like 1-pyrene butyric acid (PBA) have suppressed molecular mobility in silicone films [3]. Our study of the photophysical characteristics of several commercially available pyrene derivatives had shown that the silicone coatings of these compounds had poor oxygen quenching for excimer emission [11]. Hence efforts were made to prepare a new pyrene derivative with high oxygen quenching sensitivity and lower diffusion coefficient in silicone polymer matrix. These investigations have resulted in the synthesis of a promising compound, namely 1-decyl-4-(1-pyrenyl) butanoate (DPB). In the present study, photophysical characteristics of DPB were examined with a view to use it as a substitute for pyrene in the pressure-sensitive paint and the results are discussed.
2. Experimental 2.1. Materials Pyrene and 1-pyrene butyric acid were procured from Sigma Aldrich Corporation. 1-Decanol was obtained from BDH and p-toluene sulfonic acid from Burgoyne and Co. The silicone resins, RTV IS 9188, RTV IS 8002, RTV 615 (clear), TSE 3455T and RTV 11 were obtained from GE Silicones and Silastic 733 from Dow Corning. RTV IS 9188, RTV IS 8002 and Silastic 733 are one-component silicone resins and RTV 11, RTV 615 and TSE 3455T are two-component silicone resins. These resins were used as binders for the PSP coatings. The reference luminophore, Y2 O2 S:Eu (red phosphor, type QKL63/N-C1), was procured from M/s Phosphor Technology, UK. AR grade toluene, n-hexane and ethyl acetate from Ranbaxy Fine Chemicals were used as solvents. 2.1.1. Synthesis of 1-decyl-4-(1-pyrenyl) butanoate A direct esterification reaction between PBA and 1-decanol was used for the preparation of DPB. p-Toluene sulphonic acid was used as catalyst. A higher molar ratio of decanol to PBA was used to ensure maximum conversion of PBA to ester. 0.10 g (0.346 mM) of PBA was mixed with 0.316 g (2.0 mM) of 1-decanol in 30 ml toluene in a 250 ml round bottom flask. 0.05 g of p-toluene sulphonic acid was added as catalyst. The above solution was refluxed for 6 h under nitrogen atmosphere. During the reaction, water was removed via a Dean-Stark trap. The brownish yellow reaction mixture was cooled to room temperature, then stirred with 10 g of activated silica gel (60–200-mesh) for 2 h in order to remove unreacted starting material. The filtered solution was light yellow. The filtrate containing the product
in toluene was evaporated to about 5 ml. The product was purified using silica gel column chromatography. A mixture of hexane and ethyl acetate (3:1) was used for eluting the product. DPB was eluted after toluene followed by excess decanol present in the mixture. The fraction containing DPB was evaporated to remove the elutants. The product was a brownish yellow viscous liquid soluble in toluene. It was found that yield of the product was not improved if the reaction time was longer than 6 h. TLC analysis indicated only one species. The infrared spectrum of the product showed signals at 1734.66, 1244.83 and 1178.29 cm−1 (duo to C=O group) which clearly indicates that the product is ester. The absence of signal around 3500 cm−1 shows the absence of –OH group of decanol or PBA and hence the product is pure and free from decanol and PBA. 2.1.2. Preparation of sensor coatings Oxygen sensor coatings were prepared by mixing 1 g of silicone resin with a known amount of DPB dissolved in 5 ml toluene. Other paint ingredients like red phosphor and blue pigment also were added to the mixture as described earlier [19]. The mixture was sprayed onto 15 cm ×5 cm aluminum sheets of 0.3 mm thickness coated with a white reflecting undercoat. The coating was allowed to cure for 24 h at room temperature. Five sensor specimens of 3 cm × 5 cm were cut from each aluminum sheet sprayed with each mixture. 2.2. Methods 2.2.1. Fluorescence measurements Fluorescence emission spectra were recorded using a fiber optic spectrometer, model SD1000 from Ocean Optics Inc., USA, and a 300 W Xenon Arc Lamp (model no. 6258 from Oriel Instruments) as source [19,20]. A bandpass filter (Oriel cat. no. 51650) was used to transmit UV radiation from the source in the wavelength range of 300–380 nm and to illuminate the specimens with UV radiation. The specimens were mounted in a sample chamber fabricated in our laboratory so that emission was measured by the front face technique. Purified nitrogen was passed through the sample chamber to record the emission spectra in the absence of oxygen. The fluorescence emission spectra of the coatings were recorded in air and in the presence of nitrogen.
3. Results and discussion 3.1. Photophysical characteristics of DPB in solution and in silicone coatings The absorbance spectrum of a solution of DPB in toluene is shown in Fig. 1. The spectrum was very similar to that of pyrene and pyrene butyric acid. The absorbance peaks of DPB are shifted to the longer wavelength side by about 5 nm compared to that of pyrene. The absorbance maxi-
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17
250
1 Intensity (counts)
2
Absorbance
0.8 0.6 0.4 0.2
200 150 100 1
50 0 350
0 300
400
500
600
400
(A)
450
500
550
600
650
600
650
Wavelength (nm)
Wavelength (nm) 400
mum was around 345 nm. This is in agreement with the results reported earlier for other probes based on PBA [21]. It has been reported that when pyrene is connected to other functional groups through methylene bridge on pyrene, then the absorbance spectra of the resulting probe exhibited well-resolved spectra similar to that of unsubstituted pyrene. The fluorescence emission spectrum of a solution of DPB in toluene is shown in Fig. 2. The excimer emission band at about 480 nm was more prominent than the monomer emission peaks. The solution of DPB had high UV stability and there was no decrease in the fluorescence intensity on continuous UV exposure of the solution for several minutes. The excimer fluorescence of DPB in solution was oxygen sensitive as the emission intensity increased about four times after de-oxygenation of the solution with pure nitrogen (Fig. 2). Silicone coatings of DPB were prepared as described in Section 2.1.2 using RTV IS 9188 resin as binder. Fig. 3A shows the fluorescence emission spectra of a DPB coating in air and in the presence of nitrogen. The spectra of DPB coating exhibited monomer peaks in addition to the broad excimer peak. The peaks between 600 and 650 nm (red emission) were due to reference luminophore and the red intensity was independent of the partial pressure of oxygen. The blue and red emission intensities were measured at 475 and 626 nm respectively in air and in the presence of nitrogen. The intensity ratio, Iblue /Ired , was calculated in each 120
2
Intensity (counts)
100 80
Intensity (counts)
Fig. 1. Absorbance spectrum of a solution of DPB in toluene. 300 200 2
100 1
0 350
(B)
400
450
500
550
Wavelength (nm)
Fig. 3. Fluorescence emission spectra of silicone polymer coatings of (A) DPB and (B) PBA (1) in air and (2) in the presence of nitrogen.
case. Normalized intensity was determined using the equation (Iblue /Ired ) × 100. The oxygen quenching sensitivity (OQS) or the pressure sensitivity (PS) of the sensor coating was calculated using the equation (IN2 − Iair ) × 100/IN2 , where IN2 and Iair were the normalized intensity values of the coating in nitrogen and air respectively. The OQS of DPB in the silicone coating was about 75%. This value is higher than the PS of silicone coatings containing any other commercially available pyrene derivative at the same concentration. For comparison, the fluorescence emission spectra of a silicone coating of PBA (with the same silicone resin) in air and in the presence of nitrogen are shown in Fig. 3B. It can be seen that the excimer emission of PBA coating has poor oxygen sensitivity (<10%) whereas excimer emission of DPB coatings have higher quantum efficiency and higher oxygen sensitivity. For PSP applications, the excimer emission is utilized because of its larger Stokes’ shift, higher oxygen quenching and higher quantum efficiency than that of the monomer emission [18]. Hence DPB may be used as active luminophore in PSP formulations.
60 40
3.2. Effect of DPB concentration on fluorescent intensity and pressure sensitivity
1
20 0 300
400
500
600
700
Wavelength (nm)
Fig. 2. Fluorescence emission spectra of a solution of DPB (1) before deaeration and (2) after deaeration.
Effect of DPB concentration on the photophysical characteristics of the paint coatings was studied. Paint coatings were prepared using GE RTV IS 9188 resin as described in Section 2.1.2. The concentrations of red phosphor and blue pigment in the coatings were kept constant. The DPB con-
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Table 1 Effect of DPB concentration on the fluorescent intensity and pressure sensitivity of a DPB-based binary paint using the silicone resin GE RTV IS 9188 DPB concentration (mM)
Normalized intensity in air (counts)
Pressure sensitivity (% bar−1 )
2.0 4.0 8.0 12.0 16.0
4.6 6.2 11.0 17.3 24.7
51.0 66.1 72.9 74.1 72.5
centrations in the coatings were varied from 2 to 16 mM. The blue and red emission intensities of these coatings were measured in air and nitrogen. Normalized intensity and pressure sensitivity of the coatings were calculated. The results are shown in Table 1. A comparison of the effect of luminophore concentration on fluorescent intensity and pressure sensitivity (PS) of DPB-based and pyrene-based PSP coatings is shown in Fig. 4. It was found that the normalized intensity values increased with increase in luminophore concentration for both coatings (Fig. 4A). The PS of DPB coating was maximum at a DPB concentration of 12 mM and decreased at lower and higher concentrations whereas the PS decreased with increase in pyrene concentration for pyrene-based PSP coatings as shown in Fig. 4B. The Stern–Volmer calibration plot (intensity ratio versus air pressure) of a DPB-based PSP coating was recorded using the PSP calibration chamber of the PSP system at NAL and it was found that the plot was fairly linear with a slope of 0.75 bar−1 .
Intensity (counts)
40 30
2
20 1
10 0 0
Pressure sensitivity (%/bar)
(A)
5 10 15 Luminophore concentration (mM)
20
2
I0 τ0 4πNσατ0 P(pO2 ) = = 1 + kq τ0 [O2 ] = 1 + I τ 1000
1
Table 2 Fluorescence characteristics of DPB coatings with different silicone resins
60 40
Silicone resin
Blue intensity in air (counts)
Pressure sensitivity (% bar−1 )
IS 9188 SIL 733 IS 8002 RTV 11 RTV 615 TSE 3455T
44.7 40.7 45.0 45.3 45.8 48.0
73.1 73.2 72.1 76.3 75.6 74.5
20 0 0
(B)
It has been reported that binder has an important role in deciding the oxygen sensor performance [22]. Commercially available silicone prepolymers contain solvents, fillers, low molecular weight cross-linkers, catalysts and other additives. The effect of these components on the oxygen quenching of the sensor molecules is not known. Therefore the effect of different silicone resins on the oxygen quenching of DPB-based coatings was studied. Six commercially available RTV silicone resins of different types of curing were selected for this study. These were single-component resins like GE RTV IS 9188 (clear), GE RTV IS 8002 (white), Silastic 733 (clear) (Dow Corning) and two-component resins like GE RTV 11 (white), RTV 615 (clear) and TSE 3455T. Binary PSP coatings of DPB were prepared using these silicone resins as binder and the spectral properties of the coatings were studied. A comparison was made of the photophysical characteristics of DPB-based coatings with these different resins. Six sets of PSP coupons were prepared with about 10 mM DPB using these six resins as binders. Coatings with RTV615 and TSE 3455T were prepared without blue pigment as the paint did not cure in the presence of pigment. The blue and red emission intensities of these coatings were measured in air and nitrogen. Normalized intensity and PS were calculated. The results are shown in Table 2. It was found that the blue excimer intensity of the coating was nearly independent of the type of silicone resin used. Maximum PS was obtained for DPB coatings with RTV 11 (76% bar−1 ). The DPB coatings with the three single-component silicone resins showed a slightly lower PS of about 72–73% bar−1 . The DPB coatings of RTV 615 and TSE 3455T had intermediate values of PS. It is possible to explain these results on the basis of the oxygen quenching kinetics [23]. Dynamic quenching or diffusion-controlled quenching is described by the Stern–Volmer equation:
where I and I0 are the intensity, and τ and τ 0 are the lifetimes of the luminophore, at a molar concentration of oxygen [O2 ] and in the absence of oxygen, respectively, kq the quench-
100 80
3.3. Effect of binder on the oxygen quenching of DPB-based silicone coatings
5 10 15 Luminophore concentration (mM)
20
Fig. 4. Effect of luminophore concentration on (A) intensity and (B) pressure sensitivity for (1) DPB coating and (2) pyrene coating.
B.J. Basu et al. / Sensors and Actuators B 104 (2005) 15–22
3.4. Effect of normal aging on the spectral properties of DPB-based PSP coatings The effect of normal aging on the emission intensity of DPB-based PSP coatings was studied. The emission intensities of the coatings were measured at regular intervals starting from the first day after the paint was cured. Thus the duration for which a coating can be stored without deterioration of its intensity (the shelf life of the coating) was determined. Fig. 5 shows the effect of storage time (or normal aging) on the normalized intensity of PSP coatings of four DPB concentrations for 30 days. It was seen that the intensity increased slightly for coatings with low DPB concentrations. At 4 mM DPB, the increase in normalized emission intensity was about 12% in 30 days and at 8 mM it was about 6%. When the DPB concentration was 12 mM, the normalized intensity of the paint coupon remained nearly constant for 30 days. At 16 mM DPB, the normalized intensity was constant for about 23 days and decreased after that. Further normal aging of the coupons showed that a coating with 12 mM DPB could be stored for about 60 days without much change in their emission properties.
Normalized Intensity (counts)
40
4mM 8mM 12mM 16mM
35 30 25 20 15 10 5 0 0
5
10
15 20 Time (days)
25
30
35
Fig. 5. Effect of normal aging on the intensity of coatings with different DPB concentrations.
A comparison was made of the normal aging behavior of coatings of DPB and pyrene with about 10 mM concentration prepared with the same binder, RTV IS 9188. The results are shown in Fig. 6. It was seen that the intensity of pyrene-based PSP coating degraded rapidly by about 40% in 12 days whereas the intensity of DPB-based coating remained constant. The evaporation loss of luminophore at room temperature was found to be negligible for DPB-based coatings whereas it was high for pyrene coatings. Thus it was found that DPB-based PSP coatings are more stable than pyrene-based PSP coatings under ambient storage conditions. 3.5. Effect of thermal aging on the spectral properties of DPB-based coatings In order to check the stability of DPB-based PSP coatings, a thermal aging test was carried out by exposing the coatings to 60 ◦ C and 0.1 bar for 1 h in a vacuum oven and measuring the emission intensity before and after thermal aging. The effect of thermal aging on the normalized intensity and pressure sensitivity of PSP coatings of four DPB concentra20 Normalized Intensity
ing rate constant, α the probability that a collision leads to quenching which is assumed to be equal to unity in many cases, N the Avogadro’s number, σ the collision radius of the oxygen–dye complex, pO2 the oxygen partial pressure in contact with the surface of the coating under ambient pressure and temperature conditions and P the oxygen permeability of the polymer matrix. P is the product of DO2 and sO2 where sO2 is the solubility of oxygen in the coating and DO2 is the diffusion coefficient of oxygen in the polymer. It is evident from this equation that the pressure sensitivity of a PSP coating depends on the oxygen permeability of the binder and unquenched luminescence lifetime, τ 0 , of the luminophore. It has been reported that silicone coatings with higher cross-linking exhibit decreased oxygen permeability [22]. Even though the composition of the commercial silicone resins is unknown, our results indicate that the coatings of IS 9188, IS 8002 and SILASTIC 733 may have higher cross-linking due to branched chains in their molecular structure. RTV 11 may have slightly higher oxygen permeability than the other resins as seen from these results. The effect of binder on the UV degradation of the coating was studied. A set of paint coupons prepared with the six binders were exposed to UV radiation continuously for 300 s and the intensity values were recorded before and after UV exposure. It was found that coatings with IS 9188 had better UV stability than coatings with other resins. DPB coatings prepared with resins containing white fillers (IS 8002 and RTV 11) degraded faster than coatings with clear and transparent resins (IS 9188, RTV 615 and SILASTIC 733). Based on these results, IS 9188 resin was selected as binder for preparation of the PSP coatings.
19
2
15
1
10
5
0 0
5
10 15 Time (days)
20
25
Fig. 6. Comparison of the normal aging stability of the coatings of (1) DPB and (2) pyrene.
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Table 3 Effect of DPB concentration on the thermal stability of DPB coatingsa Pressure sensitivity (% bar−1 )
DPB concentration (mM)
Normalized intensity Before thermal aging
After thermal aging
Before thermal aging
After thermal aging
4 8 12 16
6.34 11.37 18.5 27.3
6.19 10.2 16.3 22.7
66.1 72.9 74.1 72.5
63.2 71.4 74.2 73.3
tions was studied. The DPB concentrations in the coatings were 4, 8, 12 and 16 mM. Thermal aging at 60 ◦ C and 0.1 bar for 1 h was done separately for paint coupons with different DPB concentrations. The results are summarized in Table 3. It was found that thermal degradation increased with increase in DPB concentration. Thermal stability was good at low DPB concentration but the emission intensity was poor. Also there was a decrease in PS after thermal aging at low DPB concentrations whereas the change in PS was negligible at 12 mM DPB concentration (Table 3). Therefore, an optimum DPB concentration of 12 mM DPB was selected for preparation of PSP coating. Thermal aging was done for several such DPB-based PSP coatings. The average decrease in normalized intensity after thermal aging was found to be about 15%. Fig. 7 shows the fluorescent emission spectra of a typical DPB-based PSP coating in air and in the presence of nitrogen before and after thermal aging. It can be seen that there was about 15% decrease in the intensity in air after thermal aging but PS was not affected. A comparison was made with the thermal aging behavior of a DPB-based PSP coating with that of a pyrene-based PSP with 10 mM pyrene concentration. Fig. 8 shows the fluorescent emission spectra of a pyrene-based PSP coating in air and in the presence of nitrogen before and after thermal aging. The decrease in normalized intensity for pyrene coating after thermal aging was about 50–60% for a coating of 30 m thickness. 400 A1,N1- Before thermal aging A2,N2- After thermal aging
Intensity (counts)
−2.4 −10.5 −11.8 −16.6
Thickness of the coating = 35 ± 10 m.
300
A1,N1- Before thermal aging A2,N2- After thermal aging
500 N1
400 300 200
N2
100
A1 A2
0 350
400
450
500
550
600
650
Wavelength (nm)
Fig. 8. Fluorescence emission spectra of a pyrene-based coating (1) before and (2) after thermal aging in air (A1, A2) and in the presence of nitrogen (N1, N2).
3.6. Effect of thickness of the coating on thermal stability The effect of thickness of the DPB coating on thermal stability was studied. Coatings of different thickness ranging from 20 to 60 m were prepared with a paint mixture of 12 mM DPB concentration. It was seen that blue intensity of the coating increased with increase in thickness of the coating and was maximum at 60 m thickness. Pressure sensitivity was almost independent of the thickness of the coating. Coatings of different thickness were thermally aged at 60 ◦ C and 0.1 bar for 1 h and the intensities were measured before and after thermal aging. The results are shown in Table 4. It was found that the degradation in intensity after thermal aging test was independent of the thickness of the coating in
N1
200
Table 4 Effect of thickness of DPB coatings on the thermal stability
N2
100 A1
0 400
600
Intensity (counts)
a
Change in intensity after thermal aging (%)
A2
450
500
550
600
650
Thickness (m)
Normalized intensity Before thermal aging
After thermal aging
20 30 40 60
12.20 15.34 16.12 24.17
9.60 13.63 14.11 20.53
700
Wavelength (nm)
Fig. 7. Fluorescence emission spectra of DPB coating (1) before and (2) after thermal aging in air (A1, A2) and in the presence of nitrogen (N1, N2).
Change in normalized intensity (%) −21.3 −11.1 −12.5 −15.1
B.J. Basu et al. / Sensors and Actuators B 104 (2005) 15–22 Table 5 Comparison of paint properties of DPB-based and pyrene-based PSP coatings Paint properties
DPB coating
Pyrene coating
Blue intensity (counts) Pressure sensitivity (% bar−1 ) Normal aging stability/shelf life Average thermal degradation (%) UV degradation (% min−1 )
40–50 74.0 Good −15 −0.30
60–70 79.0 Poor −50 −0.20
the range of 30–60 m. When the thickness of the coating was ≤20 m, the degradation was found to be >20%. Thus the thermal stability of DPB-based PSP coating was found to be independent of thickness of the active layer at coating thickness ≥ 30 m. In the case of pyrene-based PSP coatings, the thermal stability decreased with decrease in thickness of the coating [19]. Coating thickness was a critical parameter for the thermal stability and shelf life of pyrene-based paints since pyrene molecules have high mobility due to diffusion in silicone polymer matrix. 3.7. Response time and photostability The response and recovery profile of the DPB coating when exposed to 100% oxygen followed by 100% nitrogen was studied. The 90% response and 90% recovery times of the coating to an alternating atmosphere of oxygen and nitrogen were calculated. The coating exhibited fully reversible response and recovery times of 10 and 40 s respectively. It was found that the response is faster compared to the recovery time. The photostability of the coating was evaluated. The decrease in fluorescence intensity of the coating after continuous irradiation for 1 h was <5%. There was no interference from carbon dioxide or water vapor in amounts as present in air under ambient conditions. Table 5 shows a comparison of properties between DPB coatings and pyrene coatings containing 10 mM of active luminophore. It can be seen that the stability was higher for DPB-based coatings even though the PS and quantum efficiency were higher for pyrene-based PSP.
4. Conclusions The photophysical characteristics of the new pyrene derivative were found to be similar to that of pyrene. A binary pressure-sensitive paint was developed using DPB as the active luminophore, red phosphor as reference luminophore and an RTV silicone resin as binder. The excimer emission intensity and pressure sensitivity of DPB-based PSP coatings depended on the concentration of the luminophore in the coating. The emission intensity of the coating increased linearly with increase in concentration of
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DPB. The pressure sensitivity was maximum (75% bar−1 ) at 10–12 mM DPB concentration in the coating and decreased at lower and higher DPB concentrations. The selection of binder for the paint was based on the UV stability and thermal stability of the coating. The results of the static aging tests have shown that DPB coatings have better stability than pyrene-based PSP. DPB-based PSP coatings had good shelf life for about 60 days when stored under ambient conditions. It was found that there was about 15% degradation in intensity for DPB coating after thermal aging at 60 ◦ C and 0.1 bar for 1 h. The change in pressure sensitivity after thermal aging test was <1%. Thickness of the coating in the range of 30–60 m had little effect on the thermal stability of the DPB coating. Thus DPB was found to have potential as a substitute for pyrene in pressure-sensitive coatings used for the measurement of surface pressure distribution on wind tunnel models.
Acknowledgements The authors are grateful to Dr. B.R. Pai, Director, NAL, for his support and permission to publish the work and Dr. T.S. Prahlad, Ex-Director, NAL, for his constant encouragement during the course of the study. We thank Dr. Channa Raju for calibration of a DPB coating in the PSP calibration chamber.
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Biographies Bharathibai J. Basu received her Masters degree in chemistry from Calicut University, Kerala, India, in 1973 and Ph.D. from Indian Institute of Science, Bangalore, India, in 1995. She is working as a senior scientist in National Aerospace Laboratories, Bangalore, India. Research areas of her interest are trace element analysis, electroanalytical chemistry and spectroscopy. Her current interest is in the development of pressure-sensitive paints for wind tunnel studies. A. Thirumurugan obtained his M.Sc. degree in chemistry from American College, Madurai, India, in 2001. He worked as a trainee in the project on the development of pressure-sensitive paints in National Aerospace Laboratories, Bangalore, from 2001 to 2002. Presently he is pursuing his Ph.D. in chemistry in Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India. A.R. Dinesh received his B.E. degree from Bangalore University, Karnataka, India, in 2000. He worked as a trainee in the project on the development of pressure-sensitive paints in National Aerospace Laboratories, Bangalore, from 2001 to 2002. He obtained his M.S. in chemical engineering from Department of Chemical Engineering, University of Louisiana at Lafayette, USA, in 2003. C. Anandan obtained his M.Tech. in materials science from Indian Institute of Technology, Kanpur, in 1981 and Ph.D. from University of Wales, College of Cardiff, in 1990. He has worked at National Physical Laboratory, New Delhi, India, from 1983 to 1999 in the area of thin film materials and devices and surface analysis techniques. Currently he is working in the Surface Engineering Division of National Aerospace Laboratories, Bangalore. K.S. Rajam holds a doctorate from Bangalore University. Currently she is the head of the Surface Engineering Division, National Aerospace Laboratories, Bangalore, India. Her research interests include developing surface modification technologies for aerospace and other engineering applications.