Dual sensing of pO₂ and temperature using a water-based and sprayable fluorescent paint

Abstract

Core–shell particles (CSPs) composed of a polystyrene core and a poly(vinyl pyrrolidone) shell were dyed with a luminescent platinum(II) porphyrin probe for oxygen. In parallel, microparticles were dyed with a luminescent iridium(II) complex acting as a probe for temperature. The particles were deposited (by spraying) on a surface to enable continuous imaging of the distribution of oxygen (and thus of barometric pressure) and temperature. Unlike most previous paints of this kind, a binder polymer is not needed and water can be used as a dispersant. This makes the paint environmentally friendly and reduces costs in terms of occupational health, clean-up, and disposal. Both indicator probes in the sensor paint can be excited at 405 nm using LEDs or diode lasers, whilst their emission maxima are spectrally separated by about 130 nm. Thus, two independent optical signals are obtained that allow for fluorescent imaging of barometric pressure (in fact oxygen partial pressure) and of temperature, and also to correct the oxygen signal for effects of temperature. The paint was calibrated at air pressures ranging from 50 mbar to 2000 mbar and at temperatures between 1 °C and 50 °C.

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Introduction

Optical sensing of oxygen and temperature has found various applications in medical, biological and technical fields.^1–5 One specific field of application is in aerodynamic research where the visualization of the distribution of barometric pressure (p) and temperature (T) on aircraft models in wind tunnel tests plays an important role.^5–9 Both parameters can be imaged using luminescent coatings which often are referred to as pressure-sensitive paints (PSPs) or—more precisely—“barometric paints” and as temperature sensitive paints (TSPs), respectively. Such paints usually are based on volatile and toxic organic solvents (often in large quantities) which cause a substantial effort in terms of clean-up and respiratory protection. Also, the level of toxic vapors in test facilities allowed by the Occupational Safety and Health Administration can be heavily exceeded by using such “regular” paints.¹⁰ The use of water soluble paints or aqueous dispersions therefore would represent a highly attractive alternative.

PSPs and TSPs usually consist of a luminescent compound embedded in a polymer matrix.^7,8 The most important factor governing the response characteristics of the probes is the gas permeability of the matrix. Obviously, the matrices for pressure and temperature sensing have to be different in order to achieve optimized sensitivity with respect to the desired dynamic range. Especially the temperature probe has to be shielded from undesired oxygen quenching by applying a very gas impermeable matrix polymer. This of course does not apply for thermographic phosphorus which can be used instead of dye-based temperature probes and show virtually no cross-sensitivity to oxygen. Simultaneous imaging of p and T is particularly practicable if particles are being used because this facilitates the adjustment of the ratio of probes, prevents fluorescence energy transfer to occur (which is highly undesirable), and prevents the leakage of the probes into their (micro)environment like the surface of the sensor film or into other adjacent matrices (e.g. the base coat). In contrast to the few previously reported water-based PSPs^10,18 neither dissolved nor emulsified binders have been used.

Both the intensities and lifetimes of the luminescence of these compounds change with T and p (in fact with oxygen partial pressure). Intensity measurements suffer, however, from a variety of sources of error including interferences by ambient light, varying distances between light source, paint, and camera, inhomogeneous illumination, reflections and scatter of excitation light, and variations in the thickness of the sensor paint. Lifetime measurements, in contrast, are intrinsically referenced and much less susceptible to such errors.¹¹

Several types of optical sensors for simultaneous imaging of multiple parameters (so-called dual sensors; e.g. for p and T) have been reported.^12–15 In order to be separable, their signals (usually luminescence) have to be collected at two distinctly different wavelengths at least. In this case the rapid lifetime determination (RLD)¹¹ scheme can be applied. The signals may also be separated by temporal resolution if luminescence lifetimes are adequately different.^3,16 In the second case, the two signals need not necessarily be separated by optical filters because the signal of the probe with the longer lifetime is recorded only after the luminescence intensity of the first probe has decayed to virtually zero. The dual lifetime determination (DLD) scheme¹⁷ is preferred in such cases.

Here we report on a water-based sprayable dual sensitive paint for lifetime imaging of p and T on aircraft models in subsonic wind tunnel tests (max. wind speed = 100 m s⁻¹). It consists of an aqueous dispersion of two different kinds of well accessible polymer particles that are dyed with appropriate luminescent probes. The resulting sensor paint not only possesses the advantages of being an aqueous paint, but also shows remarkable sensing ranges and response.

Results

Sensor composition

The PSP presented here consists of commercially available poly(styrene)/poly(vinyl pyrrolidone) (PS/PVP) spherical core–shell particles (CSPs) with an average size of 240 nm¹⁹ that contain the platinum complex of 5,10,15,20-tetrakis(2,3,4,5,6-pentafluorophenyl)porphyrin (referred to as PtTFPP) which acts as the oxygen (pressure) sensitive probe. The particles were dyed in a simple manner reported previously.²⁰ Briefly, the beads are swollen in the presence of tetrahydrofuran (THF) and the lipophilic oxygen indicator is added. The dye is incorporated in the core of the beads when THF is removed under reduced pressure. A brightly luminescent iridium complex, referred to as Ir(ppy)₂carbac, was used as the probe for T. It was incorporated into microparticles of poly(acrylonitrile) (PAN) with an average size of 1 µm. PAN microparticles are easily prepared and virtually impermeable to oxygen. Thus, almost no quenching by oxygen is observed. The chemical structures of the two probes are given in Fig. 1.

Fig. 1 Chemical structures of the oxygen probe PtTFPP (A) and of the temperature probe Ir(ppy)₂carbac (B).

Properties of the paint

The PS/PVP core–shell particles display good adhesion on various surfaces including metals, glass, plastic etc. Excellent adhesion on aluminium is particularly important in context with imaging barometric pressure because this material is widely used to model aircrafts. We check adhesion with the help of the so-called “tape test” where Scotch® tape is glued onto the paint and stripped off. No visible destruction was observed in case of this paint which is proof for adhesion that is adequate for applications in a subsonic wind tunnel test. Dispersions of such CSPs were prepared in concentrations ranging from 10 to 30% (w/w) in water. Homogeneous films are obtained after drying. The PAN microparticles for temperature imaging are dispersed together with the PS/PVP particles and thus are incorporated in the resulting layer (see Fig. 2). For the paint presented here we used a 10% w/w dispersion of the PS/PVP beads charged with 15 mg of the PAN particles per mL dispersion. We obtained films with a thickness of ca. 10–15 µm.

Fig. 2 Schematic of the dually sensitive layer. The poly(styrene)/poly(vinyl pyrrolidone) (PS/PVP) particles dyed with the oxygen probe PtTFPP form a homogeneous film that adheres to the surface well enough. The poly(acrylonitrile) (PAN) microparticles containing the temperature probe Ir(ppy)₂carbac are then dispersed in the layer of PS/PVP particles.

The sensing layer produced by spraying possesses excellent mechanical stability. Unlike the two previously reported water-based paints, the PSP/TSP presented here can be easily removed from the surface with plain water (see Fig. 3). The paint also can be reused by collecting the washing water and concentrating it to the desired concentration. This is a most welcome feature and distinguishes this method from others, albeit not dual sensors.^10,18 In fact, these rely on polymerizable compositions with binders that cannot be easily removed.

Fig. 3 Images of the aluminium foil before and after coating and washing. (A) Before spraying it with the PS/PVP particles; (B) after spraying it with the red paint (dyed with PtTFPP), and (C) after removing the red paint with water.

Fig. 4 shows an SEM image of the sensor paint containing the oxygen-sensitive beads only. The particles are not densely packed and are well separated. The channels and cavities formed between the individual beads promote fast diffusion of the gas and consequently fast response of the paint to alteration of pressure. The response time is too fast (<1 s) to be determined with our system.

Fig. 4 Scanning electron microscopy image of (a) the paint based on PtTFPP/PS/PVP beads and (b) of the dual sensitive paint with the (much larger) PAN particles incorporated into the film of PtTFPP/PS/PVP beads.

Signal separation

The luminescence signals for p and T can be separated by optical filters because the two probes have different emission wavelengths. The emission of Ir(ppy)₂carbac peaks at 519 nm, that of PtTFPP at 658 nm. The luminescence of Ir(ppy)₂carbac is acquired by using a bandpass filter with a 530-nm center wavelength and a 50 nm spectral band width. The barometric signal of the probe PtTFPP was isolated with a second bandpass filter (type BP 680/60). The spectral properties of the dual sensor system are displayed in Fig. 5.

Fig. 5 Absorption and emission spectra of the sensor system. (A) Absorption and (B) emission of Ir(ppy)₂carbac; (C) absorbance and (D) emission of PtTFPP; (E) transmittance of the two optical bandpass filters.

Data acquisition

The work function of the sensor was established using time-gated data which were obtained using the RLD method.^11,21 Rather than collecting data at a single point, the data of the whole sensing area were recorded with a CCD (charge coupled device) camera to yield images of the distribution of pressure and temperature. The luminescence of the dual sensor layer was excited with a 405-nm light pulse from a LED light source. The decay time of the luminescence of PtTFPP ranges from 55 µs at 50 mbar air pressure to 10 µs at 1950 mbar air pressure, that of the iridium complex from 2.6 µs at 1 °C to 1.9 µs at 54 °C. Almost ideally square shaped light pulses can be created with LED light sources in the µs scale.²² Following photo-excitation, the luminescence intensity was recorded in two precisely timed gates using a triggered CCD camera, this is followed by a dark image that is needed for substraction of dark current (background).¹¹ Instead of utilizing the luminescence intensities as sensor signal we prefer to use the ratio of the intensities in gate 1 divided by gate 2. The resulting signal consequently is internally referenced and no reference dye is needed.

Luminescence intensity is acquired in the first gate (A₁; starting at t₁). This is repeated several times over a certain period of time (the integration time) and the measured intensities are accumulated and displayed as the first image. Afterwards, the second gate (A₂) and the respective dark images are recorded likewise. Lifetime is then calculated⁷ using the integrated intensity values A₁ and A₂ according to:

The typical standard deviations for such measurements are below 4%. A diagram of the RLD method is given in the ESI.† The RLD imaging parameters applied in this work are summarized in Table 1.

Table 1 Basic settings for luminescence lifetime imaging according to the RLD scheme

Probe	Excitation pulse width/µs	Gate width/µs	t 1/µs	t 2/µs
Ir(ppy)2carbac	5	1.4	0	0.7
PtTFPP	10	10	0	5

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The RLD method assumes the luminescence decay profile to be mono-exponential. While this is not totally valid in case of the two kinds of particle probes used here, this assumption is justified because the intention here is to rapidly image any changes in T or barometric pressure, rather than the determination of precise (multiple) decay times of probes. The RLD method is fast, intrinsically referenced, and hardly affected by the sources of error sources as listed before. Indeed, we find the results to be highly reproducible and not to be adversely affected by variations in the intensity of the excitation light.

The sensing area typically investigated is 3 × 3 cm in size, with the paint deposited on an aluminium plate mounted inside a calibration chamber where barometric pressure can be adjusted between 50 and 2000 mbar. A temperature-sensing Peltier element in the chamber enables temperatures to be adjusted to values between 1 and 55 °C. Photo-excitation and optical readout are performed from top through a window in the chamber.¹¹

Response of the dually sensitive paint to oxygen (barometric pressure)

The luminescence decay time of PtTFPP decreases with rising oxygen partial pressure because oxygen acts as a dynamic quencher of luminescence (in terms of intensity and decay time). For situations like the one here (where probes are located in different microenvironments in the polymer), the Stern–Volmer equation in the form of the so-called “two-site” model⁷ is well suited to describe this effect:

The lifetimes were normalized to τ₀ at the lowest pressure (50 mbar) for each temperature. The resulting Stern–Volmer plots together with the respective two-site fits are given in Fig. 6. For calculating the fits according to the two-site model, f₁ was set to 0.9 and f₂ was set to 0.1. Table 2 summarizes the fit parameters.

Fig. 6 Stern–Volmer plots of the quenching of the emission of PtTFPP in PS/PVP core–shell particles by oxygen (expressed as air pressure) at various temperatures.

Table 2 Stern–Volmer constants (K_SV; in 10⁻⁴ mbar⁻¹; mbar refer to air pressure) of the quenching of the luminescence of PtTFPP calculated from the fits according to the two-site model in Fig. 6. Fitting parameters f₁ and f₂ were set to 0.9 and 0.1, respectively

T/°C	K ^1 SV/10^−4 mbar^−1	K ^2 SV/10^−4 mbar^−1	R ^2
1	13.8 ± 0.8	0.11 ± 0.65	0.997
12	15.8 ± 0.9	0.15 ± 0.72	0.997
24	19.1 ± 0.9	0.14 ± 0.48	0.998
36	22.1 ± 0.9	0.12 ± 0.47	0.997
48	25.3 ± 0.8	0.15 ± 0.45	0.997

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The imaging of oxygen partial pressure is demonstrated in Fig. 7. The ratio images (A₁/A₂) of an area of ca. 2 × 1.5 cm are depicted as false colour pictures at 30 °C and at different air pressures.

Fig. 7 Ratio images (A₁/A₂) of the PSP in the dual sensitive paint at a temperature of 30 °C and at different air pressures.

The influence of humidity of the gas phase on the response of the PSP is very small in general. The humidity in a regular wind tunnel test facility will not have any noteworthy influence on the luminescence lifetime of the PSP. For further details please see the ESI.†

Response of the dually sensitive paint to temperature

The calibration of the TSP was performed at six different pressures between 50 mbar and 2000 mbar. The resulting plots are given in Fig. 8. Only the calibration curves at the lowest and the highest pressure are displayed for clarity reasons. The temperature dependency of the luminescence of Ir(ppy)₂carbac is linear over a wide temperature range. The temperature coefficient of the TSP in the dual sensor is 0.01 µs (τ) per °C. The cross-sensitivity towards the air pressure is 0.02 × 10⁻³ µs (τ) mbar⁻¹ (air pressure).

Fig. 8 Temperature dependency of the luminescence lifetime of Ir(ppy)₂carbac in PAN microparticles in the dual sensor with linear fit at 1950 mbar air pressure.

An example of the evaluation of the paint at a random data point under operational conditions is given in the ESI.†

Discussion

As a matter of fact, each and any sensor is sensitive to temperature. Hence, the TSP is not only needed to visualize the temperature distribution but also to correct for the effect of the temperature on the PSP.²³ The temperature probe in turn, has to be shielded from undesired quenching by oxygen. This is achieved by incorporating it into microparticles with very low gas permeability and solubility for oxygen. In this case we used poly(acrylonitrile) (PAN).²⁴

The response of the PSP composed of PtTFPP incorporated in the PS/PVP core–shell particles covers the range from 0 to 2000 mbar of air pressure and is perfectly suited for wind tunnel applications. PtTFPP is a popular oxygen probe that has already been used in various polymer matrices.^25–28 On one hand, the cross-sensitivity towards the temperature can be minimized to a certain extent by using fluoroacrylic polymers like FIB (= poly(heptafluoro-n-butyl methacrylate-co-hexafluoroisopropyl methacrylate) as the matrix.²⁷ Such materials show oxygen permeabilities that are magnitudes higher compared to PS and PVP. The outcome is a much smaller dynamic range of the PSP,⁴ meaning the luminescence is almost fully quenched at very low air pressures, which lowers the pressure resolution of the sensor when it comes to pressure values relevant for aerodynamic research. On the other hand, if PtTFPP is applied in a polymer film (e.g. PSAN) that is less oxygen permeable, the dynamic range can be stretched to the same extent as for the PS/PVP core–shell particles. The drawback of using such a matrix material is the resulting higher cross-sensitivity towards temperature.³ Virtually any lipophilic oxygen indicator can be incorporated into PS/PVP beads²⁰ so that the sensitivity of the resulted pressure-sensitive paint can be tuned over a wide range. Particularly, materials having higher sensitivity are obtained using the corresponding palladium(II) complexes, while those with lower sensitivity can be based on ruthenium(II) polypyridyl complexes and iridium(III) cyclometallated coumarin complexes. Spectral properties can also be tuned over a wide range because NIR-emitting indicators such as metal complexes of porphyrin-ketones,²⁹ meso-porphyrin lactones³⁰ and tetraphenyl tetrabenzoporphyrins³¹ also can be incorporated into PS/PVP beads.

The luminescence of Ir(ppy)₂carbac shows a linear temperature dependency over the full range from 1 °C to 55 °C. It is assumed that the probe may also be used for values outside this range. In contrast, most TSPs show nonlinear temperature dependencies making the data evaluation and fitting less convenient.^3,32,33 The TSP exhibited a remarkably low cross-sensitivity towards the air pressure. Other TSPs may have higher temperature coefficients but these also display higher cross-sensitivities towards oxygen.^3,4,32 Thermographic phosphorus such as Mg₄FGeO₆:Mn⁴⁺;³⁴ Y₃Al₅O₁₂:Cr³⁺,³⁵ ruby,³⁶ as well as La₂O₂S:Eu^37,38 can also be used.³⁹ These show virtually no cross-sensitivity to oxygen but neither high sensitivity nor brightness. The thermographic phosphorus usually are prepared in form of micrometer-sized particles which are subsequently entrapped into a PS/PVP layer.

Conclusion

The dye-doped core–shell particles presented here for imaging of oxygen partial pressure (and thus barometric pressure) show excellent adhesion on aluminium, and its aqueous dispersions can be homogeneously sprayed onto such surfaces. The resulting films are of high mechanical stability and thus can function as binder for additional beads. The water-based dual sensitive paint for p and T presented here consists exclusively of PS/PVP core–shell particles and poly(acrylonitrile) microparticles without any dissolved binder. Besides the advantages of an aqueous paint like lower health risks for users and environmentally friendly application the paint can be easily removed with plain water. The paint has excellent sensing characteristics for both p and T over the areas of practical interest. The paint can be interrogated by the RLD method which provides all the benefits of an intrinsically referenced imaging method.

Experimental

Materials

Poly(acrylonitrile) (PAN) was purchased from Polysciences (http://www.polysciences.com/), PtTFPP from Porphyrin Systems (http://www.porphyrin-systems.de), and N,N-dimethylformamide (DMF) poly(styrene-block-vinylpyrrolidone) dispersion in water (38% w/w) from Sigma-Aldrich (http://www.sigmaaldrich.com). The temperature-sensitive probe di(2-phenylpyridinato-C2,N){1-(9H-carbazol-9-yl)-5,5-dimethylhexane-2,4-dione}-iridium(III) (Ir(ppy)₂carbac) was synthesized according to ref. 40.

Synthesis of sensor particles

Poly(acrylonitrile) (PAN) microparticles doped with [Ir(ppy)₂carbac]. 200 mg of PAN were dissolved in 40 mL of N,N-dimethylformamide (DMF) at 55 °C. After the polymer was fully dissolved (30 min), 7 mg of Ir(ppy)₂carbac were added. 140 mL of double distilled water were added dropwise under vigorous stirring of the solution. The microparticles formed a precipitate that was separated by centrifugation and washed 4 times with water in order to remove remaining DMF from the solution. As DMF has a high boiling point (153 °C), the particles were then taken up in distilled water and freeze-dried to remove remaining DMF from the polymer and to fully cure the particles. If the particles are washed with organic solvents without freeze-drying and thus curing them in advance, the amount of dye molecules washed out of the particles is higher and the brightness of the luminescence of the particles is smaller. Prior to use in the dual sensor, the microparticles were washed three times with ethanol and three times with tetrahydrofuran, taken up in distilled water, and freeze-dried again.

PS/PVP particles doped with PtTFPP. Five hundred twenty-six milligrams of the PS/PVP polymer aqueous dispersion (at 38% (w/w) this equals 200 mg of the polymer) were diluted with the mixture of 60 mL of water and 30 mL of tetrahydrofuran. PtTFPP (3 mg) was dissolved in 20 mL of THF, and the solution was added dropwise under vigorous stirring into the dispersion. This process renders the particles doped with PtTFPP. After 30 min all THF and partly water were removed under reduced pressure and the dispersion was concentrated to contain 20% w/w of the beads in water.

Fabrication of the sensor film

To a 10% w/w dispersion of the PS/PVP beads in water we added 15 mg of the temperature sensitive PAN particles per mL dispersion. This cocktail was sprayed from a distance of ca. 15 cm onto a 3 × 3 cm aluminium sample plate. We use a Walther PILOT paint spraying gun (http://www.walther-pilot.de) on a Güde compressor (http://www.guede.ws) for spraying. The pressure applied for spraying was 2.5 bar.

Setup for sensor calibration

Absorption and emission spectra were recorded on a Lambda 14p Perkin-Elmer UV-vis spectrophotometer (Waltham, MA, USA, http://www.perkinelmer.com) and an Aminco AB 2 luminescence spectrometer (Thermo Scientific Inc., Waltham, MA, USA, http://www.thermo.com), respectively. The p/T calibration chamber was provided by the German Aerospace Center (DLR) in Göttingen (http://www.dlr.de). Time-resolved measurements were performed with a PCO SensiCam 12 bit b/w CCD camera (PCO, Kelheim, Germany, http://www.pco.de) equipped with a Schneider-Kreuznach Xenon 0.95/17 lens (Jos. Schneider Optische Werke, Bad Kreuznach, Germany; http://www.schneiderkreuznach.com) and a 405-66-60 405 nm LED from Roithner Lasertechnik (Vienna, Austria; http://www.roithner-laser.com). The excitation light was focused by a PCX 18_18 MgF2 TS lens (from Edmund Optics, Karlsruhe, Germany: http://www.edmundoptics.com). It was filtered through a BG 12 long-pass filter (Schott, Mainz, Germany, http://www.schott.com) with a thickness of 2 mm. Residual light was then passed through a BP 530/50 and a BP 680/60 band pass filter (from AHF Analysentechnik; http://www.ahf.de).

Acknowledgements

We thank Dr U. Henne and Dr C. Klein from the German Aerospace Center (DLR) in Göttingen for providing the calibration unit; DI G. Mistlberger and DI K. Koren (Graz University of Technology) for the help in acquiring the SEM images; and Prof. Dr E. Holder and N. Tian from the Bergische Universität Wuppertal for providing the temperature probe.

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Footnote

† Electronic supplementary information (ESI) available: Contains information on (a) the Rapid Lifetime Determination method, (b) data evaluation, (c) effects of humidity, and 3 figures. See DOI: 10.1039/b927255k

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