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New Life of Ancient Pigments: Application in High Performance Optical Sensing Materials Sergey M Borisov, Christian Würth, Ute Resch-Genger, and Ingo Klimant Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac402275g • Publication Date (Web): 04 Sep 2013 Downloaded from http://pubs.acs.org on September 8, 2013

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Analytical Chemistry

New Life of Ancient Pigments: Application in High Performance Optical Sensing Materials Sergey M. Borisov,*a Christian Würth,b Ute Resch-Genger,b and Ingo Klimanta a

Institute of Analytical Chemistry and Food Chemistry, Graz University of Technology,

Stremayrgasse 9, 8010, Graz, Austria b

BAM Federal Institute for Materials Research and Testing, Division 1.10 – Biophotonics,

Richard-Willstätter-Str. 11, D-12489 Berlin, Germany

Abstract: Calcium, strontium and barium copper silicates are demonstrated to possess valuable photophysical properties which make them particularly attractive for application in optical chemosensors. Several examples of sensing materials based on these phosphors are provided. Particularly, broad excitation and near infrared (NIR) emission makes them ideal candidates for the preparation of ratiometric sensors based on absorption-based indicators. Due to their excellent chemical and photochemical stability and high brightness, these phosphors can serve as reference for fluorescent indicators to enable ratiometric intensity or dually lifetime referenced measurements. Finally, the moderate temperature dependence of the luminescence decay time enables intrinsic temperature compensation of the sensing materials at ambient temperatures. The improved sensitivity at temperatures above 100 °C makes these new materials promising candidates for high temperature thermographic phosphors.

Introduction Calcium copper silicate Egyptian Blue CaCuSi4O10 and barium copper silicate Han Blue BaCuSi4O10 are ancient pigments which were used already thousands of years ago in the Old Kingdom in Egypt and during the Western Zhou period in China, respectively.1 Their high chemical stability is emphasized by the fact that even after such a long time, the colors of ancient monuments and artifacts retained their original splendor. The luminescent properties of numerous minerals are well known for many decades, but the NIR luminescence of Egyptian blue and Han Blue remained astonishingly undiscovered until very recently. Accorsi et al. reported the interesting photophysical properties of Egyptian blue (absorption in the

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green-red part of the spectrum, NIR emission at about 900 nm and good brightness) and suggested that these properties can be useful for application of this pigment in optical materials.2 Pozza et al. indicated that Han Blue is also luminescent in the near-infrared (NIR) region.3 Very recently Johnson-McDaniel et al. showed that Egyptian Blue nanosheets can be produced upon prolong treatment of the phosphor with hot water, thereby preserving its favorable luminescence properties.4 However, despite these exciting reports, the phosphors have not yet found any application in advanced optical materials. Optical chemosensors represent a very important and constantly growing field of analytical chemistry.5,6 They often provide distinct advantages over conventional analytical techniques due to the absence of electromagnetic interferences, minimal invasiveness, suitability for miniaturization and continuous monitoring of analyte concentration as well as their typically lower costs. Apart from an indicator (which responds to the analyte of interest by altering its optical properties such as absorption, luminescence intensity, luminescence decay time, anisotropy etc.) and a polymer or a sol-gel matrix, optical chemosensors often require additional components to ensure reliable sensing. For example, fluorescence intensity alone is a rather ambiguous parameter, which is affected also by fluctuations of the excitation light intensity and indicator concentration, and is thus rarely used. Fluorescence lifetime measurements are much more reliable,7 but the required instrumentation is more expensive. Thus ratiometric wavelength referencing, requiring either two spectrally distinguishable absorption (excitation) or emission bands, is rather popular, particularly in (microscopic) imaging applications.8,9 Since most probes emit only at a single wavelength, typically a second dye excitable at the same wavelength but emitting in a different part of the spectrum is added for signal referencing. Alternatively, referencing of the fluorescence intensity can also be realized with the help of the so-called Dual Lifetime Referencing (DLR) approach.10-13 DLR relies on the use of a phosphorescent reference which has a significantly longer decay time than the fluorescent indicator (typically µs-ms and ns time domains, respectively). In the frequency domain, for example, the overall phase angle is a function of the amplitudes and the phase angles of both luminescent components. Recently, we reported a new approach for referencing absorption-based indicators.14 In this scheme, a broadband absorbing or a broadband emitting phosphor acts as a second luminophore, the luminescence intensity of which is modulated by changes in the absorption of the indicator due to the inner filter effect. Finally, it is well known that the response of all optical chemosensors can be affected by temperature as the optical properties of most indicators and fluorophores are temperaturedependent. Although temperature can be measured independently with e.g., a thermocouple, it 2 ACS Paragon Plus Environment

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is often more attractive to use an optical probe for this purpose. In this case, both parameters, i.e., analyte concentration and temperature, are measured at the same place which improves the robustness of the sensor against fluctuations in temperature and enables miniaturization (e.g., fiber-optic microsensor). A number of luminescent metal complexes15-17 and thermographic phosphors18-21 have been employed so far in optical temperature sensors and in dually- or multi-analyte sensing materials operating at ambient temperature.22-24 However, high temperature thermographic phosphors are also of much interest.25 In this contribution, we will demonstrate that Egyptian blue, Han blue and strontium copper silicate represent highly versatile materials for application in optical sensors. Particularly, it will be shown that these phosphors are promising as references for fluorescent indicators utilizing both ratiometric and DLR schemes as well as for absorption-based indicators and can also serve as internal temperature probes.

Experimental part Materials. Egyptian Blue and Han Blue were obtained from Kremer Pigmente (Aichstetten, Germany). Ethyl cellulose (ethoxyl content 49%), m-Cresol purple, calcium carbonate, strontium carbonate, trimethylsilyl chloride, tetraoctylammonium hydroxide solution (TOAOH, 20 % in methanol) were obtained from Sigma-Aldrich. Silicon oxide (99.9 %, 1µm particles), vinyl-terminated polydimethylsiloxane (viscosity 1000 cSt.), methylhydrosiloxane dimethylsiloxane copolymer (viscosity 25-35 cSt.), tetravinyltetramethyl cyclotetrasiloxane and platinum divinyltetramethyldisiloxane complex were purchased from ABCR (Karlsruhe, Germany). Basic copper carbonate was obtained from Fischer Scientific. The solvents were from VWR (Austria). Barium carbonate and the buffer salts N-cyclohexyl-2aminoethanesulfonic acid, 2-(N-morpholino)ethanesulfonic acid and N-cyclohexyl-3aminopropanesulfonic acid were supplied by Carl Roth (Germany). Polyurethane hydrogel (HydromedTM D4) was purchased from AdvanSource biomaterials (Wilmington, USA). Poly(ethylene terephthalate) (PET) support Melinex 505 was from Pütz (Taunusstein, Germany).The gases for the calibration – carbon dioxide and nitrogen were supplied by Linde (Austria). [5-(4-hydroxyphenyl)-3-phenyl-1H-pyrrol-2-yl]-[5-phenyl-3-phenylpyrrol-2-ylidene]amine “aza-BODIPY” was prepared as reported previously.26

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High temperature solid state synthesis of the phosphors. The synthesis is exemplified for calcium copper silicate (SrCuSi4O10). The phosphor was prepared via a high temperature solid state reaction using silicon oxide, strontium carbonate and basic copper carbonate. Stoichiometric amounts of these substances were thoroughly homogenized in an agate mortar and sintered at 1000 °C for 24 h. The resulting material was homogenized to powder in an agate mortar. The samples of Egyptian Blue (CaCuSi4O10) and Han Blue (BaCuSi4O10) were prepared analogously using calcium carbonate and barium carbonate, respectively.

Lipophilization of Egyptian blue. For application in the sensing materials, Egyptian blue was ground to microparticles (1-5 µm) in a ball mill. The surface of the particles was rendered lipophilic via silanization using the following procedure: 1.6 g of Egyptian blue microparticles were dispersed in 5 mL of anhydrous tetrahydrofuran and 0.5 mL of trimethylsilyl chloride were added. The suspension was stirred for 30 min, the sediment was separated via centrifugation, washed four times with acetone and twice with ethanol, and dried.

Preparation of the pH sensors 250 mg of hydrogel D4, 0.5 mg of the pH indicator aza-BODIPY and 250 mg of Egyptian blue lipophilic microparticles were dissolved/dispersed in 2.5 g of an isopropanol:H2O mixture (9:1 v/v). The “cocktail” was knife-coated on a polyethylene terephthalate support to give about 8 µm-thick sensor layers after solvent evaporation. Alternatively, the fiber-optic sensors were prepared by coating the “cocktail” on a distal end of a PMMA waveguide (∅ 1 mm, length 1 m, Ratioplast, Lübbecke, Germany).

Preparation of the CO2 sensors 100 µL of tetraoctylammonium hydroxide were added to 2 mg of m-Cresol Purple and the resulting solution was saturated with carbon dioxide. In the second vial, 0.2 g of ethyl cellulose were dissolved in a mixture of 1.52 g of ethanol and 2.28 g of toluene. The content of both vials was combined and the obtained “cocktail” was knife-coated on a polyethylene terephthalate support and was allowed to dry for 1 h at room temperature. The thickness of the layer after solvent evaporation was estimated to be about 3.5 µm. A “cocktail” of silicone primers was obtained by mixing 800 mg of vinyl-terminated polydimethylsiloxane, 32 µl of methylhydrosiloxane dimethylsiloxane copolymer, 2 µl of tetravinyltetramethyl cyclotetrasiloxane and 800 mg of hexane. Micrometer-sized lipophilic 4 ACS Paragon Plus Environment

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particles of Egyptian Blue (400 mg) were dispersed in the “cocktail”. Finally, 4 µl of the platinum complex catalyst were added and the composition was knife-coated onto the CO2sensitive layer (75 µm spacer) and was allowed to cross-link at 60 °C for 20 min.

Measurements The absorption spectra were acquired on a Cary 50 UV-VIS spectrophotometer from Varian. Emission spectra of individual phosphors were measured in front face geometry with a calibrated FSP 920 fluorescence spectrometer from Edinburgh Instruments equipped with double monochromators and a liquid nitrogen cooled NIR-sensitive InP/InGaAs photomultiplier from Hamamatsu (R5509-72). All measurements were performed with polarizers in the excitation (0°) and the emission channel (54.7°). All emission spectra presented are corrected for the spectral responsivity of the instrument´s detection channel (instrument characterization traceable to radiometric units, i.e., to the spectral radiance scale.27,28 Luminescence excitation spectra and emission spectra of the sensing materials were acquired on a Fluorolog 3 fluorescence spectrometer (Horiba) equipped with a photomultiplier R2658 (Hamamatsu) optimized for the spectral range of 300-1050 nm. About 10 µm thick layers of the phosphors in polystyrene (1:1 w/w) on a glass support were used for the measurement of the excitation spectra in order to minimize wavelength-dependent scattering effects. The excitation spectra were corrected for the wavelength-dependent output of the Xe arc lamp. Planar carbon dioxide and pH optodes were read out with a Fluorolog 3 spectrometer. A home made flow-through cell was used. In case of the carbon dioxide sensors, the gas mixtures were adjusted using Red-y gas flow controllers (Vögtlin Instruments AG, Aesch, Switzerland). The gas mixtures were adjusted to 100 % relative humidity. The pH of the buffer solutions was controlled by a digital pH meter (InoLab pH/ion, WTW GmbH & Co. KG, Germany) calibrated at 25 °C with standard buffers of pH 7.0 and pH 4.0 (WTW GmbH & Co. KG). The buffers were adjusted to constant ionic strength (IS = 0.15 M) using sodium chloride as the background electrolyte. The temperature was kept constant at 37 °C using a cryostat ThermoHaake DC50 (Thermo Fisher Scientific Inc). The fiber-optic pH sensors were read-out with a Firesting oxygen meter from Pyroscience (Aachen, Germany) which is equipped with a read 625 nm LED and a long-pass RG 9 filter from Schott. A modulation frequency of 1.5 kHz was used. The carbon dioxide sensors were also read out using a dually-emitting LED (590/650 nm) from PUR-LED® GmbH & Co. KG (Selzen, Germany). A short-pass Calflex X filter 5 ACS Paragon Plus Environment


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from Linos was used in front of the LED. The luminescence was detected at a modulation frequency of 916 Hz using a lock-in amplifier from PreSens (Germany) equipped with a photodiode and a long-pass RG-9 filter from Schott (Germany). The temperature sensitivity of the phosphors was investigated using a Firesting-mini device from Pyroscience. A glass waveguide (∅ 3 mm, length 15 cm) was used as an extender. The distal end of the waveguide was brought into direct contact with the phosphor powder kept in a 4 mm glass vial. The vial was heated in a home-made metal block using a CMAG HS 7 heating plate from IKA®-Werke GmbH & Co. KG (Staufen, Germany). The modulation frequency of 1.5 kHz was used to access the decay times.

Results and Discussion Photophysical properties Luminescence excitation and emission spectra of Egyptian blue, Han blue and strontium copper silicate are shown in Fig. 1. The phosphors have a broad absorption / excitation band located in the green-red part of the spectrum which is attributed to 2B1g → 2A1g and 2B1g → 2

Eg transitions.2 Excitation due to 2B1g → 2B2g transition occurs in the NIR part of the

spectrum. As follows from Fig. 1, the excitation bands in the visible region are very similar for all phosphors, but a small bathochromic shift (∼ 10 nm, 280 cm-1) of the 2B1g → 2A1g band is observed on going from calcium copper silicate to barium copper silicate. The NIR excitation is affected more significantly and shifts from 806 nm for calcium copper silicate to 842 nm (530 cm-1) for barium copper silicate. All phosphors emit in the NIR above 800 nm (Fig. 1). Similarly to the NIR excitation, the emission associated with the 2B2g → 2B1g transition shifts bathochromically from calcium to barium copper silicates (λmax = 909, 914 and 948 nm for calcium, strontium and barium copper silicates, respectively). Precise determination of the luminescence quantum yields in the NIR region, which is not possible with a spectrofluorometer in a 0°/90° or front face measurement geometry due to size-dependent scattering of the micrometer-sized phosphor particles and requires an integrating sphere setup, was beyond the scope of this study.27 Previous results suggest a luminescence quantum yield of 10.5 % for Egyptian blue.2 Measurements under red excitation (625 nm LED) and a photodiode as a detector revealed that the brightnesses of the strontium copper silicate and Han blue are comparable to that of Egyptian blue. It should be kept in mind here, however, that the brightness equals the product of the luminescence quantum yield and the molar absorption coefficient. Considering a rather 6 ACS Paragon Plus Environment

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high amount of the chromophore in the phosphors, which is about 10-20-fold higher than for typical phosphors (e.g., chromium(III)-activated hosts such as yttrium aluminum garnet, yttrium aluminum borate, spinel, aluminum oxide (ruby) or manganese(IV)-activated magnesium fluorogermanate; typical doping of 0.5-5% of the activator), the overall brightness of our materials is excellent. This is particularly true for thin layers of the phosphor materials since in this case a significantly more efficient absorption can compensate for lower quantum yields. For example, excitation of a thin sensing layer of a mixture of chromium(III)-activated gadolinium aluminum borate (Cr-GAB) and Egyptian blue (25 % w/w of each phosphor in polystyrene) with red light (600 nm, corresponding to the excitation maxima of both phosphors) results in about 3-fold higher signals for Egyptian blue compared to Cr-GAB. CaCuSi4O10

Norm. Luminescence intensity

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Figure 1. Excitation (lines) and emission spectra (dashed lines) of Egyptian blue (black), strontium copper silicate (red) and Han blue (blue line). The insert shows photographic images of the measured µm-sized phosphor particles, i.e., the powdered pigments under ambient light.

High homogeneity of the sensing materials is important to ensure reproducibility of the calibration and sensor response. This is particularly true if the phosphors or other luminescent species are used for referencing purposes in the composite materials. Therefore, the phosphors were ground to microparticles (∅ 1-5 µm) using a ball mill to enable a better reproducible sensor production. Although grinding does not appreciably affect the optical properties of these phosphors, we note a slight shortening of the decay time of the ground phosphors compared to the larger crystals obtained after sintering. For example, the decay times of the Egyptian Blue at 20 °C were 159 µs and 126 µs after sintering and after grinding, respectively. For comparison, the decay time of commercially available Egyptian blue powder (Kremer Pigmente) was measured to be 139 µs. The decrease of the decay time can be explained by accumulation of the defects induced by mechanical stress during grinding. 7 ACS Paragon Plus Environment


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Ratiometric pH-sensing materials The broad excitation bands in the visible part of the spectrum of the phosphors (Fig. 1) are ideally compatible with the absorption of a large number of fluorescent indicators and very favorable for ratiometric (2 wavelength) referencing of luminescence intensity. Since fluorescent indicators typically possess rather small Stokes shifts, the emission of the phosphors at 900 -1100 nm can be easily spectrally separated from that of the analyteresponsive probe. High chemical and photochemical stability of the phosphors is of course also advantageous. Fig. 2 shows the absorption and emission spectra of the tetraphenylazadipyrromethene pH indicator (“aza-Bodipy”)26 and the emission spectra of the pH-sensitive material. The sensor is prepared by dissolving/dispersing the pH indicator and Egyptian blue micropowder in a polyurethane hydrogel D4. In order to render the surface of the microparticles stable in aqueous media (hydrogel) end-capping with trimethylsilyl groups was performed. The indicator and the phosphor are excitable with the red light. Although both emit in the NIR part of the spectrum, the emissions are well separated. As expected, the fluorescence of the pH probe is quenched as the pH increases but the luminescence of Egyptian blue is not affected by pH. A minor increase of the luminescence intensity of the phosphor (about 10 % for going from pH 5.2 to 10.3) is due to a change in absorption of the pH indicator and thus, a decrease of the inner-filter effect. In fact, the absorption maxima of the protonated and deprotonated forms of the pH indicator are located at 687 and 743 nm, respectively (Fig. 2a), and the absorption at the excitation wavelength (625 nm) is about 3times more efficient for the protonated form. Evidently, the system is ideal for ratiometric emission measurements. The apparent pKa value calculated from the respective calibration curve for the luminescence intensity ratio is 7.53 which is adequate for measurements in the physiological probes and for application in seawater. It should be mentioned that simple synthetic modification allow to tune the pKa value of the indicator in a broad range.26 Considering the spectral properties of the two other phosphors (Fig. 1), they can be used for ratiometric referencing of other indicators with even more bathochromically shifted excitation and emission spectra.


Absorption

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750

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850

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Wavelength, nm 2.0 1.5 1.0

pKa = 7.53

0.5 0.0 5

6

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Figure 2. A: Spectral properties of the aza-Bodipy pH indicator in hydrogel D4 (black thick line – absorption spectrum at pH 5.2; blue thin line – absorption spectrum at pH 10.3; red dotted line – emission spectrum at pH 5.2 and λexc = 625 nm); B: pH dependence of the emission spectra of the sensing material consisting of the pH indicator and µm-sized Egyptian blue particles (37 °C, 150 mM ionic strength, λexc = 625 nm) dissolved/dispersed in hydrogel D4; C: Respective calibration for the ratio of the luminescence intensities (squares) and a sigmoidal fit (line).

Dually lifetime referenced pH sensors Dual Lifetime Referencing (DLR) represents another technique for referencing the fluorescence of indicators. Here, an inert analyte-insensitive luminescent material is dispersed in the polymer matrix along with the analyte-responsive fluorescent indicator. A reference suitable for this application should be chemically and photochemically inert and should possess a relatively long luminescence decay time (typically in µs – ms time domain). This technique can be used both in the time domain12 and in the frequency domain.11,29 In the frequency domain, the overall phase shift is measured; both the fast-decaying fluorescence of the indicator and the long-lived luminescence of the reference contribute to this value. The 9 ACS Paragon Plus Environment


most frequently used DLR references are hence metal complexes .30,31 Due to their disadvantageous liability to quenching by oxygen, these dyes, however, have to be encapsulated in gas-blocking polymers (e.g., polyacrylonitrile)11,30 which may be challenging. Application of phosphors as DLR-reference materials is rather rare.32 As we demonstrate here, Egyptian Blue and other copper silicate phosphors represent excellent DLR references since they combine bright long-lived luminescence, excellent (photo)stability, and inertness to oxygen. Fig. 3 shows the calibration curve for a fiber-optic pH sensor. The material used to coat the plastic PMMA fiber is identical to that employed in the planar optodes with ratiometric read-out. The fluorescence amplitude of the indicator is proportional to the difference of the cotangents of the overall phase shift and the phase shift of the reference:30

cotΦ − cotΦ ref =

1 A ⋅ ind sin Φ ref A ref

where Φ and Φ ref are the phase angles of the overall signal and of the luminescent reference, respectively; Aind and Aref – are the amplitudes of the fluorescent indicator and of the luminescent reference, respectively. As follows from Fig. 3, the apparent pKa value obtained for the DLR-sensor (7.47) is very close to that calculated from the ratiometric read-out. The individual sensors have slightly varying ratios of the fluorescent dye and the reference and are not calibration-free (Fig. 3, phase shift plots). Thus, a two point calibration is recommended (low pH and high pH buffer). The same refers also to the ratiometric sensor.

1.0

50 pKa = 7.47

0.8

40

0.6 30 0.4 20

0.2

Phase shift Φ , °

Normalized CotΦ Φ -CotΦ Φ0

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10

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8

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pH

Figure 3. Calibration plots for the fiber-optic DLR-pH sensor (37 °C, 150 mM ionic strength) based on an aza-Bodipy pH indicator and Egyptian blue in hydrogel D4. Symbols represent experimental data and lines sigmoidal fits. The phase shifts for three individual sensors are shown. The values CotΦ-CotΦ ref were normalized to enable the comparison of all individual sensors. The inserts show the photographic images of a fiber-optic sensor without LED illumination and during the readout. 10 ACS Paragon Plus Environment

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Ratiometric inner filter effect sensors Very recently, we presented a new scheme for the referencing of absorption-based indicators.14 Here, a reference acts a secondary emitter, the luminescence intensity of which is modulated by an indicator with an analyte-responsive absorption via inner filter effects (in absorption and /or emission). A suitable reference for this application should feature either a broad excitation spectrum or a broad emission spectrum which overlap with at least one absorbing form of the indicator. Such a scheme allows also a ratiometric readout. In addition to the general versatility of this approach, which can utilize all types of broad available colorimetric indicators and probes, and its inertness to fluorescence quenchers, the use of phosphors with long luminescence decay times as reference allows complete elimination of autofluorescence originating from the sample, indicator and/or optical components of the detection system. As follows from Fig. 1, the optical properties of Egyptian blue and other copper silicate phosphors make them ideal references for this new sensing scheme. Currently the number of available NIR colorimetric probes is rather limited, particularly those absorbing above 850 nm, but the situation is likely to change in the future. We therefore demonstrate here only the applicability of the phosphors for this versatile sensing scheme using an indicator-modulated inner filter effect in excitation, see Fig. 4. The broad luminescence excitation spectra of the phosphors almost perfectly overlap with the absorption of the deprotonated form of triphenylmethane indicators applied as pH, carbon dioxide, and ammonia probes. A two layer carbon dioxide sensor was realized, with the first layer containing m-Cresol purple and tetraoctylammonium hydroxide dissolved in ethylcellulose and the second layer Egyptian blue microparticles dispersed in silicone rubber (Fig. 4a). In this sensor design, silicone rubber acts not only as a matrix for the immobilization of the reference phosphor but also as a permeation-selective membrane which protects the sensitive layer from protons and other ions. As can be observed in Fig. 4c, the excitation spectrum of the sensing material is dependent on pCO2, thereby reflecting the modulation of the excitation light intensity by the absorption of m-Cresol purple (Fig. 4b) as change in the emission intensity of the phosphor. This also demonstrates that ratiometric readout in luminescence excitation is possible. Dually emitting LEDs are available matching both the excitation wavelength of the phosphors and the indicator absorption. Thus, a very compact, simple, and inexpensive setup for the ratiometric readout of the optical sensors can be realized (Fig. 4a), using a dually emitting LED, two optical filters, and a silicon photodiode. Fig. 4d shows the calibration plot 11 ACS Paragon Plus Environment


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obtained using a dually emitting red-orange (650/590 nm) LED. Relatively high standard deviations are observed since the thickness of both layers can vary significantly for individual sensors. Thus, the sensors realized according to the above scheme are not calibration-free, and 2 point calibration is required before the measurement. Nevertheless, the combination of Egyptian blue (or other copper silicate phosphors) with absorption-based indicators represent an excellent way to realize simple and robust optical sensors for various analytes. Indeed, the pH and ammonia sensors based on triphenylmethane dyes can be designed in a similar manner. Referencing of irreversible colorimetric probes can also be performed. Notably, very large effective shift between excitation and emission of the phosphors (> 200 nm) favors elimination of potential interferences such as autofluorescence and Raman scattering. Further simplification can include a photodiode with an integrated daylight filter (e.g. BPW 34 FAS available from Osram), rendering other optical filters unnecessary. Alternatively, the filterless set-up can be realized by eliminating potential interferences with time-resolved measurement. It is interesting to compare the properties of the sensing materials based on the new phosphors which employ three different read-out schemes presented above (Table 1). Generally, the material relying on the inner filter effect-modulated excitation read-out is significantly more robust towards potential interferences (fluorescence quenchers, background fluorescence from the sample) which can be even further minimized in the time-resolved measurement. Along with the DLR scheme this method requires only a very simple instrumentation with few optical components. Indeed, only a simple long-pass filter is necessary for the inner filter read-out, a long-pass and a short-pass filter for the DLR method, and at least three filters (short-pass for the excitation, a long-pass and a band pass for the emission, and an additional dichroic mirror) for the ratiometric scheme. On the other hand, the variation in the layer thickness only minor affects the calibration in the DLR and ratiometric scheme, but more severely in the inner-effect read-out.


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Figure 4. A: Cross-section of the carbon dioxide sensor and optical setup for the readout of the sensor with a dually emitting LED; B: Absorption spectra of m-Cresol purple (in ethyl cellulose) in the deprotonated (low pCO2, 1) and protonated (high pCO2, 2) forms and the excitation spectrum of Egyptian blue (3); C: Excitation spectra (λem = 900 nm) of the carbon dioxide sensor at varying pCO2 (25 °C) and the emission spectra of the dually emitting LED (590/650 nm); D: calibration plot obtained for the excitation of the carbon dioxide sensor with a dually emitting LED, the emission monitored at λ > 730 nm (RG 9 filter) with a photodiode (25 °C) Average values for 3 individual sensors are shown. The chemical structure of the indicator used here is shown in Fig. 4D.



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Table 1. Comparison between different read-out schemes used in this work. Sensing scheme Property Influence of the fluorescence quenchers on the calibration Influence of the thickness of the sensing layer on the calibration Influence of background fluorescence on the calibration Possibility of the timeresolved measurements Versatility of formats (planar optodes, paints, microsensors etc.) Complexity of optical set-up Intrinsic optical temperature compensation

Ratiometric (2-λ) emission

Dual Lifetime Referencing

strong

strong

Inner filter effectmodulated excitation no influence

minor

minor

large

minor

minor

negligible

not possible

not possible

possible

high

high

low

moderate possible (more complex calibration algorithms)

low possible (more complex calibration algorithms)

very low possible (calibration is not affected)

Phosphors as optical temperature probes The response of almost every optical sensor is temperature dependent. Hence, the temperature of the sensor environment should be known or be kept constant in order to obtain reliable data. To circumvent this additional source of measurement uncertainties, either a constant monitoring of the ambient temperature is required or a tool that enables an intrinsic temperature compensation of the optical sensor. As follows from Fig. 5, which shows the temperature dependence of the luminescence decay time (a) and luminescence intensity (c) of the phosphors, our NIR-emissive phosphors are also excellently suited for this task. Evidently, both parameters decrease with temperature for all materials. The temperature sensitivity of all phosphors (Fig. 5b) is moderate at ambient temperature (dτ/dT 0.25, 0.24 and 0.21 %/K at 25 °C for Han blue, strontium copper silicate and Egyptian blue, respectively). These values are significantly lower than those for recently reported chromium(III)-activated yttrium aluminum borate (0.8%/K)21 but are comparable to e.g. Ruby (∼0.22 %/K)20 or manganese(IV)-activated magnesium fluorogermanate (∼0.25 %/K)33 which are used as thermographic phopshors. Interestingly, the dynamic range of the phosphors is significantly different: Han blue operates at lower temperatures than Egyptian blue and strontium copper 14 ACS Paragon Plus Environment

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silicate. The luminescence of Han blue is almost completely quenched at 220 °C but the other phosphors are still luminescent at temperatures above 300 °C. Importantly, the sensitivity significantly improves at higher temperatures (Fig. 5c). For example, the sensitivities as high as 1 %/K are achieved for Han blue, strontium copper silicate and Egyptian blue at 154, 283 and 303 °C, respectively. About 30-34 % of the original luminescence intensity is retained at these temperatures which insures good signal-to-noise ratios. These observations underline the excellent suitability of the investigated materials as thermographic phosphors operating at 100-400 °C. Importantly, no hysteresis of the calibration was observed and the plots were identical for the cycles of increasing and decreasing temperature. Despite moderate sensitivities at ambient temperatures, the phosphors enable the temperature compensation of optical sensors (e.g. pO2, pH, pCO2 etc.). They are particularly attractive as temperature probes for ratiometric sensing relying on absorption-based indicators since variations of the luminescence intensity due to temperature-induced quenching are not relevant in this sensing scheme.

Decay time, µs

200 Han Blue

(a)

SrCuSi O

100

Egyptian Blue

dττ/dT, %/K

150

4

10

50 0 4

(b)

3 2 1 0

Norm. Intensity, a.u.

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1.0

(c)

0.8 0.6 0.4 0.2 0.0 -50

0

50 100 150 200 250 300 350 400

Temperature, °C



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Figure 5. Temperature dependence of the luminescence decay time (a) and luminescence

intensity (c) and the temperature sensitivity of the luminescence decay time (b) for the phosphors.

Conclusions We have demonstrated that calcium/strontium/barium copper silicates represent very promising phosphors for application in optical sensing materials. Particularly, they can be used as inert references for ratiometric and DLR readouts in combination with NIR fluorescent indicators and as secondary emitters for the ratiometric luminescence readout of absorption-based indicators. The latter scheme provides unique possibility of using very simple and inexpensive instrumentation for the sensor readout. The phosphors favorably compare to other materials used as references in optical sensors due to the combination of several unique features such as broad excitation spectra in green-red part of the electromagnetic spectrum, strong NIR emission, long luminescence decay times, high chemical and photochemical stability, and inertness to oxygen. It is also important to note that all the phosphors can be easily prepared from low cost materials and Egyptian blue and Han blue are also commercially available. The phosphors can be additionally used to optically access temperature for such sensors (albeit with moderate sensitivity at ambient temperatures) or as high temperature thermographic phosphors if used without other additives. It can be concluded that the ancient pigments finally found a new life as components of high performance sensing materials. Other potential applications of these exciting phosphors in optical materials are yet to be discovered.

AUTHOR INFORMATION Corresponding author. Tel.: +43 316 873 32516. Fax: +43 316 873 32502. E-mail:

[email protected]

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Graphical Table of Content Figure


New Life of Ancient Pigments: Application in ... - ACS Publications

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