NIR optical carbon dioxide sensors based on highly photostable dihydroxy-aza-BODIPY dyes

Article abstract of DOI:10.1039/c5tc00346f

A new class of pH-sensitive indicator dyes for optical carbon dioxide sensors based on di-OH-aza-BODIPYs is presented. These colorimetric indicators show absorption maxima in the near infrared range (λ_max 670-700 nm for the neutral form, λ_max 725-760 nm for the mono-anionic form, λ_max 785-830 nm for the di-anionic form), high molar absorption coefficients of up to 77 000 M^-1 cm^-1 and unmatched photostability. Depending on the electron-withdrawing or electron-donating effect of the substituents the pK_a values are tunable (8.7-10.7). Therefore, optical carbon dioxide sensors based on the presented dyes cover diverse dynamic ranges (0.007-2 kPa; 0.18-20 kPa and 0.2-100 kPa), which enables different applications varying from marine science and environmental monitoring to food packaging. The sensors are outstandingly photostable in the absence and presence of carbon dioxide and can be read out via absorption or via the luminescence-based ratiometric scheme using the absorption-modulated inner-filter effect. Monitoring of the carbon dioxide production/consumption of a Hebe plant is demonstrated.

Full text of DOI:10.1039/c5tc00346f

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Materials Chemistry C
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NIR optical carbon dioxide sensors based on
highly photostable dihydroxy-aza-BODIPY dyes†
Cite this:DOI: 10.1039/c5tc00346f
Susanne Schutting,a Tijana Jokic,a Martin Strobl,a Sergey M. Borisov,*a Dirk de Beerb
and Ingo Klimanta
A new class of pH-sensitive indicator dyes for optical carbon dioxide sensors based on di-OH-aza-
BODIPYs is presented. These colorimetric indicators show absorption maxima in the near infrared range
(lmax 670–700 nm for the neutral form, lmax 725–760 nm for the mono-anionic form, lmax 785–830 nm
for the di-anionic form), high molar absorption coefficients of up to 77 000 Mꢀ1 cmꢀ1 and unmatched
photostability. Depending on the electron-withdrawing or electron-donating effect of the substituents
the pKa values are tunable (8.7–10.7). Therefore, optical carbon dioxide sensors based on the presented
dyes cover diverse dynamic ranges (0.007–2 kPa; 0.18–20 kPa and 0.2–100 kPa), which enables different
applications varying from marine science and environmental monitoring to food packaging. The sensors are
outstandingly photostable in the absence and presence of carbon dioxide and can be read out via absorp-
tion or via the luminescence-based ratiometric scheme using the absorption-modulated inner-filter effect.
Monitoring of the carbon dioxide production/consumption of a Hebe plant is demonstrated.
Received 4th February 2015,
Accepted 18th April 2015
DOI: 10.1039/c5tc00346f
www.rsc.org/MaterialsC
indicator dye, referencing of the fluorescence intensity is
necessary to achieve reliable results. This can be realized by
Introduction
Carbon dioxide is one of the most important parameters in using an analyte-insensitive reference dye with different spec-
many scientific and industrial fields, such as medicine,1–3 tral properties from the indicator, either a different emission
marine science,4–7 food packaging,8 bio processing9,10 or environ- spectrum or a different luminescence decay time. Although
mental and industrial monitoring.11 Routine techniques of CO2 necessary in many cases, the addition of reference materials
quantification like infrared (IR) spectroscopy, gas chromato- can cause dramatic ratio changes when photobleaching of one
graphy (GC) or the Severinghaus electrode are well established, dye (reference or indicator) or both occurs. Therefore, self-
but suffer from different drawbacks.12,13 For instance IR spectro- referencing indicator dyes are highly desired for ratiometric
scopy is mainly used for gas samples,14,15 because of strong read-out. However, only a few self-referencing dyes (absorption
interferences of water and the Severinghaus electrode is and fluorescence intensity) have been published so far.32–38
strongly affected by osmotic effects. Carbon dioxide chemo- Optical carbon dioxide sensors are also of great interest in
sensors are a promising alternative,16–23 among which the biological applications.
so-called ‘‘plastic type’’ sensors are the most common ones.24–31
Here, indicators with absorption/fluorescence intensity
The core of this type of sensor is a pH-sensitive indicator dye. It maxima in the infrared (IR) or near-infrared (NIR) region are
is embedded in a polymer matrix together with a base (mainly a preferred, because of several advantages, e.g. low light scatter-
quaternary ammonium base) and responds to carbon dioxide ing, dramatically reduced autofluorescence and availability of
by changing its spectral properties according to the degree of low-cost excitation sources and photodetectors. Recently our
protonation. Read-out of these sensors can be carried out via group has presented pH-sensitive BF2-chelated tetraarylazadi-
absorption or luminescence intensity. In the case of a fluorescent pyrromethene indicators (aza-BODIPYs).38 This indicator class
represents an interesting alternative to the state-of-the-art dyes
for biological applications as SNARF indicators39 or cyanine
a Institute of Analytical Chemistry and Food Chemistry, Graz University of
dyes.40,41 The aza-BODIPYs showed absorption/fluorescence
Technology, NAWI Graz, Stremayrgasse 9, 8010 Graz, Austria.
E-mail: sergey.borisov@tugraz.at; Fax: +43 316 873 32502;
Tel: +43 316 873 32516
intensity spectra in the near-infrared region and were highly
photostable. Although pH-sensitive, these dyes could not be
used for optical carbon dioxide sensors. Here, the formation of
the ion pair was irreversible and the indicator could not be
protonated anymore, even at 100% CO2. In this study we
present a new class of pH-sensitive indicator dyes suitable for
b Max-Planck-Institute of Marine Microbiology, Celsiusstrasse 1, 28359 Bremen,
Germany
† Electronic supplementary information (ESI) available: 1H NMR and mass
spectra for the di-OH-aza-BODIPY dyes. Details on sensing properties. See DOI:
10.1039/c5tc00346f
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optical carbon dioxide sensors, the di-OH-aza-BODIPYs con- solution/suspension was acidified with 0.1 M HCl and the
taining two de-/protonatable hydroxyl groups. It will be resulting precipitate was collected by filtration and was washed
shown that the pKa values of the di-OH-aza-BODIPYs can be with water three times (3 ꢁ 100 ml). The precipitate was dried
tuned by using different substituents with either the electron- on a rotary evaporator and was used for the next step without
withdrawing or the electron-donating effect. Luminescence-based further purification (3.87 g, 89.2%).
ratiometric read-out can be realized via the absorption-
1-(4-Butoxyphenyl)-4-nitro-3-phenylbutan-1-one (compound b).
A solution of (E)-1-(4-butoxyphenyl)-3-phenylprop-2-en-1-one
(1 eq., 3.87 g, 13.8 mmol), nitromethane (20 eq., 14.8 ml,
276 mmol) and KOH (0.3 eq., 0.232 g, 4.1 mmol) in 30 ml of
absolute ethanol was heated under reflux for 12 h. After cooling
to room temperature the solvent was removed on a rotary
modulated inner-filter effect.42–45 An application example
demonstrating the carbon dioxide production/consumption of
a Hebe plant will also be presented.
Experimental
evaporator. The resulting oily residue was acidified with
Materials
0.1 M HCl and was partitioned with ethyl acetate and water
in a separating funnel. The organic layer was separated, dried
over sodium sulfate and the solvent was removed under
reduced pressure (4.035 g, 85.7%).
Ethyl cellulose (EC49, ethoxyl content 49%), thymol-blue (A.C.S.
reagent), m-cresol-purple (indicator grade), poly(styrene-co-
divinylbenzene)microspheres (8 mm mean particle size; PS-
microparticles), 30-chloro-40-hydroxyacetophenone, ammonium
acetate, benzaldehyde, N,N-diisopropylethylamine (DIPEA), dry
dichloromethane, boron trifluoride diethyl etherate, MOPS
buffer salt, sodium sulfate (anhydrous) and tetraoctylammo-
nium hydroxide solution (TOAOH, 20% in methanol) were
obtained from Sigma-Aldrich (www.sigmaaldrich.com). Deuter-
ated dimethyl sulfoxide (DMSO-d6) was purchased from Euriso-
top (www.eurisotop.com). Perfluorodecalin (98%; cis and trans),
1-butanol (99%, BuOH) and nitromethane were received
from ABCR (Germany, www.abcr.de), Hyflon AD 60 from Solvay
(www.solvay.com). Nitrogen, 2% oxygen in nitrogen, 5% carbon
dioxide in nitrogen, 0.2% carbon dioxide in nitrogen, argon
and carbon dioxide (all of 99.999% purity) were obtained
from Air Liquide (Austria, www.airliquide.at). Toluene, ethanol
(EtOH), tetrahydrofuran (THF), hydrochloric acid (37%),
dichloromethane (DCM) and hexane were purchased from
VWR (Austria, www.vwr.com). 30-methyl-4-hydroxyaceto-
phenone, 40-butoxyacetophenone and 2-fluoro-4-hydroxy-
(5Z)-5-(4-Butoxyphenyl)-N-(5-(4-butoxyphenyl)-3-phenyl-2H-pyrrol-
2-ylidene)-3-phenyl-1H-pyrrol-2-amine (c). Compound b (1 eq.,
2.5 g, 7.33 mmol) and ammonium acetate (35 eq., 19.77 g,
256 mmol) were dissolved in 50 ml of 1-butanol and the
reaction solution was heated under reflux for 24 h. The reaction
was cooled to room temperature and the solvent was removed
under reduced pressure. Then, the solid was redissolved in
DCM and washed with water three times (3 ꢁ 100 ml). The
crude solid was purified by column chromatography on silica
gel, eluting with DCM/cyclohexane (1 : 1 v/v). The product was
recrystallized from a hexane–THF mixture to give metallic
blue crystals (983 mg, 45.4%). 1H NMR (300 MHz, CDCl3)
d 8.06 (d, J = 7.1 Hz, 4H), 7.88–7.83 (m, 4H), 7.44–7.31 (m, 6H),
7.11 (s, 2H), 7.02 (d, J = 8.8 Hz, 4H), 4.06 (t, J = 6.5 Hz, 4H),
1.91–1.72 (q, J = 8.5 Hz, 4H), 1.6–1.48 (m, 4H), 1.01 (t, J =
7.4 Hz, 6H).
3,7-Bis(4-butoxyphenyl)-5,5-difluoro-1,9-diphenyl-5H-4l4,5l4-
acetophenone were from TCI Europe (www.tcichemicals.com). dipyrrolo[1,2-c:20,10-f][1,3,5,2]triazaborinine. Compound c (1 eq.,
Poly(ethylene terephthalate) (PET) support Melinex 505 was 300 mg, 0.51 mmol) was dissolved in 200 ml of dry DCM. N,N-
obtained from Pu¨tz (Germany, www.puetz-folien.com). Potas- Diisopropylethylamine (DIPEA, 10 eq., 839 ml, 5.06 mmol) and
sium chloride, potassium carbonate (pro analysi), potassium boron trifluoride diethyl etherate (15 eq., 953 ml, 7.58 mmol)
hydroxide and silica gel 60 (0.063–0.200 mm) were received were added and the reaction solution was stirred under nitro-
from Merck (www.merck.at). Sodium hydroxide, ethyl acetate, gen for 12 h. The green solution was washed with water three
the buffer salts CHES, MES and CAPS were purchased from times (3 ꢁ 200 ml) and dried over anhydrous sodium sulfate.
Roth (www.carlroth.com). De-ionized water was filtered via a The crude product was purified by column chromatography on
Barnstead NANOpure ultrapure water system. 1-(4-Hydroxy- silica gel, eluting with DCM/cyclohexane (1 : 1 v/v). The product
phenyl)-4-nitro-3-phenylbutan-1-one (compound 1b) was synthe- was recrystallized from a hexane–THF mixture to give metallic
sized according to Jokic et al.38 Silanized Egyptian blue particles red needles (179 mg, 55.2%). 1H NMR (300 MHz, CDCl3) d 8.09–
were prepared according to the literature procedure.46
8.05 (m, 8H), 7.49–7.38 (m, 6H), 7.04–6.98 (m, 6H), 4.07–4.02
Synthesis of 3,7-bis(4-butoxyphenyl)-5,5-difluoro-1,9-diphenyl- (t, J = 6.4 Hz, 4H), 1.58–1.45 (q, J = 14.4 Hz, 4H), 1.62–1.42
5H-4k4,5k4-dipyrrolo[1,2-c:20,10-f ][1,3,5,2]triazaborinine (di-butoxy- (m, 4H), 1.02–0.97 (t, J = 7.4 Hz, 6H). Electron impact-direct
complex)
insertion-time of flight (DI-EI-TOF): m/z of [M]+ was found to be
(E)-1-(4-Butoxyphenyl)-3-phenylprop-2-en-1-one (compound a). 641.3007, calc. 641.3032.
40-Butoxyacetophenone (1 eq., 2.00 g, 15.5 mmol) was dissolved
Synthesis of 4,40-(5,5-difluoro-1,9-diphenyl-5H-4k4,5k4-dipyrrolo-
in absolute ethanol (15 ml). Benzaldehyde (1 eq., 1580 ml, [1,2-c:20,10-f][1,3,5,2]triazaborinine-3,7-diyl)bis(2-chlorophenol)
15.5 mmol) and potassium hydroxide (3 eq., 2.61 g, 46.4 mmol (di-Cl-di-OH-complex)
in 5 ml H2O) were added to the solution and the reaction was
1-(3-Chloro-4-hydroxyphenyl)-3-phenylpropenone (compound 1a).
stirred at room temperature for 12 h. Then, the reaction 30-Chloro-40-hydroxyacetophenone (1 eq., 2 g, 11.7 mmol) and
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benzaldehyde (1 eq., 1.24 g, 11.7 mmol) were dissolved in 10 ml of 20-fluoro-40-hydroxyacetophenone (728 mg). Steps 1 and 2
ethanol absolute. 10 ml of aqueous potassium hydroxide solution yielded 1.033 g (90.2%) and 1.28 g (99.4%) of the crude product,
(3 eq., 1.96 g, 35.1 mmol) were added dropwise. The resulting respectively. After synthesis step 3, the crude product was
solution was stirred for 8–12 h, during which the product pre- purified by column chromatography on silica gel, eluting with
cipitated as a potassium salt. The solution/suspension was poured 4% THF/DCM to yield [5-(2-fluoro-4-hydroxyphenyl)-3-phenyl-
into 10 ml of hydrochloric acid (1 M) and further concentrated 1H-pyrrol-2-yl]-[5-(2-fluoro-4-hydroxyphenyl)-3-phenylpyrrol-2-
hydrochloric acid was added until the solution became acidic. The ylidene]amine 2c as a blue-black solid. The product was
obtained yellow solid was washed with water and used for further recrystallized from a hexane–THF mixture to give metallic green
1
synthesis without purification (2.33 g, 77%).
crystals (0.23 g, 20.9%). H NMR (300 MHz, DMSO-d6) d 8.04–
8.06 (d, J = 7.0 Hz, 6H), 7.35–7.47 (m, 8H), 6.78–6.86 (m, 4H).
DI-EI-TOF: m/z of [M]+ found 517.1626, calc. 517.1602.
1-(3-Chloro-4-hydroxyphenyl)-4-nitro-3-phenylbutan-1-one
(compound 1b). A solution of compound 1a (1 eq., 2 g,
7.7 mmol), nitromethane (20 eq., 8.35 ml, 154.7 mmol) and
potassium hydroxide (1.2 eq., 0.52 g, 9.28 mmol) in 10 ml of
ethanol was heated at 60 1C under reflux for 12 h. After cooling
to room temperature, the solvent was removed in vacuo and the
oily residue obtained was acidified with hydrochloric acid (4 M)
and partitioned between ethyl acetate (50 ml) and de-ionized
water (50 ml). The organic layer was separated, dried over
anhydrous sodium sulfate and evaporated under reduced pres-
sure. The obtained product was used for further synthesis
without purification (2.4 g, 97.2%).
4,40-(5,5-Difluoro-1,9-diphenyl-5H-4l4,5l4-dipyrrolo[1,2-c:20,10-f]-
[1,3,5,2]triazaborinine-3,7-diyl)bis(3-fluorophenol) (di-F-di-OH-complex).
4,40-(5,5-Difluoro-1,9-diphenyl-5H-4l4,5l4-dipyrrolo[1,2-c:20,10-f]-
[1,3,5,2]triazaborinine-3,7-diyl)bis(3-fluorophenol) (di-F-di-OH-
complex) was synthesized using the same procedure. Purification
was carried out via column chromatography on silica gel, eluting
with 4% THF/DCM. Recrystallization from hexane–THF gave
the final product di-F-di-OH-complex as a metallic red solid
(0.044 g, 19.6%). 1H NMR (300 MHz, DMSO-d6) d 8.12 (d, J =
7.0 Hz, 4H), 7.82 (t, J = 8.7 Hz, 2H), 7.56–7.48 (q, J = 9.4, 7.9 Hz,
6H), 7.31 (d, J = 3.4 Hz, 2H), 6.77 (d, J = 10.7 Hz, 4H). DI-EI-TOF:
m/z of [M]+ found 565.1615, calc. 565.1591.
[5-(3-Chloro-4-hydroxyphenyl)-3-phenyl-1H-pyrrol-2-yl]-[5-(3-chloro-
4-hydroxyphenyl)-3-phenylpyrrol-2-ylidene]amine (compound 1c).
Compound 1b (1 eq., 2.02 g, 8.76 mmol) and ammonium
acetate (35 eq., 19.03 g, 307 mmol) in 50 ml of 1-butanol were
heated under reflux for 24 h. The reaction was cooled to room
temperature, the salt was removed by extraction with de-ionized
water/DCM and the product was purified by column chromato-
graphy on silica gel, eluting with 5% ethyl acetate/DCM (after
eluting impurities with hexane, 20% DCM/hexane, 1% ethyl
acetate/DCM and 2% ethyl acetate/DCM) to yield 1c as a blue-
black solid. The product was recrystallized from a hexane–THF
mixture to give metallic green crystals (0.097 g, 5.77%). 1H NMR
(300 MHz, DMSO-d6) d 8.09 (d, J = 7.6 Hz, 6H), 7.90 (d, J =
8.5 Hz, 2H), 7.62 (s, 2H), 7.54–7.32 (m, 6H), 7.19 (d, J = 8.5 Hz,
2H). EI-DI-TOF: m/z [MH]+ found 549.1007, calc. 549.1011.
Synthesis of 4,40-(5,5-difluoro-1,9-diphenyl-5H-4k4,5k4-dipyrrolo-
[1,2-c:20,10-f][1,3,5,2]triazaborinine-3,7-diyl)diphenol (di-OH-
complex). Compound 3a 1-(4-hydroxyphenyl)-3-phenylpropenone
was commercially available. Further synthesis steps were per-
formed analogously to the synthesis steps of the di-Cl-di-OH-
complex. The synthesis of compound 3b gave 2.06 g (80%) of
the crude product, starting from 2.02 g of 40-hydroxychalcone.
The crude 3c was purified by column chromatography on silica
gel, eluting with 4% THF/DCM to yield [5-(4-hydroxyphenyl)-3-
phenyl-1H-pyrrol-2-yl]-[5-(4-hydroxyphenyl)-3-phenylpyrrol-2-ylidene]-
amine as a blue-black solid. The product was recrystallized
from a hexane–THF mixture to give metallic green crystals (0.41 g,
23.5%). 1H NMR (300 MHz, DMSO-d6), d 8.09 (d, J = 7.3 Hz, 4H),
7.93 (d, J = 8.6 Hz, 4H), 7.53 (s, 2H), 7.42 (m, 6H), 7.01 (d, J =
4,40-(5,5-Difluoro-1,9-diphenyl-5H-4l4,5l4-dipyrrolo[1,2-c:20,10-f]- 8.6 Hz, 4H). EI-DI-TOF: m/z [MH]+ found 481.1773, calc. 481.179.
[1,3,5,2]triazaborinine-3,7-diyl)bis(2-chlorophenol) (di-Cl-di-OH-
complex). Compound 1c (0.017 g, 0.07 mmol) was dissolved in
Purification of 4,40-(5,5-difluoro-1,9-diphenyl-5H-4l4,5l4-dipyrrolo-
50 ml of dry DCM, treated with DIPEA (10 eq., 0.053 ml, [1,2-c:20,10-f][1,3,5,2]triazaborinine-3,7-diyl)diphenol. Purification
0.32 mmol) and boron trifluoride diethyletherate (15 eq., of 4,40-(5,5-difluoro-1,9-diphenyl-5H-4l4,5l4-dipyrrolo[1,2-c:20,10-f]-
0.061 ml, 0.48 mmol) and stirred under argon for 24 h. [1,3,5,2]triazaborinine-3,7-diyl)diphenol was carried out via
Purification was carried out via column chromatography on silica column chromatography on silica gel, eluting with DCM/ethyl
gel, eluting with 2% ethanol/DCM (after eluting impurities with acetate (4 : 1). Recrystallization from hexane–THF gave the final
DCM). Recrystallization from hexane–THF gave the final pro- product di-OH-complex as a metallic red solid (0.26 g, 57.8%).
duct di-Cl-di-OH-complex as a metallic red solid (0.008 g, 44%). 1H NMR (300 MHz, DMSO-d6) d 8.13 (dd, J = 18.6, 8.0 Hz, 8H),
1H NMR (300 MHz, DMSO-d6) d 8.29 (s, 2H), 8.16 (d, J = 7.0 Hz, 7.65–7.39 (m, 8H), 6.95 (d, J = 8.8 Hz, 4H). EI-DI-TOF: m/z [MH]+
4H), 8.03 (d, J = 8.7 Hz, 2H), 7.65 (s, 2H), 7.60–7.41 (m, 6H), found 529.1771, calc. 529.1779.
7.13 (d, J = 8.7 Hz, 2H). EI-DI-TOF: m/z [MH]+ found 597.0968,
calc. 597.0999.
Synthesis of the 4,40-(5,5-difluoro-1,9-diphenyl-5H-4k4,
5k4-dipyrrolo[1,2-c:20,10-f][1,3,5,2]triazaborinine-3,7-diyl)bis-
Synthesis of 4,40-(5,5-difluoro-1,9-diphenyl-5H-4k4,5k4-dipyrrolo- (2-methylphenol) (di-CH3-di-OH-complex). The synthesis was
[1,2-c:20,10-f][1,3,5,2]triazaborinine-3,7-diyl)bis(3-fluorophenol) carried out analogously to that of the di-Cl-di-OH-complex,
(di-F-di-OH-complex). The synthesis was performed analo- starting from 30-methyl-40-hydroxyacetophenone (1.88 g). Steps
gously to that of the di-Cl-di-OH-complex, but starting from 1 and 2 yielded 2.77 g (93%) and 1.84 g (53%) of the crude
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product, respectively. After synthesis step 3, the crude product the Hyflon solution. The thickness of this layer after evapora-
was purified by column chromatography on silica gel, eluting tion of the solvent was estimated to be B4.5 mm.
with 5% ethyl acetate/DCM (after eluting impurities with
Methods
hexane/DCM 1 : 1, DCM) to yield [5-(3-methyl-4-hydroxyphenyl)-3-
phenyl-1H-pyrrol-2-yl]-[5-(3-methyl-4-hydroxyphenyl)-3-phenylpyrrol-
2-ylidene]amine 4c as a blue-black solid. The product was
recrystallized from a hexane–THF mixture to give metallic green
crystals (0.091 g, 5.8%). 1H NMR (300 MHz, DMSO-d6) d 8.10
(d, J = 7.3 Hz, 4H), 7.91–7.75 (m, 4H), 7.57–7.33 (m, 8H), 7.00
(d, J = 8.4 Hz, 2H), 2.29 (s, 6H). EI-DI-TOF: m/z [MH]+ found
509.2086, calc. 509.2103.
1H NMR spectra were recorded on a 300 MHz instrument
(Bruker) in DMSO-d6 with TMS as standard. Absorption spectra
were recorded on a Cary 50 UV-Vis spectrophotometer (Varian).
The determination of the molar absorption coefficients was
carried out as an average of three independent measurements.
Fluorescence spectra were recorded on a Fluorolog3 fluores-
cence spectrometer (Horiba) equipped with a NIR-sensitive
photomultiplier R2658 from Hamamatsu (300–1050 nm).
Photobleaching experiments were performed by irradiating
the sensor foils in a glass cuvette with the light of a high-
power 10 W LED array (lmax 458 nm, 3 LEDs, www.LED-TECH.
de) operated at 6 W input power. A lens (Edmund Optics) was
used to focus the light of the LED array on the glass cuvette
(photon flux: B3900 mmol sꢀ1 mꢀ2 as determined using a
Li-250A light meter from Li-COR). The photodegradation pro-
files were obtained by monitoring the absorption spectra of a
sensor foil based on the respective dye. The cuvette was flushed
with either carbon dioxide gas for the mono-anionic form, or
with nitrogen for the di-anionic form and sealed. Thymol-blue
and m-cresol-purple were used for comparison. In the case of
the anionic form of thymol-blue, m-cresol-purple, the di-OH-
complex and the di-CH3-di-OH-complex (di-anionic forms)
sodium hydroxide was placed at the bottom of the cuvette to
capture carbon dioxide traces. Gas calibration mixtures were
obtained using a gas mixing device from MKS (www.mksinst.com).
The gas mixture was humidified to about 85% relative humidity,
using silica gel soaked with a saturated potassium chloride
solution, prior to entering the calibration chamber. Temperature
was controlled by a cryostat ThermoHaake DC50. Dyed parti-
cles were filtered through cellulose filters type 113A from Roth
(www.carlroth.com). Particle milling was carried out using
an 80 ml milling cup, zirconia spheres (+ 5 mm) and a
Pulverisette 6 planet mill from Fritsch (www.fritsch.de). For
the determination of the pKa values titration curves in aqueous
buffer/ethanol mixtures (1 : 1) were measured and the average
value of the point of inflection obtained was used. For the
production/consumption measurements of carbon dioxide
two Firesting-Mini devices and an oxygen trace sensor from
PyroScience (www.pyro-science.com) were used. Illumination
of the sample was performed using 3 commercially avail-
able halogen bulb lamps with an averaged photon flux of
B217 mmol sꢀ1 mꢀ2 per lamp. The plant was positioned in
a glass desiccator, which was flushed with a gas mixture of 2%
oxygen in nitrogen.
Purification of 4,40-(5,5-difluoro-1,9-diphenyl-5H-4l4,5l4-dipyrrolo-
[1,2-c:20,10-f][1,3,5,2]triazaborinine-3,7-diyl)bis(2-methylphenol).
Purification of 4,40-(5,5-difluoro-1,9-diphenyl-5H-4l4,5l4-dipyrrolo-
[1,2-c:20,10-f][1,3,5,2]triazaborinine-3,7-diyl)bis(2-methylphenol)
was carried out via column chromatography on silica gel,
eluting with 5% ethyl acetate/DCM (after eluting impurities
with DCM, 1% ethyl acetate/DCM and 2% ethyl acetate/DCM).
Recrystallization from hexane–THF gave the final product di-CH3-
1
di-OH-complex as a metallic red solid (0.04 g, 40%). H NMR
(300 MHz, DMSO-d6) d 8.16 (d, J = 7.2 Hz, 4H), 8.08–7.90
(m, 4H), 7.52 (m, 8H), 6.95 (d, J = 8.5 Hz, 2H), 2.22 (s, 6H).
EI-DI-TOF: m/z [MH]+ found 557.2082, calc. 557.2092.
Staining of PS-microparticles
0.50 g of PS-particles were dispersed in a solution of 5 mg of
(1% w/w in respect to the PS-particles) the di-butoxy-complex in
10 ml of tetrahydrofuran (THF) and stirred for 1.5 h. Then 8 ml
of de-ionized water were added dropwise. After 10 min of
stirring the dispersion was transferred rapidly into a beaker
with 50 ml of de-ionized water. The dispersion was again stirred
for 10 min. Then the particles were filtered through a cellulose
filter and rewashed with 20 ml of ethanol. Afterwards the
particles were transferred into a milling cup and were overlaid
with a mixture of ethanol/de-ionized water (1 : 1) to reduce
friction and to enhance heat dissipation during grinding in
the planet mill. The particles were washed with ethanol and
dried in the oven at 70 1C.
Sensor preparation
Absorption: a ‘‘cocktail’’ containing 100 mg of ethyl cellulose
and 1 mg of dye (1% w/w with respect to the polymer) dissolved
in a toluene : ethanol mixture (6 : 4 v/v, 1.9 g) was purged with
carbon dioxide. Afterwards 100 ml of tetraoctylammonium
hydroxide solution (20% w/w TOAOH in MeOH) were added.
The ‘‘cocktail’’ was knife-coated on a dust-free PET support. A
sensing film of B7 mm thickness was obtained after evapora-
tion of the solvent. For luminescence-based measurements a
second layer was added to the absorption-based sensing foils.
To prepare the ‘‘cocktail’’ for the second layer, 0.180 g of Hyflon
AD 60 were dissolved in 2.820 g of perfluorodecalin, which was
washed prior to use with 1 mol lꢀ1 potassium carbonate solution.
Results and discussion
Synthesis
Then 0.360 g of Egyptian blue powder (200% w/w with respect As reported by Jokic et al.38 there are two ways for the prepara-
to the polymer) and 0.054 g of stained PS-particles (30% w/w tion of azadipyrromethenes to obtain either asymmetrical47
with respect to the polymer) were dispersed homogeneously in or symmetrical48 chromophores. The previously published
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asymmetrical dyes bearing one hydroxyl group were proved to
be promising pH indicators.38 Unfortunately, they were found
to be unsuitable for optical carbon dioxide sensors. The ion
pair built between the hydroxyl group and the tetraoctylammo-
nium base was comparatively strong. Even after exposure of the
sensors to pure carbon dioxide, the amount of CO2 was not
enough to achieve re-protonation of the indicators and only
after exposure to strong acid vapors (e.g. hydrochloric acid) the
indicators were irreversibly re-protonatable.
Dissociation of both hydroxyl groups in symmetrical azadi-
pyrromethenes was expected to be more difficult compared to
the mono-hydroxy derivatives since two ion pairs with bulky
cations would be built upon deprotonation. Therefore, this new
class of pH indicators potentially suitable for CO2 sensing was
investigated. The starting point of our synthesis was diaryl-
a,b-unsaturated ketones (chalcones) either commercially available
or readily made by an aldol condensation of the corresponding
aldehyde and acetophenone (Scheme 1). The Michael addition
of nitromethane to the chalcones, using KOH as a base, yields
1,3-diaryl-4-nitrobutan-1-ones in essentially quantitative yields.
Their condensation with ammonium acetate in refluxing butanol
gave azadipyrromethenes. Finally, the azadipyrromethenes were
Fig. 1 (A) Absorption spectra of the neutral (black line, pH 6.4), the mono-
anionic (red line, pH 9.3) and the di-anionic (blue line, pH 12.8) forms of the
di-OH-complex dissolved in the ethanol/aqueous buffer mixture (1 : 1) at
25 1C and (B) the titration curves for pKa1 determined at 745 nm (red) and
pKa2 at 805 nm (blue), respectively.
converted to the corresponding BF2 chelates via reaction with
BF2-etherate using diisopropylethylamine as a base at room
temperature for 24 h, giving yields ranging from 20 to 70%.
Photophysical properties
Dissolved in an ethanol/aqueous buffer mixture (1 : 1) all di-OH-
aza-BODIPY dyes showed absorption spectra corresponding to
the two protonation steps of the hydroxyl groups (Fig. 1). The
neutral forms showed maxima at 670–700 nm and the mono-
anionic forms at 725–760 nm. The di-anionic forms were again
bathochromically shifted for approximately 60 nm (lmax 785–
830 nm). Generally, the di-CH3-di-OH-complex bearing electron-
donating methyl groups showed absorption maxima shifted to
higher wavelengths compared to the non-substituted di-OH-
complex (Table 1). In contrast, complexes bearing electron-
withdrawing groups displayed absorption maxima at shorter
wavelengths.
An overview of the spectroscopic properties of the presented
dyes is given in Table 1. Molar absorption coefficients are rather
high: 55 000–77 000 Mꢀ1 cmꢀ1. pKa determination was carried
out in a mixture of ethanol/aqueous buffer solution (1 : 1). For
all di-OH-aza-BODIPY dyes two protonation steps can be
observed (Table 1). Here, the complex with electron-donating
methyl groups in the ortho-position to the hydroxyl groups
showed the highest pKa values, followed by the non-substituted
di-OH-complex. Electron-withdrawing chlorine atoms located
in the proximity of the hydroxyl groups (ortho-position) have the
strongest impact on the pKa values, whereas the electron-
withdrawing effect of fluorine atoms in the meta-position is
significantly lower.
Carbon dioxide sensors
The pH-sensitive di-OH-aza-BODIPY dyes were embedded in an
ethyl cellulose matrix (EC49) along with tetraoctylammonium
hydroxide as a base to obtain optical carbon dioxide sensors.
Clearly, the sensors showed well-observable spectral changes in
the near infrared (NIR) range according to the protonation of
the di-anionic form giving the mono-anionic form (Fig. 2). The
neutral forms of the di-OH-complexes showed slight fluores-
cence in solution, and the sensors based on the indicators did
not show detectable fluorescence, neither for the mono-anionic
Scheme 1 4-Step-synthesis of the di-OH-aza-BODIPY chelates.
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nor for the di-anionic form. Compared to the measurements in
solution the CO2 sensors based on the di-OH-aza-BODIPY dyes
showed significantly bathochromically shifted absorption spec-
tra with maxima at 760–780 nm for their mono-anionic forms
and maxima at 860–910 nm for their di-anionic forms.
Generally, a similar trend for the absorption maxima of the
CO2 sensors based on the respective complexes was observed
(Table 1). Here, the shift between the mono-anionic and the
di-anionic form was enlarged to over 100 nm. This fact is
advantageous for optical sensors, because peak separation
becomes easier.
Despite the fact that all di-OH-aza-BODIPY dyes showed two
protonation steps in aqueous solution, only one protonation
step was observed in optical sensors based on these indicators.
In fact, in the absence of carbon dioxide both hydroxyl groups
build ion pairs with the quaternary ammonium base and the
spectra of the di-anionic forms are observable. The mono-
anionic form is built in the presence of carbon dioxide and is
stable even at 100% CO2. Spectra of the neutral forms are not
observable anymore (Fig. 2). Therefore, the pKa2 values mea-
sured in solution are most relevant for optical carbon dioxide
sensors based on the di-OH-aza-BODIPY dyes. Indeed, these
values correlate very well with the sensitivities of the optical
CO2 sensors (Table 1 and Fig. 3). The sensitivity increases in the
following order: di-Cl-di-OH o di-F-di-OH o di-OH o di-CH3-
di-OH. Notably, for the most sensitive sensor based on di-CH3-
di-OH about 25% of the overall signal change is observed already
at atmospheric pCO2 which is, to the best of our knowledge,
one of the highest sensitivities reported so far. Di-F-di-OH and
di-Cl-di-OH complexes bearing electron-withdrawing groups
displayed diminished pKa2 values of 9.35 and 8.72, respectively,
Fig. 2 Absorption spectra of optical carbon dioxide sensors based on (A)
the di-Cl-di-OH-complex and (B) the di-CH3-di-OH-complex in EC49
(base: TOAOH) at 25 1C at various pCO2 values.
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Fig. 3 (A) Calibration curves (absorption–absorption at 0 kPa of the mono-anionic form) for the carbon dioxide sensors based on the di-OH-complex
(black) and the di-CH3-di-OH-complex (green) and (B) di-Cl-di-OH-complex (blue) and the di-F-di-OH-complex (red) 25 1C under humid conditions
with the respective ‘‘zoom-in’’ sections (C and D).
and enabled measurements up to 100% CO2. These great with pure carbon dioxide (mono-anionic form) or with pure
differences in sensitivity lead to a broad range of applications, nitrogen (di-anionic form). Clearly, all of the di-OH-aza-BODIPY
varying from food packaging and capnography for di-F-di-OH dyes showed outstanding photostability, much better than the
and di-Cl-di-OH complexes to environmental monitoring for reference indicators embedded in the same ethyl cellulose
di-OH and di-CH3-di-OH complexes.
matrix. After 1.5 h of illumination the presented dyes showed
Fig. 3 shows the increasing absorption of the mono-anionic hardly any photobleaching effects for both the mono-anionic
form in relation to the absorption at 0 kPa (Abs–Abs0) carbon and di-anionic forms, whereas m-cresol-purple (neutral form),
dioxide (see Fig. 2B). The higher the sensitivity of the sensor, thymol-blue (neutral form) and HPTS (only anionic form) were
the steeper the respective calibration curves, as shown in Fig. 3, degraded to less than 80%. The anionic forms of m-cresol-
and the lower the amount of carbon dioxide necessary to fully purple and thymol-blue were even less photostable than there
protonate the sensor. Sensors based on the di-Cl-di-OH-complex neutral forms (ESI,† Fig. S1).
and the di-F-di-OH-complex showed limits of detection (LOD)
of 0.19 kPa and 0.18 kPa, respectively. However, determining
the LOD value for very sensitive sensors can be very challen-
Luminescence-based ratiometric read-out using IFE
(inner-filter effect) based sensors
ging. For measurements of low levels of carbon dioxide the Read-out of the planar optodes and fibre-optic sensors based
measuring system (gas mixer, gas lines, flow cell, etc.) has to be on colorimetric systems is significantly more challenging than
completely decarbonized, which is very difficult to achieve in that of the luminescent systems. The inner-filter effect (IFE)
reality. Especially for atmospheric levels of pCO2 (0.04 kPa E was made use of in order to enable read-out via luminescence.
400 ppm in the gas phase E13.6 mmol lꢀ1 in water at 298.15 K) Additionally to the first layer containing the absorption-based
and below, traces of environmental carbon dioxide disturbed indicator dye along with TOAOH in ethyl cellulose, a second
the measurements for sensors based on the di-CH3-di-OH- layer containing the secondary emitters was used.42 It included
complex and the di-OH-complex. Hence, the determined LODs the pH-insensitive di-butoxy-complex entrapped in PS-particles
for these two complexes (0.011 kPa for di-OH and 0.007 kPa for as a fluorophore and Egyptian blue (EB) as a phosphor, both
di-CH3-di-OH) were only rough estimations.
embedded in Hyflon AD 60. The absorption spectra of the
Photodegradation profiles of the carbon dioxide sensors di-OH-complex and the emission spectra of the secondary
were obtained from the absorption spectra after illuminating emitters are shown in Fig. 4A. Here, the broad emission band of
the sensor foils with a high-power LED array (lmax 458 nm). For the phosphor (EB) overlaps with the absorption spectrum of
comparison, the state-of-the-art indicator dyes such as m-cresol- the di-anionic form of the indicator and the emission band of the
purple, thymol-blue and HPTS were used (ESI,† Fig. S1). The fluorophore (di-butoxy-aza-BODIPY-complex) overlaps with the
photostability of both the mono-anionic and the di-anionic absorption spectrum of the mono-anionic form of the indica-
form were investigated. Therefore, the cuvette was filled either tor. The isosbestic point of the di-OH-indicator is located at
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Fig. 5 (A) Experimental set-up and (B) carbon dioxide and oxygen
dynamics in a desiccator containing a Hebe plant during illumination
(yellow zones) and in darkness (gray zones).
Fig. 4 (A) Emission spectra (lexc 610 nm) of Egyptian blue (EB; blue
dashed line; ‘‘Emission Phosphor’’) and the di-butoxy-complex (dissolved
in tetrahydrofuran; magenta dashed line; ‘‘Emission Fluorophore’’);
absorption spectra of the sensor based on the di-OH-complex in the
absence (black line) and presence (red line) of carbon dioxide at 25 1C.
(B) Emission spectra (lexc 610 nm) of a carbon dioxide sensor based on
the di-OH complex and inner-filter effect read-out at 25 1C under humi-
dified conditions.
carbon dioxide was observed (Fig. 5). In darkness an increase of
carbon dioxide occurred due to respiration, whereas during
illumination the concentration of CO2 decreased. Over the
whole measurement more CO2 was produced than consumed
which may be due to stress-induced respiration of the Hebe
plant. Corresponding to the applied light sequences also the
oxygen concentration was affected. Note that over the whole
experiment the oxygen concentration was increasing which can
be explained by slow diffusion of oxygen into the desiccator.
Surprisingly, oxygen concentration did not increase signifi-
cantly during the light phase, but it increased abruptly imme-
diately after the light was switched off, reaching a plateau after
about 30 min (Fig. 5). This effect may be attributed to the
storage of generated oxygen in the plant and its release in the
beginning of the dark phase. After this time the equilibrium
between oxygen consumption during respiration and oxygen
diffusion from outside is reached. As can be seen, the same
phenomenon is observed if both the light and dark cycles are
extended to 60 min (see hours 2 to 4 in Fig. 5). The above plant
behavior is beyond the scope of the paper and calls for more
detailed investigation.
610 nm and represents an ideal wavelength for exciting the
secondary emitters matching the maxima of the red LEDs
available (605, 617 nm). The fluorescence spectra of the sensor
based on the combination of EB, the di-butoxy-complex and the
non-substituted di-OH indicator are shown in Fig. 4B. Emission
peaks in the absence (lmax 913 nm) and presence (lmax 738 nm)
of carbon dioxide were well observable. Hence, luminescence-
based ratiometric read-out becomes possible. This can be realized
either by using two emission filters isolating the respective
components or by measuring the luminescence phase shift. In
fact, the phase shift for luminescent Egyptian blue is 55.81 at
2000 Hz and the phase shift of the fluorophore is 0 at this
modulation frequency.
Carbon dioxide production/consumption of a Hebe plant
The applicability of the presented carbon dioxide sensors is
demonstrated by showing the respiration of a Hebe plant. The
Conclusion
Hebe plant in soil, a carbon dioxide sensor based on the di-CH3- A new class of colorimetric pH-sensitive indicators for carbon
di-OH-complex and an oxygen trace sensor were placed in a dioxide sensors has been presented. The di-OH-aza-BODIPY
desiccator. The desiccator was purged for 15 min with a gas dyes show characteristic CO2-dependent absorption spectra in
mixture of 2% oxygen in nitrogen (in order to achieve better the near-infrared region. In addition to the remarkable photo-
dynamics when measuring with an optical oxygen sensor com- stability of the indicators and the high molar absorption
pared to air saturation) and then closed tightly. During the coefficients, the dynamic ranges of the sensors can be tuned
measurement the setup was alternately kept in darkness and via electron-donating and electron-withdrawing substituents.
illuminated for 30 min using three halogen lamps. According to This enables a broad range of applications from environmental
these illumination sequences the production/consumption of monitoring to food packaging or capnography. The sensors
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18 R. Ali, T. Lang, S. M. Saleh, R. J. Meier and O. S. Wolfbeis,
Anal. Chem., 2011, 83, 2846–2851.
19 S. Pandey, S. N. Baker, S. Pandey and G. A. Baker, Chem.
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20 W. Hong, Y. Chen, X. Feng, Y. Yan, X. Hu, B. Zhao, F. Zhang,
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8229–8231.
based on di-CH3-di-OH-aza-BODIPYs are the most sensitive
sensors ever reported and resolve well below atmospheric CO2
levels. The absorption-modulated inner-filter effect was used to
enable referenced luminescence-based ratiometric read-out. As
an example the production/consumption of carbon dioxide of a
Hebe plant was demonstrated.
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Acknowledgements
This work was supported by 7th framework EU projects
ECO2 (project number 265847) and SenseOCEAN (project
number 614141).
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