AZOCALIXARENE PROBE AND ITS USE FOR DETECTING CARBON DIOXIDE

Abstract

The disclosure describes a probe comprising an azo-calixarene complexed with an anion. The anion may be a fluoride ion or a carbonate ion. The probe may be used to sense and/or capture carbon dioxide.

Claims

1. A probe comprising an azo-calixarene complexed with an anion.

2. The probe according to claim 1, wherein the anion is a fluoride ion or a carbonate ion.

3. The probe according to claim 1, wherein the probe comprises at least 1 mole of the anion for each mole of azo-calixarene.

4. The probe according to claim 1, wherein the azo-calixarene complexed with the anion is dissolved in a solvent.

5. The probe according to claim 4, wherein the amount of the azo-calixarene dissolved in the solvent comprises a concentration of between 1×10.sup.−6 and 1×10.sup.−3 mol dm.sup.−3 of the azo-calixarene.

6. The probe according to claim 4, wherein the amount of the anion dissolved in the solvent comprises a concentration of between 1×10.sup.−3 and 0.1 mol dm.sup.−3 of the anion.

7. The probe according to claim 1, wherein the probe comprises a solid.

8. The probe according to claim 1, wherein the azo-calixarene is an azo-calix[4]arene.

9. The probe according to claim 1, wherein the azo-calixarene is a compound of formula (I): ##STR00012## wherein R.sup.1 is selected from the group consisting of hydrogen, an optionally substituted C.sub.1-C.sub.10 alkyl, an optionally substituted C.sub.2-C.sub.10 alkenyl, an optionally substituted C.sub.2-C.sub.10 alkynyl, NR.sup.11R.sup.12, an optionally substituted C.sub.5-C.sub.10 aryl and an optionally substituted 3 to 10 membered heteroaryl; R.sup.2 is selected from the group consisting of hydrogen, an optionally substituted C.sub.1-C.sub.10 alkyl, an optionally substituted C.sub.2-C.sub.10 alkenyl, an optionally substituted C.sub.2-C.sub.10 alkynyl, a halogen, OH, (CR.sup.9R.sup.10).sub.aSO.sub.2OR.sup.9, (CR.sup.9R.sup.10).sub.aSO.sub.2NR.sup.9R.sup.10, NO.sub.2, (CR.sup.9R.sup.10).sub.aPO.sub.2OH, (CR.sup.9R.sup.10).sub.aCOOR.sup.9, (CR.sup.9R.sup.10).sub.aSS(CR.sup.9R.sup.10).sub.bCOOR.sup.9, (CR.sup.9R.sup.10).sub.aNR.sup.9R.sup.10 and N═NR.sup.9; R.sup.3 and R.sup.4 are each independently selected from the group consisting of hydrogen, an optionally substituted C.sub.1-C.sub.10 alkyl, an optionally substituted C.sub.2-C.sub.10 alkenyl, an optionally substituted C.sub.2-C.sub.10 alkynyl, (CR.sup.9R.sup.10).sub.aNR.sup.9R.sup.10), (CR.sup.9R.sup.10).sub.aOR.sup.9, (CR.sup.9R.sup.10).sub.aCOR.sup.9, (CR.sup.9R.sup.10).sub.aCOOR.sup.9, (CR.sup.9R.sup.10).sub.aCONR.sup.9R.sup.10, (CR.sup.9R.sup.10).sub.aSO.sub.2OR.sup.9, (CR.sup.9R.sup.10).sub.aCOO(CR.sup.9R.sup.10).sub.bOR.sup.9, (CR.sup.9R.sup.10).sub.aCOO(CR.sup.9R.sup.10).sub.bCOR.sup.9 and (CR.sup.9R.sup.10).sub.aCOO(CR.sup.9R.sup.10).sub.bSR.sup.9; R.sup.5, R.sup.6, R.sup.7 and R.sup.8 are each independently selected from the group consisting of hydrogen, a halogen, an optionally substituted C.sub.1-C.sub.10 alkyl, an optionally substituted C.sub.2-C.sub.10 alkenyl and an optionally substituted C.sub.2-C.sub.10 alkynyl; the or each R.sup.9 and R.sup.10 are independently selected from the group consisting of hydrogen, an optionally substituted C.sub.1-C.sub.10 alkyl, an optionally substituted C.sub.2-C.sub.10 alkenyl, an optionally substituted C.sub.2-C.sub.10 alkynyl, NR.sup.11R.sup.12, an optionally substituted C.sub.5-C.sub.10 aryl and an optionally substituted 3 to 10 membered heteroaryl; an optionally substituted C.sub.5-C.sub.10 aryl or an optionally substituted 3 to 10 membered heteroaryl is unsubstituted or substituted with one or more substituents selected from the group consisting of an optionally substituted C.sub.1-C.sub.10 alkyl, an optionally substituted C.sub.2-C.sub.10 alkenyl, an optionally substituted C.sub.2-C.sub.10 alkynyl, a halogen, ORE, NO.sub.2, CN, COOR.sup.11 and NR.sup.11R.sup.12; each R.sup.11 and R.sup.12 are independently selected from the group consisting of hydrogen, an optionally substituted C.sub.1-C.sub.10 alkyl, an optionally substituted C.sub.2-C.sub.10 alkenyl and an optionally substituted C.sub.2-C.sub.10 alkynyl; an optionally substituted C.sub.1-C.sub.10 alkyl, an optionally substituted C.sub.2-C.sub.10 alkenyl or an optionally substituted C.sub.2-C.sub.10 alkynyl is unsubstituted or substituted with one or more substituents selected from the group consisting of halogen, OH, NH.sub.2, CONH.sub.2, COOH, CN, a C.sub.5-C.sub.10 aryl, a 3 to 10 membered heteroaryl, a C.sub.3-C.sub.6 cycloalkyl and a 3 to 8 membered heterocycle; a and b are each independently an integer between 0 and 6; and m is an integer between 1 and 8; n is an integer between 0 and 7; and p is an integer between 1 and 4; wherein the total of (m+n)xp is an integer between 4 and 8; or a salt, solvate or tautomeric form thereof.

10. The probe according to claim 9, wherein R.sup.1 is an optionally substituted C.sub.5-C.sub.10 aryl or an optionally substituted 3 to 10 membered heteroaryl.

11. The probe according to claim 10, wherein R.sup.1 is a C.sub.5-C.sub.10 aryl or a 3 to 10 membered heteroaryl wherein the aryl or heteroaryl are substituted with COOR.sup.11 and/or NO.sub.2.

12. The probe according to claim 9, wherein R.sup.3 is selected from the group consisting of hydrogen, an optionally substituted C.sub.1-C.sub.10 alkyl, an optionally substituted C.sub.2-C.sub.10 alkenyl and an optionally substituted C.sub.2-C.sub.10 alkynyl.

13. The probe according to claim 9, wherein R.sup.5 and R.sup.6 are each independently selected from the group consisting of hydrogen, an optionally substituted C.sub.1-C.sub.10 alkyl, an optionally substituted C.sub.2-C.sub.10 alkenyl and an optionally substituted C.sub.2-C.sub.10 alkynyl.

14. The probe according to claim 9, wherein the azo-calixarene is a compound of formula (II): ##STR00013## wherein m is an integer between 4 and 8.

15. The probe according to claim 14, wherein the azo-calixarene is a compound of formula (III) or (VIII): ##STR00014##

16. A method of producing a probe, the method comprising contacting an azo-calixarene with a salt.

17. The method according to claim 16, wherein prior to contacting the azo-calixarene with the salt, the method comprises contacting a compound of formula (V): ##STR00015## with a compound of formula (VI): ##STR00016## thereby synthesising the azo-calixarene, wherein R.sup.1, R.sup.3, R.sup.5, R.sup.6 and m are as defined in claim 9.

18. The method according to claim 17, wherein prior, or simultaneously, to contacting the compounds of formula (V) and (VI), the method comprises contacting a compound of formula (VII): ##STR00017## with a nitrite and thereby synthesising the compound of formula (V).

19. A method of sensing or capturing carbon dioxide, the method comprising using the probe as defined in claim 1 to sense or capture carbon dioxide.

20. (canceled)

Description

[0076] For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying Figures, in which:—

[0077] FIG. 1 is a picture showing the a) para-ester diazophenylcalix[4]arene (CA-AZ) in dimethyl sulfoxide (DMSO); b) para-ester diazophenylcalix[4]arene-fluoride (CA-AZ-F) in DMSO; c) CA-AZ-F in DMSO after having been exposed to CO.sub.2; and d) CA-AZ-F in DMSO after having been exposed to CO.sub.2 and then flushed with nitrogen (N.sub.2) at room temperature;

[0078] FIG. 2 shows the UV-vis spectra for each of the samples shown in FIG. 1;

[0079] FIG. 3 shows the NMR spectra at 298.15 K for a) CA-AZ in deuterated DMSO (DMSO-d.sub.6); b) CA-AZ-F in DMSO-d.sub.6; c) CA-AZ-F in DMSO-d.sub.6 after having been exposed to CO.sub.2; and d) CA-AZ-F in DMSO-d.sub.6 after having been exposed to CO.sub.2 and then flushed with N.sub.2 at room temperature;

[0080] FIG. 4 shows the structure of CA-AZ;

[0081] FIG. 5 shows the NMR spectra at 298.15 K for 5,11,17,23-tetrakis(4-nitrophenyeazocalix[4] arene (CA-AZ′) in deuterated Chloroform (CDCl.sub.3);

[0082] FIG. 6 shows the structure of CA-AZ′;

[0083] FIG. 7 is a picture showing the a) CA-AZ′ in DMSO; b) 5,11,17,23-tetrakis(4-nitrophenyl)azocalix[4] arene-carbonate (CA-AZ′-CO.sub.3.sup.2−) in DMSO; and c) 5,11,17,23-tetrakis(4-nitrophenyl)azocalix[4] arene-fluoride (CA-AZ′-F) in DMSO; and

[0084] FIG. 8 is a picture showing a) CA-AZ′-CO.sub.3.sup.2− in DMSO; and b) CA-AZ′-CO.sub.3.sup.2− in DMSO after having been exposed to CO.sub.2.

EXAMPLE 1: SYNTHESIS OF CA-AZ

[0085] Materials

[0086] All chemicals used throughout the work were of analytical grade and were obtained from recognized chemical suppliers.

[0087] Salts used throughout the study were kept in vacuum oven and then stored in vacuum desiccators over phosphorus pentoxide, P.sub.4O.sub.10 for several days to remove water, before being used for experimental purposes.

[0088] Solvents

[0089] Methanol (CH.sub.3OH), Sigma-Aldrich, HPLC grade, 99.7%.

[0090] N,N-Dimethylformamide (C.sub.3H.sub.7NO), Sigma-Aldrich, anhydrous 99.8%.

[0091] Deuterated Solvents Used in NMR Experiments

[0092] Chloroform-d (CDCl.sub.3), Cambridge Isotope Laboratories, Inc, (D, 99.8%)+0.05% v/v TMS.

[0093] Dimethyl sulfoxide-d.sub.6 (C.sub.2D.sub.6OS) Cambridge Isotope Laboratories, Inc. (D, 99.9%).

[0094] Analytical Reagents

[0095] Hydrochloric acid (HCl), Fisher Scientific, 35-38%.

[0096] Reagents Used in Synthesis without Further Purification

[0097] Ethyl 4-aminobenzoate (H.sub.2NC.sub.6H.sub.4CO.sub.2C.sub.2H.sub.5), Sigma-Aldrich, 98%, 112909.

[0098] Calix[4]arene-25,26,27,28-tetrol, Sigma-Aldrich, 95%.

[0099] Salts

[0100] Sodium ethanoate trihydrate (C.sub.2H.sub.3NaO.sub.2.3H.sub.2O), Sigma-Aldrich, ≥99%

[0101] Sodium nitrite (NaNO.sub.2), Sigma-Aldrich, 97+ %.

[0102] Methods

[0103] In a 500 cm.sup.3 round-bottomed flask, ethyl 4-aminobenzoate (1.29 g, 7.8 mmol), sodium nitrite (0.41 g, 6.00 mmol) and conc. HCl (14 cm.sup.3) in water (25 cm.sup.3) was added gradually to a cold solution (0-5° C.) of 25, 26, 27, 28-tertrahydroxy calix[4]arene (0.64 g, 1.5 mmol) and sodium ethanoate trihydrate (1.17 g, 8.6 mmol) in a DMF/MeOH (2:1) mixture to obtain a dark orange coloured suspension. The mixture was stirred for 18 hours and a red precipitate was observed where the stirring was stopped. The mixture was filtered and the residue was washed with cold water then methanol several times. The product was left on a Schlenk line for one week. (98% yield).

[0104] The product was characterised by .sup.1H NMR (500 MHz) at 298 K.

[0105] .sup.1H NMR (500 MHz, CDCl.sub.3, δ in ppm); 10.25 (s, OH, 4H (1)); 8.13 &8.14 (d, Ar—H, 8H (5 & 5′)); 7.86 (d, Ar—H, 4h (4&4′)); 7.85 (s, Ar—H, 4H (2 & 2′)); 4.39 (q, COO—CH.sub.2-CH.sub.3, 8 H (6)); 4.4 (d, H-axial, 4H (3)); 3.87 (d, H-equatorial, 4H (3′)); 1.4 (t, CH.sub.3, 12 H (7)).

[0106] Elemental analysis was carried out in duplicate at the University of Surrey; (C.sub.64H.sub.56N.sub.8O.sub.12) MW. (1129.20); Calculated %; C, 68.08; H, 5.0; N, 9.92. Found %; C, 68.14; H, 4.93; N, 10.2.

##STR00011##

EXAMPLE 2: DETECTION OF CO.SUB.2 .USING PARA-ESTER DIAZOPHENYLCALIX[4]ARENE-FLUORIDE (CA-AZ-F) IN DMSO

[0107] Materials

[0108] Solvents

[0109] Dimethyl sulfoxide (C.sub.2H.sub.6OS), Fisher Scientific, 99%.

[0110] Salts

[0111] Tetra-n-butylammonium fluoride hydrate ((C.sub.4H.sub.9).sub.4NF.H.sub.2O), Sigma-Aldrich, 98%.

[0112] Methods

[0113] CA-AZ prepared according to the method described in Example 1 was dissolved in dimethyl sulfoxide (DMSO) at a concentration of 10.sup.−5 mol.Math.dm.sup.−3. Tetra-n-butylammonium fluoride (TBAF) was then added at a concentration of 10.sup.−3 mol.Math.dm.sup.−3. Dry ice (solid CO.sub.2) was then added to the solution. Finally, nitrogen gas (N.sub.2) was bubbled through the solution for three minutes.

[0114] Results

[0115] Colour Change

[0116] As can be seen in FIG. 1, the addition of the TBAF to the CA-AZ solution induces a colour change from pale yellow to orange. When the solution comprising the CA-AZ-F complex was then exposed to CO.sub.2 an immediate colour change from orange to yellow was observed. Finally, when the solution was purged with N.sub.2 for three minutes the colour returned to orange.

[0117] This behaviour of CA-AZ was further investigated using a UV-vis spectrometer, and the results can be seen in FIG. 2. In particular, a bathochromic shift from 383 nm to 500 nm with a shoulder at 544 nm was obtained with F− addition to CA-AZ. When the solution was exposed to CO.sub.2, the original spectrum of CA-AZ was substantially revived (λ.sub.max=393 nm). However, the N.sub.2 purge led to another bathochromic shift to 418 nm with a shoulder to 511 nm, where the spectrum pattern resembles the one of the solution comprising the CA-AZ-F complex. This suggests that the CA-AZ-F complex has reformed and CO.sub.2 has been predominantly removed from the solution.

[0118] The inventors found that after purging with nitrogen, the solution can be reused to detect CO.sub.2, and the colour changes and bathochromic shifts described above are again observed.

[0119] .sup.1H NMR Studies

[0120] .sup.1H NMR spectra were obtained for CA-AZ in DMSO-d.sub.6, CA-AZ-F in DMSO-d.sub.6, CA-AZ-F in DMSO-d.sub.6 after having been exposed to CO.sub.2 and CA-AZ-F in DMSO-d.sub.6 after having been exposed to CO.sub.2 and then flushed with N.sub.2 at room temperature, and the results are shown in FIG. 3. The .sup.1H NMR chemical shift relative to the free ligand (the CA-AZ sample) was calculated and the results are provided in Table 1.

TABLE-US-00001 TABLE 1 .sup.1HNMR chemical shift relative to the free ligand in deuterated DMSO at 298 K δ (ppm) H-2 H-4 H-5 and and and H-1 H-2′ H-3 H-3′ H-4′ H-5′ H-6 H-7 δ .sub.Ref —  7.82 3.51 4.3  7.87  8.05 4.3  1.32 F.sup.− — −0.23 — — −0.18 −0.26 0 −0.39 CO.sub.2 — −0.02 — 0.1 −0.01 −0.01 0 −0.39 CO.sub.2 — −0.23 — — −0.18 −0.26 0 −0.39 purged with N.sub.2

[0121] FIG. 4 identifies the positions of H-1 to H-7. In all samples, the concentration of CA-AZ was 1×10.sup.−5 mol dm.sup.−3, and in the final three samples the concentration of the fluoride was 1×10.sup.−4 mol dm.sup.−3.

[0122] The peaks for H-3 and H-3′ protons did not appear in all of the spectra because they were too broad. Accordingly, it was not always possible to determine the shift for these aromatic protons.

[0123] Up-field shift in the aromatic protons (H-2, H-4, H-4′, H-5 and H-5′) was observed in the CA-AZ-F complex, and the relevant peaks are circled in FIGS. 3 (a) and (b). This suggests the deprotonation of the azophenol units in CA-AZ where the hydrogen is being abstracted from the —OH to the azo unit —N═N— resulting in the formation of —N . . . H . . . F—.

[0124] It will be noted that after exposure to CO.sub.2 the .sup.1H NMR chemical shift for the aromatic protons was almost identical to that of CA-AZ in DMSO-d.sub.6. This indicates that the fluoride ion is no longer complexed with the CA-AZ ligand. It is noted that while the chemical shifts for CA-AZ aromatic protons were substantially returned to their original positions after addition of CO.sub.2, the peaks were broader than they were for the CA-AZ ligand, suggesting the formation of nucleophilic [CA-AZ].sup.−.

[0125] Up-field shift in the aromatic protons was noticed again after the solution had been purged with N.sub.2. This indicates that the CA-AZ ligand was once again complexed with the fluoride ion.

[0126] In conclusion, the .sup.1H NMR findings are in agreement with the UV-vis results.

[0127] Thermodynamic Studies of CA-AZ-F Complex Interacting with CO.sub.2 in DMSO at 298.15 K

[0128] Thermodynamic parameters of complexation of CA-AZ with the fluoride and complexation of CA-AZ-F with CO.sub.2 in DMSO at 298.15 K are listed in Table 2.

TABLE-US-00002 TABLE 2 Thermodynamic parameters of complexation of CA-AZ with the fluoride and complexation of CA-AZ-F with CO.sub.2 in DMSO at 298.15K Δ.sub.cS° Δ.sub.cG° Δ.sub.cH° (J mol.sup.−1 Guest ligand:guest log K.sub.S (kJ mol.sup.−1) (kJ mol.sup.−1) K.sup.−1) F.sup.− 1:1  5.9 ± 0.1 −33.7 ± 0.2   −12 ± 0.3 72 CO.sub.2 1:1 5.76 ± 0.07 −32.9 ± 0.3 −38.4 ± 0.3 −18

[0129] The stability constant (log K.sub.s) of CA-AZ-F complex is quite similar to the stability constant value of the CA-AZ-F complex with CO.sub.2, However, the complexation process of the anion probe with CO.sub.2 is enthalpically controlled whereas the case of F.sup.− complexation, both enthalpy and entropy contribute favourably to complex stability.

EXAMPLE 3: DETECTION OF CO, USING A SOLID PROBE

[0130] Materials

[0131] Solvents

[0132] Tetrahydrofuran (C.sub.4H.sub.8O), Fisher Scientific, ≥99.85%.

[0133] Salts

[0134] Tetra-n-butylammonium fluoride hydrate ((C.sub.4H.sub.9).sub.4NF.H.sub.2O), Sigma-Aldrich, 98%.

[0135] Methods

[0136] CA-AZ prepared according to the method described in Example 1 was dissolved in tetrahydrofuran (THF) at a concentration of 10.sup.−5 mol.Math.dm.sup.−3. Tetra-n-butylammonium fluoride (TBAF) was then added at a concentration of 10.sup.−3 mol.Math.dm.sup.−3. The resultant solution was poured into a dust free Pyrex Petri dish and the solvent was evaporated off at room temperature yielding a dark red film.

[0137] The dried probe was exposed to carbon dioxide at different time intervals.

[0138] Results

[0139] As the dried probe was exposed to the carbon dioxide a continual change in the colour of the probe from maroon red to red to orange and then yellow was observed as a result of the gas exposure

EXAMPLE 4: SYNTHESIS OF 5,11,17,23-TETRAKIS(4-NITROPHENYL)AZOCALIX[4] ARENE (CA-AZ′)

[0140] The synthetic procedure is similar to the one given for example 1.

[0141] In a 500 cm.sup.3 round-bottomed flask, 4-nitroaniline (5.52 g, 40 mmol), sodium nitrite (1.69 g, 25 mmol) and conc. HCl (7 cm.sup.3) in water (25 cm.sup.3) was added gradually to a cold solution (0-5° C.) of 25, 26, 27, 28-tertrahydroxy calix[4]arene (4.24 g, 10 mmol) and sodium ethanoate trihydrate (4.08 g, 30 mmol) in a DMF/MeOH (8:5) mixture to obtain a red coloured suspension. The mixture was stirred for 18 hours; a red precipitate was observed where the stirring was stopped. The mixture was filtered and the residue was washed with cold water then methanol several times. Crystallisation was performed using DMF-Methanol.

[0142] Results

[0143] The structure of CA-AZ′ is shown in FIG. 6 and the 1H NMR spectrum is shown in FIG. 5.

[0144] .sup.1H NMR (500 MHz, CDCl.sub.3, δ in ppm); 9.49 (s, OH, 4H (1)); 7.58 (d, Ar—H, 8H (4′)); 8.1 (s, Ar—H, 4H (2)); 8.34 (d, Ar—H, 8H (4)); 5.24 (d, H-axial, 4H (3)); 2.91 (d, H-equatorial, 4H (3′)).

EXAMPLE 5: PREPARATION OF 5,11,17,23-TETRAKIS(4-NITROPHENYL)AZOCALIX[4] ARENE-CARBONATE (CA-AZ′-CO.SUB.3..SUP.2−.) IN DMSO AND 5,11,17,23-TETRAKIS(4-NITROPHENYL)AZOCALIX[4] ARENE-FLUORIDE (CA-AZ′-F) IN DMSO

[0145] Materials

[0146] Solvents

[0147] Dimethyl sulfoxide (C.sub.2H.sub.6OS), Fisher Scientific, 99%.

[0148] Salts

[0149] Potassium carbonate and sodium fluoride.

[0150] Methods

[0151] CA-AZ′ prepared according to the method described in Example 4 was dissolved in dimethyl sulfoxide (DMSO) at a concentration of 10.sup.−5 mol.Math.dm.sup.−3. Potassium carbonate or sodium flouride was then added at a concentration of 10.sup.−3 mol.Math.dm.sup.−3.

[0152] Results

[0153] As can be seen in FIG. 7, the addition of the potassium carbonate to the CA-AZ′ solution induces a colour change from pale yellow to dark brown. Similarly, the addition of the sodium fluoride to the CA-AZ′ solution induces a colour change from pale yellow to dark orange.

EXAMPLE 6: DETECTION OF CO.SUB.2 .USING CA-AZ-CO.SUB.3..SUP.2− IN DMSO

[0154] A solution of CA-AZ-CO.sub.3.sup.2− in DMSO was prepared according to example 5. Dry ice (solid CO.sub.2) was then added to the solution.

[0155] Results

[0156] Colour Change

[0157] As can be seen in FIG. 8, when the solution comprising the CA-AZ′-CO.sub.3.sup.2− complex was exposed to CO.sub.2 an immediate colour change from dark brown to yellow was observed.

[0158] Conclusion

[0159] The inventors have shown that it is possible to use a probe comprising an azo-calixarene complexed with an anion. In particular, the inventors have synthesised para-ester diazophenylcalix[4]arene-fluoride (CA-AZ-F), 5,11,17,23-tetrakis(4-nitrophenyl)azocalix[4] arene-carbonate (CA-AZ′-CO.sub.3.sup.2−) and 5,11,17,23-tetrakis(4-nitrophenyl)azocalix[4] arene-fluoride (CA-AZ′-F), and shown that these complexes can be used to detect and sense CO.sub.2. The inventors have shown that the probe may be used in solution or as a solid.

[0160] Since the probe changes colour upon exposure to CO.sub.2, the presence of CO.sub.2 can be confirmed by the naked eye. Furthermore, the inventors have shown that the captured CO.sub.2 can be recovered from the probe without the need to use elevated temperatures. This enables the probe to be used multiple times.