Sensors and Actuators B, 13-14 (1993) 420-423
Optical humidity and ammonia gas sensor using calcein-based
Yoshihiko Sadaoka*, Yoshiro Sakai and Yu-uki Murata Deportmentof Applied Chtity,
Faculty of [email protected]
Ehikne Vnivek& Matruyamn 790 (Japan}
Abstract For calcein-poly(acrylonitrile), the optical intensity at around 530 nm increases with humidity and decreases with ammonia concentration in ambient. The sensitivity to ammonia is enhanced by humidiication. A lack of reversibility for ammonia is confirmed, while a good reversibility for humidity is observed at room temperature. The reversibility for ammonia is improved by using a polymer prepared by polycondensation of calcein and hexamethylene diamine; however, the sensitivity is reduced.
structure for determination of humidity and ammonia concentration in ambient air.
Most optical-fibre sensors have been devised for the measurement of physical parameters like temperature and displacement. There has been an increasing interest in optical sensors to detect gas species, such as humidity, ammonia,nitrogen dioxide and organic vapours, in the atmosphere [l-lo]. Most of the optochemical sensors are composed of a dye-dispersed polymer in which the absorption or fluorescence spectrum is influenced by the chemical species and the concentration in ambient. For a sensing film prepared by the dispersion of dye into a polymer matrix, migration or condensation of dyes proceeds when the film is exposed to organic or water vapour. The migration of dyes induced permanent drifts in the sensitivity. To improve the long-term stability of the sensitivity, the use of a polymer with a gassensitive molecular structure was considered. Calcein has a high fluorescence quantum yield and an absorption band in the wavelength range 400-600 nm. Fluorescence is observed in an acidic solution and diminishes in a strong basic solution. It is expected that the optical characteristic of the dye entrapped in a polymer matrix is also influenced by the acidity of the polymer and by sorption of ambient gases. Cal&n is a stable compound decomposing at 300 “C; it is therefore a preferred material for a gas sensor requiring long-term stability and good reversibility. Calcein has -COOH groups which can be covalently bound to several organic compounds with an -NH5 group (such as hexamethylene diamine) or -CH,Cl group, and forms an insoluble polymer thin film. We propose a new type of optochemical sensor using a polymer thin film with a gas-sensitive molecular ‘Author
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2. Experimental Calcein (3,3’-bis[N,N-di(carboxymethyl)aminomethyllfluorescein), hexamethylene diamine @MDA) and poly(acrylonitrile) (PAN) were used. Calcein and PAN were dissolved in N,N-dimethyl formamide (DMF). Films were formed on alumina substrates by coating with a solution of calcein-PAN. The precipitate (salt) was obtained from the 1:l mixture of calcein and HMDA in DMF and then washed with DMF and ethyl ether. A mixture of the salt and an appropriate amount of water was coated on the substrate, dried in vacua and then heated in a nitrogen atmosphere. The starting salt was deliquescent, but after heating, the obtained films became insoluble in water and even in several organic solvents. After seating the element in the chamber, a Y-type glass fibre was fixed just in front of the film. Selected light from a DJ12 lamp (400-800 nm) was guided into the fibre and directed to the film; the reflected and modulated light was collected by the same optical fibre. The collected light was analysed using a spectromultichannel photodetector (MCPD-1000, Otsuka Electronics) in the region 4OO-gOOnm. In addition, fluorescence spectra were measured using a spectrofluorophotometer (RF-5000, Shimazu) in the region 400-600 nm. All measurements were made at 30 “C. The humidity in the chamber was controlled by
mixing standard air and humid air prepared by allowing standard air to bubble through water at 30 “C. The concentration of ammonia was controlled by the mixing
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of 0.5% ammonia/air and standard air (O,=Zl%, N, = 79%, CO < 1 ppm, CO, < 2 ppm, HCl < 1 ppm and H,O < 10 ppm).
ammonia cont./% 0
3. Results and discussion 3.1. Reflection and fluorescence spectm in the solid state The reflection spectra ina 30% RH (relative humidity) atmosphere were measured. The reflection spectrum of the alumina substrate (plate) was used as a reference. The wavelength of the reflection peak observed in air is 495 run with a shoulder at 460 nm for calcein-PAN, and 450 nm with a shoulder at 550 nm for calcein-HMDA polymer. Fluorescence spectra of a calcein-PAN composite and calcein-HMDA polymer films in air were also measured. A fluorescence peak at 535 run with a shoulder at 520 nm was observed (excitation wavelength; 450 nm) for the calcein-PAN composite. For the film of calcein polymerized with HMDA, fluorescence peaks at 520 and 540 nm were observed. The former peak was strengthened by 436 nm excitation and the latter by 455 nm excitation. 3.2. Optical sensing characteristics of calcein-PAN composite In Figs. 1 and 2, the spectrum changes of calcein-PAN are shown as a function of humidity and ammonia concentration in ambient, respectively. In these measurements, the spectrum observed in dry air was used as a reference. The optical intensity at 460 nm (absorption peak) decreased and that at 520 nm (fluor-
‘. . .. .
wavelength/nm Fig. 1. Spectrum changes of calcein-PAN composite: (a) 50% RH; (b) 38% RH; (c) 0.20% ammonia in dry air; (d) 0.10% ammonia in dry air; (e) 0.20% ammonia in 50% RH air; (f) 0.05% ammonia in 50% RH air.
%RH Fig. 2. Concentration dependence of optical intensity (A=520 nm) of calcain-PAN composite. 0, relative humidity;A, ammonia in dry air; 0, ammonia in 50% RH air. Closed symbols denote the results observed in the decreasing process.
escence) monotonically increased with an increase in humidity up to 50% RH; above 50% RH the increase was greater. The increase in the fluorescence by humidification is caused by the enhancement of the dissociation of the dye. In addition, a good response behaviour was confirmed. The exposure to dry ammonia diluted with air induced a decrease in the optical intensity at about 520 nm for the calcein-PAN film. The sensitivity to ammonia was enhanced by humidification of the test gas. The spectra were broadened, possibly as a result of the superposition of an absorption band with a peak at 480 nm. For ammonia detection, the optical intensities observed in dry and in 50% RH air after exposure to test gas containing 0.2% ammonia are different from the result observed before exposure to ammonia. At least two kinds of interactions between calcein and ammonia may be considered. The response behaviour is apparently improved when the temperature is raised. 3.3. Gas-sorption abilities of the composite For the calcein-dispersed PAN film, water and ammonia contents were measured (Fig. 3). PAN is a hydrophobic polymer (insoluble in water) with no strong polar group. In this case the water-sorption ability is primarily controlled by the dye having two phenolic -OH and four -COOH groups. The number of sorbed water molecules per dye is estimated to be 10 for a calcein-PAN film at 45% RH. The dissociation of the acidic protons is enhanced by the sorption of water and results in the enhancement of fluorescence. Tbe concentration dependence of ammonia content is shown in Fig. 3. Under dry conditions, exposure to 0.01% ammonia induced the increment of 2 mg/g, suggesting one molecule of ammonia is attached to one molecule
%RH Fig. 3. Concentration dependence of the gain for cakein-PAN composite: 0, relative humidity; A, ammonia in dry air; 0, ammonia in 50% RH air.
of dye. At higher concentrations, the content gradually increased with concentration. The coexistence of water vapour enhanced the ammonia-sorption ability. The ammonia content sharply increased with ammonia concentration and then levelled off. At 0.2% ammonia in 50% RH air, the number of sorbed ammonia molecules per dye is estimated to be approximately three for the composite. It seems that ammonia molecules interact directly with the acidic sites of the dye and form a salt (chemical effect). As mentioned above, at room temperature the spectrum could not recover to the initial state in ammonia-free air after exposure to the test gas containing ammonia. By the comparison of the optical and the gain behaviours, it is possible to suggest that one molecule of ammonia per dye molecule remains when the ambient is changed to ammonia-free air. 3.4. Optical sensing characteristics of calcein-HMDA polymer For dye-dispersed films, migration and condensation of the dye occur. When the composite film is immersed in water solution, the dye is soluble in water and the film loses sensitivity. To depress these phenomena, the use of a polymer (functional polymer) having a gassensitive molecular structure was considered. The film formed on the substrate by polycondensation with calcein and HMDA is insoluble in water and some organic solvents. The pH dependence of the spectrum was measured by immersion of the flIm in a number of standard pH solutions. pH changes, in particular in acidic solution (<7.0), are responsible for the absorbance at 520 ML Hysteresis, absorbance and the sensitivity were not influenced by immersion in the solutions for 90 min. In Fig. 4, the spectrum changes in reflection mode with humidity are shown for a calcein-HMDA polymer film prepared by heat treatment at 170 “C in
wavelength/nm Fig. 4. Spectrum changes of calcein-HMDA polymerized at 170 “C: (a) 56% RH, (b) 29% RH; (c) 0.25% ammonia in dry air; (d) 0.10% ammonia in dry air; (e) 0.25 ammonia in 50% RH air; (f) 0.05% ammonia in 50% RH air.
500 600 wavelength/nm Fig. 5. Spectrum changes of calcein-HMDA polymerized at 240 “C: (a) 71% RH; (b) 29% RH, (c) 0.25% ammonia in dly air; (d) 0.10% ammonia in dry air; (e) 0.25% ammonia in 50% RH air; (f) 0.05% ammonia in 50% RH air.
nitrogen for 12 h (polycondensation). In this case, the spectrum observed in dry air was used as a reference. Humidiication induced an increase in optical intensity at 565 nm. The optical intensity at 540 nm is somewhat sensitive to dry ammonia. The sensitivity to ammonia is increased by humidification. The optical intensity at 530 nm increases with the ammonia concentration of 50% RH test gas and that at 565 nm decreases (Fig. 4). T’he sensing characteristics of the fihn polymerized at 240 “C were also examined. The results are shown in Fig. 5. The optical intensity at 570 mn is sensitive to humidity and that at a530 nm is due to ammonia, in which the sensitivity is also enhanced by humidiflcation. Concentration dependences of optical intensities are shown in Fig. 6. Changes of humidity and ammonia levels were responsible for the observed changes in optical intensity. The sensitivity reduced by polymer-
%RH Fig. 6. Concentration dependence of optical intensity of calcein-HMD polymerized at 240 “c: 0, relative humidity (A= 570 nm); A, ammonia in dry air (A=540 nm); 0, ammonia in 50% RH air (A =530 nm). Closed symbols denote the results observed in the decreasing process.
ization, but the response time and reproducibility are improved. Currently, it is difficult to obtain a polymer film with a well-defined molecular structure. The optical characteristics are also influenced by the polymerization conditions. The selection ‘of well-designed starting materials and conditions may allow this ditliculty to be largely overcome.
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