colorimetric polymer film sensors for dissolved carbon dioxide

ELSEVIER Sensors and Actuators B 21 (1994) 83-89

Calorimetric polymer film sensors for dissolved carbon dioxide

Andrew Mills *, Qing Chang Departmen of Chemishy, University College of SwanFea, Singleton Pa&, Swanwa SA2 8PP, UK

Received 17 August 1993; in revised form 25 November 199% accepted 15 December 1993

Abstract

Plasticized and unplasticized polymer calorimetric film sensors for gaseous COz, containing the dye m-cresol purple, are tested as sensors for dissolved CO,. The plasticized polymer m-cresol purple 6lm sensor develops a measurable degree of opacity when exposed to aqueous solution, especially in neutral, compared with alkaline, solution. However, it is shown that a pre- soaked, fogged plasticized polymer m-cresol purple film does function as a quantitative sensor for dissolved COz over the range a% COz. An unplasticixed polymer m-cresol purple film remains largely clear upon exposure to aqueous solution and also functions as a quantitative sensor for dissolved CO2 over the range @4% CQ. However, in both types of films the dye interacts with electrolytes present in solution; invariably the dye appears to be converted from its initial deprotonated form (blue) to its protonated form (yellow) and the rate of this process appears to increase with increasing ionic strength, anionic charge and decreasing pH. The 90% response gnd recovery times for an unplasticized tilm are determined as 19 s (COz:O+5%) and 21 s (CO,:5 +3.6%), respectively.

Krywonls: Carbon dioxide sensor; Calorimetric sensor; Polymer films

1. Intmduetion

There is increasing interest in the development of optical sensors of CO2 for biomedical applications, such as in blood- and breath-gas monitoring [l-3]. To date, such CO, sensors usually utilize a calorimetric or fluorescence pH indicator as the key element of the sensing chemistry, and have a typical configuration of support/ indicator/bicarbonate solution/gas-permeable protective membrane [4-g]. This type of sensor is often very small, cheap, disposable and, through the incorporation of fibre optics, can be used in the remote measurement of gaseous and dissolved COz. However, they also tend to dry out easily, especially in gas-phase measurement, and have long response and recovery times, due to the introduction of the protective membrane that serves to separate the chemical sensor element of the detector from the sample under test.

Recently, our group has developed a novel range of calorimetric and fluorimetric plastic film sensors for gaseous CO2 [9,10] that do not require the usual protective, gas-permeable, hydrophobic membrane found in most other CO, optodes. In addition, we have demonstrated that these films can be used for the

* Corresponding author.

quantitative detection of gaseous CO, in the clinically significant range O.l-10% COP The films exhibit fast response and recovery times to alternating atmospheres of 5% CO, and N,, i.e., typically less than 3 s, and have a very long shelf life, over one year. Further work has demonstrated that the film sensors do not undergo dye leaching when placed in water [ll], but others have noted that they become slightly opaque, i.e., fog up, in the presence of water [ll]. Clearly, it would be a valuable feature of these films if they did not fog up in water, since they could then be used to monitor levels of CO* dissolved in aqueous solution or blood. In this paper we address these problems in a detailed study of one of our typical calorimetric films as a sensor for CO, dissolved in aqueous solution.

2. Experimental

2.1. Materials

Ethyl cellulose (ethoxyl content 46%), tributyl phosphate, m-cresol purple and tetraoctylammonium bromide were obtained from Aldrich Chemicals UK The tetraoctylammonium hydroxide solution in methanol was prepared from the corresponding bromide solution

0925-4005194/$07.00 0 1994 Elsevier Science S.A. All rights reserved .~.~T)I n92s.4nnsf94m233-8

84 A. Mills, Q. Chang / &mm and Actuators B 21 (1994) 83-89

using wet silver oxide to effect the anion exchange [9]. The gases used (i.e., N2 and CO,) were of high purity ( > 99%), and were purchased from BOC, UK The gas mixtures of CO, in N2 at various percentages were generated using a gas blender (model no. 852Vl-B, Signal Instruments Co., UK). The water used to make up solutions was doubly distilled and deionized. All the other chemicals were purchased from Aldrich Chem- icals,UK.

The thin polymer fihn calorimetric sensors for dissolved CO* used in the initial part of this work had the general composition: indicator dye/phase-transfer agent/polymer/plasticizer/support, i.e., m-cresol purple/ tetraoctyl ammonium hydroxide/ethylcellulose/tributyl- phosphate/glass slide. Component solution (I) was prepared by adding 0.012 g of m-cresol purple to 1 cm3 of a 0.5 mol dm-3 tetraoctyl ammonium hydroxide methanolic solution; a further 2.5 cm3 of methanol was then added to provide a final dye concentration ap- propriate for making absorbance measurements. The second component solution, i.e., solution (II), was prepared by dissolving 10 g of ethyl cellulose into a solution comprising 20 cm3 of ethanol and 80 cm3 of toluene. The final solution used to produce the plasticized rn- cresol purple film was made up of 1 cm3 of solution (I), 10 g of solution (II), 1 cm3 of tributyl phosphate and 1. cm3 of the 0.5 mol dme3 tetraoctyl ammonium hydroxide methanolic solution.

The dry, thin plasticized polymer m-cresol purple film CO, gas sensors were made by casting the final film solution through a 100 pm thick brass sheet with a rectangular hole (0.5 cmX 1 cm). The solid support was always a glass slide and the thickness of the dried film was typically 20 pm.

As indicated above, in the initial part of this study the polymer m-cresol purple films incorporated a plasticizer, tributyl phosphate, in the plastic tihn formulation to improve the rate of dithtsion of CO, through the film and, therefore, the response and recovery times of the film sensors. However, in the bulk of the experiments described in this paper the plasticizer was omitted from the films, for reasons that will be given later. These latter films are referred to as unplasticized polymer m-cresol purple films and had the additional feature of having three times as much dye (i.e., 0.036 g m-cresol purple in solution (I)) as the plasticized polymer m-cresol purple films in order to improve the magnitude of the observed optical changes.

2.2. Methodr

UV-Vis absorption spectra and single-wavelength absorbance versus time measurements were carried out using a double-beam scanning spectrophotometer (Per- kin Elmer, model no. Lambda 3) and an optical bench

system that has been described elsewhere [9], respectively.

3. Results and discussion

3.1. Film sensor with plasticizer

Previous work carried out by our group has established that the plasticized polymer m-cresol purple tlhns can act as quantitative sensors for gaseous CO, [9,10]. The quaternary phase-transfer agent, tetraoctyl ammonium hydroxide or Q’OH- *xH,O, not only solubilixes the blue anionic form of the m-cresol purple dye, D-, but also appears to provide the water molecules necessary for the otherwise largely hydrophobic sensor to work. The overall process can be summarixed as follows [9]:

{Q’D-.xH,O}+CO,(g) s (Am,=593 nm)

{Q+HCO,- .(x- l)H,O.I-ID} (1) (Amax = 406 nm)

It follows from Eq. (1) that

a’P,,=R=[{Q+HCO,- .(x- l)H,O.HD}]/

UQ’D- 6011 (2) where P,, is the partial pressure of CO, in the gas phase. For any particular Pm, the value of the parameter R for the film can be determined from absorbance measurements made on the film at a wavelength where only the deprotonated form of the dye absorbs (600 mn in this work). The key expression is as follows:

R= pbs(Q+D--xH,O},-Abs(Q+D-.xH,O}] pbs{Q+D- *xH,O}-Abs@lank)] (3)

where A~~(Q’D-*xH,O}~ is the value of Abs(Q’D- .xH,O} when the saturating gas contains 0% CO2 and

Abs(blank) is the absorbance of the film when the saturating gas is 100% CO, and, as a result, the dye is in its protonated form, i.e., {QfHC03-. (x- l)H,O.HD}.

To date our research on the plasticized film sensors for CO* has involved using them for dry gas-phase measurements; under these conditions the films are clear [9,10]. Given the overall hydrophobic nature of the films, it should be possible to use then directly in aqueous solution, i.e., with no extra coating of a hydrophobic gas-permeable membrane material. However, it is well established that many plasticized polymers turn opaque when exposed to electrolyte solutions due to the formation of small water droplets in the polymer matrix [ll]. It appears that this is also the case for the plasticized film Co, sensors described in this work,

A. Mills, Q. Chang I Senms and Acfuators B 21 (lW4) 83-89 I35

as illustrated by the results in Fig. 1, i.e., the observed changes in absorption spectrum of the plasticized polymer m-cresol purple film with time of exposure to an aqueous solution containing 8.25~10-~ mol dm-3 of NaOH.

A measure of the opacity of any of the optical films is provided by its absorbance at some wavelength where the dye does not absorb (e.g., 800 nm), i.e., Abs, The fogging up of a film with time of exposure to an aqueous solution can be monitored through the change of the ratio Abs,,(f)lAbs,(air) as a function of time, where &,&air) is the absorbance of the hhn in air prior to immersion in the aqueous solution. Fig. 2 illustrates the results of such a study for the plasticized

200 300 800 900

Fig. 1. Change in W-V& absorption spectrum of a plasticized polymer m-cresol purple tilm as a function of time of immersion in an aqueous solution containing 8.25 X lo-’ mol dm-” NaOH.

2.5

2.0

1.5

1.0 ,c +

0 a 16 24 32 40 48 56 tlmin

Fig. 2 Degree of fogging, as measured by the experimental parameter Ah~&&4&~(air), exhibited by the plasticized (solid lines) and unplasticized (broken lines) polymer m-cre.4 purple Iilms as a function of time of exposure to a neutral (m) or alkaline (A) aqueous solution.

(solid lines) and unplasticized (broken lines) polymer m-cresol purple films when exposed to neutral (deionized) and alkaline (8.25X 1V3 mol dmV3 NaOH) aqueous solutions. From the results of this work it appears that the unplasticized polymer m-cresol purple films are not prone to foogging upon exposure to either of the aqueous test solutions, whereas the plasticized films are, with the latter showing a greater tendency to fog under neutral, rather than alkaline, conditions. Additional experiments show that the unplasticized film does not fog up in neutral solution of high ionic strength, whereas the plasticized film does. The lack of plasticizer does, however, restrict the rate of ditlirsion of gaseous CO, into and out of the fihns, as evidenced by the larger 50% response (= 2 s) and recovery (~23 s) times for unplasticized fihns compared with plasticized films (= 0.5 and 7 s, respectively).

Although fogging of the fihn is undesirable, it can be seen from the spectral changes illustrated in Fig. 1 that after about 1 h the opacity of the films is near to reaching a limiting value. In addition, the component absorbance due to the dye in the tilm is largely unaffected by the process of fogging. Thus, a pre-soaked, fogged plasticized polymer m-cresol purple film may still make a sensor for dissolved CO,. In order to test this hy- pothesis, such a fihn was prepared, using a pre-soaking time of 2 h, and placed in an aqueous solution containing 8.25~ 10e3 mol drnm3 NaOH. The aqueous NaOH solution was tonometered with gases of various CO, percentages and the visible absorption spectrum of the fihu, in contact with the solution, was recorded for each COz percentage used; the results of this work are illustrated in Fig. 3(a).

For quantitative analysis, a plot of the absorbance due to the film at &,,= of the dye, in this case 600 nm, as a function of percentage CO, is needed, since from this latter plot the parameter R can be calculated as a function of percentage CO,. The results of such an analysis for the data illustrated in Fig. 3(a) are illustrated in Fig. 3(b), from which it appears that R is proportional to percentage CO*, as predicted by Eq. (2). Similar results were obtained for a pre-soaked, fogged plasticized polymer m-cresol purple film in neutral aqueous solution. From the results of this work it appears that a pre-soaked, fogged plasticized polymer m-cresol purple Glm can be used as a quantitative sensor for dissolved CO,.

3.2. Film sensor without plasticizer

Although the plasticized films can be used as quantitative sensors of dissolved CO,, the process of fogging and the consequent need to pre-soak the fihns represent an undesirable feature. From the data illustrated in Fig. 2, it appears that fogging of the polymer m-cresol purple films in aqueous solution is not a problem if

A. Mills, Q. Chug I Sensors and Actuators B 21 (1994) 83-89

(8)

l- 400 560 600 700

1 (nm)

@I %CO,

Fig. 3. (a) Visible absorption spectra of the plasticized power m- cresol purple film immersed in an aqueous solution containing 8.25x10-‘” mol dtrJ NaOH as a function of percentage CO* in the gas used to saturate the solution. The film had been pre-soaked for 2 h before ase in the same solution. The CO, levels are. (from top to bottom): 0, 0.1, 0.3, 0.5, 0.8, 1.0, 2.0, 3.0, 5.0 and 100%. (b) Plot of the absorbance data at 600 nm taken from (a) due to the m-creaol purple dye in the pre-soaked plasticized film and the parameter R, defined by Eq. (2). as a function of %C&.

the plasticizer component of the films is omitted. The observed variation in absorbance and R as a function of percentage CO, for an unplasticized polymer m- cresol purple film in an aqueous solution containing 8.25~ lo-’ mol dmm3 NaOH is illustrated in Fig. 4, from the results of which it appears that R is proportional to percentage COz, as predicted by Eq. (2). Thus, it appears that an unplasticixed polymer m-cresol purple fihn is also a suitable quantitative sensor for dissolved co,.

%CO,

Fig. 4. Plot of the absorbance at 600 nm, due to the m-cresol purple dye in an onplasticized polymer ti, and the parameter R, defined by JZq. (Z), as a function of %COs in the gas used to saturate the aqueous solution, containing 8.25 X lo-’ mol dm-’ NaOH.

Ideally, the stability and response of a plastic film optical sensor for dissolved CO, should be independent of the nature and concentration of any chemical species dissolved in the aqueous solution. In order to assess the extent of interference exerted by common electrolytes in the aqueous solution on the unplasticixed polymer m-cresol purple Elm, a study of the stability of the film as a function of ionic strength (in neutral solution), type of anion (in neutral solution) and pH (in 0.01 mol dmm3 KNO,) was carried out. The observed relative change in absorbance of a film is defined as follows:

(4) where the subscripts ‘aq’ and ‘air’ refer to absorbance measurements made in aqueous solution and air, respectively. A decrease in rel.rlAbs as a function of time of exposure is indicative of a change in concentration of the anionic form of the dye in the film, which absorbs at 600 nm. In all our work, the decrease in rel&tibs can be shown to be associated with the conversion of the dye from its anionic (blue) form to its protonated (yellow) form and not to the leaching of the dye from the film into the aqueous solution @lue+colourless). In addition, in all cases the initial anionic (blue) form of the dye in the film could be regenerated by soaking the film in an aqueous alkaline solution.

From the results of this work it appears that the rate of fall in rel.A&s is slow but increases with increasing ionic strength (Fig. S(a)), increasing anionic charge (Fig. 5(b)) and increasing pH (Fig. 5(c)) for the unplasticixed polymer m-cresol purple film; similar results were obtained with a plasticized film. The process behind the observed decrease in rel.A& of the unplasticized and plasticized films is most probably a

A. MiuC, Q. Chang I Senwrs and Achutors B 21 (1994) 83-89 87

(d

simple anion-exchange mechanism that takes place within the film as well at the film/electrolyte interface, i.e.,

{Q+D-.xH,O}+(H+X-}-

{Q’X-.xH,O}+(H+D-} (5)

Further work is required before a more detailed mechanism can be proposed, but it is clear that reaction (5) can be reversed by alkali.

From the results of this study it appears that there is a high degree of interaction between the dye in the film and any electrolytes in solution, despite the assumed high degree of hydrophobic@ of the films. Thus, dis-

Fig. 5. Plot of the relative change in absorbance, rel.Mbs, as defined by Eq. (4), for ee unplasticiid polymer m-cresol purple Chn as a function of exposure time to equeow soh~tions di5ering in: (a) ionic strength, (b) type of anion and (c) PH. IO (a) and (b) the sohttion was neutral and in (c) the solution had 0.01 mol dm-’ KNO, as en ionic streogth buffer.

appointingly these films may only find application as sensors for dissolved CO, in aqueous solution in which the ionic strength is low (typically< 0.01 mol dmW3) and the pH is 26.

The response and recovery times of the unplasticized polymer m-cresol purple f&r as a sensor of dissolved COz were measured. In this study, 0.125 cm3 of 100% CO,-saturated water solution was injected into 2.5 cm3 of N,-saturated water solution containing the f&n sensor. The 50 and 90% response times of the fibn sensor to this 0 --) 5% change in dissolved CO, were found to be 9 and 19 s, respectively. The partial recovery of the film was achieved through the subsequent addition of 1 cm3 of N,-saturated water solution to the test solution. The 50 and 90% recovery times corresponding to the

88 A. Milk, Q. Chang I Sensors and Achutors B 21 (1994) 83-89

change in dissolved CO2 from 5 to 3.6% were found to be 6 and 21 s, respectively.

When an 8.25~10~~ mol dmm3 NaOH solution is equilibrated with various gas mixtures, ranging from 0 to 5% COu the solution will display pH values ranging from 11 to 7, respectively. Given that the Mm sensors described in this work also respond to pH changes, it could be argued that the results illustrated in Figs. 3(b) and 4 for the plasticized and unplasticized films, respectively, do not show conclusively that the films are suitable sensors for dissolved CO,, since they may simply be acting as pH sensors.

Clearly, the cross-sensitivity of the films is a problem and a concern, as indicated earlier. However, a brief examination of the results illustrated in Fig. 5 for the unplasticized fihn shows that its response to aqueous solutions that are either acidic or of high ionic strength is usually very slow (e.g., r(O-50%)> 15 min upon exposure to an aqueous solution of pH 4), compared to the response and recovery times of the films upon exposure to an aqueous solution containing 5% CO, (typically<30 s); similar results were obtained for the plasticized films. In addition, the same absorbance versus percentage CO, profiles as illustrated in Figs. 3(b) and 4 for the unplasticized and plasticized films, respectively, were obtained using aqueous solutions that were first saturated with CO, of various levels and then introduced to the film, rather than saturated with CO, (for 2-3 min) in the presence of the film, before taking an absorbance measurement. The same absorbance versus percentage CO, profiles illustrated in Figs. 3(b) and 4 for the unplasticized and plasticized films were also obtained in alkaline and neutral starting solutions.

The above results provide strong evidence that the unplasticized and plasticized films can be used to measure dissolved do, levels in aqueous solution, albeit under limited conditions of pH and ionic strength. This situation arises because the cross-sensitivity of the films towards ionic species is a slow process that is not marked at low ionic strengths and neutral/alkaline pH values. Even under the latter conditions, it is clear that the films are best suited for a single dissolved percentage CO2 measurement, rather than continuous monitoring.

4. Conclusions

Plasticized and unplasticized polymer calorimetric film sensors for gaseous Co,, containing the dye m- cresol purple, previously developed for detecting gaseous CO, can be used for making quantitative measurements of dissolved CO,. The plasticized form of the polymer m-cresol purple film sensor suffers from a disadvantage compared with the unplasticized form in that it develops a measurable degree of opacity when exposed to aqueous solution, especially in neutral, compared with alkaline,

solution. With both types of film sensors the dye appears to be slowly converted from its initial deprotonated form (blue) to its protonated form (yellow) if the aqueous solution is of a high ionic strength (i.e., >O.l mol dme3) and/or low pH (i.e., <pH 4). The rate of this process increases with increasing ionic strength, anionic charge and decreasingpH. This latter feature represents a severe limitation to using these fihns as sensors for making continuous dissolved CO, measurements. It should be possible to overcome this limitation by using a gas-permeable ion-impermeable membrane cover, such as PTFE; however, the use of more hydrophobic polymers in the ti formulation may also achieve this goal. Both these areas are under current investigation.

Acknowledgements

We gratefully acknowledge support of this research by Johnson and Johnson Professional Products Ltd. We thank Prdfessor Wolfbeis and his co-workers for sending us copies of draft manuscripts detailing their work in this area.

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A. Mills, Q. Chang I Sensors and Achutors B 21 (1994) 83-89 89

Biographies

Andrav MilLr received his B.Sc. (Hons) in chemistry from the University of London in 1979 and his Ph,D. from the Royal Institution in 1982. Since 1982 he has held a lectureship at the University of Swansea. His fields of interest include homogeneous and heteroge- neous photochemistry, solar to chemical energy conversion, optical and electrochemical gas sensors, redox catalysis and corrosion science.

Q&g Chang received a B.Sc. in chemistry from Peking University, Beijing, China, in 1982, an M.Sc. from the Institute of Photographic Chemistry, Academia Sinica, Beijing, China, in 1985 and a Ph.D. from the University of Wales, UK, in 1993. His fields of interest include chemical sensors, the kinetics and mechanism of chemical reactions, especially electrochemical reactions cat- alysed by metal colloids, and the applications of com- puter simulation technique to chemistry.

colorimetric polymer film sensors for dissolved carbon dioxide

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