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Anal. Chem. W Q l , 63, 970-974
Hydration of Trifluoroacetophenones as the Basis for an Optical Humidity Sensor Kemin Wang,' K u r t Seiler, Jean-Pierre Haug, Beatrice Lehmann, Steven West? Karel Hartman? a n d Wilhelm Simon* Swiss Federal Institute of Technology (ETH), Department of Organic Chemistry, Universitatstrasse 16, CH-8092Zurich, Switzerland The reversible hydration of trifluoroacetophenoneswas taken ' as the basis for the construction of a new kind of humidity sensor. Since this hydration is followed by a change in the absorption spectrum of the acetophenone isologues, a straightforward optical transduction in the UV range was possible. Plasticized poly( vinyl chloride) (PVC) membranes that incorporate such electrically neutral and lipophilic compounds together wHh a quatemary ammonium sail (MTDDACI) as a catalyst exhibit a series of impressive characteristics. These so-called optode membranes show an excellent selectivity for humidity over other gases, a high stability, good reproducibility, a very short response time In the range of seconds, and a wide measuring range, which can easily be changed by the introduction of different substituents In the para position to the trifiuoroacetyi group. With two isologue ligands, a determinatlon of relative humidity (RH) in the range from 1% to 100% RH with a high precision was possible.
INTRODUCTION Humidity sensing is important in the control of relative humidity, both for the promotion of human comfort and especially in industrial processes, where the efficiency of drying operations can be improved. Due to the widespread applications of humidity sensors, the demands made on the characteristics of the device, such as sensitivity, response time, reproducibility, lifetime, dependence on temperature, etc., are specific for the field of application and therefore quite different (I). An impressive number of humidity sensing systems exploit physical methods. Many sensors, which make use of the dependence of electrical properties (e.g., resistance, capacitance, or conductivity) of different materials (e.g., porous ceramics, organic polymers, electrolytes) on humidity, can be found in the literature, and some of them are commercially available (for a review, see refs 2-4). Recently, optical sensors (optodes) for various chemical species and physical parameters have drawn considerable attention because of certain advantages especially over electrochemical devices ( 5 , 6 ) . Three different types of humidity sensors with an optical transduction have been realized so far. The first is based on the physi- or chemisorption of water directly on the surface of the optical device or in a very thin and porous film (7-9), leading to a detectable change of the refractive index. Of course, other gases can also influence the refractive index and therefore can heavily contribute to the response signal. Other optical humidity sensors make use of the color change of certain metal ion complexes, when water is participating in the coordinating sphere. So the absorption band of the often used salt CoC12undergoes a dramatic hypsochromic shift, when exposed to humid air. CoC12was embedded in various matrices, such as gelatine (IO),poly(vinylpyrro1idone) (11), On leave from the Hunan University, Changsha, China. 2 0 n leave from Orion Research Incorporated, 529 Main St, Boston, MA 02129. 3 0 n leave from the University of Victoria, P.O. Box 1700, Victoria, BC. Canada V8W 2Y2.
and cellulose matrix (121, or directly deposited on a porous optical fiber (13). The selectivity of these devices seems to be quite good, whereas generally the response time as well as in some cases the hysteresis is not yet satisfactory. The third type was derived from the phenomenon that water molecules are able to influence the fluorescence emission of certain compounds. With increasing relative humidity, either an enhancement or a quenching of the fluorescence intensity of an indicator dye, such as umbelliferon (14), rhodamine 6G (15), or perylenedibutyrate (16), was observed. Remarkably short response times have been obtained with these optodes, but other gases, especially molecular oxygen, can seriously interfere. Here we report on a new kind of humidity sensor, which is based on a reversible reaction of water with certain organic molecules, namely the trifluoroacetophenones. Due to the change of the absorption spectrum produced by the hydration of these compounds ( I 7) and due to the fact that the reaction step can easily be accelerated (18),an optical humidity sensing system with a respectable performance could be realized. In the past, electrically neutral trifluoroacetophenones have been found to exhibit in ISEs a remarkable preference for carbonate over other anions (17, 19). Behringer et al. conducted investigations on the selectivity behavior of trifluoroacetophenones with different substituents (17). The introduction of strong electron acceptors in the para position to the trifluoroacetyl group favors a nucleophilic attack at the carbonyl carbon atom and as a consequence leads to a higher preference for CO2- over other anions. Furthermore, it was found that not only the selectivity but also the hydration of the carbonyl group is mainly dictated by the Hammett constant u of the substituent. Now, this observation was exploited for humidity sensing. Plasticized poly(viny1 chloride) (PVC) was used as the membrane matrix since this material already met the requirements for optode membranes selective for a large variety of electrically charged (20) and neutral substrates (21). The additional incorporation of methyltridodecylammonium chloride (MTDDACI) in the membrane phase led to a dramatic acceleration of the hydration step, so that the response time was mainly determined by diffusion processes. Not only a complete characterization of the sensor performance is given but also the fundamentals are discussed in detail.
EXPERIMENTAL SECTION Reagents. For membrane preparation, poly(viny1 chloride) (high molecular weight), bis(2-ethylhexyl) sebacate (DOS), and tetrahydrofuran (THF) were obtained from Fluka AG (Buchs, Switzerland); methyltridodecylammonium chloride (MTDDACI) was obtained from Polysciences (Warrington,PA). The syntheses of heptyl p(trifluoroacety1)benzoate (ETH 6010) and 4-(n-dodecylsulfony1)-1-(trifluoroacety1)benzene(ETH 6019) me described in ref 17. Membrane Preparation. The optode membranes consisted of 50 mg of PVC, 100 mg of plasticizer DOS, 3.0 mg of MTDDACl, 5.6 mg of ETH 6010 for membrane I, and 7.2 mg of ETH 6019 for membrane 11. The composition for membrane I1 with varying concentrations of MTDDACI was used for investigations of the response time. The membrane components were dissolved in 1
0003-2700/91 /0363-0970$02.50/0 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 10, MAY 15, 1991
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A
9.
DRY AIR
A
MEMBRANE
D
'1
I
PVC
DOS RELATIVE HUMIDITY[xi
0.8
0.6
u
1
EXHAUST
L
-__-_____-__ 2
0.4
MEASURING CHAMBER
Figure 1. Schematic representation of the gas flow system used as the experimental setup to provide different RHs: (1) rotameter; (2) reference cell (22);(3)measuring cell (22);(4) series of wash bottles.
mL of freshly distilled THF. By means of a spin-on device, two identical membranes of approximately 4-pm thickness were cast on two quartz glass plates, which were then mounted in a specially designed measuring cell (22). Before use, the membranes were conditioned for a few minutes in distilled water. Apparatus. A gas flow system (Figure 1)was designed and constructed to get the different required relative humidities (RHs) at a constant gas flow. In this system, water-saturated air, which was obtained by bubbling dry air through several wash bottles, was mixed with dry air in a well-defined proportion controlled by three rotameters (Series 1100, WISAG AG, Zurich) to obtain a total flow rate of 3 L/min. The mixed air stream passed sequentially through the sample and the reference cell. The total pressure in the cells did not perceptibly deviate from the one outside. The humidity of the exhaust was measured by a commercial electronic hygrometer (SA-100, Rotronic AG), which functioned as an on-line calibration device. For this instrument, an accuracy of *l% RH and a precision of 0.5% RH within the range of 0-95% RH are claimed. In the reference cell, two quartz glass plates without membranes were mounted. The two cells were introduced into a UV/vis spectrophotometer (Perkin-Elmer, Model Lambda 2, Ueberlingen, Germany) to measure the change of the absorption spectra in the transmission mode. Calculations. The calculated curves in Figure 3 were fitted to the experimental points by changing Kq The absorbance values AI (0.798 for the membrane incorporating ETH 6019 and 0.889 for the membrane incorporatingETH 6010) were determined with both optode membranes in contact with dry nitrogen after conditioning in distilled water. The absorbance values A,, were determined with the membrane incorporating ETH 6019 in contact with distilled water to 0.051 and with the membrane incorporating ETH 6010 in contact with 0.1 M NaOH to 0.059, at the end of the measurements. Selectivity Measurements. The selectivity measurements were performed by determining the calibration curve in the presence of different but constant concentrations of the respective interfering gases. These concentrations were obtained by bubbling dry air through the liquid of the interferinggas and mixing it with the gas stream of the different RHs.
RESULTS AND DISCUSSION Principle of Operation. The highly electron-withdrawing trifluoromethyl group of the discussed acetophenone derivatives induces a drastic increase of the electrophilicity of the carbonyl carbon atom, and as a consequence, these compounds undergo reversible hydration (I7):
Obviously this interaction leads to a loss in the electron de-
0.2
0
I 230 260 290 320 h[nn A
0
0
E
MEMBRANE
m i - C ~ ~ H 2 a ETH 6019 F3C 0 MTDDACI PVC
DOS
1.
0.8.
RELATIVE HUMIDITY [%]
0.6.
0.4.
0.2-
01
1
230 Flgm 2.
1
1
1
260
/
I
I
290
I
I
I
,
320 h[nn
(a)Absorption spectra of two 4-pmthidc optode membranes
I after equilibration with air streams of different relative humidities. The dehydrated form of ETH 6010 shows an absorbance maximum at 261 nm, the hydrated form at 231 nm. (b) Absorption spectra of two 4-pm-thick optode membranes I1 after equllibratlon with air streams of different relative humidities. The dehydrated form of ETH 6019
shows an absorbance maximum at 253 nm, the hydrated form at 220 nm.
localization of the carbonyl group with the aromatic ring and, therefore, to a hypsochromic shift of the absorption band. As can be seen from Figure 2 the absorption band of ETH 6010 shifts from 261 to 231 nm and the one of ETH 6019 from 253 to 220 nm when hydrated directly in the plasticized PVC membrane. For both ligands, the maximum change in absorbance with varying relative humidity is observed a t the wavelength of the dehydrated form.
ANALYTICAL CHEMISTRY, VOL. 63, NO. 10, MAY 15, 1991
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1
dl
ABSORBANCE
1-
MEMBRANE I
I
MEMBRANE
ETH 6010 MTDDACl PVC DOS
II
0.5 -
12 min
MTDDACl PVC DOS
I
0 I
I
I
-1
0
1
,
2
I
log(RH[%])
Flgure 3. Relative absorbance values at 261 nm for membranes I and at 253 nm for membranes I1 as a function of log RH [ % ] at 20.5 "C. The curves fitting the experimental points were calculated from eq 2b.
I k.2 I I I I I
I
i
For a complete description of the relevant processes, the above-mentioned hydration reaction has to be extended by the vaporization equilibrium of water between the gas phase (9) and the organic membrane phase (org). The overall equilibrium may be written as H,OL(org) + H,O(g) + L(org) (1) where L denotes the electrically neutral ligand. The corresponding equilibrium constant Kq depends, therefore, on the complex formation constant and on the distribution of water between the gas phase and the organic membrane phase. Since the activity of a nonionic species does not differ largely from its concentration, the activities of the involved species can be replaced by its concentrations. Furthermore, if the moist air is treated as a mixture of independent perfect gases, i.e., the validity of the ideal gas law is assumed, the fugacity of water can be replaced by the partial pressure pH@ and hence can directly be described as a function of the relative humidity. Fortunately, this assumption for water gas leads for most applications to negligible errors (23). Assuming an ideal behavior of the membrane (constant values of the distribution coefficient and of the complex formation constant as well as no aggregations of the species involved in the equilibrium (I)), the response function can be described by 1-CY P H ~ O= K e q T or
[%I
1-CY
loOK,,
with Kjq = - (2b) P* where p* is the vapor pressure of water a t the specified temperature and a is the ratio of the concentration of the dehydrated ligand [L] to the total amount L T in the membrane. The absorbance A of this system is therefore directly related to a: a = -[LI =- A -A, (3)
RH
=
LT
where AI and A. are the limiting absorbance values for a = 1 (fully dehydrated ligand L) and a = 0 (fully hydrated ligand L), respectively. If interfering, ideally behaving gases G with the partial pressure p~ compete with water for a complexation by the ligand, the left-hand side of eq 2a has to be extended by selectivity weighted partial pressures:
(4) where the optical selectivity coefficients Kg$G are equal to
d
1 min
\ TIME
Flgure 4. Absorbance response vs time curves for two 4-pm-thick optode membranes I 1 at 253 nm after relative humklity step changes between 95% RH and 0% RH and vice versa. (a)No MTDDACI, (b) molar ratio (MTDDACVETH 6019) 5-30%. the ratio of equilibrium constant Kq of water gas to the one of the respective interfering gas. Measuring Range. In Figure 3, the relative absorbance values a,measured a t 261 nm for ETH 6010 and a t 253 nm for ETH 6019, are given as a function of log (RH [ %]) at 20.5 "C. The curves fitting the experimental points were calculated from eq 2b by using log K6, = 1.65 for ETH 6010 and log K6, = 0.82 for ETH 6019. The correlation between the simulated curve and the experimental data is quite good and thus supports the validity of the derived expression. If the distribution equilibrium of water is assumed to be constant, the difference in Klgq of the two calibration curves can directly be related to the difference in the complex formation constant of the two trifluoroacetophenones. The substituent R of ETH 6019 exhibits stronger electron-acceptor properties than the one of ETH 6010 and builds up stronger complexes with water, and thus, optode membrane I1 is suitable for measurements in the low R H range (1-53%), whereas optode membrane I incorporating ETH 6010 may be used at higher RH (&loo%). Response Speed. It is well-known from organic chemistry that hydration reactions are in general slow processes but can be accelerated in the presence of bases or acids (18). Indeed, the response time of these optode membranes without any additive was very slow (see Figure 4a) but could dramatically be enhanced by the additional incorporation of the quaternary ammonium salt MTDDACl. An increasing concentration of MTDDACl significantly shortened the response time up to a molar ratio of MTDDACl to the ligand of 5% (see Figure 4b). Since the ammonium salt was only able to develop its catalytic activity, if before use the optode membrane was brought in contact with pure water, MTDDACl was assumed to partially exchange the chloride for a hydroxide anion during this conditioning process. The formed MTDDAOH, most of which is of course complexed by the ligand, seems to act as a very strong base. As a further corroboration of our conclusion, OH- was exchanged for the very lipophilic and less basic SCN- by immersing the membrane for a few minutes into a 0.1 M NaSCN solution. As expected, after this treatment, the response time was dramatically prolonged and became comparable to the response behavior of the not conditioned membrane and of the membrane without MTDDACl. As depicted in Figure 5, optode membrane I incorporating
ANALYTICAL CHEMISTRY, VOL. 63, NO. 10, MAY 15, 1991
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0 ‘1. RH 0.8632
0.8632
0.8621
0.8615
/
d 1-
0.8594 ppm) ETHANOL ppm )ETHANOL
0.5 -
ppm )ETHANOL ppm) ETHANOL
1
0.3053
0.3063 2 min
0.3068
95
‘/a
RH
0.3063
0.3071
I
I
7
I
I TIME
Figure 5. Short-term reproducibilkiesof the absorbance response of two 4-Mm-thick optode membranes I at 261 nm for relative humidity changes between 95% RH and 0 % RH.
ETH 6010 reaches the final steady-state absorbance values surprisingly fast after a relative humidity change from dry (0% RH) to wet (95% RH) and vice versa. For this membrane, the response times tg5%were in the range of seconds; membrane I1 incorporating ETH 6019 required only 5 s to go from dry to wet and 30 s to go from wet to dry air. The observation that in an organic membrane phase a reaction involving covalent interactions can be speeded up so dramatically opens up further possibilities in the realization of sensors with chemical recognition. Short-Term Reproducibility and Stability. When optode membrane I was exposed to repeated relative humidity step changes between 0% RH and 95% RH, a remarkably high reproducibility of the optical signals was observed as is shown in Figure 5. The mean absorbance values with standard deviations, as obtained from the measured signals after 1min, are in the case of membrane 10.8619 f 0.0016 for 0% RH and 0.3064 f 0.0007 for 95% RH. The precision of the values at 95% RH correspond to a standard deviation in RH of less than 0.4%. The absorbance signal at 261 nm of optode membrane I in contact with an air stream of 50% RH was recorded during a time period of 5 h. A mean value of 0.4511 and a standard deviation of 0.0005 were calculated from values taken every 30 min (n = 11) and correspond to a relative humidity of 49.8% and to a standard deviation of only 0.2% RH, respectively. The absorption spectra taken before and after this measurement were nearly identical. After a longer period of 10 days, a decrease in the absorbance of about 10% was detected for the optode membrane incorporating ETH 6010. Such a reduction of the concentration of the ligand in the membrane with time, which was observed for both membranes, was already described in ref 17 for an isologue (pbutyl(trifluoroacety1)benzene). It was shown to be due to an evaporation of the ligand from the plasticized PVC membrane. In order to eliminate such a detrimental loss of ligand and to provide a higher lifetime, more lipophilic compounds have to be applied (17). Whereas the precision of the sensor is diminished by a loss of the ligand, the accuracy is not affected, if the loss is taken care of by a compensation of the absorbance value at the isosbestic point. Response to Other Compounds. The exceptional feature of this humidity sensing system is the fact that only nucleophilic molecules are able to interact with the carbonyl group of the ligand. The calibration curves of membrane I were obtained in the presence of different gases, such as acetone, chloroform, tetrahydrofuran, and ethanol. As expected, gases, which do not contain any nucleophilic group, led to no observable interference up to concentrations of 1250 ppm. On
2
3
4
ios(PH20[pa1)
Figure 6. Relative absorbance values at 261 nm for membranes I as a function of log (pW [Pa]) in the presence of different concentrations of ethanol. The curves fitting the experimental points were calculated from eq 4.
the contrary, ethanol, which is able to form hemiacetals and acetals with carbonyl groups, can act as a real competitor with water. Since a conversion of the hemiacetal to the acetal requires specific acid catalysis, the formation of the acetal under these basic conditions has to be excluded, and thus, only 1:l complexes were considered (18).The selectivity coefficient Kxb,ethanol obtained by fitting the simplified relationship given in eq 4 to the experimental points was 2.8 for optode membrane I (see Figure 6) and 0.5 for membrane 11, respectively. As outlined above, the hydration equilibrium is susceptible to both acids and bases, and thus, selectivity over other gases, such as HAC (2000 ppm), ammonia (100 ppm), NO2 (10 ppm), and SO2 (10 ppm), was investigated. In the case of optode membrane I, they contributed to the intensity of the optical signal at 0% RH by less than 1 % , and at 10% RH,no influence at all was observed. With one exception, all the tested gas concentrations are above the limits for human exposure proposed by the American Conference of Government Industrial Hygienists (24), and hence, optode membrane I fulfills these requirements set on the selectivity. Ethanol, for which this limit is 1000 ppm, will interfere at low RH. Because trifluoroacetophenones exhibit extraordinary selectivities for carbonate in ISEs and because COP is an important component of the atmosphere, the effect of C02 on the response of the presented humidity optodes was studied in detail. When dry Nz was used instead of dry lab air in the gas flow system in Figure 1,no change in absorption spectra of the optode membranes could be observed. Obviously, the low concentration of COz in normal air (0.033 f 0.001 vol %) does not interfere. Thus, higher concentrations of C02 were introduced into the gas flow system. For optode membrane 11, at 45% RH the amount of COz can be up to 100 times higher than the normal level and at a very low relative humidity (
