Anal. Chem. 1990, 62, 1935-1942
1935
Amperometric Microsensor for Water Huiliang Huang and Purnendu K. Dasgupta* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
A thh fHm of a perfluorosulfonate lonomer (PFSI) is supported upon two electrodes. The posltlve e M r o d e Is c6mposed of a noble metal. The polymer fllm has high afflnHy for water and certain other highly polar molecules, notably alcohols. When a voltage above the threshold d electroiytk breakdown potentlal of such a compound Is applied across the film, the compound partltloned Into the film can be electrolytically demnposed. Provkled that the electroiytk breakdown products have a high enough vapor pressure to spontaneously leave the film, a versatlle sensor, with a current output related to the analyte concentration, is formed. Sensors fabrkated from available PFSI materials and functioning In this manner behave essentlally as sensors speclflc for water and the lower alcohols.
Water is omnipresent on this planet. In our present state of existence, the determination of water in a variety of matrices is routinely necessary. Volumes have been written regarding the measurement of water, whether present as a major or a trace constituent ( I , 2). Among chemical methods, the Karl Fischer reaction is the best known; new improvements in exploiting this chemistry continue to be reported (3,4). A plethora of physical methods has been utilized for the measurement of water: IR and near-IR absorption spectroscopy including the use of diode laser sources, photoacoustic detection of surface plasmon resonance (5-8), Kr or H glow-tube hygrometry (9),thermooscillometry (lo),direct potentiometry (1I), cyclic voltammetry (121,etc. represent some recent examples. With some samples, direct determination is impossible and prior chromatographic separation from the sample matrix is essential (13-15);another often used approach is to generate acetylene by reaction with CaC2,followed by chromatographic determination of the evolved gas (16-18).The reaction of dimethoxypropane with water to form acetone and methanol is well-known and has been utilized in the past for dehydrating various samples (19-22). In favorable cases, the products can be measured directly by absorption spectroscopy (23);otherwise chromatographic separation is necessary (24, 25). The enthalpimetric sensing of the dimethoxypropane reaction was first developed by Wilson (26)and has since been widely exploited in commercial thermometric titrators (27). In many cases, direct measurement of water is possible. Chilled mirror hygrometry (28)is widely believed to be the most reliable approach but cannot be applied in condensed phases or when other condensable species are present in large concentrations. Reported optical fiber based sensors generally involve a sensing element based on a phase-transition (accompanied by a change of color or opacity) (29-31)but such sensors are still to be proven commercially viable. One particularly interesting approach utilized perfluorosulfonate ionomer (PFSI) immobilized Rhodamine 6G; an increase in fluorescence intensity and decrease in fluorescence lifetime are observed with increasing humidity (32).Surface acoustic wave moisture sensors, regarded as holding much promise (33), are also in the experimental stage. In presently popular water sensors, one of two basic principles is exploited. Typically, two electrodes are in contact
with a substrate with a high affinity for water. Synthetic organic polymers or inorganic ceramic type materials are used as substrates. Sorption of water causes a change in electric properties of the polymer, such as the capacitance, which may be measured by determining the ac impedance of the device. Aluminum oxide is best known for this application (34); however, because of superior performance characterics, a silicon-based sensor is rapidly gaining popularity (2,351.Many other substrates have also been described (36-42).With some substrates, it is possible to use the change in dc resistance, rather than ac impedance, as an index of moisture content. However, such an approach is meritorious only if sufficient potential can be applied to the sensor to electrolytically decompose the sorbed water. The solid electrolyte substrate must resist electrolytic breakdown; only two substances have been shown to be applicable for this purpose. Phoshorus pentoxide is well-known for its affinity for water. The electrolysis of syrupy H3P04 as a film between two electrodes yields Pz05as a paste with H P 0 3 presumably formed as the intermediate product. The present commercially available versions of this sensor have changed little from the original description by Keidel (43);the principle was shown to be applicable to the measurement of moisture in organic liquids as well (44).While its ability to measure trace levels of water is an attractive feature, the relative fragility of the P205film is a problem. The sensor is generally irreversibly affeded upon exposures to high levels of humidity. The search for a better sensor therefore continues. Within a decade of commercialization of Nafion by Du Pont, this PFSI was shown to be applicable as a solid electrolyte substrate material for an electrolytic water sensor (45). Aside from tubular membranes attached to coillike electrodes on the interior and exterior (451,planar Nafion membranes have been used with painted-on or pressed-on electrodes (46, 47). Because of the thick film characteristics intrinsic to these designs, the response time to a step change in humidity is diffusion limited to several minutes. The attainable detection limit for water with such sensors or the effects of other compounds or temperatures are not known. Similar devices are, however, being explored for commercial use as a “microdryer”, designed to electrolytically “burn off“ the water (48). In recent years, a new cation exchanger PFSI with properties significantly different from Nafion has been reported by Dow scientists (49,50).In particular, this PFSI can be synthesized with much lower equivalent weights than Nafion, with correspondingly higher water absorption (49-51).In this paper, we report a new class of inexpensive, sensitive, and fast, thin film sensors made from the Dow PFSI and Nafion and characterize them in detail.
EXPERIMENTAL SECTION
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Reagents and Supplies. Nafion was obtained as a 5 w t % solution (equivalentweight (EW) 1100, in a medium of 90% lower alcohols/lO% water, Aldrich). The solution was diluted 1:l with 95% ethanol before use. The Dow PFSI, 1,1,2,2-tetrafluoro-2-( (trifluoroetheny1)oxy)ethanesulfonicacid, was obtained either as a film (EW ranging from -600 to -1200) or as a 2.5 wt 7% solution in alcohols as a gift from the Dow Chemical Co., Freeport, TX. The sulfonyl fluoride form, the precursor form of the Dow ionomer, was also obtained as a powder. Like Ndion, only the sulfonyl fluoride form can be extruded or pressed into
0003-2700/90/0362-1935$02.50/0 0 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990
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PTFE tubing
1
I
1
Stainless steel tubing Pt w i r e
I
Nafion membrane
Pt-wire
Flow
Plastic Tube /
/ PFSI
Figure 1. Needle-type RH sensor.
a film. Attempts to form films at the tip of needle type microsensors (vide infra) by thermoforming the sulfonylfluoride powder did not lead to good sensors and this approach was not further pursued. The sulfonic acid polymer was used in solution form, after dilution with 95% ethanol to 51.25 w t %. The preformed film material was put in solution essentially following the procedure of Martin et al. (52): weighed amounts of the polymer were dissolved in ethanol-water mixtures by prolonged heating in a poly(tetrafluoroethy1ene) (PTFE) high pressure bomb (Paar Instrument Co.) at temperatures of 150-240 "C depending on the EW of the polymer. Noble metal wires (e.g., platinum wires in diameters of 25,50, and 100 pm) were obtained from Aesar, Inc. Stainless steel needle tubing (type 304) was obtained either in the form of standard hypodermic needles or as lengths of tubing (Small Parts, Inc., Miami, FL). Instrumentation. Potenial applied to the sensor was supplied from 9-V alkaline batteries, a Harrison 6106A dc power supply (0-120 V, Hewlett-Packard), or an electrophoresis dc power supply (25-310 V, Fotodyne, Inc.). The current flowing through the sensor was monitored in a variety of ways: measuring the voltage drop on a resistor connected in series to the sensor, using an operational amplifier based current voltage converter, or using a Model 421 picoammeter or a Model 427 current amplifier (both from Keithley instruments). Sensor output was recorded on a multichannel strip chart recorder (Knauer, FRG). The effect of temperature upon the sensor response was determined by locating the sensor (and a length of in. diameter copper tubing upstream of the sensor for thermal equilibration of the test gas) inside a liquid chromatography column heater (Model CH-30, Fiatron Systems, Inc., Oconomowoc, WI). Commercial capacitance or ac impedance type sensors were tested respectively with their own electronics or were connected to a Model 213 conductivity detector (Wescan Instruments, Santa Clara, CA) for excitation at 10 kHz. Sensor Fabrication. Although a number of different sensor configurations were explored, two designs emerged as preeminent and are described here. The first, hereinafter called the needle sensor, utilizes a coaxial design as shown in Figure 1. The end of a hypodermic needle (18-22 gauge) is cut to a blunt terminus and a snugly fitting PTFE insulator tubing is inserted within. A platinum wire, 100 pm in diameter, forms the central electrode. At the rear end of the needle (e.g., inside the hub), an insulating epoxy-based adhesive is applied to hold these components in place and a sturdier lead wire is also soldered to the central Pt conductor and held in place by the adhesive. A PTFE sleeve is then placed outside the needle, this fits snugly around the needle and nearly flush with the needle terminus. A small amount (typically 3 pL) of the PFSI solution is then applied to the top of the needle assembly with a microliter syringe. After the solvent evaporates at room temperature, it is thermally treated by placing in an oven at 70-100 "C for 4-12 h. It has been previously noted that unless thermally cured, solution cast PFSI films of this type can have considerably different properties from films obtained by thermoforming the sulfonyl fluoride and subsequent hydrolysis (53). Accordingly, we noted that without the thermal treatment step the sensor response may initially be higher but tends to continually decrease during operation. The thermal treatment step was therefore incorporated in all subsequent experiments. As shown in Figure 1, the PFSI film is formed not just as a planar sheet at the tip of the sensor but also between the outer sleeve and the needle and inside the inner PTFE sleeve. While these aggregations improve the integrity of the sensor by increasing the adherence of the film to the electrodes, it becomes difficult to exactly estimate
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Figure 2. Cross section of flow-through RH sensor. A longitudinal cutaway view is shown. The gas flow is horizontal, along the plane of the paper.
the mean thickness of the planar portion of the film. On the basis of microscopic examination, we estimate that the aggregate areas can contain as much as 50% of the applied PFSI, the other half being responsible for the planar fiim. The reported film thickness data are based, however, on the assumption of a planar film only and assumes a polymer density of 2 g/cm3 (54). Sensors with increasing film thicknesses are fabricated by repeating the steps of coating, room temperature evaporation, and thermal treatment. Unless otherwise stated, the results reported are for a needle sensor. The second design, hereinafter called the flow-through sensor, is shown in cross section in Figure 2. This design utilizes a 1.5 mm i.d., 3 mm 0.d. polyurethane tube segment, ca. 2 cm in length. A narrow gauge (227) hypodermic needle is used to puncture the wall of the tubing radially. A segment of 50 or 25 pm diameter Pt wire is inserted through the bore of the needle and the needle is then removed. The process is then repeated to place a second wire parallel to and as close to the existing wire as possible. The excess lengths of platinum wires are wrapped around the polymer tube, lead wires are attached to each platinum wire electrode, and the assembly is secured and insulated with parafilm. As with needle sensors, the PFSI fib is formed by applying a microaliquot (2-5 pL) of the PFSI solution on the electrode wires inside the tube. As the solvent evaporates, the surface tension of the film pulls the wire electrodes closer; the typical distance between the wires after the film is thermally cured is 100-300 pm. Microscopic examination indicates that much of the polymer material aggregates near the supporting peripheries of the film structure, resulting in a particularly thin central film. In use, the test gas flows through the tube around the sensing film. The results from two other designs are briefly cited in this work. One design utilizes intedigitated rhodium film electrodes spaced 150 pm apart, deposited on a 1 x 1 cm alumina wafer. The second design contains a cylindrical ceramic element (6.3 mm diameter) on which two rhodium wires (0.2 x 200 mm) are wound in parallel without contacting each other (center-to-center spacing 320 pm). The film was formed on both of these sensors by dip coating several times. The film thickness of these sensors was 2100 pm. The first of these is referred to as the wafer-type sensor and the second as the cylindrical sensor. The base material for these devices was obtained from EG&G, Inc., Environmental Equipment Division (Burlington, MA) and Chandler Engineering Division (Tulsa, OK), respectively. Low-level humidity testing of these sensors was carried out at these respective facilities. Test Arrangements. Preliminary results have indicated that while the sensor can be deployed in liquid samples, film adhesion needs to be improved for reproducible results. In the present paper only gas-phase applications of the sensor will be reported. Experiments to determine sensor longevity, stability, response times, etc. were conducted by using the test setup shown in Figure 3. Compressed house air dried through silica gel (relative humidity (RH) ca. 5 % ) or cylinder nitrogen passed through M g (C10& (RH 0%) flows, regulated by regulator R, through two metallic solenoid-operated three-way valves, V1 and V2 (12 V dc, Skinner valve MBD 002). These valves are eontrolled by a microprocessor-driven process controller (Micromaster LS, 100 ms resolution, Minarik Electric, Los Angeles, CA); the value acutation time is l?
0
2
4
6
8
10
Figure 7 . Response behavior of a needle type sensw (1) compared lo a commercial ac impedance type senm (2) and a capacitive Senm (3);5-75% RH test swing.
MICROLITERS OF 1.25 I DOW PFSI Ew 850 SOLUTION ON SENSI
Flgure 5. Sensitivity and response time as a function of the amount of polymer on a needle-type sensor.
produced by devices of such size (active film area > 1 ) present in the straight line connecting them. (It can be easily shown that the specific number of clusters considered in such a model is immaterial.) Let the field strength across the water cluster be E , and the field strength across the polymer backbone be El.We assume
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1940
ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990
Q@QO Needle Sensor
'
CGCCG Needle Sensor 2 nnnoD Cylindrical Sensor nnaM Wafer Sensor
i
-26
1
-13
Figure 9. Response of four different electrolytic thin film PFSI sensors
Figure 10. Response times and sensitivity for a needle-type sensor
at low humidity levels.
as a function of applied voltage.
that the current is linearly proportional to the field strength across the polymer backbone
these macrosensors behave as an assemblage of microsensors in parallel. This essentially results in statistical consequences. The larger fluctuations observed for an individual single needle-type microsensor are understandable in that there are likely a multitude of cluster sites where water aggregation can occur. If this occurs in a chaotic manner, specific sites which are occupied (the geometric location of which, vis-a-vis the electrodes, governs the resistance) may not be the same each time the device is brought from a drier to a wetter environment. At higher humidities this does not pose a problem because of the much larger number of clusters (and hence the statistically larger number of pathways for current conduction) that exist. An assessment of the preliminary reproducibility data indicates that a t an applied voltage of 15 V, the lower practical limit of an 18 gauge needle sensor is likely to be a dew point of -40 "C (-130 ppm H20) and that for a macrosensor like the wafer sensor is likely to be -55 "C (- 20 ppm H,O). These figures should improve with higher applied field strengths (reduced interelectrode distance and/or higher applied voltage. It is possible that a constant current mode of operation with a series resistor for safety protection, with the applied voltage being the output parameter, may have advantages for low humidity sensing. The performance of the thin film electrolytic PFSI sensors is notable especially in view of Huang's recent assessment, based on his long-standing work at NIST in this area, of the present status of polymeric RH sensors-such sensors are "generally characterized by low sensitivity a t high levels of relative humidity" (60). The magnitude of the applied voltage has consequences on the response time as well. This is true even a t relatively high RH. The current level, rise time and fall time for a 19-gauge needle sensor is shown for a 5 75% RH test cycle in Figure 10 as a function of applied voltage. It seems that the ultimate response times are obtained only a t applied voltages large enough to reach the plateau current and this is consistent with the general model. Although Figure 10 does not show any marked change in the fall time, the fall time is also a function of applied voltage at lower humidity levels. Thermodynamic Behavior. Effect of Temperature. Huang (40, 41, 60) has studied thin film capacitive sensors fabricated from PVC-styrenesulfonate and PFSI's bearing both sulfonic and carboxylic acid groups. In particular, response to different water vapor concentrations was tested a t various temperatures and the values of AG, AH, and A S for the process of water sorption by the polymer thence computed. A H was found to be essentially temperature independent but strongly dependent on the water vapor concentration, de-
i = KE2
(4)
where i is the current and K is a constant of proportionality. The dimension of the water cluster is dependent on the humidity level. The total applied voltage V is given by
V = 1EI
+ (X - l)E2
(5)
When the first term in eq 5 is negligible
V=
1)Ez
(6)
i = K V / ( x - 1)
(7)
(X
-
Equations 4 and 6 yield A logarithmic transformation of eq 7 yields
In i = In ( K V / x ) - In (1 - - 1 / x ) since l / x
