Quartz Resonator for Simultaneously Measuring Changes in the Mass

Aug 30, 2012 - A novel quartz resonator was developed to measure, simultaneously, changes in the mass and electrical resistance of a polyaniline film ...
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Quartz Resonator for Simultaneously Measuring Changes in the Mass and Electrical Resistance of a Polyaniline Film Changyong Yim, Minhyuk Yun, Namchul Jung, and Sangmin Jeon* Department of Chemical Engineering, Pohang University of Science and Technology, San 31 Namgu Hyojadong, Pohang, Republic of Korea S Supporting Information *

ABSTRACT: A novel quartz resonator was developed to measure, simultaneously, changes in the mass and electrical resistance of a polyaniline film during the absorption of water vapor. Interdigitated gold electrodes were vacuum-deposited on the sensing surfaces of the quartz crystals, and polyaniline films were drop-cast on the electrodes used to measure the changes in the electrical resistance. Two symmetric semicircular gold electrodes were deposited on the bottom surface of the quartz crystal. These electrodes were used to measure the changes in the mass of absorbed water based on the changes in the resonance frequency. The simultaneous measurements of mass and electrical resistance shed light on the interactions between the water vapor and the polyaniline film. The resonator was exposed to various organic gases, including ethanol, acetone, or chloroform, and each gas was found to produce characteristic changes in the normalized electrical resistance.

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The separation of the sensors introduced time delays between the measurements, thereby requiring larger devices. In this study, we developed a novel conducting polymer sensor integrated with lateral field excited (LFE) piezoelectric resonators. In contrast to conventional QCMs, which include two electrodes deposited on the top and bottom surfaces, an LFE resonator included two symmetric semicircular electrodes on the bottom surface with a small gap that left the top surface (a sensing surface) uncoated.16−18 Interdigitated electrodes (IDEs) were deposited on the top surface, and a polyaniline film was drop-cast onto the electrodes. Changes in the mass and electrical resistance were monitored simultaneously during the adsorption of various gases onto the conducting film. This study presents, to the best of our knowledge, the first development of IDE-LFE quartz resonators and their applications to gas sensors.

onducting polymers, such as polyaniline, polypyrrole, or polythiophene, have attracted much interest because of their potential use in applications, including solar cells,1 electrochromic devices,2,3 and chemical sensors.4−8 In the area of chemical sensor applications, conducting polymer films have been extensively investigated for use as the sensitive layers in gas sensors. As gas molecules interact with conducting polymer films, the electrical resistance varies depending on the amount of gas adsorbed. Conducting polymer gas sensors have many advantages over commercially available inorganic semiconducting material-based sensors that function only at high temperatures. Conducting polymer sensors can operate at room temperature with high sensitivity. The nanostructured or porous conducting polymer films facilitate the diffusion of gas molecules, which shortens the response time of the sensors.5,6,9,10 Conducting polymer gas sensors display several drawbacks, including poor selectivity.11 Further, they require a tedious calibration process prior to use because the resistance change is not linearly proportional to the gas concentration over the entire concentration range.12 These problems can be addressed by combining conducting polymer sensors with sensors that can measure other changes that are complementary to the electrical resistance change due to gas adsorption. Quartz crystal microbalances (QCMs) are gravimetric sensors that accurately measure changes in mass. Some studies have attempted to combine QCMs with conducting polymer-based resistance sensors for the detection of various gases; however, previous studies employed QCMs and conducting polymer sensors separately.13−15 The two sensors were not successfully integrated into a single chip because the top and bottom surfaces of conventional QCMs are coated with gold electrodes. © 2012 American Chemical Society

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EXPERIMENTAL SECTION Materials. Polyaniline emeraldine salt in xylene, formic acid, ethanol, acetone, chloroform, or aqua regia was purchased from Aldrich (St. Louis, MO) and used without further purification. Deionized water (18.3 MΩ cm) was obtained from a reverse osmosis water system (Human Science, Korea). The 5 MHz quartz crystals (1.27 cm in diameter) were purchased from ICM (Oklahoma City, OK), and their gold electrodes and chromium adhesion layers were removed using aqua regia and a mixture of ceric ammonium nitrate and nitric acid, respectively. Received: May 21, 2012 Accepted: August 30, 2012 Published: August 30, 2012 8179

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Figure 1. Schematic illustrations of the electrode patterns on (a) the top surface and (b) the bottom surface of a quartz resonator. (c) An optical microscopy image of an IDE-coated LFE quartz resonator.

Figure 2. Schematic diagram showing the experimental setup.

Figure 3. Variations with relative humidity (black) in (a) resonance frequency only, (b) electrical resistance only, and (c) both resonance frequency (red) and resonance frequency (blue) of a polyaniline-coated quartz crystal.

Patterning of the Gold Electrodes on the Quartz Crystals. Shadow metal masks were obtained from Yesung (Gyeonggi-do, Korea) and were used to pattern the gold electrodes on bare quartz crystals. Parts a and b of Figure 1 show schematic diagrams of the gold patterns on the top and bottom surfaces of the quartz crystals, respectively. The lengths, widths, and gap distances among the 21 IDEs on the front side were 2.5 mm and 100 and 100 μm, respectively. The diameter and gap distance of the LFE electrodes on the back side were 10 and 1 mm, respectively. After cleaning the quartz crystals with, sequentially, aqua regia, ethanol, and deionized water, the electrodes were deposited as 10 nm chromium and 100 nm gold layers on quartz crystals via thermal evaporation. An image of a resulting quartz crystal is shown in Figure 1c. Preparation of Polyaniline Films on the Patterned Quartz Crystals. Polyaniline emeraldine salt in xylene was heated in an oven to evaporate the xylene, and the remaining polyaniline was dissolved in formic acid (1 mg/mL). A 100 μL aliquot of the solution was added dropwise to an IDE-coated quartz crystal and allowed to dry under nitrogen flow. The resistance of the polyaniline film depended on the film thickness and varied from 100 to 1000 kΩ. This resistance

range could be measured using a simple digital multimeter. The film thickness used in this study was measured to be 60 nm using a surface profiler (Alpha-Step 500, Tencor Instruments, CA). Instrument Setup. Figure 2 shows a schematic diagram of the instrument setup used in this study. A patterned quartz crystal was mounted in a temperature-controlled chamber. Dry nitrogen was used as a carrier gas and was passed through a gas bubbler containing deionized water or organic solvent to generate vapor. Pure liquid was used to generate organic solvent vapor, and the concentration of each vapor was determined from the equilibrium vapor pressures at room temperature (ethanol, 72 000 ppm; acetone, 27 000 ppm; chloroform, 23 400 ppm). By contrast, the relative humidity was varied by controlling the flow rates of the dry and wet nitrogen using mass flow controllers (Brooks Instruments, Hatfield, PA) at a fixed total flow rate of 100 mL/min. The final relative humidity was measured at the outlet of the chamber using a commercial moisture sensor (Picotech, Cambridgeshire, U.K.). The IDEs were connected to a digital multimeter (Agilent 34410) to measure the change in the electrical resistance. The LFE electrodes on the bottom surface were 8180

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resonators is sensitive to temperature, the resonance frequency of the polyaniline-coated quartz crystal decreased by ∼15 Hz as temperature increased from 15 to 35 °C under nitrogen atmosphere (see the Supporting Information). The resonance frequencies did not return to their original values after rinsing with nitrogen, showing hysteretic behavior. The degree of hysteresis increased with decreasing temperature, indicating that more water remained in the polyaniline film at lower temperatures. Below 20 °C, water condensed onto the polyaniline film and the frequency decreased at a constant relative humidity (∼80% at 20 °C and ∼70% at 15 °C). The relative humidity could not be increased above 70% at 15 °C due to the presence of substantial condensation. The decrease in the electrical resistance of the polyaniline film in the presence of water vapor arose from the proton exchange between the polyaniline and the absorbed water. The nitrogen groups in the polyaniline chains are present in various states, including NH2+, NH, N, NH+, and N+, depending on the oxidation state. Water binds to charged nitrogen groups (NH2+) in polyaniline, and a proton transfer takes place from a polymer to the absorbed water.21

connected to an impedance analyzer (QCM Z500, KSV Instruments Inc., Finland) to measure changes in the resonance frequency of the quartz crystal.

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RESULTS AND DISCUSSION Figure 3 shows the variations with relative humidity in the electrical resistance and resonance frequency of a polyanilinecoated quartz crystal. To investigate the interference between the IDE and LFE measurements, variations in resonance frequency only, variations in electrical resistance only, and simultaneous changes in resonance frequency and electrical resistance were measured when the quartz crystal was exposed to 60% relative humidity. Each measurement was conducted three times, confirming the reproducibility and reversibility of the measurements. The resonance frequency of a conventional LFE is known to be affected by the conductivity of a medium because no shielding electrode exists on its sensing surface.19 However, negligible crosstalk was observed between the IDE and LFE measurements possibly due to the shielding effect of the conducting polymer film. Parts a and b of Figure 4 show the variations in the resonance frequency and electrical resistance of a polyaniline-coated

NH 2+ + H 2O → NH + H3O+

Subsequent charge transfer between adjacent nitrogen atoms returns N−H to its original state, making the polyaniline chains conductive. As with the frequency changes, the degree of hysteresis increased with decreasing temperature, confirming that higher amounts of water vapor remained in the film at lower temperatures. The decrease in the electrical resistance saturated at high relative humidity values, and negligible changes were observed during water condensation at 15 and 20 °C. The influence of the sensing areas of the IDE and LFE on the measurements was investigated by conducting a control experiment using an IDE-coated quartz crystal without a polyaniline film. Exposure of the quartz crystal to a 75% relative humidity environment at 25 °C produced a frequency change of only 4 Hz as shown in Figure 5. A negligible change in the

Figure 4. Variations in (a) the resonance frequency and (b) the electrical resistance of a polyaniline-coated quartz crystal as a function of the relative humidity at various temperatures (black, 15 °C; red, 20 °C; green, 25 °C; blue, 30 °C; sky blue, 35 °C).

quartz crystal, respectively, as functions of the relative humidity and temperature. Both the resonance frequency and electrical resistance decreased with increasing relative humidity. The decrease in the resonance frequency indicated an increase in the mass loading due to the absorption of water vapor onto the polyaniline film. The changes in the resonant frequency of a quartz crystal (Δf) vibrating in a thickness-shear mode can be related to the change in mass (Δm) due to absorption using the Sauerbrey equation:19,20 Δf = −

2f0 2 A ρq μq

Δm

Figure 5. Variations in the relative humidity, electrical resistance, and resonance frequency of an IDE-coated quartz crystal without a polyaniline film at 25 °C (black, relative humidity; blue, resonance frequency; red, relative resistance).

(1)

where f 0 is the resonant frequency of the unloaded crystal, ρq is the density of quartz (2.648 g/cm3), μq is the shear modulus of quartz (2.947 × 1011 g/cm·s2), and A is the active area of the crystal between the electrodes. Because eq 1 is applicable only when the added mass forms uniform and dense layers on the quartz crystals, it is not applicable to gas absorption onto a drop-cast polymer film. As a result, the frequency changes were used to monitor the relative amounts of absorbed gas, rather than the mass changes. The change in the frequency as a function of the relative humidity decreased with increasing temperature due to the lower levels of water absorption onto the polyaniline film at higher temperatures. Because the resonance frequency of quartz

electrical resistance was observed, indicating that most changes in the frequency and electrical resistance were caused by the absorption of water onto the polyaniline film. This result suggested that the sensing areas of both IDE and LFE were determined not by the size of each electrode but by the polyaniline film coated on the resonator surface due to its large surface area compared to the areas of the bare quartz surfaces. The advantages of the two orthogonal measurements became apparent by plotting the measurements on a single graph. Figure 6a shows the variations in the relative electrical 8181

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film was attributed to the neutralization of NH2+ due to the absorption of chloroform.22 Figure 8 summarizes the relative resistance changes normalized by the frequency changes due to the absorption

Figure 6. (a) Variations in the electrical resistance as a function of the resonance frequency of a polyaniline-coated quartz crystal at various temperatures (black, 15 °C; red, 20 °C; green, 25 °C; blue, 30 °C; sky blue, 35 °C). (b) Variations in the electrical resistance curves as a function of the resonance frequency were superimposed on a master curve, regardless of the temperature.

Figure 8. Ratio of the change in the relative electrical resistance to the change in the resonance frequency, due to the absorption of each gas on the polyaniline-coated quartz resonator.

resistance alongside the resonance frequencies at various temperatures. The electrical resistance decreased as the frequency change increased (as water absorption increased). The electrical resistance decreased almost linearly with the absorption of water at high temperatures, saturating at low temperatures due to the water condensation. These trends demonstrated that the absorbed water did not affect the electrical resistance once the water vapor had condensed onto the film. Figure 6b shows a 2-D graph of the results in Figure 6a, revealing that the variations in the electrical resistance as a function of the resonance frequency (prior to water condensation) could be superimposed on a master curve, regardless of the temperature. This indicated that the changes in the electrical resistance were governed by the absorbed water rather than by the temperature under the experimental conditions. Similar experiments were conducted using a variety of gases, including ethanol, acetone, and chloroform, in order to investigate the resonance frequency and electrical resistance changes upon absorption of each vapor. Parts a and b of Figure 7 show, respectively, the variations in the resonance frequency

of each vapor. The greatest change was observed for water, indicating that smaller amounts of absorbed water resulted in larger changes in the electrical resistance. This trend was less pronounced for acetone than for ethanol, and the opposite trend was observed for chloroform: the relative resistance increased as the absorption of chloroform vapor increased. Although further studies are needed, this result indicates that the ratio of the two measurements may be used to identify the absorbed vapors.

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CONCLUSIONS We developed a novel quartz crystal resonator that could measure both the changes in the resonance frequency and in the electrical resistance. Simultaneous measurements were collected by depositing IDE and LFE gold electrodes on the top and bottom surfaces, respectively, of the quartz crystal. After coating the polyaniline film on the IDE surface, changes in the resonance frequency and electrical resistance of the film were measured under different relative humidity conditions and temperatures. The simultaneous measurements clearly showed the dependence of the electrical resistance on the amount of water absorbed, in contrast with the individual measurements. In particular, the absorbed water did not affect the electrical resistance once water vapor had condensed onto the film. The quartz resonators were exposed to the various organic vapors, yielding a ratio between the two measurements that differed depending on the identity of the vapor, indicating that the IDELFE integrated quartz resonator could be used to identify the adsorbed gases.

Figure 7. Variations in (a) the resonance frequency and (b) the electrical resistance of a polyaniline-coated quartz crystal due to the absorption of the various gases (black, water; red, ethanol; green, acetone; blue, chloroform).

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ASSOCIATED CONTENT

* Supporting Information S

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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changes and the relative electrical resistance values during the absorption of each vapor onto the polyaniline film. Pure liquids were used for the vapor generation, and the concentration of each vapor was determined by its equilibrium vapor pressure at room temperature. The resonance frequencies decreased during the absorption of vapors, regardless of the vapor type. More interesting results were observed in the measured relative resistance changes. The absorption of water, ethanol, and acetone caused the resistance to decrease, whereas the absorption of chloroform resulted in an increase in the resistance. The increase in the resistance of the polyaniline

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea 8182

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(NRF) funded by the Ministry of Education, Science and Technology (20120001577).

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