Analysis of Binary Mixtures of Aqueous Aromatic Hydrocarbons with

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Analysis of Binary Mixtures of Aqueous Aromatic Hydrocarbons with Low-Phase-Noise Shear-Horizontal Surface Acoustic Wave Sensors Using Multielectrode Transducer Designs Florian Bender,† Rachel E. Mohler,§ Antonio J. Ricco,‡ and Fabien Josse*,† †

Department of Electrical and Computer Engineering, Marquette University, Milwaukee, Wisconsin 53201-1881, United States Chevron Energy Technology Co., 100 Chevron Way, Richmond, California 94801, United States ‡ Department of Electrical Engineering, Center for Integrated Systems, Stanford University, Stanford, California 94305-4075, United States §

ABSTRACT: The present work investigates a compact sensor system that provides rapid, real-time, in situ measurements of the identities and concentrations of aromatic hydrocarbons at parts-perbillion concentrations in water through the combined use of kinetic and thermodynamic response parameters. The system uses shearhorizontal surface acoustic wave (SH-SAW) sensors operating directly in the liquid phase. The 103 MHz SAW sensors are coated with thin sorbent polymer films to provide the appropriate limits of detection as well as partial selectivity for the analytes of interest, the BTEX compounds (benzene, toluene, ethylbenzene, and xylenes), which are common indicators of fuel and oil accidental releases in groundwater. Particular emphasis is placed on benzene, a known carcinogen and the most challenging BTEX analyte with regard to both regulated levels and its solubility properties. To demonstrate the identification and quantification of individual compounds in multicomponent aqueous samples, responses to binary mixtures of benzene with toluene as well as ethylbenzene were characterized at concentrations below 1 ppm (1 mg/L). The use of both thermodynamic and kinetic (i.e., steady-state and transient) responses from a single polymer-coated SH-SAW sensor enabled identification and quantification of the two BTEX compounds in binary mixtures in aqueous solution. The signalto-noise ratio was improved, resulting in lower limits of detection and improved identification at low concentrations, by designing and implementing a type of multielectrode transducer pattern, not previously reported for chemical sensor applications. The design significantly reduces signal distortion and root-mean-square (RMS) phase noise by minimizing acoustic wave reflections from electrode edges, thus enabling limits of detection for BTEX analytes of 9−83 ppb (calculated from RMS noise); concentrations of benzene in water as low as ∼100 ppb were measured directly. Reliable quantification of BTEX analytes in binary mixtures is demonstrated in the sub-parts-per-million concentration range.

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as a result, among the BTEX compounds, benzene has the lowest polymer−water partition coefficients6,13 and generally exhibits the poorest sensitivity for polymer-based sensor devices. Thus, a sensor system for quantification of benzene in the presence of other BTEX compounds must overcome fundamental challenges to provide both high sensitivity and high selectivity. The current measurement approach for contaminated groundwater involves periodic sampling at strategically placed water-monitoring wells,1 then transferring collected samples to a laboratory for analysis.2 This procedure is too timeconsuming and labor-intensive for continuous monitoring. To address the dearth of suitable monitoring strategies for groundwater contaminated by fuel or oil, we are developing a sensor platform that will be the heart of a compact system

ccidental releases of fuel and oil into water systems are sometimes difficult to detect and monitor, particularly in the case of underground storage tanks, pipelines, and other sources that are hidden from view.1−3 Such releases pose potential threats to public health and the environment; their timely detection can reduce these risks and associated cleanup costs. The BTEX compounds (benzene, toluene, ethylbenzene, and xylenes), present in crude oil and its refined products in significant concentrations,9,10 provide a signature of fuel and oil releases; these compounds are regulated by government agencies.11 Among them, benzene is of particular concern due to its carcinogenicity;11 it is therefore a standard requirement in groundwater monitoring to measure its concentration. This can be challenging because relevant concentrations are in the low parts-per-million (mg/L) to low parts-per-billion (μg/L) range,4,11 and also because various similar aromatic compounds are usually present as well, including toluene, ethylbenzene, and xylenes. These compounds all have lower solubilities in water than benzene12 and, © 2014 American Chemical Society

Received: October 2, 2014 Accepted: October 27, 2014 Published: October 27, 2014 11464

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MULTIELECTRODE SH-SAW TRANSDUCER DESIGN Metal electrodes in a transducer structure on the surface of a piezoelectric SH-SAW device substrate represent changes in the boundary conditions at the surface and cause perturbation of the propagating acoustic wave. Hence, in an interdigital transducer (IDT), every edge of each electrode finger represents a potential source of acoustic wave reflection and signal distortion due to the changes in mechanical and electrical boundary conditions. For materials with large piezoelectric coupling factors, such as LiNbO3 and LiTaO3,18 these reflections can become particularly large. In an attempt to reduce such unwanted reflections, it has long been proposed to employ the double-electrode (“split-finger”) IDT,19 which has four electrode fingers per electrical period (Se = 4); see Figure 1a. This causes reflections from adjacent electrode fingers to

suitable for installation and operation in existing groundwater monitoring wells to enable frequent or continuous, automated, in situ monitoring of groundwater for the BTEX compounds.4 The SH-SAW (shear-horizontal surface acoustic wave) device is a promising sensor platform for this application.5,6 When properly designed, the device propagates a SH-SAW that is closely confined to the solid−liquid interface without suffering prohibitive attenuation, facilitating high sensitivity to analytes dissolved in the liquid phase.7,8 The deposition on the device surface of an appropriate thin polymer film enhances sensitivity due to the partitioning of organic analytes from the aqueous phase into the film, as well as providing partial selectivity to the analytes of interest. The present work combines two approaches to achieve the required selectivity. The first is the use of an array of sensors with different coatings, each with a degree of partial selectivity for each analyte based on weak physicochemical interactions. (Note that ethylbenzene and o-, m- and p-xylene are all chemical isomers, a consequence of which is that the selectivity associated with weak physical interactions with polymer films is inadequate to distinguish among them.) This approach exploits the significant differences in both the aqueous solubilities of the BTEX compounds12 and the partition coefficients of these molecules in various common polymers in contact with the aqueous phase.6,13 Although the use of variable weak affinity of an array of polymers is often selected for analytes lacking unique functional groups,14,15 it results in limited differentiation between chemically similar analytes, such as the BTEX compounds, if only the steady-state (or equilibrium) sensor responses are evaluated.6 In addition to the thermodynamically determined equilibrium partition coefficient, therefore, this work also employs the kinetically controlled response time as a sensing parameter for analyte identification. At the comparatively low analyte concentrations involved, Henry’s law is obeyed and the equilibrium concentration of organic analyte in a given polymer film is linearly related to its concentration in the contacting aqueous phase. Sensor responses are measured following a step change from pure water to a water sample that includes the BTEX analyte(s). Under these conditions, the dynamics of analyte partitioning between aqueous and polymer phases is controlled by the diffusion coefficient of the analyte in the polymer, a property that is generally (at low concentrations) a unique, analyte-concentration-independent characteristic of a given analyte/polymer pair.6 As we reported recently in this journal, use of sorption response times in combination with ratios of equilibrium responses leads to six concentrationindependent response parameters from an array of three polymer-coated SH-SAW devices that can accurately identify the BTEX analytes.5 Here the focus of the present study is primarily on demonstrating how unwanted acoustic wave reflections and phase distortions in the sensor signal from an SH-SAW device can be significantly reduced by employing multielectrode transducers16 wherein the number and polarities of the electrode fingers are arranged such that most of the reflected waves cancel one another. The presented data shows how the resulting low phase distortion and low root-mean-square (RMS) noise level in the sensor signal lead to significantly improved limits of detection, increased accuracy in the quantification of (BTEX) analytes in binary mixtures, and improved reproducibility in liquid-phase sensor operation.

Figure 1. Illustration of the spatial sampling concept for the cases (a) Se = 4 and (b) Se = 12. IDT finger patterns are shown on the left, and the corresponding spatial frequency spectra, described below, on the right. U is the sinusoidal electrical signal generated by a given IDT as it samples propagating acoustic wave. Note that the center-to-center distance between adjacent electrode fingers, p, is the same for both IDTs, and the two IDT patterns are compared in the left-hand panels for the same wavelength λ = 4p. In the right-hand panels, λ varies and is represented by the parameter s = p/λ; the variation of s reveals, for a given IDT pattern with fixed p, which harmonics (i.e., fractions of the fundamental wavelength) are supported. The case s = 1/4 (or 3/12) at right corresponds to the situation depicted in the left-hand panels. The vertical axes of the spatial frequency spectra only indicate if mode generation occurs.

have a phase difference of 180°, effectively canceling one another. This holds true as long as the amplitudes of the waves reflected from adjacent electrode fingers are approximately the same. Although this is a good approximation for weak piezoelectric coupling materials, significant reflections can remain in high piezoelectric coupling materials even when using double-electrode IDTs. In liquid-phase sensing, piezoelectric coupling materials such as LiTaO3 and LiNbO3 are often preferred for acoustic wave devices due to their high dielectric constants,18 which permit immersion of the polymer-coated IDTs in the liquid to be probed, eliminating the need for a gasket on the acoustic wave path between the IDTs. Such a gasket might distort and attenuate the acoustic wavefront, compromising the reproducibility and sensitivity of the sensor response.17 For this reason, 36° YX-LiTaO3 was selected as the piezoelectric substrate material for this work. 11465

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electrodes, which is defined as k0 = 2π/p, and the spatial frequency, k = 2π/λ (k is sometimes referred to as the wavenumber). If s is defined as the ratio of k to ko (i.e., s = k/k0 = p/λ = f p/v where f is the frequency and v is the wave velocity), then the transmission peak locations for these specific IDT patterns in the region 0 < s < 1, are given by n speak = Se (1)

As indicated earlier, however, even using double-electrode IDTs, a significantly distorted SH-SAW passband might still be obtained with LiTaO3. A slight improvement in the uniformity of the passband can be achieved by reducing the number of electrode fingers in the IDT, thus reducing the number of reflecting edges. However, this comes at the expense of a larger SH-SAW bandwidth that leads to unwanted overlap with adjacent bulk modes. Fortunately, many alternative ways exist to modify the design of an IDT to achieve the desired transfer function characteristics.20 For this work, a design approach was selected based on multielectrode IDTs:16,21 some of the electrode fingers are given an electrical polarity in antiphase with the rest of the IDT. Figure 1a,b shows examples of two structures that have Se = 4 and Se = 12 electrode fingers per electrical period, respectively. Note that the double-electrode IDT with Se = 4 (Figure 1a) has two electrode fingers of each polarity per wavelength (λ), which is the same as the periodicity (PIDT) of the electrode pattern. In comparison, for the Se = 12 IDT (Figure 1b), there are 2 electrode fingers of one polarity and 10 fingers of the other polarity per period; for this IDT, PIDT = 3λ. Because the electrical period, PIDT, of a multielectrode IDT contains many electrode fingers, and because electrode finger widths significantly larger than the wavelength range of visible light are preferred for ease of manufacturing, the frequency of operation of the fundamental SH-SAW mode in a multielectrode IDT is usually low ( ethylbenzene, in agreement with Figures 6 and 7. Nevertheless, the observed average correlation between extracted and nominal concentration is good. This demonstrates that it is possible to extract analyte concentrations from responses of a single sensor device to binary mixtures of BTEX compounds in the subppm concentration range.

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SUMMARY AND CONCLUSIONS

Various design improvements were investigated for their potential to reduce the limits of detection for BTEX compounds in aqueous phase using polymer-coated SH-SAW sensors. It was observed that a well-chosen multielectrode IDT design not only minimizes distortion in the SH-SAW passband but also significantly reduces the RMS noise levels of the sensor signals. As a result, limits of detection for single BTEX compounds in water in the range of 9−80 ppb were achieved,

Figure 5. Responses of a SH-SAW sensor with Se = 12 IDTs, coated with 0.6 μm of PECH, successively exposed to various binary mixtures in water of (a) benzene and ethylbenzene and (b) benzene and toluene. Concentrations are indicated in the graph in parts-per-billion by weight. 11469

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Figure 6. Concentrations extracted from dual-exponential fits vs nominal concentrations for a SH-SAW sensor with Se = 12 IDTs, coated with 0.6 μm of PECH, exposed to various binary mixtures of benzene and ethylbenzene in water. For comparison, the lines labeled “ideal” are shown for which extracted concentration and nominal concentration are identical. Error bars are ± standard deviation based on n = 3 measurements.

Figure 7. Concentrations extracted from dual-exponential fits vs nominal concentrations for a SH-SAW sensor with Se = 12 IDTs, coated with 0.6 μm of PECH, exposed to various binary mixtures of benzene and toluene in water. For comparison, the lines labeled “ideal” are shown for which extracted concentration and nominal concentration are identical. Error bars are ± standard deviation based on n = 4 measurements.

with a best estimated LD for benzene of 57 ppb. For the analysis of binary analyte mixtures in water, it is particularly important to measure the transient sensor response with high accuracy because this part of the response curve contains important information about the identities of the two analytes in the mixture. Therefore, care was taken to keep the response curve free of spurious influences, and the data sampling rate was increased relative to previous work.5 Through the combined use of transient (kinetic) and equilibrium (thermodynamic) responses, binary mixtures of BTEX analytes in water were successfully analyzed by individual sensors in the concentration range of 200−1000 ppb. It should be noted that in the final approach, the responses of n sensors in an array will be evaluated in combination. This approach is expected to further improve the accuracy in the estimated concentrations by providing n nominally independent measurements of the same parameter, provided suitable methods of signal analysis are used.14,25 Compared to legal maximum contaminant levels for the BTEX compounds in drinking water,4,11 the observed limits of detection and accuracy in estimating the concentrations of BTEX analytes in binary mixtures are sufficient for toluene and ethylbenzene, and will also be sufficient for xylenes, which have about the same sensitivity as ethylbenzene.6 However, further efforts are still needed to meet the very low contamination limit for benzene of 5 ppb. To address this challenge, various sensor coatings are currently under investigation, specifically designed for high sensitivity to benzene and improved signal-to-noise ratios in SH-SAW sensor measurements (we point out that the

normalized limit-of-detection parameters reported in Tables 1 and 2 are already superior (smaller) for benzene than the other two compounds as a result of our selection process to date, but this clearly is not yet sufficient to reach 5 ppb). In addition, improved methods of signal processing are under investigation25,26 that will eventually be used together with an array of sensors with different sorbent coatings to further improve both selectivity and accuracy in the analysis of multicomponent aqueous samples. Finally, it should be noted that an optimized signal-to-noise ratio will play a critical role in all sensing applications where low detection limits are required. Therefore, the approach in transducer design described in this work could benefit many applications involving other chemical sensors and biosensors.

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AUTHOR INFORMATION

Corresponding Author

*F. Josse. E-mail: [email protected]. Fax: +1 414 288 3951. Phone: +1 414 288 6789. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The authors thank Urmas Kelmser from Chevron Energy Technology Co. for helpful discussions and Edwin Yaz and Karthick Sothivelr from Marquette University for valuable assistance. 11470

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