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Solid Contact Ion Selective Electrodes for In Situ Measurements at High Pressure Andrew W Weber, Glen D. O'Neil, and Samuel P. Kounaves Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b00366 • Publication Date (Web): 12 Apr 2017 Downloaded from http://pubs.acs.org on April 17, 2017

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Analytical Chemistry

Solid Contact Ion Selective Electrodes for In Situ Measurements at High Pressure Andrew W. Weber,† Glen D. O’Neil,§ and Samuel P. Kounaves†* †

Department of Chemistry, Tufts University, Medford, MA 02115, United States

§

Department of Chemistry and Biochemistry, Montclair State University, Montclair, NJ 07043, United States

*To whom correspondence should be addressed: [email protected]

ABSTRACT: Solid contact polymeric ion-selective electrodes (SC-ISEs) have been fabricated using microporous carbon (µPC) as the ion-to-electron transducer, loaded with a liquid membrane cocktail containing both ionophore and additive dissolved in plasticizer. These SC-ISEs were characterized and shown to be suitable for analysis in aqueous environments at pressures of 100 bar. Potassium ISEs, prepared in this manner, showed excellent performance at both atmospheric and elevated pressures, as evaluated by their response slopes and potential stability. These novel SC-ISEs were shown to be capable of measuring K+ at pressures under which traditional liquid-filled ISEs fail. Furthermore, the effect of pressure on the response of these sensors had little or no effect on potential, sensitivity, or limit of detection. High pressure sensor calibrations were performed in standard solutions as well as simulated seawater samples to demonstrate their usefulness as sensors in a deep-sea environment. These novel SCISE sensors show promise of providing for the first time the ability to make in-situ real-time measurements of ion-fluxes near deep-ocean geothermal vents. Hydrothermal vents have generated intense interest since their discovery in 1977,1–3 partially because they are responsible for the mass balance of several minerals in the open ocean.3 These vents are formed along the sea floor between spreading tectonic plates, and exist as chimney like structures that release geothermally heated, mineral enriched water into the ocean.4 They are also a significant source of many ionic species and thus a key component of understanding marine geochemistry. Besides their geophysical and chemical interest, the emitted nutrient rich effuse supports chemoautotrophic ecosystems that are of biological and evolutionary importance.5 Performing robust analyses in these environments is often an enormous challenge due to their presence in most cases at depths greater than 1,000 meters below sea level,6 where pressures can exceed 100 bar, and several of the most well-studied vent system are at depths approaching 4000 m below sea level (~400 bar). In addition, the geothermally heated liquid can reach temperatures of 350 °C at the source before quickly mixing with surrounding ocean water. This creates extreme temperature gradients of more than 300 degrees within 1-2 meters of the vent. To complicate the situation even further, convection caused by expulsion of the vent fluid and its subsequent mixing with

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ocean water is a highly complex dynamic process, with chemical changes happening rapidly. As a result, methodologies which rely on batch sampling and ex situ analysis provide an incomplete picture of the dynamic processes and rich geochemistry of these environments.7–10 To provide more accurate real-time analysis requires chemical sensors which are stable under high pressures and elevated temperatures that typically surround deep-ocean hydrothermal vents. Commercial sensors for measuring temperature, conductivity, pH, O2, CO2, and selected heavy metals have been commercially available since the mid-1970s. Literature reports also describe sensors for Mn2+ (ref. 11) and a limited number of chemical species such as SiO2, O2, H2S/HS-, Fe, Br-, and NO3-.12–17 Several groups have demonstrated novel sensors for Mn2+, Fe2/3+, and several sulfur-based species7,18–21, conductivity (as a proxy for Cl-),22 NO3- and Br- using UV sensors,13,14 and SO42- using Raman spectroscopy.23,24 Several recent reviews describe a variety of such sensors,18,25–27 but the underlying issues with many of them are the limited detection limits, limited ionic species detection, vulnerability to high pressures and temperatures, and size. Ion-selective electrodes (ISEs) are commonly employed in environmental analysis8,28,29 because they are selective, sensitive over a large dynamic range (with responses typically from µM to M concentrations), and are well-adapted for in situ analysis.30–32 However, they have not been widely used in deep ocean analyses because of their perceived inability to operate at increased pressures, temperatures, and for extended periods of time, mainly due to the use of internal liquid filling solutions, sensing membrane properties, and an external reference electrode, which typically requires a porous junction. There is little available literature regarding polymeric ISE construction and testing for use at pressures above 30 bar33. This study addresses a critical need, showing that ISEs can be fabricated which are capable of operating at the pressures required for analyses at deep ocean hydrothermal vents. The past decade has seen the emergence of solid contacts used to improve the robustness of traditional ‘liquid filled’ ISEs by functioning as ion-to-electron transduction elements. Both conductive polymers34–36 and carbon-based nanomaterials have been employed as solid contacts in chemical sensors, bio sensors, and reference electrodes with great success.37–41 Several years ago, we reported on a method for producing sensors with long lifetimes by impregnating micro-porous carbon with an ionophore and utilizing it as the ion-to-electron transducer.42 This configuration provides several advantages for sensors required to function in high pressure environments: the rigid carbon pellet provides physical support to the ISE membrane; the sensors remain stable for long periods due to the capacitance of the microporous carbon structure; the ionophore-impregnated porous carbon acts as an ionophore reservoir, prolonging the lifespan of the sensor. Using a microporous carbon-based K+ SC-ISE as a model system, the work described here shows that this type of sensor can be used at pressures equivalent to a depth of 1,000 meters below sea level, thus paving the way for in-situ studies in deep-ocean environments and enabling


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real-time measurements of ion-fluxes near geothermal vents.

EXPERIMENTAL SECTION Materials. High molecular weight poly(vinyl chloride), potassium ionophore I (valinomycin), sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB), bis(2-ethylhexyl) sebacate (DOS), tetrahydrofuran (anhydrous, ≥99.9%), potassium chloride, sodium chloride, magnesium sulfate heptahydrate, and magnesium chloride hexahydrate were purchased from Sigma-Aldrich. Calcium chloride dihydrate was purchased from Fisher Scientific. Grade 1 spectroscopically pure, 3/16" diameter graphite rods (porosity 29%) were purchased from Ted Pella, Inc. Epoxy 907 adhesive system was purchased from Miller-Stephenson. All solutions were prepared using 18.2 MΩ/cm water (Barnstead Nanopure, Massachusetts).

Electrode construction. Figure 1a shows the sensor preparation process. Sections of a graphite rod (~0.5 cm long) were polished with p600, p1500, and p2500 grit carbide paper (McMaster-Carr), and a 1 mm diameter hole was bored ~3 mm into the backside of the graphite rod (Figure 1a, i). The graphite transducers were thoroughly rinsed with THF to remove any impurities, which may be soluble in the solvent, and subsequently dried at 29 in. Hg vacuum to remove the THF. Cleaned carbon was then impregnated with the ionophore/additive cocktail by soaking in a solution consisting of 3% valinomycin, 1.2% NaTFPB, and 95.8% DOS (by weight) for 4 hours. A 29 in. Hg vacuum was applied to the carbon containing solution for 30 minutes in order to remove gas from the microporous carbon (µPC) structure, and fill the evacuated pores with the ionophore/additive mixture (Figure 1a, ii) . After soaking, electrical contact was made to the graphite rod by securing a 1 mm diameter silver wire (Alpha-Aesar, 99.999%) using conductive silver epoxy cured at 60°C overnight (Epo-Tek EJ2189; Billerica, MA) as shown in Figure 1b, iii. The electrode assembly was inserted into a flexible PVC housing (Nalgene 180 PVC), which was back-filled with insulating epoxy (Miller-Stephenson 907) to minimize the risk of the test solution making contact with the silver lead wire (Figure 1a, iv). A membrane solution consisting of 1% ionophore, 0.4% additive, 33% PVC, and 65.6% plasticizer (by weight) was drop cast onto the exposed front carbon face to a final membrane thickness of ~150 um (Figure 1a, v). Once dried the inner reference lead protruding out the back of the electrode body was soldered to a water-tight underwater electrical connector (Seacon).

Electrochemical Measurements. Potentiometric measurements were performed in a high-pressure reaction vessel (vide infra) using a high impedance voltmeter (Lawson Labs EMF16 interface; Malvern, PA) in the differential configuration. A platinum wire was used as the ground electrode with an Orion Sure-flow double junction reference electrode (model 900200; Thermo Scientific). All solutions were continuously stirred during analysis.



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Figure 1. Schematic of the (a) sensor fabrication and (b) the high-pressure test cell. A schematic of the pressure vessel and electrode set up is shown in Figure 1b. A high-pressure reaction vessel (Parr 4605; Parr Instrument Company, Illinois) was connected directly to ultra-high purity nitrogen gas (Airgas). Pressure and temperature were controlled and recorded using a Parr 4838 reaction controller. Calibration solutions were added to the pressure vessel using a Milton-Roy (Milton Roy Company, Florida) 5,000 psi mini-Pump, which was able to overcome the pressure inside the chamber and pump solutions into the reaction vessel.

RESULTS AND DISCUSSION Characterization of SC-ISEs at atmospheric pressure. The performance characteristics of ISEs prepared with a microporous carbon solid-contact were evaluated at standard temperature and pressure in order to establish a baseline for the effects of pressure on the sensor response. Figure 2 shows a typical calibration curve for a K+ SC-ISEs acquired under ambient laboratory conditions. The ISEs responses are similar to those reported previously,42,43 with near-Nernstian calibration slopes (= 54.1±2.2 mV dec-1, n = 6) and detection limits of 3.4 µM. In addition, the response time was less than 5 seconds, which is comparable to conventional ISEs. One of the critical issues regarding solid-contact ISEs is the development of the so-called water-layer between the membrane and transducer.44 This thin layer causes potential instability because it is an illdefined interface at the transducer, which can be easily affected by environmental factors, including concentration changes and oxygen permeation. We hypothesized that a water layer was not likely to form on the surface of our transducer because of the lipophilic nature of the plasticizer which is loaded into the micropores, and also the hydrophobic nature of the graphite surface. Figure 2b shows the results of the water-layer experiment, in which the potential of the SC-ISE is measured in solutions containing the primary ion (K+) and an interfering ion (Na+). Fibbioli et al. suggest that if a water-layer is present, a


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significant drift is expected when the solution is changed from the primary ion to the interfering ion, and also when the solution is changed back to a solution containing the primary ion.44 The data in Figure 2b show a positive drift when the K+ ISE is first placed in the Na+ sample, which is likely caused by K+ diffusing from the membrane into the potassium-deficient solution. The most important portion of the trace in Figure 2b is when the solution is changed from Na+ back to K+, which shows no negative drift. This result shows that there is no water-layer present in these electrodes, which is consistent with other reports carbon-based transducers.37,45

Figure 2. Characterization of solid-contact potassium ISEs at ambient pressure and temperature. (a) calibration curve for a solid-state potassium ion-selective electrode at ambient pressure; (b) water-layer test for a solid-state potassium ion-selective electrode at ambient pressure.

Another important characteristic of these sensors is sensor drift, which is especially critical because we intend on employing these sensors to monitor ion fluxes in the deep ocean over a couple of hours. The water-layer test shown in Fig. 2b shows a 12-hour K+ measurement that was used as a measure of the sensor drift. The average drift over this window was 150 µV hr-1, which is somewhat higher than ISEs fabricated using 3D ordered macroporous carbon (3DOM),45 but on par with electrodes prepared with hydrogel contacts46 and electrodes based on carbon nanotubes used for measuring NH4+ concentrations in Lake Rotsee.47 However, if measuring over a four-hour period, the measured drift would contribute 1.0% error to our measurements. Characterization of SC-ISEs at pressures of 100 bar. In order to understand the effects of pressure on the sensor response, both traditional inner-filled and solid-contact ISEs were subjected to elevated pressures simulating 1,000 meters below surface level (103.4 bar). In order to reach the desired pressure, nitrogen gas was forced into the pressure chamber containing the fully submerged ISEs until the desired pressure was reached. The rate of increase was set to ~20 bar min-1 using an analog pressure



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valve. In order to understand the transient effects of pressure on the ISEs, the pressure over a K+ solution was increased while continuously recording the SC-ISE response (Figure 3a). There was a small change in potential (~10 mV) as the chamber pressure increased from 0 to 100 bar over 5 minutes. This drift is comparable to prior work which showed a ~1 mV drift as pressure was increased from 1 to 30 bar.33 Traditional liquid-filled ISEs showed tremendous variation in emf (>40 mV) and significant noise during pressurization, suggesting that the mechanical stability of these electrodes in not sufficient for analysis at elevated pressures (Fig. S1 in the supporting information). The experiments described here simulate a rapid sensor descent, and were designed to provide a high mechanical stress level to the sensors. In a real environment, the sensors would likely not experience such rapid pressure changes, although our results demonstrate that the sensors handled the mechanical stress very well.

Figure 3. Characterization of solid-contact potassium ISEs at 100 bar. (a) two-axis plot showing pressure and emf as a function of time; (b) calibration curve of a solid-contact K+ ISE at 100 bar; (c) atmospheric pressure calibration after three successive pressurizations at 105 bar; (d) measurement of K+ in simulated


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seawater using a standard addition. After reaching the maximum pressure of ~100 bar, the electrodes were calibrated by adding aliquots of a potassium chloride standard solution (Figure 3a). The solid-contact sensors showed a linear response over the entire calibration range of ~10 µM to ~100 mM with the response time remaining at 2.5, from 0.4 mV hour-1 to 0.15 mV hour-1. Another important property of these sensors is their apparent ability to rapidly recover their performance after they have been pressure cycled. Figure 3c shows a calibration curve acquired at ambient pressure for a K+ SC-ISE after 3 cycles where the pressure was changed from 1 to 105 bar. Compared to the initial readings at atmospheric pressure, the sensitivity changed by only 1 mV dec-1, the intercept drifted by fewer than 5 mV, and the limit of detection was 3.7 µM. These results show that the sensor is able to maintain its mechanical integrity over multiple cycles of pressurization and depressurization. In order to demonstrate that the sensor can function at elevated pressure in a complicated matrix like seawater, artificial seawater was used as a model system to test the SC-ISEs ability to quantify K+ in a simulated deep sea environment. Artificial seawater was prepared following the procedure described by Dickson and Goyet.48 The primary interfering species in seawater is Na+, which is present at very high concentrations (0.486 M). At 103 bar the SC-ISE exhibits a Nernstian slope of nearly 56 mV with a linear correlation coefficient of R=0.998. The limit of detection is 17 µM, which is higher than in solutions containing no interfering ion. This can be attributed to the presence of interfering ions, and is consistent with the offset expected by the potentiometric selectivity coefficient for the membrane recipe employed.

CONCLUSIONS A class of solid contact ISEs have been demonstrated which are capable of operation at pressures corresponding to a depth of 1000 m (105 bar). The ISEs were fabricated using a robust microporous



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graphitic carbon as the ion-to-electron transducer, which had been impregnated with an ionophore/additive cocktail to prevent premature sensor aging. The SC-ISEs showed a change in potential of less than ten percent when the pressure was increased from 1 to 100 bar and maintained a nearNernstian response to the primary ion. In addition, the sensor response and limit of detection remained unchanged after being pressurized to 105 bar, when compared to measurements made at atmospheric pressure. The sensors were mechanically stable enough to survive multiple pressurization cycles, with only small changes in sensor response observed after three cycles to 105 bar. These novel sensors extend the advantages of in situ polymeric ISE analysis to the high pressure environment found at deep-ocean locations.

SUPPORTING INFORMATION (1) Plot comparing the emf response during pressurization for a SC-ISE and a liquid filled ISE. This information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was partially supported by awards from the NSF (OCE-1060945) and NASA (NNX10AJ93G). The authors thank Victoria Hansen, Kyle McElhoney, and Nikos Chaniotakis for helpful discussions.

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For TOC only:


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Solid Contact Ion-Selective Electrodes for in Situ Measurements at

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