Planar Electrochemical Sensor Research Articles

Single-Frequency Impedance Studies on an Ionic Liquid-Based Miniaturized Electrochemical Sensor toward Continuous Low-Temperature CO2 Monitoring

Continuous greenhouse gas monitoring at sub-zero temperatures is needed for monitoring greenhouse gas emission in cold environments such as the Arctic tundra. This work reports a single-frequency electrochemical impedance sensing (SF-EIS) method for real-time continuous monitoring of carbon dioxide (CO2) at a wide range of temperatures (-15 to 40 °C) by using robust ionic liquid (IL) sensing materials and noninvasive, low-power, and low-cost impedance readout mechanisms since they cause minimal changes in the sensing interface, avoiding the baseline change for long-term continuous sensing. In addition, a miniaturized planar electrochemical sensor was fabricated that incorporates a hydrophobic 1-butyl-1-methylpyrrolidinium bis(trifluromethylsulfonyl)imide ([Bmpy][NTf2]) IL electrolyte and Pt black electrode materials. The high viscosity of the ILs facilitates the formation of thin, ordered, and concentrated layers of ionic charges, and the inverse relationship of IL viscosity with temperature makes them especially suited for impedance sensing at low temperatures. The unique low-temperature properties of ILs together with EIS transduction mechanisms are shown to be sensitive and selective for continuously monitoring CO2 at a -15 to 40 °C temperature range via impedance changes at a specifically selected frequency at the open circuit potential (OCP). Molecular dynamics simulations revealed insights into the structure and dynamics of the IL at varying temperatures in the presence of methane and CO2 and provided potential explanations for the observed sensing results. The miniaturized and flexible planar electrochemical sensor with the [Bmpy][NTf2] electrolyte was tested repeatedly at subzero temperatures over a 58-day period, during which good stability and repeatability were obtained. The CO2 impedance sensor was further tested for sensing CO2 from soil samples and shows promising results for their use in real-time monitoring of greenhouse gas emissions in cold temperatures such as permafrost soils.

Modeling of Transferring Processes in the Planar Electrochemical Sensors

Planar ECTs (electrochemical transducers) are the new type of sensors, which are very effective for measuring pressure and motions (velocity and acceleration). These sensors are characterized by an exceptionally high sensitivity to mechanical action due to high rate of transducing of the mechanic signal into electric current. In this work, we develop a mathematical model of a planar electrochemical system, which detects mechanical signals. We simulated the processes of mass and charge transfer in planar electrochemical transducer, obtained the main system characteristics and investigated the dependence on geometric parameters.

Platinum–Nickel Bimetallic Nanosphere–Ionic Liquid Interface for Electrochemical Oxygen and Hydrogen Sensing

By exploiting the benefits of bimetallic platinum–nickel (Pt–Ni) alloy nanosphere and an ionic liquid (IL) (i.e., 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BmpyNTf2), an organic–inorganic hybrid interface of IL/Pt–Ni was designed and characterized for electrochemical sensing of oxygen and hydrogen gases for miniaturized electrochemical gas sensor development. The spherical Pt–Ni alloy nanoparticles (NPs) were synthesized through template-free, one-pot solvothermal method. The morphology, crystal structure, and chemical composition of Pt–Ni alloy NPs were thoroughly characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The obtained Pt–Ni alloy NPs were used to fabricate the planar sensor devices and tested for oxygen and hydrogen sensing. The oxygen-sensing performance of the resulting planar electrochemical sensor was investigated over a low concentration range of 500–5000 ppm of O2 at room temperature by using constant potential amperometry. The planar electrochemical sensor device exhibited a high sensitivity to O2 ((3.04 ± 0.18) × 10–5 mA cm–2 ppm–1) compared to commercial Pt/C-based sensor ((2.57 ± 0.22) × 10–5 mA cm–2 ppm–1). The planar electrochemical sensor device also showed good reproducibility and selectivity for oxygen detection during sensing tests. Moreover, the sensor device based on the obtained Pt–Ni alloy NPs was investigated for hydrogen detection with excellent analytical performance in hydrogen sensing. The outstanding gas sensing properties were attributed to unique interface properties and highly efficient catalytic reaction of gas species of oxygen and hydrogen at the interface of IL/Pt–Ni alloy NPs. This work demonstrated that the integration of Pt–Ni alloy NPs with ILs enabled beneficial electrode interface for O2 and H2 gases sensing with high sensitivity, rapid gas response, and superior reproducibility based on a novel planar electrochemical sensor platform.

Ionic Liquid Impedance Sensor for Continuous Low Temperature CO2 monitoring

There is a significant need to develop sensor technology for applications in extreme environmental conditions. Robust gas sensors that can continuously monitor specific gaseous analytes in harsh environments are essential for a variety of applications such as environmental monitoring and greenhouse gas emissions in Arctic where temperatures are constantly in the sub-zero range. For efficient operation and long-term stability, these devices require sensing materials that can function at subzero temperatures as well as non-invasive readout mechanisms. Electrochemical techniques are advantageous for continuous gas sensing due to their cost effectiveness, simplicity, portability and easy miniaturization of the instrumentation. However, the application of constant or varying DC potential in commonly used voltametric and amperometric detection methods may result in interfacial redox reactions and formation of new products that could poison the sensor materials and take part in unfavorable side reactions. Electrochemical impedance spectroscopy (EIS) is a non-invasive alternate technique which measures impedance changes originating from change in polarization of the electrode-electrolyte interface upon addition of analyte gases. Compared to traditional aqueous electrolytes, the unique solvent and electrolyte properties of ionic liquids (ILs) enable them to operate in extreme environments without undergoing physical or chemical changes. In this presentation, we will discuss our results for the development of an ionic liquid (IL) based miniaturized impedance sensor for continuous CO2 monitoring in the -15 to 40oC temperature range using a microfabricated electrochemical sensor designed and fabricated with Pt black electrodes and hydrophobic [Bmpy][NTf2] ionic liquid electrolyte. The large hydrophobic NTf2 anion is shown to weaken cation−anion interactions, enabling a higher absorptivity of CO2. It also weakens water−anion interactions and thus impedes the absorption of H2O, minimizing the interference from humidity at the ambient conditions. Pt black provides a high surface area that allows simple and robust fabrication of planar electrodes on flexible polymer membranes. A non-invasive single frequency impedance spectroscopy (SF-EIS) method was demonstrated for the continuous and quantitative detection of CO2. In this method, impedance changes arising from increasing amounts of added CO2 analyte and [Bmpy][NTf2] IL at open circuit potential were measured at a set frequency of 1 Hz at different temperatures. It was seen that the impedance increased with decreasing temperature, and decreased with increasing amounts of added CO2. A linear relationship between the CO2 gas concentration and impedance was obtained, and increased sensitivity for CO2 detection was observed at low temperatures with a maximum slope of -5.092 ohm/%CO2 seen at -15oC. The same miniaturized and flexible planar electrochemical sensor with [Bmpy][NTf2] IL electrolyte was tested repeatedly for continuous sensing of CO2 at subzero temperatures over a two and a half month period, during which good stability and repeatability were obtained. To explain our impedance trend, we hypothesize that the high viscosity of IL at low temperature will result in the formation of a more packed, concentrated layer of ionic charges on the electrode surface. In the presence of CO2, the relatively rigid IL-electrode double layer at low temperature can be beneficial for chemical sensing since the molecular interactions between the CO2 analyte and [Bmpy][NTf2] could result in decreasing resistance and increasing conductivity by decreasing ionic attractions in the IL. Consequently, the high viscosity of ILs at low temperature, which is usually considered a limitation to practical in amperometric sensing, converts to an advantage in impedance sensing since ordered and concentrated electric double layer could provide high sensitivity and selectivity. In addition, the increased initial impedance at low temperature due to high IL viscosity and density is beneficial for our signal-off sensing mechanism. This reported CO2 sensor is among the first impedance based gas sensors shown to function continuously at sub-zero temperatures, providing CO2 measurements with good sensitivity at low cost and low power. These results obtained from our sensor show promise for potential use in real-time monitoring of carbon dioxide emissions in harsh low temperature environments, uniting the benefits of minimally invasive impedance transduction methods and robust IL sensing materials. Figure 1

Incorporating a Novel Hexaazatriphenylene Derivative to a Flexible Screen-Printed Electrochemical Sensor for Copper Ion Detection in Water Samples

A novel hexaazatriphenylene derivative, 6,7,16, 17,26,27-hexahydronaphtho[2,3-h]naphtho[2',3':7,8] quinoxalino[2,3-a]naphtho[2',3':7,8] quinoxalino[2,3-c]phenazine-5, 10,15,20,25,30-hexaone (HNQP), was successfully synthesized to selectively detect copper (Cu <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ) ions over other metal ions in aqueous solutions. A flexible and planar electrochemical sensor with silver, carbon and silver/silver chloride as counter, working and reference electrodes, respectively was fabricated on a polyimide substrate using an additive screen printing process. The HNQP was drop casted on the electrochemical sensor and its capability to selectively detect Cu <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions was investigated by performing differential pulse voltammetry (DPV). A wide peak domain with average currents increasing from 0.16 μA to 17.97 μA was observed as the concentration of Cu <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions was increased from 0 μM to 100 μM, respectively, at reduction potential ~0.05V. A sensitivity of 135 μA/mM and a correlation coefficient of 0.99 was calculated for the HNQP drop casted electrochemical sensor. Further tests revealed selective detection of Cu <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions over other interfering metal ions including Co <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> , Zn <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> , Ni <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> , Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> , Li <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1+</sup> , Pb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> , Hg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> and Ag <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1+</sup> using the HNQP drop casted electrochemical sensor. In addition, a limit of detection (LOD) and limit of quantification (LOQ) of 142 nM and 430 nM, respectively, was calculated for the sensor which is lower than the specified toxicity levels of Cu <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions in drinking water (20 - 30 μM) set by the U.S. environmental protection agency (EPA) and world health organization (WHO).

Synthesis of a novel hexaazatriphenylene derivative for the selective detection of copper ions in aqueous solution.

A hexaazatriphenylene (HAT) derivative, naphtho[2,3-h]naphtho[2′,3':7,8]quinoxalino[2,3-a]naphtho[2′,3′:7,8]quinoxalino[2,3-c]phenazine-5,10,15,20,25,30-hexaone (NQH) was synthesized, characterized, and found to have novel properties in being selective toward the detection of copper (Cu2+) ions. The capability of NQH to be employed as a colorimetric, chemo-fluorescence and electrochemical sensor for the detection of Cu2+ was demonstrated by performing UV-Vis absorbance, fluorescence intensity, and cyclic voltammetry (CV) measurements. The interaction between NQH and Cu2+ was initially observed with an obvious color change from yellow to brown upon the addition of Cu2+ ions to NQH. The interaction was also confirmed by UV-Vis absorbance, fluorescence intensity, and mass spectroscopy (MS/MS) measurements. UV absorbance, fluorescence and CV of NQH toward Cu2+ showed good linearity with a detection limit of 3.32 μM, 2.20 μM and 0.78 μM, respectively, which are lower than the toxicity levels of copper in drinking water (20–30 μM) set by the U.S. Environmental Protection Agency (EPA) and World Health Organization (WHO). A 1 : 2 stoichiometry complexation between NQH and Cu2+ was confirmed by Job's plot and MS/MS. In addition, the selectivity and sensitivity of the NQH compound towards Cu2+ ions were further confirmed by performing CV on a screen printed flexible and planar electrochemical sensor.

In-situ grown flower-like nanostructured CuO on screen printed carbon electrodes for non-enzymatic amperometric sensing of glucose

A planar electrochemical sensor, based on flower-like CuO nanostructures growth “in situ” on a commercial screen printed carbon electrode, was fabricated by an easy and effective technique and employed for the non-enzymatic determination of glucose. The prepared CuO nanostructures were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The carbon electrode modification with CuO was optimized by investigating the effect of the number of deposition cycles of precursor and their concentration. The electrodes modified by in situ growth of CuO were compared to an electrode prepared by simple deposition of CuO powder previously synthesized by the same technique. Cyclic voltammetric and chronoamperometric tests demonstrated that the in situ growth of CuO leads to excellent electrochemical performance toward glucose oxidation in 0.1 M KOH solution. The best sensor, if operated at an applied potential of 0.6 V, has a sensitivity of 1460 μA·mM−1·cm−2 and a 2.5 μM detection limit (at an S/N ratio of 3). Tests carried out within six months revealed an excellent long-term stability. This suggests that the method applied to modify the carbon electrode represents a useful tool for fabrication of an inexpensive and reliable non-enzymatic glucose sensor.

Measurement of Plasma Clotting Using Shear Horizontal Surface Acoustic Wave Sensor

The monitoring of blood coagulation is important during operation. In this study, a shear horizontal surface acoustic wave (SH-SAW) sensor is applied to monitor plasma clotting. An SH-SAW sensor with a metallized surface for mechanical perturbation detection can detect plasma clotting. As plasma clotting is a gel formation reaction, the SH-SAW sensor detects viscoelastic property changes. On the other hand, an SH-SAW sensor with a free surface for electrical perturbation detection detects only the liquid mixing effect. No electrical property changes due to plasma clotting are obtained using this sensor. A planar electrochemical sensor is also used to monitor plasma clotting. In impedance spectral analysis, plasma clotting is measured. However, in the measurement of time responses, no differences between clotting and nonclotting are obtained. Therefore, the SH-SAW sensor is useful for monitoring plasma clotting.

Voltammetric Sensor for Direct Insulin Detection

This work covers an area of planar electrochemical sensors for fast and reliable detection of species in environment under field conditions or in vitro/vivo biodetection. In this work, there were fabricated several planar carbon working electrodes using standard thick film technology which were modified with multiwalled carbon nanotubes (MWCNTs) to promote the electrochemical oxidation of insulin. These standalone planar working electrodes were successfully used for direct detection of insulin using cyclic voltammetry in electrochemical cell against conventional Ag/AgCl reference electrode and platinum auxiliary electrode.

Influence of the Interferers in the Nitrite Detection By using Planar Electrochemical Sensors

Reproducibility, sensibility and selectivity are standard parameters on the development of sensors. In this article we described a planar electrochemical sensor based silk-screened to detect nitrite and its interferers - paracetamol, ascorbic acid and uric acid. We modified the working electrode gold surface by using 1-2 diaminobenzeno (DAB) membrane in order to reduce the interferers. Preliminary results showed which using DAB there is decrease of interference signal about three times. We obtained a sensibility about 310 μA/(mol.L-1.mm2) for nitrite, using plain electrode and 90 μA/(mol.L-1.mm2) after DAB membrane. Although the actual results are promising, we are evaluating several combinations of membranes to reduce more the influence of the interferers in the nitrite detection.

Planar electrochemical sensors for biomedical applications

Electrochemistry has superior properties respect to the other measurement systems because of the rapid, simple and sensitive characteristics. For all these reasons, electrochemical sensors are playing a key role in many scientific sectors. Planar electrochemical sensors present many advantages respect to the three-dimensional devices, but this technology requires the use of specialty chemicals to meet the new functional requirements of planarization and miniaturization. In this paper, some production techniques and procedures to obtain planar electrochemical sensors are reported, together with their applications.

Planar electrochemical sensors based on YSZ with WO 3 electrode prepared by different chemical routes

Planar electrochemical sensors based on tape-cast yttria stabilized zirconia (YSZ, 8 wt.% of Y 2O 3) layers with two Pt parallel electrodes, one coated with WO 3 powders were prepared. The WO 3 powders were synthesized in the laboratory using different chemical routes. The NO 2 and CO potentiometric response of the sensors was measured at high temperature and compared to the response of the sensors fabricated using commercial WO 3 powders. Promising results in terms of selectivity, sensitivity, stability, reproducibility and response time were obtained for synthesized WO 3-based sensors in comparison to the response of commercial WO 3-based sensors.

Planar electrochemical sensors based on tape-cast YSZ layers and oxide electrodes

Electrochemical planar sensors based on yttria-stabilized zirconia (YSZ) with semiconducting oxides (WO 3 and LaFeO 3) and mixed conductors (La 0.8Sr 0.2FeO 3) as sensing electrodes were investigated. The electromotive force (EMF) of the sensors was measured at fixed temperature (450–700 °C) and different concentrations of NO 2 and CO in air in the range 20–1000 ppm. The sensors were wholly exposed to the same gas atmosphere, alternatively air and different gases in air, without using reference air. Fast and stable response was measured for all the different sensors. The EMF of p- and n-type semiconducting-based sensors was measured in opposite directions upon the same gas exposure. These results obtained for the response of planar sensors with different semiconductor sensing electrodes showed that the sensing mechanism cannot be explained by mixed potential theory. The response of these sensors is better described by a differential electrode equilibria sensing mechanism.

Nano-structured perovskite oxide electrodes for planar electrochemical sensors using tape casted YSZ layers

Solid-state NO 2 sensors based on yttria stabilized zirconia (YSZ), an O 2- conductor, combined either with an n- (WO 3), or a p-type semiconducting oxide (LaFeO 3), or a mixed electronic and ionic conductor (La 0.8Sr 0.2FeO 3) were investigated. Platinum parallel finger electrodes were applied on the surface of tape-casted YSZ layers and attached with gold wires for current collection. Nanocrystalline perovskite powders were prepared using different chemical methods: LaFeO 3 by the thermal decomposition of the LaFe–hexacyanide complex, and La 0.8Sr 0.2FeO 3 by a sol-gel route. A sub-micrometric commercial WO 3 powder was used. The oxide powders were mixed with a screen-printing oil and deposited on one Pt finger electrode. The presence of the oxide powder makes one of the electrodes different from the other in terms of catalytic activity, specific surface area, gas adsorption and reaction kinetics. Both electrodes were wholly exposed to the same gas atmosphere, without using reference air. The sensors were investigated at fixed temperature (450–700 °C) by measuring the electromotive force (EMF) at different concentrations of NO 2 and CO in air in the range 20–1000 ppm. A fast and stable response was measured for all the tested sensors. An EMF of opposite sign was measured for p- and n-type semiconducting based sensors upon exposure to the same gas. After increasing the grain size of the nano-structured La 0.8Sr 0.2FeO 3 powder by a heat treatment at 900 °C for 4 h, the response to NO x became small, slow and unstable.

Amperometric determination of ascorbic acid on screen-printing ruthenium dioxide electrode

Ruthenium oxide (RuO 2) is commonly used in resistive pastes for screen printing. The electrochemical properties of a screen-printing planar RuO 2 electrode have hardly been studied. In this communication, planar electrochemical sensors with a RuO 2 working electrode, an Ag|AgCl reference electrode and a RuO 2 counter electrode are fabricated by screen printing. The electro-oxidation of ascorbic acid, uric acid, and hydrogen peroxide on these RuO 2 electrodes is investigated by means of cyclic voltammetry. Compared to uric acid and hydrogen peroxide, ascorbic acid can be easily oxidized at the low operating potential (<150 mV versus Ag|AgCl). The amperometric measurement of ascorbic acid at 100 mV on a RuO 2 electrode shows fast response and good linearity in the 0–4 mM range. Meanwhile, the electrochemical interference from uric acid and hydrogen peroxide at this potential is very small.

A Planar Electrochemical Carbon Monoxide Sensor

A planar electrochemical sensor, comprised of three electrodes (working, counter, and reference) on an alumina substrate and a cast Nafion® film on them, was built and studied. The working and counterelectrodes were made with Pt, while the reference electrode was made with Au. The response was studied with both cyclic voltammetry and constant‐potential amperometry. The highest oxidation current was observed at 0.4 V vs. built‐in Au reference and was 0.04 nA/ppm. permeability of a Nafion® film was measured with a volumetric method. The sensitivity was calculated from the measured permeability. Comparing the observed and calculated sensitivities, the sensitivity of the present sensor was assumed to be governed by the permeation of through the Nafion® film.