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Self-Powered Potentiometric Sensors with Memory
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ACS Sensors

Cite this: ACS Sens. 2021, 6, 10, 3650–3656
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https://doi.org/10.1021/acssensors.1c01273
Published September 28, 2021

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Abstract

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Potentiometric sensors induce a spontaneous voltage that indicates ion activity in real time. We present here an advanced self-powered potentiometric sensor with memory. Specifically, the approach allows for one to record a deviation from the analyte’s original concentration or determine whether the analyte concentration has surpassed a threshold in a predefined time interval. The sensor achieves this by harvesting energy in a capacitor and preserving it with the help of a diode. While the analyte concentration is allowed to return to an original value following a perturbation over time, this may not influence the sensor readout. To achieve the diode function, the sensor utilizes an additional pair of driving electrodes to move the potentiometric signal to a sufficiently high base voltage that is required for operating the diode placed in series with the capacitor and between the sensing probes. A single voltage measurement across the capacitor at the end of a chosen time interval is sufficient to reveal any altered ion activity occurring during that period. We demonstrate the applicability of the sensor to identify incurred pH changes in a river water sample during an interval of 2 h. This approach is promising for achieving deployable sensors to monitor ion activity relative to a defined threshold during a time interval with minimal electronic components in a self-powered design.

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Potentiometric measurement tools occupy a significant portion of the sensor market due to their simple underlying operation principle. (1,2) Ion-selective electrodes (ISE) are a dominant class of these sensors and are used for detecting pH and various analytes of interest, especially in healthcare and environmental sectors. (3,4) pH probes and other ISEs routinely show lifetimes on the order of months, (1,5) even when measured in highly complex samples. In the simplest configuration, so-called direct potentiometry, the potential induced by the ISE is a direct indicator of the analyte activity. As this spontaneous response requires no power source, they become potential candidates for developing energetically autonomous, or self-powered sensors.
Self-powered sensors omit external power sources and achieve detection by solely utilizing the harvested energy from their operation. (6−8) As a result, they integrate a two-fold approach by dealing with aspects related to both detection and energy. Usually, such an integration is possible by exploiting the sample under investigation to provide the required energy for sensing. A large proportion of self-powered electrochemical sensors have resulted from this approach, primarily in the field of biosensors (using biofuel cells and battery systems), where the analyzed sample induces a redox reaction across electrodes to produce a galvanic cell. (9) Often, the use of enzymes, organelle substrates, and microorganisms have helped to achieve amperometric operation of self-powered sensors for detection of analytes such as glucose, lactate, cholesterol, ethanol, toxins, and biological oxygen demand. (10−15) The electrical signal generated from these redox reactions serves either directly as the indicator of the sensor response or is further processed to other output forms. (16)
Because potentiometric sensors are usually read out at nearly zero-current conditions, self-powered approaches can only be conceived by subjecting the sensor to a transient state where it supplies a transient current. To achieve such a transition, the sensor may take advantage of a capacitive element. For instance, in the presence of an external power source, coulometric sensors accumulate charge across a capacitive element owing to a transient current generated in the process of the system attaining a new equilibrium state. Chemical substrates such as polymer films and electronic capacitors proved to be useful in realizing electric and optical readout of ions using this transduction principle. (17−20) Based on these developments, we recently demonstrated a self-powered version of a potentiometric sensor by placing a capacitor directly between the indicator and reference probes. (21) The induced potential difference across the probes generated a transient current flow and charged the capacitor to nearly the open-circuit voltage. As a result, no high-end instruments were necessary because the capacitor voltage could be read out later with a simple hand-held multimeter to reveal ion activity. This is different from the constant potential coulometric method of ion-sensing where the measured current transient observed over a time period allows for one to achieve a better resolution in ion activity measurements. (22,23) Instead, this approach aims to obtain only a single value of capacitor voltage, as in a traditional potentiometric principle. Here, the capacitor serves as a memory element and the existence of a transient current in the circuit is due to the nature of the element and is not the parameter of interest.
In the approach discussed above, the induced voltage in the potentiometric sensor corresponds to the state of analyte at the actual time of measurement or disconnection from the cell. For monitoring analyte concentration during a certain interval, these sensors would require sophisticated supplementary electronics. This would demand a substantial voltage and power that are often not available with potentiometric sensing probes to make the complete sensor self-sufficient in operation. Recent developments in galvanic bipolar electrochemical methods involving cascading strategies can prove to be useful to push the response voltage higher. Typically, a bipolar electrode is an electrically conducting element that undergoes faradaic reactions to develop a potential difference between its ends on applying a suitable voltage across it. A pair of driving electrodes that are connected to an external power source usually provides this voltage without actually establishing a physical contact with the bipolar element in an ionic medium. (24) Alternatively, an electrochemical system can achieve a similar potential difference through oxidation–reduction reactions (or with any electron-exchange process) at separate electrodes that are electrically short-circuited. In this case, the process becomes spontaneous and thereby involves no driving electrodes. However, by incorporating an additional pair of driving electrodes (connected to an external power source) in these systems, the induced potential difference can be further modulated in magnitude. In fact, a cascade of two bipolar electrode systems can eliminate the dependency on an external power source, with one of them driving the other through the generated induced voltage. (25) Note, however, that many of these systems continuously draw a substantial steady current due to the short-circuited sensing electrodes and may quickly affect the sensor stability during prolonged operation.
We present here a self-powered potentiometric ion sensor capable of tracking perturbations in ion activity over time to reveal the deviation of analyte concentration from its original value (Scheme 1). The sensor uses a four-electrode configuration where a pair of driving electrodes exhibit a sufficiently large potential to shift the potentiometric signal to a sufficient voltage for operating a basic diode. In the absence of the diode, the capacitor voltage at a specific measurement point in time misses the information about any prior concentration fluctuations. However, by introducing the diode, the capacitor charges based on the transient current that is allowed by it. Importantly, when the analyte concentration returns to its original value, the diode prevents the discharge of the capacitor. In this manner, the measurement of the capacitor voltage at a single time point may reveal important fluctuations in the ion concentration without requiring to perform a continuous measurement. We demonstrate a sensor application for tracking pH variation for a duration of 2 h in a river water sample. The current sensing approach integrates the memory effect in a potentiometric sensor with minimal electronic components that operate in a self-sufficient manner.

Scheme 1

Scheme 1. Schematic Illustration of the Approach Adopted to Track a Deviation in the Activity of an Analyte During a Time Interval by Reading Out the Response Stored in a Capacitor at a Specific Measurement Point in Time
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Experimental Section

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Materials

Potassium chloride (KCl), hydrogen ionophore I, poly(vinyl chloride) (PVC) (high molecular weight), 2-nitrophenyloctylether (o-NPOE), sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB), tetrakis(4-chlorophenyl)borate tetradodecylammonium salt (ETH 500), tetrahydrofuran (THF), sodium chloride (NaCl), boric acid, zinc chloride (ZnCl2), phosphoric acid, and acetic acid were purchased from Sigma-Aldrich.
Hydrochloric acid (HCl, Thermo Fisher Scientific) and sodium hydroxide (NaOH, Alfa Aesar) were used as purchased. All experiments were performed with deionized water (resistivity > 18 MΩ cm) at room temperature. 10 μF metallized polyester capacitors (WIMA) and ESD Suppressors/TVS Diodes (PESD5Z3.3,115) were purchased from Distrelec, Switzerland, and Mouser Electronics.

Sample Preparation

Universal buffer solution was prepared by a mixture of phosphoric acid, acetic acid, and boric acid. NaCl and KCl solutions were prepared with the respective salts using deionized water. River water samples were collected from Arve river in Geneva, Switzerland.

Electrodes

Double junction Ag/AgCl (3 M KCl/1 M LiOAc), single junction Ag/AgCl (3 M KCl), and Pt electrodes were purchased from Metrohm, Switzerland. Zn electrodes (of diameter 1 mm, 99.995% trace metals basis) were obtained from Sigma-Aldrich.
Ag/AgCl electrodes were made by electrochemical oxidation of an Ag electrode (of diameter 3 mm, order no. 6.1204.330, Metrohm, Switzerland) using a similar procedure described in our previous work. (21)
A H+-selective membrane was prepared by dissolving a mixture made up of 58.36 mg of PVC, 116.73 mg of o-NPOE, 0.90 mg of NaTFPB, 20.67 mg of ETH 500, and 3.33 mg of hydrogen ionophore I in 2 mL of THF. The dissolved solution was cast into a glass ring (of inner diameter 22 mm) attached to a glass slide and was left overnight to form a membrane. A circular piece of the membrane (diameter 8 mm) was then loaded into the ISE body (Oesch Sensor Technology, Sargans, Switzerland) with an inner filling solution comprising 10 mM universal buffer (at pH 7) and 10 mM NaCl to obtain H+-ISE. The H+-ISEs were conditioned overnight in a solution, similar to the composition of inner filling solution.

Sensor Configuration

The sensor contains two separate compartments (of diameter 5.5 cm) to place the reference and sample solutions. The reference compartment contained 50 mL of 100 mM ZnCl2 solution (pH 2.6). On the other hand, the sample compartment featured 50 mL of solution containing 10 mM universal buffer and 10 mM NaCl. Zn and Ag/AgCl electrodes were immersed in the reference solution. The H+-ISE and Ag/AgCl (3 M KCl) electrodes were placed in a sample compartment. The separation between electrodes in each compartment is 2.5 cm (center to center). The connection between the compartments is established by making an electrical contact between the electrodes. While the Zn and Ag/AgCl (3 M KCl) electrodes were directly connected to each other, the Ag/AgCl and H+-ISE were connected through a DIP (dual in-line package) switch, diode, and capacitor.
For certain experiments, the solutions and the electrodes were modified. These were mentioned in the text wherever appropriate.

Sensing Measurements

The circuit connection, or activation of the sensor is made using a DIP switch. During sensing measurements, the concentration of the reference solution is kept constant. Small and defined changes in pH of the sample solution were made using 1 M HCl and 1 M NaOH solutions. The voltage measurements of the sensor components (capacitor and electrodes) were carried out with EMF16 data acquisition system (input impedance of ∼1013 Ω; Lawson Labs, Inc., Malvern, PA).
In certain experiments, a source meter (Keithley 2450) was used to supply voltage across the electrodes. We mentioned this at appropriate places in the text.

Results and Discussion

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The approach adopted to detect a deviation occurring in activity of an analyte during a predefined time interval without requiring continuous monitoring is illustrated in Scheme 1. The sensor system uses two compartments containing two electrodes, each electrically connected in a closed bipolar electrode configuration. The sample compartment contains the ISE, E1, selectively responsive to the analyte, along with the reference electrode E2, as in a traditional measuring cell. The second compartment contains a driving electrode E3 and a second reference element E4 in a solution whose composition is known and constant. Typically, the potential between the indicator and reference electrodes produces a potentiometric response for the analyte ion activity in the sample in analogy to classical measurements. However, the response relevant to this sensing approach is the induced voltage between electrodes E1 and E4. Therefore, in the absence of electrodes E2 and E3 that form the driving circuit, the ISE response can be obtained against the electrode E4 by establishing a salt bridge connection. Further, this response can be transferred to a capacitor by connecting it across these electrodes in a self-powered manner. (21) However, the incorporated two additional electrodes (E2 and E3) in the present work act to increase this response signal by a voltage increment, given by the asymmetry of the driving and reference electrode pair. Because these additional electrodes work by utilizing the resources already available in the original system, the system may be considered self-powered. (25) In this configuration, the open circuit potential difference (Ecell = −Ve) for the entire cell can be expressed as
(1)
where E1, E2, E3, and E4 are the potentials at the respective electrodes mentioned above. Hereafter, to explain the circuit behavior, a positive voltage Ve is referred to denote an open circuit voltage as it represents Ecell with an opposite polarity.
A capacitor can be charged to this induced potential difference, Ve, when connected as the only circuit element between E1 and E4 according to the relation
(2)
where Vc is the voltage across the capacitor with a capacitance C and i is the current flowing through the closed circuit over time t during the charging process. This strategy allows one to capture the charge redistributed upon the transient electrochemical imbalance in the capacitor, which acts as an energy storage element that can be read at any time. The voltage built up across the capacitor thereby remains a direct function of the cell potential with time. Because of this, the capacitor cannot retain information about past events but reflects the potential at the time of readout or disconnection from the cell.
To overcome this, the circuit may be endowed with a concentration memory by introducing a diode in series with the capacitor. In that case, the voltage Vc does not rapidly assume the value of the steady-state open-circuit voltage, Ve, because the diode now dictates the charging behavior of the capacitor. To illustrate this, the typical current–voltage characteristics for a unidirectional diode is shown in Figure S1. The curve can be explained with the help of a Shockley ideal diode equation (eq 3) that exponentially relates the voltage across the diode, Vd, with the current, i, flowing through it.
(3)
where is is the reverse bias saturation current (typically less than 1 nA) observed at Vd < 0. η is the ideality factor (or emission coefficient) that accounts for recombination of charge carriers. Its value for an ideal diode is 1, but it usually varies between 1 and 2 depending on the operating conditions, physical construction, and the involved fabrication process. VT = kT/q = 25.69 mV (at 25 °C) is the thermal voltage, with k being the Boltzmann constant, T being the absolute temperature, and q being the elementary charge.
As indicated by the significant exponential term in eq 3, the diode conducts with a very low resistance at higher voltages to allow for a substantial current (Figure S1a,b). However, at low voltages, the exponential term becomes negligibly small, and the current is approximately equal to is. The diode therefore becomes more resistive and almost ceases the current flow at lower voltages. For instance, for an input voltage appearing across the circuit containing a diode and capacitor connected in series, a transient current prevails. This is an advantage, as significant current is not drawn from the system continuously, which otherwise would happen if the sensing electrodes were short-circuited or connected through a low-value resistor. Because the capacitor voltage is low initially, the diode experiences almost the input voltage and offers a very low resistance. As a result, the response time is quicker at this moment due to a very low time constant to charge the capacitor. However, as time progresses, the gradual increase in capacitor voltage causes a decrease in voltage across the diode. This eventually leads to a longer response time due to a larger time constant by virtue of the diode becoming highly resistive (Figure S1c). This behavior makes the diode to allow for current flow only in one direction, that is, when the voltage across it is sufficiently high. As a result, upon activating the sensor in Scheme 1 by closing the switch S1, the capacitor initially charges quickly owing to a less resistive diode. With time, however, the gradual buildup of Vc results in a decreasing voltage across the diode (Vd = Ve – Vc). The diode starts to provide a high resistance for charging the capacitor further. Importantly, the diode allows for the capacitor to maintain its charge even when Ve is found to decrease again. This is driven by its ability to restrict current in the reverse direction.
Before establishing the pH sensing system as described in Scheme 1, the effect of driving circuit was evaluated using an initial model system with symmetrical reference elements that was driven by an external applied voltage. It comprised four identical Ag/AgCl electrodes placed in two separate solutions containing 100 mM KCl each (Figure 1a). On applying a bi-directional staircase signal from 0 to 1 V in steps of 100 mV with a step time of 10 s to the driving electrodes (E2, E3), the observed potential difference between the other two electrodes in the sensing circuit is found to have the same value (Figure 1b). This result shows that the electrodes forming the detection circuit experience an additional voltage whose value corresponds to that between the two driving electrodes. Importantly, this behavior was not altered by the insertion of a capacitor between the electrodes in the sensing circuit. Instead, the circuit charged the capacitor to a voltage equivalent to the value between the electrodes. Here, the RC time constant for the capacitive element was on the order of 50 ms, sufficiently small in the time scale of the experiment to not appreciably alter the time response of the potential readings. The use of ion-selective membranes of high resistance may result in larger values.

Figure 1

Figure 1. (a) Model system to study the influence on the capacitor connected between indicator and reference electrodes due to a voltage applied across the driving electrodes. (b) Measured voltages for an applied bi-directional staircase signal.

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The capability to influence the voltage across the electrodes in the sensing circuit with the help of two independent driving electrodes may be used to shift the level of the potentiometric signal obtained conventionally in the measurement compartment. This was demonstrated with an electrochemical sensing cell for the measurement of pH for its wide practical applicability and because sample acidity can be reliably changed in a very wide range. For sensing pH, electrode E1 was replaced with an H+-ISE while keeping the other electrodes as Ag/AgCl elements, using 10 mM universal buffer and 10 mM NaCl solution for the sample and 100 mM KCl for the reference solution (Figure 2b). The pH probe gave a Nernstian response against a double junction Ag/AgCl (3 M KCl/1 M LiOAc) electrode as confirmed in separate experiments (Figures 2a and S2). For an applied external voltage of 0.9 V between the driving electrodes, the acquired voltage response for pH was shifted by this value relative to measurements against a double junction Ag/AgCl electrode, but still showed a Nernstian behavior (Figure 2c). The opposite slope for pH shown in Figure 2c (when compared to Figure 2a) comes from presenting the voltage with reference to ISE as denoted in the circuit (Figure 2b). On reversing the polarity of the applied voltage, a difference of −0.9 V in measured voltages was equally observed from conventional potentiometric measurements.

Figure 2

Figure 2. (a) Response of H+-ISE against a double junction Ag/AgCl (3 M KCl/1 M LiOAc) electrode. (b) Model system involving an external power source to shift the level of a conventional potentiometric signal and to a charge a capacitor to the translated voltage. (c,d) Voltage across capacitor at different pH values for an applied voltage of 0.9 and −0.9 V, respectively, between the driving electrodes.

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So far, a source meter provided the required voltage across the driving electrodes for imposing the voltage levels in the response curves. To eliminate the dependence on external power sources while shifting the level of response voltage, a potential difference between driving electrodes was induced electrochemically. The modified system contained a Zn electrode as E3 is immersed in a 0.1 M ZnCl2 reference solution at pH 2.6 (Figure 3a). In this configuration, a Nernstian response for pH with a potential shift of ∼1.065 V relative to the potentiometric measurement with a double junction Ag/AgCl (3 M KCl/1 M LiOAc) electrode was observed (Figure 3b). This shift is due to the asymmetry induced by the Zn electrode E3, which acts in some analogy to the voltage applied with an external source meter. As shown in Figure 3b, the voltage response again remained identical when a capacitive element was introduced between electrodes E1 and E4. Importantly, the Zn electrode remained stable during the pH measurement for at least 17 h within a variation of ±1 mV when monitored against a double junction Ag/AgCl electrode (Figure 3c).

Figure 3

Figure 3. (a) pH sensing configuration without an external power source to shift a potentiometric signal and to store the response by charging the capacitor. (b) Response of the sensor at different pH values in open circuit and with a capacitor. (c) Stability of Zn electrode over time.

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The capacitor in the circuit (Scheme 1) is initially in a discharged state. On activating the sensor by closing switch S1, Ve aims to approach the open-circuit voltage. Because the initial capacitor voltage is zero, the diode offers a very low resistance because the potential across this circuit element is close to Ve. As the capacitor is charging in the process of approaching Ecell, the voltage across the diode is continuously decreasing, making the diode more resistive and increasing the charging time of the capacitor. The evolution of Vc at different pH sample values is shown in Figure 4a. Within a measurement period of 600 s, there is a clear distinction in the time evolution of Vc for different pH values.

Figure 4

Figure 4. (a) Voltage evolution across the capacitor at different pH of the sample. The solid lines represent the capacitor voltage, and the dashed lines indicate the induced voltage, Ve. (b) Response across the capacitor for voltage applied using source meter. The sample is maintained at pH 7. The dashed lines represent the applied voltage, and the solid lines represent the capacitor response. (c) Voltage response of capacitor for pH changes incorporated in a sample (originally at pH 7) to deviate from its initial state. The solid lines represent the capacitor voltage, and the dashed lines indicate the induced voltage, Ve. (e) Voltage across capacitor measured at the end of 2 h using a hand-held multimeter. Here, the river samples are spiked to a higher pH for an abrupt interval of 5 min, before returning it to original value. The second y-axis (Q) in the figure corresponds to the capacitor curves.

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Achieving a self-powered memory function is possible if the diode obstructs current flow in reverse direction to preserve the accumulated charge across the capacitive element when Ve returns to baseline potential. To illustrate this, we initially applied a voltage of 148 mV with the source meter in the driving circuit to mimic a pH change of 2.5 in the sample (Figure 4b), while leaving the solutions undisturbed with a sample pH of 7. During a measurement time of 10 min, the capacitor voltage deviated from its normal course observed at pH 7 because of the applied voltage at the indicated intervals. Notably, irrelevant of the time instance at which the voltage change was induced, the capacitor maintained the charge across it until the end of the time period. The longer the duration of the superimposed change, the greater the capacitor voltage owing to the longer charge buildup time. A similar behavior could be observed with smaller voltage changes of 29.6 and 59.2 mV (Figure S3). A single measurement of the capacitor voltage at a specific time point could reveal a prior potential excursion by comparing the value with that obtained for the sample in its original state. In the absence of the approach described here, this information could only be achieved by performing a continuous potentiometric measurement. Based on the data obtained with applied voltages, the sensor behavior was further explored without a source meter, but by imposing intermittent pH changes of either 1 or 1.5 units in the sample (10 mM universal buffer and 10 mM NaCl solution) using the configuration shown in Scheme 1. As shown in Figure 4c, the capacitor voltage deviated from its normal course (observed with sample at pH 7) in a similar manner as with voltages applied using the source meter.
We further applied the sensor to probe for adverse pH changes in water from the river Arve in Geneva. The pH of the river water sample was found to vary between 8.18 and 8.43 when left in laboratory air for a duration of 2 h. The sensor delivered a capacitor voltage of 0.69735 ± 0.00265 V at the end of 2 h with these samples (Figure 4d). Due to the time variation of pH of river water, the capacitor voltage was found to be less stable in comparison to the value obtained with universal buffer solution (0.685 V at pH 8.18) (Figure S4). Initial experiments superimposed a voltage of 88.8 mV between the driving electrodes using a source meter to mimic an intermittent pH change of 1.5 units. The required shortest duration of this superimposed change is about 3 min to reliably detect the pH change. The difference in measured capacitor voltage was found to exceed ∼9 mV when the duration of applied voltage was at least 5 min (Figure S5). To evaluate the memory function of the circuit during an actual measurement, the river water pH was perturbed with NaOH to introduce a pH variation of about 2.5 for a period of 5 min before returning to baseline. As expected, the capacitor voltage was found to remain distinctively different throughout the duration of the measurement period relative to baseline (Figure 4d). The potential across the Zn electrode when measured against a double junction Ag/AgCl electrode was within ±1 mV (Figure S6). Upon acquiring a single capacitor voltage measurement at the end of 2 h with the help of a simple hand-held multimeter, the sensor revealed the introduced pH perturbations in river water despite returning the sample to the original pH (Figure 4e). In this manner, the capacitor voltage in combination with the diode served as a memory element to recognize that the pH had been altered within the measurement period. The sensor configuration shown in Scheme 1 allows one to track deviations corresponding to pH increments in one direction only. An expansion of the cell components would be required to develop sensors capable of detecting both upper and lower threshold limits.

Conclusions

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The work demonstrates a self-powered potentiometric sensor capable of reporting an output voltage that can reveal deviations in an analyte concentration during a desired time interval. By incorporating a diode alongside an electronic capacitor, it is possible to preserve the harvested energy in the capacitor due to the diode’s ability to restrict the discharge of the capacitor. A measurement at the end time using a simple hand-held multimeter would allow one to identify that the ion activity had increased in the time period of interest. By incorporating a pair of asymmetric driving electrodes, the potentiometric signal can be shifted to a higher voltage, which allows one to operate the diode. Owing to the diode–capacitor combination, no substantial net current is flowing over prolonged time in the sensing circuit, which allows for a high potential stability of all electrodes. Without sophisticated electronics and dependence on external powers sources, the sensor allowed for the detection of a short-lived pH perturbation in a river water sample within a time interval of 2 h. The sensing approach in this work may form the technological basis to achieve a completely self-sufficient potentiometric sensor that could either assess ion activity above a given threshold, for instance in pollution events. It may also be useful for retaining the sensing signal in point of care analytical devices after the underlying measurement step is complete.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acssensors.1c01273.

  • Details related to diode characteristics, response of H+-ISE, capacitor voltage curves under different conditions, and stability of the Zn electrode (PDF)

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Author Information

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  • Corresponding Authors
  • Author
    • Sunil Kumar Sailapu - Department of Inorganic and Analytical Chemistry, University of Geneva, Quai Ernest-Ansermet 30, CH-1211 Geneva, Switzerland;  Instituto de Microelectrónica de Barcelona, IMB-CNM (CSIC), C/del Til·lers, Campus UAB, Bellaterra, 08193 Barcelona, Spain;  Orcidhttps://orcid.org/0000-0002-0923-0879
  • Author Contributions

    S.K.S., N.S., and E.B. conceived the idea. S.K.S. and E.B. designed the experiments. S.K.S. performed the experiments. S.K.S. and E.B. analyzed the data. S.K.S., N.S., and E.B. wrote the paper.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors acknowledge funding received from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 712949 (TECNIOspring PLUS) and from the Agency for Business Competitiveness of the Government of Catalonia. E.B. acknowledges financial support from the Swiss National Science Foundation. The authors thank Stéphane Jeanneret for his contribution to the work.

References

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    Zdrachek, E.; Bakker, E. Potentiometric Sensing. Anal. Chem. 2021, 93, 72– 102,  DOI: 10.1021/acs.analchem.0c04249
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    3
    Potentiometric Sensing
    Zdrachek, Elena; Bakker, Eric
    Analytical Chemistry (Washington, DC, United States) (2021), 93 (1), 72-102CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
    A review. This review, with ∼200 refs., describes and comments key highlights of the progress in the field of potentiometric sensing between Jan. 2019, date of the last fundamental review on the topic in the Anal. Chem. special issue, and Oct. 2020. The refs. were mainly retrieved from 2 scientific citation indexing services, Web of Science and SciFinder, by searching for relevant keywords and recognized authors in the field. Addnl., key journals were screened manually. The authors also selected relevant review papers. Given the large no. of potentiometric sensing publications and the competing need for highlighting the most relevant developments, the authors decided to limit to 200 the no. of refs. included in this review. For this reason, this review reflects the opinion on the current state of the field and does not aim at covering the full potentiometric sensing literature. With the aim to stimulate scientific discussion in the field, the authors critically analyzed the content of the cited refs., pointing out what the authors feel are their strengths and weaknesses. The authors hope that no author will feel offended by any of comments. It is exciting to see that the field of potentiometric sensing is experiencing solid growth. The current research blurs interdisciplinary boundaries and explores new sensor readout principles. Materials science offers researchers new ion-to-electron transducing materials that advance miniature sensors. This opens up new opportunities for the application of potentiometric sensors. In particular, wearable potentiometric sensors are gaining attention. The research in the domain of scalable and disposable paper-based potentiometric sensors is also very active and brings potentiometry closer to point-of-care and in-field applications. This review starts with describing the recent progress with ref. electrodes. The improvements of ref. electrodes with liq. junction and the developments of liq. junction free ref. electrodes are discussed. Next, it covers recent advances in the modern theory of potentiometry, including numerical simulations used to predict their dynamic response. New protocols were described for detg. lower detection limit and the description of the selectivity coeffs. where primary and interfering ions form complexes of multiple stoichiometries. The following section discusses new and nonclassical readout principles for ion-selective electrodes, including ion-transfer voltammetry, coulometry, amperometry, chronopotentiometry and various optical readouts. This is followed by a section dedicated to new materials for ion-selective electrodes (ISEs). It starts with an overview of the ion-to-electron transducing materials, continues with new membrane materials and approaches to overcome membrane biofouling and ends by describing mol. imprinted polymers and new ionophores. Finally, the review discusses recent developments in the area of miniaturized ISEs, including wearable sensors, paper-based devices, microfluidic devices, thread-based ISEs, ion-selective microelectrodes and ion-selective field effect transistors (ISFET).
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  4. 4
    Shao, Y.; Ying, Y.; Ping, J. Recent advances in solid-contact ion-selective electrodes: functional materials, transduction mechanisms, and development trends. Chem. Soc. Rev. 2020, 49, 4405– 4465,  DOI: 10.1039/c9cs00587k
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    Recent advances in solid-contact ion-selective electrodes: functional materials, transduction mechanisms, and development trends
    Shao, Yuzhou; Ying, Yibin; Ping, Jianfeng
    Chemical Society Reviews (2020), 49 (13), 4405-4465CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    From environmental monitoring to point-of-care biofluid anal., rapid ion detn. requires robust anal. tools. In recent years, driven by the development of materials science and processing technol., solid-contact ion-selective electrodes (SC-ISEs) with high-performance functional materials and creative structures have shown great potential for routine and portable ion detection. In particular, the introduction of nanomaterials as ion-to-electron transducers and the adoption of different performance enhancement strategies have significantly promoted the development of SC-ISEs. Besides, with the increasing miniaturization, flexibility, and dependability of SC-ISEs, this field has gradually begun to evolve from conventional potentiometric ion sensing to integrated sensing systems with broader application scenarios. This comprehensive review covers pioneering research on functional materials and state-of-the-art technologies for the construction of SC-ISEs, with an emphasis on new development trends and applications.
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    Oesch, U.; Simon, W. Lifetime of neutral carrier based ion-selective liquid-membrane electrodes. Anal. Chem. 2002, 52, 692– 700,  DOI: 10.1021/ac50054a024
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    Grattieri, M.; Minteer, S. D. Self-Powered Biosensors. ACS Sens. 2018, 3, 44– 53,  DOI: 10.1021/acssensors.7b00818
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    Self-Powered Biosensors
    Grattieri, Matteo; Minteer, Shelley D.
    ACS Sensors (2018), 3 (1), 44-53CODEN: ASCEFJ; ISSN:2379-3694. (American Chemical Society)
    A review. Self-powered electrochem. biosensors utilize biofuel cells as a simultaneous power source and biosensor, which simplifies the biosensor system, because it no longer requires a potentiostat, power for the potentiostat, and/or power for the signaling device. This review article is focused on detailing the advances in the field of self-powered biosensors and discussing their advantages and limitations compared to other types of electrochem. biosensors. The review will discuss self-powered biosensors formed from enzymic biofuel cells, organelle-based biofuel cells, and microbial fuel cells. It also discusses the different mechanisms of sensing, including utilizing the analyte being the substrate/fuel for the biocatalyst, the analyte binding the biocatalyst to the electrode surface, the analyte being an inhibitor of the biocatalyst, the analyte resulting in the blocking of the bioelectrocatalytic response, the analyte reactivating the biocatalyst, Boolean logic gates, and combining affinity-based biorecognition elements with bioelectrocatalytic power generation. The final section of this review details areas of future investigation that are needed in the field, as well as problems that still need to be addressed by the field.
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  7. 7
    Sailapu, S. K.; Macchia, E.; Merino-Jimenez, I.; Esquivel, J. P.; Sarcina, L.; Scamarcio, G.; Minteer, S. D.; Torsi, L.; Sabaté, N. Standalone operation of an EGOFET for ultra-sensitive detection of HIV. Biosens. Bioelectron. 2020, 156, 112103,  DOI: 10.1016/j.bios.2020.112103
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    Standalone operation of an EGOFET for ultra-sensitive detection of HIV
    Sailapu, Sunil Kumar; Macchia, Eleonora; Merino-Jimenez, Irene; Esquivel, Juan Pablo; Sarcina, Lucia; Scamarcio, G.; Minteer, Shelley D.; Torsi, Luisa; Sabate, Neus
    Biosensors & Bioelectronics (2020), 156 (), 112103CODEN: BBIOE4; ISSN:0956-5663. (Elsevier B.V.)
    A point-of-care (POC) device to enable de-centralized diagnostics can effectively reduce the time to treatment, esp. in case of infectious diseases. However, many of the POC solns. presented so far do not comply with the ASSURED (affordable, sensitive, specific, user-friendly, rapid and robust, equipment free, and deliverable to users) guidelines that are needed to ensure their on-field deployment. Herein, we present the proof of concept of a self-powered platform that operates using the analyzed fluid, mimicking a blood sample, for early stage detection of HIV-1 infection. The platform contains a smart interfacing circuit to operate an ultra-sensitive electrolyte-gated field-effect transistor (EGOFET) as a sensor and facilitates an easy and affordable readout mechanism. The sensor transduces the bio-recognition event taking place at the gate electrode functionalized with the antibody against the HIV-1 p24 capsid protein, while it is powered via paper-based biofuel cell (BFC) that exts. the energy from the analyzed sample itself. The self-powered platform is demonstrated to achieve detection of HIV-1 p24 antigens in fM range, suitable for early diagnosis. From these developments, a cost-effective digital POC device able to detect the transition from "healthy" to "infected" state at single-mol. precision, with no dependency on external power sources while using minimal components and simpler approach, is foreseen.
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  8. 8
    Zhu, M.; Yi, Z.; Yang, B.; Lee, C. Making use of nanoenergy from human – Nanogenerator and self-powered sensor enabled sustainable wireless IoT sensory systems. Nano Today 2021, 36, 101016,  DOI: 10.1016/j.nantod.2020.101016
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    Wang, L.; Wu, X.; Su, B. S. Q. w.; Song, R.; Zhang, J.-R.; Zhu, J.-J. Enzymatic Biofuel Cell: Opportunities and Intrinsic Challenges in Futuristic Applications. Adv. Energy Sustain. Res. 2021, 2, 2100031,  DOI: 10.1002/aesr.202100031
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    There is no corresponding record for this reference.
  10. 10
    Shitanda, I.; Morigayama, Y.; Iwashita, R.; Goto, H.; Aikawa, T.; Mikawa, T.; Hoshi, Y.; Itagaki, M.; Matsui, H.; Tokito, S.; Tsujimura, S. Paper-based lactate biofuel cell array with high power output. J. Power Sources 2021, 489, 229533,  DOI: 10.1016/j.jpowsour.2021.229533
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    Paper-based lactate biofuel cell array with high power output
    Shitanda, Isao; Morigayama, Yukiya; Iwashita, Risa; Goto, Himeka; Aikawa, Tatsuo; Mikawa, Tsutomu; Hoshi, Yoshinao; Itagaki, Masayuki; Matsui, Hiroyuki; Tokito, Shizuo; Tsujimura, Seiya
    Journal of Power Sources (2021), 489 (), 229533CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    Printable wearable lactate biosensors have attracted significant attention, and printable lactate biofuel cells have gained popularity as suitable power supplies for these sensors. However, realizing an appropriate power supply for the practical application of these sensors as wearable devices, it is necessary to improve the output of lactate biofuel cells. In this study, we fabricated a lactate biofuel cell that employed paper substrate using screen printing. The proposed paper-based biofuel cell (PBFC) features a novel electrode design that affords an open circuit voltage of approx. 3.4 V when using an array with six cells in series. Furthermore, a 6 x 6 array of the lactate biofuel cell, i.e., an array comprising six cells in series and six cells in parallel, yielded a power output of 4.3 mW. To the best of our knowledge, this output of the proposed design is higher than those of previously reported lactate biofuel cells. Arrays of the proposed cell were capable of driving a Bluetooth low-energy device for wireless transmission, without requiring a booster circuit. A com. available activity meter could be driven for 1.5 h using artificial sweat to fuel a 6 x 6 array of the PBFCs.
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  11. 11
    Ruff, A.; Pinyou, P.; Nolten, M.; Conzuelo, F.; Schuhmann, W. A Self-Powered Ethanol Biosensor. ChemElectroChem 2017, 4, 890– 897,  DOI: 10.1002/celc.201600864
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    11
    A Self-Powered Ethanol Biosensor
    Ruff, Adrian; Pinyou, Piyanut; Nolten, Melinda; Conzuelo, Felipe; Schuhmann, Wolfgang
    ChemElectroChem (2017), 4 (4), 890-897CODEN: CHEMRA; ISSN:2196-0216. (Wiley-VCH Verlag GmbH & Co. KGaA)
    We describe the fabrication of a self-powered ethanol biosensor comprising a β-NAD+-dependent alc. dehydrogenase (ADH) bioanode and a bienzymic alc. oxidase (AOx) and horseradish peroxidase (HRP) biocathode. β-NAD+ is regenerated by means of a specifically designed phenothiazine dye (i.e. toluidine blue, TB) modified redox polymer in which TB was covalently anchored to a hexanoic acid tethered poly(4-vinylpyridine) backbone. The redox polymer acts as an immobilization matrix for ADH. Using a carefully chosen anchoring strategy through the formation of amide bonds, the potential of the TB-based mediator is shifted to more pos. potentials, thus preventing undesired O2 redn. To counterbalance the rather high potential of the TB-modified polymer, and thus the bioanode, a high-potential AOx/HRP-based biocathode is suggested. HRP is immobilized in a direct-electron-transfer regime on screen-printed graphite electrodes functionalized with multi-walled carbon nanotubes. The nanostructured cathode ensures the wiring of the iron-oxo complex within oxidized HRP, and thus a high potential for the redn. of H2O2 of about +550 mV vs. Ag/AgCl/3 M KCl. The proposed biofuel cell exhibits an open-circuit voltage (OCV) of approx. 660 mV and was used as self-powered device for the detn. of the ethanol content in liquor.
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  12. 12
    Kai, H.; Kato, Y.; Toyosato, R.; Nishizawa, M. Fluid-permeable enzymatic lactate sensors for micro-volume specimen. Analyst 2018, 143, 5545– 5551,  DOI: 10.1039/c8an00979a
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    12
    Fluid-permeable enzymatic lactate sensors for micro-volume specimen
    Kai, Hiroyuki; Kato, Yuto; Toyosato, Ryoma; Nishizawa, Matsuhiko
    Analyst (Cambridge, United Kingdom) (2018), 143 (22), 5545-5551CODEN: ANALAO; ISSN:0003-2654. (Royal Society of Chemistry)
    Sensing of lactate in perspiration provides a way to monitor health and control exercise. The vol. of perspiration is miniscule, and the efficient collection of perspiration is desired for its effective sensing. The authors developed mesh-type enzymic electrodes fabricated on textile meshes and integrated the meshes into an enzymic biofuel cell. The authors tested them as self-powered lactate sensors for a small vol. of lactate soln. A fluid-permeable enzymic anode was fabricated based on an insulating textile mesh that was coated with carbon nanotubes (CNTs) and lactate oxidase. The anode was further coated with polyurethane to increase the linear range by limiting the diffusion of lactate while maintaining the advantages of the original textile mesh, such as flexibility, stretchability, and permeability. Permeability of the mesh-type lactate-oxidizing anode allowed a vertically stacked structure of the anode and a previously developed air-breathing cathode. This resulted in a small overall device size (1 cm2). The mesh-type sensor was tested using a small flow rate of lactate soln., and a moderate linearity of amperometric response for a wide concn. range (5 to ≥20 mM) was confirmed. The fluid-permeable anode and enzymic biofuel cell show the potential of the sensor for continuous monitoring of lactate in perspiration on skin.
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    Cui, Y.; Lai, B.; Tang, X. Microbial Fuel Cell-Based Biosensors. Biosensors 2019, 9, 92,  DOI: 10.3390/bios9030092
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    Microbial fuel cell-based biosensors
    Cui, Yang; Lai, Bin; Tang, Xinhua
    Biosensors (2019), 9 (3), 92CODEN: BIOSHU; ISSN:2079-6374. (MDPI AG)
    The microbial fuel cell (MFC) is a promising environmental biotechnol. that has been proposed mainly for power prodn. and wastewater treatment. Though small power output constrains its application for directly operating most elec. devices, great progress in its chem., electrochem., and microbiol. aspects has expanded the applications of MFCs into other areas such as the generation of chems. (e.g., formate or methane), bioremediation of contaminated soils, water desalination, and biosensors. In recent decades, MFC-based biosensors have drawn increasing attention because of their simplicity and sustainability, with applications ranging from the monitoring of water quality (e.g., BOD (BOD), toxicants) to the detection of air quality (e.g., carbon monoxide, formaldehyde). In this review, we summarize the status quo of MFC-based biosensors, putting emphasis on BOD and toxicity detection. Furthermore, this review covers other applications of MFC-based biosensors, such as DO and microbial activity. Further, challenges and prospects of MFC-based biosensors are briefly discussed.
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  14. 14
    Sekretaryova, A. N.; Beni, V.; Eriksson, M.; Karyakin, A. A.; Turner, A. P. F.; Vagin, M. Y. Cholesterol self-powered biosensor. Anal. Chem. 2014, 86, 9540– 9547,  DOI: 10.1021/ac501699p
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    14
    Cholesterol Self-Powered Biosensor
    Sekretaryova, Alina N.; Beni, Valerio; Eriksson, Mats; Karyakin, Arkady A.; Turner, Anthony P. F.; Vagin, Mikhail Yu.
    Analytical Chemistry (Washington, DC, United States) (2014), 86 (19), 9540-9547CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
    Monitoring the cholesterol level is of great importance, esp. for people with high risk of developing heart disease. Here the authors report on reagentless cholesterol detection in human plasma with a novel single-enzyme, membrane-free, self-powered biosensor, in which both cathodic and anodic bioelectrocatalytic reactions are powered by the same substrate. Cholesterol oxidase was immobilized in a sol-gel matrix on both the cathode and the anode. Hydrogen peroxide, a product of the enzymic conversion of cholesterol, was electrocatalytically reduced, using Prussian blue, at the cathode. In parallel, cholesterol oxidn. catalyzed by mediated cholesterol oxidase occurred at the anode. The anal. performance was assessed for both electrode systems sep. The combination of the two electrodes, formed on high surface-area carbon cloth electrodes, resulted in a self-powered biosensor with enhanced sensitivity (26.0 mA M-1 cm-2), compared to either of the two individual electrodes, and a dynamic range up to 4.1 mM cholesterol. Reagentless cholesterol detection with both electrochem. systems and with the self-powered biosensor was performed and the results were compared with the std. method of colorimetric cholesterol quantification.
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  15. 15
    Valdés-Ramírez, G.; Li, Y.-C.; Kim, J.; Jia, W.; Bandodkar, A. J.; Nuñez-Flores, R.; Miller, P. R.; Wu, S.-Y.; Narayan, R.; Windmiller, J. R.; Polsky, R.; Wang, J. Microneedle-based self-powered glucose sensor. Electrochem. Commun. 2014, 47, 58– 62,  DOI: 10.1016/j.elecom.2014.07.014
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    15
    Microneedle-based self-powered glucose sensor
    Valdes-Ramirez, Gabriela; Li, Ya-Chieh; Kim, Jayoung; Jia, Wenzhao; Bandodkar, Amay J.; Nunez-Flores, Rogelio; Miller, Philip R.; Wu, Shu-Yii; Narayan, Roger; Windmiller, Joshua R.; Polsky, Ronen; Wang, Joseph
    Electrochemistry Communications (2014), 47 (), 58-62CODEN: ECCMF9; ISSN:1388-2481. (Elsevier B.V.)
    A microneedle-based self-powered biofuel-cell glucose sensor is described. The biofuel cell sensor makes use of the integration of modified carbon pastes into hollow microneedle devices. The system displays defined dependence of the power d. vs glucose concn. in artificial interstitialfluid. An excellent selectivity against common electroactive interferences and long-term stability are obtained. The attractive performance of the device indicates considerable promise for subdermal glucose monitoring.
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    Sode, K.; Yamazaki, T.; Lee, I.; Hanashi, T.; Tsugawa, W. BioCapacitor: A novel principle for biosensors. Biosens. Bioelectron. 2016, 76, 20– 28,  DOI: 10.1016/j.bios.2015.07.065
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    16
    BioCapacitor: A novel principle for biosensors
    Sode, Koji; Yamazaki, Tomohiko; Lee, Inyoung; Hanashi, Takuya; Tsugawa, Wakako
    Biosensors & Bioelectronics (2016), 76 (), 20-28CODEN: BBIOE4; ISSN:0956-5663. (Elsevier B.V.)
    Studies regarding biofuel cells utilizing biocatalysts such as enzymes and microorganisms as electrocatalysts have been vigorously conducted over the last two decades. Because of their environmental safety and sustainability, biofuel cells are expected to be used as clean power generators. Among several principles of biofuel cells, enzyme fuel cells have attracted significant attention for their use as alternative energy sources for future implantable devices, such as implantable insulin pumps and glucose sensors in artificial pancreas and pacemakers. However, the inherent issue of the biofuel cell principle is the low power of a single biofuel cell. The theor. voltage of biofuel cells is limited by the redox potential of cofactors and/or mediators employed in the anode and cathode, which are inadequate for operating any devices used for biomedical application. These limitations inspired us to develop a novel biodevice based on an enzyme fuel cell that generates sufficient stable power to operate elec. devices, designated "BioCapacitor.". To increase voltage, the enzyme fuel cell is connected to a charge pump. To obtain a sufficient power and voltage to operate an elec. device, a capacitor is used to store the potential generated by the charge pump. Using the combination of a charge pump and capacitor with an enzyme fuel cell, high voltages with sufficient temporary currents to operate an elec. device were generated without changing the design and construction of the enzyme fuel cell. In this review, the BioCapacitor principle is described. The three different representative categories of biodevices employing the BioCapacitor principle are introduced. Further, the recent challenges in the developments of self-powered stand-alone biodevices employing enzyme fuel cells combined with charge pumps and capacitors are introduced. Finally, the future prospects of biodevices employing the BioCapacitor principle are addressed.
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    Jansod, S.; Bakker, E. Self-Powered Electrochromic Readout of Potentiometric pH Electrodes. Anal. Chem. 2021, 93, 4263– 4269,  DOI: 10.1021/acs.analchem.0c05117
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    17
    Self-powered electrochromic readout of potentiometric pH electrodes
    Jansod, Sutida; Bakker, Eric
    Analytical Chemistry (Washington, DC, United States) (2021), 93 (9), 4263-4269CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
    An absorbance-based colorimetric sensor array that is self-powered by an ion-selective electrode (ISE) in a short-circuited cell is presented. As the cell voltage is maintained at zero, the potential at the ISE serves as the power generator to directly transfer its power to a potential-dependent Prussian blue (PB) film in contact with an electrolyte soln. in a sep. detection compartment. This allows one to activate the color change of the PB film without the need for an external power supply. The potential of the PB detection element is optimized to change color between 50 and 250 mV (vs Ag/AgCl). Because the potential originates at the ISE, it is proportional to the ion activity in the sample in agreement with the Nernst equation. In this way, a higher cation activity in the sample generates a more pos. potential, which enhances the PB absorbance that serves as the anal. signal. A self-powered optical sensor array coupled to poly(vinyl-chloride)-based pH electrodes based on two different ionophores is utilized here as a model. The measuring range is tuned chem. by varying the pH of the inner filling soln. of each ISE, giving a measuring range from pH 2 to 10.5. As the optical sensor is driven by a potentiometric probe, the sensor output is independent of soln. ionic strength. It is successfully applied for quant. anal. in unmodified turbid/colored samples that included red wine, coke, coffee, baking soda, and antacid. The colorimetric output correlates well with the ref. method, a calibrated pH electrode. Compared to earlier systems where the cell potential is dictated by an external power source, the PB film exhibits excellent reproducibility and a rapid response time of about 44 s.
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  18. 18
    Kraikaew, P.; Sailapu, S. K.; Bakker, E. Rapid Constant Potential Capacitive Measurements with Solid-Contact Ion-Selective Electrodes Coupled to Electronic Capacitor. Anal. Chem. 2020, 92, 14174– 14180,  DOI: 10.1021/acs.analchem.0c03254
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    18
    Rapid Constant Potential Capacitive Measurements with Solid-Contact Ion-Selective Electrodes Coupled to Electronic Capacitor
    Kraikaew, Pitchnaree; Sailapu, Sunil Kumar; Bakker, Eric
    Analytical Chemistry (Washington, DC, United States) (2020), 92 (20), 14174-14180CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
    A const. potential capacitive readout of solid-contact ion-selective electrodes (SC-ISE) allows one to obtain easily identifiable current transients that can be integrated to obtain a charge vs logarithmic activity relationship. The resulting readout can therefore be much more sensitive than traditional open-circuit potentiometry. Unfortunately, however, comparatively long measurement times and significant baseline current drifts make it currently difficult to fully realize the promise of this technique. We show here that this challenge is overcome by placing the SC-ISE in series with an electronic capacitor, with pH probes as examples. Kirchhoff's law is shown to be useful to choose an adequate range of added capacitances so that it dominates the overall cell value. Two different ion-to-electron transducing materials, functionalized single-wall carbon nanotubes (f-SWCNTs) and poly(3-octylthiophene) (POT), were explored as solid-contact transducing layers. The established SC-ISE-based f-SWCNT transducer is found to be compatible with a wide range of external capacitances up to 100μF, while POT layers require a narrower range of 1-4.7μF. Importantly, the time for a charging transient to reach equil. was found to be less than 10 s, which is dramatically faster than without added electronic component. Owing to the ideal behavior of capacitor, the response current decays rapidly to zero, making the detn. of the integrated charge practically applicable.
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    Han, T.; Mattinen, U.; Mousavi, Z.; Bobacka, J. Coulometric response of solid-contact anion-sensitive electrodes. Electrochim. Acta 2021, 367, 137566,  DOI: 10.1016/j.electacta.2020.137566
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    Coulometric response of solid-contact anion-sensitive electrodes
    Han, Tingting; Mattinen, Ulriika; Mousavi, Zekra; Bobacka, Johan
    Electrochimica Acta (2021), 367 (), 137566CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    The coulometric response of nitrate, perchlorate, and sulfate solid-contact anion-sensitive electrodes was studied. The coulometric transduction method was originally introduced and so far mainly studied for solid-contact cation-selective sensors using poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrene sulfonate) as transducer. Here the authors provide addnl. proof-of-concept for the coulometric transduction method by focusing on the electrochem. characteristics of anion sensors. PEDOT was electrodeposited in the presence of small anions, including chloride, nitrate, sulfate, and perchlorate. The counterion influences the yield of electroactive PEDOT, which is an important parameter for the coulometric response. Anion-sensitive electrodes were prepd. by coating the PEDOT solid contact with plasticized PVC-based anion-sensitive membranes by drop-casting or spin-coating. The influence of the thickness of the PEDOT film and the anion-sensitive membrane on the coulometric response was studied. The solid-contact anion sensors showed fast charge transfer and ion transport properties, making them suitable for coulometric sensing. Also for anion-sensitive electrodes, the anal. signal was amplified by increasing the thickness of the PEDOT solid contact, which is fully consistent with earlier works on cation-selective electrodes. The coulometric transduction principle is feasible and robust for various types of solid-contact ISEs.
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    Wang, H.; Yuan, B.; Yin, T.; Qin, W. Alternative coulometric signal readout based on a solid-contact ion-selective electrode for detection of nitrate. Anal. Chim. Acta 2020, 1129, 136– 142,  DOI: 10.1016/j.aca.2020.07.019
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    Alternative coulometric signal readout based on a solid-contact ion-selective electrode for detection of nitrate
    Wang, Hemin; Yuan, Baiqing; Yin, Tanji; Qin, Wei
    Analytica Chimica Acta (2020), 1129 (), 136-142CODEN: ACACAM; ISSN:0003-2670. (Elsevier B.V.)
    Traditional potentiometric NO-3-selective electrodes suffer from a fundamental limitation of the Nernst slope (59.1 mV/dec at 25°C) due to the relationship between the potential and the logarithmic of ionic activity. Herein, a coulometric signal readout is proposed instead of the potentiometric response for detection of NO-3 based on an ordered mesoporous carbon (OMC)-based solid-contact ion-selective electrode (ISE). The mechanism for obtaining the coulometric signal is based on the elec. double layer capacitance of OMC compensating the potential change at the ion-selective membrane/soln. interface during the measurements under the control of a const. applied potential. Under the optimized conditions, the coulometric signal for the OMC-based solid-contact NO-3-ISE shows two linear responses in the activity range of 1.0 x 10-6-8.0 x 10-6 M and 8.0 x 10-6-8.0 x 10-4 M, and the detection limit is 4.0 x 10-7 M (3σ/s). The proposed coulometric response also shows excellent reproducibility and stability in the presence of O2 and CO2 and light on/off. Addnl., the coulometric response shows acceptable and reliable results for detection of NO-3 in mineral water as compared to the traditional potentiometric response and the ion chromatog. This work provides a promising alternative signal readout for detection of ions by using solid-contact ion-selective electrodes.
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    Sailapu, S. K.; Kraikaew, P.; Sabaté, N.; Bakker, E. Self-Powered Potentiometric Sensor Transduction to a Capacitive Electronic Component for Later Readout. ACS Sens. 2020, 5, 2909– 2914,  DOI: 10.1021/acssensors.0c01284
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    21
    Self-Powered Potentiometric Sensor Transduction to a Capacitive Electronic Component for Later Readout
    Sailapu, Sunil Kumar; Kraikaew, Pitchnaree; Sabate, Neus; Bakker, Eric
    ACS Sensors (2020), 5 (9), 2909-2914CODEN: ASCEFJ; ISSN:2379-3694. (American Chemical Society)
    Potentiometric sensors operate as galvanic cells where the voltage is spontaneously generated as a function of the sample compn. The authors show here that energy can be harvested, stored during the sensing process without external power, and phys. isolated from the sensor circuit for later readout. This is accomplished by placing an electronic capacitor as a portable transduction component between the indicator and the ref. electrode at the point where one would ordinarily connect the high-input-impedance voltmeter. The voltage across this isolated capacitor indicates the originally measured ion activity and can be read out conveniently, for example, using a simple handheld multimeter. The capacitor is shown to maintain the transferred charge for hours after its complete disconnection from the sensor. The concept is demonstrated to detect the physiol. concns. of K+ in artificial sweat samples. The methodol. provides a readout principle that could become very useful in portable form factors and opens possibilities for potentiometric detection in point-of-care applications and inexpensive sensing devices where an external power source is not desired.
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    Hupa, E.; Vanamo, U.; Bobacka, J. Novel Ion-to-Electron Transduction Principle for Solid-Contact ISEs. Electroanalysis 2015, 27, 591– 594,  DOI: 10.1002/elan.201400596
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    Novel Ion-to-Electron Transduction Principle for Solid-Contact ISEs
    Hupa, Elisa; Vanamo, Ulriika; Bobacka, Johan
    Electroanalysis (2015), 27 (3), 591-594CODEN: ELANEU; ISSN:1040-0397. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Solid-contact ion-selective electrodes (SC-ISEs) are traditionally employed as potentiometric sensors, where the ion activity is related to the zero-current potential of the sensor vs. the ref. electrode. An alternative ion-to-electron transduction principle for SC-ISEs is introduced. The suggested signal transduction principle resembles const.-potential coulometry using the redox capacitance of the internal solid contact to convert changes in ion concn. (activity) into elec. current and charge. This short communication provides proof-of-concept for the suggested signal transduction method for SC-ISEs using poly(3,4-ethylenedioxythiphene) (PEDOT) as the solid contact that was coated with a cation-sensitive polymeric membrane.
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    Vanamo, U.; Hupa, E.; Yrjänä, V.; Bobacka, J. New Signal Readout Principle for Solid-Contact Ion-Selective Electrodes. Anal. Chem. 2016, 88, 4369– 4374,  DOI: 10.1021/acs.analchem.5b04800
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    New Signal Readout Principle for Solid-Contact Ion-Selective Electrodes
    Vanamo, Ulriika; Hupa, Elisa; Yrjana, Ville; Bobacka, Johan
    Analytical Chemistry (Washington, DC, United States) (2016), 88 (8), 4369-4374CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
    A novel approach to signal transduction concerning solid-contact ion-selective electrodes (SC-ISE) with a conducting polymer (CP) as the solid contact was studied. The method presented here is based on const. potential coulometry, where the potential of the SC-ISE vs. the ref. electrode is kept const. using a potentiostat. The change in the potential at the interface between the ion-selective membrane (ISM) and the sample soln., due to the change in the activity of the primary ion, is compensated with a corresponding but opposite change in the potential of the CP solid contact. This enforced change in the potential of the solid contact results in a transient reducing/oxidizing current flow through the SC-ISE. By measuring and integrating the current needed to transfer the CP to a new state of equil., the total cumulated charge that is linearly proportional to the change of the logarithm of the primary ion activity was obtained. Different thicknesses of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonate) (PSS) were used as solid contact. Also, coated wire electrodes (CWEs) were included in the study to show the general validity of the new approach. The ISM employed was selective for K+ ions, and the selectivity of the membrane under implementation of the presented transduction mechanism was confirmed by measurements performed with a const. background concn. of Na+ ions. A unique feature of this signal readout principle is that it allows amplification of the anal. signal by increasing the capacitance (film thickness) of the solid contact of the SC-ISE.
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    Rahn, K. L.; Anand, R. K. Recent Advancements in Bipolar Electrochemical Methods of Analysis. Anal. Chem. 2021, 93, 103– 123,  DOI: 10.1021/acs.analchem.0c04524
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    Recent Advancements in Bipolar Electrochemical Methods of Analysis
    Rahn, Kira L.; Anand, Robbyn K.
    Analytical Chemistry (Washington, DC, United States) (2021), 93 (1), 103-123CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
    The goal of this review is to provide an overview of the advancements made in the field of bipolar electrochem. over the past two years, with an emphasis on anal. Bipolar electrodes (BPEs) are versatile-in electroanal., they have been used extensively to screen electrocatalysts- and to sense biomarkers.- Their ability to modulate local elec. fields lends them to the manipulation of cells and to the enrichment and sepn. of analytes.- And finally, by virtue of the polar and often graded profile of the interfacial potential across BPEs, they provide a platform for synthesis of Janus particles, useful as sensors and as microswimmers,- and other materials with compositional gradients., BPEs are particularly well-suited to anal. challenges that demand multiplexing or amenability to point-of-need (PON) application because even large arrays of BPEs can be controlled with simple equipment, yet yield quant. information about a system. In this review, we discuss recent progress in reactions that transduce current to a visible signal, sensing mechanisms, bipolar electrochem. cell design, integration of bipolar electrochem. with spectroscopic techniques, BPEs at the nanoscale, and the application of BPEs to electrokinetics and materials prepn. Throughout the discussion, we identify promising trends, innovative directions, and remaining challenges in the field. A BPE is a conductive object, such as a metal strip or bead, that facilitates elec. coupled faradaic reactions at its opposing poles. Unlike a conventional electrode, a BPE has a floating potential and lacks direct contact to a power supply. Therefore, its potential (EBPE) floats to a value intermediate to that of the electrolyte it contacts. When the BPE is part of an electrolytic cell, a pair of driving electrodes applies a voltage bias across the electrolyte, resulting in the development of interfacial potential differences with opposite signs at the ends of the BPE as indicated in Scheme a. These interfacial potential differences are the anodic (ηα) and cathodic (ηc) overpotentials available to drive faradaic reactions. The anodic (ic) and cathodic (ic) currents are opposite in sign and equal in magnitude such that the current through the BPE, iBPE, is related to both by the equation, iBPE = ic=-ia. In this review, we will also discuss bipolar electrochem. systems that operate galvanically. Galvanic bipolar electrochem. shares a common mechanism with corrosion-the BPE interconnects electrolytes that establish a p.d. poised by available half-reactions in each phase. The oxidn. reaction occurs at a more neg. potential than the redn. reaction, to which it is elec. coupled by the conductive substrate. Therefore, the net processes are spontaneous, and driving electrodes are not required. The concept of bipolar electrochem. was first introduced in the 1960s by Fleischmann and co-workers, who described fluidized bed electrodes comprising Cu microscale particles, which they used in aq. electrochem. reactors. BPEs were then popularized by Manz and co-workers in the early 2000s when they combined these electrodes with electrochemiluminescence (ECL), which reported the current, thereby allowing them to create electrochem. detectors compatible with the high elec. field strengths and microscale compartments employed in micellar electrokinetic chromatog. Crooks and co-workers then established a theor. framework for bipolar electrochem. and broadened its application with a series of ground-breaking advancements. They described the interplay of controllable exptl. parameters such as the length of the BPE along the applied elec. field with intrinsic features of electrochem. processes to det. signal intensity. This description was then further refined to account for the impact of the distribution of interfacial potential along the BPE and the threshold current required for ECL on the obsd. luminescence. Their work uncovered the potential for BPEs to be leveraged to form arrays of either identical sensors to achieve spatial mapping of analytes or sensors with distinct surface chemistries for multiplexed anal., They then took advantage of the compatibility of BPEs with microfluidic length scales to drive the localized depletion of electrolyte ions required for electrokinetic enrichment and sepn., which in the context of anal., is particularly useful for sample prepn. In this literature review, we summarize advancements in the field of bipolar electrochem. that have been made over the past two years. Our discussion centers around BPEs as a tool for anal. and therefore covers the development of novel BPE configurations and their integration with sensing and reporting mechanisms, voltammetric methods, and spectroscopic techniques to address current challenges in sensing. For example, because BPEs are readily arrayed, shrinking their scale and boosting sensitivity is expected to allow for the characterization of single catalytic particles in parallel. We also examine advances in sample prepn. methods that employ BPEs, including electrokinetic enrichment and sepn. of chem. species and the selective manipulation of biol. cells by dielectrophoresis (DEP). There are many recent reports of the integration of existing bipolar electrochem. methods with a wide range of surface chemistries, esp. in the context of biosensing. Since these advancements are made not to bipolar electrochem. per se, they are not discussed in detail in this review., We begin with a brief overview of bipolar electrochem. in both open and closed BPE configurations. The reader is referred to a few comprehensive review articles- for a more thorough understanding of BPEs.
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    Jaworska, E.; Michalska, A.; Maksymiuk, K. Self-Powered Cascade Bipolar Electrodes with Fluorimetric Readout. Anal. Chem. 2019, 91, 15525– 15531,  DOI: 10.1021/acs.analchem.9b03405
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    Self-Powered Cascade Bipolar Electrodes with Fluorimetric Readout
    Jaworska, Ewa; Michalska, Agata; Maksymiuk, Krzysztof
    Analytical Chemistry (Washington, DC, United States) (2019), 91 (24), 15525-15531CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
    Bipolar electrodes working in a self-powered mode are a basis for the development of easy to use electrochem.-optical sensors, these systems are very promising due to their simplicity and no need of external polarization. However, the self-powered mode can be used only in cases when the redox p.d. of reactions occurring at opposite poles of the electrode is sufficiently high. To overcome this limitation, we propose the development of a system working spontaneously, but involving two bipolar electrodes, forming a cascade system. One of electrodes ("driving" electrode) works in self-powered mode and triggers charge transfer processes in the second ("sensing") bipolar electrode. For the sensing electrode, an electrochem. process of an analyte occurs at one pole, accompanied by a complementary process at the second pole, inducing an optical (fluorimetric) anal. signal. This concept was successfully tested on a model system of a sensing bipolar electrode with a platinum electrode participating in oxidn. of an analyte, L-ascorbic acid, connected with electrode coated by poly(3-octylthiophene), where redn. of the polymer results in formation of fluorimetrically active neutral form. As the driving system, bipolar electrodes with zinc wire as one pole, characterized by a low redox potential, were used.
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  • Abstract

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    Scheme 1

    Scheme 1. Schematic Illustration of the Approach Adopted to Track a Deviation in the Activity of an Analyte During a Time Interval by Reading Out the Response Stored in a Capacitor at a Specific Measurement Point in Time
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    Figure 1

    Figure 1. (a) Model system to study the influence on the capacitor connected between indicator and reference electrodes due to a voltage applied across the driving electrodes. (b) Measured voltages for an applied bi-directional staircase signal.

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    Figure 2

    Figure 2. (a) Response of H+-ISE against a double junction Ag/AgCl (3 M KCl/1 M LiOAc) electrode. (b) Model system involving an external power source to shift the level of a conventional potentiometric signal and to a charge a capacitor to the translated voltage. (c,d) Voltage across capacitor at different pH values for an applied voltage of 0.9 and −0.9 V, respectively, between the driving electrodes.

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    Figure 3

    Figure 3. (a) pH sensing configuration without an external power source to shift a potentiometric signal and to store the response by charging the capacitor. (b) Response of the sensor at different pH values in open circuit and with a capacitor. (c) Stability of Zn electrode over time.

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    Figure 4

    Figure 4. (a) Voltage evolution across the capacitor at different pH of the sample. The solid lines represent the capacitor voltage, and the dashed lines indicate the induced voltage, Ve. (b) Response across the capacitor for voltage applied using source meter. The sample is maintained at pH 7. The dashed lines represent the applied voltage, and the solid lines represent the capacitor response. (c) Voltage response of capacitor for pH changes incorporated in a sample (originally at pH 7) to deviate from its initial state. The solid lines represent the capacitor voltage, and the dashed lines indicate the induced voltage, Ve. (e) Voltage across capacitor measured at the end of 2 h using a hand-held multimeter. Here, the river samples are spiked to a higher pH for an abrupt interval of 5 min, before returning it to original value. The second y-axis (Q) in the figure corresponds to the capacitor curves.

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  • References

    This article references 25 other publications.

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      Özbek, O.; Berkel, C.; Isildak, Ö. Applications of Potentiometric Sensors for the Determination of Drug Molecules in Biological Samples. Crit. Rev. Anal. Chem. 2020, 1– 12,  DOI: 10.1080/10408347.2020.1825065
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      Potentiometry is extensively studied by researchers as one of the electrochemical methods due to its multiple advantages. Until today, thousands of potentiometric sensors have been developed and applied successfully in many fields such as medicine, environmental monitoring, agriculture, industry and pharmaceutical sciences. Clinical drug analyses and determination of drugs in biological samples are highly important from a medical point of view. These analyses are carried out using various analytical devices including potentiometric sensors. These potentiometric sensors are superior to other devices in terms of several performance parameters, and thus present a good alternative for researchers. Using potentiometric sensors, very successful results in the identification of drug molecules in body fluids have been obtained and reported in the literature up to now. In this study, we review potentiometry-based sensors developed for the determination of drug molecules in various biological samples such as blood serum and urine, and touch upon their performance features in these applications.
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      A review. Potentiometric-based biosensors have the potential to advance the detection of several biol. compds. and help in early diagnosis of various diseases. They belong to the portable anal. class of biosensors for monitoring biomarkers in the human body. They contain ion-sensitive membranes sensors can be used to det. potassium, sodium, and chloride ions activity while being used as a biomarker to gauge human health. The potentiometric based ion-sensitive membrane systems can be coupled with various techniques to create a sensitive tool for the fast and early detection of cancer biomarkers and other crit. biol. compds. This paper discusses the application of potentiometric-based biosensors and classifies them into four major categories: photoelectrochem. potentiometric biomarkers, potentiometric biosensors amplified with mol. imprinted polymer systems, wearable potentiometric biomarkers and light-addressable potentiometric biosensors. This review demonstrated the development of several innovative biosensor-based techniques that could potentially provide reliable tools to test biomarkers. Some challenges however remain, but these can be removed by coupling techniques to maximize the testing sensitivity.
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      Zdrachek, Elena; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2021), 93 (1), 72-102CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      A review. This review, with ∼200 refs., describes and comments key highlights of the progress in the field of potentiometric sensing between Jan. 2019, date of the last fundamental review on the topic in the Anal. Chem. special issue, and Oct. 2020. The refs. were mainly retrieved from 2 scientific citation indexing services, Web of Science and SciFinder, by searching for relevant keywords and recognized authors in the field. Addnl., key journals were screened manually. The authors also selected relevant review papers. Given the large no. of potentiometric sensing publications and the competing need for highlighting the most relevant developments, the authors decided to limit to 200 the no. of refs. included in this review. For this reason, this review reflects the opinion on the current state of the field and does not aim at covering the full potentiometric sensing literature. With the aim to stimulate scientific discussion in the field, the authors critically analyzed the content of the cited refs., pointing out what the authors feel are their strengths and weaknesses. The authors hope that no author will feel offended by any of comments. It is exciting to see that the field of potentiometric sensing is experiencing solid growth. The current research blurs interdisciplinary boundaries and explores new sensor readout principles. Materials science offers researchers new ion-to-electron transducing materials that advance miniature sensors. This opens up new opportunities for the application of potentiometric sensors. In particular, wearable potentiometric sensors are gaining attention. The research in the domain of scalable and disposable paper-based potentiometric sensors is also very active and brings potentiometry closer to point-of-care and in-field applications. This review starts with describing the recent progress with ref. electrodes. The improvements of ref. electrodes with liq. junction and the developments of liq. junction free ref. electrodes are discussed. Next, it covers recent advances in the modern theory of potentiometry, including numerical simulations used to predict their dynamic response. New protocols were described for detg. lower detection limit and the description of the selectivity coeffs. where primary and interfering ions form complexes of multiple stoichiometries. The following section discusses new and nonclassical readout principles for ion-selective electrodes, including ion-transfer voltammetry, coulometry, amperometry, chronopotentiometry and various optical readouts. This is followed by a section dedicated to new materials for ion-selective electrodes (ISEs). It starts with an overview of the ion-to-electron transducing materials, continues with new membrane materials and approaches to overcome membrane biofouling and ends by describing mol. imprinted polymers and new ionophores. Finally, the review discusses recent developments in the area of miniaturized ISEs, including wearable sensors, paper-based devices, microfluidic devices, thread-based ISEs, ion-selective microelectrodes and ion-selective field effect transistors (ISFET).
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    4. 4
      Shao, Y.; Ying, Y.; Ping, J. Recent advances in solid-contact ion-selective electrodes: functional materials, transduction mechanisms, and development trends. Chem. Soc. Rev. 2020, 49, 4405– 4465,  DOI: 10.1039/c9cs00587k
      4
      Recent advances in solid-contact ion-selective electrodes: functional materials, transduction mechanisms, and development trends
      Shao, Yuzhou; Ying, Yibin; Ping, Jianfeng
      Chemical Society Reviews (2020), 49 (13), 4405-4465CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      From environmental monitoring to point-of-care biofluid anal., rapid ion detn. requires robust anal. tools. In recent years, driven by the development of materials science and processing technol., solid-contact ion-selective electrodes (SC-ISEs) with high-performance functional materials and creative structures have shown great potential for routine and portable ion detection. In particular, the introduction of nanomaterials as ion-to-electron transducers and the adoption of different performance enhancement strategies have significantly promoted the development of SC-ISEs. Besides, with the increasing miniaturization, flexibility, and dependability of SC-ISEs, this field has gradually begun to evolve from conventional potentiometric ion sensing to integrated sensing systems with broader application scenarios. This comprehensive review covers pioneering research on functional materials and state-of-the-art technologies for the construction of SC-ISEs, with an emphasis on new development trends and applications.
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    5. 5
      Oesch, U.; Simon, W. Lifetime of neutral carrier based ion-selective liquid-membrane electrodes. Anal. Chem. 2002, 52, 692– 700,  DOI: 10.1021/ac50054a024
      There is no corresponding record for this reference.
    6. 6
      Grattieri, M.; Minteer, S. D. Self-Powered Biosensors. ACS Sens. 2018, 3, 44– 53,  DOI: 10.1021/acssensors.7b00818
      6
      Self-Powered Biosensors
      Grattieri, Matteo; Minteer, Shelley D.
      ACS Sensors (2018), 3 (1), 44-53CODEN: ASCEFJ; ISSN:2379-3694. (American Chemical Society)
      A review. Self-powered electrochem. biosensors utilize biofuel cells as a simultaneous power source and biosensor, which simplifies the biosensor system, because it no longer requires a potentiostat, power for the potentiostat, and/or power for the signaling device. This review article is focused on detailing the advances in the field of self-powered biosensors and discussing their advantages and limitations compared to other types of electrochem. biosensors. The review will discuss self-powered biosensors formed from enzymic biofuel cells, organelle-based biofuel cells, and microbial fuel cells. It also discusses the different mechanisms of sensing, including utilizing the analyte being the substrate/fuel for the biocatalyst, the analyte binding the biocatalyst to the electrode surface, the analyte being an inhibitor of the biocatalyst, the analyte resulting in the blocking of the bioelectrocatalytic response, the analyte reactivating the biocatalyst, Boolean logic gates, and combining affinity-based biorecognition elements with bioelectrocatalytic power generation. The final section of this review details areas of future investigation that are needed in the field, as well as problems that still need to be addressed by the field.
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    7. 7
      Sailapu, S. K.; Macchia, E.; Merino-Jimenez, I.; Esquivel, J. P.; Sarcina, L.; Scamarcio, G.; Minteer, S. D.; Torsi, L.; Sabaté, N. Standalone operation of an EGOFET for ultra-sensitive detection of HIV. Biosens. Bioelectron. 2020, 156, 112103,  DOI: 10.1016/j.bios.2020.112103
      7
      Standalone operation of an EGOFET for ultra-sensitive detection of HIV
      Sailapu, Sunil Kumar; Macchia, Eleonora; Merino-Jimenez, Irene; Esquivel, Juan Pablo; Sarcina, Lucia; Scamarcio, G.; Minteer, Shelley D.; Torsi, Luisa; Sabate, Neus
      Biosensors & Bioelectronics (2020), 156 (), 112103CODEN: BBIOE4; ISSN:0956-5663. (Elsevier B.V.)
      A point-of-care (POC) device to enable de-centralized diagnostics can effectively reduce the time to treatment, esp. in case of infectious diseases. However, many of the POC solns. presented so far do not comply with the ASSURED (affordable, sensitive, specific, user-friendly, rapid and robust, equipment free, and deliverable to users) guidelines that are needed to ensure their on-field deployment. Herein, we present the proof of concept of a self-powered platform that operates using the analyzed fluid, mimicking a blood sample, for early stage detection of HIV-1 infection. The platform contains a smart interfacing circuit to operate an ultra-sensitive electrolyte-gated field-effect transistor (EGOFET) as a sensor and facilitates an easy and affordable readout mechanism. The sensor transduces the bio-recognition event taking place at the gate electrode functionalized with the antibody against the HIV-1 p24 capsid protein, while it is powered via paper-based biofuel cell (BFC) that exts. the energy from the analyzed sample itself. The self-powered platform is demonstrated to achieve detection of HIV-1 p24 antigens in fM range, suitable for early diagnosis. From these developments, a cost-effective digital POC device able to detect the transition from "healthy" to "infected" state at single-mol. precision, with no dependency on external power sources while using minimal components and simpler approach, is foreseen.
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    8. 8
      Zhu, M.; Yi, Z.; Yang, B.; Lee, C. Making use of nanoenergy from human – Nanogenerator and self-powered sensor enabled sustainable wireless IoT sensory systems. Nano Today 2021, 36, 101016,  DOI: 10.1016/j.nantod.2020.101016
      There is no corresponding record for this reference.
    9. 9
      Wang, L.; Wu, X.; Su, B. S. Q. w.; Song, R.; Zhang, J.-R.; Zhu, J.-J. Enzymatic Biofuel Cell: Opportunities and Intrinsic Challenges in Futuristic Applications. Adv. Energy Sustain. Res. 2021, 2, 2100031,  DOI: 10.1002/aesr.202100031
      There is no corresponding record for this reference.
    10. 10
      Shitanda, I.; Morigayama, Y.; Iwashita, R.; Goto, H.; Aikawa, T.; Mikawa, T.; Hoshi, Y.; Itagaki, M.; Matsui, H.; Tokito, S.; Tsujimura, S. Paper-based lactate biofuel cell array with high power output. J. Power Sources 2021, 489, 229533,  DOI: 10.1016/j.jpowsour.2021.229533
      10
      Paper-based lactate biofuel cell array with high power output
      Shitanda, Isao; Morigayama, Yukiya; Iwashita, Risa; Goto, Himeka; Aikawa, Tatsuo; Mikawa, Tsutomu; Hoshi, Yoshinao; Itagaki, Masayuki; Matsui, Hiroyuki; Tokito, Shizuo; Tsujimura, Seiya
      Journal of Power Sources (2021), 489 (), 229533CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      Printable wearable lactate biosensors have attracted significant attention, and printable lactate biofuel cells have gained popularity as suitable power supplies for these sensors. However, realizing an appropriate power supply for the practical application of these sensors as wearable devices, it is necessary to improve the output of lactate biofuel cells. In this study, we fabricated a lactate biofuel cell that employed paper substrate using screen printing. The proposed paper-based biofuel cell (PBFC) features a novel electrode design that affords an open circuit voltage of approx. 3.4 V when using an array with six cells in series. Furthermore, a 6 x 6 array of the lactate biofuel cell, i.e., an array comprising six cells in series and six cells in parallel, yielded a power output of 4.3 mW. To the best of our knowledge, this output of the proposed design is higher than those of previously reported lactate biofuel cells. Arrays of the proposed cell were capable of driving a Bluetooth low-energy device for wireless transmission, without requiring a booster circuit. A com. available activity meter could be driven for 1.5 h using artificial sweat to fuel a 6 x 6 array of the PBFCs.
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    11. 11
      Ruff, A.; Pinyou, P.; Nolten, M.; Conzuelo, F.; Schuhmann, W. A Self-Powered Ethanol Biosensor. ChemElectroChem 2017, 4, 890– 897,  DOI: 10.1002/celc.201600864
      11
      A Self-Powered Ethanol Biosensor
      Ruff, Adrian; Pinyou, Piyanut; Nolten, Melinda; Conzuelo, Felipe; Schuhmann, Wolfgang
      ChemElectroChem (2017), 4 (4), 890-897CODEN: CHEMRA; ISSN:2196-0216. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We describe the fabrication of a self-powered ethanol biosensor comprising a β-NAD+-dependent alc. dehydrogenase (ADH) bioanode and a bienzymic alc. oxidase (AOx) and horseradish peroxidase (HRP) biocathode. β-NAD+ is regenerated by means of a specifically designed phenothiazine dye (i.e. toluidine blue, TB) modified redox polymer in which TB was covalently anchored to a hexanoic acid tethered poly(4-vinylpyridine) backbone. The redox polymer acts as an immobilization matrix for ADH. Using a carefully chosen anchoring strategy through the formation of amide bonds, the potential of the TB-based mediator is shifted to more pos. potentials, thus preventing undesired O2 redn. To counterbalance the rather high potential of the TB-modified polymer, and thus the bioanode, a high-potential AOx/HRP-based biocathode is suggested. HRP is immobilized in a direct-electron-transfer regime on screen-printed graphite electrodes functionalized with multi-walled carbon nanotubes. The nanostructured cathode ensures the wiring of the iron-oxo complex within oxidized HRP, and thus a high potential for the redn. of H2O2 of about +550 mV vs. Ag/AgCl/3 M KCl. The proposed biofuel cell exhibits an open-circuit voltage (OCV) of approx. 660 mV and was used as self-powered device for the detn. of the ethanol content in liquor.
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    12. 12
      Kai, H.; Kato, Y.; Toyosato, R.; Nishizawa, M. Fluid-permeable enzymatic lactate sensors for micro-volume specimen. Analyst 2018, 143, 5545– 5551,  DOI: 10.1039/c8an00979a
      12
      Fluid-permeable enzymatic lactate sensors for micro-volume specimen
      Kai, Hiroyuki; Kato, Yuto; Toyosato, Ryoma; Nishizawa, Matsuhiko
      Analyst (Cambridge, United Kingdom) (2018), 143 (22), 5545-5551CODEN: ANALAO; ISSN:0003-2654. (Royal Society of Chemistry)
      Sensing of lactate in perspiration provides a way to monitor health and control exercise. The vol. of perspiration is miniscule, and the efficient collection of perspiration is desired for its effective sensing. The authors developed mesh-type enzymic electrodes fabricated on textile meshes and integrated the meshes into an enzymic biofuel cell. The authors tested them as self-powered lactate sensors for a small vol. of lactate soln. A fluid-permeable enzymic anode was fabricated based on an insulating textile mesh that was coated with carbon nanotubes (CNTs) and lactate oxidase. The anode was further coated with polyurethane to increase the linear range by limiting the diffusion of lactate while maintaining the advantages of the original textile mesh, such as flexibility, stretchability, and permeability. Permeability of the mesh-type lactate-oxidizing anode allowed a vertically stacked structure of the anode and a previously developed air-breathing cathode. This resulted in a small overall device size (1 cm2). The mesh-type sensor was tested using a small flow rate of lactate soln., and a moderate linearity of amperometric response for a wide concn. range (5 to ≥20 mM) was confirmed. The fluid-permeable anode and enzymic biofuel cell show the potential of the sensor for continuous monitoring of lactate in perspiration on skin.
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    13. 13
      Cui, Y.; Lai, B.; Tang, X. Microbial Fuel Cell-Based Biosensors. Biosensors 2019, 9, 92,  DOI: 10.3390/bios9030092
      13
      Microbial fuel cell-based biosensors
      Cui, Yang; Lai, Bin; Tang, Xinhua
      Biosensors (2019), 9 (3), 92CODEN: BIOSHU; ISSN:2079-6374. (MDPI AG)
      The microbial fuel cell (MFC) is a promising environmental biotechnol. that has been proposed mainly for power prodn. and wastewater treatment. Though small power output constrains its application for directly operating most elec. devices, great progress in its chem., electrochem., and microbiol. aspects has expanded the applications of MFCs into other areas such as the generation of chems. (e.g., formate or methane), bioremediation of contaminated soils, water desalination, and biosensors. In recent decades, MFC-based biosensors have drawn increasing attention because of their simplicity and sustainability, with applications ranging from the monitoring of water quality (e.g., BOD (BOD), toxicants) to the detection of air quality (e.g., carbon monoxide, formaldehyde). In this review, we summarize the status quo of MFC-based biosensors, putting emphasis on BOD and toxicity detection. Furthermore, this review covers other applications of MFC-based biosensors, such as DO and microbial activity. Further, challenges and prospects of MFC-based biosensors are briefly discussed.
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    14. 14
      Sekretaryova, A. N.; Beni, V.; Eriksson, M.; Karyakin, A. A.; Turner, A. P. F.; Vagin, M. Y. Cholesterol self-powered biosensor. Anal. Chem. 2014, 86, 9540– 9547,  DOI: 10.1021/ac501699p
      14
      Cholesterol Self-Powered Biosensor
      Sekretaryova, Alina N.; Beni, Valerio; Eriksson, Mats; Karyakin, Arkady A.; Turner, Anthony P. F.; Vagin, Mikhail Yu.
      Analytical Chemistry (Washington, DC, United States) (2014), 86 (19), 9540-9547CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      Monitoring the cholesterol level is of great importance, esp. for people with high risk of developing heart disease. Here the authors report on reagentless cholesterol detection in human plasma with a novel single-enzyme, membrane-free, self-powered biosensor, in which both cathodic and anodic bioelectrocatalytic reactions are powered by the same substrate. Cholesterol oxidase was immobilized in a sol-gel matrix on both the cathode and the anode. Hydrogen peroxide, a product of the enzymic conversion of cholesterol, was electrocatalytically reduced, using Prussian blue, at the cathode. In parallel, cholesterol oxidn. catalyzed by mediated cholesterol oxidase occurred at the anode. The anal. performance was assessed for both electrode systems sep. The combination of the two electrodes, formed on high surface-area carbon cloth electrodes, resulted in a self-powered biosensor with enhanced sensitivity (26.0 mA M-1 cm-2), compared to either of the two individual electrodes, and a dynamic range up to 4.1 mM cholesterol. Reagentless cholesterol detection with both electrochem. systems and with the self-powered biosensor was performed and the results were compared with the std. method of colorimetric cholesterol quantification.
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    15. 15
      Valdés-Ramírez, G.; Li, Y.-C.; Kim, J.; Jia, W.; Bandodkar, A. J.; Nuñez-Flores, R.; Miller, P. R.; Wu, S.-Y.; Narayan, R.; Windmiller, J. R.; Polsky, R.; Wang, J. Microneedle-based self-powered glucose sensor. Electrochem. Commun. 2014, 47, 58– 62,  DOI: 10.1016/j.elecom.2014.07.014
      15
      Microneedle-based self-powered glucose sensor
      Valdes-Ramirez, Gabriela; Li, Ya-Chieh; Kim, Jayoung; Jia, Wenzhao; Bandodkar, Amay J.; Nunez-Flores, Rogelio; Miller, Philip R.; Wu, Shu-Yii; Narayan, Roger; Windmiller, Joshua R.; Polsky, Ronen; Wang, Joseph
      Electrochemistry Communications (2014), 47 (), 58-62CODEN: ECCMF9; ISSN:1388-2481. (Elsevier B.V.)
      A microneedle-based self-powered biofuel-cell glucose sensor is described. The biofuel cell sensor makes use of the integration of modified carbon pastes into hollow microneedle devices. The system displays defined dependence of the power d. vs glucose concn. in artificial interstitialfluid. An excellent selectivity against common electroactive interferences and long-term stability are obtained. The attractive performance of the device indicates considerable promise for subdermal glucose monitoring.
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    16. 16
      Sode, K.; Yamazaki, T.; Lee, I.; Hanashi, T.; Tsugawa, W. BioCapacitor: A novel principle for biosensors. Biosens. Bioelectron. 2016, 76, 20– 28,  DOI: 10.1016/j.bios.2015.07.065
      16
      BioCapacitor: A novel principle for biosensors
      Sode, Koji; Yamazaki, Tomohiko; Lee, Inyoung; Hanashi, Takuya; Tsugawa, Wakako
      Biosensors & Bioelectronics (2016), 76 (), 20-28CODEN: BBIOE4; ISSN:0956-5663. (Elsevier B.V.)
      Studies regarding biofuel cells utilizing biocatalysts such as enzymes and microorganisms as electrocatalysts have been vigorously conducted over the last two decades. Because of their environmental safety and sustainability, biofuel cells are expected to be used as clean power generators. Among several principles of biofuel cells, enzyme fuel cells have attracted significant attention for their use as alternative energy sources for future implantable devices, such as implantable insulin pumps and glucose sensors in artificial pancreas and pacemakers. However, the inherent issue of the biofuel cell principle is the low power of a single biofuel cell. The theor. voltage of biofuel cells is limited by the redox potential of cofactors and/or mediators employed in the anode and cathode, which are inadequate for operating any devices used for biomedical application. These limitations inspired us to develop a novel biodevice based on an enzyme fuel cell that generates sufficient stable power to operate elec. devices, designated "BioCapacitor.". To increase voltage, the enzyme fuel cell is connected to a charge pump. To obtain a sufficient power and voltage to operate an elec. device, a capacitor is used to store the potential generated by the charge pump. Using the combination of a charge pump and capacitor with an enzyme fuel cell, high voltages with sufficient temporary currents to operate an elec. device were generated without changing the design and construction of the enzyme fuel cell. In this review, the BioCapacitor principle is described. The three different representative categories of biodevices employing the BioCapacitor principle are introduced. Further, the recent challenges in the developments of self-powered stand-alone biodevices employing enzyme fuel cells combined with charge pumps and capacitors are introduced. Finally, the future prospects of biodevices employing the BioCapacitor principle are addressed.
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    17. 17
      Jansod, S.; Bakker, E. Self-Powered Electrochromic Readout of Potentiometric pH Electrodes. Anal. Chem. 2021, 93, 4263– 4269,  DOI: 10.1021/acs.analchem.0c05117
      17
      Self-powered electrochromic readout of potentiometric pH electrodes
      Jansod, Sutida; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2021), 93 (9), 4263-4269CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      An absorbance-based colorimetric sensor array that is self-powered by an ion-selective electrode (ISE) in a short-circuited cell is presented. As the cell voltage is maintained at zero, the potential at the ISE serves as the power generator to directly transfer its power to a potential-dependent Prussian blue (PB) film in contact with an electrolyte soln. in a sep. detection compartment. This allows one to activate the color change of the PB film without the need for an external power supply. The potential of the PB detection element is optimized to change color between 50 and 250 mV (vs Ag/AgCl). Because the potential originates at the ISE, it is proportional to the ion activity in the sample in agreement with the Nernst equation. In this way, a higher cation activity in the sample generates a more pos. potential, which enhances the PB absorbance that serves as the anal. signal. A self-powered optical sensor array coupled to poly(vinyl-chloride)-based pH electrodes based on two different ionophores is utilized here as a model. The measuring range is tuned chem. by varying the pH of the inner filling soln. of each ISE, giving a measuring range from pH 2 to 10.5. As the optical sensor is driven by a potentiometric probe, the sensor output is independent of soln. ionic strength. It is successfully applied for quant. anal. in unmodified turbid/colored samples that included red wine, coke, coffee, baking soda, and antacid. The colorimetric output correlates well with the ref. method, a calibrated pH electrode. Compared to earlier systems where the cell potential is dictated by an external power source, the PB film exhibits excellent reproducibility and a rapid response time of about 44 s.
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    18. 18
      Kraikaew, P.; Sailapu, S. K.; Bakker, E. Rapid Constant Potential Capacitive Measurements with Solid-Contact Ion-Selective Electrodes Coupled to Electronic Capacitor. Anal. Chem. 2020, 92, 14174– 14180,  DOI: 10.1021/acs.analchem.0c03254
      18
      Rapid Constant Potential Capacitive Measurements with Solid-Contact Ion-Selective Electrodes Coupled to Electronic Capacitor
      Kraikaew, Pitchnaree; Sailapu, Sunil Kumar; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2020), 92 (20), 14174-14180CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      A const. potential capacitive readout of solid-contact ion-selective electrodes (SC-ISE) allows one to obtain easily identifiable current transients that can be integrated to obtain a charge vs logarithmic activity relationship. The resulting readout can therefore be much more sensitive than traditional open-circuit potentiometry. Unfortunately, however, comparatively long measurement times and significant baseline current drifts make it currently difficult to fully realize the promise of this technique. We show here that this challenge is overcome by placing the SC-ISE in series with an electronic capacitor, with pH probes as examples. Kirchhoff's law is shown to be useful to choose an adequate range of added capacitances so that it dominates the overall cell value. Two different ion-to-electron transducing materials, functionalized single-wall carbon nanotubes (f-SWCNTs) and poly(3-octylthiophene) (POT), were explored as solid-contact transducing layers. The established SC-ISE-based f-SWCNT transducer is found to be compatible with a wide range of external capacitances up to 100μF, while POT layers require a narrower range of 1-4.7μF. Importantly, the time for a charging transient to reach equil. was found to be less than 10 s, which is dramatically faster than without added electronic component. Owing to the ideal behavior of capacitor, the response current decays rapidly to zero, making the detn. of the integrated charge practically applicable.
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    19. 19
      Han, T.; Mattinen, U.; Mousavi, Z.; Bobacka, J. Coulometric response of solid-contact anion-sensitive electrodes. Electrochim. Acta 2021, 367, 137566,  DOI: 10.1016/j.electacta.2020.137566
      19
      Coulometric response of solid-contact anion-sensitive electrodes
      Han, Tingting; Mattinen, Ulriika; Mousavi, Zekra; Bobacka, Johan
      Electrochimica Acta (2021), 367 (), 137566CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      The coulometric response of nitrate, perchlorate, and sulfate solid-contact anion-sensitive electrodes was studied. The coulometric transduction method was originally introduced and so far mainly studied for solid-contact cation-selective sensors using poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrene sulfonate) as transducer. Here the authors provide addnl. proof-of-concept for the coulometric transduction method by focusing on the electrochem. characteristics of anion sensors. PEDOT was electrodeposited in the presence of small anions, including chloride, nitrate, sulfate, and perchlorate. The counterion influences the yield of electroactive PEDOT, which is an important parameter for the coulometric response. Anion-sensitive electrodes were prepd. by coating the PEDOT solid contact with plasticized PVC-based anion-sensitive membranes by drop-casting or spin-coating. The influence of the thickness of the PEDOT film and the anion-sensitive membrane on the coulometric response was studied. The solid-contact anion sensors showed fast charge transfer and ion transport properties, making them suitable for coulometric sensing. Also for anion-sensitive electrodes, the anal. signal was amplified by increasing the thickness of the PEDOT solid contact, which is fully consistent with earlier works on cation-selective electrodes. The coulometric transduction principle is feasible and robust for various types of solid-contact ISEs.
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    20. 20
      Wang, H.; Yuan, B.; Yin, T.; Qin, W. Alternative coulometric signal readout based on a solid-contact ion-selective electrode for detection of nitrate. Anal. Chim. Acta 2020, 1129, 136– 142,  DOI: 10.1016/j.aca.2020.07.019
      20
      Alternative coulometric signal readout based on a solid-contact ion-selective electrode for detection of nitrate
      Wang, Hemin; Yuan, Baiqing; Yin, Tanji; Qin, Wei
      Analytica Chimica Acta (2020), 1129 (), 136-142CODEN: ACACAM; ISSN:0003-2670. (Elsevier B.V.)
      Traditional potentiometric NO-3-selective electrodes suffer from a fundamental limitation of the Nernst slope (59.1 mV/dec at 25°C) due to the relationship between the potential and the logarithmic of ionic activity. Herein, a coulometric signal readout is proposed instead of the potentiometric response for detection of NO-3 based on an ordered mesoporous carbon (OMC)-based solid-contact ion-selective electrode (ISE). The mechanism for obtaining the coulometric signal is based on the elec. double layer capacitance of OMC compensating the potential change at the ion-selective membrane/soln. interface during the measurements under the control of a const. applied potential. Under the optimized conditions, the coulometric signal for the OMC-based solid-contact NO-3-ISE shows two linear responses in the activity range of 1.0 x 10-6-8.0 x 10-6 M and 8.0 x 10-6-8.0 x 10-4 M, and the detection limit is 4.0 x 10-7 M (3σ/s). The proposed coulometric response also shows excellent reproducibility and stability in the presence of O2 and CO2 and light on/off. Addnl., the coulometric response shows acceptable and reliable results for detection of NO-3 in mineral water as compared to the traditional potentiometric response and the ion chromatog. This work provides a promising alternative signal readout for detection of ions by using solid-contact ion-selective electrodes.
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    21. 21
      Sailapu, S. K.; Kraikaew, P.; Sabaté, N.; Bakker, E. Self-Powered Potentiometric Sensor Transduction to a Capacitive Electronic Component for Later Readout. ACS Sens. 2020, 5, 2909– 2914,  DOI: 10.1021/acssensors.0c01284
      21
      Self-Powered Potentiometric Sensor Transduction to a Capacitive Electronic Component for Later Readout
      Sailapu, Sunil Kumar; Kraikaew, Pitchnaree; Sabate, Neus; Bakker, Eric
      ACS Sensors (2020), 5 (9), 2909-2914CODEN: ASCEFJ; ISSN:2379-3694. (American Chemical Society)
      Potentiometric sensors operate as galvanic cells where the voltage is spontaneously generated as a function of the sample compn. The authors show here that energy can be harvested, stored during the sensing process without external power, and phys. isolated from the sensor circuit for later readout. This is accomplished by placing an electronic capacitor as a portable transduction component between the indicator and the ref. electrode at the point where one would ordinarily connect the high-input-impedance voltmeter. The voltage across this isolated capacitor indicates the originally measured ion activity and can be read out conveniently, for example, using a simple handheld multimeter. The capacitor is shown to maintain the transferred charge for hours after its complete disconnection from the sensor. The concept is demonstrated to detect the physiol. concns. of K+ in artificial sweat samples. The methodol. provides a readout principle that could become very useful in portable form factors and opens possibilities for potentiometric detection in point-of-care applications and inexpensive sensing devices where an external power source is not desired.
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    22. 22
      Hupa, E.; Vanamo, U.; Bobacka, J. Novel Ion-to-Electron Transduction Principle for Solid-Contact ISEs. Electroanalysis 2015, 27, 591– 594,  DOI: 10.1002/elan.201400596
      22
      Novel Ion-to-Electron Transduction Principle for Solid-Contact ISEs
      Hupa, Elisa; Vanamo, Ulriika; Bobacka, Johan
      Electroanalysis (2015), 27 (3), 591-594CODEN: ELANEU; ISSN:1040-0397. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Solid-contact ion-selective electrodes (SC-ISEs) are traditionally employed as potentiometric sensors, where the ion activity is related to the zero-current potential of the sensor vs. the ref. electrode. An alternative ion-to-electron transduction principle for SC-ISEs is introduced. The suggested signal transduction principle resembles const.-potential coulometry using the redox capacitance of the internal solid contact to convert changes in ion concn. (activity) into elec. current and charge. This short communication provides proof-of-concept for the suggested signal transduction method for SC-ISEs using poly(3,4-ethylenedioxythiphene) (PEDOT) as the solid contact that was coated with a cation-sensitive polymeric membrane.
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    23. 23
      Vanamo, U.; Hupa, E.; Yrjänä, V.; Bobacka, J. New Signal Readout Principle for Solid-Contact Ion-Selective Electrodes. Anal. Chem. 2016, 88, 4369– 4374,  DOI: 10.1021/acs.analchem.5b04800
      23
      New Signal Readout Principle for Solid-Contact Ion-Selective Electrodes
      Vanamo, Ulriika; Hupa, Elisa; Yrjana, Ville; Bobacka, Johan
      Analytical Chemistry (Washington, DC, United States) (2016), 88 (8), 4369-4374CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      A novel approach to signal transduction concerning solid-contact ion-selective electrodes (SC-ISE) with a conducting polymer (CP) as the solid contact was studied. The method presented here is based on const. potential coulometry, where the potential of the SC-ISE vs. the ref. electrode is kept const. using a potentiostat. The change in the potential at the interface between the ion-selective membrane (ISM) and the sample soln., due to the change in the activity of the primary ion, is compensated with a corresponding but opposite change in the potential of the CP solid contact. This enforced change in the potential of the solid contact results in a transient reducing/oxidizing current flow through the SC-ISE. By measuring and integrating the current needed to transfer the CP to a new state of equil., the total cumulated charge that is linearly proportional to the change of the logarithm of the primary ion activity was obtained. Different thicknesses of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonate) (PSS) were used as solid contact. Also, coated wire electrodes (CWEs) were included in the study to show the general validity of the new approach. The ISM employed was selective for K+ ions, and the selectivity of the membrane under implementation of the presented transduction mechanism was confirmed by measurements performed with a const. background concn. of Na+ ions. A unique feature of this signal readout principle is that it allows amplification of the anal. signal by increasing the capacitance (film thickness) of the solid contact of the SC-ISE.
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    24. 24
      Rahn, K. L.; Anand, R. K. Recent Advancements in Bipolar Electrochemical Methods of Analysis. Anal. Chem. 2021, 93, 103– 123,  DOI: 10.1021/acs.analchem.0c04524
      24
      Recent Advancements in Bipolar Electrochemical Methods of Analysis
      Rahn, Kira L.; Anand, Robbyn K.
      Analytical Chemistry (Washington, DC, United States) (2021), 93 (1), 103-123CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      The goal of this review is to provide an overview of the advancements made in the field of bipolar electrochem. over the past two years, with an emphasis on anal. Bipolar electrodes (BPEs) are versatile-in electroanal., they have been used extensively to screen electrocatalysts- and to sense biomarkers.- Their ability to modulate local elec. fields lends them to the manipulation of cells and to the enrichment and sepn. of analytes.- And finally, by virtue of the polar and often graded profile of the interfacial potential across BPEs, they provide a platform for synthesis of Janus particles, useful as sensors and as microswimmers,- and other materials with compositional gradients., BPEs are particularly well-suited to anal. challenges that demand multiplexing or amenability to point-of-need (PON) application because even large arrays of BPEs can be controlled with simple equipment, yet yield quant. information about a system. In this review, we discuss recent progress in reactions that transduce current to a visible signal, sensing mechanisms, bipolar electrochem. cell design, integration of bipolar electrochem. with spectroscopic techniques, BPEs at the nanoscale, and the application of BPEs to electrokinetics and materials prepn. Throughout the discussion, we identify promising trends, innovative directions, and remaining challenges in the field. A BPE is a conductive object, such as a metal strip or bead, that facilitates elec. coupled faradaic reactions at its opposing poles. Unlike a conventional electrode, a BPE has a floating potential and lacks direct contact to a power supply. Therefore, its potential (EBPE) floats to a value intermediate to that of the electrolyte it contacts. When the BPE is part of an electrolytic cell, a pair of driving electrodes applies a voltage bias across the electrolyte, resulting in the development of interfacial potential differences with opposite signs at the ends of the BPE as indicated in Scheme a. These interfacial potential differences are the anodic (ηα) and cathodic (ηc) overpotentials available to drive faradaic reactions. The anodic (ic) and cathodic (ic) currents are opposite in sign and equal in magnitude such that the current through the BPE, iBPE, is related to both by the equation, iBPE = ic=-ia. In this review, we will also discuss bipolar electrochem. systems that operate galvanically. Galvanic bipolar electrochem. shares a common mechanism with corrosion-the BPE interconnects electrolytes that establish a p.d. poised by available half-reactions in each phase. The oxidn. reaction occurs at a more neg. potential than the redn. reaction, to which it is elec. coupled by the conductive substrate. Therefore, the net processes are spontaneous, and driving electrodes are not required. The concept of bipolar electrochem. was first introduced in the 1960s by Fleischmann and co-workers, who described fluidized bed electrodes comprising Cu microscale particles, which they used in aq. electrochem. reactors. BPEs were then popularized by Manz and co-workers in the early 2000s when they combined these electrodes with electrochemiluminescence (ECL), which reported the current, thereby allowing them to create electrochem. detectors compatible with the high elec. field strengths and microscale compartments employed in micellar electrokinetic chromatog. Crooks and co-workers then established a theor. framework for bipolar electrochem. and broadened its application with a series of ground-breaking advancements. They described the interplay of controllable exptl. parameters such as the length of the BPE along the applied elec. field with intrinsic features of electrochem. processes to det. signal intensity. This description was then further refined to account for the impact of the distribution of interfacial potential along the BPE and the threshold current required for ECL on the obsd. luminescence. Their work uncovered the potential for BPEs to be leveraged to form arrays of either identical sensors to achieve spatial mapping of analytes or sensors with distinct surface chemistries for multiplexed anal., They then took advantage of the compatibility of BPEs with microfluidic length scales to drive the localized depletion of electrolyte ions required for electrokinetic enrichment and sepn., which in the context of anal., is particularly useful for sample prepn. In this literature review, we summarize advancements in the field of bipolar electrochem. that have been made over the past two years. Our discussion centers around BPEs as a tool for anal. and therefore covers the development of novel BPE configurations and their integration with sensing and reporting mechanisms, voltammetric methods, and spectroscopic techniques to address current challenges in sensing. For example, because BPEs are readily arrayed, shrinking their scale and boosting sensitivity is expected to allow for the characterization of single catalytic particles in parallel. We also examine advances in sample prepn. methods that employ BPEs, including electrokinetic enrichment and sepn. of chem. species and the selective manipulation of biol. cells by dielectrophoresis (DEP). There are many recent reports of the integration of existing bipolar electrochem. methods with a wide range of surface chemistries, esp. in the context of biosensing. Since these advancements are made not to bipolar electrochem. per se, they are not discussed in detail in this review., We begin with a brief overview of bipolar electrochem. in both open and closed BPE configurations. The reader is referred to a few comprehensive review articles- for a more thorough understanding of BPEs.
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    25. 25
      Jaworska, E.; Michalska, A.; Maksymiuk, K. Self-Powered Cascade Bipolar Electrodes with Fluorimetric Readout. Anal. Chem. 2019, 91, 15525– 15531,  DOI: 10.1021/acs.analchem.9b03405
      25
      Self-Powered Cascade Bipolar Electrodes with Fluorimetric Readout
      Jaworska, Ewa; Michalska, Agata; Maksymiuk, Krzysztof
      Analytical Chemistry (Washington, DC, United States) (2019), 91 (24), 15525-15531CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      Bipolar electrodes working in a self-powered mode are a basis for the development of easy to use electrochem.-optical sensors, these systems are very promising due to their simplicity and no need of external polarization. However, the self-powered mode can be used only in cases when the redox p.d. of reactions occurring at opposite poles of the electrode is sufficiently high. To overcome this limitation, we propose the development of a system working spontaneously, but involving two bipolar electrodes, forming a cascade system. One of electrodes ("driving" electrode) works in self-powered mode and triggers charge transfer processes in the second ("sensing") bipolar electrode. For the sensing electrode, an electrochem. process of an analyte occurs at one pole, accompanied by a complementary process at the second pole, inducing an optical (fluorimetric) anal. signal. This concept was successfully tested on a model system of a sensing bipolar electrode with a platinum electrode participating in oxidn. of an analyte, L-ascorbic acid, connected with electrode coated by poly(3-octylthiophene), where redn. of the polymer results in formation of fluorimetrically active neutral form. As the driving system, bipolar electrodes with zinc wire as one pole, characterized by a low redox potential, were used.
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