This work is a part of the STW project "Integral solution for sustainable construction (IS2C, Fleur van Rossem for her support during the chip fabrication, Justyna Wiedemair for the chip design and Allison Bidulock for her support during the manuscript preparation.

1. Chip Fabrication
Note: The chip consists of an Ag/AgCl working electrode (WE), an Ag/AgCl pseudo-reference electrode (pseudo-RE) and a platinum counter electrode on a glass chip. The silver metal is deposited on a glass chip, using standard cleanroom processes16. It is then chloridized in 0.1 M FeCl3 solution for 30 sec to form an AgCl layer over the surface. The Ag/AgCl WE (area = 9.812 mm2) is located in the center, surrounded by the Ag/AgCl pseudo-RE as shown in Figure 1.
<ol>
	<li>Deposit the silver metal on glass to form a planar silver electrode. The step-by-step chip fabrication in a cleanroom is given as follows:
	<ol>
		<li>Lithography
		<ol>
			<li>For wafer cleaning, immerse the glass wafer first into conc. HNO3 bath to remove any organic contamination and then into 1 M KOH to clean the surface.</li>
			<li>Apply a positive photo resist (200 nm thick) to the wafer by spin coating and prebake the resist at 95 &#176;C for 15 min.</li>
			<li>Expose the wafer to UV light for 5 sec through a glass mask.</li>
			<li>Develop the photoresist in developer solution. This will expose the area of glass where electrodes will be deposited and the remaining area will be covered by the unexposed resist.</li>
		</ol>
		</li>
		<li>Glass Etching
		<ol>
			<li>Immerse the developed glass wafer in the KOH bath to etch the exposed area of the glass.</li>
		</ol>
		</li>
		<li>Metal Deposition (Sputtering)
		<ol>
			<li>Deposit silver metal on the exposed glass area.
			<ol>
				<li>First deposit a 20 nm titanium (Ti) layer (by sputtering) as an adhesion layer for glass.</li>
				<li>Then deposit 70 nm palladium (Pd) layer (by sputtering) as a diffusion barrier between silver and Ti.</li>
				<li>Finally deposit 500 nm of silver metal. This is the thickness of the deposited metal. The area of the working electrode is 4 mm2.</li>
				<li>Remove the remaining resist by lift-off process in an acetone bath.</li>
			</ol>
			</li>
			<li>Dice the wafer into the single chip size. The chip is now ready to be chloridized to form an on-chip Ag/AgCl electrode.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Place the chip in an in-house designed Polytetrafluoroethylene chip holder, as shown in Figure 1. The holder provides easy electrical connections via push-pins (or spring-loaded pins) and contains an electrochemical cell.</li>
	<li>Chloridize the silver electrode to form an Ag/AgCl electrode on the chip. For that, pour 0.1 M FeCl3 solution in the cell for 30 sec. or till the silver color of the electrode becomes dark grayish. Longer immersion is not critical whereas shorter immersion time can lead to no deposition of Ag/AgCl.</li>
	<li>Rinse the chip with deionized water to clean the remaining FeCl3.The chip is now ready to be used for the chronopotentiometric measurement.</li>
</ol>
2. Electrolyte Preparation
Note: The electrolytes were prepared with different concentration of potassium chloride in 0.5 M KNO3.
<ol>
	<li>Prepare 50 ml of 50 mM KCl electrolyte stock solution (0.186 g of KCl in 50 ml of solution).</li>
	<li>In the same solution add 2.52 g of KNO3 to form a 0.5 M KNO3 background electrolyte.</li>
	<li>Keep the starting concentration of chloride to zero and KNO3 to 0.5 M. In other words add 2.52 g of KNO3 in a 50 ml of deionized water.</li>
	<li>Add 50 mM KCl to increase the chloride ion concentration (from 1 mM to 6 mM) of the electrolyte, systematically, as shown in the following steps.
	<ol>
		<li>Add 100 &#181;l of the 50 mM KCl into a 4.9 ml of 0.5 M KNO3 to get 1 mM [Cl&#713;].</li>
		<li>Add 104 &#181;l of the 50 mM KCl solution into the solution of step 2.4.1 to get 2 mM [Cl&#713;].</li>
		<li>Add 108.41 &#181;l of the 50 mM KCl solution into the solution of step 2.4.2 to get 3 mM [Cl&#713;].</li>
		<li>Add 113.19 &#181;l of the 50 mM KCl solution into the solution of step 2.4.3 to get 4 mM [Cl&#713;].</li>
		<li>Add 118.2 &#181;l of the 50 mM KCl solution into the solution of step 2.4.4 to get 5 mM [Cl&#713;].</li>
		<li>Add 123.3 &#181;l of the 50 mM KCl solution into the solution of step 2.4.5 to get 6 mM [Cl&#713;].</li>
	</ol>
	</li>
</ol>
3. Experimental Setup
Note: The measurements were performed using a potentiostat (Biologic Science Instruments, France).
<ol>
	<li>Place the chip in the chip holder and connect the corresponding terminal of the potentiostat.</li>
	<li>Connect the working electrode terminal of the potentiostat to the Ag/AgCl electrode.</li>
	<li>Connect the reference electrode terminal of the potentiostat to another Ag/AgCl electrode.</li>
	<li>Connect the counter electrode terminal of the potentiostat to the platinum electrode.</li>
	<li>Place the complete experiment setup in a faraday cage to avoid ambient noise.</li>
</ol>
4. Instrument Operational Settings
Note: The operational parameters of the electrochemical cell are controlled via the user interface of the potentiostat. The potential response of the Ag/AgCl WE during an applied current pulse is measured for different concentrations of chloride ion.
<ol>
	<li>In the potentiostat program, open the technique chronopotentiometry and set the operational parameters.
	<ol>
		<li>Set the following parameter according to the requirements:
		<ol>
			<li>Current (Is)</li>
			<li>Time of applied current (ts)</li>
		</ol>
		</li>
		<li>Start this technique by clicking the play button in the potentiostat program. 
		Note: The potentiostat program displays the potential response as a function of time under the applied current pulse.</li>
	</ol>
	</li>
</ol>
5. Measurements and Data Analysis
<ol>
	<li>Draw the calibration curve of the transition time vs. Cl&#713; ion concentration in electrolyte.
	<ol>
		<li>Change the Cl&#713; concentration in the KCl electrolyte, systematically.
		<ol>
			<li>Start the measurement with 5 ml of 1 mM KCl with 0.5 M KNO3 background electrolyte in the electrochemical cell.</li>
			<li>Start the chronopotentiometric experiment by using the potentiostat and apply a current of 10 mA/cm-2 for 10 sec and store the data.</li>
			<li>Systematically change the concentration to 6 mM with 1 mM increments and repeat the measurements.</li>
		</ol>
		</li>
		<li>Store the measured data as a.mpt file and analyze the data file in data processing program.
		<ol>
			<li>Analyze the data using an in-house developed data processing program to calculate the first derivative of the potential response and calculate the peak of the first derivative. The peak of the first derivative is the transition time. 
			Note: The complete coding of data processing program is given in the supplementary information of Abbas et al., 2014.</li>
		</ol>
		</li>
		<li>Repeat the measurement three times with the interval of 1 hr each.</li>
		<li>Open all the data files in data processing program and calculate the transition time for each measurement. Detail steps of transition time calculation is given below:
		<ol>
			<li>Plot the potential difference vs. time obtained from the data files of chronopotentiometric measurement.</li>
			<li>Calculate the first derivative of the potential response.</li>
			<li>Indicate the maximum of the first derivative and the time of it. The time of the maximum of first derivative is the transition time.</li>
		</ol>
		</li>
		<li>For the calibration curve, plot the square root of the transition time with respect to the concentration of Cl&#713; ions.</li>
		<li>Along the measured data plot the theoretical curve based on the Sand equation25. Calculate back the diffusion coefficient from the data plot.
		<ol>
			<li>In the measured data plot between transition time and the [Cl&#713;] take any point on the line and record the value of transition time and the [Cl&#713;]. The Sand equation is given as: 
			&#964; = (FC*/2j(1-tCl-))2 D&#960; 
			Here, C* is the bulk Cl&#713; ion concentration, F is the Faraday constant, j is the current density, tCl- is the transport number and D is the diffusion coefficient. Neglect the tCl- since it approaches zero for higher background electrolyte concentration.</li>
			<li>The only remaining variable in the Sand equation in the diffusion coefficient; put the value of transition time, the chloride ion concentration and the applied current in the Sand equation getting the value of the diffusion coefficient.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Drift Measurement
	<ol>
		<li>Pour 5 ml of 1 mM of the KCl in the electrochemical cell.</li>
		<li>In the potentiostat, set the applied current to be 10 Am-2 and time to be 10 sec.</li>
		<li>Measure the potential response for 2 weeks with three measurement each day with an interval of 3 hr between measurements.</li>
		<li>Refresh the electrolyte, i.e.,&#160;1 mM KCl, every day before performing the measurements.</li>
		<li>Plot the transition time over two weeks of measurements. The change in the transition time per day is the drift of the transition time response.</li>
	</ol>
	</li>
	<li>Effect of the pseudo-reference electrode on the transition time measurements 
	Note: Various pseudo-references such as a Ag/AgCl wire, a platinum wire and a steel wire are tested to measure the transition time.
	<ol>
		<li>Pour 4 mM of KCl electrolyte in the electrochemical cell.</li>
		<li>Use Ag/AgCl as a pseudo-reference electrode and connect it to the reference electrode terminal of the potentiostat. Due to anodic current pulse the Cl&#713; ions are depleted near the Ag/AgCl working electrode.</li>
		<li>Perform the chronopotentiometric measurement by applying a current density of 15 Am-2 for 10 sec.</li>
		<li>Repeat the measurements with platinum as a pseudo-reference electrode and record the data.</li>
		<li>Repeat the measurements with steel bar as a pseudo-reference electrode and record the data.</li>
		<li>Plot the measured transition time for various pseudo-reference electrodes used.</li>
		<li>Repeat the complete set of measurement with 5 mM of KCl and plot the transition time using various pseudo-reference electrodes.</li>
	</ol>
	</li>
</ol>

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The authors have nothing to disclose.

The transition time is the moment of inflection; it is theoretically independent of the reference potential i.e., the reference electrode. Therefore any metal wire can be used as a pseudo-reference electrode for transition time measurements. In contrast to the existing potentiometric measurement of chloride ions in concrete this method enables a long-term and calibration free measurement. Furthermore the sensitivity and the detection range of concentration can be tuned by adjusting the applied current pulse. For higher Cl&#713; concentrations, which are the case in concrete, higher current pulse should be applied to keep the transition time within 6 sec.
Although the dynamic measurement using the transition time of an Ag/AgCl electrode seems an attractive alternative for reference and calibration free detection of Cl&#713; ions. The interference from other anions, such as hydroxides and halides, can limit it feasibility especially when the concentration of interfering ions is higher than the Cl&#713; ions. For example a mixed (broaden) peak of transition time is observed if the [OH&#713;] is 10 times higher than the [Cl&#713;] in the electrolyte. In this case the transition time peaks for OH&#713; and Cl&#713; ions cannot be differentiated. The detection range of [Cl&#713;] depends upon the concentration of interfering ions such as OH&#713; ions7. We have tested down to 1 &#181;M KCl in the absence of interfering ions. Moreover, in the presence of iodide (I&#713;) and bromide (Br&#713;) ions the sensor will preferentially detect the transition time for I&#713; and Br&#713; ions rather than chloride. Typically in concrete interference from I&#713; and Br&#713; ions is irrelevant due to their absence or negligible amount. Besides, interference from hydroxide and halides ions can be compensated by covering the Ag/AgCl electrode with chloride ion-selective-polymer membrane (ionophores)26.
The applied current pulse is a critical parameter in this determining the transition time and should be carefully selected. Higher current pulse can rapidly degrade the AgCl surface with the formation of silver oxide. Whereas lower current pulse results in a longer transition time, consequently inducing errors in transition time due to undesired convection. The current pulse should be adjusted in such a way that the transition time is within 6 sec. This can be evaluated by putting transition time value and concentration limits in the Sand equation16. The surface degradation due to higher current pulses can be remedied by applying a reverse current (cathodic charge, Ic x tc) after each measurements (anodic charge, Ia x ta). The cathodic current will remove the excess AgCl deposited during the anodic current pulse. The cathodic current should be lower in amplitude and longer in time, tc, such that the charge of anodic and cathodic current should be equal, i.e., Ia ta = Ic tc. Furthermore, a rough range of the [Cl&#713;] ion concentration should be determined in advance and the current pulse is applied in such a way that the transition time should reach within 5 to 10 sec. Furthermore, the potentiostat program should be adjusted such that the anodic current stops once the transition time is reached. This will avoid the formation of AgO.
This method can be used for long term measurement of Cl&#713; ions in concrete giving long-term and reliable measurements. It can be used to monitor Cl&#713; ions in in drinking water, swimming pools and biological samples. Furthermore, it can be extended to monitor other halide ions like iodide and bromide in drinking water.

A chloride ion sensor based on the transition time measurement of an Ag/AgCl electrode is presented. The aim is to avoid the inherent drifts during the long-term continuous monitoring of the chloride ions in the electrolyte. Chronopotentiometric measurement, which is a dynamic measurement approach, of an Ag/AgCl electrode is used for this purpose. Here a rate of change of potential of an Ag/AgCl electrode is measured during a stimulus (galvanostatic pulse). The advantage of this approach is demonstrated by eluding the liquid-junction reference electrode and instead using any metal wire as a pseudo-reference electrode, therefore allowing the detection of Cl&#713; ions concentration for long-term (years) and in situ applications, such as measurement inside concrete structures.
Chloride ions in concrete structures is one of the major causes of degradation1,2. It initiates pitting corrosion in the reinforcement steel and results in the ultimate failure of the structure3. Therefore, measuring Cl&#713; ions in concrete is inevitable to predict the service life and maintenance cycle of a structure4,5. Different sensing principles have been reported for chloride ion measurement in concrete such as electrochemical6,7, optical8,9 and electromagnetic10,11. However, optical and electromagnetic methods have bulky setups, are difficult to integrate as a stand-alone system and have issues with selectivity12. In electrochemical technique, potentiometric measurement of an Ag/AgCl electrode is the state of the art approach6,7,13. Despite promising results, this approach is limited to lab-scale measurement since the drifts in reference potential and diffusion potential drop results in flawed data14,15. A transition time approach based on the dynamic electrochemical measurement (DEM) could alleviate the problem due to potential drift16.
In the DEM, a system&#39;s response to an applied stimulus is measured17-19. The example of such a system is chronopotentiometry. Here an applied current pulse is used as a stimulus depleting ions near the electrode surface and the corresponding potential response is measured. An anodic current at an Ag/AgCl electrode initiates a faradaic reaction (Ag + Cl&#713; <img alt="Figure 1" src="/files/ftp_upload/53312/arrow.jpg" />AgCl + e&#713;) resulting in a depletion of Cl&#713; ions near electrode surface. The potential change is a function of the applied current and the concentration of the (selective) ions in the electrolyte12,20. The moment these ions deplete completely near the electrode surface the rate of change of potential rises rapidly, giving an inflection point21. The inflection point on the potential-time response curve (chronopotentiogram) shows the transition time and can be determined from the maximum of the first derivative of the potential response22. The transition time is a characteristic of the ion concentration. This approach has been used to determine different ions concentration17 and the pH of electrolytes23,24. In case of an Ag/AgCl electrode as a working electrode (to which current is applied) the depleting ions will be chloride ions17. Therefore measuring its transition time will determine its concentration.

<table border="0" cellpadding="0" cellspacing="0" width="1009">
	<tbody>
		<tr height="25">
			<td height="25" style="height:25px;width:373px;">Platinum wire (&#8805;99.99% trace metals)</td>
			<td style="width:292px;">Sigma Aldrich, the Netherlands</td>
			<td style="width:172px;">EP1330-1EA</td>
			<td style="width:172px;"></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">Potassium chloride (BioXtra, &#8805;99.0%)</td>
			<td>Sigma Aldrich, the Netherlands</td>
			<td>P9333-500G</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">Potassium hydroxide (90% pure reagent grade)</td>
			<td>Sigma Aldrich, the Netherlands</td>
			<td>484016-1KG</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">Ferric chloride&#160;</td>
			<td>Sigma Aldrich, the Netherlands</td>
			<td>451649-1G</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">potassium nitrate (&#62; 99% reagent grade)&#160;</td>
			<td>Sigma Aldrich, the Netherlands</td>
			<td>P6083-500G</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">Ag/AgCl liquid junction reference electrode&#160;</td>
			<td>BASi, USA</td>
			<td>model MF-2079</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">VSP potentiostat</td>
			<td>Biologic Science Instruments, France</td>
			<td>VSP 300</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">Steel wire</td>
			<td>Microlab TU Delft</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">Silver wire&#160;</td>
			<td>Sigma Aldrich, the Netherlands</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

dynamic electrochemical measurement of chloride ions

This protocol describes the dynamic measurement of chloride ions using the transition time of a silver silver chloride (Ag/AgCl) electrode. Silver silver chloride electrode is used extensively for potentiometric measurement of chloride ions concentration in electrolyte. In this measurement, long-term and continuous monitoring is limited due to the inherent drift and the requirement of a stable reference electrode. We utilized the chronopotentiometric approach to minimize drift and avoid the use of a conventional reference electrode. A galvanostatic pulse is applied to an Ag/AgCl electrode which initiates a faradic reaction depleting the Cl&#713; ions near the electrode surface. The transition time, which is the time to completely deplete the ions near the electrode surface, is a function of the ion concentration, given by the Nernst equation. The square root of the transition time is in linear relation to the chloride ion concentration. Drift of the response over two weeks is negligible (59 &#181;M/day) when measuring 1 mM [Cl&#713;]using a current pulse of 10 Am-2. This is a dynamic measurement where the moment of transition time determines the response and thus is independent of the absolute potential. Any metal wire can be used as a pseudo-reference electrode, making this approach feasible for long-term measurement inside concrete structures.

The Ag/AgCl electrode is fabricated on a glass chip (Figure1) using a standard cleanroom process. The chronopotentiometric measurement setup (Figure 2) was used and response was measured using a potentiostat. To observe the effect of Cl&#713; ion concentration on the transition time, solutions containing 4, 5 and 6 mM of Cl&#713; ions in a 0.5 M KNO3 background are measured (Figure 3). The calibration curve of the square root of the transition time versus Cl&#713; ion concentration is plotted along with theoretical curve (Figure 4). Transition time response was measured for two weeks to evaluate the drift in the measurement (Figure 5). Transition time is measured in 4 and 5 mM Cl&#713; ion concentration for different pseudo-reference electrode to observe the effect of reference system on transition time (Figure 6). The range of current density over the range of 1-6 mM [Cl&#713;] is evaluated from the Sand equation (Table 1).
The transition time was measured in various concentrations of Cl&#713; ions, namely 4, 5 and 6 mM, as shown in Figure 3. The corresponding potential response is visible along with its first derivative. The peaks of the first derivatives give the transition times. The time instant of the peak is shifting to higher values with the increase in [Cl&#713;]. This is expected as more Cl&#713; ions present in the bulk electrolyte means it will take longer to completely deplete the Cl&#713; ions near the WE surface. The measured transition times for mentioned concentrations are 2.69, 4.28 and 5.92 sec, respectively.
The transition time measurements were calibrated against the known concentration of chloride ions. The measured data and its linear fit is presented in Figure 4. The square root of the transition time is in linear correlation to the [Cl&#713;], as predicted by the Sand equation. The apparent diffusion coefficient of Cl&#713; ions from the measured data is found to be 2.280 x 10-9 m2sec-1, which is in good agreement with the theoretical value, D = 2 x 10-9 m2sec-1. The deviation can be due to the uncertainty in the current density value attributed to the changing surface area during the current pulse. Furthermore the deviation is relatively larger for higher concentrations, e.g.,&#160;7 mM. This is attributed to the relatively large thickness of the concentration profile, which makes it more sensitive to convection.
The dynamic nature of transition time ensures a drift free measurement. To analyze the drift, the transition time for 1 mM Cl&#713; was measured over two weeks, as shown in Figure 5. There is a decreasing trend in transition time, 0.23 msec/day (corresponds to 0.8 &#181;M/day), in the linear fit of the data. Nonetheless, this change is small and could be attributed to handling errors, changing Cl&#713; ion concentration due to evaporation, change in apparent current density and temperature variation over the measurement period. It is therefore difficult to give a concrete conclusion about the drift of the sensor; either there is no inherent drift or the drift is relatively small.
Another aspect of the dynamic nature of transition time measurement is its independence on a reference system. Various pseudo-references (an Ag/AgCl wire, a platinum wire and a steel wire) were set up to measure the transition time of an Ag/AgCl electrode in 4 and 5 mM [Cl&#713;]. The measurements are shown in Figure 6. For various pseudo-references the response does not change significantly, transition time varies within 80 msec (&#177; 75 &#181;M at 4 and 5 mM Cl&#713; ions). Therefore the reference system has no systematic effect; any metal wire can be a used as a pseudo-reference electrode for transition time measurements.
<img alt="Figure 1" src="/files/ftp_upload/53312/53312fig1.jpg" /> 
Figure 1. Chloride sensor chip along with the electrochemical cell. (A) The Cl&#713; ion sensor chip with Ag/AgCl electrodes on a glass substrate and electrical connection pads. The dimension of the glass chip is 15 x 20 mm2 whereas the exposed area in the electrochemical cell is 78 mm2. (B) The complete electrochemical cell (chip holder) with the mounted chip. The middle circular area shows the area of the chip exposed to the electrolyte16. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53312/53312fig1large.jpg" target="_blank">Please click here to download this file.</a>
<img alt="Figure 2" src="/files/ftp_upload/53312/53312fig2.jpg" /> 
Figure 2. Schematic of transition time approach. (A) Schematic of the Cl&#713; ion detection approach. During an applied current pulse at the WE with respect to a CE (not included in this Figure), chloride ions deplete at the Ag/AgCl WE, resulting in the illustrated Cl&#713; ion concentration profile. (B) Schematic of the &#916;V and d&#916;V/dt response. The solid line (-) and dashed line (--) represent &#916;V and d&#916;V/dt, respectively. &#964; is the transition time and t is the duration of the applied current pulse. Here, t/&#964; is the ratio of the applied current time and the transition time when t/&#964; = 1, the Cl&#713; ions at the surface of the working electrode depletes completely16. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53312/53312fig2large.jpg" target="_blank">Please click here to download this file.</a>
<img alt="Figure 3" src="/files/ftp_upload/53312/53312fig3.jpg" /> 
Figure 3. Transition time measurements. The chronopotentiograms (solid line) and their first derivatives (dashed line) of the Ag/AgCl electrode with respect to another Ag/AgCl pseudo-RE electrode in solutions of 4, 5 and 6 mM Cl&#713; ions. The background electrolyte is 0.5 M KNO3, the applied current pulse is 10 Am-2 and the ambient temperature is 20.9 &#176;C. The maximum slopes of the potential responses are indicated by the dotted lines16. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53312/53312fig3large.jpg" target="_blank">Please click here to download this file.</a>
<img alt="Figure 4" src="/files/ftp_upload/53312/53312fig4.jpg" /> 
Figure 4. Calibration curve. A Calibration curve showing the square root of the transition time versus the Cl&#713; ion concentration, in a 0.5 M KNO3 background electrolyte. The circular points (o) are the measured data and the solid line (-) is the linear fit. The dashed line (--) is the theoretical curve from the Sand equation, eq. (2). The applied current pulse is 10 Am-2 and the ambient temperature is 20.8 &#176;C. D and Da are the theoretical and apparent (measured) diffusion coefficients, respectively16. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53312/53312fig4large.jpg" target="_blank">Please click here to download this file.</a>
<img alt="Figure 5" src="/files/ftp_upload/53312/53312fig5.jpg" /> 
Figure 5. Drift analysis. Subsequent measurements of the transition time over two weeks. Each measurement was carried out every other day, with 3 measurements performed each day and 3 hr between each same-day measurement. The electrolyte contains 1 mM of Cl&#713; ions in a 0.5 M KNO3 background and the applied current pulse is 10 Am-2. The data points and linear fit are denoted by (+) and a dashed line, respectively16. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53312/53312fig5large.jpg" target="_blank">Please click here to download this file.</a>
<img alt="Figure 6" src="/files/ftp_upload/53312/53312fig6.jpg" /> 
Figure 6. Effect of different pseudo-reference electrodes. Measured transition times using pseudo-REs composed of a Ag/AgCl liquid junction RE, Ag/AgCl wire, platinum wire and steel wire. The applied current pulse is 15 Am-2 and the ambient temperature is 21.2 &#176;C. Here, + and o marks show the measured data for 4 and 5 mM of Cl&#713; ions in a 0.5 M KNO3 background, respectively16. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53312/53312fig6large.jpg" target="_blank">Please click here to download this file.</a>
<table border="0" cellpadding="0" cellspacing="0" width="1048">
	<tbody>
		<tr>
			<td>[Cl&#713;] range 
			(mM)</td>
			<td>Current density, j 
			(Am-2)</td>
			<td>(j/C*)min 
			(A&#8729;m&#8729;mol-1)</td>
			<td>(j/C*)max 
			(A&#8729;m&#8729;mol-1)</td>
			<td>&#964;min 
			(s)</td>
			<td>&#964;max 
			(s)</td>
		</tr>
		<tr>
			<td>1 to 6</td>
			<td>10</td>
			<td>1.66</td>
			<td>10</td>
			<td>0.146</td>
			<td>5.26</td>
		</tr>
	</tbody>
</table>
Table 1. The selected current density for a Cl&#713; ion concentration range of 1 to 6 mM, evaluated from eq. (3). The corresponding values of the ratio j/C and transition time are also given16.

Watch this Scientific Journal Video about Dynamic Electrochemical Measurement of Chloride Ions at JoVE.com

Dynamic Electrochemical Measurement of Chloride Ions

Dynamic measurement of chloride ions is presented. Transition time of an Ag/AgCl electrode, during a chronopotentiometric technique, can give the concentration of chloride ions in electrolyte. This method does not require a stable conventional reference electrode.

This protocol describes the dynamic measurement of chloride ions using the transition time of a silver silver chloride (Ag/AgCl) electrode. Silver silver ...

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11.4K Views.  University of Twente. Dynamic measurement of chloride ions is presented. Transition time of an Ag/AgCl electrode, during a chronopotentiometric technique, can give the concentration of chloride ions in electrolyte. This method does not require a stable conventional reference electrode.

Video: Dynamic Electrochemical Measurement of Chloride Ions

Activating Molecules, Ions, and Solid Particles with Acoustic Cavitation

The chemical and physical effects of ultrasound arise not from a direct interaction of molecules with sound waves, but rather from the acoustic cavitation: the nucleation, growth, and implosive collapse of microbubbles in liquids submitted to power ultrasound. The violent implosion of bubbles leads to the formation of chemically reactive species and to the emission of light, named sonoluminescence. In this manuscript, we describe the techniques allowing study of extreme intrabubble conditions and chemical reactivity of acoustic cavitation in solutions. The analysis of sonoluminescence spectra of water sparged with noble gases provides evidence for nonequilibrium plasma formation. The photons and the "hot" particles generated by cavitation bubbles enable to excite the non-volatile species in solutions increasing their chemical reactivity. For example the mechanism of ultrabright sonoluminescence of uranyl ions in acidic solutions varies with uranium concentration: sonophotoluminescence dominates in diluted solutions, and collisional excitation contributes at higher uranium concentration. Secondary sonochemical products may arise from chemically active species that are formed inside the bubble, but then diffuse into the liquid phase and react with solution precursors to form a variety of products. For instance, the sonochemical reduction of Pt(IV) in pure water provides an innovative synthetic route for monodispersed nanoparticles of metallic platinum without any templates or capping agents. Many studies reveal the advantages of ultrasound to activate the divided solids. In general, the mechanical effects of ultrasound strongly contribute in heterogeneous systems in addition to chemical effects. In particular, the sonolysis of PuO2 powder in pure water yields stable colloids of plutonium due to both effects.

Acoustic cavitation in liquids submitted to power ultrasound creates transient extreme conditions inside the collapsing bubbles, which are the origin of unusual chemical reactivity and light emission, known as sonoluminescence. In the presence of noble gases, nonequilibrium plasma is formed. The "hot" particles and the photons generated by collapsing bubbles are able to excite species in solution.

The chemical and physical effects of ultrasound arise not from a direct interaction of molecules with sound waves, but rather from the acoustic ...

Measurement of Particle Size Distribution in Turbid Solutions by Dynamic Light Scattering Microscopy

A protocol for measuring polydispersity of concentrated polymer solutions using dynamic light scattering is described. Dynamic light scattering is a technique used to measure the size distribution of polymer solutions or colloidal particles. Although this technique is widely used for the assessment of polymer solutions, it is difficult to measure the particle size in concentrated solutions due to the multiple scattering effect or strong light absorption. Therefore, the concentrated solutions should be diluted before measurement. Implementation of the confocal optical component in a dynamic light scattering microscope1 helps to overcome this barrier. Using such a microscopic system, both transparent and turbid systems can be analyzed under the same experimental setup without a dilution. As a representative example, a size distribution measurement of a temperature-responsive polymer solution was performed. The sizes of the polymer chains in an aqueous solution were several tens of nanometers at a temperature below the lower critical solution temperature (LCST). In contrast, the sizes increased to more than 1.0 &#181;m when above the LCST. This result is consistent with the observation that the solution turned turbid above the LCST.

A protocol for the direct measurement of particle size distribution in concentrated solutions using dynamic light scattering microscopy is presented.

A protocol for measuring polydispersity of concentrated polymer solutions using dynamic light scattering is described. Dynamic light scattering is a ...

Fabrication and Testing of Photonic Thermometers

In recent years, a push for developing novel silicon photonic devices for telecommunications has generated a vast knowledge base that is now being leveraged for developing sophisticated photonic sensors. Silicon photonic sensors seek to exploit the strong confinement of light in nano-waveguides to transduce changes in physical state to changes in resonance frequency. In the case of thermometry, the thermo-optic coefficient, i.e., changes in refractive index due to temperature, causes the resonant frequency of the photonic device such as a Bragg grating to drift with temperature. We are developing a suite of photonic devices that leverage recent advances in telecom compatible light sources to fabricate cost-effective photonic temperature sensors, which can be deployed in a wide variety of settings ranging from controlled laboratory conditions, to the noisy environment of a factory floor or a residence. In this manuscript, we detail our protocol for the fabrication and testing of photonic thermometers.

We describe the process of fabrication and testing of photonic thermometers.

In recent years, a push for developing novel silicon photonic devices for telecommunications has generated a vast knowledge base that is now being ...

Electrochemical Impedance Spectroscopy as a Tool for Electrochemical Rate Constant Estimation

Electrochemical impedance spectroscopy (EIS) was used for advanced characterization of organic electroactive compounds along with cyclic voltammetry (CV). In the case of fast reversible electrochemical processes, current is predominantly affected by the rate of diffusion, which is the slowest and limiting stage. EIS is a powerful technique that allows separate analysis of stages of charge transfer that have different AC frequency response. The capability of the method was used to extract the value of charge transfer resistance, which characterizes the rate of charge exchange on the electrode-solution interface. The application of this technique is broad, from biochemistry up to organic electronics. In this work, we are presenting the method of analysis of organic compounds for optoelectronic applications.

Electrochemical impedance spectroscopy (EIS) of species that undergo reversible oxidation or reduction in solution was used for determination of rate constants of oxidation or reduction.

Electrochemical impedance spectroscopy (EIS) was used for advanced characterization of organic electroactive compounds along with cyclic voltammetry (CV). ...

Preparation of Stable Bicyclic Aziridinium Ions and Their Ring-Opening for the Synthesis of Azaheterocycles

Bicyclic aziridinium ions were generated by the removal of an appropriate leaving group through internal nucleophilic attack by nitrogen atom in the aziridine ring. The utility of bicyclic aziridinium ions, specifically 1-azoniabicyclo[3.1.0]hexane and 1-azoniabicyclo[4.1.0]heptane tosylate highlighted in the aziridine ring openings by the nucleophile with the release of the ring strain to yield the corresponding ring-expanded azaheterocycles such as pyrrolidine, piperidine and azepane with diverse substituents on the ring in regio- and stereospecific manner. Herein, we report a simple and convenient method for the preparation of the stable 1-azabicyclo[4.1.0]heptane tosylate followed by selective ring opening via a nucleophilic attack either at the bridge or at the bridgehead carbon to yield piperidine and azepane rings, respectively. This synthetic strategy allowed us to prepare biologically active natural products containing piperidine and azepane motif including sedamine, allosedamine, fagomine and balanol in highly efficient manner.

Bicyclic aziridinium ions such as 1-azoniabicyclo[4.1.0]heptane tosylate were generated from 2-[4-tolenesulfonyloxybutyl]aziridine, which was utilized for the preparation of substituted piperidines and azepanes via regio- and stereospecific ring-expansion with various nucleophiles. This highly efficient protocol allowed us to prepare diverse azaheterocycles including natural products such as fagomine, febrifugine analogue and balanol.

Bicyclic aziridinium ions were generated by the removal of an appropriate leaving group through internal nucleophilic attack by nitrogen atom in the ...

Disentangling High Strength Copolymer Aramid Fibers to Enable the Determination of Their Mechanical Properties

Traditionally, soft body armor has been made from poly(p-phenylene terephthalamide) (PPTA) and ultra-high molecular weight polyethylene. However, to diversify the fiber choices in the United States body armor market, copolymer fibers based on the combination of 5-amino-2-(p-aminophenyl) benzimidazole (PBIA) and the more conventional PPTA were introduced. Little is known regarding the long-term stability of these fibers, but as condensation polymers, they are expected to have potential sensitivity to moisture and humidity. Therefore, characterizing the strength of the materials and understanding their vulnerability to environmental conditions is important for evaluating their use lifetime in safety applications. Ballistic resistance and other critical structural properties of these fibers are predicated on their strength. To accurately determine the strength of the individual fibers, it is necessary to disentangle them from the yarn without introducing any damage. Three aramid-based copolymer fibers were selected for the study. The fibers were washed with acetone followed by methanol to remove an organic coating that held the individual fibers in each yarn bundle together. This coating makes it difficult to separate single fibers from the yarn bundle for mechanical testing without damaging the fibers and affecting their strength. After washing, fourier transform infrared (FTIR) spectroscopy was performed on both washed and unwashed samples and the results were compared. This experiment has shown that there are no significant variations in the spectra of poly(p-phenylene-benzimidazole-terephthalamide-co-p-phenylene terephthalamide) (PBIA-co-PPTA1) and PBIA-co-PPTA3 after washing, and only a small variation in intensity for PBIA. This indicates that the acetone and methanol rinses are not adversely affecting the fibers and causing chemical degradation. Additionally, single fiber tensile testing was performed on the washed fibers to characterize their initial tensile strength and strain to failure, and compare those to other reported values. Iterative procedural development was necessary to find a successful method for performing tensile testing on these fibers.

The primary goal of the study is to develop a protocol to prepare consistent specimens for accurate mechanical testing of high strength copolymer aramid fibers, by removing a coating and disentangling the individual fiber strands without introducing significant chemical or physical degradation.

Traditionally, soft body armor has been made from poly(p-phenylene terephthalamide) (PPTA) and ultra-high molecular weight polyethylene. However, to ...

Accumulation and Analysis of Cuprous Ions in a Copper Sulfate Plating Solution

Knowledge of the behavior of cuprous ions (monovalent copper ion: Cu(I)) in a copper sulfate plating bath is important for improving the plating process. We successfully developed a method to quantitatively and easily measure Cu(I) in a plating solution and used it for evaluation of the solution. In this paper, a quantitative absorption spectrum measurement and a time-resolved injection measurement of Cu(I) concentrations by a color reaction are described. This procedure is effective as a method to reproduce and elucidate the phenomenon occurring in the plating bath in the laboratory. First, the formation and accumulation process of Cu(I) in solution by electrolysis of a plating solution is shown. The amount of Cu(I) in the solution is increased by electrolysis at higher current values &#8203;&#8203;than the usual plating process. For the determination of Cu(I), BCS (bathocuproinedisulfonic acid, disodium salt), a reagent that selectively reacts with Cu(I), is used. The concentration of Cu(I) can be calculated from the absorbance of the Cu(I)-BCS complex. Next, the time measurement of the color reaction is described. The color reaction curve of Cu(I) and BCS measured by the injection method can be decomposed into an instantaneous component and a delay component. By analysis of these components, the holding structure of Cu(I) can be clarified, and this information is important when predicting the quality of the plating film to be produced. This method is used to facilitate the evaluation of the plating bath in the production line.

Here, accumulation of cuprous ions in a copper sulfate plating solution in a model experiment and an analysis based on quantitative measurements are described. This experiment reproduces the accumulation process of cuprous ions in the plating bath.

Knowledge of the behavior of cuprous ions (monovalent copper ion: Cu(I)) in a copper sulfate plating bath is important for improving the plating process. ...

Electrochemical Roughening of Thin-Film Platinum Macro and Microelectrodes

This protocol demonstrates a method for electrochemical roughening of thin-film platinum electrodes without preferential dissolution at grain boundaries of the metal. Using this method, a crack free, thin-film macroelectrode surface with up to 40 times increase in active surface area was obtained. The roughening is easy to do in a standard electrochemical characterization laboratory and incudes the application of voltage pulses followed by extended application of a reductive voltage in a perchloric acid solution. The protocol includes the chemical and electrochemical preparation of both a macroscale (1.2 mm diameter) and microscale (20 &#181;m diameter) platinum disc electrode surface, roughening the electrode surface and characterizing the effects of surface roughening on electrode active surface area. This electrochemical characterization includes cyclic voltammetry and impedance spectroscopy and is demonstrated for both the macroelectrodes and the microelectrodes. Roughening increases electrode active surface area, decreases electrode impedance, increases platinum charge injection limits to those of titanium nitride electrodes of same geometry and improves substrates for adhesion of electrochemically deposited films.

This protocol demonstrates a method for electrochemical roughening of thin-film platinum electrodes without preferential dissolution at grain boundaries. The electrochemical techniques of cyclic voltammetry and impedance spectroscopy are demonstrated to characterize these electrode surfaces.

This protocol demonstrates a method for electrochemical roughening of thin-film platinum electrodes without preferential dissolution at grain boundaries ...

Application of Voltage in Dynamic Light Scattering Particle Size Analysis

Dynamic light scattering (DLS) is a common method for characterizing the size distribution of polymers, proteins, and other nano- and microparticles. Modern instrumentation permits measurement of particle size as a function of time and/or temperature, but currently there is no simple method for performing DLS particle size distribution measurements in the presence of applied voltage. The ability to perform such measurements would be useful in the development of electroactive, stimuli-responsive polymers for applications such as sensing, soft robotics, and energy storage. Here, a technique using applied voltage coupled with DLS and a temperature ramp to observe changes in aggregation and particle size in thermoresponsive polymers with and without electroactive monomers is presented. The changes in aggregation behavior observed in these experiments were only possible through the combined application of voltage and temperature control. To obtain these results, a potentiostat was connected to a modified cuvette in order to apply voltage to a solution. Changes in polymer particle size were monitored using DLS in the presence of constant voltage. Simultaneously, current data were produced, which could be compared with particle size data, to understand the relationship between current and particle behavior. The polymer poly(N-isopropylacrylamide) (pNIPAM) served as a test polymer for this technique, as pNIPAM's response to temperature is well-studied. Changes in the lower-critical solution temperature (LCST) aggregation behavior of pNIPAM and poly(N-isopropylacrylamide)-block-poly(ferrocenylmethyl methacrylate), an electrochemically active block-copolymer, in the presence of applied voltage are observed. Understanding the mechanisms behind such changes will be important when trying to achieve reversible polymer structures in the presence of applied voltage.

Here, a protocol to apply voltage to solution during dynamic light scattering particle size measurements with the intent to explore the effect of voltage and temperature changes on polymer aggregation is presented.

Dynamic light scattering (DLS) is a common method for characterizing the size distribution of polymers, proteins, and other nano- and microparticles. ...

Asymmetric Thermoelectrochemical Cell for Harvesting Low-grade Heat under Isothermal Operation

Low-grade heat is abundantly available in the environment as waste heat. The efficient conversion of low-grade heat into electricity is very difficult. We developed an asymmetric thermoelectrochemical cell (aTEC) for heat-to-electricity conversion under isothermal operation in the charging and discharging processes without exploiting the thermal gradient or the thermal cycle. The aTEC is composed of a graphene oxide (GO) cathode, a polyaniline (PANI) anode, and 1M KCl as the electrolyte. The cell generates a voltage due to the pseudocapacitive reaction of GO when heating from room temperature (RT) to a high temperature (TH, ~40-90 &#176;C), and then current is successively produced by oxidizing PANI when an external electrical load is connected. The aTEC demonstrates a remarkable temperature coefficient of 4.1 mV/K and a high heat-to-electricity conversion efficiency of 3.32%, working at a TH = 70 &#176;C with a Carnot efficiency of 25.3%, unveiling a new promising thermoelectrochemical technology for low-grade heat recovery.

Low-grade heat is abundant, but its efficient recovery is still a great challenge. We report an asymmetric thermoelectrochemical cell using graphene oxide as a cathode and polyaniline as an anode with KCl as the electrolyte. This cell works under isothermal heating, exhibiting a high heat-to-electricity conversion efficiency in low-temperature regions.

Low-grade heat is abundantly available in the environment as waste heat. The efficient conversion of low-grade heat into electricity is very difficult. We ...