Overview

This aspect of the CENS Sensor Group effort builds on an ion selective electrode (ISE) microsensor for nitrate that was reported on in previous years.  This ISE is created using a conducting polymer (polypyrrole, Ppy) doped with nitrate ions on a carbon electrode.  In the past year work has emphasized (1) creating a scaleable reference electrode to couple with the PPy-based working electrode, and (2) testing the combined electrode under environmental conditions.

Approach

A direct potentiometry technique has been applied to determine nitrate concentrations under laboratory and real conditions. Various kinds of equipment (voltmeter to take direct potential readings, data loggers for real time potential change monitoring) were used throughout the experiments.

Systems/Experiments

Potentiometric measurements

All measurements were carried out using direct potentiometry technique. An Ag/AgCl saturated no-leak electrodes model EE-0009 from Cypress Systems (a division of ESA Inc,USA) was used as a reference electrodes in potentiometric cells. Potentiometric measurements were conducted using Fluke 111 True RMS multimeter and HOBO U12 4 channel Data logger.

 

Design and construction of polypyrrole based nitrate-selective electrodes

Ion-selective electrodes for the determination of nitrate ion in different environmental objects (soil and water) were constructed following the procedure previously described by Bendikov and Harmon: electrodes were prepared from a pencil lead (soft kind 2B, diameter 0.5 mm, approximately 2.5 cm in length) by connecting it to a piece of a copper wire with another flexible and thin wire as shown on Figure 1. Polymerization of pyrrole doped with nitrate was performed electrochemically. A silver wire/disk electrode and platinum wire/disk electrodes were used as a reference and counter electrodes respectively.

Figure 1: Samples of prototypical nitrate microsensors (left) and protected microsensors (right) fabricated for this investigation.

Problems Encountered

The nitrate-doped polypyrrole microelectrode is not long-lasting as currently fabricated.  Under static (no flow) conditions, the sensor retains sensitivity for as long as a month, but when maintained in flowing streams, the sensor loses sensitivity in approximately 1 day.  Efforts aimed at improving sensor longevity either through sample preparation and/or alternative fabrication techniques.  Meanwhile, short-term experiments in distributed networked sensing are planned to develop and demonstrate this technology an end-to-end.

Accomplishments

The main accomplishments in the potentiometric nitrate microsensor development project include:

• Fabrication of a working combined prototype (working and reference electrode) of the nitrate microsensor
• Interfacing of working nitrate microsensor prototype with commercial environmental data-logging device
• Testing of microsensor prototypes under environmental conditions in flowing water and soil test beds. More detailed description of the obtained results is given below.

General characteristics of the electrodes

Electrode response

Typical calibration curves of polypyrrole based nitrate-selective electrodes made on different days are presented in Figure 2.

Figure 2: Potentiometric response of PPy(NO)₃ electrodes made on different days to NO_3- - ion (T=22° C). Each curve title indicates the date electrode was made (MM/DD/YY format and the number of the electrode.

As one can see from Figure 2, doped PPy based nitrate-selective electrodes exhibit near-Nernstian response ((-49)-(-55) mV/decade of nitrate concentration) with linear range  0.01-100 mM (10^-5 - 10^-1 mol/L or 0.62-6200 ppm) NO_3- and detection limit (3±1) x 10^-6 M (0.124-0.248 ppm). The electrodes responses remained stable over 20 day laboratory test period (calibration curves were taken before and after storing the electrodes in 10^-4 M NaNO₃).

pH-response of the electrodes

The effect of pH on the response of the proposed electrodes was checked by recording the emf of the cell, using a potentiometric (zero current) technique, which contained 10^-4 mol/L and 10^-5 mol/L NaNO₃ at different pH values (pH 3-10). The plots of E(mV) versus pH (shown in Figure  3) indicate, that the response of the electrodes does not significantly depend on the pH changes from 3 to 8 for all studied electrodes (the potential change in pH interval from 3 to 8 is 4-6 mV in case of using 10^-4 M NaNO₃ as a background electrolyte and 3-5 mV in case of using 10^-3 M NaNO₃ as a background electrolyte). After pH 9 potential decreases dramatically, therefore the best performance for NO_3-selective electrode should be achieved in the pH range from 3-8.

Figure 3: Influence of pH on the response of doped PPy based nitrate selective electrodes (background electrolyte –10^-4 M NaNO₃ each curve title indicates the date electrode was made on (MM/DD/YY format and the number of the electrode).

Potentiometric selectivity

The potentiometric selectivity coefficients were evaluated by fixed interference method. All studied solutions contained fixed amount of interferent anion (10^-2 M) and varying amounts of nitrate ion. Values of selectivity coefficients () doped PPy based electrode towards nitrate in presence of various interfering anions are presented in Table 1. It is interesting to note that selectivity of electrodes based on pencil leads towards nitrate in presence of perchlorate is significantly different from other reported and commercially available nitrate selective electrodes (Table 2). All three commercial electrodes suffer from a strong interference from perchlorate and respond preferentially to these anions rather than to nitrate, while pencil lead based electrode still responds to nitrate in presence of ClO_4-.

Table 1: Values of selectivity coefficients of proposed electrodes towards nitrate ion in presence of various interferent anions. Each value for doped PPy based sensors is a maximum value of out of 6 electrodes studied (concentration of interfering anion was 1 x 10^-2 M).

Table 2: Comparison of the observed for perchlorate using three commercial nitrate electrodes and PPy(NO₃) based electrode.

Design and construction of PU coated reference electrodes

Due to high cost of commercially available nitrate-selective electrodes and advances of micro-fabricated electrochemical sensors, in particular, miniaturized ion-selective electrodes, some efforts were also focused on creation of a cheap alternative version of reference electrode system that will be easily integrated with other working electrodes and allow to conduct direct potentiometric measurements of nitrate under environmental conditions (in underground waters, river waters and soil). We tried to create polyurethane protected Ag/AgCl reference electrode by dip-coating chloridized silver wire (diameter 0.8 mm) into a polyurethane (5 wt%) solution containing 98 vol.% of THF and 2% of DMF. However, when we tried to measure potential difference between PU coated and commercial reference electrodes in nitrate calibration solutions, PU-coated electrodes exhibited significant potential drift (25-100 mV) which might be due to non-uniformity of polyurethane coating of the electrodes (Figure 4).

Figure 4: PU-coated reference electrode versus commercial (EE-0009) reference electrode in NO_3- - calibration solutions.

Another approach that has been used to construct reference electrodes involved silver mirror reaction (SMR). Glass Pasteur pipette tips were chosen to be a substrate for SMR. Silver coated pipette tips were chloridized galvanostatically at E=1.2 V. Copper wire was attached to pipette tip using conducting silver paint to produce electric contact.

Obtained electrodes were stored in saturated KCl solution overnight before measurements. Each of obtained electrodes as well as commercially available EE-0009 reference electrode from ESA Inc. was calibrated separately with developed PPY sensors (Figure 5).

Figure 5: Calibration curves of doped PPy vased nitrate-selective electrode with different reference electrodes (SMR, SMR 2^nd try – electrodes produced using silver mirror reaction, ESA – EE-0009 “no leak” reference electrode from ESA Inc., PU – polyurethane coated Ag/AgCl reference electrode).

Testing of microsensor prototypes under environmental conditions

To estimate sensitivity of nitrate microsensors prototypes under environmental conditions experiments were performed by placing sensors in continuous flow water in both simulated stream and soil test bed settings. Sensitivity of the electrodes was estimated by “spiking” (injecting) 5 ml of 5M NaNO₃ solution into the flow. The plots in Figure 6 demonstrate that for each “spike” the nitrate concentration in the solution increases from ~10^-4 M to ~10^-2 M and then returns to 10^-4 M due to continuous dilution of the solution. According to absolute values of potentials, “recovery” time (time period after each spike when the potential of the electrode reaches its previous value (before the spike)) is 20-30 minutes for both doped PPy-based electrodes and commercial nitrate-selective electrode.

Figure 6. Deionized water flow experiment (duration 3 hours). Each of three PPy sensors (the legend indicates the date sensor was made) was coupled with EE-0009 reference electrode. 6 “spikes” (5 ml injections of 5M NaNO₃) were made throughout the experiment.

In the long term flow experiments in continuous tap water flow and soil test bed showed that PPy sensors lose their sensitivity after about 6 hours.  However, recent results have demonstrated that these sensors can be reconditioned with overnight exposure to dilute nitrate solutions.  Thus, the deactivation of PPy-doped electrode might be due to de-doping of polymeric matrix.

Future Directions

• Nitrate sensor field deployment (involving both commercially available and “home-made nitrate sensors
• Laboratory testing of nitrate sensors based on different conducting polymers
• Development of different types of sensor packaging for field deployment (javelin, roboduck and etc.)

People

Thomas Harmon, UC Merced, Professor
Michael Bendikov, Weizmann Institute, Assistant Professor
Alexander Rat’ko, UC Merced, Postdoctoral Scholar
Yair Wisjboom, Weizmann Institute, Doctoral Student
Christopher Butler, UC Merced, Undergraduate Environmental Engineering Major