Overview

During the first few years of CENS, this project focused on developing contaminant transport inverse models and sensor network design strategies using off-the-shelf sensor technology (mainly thermocouples).  In the past year, emphasis on this project has shifted toward testing CENS-fabricated sensors (see Sensor Group section on potentiometric nitrate sensor fabrication efforts).  The objectives of this project are:

• Testing of prototypical combination potentiometric nitrate microsensors in flowing water and soils using commercial data acquisition interface hardware
• Side-by-side comparison of commercial nitrate sensors with CENS-fabricated microsensors in on-going contam test beds in UCLA, UC Merced, and Palmdale
• Deployments in support of ecological experiments at the Sevietta LTER in New Mexico, and La Selva Biological Station in Costa Rica

Approach

Expanded Water Quality Sensors and MDA300 Calibration.  This task examined a broader

range of sensors using CENS sensor boards (MDA300) and wireless transceivers (MICA2 Motes).  In January 2006, we deployed a wireless sensor network in a rice paddy in Bangladesh to measure parameters correlated with arsenic in the groundwater.  Sensors measured ammonium, calcium, nitrate, chloride, carboante, pH, and oxygen redox potential (ORP).  Sensor interface issues (sampling rate and data aggregation) were explored along with pre- and post-deployment calibration issues. 

CENS Microsensor Environmental Testing.  This project interfaces with the CENS Sensor Group effort aimed at creating a scaleable potentiometric nitrate microsensor suitable for distributed, embedded soil and aquatic deployments.  The microsensor for nitrate is described in more detail in the Sensor Group’s section of this report.  This ISE is created using a conducting polymer (polypyrrole, Ppy) doped with nitrate ions on a carbon electrode.  In the past year Contam has focused efforts on testing scaleable testing the combined (working and reference) microelectrode under environmental conditions in a series of water and soil flow-through experiments.

Systems/Experiments

Expanded Water Quality Sensors and MDA300 Calibration.  In January 2006, we deployed a wireless sensor network in a rice paddy in Bangladesh to measure parameters correlated with arsenic in the groundwater.  Sensors measured ammonium, calcium, nitrate, chloride, carboante, pH, and oxygen redox potential (ORP)—see deployment description in the section below.  Prior to the deployment, calibration curves were used to test the sensors and motes.  First, five-point calibration curves for each sensor type were measured using a wireless mote and pH meter, for comparison.  Second, minimum detection levels were determined graphically for each sensor type, using ten-point calibration curves.  Third, in Bangladesh, all sensors were calibrated prior to deployment using six-point calibration curves, based on the linear portion of the longer curves.  Following deployment, all sensors were re-calibrated, to determine changes in sensitivity.   For each calibration curve, all sensors were measured consistently with the same mote. 

 

The following graphs are for ammonium only, but are representative of the other sensor types, unless indicated. Results for five-point calibration curves indicated similar slopes between the pH meter and motes, with greater intercept values (or offsets) for the motes.    

 

Figure 1. Comparison of five-point calibration curves for ammonium (NH4), measured using a pH meter (left) and the MDA300/Mote system (right).  

Figure 2. Ten-point calibration curve for the ammonium sensor connected to mote 11 (the 0 point was dropped when concentration was converted to log units). 

Comparisons of pre- and post-deployment calibration curves indicated that some sensors withstood exposure to the subsurface environment over the 6 to 11 day deployment, while others lost sensitivity—see figures below for ammonium pre- and post-deployment calibration curves.     

Figure 3. Pre- (left) and post- (right) deployment calibration curve for using the same MDA300/Mote combination for an ammonium sensor. 

CENS Microsensor Environmental Testing.  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.

 

All measurements were carried out using direct potentiometry technique. Ag/AgCl saturated no-leak electrodes model EE-0009 from Cypress Systems (a division of ESA Inc, USA) were 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.

 

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 [1, 2]: 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. Silver paint was applied on top of the connection area to guarantee a good contact between pencil lead and silver wire. 1.5 cm length of pencil lead was immersed to a pyrrole solution (1.67 g of concentrated pyrrole solution mixed with 25 ml of 10-1 M NaNO3 solution) to perform electrochemical deposition of polypyrrole. Thus the total working (sensitive) surface of the electrode was 0.236 cm2.

 

Polymerization of pyrrole doped with nitrate was performed electrochemically; a Princeton Applied Research Potentiostat/Galvanostat, Model 363 A was used to supply constant currents running from 400 to 700 mA (current densities from 1.69 to 2.97 mA/cm2 respectively for 20 minutes). A silver wire/disk electrode and platinum wire/disk electrodes were used as a reference and counter electrodes respectively. Electropolymerization solution was purged with nitrogen for 5-10 minutes before deposition to remove oxygen. During preliminary testing experiments it was established that current strength 550 mA (current density 2.331 mA/cm2) is the best in terms of basic electrochemical characteristics of produced sensors (slope of electrode function and sensitivity (results are not shown)). In order to protect the electrode body during the experiments epoxy glue was applied on a copper wire and connection area leaving only sensitive polypyrrole covered tip exposed (plastic pipette tip was used as a protection cover for electrode body in preliminary experiments but it appeared to be less suitable for electrode minituarization purposes).  After polymerization the electrodes were rinsed with deionized water and placed into the conditioning solution (10-2 M NaNO3) for at least 24 hours prior to potentiometric measurements (the slopes of the electrodes conditioned for less than 24 hours were 25-30 mV/decade of nitrate concentration whereas the slopes of the electrodes conditioned for 24 hours were closer to the theoretical slopes expected on the basis of a Nernst equation (results are not shown). Between the measurements electrodes were stored in the dark in 1 x 10-4 M NaNO3.

Figure 4. Samples of prototypical nitrate microsensors (left) and protected microsensors (right) fabricated for this investigation.

In order to get an estimation of sensitivity behavior of nitrate microsensors prototypes under environmental conditions 3 types experiments were performed: sensors were put into continuous flow of deionized water for 3 hours (in 500 ml beaker), tap water for 28 hours, and soil test bed with continuous tap water flow (flow rate 1-1.5 liters per hour, in 5 l laboratory bath). Sensitivity of the electrodes was estimated by “spiking” (injecting) 5 ml of 5M NaNO3 solution into the flow. The results of the experiments are presented in figures 7-9 (HOBOÒ U12 4 channel data logger was used for data collection).

It can be seen from Figure 5, that with each “spike” nitrate concentration in the solution increases from ~ 10-4 M to ~10-2 M and then it becomes 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 nitrate-selective electrode purchased from Sentek.

Figure 5. 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.

Data presented above indicate, that doped PPy-based sensors demonstrate good reproducibility of the potential and their behavior in short-time flow experiments is very similar to that demonstrated by nitrate ISE purchased from Sentek. However, long term flow experiments in continuous tap water flow and soil test bed showed that PPy sensors loose their sensitivity after about 6 hours (while exposed to tap water continuous flow, see Figure 8) and 20 hours (when put into a soil test bed with continuous tap water flow, see Figure 9) while Sentek nitrate ISE keeps its sensitivity. During these experiments values of potential decrease after the spike with 5 M NaNO3 were found to be lower than in deionized water (30-50 mV for PPy based sensors and 30-70 mV for Sentek sensor) because of the presence of nitrate in local tap water. The passivation of PPy doped electrode might be due to dedoping of polymeric matrix (nitrate ions come out of the electrodes’ membrane (PPy(NO3) layer in this case) due to exchange processes taking place on the boarder membrane/studied solution).

Accomplishments

• The following are the major accomplishments in this area:
• Successful testing of CENS-fabricated combination nitrate selective electrode under environmentally relevant conditions
• Successful integration of a large variety of water quality sensors into the pylon configuration