OVERVIEW

Small, low-cost, robust, reliable, and sensitive sensors are needed to enable the realization of practical and economical sensor networks. Although there are a large number measurands that are of interest for sensor-network applications (e.g., seismic, temperature, light, sound, magnetic, chemical, etc.), appropriate commercial sensors exist for many of these measurands. However, one prominent exception is the fact that appropriate chemical sensors are not available. It is for this reason that the sensor technology effort within CENS is researching the design, fabrication, and implementation of chemical sensors that have the specifications needed for sensor networks.

In order to have a targeted effort to develop chemical sensors for sensor networks, we have focused on one sensor-network application, namely monitoring soil contamination / habitat monitoring. To model the flow of contaminants in soil, miniaturized lab-based soil systems (i.e., designed on the scale of meters) are first used to simulate real-world macroscopic field tests. These lab-based test systems need an array of miniaturized chemical sensors to accurately monitor the flow of contaminants in the model. Microsensors developed for this application will first be used in the lab-based system that has a controlled environment. Once the sensors have the robustness needed they will be used for field tests. To further focus and simplify the initial sensor technology development, the specific chemical contaminant compound that will be monitored is nitrate.

APPROACHes

It has recently been shown that doped polypyrrole films can be used as highly selective membrane material in potentiometric ion selective electrodes (ISE) [see Hutchins and Bachas, Anal. Chem. 1995, 67, 1654]. These ISEs show high selectivity toward the dopant ion which is not related to their lipophylicity. When pyrrole is doped with nitrate ions during polymerization process, a truly selective nitrate ISE can be prepared. These ISEs exhibit a few orders of magnitude higher selectivity towards nitrate ion with the same sensitivity level relative to commercial analogs. This portion of the Micro/Nano Sensor Technology research has been focused on incorporating highly selective interactions between electrode sensitive layer (doped polypyrrole) and target ion (NO3-) into a nitrate-selective chemical sensor, and developing a method for mass production of this type of sensor using micro-fabrication techniques.

Polypyrrole is one of the most widely studied conducting polymers because of its ease of preparation, high conductivity and relative stability. The oxidative polymerization of pyrrole is shown in Scheme I.

Scheme I

In the oxydized state, polypyrrole exist as a polyradical cation, and at the oxidation stage anions (NO3- in our case) are attracted electrostatically into the polymerized film as a counter ions (dopant) as illustrated in Scheme II. This particular structure, involving an exchangeable counter ion, has led to the application of polypyrrole as a membrane component for ISEs.

Scheme II

Polypyrrole doped electrodes show high selectivity towards the dopant ion. A layer of polypyrrole doped with nitrate is expected to have cavities that are complementary to the size of the target ion (NO₃^-). Thus, when expose to a nitrate solution, NO₃^- will move from the region of high nitrate activity to the region of low nitrate activity (from the polymer film to the studied solution or vice versa). As a result a constant potential difference across the interface is formed (eq 1).

(1) E = RT / nF ln(A_in/A_out)

R is the gas constant , F is the Faraday constant, T is the temperature, n is the ion charge (in the case of NO₃^-, n = 1) and A_in and A_out are the activities of the NO₃^- ion in the polymer film and in the solution, respectively. Since the concentration of nitrate within the polymer does not change it can be represented as a constant. Converting the natural logarithm in eq 1 to the base 10 logarithm, and inserting T = 298.15 K gives the most useful form of the equation, eq 2,

(2) E = constant - b (0.059) log A(NO₃^-)_out

where b is the electromotive efficiency with a value is close to 1 for most electrodes. According to eq 2, potential is expected to change by 59.16 mV for every factor-of-10 difference in nitrate activity.

PROBLEM ENCOUNTERED

• PI Harmon’s move to UC Merced delayed progress for in the fall of 2004 as his new laboratory was being constructed and cleared for use by health and safety.  This lab is now fully operational.

SYSTEMS / EXPERIMENTS

Tests over the past year have been executed in batch water and soil systems.  We have focused on a fibrous form factor in order to create a microsensor more suitable for deployment in porous media, such as soil, and continue to explore new strategies for creating a sensor package that facilitates intimate contact with the media while protecting the sensor.  In addition, we tested a modified fabrication procedure and found that it fails for nitrate, yet does yield a viable sensor for perchlorate.

ACCOMPLISHMENTS

The main accomplishments associated with this aspect of the nitrate sensor project are:

• Fabrication and testing of parylene C and epoxy-sheathed nitrate sensors, and testing of fibrous form factor to directly probe nitrate concentrations in residual soil moisture from real samples
• Proof of concept in extending nitrate-selective sensor fabrication process to another prominent contaminant, perchlorate
• Attainment of additional research funding to expand upon our on-going environmental packaging, testing, and test bed deployments of potentiometric nitrate microsensors

Potentiometric Nitrate Microsensor: Protective Sheaths and Residual Soil Moisture Probing

CENS REU summer interns were learned the microsensor fabrication techniques detailed in our previous annual report and then proceeded to innovate in terms of creating protective designs to facilitate deployment of these fragile sensors in the environment, specifically soils. First, the process initiated last year was perfected and tested. In this process, the fiber is first coated with parylene C to provide structural integrity (Figure 3.C.5-11) and protection from environmental conditions.

An advantage of using the carbon fibers as a substrate for pyrrole polymerization process is that these fibers are relatively easy to manipulate, lending themselves to root-like electrode designs which may be ideal for observing the water chemistry of soil moisture. Using prototypical PPy-coated microfibers, we were able to directly measure nitrate concentrations in residual soil water contents as low as 8% by weight for soil samples collected at the Palmdale test bed. Physical testing of the same soil’s water retention characteristics indicated that the moisture was accessible to the probes at moisture contents above those constituting highly drained conditions. For nitrate levels determined using the direct probing technique were shown to be spatially distributed on even small (roughly 20 g) soil samples, an observation which would have been difficult to make using traditional “wet chemistry” techniques.

Potentiometric Perchlorate Microsensor

An alternative fabrication process was examined in an effort to improve the upon the longevity of the PPy-based microsensors.  Unfortunately, the resulting fabrication process did not work well for nitrate.  The process, based on doped poly(3,4-ethylenedioxythiophene) (PEDOT) films, did however yield stable potentiometric solid state sensors for the perchlorate ion (ClO₄^-). PEDOT, one of the most promising conducting polymers, is extremely stable in its oxidized state. Using PEDOT(ClO₄^-) films as sensing material in ion selective electrodes presents a unique opportunity to create sensors having a longer lifetime compared to analogous sensors, such as those created using doped polypyrrole. Over the eight month period of this study, the PEDOT(ClO₄^-) sensors exhibited a stable, linear response spanning at least five orders of magnitude in concentration (1 M – 1 ´10^-5 M perchlorate) with near-Nernstian slopes approaching -50 mV/decade of ClO₄^- concentration and a limit of detection of 5 ´ 10^-6 M. Carbon fibers and pencil leads were employed as alternative and inexpensive substrates for EDOT polymerization

FUTURE DIRECTIONS and Objectives

The two proposed collaborative efforts discussed in last year’s report have both been awarded funding.  Through these new projects, we will be able to further the development and application of these types of microsensors.  The first involves an NSF-funded effort with the CENS investigator Michael Hamilton and several other UC-Riverside biologists entitled “Automated-Minirhizotron and Arrayed Rhizosphere Soil Sensors” (A-MARSS, PI. Michael Allen, UCR).  The second effort will be a collaboration with the University of Texas-Austin Center for Subsurface Modeling, on a research project entitled “Origin and Scale-dependence of Dispersivity: Implications for Miscible Flooding”, which was awarded funding by the Department of Energy’s National Energy Technology Laboratory.  Harmon’s group will modify the nitrate microsensors and make them available to Prof. Steve Bryant and others at UT Austin will use them to study hydrodynamic dispersion processes in porous media.

PEOPLE

Faculty:

Prof. Thomas C. Harmon

Staff:

Dr. Tatyana Bendikov