A nitrate-sensing system that consists of a micromachined sensor substrate, anion-permeable membrane, integrated microfluidic channels, and standard fluidic connectors has been designed, fabricated, assembled, and tested. Our microsensor was designed for in-situ monitoring of nitrate concentrations in ground water. A silver electrode was patterned for amperometric nitrate detection. An electrochemically oxidized silver electrode was used as a reference electrode. Microfluidic channels were fabricated as flow paths to the microelectrochemical (MEC) cell for the eluent and ground-water sample. The sensor also incorporates an anion-permeable membrane that is used for selective measurement of nitrate. With standard addition methods, linear calibration curves have been obtained and the detection limit is ~1 μM. To test selectivity, the sensor response to a 100-μM-nitrate solution and a mixture of nitrate and interfering ions (100 μM each of NO₃⁻, PO₄⁻², SO₄⁻³, F⁻, and Cl⁻) were compared. Despite the 400% increase in total ionic concentration, the sensor output only increased 13.9%.

Nitrate-sensing system that consists of a micromachined sensor substrate, nitrate-permeable membrane, integrated microfluidic channels, and standard fluidic connectors has been designed, fabricated, assembled, and tested. Our microsensor was designed for in-situ monitoring of nitrate concentrations in ground water. A silver electrode was patterned for amperometric nitrate detection. An electrochemically oxidized silver electrode was used as a reference electrode. Microfluidic channels were fabricated as flow paths for the eluent and ground-water sample to the microelectrochemical (MEC) cell. The sensor also incorporates a nitrate-permeable membrane that is used for selective measurement of nitrate. The sensor has a linear response over concentration ranging from 1 μM to 1 mM.

To enable the field testing of sensors developed by the CENS Sensor group, a sample-preparation and data-acquisition system must be developed. We have found and tested compact, low power valves that use only 5.5mJ per switch, and have been combined into a fluidic multiplexer for calibration. We have made a potentiostat out of a few dollars of chips to replace a large bench top potentiostat that cost $20,000. We have designed a nitrate diffusion system to prevent clogs but still allow nitrate ions to diffuse. Finally, we have created a new board for sensor control and measurement that adds functionality to the previous board and corrects the errors of the old one.

A sensitive and miniaturized amperometric nitrate (NO3-) sensor for groundwater monitoring was designed, fabricated, and tested. The thin-film microelectrodes were patterned on the silicon substrate with microfabrication techniques. The sensor shows very promising performance, compared to commercial nitrate sensors. A low detection limit of 4 ?M and wide dynamic range of 10 mM with excellent linearity (r2=0.99) are achieved and yet sensor is in a small form factor (4×4×3 cm). Many efforts have been being made to improve the sensor reliability and to realize stand-alone field measurements. Reference electrode was treated with polyurethane coating. An automated sample-handling and sensor calibration system was studied. A miniaturized potentiostat with built-in pumps and valves controller have also been made. In order to overcome short life-time of the thin-film sensing electrode, a palm-sized sensor that consists of Plexiglass housing, macro-scale electrodes, and microfluidic channels have been newly developed. The interference from common ions in groundwater was characterized and major interferents are Sr+, Ca2+, and PO¬43-. An ion specific membrane is employed in the sensor to minimize interference, and selectivity of the sensor is currently being analyzed.

An electrochemical system that consists of a silver electrode in 0.01 M sodium hydroxide electrolyte was investigated in an effort to develop a sensitive in situ analytical method for nitrate. Cyclic voltammetry demonstrated that the proposed system has a high normalized sensitivity (2.47 A s^½ V^−½ M⁻¹ cm⁻²), compared to more complex electroanalytical schemes. Double- potential-step chronocoulometry was used to maximize the signal-to-noise ratio (SNR), and minimize interference from dissolved oxygen in the electrolyte. The integration period for double- potential-step chronocoulometry was determined by optimizing the extended Cottrell equation. The integrated current is proportional to nitrate up to 10 mM and the average detection limit is approximately 1.7 µM. Dissolved oxygen does not degrade performance. To examine the potential interference of other ions when analyzing nitrate, we measured the electrode response to 1000 µM each of NO₂⁻, Cl⁻, PO₄³⁻, SO₄²⁻, F⁻, CO₃²⁻, BO₂⁻, K⁺, Ca²⁺, and Sr²⁺ with and without 1000 µM nitrate. Interference is negligible for most of the ions when nitrate is absent (i.e., 1% of the response to equimolar nitrate). However, interference is substantial (20% increase or decrease in the electrode response to nitrate) for PO₄³⁻, Ca²⁺, and Sr²⁺ when equimolar nitrate is present.

Inorganic nitrogen (nitrate, NO3-) is a major source of pollution in groundwater, surface water and the air. Application of nitrate-containing fertilizers can create distributed or non-point source pollution problems. Scalable nitrate sensors (sensors which are small and inexpensive) would enable us to better assess non-point source pollution processes in agronomic soils, groundwater and rivers. Sensor research groups in the CENS have been working toward high-performance scalable nitrate sensors using (1) potentiometric, (2) amperometric method, and the recent addition is (3) spectrochemical sensor. 1. Potentiometric Nitrate Sensor. This work describes the fabrication and testing of inexpensive PVC-membrane-based ion selective electrodes (ISEs) for monitoring nitrate levels in soil water environments. Over the past year, we emphasized testing of the in situ behavior of fabricated sensors in soils subject to irrigation with dairy manure water. Observed temporal responses for the nitrate sensors exhibit diurnal cycling with elevated nitrate levels at night and depressed levels during the day; these cycles are prominent even after rigorous temperature corrections. We have thus far concluded that while modern ISEs are not yet ready for long-term, unattended deployment, short-term installations (on the order of 2 to 4 days) are viable and may provide semi-quantitative insights into nitrogen dynamics in complex soil systems. 2. Amperometric Nitrate Sensor. A simple and sensitive amperometric nitrate sensor is microfabricated on the silicon chip and uses a nitrate-sensitive silver electrode. The concentration is determined by double-potential-step chronocoulometry because of high SNR and rejection of oxygen background current. The limit of detection ranges from 4 to 75 µM, and the upper limit of the linear range varies between 500 and 2000 µM. HPO42-/PO34- , Ca2+, and Sr2+ show significant interference (> 20 % signal distortion). We proposed a compact sample-preparation system based on Donnan dialysis to minimize the interference. The novel dialyzer uses small-bore tubes in containing solutions and a peristaltic pump in circulating the solutions to promote agitation. By using an anion-exchange membrane (AEM), the dialyzer removes cations completely and reduces interfering anions significantly from groundwater samples. Major advantages of the novel dialyzer are (1) high dialysis efficiency (~ 90 %), (2) linear response, and (3) reasonable throughput (1 samples/hr).A numerical analysis based on the one-dimensional bi-ionic-system-in-series model provides a good prediction of dialyzer performance. 3. Spectrochemical Nitrate Sensor. A multiplexible, miniaturized, and spectrochemical sensor is proposed because optical methods usually provide superior long-term reliability. UV light is absorbed by nitrate while propagating through groundwater sample in a liquid-core capillary waveguide, and absorbance is analyzed to quantify nitrate using a fiber-optic spectrometer. Using fiber-optic cables and optical multiplexers, multiple sensors can be operated remotely by single spectrophotometer and light source. Multivariate analysis is used to compensate these interferences from ionic and neutral species and baseline from dissolved organic species in groundwater, and calculate nitrate concentration precisely. A series of numerical analyses were performed to assess effects of spectrometric noises and interferences. According to the analyses, a large sharp Gaussian noise with a peak located near the peak of nitrate spectra will generate a significant error. Using a bench-top spectrometer, nitrate spectra in wide concentration range (10-7 to 1-4 M) were recorded to obtain concentration. It is observed that spectra were linear over the range.

Although many sensors (e.g., temperature, light level, acceleration, etc.) that are compatible with sensor networks (i.e., sensitive, small, low power, etc.) are now commercially available, two important classes of sensors are not as technologically mature and remain an area of active research: chemical sensors and biological sensors. The sensor-technology-development efforts in the CENS center are focused on these very challenging classes of sensors. Successful development of chemical and biological sensors will enable wireless-sensor-network technology to span the full range of possible classes of measurements. In addition to better performance, the technological emphases are miniaturization and automation of the developed sensors. Specific sensor-technology-development efforts include: (1) amperometric and potentiometric electrochemical sensors for monitoring nitrate-ion detection in ground water; (2) lab-on-a-chip aquatic microorganism analysis system; and (3) ultra sensitive field operational sensor for marine environmental monitoring.