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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 highly sensitive and miniaturized amperometric nitrate sensor for groundwater monitoring was successfully designed, fabricated, and tested. The silver sensing electrodes were patterned on the silicon substrate with microfabrication techniques. The sensor was thoroughly characterized and the performance is very promising. A low detection limit of 4 ,#61549;M, wide dynamic range of 10000 ,#61549;M with excellent linearity (r2=0.99), and a good selectivity (13.9 % increase in signal for 400% increase of interfering ion concentration) were achieved and yet sensor is in a small form factor (4��×4��×3 cm without electronics). Many efforts have been being made to improve the sensor reliability and operational lifetime, such as including polymer-coated Ag/AgCl reference electrode. To enable real-time field measurements, a sample-preparation system, an automated sample-handing system, and a micropotentiostat with built-in pumps and valves controller have also been studied. In order to overcome limited life-time of the thin-film working electrode, a palm-sized sensor system that consists of macro-scale commercial electrodes, Plexiglass housing, built-in microfluidic channels, and anion-permeable membrane, have been developed.

Before the miniature nitrate sensors designed by the CENS Embedded Sensor Project can be integrated into a wireless sensor network, each sensor must first have a sample preparation system that filters the incoming ground water for substances that could damage the sensor or cause faulty results, while still preserving the substance to be measured. Therefore, we are designing a compact, low-power system to provide a continuous stream of segregated acceptable samples to the sensors to enable them to work in remote environments.

A nitrate-sensing system that consists of a micromachined sensing chip, anion-permeable membrane, and integrated microfluidic channels has been designed, fabricated, assembled, and tested. Our microsensor was designed for in-situ monitoring of nitrate concentrations in ground water. Cyclic voltammery of nitrate demonstrates high sensitivity of silver in 0.01 M NaOH electrolyte. Thus, a silver microelectrode 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 cell for the eluent (0.01 M NaOH) and ground-water sample. A polyimide insulation layer was deposited to protect lead-wires and electrodes from corrosion. The sensor also incorporates an anion-permeable membrane that is used for selective measurement.

A 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 an anion-permeable membrane that is used for selective measurement of nitrate. With standard addition methods, linear calibration curves (1NlM to 1 mM) are obtained for several sensing chips. We observed that the sensor output increased only 13.9 %, even though a 100NlM nitrate sample solution contains interfering ions of the same concentration (100NlM each of PO 4-2, SO4-3, F-, Cl-).

We have conducted feasibility studies based on chronocoulometry of nitrate for the purpose of fabricating a miniaturized sensor. Detection and monitoring of nitrate ion in surface water are important because of environmental and ecological influence of nitrate. For this application, sensors should be small, stand-alone, and have a low detection limit. Conventional nitrate sensing methods (spectroscopy, chromatography, electrophoresis, etc.) Do not meet aforementioned requirements because of the cumbersome sample preparation and complex implementation. Electrochemical detection is simple and a sensor can be easily miniaturized when using this method. The proposed electrochemical cell (silver working electrode, platinum counter electrode, ag/agcl reference electrode, and sodium hydroxide supporting electrolyte) shows high sensitivity for the nitrate ion detection. Oxygen dissolved in aqueous electrolyte interferes with the nitrate detection, but was successfully removed by a simple differential approach.