Overview

Small, low-cost, robust, reliable, and sensitive sensors are needed to enable the realization of practical and economical sensor networks. The strategy of the sensor group is to work on application-driven and commercially unavailable sensors. In addition to better performance, the technological emphases are miniaturization and automation (ease of use) of the developed sensors.

Sensor-network applications of interest call for different types of sensors (e.g., seismic, temperature, light, sound, magnetic, chemical, etc.). Although many sensors are commercially available (e.g., seismic, meteorological, acoustic), small, inexpensive, and high-performance sensors for other critical measurands are not (e.g., chemical and biological). Thus, the sensor group will only emphasize important CENS sensors and sensor-related technologies currently not available from the market.  For example, for the last three years, we have focused on developing sensors for soil. These applications require an array of miniaturized chemical sensors to accurately monitor the flow of contaminants in environmental media. Nitrate was identified as a critical molecule to detect in a number of different applications, and so we have developed several novel technologies that cover a broad range of nitrate sensitivity from ~ppm to ppb.  Two of these are ready to be packaged for field deployment: a potentiometric sensor that uses nitrate-doped electropolymerized pyrrole on carbon fibers, and a micro-machined amperometric nitrate sensor with partially integrated and assembled microfluidics. Using prototypical PPy-coated microfibers, we have directly measured nitrate concentrations in residual soil water found in soil previously irrigated with recycled wastewater (the first known direct assessment of nitrate concentrations in residual soil moisture of its kind) and have observed local nitrate concentrations in surface sands at the Palmdale experimental irrigation site.  In a laboratory setting, we observed spatial variability on a scale of cm, which has large implications with respect to mapping soil nitrogen biogeochemical cycling activity in the subsurface.  The micro-machined amperometric nitrate sensor has a detection limit of 1 μM with good linearity over several orders of magnitude. To improve stability, Polyurethane-Coated Ag/AgCl reference electrodes are used. Other CENS nitrate-sensor technologies include a High-Performance Liquid Chromatography (HPLC) sensor, one of the most powerful, versatile, and widely used techniques for chemical analysis; and an entirely novel sensor, based on the propagation of surface plasmon waves through periodic nanostructures, is under development.  In the second phase, we plan to improve, deploy, and diversify our sensor technologies for CENS-related applications.

Looking forward, we have also started a new initiative to develop lab-on-a-chip biotechnology. The development and deployment of biologically relevant sensors remain fundamental hurdles when constructing observing systems for environmental research.  Our research goal is to develop chip-based sensors for the identification and enumeration of a variety of aquatic microbes (viruses, bacteria, algae, protozoa), photopigment analyses to obtain information on major classes of algal taxa, and the measurement of specific chemical constituents of ecological or societal importance (e.g., algal neurotoxins and carcinogens). Flow cytometry and HPLC have become fundamental tools of aquatic ecologists, but their usefulness has been limited by an inability to embed these large, power-hungry instruments into ecosystems. HPLC-on-a-chip and flow cytometry-on-a-chip constitute a new generation of technologies that will tremendously expand our ability to make observations and test hypotheses relating to microbial distribution, abundance and activity in aquatic ecosystems. The main challenge resides in the integration of various functions on-a-chip including microfluidics, sample collection, sample preparation, sensing and control.  The payoff will be low power, small volume, high sensitivity and, most importantly, the ability to deploy these sensors in the environment, making a significant contribution to marine and freshwater observing systems now coming on line nationally and internationally (e.g. Global Ocean Observing System).

Illustration of a chemical sensor network used to monitor aqueous chemical contaminants
in the soil and ground water.

Approaches

The overall sensor research in CENS can be categorized into 6 efforts. Two efforts are focused on developing miniaturized and selective electrochemical sensors and one effort is focused on developing a surface-plasmon band gap (SPRBG) sensor. The development of the electrochemical and SPRBG sensors are near-term projects but will be highly tailored for nitrate detection. In addition, there are two new initiatives focused on sample preparation technology which can serve general purposes of sample collection for nitrate sensor analysis.  One effort is to develop a compact low power sample preparation system.  The rest two efforts are to develop a lab-on-a-chip aquatic microorganism analysis system and new technology for marine microorganism detection and identification.