Overview

During this past year, the sensors research group made several significant advances in sensor development, including (1) Amperometric Nitrate Sensors; (2) Potentiometric Nitrate Sensors; (3) Aquatic Microorganism Analysis System; and (4) Ultra Sensitive Laser Induced Fluorescence Sensors. In future work, we will emphasize making sensors suitable for rugged field use. Indeed, we have already begun by packaging sensors for field use, and designing and fabricating a common electronic sensor-interface board that can link the packaged sensors with a wireless network. As the sensor group continues to improve and advance its sensor technology, it is also aggressively moving toward system integration for real field use.

Amperometric nitrate sensors
Earlier, we tested the newly designed nitrate sensor chip calibration curve and performed quantification of nitrate sensor selectivity to interfering ions.  This year, we characterized ionic interference with the sensor and made the system more reliable.  After testing six ionic species [nitrite (NO₂-), fluoride (F-), chloride (Cl-), sulfate (SO₄²-), phosphate (PO₄³-), and potassium (K+)], we found that interference is, in general, minimal (i.e., less than 1% of response relative to 1000 μM nitrate). The largest interference signal came from PO₄³- (equivalent to 6.9 μM nitrate) but even this is relatively small. The signals from SO₄²-, F-, CO₃²-, BO₂-, K+, and Sr²+ were smaller than the average nitrate detection limit. Two significant roadblocks to field deployment of our sensors are the short working life of electrodes and the drift of reference electrodes. To alleviate these problems we modified our nitrate sensing system. The system consists of sensor housing, miniature peristaltic pumps, calibration standards, valve manifolds with control circuitry, data acquisition board, and LABVIEW software. The calibration curve was obtained automatically with the setup, the detection limit was 1.2 μM, and the response was linear up to 1000 μM (r²=0.999).

Potentiometric nitrate sensor
The main accomplishments in this project include (1) successful field-testing of the potentiometric nitrate microsensor in field deployments in Merced, Palmdale, and the James Reserve and (2) developing the bi-layer approach to protecting the polypyrrole membrane of these sensors in an effort to extend their longevity.  In field deployments prototype sensors were buried in the soil adjacent to commercial nitrate sensors, as shown in figure 3.  Typical results for the Palmdale test are plotted in figure 4.  To address the longevity problem, we have created double-layer deposits, where an insulating layer of bis-3.4-ethylenedi-oxyhtiophene (bis-EDOT) is deposited atop the PPy surface in an effort to retain the dopant nitrate anions, which may be lost in flowing water.  The resulting performance in terms of calibration with the double-layer added was found to be as good as or better than the microsensors fabricated with a single layer of PPy.

Lab-on-Chip aquatic microorganism analysis
This project addresses needs in marine biology research using chip-based technology, which reduces the total sample needed and the detection time.  Our project is organized into two main research areas: (1) a chip to monitor the content of the sea water, and (2) a chip that can culture algae under combination of different conditions to screen for factors inducing toxin production.  Earlier we built a chip to separate particles based on size.  This year, we fabricated an impedance sensor chip that counts particles and made a cell culture chip with integrated combinatorial mixer, which can take in three inputs and generate all the possible combinations of outputs.  We will start culturing algae using this chip and expose cells to combinations of different compounds.

Laser-induced fluorescence detection
This system is intended to detect domoic acid (DA), a toxic by product of harmful algal blooms, downstream of the algae culture chip described above. The ultra sensitive optical sensor is based on laser induced fluorescence combined with confocal microscopy and will detect concentrations down to fM in extremely small sample volumes.  In this method, molecules of interest are labeled with fluorophores and when each molecule passes through the detection volume a photon burst is observed. The number of peaks per unit time indicates the concentration of domoic acid in the sample. What’s more, any type of molecule tagged with fluorophores can be detected in this manner.