Remote underwater electrochemical sensing system for detecting explosive residues in the field

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Abstract

A mobile remote underwater electrochemical sensing system is described in this paper for detecting the explosive 2,4,6-trinitrotoluene (TNT) in the marine environment. The detection technology relies on using a carbon fiber as the working electrode, and applying a rapid and sensitive square-wave voltammogram for scanning. This submersible, pulsed-voltammetric, 3-electrode electrochemical sensor is mounted to a remotely operated surface vehicle with vision detection capability. The use of vision enhances the sensor package by allowing detection of and placement below detection limits. A wireless communication unit provides the ability to monitor and detect in real time. The detection scheme is based upon a stepwise reduction of the nitro aromatic groups on the explosive molecule first to hydroxylamines, and then followed by the conversion of the later to amine groups. The reduction potentials provide the method selectivity for a specific explosive, and the current required per unit time determines the concentration of the targeted explosive in an aqueous media. Successful field test data have been obtained which are comparable to those measured with laboratory instruments.

Introduction

The presence of explosive residues in water, mostly caused by the use, and disposal of explosive substances for the past 100 years both as expended ammunition and as waste from the manufacture of military ordinance [1], represents a significant environmental problem. Because of the toxicity and the toxicity of its decomposition products, 2,4,6-trinitrotoluene (TNT) among the explosive residues is of great concern. Also, the increasing anti-terrorism surveillance activity has put the detection of TNT in marine environment high priority. Based on the requests from both environmental and security applications, a fast, simple, sensitive, reliable and cost-effective in-situ remote sensing system for detecting TNT in the field (both in marine environment and other natural water) is in highly demand [2]. A recent electrochemical flow system for on-line voltammetric monitoring of trace TNT has been reported [2]. Other emerging detection technologies including the development of immunochemical based optical sensors [3], [4], and the application of amplifying fluorescent polymer (AFP), for long-term monitoring for explosive contaminated ground water [5].

This article describes a self-contained electrochemical detection system for remote sensing of trace TNT in the field. A submersible square wave potentiostat and a 3-electrode electrochemical sensor is mounted to a remotely controlled surface vehicle which also contains a vision detection capability, an on board wireless communication unit enables remote monitoring of trace TNT in seawater from on shore. The success of this submersible probe for continuous monitoring of TNT stems from a judicious choice of the carbon fiber working microelectrode, the use of rapid square-wave scanning voltammetry, and the low potential selective cathodic detection of this explosive [6].

Section snippets

Construction of remote sensing system

All experiments were performed with a PalmSens (Palm Instruments BV, Electrochemical Sensor Interfaces, Netherlands), which is a hand-held battery powered potentiostat instrument for use with electrochemical sensors or electrochemical cells. The PalmSens is connected to a Pocket PC (Compaq, USA) to specify the parameters of the measurement, to display the measured curves and calculate the results of the measurements.

The remote sensing system consists of three major components. (I) The remote

Oxidation–reduction chemistry of explosive compounds

Most explosives can be classified into one of several groups represented by nitro compounds, nitric acid esters, nitramines, salts of perchloric, nitric, and chloric acids, azides and other miscellaneous compounds capable of producing an explosion, and mixtures of explosives from the above groups [7]. Selective representatives of common commercial and military explosive compounds along with their common abbreviations and molecular structures are listed in Table 1.

Square-wave voltammetry can be

Conclusions

The above experiments have illustrated that an integrated TNT sensor can be practically realized and configured with guided vehicle technology for remote sensor access and control. This mobile, intelligent sensor platform permits an “operator in the loop”, which facilitates real time decisions during chemical survey operations by having an analyst perform the signal processing yielding maximum adaptive sensing. The construction of a remote explosives monitoring unit using a recently designed

Acknowledgements

This work was supported by the Navy (Grant number N00014-03-1-0480), and US Army Space & Missile Defense Command (Grant number DASG60-00-C-0089). The authors are grateful to Dr. Eric Steimle and George Steimle from ENG Concept, Inc., for their contribution on completing this project, to Chad Lembke for providing the vivid 3D drawings, to Joe Kolesar for his elegant design for making a successful sensor. Technical support from Charles Jones, Mike Kerr, useful discussion with Arben Merkoci and

Xiaojuan Fu is a Chemical Sensor Development Engineer in the Center for Ocean Technology, College of Marine Science, University of South Florida. She received her Ph.D. in Chemical Engineering in December 2003 from University of South Florida. Her research activity is the development of electrochemical sensor for explosive residue detection (both gas and aqueous phase), hypergolic fuel leaking detection and other environmental application; development of optical sensor for identifying and

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Xiaojuan Fu is a Chemical Sensor Development Engineer in the Center for Ocean Technology, College of Marine Science, University of South Florida. She received her Ph.D. in Chemical Engineering in December 2003 from University of South Florida. Her research activity is the development of electrochemical sensor for explosive residue detection (both gas and aqueous phase), hypergolic fuel leaking detection and other environmental application; development of optical sensor for identifying and detection for microorganisms in water, and microtechnology for senor application.

Robert Benson is a Chemical Sensor Development Engineer in the Center for Ocean Technology, College of Marine Science, University of South Florida. He received his Ph.D. in Physical Chemistry from the University of Arkansas in 1978. He earned a Master of Science in Chemistry from the University of South Florida and a Bachelor of Science in Chemistry from the University of Florida. He has worked in industry at the Texaco Port Arthur Research Laboratory, Reynolds Metals Alumina Research Laboratory, Jim Walter Research Corp. and Honeywell Military Avionics Division.

Joseph Wang is a Regents Professor of Chemistry at New Mexico State University (NMSU). He holds a Manasse Chair at NMSU and serves also as the Chief Editor of Electroanalysis. Dr. Wang obtained his higher education at the Israel Institute of Technology (Haifa), being awarded his D.Sc. in 1978. The research interests of Dr. Wang include the development of microfluidic (‘Lab-on-Chip’) devices, DNA recognition and diagnostics, electrochemical sensing devices for environmental, security, and clinical monitoring, microfabrication and miniaturization, counterterrorism-detection, nanomaterials-based sensors, the development and characterization of new surfaces for electroanalysis, sensor/recognition coatings, remote sensing, the development of techniques for ultratrace measurements and the design of on-line flow detectors.

David Fries is a Senior Developmental Engineer in the College of Marine Science at the University of South Florida. His primary research interest is the development of microsystems and microtechnologies for sensor applications, advanced sensor development (e.g. inline mass spectrometers and molecular probes technologies) and mobile robotic systems for field applications. He has a BSc in Chemistry from the University of Pittsburgh, and Masters in Chemistry from the University of South Florida. He was previously a Staff Scientist in the Specialty Components Division of Sandia National Labs/Lockheed Martin Corp., specializing in instrumentation development for process analytical chemistry.

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