The established methods for the detection of pollutants in waters are based on sampling and analysis of discrete water samples. The analysis is performed in laboratories located remotely away form the sampling sites and frequently the chemical analysis is carried out with expensive apparatus, such as bench-top spectrometers and chromatographs. The issue with these methods is that, although they function well and provide reliable chemical data, they are laboratory-based, personnel-dependent, time-consuming and expensive. However an emerging research trend in the last 1-2 decades has been the growing interest in research in chemical sensors and biosensors which can meet the monitoring needs of those interest in polluted water analysis.
Oxygen levels can be a simple, non-specific indication of biodegradable pollutant levels. The best-known oxygen sensor is based on the amperometric detection by electrochemical reduction of oxygen at a platinum, gold or silver cathode (the so-called Clark electrode). This sensor or probe is a complete electrochemical cell consisting of cathode, anode (usually a silver reference electrode), electrolyte solution and gas-permeable membrane. The current flow in the electrochemical cell due to the reaction
O2 + 2H + 2e- > H2O2
is proportional to the concentration of oxygen which permeates the membrane and diffuses through the electrolyte solution to the cathode. Recent advances in this approach have included the use of thick film (screen-printing) fabrication and photolithographic microfabrication technologies for preparation of low-cost or micromachined probes.
Various approaches to miniaturised oxygen sensors have been examined, such as by Atkinson et al (University of Southampton) using screen-printed electrodes for amperometric determination of dissolved oxygen. This work included development of a 200 Âµm-square gold electrodes with a silver-silver chloride reference on an alumina substrate; both the potentiostat (to control the potential applied) and a signal generator were also fabricated on-chip enabling the complete system to be packaged in a highly-portable housing with a small battery as power source. This thick-film electrochemical instrument was evaluated for dissolved oxygen detection. Thick-film (screen-printing) technology is a rapid and low-cost method for the preparation of sensors. The low-cost of the fabrication enables devices to be used as “one-offs” and hence disposable, or at least non re-usable by design. This would be a disadvantage for a continuous (or semi-continuous) remote autonomous water monitoring system. Another disadvantage is in size of electrode devices that can be achieved. Photolithographically-fabricated electrodes can reach smaller dimensions and Tyndall National Institute has been active in this area, initially looking at metals detection in environmental samples. Wittkampf et al developed a three-electrode Clarke-type oxygen sensor compatible with CMOS technology. The working electrode (cathode) was actually an array of microelectrodes, meaning that a greater signal can be obtained than by using a single microelectrode. On top of the microfabricated electrodes, a gel electrolyte within a calcium alginate layer was constructed and the oxygen-permeable membrane was then fabricated on top of this. This latter membrane consisted of a room-temperature vulcanising silicone rubber. This system could operate continuously for 12 hours with no signal decay. Optical approaches to oxygen detection are also of interest. Various optical transduction mode (fluorescence intensity quenching, phase fluorometry, room temperature phosphorescence) were investigated. Many such sensors have been studied in the lab and shown to be stable and to be operable in complex multicomponent liquids (wastewaters, biological cell growth media)
Nutrient sensors have been the subject of some interest. A number of publications have reported the development and in situ evaluation of nitrate selective electrodes. These electrode devices are traditional membrane sensor electrodes but the novelty is in the ion-recognition component used and in particular its covalent attachment to the polymer membrane support thus ensuring adequate lifetime. On-site continuous measurement of nitrate in a river was performed automatically for two months using the developed ion-selective electrode. A non-submersible electrode support system was developed and a field instrument was constructed comprising the nitrate electrode, a reference electrode and a temperature probe connected through a pre-amplifier to a data-logger and battery supply. The nitrate-ISE was constructed using a commercially available electrode body, with a membrane incorporated into the tip. This rubber membrane contained the sensor molecule covalently bound to polystyrene-block-polybutadiene-block-polystyrene. A temperature correction algorithm was also used to accommodate the temperature changes encountered in the river. The river nitrate results obtained with the sensors at hourly intervals compared very favourably with those obtained with laboratory automated chemical determinations made on contemporaneous river samples. The sensors did not require re-calibration and no deterioration in performance or fouling of the membrane surface was observed during 40 days.
Unisense offers an on-line nitrate sensor operating in a wide variety of matrices. This is a biosensor employing nitrate-reducing bacteria to produce nitrogen oxide which is then detected amperometrically as the same sort of device as used for DO detection. Its biological element content makes its use for long term remote deployment untenable. Phosphate determinations by selective electrodes are also possible. For example, a cobalt-wire electrode was developed as a phosphate-selective sensor for monitoring of hydroponic solutions; interferences from chloride, nitrate and sulphate were negligible. Diamond has investigated the determination of phosphate in rivers in an analytical microfuildic system and sought to demonstrate stability of the chemical reagents needed over a 12-month period, as part of development of autonomous sensors for environmental monitoring. Oil sensors would obviously be of interest for the direct detection in seawater of oil spills/leaks from ships. However, optical methods based on surface scanning from an aircraft, are the nearest and a recent review deals with lab-based methods for the detection of organic substances in sea water, meaning that the area of sensing of oil in seawater is wide open for exploration of new sensing methods and technologies.
TNI-UCC will explore and develop novel chemical sensor subsystems which can be integrated with the overall robot concept being developed. Thus miniaturised sensors and sensor arrays as well as novel membrane strategies for provision of chemical sensitivity and anti-fouling behaviour will be examined. In particular the group will examine:
- The development of characterisation of interdigital microelectrode array sensors-on-chips for the measurement of electrical properties of chemically-selective membranes (e.g. membrane conductivity changing with partitioning of pollutant such as nitrate in to the membrane bulk phase);
- The development of integrated microelectrode array amperometric sensors for dissolved oxygen measurements
- The development of specialised signal and data processing algorithms capable of delivering output data in accordance with end-user requirements
- The development of miniaturised and integrateable instrumentation (hardware and software) for use with the developed sensor systems
- An assessment of optical or electrochemical transduction methods compatible with integrated sensors and instrumentation for oil on water detection.
The above investigations will provide the consortium with prototype devices designed specially to be integrated within a robotic autonomous system and to be capable of measuring sea water chemical parameters directly in situ. Given that the state of the art is all lab-based methods, the proposed suggestions will go far beyond the state of the art by implementing these lab-based methods in situ on board the robotic fish.