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Published: Kem. Ind. 51 (11) (2002) 465–480
Paper reference number: KUI-45/2001
Paper type: Review

Optical Sensors in Environmental Analysis

S. Miljanić


Rapid population growth combined with industrialization, urbanization and agricultural intensification,has resulted in an increased consumption of natural resources and energy, massive waste production,and steadily growing environmental pollution. Availability and use of chemicals, exposure to pesticides, heavy metals and small particulates, emission of hazardous substances in the air, soil and water, pose an increasing threat to the health of humans and their environment. Owing to governmental and public concern over the extent of environmental pollution, considerable effort has recently been directed towards developing advanced sensing instrumentation capable of continuous in situ and real-time monitoring. A chemical sensor is a small analytical device, which can generate a measurable signal on the basis of a certain chemical characteristic of an analyte (Fig. 1). Optical chemical sensors (optodes) are used with optical methods of chemical analysis, mainly spectroscopy in UV/VIS and IR regions, measuring changes in optical property of an analyte or indicator as a function of analyte concentration. Optodes are usually coupled to optical fibers, which on the basis of total internal reflection allow the transmission of light over large distances, typically from 10 to 1000 m (Fig. 3).2, 3 There is a diversity of configurations of fiber optic chemical sensors, which differ depending on the arrangement of sensing material and of optical fibers (Fig. 4, 5, 6).4, 5 The ion-selective compound, ionophore, and the proton-selective indicator, chromoionophore, immobilized in plasticized poly(vinyl chloride), are frequently employed in the fabrication of optodes for water monitoring. Based on the cation exchange mechanism between the polymer membrane and aqueous solution (Fig. 7), optodes for sensing lead,8 silver,9,10 mercury10 and uranyl cations11 have been developed. Depending on the basicity of the chromoionophore and on the pH of the sample solution, reversibly working optodes show different dynamic ranges and detection limits (Fig. 10). Good sensitivity and selectivity towards lead in river water have been ascribed to an optical sensor with ionophore ETH 5435 and chromoionophore ETH 5418 (Tables 1, 2). Optodes for uranium determination in the nuclear fuel cycle are based on the fluorescence of uranyl cation in the presence of phosphoric acid. In a setup with a chemically passive optode12, optical fibers transmit light to and from the cell in which phosphoric acid has been manually added to a sample (Fig.13, 14). For remote monitoring a flow optode13 has been designed. UO22+ions diffuse through a Nafion membrane into a reaction/analysis chamber, filled up with H3PO4 (Fig. 15, 16). This setup allows environmental monitoring of uranium near a nuclear waste disposal site over concentration range of 10-4 - 10-9 mol dm-3.An irreversible chemical reaction between chlorinated hydrocarbons and basic pyridine is a response mechanism of an optical sensor for chloroform and trichloroethylene determination.14 The reaction products in a semipermeable tube are detected by their absorption at 555 nm (Fig. 17) and can be monitored remotely with optical fibers (Fig. 18, 19). On this working principle a portable optical chemical sensor for sensing ppb levels of trichlorethylene, carbon tetrachloride and chloroform in water, soil and gaseous samples has been designed.15 For seawater monitoring a fiber optic evanescent wave sensor operating in the range 3-20 mm, been developed. 16 The evanescent field, formed by interaction of the incident and reflected light in zinc selenide crystal, penetrates into a polymer medium where absorption by hydrocarbons, extracted from the seawater,occurs. The molecule-specific absorption pattern in the fingerprint region allows the simultaneous determination of chemically similar substances down to the low ppb concentrations (Fig. 22, 23). Seawater salinity can be determined using an optode based on a chloride-quenchable fluorescent indicator immobilized on a Nafion film (Fig. 24).17 By addition of an inert phosphorescent reference luminophore into a film, the fluorescence intensity information is converted into phase angle information (Fig.25). The sensor has been successfully used to determine the salinity in seawater and brackish water of the North Sea (Fig. 26). Carbon dioxide in marine and freshwater sediments is determined by a pCO2 fiber optic microsensor.18 The sensor is based on a change of fluorescence of a pH dependent fluorophore built in ethyl cellulose, covered with a gas permeable Teflon layer (Fig. 27). A significant increase in pCO2 has been found in the upper 5 mm of the sediment, caused by respiration of the biological system (Fig. 28). In summary, optical sensors offer a variety of new aspects and possibilities of sensing numerous species using well known spectrophotometric and spectrofluorimetric methods, with newly synthesized and appropriately immobilized chromoionophores and with advanced fiber optic techniques.

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environmental analysis, optical sensor, optical fiber, determination of metal ions, determination of chlorinated hydrocarbons, monitoring of seawater salinity, sensing of CO2 in sediment