In this article we present some examples illustrating advances in optical sensing technology, with regards to its application to water quality monitoring, that are being made in the BEST Centre at DCU. The development of optical chemical sensors is essentially multi-disciplinary in nature, requiring inputs from analytical chemistry, material science, optoelectronics and even mechanical engineering. Optical Chemical sensing technology has available to it a multiplicity of techniques already developed for routine chemical analysis. In addition, new analytical methods based on novel spectroscopic techniques and advances in molecular recognition are emerging all the time. The application of optical techniques to pollutant detection in a q u e o u s environments is driven not only by the large potential demand for water m o n i t o r i n g devices, but also by the possibility of remote sensor operation afforded by the utilization of optical fibres. Real time monitoring from portable test units that can be left in situ for long periods of time is becoming an achievable goal, which will reduce the large laboratory costs involved in current methods of analysis. An increasing number of water quality parameters are being investigated using optical chemical sensors in both marine and potable waters, and this article is intended as an overview of activity in the BEST Centre at DCU within this field. 

Sensor Design

The construction of an optical chemical sensor brings together three distinct areas of interest: 

Optoelectronics

Apart from the sensor chemistry, the principal components of an optical chemical sensor are from the area of optoelectronics (e.g. optical sources, detectors, spectral selection (filters / spectrometers), optical fibres / waveguides, optical materials signal processing). Because of the major expansion of optoelectronics in areas such as information technology and telecommunications, significant developments in all the above areas are continually emerging and are having a significant impact on optical chemical sensors. 

Sensor Chemistry

With reference to sensor chemistry, the critical issues are selectivity, stability and reversibility. Selective sensors display minimal interference from other species, which are likely to be found in the measurement environment. When reagents are used they should exhibit long-term stability and the sensing reactions should ideally be reversible. Advances in molecular recognition (both chemical and biological) are emerging all the time and some of these are likely to have an impact on chemical sensor development. 

Reagent Immobilization

The support matrix for reagent-mediated sensors should be stable in a wide range of environments and should retain the sensor reagent effectively. Furthermore, the matrix should have a high permeability for the analyte so that the response time of the sensor is not compromised. The typical materials used are polymers and sol-gel derived porous glasses 

Prototype Devices

Multi-parameter water monitor

Many methods for the determination of water quality parameters rely on standard colourimetric tests. The optical sources used in these tests are usually incandescent filament bulbs combined with interference filters. Such sources suffer from a number of problems, which can degrade performance over time. Utilization of high brightness light emitting diodes (LED’S) offers the opportunity both to enable overcome these problems and the miniaturization of the device. LED’s are inexpensive, have relatively narrow bandwidths and consequently may not require the use of filters. Under normal operating conditions they generate no electrical heating and their output is stable and easily modulated. Hence they are suited to on-line operation and their small size and low power consumption makes them ideal sources for miniature portable systems.

A LED-based portable sensor system capable of using standard colourimetric tests to monitor a wide range of water quality parameters has been developed at the BEST Centre. The system employs a multipurpose flow cell, which can provide a range of path lengths. An array of LED sources, as required by the v a r i o u s colourimetric tests, enables measurements to be made of aluminium, iron, manganese and phosphate. The system also measures sample of how taking advantage of recent advances inoptoelectronics can greatly improve on what is seen as a standard device. 

LED-based dissolved oxygen (DO) sensor

Optical oxygen sensors are more attractive than conventional amperometric devices because they do not consume oxygen, and are not easily poisoned. Sensor operation is usually based on the quenching of fluorescence in the presence of oxygen. The fluorescent complex, tris-(4, 7 – dipheny1-10, phenanthroline) ruthenium (II) chloride, is widely used as an oxygen-sensitive indicator because of its specific sensitivity to oxygen, large Stokes shift, and strong absorption in the blue/green region of the spectrum (compatible with blue LED’s). Oxygen sensors based on the immobilization of this ruthenium complex in nanoporous silica films prepared via the sol-gel process have been developed in the BEST Centre at DCU and the work is summarized here. 

The sol-gel process is a low t e m p e r a t u r e f a b r i c a t i o n technique, which can be used to produce high quality thin films in which analyte sensitive dyes are immobilized. The dye molecules are entrapped in the nanometre-scale cages formed by the cross-linking silicon and oxygen units. Smaller analyte molecules can permeate the matrix and access the dye complex in the pores. By selecting fabrication parameters appropriate to the size of the dopant molecule, leaching is negligible.

The configuration of the sensor is shown in figure 1. The ruthenium complex is excited using a high-brightness blue LED whose spectral output peaks at 450nm. Light passes through a wide-band pass filter before impinging on the coated substrate, which is held at 45 degrees to the excitation beam in the sample chamber. The fluorescence from the coated substrate passes through a long-pass filter and is focused onto a low-cost silicon photodiode detector. This all solid-state system serves not only to provide a high signal to noise ratio (see figure 2), but also allows for miniaturization and facilitates portability. A limit of detection of 10 ppb is routinely achievable with this system. By virtue of the dynamics of the fluorescence quenching process the sensor also has the benefits of complete reversibility and increased sensitivity at lower concentrations.

Sol-gel pH sensor using CCD-array spectrometer

We have constructed an extended range optical pH sensor based on two pH indicator dyes co-immobilized in a sol-gel-derived film. The film is coated on an optical fibre core so that the pH dependent colour changes of the film can be monitored by evanescent wave absorption. The two indicators used were bromophenol blue and bromocresol purple which, when combined, provide a pH 10. Using a tungsten halogen source the absorbance spectra of the coated fibre in a range of pH buffers were recorded by the Ocean Optics compact spectrometer (see figure 3). The system software can be configured to process the measured data in a number of ways. Such a system is generic in that it enables sensor characterization over a broad spectral range and applies to any absorption-based indicator operating in that range.

Infrared sensor for chlorinated hydrocarbons and pesticides

A fibre optic sensor for the in-situ detection of chlorinated hydrocarbons (CHC’s) and some pesticides in water has been developed at DCU. The sensing element consists of a sliver halide optical fibre coated with an appropriate polymer cladding (polyisobutylene or Teflon). The polymer both enriches the chemical species to be measured in the evanescent wave region of the fibre and minimizes water I n t e r f e r e n c e. Evanescent wave spectrometry is then used to quantify the concentration of the enriched species, such as CHC’s which have their s t r o n g e s t absorption bands at wavelengths above 10um where sliver halide fibres transmit. The desirable properties for the polymer are high enrichment for a large range of analytes, hydrophobicity, fast response time, good reversibility and minimum number of absorption features in the spectral region of interest. 

Prospects

It is clear that water pollution monitoring instrumentation can exploit many of the advances emerging from the rapidly expanding optoelectronics industry. These advances are already making an impact and projects are underway at the BEST centre to further develop our prototype sensors for commercial e x p l o i t a t i o n. N e v e r t h e l e s s, S i g n i f i c a n t technical barriers remain to be overcome before this technology can yield a wide range of reliable sensors.  

The principal areas of concern are:

Stability: in terms of calibration and materials .

Molecular recognition : this is case specific but the key issues are selectivity and reversibility.

Fouling: biofilm formation and turbidity cause problems for most optical sensors in field operation.

It is expected, however, that these problems can be overcome in many instances by a combination of good science and engineering.