Total costs: FIM 369 000 Funding: Tekes FIM 369 000
Summary
The project was focused on the design and study of new solid-state luminescent materials to be employed for environmental analysis purposes. The research included the following main parts:
1. Design of new solid-state phosphorescent materials sensitive to SO2, O2, and NOx, and analytical systems based on these materials
2. Design of new materials for luminescence lifetime-based oxygen sensors;
3. Studies of the possibilities of electrogenerated chemiluminescent detection of long-lived luminescent dyes and its use for the development of trace analysis of environmental samples.
In the course of the 2-year study significant progress has been achieved. The main results obtained are summarised below.
1. Results
1.1 Optical system for sensing of SO2/sulfite using a phosphorescent probe
Covalent conjugates of water-soluble phosphorescent dye – platinum(II) complex of coproporphyrin I (PtCP) and bovine serum albumin (BSA) were synthesised, isolated in a pure form and their spectral-luminescent properties and quenching behaviour was investigated in aqueous solutions. In the presence of sulfite, strong dynamic quenching of phosphorescence of PtCP-BSA conjugates was observed in the acidic pH range. The quenching appears at pH below 4.0, reaches its maximal level at pH of about 1.5, with Stern-Volmer quenching constants being as high as 2000 M-1. Detailed investigation showed that the quenching is associated with the formation of SO2. Due to the exceptionally long lifetime of the phosphorescent probe, the efficiency of quenching by SO2 of the PtCP-BSA was much higher (10 times and more) than for other systems reported. The phosphorescence of PtCP-BSA was strong enough in the presence of ambient oxygen (oxygen is also strong quencher) which enabled to use it in air-saturated aqueous solutions, particularly for quantitation of SO2/sulfite. Only few other compounds, namely Fe(3+) and nitrite ions were found to cause quenching interferences for the SO2 probe based on the phosphorescent dye.
The above findings allowed creation of sensitive analytical system for quenched-phosphorescence detection of SO2/sulphite (Figure 1). A simple flow-injection system for the determinations of SO2/sulfite was developed based on the
PtCP-Figure 1. New FIA system and oxygen sensor set-up.
BSA conjugate immobilised on a preactivated microporous membrane (Biodyne ABC, Pall). Such a phosphorescent membrane was then placed in a flow-through cell in the stream of the acidic carrier buffer and linked to the luminescent detector (AB2 fluorometer, SLM-Aminco) by means of a bifurcated fibre-optic bundle to monitor continuously the phosphorescence intensity from the sensor. Such a solid-state phosphorescent probe allowed rapid detection of SO2/sulphite in aqueous samples with maximal sensitivity of about 10 •M (0.64 ppm) and analytical range up to 10 mM. Main characteristics and performance of the new analytical system for SO2 detection were evaluated. Typical sensor response curve and calibration graph are shown in Figure 2 and Table 1.
Figure 2. A typical sensor response curve and calibration graph.
Table 1. Main characteristics of the FIA sulphite sensor with PtCP-BSA on a Biodyne membrane as an active element.
Parameter Description
5-10 minutes (5 - 10 assays/hour) min. 4 weeks of continuous use 3 points daily
The new FIA system was tested with real samples and showed its usefulness, although turbid and coloured samples were found to have significant interfer-ence on the luminescinterfer-ence intensity signal from the sensor. Further development of the above system is envisioned which will include its integration with the phosphorescence phase detector. This will enable to realise luminescence life-time-based sensing approach which promises elimination of the above interfer-ences and significant improvement of overall sensor performance. The main results are under patenting process.
1. 2 New materials for luminescence lifetime based oxygen sensors: the use of microporous light-scattering support New materials for luminescence based oxygen sensors and technology of their preparation were worked out. In contrast to conventional technologies, which utilise flat transparent oxygen-impermeable support materials for sensor fabrica-tion, such as glass surface or polyester film, our approach was based on the use of microporous light-scattering supports such as membrane filters and fibrous structure depth filters.
Using the oxygen-sensitive polymer compositions with of platinum(II) complex of octaethylporphine-ketone (PtOEPK, phosphorescent dye), a number of
differ-ent oxygen-sensitive coatings and support materials were studied comparatively, so as to demonstrate the regularities of the new approach and its practical ad-vantages. It was demonstrated that the new technology and sensing materials display a number of advantages over conventional oxygen sensor membranes.
These advantages are:
• Highly scattering media of the microporous support provide a significant (8-10 times) increase of the luminescence intensity signal obtained from the sen-sor element achieved with the same amount of sensitive coating. This allows significant saving of the sensing materials, bringing down sensor photo-bleaching, improvement of the reliability of measurements and signal resolu-tion.
• Good mechanical protection of the oxygen-sensitive layer and of additional biolayer(s), such as enzyme or antibody coatings, from erosion by liquid flow and mechanical particles. Problem of efficient adhesion of the oxygen-sensitive layer and biolayer to the support material was essentially eliminated.
• The oxygen membrane with microporous support provides protection for the measurement system (luminescence detector) from background interferences by ambient light and sample.
• Simple and robust procedure of sensor fabrication in which the sensitive coating is applied very uniformly. This is defined by the uniform thickness of the microporous sorbents such as membranes, and their uniform capacity for liquids and in particular for the coating mixtures. Simple "drop technology"
can be efficiently used for membrane preparation, and thus there is no need of special casting equipment.
• With selecting the material of the microporous support, it is possible to preserve the sensitivity of the coating almost unchanged; therefore the new technology is applicable to a large variety of luminescent sensitive coatings, not only oxygen sensors.
• Despite of significant thickness of resulting sensor membranes, high surface area and 3D-microstructure of the oxygen-sensitive layer provide the sensor fast response. The membranes have high capacity for the oxygen-sensitive coating and for additional recognition species. For example, enzymes and antibodies can be trapped inside the micropores using crosslinking or passive adsorption.