A selective pool optode for mercury ion sensing has been developed. The selectivity of sensor operation is based on the reagent pool of safranin–iodide solution trapped behind the Nafion membrane, which is highly permeable to cations and excludes all anions electrostatically. Further selectivity towards cations is governed by safranin–iodide reaction, which occurs specifically in the presence of Hg(II) ions as catalyst. In the absence of Hg(II), no reaction occurs between safranin and iodide. However, when trace of Hg(II) ions are present the reaction take place within a few minutes and a colour change from red to colourless occurs, which can be spectroscopically detected via optical fibres. This kind of sensor scheme would be suitable for vast number of irreversible reactions and also shows the possibility of optode development in cases where the immobilisation of reagent is not suitable. Furthermore, for simple measurements the optode can be used as a disposable sensor, since it preparation is simple and inexpensive. Since a membrane is employed in the sensor system where the reagent and sample are separated, the sensor can be applied for in situ measurements, as sample conditioning (e.g. buffer and ionic strength) would not be necessary. Various experimental parameters, such as reagent concentration, pH, temperature and volume have been optimised to obtain high sensitivity and selectivity of measurements. The optode has a dynamic range 0–50 μM with limit of detection of 5 μM toward Hg(II) ions, with only Ag(I) as the main interferent.
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