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Ammonium sensing with plastic optical fiber modified by molecular imprinting

Sequeira, F. ; Rudnitskaya, Rudnitskaya, A. ; S. R. Gomes, Gomes, M. T. S. R. ; Nogueira, R.N. ; Bilro, L.

Ammonium sensing with plastic optical fiber modified by molecular imprinting, Proc EUROPT(R)ODE XIII, Graz, Austria, Vol. 1, pp. 184 - 184, March, 2016.

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We report the development of a plastic optical fiber (POF) sensor, modified by molecular imprinting, for ammonium detection in water.
Monitoring of surface water status is of fundamental importance, being turbidity, pH and ammonia important water quality indicators. Nowadays, the monitoring of relevant parameters is constrained by the availability and cost of commercial sensors. Despite the innumerous works reported in the literature almost all target the presence of ammonia in gaseous state and consequently they do not detect ammonium ion in aqueous medium.
The selected fibers have 1 mm diameter and a PMMA (polymethyl methacrylate) core. The cladding of the fiber was removed, after which a molecular imprinted polymer (MIP) was grafted by radical polymerization with thermal initiation, to act as a selective sensing layer.
For the polymerization, 2,2’-Azobis(2-methylpropionamidine)dihydrochloride (AAPH) was used as initiator, methacrylic acid (MAA) as a monomer, ethylene glycol dimethacrylate (EDMA) as a cross-linker, ammonium chloride (NH4Cl) as a template and 30% of ethanol in water was the solvent.
The optical system consisted on an intensity based scheme using an optical coupler (90:10), a LED (660 nm) and two photodetectors. The signal output was monitored in real time with LabView software. The signal was self-referenced, avoiding source fluctuations and variations due to external factors, namely temperature.
The response of the developed sensor to different concentrations of ammonium solutions in water was registered. The response was very fast and reversible. The tests were repeated with solutions of D-(+)-Glucose with the same refractive index of the tested samples.
Further developments will be focused on optimization of MIP grafting procedure and sensor performance, in order to increase sensitivity and selectivity.