A colorimetric iron sensor based on the partition of phenanthroline complexes into polymeric hydrogels. Combinatorial synthesis and high throughput screening of the hydrogel matrix

- A combinatorial library of 21 polymeric hydrogels is synthesized.
- A colorimetric high throughput screening allows to find out the materials which absorb the target complex: tris(phenanthroline)iron(II).
- A colorimetric sensor for iron(II) is built by loading bathophenathroline inside a poly(acrylamide-co-(-acrylamido-2-methylpropanesulfonic acid)) hydrogel.
- The sensor shows a detection limit of 0.1 ppm (colorimetric) and 0.01 ppm (spectrophotometric) for iron (II).
- The sensor is applied in the direct detection of iron content in milk, where direct colorimetric detection in solution is precluded due to the opacity of the sample.

A combinatorial library of hydrogels is produced by radical polymerization and tested for retention of iron (II) complexes, using a simple colorimetric high throughput screening (HTS). Six acrylamide monomers are used to produce a library of 21 cross-linked hydrogel matrixes. The swelling of all hydrogels is determined and related with the structure of each monomer unit present. The HTS shows that some materials are able to retain the tris(phenanthroline)iron(II) (Fe(Phen)32+) complex. The partition coefficient of Fe(H2O)62+ and Fe(Phen)32+ in all hydrogels is determined by UV-vis spectrophotometry and related with the presence of anionic (e.g. −SO3−), hydrophobic (e.g. isopropyl) and hydrophilic (e.g. −NH2 of acrylamide) groups. Hydrogels with sulfonic (−SO3−) groups show large values of partition coefficients of Fe2+ into the hydrogel (>500). Based on the data and HTS, poly(acrylamide-co-50%(2-acrylamido-2-methylpropansulfonic acid)) (PAAm-co-50%AMPS) is selected to build a colorimetric sensor. Since the phenanthroline could be released from the hydrogel, bathophenanthroline (BPhen) which is not released is used as ligand. The hydrogel is loaded with BPhen and exposed to Fe2+ in water. The visual detection of color gives a detection limit of 0.1 ppm. Little interference of different common ions is found. A special set-up is made which allows spectrophotometric measurement of the complex inside the gel. The detection limit is found to be of ca. 0.01 ppm by spectrophotometry. Using the sensor, the iron content in milk is measured directly demonstrating the use of the sensor to measure free iron in opaque liquids where solution spectrophotometry is not effective without complex sample pretreatments. The non-covalent retention of ligands and complexes inside polymeric hydrogels seems to be a suitable method for building specific ion sensors.

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