Hybrid functionalised mesoporous silica–polymer composites for enhanced analyte monitoring using optical sensors

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Abstract
We report herein a new optical sensor for the colorimetric monitoring of aqueous amines designed by anchoring a suitable reactand on a hybrid material that was further incorporated on a polymeric matrix. An azo chromophore containing a trifluoroacetophenone moiety was used as signalling reporter. In this system colour modulation is a consequence of the reversible reaction of the amine with the trifluoroacetyl group that results in the formation of a hemiaminal. Based on this approach but at the same time trying to enhance selectivity the chromoreactand was anchored in a first step onto suitable mesoporous nanoscopic hydrophobic pockets. As a mesoporous system we selected a UVM-7 solid (a MCM41-type material) containing a homogeneous distribution of pores of about 3 nm and a specific surface area of over 1000 m2/g. This material was chemically modified to yield a trifluoroacetylazobenzene indicator dye within the pores, resulting in the hybrid material S3. It was found that whereas the analogous trifluoroacetylazobenzene dye in solution gives an unspecific response, the hybrid material S3 shows enhanced features in terms of selectivity, only reacting with the not-too-large but lipophilic enough aminesn-octylamine and n-decylamine. The material S3 reacts with neither small hydrophilic amines (due to the hydrophobicity of the inner pores in S3) nor very large aliphatic amines (due to the size of the pore). In the second part of this work a composite material (P–S3) was prepared by mixing hydrophilic polyurethane polymer and S3. The X-ray diffraction patterns of P–S3 displays the characteristic intense peak of the MCM-41 type mesoporous material and TEM images show a homogeneous dispersion of the nanometric mesoporous particles into the polymeric matrix. The measurements were carried out in a flow-through module connected to an optical fibre via a Y-probe at pH 9.5 (borax buffer). The P–S3 composite retains the favourable features in terms of selectivity shown by S3 and only displays a clear change in colour for n-octylamine or n-decylamine, but no response to the remaining linear primary amines. The response time of the sensor P–S3 to solutions of these amines was in the range of a few seconds, the regeneration time was between 5 to 10 min and the detection limit was 6 × 10−4 mol L−1. The sensor response was reproducible and reversible.

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