Selective detection of organophosphate through molecularly imprinted GERS-active hybrid organic–inorganic materials

A selective sensing platform for the organophosphate paraoxon, a highly toxic organic pollutant, has been designed and tested on water samples. A porous hybrid organic–inorganic film, synthesised using tetraethoxysilane, 1,8-bis(triethoxysilyl)octane and cetyltrimethylammonium bromide, has been molecularly imprinted with a structural analogue of paraoxon, the diethyl(4-nitrobenzyl)phosphonate, to induce selective recognition. Exfoliated graphene has been incorporated into the porous matrix to provide enhancement of the Raman scattering signal. The Raman sensor has been tested on different concentrations of paraoxon in both ethanol and water/ethanol mixture. The molecular selectivity has been assessed by comparing the Raman signal enhancement of paraoxon with a similar organophosphate, the bis-(4-nitrophenyl) phosphate. The molecularly imprinted film has shown a fourfold increase of the paraoxon signal, when compared with the corresponding not-imprinted. The evaluation of the density of molecular cavities into the molecularly imprinted samples (4.50 * 10−10 μmol μm−3) has allowed assuming that each molecular cavity is capable of providing a remarkable signal enhancement of 1.47 * 1012 count * μmol−1 only when recognising paraoxon. The material design has allowed coupling the sensitivity of the graphene-mediated enhancement of Raman scattering with the selectivity of molecular imprinting into a single and potentially portable, analytical system.