Sulphate sensing in self-assembled monolayers by surface infrared and Raman spectroscopy techniques

In this paper we demonstrate the applicability of surface vibrational spectroscopy techniques in sensing sulphate anions, using self-assembled monolayers of hydrogen bonding receptors. Three uncharged, diamidocarbazole receptors were synthesised and attached to gold and silver surfaces via one or two disulphide arms. Their ability to bind anions on the metal surfaces was investigated with polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and surface enhanced Raman scattering (SERS) spectroscopy. Contrary to other techniques applied in optical sensors, such as UV–vis absorption, fluorescence or surface plasmon resonance, surface vibrational spectroscopy techniques used in this work not only display spectral changes proportional to the concentration of sulphate, but also provide an insight into the state of the receptor monolayer and its interactions with the analyte. While SERS measurements confirm the formation of anion-receptor complexes on metal surfaces, PM-IRRAS spectra were further employed to quantify the optical response of the sensor. The system can detect SO42− anions within a concentration range of 10-6 – 10-1 M (in acetonitrile). An effect of the hydrogen bonds formed at the binding site of the diamidocarbazole molecules on the symmetry of sulphate anions is observed, which ultimately proves the anion-receptor binding.