Sensing properties of buffered and not buffered carbon nanotubes by fibre optic and acoustic sensors

Abstract

Single-walled carbon nanotubes (SWCNTs) are advanced nanostructured materials with promising sensing properties in terms of sensitivity, low sub-ppm limit of detection, on-line and real-time vapour detection, at room temperature. This work is focused on the study of the sensitivity to aromatic volatile organic compounds (VOCs) of standard silica optical fibre (SOF) and quartz crystal microbalance (QCM) sensors incorporating Langmuir–Blodgett multilayers of SWCNTs. Multilayers of SWCNTs with different thicknesses and successfully transferred directly onto the sensors' surface were tested for the detection of toluene and xylene at room temperature and compared with the sensing performances of SWCNT multilayers buffered by a linker multilayer of cadmium arachidate. The optical and acoustic sensors' principle of operation relies respectively on the complex dielectric function and mass change induced by target analyte molecules adsorbed into the sensitive nanomaterials. A time division multiplexing approach for both optical and acoustic chemical sensors has been exploited in order to simultaneously test up to eight SOF and six QCM sensors. The results obtained demonstrate that the sensors based on SWCNTs provide high sensitivity, very low limits of VOC detection and fast response, at room temperature, with a clear dependence of the sensors' sensitivities on the nanomaterial thickness. Furthermore, higher sensitivity was observed in the case of optical fibre sensors exposed to xylene; in addition, behaviour with the opposite sign in the optical response occured between buffered and not buffered SWCNTs overlayers. Also, effects of humidity have been investigated in the case of optical fibre sensors demonstrating a linear dependence of the response at a constant temperature of 28 °C.