Phospholipids are widely used as mimetic systems to exploit interactions involving biological membranes and pharmacological drugs. In this work, the layer-by-layer (LbL) technique was used as a new approach to produce multilayered thin films containing biological phospholipids applied as transducers onto Pt interdigitated electrodes forming sensing units of an electronic tongue system. Low concentrations (nM level) of a phenothiazine compound were detected through impedance spectroscopy. Both negative 1,2-dipalmitoyl-sn-3-glycero-[phosphor-rac-(1-glycerol)] (DPPG) and zwitterionic l-alpha-1,2-dipalmitoyl-sn-3-glycero-phosphatidylcholine (DPPC) phospholipids were used to produce the LbL films, whose molecular architecture was monitored combining spectroscopy and microscopy at micro and nanoscales. The sensor array was complemented by Langmuir-Blodgett (LB) monolayers of DPPG and DPPC deposited onto Pt interdigitated electrodes as well. It was found that the distinct molecular architecture presented by both LbL and LB films plays a key role on the sensitivity of the sensor array with the importance of the LbL films being demonstrated by principal component analysis (PCA).

The possibility of generating distinct film properties from the same material is crucial for a number of applications, which can only be achieved by controlling the molecular architecture. In this paper we demonstrate as a proof-of-principle that ultrathin films produced from iron phthalocyanine (FePc) may be used to detect trace amounts of copper ions in water, where advantage was taken of the cross sensitivity of the sensing units that displayed distinct electrical properties. The ultrathin films were fabricated with three methods, namely physical vapor deposition (PVD), Langmuir-Blodgett (LB), and electrostatic layer-by-layer (LbL) techniques, where for the latter tetrasulfonated phthalocyanine was used (FeTsPc). PVD and LB films were more homogeneous than the LbL films at both microscopic and nanoscopic scales, according to results from microRaman spectroscopy and atomic force microscopy (AFM), respectively. From FTIR spectroscopy data, these more homogeneous films were found to have FePc molecules oriented preferentially, tilted in relation to the substrate surface, while FeTsPc molecules were isotropically distributed in the LbL films. Impedance spectroscopy measurements with films adsorbed onto interdigitated gold electrodes indicated that the electrical response depends on the type of film-forming method and varies with incorporation of copper ions in aqueous solutions. Using principal component analysis (PCA), we were able to exploit the cross sensitivity of the sensing units and detect copper ions (Cu 2+ ) down to 0.2 mg/L, not only in ultrapure water but also in distilled and tap water. This level of sensitivity is sufficient for quality control of water for human consumption, with a fast, low-cost method.