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Environmental monitoring of novel important pollutants in air and water currently is performed with expensive instruments in a laboratory conditions. Currently, there is a high demand for real-time measurements in the field. Biosensors are recognized by both the scientific and industrial communities as the most promising alternative to fill this gap. For the preparation of the sensing layer in a chemical sensor, the Langmuir-Blodgett (LB) method is considered the best alternative for supramolecular architecture. An insoluble monolayer from organic molecules is formed and investigated at the air-water interface (Langmuir film). Subsequently, a transfer layer by layer on a substrate with controlled molecular orientation, density, and phase is performed to enhance sensor sensitivity to the agents of interest. In the past 30 years we have systematically investigated fluorescently labeled phospholipids, mainly Dipalmitoyl Phosphatidyl Ethanolamine head labeled with NitroBenzoxaDiazole (DPPE-NBD). Mimicking the molecules in biological membranes, these molecules are a suitable matrix for insertion of selectively reacting proteins, enzymes, aptamers in the sensing layer of a biosensor. LB layers from pure phospholipids and lipids show promising gas sensitivity to volatile organic compounds (VOCs) with very fast reacting times, excellent reproducibility and complete reversibility on gas removal. Work on using the well-studied Arachidic Acid (AA) for VOC sensing applications is reported here with emphasis on the deposition quality, estimated by measuring the deposited mass from layer to layer. Possibility to detect heavy metal ions (e.g. Cadmium) in water is also demonstrated. Three new effects were discovered by us in Langmuir and LB films from DPPE-NBD which further enhance their use in nano biosensor applications. Fluorescence self-quenching was observed when molecules are in a condensed phase close to each other. Insertion of large bivalent ions in the water subphase increases the distance between the molecules and thus the fluorescence signal was recovered. This effect was used for the detection of the very harmful Cadmium ions in water at very low concentrations. Finally, formation of 3D needle-like structures at thermodynamic equilibrium for single component monolayers was first observed for this molecule. This new effect is especially important for biosensor applications because the surface of the sensing layer is significantly increased while keeping the volume to a minimum. In that way, fast and very sensitive sensors are possible and this is an alternative method of increasing the surface-to-volume ratio in sensing layers. Transduction of signal from the sensing layer to a measurable output in our research was performed with all 3 main methods: optically by measuring fluorescence intensity and kinetics; electrically by electrical impedance spectroscopy; and gravimetrically by using a two-port Rayleigh type Surface Acoustic Wave resonators operating at around 430 MHz. These SAW resonators exhibit 4000 times higher sensitivity compared to standard 10 MHz quartz crystal microbalance (QCM) devices. [ABSTRACT FROM AUTHOR]

Fluorescently head labelled phospholipids form stable organized organic monolayers at the air-water interface (Langmuir films) and can be deposited on solid supports (glass or Surface Plasmon Resonance (SPR)) substrates as Langmuir-Blodgett (LB) films. In a subsequent work LB mono- and multilayers were deposited on Surface Acoustic Wave (SAW) devices. LB monolayers are very stable and hard to remove while the multilayers, typically for phospholipids, are difficult to deposit and can be removed more easily. Several new effects were discovered in Dipalmitoyl Phosphatidyl Ethanolamine head labelled with NitroBenzoxaDiazole (DPPE-NBD) molecule in the past which still need detailed explanation. Previous research of the Langmuir film was combined with the present study of fluorescence spectra and decay kinetics of LB monolayers and a 15 layer SPR substrate, which is a plastic substrate with gold evaporated electrodes. Langmuir-Blodgett films were deposited as monolayers or multilayers on glass or gold structured supports at different surface pressures and from either pure water subphase or with presence of Cadmium ions (Cd2+) in the water subphase. Atomic force microscopy reveals phase coexistence of liquid phase, solid phase and bilayer or higher height hundreds of nanometers in diameter cylinders. This rich polymorphism leads to complicated fluorescence lifetime spectra. Understanding of the structure and aggregation in these films can help with their possible future applications. [ABSTRACT FROM AUTHOR]

Organic monolayers from a fluorescently labeled phospholipid (DPPE-NBD) were deposited on solid supports under special conditions that form stable nanometer wide bilayers cylinders that protrude from the monolayer. This molecule was frequently used in sensor applications due to its sensitivity to environment changes. The proposed configuration should provide both fast response times (ultra thin film) and increased sensitivity (greatly increased surface area). AFM can clearly distinguish between the different phases. The height difference between the solid-expanded and the liquid-expanded phase was measured to be 1.4 nm while the bilayer thickness was 5.6 nm. The solid domains show a 20 % decrease in fluorescence lifetime in comparison to the monolayer as measured by FLIM. This difference in lifetimes is explained in the model of fluorescence self quenching in the solid phase due to the molecules being closer to each other. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]