New effects in Langmuir and Langmuir-Blodgett monolayers from fluorescently labelled phospholipids and their possible use for water quality control

Abstract

Abstract. Secondary water contamination poses significant challenges to the sensitivity and selectivity of sensors used for its detection and monitoring. Currently only lab tests can detect these contaminants and by the time this happens the contaminated water has entered the city water supply system. Fluorescent chromophore NitroBenzoxaDiazole (NBD) is very suitable and had been successfully used in biosensor applications due to its high sensitivity to close proximity polarity of the medium. Over the years we have discovered 3 new effects in NBDlabelled phospholipids which can significantly improve the performance of biosensors. The phospholipid matrix provides flexible biocompatible environment for immobilization of selectively reacting enzymes, microorganisms, DNA, immunoagents, whole cells. Use of single layer (3.1 nm thickness) films at the air-water interface (Langmuir films) or deposited on solid support as Langmuir-Blodgett (LB) film gives fast response times for real time monitoring (no slow diffusion processes) and precise molecule ordering and orientation. The first new effect was fluorescence self-quenching in Langmuir and LB films. In the liquid phase films exhibit normal fluorescence. Upon transition to solid phase fluorescence intensity is almost completely self-quenched and fluorescence lifetimes in the nanosecond region decrease 2 times. This is easily measured. Usually large heavy metal atoms quench fluorescence. We observed the opposite new effect when LB film is deposited in the solid phase from a subphase containing heavy metals. The third new effect is the obtaining of nanosized cylinders with bilayer thickness, which remain stable at least for months, when LB monolayer is deposited in the phase coexistence region at thermodynamic equilibrium. This greatly increases reacting surface and sensitivity of possible sensors. Almost all possible optical experimental methods were used for this research. This includes polarized ATR FTIR and polarized UV-VIS spectroscopies, Fluorescence Lifetime Imaging Microscopy (FLIM), Scanning Near-field Optical Microscopy (SNOM).

First observation of 3D aggregates in a single-component Langmuir film below the equilibrium spreading pressure

“…Initially domains are circular growing from a single nucleation centre. Afterwards at increasing surface pressure smaller domains start to attach to the bigger central domain [10]. This mechanism was observed also in a Langmuir film from DP-NBD-PE at 5 O C [6] where higher surface pressure was needed to overcome the repulsion between the domains and they start to merge.…”

“…The domains are with different diameters, suggesting a second generation of crystallization centres. On a higher resolution inspection with fluorescence microscopy [10] and AFM [6] the domains are initially almost circular but at higher pressure become dendridic. This shape change is due to a balance between the line surface tension which tries to keep domains circular minimizing their perimeter and the repulsive forces between similarly oriented molecular dipoles which tries to separate molecules further apart.…”