Langmuir and Langmuir−Blodgett Film Characterization of a New Amphiphilic Coumarin Derivative

In this paper, we report the synthesis and the monolayer behavior (surface pressure and surface potential) of a new amphiphilic coumarin dye, 7-aminocoumarin-4-acetic acid octadecylamide (ACO), at the air−water interface. The spectroscopic characteristics (UV−vis, fluorescence, and fluorescence imaging) of pure and mixed films (1:20, ACO/stearic acid and ACO/oleic acid) at the air−water interface as well as Langmuir−Blodgett (LB) films have been investigated and compared with the spectroscopic characteristics of ACO in solution. These experiments provide evidence of aggregate formation during compression of the monolayer at the air−water interface. Surface pressure dependent in situ fluorescence imaging confirms that the fluorescent quenching of the dye with increasing surface pressure originates as a result of formation of nonfluorescent aggregates. Atomic force microscopy imaging of a pure ACO LB film shows that the size of these aggregates is in the nanometer scale. This work provides information that ACO forms a stable monolayer and may be utilized as an efficient molecular probe for monolayer studies

Reexamination of the Monolayer Properties of a Fluorescein Amphiphile in Langmuir and Langmuir−Schaefer Films

We herein report a reexamination of Langmuir and Langmuir−Schaefer films of (5-octadecanoylamino)fluorescein (ODFL) prepared from three different spreading solvents:  ethanol (EtOH), CHCl3/EtOH (9:1, v/v), and CHCl3/trifluoroacetic acid (9:1, v/v). It was found that the limiting molecular area of the ODFL monolayer is strongly spreading solvent-dependent, ranging from 15 to 64 Å2molecule-1. These results strongly disgree with reported results from Tsukanova et al. (Langmuir 2000, 16, 1167) of the same molecule by using ethanol as spreading solvent. Our study also shows that this fluorescein amphiphile significantly aggregates at the air−water interface. The UV−vis spectra of the monolayer are redshifted about 20 nm compared to that in the solution. Fluorescence quenching was observed from both a pure ODFL monolayer and a monolayer of ODFL mixed with stearic acid. However, fluorescence emission spectra were readily observed from mixed monolayers of ODFL with three synthetic peptide lipid molecules. This study shows that one should be extremely cautious when mixing fluorescence lipids as probes for Langmuir and Langmuir−Blodgett film investigations

Interaction between organophosphorous hydrolase and paraoxon studied by surface chemistry in situ at air–water interface

Subphase conditions have been optimized to obtain stable organophosphorous hydrolase (OPH-EC 3.1.8.1) as Langmuir films. The Langmuir film was characterized by surface pressure and surface potential-area isotherms and UV-Vis spectroscopy in situ. The interaction of an organophosphorous compound, namely Paraoxon, with the OPH film was investigated for various surface pressures. The stability of the monolayer and the evidence of the enzyme activity at air–water interface support the use of enzyme LB films as biosensor

Detection of paraoxon by immobilized organophosphorus hydrolase in a Langmuir–Blodgett film

Langmuir–Blodgett (LB) film deposition technique was employed for the immobilization of organophosphorus hydrolase (OPH). OPH enzyme was covalently bonded to a fluorescent probe, fluorescein isothiocyanate (FITC), and used as a biological recognition element. Under optimal experimental conditions, OPH monolayers were deposited onto the surface of silanized quartz slides as LB film and utilized as a bioassay for the detection of paraoxon. Two different methods were employed for detection of paraoxon: the fluorescence quenching of the fluorescence probe (FITC) covalently bonded to OPH and the UV–vis absorption spectrum of the paraoxon hydrolysis product. The UV–vis absorption measurement demonstrated a linear relationship between the absorbance at 400 nm and the concentration of paraoxon solutions over the range of 1.0 × 10 −7–1.0 × 10 −5 M (0.27–27 ppm). By observing the FITC fluorescence quenching, the concentration of paraoxon can be detected as low as 10 −9 M (S/N = 3). The research described herein showed that the LB film bioassay had high sensitivity, rapid response time and good reproducibility

The interaction between OPH and paraoxon at the air–water interface studied by AFM and epifluorescence microscopies

The paraoxon hydrolysis reaction catalyzed by organophosphorus hydrolase (OPH) monolayer at the air–water interface was studied. OPH–paraoxon interactions, occurring at the two-dimensional interface, by close-packed, highly orientated OPH monolayer, were investigated by several different surface chemistry techniques; e.g. surface pressure area isotherms, atomic force microscopy (AFM), and in situ epifluorescence microscopy. The characterization of OPH Langmuir and Langmuir–Blodgett films prepared in both the presence and absence of paraoxon, demonstrated significantly distinctive feature when compared with one another. Continuous growth of the OPH aggregates is a distinct phenomenon associated with hydrolysis, in addition to the pH changes in the local environment of the enzyme macromolecules