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We studied the interactions of tetracycline (TC) antibiotic molecules with phospholipid monolayers with the two-fold aim of elucidating the mechanism of action and providing a first step for the realization of bio-mimetic sensors for such drugs by means of the Langmuir–Blodgett technique.We examined spreading monolayers of three phospholipids in the presence of tetracycline in the subphase by means of surface pressure–area and surface potential–area isotherms as a function of bulk pH.We selected phospholipids with hydrophobic chains of the same length but polar head groups differing either in dimensions and protonation equilibria, i.e. dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylethanolamine (DPPE) and dipalmitoylphosphatidic acid (DPPA).The interaction of tetracycline with the three phospholipids was found to be highly dependent on the electric charge of the antibiotic and on the ionization state of the lipid. Significant interactions are established between the negatively charged form of dipalmitoylphosphatidic acid and the zwitterionic form of tetracycline. The drug was found to migrate at the interface where it is adsorbed underneath or/and among the head groups, depending on the surface pressure of the film, whereas penetration through the hydrophobic layer was excluded for all the three phospholipids. [Copyright &y& Elsevier]

This paper is part of a systematic study of the interactions of tetracycline antibiotics with phospholipid monolayers at the water–air interface. Tetracyclines are widespread antibiotics that undergo a series of protonation equilibria in solution, depending on the pH. The surface activity of tetracyclines was determined by means of surface tension measurements for three different systems, i.e. water, TRIS and McIlvaine-EDTA buffer. Surface pressure–molecular area and surface potential–molecular area isotherms were acquired for dipalmitoylphosphatidic acid monolayers on TRIS buffer (pH=7.0) and McIlvaine-EDTA buffer (pH=4.0) solution as a function of tetracycline concentration in the subphase. Comparative analysis of surface potential data, with the molecular dipole moment of tetracycline obtained from semiempirical calculations, provided information on the orientation of tetracycline at the interface. Surface pressure measurements as a function of monolayer compression were described, applying either a continuous partition model or Langmuir adsorption isotherms. The results obtained in the case of buffer solutions were compared to those obtained for tetracycline in water subphases. The analysis of the results indicated that electrostatic interactions dictate the migration of tetracycline to the monolayer interface. Penetration of the molecule to the lipophilic portion of the monolayer was unlikely, especially at high surface pressures.The results showed that stronger interactions are established between the zwitterionic tetracycline and the deprotonated phosphatidic group in TRIS buffer solution; in this case, tetracycline binds at the monolayer interface following a Langmuir type adsorption. In the case of water, where the monodeprotonated acid and the tetracycline zwitterions are the only species involved, the data can be described by continuous partition of tetracycline between interfacial and bulk phases. The same holds for McIlvaine-EDTA buffer subphases, although the high concentrations of citrate ions in this buffer competitively interfere with tetracycline association at the monolayer interface. [Copyright &y& Elsevier]

We studied the interactions of tetracycline antibiotics, TCs, with phospholipid monolayers with the two-fold aim of elucidating the mechanism of action of TCs and to provide a first step for the realization of bio-mimetic sensor for such drugs by means of the Langmuir–Blodgett technique. Preliminary surface tension studies demonstrated that surface activity of tetracycline is moderate and dependent on the pH of the subphase. We selected three phospholipids having hydrophobic chains of the same length but differing in the polar head structures, i.e. dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylethanolamine, and dipalmitoylphosphatidic acid. Surface pressure- and surface potential- area isotherms were employed to investigate the behavior of the phospholipid monolayers at the water–air interface when tetracycline was added to the aqueous subphase. Analysis of the results indicated that the electrostatic interaction is the driving force for migration of tetracycline towards the interface where localized adsorption to the head groups occurs. Nevertheless, such interactions appear to be insufficient to promote penetration of tetracycline through the hydrophobic layer. [Copyright &y& Elsevier]

We immobilized a hemicyanine dye, Di-8-ANEPPS, in ordered thin films of an organic matrix, dihexadecyl phosphate (DHP), and we transferred the mixed monolayers onto solid support by the Langmuir-Blodgett technique. We used gold and quartz slides and indium tin oxide (ITO) evaporated on glass slides as substrates. The multilayers formation was confirmed by ellipsometric and contact angle measurements. The optical response of the nanostructures was investigated collecting UV–Vis absorption and fluorescence emission intensity profiles. [Copyright &y& Elsevier]

Redox mediators were immobilized in ordered thin films of an organic matrix. We preliminarily prepared and characterized Langmuir–Blodgett (LB) films of dipalmitoyilphosphatidic acid (DPPA), Self-Assembled (SA) monolayer of octadecylthiol (ODM) and Hybrid Films (HF) combining LB and SA technologies.Several redox mediators were tentatively co-spread with DPPA molecules at the water–air interface and surface pressure-area (π–A) isotherms were recorded. On the basis of its evident tendency to remain incorporated into the monolayer, we select 3-3′,5-5′-tetramethylbenzidine (TMB) and we prepared LB and HF films containing TMB on different solid supports. The resulting structures were characterized by means of ellipsometry and contact angle measurements as a function of mediator content.Electrochemical characterization was performed on the same systems, recording cyclic voltammograms as a function of the number of layers constituting the film at two different transfer surface pressures (πtr).LB films containing TMB were also characterized by means of UV–VIS spectroscopy, as a function of πtr and of the number of layers.The two sets of independent results evidenced that TMB is quantitatively immobilized in LB nanostructures of DPPA maintaining its electroactive behaviour. [Copyright &y& Elsevier]