Optical methods are widely used to study thin films. Among them is the optical waveguide lightmode spectroscopy (OWLS) that we were one of the first to apply to polyelectrolyte multilayer films. These new kinds of films are now widely employed in a large range of applications. In our lab, we mainly focus on the biomedical applications of such films and in particular, in the coating of biomaterial surfaces to render the films bioactive and biofunctionalized.

“The article describes a new in situ methodology and the corresponding theoretical analysis of the raw data, useful for those who wish to measure film thickness up to about 400 nm.”

We also investigated protein adsorption onto these films. So, we had to develop the exact model for OWLS data analysis for a monolayer as well as for a double layer composed, for example, of a first polyelectrolyte multilayer layer and of a second protein layer. Using these appropriate models, we found a good correlation between OWLS data and scanning angle reflectometry data. I assume that the article has drawn a great deal of attention because it provides a direct comparison of polyelectrolyte multilayer (PEM) films by two optical techniques and because it presents an exact model allowing us to treat OWLS data for films of thickness up to several hundreds of nanometers.

I became involved in this research when I started to work on the polyelectrolyte multilayer films at the INSERM U595, whose Director was Jean-Claude Voegel. The first step in our project was to validate the OWLS apparatus for the study of PEM films. I, along with my colleagues Frederic Cuisinier, Csilla Gergely, Guy Ladam, Pierre Schaaf, Jean-Claude Voegel, and Bernard Senger were deeply involved in this project. Thus, we decided to use the already well-characterized PSS/PAH system as a reference, because its thickness in different conditions was already evaluated by other techniques such as scanning-angle reflectometry. We then had to fully solve the optical phase shift equations which, so far, had only been solved using a thin film approximation (d_Α<< λ, where d_Α is the thickness of the film and λ is the wavelength of the laser light). This method is now extremely useful for everybody in our lab and every other OWLS user who wishes to investigate, for example, polyelectrolyte multilayers buildup.

The article describes a new in situ methodology and the corresponding theoretical analysis of the raw data, useful for those who wish to measure film thickness up to about 400 nm.

An important aspect is that the article shows that thickness determination by OWLS gives similar results to that obtained using scanning-angle reflectometry data. Also, one of the great advantages of OWLS, as compared with many other techniques, is that it requires only 100 µL of solution per injection. Such a small volume renders the technique attractive for investigating the adsorption of proteins, peptides, or other expensive molecules. Furthermore, different types of waveguides with different coatings are now available and sold by the manufacturer. Also, all the researchers working in the growing field of polyelectrolyte multilayers could benefit from the OWLS apparatus and from the new model developed.

Dr. Catherine PICART
Maître de Conférence à l'ECPM
(Ecole Européenne de Chimie, Polymères, Matériaux)
INSERM U595
Faculté de Médecine
Strasbourg, France

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