Surface plasmon resonance ellipsometric transducer for surface biomolecules interaction

From fundamental and technological points of view, there is interest in developing optical transducers exploiting the localized surface plasmon resonance (LSPR) of gold nanoparticles (Au NPs) for the study of interaction of biomolecules as well as of their functionalization mechanism of inorganic surfaces. Spectroscopic ellipsometry monitoring the LSPR change gives the possibility to detect with good resolution and accuracy time-dependent changes of amplitude ?(t) and phase ?(t) [1] during the immobilization of biomolecules at the solid-liquid interfaces. The simultaneous measurements of kinetics of both ellipsometric parameters ?(?,t) and phase ?(?,t) enable to reach advanced sensitivity in a wide range of the binding process, even up to the complete formation of a biomolecular layer [2]. One of the problems encountered in developing Au NPs LSPR sensor is the change of the sensor itself due to mobility of the Au NPs depending on the interaction with the functionalizing molecules. As an example in the case of the most investigated thiols functionalization, depending on the molecular structure of the thiol and Au NPs size, interparticle aggregation may occur resulting in instability of the sensor itself. The aim of this work is to demonstrate the advantage of simultaneous measurements of ?(t) and ?(t) for the evaluation of the interaction of biomolecules with Au NPs inducing their aggregation and temporal instability. Once ellipsometry has been useful to identify a stable Au NPs sensing substrate (in various conditions of solvents, pH, molecules, etc..), parameters characterizing the immobilization kinetics of porphirins and antigen interaction with immobilized antibodies is shown. The ellipsometric characterization is corroborated by morphological analysis with atomic force microscopy (AFM) also operating in electric force mode (Kelvin probe) and by structural analysis ith Raman spectroscopy.