Stereoselective and chiroselective surface plasmon resonance (SPR) analysis of amino acids by molecularly imprinted Au-nanoparticle composites.

Au nanoparticles (NPs) functionalized with thioaniline and cysteine are used to assemble bis-aniline-bridged Au-NP composites on Au surfaces using an electropolymerization process. During the polymerization of the functionalized Au NPs in the presence of different amino acids, for example, L-glutamic acid, L-aspartic acid, L-histidine, and L-phenylalanine, zwitterionic interactions between the amino acids and the cysteine units linked to the particles lead to the formation of molecularly imprinted sites in the electropolymerized Au-NP composites. Following the elimination of the template amino acid molecules, the electropolymerized matrices reveal selective recognition and binding capabilities toward the imprinted amino acid. Furthermore, by imprinting of L-glutamic or D-glutamic acids, chiroselective imprinted sites are generated in the Au-NP composites. The binding of amino acids to the imprinted recognition sites was followed by surface plasmon resonance spectroscopy. The refractive index changes occurring upon the binding of the amino acids to the imprinted sites are amplified by the coupling between the localized plasmon associated with the Au NPs and the surface plasmon wave.