Molecular Recognition by Self-Assembled Monolayers of Cyclodextrin on Ag

Abstract

Self-assembled monolayer (SAM) of a thiolated cyclodextrin (CD) derivative was prepared, and stereoselective inclusional complexation of optically active azo dyes (p- or o-methyl red conjugates with chiral 1-phenylethylamine, p- or o-MR−PEA) with both free CD and the SAM of the CD derivative on silver surface was investigated by using resonance Raman (RR) and surface-enhanced resonance Raman spectroscopy (SERRS), respectively. Characteristic molecular vibrations of functional groups of MR−PEA were used to describe the complexation. To our knowledge, this is the first report which compares the complexations of free and surface-confined CD by Raman spectroscopy. Association constants for the complexation (K) were determined from the relative intensity of Raman scattering of the dye complexed with CD. The K values for the complexation between MR−PEA and free CD determined by Raman spectroscopy were quite similar to those measured by UV−visible spectroscopy, which confirmed the validity of Raman spectroscopy for the analysis of the inclusion phenomena. Though the K values of the SAM system were much larger than that of the free system, similar stereoselectivity was observed in both of the systems. The R enantiomer of o-MR−PEA was more preferentially included both by free and surface-confined CD than the S-enantiomer, whereas a slight selectivity was observed for p-MR−PEA. The importance of the position of optically active carbon atom with respect to the azobenzene moiety, which penetrated into the cavity of CD, was suggested. Our findings show that the enantioselectivity of CD is preserved after immobilization onto solid surface.