Sensitive detection of a pseudo-polyrotaxane ultrathin film by SPR and QCM-D methods

Surface plasmon resonance spectroscopy (SPR) and quartz crystal microbalance with dissipation (QCM-D) are powerful techniques to analyze structure of a thin film in sub-nanometer thickness and 10 ng cm−2 mass per area level, respectively. In this study, the pseudo-polyrotaxane in the film state was detected by both methods. The pseudo-polyrotaxane ultrathin film was composed of α-CD molecules and a self-assembled-monolayer (SAM) in which the SAM was fabricated with chains consisting of alkane thiol, a PEG chain, and an azobenzene derivative (MeAzoPEGn; PEG chain length n = 6-, 10-, and 16-mer). Though polyethylene glycol (PEG) is known to form a polyrotaxane with α-cyclodextrin (α-CD) in aqueous solution and the structure of polyrotaxane was investigated in detail, the pseudo-polyrotaxane in the film state was not studied well. SPR measurement in situ indicated resulting increases in layer thickness for a longer PEG chain in a state of equilibrium between the α-CD and the SAM. The results detected by QCM-D, showed that the film mass and rigidity increased with PEG chain length. They also provide evidence for pseudo-polyrotaxane formation. To evaluate it quantitatively, titration of the MeAzoPEGn SAMs with α-CD was measured by SPR. The increase in film thickness was found to depend on the mth order of the α-CD concentration in equilibrium state. SPR and QCM-D were found to be useful to detect the pseudo-polyrotaxane in the film state.