Amino β-cyclodextrins immobilized on gold surfaces: Effect of substituents on host-guest interactions

In the present paper we describe a simple method to immobilize amino cyclodextrins (CDs) and methylated-amino cyclodextrins on gold surfaces. We also report on the effect that the presence of methyl groups in the broader rim of the cyclodextrin causes on the interaction with the guest molecule bentazon. By means of electrochemical measurements, X-ray photoelectron spectroscopy and contact angle experiments we have demonstrated that the CDs attach covalently to the gold surfaces by amide bond formation and that the CD cavity is oriented opposite to the gold surface. We have shown that methylated-CD/Au modified surfaces are more sensitive towards the recognition of the herbicide bentazon than the non-methylated variants. The association constants for the corresponding interactions of the immobilized CD with the guest molecule have been determined from surface plasmon resonance experiments. The magnitudes of these constants (30.8 ± 1.0 M⁻¹ and 80.5 ± 4.2 M⁻¹ for amino-CD and methylated-amino cyclodextrins, respectively) are consistent with the change of hydrophobicity caused by methyl groups. The results demonstrate the feasibility of using CD-gold modified surfaces to encapsulate herbicides such as bentazon within the macrocyclic receptor without necessity of carrying out the experiments in solution.