Evidence of the Disassembly of alpha-Cyclodextrin-octylamine Inclusion Compounds Conjugated to Gold Nanoparticles via Thermal and Photothermal Effects

Abstract

Cyclodextrin (CD) molecules form inclusion compounds (ICs), generating dimers that are capable of encapsulating molecules derived from long-chain hydrocarbons. The aim of this study is to evaluate the structural changes experienced by ICs in solution with increasing temperatures. For this, a nuclear magnetic resonance (H-1-NMR) titration was performed to determinate the stoichiometric alpha-cyclodextrin (alpha-CD):octylamine (OA) 2:1 and binding constant (k = 2.16 M-2) of ICs. Solution samples of alpha-CD-OA ICs conjugated with gold nanoparticles (AuNPs) were prepared, and H-1-NMR spectra at different temperatures were recorded. Comparatively, H-1-NMR spectra of the sample irradiated with a laser with tunable wavelengths, with plasmons of conjugated AuNPs, were recorded. In this work, we present evidence of the disassembly of ICs conjugated with AuNPs. Thermal studies demonstrated that, at 114 degrees C, there are reversible rearrangements of the host and guests in the ICs in a solid state. Migration movements of the guest molecules from the CD cavity were monitored via temperature-dependent H-1-NMR, and were verified comparing the chemical shifts of octylamine dissolved in deuterated dimethylsulfoxide (DMSO-d(6)) with the OA molecule included in alpha-CD dissolved in the same solvent. It was observed that, at 117 degrees C, OA exited the alpha-CD cavity. CD IC dimer disassembly was also observed when the sample was irradiated with green laser light.